de thèse THÈSE (UNISTRA) par Chang-Bo HUANG V contrainte, le matériau actif est soit comprimé (sous contrainte de compression) soit étiré (sous contrainte de traction), remodelant ainsi les ligands organiques en raison de leur nature flexible.

La distance interparticulaire, qui peut être considérée comme la barrière de potentiel de l'onde électronique, sera respectivement diminuée/augmentée sous la contrainte de compression/traction, augmentant/diminuant ainsi le courant tunnel de façon exponentielle. D'après ces résultats, le capteur de déformation a démontré un facteur de jauge élevéet une réponse très fine à la contrainte de traction, à la contrainte de compression, au mouvement mixte et même aux vibrations. D'intenses efforts ont été consacrés à la conception de la structure de l'appareil pour réaliser une détection du signal sans fil. Au cours des dernières années, les antennes d'identification par radiofréquence (RFID) ont fait l'objet de nombreuses recherches dans le domaine de l'Internet des objets et des systèmes cyber-physiques (CPS) en raison de leurs propriétés intéressantes telles que la passivité de l'objet, fonctionnant sans fil, simple et compacte, et de nature multimodale.[12, 16] Dans ce projet, la méthode de transmission de données sans fil, c'est-à-dire une puce passive RFID, a été utilisée pour construire un système de détection de contrainte sans fil avec des réseaux AuNPs-TEG comme matériau de détection actif. Le système est composé de deux circuits, qui sont respectivement le circuit à puce et le circuit de mesure. Grâce à l'inductance mutuelle, ces circuits peuvent établir une communication et échanger des informations. Le circuit de mesure est un circuit d'initiative avec une entrée d'alimentation alternative, alors que le circuit à puce est un circuit passif qui répond par inductance mutuelle. Le capteur de contrainte a été intégré dans le circuit à puce en tant que résistance, dans laquelle la résistance changera avec la contrainte, affectant ainsi le courant dans le circuit de mesure. La fabrication et la mesure du capteur de déformation ont été réalisées, alors que la détection sans fil est toujours en cours de test. Le capteur de pression hybride de type résistif à base de graphène est décrit au chapitre 7. Un nouveau type de capteur de pression dont la sensibilité peut être réglée avec la lumière UV a également été conçu dans ce projet. Le principe consiste à greffer un polymère photosensible sur des feuillets de graphène, dont la rigidité augmentera avec la lumière UV due à la polymérisation de celui-ci. Le courant tunnel passant à travers les feuillets de graphène est Résumé de thèse THÈSE (UNISTRA) par Chang-Bo HUANG VI directement régi par la distance entre les couches de graphène. Cette dernière est ainsi contrôlée par la pression appliquée au dispositif. En irradiant le dispositif à l'aide d'une lumière UV avec une puissance ou pendant un temps différent, le composite présentera une compressibilité différente. De ce fait, la sensibilité du dispositif sera modulée via cette méthode. Dans ce travail, la résine photosensible négative SU-8 2002 a été choisie pour former un matériau composite avec du graphène. La résine photosensible négative est largement utilisée en photolithographie et sa rigidité augmente en raison de la polymérisation déclenchée par les UV. Le SU-8 2002 est un photorésiste à base d'époxy à contraste élevé, conçu pour le micro-usinage et pour d'autres de pression/déformation très sensibles. Les caractéristiques supplémentaires de l'appareil telles que la faible consommation énergétique (tension de fonctionnement de 0,2 V), le processus de fabrication à grande échelle, les matières premières disponibles commercialement, le faible coût de production et, plus important encore, la détection sans fil, en font un candidat attrayant pour les applications technologiques portatives contrôlant la santé, pour des dispositif de surveillance, dans la robotique de détection multi-mouvement et pour l'IoT.

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Résumé de thèse THÈSE (UNISTRA) par Chang-Bo HUANG I

Résumé de thèse

L'Internet des objets (IoT), sans aucun doute considéré parmi les technologies les plus importantes et les plus prometteuses aujourd'hui, a été estimé comme étant un énorme marché pour les prochaines décennies. Cela comprend des milliards d'appareils connectés tels que les smartphones, les PC, les appareils portables, les tablettes et autres appareils synergiques. [1-4] Que ce soit pour l'utilisation d'infrastructures urbaines, d'usines, de soins personnels ou dans d'autres scénarios d'application, les capteurs (sous forme de matrice) sont connectés directement ou indirectement aux réseaux IoT servant d'interface frontale pour collecter les données de l'environnement telles que la température, l'humidité, la pression, le mouvement, l'accélération, la présence de gaz, des image, etc. [2, 4] Par conséquent, la conception de capteurs possédant des propriétés intéressantes est d'une importance capitale pour le développement de l'IoT.

Les capteurs ont été traditionnellement définis comme des appareils capables de convertir des variables physiques en signaux électriques ou de modifier les propriétés électriques de matériaux actifs. Pour répondre aux exigences des technologies modernes telles que l'IoT, diverses propriétés doivent être présentes, telles que l'habilité à traiter des informations complexes, le faible coût de production, la transmission sans fil, la flexibilité, la faible puissance de fonctionnement, la robustesse, l'auto-étalonnage, etc. [5] Résumé de thèse THÈSE (UNISTRA) par Chang-Bo HUANG II Cette thèse se concentre sur la conception et la fabrication de capteurs de pression et de déformation en utilisant des nanomatériaux fonctionnels préprogrammés, appliqués à la surveillance via des appareils portatif analysant la santé. Ainsi, du graphène et des nanoparticules d'or (AuNPs) ont été utilisés comme nanomatériaux fonctionnels pour fabriquer des capteurs de pression et des capteurs de contraintes mécaniques, respectivement. La puissance électrique (résistance électrique en général) a été collectée lors de pression/déformation. En raison de la compressibilité/extensibilité limitée du matériau luimême, un ressort moléculaire a été introduit dans les matériaux actifs pour la détection, via une fonctionnalisation chimique pour améliorer la sensibilité de ces capteurs de pression/déformation.

L'utilisation de ressorts moléculaires intégrés dans les matériaux de détection est une nouveauté fondamentale dans ce travail, il est donc nécessaire d'introduire cette approche sous la forme d'un chapitre indépendant (chapitre 2). Les ressorts (supra)moléculaires sont des objets nanométriques artificiels possédant des structures bien définies et des propriétés physicochimiques modulables. Comme les ressorts macroscopiques, les ressorts supramoléculaires sont capables de changer leur conformation à l'échelle nanométrique en réponse à des stimuli externes (lumière, ions, chaleur, pH, solvant, redox, etc.), induisant alors des mouvements mécaniques de type ressort. [6-9] Une telle action dynamique offre des opportunités intéressantes pour l'ingénierie des nanomachines moléculaires, en traduisant les mouvements nanoscopiques sensibles aux stimuli en un travail macroscopique. [10-11] Ces objets nanoscopiques sont des architectures multifonctionnelles dynamiques réversibles qui peuvent exprimer une variété de nouvelles propriétés et se comporter comme des systèmes nanoscopiques adaptatifs. Dans ce chapitre, la conception et la relation structure/fonction des ressorts supramoléculaires ont été discutées. De plus, leur (auto)assemblage en tant que prérequis pour la génération de nouveaux matériaux dynamiques comportant des mouvements contrôlés, facilement intégrables dans des dispositifs macroscopiques pour des applications en détection, en robotique et IoT, a également été démontré.

Résumé de thèse THÈSE (UNISTRA) par Chang-Bo HUANG III Dans le chapitre 3, une introduction sur les capteurs de pression et de contrainte a été discutée, comprenant les aspects fondamentaux (tels que la définition, les mécanismes de détection, les paramètres clés, etc.) et les matériaux actifs pour la détection (par exemple les nanoparticules d'or, les nanotubes de carbone, le graphène, etc.). Le capteur de pression et le capteur de contrainte ont été des sujets de recherche extrêmement attrayants ces dernières années du fait de leurs vastes applications dans des domaines tels que la surveillance de la santé humaine via un appareil portatif, la peau électronique (e-skin), l'interface homme-machine, la robotique, l'automatisation, etc. [1, 5, 12-15] Ce chapitre offre au lecteur le contexte de recherche de cette thèse et décrit la conception de la recherche développée dans ce travail de thèse, ainsi que l'importance du développement d'un capteur de pression/déformation basé sur ces nouveaux matériaux hybrides.

Dans le chapitre 4, nous avons discuté des méthodes et techniques expérimentales utilisées dans la réalisation de ce travail de thèse, telles que la synthèse de matériaux actifs, les méthodes de fabrication de dispositifs, ainsi que diverses techniques de caractérisation. Une méthode de synthèse efficace (par exemple via un processus simple, avec un rendement élevé, un faible coût des matières premières, etc.) et propre (faible toxicité, respectueuse de l'environnement, traitement facile des déchets, etc.) des matières actives est la condition préalable à la production de masse pour des applications futures. Dans ce chapitre, nous avons discuté de la synthèse efficace des matériaux de détection et de la fabrication du dispositif plus élaboré, afin de le rendre plus adapté à la fabrication à grande échelle. De plus, une introduction complète de diverses méthodes de caractérisation comprenant les mesures électriques, la configuration du dynamomètre, les caractérisations spectroscopiques (la spectroscopie Raman, la spectroscopie photonique aux rayons X, la diffraction des rayons X sur poudre) et les caractérisations de surface (la microscopie à force atomique et la microscope électronique à balayage) a été fourni pour acquérir une connaissance précise et intrinsèque de la structure et de la composition chimique des matériaux et des dispositifs.

Résumé de thèse THÈSE (UNISTRA) par Chang-Bo HUANG IV

Les capteurs de pression représentent une catégorie d'appareils qui peuvent donner des informations de pression via un signal électrique. Sur la base des mécanismes de fonctionnement, ils peuvent être divisés en capteur de pression piézorésistif, capteur de pression piézoélectrique, capteur de pression capacitif, etc. Le capteur de pression piézorésistif, dont la résistance change avec la pression, a été largement rapporté dans la littérature, en raison de sa haute sensibilité, de sa réponse rapide, et de sa facilité de fabrication. Dans le chapitre 5, nous avons discuté d'un type radicalement nouveau de capteur de pression piézorésistif basé sur une architecture de type millefeuille, composé de couches d'oxyde de graphène réduit (rGO) intercalé par des piliers moléculaires attachés de manière covalente (R1: aminotriéthylène glycol (TEG), R2: 1-octylamine et R3: 4-aminobiphényle) possédant des propriétés mécaniques modulables. En appliquant une petite pression sur la structure multicouche, le courant tunnel régissant le transport de charge entre les feuillets de rGO successifs entraîne une diminution colossale de la résistance électrique du matériau. De manière significative, la rigidité intrinsèque des piliers moléculaires utilisés permet un réglage fin de la sensibilité du capteur, atteignant des sensibilités aussi élevées que 0,82 kPa -1 dans une région de basse pression (0-0,6 kPa), avec des temps de réponse courts (≈ 24 ms) et une limite de détection faible (7 Pa). Nos capteurs de pression permettent une surveillance efficace du rythme cardiaque et peuvent être facilement transposés à une matrice capable de fournir une carte 3D de la pression exercée par différents objets.

Le capteur de déformation basé sur le réseau AuNPs-TEG a été discuté dans le chapitre 6. En nous inspirant des capteurs de pression modulant le courant d'effet tunnel décrit dans le chapitre 5, nous avons conçu un nouveau modèle de détecteur qui utilise des nanoparticules d'or d'une dizaine de nanomètres comme unités de transmission d'électrons, qui est donc le matériau actif du capteur de déformation. Les nanoparticules d'or sont interconnectées par des molécules de dithioltétra(éthylène glycol) formant des réseaux 3D déposés directement sur des électrodes interdigitées flexibles (substrat en polyimide). Sans contrainte appliquée, les AuNP sont séparées par les ligands organiques isolés qui définissent un transfert de charge efficace entre les nanoparticules d'or dans une certaine gamme de distance. Lors de l'application d'une for example consists of multiple built-in sensors including accelerometer, magnetometer, barometric pressure sensor, ambient temperature sensor, heart rate monitor, oximetry sensor, etc. 1,2 These sensors can refactor a virtual environment based on the collected ambient parameters and monitor the health conditions of the subject. After the overall analysis of the subject, the smart watch can provide personalized instructions such as reminding a break during sedentary work, alert of bedtime, making a suggestion of medical examinations, etc. to promote a healthier lifestyle.

Electronic sensing devices are among the key components of the smart wearable electronics, in which pressure and strain sensors are one of the most popular research directions in recent years for their wide applications in motion detection, health monitoring, robotics, humanmachine interaction, electronic skin, etc. [3][4][5][6] In addition to basic pressure/strain sensing properties such as high sensitivity, fast response, good robustness, etc., unique characteristics including mechanical durability, stretchability, and biodegradability have been imparted to novel sensing devices to improve the device-human body integration through the development of new materials and fabrication strategies. [7][8][9][10] A very important space-interval engineering concept, supramolecular springs, has been introduced (Chapter 2) before the general overview of pressure and strain sensors (Chapter 3).

This concept has been employed in the design principle of pressure and strain sensors (Chapter 5, 6, and 7) discussed in this thesis. Supramolecular springs are artificial nanoscale objects possessing well-defined structures and tunable physico-chemical properties. Like a macroscopic spring, supramolecular springs are capable of switching their nanoscale conformation as a response to external stimuli such as light, humidity, pH, pressure, etc. by undergoing mechanical spring-like motions. In this thesis, oligomer-based molecular springs have been used to fabricate pressure and strain sensors in which the dynamic motion of molecular springs triggered by external pressure or strain can be monitored by the tunneling current change under bias voltage.

Overall, this Thesis is dedicated to the design and fabrication of flexible pressure and strain sensors featuring superior sensing properties by using low-dimensional hybrid nanomaterials as active components, such as graphene and gold nanoparticles based networks. These novel sensing devices holding superior sensing properties offer unique opportunities for the emerging classes of smart wearable electronics.

Structure of the thesis

This thesis is given in 8 chapters as follows:

Chapter 1: Brief introduction of the Thesis has been discussed including the inspiration of the projects, the design principle of pressure/strain sensor, and the outline of the Thesis.

Chapter 2:

The core concept utilized in the Thesis, supramolecular spring, has been thoroughly discussed including the concept, structure vs. property relationship, and novel dynamic materials featuring controlled movements for applications in sensing, robotics, etc.

Chapter 3: An introduction of pressure and strain sensors has been discussed including the fundamental aspects (e.g. definition, sensing mechanisms, key parameters, etc.) and active sensing low-dimensional materials. Research background information and significance of this thesis has also been discussed, which is helpful for readers to gain a better understanding of this Thesis.

Chapter 4: Experimental techniques of synthesis process of active materials involved in this thesis (i.e. graphene and gold nanoparticles) have been discussed. In addition, working principles of different characterization methods including SEM, AFM, Raman, and XPS etc.

have been discussed in order to have an accurate and comprehensive understanding of the novel functional materials.

Chapter 5: Graphene-molecules hybrid materials based pressure sensors have been devised

and fabricated. The working principle, synthesis and characterization of functionalized graphene materials, fabrication of pressure sensing device, electrical test of pressure sensing performance, and the demonstration of health monitoring applications have been discussed thoroughly.

Chapter 6: A novel strain sensor based on gold nanoparticle network has been devised. Gold nanoparticles are interconnected by tetra (ethylene glycol) dithiol (TEG) to form 3D networks and have been employed as strain sensing materials. Synthesis and characterization of active materials and fabrication of strain sensing device have been discussed thoroughly. The strain sensor has demonstrated high gauge factor and highly sensitive response to tensile strain, compressive strain, the mixed motion and even vibration. Extensive effort has been paid to the exploration of the structure design of device to achieve wireless sensing for broader applications, and the optimization of data transmission is underway. property relationship of supramolecular springs and their (self-) assembly as a prerequisite towards the generation of novel dynamic materials featuring controlled movements to be readily integrated into macroscopic devices for applications in sensing, robotics and IoT (Internet of Things).

Introduction

During the last three decades, a great effort has been devoted to the design and synthesis of more and more complex supramolecular architectures [1][2][3] relying on the use of non-covalent interactions to assemble individual chemical entities with a sub-nanometer precision as elements for the generation of systems and materials with unprecedented and often unique chemical and physical properties. 4,5 Supramolecular chemistry, which combines reversibility, directionality, specificity and cooperativity, offers highest control over the process of molecular assembly. 6,7 Among the various architectures which have been realized, there has been recently an increasing interest on the fabrication of functional supramolecular structures with a nanoscale control over their mechanical properties, targeting at the development of supramolecular machines such as shuttles, 8-10 rotors, [11][12][13][14] switches, [15][16][17] ratchets, 18-20 springs 21,22 etc., to mimic biological activities via the implementation of controllable molecular-level motions. While the development of supramolecular shuttles, rotors etc. have been the subject of comprehensive review articles, [23][24][25][26][27] only a few works discuss the progress in the field of (supra) molecular springs. [27][28][29][30] In the macroscopic Hooke's helical springs, the spring constant, which indicates the resistance to elongation, is ruled by several parameters such as material shear modulus, the spring diameter, the number of coils, and the thickness of the coiled wire.

At the nanoscale, helically coiled and folded molecular conformations are well-known and have attracted significant attention. However, the potential likeness between the supramolecular springs and Hooke's springs is still largely unexplored, except for DNA. In this chapter we highlight the advances in design, synthesis and structure vs. property relationship of supramolecular springs (Section 2.2). (Self-) assembled springs based dynamic materials, in which the motion was controlled by stimuli like light and heat have been discussed in Section 2.3. We also extended the scope of supramolecular spring concept to spring-like macroscopic devices which are highly relevant for practical applications such as actuators, pressure sensing, humidity sensing etc. (Section 2.4).

Supramolecular springs widely exist in biological systems to serve as engines for motility. 21 Conformational energy is stored (contracted conformation) in such active mechanochemical systems and released (extended conformation) with certain non-covalent bonds broken to trigger mechanical movement or other functions such as infection and mitosis. In biological systems, supramolecular springs are generally constructed with proteins (e.g. actin bundle in a Limulus sperm, 31,32 etc.) and DNAs. [33][34][35] The research on a spring-like protein complex NOMPC, [36][37][38][39] changing its conformation under pressure to control the ion channel for electromechanical signal transferring, has attracted significant attention due to the potential mechanism interpretation of mechanoreceptor, which is related to the detection of touch, hearing or even the blood pressure.

In chemistry, (supra)molecular springs have been widely reported in the form of (single or double) helicates, 22,29,40 π-expanded helicenes, 41 rotaxanes, 42 catenanes, 43 clathrates, 44 peptides, 45 helices, 46,47 helical (coordination) polymers, 48,49 and other molecules assemblies, [50][51][52] which can reversibly extend or/and contract their conformations upon external stimuli accompanied with energy storage and release process. 22,27,53 First of all, it is worth pointing out that the contraction/extension motion of a (supra)molecular spring can be fueled either by means of a mechanical input, i.e. a force applied, or of a physical/chemical stimulus such as light irradiation, heat, change of pH, ion complexation, solvation, etc. Under applied force, supramolecular springs are extended or contracted, and the conformational equilibrium is lost to reach a high energy state. Once the applied force is released, the springs automatically recover to the initial equilibrium state. 27 Depending on the chemical system and stimuli employed to trigger its spring-like motion from helical to non-helical conformation, the equilibrium state could either be the helical or the non-helical state. It should be noted that supramolecular springs differ from the largely existing "spring-like" molecules, 54 which exhibit the spring-like structure, but do not undergo directional contraction-extension movements. Moreover, molecules or assemblies thereof, 55 whose conformations can be extended and/or contracted as a response to external stimuli should not be considered as supramolecular springs. Therefore, several supramolecular foldamers, 56 supramolecular polymerized helical structures, 57 and coordination helical structures, 58 which can be controllably assembled/disassembled into helical conformations by changing the solvent or other conditions are not within the discussion scope of supramolecular springs. Finally, based on the definition of spring structure, supramolecular springs should also be distinguished from (supra) molecular muscle 55,59,60 or actuators, 61,62 even though both of them feature contraction-extension motion under stimuli.

The subtle design of molecular structure makes it possible to control the structural transformations of supramolecular springs in terms of relative position, motion speed, frequency, etc., as a result of different inputs such as ion coordination, 22,40,53,63,64 light irradiation, 51,65 pH, 40,50 heat, 48,66,67 and solvent. 44,45 Even though the mechanistic details and kinetics of conformational changes have been comprehensively studied by means of NMR, UV-Vis, CD, STM and even X-ray crystallography, most of the research have been conducted

in the solution and at the single crystal level thus limiting to a large extent the application in real macroscopic devices. 23,26 To fill this gap, in this chapter, in addition to the discussion of the chemistry of supramolecular springs we will reveal the challenging, yet valuable connection between molecular scale supramolecular springs and macroscopic functional entities according to a few enlightening reports.

Supramolecular spring in solution and in single crystals

In analogy with natural proteins which (typically) possess folded structure, the design and synthesis of molecules that can adopt helical conformations or assemble into helical architectures has attracted considerable attention. These artificial molecules can form supramolecular spring-like helical structures which are held together through different intraor inter-molecular non-covalent interactions such as metal-ligand coordination, hydrogen bonding, solvophobic effects, etc. The responsive nature of non-covalent interactions to external stimuli has provided tools to tune the extension-contraction mechanism of the helical structure in a well-controlled manner, yielding a modification of the properties of supramolecular springs. In this section, the chemistry of supramolecular springs has been discussed based on the different inputs exploited to activate the spring's mechanochemical motion at the molecular level, in order to shed light onto their working principle.

Coordination driven helicates as supramolecular springs

In 1987 the term helicate was introduced by Lehn and co-workers to describe the discrete oligo-nuclear supramolecular complexes, in which at least two metal ions are wrapped by two or more oligo-dentate ligands along the helical axis. 68 Since then, helicates have been intensively studied and became the most important supramolecular springs due to the versatile ligand design with different self-assembly strategies. [69][70][71] In 2000, Jung and co-workers reported the first example of supramolecular spring 72 whose helical pitch could be reversibly modulated by incorporating guest anions with a different size.

As portrayed in Figure 2-1, the Py2O (3, 3'-oxybispyridine) ligands (curve stripe in Figure 2-1a) are linearly coordinated with the silver (I) ion (black and white squares in Figure 2-1b) to form self-assembled cylindrical helices with counteranions pinched in the helical pitch of the single strand column. Each turn consists of two Py2O units and two silver ions to form the infinite helical structure. Interestingly, the skeletal structures constructed by different counteranions exhibit similar helical motif only differing in their pitch, which is proportional to the volume of the counteranion guest (NO3 -, 36.0 cm 3 /mol; BF4 -, 51.0 cm 3 /mol; ClO4 -, 52.1 cm 3 /mol; and PF6 -, 56.2 cm 3 /mol). Upon using counteranions having a different size, the spring pitch can be reversibly tuned from 7.430 to 9.621 Å (as revealed by X-ray crystallographic characterization), meaning that the supramolecular spring can feature anion dependent contraction-extension movement.

A very interesting strategy to control the supramolecular spring conformation through coordination driven interaction has been developed by Numata and co-workers 49 Regulated by precise molecular recognition events through Zn-N coordination, the helical polysaccharide was endowed with spring-like motion, in which the degree of elongation could be tuned by varying the contour length of the added ligand (Figure 2-4b-c). Reproduced from Ref. 49 .

(a) (b) (c)

Solvent driven supramolecular springs

A supramolecular spring formed by self-assembled helicenediol has been reported by Tanaka and co-workers 44 The helicenediol 2,13-bis(hydroxymethyl)dithieno [3,2- Scrimin and co-workers have reported a peptide made with seven amino acids which exhibit solvent dependent spring like motion. 45 The peptide was reversibly switched between contracted mode (α-helix, 15 Å) and extended mode (310 helix, 17 Å) according to the solvent polarity. The molecular calculation results suggested that the hydrophobic interactions between the side arms of the amino acids provide a major driving force for the stabilization of the α helix in aqueous solution.

Redox-responsive supramolecular springs

An oligomeric ο-phenylene with highly condensed π-cloud that undergoes a redox-responsive dynamic motion has been reported by Aida et al. 47 As shown in Figure 2-8, the polymeric οphenylenes are forced to adopt a helical structure as dictated by the heavily angled aromatic connection. Different from the solid-state in which enantiomerically pure crystals are formed due to the chiral symmetry-breaking process, the helices have a rapid helical inversion as an extended conformation with interplanar distances ca. and in the solid state. Reproduced from Ref. 47 .

By and large, in solution, supramolecular springs can be constructed in various forms (e.g.

helicates, helicene assemblies, etc.) and their elastic motion can be controlled via different stimuli such as metal ions, temperature, pH, solvent, redox processes, etc. The contractionextension motion can be reversibly triggered in a highly controlled manner, which renders supramolecular springs extremely appealing for various applications such as nanoscale mechanical devices 22 or in the separation of chiral compounds. 47

Self-assembled springs based dynamic materials

The chemistry of supramolecular springs, introduced in the previous section, enables the precise control over the contraction-extension dynamics as a response to different external stimuli to tune intra-and intermolecular interactions such as metal-ligand coordination, hydrogen bond, π-π stacking, solvophobic effect, etc. Although versatile tools have been developed to modulate the supramolecular spring motions, most of the research has been conducted in solutions or single crystals, thus representing a severe limitation for their further applications in everyday life. The exploitation of supramolecular machines including molecular springs in the macroscopic world, by controlling the contraction-extension motions at the molecular level, represents a grand challenge. 23,26 Integrating supramolecular springs in macroscopic materials is a mandatory step for the development of novel functions with highlevel control over the macroscopic movements under the effect of different stimuli. Hitherto, very few examples on the solid-state supramolecular spring macroscopic materials have been reported and are discussed in this section.

Light-responsive dynamic materials

Light is a straightforward and attractive stimulus to trigger macroscopic motions because it is a non-invasive and controllable energy source featuring a high spatio-temporal resolution which can be used to drive different nanoscale molecular systems. A spring-like material capable of converting light energy into macroscopic mechanical movement has been reported by Katsonis et al. 51 The prototypical photochromic molecule, i.e. an azobenzene-based molecular switch, (Figure 2-9a) has been embedded in the prototypical nematic liquid crystal E7 together with chiral (S) or (R) octan-2-yl 4-((4-(hexyloxy)benzoyl)oxy)benzoate (S-811 and R-811 in Figure 2-9b) to confer a chiral nature to the polymeric ribbons. The mixture was then introduced into a glass cell to promote a twist geometry, in which the orientation of the liquid-crystal director changed smoothly by 90° from the bottom surface to the top surface.

The polymer film was cut into ribbons with different angular offset (i.e. angle between the orientation of the molecules at mid-plane and the cutting direction). The as prepared ribbons intrinsically adopt different coiling modes, i.e. winding, unwinding, and twisting, in which the pitch and handedness as well as the photoresponsive motion is determined by the angular offset.

the right-handed ribbon with certain angular offset showed helix inversion under the same condition. All the photoresponsive motions above were fully reversible within a few seconds upon exposure to visible light. The mechanism of the reversible contraction-extension was found to depend on the handedness of the director twist and their angular offset. As displayed in Figure 2-9d, the isomerization of azobenzene under UV light induces disorder and causes contraction along the direction of the director and elongation in perpendicular direction. As the top and bottom surfaces adopt different orientation of the director due to the helical twisting, the ribbons need to deform in order to accommodate the anisotropic deformation of both sides. The deformation types (winding, unwinding, and helix inversion) are determined by the orientation of the molecules, which is in turn ruled by the cutting direction.

A proof-of-concept for a photomechanical engine has also been demonstrated to prove the capability of molecular spring material to execute useful work. The detailed device fabrication process 74 relying on a more delicate composition and micropattern design 65 have been reported to demonstrate this bio-inspired strategy could be widely employed for the design of novel light-driven smart materials.

Thermal fueled dynamic materials

Dendronized polymers are a class of linear polymers exposing dendrons in the side groups. 75 The shape-inducing effect of dendritic substituents can be explored to create nanoobjects with a cylindrical shape, which not only considerably widens the range of applications for dendrimers but also opens up new perspectives for various applications. In particular, their self-organization into helical motifs has been reported as a novel strategy for constructing supramolecular spring materials capable of undergoing large-scale motion under external stimuli, such as the thermal treatment. 66 The thermoreversible cisoid-to-transoid conformational isomerism between hexagonal columnar phase (ϕhio) and a hexagonal columnar (ϕh) liquid crystal phase has been observed in dendronized helical polyarylacetylenes by means of differential scanning calorimetry (DSC) and XRD. As displayed in Figure 2-10a, macroscopic samples of the cylindrical polymers are produced by extrusion of the liquid crystalline melt, with the fiber axis being aligned with the cylinder axis.

The extruded fiber obtained from the self-organized dendronized helical polymer undergoes extension along its fiber axis upon heating above certain temperature (Figure 2-10b), which is caused by the ϕhio-to-ϕh phase transition. Conversely, upon successive cooling down to room temperature, the fiber contracts back to its original length.

The collective thermal responsive elastic motion of the cylindrical dendronized polymer actuators on the macroscopic scale were demonstrated by object lifting experiment as reveled

Spring-like macroscopic devices

While hitherto supramolecular or self-assembled springs have not been yet integrated in macroscopic devices, spring-like composites and hybrid structures comprising molecular springs and low-dimensional functional nanostructures, including 0D (like nanoparticles and networks thereof), 1D (e.g. nanotubes, nanorods, nanoribbons and nanofibers) and 2D (such as graphene and related layered materials) have attracted a great attention due to their structural and functional versatility which opens perspective towards different technological applications. In particular, their superior electrical, optical and mechanical properties of these starting low-dimensional nanomaterials, their controlled functionalization and the possibility of producing them in the bulk quantities make them key components for the generation of functional hybrid material at both lab and industrial scale. Inspired by the supramolecular springs generated via self-assembly at the molecular level, the elastic spring-like motion can also be achieved by the integration of molecular springs and materials, in which macroscale motions or effects of practical applications can be achieved. In this section we will briefly discuss the use of molecular spring containing materials capable to accomplish macroscopic tasks which are integrated into devices such as electromechanical actuators, pressure sensor, and humidity sensor.

Spring-like materials based on 0D nanomaterials

Gold nanoparticles are unique building blocks exhibiting a wide variety of electrical and optical characteristics resulting from their size and electronic interactions. 76,77 Molecular springs can be co-assembled with gold nanoparticles (AuNPs) to form 2D networks capable of sensing humidity in air. 78 In this case, 8 nm sized AuNPs are covalently interconnected by dithiol PEGs (Figure 2 the absorbance at a fixed wavelength of 600 nm over time. Reproduced from Ref. 78 .

1D-based spring-like materials

CNTs have been widely utilized to fabricate spring-like materials due to its remarkable properties 80 such as high tensile strengths, electrical conductivities, and most importantly, the ability to further assemble into macroscopic structure in a highly aligned manner. Single-layer fabric ribbons with tunable electromechanical actuations have been reported by Peng et al., 81 in which spring-like CNT fibers were integrated through a sewing method. The CNT fibers were dry-spun from spinnable CVD CNT arrays with a helical structure as shown in Figure 2-12a, in which the chirality depended on the spinning direction during preparation. By applying a voltage, electromechanically contractive and rotary actuations had been generated by the CNT fibers, in which the rotary direction could be tuned by varying the helical chirality. The contractive actuation could be enhanced by increasing the number of fibers to leverage the electromagnetic effects among the electrically conducting fibers. In order to demonstrate the macroscopic motion control, these spring-like fibers were woven into Kapton film which produces reversibly electromechanical bending-recovery motion resembling crawling robot upon current. Furthermore, these fiber-integrated fabric ribbons could be reshaped into helical structures with different chirality by heat setting as shown in Figure 2-

(a) (c) (b)

Conclusion and perspectives

Spring-like structures are very common in Nature and include systems like DNA and tendrils, yet, mimicking the spring-like function at the nanoscale in artificial molecules is highly challenging. Since the first report on supramolecular spring appeared in 2000, increasing efforts have been dedicated to the use of different external stimuli such as light, heat, ion, pH, solvent, etc., to control of the elastic extension-contraction motion in solution and single crystal. Various stimuli-responsive characteristics have been encoded in the helical molecules or assemblies through supramolecular interactions (e.g. metal-ligand coordination, π-π, hydrogen bonding, van der Waals interaction, etc.). A few attempts have been made to integrate molecular springs into polymers and aggregates to perform extension-contraction on the macroscopic scale for real applications, e.g. actuators and molecular muscles. Selfassembly provides promising way to create smart materials possessing stimuli-response behaviors in micro/macroscopic scale, in which individual motions could be harnessed to orient the nanomechanical actuators thus accomplishing tasks in the macroscopic world.

Spring-like materials, constructed by the integration of molecular springs and lowdimensional nanostructures (e.g. CNT, graphene, etc.), have also been discussed in this chapter as a route to realize functional (nano) devices. The combination of adaptive (supra) molecular springs and materials with versatile properties offers a multitude of potential solutions towards the development of a broadest arsenal of smart materials for sensing, chemical separation and other applications. In the future, the development of artificial machines which can convert molecular movement into macroscopic work will be the greatest challenge in the field, by taming functional complexity in chemical programming of dynamic materials to be readily integrated in functional devices.

Chapter 3

Pressure sensors and strain sensors

This chapter provides an introduction to pressure and strain sensors by discussing the fundamental aspects (e.g. definition, sensing mechanisms, key parameters, etc.) and active sensory materials. Both pressure sensor and strain sensor are extremely appealing research topics holding a true potential for technological applications in wearable human health monitoring, e-skin, human-machine interface, robotics, automation, etc. This chapter will guide the reader to fully understand the research background of this thesis and to outline the research design and significance of the development of pressure/strain sensor with novel hybrid materials.

Introduction

Pressure is one of the most frequently used physical parameter either in industrial production or personal electronic devices as an indicator of the physical environment of vapor, liquid, and contact etc. In the past few decades, considerable attention has been paid to the design and fabrication of pressure sensor for actual electronic applications such as touch display, 1,2 e-skin, [3][4][5] real time human health monitoring, [6][7][8] motion detection, 9 and others. Together with strain sensor, pressure sensors have been widely integrated into wearable devices to monitor physiological parameters such as pulse, blood pressure, temperature, heart rate, sleeping quality, oxygen saturation etc. [10][11][12] With the development of nanomaterials, novel pressure sensors and strain sensors have been developed with superior properties such as light weight, flexible, robust, high sensitivity, fast response, biocompatibility, low power consumption etc. [13][14][15][16] In this chapter, we will briefly introduce the fundamental aspects of pressure sensors and strain sensors as well as the most widely used active sensing materials within different dimensions.

Pressure sensors

Fundamental aspects of pressure sensor

Definition and key parameters

Pressure sensors are devices which capable to quantify a pressure change through an electrical readout. Pressure sensors are extensively used in our daily life either directly (e.g. touch screen, microphone, etc.) or indirectly (e.g. altitude, speed of fluid/gas flow, etc.). To evaluate the performance of pressure sensors, key parameters need to be taken consideration including sensitivity, linearity, detection range, response time, power consumption, flexibility etc.

Among all the parameters, sensitivity is the most important one because it quantifies the effectiveness of pressure sensor. Sensitivity is defined as the ratio between the change of output electrical signal and the change of input pressure of the device. The output signal might be different based on the sensing mechanisms. For examples, relative capacitance change (C/C0) is used for capacitive pressure sensor. For the most common reported piezoresistive pressure sensor, the sensitivity is defined as:

𝑆 (𝑠𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦) = ∆𝑅/𝑅 0 ∆𝑃 (3 -1)
where R/R0 represents the relative resistance change and P represents the pressure change. 13 Linearity is also an important parameter to determine a pressure sensor's static characteristics.

The pressure sensor has a good linearity if the output signal can fit the input pressure change in a straight line, and it is more reliable and accurate in the linear response range. For certain applications, linearity would be considered as the priority parameter such as weight balance. 14 The design of detection range of pressure sensor is usually related to specific applications. For the daily activities of human beings like touch, object manipulation etc., the pressure mainly located below 100 kPa. 17 As shown in the Figure 3-1, normally the pressure can be divided into four ranges, which are the ultra-low-pressure (below 1 Pa), subtle-pressure (1 Pa-1 kPa), low-pressure (1 kPa-10 kPa), and medium-pressure (10 kPa-100 kPa). 14 To design pressure sensor devices with specific applications, one must consider the relevant detection range.

Noteworthy, the ultra-low-pressure range is normally related to sound. The development of microelectromechanical system (MEMS) based ultra-sensitive pressure sensor is of great importance to microphones, hearing aids, and other intelligent products. 18 Figure 3-1. The detection range and relevant applications. Reproduced from Ref. 14 .

The response time is an important parameter to evaluate a pressure sensor, especially for dynamic real-time sensing devices. The time requires by device once received the input pressure to generating a stable output signal is defined as response time. The smaller is the value, the faster is the response. Normally a response time below 100 ms is widely acceptable, while a shorter response time is necessary for certain applications such as touch screen or health monitoring systems.

Other parameters to assess pressure sensor such as power consumption, flexibility, stability etc. are also important and need to be taken careful consideration by researchers when choosing suitable materials (active sensing materials, electrode materials etc.) and configurations to get access to various practical applications with comprehensive and balanced qualities.

Sensing mechanisms

The "leit motiv" of the pressure sensors, regardless the mechanism employed, is the existence of compressible active materials whose properties are changed when they are subjected to a pressure. Various transduction mechanisms have been employed in pressure sensors including the most frequently reported piezoresistive sensing, capacitive sensing, piezoelectric sensing, and other sensing mechanisms such as resonant sensing, optical sensing, and triboelectric sensing. The difference and characteristics have been summarized below. (nanoparticles), 19,20 PDMS (polydimethylsiloxane) /SWNTs, 12,21,22 (functional) graphene (composites), 23,24 polypyrrole, 25 etc.

A very simple yet effective piezoresistive pressure sensor has been reported by Suh and coworkers. 26 The highly flexible device is consisting of two interlocked arrays of high-aspect-ratio Pt-coated polymeric nanofibers and thin PDMS supporting layers (Figure 3-3). Under applied pressure, the degree of interconnection of the conductive nanofibers changes, in which the contact resistance decreases reversibly. When

a) b) c)   𝐶 =  �  � 𝐴 𝑑 (3 -2)  �  � a) b) c)
ionic conductive layer, 28 air-dielectric graphene transistor, 29,30 AgNWs/polyurethane (PU) composite, 31 etc.

In 2010, a highly sensitive flexible pressure sensor with a microstructured rubber dielectric layer has been reported by Bao and coworkers. 17 Reproduced from Ref. 17 .

The change of capacitance is usually very small (picofarads), so it is usually integrated into organic field-effect transistors (OFET) to amplify its sensing signal. A typical OFET structure consists of a semiconductor layer, a gate dielectric layer, source-drain electrodes, and a gate electrode. The dielectric layer sandwiched by semiconductor layer and the gate electrode can be seen as a capacitor in which the charge status is affected by external pressure. The subtle capacitance change will be amplified by the integration of capacitive pressure sensor with OFET. 32 3) The third most commonly reported pressure sensing mechanism is piezoelectric sensing. Under applied mechanical stresses, certain inorganic materials like crystals, ceramics or other piezoelectric polymers can generate electrical charge which is called piezoelectricity. The working principle of piezoelectric pressure sensors is based on the dipole moments generated under external pressure stimuli in the anisotropic crystalline materials (Figure 3-2c). This kind of sensing mechanism features a high sensitivity to dynamic pressure change, fast response, low power consumption while suffering from the poor stretchability, temperature sensitivity.

Polyvinylidene fluoride or polyvinylidene difluoride (PVDF) nanofibers 33 , ZnO nanoplatelets 34,35 and other inorganic piezoelectric materials such as BaTiO3, Pb[Zrx,Ti1-x]O3 (PZT) 36 have been widely used as active sensing materials for piezoelectric sensing.

In 2013, Rogers and coworkers 33 For pressure sensing materials, metal nanoparticles, 20 CNTs, 43,44 carbon fibers, 45 graphene, 46- 49 microstructured rubber, 17 conductive composites [50][51][52] etc. have been widely reported and nicely reviewed 14,[53][54][55] in recent years. In the following sections, typical and highly cited works will be discussed based on the dimensionality of active sensing materials.

0D materials

0D materials usually consist of metal nanoparticles, quantum dots, C60, etc. Among them, gold nanoparticles (AuNPs) have attracted large amount of attention due to its stability in air, the easiness to tune its size/shape/composition and characteristics. The size-related electronic, magnetic and optical properties, which are also known as quantum size effect, make it attractive in the applications of catalysis and biology. In 2016, Lee and coworkers 20 have reported a highly sensitive, transparent and durable pressure sensor based on sea-urchin shaped metal (gold and silver) nanoparticles (SSNPs) as shown in Figure 3 properties, are novel building blocks for the fabrication of pressure sensor. In 2014, Cheng and co-workers 58 have reported a highly sensitive and flexible pressure sensor by sandwiching ultrathin gold nanowire-impregnated tissue paper between two thin PDMS sheets. More conductive pathways between AuNWs and finger electrodes were formed upon applied pressure, leading to an increasing of current. This piezoresistive pressure sensor has a high sensitivity (1.14 kPa -1 ), fast response time (17 ms), high stability (50000 loading cycles)

and can be applied in the real-time blood pulses monitoring and the detection of small vibration from sound.

2D materials

2D materials such as 2D flakes like graphene which can be obtained by vacuum filtration, CVD, chemical synthesis are the most widespread pressure sensing building blocks for all transduction mechanisms. 47,[59][60][61][62] Most 2D materials based sensor display negligible hysteresis, good linearity, and above-average sensitivity. 54 In 2017, Wu and coworkers 49 have reported an ultrafast dynamic pressure sensors based on graphene hybrid structure. As shown in Figure 3-9a, the GO/graphene composite film is obtained by vacuum filtration of graphene oxide and graphene mixture solution. In this case, the 2D graphene layers are the conductive element and GO is served as surfactant to prevent the restacking and aggregation of graphene in aqueous solution. The pressure sensor adopts a lateral structure, in which the planar film of GO/Graphene is placed on top of the electrodes surface as shown in Figure 3-9b. The working principle is based on the conductive graphene layers in the composite film being squeezed under pressure, in which more contacting points are created thus leading a decreasing of resistance. The pressure sensor has demonstrated good sensitivity (0.032 kPa -1 ) in the lowpressure range (1kPa) with a very low detection limit (0.15 g) (Figure 3-9c). It also showed excellent cycling stability (8000 cycles) and impressive response time which can detect the dynamic pressure signal as high as 10 kHz. This graphene based resistive pressure sensor can be applied in many fields including health monitoring or robotic arms.

3D materials

3D porous structures, usually obtained by freeze drying, skeleton erosion, elastomer casting, represent another widely used pressure sensing materials. 23,48,63 The intrinsic porous and elastic structure makes is compressible under pressure, in which the electrical properties have changed to form the pressure-electrical signal relationship. Normally the 3D structures can be produced in a large quantity and be used in all the transduction mechanisms. 54 In 2013, Yu and coworkers 23 Ref. 23 .

  𝐺𝐹 = � ∆𝑅/𝑅 � 𝜀 � (3 -3)

Sensing mechanisms

The sensing mechanisms of strain sensor can be mainly divided into 5 categories: geometrical effect, piezoresistive effect, disconnection mechanism, crack propagation and tunneling effect. 64 The working principle and differences of these sensing mechanisms will be discussed briefly in the following paragraphs.

1) Geometrical effect is the main working principle of piezoresistive and piezo capacitive strain sensor, in which the resistance or capacitance change due to the size changes caused by the strain. The resistance of a conductor is calculated as:

𝑅 = 𝜌𝐿 𝐴 (3 -4)
where ρ is the electrical resistivity, L is the length and A is the cross-sectional area. L increases and A decreases upon stretching which increases the resistance of the conductor. While for capacitor, the capacitance can be expressed as:

𝐶 = 𝜀 𝐴 𝑑 (3 -5)
where  is the electric constant, A is the overlap area of the two plates and d is the separation distances. A increases and d decreases upon stretching which increases the capacitance of the capacitor.

2) Piezoresistivity refers to the resistance change (R) of materials caused by structural deformations. The relative resistance change can be expressed as:

∆𝑅 𝑅 0 = (1 + 2) +   0 (3 -6)
where  is Poisson's ratio, ρ/0 is the relative change of electrical resistivity. In this equation, the former term is caused by geometrical change while the latter one means the piezoresistivity of the material. Piezoresistivity of metal materials could be increased of a few fold while some semiconductor materials like silicon can be increased by several orders of magnitudes. 72,73 However, these semiconducting materials and other nanoscale materials like CNTs and zinc oxide nanowires (ZnONWs) featuring high piezoresistivity exhibit a very narrow strain sensing range.

For this reason, composites of functional materials and stretchable polymers are commonly employed for strain sensing. 74,75 3) Disconnection mechanism is often employed in the conductive thin films made of nanomaterial network, in which electrons can travel through percolation pathways.

Upon stretching the thin films, the percolation pathways get worsen yielding an increase in resistance. Nanowires networks [76][77][78] and graphene 79,80 based thin films are widely exploited as stretchable and flexible strain sensors employing this detection mechanism.

4) Crack propagation is very similar to the disconnection mechanism. Conductive brittle thin film such as metal film, 81 CNT, 71 AuNWs, 82 AgNPs, [START_REF] Lee | A stretchable strain sensor based on a metal nanoparticle thin film for human motion detection[END_REF] graphene [START_REF] Wang | Wearable and Highly Sensitive Graphene Strain Sensors for Human Motion Monitoring[END_REF][START_REF] Park | Highly Stretchable and Wearable Graphene Strain Sensors with Controllable Sensitivity for Human Motion Monitoring[END_REF] are adsorbed onto stretchable substrates and (microscale) cracks are formed and spread upon stretching. As a result, the electrical conduction of the conductive film decreases due to the separation of conductive element. Noteworthy, the resistance usually decreases dramatically under strain thus leading a very high gauge factor of the strain sensor. As shown in Figure 3-12, cracks are generated and enlarged in the silver nanoparticle thin film under increasing strain, [START_REF] Lee | A stretchable strain sensor based on a metal nanoparticle thin film for human motion detection[END_REF] thus increasing the electrical resistance of the film. Upon releasing the strain, the opened cracks of the film are gradually closed, and the resistance is decreased accordingly. Simmons's theory [START_REF] Obitayo | A Review: Carbon Nanotube-Based Piezoresistive Strain Sensors[END_REF][START_REF] Alamusi; Hu | Piezoresistive Strain Sensors Made from Carbon Nanotubes Based Polymer Nanocomposites[END_REF] as follows:

𝑅 𝑡𝑢𝑛𝑛𝑒𝑙 = 𝑉 𝐴𝐽 = ℎ 2 𝑑 𝐴𝑒 2 √2𝑚 𝑒𝑥𝑝 ( 4𝜋𝑑 ℎ √2𝑚) (3 -7)
where V is the electrical potential difference, A is the cross sectional area of the tunneling junction, J is the tunneling current density, h is Plank's constant, d is the distance between adjacent nanomaterials, e is the single electron charge, m is the mass of electron, and λ is the height of energy barrier for polymers. For example, in the CNT-polymer composite materials, [START_REF] Hu | Tunneling effect in a polymer/carbon nanotube nanocomposite strain sensor[END_REF] electron can pass through very close distance between CNTs (as shown in Figure 3-13), in which the tunneling current is affected by the inter nanotube distance modulated by strain.

1D materials

Nanowires (or nanofibers) 26,74,76,77,[START_REF] Yao | Wearable multifunctional sensors using printed stretchable conductors made of silver nanowires[END_REF][START_REF] Gong | Tattoolike Polyaniline Microparticle-Doped Gold Nanowire Patches as Highly Durable Wearable Sensors[END_REF][START_REF]Highly stretchable and sensitive strain sensor based on silver nanowire-elastomer nanocomposite[END_REF] and carbon nanotubes 43,71,[START_REF] Shin | Highly stretchable conductors and piezocapacitive strain gauges based on simple contact-transfer patterning of carbon nanotube forests[END_REF][START_REF] Cohen | A Highly Elastic, Capacitive Strain Gauge Based on Percolating Nanotube Networks[END_REF][START_REF] Roh | Stretchable, Transparent, Ultrasensitive, and Patchable Strain Sensor for Human-Machine Interfaces Comprising a Nanohybrid of Carbon Nanotubes and Conductive Elastomers[END_REF] are the most widely used materials for strain sensing because of the flexibility and stretchability to develop wearable devices. These 1D structures can be integrated into the strain sensing devices in different ways. Common fabrication methods of 1D strain sensing materials have been summarized as follows. Pt-coated polymer nanofibers 26 were placed in a reversible interlocking way, in which the resistance changes differently under the effect of pressure, shear, and torsion as shown in Figure 3-15a. The nanowires can also be integrated with polymer to form conductive composite, [START_REF]Highly stretchable and sensitive strain sensor based on silver nanowire-elastomer nanocomposite[END_REF] in which the resistance will change with strain ruled by the tunneling effect as shown in Figure 3-15b. CNTs can be directly spray coated onto flexible substrate 43 as conductive electrodes to fabricate capacitive strain sensor as shown in Figure 3-15c, in which the separation distance d decreases under lateral strain thus increasing the capacitance. Another way to fabricate strain sensor is by using well-aligned CNTs as strain sensing materials, 71 in which the CNTs film fractures into gaps and islands with bundles bridging the gaps as shown in Figure 3-15d. This method has increased the sensing range effectively up to 280%, 50 times higher than conventional metal strain gauges.

3D materials

3D strain sensing materials normally refer the composite materials consisting conductive fillers (e.g. graphene, CNTs, etc.) and elastic frameworks. 79,101 These polymer frameworks not only strengthen the flexibility and stretchability of the conductive fillers, but also improve the compressibility thus broaden the sensing region dramatically. In 2014, Coleman and coworkers 79 have reported a graphene-rubber composites based strain sensor which has a high gauge factor and the ability of detecting high frequency dynamic strain (Figure 3

Applications and Development

Both pressure sensors and strain sensors have very broad applications in health monitoring (e.g. heartbeat, 26,[START_REF] Wang | Wearable and Highly Sensitive Graphene Strain Sensors for Human Motion Monitoring[END_REF] artery pulse, 36,60,68 respiration, 58,71 etc.), motion detection (by attaching on limbs, [START_REF] Yao | Wearable multifunctional sensors using printed stretchable conductors made of silver nanowires[END_REF] joints, 61 fingers, [START_REF] Benniston | A Light-Induced Molecular Shuttle Based on a []Rotaxane-Derived Triad[END_REF] face, [START_REF] Wang | Wearable and Highly Sensitive Graphene Strain Sensors for Human Motion Monitoring[END_REF]103 etc.), human-machine interface (e.g. robotics, [START_REF] Gong | Tattoolike Polyaniline Microparticle-Doped Gold Nanowire Patches as Highly Durable Wearable Sensors[END_REF] artificial limb, smart glove, [START_REF]Highly stretchable and sensitive strain sensor based on silver nanowire-elastomer nanocomposite[END_REF]104 virtual reality and interactive gaming, 105 etc.). Flexible and wearable pressure sensors and strain sensors for sure will be fully integrated into our daily electronic gadgets in the near future just as smart watch/band nowadays. Wearable personal health monitoring system has been widely believed as the key solution of next generation of health care technology. The sensing devices can continually sense, analysis, store, and transmit the health data of our body, providing accurate and long-term health evaluation. To achieve this goal, on-skin, light weight, non-invasive and wireless sensing devices will be the best option.

The trend of personalized health monitoring will push forward the research, more importantly, commercial implementation of new materials-based sensing devices with high sensitivity, high flexibility, and high accuracy.

However, with such high expectation on the new materials (e.g. graphene, CNTs, nanowires, nanoparticles, etc.) based sensing devices for health monitoring and the endless effort of global research institutes and numerous start-ups, the dominant products in the market is still silicon-based MEMS devices. Insufficient accuracy and lack of long-term stability of these novel material based pressure/strain sensors are nowadays the main limitations. Normally the accuracy of commercial pressure sensors for industrial applications (e.g. LPS33W-STMicroelectronics, MLH series-Honeywell, etc.) should be controlled below  (0.1-0.5%). While the accuracy of the novel materials based pressure sensors can easily reach over 5-10%, which is too large for any practical applications.

The reason for this is largely depends on the quality of the active sensing materials and the fabrication methods. If one considering graphene for an example, most of the superior properties such as high conductivity, high mechanical strength, and high thermal conductivity are depending on the crystal structure. But it is difficult to scale up the production while maintaining the high quality. For example, mechanical exfoliated graphene flakes possess perfect single-layer crystal structure yet the size of it is usually small. Solution exfoliation process (e.g. reduced graphene oxide etc.) can increase the yield effectively while defects are inevitable. CVD method produces the highest quality graphene, but it suffers high cost, low efficiency, and inevitable grain boundaries. Graphene is undoubtedly a great material regarding of all its superior properties, and tremendous efforts have been devoted in the last

General information

Graphene oxide (GO) (4 mg/ml, water dispersion) was purchased from Graphenea. Before the synthesis of functionalized graphene in Chapter 5 and Chapter 7, it was diluted in ethanol and sonicated for 30 min to have a better dispersion.

All the chemicals and reagents were purchased from Sigma-Aldrich and used without further purification.

ITO-PET (surface resistivity: 60 Ω/sq, L × W × thickness 1 ft × 1 ft × 5 mil, thickness of ITO coating: 130 nm) was purchased from Sigma-Aldrich. No further surface treatment was needed due to the protection film, which was removed before using.

Silicon substrate (230 nm  10 nm SiO2, 1515 mm 2 ) was purchased from Fraunhofer.

Substrate was cleaned with sonication bath in acetone and isopropanol for 20 min respectively, followed by UV ozone treatment to further remove the surface contaminant.

Synthesis of active materials 4.2.1. Functionalization of graphene oxide

Graphene oxide is a great 2D platform that can be readily functionalized thereby modifying its structural, electronic, chemical properties in view of its applications in the field of catalysis, [1][2][3][4] bio-sensing, [5][6][7][8][9][10][11] drug-delivery [12][13][14][15][16] etc. In recent years, graphene and graphene oxide have been widely functionalized with chromophores, [17][18][19][20] organic compounds, 21,22 polymers, 23,24 nanoparticles. [25][26][27][28] These approaches have been extensively reviewed. [29][30][31][32][33][34] As shown in Figure 4-1, the functionalization of graphene or graphene oxide can be achieved through covalent bonding (e.g. nucleophilic substitution reaction, electrophilic substitution reaction, condensation reaction, addition reaction etc.) or noncovalent bonding (e.g. - interaction, cation- interaction, anion- interaction, H- interaction etc.). Substitution reaction, usually through the attack of epoxy groups with amine (-NH2) containing organic compounds, has been considered as a promising method for large scale production due to its high yield at room temperature, in absence of catalyst in aqueous medium. Therefore, we employed this method to functionalize GO (Chapter 5 and Chapter 7). The related synthesis details are discussed in each chapter.

Synthesis and functionalization of gold nanoparticles (AuNPs)

Gold nanoparticles have attracted great interest for applications in the field of catalysis, [35][36][37][38][39] biosensing, [40][41][42][43][44] biomedicine, [45][46][47][48] bioimaging, [49][50][51][52] etc. (see Figure 4-2) because of their unique optical, electrical, magnetic properties. Although AuNPs has been first synthesized over 100 years ago, much research focus has been so far addressed to its functionalization as well as to the various size and shape controlled synthesis methods to enhance and broaden potential applications in different fields. To meet the requirements for various applications, versatile ligands, polymers, biomolecules, etc. can be easily functionalized on AuNPs, thus making it one of the most active dynamic research areas. 53,54 In this section, experimental details of the synthesis and functionalization of AuNPs utilized in Chapter 6 are discussed. because they possess the highest energy level meanwhile not too strong to destroy the gold layer. But the experiment showed that the IR laser scanning with the two sets of parameters also did not work well due to the unstable irradiation, which made the IR laser method not suitable in this case.

√ √ √ 50% × × × × × × × × × 60% × × × × × × × × × 70% × × × × × × × × × 80% × × × × × × × × × 90% × × × × × × × × × 100% × × × × × × × × ×
Due to the failure of IR laser irradiation experiment as well as the multistep of photolithography technique, this fabrication design has been modified to have a more efficient fabrication process. Also, the fully flexible structure might cause unstable electrical performance during the operation process, thus commercially available capacitor and inductor were employed to simplify the fabrication process and to improve the operation stability. The new structure is discussed as below: In this thesis, XPS was used to characterize the functionalized graphene materials in Chapter 5 and Chapter 7. XPS analysis was performed with a Thermo Scientific K-Alpha X-ray photoelectron spectrometer. Details of the materials characterization were discussed in the following chapters.

UV-Vis Spectroscopy

Ultraviolet-visible (UV-Vis) spectroscopy is one of the most commonly used techniques in analytical chemistry for the quantitative determination of different organic and inorganic compounds in solution. The principle of UV-Vis spectroscopy is based on the interaction of UV or visible light with molecules, in which the bonding or non-bonding electrons can absorb energy and be excited to higher anti-bonding molecular orbitals (e.g. σ-σ*, n-σ*, π-π*, n-π*).

UV-Vis spectroscopy follows the Lambert-Beer law, in which the absorbance of a solution is proportional to the concentration of absorbing species (c) and the path length (l):

A = log 10 𝐼 0 𝐼 = 𝑐𝑙 (4 -3)
Where A is the measured absorbance, I0 is the intensity of the incident light at a given wavelength, I is the transmitted intensity, and  is the extinction coefficient. Based on this equation, the concentration of the absorber in a solution can be calculated with a known path length and given extinction coefficient.

As shown in Figure 4

Atomic Force Microscopy

Atomic Force Microscopy (AFM) is one of the Scanning Probe Microscopes (SPM) invented in the 1980s. 57,58 It is an important technique to acquire topographic information of conducting and insulating surfaces with high resolution (atomic resolution in some cases).

Not only for the application of surface topography imaging, AFM can also be used to measure local material properties such as elasticity, hardness, adhesion and surface charge densities etc. by plotting force curves (i.e. force vs distance curves). 59 As shown in Figure 4 In piezoresistive pressure sensor, the electrical resistance changes as a function of an applied pressure. Due to its high sensitivity, fast response, and ease of fabrication, it has been the subject of various studies. In this chapter, we discuss a radically new type of piezoresistive pressure sensor based on a millefeuille-like architecture of reduced graphene oxide (rGO) intercalated by covalently tethered molecular pillars holding on-demand mechanical properties. By applying a tiny pressure to the multilayer structure, the electron tunneling ruling the charge transport between successive rGO sheets yields a colossal decrease in the material's electrical resistance. Significantly, the intrinsic rigidity of the molecular pillars employed enables the fine-tuning of the sensor's sensitivity, reaching sensitivities as high as 0.82 kPa -1 at low pressure region (0-0.6 kPa), with short response times (≈24 ms) and detection limit (7 Pa). Our pressure sensors enable the efficient heartbeat monitoring and can be easily transformed into a matrix capable of providing a 3D map of the pressure exerted by different objects.

Introduction

The basic concepts and fundamental aspects of pressure sensor have been discussed in Chapter 3. Pressure sensors are devices capable of generating electrical signals in response to a change in pressure. 1,2 The sensitivity of the pressure sensor, defined as the ratio between the change in the electrical signal output and the applied pressure, is the most important parameter to evaluate the performance of pressure sensor. Sensors featuring high sensitivities are capable of detecting extremely small changes in the pressure, and can be exploited even to transduce muscle movements 3,4 as well as the subtle vibrations of sound [5][6][7][8] into electrical outputs.

Compared to the complicated fabrication methods such as microelectromechanical systems (MEMS) 9,10 and microfluidics techniques 11 , the engineering of the structure of active material represents the simplest and the most straightforward approach for the fabrication of pressure sensors with high sensitivity. For example, upon applying a pressure, cracks and structural defects can be generated, which results in modification of the percolation pathways for charge transport, and can ultimately result in large variations in the electrical output. [12][13][14] Moreover, the contact resistance at the electrode-active layer interface can be modulated by pressure resulting into an improvement of the sensitivity. [15][16][17][18] Furthermore, a change in capacitance can be induced by pressure, which is the working principle of capacitive pressure sensor. In that case the sensitivity can be improved by micro structuration of the dielectric layer between electrodes. [19][20][21] Although these methods can indeed improve the sensitivity of the device, their applications are generally limited due to the complicated nature of the fabrication process, high production cost and high operating voltage. Conversely, piezoelectric sensors are usually not chosen because they display lower pressure sensitivities and can be hardly integrated into flexible electronic skins. 21

Results and discussion

In this chapter, we have developed a novel and simple method to drastically improve the sensitivity of a piezoresistive pressure sensor. The active material has been assembled by reacting commercially available GO with amino functionalized molecules in order to form covalent bonds on the basal plane of GO through the epoxy ring-opening reaction (Figure 5-1a). Three organic molecules characterized by a similar contour length and increasing rigidity, namely triethylene glycol (TEG) amine 1-octylamine (R2) and 4-aminobiphenyl (R3)

were chosen as the molecular units. Upon condensation of R1-R3 with GO, hybrid structures are obtained in the form of ink dispersed in ethanol. In this way the growth of molecular pillars occurs specifically along the axis perpendicular to the graphene basal plane, 22 yet, the amidation of carboxyl groups, present at the edges of GO sheets may occur as a side reaction.

Such chemically modified GO (CMGO) has been chemically reduced with hydrazine 23 to restore high electrical conductivities (for details see experimental methods). The as-obtained conducting ink can be deposited onto arbitrary substrates by spray-coating, yielding multilayer structures with spacing between adjacent GO sheets which are dictated by the employed molecular pillars, i.e. R1-R3 molecules. Significantly, the latter possess different compressibility, resulting from the intrinsic flexibility of the chosen molecules. The CMGO containing molecules possessing the highest flexibility should display the largest compressibility at a given pressure, thus leading to the highest sensitivity to detect changes of pressure. In analogy to the Hooke's law ruling the compressibility of macroscopic springs, we demonstrate that the sensitivity of the pressure sensor can be improved from rGO-R3 (0.32 kPa -1 ) to rGO-R1 (0.82 kPa -1 ) by using more flexible linkers acting as molecular springs separating rGO layers (Figure 5-1b). Compared to the other approaches employed so far, 12,18,21 this method shows several additional advantages (Table 1) that render it of potential interest for technological applications including the low cost production, simple device fabrication and low operating voltage (0.2 V). Chapter 5 Graphene-molecules hybrid materials based pressure sensor for human health monitoring the interlayer distance between the rGO sheets, which is in turn dictated by the flexibility (or conformation) of the molecular bridges (Figure 5-2b). Laser Scanning Confocal Microscopy (LSCM) measurements revealed that the interlayer distance of functionalized graphene materials decreases when a pressure is applied (Figure 5-14), thus yielding an increase in the tunnelling current. The contribution to resistance variation determined by the contact resistance has been found being negligible as revealed by performing blank experiment with neat rGO as active material (Figure 5-3a, green line) and the effect caused by pore structure has been excluded by BET analysis (Table 2, Figure 5-13). The electron transport mechanism has been proved to be direct electron tunnelling by temperature dependent electrical conductivity measurement (see Figure 5-17 for detailed analysis). The molecular flexibility is related to the intrinsic conformational degree of freedom of carbon and/or oxygen atoms around their chain bonds. Persistence length (Lp) has been widely used to quantify the flexibility or stiffness of polymers, in which the smaller is the Lp, the more flexible is the polymer. [29][30][31] Here the three molecular linkers R1, R2 and R3 can be considered as small oligomers of polyethylene glycol (PEG), polyethylene (PE) and poly-(p-phenylene) (PPP) polymers, respectively. Thus, by considering the Lp of each polymer, which amounts to 0.37 nm for PEG 29 , 0.65 nm for PE 30 and 28 nm for PPP 31 , R1 should be the most flexible linker while R3 the most rigid one. The trend we have observed for the sensitivity is in excellent agreement with the different Ln of the molecular linkers suggesting that the more flexible is the linker the higher is the sensitivity. Furthermore, the vertical amplitude of the error bar associated to ΔR/R0 (Figure 5-3a) is increasing as Ln is decreasing, suggesting that flexible linkers can possess different conformations at the same pressure.

The response of the devices to a dynamic force has also been investigated through finger press, bending test, light object trigger and fatigue test. For these experiments, the pressure sensor containing rGO-R1 as active material has been employed due to its highest sensitivity. monitor wrist pulse and carotid artery pulse of an adult male, which are generally used to evaluate the health condition of human body. 19,34 As shown in Figure 5-4a, the pressure sensor has been placed on an adhesive bandage and fixed to the wrist or to the neck of the tested human object, who voluntarily took part in this proof-of-concept demonstration with full understanding of the process and any risks involved. A stationary signal has been recorded from which the radial artery frequency (65 pulses min -1 ) and carotid artery frequency (61 pulses min -1 ) can be obtained, indicating that the subject in good health condition as an adult male. More importantly, due to the high resolution of the radial artery pulse curve, we can clearly observe two distinguish peaks, P1 which refers to the sum of ejected wave and reflected wave, and P2 which is the peak of the reflected wave from the lower body minus end-diastolic pressure. Thus we can calculate two important parameters, i.e.

As shown in

the radial artery augmentation index (AIr) and the time between two peaks (ΔTDVP), which are commonly used for arterial stiffness diagnosis. [44] According to the measured results, AIr ≈ 61%, ΔTDVP ≈ 0.26 s further verify the good health condition of the subject. All these results

indicate that the pressure sensor (rGO-R1) can be successfully applied to human health monitoring.

Furthermore, we provide a demonstration of a sensor matrix by assembling the PET-ITO thin strips to a network as shown in Figure 56. In this way the sensor can be used not only to measure the pressure, but to capture spatial information. Each cross point will indeed act as a pressure sensor and used as pixel to form a rough 3D mapping. The sensing matrix is made with a similar architecture of the pressure sensors discussed above, which is composed by ITO-PET as electrodes and rGO-R1 as active material in between. The pressure is determined by measuring the change of relative resistance (ΔR/R0) of each pixel. As shown in Figure 5-4d, a bolt (mass = 6.1 g) with an annular bottom was placed on the matrix and its contact area (white dash circle) corresponds to the pixels plotted by the electrical output. The highest relative resistance change (ΔR/R0) occurs in pixel "3D", which indicates the centre of gravity of the bolt. This pressure sensor matrix is also applicable for objects with irregular contact area. Chapter 5 Graphene-molecules hybrid materials based pressure sensor for human health monitoring

Experimental methods

Material

Graphene oxide (GO) (4 mg/ml, water dispersion) was purchased from Graphenea.

Triethylene glycol amine has been synthetized as reported. All the chemicals and reagents were purchased from Sigma-Aldrich and used without further purification. All the solvents and chemicals were used as received from Aldrich without any further purification. 1 H NMR spectra were recorded on a Bruker Avance 400 spectrometer. The 1 H NMR chemical shifts (δ) are given in ppm and referred to residual protons on the corresponding deuterated solvent. All deuterated solvents were used as received without any further purification.

Synthesis of triethylene glycol amine

2-[2-(2-Methoxyethoxy)ethoxy]ethyl p-toluenesulfonate

(2). 35 Triethylene glycol monomethyl ether (1) (8.0 g, 48.7 mmol, 1 eq) was dissolved in 4 mL of THF. An aqueous NaOH (1.8 g, 45 mmol, 0.9 eq) solution (4 mL) was added to the reactant at 0 °C and then a THF solution (15 mL) of p-toluenesulfonyl chloride (TosCl) (8.6 g, 46 mmol, 0.9 eq) was added while stirring (1 h). After stirring at 0 °C (2 h), the reaction mixture was diluted with cold water (60 mL). The organic layer was separated, and the aqueous layer was extracted with DCM (3 × 50 mL). The combined organic layers were extracted 3 times with H2O (50 mL). The organic layer was dried with Na2SO4 and evaporated in vacuum to yield 10.45 g (67%) of 2 as a yellow oil. Triethylene glycol amine (3). 35,36 5 g of 2 (15.7 mmol, 1 eq) was dissolved in 70 mL of EtOH, then sodium azide (1.8 g, 25.2 mmol, 2 eq) was added and the solution was stirred under reflux for 16 h. The solution was concentrated to ca. 1/3 of volume under vacuum and 80 mL of water were added, then the ethanol was completely removed in vacuum. The remaining water solution was extracted with DCM 4 times. The organic layer was dried with Na2SO4

and evaporated in vacuum, The obtained yellow oil (2.98 g, 15.7 mmol, 1 eq) was added dropwise to a two neck flask containing 80 mL of anhydrous THF, triphenyilphosphine (5.9 g, 23.6 mmol) under N2 at 0 °C, and the reaction was stirred overnight under N2. Then 50 mL of water were added, and the reaction was stirred for 8 hours. Then the organic phase was evaporated, and the water phase was extracted 3 times with toluene. Finally, the water phase was dried under vacuum to obtain 3 as pure colorless oil (1.95 g; 75.9 % yield).

Synthesis of functionalized graphene rGO-R1-3

50 ml water dispersion of GO (4 mg/ml) was diluted with 120 ml ethanol in a 250 ml roundbottom flask and sonicated for 30 min. Then 200 mg of the corresponding organic amine compound was weighed into a 10 mL glass vial and dissolved in 5 ml ethanol. The amine solution was then added into the GO dispersion and refluxed while stirring overnight at 90 °C under N2 atmosphere. After cooling down the solution to room temperature, the dispersion was centrifuged for 10 min at 2000 rpm. Then the supernatant was removed, and 200 ml of ethanol were added to the black sludge. The mixture was sonicated for 10 min and centrifuged again. The sonication-centrifugation process was repeated 3 times. After the last centrifugation the black sludge (GO-R1-3) was dispersed in 200 ml ethanol and sonicated 10 min.

The reduction process was carried out by adding 2 ml of hydrazine hydrate to GO-R1-3 ethanolic dispersion followed by refluxing overnight at 90 °C under N2 atmosphere. The purification was performed through sonication-centrifugation cycles as described above for GO-R1-3. The resulting dispersion was decanted and used for characterization and device fabrication.

Device fabrication

ITO-PET (surface resistivity: 60 Ω/sq, L × W × thickness 1 ft × 1 ft × 5 mil, thickness of ITO coating: 130 nm) was purchased from Sigma-Aldrich.

5.3.2.1.

Pressure sensor fabrication process ITO-PET foil was cut into rectangular pieces (13 mm × 20 mm). A part of the ITO was protected from the rGO deposition with a shadow mask (13 mm × 5 mm) to ensure a good electrical contact. After fixing the PET substrates on the hotplate (80 °C), the ethanolic dispersion of rGO-R1-3 was spray coated onto the surface and the amount of deposited material was monitored by UV-Vis absorption in order to obtain a film transmittance of 20% at the wavelength of 500 nm. Afterwards the mask was removed, and two electrodes were fixed together using polyimide (PI) tape keeping the active layer sandwiched in between. The clean ITO strips were then wired out with cooper wire and silver paste. strips were wired out with copper wires and silver paste.

Pressure sensor matrix fabrication process

Characterization

XPS analysis was performed with a Thermo Scientific K-Alpha X-ray photoelectron spectrometer. X-ray powder diffraction (XRD) has been acquired on a Bruker ASX D8

Advanced equipped with Cu anode. FT-IR spectra were recorded using a FT-IR-4700 Fourier Transform Infrared Spectrometer (JASCO) equipped with ATR Diamond. Raman spectra were acquired with a Renishaw InVia RE04 combined with a MS20 Encoded stage 100 nm and a 532 nm laser maintained at a power excitation below 1 mW. The measurements were carried out in ambient conditions with a 100x lens affording a beam spot of 800 nm. Scanning

Electron Microscopy (SEM) images were recorded with a FEI Quanta FEG 250 instrument S3

(FEI corporate, Hillsboro, Oregon, USA). The sample was prepared by spray coating the rGO-R1-3 ethanol dispersion onto a silicon substrate. The UV-Vis absorption spectra were recorded using a Jasco V-670 UV-Vis spectrophotometer. Thicknesses of the spray-coated layers were determined by means of an Alpha-step KLA-Tencor Alpha-Step IQ Profilometer.

The test system consists of a digital force gauge (Mark-10, M7-025E, ~1.0 N), a motorized test stand (Mark-10, ESM-303E), and a system source meter (Keithley, 2635B). Pressure sensor was fixed on the sample stage (diameter of round compression plate: 1.15 cm) and linked with source meter. Set points of pressure and bias voltage were controlled by MESUR gauge Plus software and Labtracer respectively.

XPS spectra of GO, GO-R1-3, rGO-R1-3

X-ray photoelectron spectroscopy (XPS) has been used to study the chemical composition of hybrid material by identifying the relevant chemical elements present in the neat and functionalized GO. In the wide energy spectrum of pristine GO, only two peaks at 286.7 eV and 532.3 eV are found, and are attributed to C1s and O1s, respectively (Figure 567). In contrast, the appearance of the N1s peak is observed in the wide energy spectrum of functionalized GO. Furthermore, a strong decrease in the oxygen signal is observed upon reduction of the samples with hydrazine.

(a) (b) (c) (d) (e) (f) (g)
The high-resolution N1s spectrum of GO sample does not show any nitrogen signal (Figure 56789). Conversely, clear signal is observed in GO-R1-3 samples, and it can be deconvoluted into two peaks, i.e. at 399.6 eV and 401.5 eV, which can be ascribed to secondary and protonated primary amines, respectively. Noteworthy, the existence of the latter can be associated with the formation of the ammonium-carboxylate species.

Notably, presence peaks at 401.5 eV suggest that not all the molecules undergo the covalent grafting onto GO sheets but instead are physisorbed mostly at the edges of GO surface, where carboxylic groups are being present. Based on the intensity of the 401.5 eV peaks, we concluded that the largest portion of unreacted amine molecules was found in the GO-R1 samples.

Interestingly, the unreacted molecules were easily removed from the hybrid material during the reduction step. The N1s spectra of all samples display only one signal, which correspond to secondary amines. Interestingly, the C/N ratio, which amounts to 23.3, 16.7 and 20.9 for rGO-R1, rGO-R2 and rGO-R3, respectively, supports our initial assessment and proves that the functionalization degree can be presented as following: rGO-R2 > rGO-R3 > rGO-R1.

XRD spectra of GO and GO-R1-3, rGO and rGO-R1-3

Wide-angle X-ray scattering (WAXS) of the pristine GO displays only a typical sharp peak at ≈10.01° (see Figure 5-10), corresponding to an interlayer spacing of 0.87 nm due to the (002) reflection of stacked GO sheets, in accordance with the previously reported value.

Interestingly after reduction with hydrazine, such peak disappears and a new broad peak at 23.94° appears corresponding to an interlayer distance of 0.38 nm, which has been attributed to π-π stacking between the rGO sheets due to the partial restore of the graphitic structure.

When GO is reacted with organic aliphatic amines (R1 and R2), a bimodal packing has been observed with the presence of a first broad peak at 18.98° (0.47 nm) and 21.55° (0.41 nm) and a second sharp peak at ≈ 9.55° (0.93 nm) and 8.36° (1.06 nm) for GO-R1 and GO-R2

respectively. These peaks suggest that the functionalization of GO by aliphatic amines is occurring concomitantly to a partial reduction of the GO sheets thus the spacing is dictated by both the aliphatic amine (≈ 1 nm) and π-π stacking interactions (≈ 0.4 nm) of graphitic domains. Interestingly such bimodal packing is not observed for the case of the aromatic amine (R3) which is instead characterized by a single peak at 9.10° (0.97 nm) indicating that the spacing is only dictated by the aromatic molecular spacer (4-aminobiphenyl) while the reduction process is not taking place at a reasonable extent to give rise to observable π-π stacking interactions. Such observations have been further corroborated by WAXS analysis performed on the samples after reduction with hydrazine. Indeed a bimodal packing is observed for all the samples in particular the first peak for rGO-R1, rGO-R2 and rGO-R3

was observed at 23.74°, 23.12° and 24.71° which correspond an interlayer distance of 0.37 nm, 0.38 and 0.36 nm, respectively and ascribed to π-π interactions 37,38 , while the second peaks are shifted toward lower angles, from 11.38° (0.78 nm) for rGO-R1, 11.98° (0.74 nm) for rGO-R2 to 12.51° (0.71 nm) for rGO-R3 due to the presence of the organic spacer.

IR spectra of GO, rGO, rGO-R1-3

The successful functionalization and reduction of GO has been further confirmed by infrared spectroscopy (FTIR). As shown in Figure 5-12, pristine GO displays the typical peaks corresponding to a variety of oxygen-containing functional groups. In particular, the broad band around 3390 cm -1 is typical of O-H stretching vibrations, the peak at 1724 cm -1 has been attributed to the C=O stretching of carboxylic acid group; the peak at 1624 cm -1 to the sp 2 C=C bonds, while the peaks at 1216 cm -1 and 1054 cm -1 have been attributed to C-O stretching of epoxy and alkoxy groups, respectively. The disappearance of the peak at 1724 cm -1 and a significant decrease in peak intensity within 3200-3500 cm -1 indicate the successful reduction of the functionalized GO upon hydrazine treatment. The sp 2 C=C (1623 cm -1 ) and =C-H (999 cm -1 ) bonds still can be observed. Typically, cross-linking reaction between GO and amine derivatives results in the formation of amides (at the edges of GO flakes) and opening the epoxy groups (located on the plane of GO flakes). After reaction, the epoxy groups in the GO disappear, and a new band at 1169 cm -1 , 1192 cm -1 , 1163 cm -1 appears in rGO-R1, rGO-R2 and rGO-R3, respectively. These bands are attributed to the C-N stretching mode. The most important change observed after amine treatment is a strong decrease in intensity of the C=O band of COOH groups along with appearance of strong absorption at 1560-1580 cm -1 , which is corresponding to N-H stretching vibration. In the case of rGO-R2, the asymmetric peaks at 2911 and 2842 cm -1 are assigned to the C-H stretching vibrations of the alkyl group -CH2 of the alkyl amine.

BET analysis of rGO, rGO-R1-3

Figure 5-13a shows the nitrogen sorption isotherms of rGO, rGO-R1, rGO-R2 and rGO-R3 samples, and the gas sorption analysis results are listed in Table S1. All of the four samples are mesopore-structured as observed from the shape of nitrogen sorption isotherms, and the pore size distribution (PSD). The average pore diameter calculated with the Barrett-Joyner-Halenda (BJH) model are amounts to 4.1, 6.2, 5.4 and 6.1 nm for rGO, rGO-R1, rGO-R2 and rGO-R3 respectively (Figure 5-13b). After functionalization and reduction, increase the porosity of the materials is observed and the specific surface area values of rGO-R1/R2/R3 are 123, 119, 128 m 2 /g which are all higher than that of rGO (29 m 2 /g). The isotherms of N2 adsorption and desorption of rGO and rGO-R1/R2/R3 are similar to type IV isotherm, which possess a distinct H4 hysteresis loop, revealing a mesoporous structure with narrow slit-like pores with irregular shape and broad size distribution. This type of isotherm is often used to describe adsorption-desorption process for 2D and 3D-carbon based materials. 45 In the present case, the isotherms revealed slow uptake followed by rapid approach to attain equilibrium.

This behaviour for gas adsorption may be due to the significant adsorption capacity.

Table 2. Specific surface area calculated from the nitrogen adsorption isotherm using the BET method. Pore volume obtained from Barret-Joyner-Halenda (BJH) desorption cumulative volume of pores and rGO, rGO-R1, rGO-R2 and rGO-R3 results obtained from BJH desorption average pore diameter.

BET surface area (m 2 g -1 ) Pore volume (cm

Temperature dependent electrical conductivity measurement

Electrical measurements at low temperature were done with a Nitrogen cryostat Oxford Instrument OptistatDN-V from 80 K to 300 K with a step of 10 K, controlled through an ITC503S Cryogenic Temperature Controller and interfaced with a dual channel Keithley 2636A sourcemeter. The electrical conductance of rGO-R1 has been measured at each temperature, from -0.1 V to 0.1 V (0.005 V steps), in a vertical geometry device based on two rigid quartz substrates covered with ITO. This alternative device was mandatory to get accurate results since the thermal expansion of PET is bigger than the quartz one, which would strongly affect the measurement. As a comparison, a lateral geometry has also been measured between -0.5 V and 0.5 V (0.01 V steps) with interdigitated gold electrodes (W = 1 cm, L = 60 µm).

In both cases, the conductivity is almost stable over the whole temperature range (Figure 5- 17), allowing excluding a hopping transport mechanism that usually demonstrates an exponential behavior of the conductivity while increasing the temperature. It indicates a direct tunneling transport due to the small voltage used, because of the low temperature dependence of this mechanism which is driven by the energy barrier width between states, i.e, the distance between reduced graphene sheets. 46 (a)

Conclusion

In summary, we have described a novel method to tune and boost the sensitivity of pressure sensors by using as active component a hybrid architecture comprising a mille-feuille assembly of rGO separated by covalently tethered molecular linkers. In analogy with the Hooke's law, our results clearly demonstrate how the sensitivity is effectively improved when flexible molecular linkers are employed. In particular, the sensitivity of pressure sensor is significantly increased by functionalization of graphene with more flexible molecules. The pressure sensor based on graphene oxide chemically modified with triethylene glycol amine (rGO-R1) exhibits a sensitivity as high as 0.82 kPa -1 , short response time (24 ms), ultralow detection limit (7 Pa), high durability (over 2000 times) and flexibility. By taking advantage of the compatibility of graphene for on-the-skin applications, we have demonstrated how our hybrid multilayer architecture can be employed for health monitoring and can be easily transformed into a matrix, which allows a 3D mapping of the pressure exerted by different object thus providing also spatial information. The device additional features such low power consumption (0.2 V operating voltage), large-scale fabrication process, commercially available raw material and low cost, makes it an appealing candidate for the technological applications in wearable health monitoring device, multimotion detection robotic and internet of functions. Furthermore, our pressure sensor is fabricated by solution processing, thus it is compatible with printed electronics solutions.

Chapter 6

AuNPs-TEG network based strain sensor

Inspired by the tunneling current ruled pressure sensor discussed in Chapter 5, we have designed a novel strain sensor based on gold nanoparticles (AuNPs) featuring a size of dozens of nanometers as conductive nanostructures in an active hybrid material of the strain sensor.

AuNPs are interconnected by tetra (ethylene glycol) dithiol (SH-TEG-SH) to form 3D networks deposited directly in between flexible interdigitated electrodes. Without applied strain, AuNPs are separated by insulated organic ligands which can effectively define the charge transfer among AuNPs at a certain range. Upon applied strain, the active material is either compressed (under compressive strain) or stretched (under tensile strain) thus having the organic ligands undergoing conformational change ruled by its flexibility. The interparticle distance, which can be seen as the insulating barrier for charge transport, will be decreased/increased under compressive/tensile strain respectively, thus increasing/decreasing the tunneling current exponentially when voltage is applied. Our results show that the developed strain sensor displays high gauge factor (GF) and highly sensitive response both to tensile strain and compressive strain as well as to the mixed motion and even vibrations.

Extensive effort has been devoted to the optimization of the structure design of device to achieve wireless sensing. In recent years, Radio Frequency IDentification (RFID) tag antennas have been widely employed in the field of Internet of Things (IoT) and cyberphysical systems (CPS) due to their superior properties such as passive, wireless, simple, compact size, and multimodal nature. 1,2 In this Chapter, the wireless data transmission method, i.e. RFID, has been employed to construct a wireless strain sensing system with

Introduction

Strain sensors, which respond to mechanical deformations providing an electrical output, have been extensively integrated into flexible and wearable electronic devices making it adapt for applications in health monitoring, 3 motion detection, 4 human-machine interface [5][6][7] and soft robotics 8,9 etc. The definition and the key performance indicators have been fully discussed in Chapter 3.3. Different materials such as metal nanoparticles, [10][11][12] carbon nanotube [13][14][15] , graphene [16][17][18] etc. have been used to fabricate strain sensors.

Nanoparticles gathered a large amount attention for application in strain sensing due to the easiness in controlling the composition, shape, size and molecular functionalization combined with a simple and cost-effective of fabrication. 19 In this Chapter, AuNPs have been chosen to serve as the active material in strain sensing devices. AuNPs have been already exploited as active sensing material of strain sensor, 10,20,21 the mechanism of which lies on the correlation between strain and electron tunneling among nanoparticles. Tunneling is a quantum mechanical effect indicating that electrons are able to move across a barrier (Figure 6-1).

According to classical models, if an object doesn't have enough energy to move across a barrier, it will be stopped. However, according to quantum mechanics, electrons are free to move in a conductor, but when they meet a barrier, their wave function decreases exponentially. If the barrier has a finite thickness, the electrons have a finite probability to pass the barrier. Based on this principle, we have designed a device architecture which utilizes AuNPs featuring a size of dozens of nanometers as electrically conducting nodes within an active hybrid material of the strain sensor. AuNPs are interconnected by SH-TEG-SH to form 3D networks sandwiched by flexible electrodes. The AuNPs-TEG network has been proved to respond to changes in the environment humidity 22,23 due to the reversible binding and unbinding of water molecules with TEG chains varying the conformation of the latter. In this way the distance between AuNPs is changed affecting the tunneling modulated charge transport through the network. As a strain sensor, the electrical properties of AuNPs-TEG network can also be modulated by strain. Without applied strain, AuNPs are separated by electrically insulating organic ligands which can efficiently limit the charge transfer through the particles network. Upon applied compressive strain, the active material is compressed thus squeezing the organic ligands due to the flexible nature. The interparticle distance which can be seen the barrier for electron transfer will decrease in size yielding to an exponential increase of the tunneling current. On the contrary, the interparticle distance increases under tensile strain, resulting in a decrease of the tunneling current.

To fully exploit such a mechanism in wearable sensing devices, all the components of the device are designed to be flexible. In this way, micro-patterned gold electrode on polyimide (PI) substrate was chosen to make the whole device flexible. Under certain bias voltage, the current passing through the device changes with strain, thus making it a piezoresistive strain sensor.

In addition to design a novel strain sensor featuring high sensitivity and flexibility, the data transmission method has also been intensively investigated to improve the sensing efficiency.

Among different wireless data transmission methods, RFID 2 has attracted a notable attention due to its potential for applications in structural health monitoring (SHM) 2,24 and IoT 25,26 .

These passive RFID tag features a simple structure, a compact size and a large-scale fabrication rendering suitable both for facile large-scale fabrication and adapt for daily life.

For this reason, the wireless data transmission method (i.e. RFID) has been exploited to construct a sensitive strain sensor that can be in simple contact to the outside world.

The system is consisting of two circuits: the chip circuit and the measure circuit (Figure 6-2).

Information can be transferred between the two circuits through mutual inductance. The measure circuit is an initiative circuit with an alternative power supply input. The chip circuit is a passive circuit which responds by the mutual inductance 𝐻. 𝑅2 is the functional part, i.e.

the sensing device with AuNPs-TEG network as the active sensing material.

𝑢 = 𝑖 � �𝑅 � + 𝑗𝜔𝐿 � -𝑗 1 𝜔𝐶 � � + 𝑖 � ⋅ 𝑗𝜔𝐻 (6 -1) 0 = 𝑖 � ⋅ 𝑗𝜔𝐻 + 𝑖 � � 𝑅 � 𝑗𝜔𝐶 � 𝑅 � + 1 + 𝑗𝜔𝐿 � � (6 -2) 𝑢 𝑖 � 𝑖 � ‖𝑉‖ = ‖𝑖 � ‖𝑅 � (6 -3) 𝑖 � 𝑅 � 𝑉 𝑅 � 𝑅 � 𝜔 = 1 �𝐿 � 𝐶 � (6 -4)

Results and discussion

Analysis of gold nanoparticles

The consecutive seed growth method 27 was applied to accomplish the synthesis of AuNPs whose details have been provide in the Experimental methods. To determine the concentration and the size of the AuNPs, a UV-Vis spectrum of the solution was recorded. The shape and morphology of the particles were investigated by SEM. To determine a distribution of the diameter of the nanoparticles, the image taken by STEM was analysed statistically. DLS was used to study the distribution profile of the hydrodynamic diameter of the nanoparticles.

UV-Vis spectrum is the most direct method to evaluate both the size and concentration of AuNPs based on the optical properties. 28 For spherical particles below 100 nm the Lambert-Beer law was used to calculate the Au(0) concentration:

𝐴 = 𝜀 𝑎𝑏𝑠 • 𝑐 • 𝑙 (6 -5)
where A is the absorption, 𝜀 𝑎𝑏𝑠 the extinction coefficient in M -1 cm -1 , c the concentration in M (mol/L) and l the width of the cuvette in cm. The concentration was determined at 400 nm, where the absorbance is mainly due to interband transition 29,30 (from 5d band to hybridized 6sp band) and the extinction coefficient is known to be 2.6 M -1 cm -1 for the estimation of the Au(0) concentration. 28 A unique photophysical response of gold nanoparticle occurs when it is exposed to light. The free electrons of AuNPs which are located on conduction band oscillate with the oscillating electromagnetic field of the light, causing charge separation and dipole oscillation along the electric field direction (Figure 6-3a). The amplitude of the oscillation reaches a peak at certain frequency, which is called surface plasmon resonance (SPR). 31 The SPR peak correlates with the size of the particles, 32 which allows an estimation of the particle size. 33 Three cycles of seed growth have been conducted during the synthesis process. As shown in Figure 6-3b, symmetric surface plasmon absorption was observed from the UV-Vis spectrum of the three samples after each cycle of nanoparticle growth. The spectra red-shifts with the increasing size of the nanoparticles, in accordance with reference results. 27 The results of the SPR peak, absorption at 400 nm and the resulting concentration and estimated size have been summarized in Table 6-1. c)

The dynamic radius increases with seed growth cycles, in accordance with the previously tested results. The hydrodynamic diameter of the final AuNPs after the 3 rd seed growth cycle is 35.20 nm. The hydrodynamic size refers to the size of a sphere that diffuses as the same rate as the particle being measured, and it is generally larger than the real radius of the particles due to the hydration layer attached to the particles in solution. Figure 6-6 shows the size distribution of the AuNPs synthesized with the seed growth method after the 1 st , 2 nd and 3 rd cycle, in which both the size and deviation increase accordingly. This might be caused by the heterogeneity of growth, which broadens the distribution after each cycle of nanoparticle growth. . This result has clearly demonstrated the inner strain of AuNPs-TEG network could be the reason of formation of the sunken lines. Also, it is easy to find that very few network bridges were formed over the electrodes without N2 blowing. This might due to the fact that the network is bonded more tightly after drying with N2, which will in turn facilitates the growth of the network. As shown in Figure 6-14, the relative resistance shows a linear relationship with strain in the range of -0.005 to 0.005. The mean value and standard deviation (SD) of the relative resistance of nine devices were measured, in which the mean gauge factor is 30.29 with a standard deviation of 10.25. The high gauge factor for both compressive and tensile strain demonstrates excellent strain sensing properties of AuNPs-TEG based strain sensor, which makes it promising for the applications in electronic skin, motion detector and health monitoring. Afterwards photoresist AZ1505 was spin coated with the same parameters as in direct laser writing of the devices (see above). IDEs with a channel length of 3 µm were patterned and the substrate was developed for 30 seconds in the developer (MIF 726).

The gold was wet etched in the KI/I2 solution for 20 seconds and the chromium layer was etched with chromium etchant (diammonium hexanitratocerate and nitric acid) for 10 seconds.

Substrate fabrication: photoresist (AZ1505) was spin coated on the substrate after gold deposition, and the mask was placed on top of it with the gold and chromium side facing downwards. Then the substrate and mask were exposed for 15 seconds under UV light and the photoresist was developed for 30 seconds with the same method. Afterwards the gold was etched in KI/I solution within 10 seconds.

Substrate releasing:

In the last step the substrates were immersed in Milli-Q water. With the sacrificial layer of PSSNa dissolved, the PI substrate was released on the water surface.  

Electrical connection

The AuNPs-TEG network being present around the interdigital part of the electrodes was carefully wiped, making sure that the network was only present in between the IDEs. The pads of the electrodes were connected to copper wires (0.1 mm diameter) with silver paste.

The connections were fixed with tape. The electrodes were encapsulated by placing a PF gel film on the interdigital part and placed in the vacuum evaporator to remove trapped air in the network. where the AuNPs network is bridging these two electrodes.

Electrical characterization

The electrical characterization was performed with a source meter (Keithley 2635 B System Source Meter) and the software LabTracer 2.0. The baseline current was measured by keeping the PET substrate relaxed and flat. Afterwards, tensile strain and compressive strain was tested through bending the PET sheets upwards and downwards respectively. Also, the effect of vibration of the device on the current was tested by tapping one end of the PET sheet while

Conclusion

In conclusion, a novel flexible strain sensor has been designed and fabricated, in which AuNPs-TEG network has been used as active sensing material. AuNPs have been synthesized by seed growth method with the diameter of 30 nm. The flexible electrode with PI substrate was fabricated by photolithography, and mask aligner method was used to increase the fabrication efficiency. The AuNPs-TEG network was formed by layer-by-layer deposition on the gold electrode. The channels of the IDEs were covered by AuNPs network uniformly which is crucial for the detection of efficient charge transport. The strain sensing properties have been tested and the device revealed a high sensitivity and fast response to both compressive and tensile strain. The gauge factor has been calculated to be 30.29 in the low strain range (-0.005 to 0.005). These excellent sensing properties have provided unambiguous evidence that the AuNPs-TEG network based strain sensors represent promising devices in the applications of electronic skin, motion detector and health monitoring. Bing present in sunlight, UV light is an electromagnetic radiation with wavelength from 10 nm to 380 nm, which has a wide application of photolithography technique in electronic industry. Triggered by UV, negative photoresist undergoes photo-induced polymerization process, in which the polymer precursors start to be polymerized or cross-linked to form a more rigid layer.

In this Chapter, a prototypical negative photoresist, i.e. SU-8 2002, is chosen to form composite material with graphene. SU-8 2002 is a high contrast, epoxy based photoresist designed for micromachining and other microelectronic applications. 1 The exposed and subsequently thermally cross-linked portions of the film are rendered insoluble to liquid developers. Upon exposure, cross-linking proceeds in two steps (1) formation of a strong acid during the exposure step, followed by (2) acid-catalyzed, thermally driven epoxy crosslinking during the post exposure bake (PEB) step. The stiffness of SU-8 2002 was determined by AFM nano indentation method, which is commonly used to gain quantitative insight into the mechanical properties of soft matter like cells. [2][3][4] The fabrication process of pressure sensor is similar to the one discussed in Chapter 5. Asprepared GO based hybrids dispersions were spray coated onto ITO coated PET substrates, followed by face-to-face assembly and wiring out by cooper wire and silver paste. The pressure sensing properties have been tested by recording the current vs bias voltage while applying a vertical pressure. Prior to the device characterization, the active material was placed under UV irradiation (365 nm) for different time to investigate the effect of UV irradiation on the device sensitivity.

fitting. Above results have demonstrated that the mechanical properties of SU-8 2002 can be finely tuned by UV light in a linear way. Chapter 7 Graphene-photoresist hybrid based pressure sensor with tunable sensitivity by UV XPS was used to characterize the composition of the product in each step. As shown in Table 7-1, the N content increased to 8.06% in GO-EDA compared to the negligible amount in GO, which indicates the successful functionalization of ethylene diamine onto graphene. After the reduction step, the oxygen content was clearly decreased from 14.37% to 10.30%, indicating a successful reduction step. The last step of the active sensing material synthesis consists in the reaction between rGO-EDA and SU-8 2002. Upon addition of polymer that comprises only carbon and oxygen atoms, the N1s signal of the final product decreases from 7.42% to 2.56%. Detailed analysis of XPS results are discussed in the Experimental methods. To test the pressure sensing characteristics of the functionalized graphene oxide, the pressure sensor devices were fabricated with the same method presented in Chapter 5. Commercially available ITO coated PET was selected as flexible electrode, on which the rGO-EDA-SU dispersion was spray coated as sensing layer. Two substrates were assembled in a face-to-face manner and wired out by cooper wire and silver paste. The whole device was encapsulated by transparent scotch tape to pass through UV light and to avoid the interference of humidity in the air. Electrical test has been conducted by applying bias voltage (0.2 V) between the electrodes while measuring the current simultaneously to the application of pressure to the system. The electrical response as a result of the applied pressure is shown in the Figure 7-6.

The black line represents the electrical signal of the as-prepared pressure sensor without any UV light treatment. Upon pressing continuously with a finger, the device shows certain electrical response which is however very unstable and irregular. All the peaks having

Synthesis details of rGO-EDA-SU:

25 ml water dispersion of GO (4 mg/ml) was diluted with 100 ml ethanol in a 250 ml roundbottom flask and sonicated for 30 min. Then 500 mg of ethylene diamine (EDA) was weighed into a 10 mL glass vial and dissolved in 5 ml ethanol. The amine solution was then added into the GO dispersion and refluxed while stirring overnight at 90 °C under N2 atmosphere. After cooling down the solution to room temperature, the dispersion was centrifuged for 10 min at 2000 rpm. Then the supernatant was removed and 200 ml of ethanol were added to the black sludge. The mixture was sonicated for 10 min and centrifuged again. The sonicationcentrifugation process was repeated 3 times. After the last centrifugation the black sludge (GO-EDA) was dispersed in 200 ml ethanol and sonicated 10 min.

The reduction process of GO-EDA was carried out by adding 2 ml of hydrazine hydrate to GO-EDA ethanolic dispersion followed by refluxing overnight at 90 °C under N2 atmosphere.

The purification performed through sonication-centrifugation cycles as described above for GO-EDA. The resulting dispersion (200 ml) of rGO-EDA was decanted and used for the next step synthesis.

The final synthesis step was carried out by the reaction of rGO-EDA and SU-8 2002. 10 ml rGO-EDA dispersion from the previous step was diluted with 15 ml ethanol and 25 ml acetone in a 100 ml round-bottom flask to increase the solubility of SU-8 2002. Then 1 ml SU-8 2002 (solid content: 29%) was added into the dispersion and refluxed while stirring overnight at 90 °C under N2 atmosphere. After cooling down the solution to room temperature, the dispersion was centrifuged for 10 min at 2000 rpm. Then the supernatant was removed and 100 ml of ethanol were added to the black sludge. The mixture was sonicated for 10 min and centrifuged again. The sonication-centrifugation process was repeated three times. After the last centrifugation the black sludge (rGO-EDA-SU) was dispersed in 10 ml ethanol and sonicated 10 min. The functionalized graphene material has been finalized and ready to be integrated into pressure sensor devices.

X-ray Photoelectron Spectroscopy

The chemical bonding of each functionalization step of GO was analyzed by X-ray photoelectron spectroscopy (XPS). The full range spectra shown in Figure 7-13 indicates the occurrence of N 1s peak at 400.11 eV after the functionalization step of EDA. The analysis of high resolution spectra of N 1s in Figure 7-14b, d, f (as well as Table 7-2, and Table 7-3) reveals the three types of nitrogen bonding at 398.85 eV, 399.85 eV and 401.67 eV, which could be assigned to be primary amine (C-NH2), secondary amine (C-NH-C), and protonated primary amine ( C-NH3 + ). After the reduction by hydrazine, the oxidation degree of GO has shown a prominent decrease from 14.37% to 10.30% with the amount of nitrogen species remains unchanged. The high resolution C1s spectra in

Chapter 8

Conclusions and perspectives

In this Chapter, we draw some general conclusions from each sub-project and provide some short-term and long-term perspectives based on our approaches and on this field of science.

Conclusions

This Thesis work has been focused on the design and fabrication of low-dimensional nanomaterials (i.e. graphene and gold nanoparticles) based flexible pressure and strain sensors featuring high sensitivity, high flexibility, fast response, etc. which can be applied in the field of human health monitoring. leading to the highest sensitivity to detect changes of pressure. In analogy to the Hooke's law ruling the compressibility of macroscopic springs, we demonstrated that the sensitivity of the pressure sensor can be improved from rGO-R3 (0.32 kPa -1 ) to rGO-R1 (0.82 kPa -1 ) by using more flexible linkers acting as molecular springs separating rGO layers. Apart from high sensitivity, the pressure sensor (rGO-R1) also demonstrated high flexibility, fast response (less than 24 ms), and good robustness (over 2000 cycles). By taking advantage of the compatibility of graphene for on-the-skin applications, we have demonstrated how our hybrid multilayer architecture can be employed for health monitoring and can be easily transformed into a matrix, which allows a 3D mapping of the pressure exerted by different object thus providing also spatial information. The device additional features such low power consumption (0.2 V operating voltage), large-scale fabrication process, commercially available raw material and low cost, makes it an appealing candidate for the technological applications in wearable health monitoring device, multimotion detection robotic and internet of functions. Furthermore, our pressure sensor is fabricated by solution processing, thus it is compatible with printed electronics solutions.

The second research project discussed in Chapter 6 has been based on the design and fabrication of a flexible strain sensor in which gold nanoparticles featuring a size of dozens nanometers were employed as conductive nanostructures in an active hybrid material. The working principle of the strain sensor was the electron tunneling through gold nanoparticles under bias voltages. Gold nanoparticles are interconnected by tetra (ethylene glycol) dithiol (TEG) to form 3D networks deposited directly in between flexible interdigitated electrodes.

Without applied strain, AuNPs are separated by insulated organic ligands which can effectively define the charge transfer among gold nanoparticles at a certain range. Upon applied strain, the active material is either compressed (under compressive strain) or stretched (under tensile strain) thus having the organic ligands undergoing conformational change ruled by its flexibility. The interparticle distance, which can be seen as the insulating barrier for charge transport, will be decreased/increased under compressive/tensile strain respectively, thus increasing/decreasing the tunneling current exponentially when voltage is applied. The strain sensing properties have been tested and the device demonstrated high sensitivity and fast response to both compressive and tensile strain. The gauge factor has been calculated to be 30.29 in the low strain range (-0.005 to 0.005). Our results show that the developed strain sensor displays high gauge factor and highly sensitive response both to tensile strain and compressive strain as well as to the mixed motion and even vibrations. Apart from the superior strain sensing properties of the strain sensor, the wireless data transmission method (i.e. RFID) has been employed to construct a wireless strain sensing system with AuNPs-TEG networks being active sensing material. The system is consisting of two circuits, which are the chip circuit and the measure circuit, respectively. The strain sensor was integrated into the chip circuit as a variable resistor, in which the resistance changes with strain thus affecting the current in the measure circuit through mutual inductance. The fabrication and measurement of strain sensor have been accomplished while the wireless sensing is under testing (see Perspectives).

In the third research project, a novel pressure sensor whose sensitivity can be tuned with UV light irradiation has been reported. The working principle is similar to the one of the first project. By employing photosensitive polymers and grafting them onto graphene flakes, the rigidity of the hybrid structure can be modulated with UV light irradiation as a result of photo-induced polymerization. The current passing through the hybrid materials is directly ruled by the distance between adjacent graphene flakes, which is controlled by the applied pressure. By irradiating with UV light at different power/time, the degree of cross-link of the photosensitive polymer can be tuned yielding ultimately to a modulation of the hybrid's compressibility. The latter determines the device sensitivity. In this project, a prototypical negative photoresist SU-8 2002 was chosen to form hybrid material with graphene. Negative photoresist is a kind of polymer widely used in photo lithography, whose rigidity will increase as a result of the polymerization triggered by UV. Prior to the synthesis of the hybrid material,

Perspectives

Short-term perspectives

Short-term attentions will be focused on the open projects which were discussed in Chapter 6 and Chapter 7.

In Chapter 6, the design and fabrication of AuNPs-TEG network based strain sensor has been accomplished and the electrical strain sensing properties under dynamic and static stimulations have been fully studied. With the high sensitivity, fast response, and high flexibility, the strain sensor is very promising for the applications of human health monitoring as demonstrated in Chapter 5. The strain sensor can be attached on the wrist or neck of subject to monitor the pulse signal by recording the current change under bias voltages.

Noteworthy, the difference with the functionalized graphene based pressure sensor is that the current change of the strain sensor is caused by the subtle strain change, in which higher flexibility and conformability is needed during the sensing process. For this reason, PET substrate might not be suitable in this case due to the lack of stretchability. In the following on-skin test, thin PDMS film (~2 mm thick) could be employed as substrate for a better attachment of the device. Efforts should also be paid on the fabrication of wireless data transmission setup of the strain sensing system. RFID technique has been employed in this project, in which the strain sensor was integrated in the chip circuit as a variable resistor.

Based on the preliminary research results, the use of commercially available RFID chip is more promising due to the complexity of the circuit design and adjustment. Realization of wireless strain sensing with multi-discipline cooperation will be one of the key goals in the following research and will largely improve the practicability and efficiency of the strain sensor.

In Chapter 7, a pressure sensor featuring tunable sensitivity by UV light was designed and fabricated. Functionalized graphene oxide with SU-8 2002 (positive photoresist) was synthesized and characterized as active sensing material of the device. The graphene hybrid materials were spray coated on ITO coated PET substrates and the substrates were assembled in a face-to-face manner with active sensing materials sandwiched in between. The major problem of this project is the unstable electrical response to pressure simulations, which is caused by the irregular stack of the aggregated hybrid materials. Dispersion of the graphene hybrid into polymer matrix could be a promising method to prevent the irregular dislocation movement of the aggregates under pressure. SU-8 2002 is a very viscous liquid photoresist in which the epoxy-based molecule SU-8 is dissolved in organic solvents such as propylene glycol methyl ether acetate (PGMEA), cyclopentanone, or gamma-butyrolactone (GBL). To improve the deposition quality of the active sensing materials, the functionalized graphene oxide aggregates can be dispersed into SU-8 2002 to form a hybrid ink which can be spray/spin coated onto the flexible substrate. Polymerization triggered by UV light can not only modulate the rigidity of the active sensing layer, but also fix the aggregates inside the polymer matrix to form a more stable stacking structure. The pressure sensor with tunable sensitivity by UV light can be fabricated with this method.

Long-term perspectives:

Long-term and more visionary perspective could include:

Development of pressure/strain sensors with the on-demand molecular linkers will enable to further improve the sensing properties such as detection range, response time, etc.

In Chapter 5, we have proved that the use of highly flexible molecular linkers can improve the sensitivity of pressure sensor. In the following research, different molecular linkers featuring high flexibility (e.g. long-chain polymers, dendrimers, etc.) will be investigated to achieve better pressure or strain sensing properties. Natural macromolecules such as DNA, proteins, cellulose, polysaccharide, etc. can also be employed to improve the biocompatibility and biodegradability of the sensing devices.

Other sensing devices such as humidity sensor, ion sensor, pH sensor, etc. will be developed with the space-interval engineering concept, i.e. molecular springs. In Chapter 2, the existence of molecular springs with different stimuli responsive properties was discussed, which can be integrated into pressure and strain sensing devices serving as dynamic sensing unit. Different molecular springs can be employed to serve as space interval between conductive units such as graphene flakes, CNTs, metal nanoparticles or nano wires, etc. For example, molecular springs featuring specific ion combination sites can be integrated with gold nanoparticles, in which the tunneling current can be monitored to give the information of the present and concentration of target ions. With the instruction of this design principle, different sensors featuring superior sensing properties can be developed.

In addition, the development of wireless pressure and strain sensing is also an important longterm goal. In 2019, Bao and coworkers 1 reported a flexible wireless pressure sensor which can be used to monitor arterial blood flow and other various surgical operations. Capacitive pressure sensor was integrated into the RFID chip as a variable capacitor, in which the change of pressure will cause a shift of the resonant frequency of the inductor-capacitor-resistor (LCR) circuit. Resistive pressure sensor has also been employed in the wireless sensing system to analyze a person's pulse, breath and body movement. 2 In our design in Chapter 6, flexible strain sensor was integrated into the RFID chip as variable resistor. Instead of monitoring the resonance frequency of the LCR circuit, the induced current was recorded as a function of strain. Future work will focus on the integration of the pressure and strain sensors into RFID system to achieve wireless sensing. The development of corresponding software or APP will be implemented through inter-discipline corporations. I also would like to express my gratitude to Dr. Artur CIESIELSKI. He is not only my cosupervisor, but also a big bro who is always willing to help and to cultivate my confidence.

He is a great model for us with his intelligence and profession for work, responsibility and love for family, optimism and enthusiasm for life.

Nanochemistry lab has always been an international group, in which I enjoy the multi-cultural environment very much. I would like to thank all my lovely colleagues (also including those who have already left this lab) for your kind help, in particular the Losers Support Team (with Yifan, Yuda, Zhaoyang), Minghao (the most talented friend of me), Ye, Verónica, Irene,

苟利国家生死以,岂因祸福避趋之

In line with the conviction that I will do whatever it takes to serve my country even at the cost of my own life, regardless of fortune or misfortune to myself.

Chang-Bo HUANG

Nanomatériaux fonctionnels à faible dimensionnalité pour la détection de pression et de tension

Résumé L'objectif de cette thèse a été la conception chimique et la fabrication de capteurs de pression/déformation avec des matériaux actifs pour une détection fine afin de surveiller la santé humaine. L'ingénierie de la compressibilité du matériau actif à travers des molécules modulables (telles un ressort moléculaire ou un polymère photorésistant) est la principale nouveauté de cette thèse. L'hybride graphène-molécule a d'abord été utilisé comme matériau actif de détection de pression dans lequel la sensibilité peut être réglée en modifiant la rigidité des ressorts moléculaires. Une stratégie de conception similaire a été appliquée au capteur de déformation basé sur le réseau AuNPs-TEG, dans lequel le signal de détection peut être transmis via un système RFID sans fil. Dans le dernier projet, la résine photosensible a été utilisée pour fabriquer un capteur de pression hybride à base de graphène possédant une sensibilité réglable par irradiation UV. Dans l'ensemble, cette nouvelle conception de matériau de détection de pression/déformation a fourni une méthode efficace pour fabriquer des capteurs de pression/déformation très sensibles. Les caractéristiques supplémentaires de l'appareil telles que la faible consommation énergétique (tension de fonctionnement de 0.2 V), le processus de fabrication à grande échelle, les matières premières disponibles commercialement, le faible coût de production et, plus important encore, la détection sans fil, en font un candidat attrayant pour les applications technologiques portatives contrôlant la santé, pour des dispositif de surveillance, dans la robotique de détection multi-mouvement et pour l'IoT.

Mots-Clés : graphène, capteur de pression, capteur de déformation, ressort moléculaire, Surveillance de la santé

Résumé en anglais

The aim of this Thesis is the chemical design and fabrication of pressure/strain sensor with delicately designed active sensing materials for human health monitoring applications. Engineering of compressibility of active material through tunable molecules (e.g. molecular spring, photoresist polymer) is the main novelty in this thesis. Graphene-molecule hybrid has been firstly employed as active pressure sensing material in which the sensitivity can be tuned by changing the rigidity of molecular springs. Similar design strategy has been applied on the AuNPs-TEG network-based strain sensor, in which the sensing signal can be transmitted through RFID system in a wireless manner. In the last project, photoresist has been utilized to fabricate hybrid graphene material-based pressure sensor possessing tunable sensitivity by UV irradiation. Overall, this novel design of pressure/strain sensing material has provided an effective method to fabricate highly sensitive pressure/strain sensors. The additional device features such as low power consumption (0.2 V operating voltage), large-scale fabrication process, commercially available raw material, low cost, and more importantly, the wireless sensing, make it an appealing candidate for the technological applications in wearable health monitoring device, multimotion detection robotic and IoT.

Key words: graphene, pressure sensor, strain sensor, molecular spring, health monitoring

Chapter 7 :Chapter 8 :

 78 In this chapter, a new type of pressure sensor of which the sensitivity can be tuned with UV light irradiation has been devised by using a negative photoresist(SU-8 2002) to form composite material with graphene. Graphene composite materials have been synthesized and fully characterized by XPS, RAMAN, SEM, etc. Electrical tests of the pressure sensing performance have been preliminarily carried out. Draws the conclusions of this thesis project and put this in the context of shortterm and long-term perspectives. molecular springs are artificial nanoscale objects possessing well-defined structures and tunable physico-chemical properties. Like a macroscopic spring, supramolecular springs are capable of switching their nanoscale conformation as a response to external stimuli by undergoing mechanical spring-like motions. Such dynamic action offers intriguing opportunities for engineering molecular nanomachines by translating the stimuli-responsive nanoscopic motions into macroscopic work. These nanoscopic objects are reversible dynamic multifunctional architectures which can express a variety of novel properties and behave as adaptive nanoscopic systems. In this chapter, we focus on the design and structure vs.

  in 2013. A semi-artificial curdlan (α β-1,3-glucan) featuring a dendritic amphiphilic Zn-chlorophyll unit attached to each glucose residue (CUR-Chl) (Figure 2-4a) has been prepared to form a onedimensional right-handed helical structure with a helical pitch of approximately 1.8 nm.

Figure 2 - 4 .

 24 Figure 2-4. (a) Chemical structure of CUR-Chl and (b) potential ligands for coordination with its chlorophyll units. (c) Schematic representation of the controllable motion (contraction/expansion) of CUR-Chl through molecular recognition of its peripheral moieties.

  e:3',2'-e']benzo[1,2- b:4,3-b']-bis[1]benzothiophene has been recrystallized into racemic mixtures (PM-1) (Figure2-7a) from ethanol or 1,2-dichloroethane yielding different host-guest stoichiometric ratios, i.e. (PM-1)•EtOH and (PM-1)2, respectively (Figure2-7b-c). As shown in Figure2-7b, two homochiral stacked columns, i.e. a right-handed strand of (P-1)•EtOH and a left-handed strand of (M-1)•EtOH, are formed by intermolecular hydrogen bonding of host molecules, in which the ethanol molecules are locked with the homochiral columns by hydrogen bonds. As governed by the interaction of π-π forces between the neighboring benzothiophene rings (ring 4-5) and by the hydrogen bond, the interplanar angle between two terminal thiophene rings of (PM-1)•EtOH results 38.0°. When the racemic helicenediol was recrystallized from 1,2dichloroethane, (PM-1)2•(1,2-dichloroethane) was formed with a 2:1 host-to-guest stoichiometry as displayed in Figure2-7c. The terminal hydroxyl groups of a single molecule are located between two hydroxyl groups of the opposite enantiomer and held by hydrogen bonding, which extends the interplanar angle from 38.0° to 54.5° as an extension mode. This helicenediol is among the earliest reported supramolecular springs though the solvent dependent contraction-extension modes are modulated in the crystalline state.

3 . 2 Å

 32 in solution. Once oxidized by one-electron oxidant such as tris(4-bromophenyl) ammoniumyl hexachloroantimonate [(4-BrC6H4)3N •+ SbCl6 -], the helices adopt a more compact conformation with interplanar distances of ca. 3.1 Å. and feature a slower switching speed thus allowing a long-lasting chiroptical memory due to a decreased π-electronic repulsion. The reversible contractionextension mode can be switched by oxidation and reduction with [(4-BrC6H4)3N •+ SbCl6 -] and Zn respectively.

Figure 2 - 8 .

 28 Figure 2-8. Schematic representation of the helical inversion dynamics of OP8NO2 in solution

79 Figure 2 -

 792 Figure 2-11. (a) Cartoon of the covalently tethered 2D network of AuNPs with di-thiolated PEG; (b) SEM image of the network; (c) reversibility of the sensing process by monitoring

Figure 3 - 2 . 1 )

 321 Figure 3-2. The major pressure sensing mechanism. a) piezoresistive pressure sensor; b) capacitive pressure sensor; c) piezoelectric pressure sensor. Reproduced from Ref. 14 .

  Micrometer scale pyramid like structures have been duplicated on the dielectric layer by casting PDMS in the silicon mould with inverse features (Figure 3-4). This novel capacitor features microstructure dielectric layers has shown high sensitivity (0.55 kPa -1 ) and much faster response time (millisecond timescale) compared to the one with unstructured dielectric layer.

Figure 3 - 4 .

 34 Figure 3-4. Schematic process for the fabrication of microstructured PDMS films.

  have reported a high performance piezoelectric devices based on aligned arrays of nanofibers of poly(vinylidenefluoride-cotrifluoroethylene) [P(VDF-TrFe)]. As shown in Figure 3-5, free standing films with highly aligned copolymer fibers have been acquired by electro spun process, in which the polymer chains adopt strongly preferential orientations. The obtained films are mechanically robust and respond to both compressive and bending forces. It shows high sensitivity even in the low-pressure region (0.1 Pa) by simply contact the ends of the aligned fibers. This method (electro spun) has provided an effective way to well align the polymer fibers as well as the main molecular chains along the fiber longitudinal axis, in which the active dipoles (C-F) have been aligned thus enhancing the piezoelectric properties of the film. This flexible, robust, and piezoelectric freestanding film is attractive for the fabrication of large area devices for human motion monitoring or robotics.



  -7a. The SSNPs and polyurethane composite, used as active sensing material, exhibited excellent pressure sensing performance with a high sensitivity of 2.46 kPa -1 and fast response/relaxation time of 30 ms. The high sensitivity of this piezoresistive pressure sensor is based on the quantum tunneling effect among the SSNPs. As shown in Figure3-7b, the nanoparticles are separated from each other in the relax state and electrons are not prone to pass from one particle to adjacent ones. Conversely, under pressure the interparticle distance decreases enabling a more efficient quantum tunneling effect. By using ITO-PET as electrodes, the pressure sensor Ultrathin metal nanowires featuring superior conductivity, robust and flexible mechanical

Figure 3 - 9 .

 39 Figure 3-9. Schematics of the synthesis procedure of the GO/Graphene composite film. The as-prepared film is sufficiently flexible such that it can be bent easily without cracking; b)Resistive variation as a function of applied pressure. From 0 to 1000 Pa, the deformation changed linearly. Insets are the sketches of resistance changes of the film with and without pressure; c) Detecting the very slight pressure and subtle motions of an ant, which reflects the low detection limit of the GO/Graphene composite film and its fast response. Reproduced from Ref.49 .

  have reported a flexible and highly sensitive graphene-polyurethane sponge based pressure sensor with fractured microstructure design. The fabrication process has been shown in Figure3-10 a-e. PU sponge was immersed into GO solution to have a coating of GO, after which the GO-PU sponge was reduced by immersing into hot hydrogen iodide (HI) solution to form a conductive rGO layer on the sponge backbone. The final fractured microctructured design, which has been shown in Figure3-10 f-h, was achieved by the further hydrothermal treatment and compression (95% strain) for 2 hours. The intact adhesion of rGO on the PU microfibers has been demonstrated in the insets. As shown in Figure3-10i, the working principle of the 3D framework is based on the pressure dependent resistance. Larger contact area of the fiber network was form under pressure and recovered while the pressure is released. This fractured structure can effectively improve the sensitivity (0.26 kPa -1 , 0-2 kPa) compared the RGO-PU sponge without fracture design (0.001 kPa -1 , 0-10 kPa). The pressure sensor also demonstrated good cycling stability (10000 cycles) and large area fabrication, which make it promising in the field of e-skin applications.

Figure 3 -

 3 Figure 3-10. a-e) Schematic illustration of the fabrication procedure of fractured RGO-PU sponges; f-h) SEM images of the RGO-PU sponge, hydrothermally treated RGO-PU sponge (RGO-PU-HT), and the compressed treated RGO-PU-HT sponge (RGO-PU-HT-P); i) Pressure-sensing models of as-prepared RGO-PU-HT-P sponge pressure sensors, showing the contact area variation of fiber network with compressive deformation. Reproduced from

Figure 3 - 5 )

 35 Figure 3-12. (a) Results of molecular dynamics based numerical simulation for the Ag NPs thin film under elongation/relaxation processes; during the elongation process by the external load, micro-cracks were opened and propagated at higher strains. During the relaxation process, opened micro-cracks become smaller and closed with decreasing strains; (b) relative change of electrical resistance of the Ag NP thin film with and without initial micro-cracks under the stretching/releasing process calculated by numerical simulation. Reproduced from Ref. 83 .

Figure 3 -

 3 Figure 3-13. Modeling of tunneling effect in the resistor network: a) SEM image of possible tunneling effect among CNTs in the nanocomposites; b) modeling of tunneling resistance in the resistor network; and c) tunneling conductivity for various distances. Reproduced from Ref. 88 .

  -17a). This cheap, lightweight, mechanically compliant strain sensor has been used as body motion detectors which can effectively monitor human motions such as joint and muscle motion, breathing and pulse. CNTs composites also play an important role of constructing strain sensing devices due to its high conductivity and flexibility. In 2017, Zhang and coworkers 101 have reported a CNTs-thermoplastic elastomer (TPE) based strain sensor which can detect stretching, bending as well as torsion sensitively. The resistance increases under stretching due to the disconnection of CNTs as shown in Figure3-17b. The special gradient TPE structure not only renders the composite high sensitivity and fast response, but also hydrophobize the material surface, which make the CNT-TPE composite suitable for waterproof strain sensing coating materials.

Figure 3 - 19 .

 319 Figure 3-19. General hype cycle for technology. Free Copyright from

Figure 4 - 1 .

 41 Figure 4-1. Different approaches for the functionalization of GO. Reproduced from Ref.34

Figure 4 - 2 .

 42 Figure 4-2. Types of functionalization of gold nanoparticles and their potential biomedical applications. Reproduced from Ref.44

Figure 4 - 6 .

 46 Figure 4-6. Photo of substrate under IR lase scan. (red dash square: scanned area)

Figure 4 - 8 . 4 . 3 . 2 .

 48432 Figure 4-8. Commercially available RFID tag with two connecting pads.

Figure 4 - 9 .

 49 Figure 4-9. The test system consists of a digital force gauge ① (Mark-10, M7-025E, ~1.0 N), a motorized test stand ② (Mark-10, ESM-303E), sample test platform ③ (sample holder and compression plate with diameter of 1.15 cm), control panel ④, and software ⑤ (MESUR gauge Plus).

Figure 4 - 11 .Figure 4 - 13 .

 411413 Figure 4-11. Schematic of an upright Raman microscope. Reproduced from Ref.55



  -14, major components of the UV-Vis spectrophotometer including light source, filter, monochromator, beam splitter, photodiode, and data processing system have been demonstrated. The UV-Vis spectrophotometer used in this thesis is the model Jasco V670, in which two light sources (i.e. a deuterium lamp producing UV light from 200-400 nm, and a halogen lamp producing visible and near-infrared light from 400-2700 nm) have been equipped. Made by a grating and slit, monochromatic light can be generated and selected with a desired wavelength and intensity. Beam splitter is used to send the light signals to the sample and reference (normally made of the same solvent as the sample solution). Light signals passing through the sample and reference are collected by photodiode and analyzed as a plot of optical absorbance of the sample as a function of the wavelength. hydrodynamic radius. The Translational Diffusion Coefficient is extracted in the autocorrelation process by the correlator. The size distribution of all the particles in a sample can be acquired with DLS by showing the intensity of scattered light, and the results can also be converted to the volume or number size distribution for different research purposes.

Figure 4 - 16 .

 416 Figure 4-16. Illustration of the dynamic light scattering principle. Reproduced from Ref.56

  -17, the major components of a MultiMode AFM head used in this thesis consist of laser, mirror, cantilever, tilt mirror, and photodetector. Apart from those components, three sets of screws are designed to adjust the position of laser, photodiode and stage respectively.

Figure 4 -

 4 Figure 4-17. MultiMode SPM Head and Major Components. Reproduced from SPM Instruction Manual. Copyright © [2006] Veeco Instruments Inc.

Figure 4 - 18 .

 418 Figure 4-18. Typical Scanner Piezo Tube and X-Y-Z Electrical Configurations. Reproduced from SPM Instruction Manual. Copyright © [2006] Veeco Instruments Inc.

Figure 4 - 20 .

 420 Figure 4-20. Schematic of Scanning Electron Microscope a) internal components and b) working process. Copyright © 2017 NANOIMAGES.

Chapter 5 8 ,Figure 5 - 9 )

 5859 Figure 5-9). The interlayer distance (d002) between modified GO sheets with R molecules is characterized by wide-angle X-ray scattering (WAXS). After functionalization the 2 peak of GO-R1-3 shifts toward lower angles compared to pristine GO, which indicates an increase in the interlayer distance (Figure 5-10). The efficiency of the reaction has also been confirmed by Raman spectroscopy (Figure 5-11) and infrared spectroscopy (FTIR) (Figure 5-12). The porosity of functionalized graphene has been investigated by the means of BET measurements (Figure 5-13). Details on the details of analysis and results obtained by XPS, WAXS, Raman, BET and FTIR are provided in the Experimental Methods.

Figure 5 - 33 Figure 5 -Chapter 5

 53355 3b, quick press and release of a finger on the device leads to a quick response, which results in sharp peaks of current. The response time (t1) and recovery time (t2) of rGO-R1 are 24 ms and 10 ms (inset in Figure5-3b) respectively, which are 3~5 times faster than those reported for pressure sensors made with reduced graphene oxide.32, 3c shows the dynamic response of pressure sensor rGO-R1 during bending test. During bending cycles (1.5 cm bending radius, 100° bending angle) sharp peaks of current have been recorded demonstrating high flexibility and robustness of the sensor. By taking full advantage of the high sensitivity, the pressure sensor can also detect ultra-small pressure fluctuations as tiny as 7 Pa. A paper-folding star (ca. 70 mg, corresponding to 7 Pa, inset in Figure 5-3d) has been placed-and-removed continuously onto the pressure sensor, causing a distinct change of current. The ability of sensing such small pressures once again represents an unambiguous Graphene-molecules hybrid materials based pressure sensor for human health monitoring THÈSE (UNISTRA) par Chang-Bo HUANG 105 | P a g e

Figure 5 -

 5 4e reveals the four contact points of a stone (mass = 67.2 g), whose weight is mostly located in position "4C". By expressing the pressure value in the Z-scale of each pixel, a 3D map of the weight distribution has been obtained as shown in Figure5-4f. Such results provide clear evidence for the potential of such arrays for being implemented into multi-touch devices.

Figure 5 - 5 .

 55 Figure 5-5. Synthesis of triethylene glycol amine.

  1 H NMR (400 MHz, CDCl3) δ 7.80 (d, J = 8.3 Hz, 2H), 7.34 (d, J = 8.3 Hz, 2H), 4.21 -4.13 (m, 2H), 3.73 -3.65 (m, 2H), 3.65 -3.56 (m, 6H), 3.57 -3.49 (m, 2H), 3.37 (s, 3H), 2.45 (s, 3H).

Figure 5 - 6 .

 56 Figure 5-6. Photo of pressure sensor matrix based on rGO-R1.

Figure 5 -

 5 Figure 5-10. XRD spectra of (a) GO and GO-R1-3; (b) rGO and rGO-R1-3.

Figure 5 -

 5 Figure 5-12. IR spectra of (a) GO; (b) rGO; (c) rGO-R1; (d) rGO-R2; (e) rGO-R3.

Figure 5 -

 5 Figure 5-13. (a) BET adsorption-desorption isotherms of rGO, rGO-R1, rGO-R2 and rGO-R3; (b) Pore volume dV/dD plots for mesoporous rGO, rGO-R1, rGO-R2 and rGO-R3.

Figure 6 - 1 .

 61 Figure 6-1. Illustration of barrier penetration in classical and quantum field. Free use with reserved copyright from http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/barr.html.

  of the Lambert-Beer Law. Further morphology information and size distribution has been obtained by STEM.

Figure 6 - 4 :

 64 Figure 6-4: AuNPs after 3 rd cycle of the seed growth method.

Figure 6 - 6 .

 66 Figure 6-6. Size distribution of AuNPs of cycle 1, 2 and 3.

Figure 6 - 8 .Figure 6 -

 686 Figure 6-8. SEM images of AuNPs-TEG network on the electrode. (Channel length: 10 m)

Figure 6 - 9 .

 69 Figure 6-9. SEM images of AuNPs-TEG network formed a) without N2 blowing, b) increased amount of SH-TEG-SH (3.5/10), and c) normal amount of SH-TEG-SH (3.5/1).

Figure 6 - 14 : 9 devices

 6149 Figure 6-14: Relative resistance versus strain. (Mean value with standard deviation of 9 devices.) Material

Figure 6 -

 6 Figure 6-15. Seed growth of AuNPs up to 30 nm in diameter. Reproduced from Ref.27

Figure 6 - 18 :

 618 Figure 6-18: IDEs pattern for direct laser writing.

  Electrical characterization of the devices at cryogenic temperature were done by using a Nitrogen cryostat Oxford Instrument OptistatDN-V working between 80 K and 300 K (temperature steps of 10 K), controlled via an ITC503S Cryogenic Temperature Controller.The whole setup is interfaced with a dual channel Keithley 2636A source meter and controlled with a homemade LabVIEW program. The electrical conductance of AuNPs has been measured at each temperature, from -2 V to 2 V (0.05 V steps), in a planar geometry device based. This device is composed by a SiO2 substrates and IDEs (L = 20 m, W = 1 cm)

  fixing the other one. The sensor was wrapped around cylinders with different well-known diameters (Figure6-20a) to measure the gauge factor. This procedure was used for both tensile and compressive strain. The stability and the robustness of pressure sensors has been investigated by fatigue test, in which the periodic press and release on the sensor surface was generated by a digital force gauge (Mark-10, M7-025E, ~25 N) equipped with a motorized test stand (Mark-10, ESM-303E) as shown in Figure6-20b. All the above-mentioned tests were performed by applying a bias voltage (0.2 V) and measuring the current over time. ①force gauge ②motorized stand ③sample stage ④control panel ⑤MASURGauge software). a) b)

  photosensitive polymer can be tuned which ultimately to a modulation of the hybrid's compressibility. The latter determines the device sensitivity. In this Chapter, a prototypical negative photoresist SU-8 2002 was chosen to form hybrid material with graphene. Negative photoresist is a kind of polymer widely used in photo lithography, whose rigidity will increase as a result of the polymerization triggered by UV. SU-8 2002 is a high contrast, epoxy based photoresist designed for micromachining and other microelectronic applications. In this Chapter, the as-prepared hybrids have been blended with SU-8 2002 and irradiated with UV light to trigger polymerization between graphene flakes. The synthesis of active material and the fabrication of devices have been accomplished, and the characterization of sensing properties is under testing.

7. 1 .

 1 IntroductionGraphene based pressure sensors, which play an important role in the field of electronic skin, health motoring or other applications have been discussed in Chapter 3. In Chapter 5, we introduced a radically new generation of pressure sensors whose sensitivity is modulated by flexible molecular linkers placed in between graphene layers. Sensitivity of the pressure sensor can be effectively improved by chemical modification of graphene with highly flexible molecular ligands. Inspired by this rigidity modulation method, we have designed a new type of pressure sensor, of which the sensitivity can be tuned by UV light.

Figure 7 - 2 .

 72 Figure 7-2. SEM images of a new AFM tip (model: SCM-PIC, spring constant: 0.2 N/m). Top: side-view; Bottom: top-view.

Figure 7 - 3 .

 73 Figure 7-3. SEM images of a used AFM tip (model: SCM-PIC, spring constant: 0.2 N/m). Top: side-view; Bottom: top-view.

Figure 7 -

 7 14a, c display that the C=O (288.1 eV) and C-O (286.34 eV) of rGO-EDA drops from 17.07% to 10.17% and from 24.84 % to 21.25 %, respectively, implying the successful reduction. The C-N peak at 285.62 eV has witnessed a slight increase from 5.85% to 7.96% as a result of losing oxygen atoms during reduction. Further connection with the SU-8 2002 results in the drastically decrease of nitrogen atomic percentage to a negligible amount (0.94%) and an enhanced oxygen and carbon content as shown in Figure 7-14e. The disappearance of C-N peak and C=O peak is due to the screening of long chains of functionalized SU-8 2002 and thereby overwhelmingly relative increased of C-C and C-O, which make the signals of C-N and C=O undetectable in XPS. However, the last functionalization step has been confirmed by the high resolution N 1s spectrum as demonstrated in Figure 7-14f, in which rGO-EDA-SU converts to a major C-NH-C peak (76.83 % atom) accompanied by a decrease of the contribution from C-NH2 and C-NH3 + , which could be explained by the conversion of C-CH2 to C-NH-C upon addition of the epoxy group of SU-8 2002. On the other hand, the evident increase in C-O peak in Figure 7-14f further justifies the success of functionalization of rGO-EDA with SU-8 2002 that took advantage of the abundance in epoxy groups.

  The first research project discussed in Chapter 5 has been based on the design and fabrication of pressure sensors with functionalized graphene as the active sensing materials. A novel and simple method has been developed to drastically improve the sensitivity of a piezoresistive pressure sensor. The active material has been assembled by reacting commercially available GO with amino functionalized molecules in order to form covalent bonds onto the basal plane of GO through the epoxy ring-opening reaction. Three organic molecules characterized by a similar contour length and increasing rigidity, namely triethylene glycol (TEG) amine (R1), 1-octylamine (R2) and 4-aminobiphenyl (R3) were chosen as the molecular units. Upon condensation of R1-R3 with GO, hybrid structures have been obtained in the form of ink dispersed in ethanol. Such chemically modified GOs (CMGOs) have been chemically reduced with hydrazine to restore high electrical conductivity. The as-obtained dispersion can be deposited onto arbitrary substrates by spray-coating, yielding multilayer structures with spacing between adjacent GO sheets which are dictated by the employed molecular pillars, i.e. R1-R3 molecules. Significantly, the latter possess different compressibility, resulting from the intrinsic flexibility of the chosen molecules. The CMGO containing molecules possessing the highest flexibility should display the largest compressibility at a given pressure, thus

  the mechanical properties of SU-8 2002 have been investigated by AFM based nano indentation experiment, showing a linear increase of the Young's modulus with the UV irradiation time. Then the hybrid material was successfully synthesized and well characterized by XPS, Raman spectroscopy, and SEM. The pressure sensor device was fabricated by spray coating of the rGO-EDA-SU hybrid onto the commercially available ITO coated PET substrates, after which two substrates were assembled in a face-to-face manner to form a sandwich structure. Electrical response of the pressure sensing device was tested by recording the current change under bias voltage of 0.2 V. Unfortunately, the pressure sensors were not able to generate stable electrical signals under pressure simulation. As revealed by the SEM results, the hybrid material forms aggregates instead of layered structures which might be characterized by a heterogeneous stacking during the deposition step. When subjected to pressure, these aggregates could undergo irregular dislocation movement which causes unstable electrical response. A stable electrical signal could be acquired by achieving a greater control onto the assembly of the hybrid materials into a layer-by-layer architecture or by depositing these dispersed aggregates into polymer matrix etc. Further modification of pressure sensor structure design is under way (see Perspectives).

  The development of 4G (the fourth generation wireless technology) enabled us to have high speed internet connections, high quality video calls, and stable connections in the past 10 years (~2009-2019). With the higher demand of Internet of Things, 4G is no longer capable of connecting the huge number of smart electronic devices. 5G (the fifth generation wireless technology) has given birth paving the way towards greater speed in the transmissions, a lower latency, greater number of connected devices and network slicing, in which the lifechanging applications including autonomous vehicles, smart city infrastructure and traffic management, and smart wearables etc. can become a reality. Electronic sensing devices, as the core components of IoT, will meet a great increasing consequently in the following decades.Since I stepped on the foreign land for the first time on 2016.09.27, my life has started a brand-new chapter. I want to firstly thank my supervisor Prof. Paolo SAMORÌ for accepting me as a PhD student in his group, and for his valuable guidance during my PhD. With his encouragement, professional guidance and unlimited support, I was guided into the fascinating nanochemistry world and able to carry out independent research. I am very thankful to be guided by a world class scientist and I am very proud of working in this topnotch lab.

  Matilde, Lili, Samanta, Miriam, Haixin, Sara, Can, Rafael, Agostino, Marco S., Stafano I., Alessandro, Lei, Tim, Cosimo, Pietro, Dawid, Haijun, Daniel, Stafano C. Stefano D-B., Marco C., Xiaoyan. I will always remember those great moments we shared in Christmas dinners, annual barbecues, music festivals, and dinners at 6 th floor for reasons and for no reason. I will always remember our amazing trips in Easter holidays, summer holidays, and ski seasons. I will always remember our great Friday football nights. I would like also to thanks Corinne, Fanny, and Nicolas for their kind help during the past years.It would take another chapter to list all the great and crazy moments with my best friend, Marc-Antoine. Thank you for your help and for introducing me a bunch of nice friends: Justine, Nicolas, Chaudière, Romain, Maxim, Justin… And yes, it's just a start... I am deeply thankful to my family, in particularly my parents, for their endless love and support. This debt will never be paid, and I will love you until the end of my life. I also would like to thank Erika & Gunther BERL who treated me as family for the past years. These great moments will always be kept in my mind.At the end, this thesis is dedicated to my wife, Ying, on the occasion of our 10 years anniversary of love. She is the meaning of my life. With the long distance (8971 km) and the long separation (1265 days), our love never fades away.

Table of contents

 of

				Symbols and Abbreviations
	PEB	3.2.2.1.	0D materials post exposure bake
	PEG	3.2.2.2.	1D materials Symbols and Abbreviations polyethylene glycol
	Résumé de thèse 3.2.2.3. PET	2D materials polyethylene terephthalate	I
	PGMEA	3.2.2.4.	3D materials propylene glycol methyl ether acetate
	Abstract 3.3. Strain sensors49 VIII AFM atomic force microscopy PI polyimide
	Table of contents 3.3.1. Fundamental aspects of strain sensor49 AI artificial intelligence PPP poly-(p-phenylene) XV Symbols and abbreviations XIX 3.3.1.1. Definition and key parameters 3.3.1.2. Sensing mechanisms AIr augmentation index APTES (3-Aminopropyl) triethoxysilane PSSNa poly(4-styrenesulfonic acid) sodium salt PU polyurethane Chapter 1
	1. Introduction 1.1. Context1 1.2. Structure of the thesis3 1.3. References 5 2. Supramolecular springs 2.1. Introduction7 2.2. Supramolecular spring in solution and in single crystals10 2.2.1. Coordination driven helicates as supramolecular springs10 2.2.2. Thermally driven supramolecular springs14 2.2.3. pH-triggered supramolecular springs15 2.2.4. Solvent driven supramolecular springs16 2.2.5. Redox-responsive supramolecular springs18 4.3.4.2. SEM MEMS microelectromechanical systems XPS X-ray photoelectron spectroscopy 3.1. Introduction33 4.3.4.1. AFM LSCM laser scanning confocal microscopy Wi-Fi wireless fidelity 3. Concepts of pressure sensors and strain sensors 4.3.4. Surface characterization89  wavelength WAXS wide-angle X-ray scattering 2.6. References27 4.3.3.4. DLS ITO VR virtual reality indium tin oxide 2.5. Conclusion and perspectives26 4.3.3.3. UV-Vis spectroscopy IR Vis visible light infrared spectroscopy 2.4.2. 1D-based spring-like materials24 4.3.3.2. XPS IPA UV ultraviolet isopropanol 2.4.1. Spring-like materials based on 0D nanomaterials23 4.3.3.1. RAMAN spectroscopy IoT  Poisson's ratio internet of things 2.4. Spring-like macroscopic devices23 4.3.3. Spectroscopic characterization81 IDEs TPE thermoplastic elastomer interdigitated electrodes 2.3.2. Thermal fueled dynamic materials21 GO graphene oxide THF tetrahydrofuran 4.3.2. Force gauge setup80 2.3.1. Light-responsive dynamic materials19 GF gauge factor TEG triethylene glycol 4.3.1. Electrical measurement73 2.3. Self-assembled springs based dynamic materials19 AuNPs gold nanoparticles PVD physical vapour deposition 3.3.2. Active sensing materials54 3.3.2.1. 0D materials 3.3.2.2. BET Brunauer-Emmett-Teller BST PVDF polyvinylidene difluoride Introduction backscattered electron detector PVP poly (4-vinyl phenol) 1D materials 3.3.2.3. CCD charge coupled device  electrical resistivity 2D materials 3.3.2.4. CD circular dichroism RFID radio frequency identification 3D materials CMGO chemically modified graphene oxide rGO reduced graphene oxide 3.4. Applications and Development60 CNTs carbon nanotubes SD standard deviation 3.5. References63 CVD chemical vapor deposition SED secondary electron detector 4. Experimental techniques DCM dichloromethane SEM scanning electron microscope 4.1. General information70 GBL gamma-butyrolactone STEM scanning transmission electron microscope 4.3. Characterization techniques73 EtOH ethanol SPR surface plasmon resonance 4.2.2. Synthesis and functionalization of gold nanoparticles (AuNPs) 72 EDA SiO2 silicon dioxide ethylene diamine 4.2.1. Functionalization of graphene71  Si silicon strain 4.2. Synthesis of active materials71 DLS dynamic light scattering SH-TEG-SH tetra (ethylene glycol) dithiol 1.1. Context
	3.2. Pressure sensors34 4.4. References94 OFET organic field-effect transistor XRD X-ray powder diffraction
	3.2.1. Fundamental aspects of pressure sensor34 5. Graphene-molecules hybrid materials based pressure sensor for human health monitoring PCB printed circuit board
	3.2.1.1. 5.1. Introduction98 Definition and key parameters PDMS polydimethylsiloxane
	PE	3.2.1.2. 5.2. Results and discussion99 Sensing mechanisms polyethylene
		3.2.2. Active sensing materials41
				THÈSE (UNISTRA) par Chang-Bo HUANG THÈSE (UNISTRA) par Chang-Bo HUANG	XVI | Page XX | Page

Since few years we are witnessing a major progress in smart wearables solutions with the development of the advanced technologies such as AI (Artificial Intelligence) and VR (Virtual Reality) being integrated into wearable devices including smart watches, smart bands, smart glasses, smart rings, smart clothes, etc. These smart electronic devices are used to collect information on subject and/or environment and can give corresponding feedback based on the data analyzed either by the devices themselves or by the center network through wireless data transmission techniques such as Wi-Fi (Wireless Fidelity), Bluetooth, RFID (Radio Frequency IDentification), etc. Smart watch (commercialized by Apple, Huawei, etc.)

  Lin, Z. S.; Xu, F.; Sun, X. X.; Li, J. J.; Yin, W. L.; Li, Y. B.; He, X. D., Artificial muscle with reversible and controllable deformation based on stiffness-variable carbon nanotube springlike nanocomposite yarn. Nanoscale 2019, 11, 8124-8132.

83. Xu, L. L.; Peng, Q. Y.; Zhu, Y.; Zhao, X.; Yang, M. L.; Wang, S. S.; Xue, F. H.; Yuan, Y.;

Table 3 -

 3 1. Dimensional classification of nanostructures

	Dimensionality (D)	Description	Examples
	Zero-dimensional (0D)	All dimensions in nanoscale	Nanoparticles, C60, Quantum Dots
	One-dimensional (1D)	One dimension not in nanoscale	Nanorods, Nanotubes, Carbon Fibers
	Two-dimensional (2D)	Two dimensions not in nanoscale	Thin nanolayers, graphene
	Three-dimensional (3D)	Three dimensions not in nanoscale	Nanocomposites

Table 4 -

 4 1. PVP layer burned with IR laser under different power lever and pulse rate (kHz)

	Pulse rate Energy level	20	30	40	50	60	70 √

Table 6 -

 6 3: Cylinders with their inner and outer radii and the corresponding compressive and tensile strain.

	Cylinder mould	Inner radius r1 (compressive)	Calculated compressive strain	Outer radius r2 (tensile)	Calculated tensile strain 
	C1	13 mm	-4.81•10 -3	15 mm	4.17•10 -3
	C2	18 mm	-3.47•10 -3	20 mm	3.13•10 -3
	C3	28 mm	-2.23•10 -3	30 mm	2.08•10 -3
	C4	38 mm	-1.64•10 -3	40 mm	1.56•10 -3
	C5	48 mm	-1.30•10 -3	50 mm	1.25•10 -3
	C6	73 mm	-8.56•10 -4	75 mm	8.33•10 -4

Table 7 -

 7 1: survey of XPS results of GO, GO-EDA, rGO-EDA, rGO-EDA-SU

		Name	Peak BE FWHM eV	Area (P) CPS.eV	Atomic %
		C1s	286,45	4,26	1697077,80	67,40
	GO	O1s	532,94	2,91	1953047,15	32,08
		N1s (negligible) 402,14	3,78	20152,85	0,52
		C1s	285,21	3,27	1683357,60	77,57
	GO-EDA	O1s	532,19	3,58	753691,31	14,37
		N1s	399,96	3,40	271537,89	8,06
		C1s	285,11	3,08	1871892,79	82,28
	rGO-EDA	O1s	532,14	3,91	566190,50	10,30
		N1s	399,91	3,53	262029,29	7,42
		C1s	285,01	3,35	1381176,13	82,50
	rGO-EDA-SU	O1s	532,86	2,79	603995,42	14,94
		N1s	399,40	3,84	66506,09	2,56

Table 7 -

 7 2. C1s survey of GO-EDA, rGO-EDA, rGO-EDA-SU.

	C1s	Name	Peak BE	FWHM eV Area (P) CPS.eV Atomic %
		C-C	284,58	1,11	106853,1	52,25
	GO-EDA	C-O C=O	286,34 288,13	1,72 3,11	50731,11 34824,02	24,84 17,07
		C-N	285,62	1,11	11947,16	5,85
		C-C	284,57	1,07	120526,2	60,62
	rGO-EDA	C-O C=O	286,26 288,19	1,85 1,96	42211,8 20176,52	21,25 10,17
		C-N	285,59	1,07	15812,88	7,96
	rGO-EDA-SU	C-C C-O	284,33 286,15	1,33 1,56	122358 67630,23	64,38 35,62

Table 7 -

 7 3. N1s survey of GO-EDA, rGO-EDA, rGO-EDA-SU.In this Chapter, we have designed and synthesized a new functionalized graphene oxide material for pressure sensing application. Negative photoresist SU-8 2002 was used as elastic material which was covalently grafted to graphene oxide. Prior to the synthesis of the hybrid material, the mechanical properties of SU-8 2002 have been investigated by AFM-based nano indentation experiment, showing a linear increase of the Young's modulus with the UV irradiation time. Then the hybrid material was successfully synthesized and well characterized by XPS, Raman spectroscopy, and SEM. The pressure sensor device was fabricated by spray coating of the rGO-EDA-SU hybrid onto the commercially available ITO coated PET substrates, after which two substrates were assembled in a face-to-face manner to form a sandwich structure. Electrical response of the pressure sensing device was tested by recording the current change under bias voltage of 0.2 V. Unfortunately, we were not able to generate stable electrical signals under pressure simulation when rGO-EDA-SU hybrid was employed as active material. As revealed by SEM imaging of the hybrid material forms large aggregates

	N1s	Name	Peak BE	FWHM eV Area (P) CPS.eV Atomic %
		-NH-	399,9	1,54	14376,05	41,82
	GO-EDA	-NH 2	398,91	1,75	10606,78	30,84
		-NH 3 +	401,51	1,75	9387,92	27,34
		-NH-	400,05	1,98	13923,38	42,64
	rGO-EDA	-NH 2	399,11	2,22	13368,39	40,91
		-NH 3 +	401,67	2,54	5363,66	16,45
		-NH-	399,93	2,42	2828,47	76,38
	rGO-EDA-SU	-NH 2	398,86	1,72	533,95	14,41
		-NH 3 +	401,88	1,96	340,55	9,21

instead of layered structures which might be characterized by a heterogeneous stacking during the deposition step. When subjected to pressure, these aggregates could undergo irregular dislocation movement which causes unstable electrical response. A stable electrical signal could be acquired by achieving a greater control onto the assembly of the hybrid materials into a layer-by-layer architecture or by depositing these dispersed aggregates into polymer matrix etc. Further modification of pressure sensor structure design is under way.

applications en microélectroniques. Les parties du film exposées et réticulées thermiquement sont rendues insolubles aux révélateurs liquides. Lors de l'exposition, la réticulation se déroule en deux étapes : (1) un acide fort est formé pendant l'étape d'exposition, puis (2) la réticulation des époxy est catalysée par l'acide et accélérée thermiquement pendant l'étape de recuit finale (PEB). Dans ce projet, le composite tel qu'il a été préparé a été mélangé avec SU-8 2002 et irradié avec de la lumière UV pour former un polymère entre les feuillets de graphène. La synthèse du matériau actif et la fabrication de dispositifs ont été réalisées, mettant en évidence une potentiel application comme capteur de pression.En conclusion, l'objectif de cette thèse a été la conception chimique et la fabrication de capteurs de pression/déformation avec des matériaux actifs pour une détection fine afin de surveiller la santé humaine. L'ingénierie de la compressibilité du matériau actif à travers des molécules modulables (telles un ressort moléculaire ou un polymère photorésistant) est la principale nouveauté de cette thèse. L'hybride graphène-molécule a d'abord été utilisé comme matériau actif de détection de pression dans lequel la sensibilité peut être réglée en modifiant la rigidité des ressorts moléculaires. Une stratégie de conception similaire a été appliquée au capteur de déformation basé sur le réseau AuNPs-TEG, dans lequel le signal de détection peut être transmis via un système RFID sans fil. Dans le dernier projet, la résine photosensible a été utilisée pour fabriquer un capteur de pression hybride à base de graphène possédant une sensibilité réglable par irradiation UV. Dans l'ensemble, cette nouvelle conception de matériau de détection de pression/déformation a fourni une méthode efficace pour fabriquer des capteurs

THÈSE (UNISTRA) par Chang-Bo HUANG

AuNPs-TEG networks being active sensing material. Strain sensor has been integrated in the chip circuit as a resistor, in which the resistance was changing with strain thus affecting the current in the measure circuit through mutual inductance. The fabrication and measurement of strain sensor have been accomplished while the wireless sensing is under testing.

Acknowledgements THÈSE (UNISTRA) par Chang-Bo HUANG Acknowledgements At this special moment, I would like to express my highest gratitude to all the medical staffs and researchers all over the world who are fighting against COVID-19 in the frontline. I would like to thank my jury members: Prof. David BELJONNE, Prof. Marta MAS-TORRENT, Prof. Yves H. GEERTS, and Prof. Alberto BIANCO for their acceptance and valuable time to evaluate my work. High gratitude has been dedicated to Solvay for the valuable support of my PhD research. I am very grateful to Dr. Patrick. MAESTRO for his kind concern of my research work, and many thanks to his invitation of Solvay lab visit both in Lyon and Shanghai. Acknowledgements THÈSE (UNISTRA) par Chang-Bo HUANG

Chapter 4

Experimental techniques

In this Chapter, the experimental techniques employed during synthesis process of active materials described in this thesis (i.e. graphene and gold nanoparticles) have been discussed.

In addition, working principles of different characterization methods including SEM, AFM, Raman, and XPS etc. have been discussed in order to provide an accurate and comprehensive understanding of these novel functional materials.

All chemicals and materials such as poly (4-styrenesulfonic acid) sodium salt, Poly (pyromellitic dianhydride-co-4,4'-oxydianiline), PET film, PF-gel film, silver paste, cooper wire etc. were purchased from Sigma-Aldrich without further treatment.

6.3.1.1.

Synthesis of gold nanoparticles

The synthesis of AuNPs was accomplished by reduction of gold (III) derivatives by citrate in water, which was introduced by Turkevitch in 1951. This has been for many years the most popular method to produce AuNPs featuring diameters of ca. 20 nm. More recently other methods have been proposed including changing the ratio between reducing/stabilizing agents, 37 two-phase synthesis, 38 seeding growth, 27,39,40 and physical methods (e.g. photochemistry, [41][42][43] sonochemistry, 44,45 radiolysis, 46,47 and thermolysis 48 etc.) as a mean to achieve a better control over the particle composition, size, shape, etc.

In this Chapter, the seeding growth method was chosen for the synthesis of AuNPs according to the Ref. 27 , to generate monodisperse citrate-stabilized AuNPs with a uniform quasispherical shape of up to 30 nm and a narrow size distribution. Focusing on the inhibition of secondary nucleation during the homogeneous growth, AuNPs was enlarged uniformly by the surface-catalyzed reduction of Au 3+ by sodium citrate.

Synthesis of Au Seeds:

As shown in Figure 6-15, a solution of 2.2 mM sodium citrate in Milli-Q water (150 mL) was warmed up with a heating mantle in a 250 mL three-necked round-bottomed flask with a condenser for 15 min under vigorous stirring. 1 mL of HAuCl4 (25 mM) was injected while the solution was boiling. The color of the solution changed from yellow to bluish gray and then to soft pink in 10 min. The resulting particles (∼10 nm, ∼310 12 NPs/mL) are coated with negatively charged citrate ions and hence are well suspended in H2O.

Seeded Growth of AuNPs of Up to 30 nm in Diameter:

Following by the previous step, the solution was cooled to 90 °C. Then, 1 mL of sodium citrate (60 mM) and 1 mL of HAuCl4 solution (25 mM) were sequentially injected (time delay 2 min). After 30 min, aliquots of 2 mL were extracted for further characterization by transmission electron microscopy (TEM) and UV-vis spectroscopy. By repeating this process twice (sequential addition of 1 mL of 60 mM sodium citrate and 1 mL of 25 mM HAuCl4), gold particles of diameters of 30 nm were synthesized. The concentration of each generation of NPs was approximately the same as the original seed particles (310 12 NPs/mL).

Closing remarks

All the experimental data reported in this thesis were performed and collected by the author (Chang-Bo HUANG) except from the following:

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