Dr Mr Himanshu Maheshwari

Dr Neus Vilà

Grégoire Herzog
email: gregoire.herzog@univ-lorraine.fr

Dr Alain Walcarius
email: alain.walcarius@univ-lorraine.fr

Selective detection of cysteine at a mesoporous silica film electrode functionalized with ferrocene in the presence of glutathione

Keywords: Cysteine detection, Ferrocene mediator, Film electrode, Mesoporous silica, Selectivity

Unmodified and ferrocene-functionalised mesoporous silica layers exhibiting vertical nanochannels have been grown by electrochemically assisted self-assembly onto indium-tin oxide (ITO) and the resulting film electrodes have been applied to the electrochemical detection of cysteine. When using the unmodified film electrode and ferrocenedimethanol (Fc(MeOH)2) as mediator in solution, both cysteine and glutathione can be oxidised by Fc(MeOH)2 + generated at the electrode surface but the electron transfer rates were three times faster with cysteine than with glutathione. This can be exploited for the reagent-free selective detection of cysteine over glutathione by using a ferrocene-functionalised mesoporous silica film on ITO electrode, based on a combination of charge transfer kinetic and mass transport limitations through the oriented nanochannels. This has been demonstrated here by cyclic voltammetry and amperometry in a flow injection analysis mode.

A sensitivity of 1.28 mA M -1 was measured for the cysteine concentration range of 3-20 µM with a repeatability of the signal between 3 and 12 %. The lowest concentration of cysteine experimentally detected was 3 µM.

Introduction

Thiol molecules, such as cysteine, play an important role as antioxidants in physiological fluids. [1,2] Cysteine concentration in whole blood is generally in the sub-millimolar range. [3] As its concentration variation has been correlated with a variety of disease, the monitoring of cysteine in physiological fluids could be used as a diagnostic of the evolution of several pathologies and clinical conditions. [4][5][6][7][8][9] High levels of thiols in blood have been associated to cancer or diabetes, [5,8,9] while high thiol concentrations were observed in patients suffering from Alzheimer and Parkinson disease. [4,6] Cysteine is a molecule that can be electrochemically oxidised with a standard redox potential of -0.22 V vs NHE. [10] The overall electrochemical cysteine oxidation process leads to the formation of a disulfide bond as described in the reaction (1). [10] 2𝐶𝑦𝑆𝐻 → 𝐶𝑦𝑆𝑆𝐶𝑦 + 2𝐻 + 2𝑒

(1)

Due to low charge transfer kinetics at commonly used bare electrodes, overpotentials close to +1 V were observed on gold, platinum and glassy carbon electrodes. [10] Oxidation of thiols at such high potentials can lead to the oxidation of other molecules present in the solution and to the passivation of the electrode surface by reaction products, which limit the possibility of analytical applications. [11] Boron-doped diamond [12] and FTO [13] electrodes were also used for the direct oxidation of cysteine and thiol compounds at lower overpotentials. However, the alkaline [12] or acidic [13] conditions limited their analytical applications. The overpotentials are partially overcome by the addition to the solution of a variety redox mediators, which include: nitrosophenyl, [14] catechol, [15] quinone, [16] ferrocene derivatives, [17] cyclotricatechylene. [18] The electrode surface has also been modified to provide electrocatalytic properties in order to lower the thiol oxidation potential. The various strategies included the immobilisation of redox-active polymers such as Prussian blue, [19] poly (3,4-ethylenedioxythiophene), [20,21] poly(eugenol), [22] caffeic acid-based polymer. [23] Redox mediators were also trapped inside polymer matrix [24,25] or sol-gel material [26] for electrocatalytic thiol detection.

Electrodes modified with vertically aligned nanoporous silica membranes can be prepared by electrochemically assisted self-assembly (EASA) [27] or by a Stöber solution growth. [28] The EASA method, developed in our group, leads to the formation of silica membranes of a thickness ranging from 50 to 150 nm [29] and with pore diameters tuneable between 2 and 3 nm. [30] Organic moieties can be further introduced to the silica membranes. [31][32][33] The interest of such mesoporous silica membranes for electrochemical sensing has raised in recent years. [34][35][36][37] Mesopore diameters, [30] membrane thickness, [38,39] the solution pH and ionic strength, [40,41] the surface charge of the pores, [37,42] and the nature of the redox probes [43] have a strong influence on the electroanalytical signal. Furthermore, vertically oriented mesoporous silica films have demonstrated permselective ion transport and molecular sieving properties. [30,35,44,45] The negatively charged nature of the silica walls for pH > 3 allowed the transport of cationic and neutral species but restricting anionic ones. [44,45] Size selectivity was also demonstrated with molecular probes (ferrocene-functionalised dendrimers, ruthenium complexes and redox proteins), which dimensions were greater than the pore diameter. [30,46,47] We have investigated the oxidation of two thiols of biological relevance, cysteine and glutathione, at electrodes modified with mesoporous silica films. In a first set of experiments, the thiols were electrocatalytically oxidised by the presence of ferrocenium derivatives generated at the electrode surface. The influence of the film on the thiol oxidation mediated by Fc(MeOH)2 was studied by a series of cyclic voltammetry experiments. In a second set of experiments, ferrocene groups were covalently attached to the silica walls. The immobilisation of the redox mediator induced the selectivity for cysteine over glutathione, as it was demonstrated by flow injection analysis. synthesised as described previously. [31] 2.2. Electrochemical measurements EASA and cyclic voltammetry experiments were carried out on a PGSTAT 100 potentiostat (Autolab, Netherlands). A three-electrode system was used with an Ag/AgCl (3M KCl) reference electrode (Metrohm) and a stainless-steel rod as the counter electrode, and ITO plates as working electrode, modified with mesoporous silica thin films (unless stated otherwise). Chronoamperometry performed in the flow cell was measured using a PGSTAT302N potentiostat (Autolab, Netherlands).

Materials and Methods

Preparation of the mesoporous silica thin films

Mesoporous silica films were prepared by EASA as described previously. [27] A sol was prepared with 100 mM TEOS, 32 mM CTAB in 0.1 M NaNO3 in an ethanol: water 1:1 v/v ratio. The pH of the sol was adjusted to 3 by the addition of HCl. The sol was left to hydrolyse for 2.5 h before electrodeposition. The sol was prepared on a daily basis. A potential of -1.3 V was applied for 20 s (using an ITO plate as working electrode of a surface area of 0.502 cm 2 ), a silver wire as a pseudo-reference electrode and a stainless-steel mesh as a counter electrode). After the film formation, the electrodes were kept at 130 °C for 16 hours to ensure good cross-linking of the silica network. Surfactant template was removed by immersion in 0.1 M HCl in ethanol for 30 minutes. Mesoporous silica films were further characterized by electrochemistry (surface area exposed for the characterisation and cysteine detection work was 0.188 cm 2 ), grazingincidence small-angle scattering (GISAXS), and transmission electron microscopy (TEM) as described in the supplementary information section (Figures S1-4 and associated text). The level of mesostructural order was evaluated by TEM using a Philips CM20 microscope at an acceleration voltage of 200 kV. The samples were prepared by removing some pieces of the films, which were supported on a carbon-coated copper grid.

Functionalisation of the mesoporous silica thin films

The ferrocene-functionalized films were prepared in a two-step procedure described previously. [31] The first step consisted of the co-condensation by electrochemically assisted self-assembly from a sol prepared with a 70 / 30 molar ratio of TEOS / AzPTMS (total silane concentration was 100 or 200 mM). The CTAB / silane ratio was kept at 0.32. Other parameters are the same as for the non-functionalised films (section 2.2). After electrogeneration, the films were cured in the oven ( 16

Flow injection analysis

The flow injection cell was purchased from BASi® (USA). An ITO electrode modified with ferrocene-functionalized mesoporous silica film, according to section 2.4, was used as a working electrode (A = 0.407 cm 2 ). An Ag / AgCl / 1 M KCl was used a reference electrode and the stainless-steel body of the flow cell was used as the counter electrode. The total volume of the flow cell chamber was 2.22 mL. Flow injection analysis was performed in amperometric mode with an applied potential of +0.65 V while injection 150 µL aliquots of thiol at the desired concentration. The flow rate of the buffer electrolyte solution (0.1 M bis-tris + 0.1 M NaCl) was 1 mL min -1 . The buffer solution and the aliquots of cysteine were degassed by bubbling N2 to remove dissolved oxygen and hence avoid oxidation of thiol.

Results and Discussion

3.1 Thiol oxidation mediated by Fc(MeOH)2 at mesoporous silica film electrodes Mesoporous silica films were electrogenerated according to the described procedure. The quality and integrity of the films were assessed by a string of characterisation techniques (Figures S1-4). The EASA method has led to the formation of about 100 nm thick films with mesopores of 2.4 nm in diameter. Those pores are organised in a hexagonal compact arrangement and are oriented perpendicular to the electrode surface, allowing efficient mass transport of molecules from the bulk of the solution to the electrode surface, through the mesoporous silica films. [29] Figure 1A shows the cyclic voltammetry of 0.5 mM of Fc(MeOH)2 at an electrode modified with mesoporous silica film, which occurred at E1/2 = +0.252 V. The addition of 1.25 mM of cysteine to the solution led to the increase of the forward current and the decrease of the reverse peak current. The Fc(MeOH)2 + formed at the electrode surface transferred an electron with cysteine, and was reduced back to Fc(MeOH)2. Fc(MeOH)2 was then regenerated at the electrode surface, causing the rise in forward peak current, in good agreement with an EC' mechanism. [48] Similar results were observed when 1.49 mM of glutathione was added, indicating that the oxidation of the thiol function of glutathione could be oxidised in the same way as the one of cysteine. The difference in thiol concentration led to a faster drop in current observed for cysteine (Figure 1A) than for glutathione (Figure 1B). The scan rate was varied to understand the effect of reaction time on the voltammetric response of a solution of 0.5 mM of Fc(MeOH)2 in the presence of 1.25 mM cysteine at a mesoporous silica electrode (Figure 2). At 10 mV s -1 , the reverse peak is much smaller with a ratio of forward peak current over reverse peak current, if / ir = 1.9. The forward current increased with the scan rate along with the appearance of a larger current in the reverse scan, indicating that the overall reaction kinetics was limited by the rate of electron transfer between the thiol group and the redox mediator and not by the electron transfer between the redox mediator and the electrode. Figure 3 showed the if / ir ratio for concentrations of cysteine of 1.25 and 4.5 mM, at scan rates varied between 10 mV s -1 to 1 V s -1 . Those were compared to the reversible signal observed in the absence of cysteine (data shown as discs on Figure 3). For both cysteine concentrations, the if / ir ratio decreased as the scan rate increased, to reach unity at the fastest scan rate of 1 V s -1 . At such a scan rate, only the reversible electrochemical response of Fc(MeOH)2 was observed and no cysteine oxidation. The if / ir ratio was also compared at both bare and modified electrode s (Figure 3). The current ratio was slightly lower at the electrodes modified with mesoporous silica film than at bare ITO electrodes, suggesting that the presence of the film impacted somewhat the overall kinetics of the reaction. The most likely reason for slower kinetics would be mass transport limitation of cysteine within the pores, despite their orientation perpendicular to the electrode surface (Figures S2-3). The apparent rate constant, kobs, of an EC' mechanism can be determined by cyclic voltammetry. [49] Based on the scan rate studies (Figure 2), we assumed that the electron transfer for the ferrocene/ferrocenium redox couple was fast and that the rate determining step was the electron transfer between the ferrocenium and the thiol group. kobs was determined in a large excess of thiol ( = 100) at a bare ITO electrode (black curves on Figure 4). In such cases, the catalytic current, ic, reached a steady-state, which can be described by equation ( 1):

𝑖 = 𝑛𝐹𝐴𝐶 ( ) 𝐷𝑘 (1)
Where n is the number of electron transferred in the reaction, F the faraday constant, A the electrode surface area (0.188 cm 2 ), 𝐶 ( ) the bulk concentration of Fc(MeOH)2 (in mol cm -3 ), D the diffusion coefficient for Fc(MeOH)2 (7 × 10 -7 cm 2 s -1 ). kobs can be deduced from equation ( 1) and the intrinsic rate constant of the electron transfer between the thiol and the ferrocenium ion, ke, can be determined from equation ( 2):

𝑘 = 𝑘 𝐶

Based on equations ( 1) and ( 2) kobs and ke were determined for the oxidation of cysteine and glutathione at the bare ITO electrode using ic values at E = +0.45 V for a series (N = 6) of scan rates between 10 and 100 mV s -1 (Table 1). The electron transfer between cysteine and ferrocenium ion was 3.5 times faster than between the glutathione and the ferrocenium ion.

Such a significant difference between the intrinsic rate constants could be attributed to the difference between the pKa of the buffer molecules (bis-Tris, pKa = 6.5) and the one of the thiol molecules (pKa(Cysteine) = 8.1; pKa(Glutathione) = 9.2). [50] Table 1: Apparent and intrinsic rate constants determined for cysteine and glutathione mediated oxidation by Fc(MeOH)2 at a bare ITO electrode. For large excess of cysteine ( = 100), ic was lower at the modified electrode than at the bare ITO electrode, throughout the 10 -100 mV s -1 scan rate range. Such results are consistent with the ones observed in lower excess of cysteine ( = 2.5 and 9) as shown in Figure 3. For glutathione, the presence of the film has little impact between on the catalytic current recorded, unlike for cysteine where the catalytic peak current was smaller at the modified electrode than at the bare one (Figure 4). The difference of impact of the presence of the film on the catalytic current for the oxidation of cysteine mediated by Fc(MeOH)2 can be attributed to the difference of rate constants for glutathione and cysteine. The cysteine catalytic peak current decreased by 32 % when a mesoporous silica film was present. The oxidised form of Fc(MeOH)2 was cationic and accumulated within the negatively charged silica walls. Diffusion of both Fc(MeOH)2 + and cysteine through the silica film was slower than in the bulk. This mass transfer limitations due to the presence of the mesoporous silica film contributed to a reduced catalytic current. In the case of glutathione, mass transfer is also limited by the presence of silica film on the electrode. However, as the electron transfer reaction between the thiol and Fc(MeOH)2 is already slow as the bare electrode experiments showed, this mass transfer limitation did not to affect the electrocatalytic reaction. Despite the different apparent reaction rates for both glutathione and cysteine, the presence of a mesoporous silica film was not sufficient to selectively detect cysteine over glutathione.

Analyte

The further step was to immobilise ferrocene groups inside the silica mesochannels to induce the desired selectivity.

Cysteine oxidation mediated by ferrocene-functionalised mesoporous silica films

The electrochemistry of mesoporous silica film functionalised with ferrocene groups is shown in Figure 5, in good agreement with previous works. [31,51,52] The successful grafting of the ferrocene groups is confirmed by the presence of reversible signal. The amount of ferrocene moieties immobilised on in the silica channels has two effects. [51] A high density of ferrocene groups ensured a good rate of electron hopping between the ferrocene groups located close to the electrode surface and the ones further away, that cannot undergo direct electron transfer with the electrode surface. Yet, the charge transfer process operating at such rigid network bearing ferrocene moieties covalently bonded to the isolating silica material (i.e., electron hopping) is much slower than the direct oxidation of soluble ferrocene derivatives on ITO electrode. 8 The oxidation of the ferrocene groups in the mesopore channels has led to the formation of ferrocenium groups and the generation of a positive charge that can only be counterbalanced by the ingress of an anion of the background electrolyte. [51] However, the high content of ferrocene attached to the silica walls can limit mass transport of redox species (such as Ru(NH3)6 3+ ) [31] or of counter anions. [51] We selected a ratio of AzPTMS:TEOS of 30:70 to form a film with a high content of ferrocene group, with the purpose of serving the following two objectives: (i) to ensure an efficient electron hopping in order to keep satisfactory sensitivity; and (ii) to allow the mass transport of cysteine within the mesochannels and to hinder the mass transport of the bulkier glutathione, in order to achieve selectivity. This was demonstrated by the cyclic voltammograms recorded upon addition of thiol to the solution.

In the case of additions of 196 and 281 µM of cysteine to the solution, the forward peak current rose from µA in the absence of cysteine to 1.13 and 1.34 µA, while the reverse peak current decreased (Figure 5A). This behaviour was similar than the one reported when Fc(MeOH)2 was diffusing freely in solution (see section 3.1), suggesting that cysteine could diffuse through the mesochannels to transfer an electron with ferrocenium, undergoing an EC' mechanism. However, in the case of the addition of the same concentrations of glutathione (Figure 5B), there was no significant changes in the cyclic voltammograms. The limited mass transport of glutathione through the functionalised pores did not allow electron transfer between ferrocenium moieties and glutathione. Another possible effect that can contribute to explain such difference is the aforementioned slower rate constants observed for the mediated oxidation of glutathione (compared to cysteine) by ferrocene dimethanol (Table 1), which should be even accentuated when using the ferrocene functionalised film characterised by slower charge transfer by electron hopping. The selective detection of cysteine was then possible using mesoporous silica films modified with ferrocene. An electrode modified with a film functionalised with ferrocene was then used as a working electrode in a flow cell that was elaborated for other purposes in a previous study. [52] A potential of +0.65 V was applied to oxidise the ferrocene moieties immobilised in the mesopores. The applied potential was determined after recording the CV of the modified electrode in the flow cell (Figure S6). When cysteine was injected in the flow cell, a rise in current was observed (Figure 6A), which was the result of the electron transfer from cysteine to the ferrocenium and causing the re-oxidation of ferrocene. The peak current increased linearly (Ipeak = 1.28 × 10 -3 [Cys] + 1.39 × 10 -2 , R 2 = 0.988) with the concentration of cysteine which ranged from 3 -20 µM. Over the 20-100 µM concentration range, the slope of the calibration curve dropped to 0.68 × 10 -3 A M -1 . Despite a large background noise originating from the flow cell, a good repeatability of the peak current was observed with a relative standard deviation for the measurements of 3 -12 %. The analytical performances of the mesoporous silica film functionalised with ferrocene compared well with the modified electrodes already reported in the literature (Table SI1). [14][15][16]18,20,21,[23][24][25][26] approximately 10 nA for a noise around 14 nA. The signal to noise ratio calculated from Figure 7 was lower than 1 while a species is considered to be statistically detectable if the signal to noise ratio is greater than 3. When the same concentration of cysteine (50 µM) was injected in the flow injection analysis system, the current measured rose by 53 nA, which gave a signal to noise ratio greater than 3, confirming the idea of a good selectivity for cysteine over glutathione as previously suggested from CV experiments (Figure 5).

Conclusions

The thiol function of cysteine and of glutathione can be oxidised by Fc(MeOH)2 + generated at the surface of a mesoporous silica electrode. The overall kinetics of this reaction was limited by the electron transfer between Fc(MeOH)2 + and the thiol. Such an electron transfer was faster with cysteine than with glutathione. By immobilising the redox mediator inside silica mesochannels, the slow kinetics of glutathione oxidation were exploited to selectively detect cysteine. Indeed, the presence of cysteine in solution was oxidised at ferrocene-functionalised films while glutathione was not. The combination of charge transfer kinetic and mass transport limitations allowed the selectivity of the ferrocene-functionalised electrode for cysteine over glutathione.

  h at 130 °C); the template was extracted in 0.1 M HCl in ethanol. The second step of the synthesis consisted on the Huisgen cycloaddition reaction leading to the coupling of the azide-and alkyne-bearing molecules, catalysed by Cu(I) ions, to form a triazole core substituted in the 4 position. The azide-functionalised films were immersed for 24 h (in the dark at room temperature) in a solution of BuOH/H2O (6/4; 20 mL) of ethynylferrocene (10.0 mg), CuSO4, H2O (2.0 mg) and ascorbic acid (10.5 mg). This reaction led to the derivatisation of the azide-based films via 'click chemistry' into ferrocenefunctionalized films. The film were then thoroughly rinsed with water, ethanol and acetonitrile (10 min in each solvent) to remove unreacted species and traces of catalyst. The films were then kept in 0.05 M Sodium diethyldithiocarbamate solution in ethanol for 30 min to remove any traces of copper. The films are subsequently dried for 30 min in the dark. IR characterization of the synthesised films are shown in Figure S5, demonstrating the successful click coupling.

Figure 1 :

 1 Figure 1: Cyclic voltammetry at a mesoporous silica film electrode of 0.5 mM of Fc(MeOH)2 in the absence (black curve) and in the presence (red curve) of (A) 1.25 mM cysteine and (B) 1.49 mM glutathione.  = 10 mV s -1 .

Figure 2 :

 2 Figure 2: Cyclic voltammetry at a mesoporous silica film electrode of 0.5 mM of Fc(MeOH)2 in the presence of 1.25 mM cysteine.  = 10; 20; 50 and 100 mV s -1 .

Figure 3 :

 3 Figure 3: Ratio of the forward peak current / reverse peak current, if / ir, as a function of the scan rate extracted from cyclic voltammograms obtained at bare ITO electrode (black solid lines) and at mesoporous silica film electrodes (red dashed electrodes) with 0.5 mM Fc(MeOH)2. Cysteine concentration was 0 (discs), 1.25 (triangles) and 4.5 (squares) mM.

  at E = +0.45 V; b : ic measured at the electrode modified with mesoporous silica.

Figure 4 :

 4 Figure 4: Cyclic voltammetry of 0.083 mM of Fc(MeOH)2 for a large excess of (A) cysteine ( = 100) and (B) glutathione at a bare electrode (black curve) and at a mesoporous silica film electrode (red curve).  = 10 mV s -1 .

Figure 5 :

 5 Figure 5: Cyclic voltammetry at an electrode modified with a mesoporous silica film functionalised with ferrocene with additions of 196 and 281 µM of (A) cysteine and (B) glutathione.  = 10 mV s -1 Black curves represent the cyclic voltammograms in the absence of thiol and dashed curves represent the electrochemical response when no ferrocene moieties were immobilised.

Figure 6 :

 6 Figure 6: (A) Chronoamperograms obtained for increasing concentrations of cysteine (3-100 µM). Eapplied = + 0.65 V; flow rate was 1 mL min -1 . (B) Calibration curve showing peak current as a function of cysteine concentration.

Figure 7

 7 Figure 7 showed the chronoamperogram for the injection of 50 µM of glutathione (two first red arrows) and of cysteine (three last black arrows). For glutathione, the current recorded by

Figure 7 :

 7 Figure 7: Chronoamperograms showing the flow injection analysis of 50 µM of glutathione (red arrows) and of cysteine (black arrows). Eapplied = + 0.65 V; flow rate was 1 mL min -1 .

Acknowledgements

This work was supported partly by the French PIA project "Lorraine Université d'Excellence" (Reference N° ANR-15-IDEX-04-LUE). M Lionel Richaudeau is thanked for his help with the GISAXS measurements.


Vincent Fournée
email: vincent.fournee@univ-lorraine.fr

Julian Ledieu

Émilie Gaudry

Hem-Raj Sharma

Ronan Mcgrath
email: mcgrath@liverpool.ac.uk

Raj Sharma

Ultra-thin films on complex metallic alloy surfaces: A perspective

Complex metallic alloys (CMAs) may be defined as those intermetallic compounds having large (>≈nm) unit cell dimensions. This includes quasicrystals as a special case, the unit cell being infinite. The discovery of quasicrystals motivated the study of CMAs, and the surface science community became active in the field once stable samples of sufficient size were produced. While the initial surface science activity centred on clean surface preparation, increasingly the formation of thin films, both metallic and molecular grew in importance. In this chapter we give a brief introduction to this topic, and then focus on several current areas of interest. These include the growth and characterisation of ultrathin metallic films of diverse architectures, the formation through deposition of novel molecular overlayers and thin films, complex intermetallics as surface alloys, and the potential use of intermetallic surfaces for catalytic reactions.

Introduction

Quasicrystals are physical systems which possess long-range order but without translational symmetry. The synthesis of the first quasicrystal, the metastable Al 6 Mn alloy by Dan Shechtman in 1982 [1] and the subsequent controversy surrounding this discovery is well-documented (e.g. [2]). In the subsequent decades many more solid quasicrystals were discovered, and a large and diverse research community came together in an effort to understand their structure, physical properties, and potential for applications [START_REF] Dubois | Useful Quasicrystals[END_REF]. Indeed, quasicrystallinity was later found in a large variety of physical systems, including dendritic liquid crystals [START_REF] Zeng | [END_REF], ABC-star poly-mers [5], binary nanoparticle superlattices [6], colloids [7], mesoporous silica [8], and oxide thin films [9]. New and surprising phenomena in quasicrystal systems continue to be reported; for example in 2018 the Al-Zn-Mg quasicrystal was found to be superconducting [10].

Once stable quasicrystals were discovered and produced in large enough samples [11,12], they became suitable for surface science studies. Among the first such studies were those of the ten-fold surface of the decagonal phase [13,14] using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The five-fold surfaces of icosahedral quasicrystals and the ten-fold surfaces of decagonal quasicrystals in particular have received most attention [START_REF] Thiel | Surfaces of Quasicrystals and Complex Metallic Alloys[END_REF].

This focus on quasicrystals in turn sparked increasing focus on their approximants [16]. Approximants are periodic crystals close in stoichiometry to quasicrystals. They tend to have large unit cells and local structure within the unit cell similar to their quasicrystalline counterparts. They are attractive to study because of their periodicity -many structural (diffraction) and theoretical (density functional theory) techniques are based on a periodic formalism. Studying approximants can yield important knowledge that can be applied to the understanding of quasicrystals themselves [17]. Again, the surfaces of approximants have received considerable interest [18][19][20].

In time, the phrase complex metallic alloys (CMAs) began to be used to describe quasicrystals, their approximants, and other alloy structures [21] with large numbers of atoms in the unit cells. Dubois has identified the presence of a pseudogap in the electronic structure at the Fermi edge as a necessary but not sufficient differentiator between CMAs and other alloy systems [START_REF] Dubois | An introduction to complex metallic alloys and to the CMA Network of Excellence[END_REF].

Once excellent surfaces could be prepared, there followed considerable interest in the nature of epitaxial processes on CMA surfaces, and in the structure and properties of the overlayers and thin films which result [23][24][25]. This interest was motivated partly by the importance of thin films in technological processes, but was also curiosity-driven: would thin films and molecular overlayers adopt the structure of the substrate, or order according to the structure of the bulk? This question has been answered in part -many elements and molecules do adopt the structure of the substrate, at least in the first layer; indeed for some elements the quasicrystalline architecture persists beyond a single layer [26].

This chapter is not intended to be a comprehensive review of the literature; rather we describe a number of topics of current interest to the CMA thin film community. We discuss growth of multi-layer quasicrystalline thin films, molecular adsorption and architectures on CMA surfaces, CMAs as surface alloys and the application of CMA surfaces in catalysis. We conclude with a perspective and future look.

Growth of multi-layer quasicrystalline thin films of Ag-In-Yb surfaces

In this section, we report studies of growth of quasicrystalline thin films of single elements on surfaces of the icosahedral (i)-Ag-In-Yb quasicrystal. All three high symmetry surfaces (2-, 3-and 5-fold) of this system have been studied [27][28][29] and used as a template to grow thin films of various elements such as Pb, Bi, In and Sb. Here we will limit our discussion on the growth of Pb and Bi on the 5-fold surface [26,30]. The i-Ag-In-Yb quasicrystal is isostructural to the binary i-Cd-Yb phase, where Cd is replaced by Ag and In. The atomic structure of this system can be explained by rhombic triacontahedral (RTH) cluster. The RTH cluster is formed by five successive atomic shells: a tetrahedron (innermost, 4 Cd atoms), a dodecahedron (12 Cd atoms), an icosahedron (30 Yb atoms), an icosidodecahedron (32 Cd atoms) and a rhombic triacontahedron (92 Cd atoms) [31]. A RTH cluster bisected perpendicular to the five-fold axis is shown in Fig. 1(a) (first shell not shown). For clarity, we have shown different atomic shells by different colours.

Although, the three-dimensional shells are believed to be the building blocks of i-Cd-Yb, the structure can still be explained using a notion of atomic plane. Fig. 1(b) shows the atomic density distribution along the five-fold axis, for the ideal (unrelaxed) Cd-Yb model [31]. For simplicity, we define a region with a non-zero density as a plane. The density of the densest atomic planes is comparable to that of the closed-packed Yb surface [27]. Some of atomic planes are marked in Fig. 1(b). These planes are important for discussion of the structure of the thin films. The atomic planes are formed by atoms of specific shells of the RTH cluster. For example, plane 1 is formed by atoms of the third and fourth shells, while plane 2 is formed by atoms of the fifth shell.

When the surface of i-Ag-In-Yb is prepared using the common sputter-annealing methods, it produces a step-terrace structure, as revealed by scanning tunneling microscopy (STM) [27]. The terraces are formed at bulk planes intersecting the centres of the RTH clusters (for example, plane 1 in Fig. 1(b)). These surface terminating planes have a moderate atomic density. The structure of a surface terminating plane, superimposed with structure of other planes, is shown in Fig. 1(c) and (d). The structure can be mapped by a Penrose P1 tiling of edge length 2.50 nm with vertices located at the centres of the RTH clusters.

The atomic sites above the surface plane are vacant. Interestingly, these sites will be occupied by the adatoms (Pb or Bi) when deposited on the surface, mimicking the structure of RTH cluster (Fig. 1(e)). The structure of the film thus can be characterised by layers of atoms, and the structure of each layer can be deduced from the bulk structure.

Fig. 2(a) and (b) show STM images taken from first and second layer of Bi, respectively [30]. It was found that the first layer is formed by fifth shell atoms (which form plane 5 and 6). The most common features observed in the first layer by STM are pentagonal stars, pentagons and crescent shapes (marked in Fig. 2(a)). These features match very well with those in plane 5 and 6 (Fig. 1(c)), in terms of height, size, orientation and location with respect to the surface plane. Similarly, the second layer Bi atoms are located at the vertices of a Penrose P1 tiling of 2.5 nm. This structure can be explained by Yb atoms of the third shell (plane 8). The autocorrelation pattern and the fast Fourier transform (FFT) of the STM image display ten-fold symmetry with maxima located at τ-scaling distances (Fig. 2(c,d)), confirming long-range quasicrystalline order of the film. The absorption sites for Bi atoms identified by STM are similar to those established by Density functional theory (DFT) calculations [32]. Lead atoms deposited on the surface also occupy the cluster sites, displaying layers of different heights. STM images of first, second and third layers of Pb are shown in Fig. 3(a), (b) and (c), respectively [26]. These layers are formed at planes 2 (fourth shell), 7/8 (third shell) and 3 (third shell), respectively. DFT calculations have identified other layers, denoted by Pb under and Pb inter in Fig. 1(b) [26]. The Pb under layer, which is underneath the first layer, is formed after the first layer is completed and thus this layer is not observed by STM. The existence of these under-and intermediate-layers is vital for the stability of the other layers, and thus the film as a whole. The formation of layered structure is also confirmed by x-ray photoemission spectroscopy (XPS) [26].

The atomic density of a single layer is very low such that the distance between Pb atoms in the layer is larger than the nearest neighbour distance of Pb in its natural face-centred cubic structure. However, the Pb-Pb distance in neighboring layers is short enough (comparable to the fcc Pb-Pb distance) to have a strong interaction between the adatoms, stabilizing such 3D films [26]. The same observation is true for Bi films [30].

Fig. 3 (a-c) STM images of the first (30 nm × 30 nm), second (20 nm × 20 nm) and third (23 nm × 15 nm) layer Pb formed on the five-fold surface of i-Ag-In-Yb [26]. A Penrose P1 tiling of 2.5 nm edge-length is superimposed in (a).

The nature of thin film growth on i-Ag-In-Yb is unique. A certain number of elements (Pb, Bi, Sb and Sn) grown on Al-based quasicrystals are also found to develop quasicrystalline structure. However, quasicrystalline order is limited up to one atomic layer [33][34][35][36], in contrast to three-dimensional quasicrystalline growth on i-Ag-In-Yb. After completion of the monolayer, the adsorbate develops its natural periodic structure [37,38], [39]. Some other systems, for example Cu or Co on the i-Al-Pd-Mn surface, yields atomic rows spaced in a Fibonacci sequence at coverage above a few atomic layers [40][41][42]. Below this critical coverage, no order is observed. The structure within the atomic rows is periodic.

Intermetallic surfaces as templates for complex molecular architectures

The formation of molecular thin films on metal surfaces typically occurs out of thermodynamic equilibrium. It involves molecular adsorption from the gas phase to the surface, random diffusion of the molecules between local minima of the potential energy surface (which is a thermally activated process to overcome surface diffusion barriers), and island nucleation or attachment to a pre-existing island. If the deposition flux is high and the substrate temperature is low, the diffusivity of the molecules is low and they will be easily trapped into metastable adsorption sites or captured by pre-existing nuclei. In this case, the corrugation of the potential energy surface cannot efficiently drive the molecular self-assembly. On the other side, if the deposition flux is low and the substrate temperature is high, then diffusivity of the molecules is high and the molecules can more easily explore the potential energy surface and reach the deepest adsorption sites before nucleating into islands. These deposition conditions are therefore more favorable to take benefit from the templating effect of the substrate and drive molecular self-assembly into long-range ordered structure. First attempts to grow molecular films on quasicrystalline surfaces were performed by room temperature deposition of fullerene C 60 on the 5-fold surface of the icosahedral Al-Pd-Mn quasicrystal [43] or the 10-fold surface of the decagonal Al-Ni-Co quasicrystal [44]. STM images of these surfaces dosed with a fraction of a monolayer showed individual immobile molecules rather than the formation of islands, suggesting that the molecules are trapped in adsorption sites and cannot diffuse further at this temperature. Nevertheless, common distances separating individual molecules were found to exhibit τ scaling relationships characteristic of the bare quasicrystalline substrate, suggesting that local ordering takes place at low coverage. However, upon further deposition, a disordered film is obtained producing no diffraction pattern. Post-annealing treatment of the as-deposited films did not produce any structural change and the molecules were found to desorb above 600 K.

Two recent studies reported the growth of ordered quasiperiodic C 60 films templated on quasicrystalline substrates obtained after high temperature deposition [45,46]. Smerdon et al. [45] reported the growth of a C 60 film deposited on the fivefold surface of the Al-Cu-Fe icosahedral quasicrystal held at a temperature comprised between 770 and 970 K. In this temperature regime, a complete monolayer cannot be obtained due to a reduced sticking coefficient and/or possible molecular desorption taking place. STM investigations of the film revealed individual molecules trapped in a set of deeper adsorption sites and forming a discontinuous film (see Fig. 4(a)). The fast-Fourier transform and autocorrelation function of the position map of individual C 60 exhibited a ten-fold rotational symmetry and τ scaling relationships characteristic of quasiperiodic systems (see Fig. 4(b)). Some of the molecules appeared brighter than others, corresponding to a height difference of 1.4 Å with respect to dimer molecules. It was concluded that this height difference is due to molecules adsorbed at two different types of adsorption sites. The complete analysis of the STM images led to a scenario in which bright C 60 molecules partially occupy a pentagonal lattice with edge length equal to 1.2 nm connecting Fe surface atoms embedded in an otherwise almost pure Al top layer [47]. The dim molecules were ascribed to C 60 adsorbed on top of subsurface Fe atoms located at about 2.1 Å below the top plane, implying some surface reconstruction.

Another study reported the growth of dense, long-range quasiperiodically ordered C 60 layers grown on various quasicrystalline surfaces held between 623 and 673 K [46]. This temperature range is still above the multilayer desorption temperature but low enough to allow saturation of the first layer. It is high enough to facilitate molecular diffusion and sampling of the potential energy surface and still low to avoid fragmentation or polymerization of the C 60 as verified by core-level photoemission spectroscopy. Molecular films were grown on the five-fold surfaces of Al-Pd-Mn and Al-Cu-Fe icosahedral quasicrystals as well as on the ten-fold surfaces of the Al-Cu-Co and Al-Ni-Co decagonal quasicrystals. The long-range quasiperiodic order was demonstrated by the LEED patterns exhibiting sharp diffraction spots and either five-or ten-fold rotational symmetry. Fast-Fourier transform and autocorrelation of STM images are characterized by rings of ten spots with diameters related by τ scaling relationships, in agreement with the LEED patterns. The structure of the film can be interpreted by the adsorption of the molecules at specific sites of the substrate exhibiting a local five-fold symmetry. Both decagonal phases are described by the packing of partially overlapping columnar clusters extending along the ten-fold axis, perpendicular to the surface plane (see Fig. 5). The cluster centers truncated by the surface constitute five-fold symmetric sites located at the nodes of a quasiperiodic tiling made of pentagonal (P), star (S) and boat (B) tiles, with an edge length equal to 2 nm [48]. Additional five-fold symmetric sites form inverted pentagons with edge length equal to 1.2 nm inside the P tiles. The decoration of these specific sites with C 60 perfectly reproduces the experimentally observed film structure. It leads to a dense molecular film, with a packing density of one C 60 per nm 2 and the shortest intermolecular distances of about 1 nm, i.e. similar to the van der Waals radius. The molecules have different contrasts in the STM images, corresponding to an height difference of 0.1 to 0.2 nm, but these values are strongly bias dependent suggesting some electronic origin. It may be due to differences in the local chemistry surrounding the various adsorption sites and/or to different adsorption configuration of the molecules.

A similar self-assembly occurs for C 60 deposited on the five-fold surfaces of Al-Cu-Fe and Al-Pd-Mn icosahedral phases, leading to dense molecular films with long-range quasiperiodic order. In these cases, the molecules appear either bright or dim at negative bias in STM images, corresponding to a height difference of about 0.1 nm. The most common motifs are 1.2 nm edge-length pentagonal tiles made by five bright C 60 centered by either a bright or a dim molecule. These bright and dim motifs are pointing in opposite direction on a same terrace and they keep the same orientation across terraces. The structure of the films can again be explained by preferential adsorption at specific sites of the substrate characterized by a local five-fold symmetry. Both icosahedral phases are described as a packing of pseudo-Bergman and pseudo-Mackay type clusters. The surface terminations of both phases correspond to dense Al-rich planes of the bulk structure. The most frequent motifs observed by STM have been termed 'white flowers' (WF) and 'dark stars' (DS). They both correspond to truncated clusters or clusters hanging down from the surface plane, but the DS motifs have a central vacancy. Both motifs exhibit local fivefold symmetry. The WF are located at the nodes of a tiling made of pentagons, stars and rhombi with an edge length equal to 1.26 nm [49][50][51]. Some of the pentagonal tiles have a DS motif in their interior, all pointing in the same direction. It was thus concluded that the molecular self-assembly occurs through preferential adsorption of the C 60 at these specific sites, the dim molecules corresponding to C 60 adsorbed at DS sites, i.e. on top of a vacancy. This growth scenario perfectly matches with all experimental observations. It was further verified by performing submonolayer deposition experiments, allowing to image both the substrate structure and the C 60 positions in order to determine the adsorption sites.

For both icosahedral and decagonal substrates, the driving force promoting the molecular self-assembly is the preferential adsorption at specific sites exhibiting local five-fold symmetry. Considering that symmetry matching between the molecules and the substrate is an important factor in determining the adsorption configuration, these results suggest that the molecules adsorb with a C pentagonal face down at these five-fold symmetric sites. Ab initio DFT calculations DFT have then been performed in the case of C 60 /Al-Pd-Mn in order to determine adsorption energies of C 60 molecules at different adsorption sites and for various adsorption configurations. These calculations were performed using a structure model of a periodic approximant which contains both the WF and DS local configurations. The most stable adsorption configurations were found to correspond to C 60 contacting the surface with a pentagonal face aligned with the five Al atoms forming the DS motif or aligned with Al pentagon centered by a Mn atom for the WF motif. The DFT calculations indicate that the bonding is quite different at DS and WF sites. At DS sites, the bonding is rather directional between the 5 C-Al pairs. The C atoms involved are not those forming the bottom pentagonal face but are C atoms involved in adjacent hexagonal faces. At WF sites, the bonding charge is localized at the interface between the pentagonal C face and the Mn atom at the center of the WF motif. The calculated densities of states show a broadening of the molecular orbitals as well as an energy shift consistent with a charge transfer from the substrate to the molecules. New electronic states appear at the Fermi level as a result of sp and spd hybridization between C and substrate atoms, the film becoming metallic. It is to be noted that C 60 adsorption with a C pentagonal face contacting the surface is rather unusual compared to noble metal surfaces but maybe quite general for quasicrystalline substrates or possibly their approximants due to the existence of local five-fold symmetric configuration.

Additional growth experiments have been reported using different types of molecular building blocks. One is the growth of corannulene C 20 H 10 on the fivefold surface of the Ag-In-Yb quasicrystal at room temperature [52]. Corannulene molecules are bowl-shape, consisting of a fragment of a C 60 and they have C 5v symmetry. This system also self-assembles into a quasiperiodic pattern which appears to be driven again by symmetry matching, in the sense that the specific adsorption sites identified are characterized by their local five-fold symmetry.

A totally different picture was found for pentacene molecules deposited on the same surface [45]. Pentacene are polycyclic aromatic hydrocarbon molecules consisting of five linearly-fused benzene rings; it is an organic semiconductor. The room temperature diffusivity of the pentacene was found high enough to lead to a quasiperiodic distribution of the molecules as demonstrated by the autocorrelation function of the molecular positions map. Pentacene molecules do not have five-fold symmetry contrary to C 60 and therefore symmetry matching between molecules and substrate does not play a significant role. From the analysis of the STM data, it was concluded that pentacene molecules are anchored on pairs of Yb surface atoms, which are quasiperiodically distributed. Therefore the chemical affinity between the molecules and the various species present in the surface planes is the key factor driving the self-assembly in this particular case.

In conclusion, quasicrystalline surfaces are both structurally and chemically complex and they have been used successfully as templates for the self-assembly of molecular building blocks into quasiperiodically ordered thin films. So far, mainly prototype molecules like C 60 have been used, but there is a full range of more complex molecules carrying various functional properties awaiting for further growth experiments. These are considered as interesting model systems to understand the structure/properties relationship linked to the long-range quasiperiodic order.

Complex intermetallics as surface alloys: an alternative

Most of the structural work performed on complex intermetallic surfaces have been achieved using centimeter-sized single crystals of high structural quality and purity. Such specimens can be obtained by conventional growth techniques. Once polished and oriented, they are investigated using surface science techniques under ultra high vacuum conditions. Depending on the intermetallics targeted, it is not always possible to obtain large enough samples due to for instance growth limitations (temperature, phase diagram constraints . . . ) or elemental costs. Hence, an alternative approach consists in the deposition of pure elements on unary, binary or even ternary systems aiming to form the desire complex phases as surface alloys. The crystallographic and electronic surface structures of the terminating layers can then be characterised in situ. Sometimes the growth of surface intermetallic compounds can be fortunate originating from the alloying between adsorbates and substrate elements. As will be described below, this has often occurred while studying the growth of atomic thin films on aperiodic systems [53][54][55][56][57].

Elemental adsorption on a binary intermetallic compound represents also a great opportunity to follow the initial nucleation and growth of complex metallic alloys, including the propagation of aperiodic long-range order. In addition to the determination of the clean surface structure, the formation of complex intermetallic compounds as surface alloys allows characterisation down to the atomic level of the interfaces between the matrix (here the substrate) and the grown phase. Such fundamental studies could in fine bring new solutions to issues encountered in the field of coating technology. Regarding complex metallic alloys, we will refer here to delamination and reactive diffusion upon post-annealing treatment often identified as technological bottlenecks [58]. Hence, an atomistic description of the interfacial structure between both antagonists could lead to the design of specific buffer layers to accommodate the strain related to lattice mismatch and/or to release internal stress due to thermal expansion coefficient differences between the two phases.

Although extensive works on single and multilayer surface alloys have been reported [59], the surface characterisation of Al-based (complex) intermetallic compounds formed upon adsorption is much more limited. We will start with studies where intermetallic compounds have been formed while trying to grow quasiperiodic thin films.

Due to its tendency to self-assemble into decagonal and icosahedral shaped clusters, the adsorption of Au atoms was carried out on the decagonal Al-Ni-Co surface [53]. Submonolayer deposition of Au adatoms at room temperature leads to the formation of a Al-Au surface alloy without any orientational order as evidenced from x-ray photoelectron diffraction (XPD) and reflection high-energy electron diffraction (RHEED) experiments. Upon annealing the dosed sample to 350-400 K, the surface structure changes and can be interpreted as an epitaxial layer consisting of twinned Au-Al 2 crystals with (110)-oriented surface (see Fig. 6(a)) [53] 1 . The twin azimuthal orientations replicate the ten-fold symmetry of the underlying template. For 10 ML thick Au layers, the thin film is described as a polycrystalline Au layer using RHEED measurements. The structure is unchanged upon annealing the Au thick film until adatom re-evaporation occurs [55]. The same scenarios apply to Au adsorption on the five-fold Al-Pd-Mn surface [54] and for Pt adsorption on the decagonal Al-Ni-Co surface with the formation of a Pt polycrystalline film regardless of the film thickness [60]. However, the Au and Pt growth modes are altered if the quasicrystalline substrate is pre-covered by an In layer [54,55,60]. Indeed, the Au polycrystalline thick film deposited at room temperature on the In pre-covered surface transforms upon annealing to 350-400 K to the AuAl 2 structure as demonstrated by RHEED and X-ray photoelectron spectroscopy (XPS) analysis. As already observed for submonolayer deposition [53], XPD images reveal a tenfold symmetry pattern consistent with an epitaxial layer model of multiply twinned AuAl 2 (110)-oriented crystals [55]. Based on XPS analysis, there is clear evidence for In diffusion across the thick Au overlayer at 300 K. With annealing, the initial In 3d core level intensity is restored. Indium is then considered here as a surfactant promoting Frank-van der Merwe over Stranski-Krastanov type growth. Similar mechanisms are observed while dosing Pt and Au adatoms on In pre-covered Al-Ni-Co and Al-Pd-Mn surfaces respectively. In the former case, the structure of the annealed Pt thick film (490 K) results in an epitaxial layer of multiply twinned Pt-Al 2 crystals [60].

Still aiming at forming pseudomorphic overlayers, another intermetallic phase has been grown on similar quasicrystalline substrates. Located in the same column of the Periodic Table as Au, the adsorption of Cu atoms on complex metallic alloy surfaces has led to the formation of a unique intermetallic compound, namely the complex Hume-Rothery Al 4 Cu 9 phase. The lattice parameter of this simple cubic γ-brass phase is equal to 8.71 Å (space group P 43m). At room temperature, the adsorption of Cu adatoms on the five-fold and two-fold Al-Pd-Mn quasicrystalline surfaces yields quasiperiodic structures [40,61]. Once annealed to 570 K, the thick Cu film (25 ML) crystallises into five periodic domains rotated from each other by 72 • , i.e. following the directional order of the five-fold substrate [56]. XPS, XPD and LEED analysis reveal that the domains correspond to the (110) surface-oriented Al 4 Cu 9 structure [57]. This is also the conclusion drawn when Cu adsorption is car-ried out on the five-fold Al-Cu-Fe surface. Contrary to the Cu/Al-Pd-Mn system, the formation of β -Al(Cu, Fe)(110) domains is initially observed at room temperature from 1 ML to 8 ML [62]. Within this coverage regime, the β -phase is also the stable phase up to 573 K. Then, the growth of five Al 4 Cu 9 (110) domains appears for 20 ML thick Cu film annealed to 558 K, i.e. there is a minimal Cu thickness required to growth the Al 4 Cu 9 phase as a surface alloy.

Interestingly, this γ-brass phase is also the one that appears upon the adsorption of Al atoms on a simple Cu(111) surface. In this case, the formation of Al 4 Cu 9 (110) domains occurs only if the nominal Al thickness is above 12 ML. The STM measurements combined with ab initio calculations demonstrate that the γ-Al 4 Cu 9 surface is bulk-terminated at puckered planes [63]. This conclusion is in agreement with the surface plane selection identified across the Al 4 Cu 9 (110) surface using a single crystal [64].

As explained at the beginning of this section, elemental adsorption on a binary complex metallic alloy could represent an alternative route to grow ternary quasiperiodic surface phases and to possibly monitor in situ the propagation of long range order. To this end, Cu adatoms have been deposited on the Al 13 Co 4 (100) surface [65]. The latter crystal which is considered as an approximant to the decagonal quasicrystal, was an apparently ideal candidate for aperiodic surface alloy formation. Instead, the results indicate the formation of a β -Al(Cu, Co) phase above 3 ML Cu deposition at 300 K. For higher temperature (473 K), the β -Al(Cu, Co) is also formed but it is replaced by a γ-Al 4 Cu 9 phase above 7.5 ML Cu deposition [65]. One of the striking features lies here in the domain orientations of both βand γ-phases. The orientation of the γ-Al 4 Cu 9 domains reported in previous studies tends to match with the atomically densest directions of the surface plane. Hence, five domains and two sets of three-fold rotational domains are formed on five-fold surfaces and on Cu(111) respectively. For the Cu/Al 13 Co 4 (100) case, the two (110) domains identified across the surface are rotated from each other by 72 • . Moreover, the growth proceeds 4±1 • off the [011] and [01 1] directions of the orthorhombic substrate. At this stage, these peculiar growth directions could only be related to the orientation of the bipentagonal motifs present on the clean Al 13 Co 4 (100) surface. These motifs correspond to the elementary pentagonal bipyramid clusters, i.e. building blocks of the bulk structure. These different studies demonstrate the stability of the γ-Al 4 Cu 9 phase as a surface alloy. It also shows that the phase will be formed as long as a critical amount of Al and Cu elements is reached within the surface region. The surface structural complexity has little influence on the γ-Al 4 Cu 9 phase growth except for the domain orientations.

Aside from model adsorption studies performed under ultra high vacuum conditions, the formation of complex intermetallic compounds is also encountered in industrial processes. To improve their oxidation and corrosion resistance, steel and cast iron can be hot-dip coated with pure Al or Al alloys of varying composition. The resulting coating which is dependent on the chemical composition of the bath consists of an outer layer and an inner intermetallic layer which can be further subdivided in two zones. Using a pure Al bath, the intermetallic layer adjacent to the cast iron (first zone) contains the Al 5 Fe 2 phase while in the second zone the Al 3 Fe is identified (Si atoms being present in both phases in different amounts) [66]. The resulting interfaces between the different antagonists vary quite drastically from an irregular tongue-like morphology between the cast iron and the Al 5 Fe 2 layer to a more regular interface between the Al outer layer and the Al 3 Fe phase. The interfacial microstructure and composition will in fine dictate the coating physical and chemical properties. Depending on the applications, protective coatings may be designed to sustain harsh conditions, including for instance high temperature conditions and/or excessive oxidizing environments. Given its high melting temperature and due to its oxidation resistance, Al-Ir based coating have been considered as protective coating in gas turbine engines [68]. Although several methods are focussing on improving deposition processes, little is know on the surface structure of Al-Ir alloys and compounds. This is true for the interfacial structure between Al-Ir and the supporting substrate also.

To this end, the adsorption of Ir atoms on the Al(100) clean surface has been recently carried out under ultra high vacuum conditions. The objectives were to determine (i) which Al-Ir phases will initially appear; (ii) the associated surface structure2 and (iii) the corresponding interface structure between the single crystal and the surface alloys formed. A reverse strategy by adsorbing Al on Ir(111) surface had been previously chosen to study the Ir-rich side of the phase diagram [69]. The layer-by-layer growth mode reported at 300 K preceded the formation of Ir 4 Al surface aluminide obtained for high temperature deposition (1100 K to 1500 K). For the Ir/Al(100) system, a higher intermixing length is already expected at room temperature [70]. This is confirmed by experimental and theoretical results revealing diffusion of Ir adatoms within the surface selvedge at room temperature [67]. Ir adsorption above 623 K leads to the growth of Al-Ir domains exhibiting an unit cell equal to √ 5 times the Al(100) surface unit cell. After a careful analysis of the Al-Ir binary system and in agreement with dynamical LEED analysis, the grown phase has been assigned to the Al 9 Ir 2 compound. The surface of the Al 9 Ir 2 (001) oriented domain is bulk-terminated at pure Al planes in accordance with ab initio calculations demonstrating that Ir adatoms located within the topmost surface layer are energetically unfavored.

Regarding the Al 9 Ir 2 growth orientation with respect to the Al(100) substrate, it can be described for one domain by the following epitaxial relation: (100) Al [67]. The absence of dislocations within the measurements is related to the low strain due to lattice mismatches close to 0.4% between the two antagonists.

From the work presented in this section, it is clear that the surface characterisation of complex intermetallic grown as surface alloys is still in its infancy. It is also true for interfacial structural characterisation, which should play a crucial role when intermetallic compounds are envisaged as protective or functionalised coating. Furthermore, the introduction of intermetallic phases as buffer layers between substrates and coatings may represent an interesting alternative to circumvent adhesion or delamination issues [62].

Complex intermetallic surfaces and recent developments in catalysis

The interest in (complex) intermetallics as catalysts has increased significantly in the past few years, as documented in recent numerous reviews [71][72][73][74]. Indeed, their surfaces present several main advantages compared to simple metals and alloys. They are stable, with surface segregation being generally suppressed, due to the strong bonding network occurring in the bulk materials. Site isolation, i.e. the spatial separation of catalytically active sites at the surface, is commonly achieved on complex intermetallic surfaces, which is known to be an important factor for reaction selectivity. Synergistic effects are also expected when combining different chemical elements in ordered alloys, like the ones highlighted using PdZn and PdCd as selective catalysts for methanol steam reforming [75,76]. While pure Cu is traditionally used for this reaction, PdZn and PdCd lead to similar high selectivity towards the formation of CO 2 . It is attributed to the similar valence electronic densities of states of Cu and the 1:1 compounds. PdZn and PdCd can then be considered as replacing Cu for this reaction. Due to the large number of intermetallic compounds known so far -more than 6000 binaries have been synthesized and characterized up to now, one expects to find new competitive intermetallic catalysts by a careful selection of their atomic and electronic surface structures. Several families of intermetallics have already been identified as stable, reactive and selective catalysts. Most of them combines noble metals -the catalytically active sites -with elements from different areas of the periodic table, for example, sp metals like Al, Ga or Sn. A few noble metal-free catalysts do exist, promising for the reduction of costs and environmental impacts.

In the following sub-sections, two families of (complex) intermetallic compounds are discussed, in relation to the hydrogenation of acetylene and butadiene. The case of quasicrystals or approximants used as catalysts after a leaching process [77][78][79][80][81][82] is not developed, since the catalytic material obtained after the chemical treatment generally consists of metallic nanoparticles dispersed on an oxide, thus leading to mechanisms for the reactivity distinct from the ones valid for ideal intermetallic surfaces.

Pd-and Pt-based intermetallic catalysts

One of the first intermetallic compound surfaces used as a catalyst is probably Pt 3 Sn(111). For the semi-hydrogenation of butadiene, the activities of its p(2 × 2) or ( √ 3 × √ 3) surfaces were found to be one order of magnitude lower than that of Pt(111) but the selectivity into butenes is largely increased by comparison to pure Pt (98-100%) [83]. Theoretical studies on surface alloys showed that the presence of Sn at the surface weakens the chemisorption of unsaturated hydrocarbon molecules [84]. More complex intermetallics -as least when evaluated with the number of atoms in the cell -were also identified as potential catalysts for selective semi-hydrogenation reactions (Fig. 7), like Pd 3 Ga 7 , Pd 2 Ga, PdGa [85][86][87][88]. Selectivity is expected to increase with the number of Ga atoms in the vicinity of the active Pd atoms, it has been demonstrated on Pd 2 Ga(010) [89]. Other compounds with similar compositions, like Ga 2+x+y Sn 4-x Pd 9 do not present attractive performance [90], highlighting the challenge to predict the catalytic properties from the knowledge of the bulk structure. Surface structure is a decisive factor for catalytic performance [91][92][93][94]. In the cases of GaPd (and AlPd), corrugated surfaces with slightly protruding transitionmetal atoms forming a triangular arrangement with two neighboring Al/Ga atoms are indeed highlighted as catalytically attractive surfaces, allowing the transition from the di-σ bonded adsorption configuration for acetylene to a π-bonded adsorption of ethylene. The mechanism follows the one proposed by Horiuti and Polanyi in 1934 [95] and consists of three steps: (i) adsorption of the unsaturated molecule on the hydrogenated catalyst surface, (ii) hydrogen migration to the β -carbon of the unsaturated molecule and formation of a σ -bond between the catalyst surface and α-C, and finally (iii) reductive elimination of the hydrogenated molecule. The large distances between the isolated active sites avoid undesired side-reactions such as oligomerization.

Hydrogenated surfaces are a clear requirement for this mechanism. On simple transition metal surfaces, atomic hydrogen is adsorbed on top sites. On GaPd or AlPd surfaces, H also generally adsorbs on transition metal top sites, with a few exceptions. For example, a Ga-H top interaction has been identified on PdGa(100) [96], highlighting that surface Ga atoms are not only a spacer, but part of the active site, as already postulated by M. Krajčí et al. [72,97].

Non-noble intermetallic catalysts

Highly active catalysts towards hydrogenation reactions are usually based on Ptgroup metals. There are not many solutions available today to replace these noble metals. Non-precious metal catalysts, especially those based on nickel (such as Raney nickel) have been developed as economical alternatives, but they are less active and/or selective than Pt-group metals and prone to deactivation [98,99].

Al-based complex intermetallic compounds like γ-Al 4 Cu 9 or Al 13 TM 4 (TM = Co,Fe) have recently been identified as potential catalysts for hydrogenation reactions [87,[100][101][102]. For the semi-hydrogenation of acetylene, Al 13 Fe 4 presents a high conversion and a high ethylene-selectivity of 81-84% (reaction conditions: 0.5% C 2 H 2 , 5% H 2 , 50% C 2 H 4 in He, 30 ml min -1 total flow, 200 o C). For the hydrogenation of butadiene, the γ-Al 4 Cu 9 (110) surface is active and 100% selective to butenes (2-20 mbar and temperatures of 110-180 o C).

As already mentioned, the presence of atomic hydrogen on the intermetallic surfaces is mandatory for the Horiuti and Polanyi mechanism. The dissociative adsorption of H 2 on γ-Al 4 Cu 9 (110) is the rate limiting step. It is likely caused by the high concentration of Cu in the topmost surface (Cu:Al is 12:6), H 2 dissociation occurring with a non negligible barrier on pure Cu (47 kJ/mol [103]). Hydrogen dissociation is much easier when transition metals protrude at the surface, as in the case of the Al 13 Co 4 (100) surface model obtained by bulk truncation (17 kJ/mol) [104]. However, the combination of surface science studies and theoretical calculations point towards a dense Al-rich topmost layer for Al 13 Co 4 (100) [105], leading to a rather high H 2 dissociation barrier ( 52 kJ/mol) [106]. These observations highlight again the importance of surface structure on catalytic performances (Table 1). In summary, intermetallic compound surfaces are promising for applications in catalysis. An enthalpic driving force usually prevents surface segregation, but sp metals are subject to oxidation and therefore the surface of the intermetallic may deviate strongly from the bulk in the presence of oxygen. Their surfaces however provide isolation for the active site, which make them at least interesting model systems to further investigate single-atom catalysts [108].

Conclusions

We have presented a topical review of several areas of current interest relating to the structure and properties of surfaces of quasicrystals and other complex metallic alloys, and the formation of ultra-thin atomic and molecular films on these surfaces. Interest in these areas varies from fundamental curiosity about the behaviour of adsorbing materials on these surfaces to their potential for applications in economically important areas such as corrosion-resistance and catalysis. We hope to have demonstrated that the surfaces of complex metallic alloy surfaces, though not receiving perhaps the same levels of attention as simpler elemental and alloys surfaces, nonetheless merit an increasing level of scrutiny from the international surface science community.

Fig. 1

 1 Fig. 1 (a) An RTH cluster, the building block of the i-Cd-Yb quasicrystal, bisected perpendicular to the five-fold axis [31]. Atoms of different shells are coloured differently. (b) Atomic density distribution along the five-fold axis deduced from the idea model structure of i-Cd-Yb. The density is estimated in slabs of 0.01 nm thickness. Plane 1 represents the surface. Other planes indicated by numbers are occupied by Pb or Bi adatoms. The number in parentheses indicates the n th shell of the RTH cluster. (c) Structure of surface (plane 1) superimposed with planes 5 and 6, which explains the first layer of Bi. (d) Structure of surface (plane 1) superimposed with plane 8, which produces the second layer of Bi. A Penrose P1 tiling of 2.5 nm edge length is superimposed in (c) and (d). (e) Surface truncated RTH clusters with Bi atoms atop. Black spheres represent Bi atoms. Only relevant shells are shown for clarity. Reprinted from [30] c 2018 Elsevier.

Fig. 2

 2 Fig. 2 (a, b) STM images of the first layer (20 nm × 20 nm) and second layer (20 nm × 20 nm) Bi formed on the five-fold surface of i-Ag-In-Yb. Common features are highlighted in (a). (c) Autocorrelation and (d) fast Fourier transform of the STM image (b). A Penrose P1 tiling of 2.5 nm edge-length is superimposed in (b). Reprinted from [30] c 2018 Elsevier.

Fig. 4

 4 Fig. 4 (a) STM image (11x13 nm 2 ) showing individual C 60 molecules deposited on the 5-fold surface of the i-Al-Cu-Fe. They are located at the node of a quasiperiodic tiling. (b) autocorrelation pattern of the position map of the molecules extracted from an STM image, showing 10-fold symmetry of the layer. Adapted with permission from J. A. Smerdon, K. M. Young, M. Lowe, S. S. Hars, T. P. Yadav, D. Hesp, V. R. Dhanak, A. P. Tsai, H. R. Sharma, and R. McGrath, Nano Letters 14, 1184 (2014). Copyright (2014) American Chemical Society.

Fig. 5

 5 Fig. 5 (a) Structure model of the d-Al-Cu-Co decagonal phase perpendicular to the 10-fold axis. Al atoms appear as light blue circles and transition metal atoms (Cu/Co) as dark blue circles. The cluster centers are highlighted as yellow circles. The yellow ring delimits the 2 nm wide cluster units. The tiling connecting the cluster centers appears as dark lines. It consists in pentagonal (P), star (S) and boat (B) tiles with 2 nm edge length. The green circles inside the P tile at the bottom of the figure indicate additional five-fold symmetric sites. They define a smaller pentagon with edge length 1.2 nm (dotted dark line). The decoration of the P tile by C 60 molecules (in grey) is illustrated. (b) A portion of the quasiperiodic tiling with 2 nm edge length superimposed on an STM image of the C 60 film. (c) LEED pattern of the C 60 film formed on that surface, recorded at a primary beam energy of 48 eV. Adapted from [46].

Fig. 6

 6 Fig. 6 (a) Stereographic projection (top) and single scattering cluster simulations (bottom) XPD images for Au 4f from the Au-dosed and annealed decagonal Al-Ni-Co surface and from an epitaxial AuAl 2 layer respectively. (b) Atomically resolved HAADF-STEM image of the precipitate/matrix interface with the superimposed structural units of Al 9 Ir 2 and Al substrate viewed in the [1 30] Al 9 Ir 2 and [001] Al projections respectively. ((a) Reprinted with permission from [53] M. Shimoda, T. Sato, A. Tsai, J.Q. Guo, Phys. Rev. B, 62, 11288 (2000), https://doi.org/10.1103/PhysRevB.62.11288, copyright (2000) by the American Physical Society, and (b) reprinted with permission from [67] J. Kadok, K. Pussi, S. Šturm, B. Ambrožič, É. Gaudry, M.C. de Weerd, V. Fournée, J. Ledieu, Physical Review Materials 2(4), 043405 (2018), https://doi.org/10.1103/PhysRevMaterials.2.043405, copyright (2018) by the American Physical Society.

  [001] Al (001) Al 9 Ir 2 [1 30] Al 9 Ir 2 As shown using STM, LEED and transmission electron microscopy (TEM) techniques, these specific domain orientations originate from the alignment of local Al motifs present in the Al 9 Ir 2 (001) planes with the Al(100) surface unit cell. Atomicresolution high-angle annular dark field scanning TEM (HAADF-STEM) images reveal a coherent growth and a sharp interface between Al 9 Ir 2 precipitates and the Al matrix (see Fig.6(b))

Fig. 7

 7 Fig. 7 Comparison of several intermetallic catalysts to supported 5% Pd/Al 2 O 3 and the unsupported substitutional alloy Ag 80 Pd 20 . The dashed line is a guide to the eye. This figure is reproduced from ref. [73] (https://doi.org/10.1088/1468-6996/15/3/034803) under the licence Creative Commons Attribution 3.0 licence.

Table 1

 1 Dissociation, activation and desorption energies for the semi-hydrogenation of acetylene on complex Al-Co intermetallic surfaces. E dis a and E i a are the activation energies for H 2 dissociation and C 2 H x hydrogenation, respectively. E C 2 H 4 d is the ethylene desorption energy. Energies are given in kJ.mol -1 .

	Surface	Model	H 2 →2H C 2 H x →C 2 H x+1 C 2 H 4 ↑
			E dis a	E x=2 a	E x=3 a	E x=4 a	E C 2 H 4 d
	Al 5 Co 2 (2 10) P B [107] Al 5 Co 2 (2 10) P B-4Co [107]	15 -	60 50 60 80 87 64	87 105
	Al 13 Co 4 (100) [104]	17	63 61 80	70
	Al 13 Co 4 (100) [106]	52	78 83 111	89

The term "epitaxy" refers here to situation where films grown on quasicrystalline surfaces preserve the substrate directional order.

The Ir cost prohibited the growth of a large single crystal here.

Acknowledgements This work was partly supported by the Engineering and Physical Sciences Research Council, and the European Integrated Centre for the Development of New Metallic Alloys and Compounds. We dedicate this chapter to the memory of our colleague, mentor and friend An Pang Tsai who passed away in early 2019 after a short illness.


Decelle Rémi
email: remi.decelle@loria.fr

Ngo Phuc

Isabelle Debled-Rennesson

Mothe Frédéric

Lon- Guetaud Fleur

Détection automatique de la moelle sur des images d'extrémités de grumes de Douglas brutes de sciage

Introduction

La qualité du bois est un élément important dans l'industrie du bois. L'un des éléments clefs à détecter est la moelle. En effet, cette information permet d'analyser de manière plus simple d'autres éléments importants à l'estimation de la qualité (aubier, duramen, largueur des cernes).

Le problème de la détection automatique de la moelle a déjà été étudié dans la littérature. De nombreuses méthodes ont été mises au point pour la tomodensitométrie [START_REF] Longuetaud | Automatic detection of pith on ct images of spruce logs[END_REF][START_REF] Boukadida | Pithextract: A robust algorithm for pith detection in computer tomography images of wood application to 125 logs from 17 tree species[END_REF]. Un tel scanner est un investissement trop lourd pour la plupart des scieries. En parallèle, les caméras bon marché comme les caméras RVB fournissent des images à moindre coût avec une bonne résolution d'image. Sur des surfaces brutes, les marques de sciage, traces de terre et/ou peinture sont parfois visibles, ce qui rend l'analyse plus difficile. Wei et al.(2012) ont publié une brève étude bibliographique sur l'estimation de la position de la moelle. À notre connaissance, seules trois études [START_REF] Norell | Estimation of pith position in untreated log ends in sawmill environments[END_REF][START_REF] Schraml | Pith estimation on rough log end images using local Fourier spectrum analysis[END_REF][START_REF] Kurdthongmee | A fast algorithm to approximate the pith location of rubberwood timber from a normal camera image[END_REF]) ont proposé des algorithmes pour cette détection. Les trois méthodes développées sont robustes et donnent la localisation de la moelle avec une bonne précision. Cependant elles ont quelques limites. [START_REF] Norell | Estimation of pith position in untreated log ends in sawmill environments[END_REF] segmentent manuellement les coupes transversales du bois. [START_REF] Kurdthongmee | A fast algorithm to approximate the pith location of rubberwood timber from a normal camera image[END_REF] et [START_REF] Schraml | Pith estimation on rough log end images using local Fourier spectrum analysis[END_REF] supposent que la section de bois est centrée sur l'image.

Dans notre étude, la section de bois n'est pas segmentée et n'a pas besoin d'être centrée sur l'image. L'idée est d'avoir une méthode utilisable partout. Nous proposons une méthode robuste pour détecter l'emplacement de la moelle.

Matériel et méthode

Jeux d'images

Nous avons deux séries d'images différentes de Douglas. Cela diffère de Norell et Borgefors (2008) qui ont traité de l'épicéa et du pin sylvestre. [START_REF] Schraml | Pith estimation on rough log end images using local Fourier spectrum analysis[END_REF] ont traité de l'épicéa. [START_REF] Kurdthongmee | A fast algorithm to approximate the pith location of rubberwood timber from a normal camera image[END_REF] ont traité de l'hévéa.

Les deux jeux d'images concernent les même grumes à des instants différents. La première série d'images (appelée Besle), prise par un appareil photo numérique (modèle Panasonic DMC-FZ45), se compose de 65 images prises juste après la coupe en forêt et. La deuxièm e série (appelée BBF), prise par un smartphone (Huawei modèle ANE-LX1), est constituée de 16 images des grumes prises sur le parc à grumes. Quelques images des jeux de données sont illustrées dans la figure 1.

Chaque grume a été prise plusieurs fois (de 3 à 5 fois) en tournant légèrement à gauche ou à droite la caméra entre chaque vue. Nous considérons que la position de la moelle est représentée par un pixel, bien que d'un point de vue biologique elle peut occuper une plus grande surface. Les vérités terrains, c'est-à-dire la position réelle de la moelle sur chaque image, ont été évaluées manuellement par deux opérateurs différents.

Première estimation de la moelle

Nous procédons en deux étapes. La première doit remédier à l'absence de segmentation et fournir une première estimation de la moelle sur la section. Nous supposons que la section peut être assimilée à un cercle. Ainsi, notre première étape repose sur une méthode de détection de cercles : transformée de Hough (HT). En pratique, la grume n'est pas un cercle parfait et ainsi plusieurs cercles sont détectés.

Une fois les cercles trouvés, pour estimer la position de la moelle nous déterminons le barycenter de l'ensemble des centres des cercles détectés. Chaque attribue un poids à chaque pixel d'autant plus important qu'il est proche du centre, ce qui conduit à une carte de probabilité de présence de la moelle. La figure 2 montre les cercles détectés et la carte de probabilité.

Vers une estimation précise de la moelle

La seconde étape est notre implémentation (itérative) de la méthode développée par [START_REF] Longuetaud | Automatic detection of pith on ct images of spruce logs[END_REF] pour des images tomographiques. L'idée est d'extraire une petite sous-image centrée sur la première détection faite précédemment et d'accumuler les normales des tangentes aux cernes. Comme ces derniers peuvent être assimilés à des cercles concentriques centrés sur la moelle, l'accumulation doit faire ressortir la position de la moelle.

Fig. 2 : À gauche les cercles détectés par la transformée de Hough. À droite, la carte de probabilité obtenue à l'aide des centres des cercles.

Résultats et discussion

Nous

Conclusions

Nous avons proposé une méthode permettant d'estimer la position de la moelle sur des images capturées par des appareils photo numériques.Notre méthode ne nécessite pas de segmentation manuelle ou d'hypothèse sur la position de la section dans l'image comme les méthodes décrites dans la littérature. Les résultats ont montré une estimation plus précise sur les deux ensembles de données, malgré des cas difficiles. Cependant, cette précision conduit à un temps de calcul légèrement plus long. A l'heure actuelle, une seule essence de bois a été traitée, à savoir le Douglas.

Les travaux futurs devraient se concentrer sur l'amélioration de la seconde étape de la détection. Nous testerons aussi si, comme nous l'espérons, la méthode peut s'appliquer sur d'autres essences. Nous nous intéressons aussi à la détection de l'aubier et du duramen.

Détection automatique de la moelle sur des images d'extrémités de grumes de Douglas brutes de sciage

Fig. 1 :

 1 Fig. 1 : Images issues des jeux de données. Deux premières lignes issues de Besle et la dernière issue de BBF.

  avons comparé notre méthode à celle proposée par Kurdthongmee et al. (2018) sur nos jeux de données. La figure 3 montre les résultats pour les deux méthodes et les deux jeux. Notre méthode est plus précise. Nous obtenons un écart moyen de 14.29mm pour notre méthode contre 25.05mm pour celle de Kurdthongmee et al. (2018) sur le jeu Belse. Notre algorithme est cependant plus lent : une image est traitée en 2956ms contre 667ms pour celui du Kurdthongmee. Nos expériences ont été réalisées sur un Intel Core i7 avec 2,5 GHz et 16 Go de RAM.

Fig. 3 :

 3 Fig. 3 : Comparaison de la distance (mm) entre la vérité terrain et la position estimée de la moelle (Besle à gauche et BBF à droite). La courbe rouge représente notre méthode et la courbe bleue celle de Kurdthongmee et al. (2018).

Étape 1 -

 1 Détecter la grume dans l'image → Hypothèse : Grume assimilable à un cercle → Solution : Détection de cercles Méthode de Hough pour la détection de cercles Étape 2 -Estimer la position de la moelle → Idée : Tracer les normales des tangentes des cernes → Initialisation : À partir des cercles précédents → Itérativement 1. Estimer l'orientation des cernes dans une sous-image centrée sur l'estimation actuelle. 2. Tracer les normales aux tangentes des cernes 3. Récupérer une nouvelle estimation (intersections) 4. S'arrêter si la position à peu varier Méthode Résultats & Conclusion

  DECELLE Rémi 1 , NGO Phuc 1 , DEBLED-RENNESSON Isabelle 1 , MOTHE Frédéric 2 , LONGUETAUD Fleur 2 1 Université de Lorraine, LORIA, UMR 7503, 54506 Vandoeuvre-lès-Nancy, France 2 Université de Lorraine, AgroParisTech, Inra, Silva, F-54000 Nancy, France

	Contexte
	Projet TreeTrace Tracabilité du bois & Estimation de la qualité du bois		Camera à bas prix Peu chère	Scanner CT Interne
	Analyses qualité et tracabilité déjà faites sur des scanner CT Problématique → Extension aux caméras à bas prix ?	Instantanée Portabilité Externe	Externe Totalité Cher
	Objectif applicatif → Aide aux petites et moyennes scieries	Partiel	Lent
	Matériel
	105 d'extrémités de grumes de Douglas
	2 jeux de données
	• En forêt après la coupe
	• Au parc à grumes
	Même grumes !
	Prises plusieurs fois en tournant la caméra !
	Vérités terrains
	2 opérateurs
	Pointage de la position de la moelle (= 1 pixel)
	Méthode	Moyenne	Écart-type	Temps
		(mm)	(mm)	(s)
	Notre méthode	3,11 3,14	1,49 1,36	1,6 2,2
	Kurdthongmee et al. (2018)	25,06 38,38	21,23 23,12	0,6 0,7
	Schraml et al. (2013)	2,58 2,39	0,98 1,12	7,2 8,6

Nous remercions l'ANR pour le financement du projet

èmes journées du GDR 3544 « Sciences du bois » -Epinal, 18-21 novembre 2019