L Henry

N Biscay

C Huguet

S Loison

C Aymonier
email: cyril.aymonier@icmcb.cnrs.fr.

A water-based process for the surface functionalisation of ceramic fibres

This research work applied the hydrothermal process to the surface treatment of ceramic fibres which are integrated into the fabrication of ceramic matrix composites (CMCs). In the conventional process, the main step consists in dissolving the oxidised phases at the fibre surface using strong acids. As a consequence, the chemical homogeneity of the surface is improved and a microporous carbon (C) film is generated at the fibre surface. The C-rich surface enhances the compatibility of the fibre with the pyrocarbon interphase that is deposited on the fibres prior matrix deposition when processing CMCs. This paper shows the possibility to substitute this conventional process using strong acids by a hydrothermal treatment. Indeed, as water displays tunable physico-chemical properties as a function of temperature and pressure, it is possible to recover fibres demonstrating similar surface characteristics as the ones obtained after the reference acid-based treatment. The efficiency of the hydrothermal treatment is assessed through optimised surface properties obtained after one single step process operating in a semi-continuous mode. Then, the mechanism investigation reveals a selective attack of the Si atoms contained in the fibre via a hydrolysis reaction. Besides, it is denoted that the process follows a zero order kinetics. This allows a fine control of the fiber surface properties to obtain CMC with high thermomechanical performance.

Introduction

The global air traffic undergoes a constant growth as the passenger number increased from few millions in 1950 to 4.3 billion in 2018. [START_REF]Annual Review 2019 released for IATA's General Meeting[END_REF] In that context, the Advisory Council for Aviation Research and Innovation in Europe (ACARE) developed a Strategic Research Agenda (SRA) aimed at meeting the goals of Vision 2050. The Vision 2050 program has been built to face global warming effects. The objectives are to decrease the NOx and CO 2 emissions by 90% and 75%, respectively, and to lower the noise pollution by 65%.

To achieve the Vision 2050 objectives, both the decrease of the total weight of planes and the enhancement of the motor performance are solutions to load and consume less fuel. Ceramic Matrix Composites (CMC) can address those two challenges by replacing the commonly-used superalloys and consequently reducing the weight and allowing new design for aircraft engines parts. They demonstrated similar mechanical properties for a 50% weight reduction [START_REF] Krenkel | Carbon fibre reinforced silicon carbide composites (C/SiC, C/C-SiC) in Handbook of ceramic composites[END_REF] and have already been used in recent aircraft engines. 3 Due to a high-volume fibre fraction, CMCs are highly resistant to fatigue at long period of times and high temperatures. [START_REF] Corman | Silicon melt infiltrated ceramic composites (hipercomp) in Handbook of ceramic composites[END_REF][START_REF] Dicarlo | SiC/SiC composites for 1200 °C and above in Handbook of ceramic composites[END_REF] Besides, they display a high resistance to chemicals and thermal shock. To conclude, CMCs are excellent candidates to be used as constructive parts in high temperature areas in aircrafts as they show high thermomechanical properties under extreme conditions (oxidizing, corrosive or reducing atmospheres). Especially, silicon carbide-based CMCs are already used for hightechnology applications as they lead to the highest performances.

6

During its fabrication, physico-chemical phenomena take place at the fibre-matrix interface. In fact, chemical, tribological and tribochemical reactions occur at high temperature leading to a dramatic drop of the CMCs performance. Moreover, adhesion phenomena exist between the fibre and the matrix. Indeed, the efficiency of both the transfer load from the matrix to the fibre and the fibre pull-out are dependent of these phenomena. That is the reason why CMCs adopt a mechanical behaviour either fragile or pseudo-ductile when submitted to stress. Hence, the surface treatment of the fibres prior to the fabrication process of the composites needs to be precisely controlled to optimise the physico-chemical properties of the interface and, in fine, the thermomechanical properties of the CMCs.

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This paper describes the hydrothermal treatment of ceramic fibres as an alternative to the conventional acid-based method. These fibres are made of small crystalline SiC particles (1-3 nm) embedded in an amorphous phase. According to Laffon et al., the crystalline phase accounts for 55 wt.% of the total weight, the amorphous phase (oxycarbidic silicon SiC x O y phase) for 40 wt.% and the free carbon organised in Basic Structural Units for 5 wt.%. 7 At their surface, an oxygen-rich layer forms when exposed to air. Although this external layer could be assimilated to a C-rich silica layer 8 , XPS measurement revealed the presence of a pure silica layer on the top of a desized Nicalon NL 200 fibres. 9 As this oxide layer is rigid, the mechanical properties of the CMCs are diminished. Hence, the deposition of one or more less rigid layers between the matrix and the fibres (interphase) leads to an increase of the composite toughness. 10 The damage tolerance of CMCs can be enhanced by using pyrocarbon and boron nitride as interphases. [START_REF] Lamon | Chemical Vapor Infiltrated SiC/SiC composites (CVI SiC/SiC) in Handbook of ceramic composites[END_REF] Indeed, their layered structure results in a weak fibre-matrix interface which develops low adhesion strength. Consequently, the interphase can deflect the matrix cracks by causing the debonding of the fibres. [START_REF] Marshall | Encyclopaedia of Aerospace Engineering, Chap. Textile Composite Materials: Ceramic Matrix Composites[END_REF] Crack growth is then limited, and the composite failure is delayed. In the case of pyrocarbon interphases the fibre surface needs to be homogeneous and chemically compatible, in order to promote the anchorage of the interphase. An industrial method to dissolve the oxide layer and to grow a C-rich interface at the surface of the Nicalon fibres up to a thickness of 100 nm was patented. The 3 step-treatment is the following: i) the fibres are desized under air at high temperatures (400-600 °C), then ii) they are soaked into a mixture of nitric, acetic and fluorhydric acid, and iii) they are rinsed out with water. [START_REF] Loison | [END_REF] After the acidic treatment, the oxide layer thickness is reduced to 5 nm and the fibres are covered with a microporous C film. However, this acidic treatment is extremely harmful for the environment as large quantity of strong acids and water are used. The treatment requires various equipment at each step generating additional costs and time.

Our recent research work demonstrated the possibility to solubilise silicon (Si) atoms from the fibre surface and to generate a C film with only water under high pressure and temperature. 14 Moreover, carbon nanomaterials have already been prepared by this type of method.

15,16

Hence, this paper clarifies the utilisation of water instead of strong acids to modify the surface of SiC fibres. The optimisation of the surface properties of the treated fibres is achieved by tailoring the interaction between water and the Si atoms. Practically, the behaviour of subcritical water is investigated with the variation of experimental conditions as temperature, pressure, time and nature of the solvent.

Reactor

The experiments were performed in a tubular reactor of 62 cm length with a capacity of 3.3 mL. The whole set-up (Fig. 1) is made of a Jasco PU 2080 Plus high-pressure pump, a preheating system and a cooling zone followed by a Jasco BP 2080 Plus back pressure regulator in order to operate in a semi-continuous mode. A 62 cm long tow is placed inside the tubular reactor which settles in the oven already at the process temperature. When the reactor is sealed, the reactive medium is injected in the circuit and the temperature of the preheater is set at the same process temperature. Then, the pressure is fixed (between the saturated vapour and 30 MPa) and the treatment is run during a determined time period (less than 2 h). At the end of the treatment preiod, the set-up is depressurised, and the heating systems and the injection pump are turned off. The reactor can be open to retrieve the dried and treated tow. The material is directly characterised.

Characterisation techniques

The surface of all the fibres was analysed by Auger Electron Spectroscopy (AES 310F from Thermofischer Scientific) and X-Ray Photoelectron Spectroscopy (XPS K-Alpha from Thermofischer Scientific). AES was performed with an acceleration voltage of 10 kV. The etching was performed with an Argon beam of 4.5 keV and the etching spot diameter was about 50 nm. XPS measurements were conducted using an Al X-ray monochromatic source (1486.6 eV). The elemental composition was calculated from the high-resolution (HR) spectra acquired with the following parameters: a pass energy of 40 eV, a step width of 0.2 eV and a dwell time of 200 ms.

The etching was realised with an Argon beam of 5 keV and the etch spot diameter was about 500 µm. For both methods, the fibres were glued on an Indium plate which was taped on a metallic support.

Chemical functions Binding energy (eV)

Full width at half maximum (eV) (Si2p) Si-C 99.5 -100.5 0.9 -1.1 converted into atomic concentration through the sensitivity factors 23,24 with an uncertainty of 5 to 10%. HR spectra of C1s

(Si2p) Si-O-C SiC + 1.0 0.9 -1.1 (Si2p) Si-O 2 -C 2 Si-O-C + 1.0 1.0 -1.2 (Si2p) SiO
were fitted using a 30 to 50% ratio of a Lorentzian and Gaussian function type lines and a Shirley type background. Please do not adjust margins Please do not adjust margins Similarly, the dissolution of the amorphous phase starts around 300 -400 °C and 100 MPa 29-30 :

SiCxOy (s) + (2-y)H2O  SiO2 (s) + xC (s) + (2-y)H2 (g) (1.3)
Hence, a C film grows on the SiC surface from 10 nm up to 1 µm depending on the treatment time and temperature. As the pressure and temperature ranges of our study are lower than these previous studies, silica was not identified at the surface of our treated fibres. From Fig. 3, the treatment, performed below 400 °C and at a pressure comprised between the saturated pressure and 30 MPa, proved the feasibility of obtaining fibres with similar surface properties than the fibres treated with the reference acid-based treatment. The subCW treated fibres were coated with a C film with a thickness of 29 nm and the O-rich layer was removed. From literature, it was reported the possibility to form hydrosoluble silica instead of This soluble silica is not able to agglomerate and to densify at the fibre surface. Accordingly, it is believed that soluble silica is also produced during our treatment. However, TEM investigation highlighted the formation of a C-rich layer of about 100 nm (Fig. 4). Hence, it is suggested that the O-rich layer formed at the fibre surface was attacked under subcritical conditions to give rise to a C-rich layer as the following: Indeed, subcritical water exhibits specific properties. For example, its ionic product (Ke) reaches its maximum between 100 and 350 °C whereas it falls dramatically above the critical temperature (Fig. 5). 32 This means that subcritical water has a high concentration of charged species in comparison with

Please do not adjust margins Please do not adjust margins supercritical water (if the pressure stays below 100 MPa). 33 At the opposite, supercritical water completely solubilises oxygen and organics. 34 That is why hydrolysis reactions are favoured in the subcritical region of water compared to oxidations reactions that dominate in the supercritical domain. Moreover, some studies pointed out high dissolution rate of Si-based materials (Si single crystal, amorphous silica and SiOC ceramics) in subcritical water. [35][36][37][START_REF] Iler | The chemistry of silica: Solubility, polymerization, colloid and surface properties, and biochemistry[END_REF][START_REF] Ingri | Proceedings of the 40 th Nobel Symposium[END_REF][START_REF] Karásek | [END_REF][41][42] In opposite to Gogotsi et al.

Mechanism and kinetics of the treatment

Firstly, Allongue et al. identified the etching mechanism of a Sisingle crystal in alkaline solution under ambient temperature and pressure. 43 The Si atoms at kink sites were directly attacked by water until it forms silicic acid, a soluble molecule.

As subcritical water displays a high concentration of HO -and H 3 O + ions, it could be possible for the Si atoms of the Nicalon fibres to undergo several nucleophilic attacks. In order to verify their fundamental role in the fibre treatment mechanism, ethanol was used as a reactive medium. Although ethanol can be involved into solvolysis reactions, it does not have the capacity to generate ions (pKa =16). [START_REF] Negishi | Handbook of organopalladium chemistry for organic synthesis[END_REF] Besides, as it demonstrates a lower dielectric constant [START_REF] Fernández | [END_REF] than water, the dissolution rate of Si atoms is expected to decrease while increasing its proportion into the reactive medium. Several experiments were performed at temperatures below 400 °C and at a pressure comprised between the saturated pressure and 30 MPa but at different volume fractions of ethanol. From Fig. 6.1 it appears that the chemical functionalities of the treated fibres are similar, except for the one treated in a pure ethanol medium.

All the fibre surfaces demonstrate a high C content and a low O content which reveals a high efficiency of the treatment. However, the use of pure ethanol only permits to remove the sizing agent from fibres. This explains the high content of oxide (SiO 4 ) and the low C content at their surface. From Fig. 6.2, it is noticed that the quantity of dissolved Si atoms drops, i.e. the kinetics slow down, when the ethanol proportion increases in the reactive medium. As the addition of ethanol resulted in the decrease of ions concentration, it can be said that the presence of ions in subcritical water enables the hydrolysis reactions to occur. To conclude, the mechanism of our treatment allowed the study of the kinetics. Regarding the understood reactions (1.5) and (1.6), either the concentration of silicic acid or the C film thickness can be followed to assess the kinetics of the hydrothermal treatment.

As the process is semi-continuous, the flowrate parameter needs to be incorporated into the kinetics calculations. For that reason, a water quantity was calculated for each experiment by multiplying the flowrate by the treatment time. This approximation can be reasonably done over our range of study because, according to Fig. 7, the quantity of dissolved Si linearly evolves with the flowrate. Hence, this parameter can be plotted versus the quantity of water. From Fig. 8, linear fits were obtained for the evolution of the Si concentration at different temperatures (250, 300, 315, 350 °C) and at a pressure comprised between the saturated pressure and 30 MPa. As silicic acid released from equation (1.5) cannot be discriminated from the one released from equation (1.6), the results will indicate a global kinetic for the formation of the silicic acid species. Similarly, an initial quantity of Si was referred as the sum of the Si atoms included in the crystalline and amorphous phases of the fibres [Si tot ].

The differential rate equation is:

(1.7)
with k the rate constant, α and β, the reaction orders regarding [Si tot ] and [H 2 O], respectively. According to the literature, the corrosion kinetics of SiC in a hydrothermal medium are limited by the reactions at the interface. 46 In this study, it is obvious that the chemical reactions occur on a 2D surface which always regenerates.

In fact, one can imagine that the reactants are consumed layer by layer and that a new interface appears constantly. So, in the studied system, the [Si tot ] quantity is considered as infinite. The same argument is applied to the [H 2 O] quantity as water is in large excess in our system. In fact, water is both the reactant and the solvent, and it is continuously renewed due to the semi-continuous mode. The [Si tot ] and [H 2 O] quantities do not vary significantly during the treatment and are therefore considered equal to 1.

Then, the equation is:

(1.8)

The solution is:

(1.9) t, the time, will be replaced by the water quantity (mg/g of fibres/g). From Fig. 8, the evolution of dissolved Si for different temperatures along with the water quantity follows a linear regime. Hence, the Arrhenius empiric law could be used from the following equation:

(1.10)

A is the pre-exponential factor, E A , the global activation energy, R, the perfect gas factor and T, the temperature. By plotting lnk versus 1/T (data in Table 2), it is possible to determine A and E A . As a linear fit was obtained (R 2 = 0.99), the E A value was calculated to be 56 kJ/mol. This value is 3 times less than the one for the SiC corrosion following the reactions (1.2) and ( 1.3) that occurred between 500 and 800 °C under 200 MPa. Please do not adjust margins

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This points out that different mechanisms are involved in the corrosion of SiC-based materials depending on the temperature and pressure ranges of water. Indeed, as oxidation kinetics are low between 200 and 350 °C, 47 hydrolysis reactions dominate in subcritical water. In addition, it is worth to mention the crystallinity difference between SiC particles and the Nicalon fibres, which contain a large amount of amorphous phase. For example, the calculated E A value is close to the E A dissolution value of amorphous silica between 25 and 100 °C at neutral pH (55 kJ/mol) 48 and in supercritical water between 400 and 600 °C at 60 MPa (60 kJ/mol). 49 Again, the global activation energy of Nicalon dissolution into subcritical water is comprised in the 28 -80 kJ/mol range corresponding to the corrosion of oxycarbidic silicon phases between 150 and 250 °C and 0.5 to 4 MPa. [START_REF] Ingri | Proceedings of the 40 th Nobel Symposium[END_REF] The large range of values is linked to the measurement of Si concentration in a closed system that tends to be saturated after a long treatment time.

To conclude, the study revealed a zero-order kinetics for the surface treatment developed in this paper. The treatment takes place at a constant rate completely independent of the reactant concentration. The semi-continuous process allowed running the treatment in specific conditions where the excess of water resulted in highly catalysed reactions without any saturation effect of the produced silica. As it is figured on the graphs (Fig. 7 and Fig. 9), the Si atoms etching can be precisely controlled during the hydrothermal treatment. Indeed, the flowrate and the treatment time parameters are easily accessible to tune the Si atoms etching and, consequently, the thickness of the C film which is the primordial key to optimise the mechanical properties of CMCs.

Conclusions

According to this study, a green and innovative semicontinuous treatment was successfully adapted to the surface functionalisation of ceramic materials. Besides, we can affirm that the hydrothermal treatment of the Nicalon fibre under subcritical conditions occurs via Si atoms hydrolysis of the crystalline and amorphous phases. The reactions lead to the production of silicic acid in the medium and the generation of a C film at the outer of the filaments. The strong potential of the process resulted from several factors: i) the acid-base properties of water in the studied ranges of pressure and temperature, as a high ions concentration catalyses the Si atoms hydrolysis, ii) the polarity and proticity of the reactive medium which helps both the hydroxylated surface of the fibres to get involved into strong interaction with water and the stabilisation and solvatation of the silicic acid, iii) the temperature and pressure ranges of water which lead to enhanced silica solubility, and iv) the semi-continuous operating mode that regenerates the reactive medium and establishes a constant rate process. Hence, the hydrolysis reactions do not seem to be limited by water diffusion through the C film but by the reactions that take place at the fibre/C interface.

To conclude, the replacement of the acidic treatment by this new method will participate to the expansion of green solutions applied to the elaboration process of ceramic-based materials.

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Fig. 1

 1 Fig.1 Scheme of the experimental set-up used to treat SiC ceramic fibres.

Fig. 2 . 1 )

 21 Fig.2. 1) Si 2p, 2) O1s and 3) C1s fitted HR spectra of the surface of a SiC Nicalon fibre. 4) Etching profile of C1s spectrum.

3 Results and discussion 3. 1 C 26 :

 126 film formation in water under pressure and temperature In a hydrothermal medium, SiC materials are subjected to corrosion. Above 500 °C and 100 MPa, crystalline phases of the Nicalon fibre interact with water if the molar ratio of H 2 O:SiC is above 2:1 according to the following equation SiC (s) + 2H2O  SiO2 (s) + CH4 (g) (1.1) | 4

Fig. 3 1 ) 29 :

 129 Fig. 3 1) Schematic representation of the chemical functionalities of the surface of sized, desized, SC (Safran Ceramics) treated and SubCW treated fibres. AES depth profiles of 2) sized, 3) desized, 4) SC treated and 4) SubWC treated fibres.

31 :

 31 the solid silica after corrosion of SiC in heated water (290 °C).Hirayama et al. proposed the formation of silicic acidSiC (s) + 4H2O (g)  Si(OH)4 (aq) + CH4 (g)(1.4)

  SiC (s) + 4H2O (aq)  Si(OH)4 (aq) + C (s) + 2H2 (g) (1.5) SiCxOy (s) + (4-y)H2O (aq)  Si(OH)4 (aq) + xC (s) + 2(2-y)H2 (g)(1.6)

and

  Yoshimura et al., the subcritical treatment of this study engenders hydrolysis reactions to form a C coating by dissolution of an O-rich layer at the fibre surface. Hence, our temperature and pressure ranges are much lower and the treatment time is largely decreased by operating in a semicontinuous mode. The next section will focus on the understanding of the Si extraction mechanism by modifying the physico-chemical properties of the reactive medium.

Fig. 4 Fig. 5

 45 Fig.4TEM micrographs of a 1), 3) cross section of a treated fibre with 2) a schematic representation and 4) a STEM analysis of Si (blue) and C (red) atoms. NPs: nanoparticles.

32

 32

Fig. 6 1 )

 1 Fig. 6 1) Schematic representation of the chemical functionalities of the surface of the treated fibres regarding the nature of the reactive medium. 2) Evolution of the quantity of dissolved Si as a function of the volume fraction of ethanol.

47 Fig. 7

 477 Fig. 7 Evolution of the quantity of dissolved Si in the medium as a function of the flowrate. The fibres were water treated with the same temperature and pressure conditions than for Fig. 3.

Fig. 8 :

 8 Fig. 8: Evolution of the quantity of dissolved Si in the water medium as a function of the quantity of used water.

Fig. 9

 9 Fig. 9 Evolution of the quantity of dissolved Si as a function of the treatment time.

Table 1 .

 1 XPS fitting parameters of the O

	17-18 , C	19-21 and Si	22-23 HR spectra.

Table 1 ,

 1 fitted HR spectra are obtained and shown in Fig.2. On the Si2p spectrum, for each p orbital, a doublet is considered (p 1/2 and p 3/2 ) due to spin-orbit coupling. The height of the p 1/2 orbital is equal to 0.56 times the p 3/2 orbital. The gap between the two orbitals is fixed at 0.6 eV. For the microstructural characterisation, Au nanoparticles were deposited on the filament surface of the treated tow. Then, a sandwich material was built with a Si wafer, at the bottom, and a glass plate, at the top. And a composite, which corresponds to the treated fibres embedded in a cured epoxy resin, was placed in between. Thin slices of the sandwich material were cut and polished before being subjected to ionic beam thinning (Cryo Ion Slicer EM 09100IS from JEOL). The C film was observed by Transmission Electron Microscopy (TEM 6700F from JEOL at 200 keV) and the O, C and Si atom distribution was revealed by Scanning Transmission Electron Microscopy (STEM). Finally, elemental analyses were intended to quantify Si atoms released in the reactive medium after treatment by ICP-OES (720ES from Varian).

25

Using

Table 2

 2 Slope (equivalent to k) and R 2 values extracted from the linear fit on Fig.8and input data for the Arrhenius plot.

	Temperature	Slope (mg/g	R	2	1/T (K	-1 )	ln k
	(°C)	de fibres/g)
	250	0.0007	0.87 0.00191	-4.96
	300	0.0204	0.90 0.00174	-3.89
	315	0.0284	0.99 0.00170	-3.56
	350	0.0503	0.99 0.00161	-2.99

Acknowledgements

The authors would like to thank the Direction Générale de l'Armement (DGA), the Région Nouvelle Aquitaine and Safran Ceramics company for their financial support. The authors would like to thank Christine Labrugère and Michel Lahaye for their support to surface analyses.

Conflicts of interest

There are no conflicts of interest to declare.


Guy Rahamim

Dr Ehud Greenberg

Dr Khalil Rajouâ

Dr Frederic Favier

Prof. D Hagay Shpaisman
email: hagay.shpaisman@biu.ac.il

David Zitoun
email: david.zitoun@biu.ac.il

Zitoun

Laser-Induced Colloidal Writing of Organometallic Precursor-Based Repeatable and Fast Pd-Ni Hydrogen Sensor

Keywords: hydrogen, laser deposition, nanoparticles, organometallics, sensors

The advent of hydrogen economy brings new challenges in terms of safety and sensing with a need for fast and low-cost monitoring of hydrogen concentration. Herein, a repeatable process for the fabrication of Pd-based hydrogen sensor is presented. First, a room-temperature reaction of organometallic precursors yields colloidal Pd/Ni alloyed nanoparticles. This organic solvent-based colloidal dispersion shows stability over months even with a relatively high metal content (≈1 wt %). Then, a laser induced microbubble deposits the nanoparticles in predetermined patterns from a microdroplet dispersion that is placed on a glass slide. An optical microscope monitors the writing process while a multimeter measures the sensor's conductance, assessing the success of the fabrication process. The fabricated sensors demonstrate excellent hydrogen detection performance in terms of response time, signal stability, and detection limit down to 100 ppm of H2 in air at room temperature.

Introduction

The high energy density of hydrogen makes it the most promising and cleanest technology to replace fossil fuels for transportation, besides the more established use of hydrogen in industry. [1][2][3][4] This high energy density comes with a downside in terms of safety, with low minimum ignition energy (0.017 mJ), high combustion heat (142 kJ g-1), wide flammability range (4-75%), and high permeability of hydrogen gas. [3] The transition toward hydrogen economy must include the deployment of hydrogen monitoring and hydrogen sensing which is not straightforward since it is a colorless, odorless, and tasteless flammable gas. There is an obvious need for fast and accurate sensing of H2 generation or accumulation to prevent explosions. Among the diverse sensing techniques, resistive gas sensors display good performance and low cost. Palladium has been investigated as hydrogen sensor material due to its high dissociative absorption of hydrogen. When coupled with semiconductor material, the dissociate hydrogen might affect the semiconductor electron depletion layer resulting with sensing capabilities. [5][6][7][8] During hydrogen absorption, the metallic phase experiences a crystallographic lattice expansion and a conductivity decrease, eventually converting to a semiconductor. [9,10] Charge pathways may also decrease due to cracks in the films after expansion. [11,12] The resistive response, as a function of hydrogen concentration, enables hydrogen sensing as reported in the literature. [13][14][15][16][17] The volume expansion of Pd, upon hydrogen absorption, may lead to another phenomenon that could be used for sensing. In nanogranular systems the volume expansion results in closure of nanogaps and provides new electric pathways thus demonstrating a decrease in resistivity. This phenomenon happens mainly in thin films and nanogap Pd sensors, which show improved sensitivity and response time compared to bulk Pd sensors. [18][19][20][21][22][23][24] Quantum tunneling may also explain the response of nanogap systems, even when the hydrogen absorption does not fully close the gaps. [16] The main drawback of Pd for hydrogen sensing is the hysteresis effect caused by the expansion (mechanical deformation) of palladium hydride during its transition from α to β phase, upon repeated exposure to hydrogen. The fabrication of very thin films partially overcomes this effect, [23,25,26] and the alloying of Pd, mainly with Ni, allows decreasing the hysteresis. [14,27,28] Most of the research in this field during recent years have targeted the fabrication of thin films and/ or alloyed Pd systems, using sputtering, [10][11][12]18,19,25,[28][29][30][START_REF] Peng | MATEC Web Conf[END_REF][START_REF] Zeng | [END_REF] electron-beam evaporation, [9,14,[21][22][23] and pulsed laser deposition [15] techniques. Alongside physical deposition techniques, wet chemical deposition methods, that are known to be low cost and versatile, can yield efficient sensors through electrodeposition [24,33] or colloidal deposition. [34] Colloidal synthesis represents the state-of-the-art in terms of control over all the parameters of nanoparticles (NPs) that could be used to construct the hydrogen sensors. Specifically, organometallic precursors yield NPs under mild conditions of temperature and pressure, giving rise to an accurate control on the kinetics of the NPs formation and growth. [35][36][37] Colloidal methods allow for the formation of composite materials which can be sprayed to form a thin film, like Pd nanocube-graphene, core-shell Pt/Pdgraphene hybrid sensors, [38,39] or Pd nanocube-MWCNTs. [40] The electrophoretic deposition (EPD) method could be used for depositing NPs from a colloidal solution, and has been demonstrated for Pd on InP and GaN. [34,41,42] Other deposition methods rely on applying an electric, [43] magnetic, or acoustic field for the spatial control on the colloids. [44] Alas, these methods are not optimized for formation of on demand arbitrary controlled micropatterns. Localized bottom-up on demand micropatterns could be formed by directing light to specific locations. A laser beam focused on a dispersion of NPs leads to the formation of a microbubble on the substrate due to heating of the solvent. Convective flows around the microbubble carries NPs to its base where some of them are pinned. [45] Moving the focused beam relative to the substrate results in material deposition along a predetermined path. This method is referred to as the laser induced microbubble technique (LIMBT). Lately Armon et al. [46] have shown that modulation of the laser significantly improves the control over the microbubble and the pinning of materials, and could be used for formation of continuous conductive micropatterns. Here we show for the first time how modulated-LIMBT is used to micropattern NPs for fabrication of hydrogen sensors. Herein, we present a repeatable process for the fabrication of Pd-based hydrogen sensors. In the first step, the room-temperature reaction of organometallic Pd and Ni precursors yield a colloidal dispersion of Pd/Ni alloyed NPs. This organic solvent-based colloidal dispersion shows stability over months even with a relatively high metal content (≈1 wt%). Then, using modulated-LIMBT, Pd/Ni NPs are deposited from a microdroplet of the above colloidal dispersion. The NPs are selectively deposited along a predetermined path, thus bridging the gap between two gold pads on top of a glass substrate. The resistivity between the two gold pads changes as a function hydrogen concentration, and these sensors were tested in air at room temperature to assess the sensitivity and reaction time in a real environment.

Results and Discussion

The overall flowchart shown in Figure 1a displays the steps for sensor fabrication: organometallicbased synthesis of Pd-Ni colloids and laser-induced deposition. Figure 1b shows an illustration of the sensing bench-testing system.

PdNi Alloy Colloidal Synthesis and Characterization

The organometallic precursors easily decompose under hydrogen through the hydrogenation of the ligands and release of the metal centers (see Supporting Information). The reaction occurs at low temperature (35 °C) and in a short time (12 min) which enables the homogeneous nucleation of small NPs. The colloidal dispersions are stored without any further treatment or purification. The yield is close to 100% and the colloidal dispersion is stable for months. The metal weight concentration in the dispersion is set to 1 wt%, a relatively high value in the field of metal NPs dispersed in an organic solvent. The long chain alkylamines stabilize the colloidal dispersion for months and facilitate the ability for small particles size. Alkylamines are surfactants that can be easily displaced or removed for the fabrication of the sensors, in contrast to other surfactants which strongly bind to the surface of Pd (for instance alkylthiols).

The NPs are characterized by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), energy dispersive X-Ray spectroscopy (EDS), and X-ray diffraction (XRD) (Figure 2). The NPs are well dispersed and homogeneous in size (Figure 2a). The particle size dispersion (PSD) is derived from the analysis of 300 particles visualized by TEM, using the ImageJ software. The colloids display a lognormal PSD of 4.6 ± 1.4 nm. The EDS measurements (Figure 2c) on the NPs confirm the initial stoichiometry of the organometallic precursors with a metallic ratio of Pd0.9Ni0.1. The atomic plane spacing (0.222 nm) shown in Figure 2b is very close to pure Pd (0.223 nm), and indicates the formation of a Pd-rich alloy. The XRD shows the presence of a single phase (Figure 2d). The lattice parameters are very close to face centered cubic (FCC) Pd (JCPDS # 05-0681; Fm-3m, a = 3.8902 Å: red lines) and do not show the presence of FCC Ni (JCPDS # 04-0850; Fm-3m, a = 3.5238 Å: blue lines). The two metals form an alloy, as expected from the bulk phase diagram. The very fast decomposition of the organometallic precursors can explain the very small particle size and the formation of a disordered statistical alloy.

Modulated-LIMBT Deposition

First, two gold pads that serve as electrodes with a 3-4 μm gap b m gap between them (Figure S1A, Supporting Information) were formed by dividing a larger gold pad (Figure 1a.2) that was evaporated on a glass substrate. This is achieved by metal dewetting using a 532 nm laser. Then, a micro droplet (10 μm gap b L) of the colloidal dispersion is placed on the electrodes. We use modulated-LIMBT [46,47] in order to bridge the gap between the gold electrodes with the Pd0.9Ni0.1 NPs. The laser beam is focused at the liquid/substrate interface. The colloidal dispersion strongly absorbs the laser beam, causing local temperature increase of the solvent. The solvent's vapor pressure rises until a microbubble is formed. The temperature gradient between the top and bottom of the microbubble leads to formation of two types of convection flows: natural (due to the temperature/density gradient) and Gibbs-Marangoni (due to changes in surface tension). These two convection mechanisms lead to pinning of the NPs at the contact line between the microbubble and the substrate. The motorized stage is set so the deposition occurs along the gap between the electrodes (Figure 3a). The optimization of the LIMBT modulation parameters (power, stage velocity, and duty cycle) was determined by connecting the two Au pads to a multimeter and measuring the conductivity of the deposited material in real time until it reaches a measurable resistance. The resistance between the Au pads monitors the formation of conductive PdNi bridges in situ. The deposited layer was observed by a SEM (Figure 3a) and consists of nanoscale particles visualized by high resolution SEM (Figure 3b). This points at the successful assembly of a conductive layer directly from the Pd0.9Ni0.1 colloidal dispersion, without any further processing. Atomic force microscopy (AFM) gives the height profile of the deposited layer. Figure 3c shows a line-scan along the deposited layer. The average thickness was found to be 360 ± 160 nm. The inhomogeneity in the deposited thickness is due to the sequential deposition of overlapping rings (contact lines between the microbubble and the substrate) that is sometimes observed in deposits of LIMBT. The modulated-LIBMT assisted colloidal fabrication protocol yields sensing layers with a broad range of initial resistivity. The sensors' activation is carried out through exposure to hydrogen [21,24,48] that results in a high increase of the sensors' initial resistance.

Sensing Performances

The sensors were tested in a homemade testing chamber connected to a gas channels system equipped with mass-flow controllers and electrical valves, with an interface to Labview software for automated measurements (Figure 1b). The Au pads were connected with Cu wires and silver paste to a potentiostat. In general, sensors with different morphological and electrical properties require optimization of the working mode (constant current or constant voltage) and a specific set of electrical input parameters. For both working modes the operation voltage and current tuning was initiated at low values and slowly increased in order to achieve maximal response to H2 while preventing Joule heating effects that might result in the sensor's failure. The optimal electrical parameters for the sensor's measurements shown in Figure 4 were set to 7 V for the constant voltage mode and 0.11 mA for the constant current mode. The H2 concentrations in air varied from 4% (the lower limit for explosion in air [2,14,24]) down to 0.15%. The electrical signals obtained were monitored and recorded. The analysis of the results reveals that the response time does not vary with the operation mode. However, the constant voltage mode shows an improved response amplitude with sharpened signals and lower detection limit (Figure 4) when compared with the constant current mode. Therefore, below we report the sensing response using the constant voltage mode, unless stated otherwise. After finding the optimal operation mode, we measured the sensors' response to lower concentrations, in the range of 0.5-0.01% of H2 with a 0.5 V operation bias. The ability to detect lower concentrations than the explosion limit (4% H2) is crucial for safety and is required in all hazard alarms. The results presented in Figure 5a indicate stable and sharp signals obtained while exposing the sensor to low H2 concentrations. While exposed to concentrations above 0.1% H2, the sensor signal displays a slight drift. Exposing to lower concentrations than 0.1% reveal a more stable baseline with longer response/recovery times. Moreover, the sensor's response to 0.01% (100 ppm) of H2 displays a high and sharp amplitude, which might indicate the sensor ability to detect even lower H2 concentration. To test the stability of the signal, the sensor was repeatedly exposed to the edges of the selected concentration range. The sensor's signal for repeated exposure to 0.5% (Figure 5b) and 100 ppm (Figure 5c) H2 does not show a significant drift. The sensor's response is consistent, repeatable, and stable.

Response and Recovery Time Measurements

The response time is defined as the time needed for the sensor to reach 90% of the maximal signal amplitude, while the recovery time is the time needed for the signal to return within 10% from the base line. The layer of NPs is expected to show short response and recovery times due to its high surface area. We calculated the response/recovery times for each concentration tested. Figure 6a shows the response and recovery times for the exposure of the sensor to 0.5% of H2 . The results shown in Figure 6c reveal an inversely proportional behavior to the H2 concentration with a maximum response time of 69 s at 200 ppm followed by a slight decrease at 100 ppm. 0.5% H2 is detected within 31 s and higher H2 concentrations are expected to result in even faster response times. The recovery time shows a similar behavior with a maximum of 156 s for 100 ppm of H2 and a minimal recovery time of 75 s for 0.5% H2 . We modified our testing system in order to measure shorter response and recovery times. Two electric valves were added very close to the testing chamber inlet, one for air flow and one for H2. Both valves were computer controlled and interfaced with Labview. This modification was crucial to avoid any unwanted source of error, arising primarily due to the mass-flow controller (MFC) response/stabilization time and leakage. The addition of the electrical valves allowed us to minimize the gas channel lengths and reach a low measurable gas channel volume. In this configuration, the gas channel volume was 2.5 cc and the flow 100 cc/min. These values were used to calculate the delay time that was found to be 1.5 s and was taken into account when determining short response times. At high H2 concentrations, the constant current operation mode provides faster responses. The current setpoint was 0.1 mA and the measurements were carried out with repeating exposure/washing of H2/Air. Figure 6b presents the response time measurements with different flow rates and 100% H2 and air cycling. The first three H2 exposures were at 100 cc/min followed by three H2 exposures at 50 cc/min. This allows us to study effect of flow rate on the sensor's response time. The results are consistent with a response time of 2.5 ± 0.7s, for 100% H2. Power fitting of the low H2 concentration response times predict very well the high H2 concentration value (Figure 6d). Moreover, it was found there is no significant influence of the flow rate. However, the recovery time increases almost linearly with the flow rate. While the recovery times were in the range of 14-21 s for the 100 cc/min flow, the recovery times were in the range of 34-37 for the lower flow rate. Selected results from the literature can be found in Table S1 and Figure S2 in the Supporting Information.

Sensing Mechanism

There are several sensing mechanisms associated with Pd-based sensors. Two of them can be related to the sensing response observed in this work. Exposing the sensor to H2 leads to the dissociation and absorption of the gas inside the NPs, resulting with two main changes that affect the device's resistance: formation of palladium hydride phase coupled with volume expansion. On one hand, palladium hydride has higher resistance compared with pure Pd, thus, the formation of hydride includes the increase of the device resistance. On the other hand, when working with nanostructures device (like our sensor), volume expansion of the NPs might induce higher contact between neighboring NPs, increasing the electrical percolation in the device, resulting with a conductive response. In our case, the increasing resistance (decreasing current) during exposure to H2 (Figures 4 and5) points at major role of the first mechanism, hydride formation. The proposed mechanism is illustrated in Figure 7.

Conclusion

Herein, we have reported a novel sensor fabrication approach, combining an organometallic-based NPs synthesis followed by modulated-LIMBT deposition. The synthesis is fast and with high yield and the formed colloidal dispersions are stable for months. The modulated-LIMBT deposits the PdNi Nps between two gold pads, thus providing conductive functional sensors. The effective deposition method allows formation of hundreds of sensors even with the small-scale synthesis described. The sensors display excellent sensitivity to a wide range of H2 concentrations with a detection limit below 100 ppm.

Acknowledgements

The authors thank Michael Mirilashvili for the fruitful discussions on the sensing mechanism.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Conflict of Interest

The authors declare no conflict of interest.


Matteo Atzori

Ivan Breslavetz

Kévin Paillot

Katsuya Inoue

Cyrille Train

A Chiral Prussian Blue Analogue Pushes Magneto-Chiral Dichroism Limits

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A Chiral Prussian Blue Analogue Pushes Magneto-Chiral Dichroism Limits

Matteo Atzori, †, * Ivan Breslavetz, † Kévin Paillot, † Katsuya Inoue, ‡ Geert L. J. A. Rikken † and Cyrille Train †, * † Laboratoire National des Champs Magnétiques Intenses (LNCMI), Univ. Grenoble Alpes, INSA Toulouse, Univ. Toulouse Paul Sabatier, EMFL, CNRS, Grenoble -Toulouse, France ‡ Department of Chemistry and Center for Chiral Science, Hiroshima University, Hiroshima, Japan

Supporting Information Placeholder

ABSTRACT: Here we report on the Magneto-Chiral Dichroism (MChD) detected with visible light on the chiral Prussian Blue Analogue [Mn II (X-pnH)(H 2 O)][Cr III (CN) 6 ]•H 2 O (X = S, R; pn = 1,2-propanediamine). Single crystals suitable for magneto-optical measurements were grown starting from enantiopure chiral ligands. X-ray diffraction and magnetic measurements confirmed the 2D structure of the material, its absolute configuration, and its ferrimagnetic ordered state below a critical temperature of 38 K. Absorption and MChD spectra were measured between 450 and 900 nm from room temperature down to 4 K. At 4 K the electronic spectrum features spin-allowed and spin-forbidden transitions of Cr III centers, spin-forbidden transitions of the Mn II centers and metal-to-metal charge transfer bands. The MChD spectra below the magnetic ordering temperature exhibit intense absolute configuration-dependent MChD signals. The temperature dependence of these signals closely follows the material magnetization. Under a magnetic field of 0.46 T, the most intense contribution to MChD represents 2.6% T -1 of the absorbed intensity, one of the highest values observed to date.

Investigating the interplay between chirality, light, and magnetism is a fascinating research challenge. All started in the 19

th century through the discoveries by Arago and Faraday of natural optical activity (NOA) and magneticallyinduced optical activity (MOA), respectively.

1 NOA originates from a nonlocal optical response of a medium that lacks mirror symmetry, i.e. a chiral system, while MOA originates from the breaking of time-reversal symmetry by the magnetic field. [START_REF] Rikken | Observation of Magneto-Chiral Dichroism[END_REF] This essential symmetry difference justifies that the direct link between the two phenomena was sought in vain by Louis Pasteur.

1 However, these symmetry arguments allowed to predict the existence of a large class of nonreciprocal magneto-chiral effects, that can be observed in chiral systems possessing a magnetization, either spontaneous or induced by a magnetic field. [START_REF] Rikken | Observation of Magneto-Chiral Dichroism[END_REF][START_REF] Barron | Magneto-Chiral Birefringence and Dichroism[END_REF][START_REF] Pop | Electrical Magnetochiral Anisotropy in a Bulk Chiral Molecular Conductor[END_REF] Their optical manifestation features an absorption or emission of unpolarized light that depends on the relative orientation of the magnetic field (B) with respect to the direction of light propagation (k), and on the absolute configuration of the system. In absorption, this effect is called Magneto-Chiral Dichroism (MChD), 2 while in refraction is called Magneto-Chiral Birefringence.

3 MChD has been observed for frequencies ranging from microwaves [START_REF] Tomita | Direct Observation of Magnetochiral Effects through a Single Metamolecule in Microwave Regions[END_REF] to hard X-rays, [START_REF] Sessoli | Strong Magneto-Chiral Dichroism in a Paramagnetic Molecular Helix Observed by Hard X-Rays[END_REF] underlining the universality of this effect. [START_REF] Rikken | Observation of Magneto-Chiral Dichroism[END_REF][START_REF] Tomita | Direct Observation of Magnetochiral Effects through a Single Metamolecule in Microwave Regions[END_REF][START_REF] Sessoli | Strong Magneto-Chiral Dichroism in a Paramagnetic Molecular Helix Observed by Hard X-Rays[END_REF][START_REF] Taniguchi | Strong Magnetochiral Dichroism for Visible Light Emission in a Rationally Designed Paramagnetic Enantiopure Molecule[END_REF][START_REF] Train | Strong Magneto-Chiral Dichroism in Enantiopure Chiral Ferromagnets[END_REF][START_REF] Rikken | Pure and Cascaded Magnetochiral Anisotropy in Optical Absorption[END_REF][START_REF] Train | The Fruitful Introduction of Chirality and Control of Absolute Configurations in Molecular Magnets[END_REF][START_REF] Nakagawa | Magneto-Chiral Dichroism of CsCuCl3[END_REF][START_REF] Kitagawa | Magneto-Chiral Dichroism of Organic Compounds[END_REF][START_REF] Taniguchi | Local-Site Dependency of Magneto-Chiral Dichroism in Enantiopure One-Dimensional Copper(II)-Chromium(III) Coordination Polymers[END_REF] To date, MChD in the Visible range was studied on a limited number of chiral molecular systems, namely nickel(II) sulfate hexahydrate, [START_REF] Rikken | Pure and Cascaded Magnetochiral Anisotropy in Optical Absorption[END_REF] a terbium(III) complex, [START_REF] Wang | Chiral Bis(Phthalocyaninato) Terbium Double-Decker Compounds with Enhanced Single-Ion Magnetic Behavior[END_REF] Jaggregate of porphyrins, [START_REF] Kitagawa | Magneto-Chiral Dichroism of Organic Compounds[END_REF] a 1D Cu II -Cr III coordination polymer, [START_REF] Taniguchi | Local-Site Dependency of Magneto-Chiral Dichroism in Enantiopure One-Dimensional Copper(II)-Chromium(III) Coordination Polymers[END_REF] and [N(CH 3 )(n-C 3 H 7 ) 2 (sec-C 4 H 9 )][Mn II Cr III (oxalate) 3 ], a 2D ferromagnet with a Curie temperature of 6 K. [START_REF] Train | Strong Magneto-Chiral Dichroism in Enantiopure Chiral Ferromagnets[END_REF] Nonetheless, two major issues call for more in-depth studies of MChD in the UV-Visible-NIR region: i) the reverse effect, called magneto-chiral photochemistry, [START_REF] Rikken | Enantioselective Magnetochiral Photochemistry[END_REF] is considered as one of the possible mechanisms for the emergence of homochirality of life on Earth, 1,12 ii) the use of MChD for optical readout of magnetic data has been envisioned but not yet demonstrated. [START_REF] Galán-Mascarós | Bring to Light[END_REF][START_REF] Barron | Chirality and Magnetism Shake Hands[END_REF] For both issues, it is crucial to shed light on the origin of this effect and determine the microscopic parameters that govern MChD to design materials with strong MChD. For the later, it is essential to enhance the temperature where the spontaneous magnetization of the material drives the effect.

To tackle these challenges, we have investigated [START_REF] Inoue | Structure and Magnetic Properties of a Chiral Two-Dimensional Ferrimagnet WithTC of 38 K[END_REF][START_REF] Yoshida | Biomimetic Transformation by a Crystal of a Chiral Mn II -Cr III Ferrimagnetic Prussian Blue Analogue[END_REF] hereafter referred as 1-(S) and 1-(R). This material is a 2D chiral molecular ferrimagnet with a Curie temperature of 38 K. [START_REF] Yoshida | Biomimetic Transformation by a Crystal of a Chiral Mn II -Cr III Ferrimagnetic Prussian Blue Analogue[END_REF] The cyanide bridge ensures an efficient exchange interaction between metal ions and the chiral ligand triggers the absolute configuration of the material. [START_REF] Train | The Fruitful Introduction of Chirality and Control of Absolute Configurations in Molecular Magnets[END_REF] In this bimetallic network, only Mn II is coordinated to a chiral ligand, thus providing nearest-neighbor chiral features to only one metal center. 19 Single crystals of 1 are obtained by reacting hexacyanidochromate(III) with Mn II and one of the two 1,2propanediamine enantiomers following published protocols. [START_REF] Yoshida | Biomimetic Transformation by a Crystal of a Chiral Mn II -Cr III Ferrimagnetic Prussian Blue Analogue[END_REF] Slow liquid diffusion of the molecular components allows to obtain single-crystals with good optical transparency (Figure S1), a key feature for quantitative evaluation of MChD. The system crystallizes in the orthorhombic chiral space group P2 1 2 1 2 1 with one [Cr III (CN) 6 ]

[Mn II (X- pnH)(H 2 O)][Cr III (CN) 6 ]•H 2 O (X = S, R; pnH = 1,2-propane- diamine),
3-anion, one Mn II ion, one chiral X-pnH and one water molecule in the asymmetric unit (Figure 1a). In this structure, Cr III is surrounded by its six cyanide ligands while Mn II is coordinated to the monodentate chiral X-pnH ligand in the apical posi-tion of a very slightly elongated octahedron (Figure 1a andS2). Accordingly, the immediate environment of both metal ions is not chiral, although Mn II is located closer to the stereogenic center than Cr III . Repetition of the asymmetric unit leads to squared 2D layers (Figure 1b andS3) that are at the origin of the long-range magnetic ordering. Powder X-ray diffraction analysis on 1 confirmed the crystalline phase with experimental patterns in perfect agreement with the one calculated from single crystal XRD (Fig- ures S4-S5). [START_REF] Yoshida | Biomimetic Transformation by a Crystal of a Chiral Mn II -Cr III Ferrimagnetic Prussian Blue Analogue[END_REF] Magnetic measurements as a function of temperature and field confirmed the magnetic behavior of 1, [START_REF] Inoue | Structure and Magnetic Properties of a Chiral Two-Dimensional Ferrimagnet WithTC of 38 K[END_REF] in particular the ferrimagnetic ordering below 38 K (Figure S6-S8).

Single crystals of ca. 0.80.83 mm (Figure S1) were used to record absorption spectra in transmission mode along the c crystallographic axis as a function of the temperature (2944.0 K) in the 450900 nm spectral window to get insights on the electronic transitions of 1. The temperature variation of the absorption coefficient (A, cm -1 ) is reported in Figure 2a, while Figure S9 shows a comparison between the spectra at 294 K and 4.0 K.

The electronic spectrum at room temperature shows two broad absorption peaks at  = 490 nm and 608 nm, the latter being shouldered, and a structured absorption spanning from 730 nm to 820 nm with two distinct peaks at  = 782 nm and 801 nm. These features are similar to those reported in the seminal work of Yoshida and coworkers. [START_REF] Yoshida | Biomimetic Transformation by a Crystal of a Chiral Mn II -Cr III Ferrimagnetic Prussian Blue Analogue[END_REF] The two broad peaks at high energy are attributed to the two low energy spin allowed (S = 0) d-d transitions of octahedral Cr III :  = 490 nm, [START_REF] Pop | Electrical Magnetochiral Anisotropy in a Bulk Chiral Molecular Conductor[END_REF] T 1 ( 4 F) ← 4 A 2 ( 4 F), and  = 608 nm, [START_REF] Pop | Electrical Magnetochiral Anisotropy in a Bulk Chiral Molecular Conductor[END_REF] T 2 ( 4 F) ← 4 A 2 ( 4 F). [START_REF] Tanabe | On the Absorption Spectra of Complex Ions II[END_REF][START_REF] Lever | Inorganic Electronic Spectroscopy[END_REF] According to previous reports on closely related compounds, the broad absorption peak at low energy can be attributed to a set of metal-to-metal charge transfer (MMCT) transitions. [START_REF] Tanabe | On the Absorption Spectra of Complex Ions II[END_REF] The energy of the lowest energy spin-allowed transition for Cr III , 615 nm, provides the value of the crystal field parameter  O , 16260 cm -1 . The ratio between the two lowest energy spin-allowed transitions is 1.26(1), which corresponds to  O /B of 37. [START_REF] Racah | Theory of Complex Spectra[END_REF] This allows to evaluate the position of the

2 T 2 ( 4 F) ← 4 A 2 ( 2 G
) transition at ca. 690 nm (E/B = 33), which is in agreement with the experimental findings and validates the assignments. Finally, the three electronic transitions between 500 nm and 550 nm correspond to spin-forbidden transition of high-spin Mn II . [START_REF] Lever | Inorganic Electronic Spectroscopy[END_REF][START_REF] Jørgensen | Absorption Spectrum of Manganese (II) Diethylenetriamine Complexes[END_REF] The other Mn II expected transitions are hidden under the spin-allowed transitions of Cr III precluding any further fine analysis.

Magnetic circular dichroism measurements for 1 were already reported [START_REF] Inoue | Structure and Magnetic Properties of a Chiral Two-Dimensional Ferrimagnet WithTC of 38 K[END_REF] while natural circular dichroism spectra are reported and commented in the SI (Figure S11). MChD measurements were performed on single crystals of 1-(S) and 1-(R) in light transmission configuration with a fixed light propagation vector k parallel to the c crystallographic axis and an alternating magnetic field B ( = 1.5 Hz) of amplitude ±1.0 T. [START_REF] Kopnov | A Multichannel Magneto-Chiral Dichroism Spectrometer[END_REF] Measurements were performed in the temperature range 4.0165 K. Figure 2b shows a survey spectrum for the two enantiomers at T = 4.0 K, featuring multiple MChD signals originating from the difference in the absorption of the electronic transitions at  = 545, 532, and 520 nm, as well as  = 770 and 802 nm (Figure S1o). Measurements performed in the same conditions for 1-(S) and 1-(R) provides signals of opposite signs resulting in perfect mirror images.

Following the interpretation of the absorption spectra, it appears that the most intense MChD signal is associated to the Mn II absorber, the less intense well-defined low energy MChD signals to the MMCT bands, while the transitions associated to Cr III weakly contribute to the MChD signal. These findings give some clues about the mechanisms of MChD in the Visible. First, in accordance with previous measurements of MChD in this spectral range [START_REF] Train | Strong Magneto-Chiral Dichroism in Enantiopure Chiral Ferromagnets[END_REF] and in contrast with what was observed in the hard X-ray region, [START_REF] Sessoli | Strong Magneto-Chiral Dichroism in a Paramagnetic Molecular Helix Observed by Hard X-Rays[END_REF] intense signals appear for metal ions with low orbital moments. Second, despite the elusive chirality of the metal centers themselves, the MChD signals are present and intense. This result assesses the possibility to exploit the non-local character of chirality to observe strong MChD signals. However, the decrease in intensity when comparing Mn and Cr signals, clearly indicates how MChD is sensitive to the distance between magnetic and chiral centers.

The temperature dependence of A MChD was investigated for the most intense signals (510560 nm) for both 1-(S) and 1-(R) and the results are reported in Figure 3. Remarkably, the strongest signal at  = 545 nm is clearly observed up to 42 K (Figure 3b), thus slightly above the ordering temperature of the material (T c = 38 K). To confirm that the enhancement of the MChD signal follows the spontaneous magnetization of the material, A MChD data were compared to magnetization data (Figure 4). Remarkably, the MChD signals are enhanced by a factor of 8 by lowering the temperature from 50 to 30 K (Figure 4a). An increase of the A MChD signal when entering into the magnetically ordered phase is due to the proportionality of A MChD to the cross-product k•M, where M is the magnetization of the medium.

2 An enhancement of a factor of 17 was observed for the MChD signal of the compound [Mn II Cr III (oxalate) 3 ] when entering in to the ferromagnetic phase. [START_REF] Train | Strong Magneto-Chiral Dichroism in Enantiopure Chiral Ferromagnets[END_REF] However, considering the ferrimagnetic ordering nature of 1, which provides a low average magnetic moment (total ground spin state S = 1) and the low anisotropic contribution to the magnetic moments involved, the enhancement is remarkable. Notably, [Mn II Cr III (oxalate) 3 ]

-ordered ferromagnetically with a total ground spin state S = 4. To provide quantitative values of the MChD anisotropy factor g MChD , [START_REF] Rikken | Pure and Cascaded Magnetochiral Anisotropy in Optical Absorption[END_REF] which is defined as

) ) )
)

))

(1)

we have determined the effective absorption coefficients A of the involved transitions by spectra deconvolution analysis (Figure S12). This allows quantification of g MChD for each observed MChD signal according to eq. 1 without the spurious contribution of superimposed absorption bands that are not intrinsically involved (Table S1). Of particular relevance is the signal at  = 545 nm, that provides a value of g MChD = 0.012(1) T -1 (T = 4 K, B = 1 T), which is comparable to that recently observed through light emission for a Tb III complex at T = 5 K under a much more intense magnetic field B = 14 T (0.011 T -1 ). [START_REF] Taniguchi | Strong Magnetochiral Dichroism for Visible Light Emission in a Rationally Designed Paramagnetic Enantiopure Molecule[END_REF] This is notable given the negligible orbital contribution to the magnetic moment of Mn II with respect to Tb III . Considering that the magnetic field dependence of A MChD shows saturation already at 0.46 T (Figures 4b andS13), the g MChD for 1

at  = 545 nm reaches a maximum value of 2.6% T -1 which represents a record for low applied fields.

In conclusion, we have investigated the magneto-optical properties of a 2D-layered chiral molecular ferrimagnet through MChD spectroscopy. Several strong MChD signals are detected in the Visible range with opposite sign for the two enantiomers. These results seem to indicate that high orbital moments are not a key ingredient for the observation of MChD in this spectral region. Moreover, it appears that independent magnetic and chiral centers can lead to the observation of MChD, the vicinity of the two centers increasing the intensity of the signal. The intensity of the MChD signals follows the temperature and field dependence of the magnetization and remains strong up to its ordering temperature (38 K) even for low applied fields (ca. 0.5 T). Pushing the observation of MChD in an ordered magnet from 6 K, as observed for the chiral magnet based on [Mn II Cr III (oxalate) 3 ] -2D-layers, to 38 K, as found here for 1, is an important step towards the use of MChD for the optical detection of magnetization using unpolarized light. These results can thus be considered a starting point for MChD investigations in rationally designed chiral molecular materials aimed at reaching high MChD anisotropy factor at temperatures viable for practical applications.

Figure 1 .

 1 Figure 1. View of the structural moiety (a) and the 2D layered structure of 1 along the a crystallographic axis (b). Color codes: violet, Mn; green, Cr; blue, N; red, O; gray, C; white, H. The asterisk indicates the stereogenic center on the ligand.

Figure 2 .

 2 Figure 2. Temperature variation (see legend) of the absorption coefficient A (cm -1 ) for a single-crystal of 1 (a). A MChD at 4.0 K recorded on single crystals of 1-(S) and 1-(R) of different thickness (b). By lowering the temperature, the shoulder of the  = 608 nm peak becomes a distinct peak at ca. 684 nm below 80 K (Figures 2a, S9, S10). Moreover, the MMCT band now appears as a series of narrow peaks. Finally, three peaks of weak intensities appear between the two spin-allowed d-d transitions of Cr III . The peak at  = 684 nm corresponds to the Cr III spinforbidden (S = ±1) 2 T 2 ( 4 F) ← 4 A 2 ( 2 G) transition. Tanabe Sugano diagrams allow to analyze the spectroscopic features of the chromium centers.[START_REF] Tanabe | On the Absorption Spectra of Complex Ions II[END_REF] The energy of the lowest energy spin-allowed transition for Cr

Figure 3 .

 3 Figure 3. A MChD at different temperatures (see legend) for both 1-(S) and 1-(R) in the  = 510560 nm range (a). Temperature dependence of the A MChD signal at  = 545 nm (b).

Figure 4 .

 4 Figure 4. Temperature (a) and magnetic field (b) dependence of the A MChD signal ( = 545 nm) (B = 1.0 T, T = 4 K, respectively) for 1-(S) compared to FC magnetization data.

ASSOCIATED CONTENT Supporting Information

Additional Figures, Tables, Equations and Experimental Section as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author matteo.atzori@lncmi.cnrs.fr cyrille.train@lncmi.cnrs.fr

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT

The French National Research Agency (ANR) is acknowledged for financial support through ChiraMolCo (ANR 15-CE29-0006-02) and Monafer (ANR-18-CE09-0032) projects.


Bernard Sellato

Benjamin Constant

AND CULTURAL REFERENTS IN BORNEO

Keywords: Borneo, material culture, basketry, decorative motif, naming system, cultural reference, variation, variability

Decoration has long suffered from relative neglect in material culture studies, now more focused on the social and cognitive processes behind the object. It has been regarded either as devoid of meaning, with motif names being simply labels, or as imbued with major socio-cultural and religious significance. This chapter, covering the field of basketry in Borneo, examines the underlying, potentially complex, relations between a motif or pattern, the way it is named, the meaning of this name, and what it denotes, both as an immediate representation and as a vector of metaphorical, symbolic, mythological, or religious referents. It stresses the need to view culture at the local level, as the current product of a given community's unique history of internal evolution and external cultural and social interaction, with high variability among communities. It concludes that a local catalogue of motifs is the result of an ongoing ad hoc collective bricolage, with possibly some kind of hierarchy among various classes of motifs, but in which norms, rules, and coherent naming and reference systems remain elusive.

INTRODUCTION

Studies in material culture, and especially technology, underwent a sort of revival in the 1980s, followed by a shift in focus, based on a view of objects as full-fledged agents with a "social life" of their own (see Appadurai 1986[START_REF] Gell | Art and Agency: an anthropological theory[END_REF], toward the social and cognitive processes behind the object (see, e.g., [START_REF] Sillitoe | The Development of Indigenous Knowledge[END_REF], Sefa Dei et al. 2000[START_REF] Ellen | Indigenous Environmental Knowledge and Its Transformations: Critical Anthropological Perspectives[END_REF][START_REF] Ingold | Beyond Art and Technology: The Anthropology of Skill[END_REF]). In the context of "ethnic" material productions, however, it appears that studies, often remaining at the descriptive stage, have neglected the underlying, potentially complex, relations between a motif or pattern, the way it is named, the meaning of this name, and what it denotes, both as an immediate representation and as a vector of metaphorical, symbolic, mythological, or religious referents.

Regarding Borneo, a lingeringand, conceivably, rather pointlessdebate, centered on the textiles of the Iban of Sarawak, eastern Malaysia, has been taking place over the relation between decorative patterns, their names, meanings, and cultural or symbolic referents. Some authors viewed patterns as meaningless decoration, and their names as a mere aide-mémoire, a set of ad hoc terms to remember them by, varying with place and time, while others recognized in patterns a system of metaphorical or symbolic references to powerful spiritual entities and associated narratives. i The field of Iban textiles is a very narrow one: a single, though numerous, ethnic groupdespite internal variationand a single medium, textiles, a flat surface with its specific technical constraints. In this chapter, I would like to expand this discussion by moving it to the field of basketry. Indeed, the decorated plaitwork of Borneo unlocks a much broader field of investigation: firstly, in terms of the wide array of techniques and combinations of techniques used, the variety of forms created, in which tri-dimensionality is prominent, and the range of functions fulfilled, from the most trivial to the highly religious; and secondly, in an island three times the size of the United Kingdom, in terms of the scores of different ethnic groups and cultural contexts examined, which allows for fruitful comparative analyses. In such a field, studies focused on decoration may further our understanding of the questions of names, meanings, and references, as well as of nuance and variability.

BASKETRY IN BORNEO

Basketry, this "humblest of crafts" [START_REF] Sentance | Basketry: A World Guide to Traditional Techniques[END_REF], which "we do not hold in high esteem" (Lévi-Strauss 1993), certainly reaches back in the history of mankind over 10,000 years [START_REF] Adovasio | Basketry Technology: A Guide to Identification and Analysis[END_REF], and probably much earlier. In Borneo, it is attested in archaeological layers dating back to a few centuries BC [START_REF] Cameron | Archaeological Textiles from the Niah Caves[END_REF])which does not preclude the production of some forms of plaitwork by the hunting-gathering peoples who had lived there since at least 50,000 BP.

The archaeological literature offers a confusing terminology whereby "basketry" and "matting" are commonly viewed as two subclasses of "textiles," while ethnographic works widely diverge in their technical definitions of "basketry"not to mention a rather indiscriminate use of the terms "plaiting" and "weaving." I adhere here to Adovasio's 1977 broad definition of basketry, applied to various kinds of items: "in addition to rigid and semirigid containers, matting, and bags, it embraces forms such as fish traps, hats, and cradles …" And I use here "basketry" and "plaitwork" as equivalent, all-encompassing terms to cover various techniques, such as plaiting proper, twining, coiling, braiding, and even those using a frame -but exclusive of cloth weaving. "Southeast Asia," Ruth [START_REF] Barnes | South-East Asian Basketry[END_REF] wrote, is one of the [world's] most prolific areas for the production of basketry. Hardly a technique for interlacing fibre in basketry has been invented that is not found in this region, and many areas have developed a particular quality and perfection […] Yet surprisingly enough, the making of baskets and their function and meaning in Southeast Asian communities have not been given much attention in recent studies. Indeed, few significant works deal with Southeast Asian basketry: Otis Mason's major work (1908) on the Malay peninsula, [START_REF] Loebèr | Het Vlechtwerk in den Indischen Archipel. Uitgegeven bij gelegenheid der tentoonstelling van de vereeniging "Oost en west[END_REF], [START_REF] Jasper | De Inlandsche Kuntsnijverheid in Nederlandsch Indië[END_REF] and, later, [START_REF] Barnes | South-East Asian Basketry[END_REF] on the Indonesian archipelago, and [START_REF] Lane | Philippine Basketry. An Appreciation[END_REF] and [START_REF] Capistrano-Baker | Containing Life: Philippine Basketry Traditions on the Cordillera[END_REF] on the Philippines. As for Borneo, which arguably is home to the world's richest, most diverse, sophisticated, and aesthetically appealing plaitwork traditions, the literature only offers brief and scattered notes for the period 1850-1920, hardly a dozen short articles in the 1920s and 1930s, and only two, quite recent, full-size books [START_REF] Bléhaut | Iban Baskets[END_REF][START_REF] Sellato | Art and Identity in the Plaited Arts of Borneo: An Introduction[END_REF].

The roots of these traditions and the successive outside influences that shaped them must be viewed in the wider insular Southeast Asian context, and beyond (see [START_REF] Sellato | Hornbill and Dragon (Naga dan Burung Enggang): Kalimantan, Sarawak, Sabah, Brunei[END_REF][START_REF] Bléhaut | Iban Baskets[END_REF]. Peoples who began settling in Borneo c. 2500 BC [START_REF] Bellwood | Prehistory of the Indo-Malaysian Archipelago[END_REF])or much earlierbrought with them a so-called "Neolithic Culture," believed to have emerged around 5000 BC in southeastern China [START_REF] Solheim | Southeast Asian Earthenware Pottery and Its Spread[END_REF]. Bléhaut stressed analogies between the arts of Borneo and those of southern China's Dian and Zhou cultures, and of Indochina (Cambodia, Dong Son culture of northern Vietnam) [START_REF] Bléhaut | Iban Baskets[END_REF]; see also [START_REF] Mcbain | Shang and Chou Influences Observed in the Ethnic Art of Borneo[END_REF]. Borneo's cultures, and their various plaitwork traditions, later responded to influences from China, India, and Persia and, later still, beginning in the 16 th and 17 th centuries, from the Islamic and Western worlds.

MATERIALS, FUNCTIONS, TECHNIQUES

This section provides a succinct overall summary of our current knowledge of plaitwork in Borneo, based on several decades of fieldwork.

Materials

Borneo's humid tropical environment has one of the world's highest levels of plant diversity. Much of the island is (or was) covered with high-canopied forest, as well as with various other ecosystems varying with elevation and soils (see MacKinnon et al. 1996), which offer a great deal of useful plant species: timber for house and boat construction, fiber plants for plaitwork and cloth weaving, and edible, medicinal, or ritual plants.

Prominent among plant families used in plaitwork are Arecaceae (rattans, fan-palms, sago palms), Poaceae/Gramineae (bamboos), Pandanaceae (screw-pines), Cyperaceae (sedges), Zingiberaceae (gingers), as well as Hypoxidaceae, Marantaceae, Gleicheniaceae, and Moraceae. Certain species, such as the Nypa palm, serve multiple purposes, from construction materials (thatch) and plaitwork fibers to the production of sugar and an alcoholic beverage.

The choice of plant species used in crafts rests mostly on practical criteriatheir local availability, qualities for a given function (durability and resistance to water and pests), ease of processing (e.g. pliability)but may also be informed by specific social and ritual requisites or personal tastes. It tends to be enduring, due to in-group transmission of botanical knowledge and processing knowhow, but it remains subject to change through cross-cultural influences and various dynamic processes (see below).

Functions

While a community's subsistence needs and the availability of resources are key factors governing the functions of plaited crafts, social and ritual functions must also be taken into consideration. Plaitwork artifacts are used in farming, hunting, fishing, and in the household and kitchen (Fig. 1), and some are multifunctional. A winnowing tray is commonly used to dry coffee beans, or a fishing basket may also serve to store pieces of clothing (see, e.g., Sellato 2012). Certain types of crafts are produced with various levels of technical complexity, either as basic serviceable objects, or in refined, decorated versions for social display or ritual use (Fig. 2).

Fig. 2.

A wide sunhat of pandanus with motifs of dragon-dog heads; Aoheng people (photo: author).

Ordinary objects, however, may also have a specific role in religious practices. Fishing utensils, such as a fish trap and fish scoop, can be used in rituals to "catch" invisible symbolic riches [START_REF] Couderc | Cultural and Literary Aspects of Uut Danum Patterned Plaiting[END_REF]. They may also be diverted from their primary function to become part of the ritual kit. For example, the garong baskets of the Iban serve no daily function as containers, but, instead, are used only during the Gawai Antu festival to memorialize the recent dead [START_REF] Sandin | Garong Baskets[END_REF][START_REF] Sather | Recalling the dead, revering the ancestors: Multiple forms of ancestorship in Saribas Iban society[END_REF][START_REF] Bléhaut | Iban Baskets: Their Production and Functions[END_REF].

Roughly put, form and size are primarily dictated by function. Large flat hats provide better protection against the sun than narrow pointed hats; small baskets tied at the waist are practical for sowing rice, whereas large burden baskets with sturdy shoulder straps and an extra forehead strap are most efficient for carrying the harvested paddy back to the village. The natural environment is also critical: different forms and sizes of fish traps are built for deep or shallow streams, for still or swift water, for small or big fish.

However, even in similar environmental conditions, forms responding to the same function may differ, based on local cultural traditions. Fish scoop baskets, for example, come in various shapes, and so do winnowing trays [START_REF] Sellato | Art and Identity in the Plaited Arts of Borneo: An Introduction[END_REF]. The forms of objects, Ingold (2000) convincingly argued, "grow from the mutual involvement of people and materials in an environment," the part played by individuals, along with their social and cultural environment, being here decisive (Lemonnier 1993[START_REF] Sellato | Crafts, culture and economics between resilience and instability: Borrowing from and trading to farmers among Borneo's nomads[END_REF].

History, thus, is a critical factor, as it accounts for the evolution of forms through various processesdiffusion, borrowing, adaptation or reinterpretation of borrowed forms, retention or abandonment of old forms, and innovationall of which are subject to explicit or tacit, collective or individual choices. The assemblage of forms that we now find in any given human community is the result of the combined effects of these processes over long periods of time, and it should be examined at this local level (see [START_REF] Ball | Knots, String and Blades": Production and Use of Organic Utility Objects by the Orang Ulu of Sarawak[END_REF]. Techniques [START_REF] Barnes | South-East Asian Basketry[END_REF] remark about Southeast Asia (above) largely applies to Borneo, resulting in much variety in the detail of techniques (see [START_REF] Dunsmore | Traditional Basketwork Techniques in Borneo[END_REF]. Different techniques, throughout the island, are used to produce similar types of crafts, while the same technique may be applied to a variety of crafts of differing functions, forms, and sizes.

A salient contrast between orthogonal and diagonal (or straight and oblique) plaitwork may point to separate plaiting traditions, as these techniques are found applied either to distinct types of crafts by the same ethno-cultural group or to the same type of craft by different ethno-cultural groups (see Sellato 2012). Moreover, a single type of object, such as the large burden basket carried with shoulder straps (Fig. 3), may involve an impressive array of widely different techniques. Choices of techniques depend not just on the availability of materials and on ethnobotanical knowledge, but also on the transmission of technological knowhow, and they tend to endure, albeit with a certain degree of inclination toward new alternatives. Knowledge about basketry encompasses three areas: botanical knowledge of plant resources and experience of their uses (see, e.g. Christensen 2012); technical expertise in the processing of fiber and dye plants and of plaitwork techniques (see [START_REF] Ellen | A modular approach to understanding the transmission of technical knowledge: Nuaulu basket-making from Seram, eastern Indonesia[END_REF][START_REF] Novellino | From Museum Collections to Field Research. An ethnographic account of Batak basket-weaving knowledge, Palawan island, Philippines[END_REF]; and the command of an inventory of decorative motifs and patterns. Transmission of knowledge is clearly achieved through word and gesture (see Leroi-Gourhan 1964); but, more often than not, it is based on tacit (or implicit) knowledge, i.e., observing the working procedures of others (see [START_REF] Sigaut | Learning, Teaching and Apprenticeship[END_REF][START_REF] Smith | Communities of Practice[END_REF], rather than on explicit knowledge, as an integral part of the learning process.

Transmission of basketry knowledge is both vertical, from parents to children within the household and, more generally, from one generation to the next within the community, and lateral, through diffusion by contact between communities (see [START_REF] Puri | Transmitting Penan basketry knowledge and practice[END_REF]. In the former case, it tends to promote conservative practices, whereas in the latter it inspires change. A new type of craft may be adopted without alteration in technique or form and be used for the same function, or be adapted to different conditions of use, or else undergo changes in function and, eventually, be physically modified, "reinterpreted," to fit such new function (see [START_REF] Sellato | Crafts, culture and economics between resilience and instability: Borrowing from and trading to farmers among Borneo's nomads[END_REF]. Changes may also result from divergent local practices and aesthetic tastes.

As technical knowledge for the production of a given craft within a community is often resident in the artisans' minds, each having his or her own specific criteria, in-group transmission of this knowledge may be achieved without a sample of that craft. In lateral transmission, except in the case of intergroup marriages, for the efficient diffusion of a new type of craft, a material sample or prototype is necessary. Regarding motifs, some groups resort to "samplers," for example, a plaited mat carrying a "catalogue" of motifs, local and imported (see [START_REF] Klokke | Plaited Motifs of the Ngaju Dayak Plaited[END_REF], serving as learning devices, without which certain motifs may simply cease to exist.

Basketry and Society

Basketry is "containing life," in the words of F.H. Capistrano-Baker (1998), and this holds true of non-container articles, such as hats or mats. In traditional contexts, plaited articles were mostly meant for local needsdaily use, prestige, or ritual purposesbut some were also bartered away. Expert artisans in the Borneo interior usually were ordinary farmers, although in urban settings they had a more professional standing. Status was gained through expertise, and the best artisans achieved regional fame. Recent years have witnessed the emergence of semi-professional artisans among hinterland people settled in an urban environment, who supply souvenir shops.

Forms of division of labor pertain to plaitwork, focusing primarily on techniques: objects or parts of objects made by orthogonal and hexagonal plaiting are often produced by men, while fine decorated diagonal plaiting is mostly made by women. But criteria of function (and physical strength) also are relevant: fish traps, heavy-duty baskets, and coarse drying mats are made (and used or carried) by men, and kitchen implements by women. These practices, however, vary widely with ethno-cultural groups, or even within a group. Moreover, not anyone may manufacture any kind of plaited work, and craft making may locally be governed by a set of obligations or taboos. Taboos focusing on age and gender often apply to decorated crafts, since certain powerful patterns entailing heavy spiritual risk can only be created by experienced and/or senior persons.

Local taxonomies of basketry, often related to the division of labor, are based on a combination of criteria of form, function, material, and technique. For example, from a standard "design-type" (see, e.g., Niessen 2009), the large sunhat (cahung), the Aoheng provide a finer designation by a second-order term identifying the material (cahung da'a, "sunhat of pandanus"), the presence of decoration (cahung karung, "sunhat with pattern"; see Fig. 2), the wearer's gender (cahung laki, "sunhat for men"), or specific function (cahung adet, "sunhat for rituals"; Sellato 2012; see also Maiullari 2011).

MOTIF, NAME, MEANING

A vast variety of plaitwork motifs and patterns is found in Borneo, as shown by inventories gathered by researchers (see, e.g., [START_REF] Woolley | Some Notes on Murut Basket Work and Patterns[END_REF][START_REF] Woolley | I. Murut Basket Work" & "II. Murut Basket Work[END_REF][START_REF] Tillema | Vlechtindustrie op Borneo[END_REF][START_REF] Klausen | Basket-Work Ornamentation among the Dayaks[END_REF], Maiullari 2011[START_REF] Klokke | Plaited Motifs of the Ngaju Dayak Plaited[END_REF]; see also [START_REF] Van Der Hoop | Indonesische Siermotiven. Ragam-ragam Perhiasan Indonesia. Indonesian Ornamental Design[END_REF]. Reflecting a contrast noted in certain vernacular categories (e.g., Bléhaut 2012), I here call "motif" a minimal or simple decorative unit, and "pattern" a larger-sized, intricate motif or an amplification by repetition or combination of one or more motifs, in a linear or bi-dimensional way (Fig. 4). Complex patterns, as bi-dimensional compositions of one or more simpler patterns, may be very elaborate (Fig. 5).

The most basic, usually geometric, motifs, made of only a few fiber strands ("dove's eye," "hook," "triangle"), occur everywhere in Borneo, as elsewhere in the world. More intricate motifs, as well as patterns, often occur in diverse forms, varying with ethno-cultural groups and their respective specific styles, through simplification, expansion, stylization or reinterpretation [START_REF] Sellato | Hornbill and Dragon (Naga dan Burung Enggang): Kalimantan, Sarawak, Sabah, Brunei[END_REF][START_REF] Bléhaut | Iban Baskets[END_REF]. Beyond basic motifs, decoration is the product of a group's singular history, allowing for the development of distinctive regional or "ethnic" styles, which actually may be even more distinctive in other media less constrained by technique, such as sculpture or mural painting. Lisum people (photo: G. Perret, by permission). This particular Punan group, the Lisum, plait mats starting from a corner, with all black or all white rattan strands (instead of alternating white and black strands); so, they end up with all-black and all-white corners.

Researchers have found that the names of basketry motifs and patterns vary widely with groups and even within the same group. Names are usually taken from the surrounding nature, manufactured articles, or the names of persons or ethnic groups believed to have created or introduced a motif. While the names for some basic motifs ("dove's eye" or "hook") are the same throughout Borneo, for others, just as basic, they differ. The triangle motif is variously called "bamboo shoot," "areca palm bud," "blade point," "thorn of the durian fruit," or "bat's elbow." Among apparently figurative motifs, close variants of the motif commonly called "person" may be found under the names "bat," "falcon." and "blades." Does the name, then, precede the pattern by intent, we may ask (after Bléhaut 1997), or the other way around, by coincidence, due to a post facto resemblance with a particular form? The answer certainly is not straightforward. [START_REF] Mashman | The Baskets of the Kelabit at Long Peluan and Their Neighbors[END_REF] reports that among the Kelabit the motif or pattern "does not represent the object after which it is named," and that the people themselves state that "the motifs have no meaning." Among Orang Ulu communities, "many motifs have lost their names completely" and are now viewed as "nothing more than decoration" (Davy Ball 2009).

It would seem, therefore, that in the cases above the relation between a motif and the name (if there is any) by which it is called is inconsistent and irrelevant, and that we should view the names of basketry motifs and patterns as just a set of "labels" forming a mnemonic device. Such mnemonic devices could vary from one ethnic group, or from one village, to the next. A "label," J.-F. Bléhaut remarked (personal communication, 2010), is what would remain after everything else about a pattern had been forgotten.

REPRESENTATION AND SYMBOLIC VALUE

Several researchers have stressed both their own perception of the importance of a motif's name for an understanding of its meaning and symbolic weight, and their frustration at being unable to interpret motifs through their names: "Enquiries as to the meaning of names […] were often fruitless" [START_REF] Woolley | Some Notes on Murut Basket Work and Patterns[END_REF]. Sometimes, a motif or pattern is found with a story, albeit mundane and possibly post facto, linked to its name: the "mad pattern" of the Murut of Sabah is said to represent the track of a woman who wandered in the forest till she lost her mind (ibid.). However, a far cry from the mundane, certain plaited artifactsmats, hats, basketscarry whole pictorial narratives about mythical heroes, spirits, and godly characters, with profound social and ritual significance. Such patterns certainly possess marked symbolic value, since these artifacts are created for ritual reasons, and are in use or in attendance during religious festivals (Fig. 6) (see, e.g., Maiullari 2011 and Klokke 2012 for the Ngaju, Couderc 2012 for the Uut Danum (or Ot Danum), Sellato 2012 for the Kenyah and the Aoheng).

Moreover, as mentioned above, creating patterns referring to divinities, spirits, or head trophies entails spiritual risk for the maker. Among the Rejang, a plaitress may become blind [START_REF] Swayne | Rejang Baskets from Sarawak[END_REF]; and an Aoheng tattooer demands a ritual payment to fortify his/her soul and defuse this risk. Clearly, therefore, some patterns have not only an explicit meaning but also a high spiritual potency, if only for the caution exerted in creating them.

Certain trivial artifacts, distinct from spiritually heavily laden artifacts featuring in religious festivals, are perplexing. Why do people travelling in the forest bother to carve a tiny stylized dragon motif on a coarse bamboo spoon or delousing blade, soon to be discarded after use? There is no formal ritual context here, and apparently no taboo involved or spiritual risk incurred, but this certainly is more than mere ornament. It is suggested that it amounts to a minor ritual act: by carving a dragon motif, a man invokes the dragon goddess, possibly creating in the process a dragon spirit, and placing the artifact (and himself? and his action?) under the protection of this goddess.

Ritual caution, mandatory in creating a spiritually powerful pattern, is also required with its name, hence the use of a milder substitute, such as "dog" for "dragon," "lizard" for "crocodile"in exactly the same way that strict Christians avoid uttering the name of God in mundane contexts. As informants may be wary of uttering sacred names and revealing to outsiders the meanings of patterns, this common use of substitute names and alternative explanations certainly accounts in part for the inconsistency and confusion in pattern names and meanings noted by foreign authors.

Beyond a pattern's name, we should therefore look at concealed cultural referents. Among the Kayan and related groups, the "dog," "deer," and "water buffalo" are avatars and aliases of the dragon goddess, and such patterns all refer to the underworld, i.e., to water, land fertility, female fecundity, and the community's overall prosperity [START_REF] Sellato | Hornbill and Dragon (Naga dan Burung Enggang): Kalimantan, Sarawak, Sabah, Brunei[END_REF][START_REF] Sellato | Art and Identity in the Plaited Arts of Borneo: An Introduction[END_REF]. Even simpler motifs may obscure spiritual concerns. Among the Iban, various leaf or flower motifs all refer to the concept of "fragrance," associated with headhunting and head trophies, which bring about fertility [START_REF] Bléhaut | Iban Baskets[END_REF]; and the occurrence of a very basic motif, the "full moon" (a simple circle), conceptually associated with a nubile woman, on wedding baskets is unlikely to be coincidental (J.-F. Bléhaut, personal communication, 2010).

DISCUSSION

Motifs and patterns, then, are of several types, and generalizations about them should be carefully avoided. Some are geometric decoration devoid of meaning, or otherwise "irrelevant," featuring on daily-use objects (or as borders or fill-ins on more ritually oriented artifacts), and others refer to prominent powerful, even dangerous, symbols, created only on critical ritual artifacts. Others still, or maybe the same, are found partaking in casual personal ritual performances that, we may surmise, allow people to surround themselves with images to protect humans and the human sphere from potentially hostile spiritual entities. It is important to take into account a decorated object's context of production, as well as its function. Place and time also are of the essence (see below).

Let us examine the motif-name-meaning-referent chain connection. Some motifs have a name while some do not (see above), but having a name does not necessarily imply having a cultural referent. For example, although I have never heard that the common "dove's eye" motif has one, the variously named triangle motif sometimes does, referring to fertility. Through time and the vicissitudes of its voyage, a motif may have remained associated with its name or been renamed. Its name may or may not still carry a meaning and/or symbolic value within a community; and these may have remained linked to a given cultural referent or been reinterpreted. A motif's cultural referent and symbolic value may vary with ethno-cultural groups and the types of artifacts carrying it. The "torso" or "person" motif, which the Aoheng routinely plait into drawstring baskets, occurs among the Ngaju on ceremonial hats, under the name of "augury bird," referring to a female celestial omen-bearing spirit. The "dog" motif of the same Aoheng baskets (Fig. 7), whether or not it refers there to the underworld dragon goddess, occurs on Ngaju ritual hats as the "golden goat," carried down to this earth by a mythological celestial ancestor (Maiullari 2011). Likewise, the motif of the squatting anthropomorphic figure with raised arms certainly does not carry the same symbolic value on a sleeping mat or ordinary basket as it does on a ritual hat or a baby carrier [START_REF] Sellato | Art and Identity in the Plaited Arts of Borneo: An Introduction[END_REF](Sellato , 2017a[START_REF] Sellato | La vannerie à Bornéo : fonctions sociales, rituelles, identitaires[END_REF].

Whatever the ultimate origin of a motif or pattern, and whether it belongs to an ancient common pool or was developed in a given community, the chain connection, for each motif, has undergone alteration through space and time, in the course of a typically complex cultural history. Indeed, linguistic and cultural features, or basketry forms, techniques, and functions (see above), may have circulated independently from one another; likewise, motifs, their names, and their meaning and cultural referents did not necessarily travel together.

In a given community, a portfolio of decorative motifs, along with their names, meanings, and cultural referents, is constantly subjected to dynamic processes of change and, at the same time, tends to integrate its constitutive elements into a somewhat coherent whole, a forever incipient "system." In such a catalogue, a few resilient motifs, among which powerful ones (gods or named spirits, crocodile, tiger, dragon, hornbill, head trophy), probably tend to endure through time along with their names and cultural referents. But scores of other motifs are subject to such a broad variation in names that their cultural referents, we assume, might not (or no longer) be crucial to the community. Even powerful motifs, however, are subject to name transpositions relative to taboos (see above). And an artisan's idiosyncratic decorative innovation may arise associated with a new name, and even a whole new story, e.g., as allegedly revealed in dreams.

At any point in time, therefore, like culture as a whole (see, e.g., [START_REF] Boas | Primitive Art[END_REF] or any subfield thereof, a local portfolio of motifs is the outcome of an ongoing ad hoc collective bricolage: ii the current end product of a given community's unique history of internal development and external cultural and social interaction. iii In such a portfolio, a hierarchy of sorts among motifs and patterns in terms of their resistance to time, erosion, and influences might be discerned, from the critically powerful to the casually relevant to the purely ornamental. In any event, while researchers often anticipate coherent systems of naming and reference, as well as rules or norms, regarding motifs in plaitwork or other techniques, these remain elusive. Impermanence and adjustment are intrinsic features of human societies, so variability is an intrinsic feature of culturesand of resources of motifs and patterns. * Senior researcher (emeritus), Centre Asie du Sud-Est (Centre National de la Recherche Scientifique, École des Hautes Études en Sciences Sociales, and Institut National des Langues et Civilisations Orientales), PSL Research University, Paris. This is a slightly revised and updated version of an article published in the Borneo Research Bulletin (Sellato 2017b). The author gratefully acknowledges permission granted by the editor of the Bulletin, and insightful comments by Clifford Sather, Michael Heppell, and anonymous Bulletin reviewers, as well as by this volume's editors and the publishers' referees.

Fig. 1 .

 1 Fig. 1. Inside a Dayak farmhouse; note the broad variety of plaited objects (photo: author).

Fig. 3 .

 3 Fig. 3. Large burden basket of rattan with shoulder straps (h. 63 cm); Kenyah people (photo: author).

Fig. 4 .

 4 Fig. 4. Long sitting mat showing separate panels with various geometric patterns; Tingalan (aka Agabag) people (photo: author).

Fig. 5 .

 5 Fig. 5. An elaborate sleeping mat of rattan combining octagonal and star patterns (l. 195 cm); Punan

Fig. 6 .

 6 Fig. 6. Ritual rattan mat with the tree of life pattern, referring to the myth of creation of the world (l. 200 cm); Ngaju people (photo: G. Perret, by permission).

Fig. 7 .

 7 Fig. 7. An old drawstring basket with rows of dog motifs (h. 35 cm); upper Mahakam river area (photo: G. Perret, by permission).