Ultrathin gold nanowires (NWs) dispersed in hexane were prepared by chemical reduction of HAuCl4 in oleylamine, along with nanospheres (NSs), side products of the reaction. X-ray photoelectron spectroscopy and small angle X-ray scattering evidenced a stabilization of these nano-objects by oleylammonium chloride surfactants. The directed assembly of these nanoobjects on surfaces was performed by atomic force microscopy (AFM) nanoxerography in a few seconds. Selective assembly of gold NWs only occured onto positively charged patterns while NSs assembled more specifically on the negatively charged ones. This sorting suggests that the strong electric field generated by the charge patterns induced a strong negative effective charge on the gold NWs and a weak positive effective charge on the NSs. Such difference could be explained by the ion organization at the colloid surface, monolayered in the case of NWs and bilayered in the case of NSs. By adjusting the design of the positive patterns and the experimental conditions of development, single gold nanowires were successfully assembled by AFM nanoxerography on pre-defined sites of surfaces without damaging them, opening the way for future electrical and mechanical characterizations.

Introduction

The chemical synthesis in mild conditions of anisotropic colloidal nano-objects such as nanorods or nanowires led to highly crystalline nano-objects exhibiting enhanced physical properties such as high magnetic remanence. [START_REF] Chen | One-Dimensional Nanostructures of Metals: Large-Scale Synthesis and Some Potential Applications[END_REF][START_REF] Wetz | Synthesis and Magnetic Properties of Co Nanorod Superlattices[END_REF] Recently, colloidal gold (Au) nanowires (NWs) were obtained by reduction of a gold chloride precursor in presence of a long chain amine. [START_REF] Feng | Simple and Rapid Synthesis of Ultrathin Gold Nanowires, Their Self-Assembly and Application in Surface-Enhanced Raman Scattering[END_REF] With their diameter below 2 nm, these ultrathin Au NWs represent fascinating systems likely to display electronic transport phenomena inerrant to the quasi one-dimensional confinement arising at the nanoscale. [START_REF] Chandni | Insulating State and Breakdown of Fermi Liquid Description in Molecular-Scale Single-Crystalline Wires of Gold[END_REF][START_REF] Roy | Single Crystalline Ultrathin Gold Nanowires: Promising Nanoscale Interconnects[END_REF] However, such electronic transport studies require the addressing of individual NWs. In this aim, highly challenging technological steps must be overcome namely on i) the deposition and localisation of individual NWs on pre-defined areas of surface and ii) their electrical addressing. If their micrometric length opens the way to electrical connections by classical microelectronic processes, [START_REF] Pud | Features of Transport in Ultrathin Gold Nanowire Structures[END_REF] their ultra-small diameters confer them an extreme fragility.

Under external stress, they tend to break into spherical particles due to Rayleigh instability. [START_REF] Lacroix | Dynamic HAADF-STEM Observation of a Single-Atom Chain as the Transient State of Gold Ultrathin Nanowire Breakdown[END_REF] Regarding the first step of deposition and localisation, studies have mainly focused on the deposition of NWs by drop casting, 8 liquid-air interphase evaporation, [START_REF] Sánchez-Iglesias | Highly Transparent and Conductive Films of Densely Aligned Ultrathin Au Nanowire Monolayers[END_REF] Langmuir-Blodgett, 10 or microfluidic techniques. [START_REF] Kisner | Probing the Effect of Surface Chemistry on the Electrical Properties of Ultrathin Gold Nanowire Sensors[END_REF] Based on these approaches, the conduction properties of Au NW bundles, were recently reported. [START_REF] Oshima | Quantized Electrical Conductance of Gold Helical Multishell Nanowires[END_REF][START_REF] Agraı̈t | Quantum Properties of Atomic-Sized Conductors[END_REF][START_REF] Yoshihira | Single Electron Transistors with Ultra-Thin Au Nanowires as a Single Coulomb Island[END_REF][START_REF] Guerin | Coulomb Blockade in a Granular Material Made of Gold Nanowires[END_REF][START_REF] Loubat | Cotunneling Transport in Ultra-Narrow Gold Nanowire Bundles[END_REF] However, the experimental demonstration of clear correlations between the high aspect ratio of the nanowires and their conduction properties still requires measurements on individual nanowires.

Different approaches, such as dip-pen lithography, [START_REF] Demers | Combinatorial Templates Generated by Dip-Pen Nanolithography for the Formation of Two-Dimensional Particle Arrays[END_REF] convective self-assembly, [START_REF] Malaquin | Controlled Particle Placement through Convective and Capillary Assembly[END_REF] dielectrophoresis, [START_REF] Khondaker | The Fabrication of Single-Electron Transistors Using Dielectrophoretic Trapping of Individual Gold Nanoparticles[END_REF] or nano-manipulation by atomic force microscopy (AFM) [START_REF] Junno | Controlled Manipulation of Nanoparticles with an Atomic Force Microscope[END_REF] have yet been used in the literature to assemble single colloidal nanoparticles on surfaces. However, these techniques suffer from at least one of the following disadvantages: high complexity, lack of versatility, time consumption or restrictions on the size and geometry of the colloidal nanoobjects. There is still a compelling need to develop a quick and versatile alternative to achieve directed assembly of any kind of single colloidal nano-objects, in particular anisotropic ones.

Over the last past years, AFM nanoxerography has emerged as a highly versatile way to perform directed assembly of charged and/or polarizable colloidal nanoparticles dispersed into various polar or apolar solvents on surfaces. [START_REF] Mesquida | Attaching Silica Nanoparticles from Suspension onto Surface Charge Patterns Generated by a Conductive Atomic Force Microscope Tip[END_REF][START_REF] Mesquida | Maskless Nanofabrication Using the Electrostatic Attachment of Gold Particles to Electrically Patterned Surfaces[END_REF][START_REF] Tzeng | Templated Self-Assembly of Colloidal Nanoparticles Controlled by Electrostatic Nanopatterning on a Si3N4/SiO2/Si Electret[END_REF][START_REF] Barry | Printing Nanoparticles from the Liquid and Gas Phases Using Nanoxerography[END_REF][START_REF] Palleau | Coulomb Force Directed Single and Binary Assembly of Nanoparticles from Aqueous Dispersions by AFM Nanoxerography[END_REF][START_REF] Ressier | How to Control AFM Nanoxerography for the Templated Monolayered Assembly of 2 Nm Colloidal Gold Nanoparticles[END_REF] It relies on selective electrostatic trapping of nano-objects onto charge patterns written by AFM on electret surfaces. A wide variety of nanoparticle assemblies such as binary, 3D or single nanoparticle assemblies with a fine control on the placement and organization of the nanoparticles have been achieved. [START_REF] Palleau | Coulomb Force Directed Single and Binary Assembly of Nanoparticles from Aqueous Dispersions by AFM Nanoxerography[END_REF][START_REF] Sangeetha | 3D Assembly of Upconverting NaYF4 Nanocrystals by AFM Nanoxerography: Creation of Anti-Counterfeiting Microtags[END_REF] Except one isolated work on carbon nanotubes, [START_REF] Seemann | Selective Deposition of Functionalized Nano-Objects by Nanoxerography[END_REF] AFM nanoxerography has been used to perform directed assembly of spherical colloidal nanoparticles with sizes ranging from a few to hundreds of nanometers.

In this work, the capability of AFM nanoxerography for directed assembly of gold colloidal nanowires of a few micrometers length, dispersed in hexane, an apolar solvent, onto pre-defined areas of surfaces was investigated. By using charge patterns of opposite polarities, the ion organization at the surface of the two types of nano-objects present in solution, the gold nanowires of interest and the spheres, side products of the chemical reaction, was evaluated. On the basis of these results, the directed assembly ofsingle gold nanowires by AFM nanoxerography, was intended by adjusting the design and surface potential of the charge patterns.

Experimental section 2.1 Synthesis of gold nanowires

Ultrathin gold nanowires were prepared following a chemical synthesis adapted from B. Xing and co-workers, [START_REF] Feng | Simple and Rapid Synthesis of Ultrathin Gold Nanowires, Their Self-Assembly and Application in Surface-Enhanced Raman Scattering[END_REF] as previously reported. [START_REF] Loubat | Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS[END_REF] All reactants were used as received, oleylamine (OY, Aldrich, 70%) was stored under Ar blanket, triisopropylsilane (TIPS, Sigma Aldrich, 99%) and gold precursor (hydrogen tetrachloroaurate(III) trihydrate, HAuCl4 . 3H2O, Alfa Aesar, 99,99%) were stored in a desiccator. Typically 10 mM solution of gold was prepared: 10 mg of HAuCl4 . 3H2O were dissolved in a solution containing 340 µL OY (400 mM, OY/Au = 40) in 1.65 mL of hexane (Alfa Aesar, >98%). 510 µL of TIPS, (1M, TIPS/Au = 100) were added to initiate the gold reduction. The solution was then kept undisturbed at 40°C for 3h30. The excess of ligands was removed by a classical precipitation process. 5 mL of absolute ethanol (Sigma Aldrich, > 99.8%) was added, the solution was centrifuged 5 min at 4000 rpm. The isolated colloids were redispersed in 1.5 mL of hexane to obtain a purified suspension with a mass concentration [Au] = 3 g.L -1 .

Characterization of the gold colloids

The colloids synthetized were characterized by transmission electron microscopy (TEM), using a 120 kV JEOL-1400 microscope. 100 µL of the purified suspension was diluted into 900 µL of hexane. Few drops of this dilution were deposited on carbon coated Cu grids. X-ray photoelectron spectroscopy (XPS) spectra were recorded using a Kratos Analytical Limited Axis ultra-limited system fitted with a microfocused monochromatic Al Kα X-ray source (1486.6 eV, 12kV , 120W). The pass energy was set at 160 eV and 20 eV for the survey and the regions spectra respectively. Small Angle X-Ray Scattering (SAXS) measurements were performed on glass sealed capillaries (1.5 mm in diameter) containing the Au nanowire suspension as prepared and after purification.

The experiments were performed using a monochromatic X-ray beam generated by a rotating anode X-ray source (Cu Kα, 50 kV, 50 mA). The instrument covers a range of scattering vectors q between 0.01 and 0.18 Å -1 . SAXS intensity was recorded on a CCD bi-dimensional detector placed inside a vacuum tube to reduce noise. For background subtraction, the capillary was measured first filled with pure solvent (hexane) before introducing the colloidal suspension (inside the same capillary).

AFM nanoxerography

Figure 1 presents the two step-protocol of AFM nanoxerography used to assemble gold NWs on surfaces. The first step consisted in writing charge patterns onto a 100 nm film of polymethylmethacrylate (PMMA), spin-coated on silicon wafers by applying 1ms voltage pulses at 50 Hz to a conductive AFM tip (Figure 1a). Two types of charge patterns were chosen in this study. The first type was used for evaluating the ion organization at the surface of NWs and studying their kinetics of assembly from the colloidal suspension onto the charge patterns. It consisted in two 3 µm squares of opposite polarities. Each square was made of multiple charge dots written using +80 V pulses for the positively charged one and -65 V for the negatively charged one, in order to ensure equivalent charging of each pattern. [START_REF] Ressier | Electrostatic Nanopatterning of PMMA by AFM Charge Writing for Directed Nano-Assembly[END_REF] The tip velocity was adjusted at 10 µm.s -1 , leading to 200 nm spacing between each charged dot. The second type of charge patterns, used to achieve a fine control of the directed assembly of single gold NWs, consisted in 10 µm positively charged dotted lines. Spacing between dots ranged from 50 nm to 400 nm, by adjustment of tip velocity from 2.5 µm.s -1 to 20 µm.s -1 . The voltage pulses were tuned between +30 V and +75 V.

For both types of charge patterns, the AFM z-feedback was adjusted to control the tip-sample distance during charge writing. Surface potential mapping of charge patterns was carried out by the AFM-based electric technique of amplitude modulation Kelvin Force Microscopy (KFM), using a lift height of 20 nm and a scan rate of 0.5 Hz.

The second step of the nanoxerography process consisted in developing the electrostatically patterned substrates into the colloidal suspension (Figure 1b). First, the purified colloidal suspension ([Au] = 3g.L -1 ) was sonicated 2 minutes and diluted 50 times leading to a gold concentration of 60 mg.L -1 . This suspension was then sonicated 30 seconds and diluted 25 times for the square patterns ([Au] = 2.4 mg.L -1 ) or 100 times for the line patterns ([Au] = 0.6 mg.L -1 ) respectively. Immersion of charge patterns into the colloidal suspension lasted from 5 to 30 seconds, followed by a 30 second rinsing in pure hexane. The samples were allowed to completely dry under ambient condition (Patm, room temperature). Finally, NWs assemblies on charge patterns were characterized by topographical observations by AFM in tapping mode.

Results and Discussion

Characterization of the gold colloids

The reduction of HAuCl4 by triisopropylsilane in presence of oleylamine led after only few hours to ultrathin Au nanowires and nanospheres as evidenced in Figure 2a. Diameters of 1.7 nm were deduced from TEM images and SAXS measurements for both, the spheres and the wires. [START_REF] Loubat | Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS[END_REF] We previously reported a detailed SAXS study on Au nanowires growth in their mother liquor, revealing the presence of a bilayer of surfactants at the surface of the wires. [START_REF] Loubat | Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS[END_REF] The same study also revealed a volume fraction of 5.3×10 -5 and 1.8×10 -5 for the wires and the spheres, respectively. Taking into account the wire mean length (4 µm) and the diameter (1.7 nm) of the NWs and NSs, one can estimate a population of spheres approximatively 1200 times larger in number than the wire population in the mother liquor.

In the present work, a purification process was necessary to remove the excess of surfactants in order to make possible their directed assembly by nanoxerography (see experimental section for further details). This purification could modify the surface chemistry of the nanowires and nanospheres and their relative population. Therefore, SAXS measurements were performed on both as prepared and purified nanowires in liquid phase (Figure 2b). In both cases, self-assembly of the nanowires in solution was observed with time, as evidenced by the presence of Bragg peaks at very small angles. The peak positions in both cases characterized an hexagonal array of parallel nanowires. [START_REF] Loubat | Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS[END_REF] The parameter of the hexagonal phases inferred from the peak positions was a = 9.7 ± 0.1 nm and a = 5.5 ± 0.1 nm respectively for the as-grown and purified nanowires.

The lattice parameter corresponds to the center to center distance and is the sum of the wire diameter and the inter-wire spacing. [START_REF] Loubat | Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS[END_REF] Taking into account that the wire diameter does not change with the purification process, as showed by TEM, one can conclude that the inter-wire distance in the self-assembled array decreased from 8 nm down to 3.8 nm after purification. This latter distance corresponds approximately to twice the oleylamine length, in agreement with a model of Au NWs surrounded by a single layer of surfactants. XPS performed on the purified Au NWs revealed the presence of Cl -ions and nitrogen in both NH2 (oleylamine) and NH3 + (oleylammonium) environment (Figure S1, supporting information). Thus, it appears that the purified Au NWs used for the nanoxerography experiments were stabilized by a single layer of oleylamine / oleylammonium chloride.

Directed assembly of the gold colloids by AFM nanoxerography

Figure 3a presents a typical KFM image of two 3 µm charge squares of opposite polarities written by AFM on a 100 nm PMMA film. Using the charging conditions described in the experimental section, the surface potential of the upper (resp. bottom) square was + 6 V (resp. -6 V). After development in the purified colloidal suspension for 10 s, AFM observations revealed that gold NWs and NSs were selectively grafted onto charge patterns (Figures 3b-d). Gold NWs were exclusively assembled onto the positively charged square. The mean length of the assembled NWs was 4.2 ± 1.2 µm, proving that NWs were not altered during their selective assembly. This is remarkable as we reported that these NWs are extremely fragile and easily segmented under a high energy electron beam. [START_REF] Lacroix | Dynamic HAADF-STEM Observation of a Single-Atom Chain as the Transient State of Gold Ultrathin Nanowire Breakdown[END_REF] We demonstrate here that AFM nanoxerography can achieve such directed assembly without damaging Au NWs: electrostatic forces generated by charge patterns are sufficient to trap them onto the surface without breaking them. It is to be noted that this is the first time that colloidal nanowires with a length of a few micrometers are successfully assembled on surfaces by AFM nanoxerography. Figure 3d shows that gold nanospheres were assembled onto the negatively charged pattern with a high density.

Nevertheless, a few NSs were also grafted on the positively charged pattern. This result shows that AFM nanoxerography allowed sorting efficiently the two kinds of nano-objects (NWs and NSs) present in the colloidal suspension.

Kinetic study of the colloid directed assembly

A study of the assembly kinetics was performed by varying the development time in the colloidal suspension. Similar charged squares as those presented in Figure 3a were written by AFM. AFM observations revealed that 2 NWs/µm² were trapped on the positive charge pattern (Figure 4a) while 56 NSs/µm² were trapped on the negative charge pattern (Figure 4d) after only a 5 s development. The density of NSs grafted on the positive charge pattern was very low and did not prevent any assembly of NWs. Similarly, the density of NWs grafted on the positive charge pattern left enough space to trap NSs. These results show that there was no competition between nanowires and nanospheres during the assembly process and that, as one could expected, the number of NWs and NSs grafted on each charge pattern strongly increase with the development time (Figures 4a-c and4d-f). Densities of 6 NWs/µm² and 442 NSs/µm² were observed after a 30s development. It is to be noted that the mean length of the assembled NWs is 4.3 ± 2.5 µm, confirming that NWs were unharmed during the trapping process.

Determination of the ion organization at the surface of the gold colloids

Directed assembly of gold NWs and NSs on charge patterns of opposite polarities (Figures 3 and4) gives direct information on the ion organization at the surface of these colloidal nano-objects.

During the development step, two electrostatic forces generated by the local electric field created by charge patterns, act indeed on the NWs and NSs: the electrophoretic forces and the dielectrophoretic forces (Figure 5). [START_REF] Palleau | Quantification of the Electrostatic Forces Involved in the Directed Assembly of Colloidal Nanoparticles by AFM Nanoxerography[END_REF] Electrophoretic forces describe the Coulomb interaction between charged nano-objects in suspension and the electric field generated by the charge patterns. They cause nano-objects carrying an effective charge to be attracted by oppositively charged patterns and repelled by like charge patterns. In addition, the presence of charge patterns induces an electric field gradient which interacts with polarizable nano-objects. The distortion of their electrical double layer yields dipoles which interact with the non-uniform electric field and result in dielectrophoretic forces. The high polarizability of the gold nano-objects in hexane leads to attractive dielectrophoretic forces, independent of the charge polarity of the patterns. [START_REF] Pohl | Dielectrophoresis: The Behavior of Neutral Matter in Nonuniform Electric Fields[END_REF] On this basis, experimental results presented in Figures 3 and4 clearly suggest that gold NWs present a negative effective charge close to the charge patterns. As depicted on figure 5, the NWs thus assembled on the positive charge pattern through both attractive electrophoretic and dielectrophoretic forces. No nanowire was assembled on the negative charge pattern because attractive dielectrophoretic forces were not sufficient to counterbalance the strong repulsive electrophoretic forces on the negative charge pattern. In the case of NSs, the difference of density observed on the patterns, high (resp. weak) density on the negative (resp. positive) pattern, indicates that the gold NSs present only a small positive effective charge, leading to a balance between dielectrophoretic and electrophoretic forces (Figure 5). On the positive charge pattern, repulsive electrophoretic forces were insufficient to fully compensate the attractive dielectrophoretic forces resulting into a weak density of grafted nanospheres. On the contrary, a high density of NSs was trapped onto the negative charge pattern where both electrophoretic and dielectrophoretic forces were attractive.

Following the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, the charge separation between the ions adsorbed at the surface of the colloids and the diffuse counter ions layer is favoured by a large dielectric constant of the solvent and by a low ionic strength. On the contrary, apolar solvent, like hexane, are well known to favour the formation of ion pairs because of their very small dielectric constant. Therefore, colloidal suspension in hexane should be strongly unfavourable for electrophoretic forces. Dielectrophoretic forces were thus expected to be the predominant driving forces for assembly in our case.

Actually, examples of nanoparticle self-assemblies driven by electrophoretic interactions from colloidal suspensions in an apolar solvent are scarce. Shevshenko et al. showed that metal and metal oxide nanoparticles in toluene could exhibit an electrophoretic mobility with a sign that depends on the nature of the organic ligand at their surface. [START_REF] Shevchenko | Structural Diversity in Binary Nanoparticle Superlattices[END_REF] Binary assemblies presenting a structure analogous to that of ionic crystals were obtained by mixing oppositely charged nanoparticles in a mixture toluene-chloroform. Martin et al. described the synthesis in water of charged nanoparticles and their transfer into hexane with dodecanethiol as coating agent. [START_REF] Martin | Charged Gold Nanoparticles in Non-Polar Solvents: 10-Min Synthesis and 2D Self-Assembly[END_REF] The resulting particles in hexane exhibited a negative charge, which played an important role for the particle self-assembly at the interface air-toluene.

In our case, it is unlikely that the NWs and NSs in suspension in hexane exhibit a net charge because of the very low polarity of the solvent. The effective charge revealed by the nanoxerography experiments may result from the presence of the strong electrical field generated by the charge patterns. For the wires in hexane, the chloride anions associated to oleylammonium cations insure their global charge neutrality and their steric stabilization as showed by XPS analysis. The effective negative charge of the gold nanowires evidenced by nanoxerography (Figures 3 and4) suggests that the chloride ions are linked to the gold surface atoms and that during the development in the colloidal suspension, the strong electric field generated by the positively charged pattern probably separates the {gold + chloride ions} core from the ammonium layer as illustrated on Figure 6a. A direct measurement of the electrophoretic mobility of the NWs would confirm this assumption, but unfortunately our attempts have not been successful so far, probably because of the non-spherical shape of the objects. More amazing is the very different behaviour of the small NSs which were present in solution together with the NWs and exhibited a small positive charge revealed by a preferential assembly on the negative pattern. An effective positive charge on gold nanospheres coated with bromide ions and quaternary ammoniums was already evidenced by AFM nanoxerography. [START_REF] Ressier | How to Control AFM Nanoxerography for the Templated Monolayered Assembly of 2 Nm Colloidal Gold Nanoparticles[END_REF] In the present case, this positive charge is probably due to a second shell of oleylammonium cations interdigitated with the first shell through strong Van der Waals interaction (Figure 6b). A bilayer of long chain ammonium around NSs has already been described in the case of ZnO particles. [START_REF] Pagès | Self-Assembly of ZnO Nanocrystals in Colloidal Solutions[END_REF] Such a bilayer could explain a difference in the electrophoretic mobility of the NSs and NWs. It suggests that the spherical geometry is more favourable for the ammonium interdigitation.

Directed assembly of single gold nanowires

On the basis of these results, directed assembly of single gold NWs by AFM nanoxerography was studied. Figure 7a displays KFM images of four patterned lines of charged dots using different voltage pulse amplitudes and spacings between each dot. From the left to the right, the surface potential, the spacing between each dot and the width of the resulting lines of charged dots were +1.2 V / 400 nm / 470 nm, +1.4 V / 200 nm / 460 nm, +1.26 V / 200 nm / 314 nm and +0.25 V / 50 nm / 205 nm respectively. Except for the left pattern, all the patterned dots appear as continuous charged lines due to a lack of lateral resolution in KFM and diffusion of charges within the PMMA film. [START_REF] Jacobs | Resolution and Contrast in Kelvin Probe Force Microscopy[END_REF] After development in the colloidal suspension, Figure 7b

Conclusion

Directed assembly of colloidal gold nanowires and nanospheres, from their hexane solution onto specific areas of surfaces, has been achieved by AFM nanoxerography in a few seconds. Despite the very low polarity of hexane that makes this solvent strongly unfavorable for charge separation, a selective assembly of the NWs only occurred onto positive charge patterns written by AFM while the NSs essentially assembled on negative charge patterns. These results suggest the appearance of an effective charge on the nano-objects due to the very strong electric field in solution at the vicinity of the charge patterns. The negative effective charge of the nanowires was interpreted by a first shell of chloride ions strongly bonded to the wire surface and a second shell of mobile ammonium cations. The positive effective charge of the spheres was interpreted as resulting from a second layer of oleylammonium cations interdigitated through strong Van der Waals interactions. AFM nanoxerography is therefore an efficient way to give information on the ion organization at the surface of colloids, and, in the present case, to sort colloidal suspension composed of two kinds of nano-objects. This study finally reveals the capabilities of AFM nanoxerography to assemble single gold NWs with very high aspect ratio (1:2100) onto pre-defined areas of surfaces without damaging them, opening the way to further investigations of the properties of single NWs or other highly anisotropic nano-objects such as carbon nanotubes or DNA strands.

FIGURES

  Directed assembly of single colloidal gold nanowires by AFM nanoxerography Pierre Moutet, a Lise-Marie Lacroix, a *Antoine Robert, a Marianne Impéror-Clerc, b Guillaume Viau a and Laurence Ressier. a * a. Université de Toulouse, LPCNO (Laboratoire de Physique et Chimie des Nano-Objets), UMR 5215 INSA-CNRS-UPS, 135 Avenue de Rangueil, F-31077 Toulouse, France lmlacroi@insa-toulouse.fr; phone +33561559652, fax +33561559697 laurence.ressier@insa-toulouse.fr; phone +33561559672, fax +33561559697 b. Laboratoire de Physique de Solides, UMR 8502, Bat. 510, Université Paris-Sud, F-91405 Orsay, France KEYWORDS. Metallic nanowires, directed assembly, AFM nanoxerography, electrostatic forces, ammonium chloride ABSTRACT.

  reveals that for charged dots of +1.2 V / 470 nm, coiling of NWs occurred on each dot, without any NW interconnections of adjacent dots. Bending radii as small as 40 nm were measured on the NWs without any visible breaking, evidencing the remarkable flexibility of these 1D gold nanowires.Below 200 nm spacing between dots, NWs were connected to each dot of the charge pattern, resulting into nice alignment of the gold NWs along the charged lines. 5 NWs were assembled along the 460 nm wide charged line presenting a +1.4 V surface potential, while only 2 NWs were assembled along the 314 nm charged line with a +1.26 V surface potential, showing that the number of grafted NWs can be controlled by a fine tuning of the surface potential and the width of the dotted line. A single NW was perfectly aligned along the charge line in the case +0.25 V / 50 nm / 205 nm charged line. Such assemblies of gold NW open the way to further investigation of their mechanical and conductive properties.

Figure 1 .

 1 Figure 1. Directed assembly of gold nanowires by AFM nanoxerography: (a) AFM charge writing, (b) development by immersion in the gold colloidal suspension

Figure 2 .

 2 Figure 2. (a) TEM image of the purified colloidal suspension exhibiting gold nanowires and nanospheres. The low magnification TEM image in the inset shows the micrometric length of the nanowires, (b) SAXS data of the hexagonal phase of as-grown (black) and purified (red) gold nanowires. The peak positions in the reciprocal space relative to the position of the first peak are 1:√3:√4:√7 for the as-grown and 1:√3 for the purified nanowires.

Figure 3 .

 3 Figure 3. Directed assembly of gold nanowire suspension by AFM nanoxerography on 3 µm charge squares of opposite polarities: (a) KFM image of two 3 µm charge squares of opposite polarities written by AFM on a 100 nm PMMA film, (b) to (d) AFM topographical images of the resulting assembly of gold nanowires and nanospheres on charge squares presented in (a) after development in the gold NW colloidal suspension for 10 s.

Figure 4 .

 4 Figure 4. AFM images of positive (a-c) and negative (d-f) charge squares after 5, 15 and 30 s of development into the colloidal gold NW suspension, and (g) evolution as a function of the development time of the NW and NS density on positive and negative charge patterns respectively.

Figure 5 .

 5 Figure 5. Schematics of the electrophoretic (Felec) and dielectrophoretic (Fdiel) forces exerted by charge patterns of opposite polarities on the gold (a) nanowires and (b) nanospheres. The size of the coloured arrows indicates the amplitude of the electrostatic forces.

Figure 6 .

 6 Figure 6. Schematics of the surface stabilisation of gold (a) nanowires and (b) nanospheres, far from the charge patterns and near them. The repartitions of chloride anions and oleylammonium are drastically modified by the presence of charge patterns, resulting in effective charges.

Figure 7 .

 7 Figure 7. (a) KFM images of four charged dotted lines written by AFM using different voltage pulse amplitudes and spacing between dots: from the left to the right, the amplitude of the voltage pulses and the spacing between each dot were +75 V / 400 nm, +70 V / 200 nm, + 55 V / 200 nm and + 30 V / 50 nm; (b) AFM topographical image of the directed assembly of gold NWs on the dotted lines presented in (a) by AFM nanoxerography.

ASSOCIATED CONTENT Supporting Information. XPS spectra of purified Au nanowires are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

* Dr Lise-Marie Lacroix. lmlacroi@insa-toulouse.fr. Tel : +33567048833 .Fax : +33561559697 * Pr Laurence Ressier. laurence.ressier@insa-toulouse.fr. Tel : +33561559657 .Fax :

+33561559697

Author Contributions

The manuscript was written through contributions of all authors. ACKNOWLEDGMENT Romuald Poteau (LPCNO, Toulouse) is warmly thanked for fruitful discussions. The authors also thank G. Antorrena (INA, Saragossa) for XPS measurements. They finally acknowledge the financial support of the Labex NEXT, N° 11 LABX 075.


Anaïs Loubat

Lise-Marie Lacroix
email: lmlacroi@insa-toulouse.fr

Antoine Robert

Marianne Impéror-Clerc

Romuald Poteau

Laurent Maron

Raul Arenal

Brigitte Pansu

Ultrathin Au Nanowires: Soft-Templating vs Liquid Phase Synthesis, a quantitative study

Keywords: SAXS, lamellar phase, reaction yield, AuCl-oleylamine precursor, gold nanowires

Ultrathin Au nanowires were prepared following two routes: the reduction of the lamellar phase [OY-Au I Cl] in an excess of pure oleylamine and the direct reduction of Au III in a solution of oleylamine in hexane. The superiority of the reduction of chloride gold precursor in isotropic environment is evidenced by SAXS measurements. A reaction yield of 75% is observed, an order of magnitude higher than the route involving the lamellar phase. Based on a detailed SAXS analysis, the soft template role of the [OY-Au I Cl] intermediate solid phase was discarded for the nanowire growth. In the case of the liquid phase synthesis, DFT calculations evidence a cooperative adsorption and organisation of ions pairs at the surface of the ultrathin Au NWs. We propose that this charged backbone plays a key role in the unique growth mechanism of such anisotropic objects.

Introduction

Ultrathin gold nanowires (Au NWs), exhibiting sub-2 nm diameter and micrometric length, have attracted expanding interests due to their unique properties, as high surface-to-volume ratio, mechanical flexibility and remarkable conductivity properties. [START_REF] Loubat | Cotunneling Transport in Ultra-Narrow Gold Nanowire Bundles[END_REF][START_REF] Pud | Features of Transport in Ultrathin Gold Nanowire Structures[END_REF] These features open the perspective towards various applications including electrical sensors, [START_REF] Cui | Ultrathin Gold Nanowire-Functionalized Carbon Nanotubes for Hybrid Molecular Sensing[END_REF][START_REF] Kisner | Probing the Effect of Surface Chemistry on the Electrical Properties of Ultrathin Gold Nanowire Sensors[END_REF] fuel cell anodes, [START_REF] Yang | Facile Fabrication of Network Film Electrodes with Ultrathin Au Nanowires for Nonenzymatic Glucose Sensing and glucose/O2 Fuel Cell[END_REF] elastic coiled springs, [START_REF] Xu | Mechanical Nanosprings: Induced Coiling and Uncoiling of Ultrathin Au Nanowires[END_REF][START_REF] Xu | Confinement of Polymer-Tethered Gold Nanowires in Polymeric Colloids[END_REF] or transparent electrodes. [START_REF] Chen | Mechanically Strong, Optically Transparent, Giant Metal Superlattice Nanomembranes From Ultrathin Gold Nanowires[END_REF][START_REF] Sánchez-Iglesias | Highly Transparent and Conductive Films of Densely Aligned Ultrathin Au Nanowire Monolayers[END_REF] To be competitive with the existing technologies, the developed alternatives should rely on cost-effective synthesis of Au NWs, i.e. with high yield and limited post-treatment requirements. Since the pioneer work of Halder and Ravishankar in 2007, [START_REF] Halder | Ultrafine Single-Crystalline Gold Nanowire Arrays by Oriented Attachment[END_REF] two main routes have been followed to synthesize these Au NWs: the first one involves the precipitation of an intermediate solid phase assuming that it acts as a soft-template for the wire growth, [START_REF] Lu | Ultrathin Gold Nanowires Can Be Obtained by Reducing Polymeric Strands of Oleylamine-AuCl Complexes Formed via Aurophilic Interaction[END_REF][START_REF] Huo | Sub-Two Nanometer Single Crystal Au Nanowires[END_REF][START_REF] Kura | Synthesis and Growth Mechanism of Long Ultrafine Gold Nanowires with Uniform Diameter[END_REF][START_REF] Kisner | The Role of Oxidative Etching in the Synthesis of Ultrathin Single-Crystalline Au Nanowires[END_REF][START_REF] Imura | Water-Dispersible Ultrathin Au Nanowires Prepared Using a Lamellar Template of a Long-Chain Amidoamine Derivative[END_REF] while the second one is a direct reaction in an isotropic solution. [START_REF] Halder | Ultrafine Single-Crystalline Gold Nanowire Arrays by Oriented Attachment[END_REF][START_REF] Feng | Simple and Rapid Synthesis of Ultrathin Gold Nanowires, Their Self-Assembly and Application in Surface-Enhanced Raman Scattering[END_REF][START_REF] Kang | Size-and Shape-Selective Synthesis of Metal Nanocrystals and Nanowires Using CO as a Reducing Agent[END_REF][START_REF] Li | Facile Synthesis of Ultrathin Au Nanorods by Aging the AuCl(oleylamine) Complex with Amorphous Fe Nanoparticles in Chloroform[END_REF] These two approaches led to the production of ultrathin Au NWs along with other nanoparticles such as spheres or rods. The quantitative analysis of the different populations of such nanostructures, and therefore the Au NWs yield estimation, even if it is crucial, cannot be easily addressed. Such problematic is far more generic than the Au NWs' synthesis. Indeed, physical properties such as plasmonic resonances, magnetism or luminescence, drastically depend on the size and the shape of nanoparticles. Though efficient separation techniques, such as centrifugation processes, are often used to narrow the size distribution of the nanoparticles obtained, size selection cannot be always applied. For instance, fragile objects or interacting particles cannot be easily sorted. Therefore, one should be able to assess, in a polymorphic synthesis, the yield of the desired objects. If transmission electron microscopy (TEM) is the technique of choice to deeply characterize the particle shape/size/composition, reliable estimation of the relative yield cannot be satisfactorily obtained due to sampling issues. Other techniques such as NMR and UV-visible spectroscopies, or dynamic light scattering can be used in specific cases to characterize nanostructures. However they cannot address the problematic of quantifying the relative yield of Au nanospheres and NWs due to the peculiar features of ultrathin NWs. Small Angle X-Ray Scattering (SAXS) is a state-of-art in situ technique which can shed light on the growth mechanism, [START_REF] Polte | Nucleation and Growth of Gold Nanoparticles Studied via in Situ Small Angle X-Ray Scattering at Millisecond Time Resolution[END_REF][START_REF] Wuithschick | Size-Controlled Synthesis of Colloidal Silver Nanoparticles Based on Mechanistic Understanding[END_REF][START_REF] Abécassis | Probing in Situ the Nucleation and Growth of Gold Nanoparticles by Small-Angle X-Ray Scattering[END_REF] and the synthesis yield of metallic nanoparticles. [START_REF] Loubat | Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS[END_REF][START_REF] Hubert | Growth and Overgrowth of Concentrated Gold Nanorods: Time Resolved SAXS and XANES[END_REF] The quantitative analysis of SAXS data on an absolute scale enables to determine the number of particles along with their size and shape. In the case of monodisperse nanoparticles, SAXS analysis is fairly straightforward, [START_REF] Mckenzie | Determining Nanoparticle Size in Real Time by Small-Angle X-Ray Scattering in a Microscale Flow System[END_REF] but for more complex populations, the interpretation becomes non univocal due to the averaging of several scattering profiles. [START_REF] Sun | In Situ Synchrotron X-Ray Techniques for Real-Time Probing of Colloidal Nanoparticle Synthesis[END_REF][START_REF] Thünemann | Superparamagnetic Maghemite Nanorods: Analysis by Coupling Field-Flow Fractionation and Small-Angle X-Ray Scattering[END_REF] Coupled TEM and SAXS characterizations enable to obtain unambiguously quantitative analysis even on polymorphic nanoparticles. [START_REF] Koerner | In Situ UV/Vis, SAXS, and TEM Study of Single-Phase Gold Nanoparticle Growth[END_REF][START_REF] Wang | Real-Time SAXS and Ultraviolet-Visible Spectral Studies on Size and Shape Evolution of Gold Nanoparticles in Aqueous Solution[END_REF] We previously studied the growth of ultrathin Au nanowires in hexane medium coupling these two techniques and could extract a reaction yield for both, the side products nanoparticles and the nanowires of interest. [START_REF] Loubat | Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS[END_REF] We report here a quantitative comparison between the two main routes for ultrathin Au NWs synthesis. The lamellar phase of the first approach is Page 3 of 30

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The Journal of Physical Chemistry carefully analysed by structural, spectroscopic and computational techniques, confirming the formation of linear Au I complexes. However, its prevalence as driving force towards unidimensional growth is questioned. Indeed, we clearly demonstrate the advantage of the liquid phase approach for a high yield synthesis of ultrathin Au NWs. Based on DFT calculations, we propose that a cooperative adsorption and organisation of ligands favors the stabilisation of such unique anisotropic objects.

Experimental section

Route 1: Soft-templating synthesis

Ultrathin Au NWs were prepared according to a slightly modified 2 steps procedure, described in the literature. [START_REF] Huo | Sub-Two Nanometer Single Crystal Au Nanowires[END_REF] Step 1: synthesis of the [OY-Au I Cl] lamellar phase. A 10 mM solution of gold was prepared: 20 mg of HAuCl 4 ,3H 2 O were dissolved in 5 mL oleylamine (OY, with a molar ratio OY/Au = 300) under ultrasonication (15 min). The solution was then kept undisturbed at 25°C for 48 h, leading to the formation of a white precipitate.

Step 2: reduction of the [OY-Au I Cl] phase. The white precipitate was separated from the yellow supernatant by centrifugation (2000 rpm, 2 min) and let to react 48 h at 45°C. A pink shade was progressively observed with time. The particles were separated from the excess oleylamine by a classical washing with mixed solvents (1:3) of toluene and ethanol. The obtained solution was centrifuged 10 min at 3500 rpm. The process was repeated 3 times. The final particles were redispersed in toluene.

Route 2: Liquid phase synthesis

The ultrathin gold nanowires (NWs) were prepared following a synthesis previously described. [START_REF] Feng | Simple and Rapid Synthesis of Ultrathin Gold Nanowires, Their Self-Assembly and Application in Surface-Enhanced Raman Scattering[END_REF][START_REF] Loubat | Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS[END_REF] Typically, 10 mM solution of gold was prepared: 40 mg of HAuCl 4 ,3H 2 O were dissolved in a solution containing 1.32 mL of oleylamine (OY, 400 mM, molar ratio OY/Au = 40) in 6.60 mL of hexane. 2.05 mL of triisopropylsilane (TIPS, 1M, molar ratio TIPS/Au = 100) were added to initiate the gold reduction. The solution was then kept undisturbed at 40°C for 3 hours. The particles were centrifuged once with ethanol (4000 rpm, 5 min) to remove the excess of reactants (OY, TIPS) and redispersed in hexane.

Characterization

The two syntheses of ultrathin gold nanowires were followed in situ by small angle X-Ray scattering (SAXS) and ex situ by transmission electron microscope (TEM). TEM images were obtained with a JEOL-1011F microscope, operating at 100 kV. HRTEM images were obtained using an imaging-side aberration-corrected FEI Titan-Cube microscope working at 80 kV, equipped with a Cs corrector (CESCOR from CEOS GmbH). The TEM samples were prepared by dispersing and depositing the raw synthesis products on a standard copper TEM grid coated with a thin amorphous carbon film. In situ SAXS measurements were performed on glass sealed capillaries (1.5 mm in diameter) containing either the white precipitate (route 1) or the hexane solution (route 2). The capillaries were heated 64 hours at 45°C (route 1) and 3 hours at 40°C (route 2). The SAXS data were expressed in terms of the scattering vector modulus ݍ = ସగ ఒ ݊݅ݏሺߠሻ, with ߣ the wavelength of the X-rays and ߠ the Bragg scattering angle. The experiments were performed for the softtemplating synthesis (route 1) at the LPS using a monochromatic X-ray beam generated by a rotating anode X-ray source (Cu Kα, 50 kV, 50 mA). The instrument covered a range of scattering vectors q between 0.01 and 0.18 Å -1 . For the liquid phase synthesis (route 2), the experiments were performed on the SWING beamline at the SOLEIL synchrotron using a monochromatic X-ray beam Page 5 of 30

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The Journal of Physical Chemistry 6 (10 keV). The accessible q-range was between 6×10 -3 and 0.6 Å -1 . For both instruments, the SAXS intensity was recorded on a CCD bi-dimensional detector placed inside a vacuum tube to reduce background. For background subtraction, the signal of a capillary filled with hexane was used. The SAXS intensities were normalized in absolute units (cm -1 ) using the signal of a capillary filled with water as a standard. This allowed quantifying the volume fractions in solution of the spheres and the nanowires from the modeling results, as recently shown in a previous publication. [START_REF] Loubat | Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS[END_REF] This method gives accurate values, and is based on the scattering contrast between gold and the organic solvent or the oleylamine (ρ gold = 4650 e/nm 3 , ρ hexane = 230 e/nm 3 , ρ oleylamine = 278 e/nm3, leading to a scattering contrast = ρ gold -ρ solvant = 4400±50 e/nm 3 in both cases). Indeed, SAXS is sensitive only to the contrast between gold and the organic matter around (solvent and organic ligands). The volume fraction (vol frac ) is related to the gold cores of the nanoparticles, and it is defined by

݈ܸ = ܰ
, with V P the particle's volume (gold core only), N the total number of particles and V the sample volume.

Wide Angle X-ray Scattering (WAXS) measurements (precipitate analysis) were performed at the LPS using a rotating anode X-ray source (Cu Kα, 40 kV, 40 mA) equipped with a MAR Image-Plate detector covering a range of scattering vectors q between 0.2 and 3.5 Å -1 .

Density Functional Theory calculations

All molecular calculations have been performed using the Gaussian09 suite of programs. [START_REF] Frisch | Gaussian 09[END_REF] Geometry optimizations and subsequent electronic properties calculations have been carried out in the framework of density functional theory (DFT), using the B3PW91 hybrid functional. [START_REF] Becke | Density-Functional Thermochemistry. III. The Role of Exact Exchange[END_REF][START_REF] Burke | Electronic Density Functional Theory: Recent Progress and New Directions[END_REF][START_REF] Perdew | Electronic Structure of Solids[END_REF][START_REF] Perdew | Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation[END_REF][START_REF] Perdew | Erratum: Atoms, Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation[END_REF][START_REF] Perdew | Generalized Gradient Approximation for the Exchange-Correlation Hole of a Many-Electron System[END_REF][START_REF] Perdew | Erratum: Generalized Gradient Approximation for the Exchange-Correlation Hole of a Many-Electron System[END_REF] A double-ζ basis set augmented by a set of polarization functions, namely the standard Pople's 6-31G(d,p) basis set, has been employed for H, C and N atoms. [START_REF] Hehre | Self-Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules[END_REF] Relativistic effective core potentials developed by the Stuttgart/Cologne groups and their associated basis sets [START_REF]Effective core potentials and associated basis sets can be downloaded from[END_REF] Page 6 of 30

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The Journal of Physical Chemistry have been used for Au [START_REF] Schwerdtfeger | Relativistic Effects in Gold Chemistry. I. Diatomic Gold Compounds[END_REF] and Cl. [START_REF] Bergner | Ab Initio Energy-Adjusted Pseudopotentials for Elements of Groups 13-17[END_REF] This basis set was augmented with a set of d-or f-polarization functions for Au and Cl atoms (Au: ξ f =1.0 ; Cl: ξ f =0.643 [START_REF] Maron | On the Accuracy of Averaged Relativistic Shape-Consistent Pseudopotentials[END_REF] ). The nature of each stationary points was characterized by Hessian calculations whereas Gibbs free energies G° at 298 K were calculated by means of the harmonic frequencies, i.e. by a straightforward application of statistical thermodynamic equations. [START_REF] Mcquarrie | Statistical Mechanics[END_REF] The electronic properties of [111] oriented bulk Au NWs were calculated within the framework of the DFT considering periodic boundary conditions and the spin unpolarized or polarized constraint, depending of the system under study. These [111] direction oriented Au NWs, were generated by cleaving fcc bulk Au with low index surfaces, similarly to Roy et al. [START_REF] Roy | Single Crystalline Ultrathin Gold Nanowires: Promising Nanoscale Interconnects[END_REF] . The exchange-correlation potential was approximated by the generalized gradient approach proposed by Perdew, Burke, and Ernzerhof (PBE). [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] Calculation of the energetic parameters as well as the geometry optimizations were carried out using the projector augmented waves (PAW) full-potential reconstruction [START_REF] Blöchl | Projector Augmented-Wave Method[END_REF][START_REF] Kresse | From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method[END_REF] implemented in the Vienna ab initio simulation package, VASP. [START_REF] Kresse | Efficiency of Ab Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set[END_REF][START_REF] Kresse | Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set[END_REF] To minimize errors arising from the frozen core approximation, we used the PAW data sets treating the 4p, 4d and 5s Ru states (14 valence electrons). A kinetic energy cut-off of 525 eV was sufficient to achieve a total energy convergence within several millielectronvolts for H adsorption. Van der Waals interactions were taken into consideration by adding a pairwise interatomic term E disp to the Kohn-Shan DFT energies, which was evaluated using the revised DFT-D3 method of Grimme with Becke-Jonson damping. [START_REF] Grimme | A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu[END_REF][START_REF] Grimme | Effect of the Damping Function in Dispersion Corrected Density Functional Theory[END_REF] For the geometry optimizations, a (1×1×5) Γ-centered 51 k-points grid was used to sample the reciprocal space combined with a Gaussian smearing of 0.02 eV width for the partial occupancies. Atoms were free to move until the residual forces on any direction were less than 0.02 eV/Å. The supercell size along the a and b directions was set to ensure a vacuum space of ca. 14 Å between periodic images of decorated Au NWs. Oleylamine and oleylammonium were replaced with methylamine and methylammonium for the sake of computational feasibility.

Results and Discussion

Comparative TEM study of the two approaches

Ultrathin Au nanowires were prepared following the two approaches reported in the literature: the method assuming a soft-templating role of the solid intermediate phase [OY-Au I Cl] (route 1) based on a 2 steps reaction, and the liquid phase synthesis (route 2) relying on a direct reduction of HAuCl 4 in hexane (Scheme 1).

The particles were purified in both cases by precipitation in ethanol combined with a smooth centrifugation. Once the excess of ligands was removed, Au nanoparticles were redispersed in adequate solvent and deposited on TEM grids. Figure 1 shows comparative TEM images. The two syntheses yielded micrometric long Au nanowires, highly crystalline (Figure S1), [START_REF] Lacroix | Dynamic HAADF-STEM Observation of a Single-Atom Chain as the Transient State of Gold Ultrathin Nanowire Breakdown[END_REF] concomitantly with Au spheres, with diameters varying from ca. 8 nm for the soft-templating approach (route 1, Figure 1a) to ca. 1.5 nm for the liquid phase synthesis (route 2, Figure 1b). Apart from this difference in the diameter of side products, TEM characterizations do not allow to conclude on the quantity of nanowires for these two reactions. Therefore, deeper analyses were undertaken to obtain quantitative elements to compare both approaches. The reaction yield (expressed in %) for one type of nanoparticles (spheres or nanowires) was defined as the ratio between the amount of gold atoms inside this type of nanoparticles and the total amount of gold atoms in presence.

Analysis of the soft-templating approach

The reduction of the gold chloride salt (here HAuCl 4 ) was performed in pure oleylamine in a two steps process (Scheme 1). In step 1, after 48 h of reaction at room temperature, a white precipitate was obtained and separated by centrifugation. In step 2, this precipitate was placed at 45°C, Step 1. Analysis of the precipitate Since this precipitate plays the role of precursor for the second step, further characterizations of the isolated precipitate were undertaken. TEM energy dispersive X-Ray spectroscopy (EDS) indicated the presence of Au and Cl ions in a 1 to 1 ratio (Figure S2), in agreement with the AuCl-oleylamine complex proposed previously. [START_REF] Huo | Sub-Two Nanometer Single Crystal Au Nanowires[END_REF][START_REF] Li | Facile Synthesis of Ultrathin Au Nanorods by Aging the AuCl(oleylamine) Complex with Amorphous Fe Nanoparticles in Chloroform[END_REF] However, both thermogravimetric (TGA) and differential scanning calorimetry (DSC) analysis revealed a gold content of solely 2% in mass (Figure S3), far from the 40% expected in case of pure AuCl-oleylamine. Therefore, free OY molecules precipitated with the the [OY-Au [START_REF] Loubat | Cotunneling Transport in Ultra-Narrow Gold Nanowire Bundles[END_REF] Cl] solid phase.

The structural characterization of the precipitate was performed using WAXS (Wide Angle X-ray Scattering). First, WAXS pattern (Figure 2a) revealed an amorphous peak around q = 1.4 Å -1 plus some weak narrow peaks at even higher scattering vectors. Thus, coexistence of two states were evidenced for the organic chains of the oleylamine molecules. Though most of oleylamine molecules were in a molten state (amorphous peak), as in liquid pure oleylamine, some were crystallised (weak narrow peaks). Moreover, at smaller scattering vectors, a series of intense diffraction peaks, consistent with (00l) peaks of a lamellar phase, was observed (Figure 2a). An interlamellar distance of 4.8 nm could be determined from the indexation of these peaks. The length of fully extended oleylamine being of 2.0 nm, [START_REF] Mourdikoudis | Oleylamine in Nanoparticle Synthesis[END_REF] the lamellar structure could be composed of Au planes separated by two molecules of oleylamine partly interdigitated (Figure 2b). This model was confirmed by further experiments where the alkyl chain length of the amine was decreased from 12 carbons down to 4 (Figure S4). The interlamellar distance varied linearly with the number of atoms of the alkyl chain. The corresponding linear fit confirmed the presence of two molecules partly interdigitated and evidenced Au planes of ca. 0.6 nm (Figure S5).

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The Journal of Physical Chemistry 10 DFT calculations were performed to address the stable configuration of AuCl-amine precursor within the lamellar phase. The precursor adopted a linear configuration as evidenced in Figure 3.

Aurophilic bounds, as long as hydrogen bounding between successive Cl and NH 2 groups, stabilized this one-dimensional organisation, which could play a decisive role in the future growth of ultrathin Au NWs.

The formation of this lamellar phase, composed of the linear Cl-Au I -RNH 2 precursor, was not total. Indeed, all the Au initially introduced was not retained in the precipitate because a large fraction was lost in the supernatant. The quantification of the total mass of the precipitate (typically 300 mg) and the mass fraction of Au (2%) revealed a 60% yield for step 1 (Scheme 1).

Step 2 : reduction of Au I precursor into Au 0

Step 2 consisted in the final reduction of the Au I precursor previously obtained into Au nanoparticles. We studied the effect of temperature on this reduction step by in-situ SAXS and exsitu TEM. At room temperature, the initial precipitate gave a diffraction peak at a scattering vector q = 0.13 Å -1 , corresponding to the (001) peak of the lamellar phase previously described (Figure 2) and a concomitant q -4 power law at small q vectors (SAXS experiment) (Figure 4a). This q -4 signal, known as Porod contribution, arisen form the scattering at the interface between the lamellar phase domains and the surrounding oleylamine. This two features, (001) diffraction peak and Porod contribution, attested the presence of the lamellar phase during the reaction. With time, this initial signal evolved, allowing following the growth of gold nanoparticles. The adjustment of the experimental curve gave access to the size and shape of the objects along with their volume fraction. Compared to the theoretic 8.5×10 -4 volume fraction of gold deduced from TGA, the reaction yield after step 2 could thus be deduced.

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Below 45°C, the reaction kinetic was very slow (Figure 4b). The modification of the scattering profile with time was fairly weak, leading to a difficult analysis of the nanoparticle populations.

Even after 70 h of reaction, a yield of only 0.6% was determined. Between 45 and 50°C, the reaction was faster (Figure 4c and S6a). The fitting of the scattering profile at different reaction time evidenced at first the growth of Au NWs, then followed by the growth of Au nanospheres (Figure S7). After 48h of reaction, TEM images confirmed the presence of both populations (Figure 5a).

While the nanospheres' volume fraction increased continuously with time, Au NWs' one saturated after 30 h at 3 x 10 -5 (Figure 6).

Increasing the reaction temperature above 50°C led to the melting of the lamellar phase (Figure 4d and S6b), as revealed by the disappearance of the (001) peak and the Porod contribution. Under such conditions, only polydisperse spheres were detected both by SAXS and TEM (Figure 5b).

Their diameters increased from 5 to 7 nm during the first 30 h, in agreement with the size distribution determined from TEM image (Figure S8). After that, the particles tended to form aggregates as evidenced by the additional correlation peak observed by SAXS. From the fit of this correlation peak, using the Percus-Yevick expression of the structure factor for hard spheres, one could deduce that, after 45 h, the spheres were all embedded in these agglomerates. The position of the correlation peak was related to the average distance between the spheres, and the typical separation distance in-between two gold-cores was found equal to 3.5 ± 1.7 nm (Figure S9).

Optimization of the Au NWs synthesis

Regarding the mastery of the soft-templating approach for the quantitative synthesis of Au NWs, the nanowires yields were fairly low and did not exceed 3% for the whole process. This low yield resulted from i) the chemical equilibrium between Au I and Au III which led to a 40% loss of gold after step 1, ii) the insignificant reduction of Au I to Au 0 during step 2, which did not further evolve with time.

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12

To improve the total yield of the soft-templating reaction, we tried to optimize these two drawbacks.

Step 1 was performed at higher temperature and/or for longer time. However, neither of these parameters led to a quantitative modification of the chemical equilibrium. They both favour the reduction of Au I into Au 0 , blurring the demarcation between step 1 and step 2. For instance, after 25 days at room temperature, Au NWs and spheres could be observed concomitantly with Au III as evidenced by the characteristic yellow colour of the supernatant (Figure S10).

Regarding the reduction Au I to Au 0 , neither the temperature, nor the reaction time could favour a quantitative step 2 reaction. In both cases, reduction yield below 10% were obtained, and Au nanospheres were formed detrimentally to Au NWs. Therefore, stronger reducing agents such as H 2 or triisopropylsilane (TIPS), which was used as reducing agent in the liquid phase synthesis, were added during step 2. None of them led to the quantitative formation of Au nanowires. A 3 bars H 2 atmosphere fastened the reaction and yielded spherical and rod like NPs, while TIPS did not drastically affect the kinetic, leading to similar particles as the ones obtained in presence of oleylamine solely (Figure S11). Thus, the synthesis of Au NWs following the soft-templating approach (route 1) may not be strictly limited by the reduction of the lamellar phase, as evidenced by the experiments in presence of stronger reducing agent, but more likely by the diffusion within the soft-template.

Route 2 : Liquid phase synthesis

This synthesis approach relied on a one step liquid phase reaction, where the gold chloride salt was solubilised in hexane in presence of oleylamine and reduced by triisopropylsilane. We followed the growth of Au NWs by in-situ SAXS and could characterize a reaction yield for Au NWs, which should be compared to the global yield obtained for the soft-templating approach. The scattering profiles could be fairly easily adjusted by summing only the contributions of spheres and wires, no

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The Journal of Physical Chemistry diffraction peak neither Porod contribution did hinder the signals. Small spheres of 1.7 nm of diameter were present since the beginning of the reaction, as evidenced by the characteristic plateau in the scattering profile. Then, a q -1 contribution, characteristics of Au NWs, appeared and grew with time. The volume fractions of both populations were deduced from the fitting parameters. [START_REF] Loubat | Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS[END_REF] The diameter of the nanowires was found constant at 1.7 nm with a polydispersity on the diameter distribution of only 2%. The reduction Au III -Au 0 was total after only 90 min of reaction at 40°C, the yield of Au NWs being of 75%, far above the 3% obtained with the route 1 (Figure 7).

Discussion

The soft-templating approach is based on the assumption that the lamellar phase plays a key role in the growth of Au NWs. Indeed, the in situ SAXS study of the route 1 shows that Au NWs are indeed obtained in presence of this lamellar phase. Moreover, for reaction temperature above 50°C, the lamellar phase melt and only spherical particles are obtained instead. Therefore, at first sight, our experiments seem to confirm the importance of the lamellar phase. However, the growth of Au NWs within the lamellar phase should have modified the XRD pattern through an increasing of the inter-lamellar distance, the 1.7 nm diameter of the Au NWs being much larger than the initial thickness of the Au plans, and/or a broadening of the (00l) lines due to a loss of crystallographic correlations. Neither the position nor the broadening of the (001) peak of the lamellar phase showed any modification during the appearance of the NWs, thus the growth may not directly occurred within the lamellar phase. Moreover, the reduction of the Au precursor is hindered in presence of the lamellar phase, leading to a final Au NWs yield below 3%. On the contrary, liquid phase reaction led to a quantitative reduction and a 75% yield. Thus, we conclude that the Au NWs growth does not take place inside a soft lamellar template, as previously assumed. The lamellar phase is actually detrimental to the growth, as revealed by the low reaction yield.

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If the growth within a lamellar phase acting as a soft template was an elegant way to explain the unique growth of Au ultrathin nanowires, new hypotheses on such anisotropic driving force should now be invoked. Oriented attachment of preformed spherical particles was previously discarded due to the fairly constant spheres' volume fraction evidenced by SAXS. [START_REF] Loubat | Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS[END_REF] The growth may therefore occur in preformed cylindrical micelles, [START_REF] Pazos-Pérez | Synthesis of Flexible, Ultrathin Gold Nanowires in Organic Media[END_REF] or thanks to a cooperative effect between surfactants organisation and Au reduction, previously described as a "zip" mechanism for the growth of Au nanorods in water. [START_REF] Gao | Dependence of the Gold Nanorod Aspect Ratio on the Nature of the Directing Surfactant in Aqueous Solution[END_REF] DFT calculations of the energetic stabilisation by polar heads (oleylamine vs. Nevertheless, the methylammonium chloride ion pair adsorption was significantly enhanced if both moieties were simultaneously adsorbed on the surface (Figure S12). Such adsorption, when occurring along the nanowire, makes an alternant positive/negative pattern that enhances adsorption strength. Figure 8 shows the stabilisation with six methylammonium chloride entities per unit cell.

The resulting adsorption energy was found to be -37.7 kcal/mol per ion pair, suggesting that a network of ion pairs could stabilize the surface of the nanowire. Thus, these preliminary DFT results tend to show that a "zip" mechanism may also be invoked in the case of ultrathin Au NWs in non polar solvent.

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Conclusion

Ultrathin Au nanowires were prepared following two routes: the reduction of the lamellar phase [OY-Au I Cl] in an excess of pure oleylamine and the direct reduction of Au III in a solution of oleylamine in hexane. In both cases nanospheres were found as side product. SAXS analyses enabled to determine with a good precision the volume fraction of the different gold particles present in solution, from which reaction yields were inferred. The superiority of the reduction of gold chloride precursor in isotropic environment was clearly evidenced. A large reaction yield of 75% was determined, an order of magnitude higher than the route involving the lamellar phase. We ASSOCIATED CONTENT Supporting Information. Additional SAXS diagrams, TEM and HRTEM images. TEM-EDS, TGA and DSC analysis of the precipitate. XRD patterns of the precipitate obtained with different amines and the evolution of the corresponding interplanar distance. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure captions

Scheme 1 : Overview of a) the soft-templating approach (route 1) based on a 2-step reaction. The step 2 uses only the white precipitate, obtained after removal of the supernatant. b) the liquid phase synthesis (route 2).

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 30 Paragon Plus EnvironmentThe Journal of Physical Chemistry ultrathin Au NWs and polydisperse nanospheres were formed upon time. These two steps have been quantitatively studied to estimate the resulting global Au NWs yield.

  oleylammonium chloride) were performed to investigate these hypotheses. Due to computational limitation, the alkyl chain was limited to one carbon. Methylamine adsorbs preferentially on top of the gold surface atom with the lowest coordination number (c.n.: 5). Its adsorption energy, -22.4 kcal/mol, was roughly twice as strong as on the close-packed Au(111) surface. Actually this value is in fair agreement with the adsorption energy on a gold adatom (c.n : 3) 56 placed in the fcc position of this surface (see TableS1 and ref 55). The methylammonium chloride ion pair, with polar ammonium head lying above an adsorbed chloride, bound similarly to methylamine at the Au NW surface, with almost the same adsorption energy (-19.1 kcal/mol). Despite the low coordination number of gold surface atoms and possible finite-size effects,[START_REF] Kleis | Finite Size Effects in Chemical Bonding: From Small Clusters to Solids[END_REF] these values remain quite low.

  demonstrated that neither the reaction temperature/time nor the addition of stronger reducing agent could improve the Au NWs yield in presence of the lamellar phase. Indeed, after 30 h of reaction and for temperature above 50°C, only polydisperse Au NPs were obtained. This study allows ruling out the assumption that the [OY-Au I Cl] intermediate solid phase acts as a soft template for the nanowire growth. In the case of the liquid phase synthesis, DFT calculations evidenced a cooperative adsorption and organisation of ions pairs at the surface of the ultrathin Au NWs. We propose that such charged backbone plays a key role in the unique growth mechanism of such anisotropic objects.ACKNOWLEDGMENTThe authors acknowledge the financial support of the Midi-Pyrénées region, of the university of Toulouse, PRES, of the Labex NEXT, N° 11 LABX 075, of the French GDR Or-Nano. Simon Cayez (LPCNO, Toulouse) is warmly thanked for the EDS analysis and the preparation of TEM grids. We thank Francis Chouzenoux (LPCNO, Toulouse) for TGA-DSC measurements. R.A. acknowledges funding from the Spanish Ministerio de Economia y Competitividad (FIS2013-46159-C3-3-P). Microscopy work was conducted in the Unité Mixte de Service Castaing, Toulouse (France) and in the Laboratorio de Microscopias Avanzadas at the Instituto de Nanociencia de de Zaragoza (Spain). The research leading to these results has received funding from the European Union Seventh Framework Program under Grant Agreement 312483 -ESTEEM2 (Integrated Infrastructure Initiative -I3). RP thanks the HPCs CALcul en MIdi-Pyrénées (CALMIP-Hyperion and CALMIP-EOS, grant P0611) and the Grand Equipement National de Calcul Intensif (GENCI-TGCC-Curie, grant 6211) for generous allocations of computer time.

Figure 1 .

 1 Figure 1. TEM images of Au nanoparticles obtained with a) the soft-templating approach (route 1) and b) the liquid phase synthesis (route 2).

Figure 2 .

 2 Figure 2. a) XRD pattern of the precipitate obtained after step 1 (red) and the corresponding oleylamine reference (black). b) Schematic view of the lamellar phase with partly interdigitated alkyl chains.

Figure 3 .

 3 Figure 3. DFT calculation of the stable configuration of Cl-Au I -RNH 2 complexes. 3 carbons alkyl chain amines were considered due to computational time limitation. In green : Cl, blue : N, Grey : C, White : H, Black : Au I .

Figure 4 .

 4 Figure 4. SAXS pattern of a) starting precipitate at room temperature and its further evolution with time at b) 43°C, c) 48°C and d) 53°C.

Figure 5 .

 5 Figure 5. TEM images of the particles obtained at a) 48°C after 45h and b) 53°C after 30 h with the soft templating approach (route 1).

Figure 6 .

 6 Figure 6. Volume fraction of nanospheres (black squares), nanowires (red circles) and their sum (blue triangles) deduced from SAXS profiles fitting at 45°C.

Figure 7 .

 7 Figure 7. Volume fractions calculated from the SAXS modelling at different times. Inset : Small angle X-ray scattering results measured at 40 °C from t=0 to t=150 min.

Figure 8 .

 8 Figure 8. DFT geometry of a gold nanowire stabilized by methylammonium chloride (the unitcell is highlighted, as well as the faceting of the NW resulting from the low-index surface cleavage). In green: Cl, blue: N, grey: C, white: H, yellow: Au.

Funding Sources

The authors acknowledge the financial support of the Midi-Pyrénées region, of the university of Toulouse, PRES, of the Labex NEXT, N° 11 LABX 075, of the French GDR Or-Nano, of the European Union Seventh Framework Program under Grant Agreement 312483 -ESTEEM2.

AUTHOR INFORMATION

Corresponding Author

* lmlacroi@insa-toulouse.fr; phone +33561559652, fax +33561559697

Author Contributions

The manuscript was written through contributions of all authors. A.L., L.-M.L, A.R. and G.V. performed the chemical synthesis. R.A. performed HRTEM study. A.L., M.I.C and B.P. performed and analyzed SAXS measurements. L.M. and R.P. performed DFT calculations.