Mechanical Properties of Au Supracrystals Tuned by Flexible Ligand

Marco A. L. Cordeiro , Edson R. Leite , and Eric A. Stach .... Yifan Wang , Pongsakorn Kanjanaboos , Sean P. McBride , Edward Barry , Xiao-Min Lin , H...
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Mechanical Properties of Au Supracrystals Tuned by Flexible Ligand Interactions Melanie Gauvin,†,‡ YanFen Wan,†,‡ Imad Arfaoui,†,‡ and Marie-Paule Pileni*,†,‡ †

Sorbonne Universités, UPMC Univ Paris 06, UMR 7070, LM2N F-75005, Paris, France CNRS, UMR 7070, LM2N, F-75005, Paris, France

‡

S Supporting Information *

ABSTRACT: Here mechanical properties of face cubic centered colloidal crystals obtained out of equilibrium by solvent evaporation of coated Au nanocrystals suspension, called supracrystals, are reported as a function ligand chain length (n) and interparticle edge-to-edge distance within the supracrystals (δpp) for two nanocrystal sizes (d). Young’s modulus (E*) and hardness (H) are independent of δpp and of the supracrystal morphology. Both E* and H are in the range of few tenths of a MPa to a few GPa. Tuning of δpp by 50% is achieved by controlling the solvent vapor pressure (Pt) during the evaporation process. For any nanocrystal size, at Pt = 0, E* and H values markedly increase with increasing n from 12 to 14. At Pt = 39% and 75%, such dependency disappears. This trend differs from classical nanocomposite materials and is attributed to a change in the conformation of flexible ligands with n and to free thiol-containing molecules trapped in the supracrystal lattices.

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arrays of close-packed Au nanoparticles,25 and 3D colloidal solids26−28 show that there is no general agreement on the role of nanocrystal size, ligand length, and interparticle distance on mechanical properties of assembled nanocrystals. Very recently, we demonstrated, at the equilibrium, that the elastic properties of highly ordered Au nanocrystals in 3D superlattices markedly change with the supracrystal growth mechanism29 and with the crystalline structure of the nanocrystals involved in the formation of the colloidal crystals.30 These experiments were conducted on Au supracrystals produced from Au nanocrystals differing by their sizes, all coated with dodecanethiol ligand and separated by rather fixed interparticle distance within the supracrystals. The results above led to the conclusion that nanocrystallinity has a marked effect on mechanical properties of supracrystals.30 Still the contributions of edge-to-edge interparticle distance, length of the coating ligand, and the way supracrystals are produced on mechanical properties of supracrystals remain unknown and need to be studied. Thus, we systematically investigate the role of number of carbon atoms in the alkyl chain (n), interparticle distance (δpp), and solvent vapor pressure during the evaporation processes for two Au nanocrystal sizes. Mechanical properties of Au supracrystals are measured using nanoindentation performed with an atomic force microscope (AFM).

INTRODUCTION Colloidal crystals of nanocrystals, called supracrystals or supercrystals, are referred to as “artificial solids”. One of the first 3D superlattices reported was produced with semiconductor nanoparticles.1,2 Since then, supracrystals of quasispherical nanometer sized metallic nanocrystals were extensively studied experimentally3−10 and theoretically.11−14 Different nanocrystals made of metallic cores coated by organic ligands can self-assemble to form different supracrystals with specific properties. For instance, one can tailor the core size, the ligand length, the ligand surface coverage, or the ligand−solvent interactions.12,15 Controlling the structure and multiscale interactions in nanocrystal assemblies is an exciting route to achieve new materials with tunable optical, magnetic, and electronic properties,16−19 referred as “meta-materials”.20 The design of new functional supracrystals relies on control and understanding of individual properties of nanocrystals as well as collective properties of nanocrystal assemblies.7,21 For instance, electronic properties markedly change from isolated nanocrystal to 3D supracrystals.22,23 As bulk properties of solids rise from the strength and the nature of interactions between the basic constituents, we address the question of mechanical properties of nanocrystal assemblies as a function of their structural properties. Determination of supracrystals mechanical properties is important for potential applications and fundamental understanding of the interactions responsible for long-range cohesion between nanocrystals in the superlattice. Mechanical characterization of films of CdSe nanocrystals randomly organized and deposited by electrophoresis,24 2D © 2014 American Chemical Society

Received: December 4, 2013 Revised: February 4, 2014 Published: February 11, 2014 5005

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EXPERIMENTAL METHODS A. Chemicals. All products and solvents were used as purchased without further purification. This includes toluene (>98%, Riedel de Haën), ethanol (99.8%, Prolabo), chlorotriphenylphosphine gold(I) (98%, STREM), 1-dodecanethiol, tetradecanethiol, hexadecanethiol, and tert-butylamine borane complex (97%, Aldrich). The water used was purified with a Millipore system (18.2 MΩ). B. Synthesis. The syntheses of 5 and 7 nm Au nanocrystals (Au5 and Au7) with 7% and 5% as size distribution respectively are described in ref 31. Au5 Nanocrystals. A solution of 0.25 mmol of chlorotriphenylphosphine Au(I) dissolved in 25 mL of toluene is mixed with 2 mmol of alkyl-thiol derivatives differing by the length of the chain (C12H25SH, C14H29SH, and C16H33SH). A second solution is 5 mmol of tert-butylamine−borane complex dissolved in 2 mL of toluene. Before mixing, the two solutions are placed in a silicon bath at 100 °C and are stirred until all the products are dissolved to produce clear solutions. The colorless and clear mixture turns quickly to brown and reaches a dark red solution. The colloidal solution is dried in a nitrogen flow, and then ethanol is added to the dark powder. After stirring, the solution is centrifuged and a black precipitate appears. The supernatant is removed. The black precipitate is dried in a nitrogen flow in order to eliminate the remaining ethanol. The Au nanocrystals are dispersed in toluene and centrifuged at 5000 rpm for 5 min. To remove impurities and the precipitate, the resulting pure solution was obtained. All the various steps were manipulated in a glovebox under nitrogen. Au7 Nanocrystals. The procedure described above remains the same, except (i) the 2 mmol of thiol derivative in the first solution is replaced by 1 mmol and (ii) the second solution is 2.5 mmol of tert-butylamine−borane complex dissolved in 15 mL of toluene. Thiol derivatives used to coat the Au nanocrystals are dodecanethiol (C12H25-SH or C12), tetradecanethiol (C14H29SH or C14), and hexadecanethiol (C16H33-SH or C16). Below the supracrystals of Au5 and Au7 nanocrystals differing by their coating ligands considered in our study will be labeled Au512, Au712, Au514, Au714, and Au516, Au716, respectively. An MA-E211 inductively coupled plasma atomic emission spectroscopy (ICP-AES) is used to quantify the mass percentage of gold (Au) and sulfur (S) of the dried powder; these values were found to be approximately 40 and 2 wt %, respectively. This estimation shows that the amount of thiol derivatives exceeds the number of thiol derivatives that coat the Au nanocrystals at full surface coverage. C. Supracrystal Growth Processes. 100 μL of Audn nanocrystals dispersed in toluene is deposited in a beaker (1.52 cm3) containing a 5 × 5 mm2 silicon wafer at the bottom, and the solvent progressively evaporated. During the evaporation process to produce Audn supracrystals, at room temperature, the partial toluene vapor pressure, Pt, is controlled:32 Pt, during evaporation time, is evaluated using the principle of a wet−dry hygrometer, using a microvoltmeter (Agilent 34401A-Digit multimeter), and is found 0%, 39%, and 75% (more information is given in ref 32). D. Structural Characterization. Average core diameter and size distribution of Au nanocrystals are measured by transmission electronic microscopy (TEM, JEM 1011, 100 kV). Supracrystal specimens were characterized by small-angle X-ray diffraction (SAXRD) to determine the superlattice structure

and interparticle distance. SAXRD experiments were conducted using a rotating copper anode generator operated with a smallsize focus (0.1 × 0.1 mm2) at 40 kV and 20 mA and photostimulable phosphor plate as a detector. Au supracrystals deposited on Si substrate were characterized by conventional scanning electron microscopy (SEM) and atomic force microscopy (AFM) in acoustic mode. E. Mechanical Characterization. AFM nanoindentation experiments were performed using an AFM 5100 System (Agilent Technologies). AFM tips were purchased from Mikromasch Inc. The radius of curvature of the tip apex was measured by SEM imaging. The spring constant of the AFM cantilever was in the range of 9−20 N/m as determined using the Thermal K method. A total of 15−20 indentations were performed, under displacement control, at different locations for each supracrystal. At least three different supracrystals were studied for each specimen. After indentation, projected area of residual indentation marks are measured by imaging the supracrystal surface in acoustic mode. When indenting a deformable surface, the cantilever deforms as well and contributes to the measurement of piezo displacement. Deformation of the cantilever is measured by pushing the tip against a nondeformable surface like mica and subtracted from load−displacement curves. Oliver and Pharr analysis33,34 was used to calculate the Young’s modulus and hardness of Au supracrystals on silicon substrate. According to the model, the reduced Young’s modulus Er is defined as in eq 1: 1 1 1 = + Er E* E*i

(1)

with E* = E/(1 − ν ) and E*i = Ei/(1 − where E and ν are the Young’s modulus and the Poisson’s ratio for the sample and Ei and νi are the Young’s modulus and the Poisson’s ratio for the indenter. Er is calculated from force−displacement curves using eq 2: 2

Er =

S π 2 A

νi2),

(2)

where S is the stiffness of the upper unloading segment of the force−displacement curve and A is the calculated projected contact area. Teflon was used as a reference sample giving an average Young’s modulus of 0.58 ± 0.28 GPa, in agreement with literature values.35,36 Hardness H is calculated using eq 3: H=

Fmax A

(3)

where Fmax is the maximal force applied to the sample during nanoindentation and A the projected contact area. Nanoindentation is a depth sensing technic that allows measuring mechanical properties as a function of indentation depth. To avoid substrate effect,37 maximum indentation depths were in the range of 10%−15% of sample thickness. The thickness of the supracrystal is determined by measuring the step height between the top surface of the supracrystal and the underlying substrate using AFM imaging. Additionally, this AFM setup has been already successfully implemented to determine mechanical properties of Au supracrystals obtained from different growth mechanisms.29

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RESULTS AND DISCUSSION As described above, Au5 and Au7 nanocrystals coated by various coating agents (C12, C14, and C16) are dispersed in toluene. 5006

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amorphous film of disordered Aud14 nanocrystals (Figure 1b,d). A further increase of the alkyl chain length to C16 induces formation of amorphous film with Au516 while for Au716 nanocrystals, additional layer-by-layer fcc supracrystals sit on an amorphous film (Figure 1e) are formed. Surface morphologies of the supracrystals shown by SEM images (Figure 1a−e) agree with those measured by AFM (Figure 1f− j). The average roughness of supracrystals is below 3 nm and should have a negligible effect on nanoindentation measurements. From SAXRD patterns (insets in Figure 1a−e) formation of fcc supracrystals is confirmed. The average interparticle distance, δpp, calculated from the d111 spacing, is reported in Table 1. δpp depends on the length of the coating chains and increases linearly with the number of carbon atoms in the alkyl chains, n (see Figure S1).32 Such linear behavior agrees with theoretical predictions of the overlapping cone model (OCM) with fourbody interaction in vacuum.14 This indicates that, at Pt =0, the length of the cone involved in the overlapping between nearestneighboring nanocrystals is directly related to the length of the alkyl chain used to passivate the nanocrystals. Mechanical properties of the supracrystals described above are measured by AFM nanoindentation measurements using standard AFM tips with cantilevers of calibrated stiffness. Young modulus (E*) and hardness (H) values are calculated from force−displacement curves using the Oliver and Pharr model33,34 and assuming that supracrystals behave like pseudocontinuous media with respect to the indenter. This approximation is justified since the size of the indenter radius, around 40 nm, is larger than both δpp and nanocrystal size. Figure 2a shows typical force−displacement curves obtained from such measurements on Au712 and Au714 supracrystals, at Pt = 0. On increasing the ligand length from 12 to 14 carbon atoms and keeping the same nanocrystal size (Au7), the maximum indentation depth decreases by about 10 times for the same applied force. This suggests that Au712 supracrystals are qualitatively much softer than Au 714 supracrystals. Determination of the contact area between the indenter and the indented surface is a well-known issue in nanoindentation experiments and needs to be addressed to provide the most accurate E* and H values. Figure 2b shows an array of welldefined and homogeneous Berkovich-like marks after indentation of the surface of Au714 supracrystal with an AFM tip and subsequently imaged in acoustic mode. Moreover, it shows that no sliding occurs in the contact area between the AFM tip and the supracrystal surface. To estimate the uncertainty on the absolute value of the contact area A, we compared the calculated contact area based on the AFM tip geometry determined by SEM imaging to the experimental residual mark area for a set of 32 indentation marks (Figure S2a). Then, the average absolute error between calculated and measured contact area is about 40%, as shown in Figure S2a. At this point, we need to address the validity of these measurements and the influence of the substrate on the values of E* and H. Indentations at various depths, x, and sample thicknesses, t, were performed. The depth of the indentation, x, is limited to 10% of the sample thickness. Supracrystals thicknesses are in the range of 500 nm to 2 μm. In this range of depth, a rather large number of E* and H values are deduced from the measurements. Then, the E* and H values of the various supracrystals studied remain constant within the range where x/t < 12% (Figure S2). Thus, the indentation measurements are not influenced by the substrate. E* and H data distribution

Supracrystals are produced by solvent evaporation under control of toluene vapor pressure (Pt). Below the supracrystals of Au5 and Au7 nanocrystals differing by their coating ligands considered in our study will be labeled Au512, Au712, Au514, Au714, and Au516, Au716, respectively. At Pt = 0, Au512 nanocrystals form highly ordered films whereas Au712 nanocrystals form well-defined shaped supracrytals (Figure 1a,c). On increasing the alkyl chain length to C14, for any nanocrystal size (5 and 7 nm), well-ordered films characterized by an fcc mesoscopic structure lying on an

Figure 1. SEM images (left) and the corresponding AFM images (right) of fcc supracrystals: (a, f) Au512, (b, g) Au514, (c, h) Au712, (d, i) Au714, and (e, j) Au716 supracrystals deposited on silicon substrate and produced under toluene vapor pressure, Pt = 0%, during the evaporation time. Insets in (a−e) show SAXRD patterns of corresponding supracrystals. (a−e) Scale bars correspond to 5 μm. (f−j) Dimensions are indicated in micrometers. 5007

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Table 1. Interparticle Distance (δpp), Average Elastic Modulus (E*), and Average Hardness (H) of Supracrystals of Au5 and Au7 Nanocrystals Differing by Their Coating Agent Cn and Their Solvent Vapor Pressure during Evaporation Process (Amorphous Samples Are Labeled A) Au5 δpp (nm)

E* (MPa)

H (MPa)

Au7

Pt (%)

C12

C14

C16

C12

C14

C16

0 39 75 0 39 75 0 39 75

1.8 3.1 3.0 465 ± 125 385 ± 114 658 ± 243 10 ± 5 9±6 11 ± 5

2.1 3.0 3.1 1538 ± 325 546 ± 192 666 ± 202 60 ± 30 16 ± 12 26 ± 17

A A A A A A A A A

1.9 2.9 2.8 63 ± 7 130 ± 60 80 ± 33 4±2 6±2 2±1

2.0 2.9 3.0 1389 ± 725 235 ± 130 216 ± 105 27 ± 17 8±7 5±2

2.3 A A 1080 ± 373 A A 46 ± 27 A A

Figure 3. E* and H averaged values of fcc Au512, Au514, Au712, Au714, and Au716 supracrystals as a function of the number of carbon atoms (n) in the coating alkanethiol chain and the corresponding interparticle distance (δpp). Pt = 0. Square and circle symbols indicate supracrystals grown from Au5 and Au7 nanocrystals.

Figure 2. (a) Typical force−displacement curves obtained by nanoindentation measurements of fcc Au712 and Au714 superlattices (Pt = 0%). Red and black colors indicate Au712 and Au714, respectively. (b) Array of tip prints obtained by AFM imaging in acoustic mode after indenting Au714 supracrystal surface. Scale bar indicates 1 μm.

chain length (n = 12, 14, or 16) and the corresponding interparticle distance (δpp). An increase in the E* and H values for both Au5n and Au7n supracrystals on increasing the alkyl chain length from C12 to C14 is observed. With Au716 supracrystals, these values remain in the same order of magnitude whereas no measurements were possible with Au516 nanocrystals (an amorphous thin film is obtained). Figure 3a,b shows no linear dependence of E* and H with δpp. Furthermore, a drop in the E* and H values is observed from Au512 to Au712 supracrystals, which is in agreement with previous results29 (Figure 3). This drop in the E* and H values remains for any chain length of the coating agent with a larger

with x/t < 12% is fitted by using a Gaussian function and allows determining the mean value and standard deviation of experimental data points. Therefore, a large number of indentations at different spots and for various supracrystals were performed in order to obtain the most reliable values for E* and H. E* and H values are measured for various supracrystals differing by their alkyl chain length (n) and nanocrystal size (d) Audn. Figure 3 and Table 1 show the average E* and H values of fcc Au5n and Au7n supracrystals as a function of the coating 5008

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for nanocrystals to find the position of lowest energy that corresponds to appearance of amorphous assembly. As mentioned in the Experimental Methods section, a rather large amount of alkanethiol molecules remains associated with the coating agent. The scheme proposed above, concerning the change in the conformation of flexible ligands with n, does not take into account presence of free organic molecules in the superlattices. That is expected to be the case at Pt = 0 where δpp varies linearly with n (Figure S1). This could be explained by assuming that the residual thiol containing molecules physisorbed on the alkyl chains used as coating agents are expelled with the solvent.32 This depletion phenomenon could be reduced if the evaporation process takes place at higher Pt values.32 The beaker containing the colloidal solution, used above at Pt = 0, is placed in confined media instead of the glovebox; the partial vapor pressure is Pt = 75% and 39% when the nitrogen flow through the sample is saturated or not by toluene, respectively. In such experimental conditions, under Pt = 39%, very few changes in the morphologies and crystalline structures are observed compared to Pt = 0 (see Figure S4): For Au512, Au514 nanocrystals the fcc layer-by-layer supracrystals remain produced and 5% of shaped supracrystals appear. With Au514 nanocrystals, the underlying disordered nanocrystals film, observed at Pt = 0, disappeared and is replaced by a thin wellordered layer-by-layer film, with an fcc crystalline structure, whereas with Au714 nanocrystals it remains disordered assemblies. Under Pt = 75%, the morphologies of the fcc supracrystals markedly change (see Figure S4). For Aud12 nanocrystals, shaped supracrystals with few films are produced. With Au514 nanocrystals, well-defined triangular supracrystals in the presence of few films are produced. For solvent vapor pressure above zero, Aud16 nanocrystals form amorphous films. At Pt = 39% and 75%, the fcc superlattice parameter expends, resulting in 50% increase of δpp as compared to the interparticle distance of fcc supracrystals deposited under Pt = 0. In addition, δpp of Au5 and Au7 supracrystals remains a “quasi” constant value of ∼3 nm (Table 1 and Figure S1).32 The E* and H values of supracrystals produced at various Pt are given in Figure 4. For Au512 and Au712 supracrystals the E* and H values remains rather fixed regardless of Pt. Thus, E* and

stiffness of Au5n supracrystals compared to the Au7n ones. Note that, for any supracrystals studied, the Young moduli span a wide range of values from MPa range to GPa range, and their hardness is in the MPa range. These values have a common features with others 3D self-assemblies as well as polymeric materials.27,28 For both nanocrystal size, the increase in the E* and H values on increasing the number of carbon atoms of the alkyl chain length used as coating agent is unexpected (Figure 3). Indeed, on increasing the ligand chain length so does the interparticle distance, and the relative ratio of the metallic core volume to the organic layer used as coating agent becomes less important. In other words, the E* and H values increase while the contribution from the soft organic layer increases as well. Such a trend is opposite to what one would expect for simple nanocomposites materials composed of inorganic nanoparticles embedded in an organic host matrix. In such conditions, one would expect an increase of E* and H values with an increase of the nanocrystal core size and/or a decrease of the δpp, i.e., when the relative fraction of metallic core volume to the organic matrix becomes more important. Thus, these Cn-coated Au supracrystals cannot be compared to nanocomposite materials as suggested by previous studies.27,28 To explain such behavior, we have to take into account the conformation of alkyl chains used to coat the nanocrystals. It partially depends on the chain coverage density at the surface of each nanocrystal. As already reported, the alkyl chain coverage decreases with decreasing curvature (i.e., core size increases) and tends toward a flat surface. This can be understood as an effect of the spherical (or quasi-spherical) geometry of the nanoparticle. Because of the curvature of Au nanocrystals, the density of alkyl chains bound to the Au surface decreases as the distance from the surface increases. Thus, for a given surface coverage density at the curved metallic surface, the density of the chains is higher close to the Au surface than at the free end of the chains. Consequently, close to the metallic core, alkyl chains stretch out to prevent overlapping of chain segments of neighboring chains. At larger distance from the Au surface, the alkyl chain density decreases and allows end-groups of the alkyl chains to occupy more space due to excluded-volume effect. This resulting flexibility of the outermost chain region increases with increasing the chain length of the coating agent. The change in chain flexibility of coating alkyl molecules with increasing n modulates ligand−ligand interactions between nanocrystals and modifies self-assembly process of nanocrystals during supracrystals formation. On increasing the flexibility of coating chains, such a process could favor alkyl chains entanglement and thereby enhance mechanical cohesion between adjacent nanocrystals within the supracrystal. Such modified assembling process could consequently increase the supracrystal stiffness. This qualitative picture has appealing similarities with the model of Ohno et al. applied to polymer chains grafted on spherical nanoparticles.38 By comparing Cn ligands that form the capping layer on Au nanocrystals to grafted polymers on spherical nanoparticles in solution, one could predict the conformal state of Cn alkyl chain as a function of surface coverage and coating chain length. Unfortunately, this model needs the determination of parameters that are not well-known in our case. Nevertheless, we introduce the scaling model in the Supporting Information with the relevant parameters assumed from the literature (see Figure S3 and Table S1). This model is compatible with the fact that by enhancing the strength of interaction between nanocrystals, flexible ligands reduce the number of configurations accessible

Figure 4. (a, b) Elastic modulus, E*, and (c, d) hardness, H, of supracrystals of Au5 and Au7 nanocrystals, respectively, coated with different C12, C14, and C16 ligands and obtained at different solvent vapor pressures (Pt). Solid, void, and striped bars indicate supracrystals obtained from C12, C14, and C16 coated Au nanocrystals, respectively. 5009

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H values of Au512 and Au712 supracrystals are insensitive to a change in the interparticle distance of ∼50%. This is quite unexpected if Au supracrystals are considered as simple nanocomposite materials. Instead, it can be attributed to the fact that C12 alkyl chains bound to the nanocrystal cores are in semicrystalline regime and their specific interactions are predominant even though some thiol containing molecules and solvent could remain in the interstices favoring an increase in the interparticle distance from 1.8 nm (at Pt = 0) to around 3 nm (at Pt = 39% and 75%).32 During the evaporation process, the mechanism process above at Pt = 0 could be partially valid with a small amount of organic molecules expelled with the solvent from the superlattices. As observed at Pt = 0 (see Figure 3), the E* and H values of these supracrystals decrease with increasing the size of the nanocrystals involved in formation of supracrystals. We already observed (Pt = 0, δpp = 2.1 nm) a large increase in the E* and H values of the supracrystals on increasing by two carbon atoms the alkyl chain length (C12 to C14) used to form such assemblies. This was attributed to the ability of long coating chains to enhance the cohesive forces between nanocrystals through entanglement of flexible long coating chains. On increasing Pt, the cohesive forces decrease by the presence of solvent and thiol containing molecules that quench interactions between flexible ligands inducing a drop of E* and H values. Even though residual molecules may screen out entanglement between more or less flexible ligands bound to nanocrystals, additional cohesive forces between nanocrystals such as van der Waals forces must play a role to maintain the morphology and long-range order in swollen superlattices. We know that by changing Pt values, the morphology of supracrystals markedly changes from layer-by-layer films (heterogeneous growth process) to shaped assemblies (homogeneous growth). Figure 4 clearly shows no drastic change in the mechanical parameters with Pt. That clearly shows that supracrystal mechanical properties are not related to their morphologies and consequently to their growth mechanisms. This claim is also supported by the fact that no linear dependence of E* and H with δpp (Figure 3) is observed whereas it linearly depends on the length of alkyl chain (Figure S1). From that, it is concluded that the mechanical properties of Au supracrystals, produced by solvent evaporation i.e., out of the equilibrium, do not change with their growth mechanism processes. This conclusion is in total contradiction with data previously obtained with Au512and Au712 supracrystals produced from the same synthetic pathway.29 The major difference between the present data and those published recently in our group29 is due to the fact that here the supracrystals are produced by solvent evaporation whereas those described previously were produced at the equilibrium state under Pt = 100 and fixed δpp (2.2 and 2.4 nm for Au512 and Au712, respectively).26 Here the layer-by-layer supracrystals are transferred from air−toluene interface and consequently were at the equilibrium with toluene solvent. Diffusion of toluene at the interstice and dispersion of excess of alkanethiol in solution could induce a drop of the mechanical properties. Note that the average values of E* and H of supracrystals produced at Pt = 0 and at the interface Pt = 100 are of the same order of magnitude. This could explain the major difference between mechanical properties obtained by different groups.26−28 However, for any method we use to growth Au supracrystals, the E* and H values decrease on increasing the nanocrystal size, and therefore the size effect of nanocrystals within supracrystals is not correlated to the growth mechanism.

Finally, indentations were performed with a large radius probe to check for possible artifacts dues to the size difference between Au712 and Au512 nanocrystals under the same AFM tip. In other words, a larger number of Au512 nanocrystals would be involved in the contact area under the AFM tip when compared to Au712 nanocrystals and could possibly lead to apparent higher mechanical properties. To check this assumption, we used a colloidal probe of 5 μm in diameter for which the radius is much larger than the nanocrystal size and δpp. Figures 5a and 5b show SEM images of Au512 and Au712 supracrystals, respectively, after indentation measurements

Figure 5. SEM images of (a) Au512 and (b) Au712 supracrystals produced at Pt =0, after indentation using a 5 μm in diameter colloidal probe and under similar loading conditions. Circled zones indicate indented areas. Scale bars correspond to 5 μm.

using a 5 μm diameter colloidal probe under similar loading conditions. Au 512 supracrystals are almost intact after indentation while Au712 supracrystal surface exhibits a deep circular hole revealing the silicon substrate at the center. Qualitatively, Au512 supracrystals are mechanically stronger than Au712 supracrystals, independently of the indenter size. Even though here the system evolves out of equilibrium (Pt = 0), these observations agree with those obtained on Au5 and Au7 supracrystals obtained through supracrystal growth processes at the equilibrium state (Pt = 100).29 Because here supracrystals are formed out of equilibrium, from evaporation of the colloidal solution, both single-domain nanocrystals and polydomain nanocrystals are incorporated in the superlattice. Single-domain Au nanocrystal supracrystals show higher Young’s modulus value as compared to supracrystals of polydomain Au nanocrystals. Thus, the difference of mechanical properties between Au5 and Au7 supracrystals can be attributed to the relative amount of single- to polydomain nanocrystals within the supracrystals for each nanocrystal size.30 5010

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Notes

CONCLUSIONS In this report, Young’s modulus and hardness of fcc supracrystals of Au nanocrystals coated with different alkanethiol chains were studied by nanoindentation measurements using an AFM. For a given coating agent, the Young’s modulus and hardness of supracrystals decrease on increasing the nanocrystal size from 5 to 7 nm. The dependence of mechanical properties of Au5n and Au7n supracrystals on the interparticle distance δpp is studied either by varying the length of the coating agent from C12 to C16 or by evaporating the colloidal solution under various solvent vapor pressure (from 0%, 39% to 75%) while keeping the same coating agent. This strategy allows us to explore two different types of selfassembled colloidal crystal: (i) We have considered Au supracrystals for which δpp varies linearly with the chain length of the nanocrystal coating agent. For a given nanocrystal size, the change by two carbon atoms the chain length of the coating agent used to coat Au nanocrystals induces a marked change in the Young’s modulus and the hardness of the supracrystals. This effect cannot be attributed to changes in the interparticle distance. By analogy with polymers, such an effect is attributed to change in the conformal state of alkyl chains with number of carbon atoms involved to coating agents used to stabilize the Au nanocrystals. (ii) We have considered Au supracrystals where the superlattice parameter expends with a resulting δpp increased by about 50%. For a given nanocrystal size, mechanical properties of Au512 and Au712 supracrystals do not change when increasing δpp by 50%. In the case of Au514 and Au714 supracrystals, E* and H values drop significantly as δpp increases by 50%. E* and H show similar values as those obtained on Au512 and Au712 supracrystals. This is interpreted as a consequence of trapped solvent and excess ligands between nanocrystals in the swollen superlattice that screen out entanglement of flexible long coating chains. These results show that mechanical properties of supracrystals rise from the nature and the strength of interactions between coated nanocrystals in 3D superlattice. Ligand chains used to coat nanocrystals mediate these interactions. The conformal state of coating chains seems to play a major role in tuning interactions between coating layers of adjacent nanocrystals and is a key parameter in building cohesive 3D assemblies of nanoparticles with specific mechanical properties. (iii) We clearly demonstrated that, from the same batch of nanocrystals, the mechanical properties markedly evolve through to way they have been produced (at or out of equilibrium).

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported from Advanced Grant of the European Research Council under Grant Agreement No. 267129. The authors thank N. Goubet and Dr. C. Yan for fruitful discussions.

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ASSOCIATED CONTENT

S Supporting Information *

Figure of the interparticle distance as a function of the chain length of the coating agent at different solvent vapor pressure; figure of the error between calculated contact area and measured indented area and of E* and H values as a function of indentation depth; description, figure, and table of parameters of the modified Daoud−Cotton model; SEM pictures of the various supracrystals obtained at different solvent vapor pressure. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.-P.P.). 5011

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