Monodisperse Copper Nanocubes: Synthesis, Self-Assembly, and

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Monodisperse Copper Nanocubes: Synthesis, Self-Assembly, and Large-Area Dense-Packed Films Hong-Jie Yang, Sheng-Yan He, Hsin-Lung Chen, and Hsing-Yu Tuan* Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan 30013, ROC S Supporting Information *

ABSTRACT: In comparison to the well-characterized bottom-up synthesis of Au and Ag nanomaterials, the synthesis of Cu nanocrystals with well-defined and controllable shapes is still in need of improvement. Among the many shapes, a cube covered by six {100} facets can be regarded as a standard model to study the surface properties of {100} facets. Herein, we have prepared monodisperse Cu nanoparticles having a slightly truncated cubic shape with an average edge length of 75.7 nm and a standard deviation of 3.87% by using CuCl as the precursor, oleylamine as the reaction solvent, and trioctylphosphine and octadecylamine as shape control agents. The as-prepared Cu nanocubes tend to self-assemble on transmission electron microscopy grids or silicon substrates. Electron microscopy and small-angle X-ray scattering reveal that the Cu nanocubes prefer to self-assemble into 2D or 3D rhombohedral structures (RS). Large-area dense-packed films (1.5 cm × 2.5 cm) composed of monodisperse Cu nanocubes were fabricated by immersing a Si substrate in a dispersion of dodecanethiol-capped Cu nanocubes in toluene and evaporating the toluene at a controlled rate while holding the substrate at an angle. The electrical properties of the Cu films with various thickness and annealing temperatures were studied.

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INTRODUCTION Monodisperse metal nanomaterials with well-defined shape have attracted extensive attention due to their size- and shaperelated properties; thus, their controlled synthesis is a widely sought goal.1−5 For example, these materials exhibit a unique optical property known as localized surface plasmon resonance (LSPR), which occurs when light interacts with free electrons at the surface, leading to their collective oscillations and enhancing light absorption and scattering.6 Obtaining uniform metal nanomaterials is critical to understanding how their size and shape influence their properties, in order to further improve their characteristics for specific applications.7−11 Various shapes of silver and gold nanomaterials including particles,12 cubes,13 wires,14 rods,15 plates,16 and polyhedra17 have been synthesized by chemical methods. The cube shape in particular has many special properties due to the dominance of (100) faces on its surface. For example, the catalytic activity of Pt nanocubes is twice as high as that of commercial Pt catalysts for oxygen reduction.18 Bulk quantities of metal nanomaterials with uniform size and shape have been prepared via colloid synthesis.1 Monodisperse isotropic or anisotropic nanocrystals may also assemble into 2D or 3D superstructures, which have been considered as building blocks for self-assembly into artificial solids.19−22 The self-assembly of nanocrystals into ordered structures requires a narrow particle size distribution (standard deviation ≤ 5%), uniform shape, compatible surface ligands, and van der Waals attraction between the nanocrystals.23−28 Usually, © 2014 American Chemical Society

spherical nanocrystals self-assemble into face-centered cubic (FCC) or hexagonal close packed structures to achieve the highest packing efficiency, i.e., 74.04%.29−32 Nanocubes with flat faces and sharp edges tend to assemble in simple cubic (SC) superlattices.33−35 Recently, Gang and co-workers reported that nanocubes with rounded corners (truncated) and nonflat faces (considering the surface ligand shell) can assemble into rhombohedral structure (RS).23 Dijkstra et al. report that a system of colloidal hard superballs, with deformation factor p > 1.1509, prefers to exhibit the C1 phase, which also corresponds to rhombohedral packing.34 The assembly of nanocrystals introduces interesting physical properties generated from collective interaction between these nanocrystals.36−38 For example, the absorption spectrum of Au nanorods shows a red shift of the transverse plasmon band and a blue shift of the longitudinal plasmon band when the Au nanorods are assembled in a side-by-side orientation. The intensity of plasmon coupling is related to the internanorod distance and the number of nanorods in the assembly.39 Copper (Cu) has the second highest electrical conductivity of all the metals40 and is more abundant and less expensive compared to gold and silver. Nanosized Cu exhibits LSPR because its dielectric function arises from the large negative real component and small imaginary component.41,42 However, Received: September 17, 2013 Revised: January 25, 2014 Published: February 11, 2014 1785

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Figure 1. (a, b) TEM images of Cu nanocubes using CuCl as a precursor by hot injection method. (c) UV−visible spectrum of Cu nanocubes dispersed in toluene at room temperature. The inset shows a photograph of Cu nanocubes solution. (d, e) HRTEM images of a single Cu nanocube. (f) SAED pattern of the Cu nanocube in (d); the electron beam is perpendicular to the face of the Cu nanocube. (g) Scheme of the Cu nanocube synthesized by our approach.

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control of the shape and size of Cu nanocrystals is still in its preliminary stages compared to what has been achieved for Ag and Au in the solution phase. The difficulty in obtaining uniform Cu nanocrystals arises from their tendency to form Cu2O or CuO. Shape-controlled synthesis of Cu nanomaterials, including nanocubes and nanowires, has recently been achieved using Cu(II) chloride dihydrate in water43 or organic solvents,44 where the capping ligand plays an important role in controlling the nucleation and growth of Cu nanostructures. Controlling the synthesis of Cu nanocubes evidently relies on finding a suitable capping ligand; further improvement in monodispersity is still needed.45 Herein, monodisperse Cu nanocubes with an average edge length of 75.7 nm and a standard deviation of 3.87% were synthesized by rapidly injecting a mixture of CuCl and octadecylamine (ODA) in squalane into a hot oleylamine/ trioctylphosphine (TOP) solution at 330 °C. TOP and ODA are labile capping agents and facilitate the formation of monodisperse Cu nanocubes. The Cu nanocubes have a slightly truncated cubic shape, with a deformation factor of p = 2.2. Upon drop casting or evaporation of the solvent from the dispersion, the monodisperse Cu nanocubes mainly organize themselves into 2D or 3D supercrystals with RS, as confirmed by electron microscopy and small-angle X-ray scattering (SAXS). This method yielded a 1.2-μm thick film of monodisperse Cu nanocubes over a large area (1.5 cm × 2.5 cm) on a silicon wafer. The optical and electrical properties of the nanocubes and the nanocube film were studied.

EXPERIMENTAL SECTION

Chemicals. All chemicals were used as received. Cu(I) chloride (99.99%) and trioctylphosphine (TOP, 90%) were purchased from Alfa Aesar. Oleylamine (OLA, 70%), octadecylamine (ODA, 97%), squalane (99%), 1-dodecanethiol (98%), and anhydrous toluene (99.8%) were purchased from Sigma-Aldrich. A stock solution of Cu was prepared by dissolving 0.1 g (1 mmol) of Cu (I) chloride and 0.2 g (0.74 mmol) of ODA in 2 mL of squalane on a hot plate at 200 °C for 30 min in an Ar-filled glovebox. Synthesis of Cu Nanocubes. To synthesize monodisperse Cu nanocubes, 19 mL of OLA and 1 mL of TOP were mixed in a 50 mL three-neck flask under inert gas (Ar). The flask was then rapidly heated to 330 °C. Next, 2 mL of Cu stock solution was quickly injected into a hot flask. The reaction was held at 330 °C for 3 min. Next, the reaction mixture was rapidly cooled to room temperature in a water bath to obtain the Cu nanocubes. The nanocubes were purified by dispersing the reaction mixture in 20 mL of toluene and then centrifuging at 8000 rpm for 5 min. The supernatant was discarded. A total of 40 mL of anhydrous toluene was then added to the sediment, and the mixture was centrifuged at 8000 rpm for 5 min. The washing procedure was repeated twice to remove unreacted precursors. The isolated Cu nanocubes were stored in anhydrous toluene before characterization. Ligand Exchange of Cu Nanocubes. Dodecanethiol-capped Cu nanocubes were prepared by addition of 1 mL of dodecanethiol into a Cu nanocubes solution (dispersed in anhydrous toluene) and under sonication for 10 min. Next, The dodecanethiol-capped nanocubes were purified by addition of 20 mL of toluene and then centrifuged at 8000 rpm for 5 min. The washing procedure was repeated twice. Preparation of Cu Nanocube Films and Large Area Cu Nanocube Superlattice. Large-area Cu nanocube superlattices were prepared by evaporating the solvent from a dispersion of dodecanethiol-capped Cu nanocubes in toluene on a silicon substrate 1786

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at room temperature. To form large area Cu nanocube superlattices, the Si substrate (1.5 cm × 2.5 cm) was immersed in a vial containing an appropriate concentration of dodecanethiol-capped Cu nanocubes dispersed in toluene. The solvent was slowly evaporated by reducing the system pressure below 150 Torr at 25 °C.46 To make a conductive Cu nanocube film, a desired amount of dodecanethiol-capped Cu nanocube solution dispersed in toluene (concentration = 10 mg/mL) was simply drop-casted on Si or glass substrate (1.5 cm × 2.5 cm) and dried under air, and the Cu nanocube film was then annealed at 150 °C, 250 °C, or 350 °C (heating rate of 1 °C/min) under a pressure of 0.1 Torr and Ar (50 sccm)/H2 (50 sccm) gas flow for 30 min. The film thickness can be adjusted by changing the nanocube concentration. Characterization. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) were performed on a JEOL JEM 2100F electron microscope operating at an accelerating voltage of 200 kV. X-ray diffraction (XRD) measurements were recorded on a Rigaku Ultima IV X-ray diffractometer using Cu Kα radiation, operating at 40 kV and 20 mA. UV−visible spectra were recorded with a Hitachi U-4100 spectrophotometer at room temperature. Surface topology of the nanocube films was characterized with a Park Systems XE-70 atomic force microscope (AFM) in contact mode under ambient conditions. Scanning electron microscopy (SEM) images were obtained on a HITACHI-S4700 field-emission SEM with 10 kV accelerating voltage. TEM, SEM, and XRD samples were prepared by drop casting the Cu nanocubes dispersed in toluene onto 200 mesh carbon-coated Cu grids for TEM and SEM or silicon substrates at room temperature for XRD. SAXS measurements were obtained using monochromatic radiation of wavelength λ = 1.24 nm on beamline 23A1 at the National Synchrotron Radiation Research Center (NSRRC) located at HsinChu, Taiwan. A two-dimensional Mar CCD detector with 512 × 512 pixel resolution was used to record the SAXS pattern. The distance of sample to the detector is 2423 mm with energy of the X-ray source of 12 keV. The beam center was calibrated using silver behenate powder standard with the primary reflection peak at 1.076 nm−1. The Fouriertransform infrared spectra (FTIR) were recorded by Perkin-Elmer Spectrum RXI FTIR spectrometer with resolution of 4 cm−1. The XPS measurements were made by a Kratos Axis Ultra DLD using a focused monochromatic Al X-ray (1486.6 eV) source. The resistivity of the Cu films (the size of the films was cut into 1 cm × 1 cm) was determined from sheet resistance using a four-point probe by a Keithley 2400 Source Meter, and the thickness of the Cu films was observed by SEM.

It should be noted that there exist many defects in the 2D arrays, as observed in the TEM image, where spherical or rodlike nanocrystals induce defects in the film. Figure 1d shows a TEM image of a single Cu nanocube, with edge lengths in the range of 74.8 nm and all corners appearing slightly truncated. A high-resolution TEM image of a single Cu nanocube (Figure 1e) clearly shows continuous fringes with a lattice distance of 0.18 nm, corresponding to the (200) planes in the facecentered cubic (FCC) structure that is characteristic of Cu, and confirms that the cube consists of a single crystal. The continuous fringes parallel to the edges of the cube indicate that the Cu nanocubes are bound primarily by facets. The SAED pattern (Figure 1f) acquired from a single Cu nanocube (Figure 2a) viewed along the [001] zone axis confirms that the nanocube is a single crystal with its surfaces bounded by {100} facets.

Figure 2. XRD pattern recorded from Cu nanocubes on silicon substrates by drop casting process. The eliminated part is the signal obtained from the silicon substrate.

Figure 2 shows the XRD pattern of the nanocubes. The peaks were assigned to the (111), (200), and (400) planes of FCC Cu (JCPDS no. 03-1018), respectively. No adventitious phases such as CuO and Cu2O were found. Supporting Information Figure S1 shows that the SAED pattern of the Cu nanocubes is also consistent with that of a FCC structure. The intensity of the (200) diffraction is significantly higher than that of other peaks, (111) and (220), which is the strongest peak and much greater than the value obtained from a conventional powder sample. This result indicates that the cubes tend to preferentially orient parallel to the supporting substrates (silicon substrate) and are consistent with prior observations by other researchers.44 The surface of FCC single-crystal particles is usually composed of low-index crystal planes. In addition, the surface energy of the low-index crystal planes largely determines the faceting and crystal growth, which has been estimated to occur in the following sequence {110} > {100} > {111}.48,49 Taking into account the surface free energy and surface area, this implies that a single-crystal seed is expected to exist as truncated octahedra bound by a mix of {111} and {100} planes. In solution synthesis, the nucleation and growth stages can be further controlled by employing capping ligands to selectively stabilize specific crystal planes. The surface state of Cu nanocubes was measured by FTIR and XPS. Symmetric and asymmetric alkane CH2 stretches can be detected at ∼2848 and ∼2908 cm−1, as well as symmetric rocking mode of terminal methyl group at ∼1454 and 1504 cm−1 and C−P stretching

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RESULTS AND DISCUSSION Cu nanocubes were synthesized by injecting Cu precursor stock solution into the mixture of OLA and TOP mixture at 330 °C. After injection, the color of the solution changed from light yellow to red-brown, indicating the formation of Cu0 species. The inset of Figure 1a shows a typical batch of Cu nanocubes dispersed in toluene; the solution is red-brown in color. Figure 1c shows the UV−vis absorption spectrum of the Cu nanocube solution. The Cu nanocubes exhibit a major LSPR peak at 588 nm, in the visible region, which agreed with the experimental observation by other groups.43 Parts a and b of Figure 1 are TEM images of Cu nanocubes, most of which show truncated cubic morphology with an average edge length of 75.7 nm and a standard deviation of about 3.87% (Figure 1b), where a monodisperse size distribution is defined as having a standard deviation of ≤5%.47 A schematic of the slightly truncated nanocube, based on the TEM results, is shown in Figure 1g. Statistical analysis indicates that 95% of the main product is cubic in shape, with the remainder of the particles being spherical or rod-shaped. It is well-known that uniform size and shape are critical for the self-assembly of structures with longrange order. The Cu nanocubes self-assemble into ordered 2D arrays on the TEM grids coated by an amorphous carbon film. 1787

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peaks at ∼1108 and 1158 cm−1.50 The N−H stretching region and NH2 scissoring region are visible at 3156 cm−1 and 1632 cm−1, respectively (Figure 3a).51 Additionally, the N 1s and P

Figure 4. (a, b) SEM images of Cu nanocubes self-assembled on silicon substrate. (c) TEM images of Cu nanocubes self-assembled in rhombohedral array on a carbon-coated Cu TEM grid. (d) SAED pattern of the Cu nanocube in rhombohedral array, as show in (c). (e) Scheme of the Cu nanocubes self-assembled into the rhombohedral structure (RS).

Figure 3. (a) FTIR spectra of Cu nanocubes. XPS spectra of Cu nanocubes: (b) N 1s; (c) P 2p.

2p peaks (Figure 3b,c) also appeared in the XPS spectrum. These results indicate that the OLA, ODA, and TOP molecules do exist on the surface of the Cu nanocube. Previous studies of Cu nanocubes have found that TOP can promote the formation of single crystal seeds44 and long carbon chain primary amines such as hexadecylamine (HDA) can fine-tune the faceting growth rate,43 although their detained mechanism is not yet completely clear. On the basis of these previous cases,43,44 we speculate that, in the present study, TOP and ODA may play a critical role in the nucleation and growth of Cu nanocubes to form single crystal seeds and selectively stabilize the {100} facets. Direct observation of the nanocube assembly was carried out via SEM and TEM (Supporting Information Figure S2 and S3). Figure 4a is a SEM image of a Cu nanocube array, where the Cu nanocubes self-assembled on the Si substrate with no size selection upon drop casting the nanocubes from toluene in ambient environment. The Cu nanocubes are close-packed in the form of mainly rhombohedral arrays and a small part of the SC arrangement over a short-range. Figure 4b,c also shows that the nanocube packing is characteristic of RS. Figure 4d shows the SAED pattern corresponding to over 20 nanocubes from the sample in Figure 4c, in the projection direction of [001]. The pattern shows fourfold symmetry, which indicates that the preferred crystal orientation of self-assembled Cu nanocubes is with the {100} planes parallel to the substrate. Figure 4e shows a schematic of a rhombohedral array. Figure 5 shows the self-assembled patterns of Cu nanocubes that form upon drop casting the Cu colloid sample in different positions on the carbon coated Cu grid. Parts a and b of Figure 5 show a typical TEM image and schematic drawing of nanocubes in a RS assembly. Figure 5c shows the FFT image

obtained from Figure 5a. It reveals RS consistent with the simulation results in Figure 5d that show a lattice angle α = 73° projected from the [001] direction. Parts e and f of Figure 5 show a TEM image and schematic drawing, respectively, of nanocubes in a RS assembly. Figure 5g shows the FFT image obtained from Figure 5e, revealing rhombohedral symmetry consistent with the simulation results that show RS with a lattice angle α = 73° projected from the [11̅0] direction (Figure 5h). The RS projected from the [111̅] direction is shown in Supporting Information Figure S4. In summary, the TEM images of Cu nanocube superlattices show that the Cu nanocubes tend to self-assemble into rhombohedral packing structures. SAXS was used to further identify the structure of the Cu nanocube superlattice. The diffraction peaks associated with the superlattice, observed in the SAXS profile (Figure 6) of the Cu nanocube film, can be indexed to RS, which is the intermediate phase in a continuous transformation path from a FCC (α = 60) to SC (α = 90) lattice, where α is the lattice angle. FCC-toSC transformation has been observed in several systems23,52 For instance, Zhang et al. observed a continuous transformation from SC to rhombohedral phases during the assembly of Pd nanocubes by solvent evaporation.23 The ligand thickness and the particle sphericity both govern the value of α associated with RS. According to Zhang et al., α can be determined by first plotting the ratios of the q positions of the diffraction peaks over q(100) against α via the equation q(hkl)/q(100) = [(h2 + l2 + k2) sin2 α + 2(hk + kl + lh)(cos2 α − cos α)]1/2/sin α (Figure 7a). The value of α is then determined by locating in the plot the value corresponding to the experimentally observed 1788

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Figure 5. (a) TEM image of of the [100] projection of RS stacking structure and (b) its stacking structure by the drawing. (c) FFT and (d) simulation of the TEM image in (a). (e) TEM image of of the [110̅ ] projection of RS stacking structure and (f) its stacking structure by the drawing. (g) FFT and (h) simulation of the TEM image in (e).

Figure 6. SAXS spectra of the superlattices made of Cu nanocrystals. Inset shows a schematic of a ligand-capped Cu nanocube with slight truncation.

reduced peak position. The lattice angle thus obtained from the experimentally observed value of q(200)/q(100) was 72.5°. Perfect cube-like nanocrystals tend to assemble into a SC packing pattern, which is the most energetically favorable.53 The Cu nanocubes method contain a Cu core with some degree of truncation and surface organic ligands (Figure 6 inset) that cause slight deviations from a perfectly cubic shape. A wide variety of particle shapes, from octahedron (q = 0.5) to sphere (q = 1), to cube (q = ∞), can be described by a superball model, |X|2P + |X|2P + |X|2P ≤ 1, where x, y, and z are Cartesian coordinates and p is the deformation parameter.23,34,54 The relationship of lattice vectors and lattice angle for a RS can be expressed as α(p) = arcos[ei·ej/(|ei||ej|)], i, j = 1, 2, 3 (i ≠ j), where e1 = −2(s + 2−1/2p)i + 2sj + 2sk, e2 = −2si +2sj +2(s + 2−1/2p)k, and e3 = −2si +2(s + 2−1/2p)j + 2sk, where i, j, and k are unit vectors along the x, y, and z directions and s is the smallest positive root of the equation (s + 2−1/2p)2p + 2s2p − 1 = 0. We plotted 1/p against α (Figure 7b) and obtained a value of 2.2 for p from the known value α. The RS assembly with lattice angle variable is proposed by nanocube sliding under a certain extent without rotation.23 The studies of theory

Figure 7. (a) Plot of the reduced position of the diffraction peak [q(111)/q(100), q(200)/q(100), and q(220)/q(100)] vs α (the relationship of lattice vectors and lattice angle). (b) Plots of 1/p vs α. The bottom images shows schematic pictures of Cu nanocubes with obtained p and α.

and simulation on self-assembled of nonspherical particles are still in the early stages of development. But basically, the final self-assembling pattern originates from repulsive and attractive interactions between adjacent particles. In our case, the nanocubes can be considered as quasispherical shape due to geometrical effects (truncated shapes) and soft molecular shells, 1789

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Figure 8. (a) SEM and (b) photographic image of the Cu nanocube films prepared by immersion process. (c) AFM images and height profile of Cu nanocube films fabricated by immersion process on a silicon substrate. (d) Reflectance spectra of the Cu nanocube films (red line) and Cu foil (black line).

Cu is a good alternative material as it possesses high conductivity and is significantly cheaper than Au and Ag. In addition, Cu nanocubes can be coated by a layer of organic molecule which can be dispersed in organic solvent as conductive inks. In order to obtain better dispersibility, we perform ligand exchange of Cu nanocubes with dodecanethiol because the amine is a relatively weak capping agent.55,56 The shape of Cu nanocubes did not changed after ligand exchange (Supporting Information Figure S5). Figure 8 shows a SEM image of a high-quality film of Cu nanocubes formed over a large area (1.5 cm × 2.5 cm). The Si substrate was immersed aslant in a solution of dodecanethiol capped Cu nanocubes, and the solvent (toluene) was slowly evaporated by controlling the pressure under 150 Torr (Supporting Information Figure S6).46 The image reveals that the surface of the Cu nanocube film is flat and dense (Figure 8a, Supporting Information Figures S7 and S8). When the Si substrate was slowly dried aslant in the

which lead to RS self-assembly. We think that the van der Waals force is the only force between nanocubes due to their characteristic of short-range order. For small 1/p < 0.12 (perfect cube or cubelike particles), the van der Waals force is obvious (>4 kBT); therefore, SC arrangement is favored. When 1/p > 0.25, the influence of the van der Waals force declines (∼kBT), leading to the RS assembly.23 According to a study by Torquato and co-workers, the densest packing for all convex and concave particles is the cube-like superball; the deformed SC structure, corresponding to RS, becomes the densest when p > 1.1509.54 A study by Gang et al., on the relationship between volume fraction and p, shows that the packing of RS becomes denser packing than that of SC, due to a rapid increase in the packing density of RS 1/p > 0.12.23 In our system, 1/p is about 0.4, so it is quite reasonable that these truncated and ligand coated nanocubes tend to exhibit RS arrangement 1790

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Figure 9. (a) SEM image and (b) cross-sectional morphology of the Cu nanocube film made by drop casting method after heat treatment at 350 °C. (c) The resistivity of the Cu nanocube film with different thicknesses after heat treatment at 350 °C. (d) The resistivity of the Cu nanocube film (black) and Cu nanoparticle film (red) with a thickness of 1.2 μm after heat treatment.

to the substrate after polishing, like polishing Cu foil or polishing silicon substrate. The contact-mode AFM images of the Cu nanocube films (Figure 8c) also reveals that the surface roughness of the thin film (the root-mean-square deviation of its topography) is only 4.6 nm, which means that the Cu nanocubes can easily be made into smooth Cu nanocube film. On the basis of Figure 8a, the two-dimensional (2D) packing density is estimated as ψ2D = N × L2/A, where N and L are the number of Cu nanocubes in Figure 8a and the average edge length of Cu nanocubes, respectively. A is the surface area of substrate in Figure 8a. The calculated two-dimensional packing density is about 0.723. In addition, we also can calculate the packing density ϕRS(p) when the truncated nanocube selfassemble into a RS arrangement with deformation factor p = 2.2 by the following equation, ϕRS(p) = Vsb(p)/VRS(p), where Vsb is the volume of superballs and VRS = e1 × e2·e3 is the unit cell volumes of the RS phase. The calculated ϕRS(2.2) is approximately 0.88.23,54 The packing density of the Cu nanocube film is reasonably close to the theoretical value. A metal film composed of nanoparticles with high packing density has high film conductivity.61 Considering the high conductivity of natural Cu and the theoretically high packing density of Cu nanocubes, Cu nanocubes serve as a good material to be made into conductive film; therefore, the resistivity of the Cu nanocube film was also measured. The dense Cu nanocube film prepared by drop casting method has large resistivity before the annealing process. The surface ligand length is known to play a critical role in the conductivity of the nanocrystal film. When the length of the bifunctional cross-linkers < 0.8 nm, the nanocrystal film becomes very conductive; when the linkers are longer, the nanocrystal film becomes highly resistant.59 It is well-known that heat treatment can improve the metal film conductivity. Figure 9a shows SEM and optical images (inset) of Cu thin films annealed for 30 min at 350 °C under a pressure

solution of dodecanethiol capped Cu nanocubes, the red− brown solution deposited as a shiny Cu layer on the substrate (Figure 8b). The reflectance spectra of the Cu nanocube film, as shown in Figure 8d, exhibits a collective plasmon band at 603 nm. The change in color for the Cu nanocube film can be attributed to (1) the difference in refractive indices of air and toluene, because the plasmon resonance is highly sensitive to the dielectric constant of the surrounding medium,1 and (2) collective plasmon resonance in self-assembly of Cu nanocubes. Some studies report that the collective plasmon resonance peak position is very sensitive to the gap spacing between Cu nanocubes. For Cu nanocubes solution, it exhibits a unique plasmon response due to their cube geometries, but for selfassembling Cu nanocubes, collective plasmon resonance between Cu nanocubes is expected to control the optical response; therefore, the color of Cu nanocubes film is significantly different from Cu nanocubes dispersed in solution.57,58 We also compare the reflectance spectra of the Cu nanocube film (red line) and Cu foil (black line) in the visible region (Figure 8d). The reflectance of the Cu nanocube film is nearly identical to that of Cu foil in the low-reflectance region (400−550 nm) except the reflectance dips in Cu nanocube film. The reflectance of the two materials diverges above 500 nm; Cu foil exhibits relatively high reflectance in the 550−800 nm wavelength range, while the Cu nanocube film exhibits relatively low reflectance (reflectance dips) at about 631 nm. These reflectance dips probably were induced from the interlayer collective plasmon resonance when the layer number of Cu nanocubes on substrate is large enough. In addition, on the basis of the results for other thin films of nanocrystals, the optical properties of Cu nanocube film should be related to the film thickness (layer number).58,59 Comparing with Cu film made by Cu nanoparticles synthesized by the polyol process,60 the Cu nanocube film exhibits shiny color which is very similar 1791

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of 0.1 Torr. The flow rates of the Ar and H2 carrier gases were 50 and 50 sccm, respectively. After annealing, the film becomes dense but still exhibits some cracks. The cross-sectional SEM image (Figure 9b) shows that the Cu film has a thickness of around 1.2 μm. Heat treatment reduced the resistivity of the Cu nanocube films, as shown in Figure 9c,d. First, we compare the resistivity of Cu film made by Cu nanocrystals and Cu nanocubes after annealing. According to Figure 9d, the resistivity (1.33 × 10−7 Ω m) of the Cu nanocube film is significantly lower than that of the Cu nanocrystals (6.215 × 10−6 Ω m) film. We think that the packing density is one of the main reasons leading to the differences in resistivity. In addition, the contact area between nanocubes should be significantly greater than that of the nanoparticles due to the geometry itself. Next, we also compare the effect of resistivity of Cu nanocube film on film thickness and annealing temperature. Figure 9c shows the resistivity of Cu nanocube annealed at 350 °C with different thicknesses which are of the same order (∼10−7 Ω m), and the film resistivity of all samples decreases with increasing annealing temperature (Figure 9d, from 12.22 × 10−7 Ω m at 150 °C to 1.33 × 10−7 Ω m at 350 °C). The highest resistivity of Cu nanocube films is only 1 order of magnitude higher than that of bulk Cu, whose resistivity is 1.68 × 10−8 Ω m. Heat treatment removes the organic ligands on the Cu nanocube surface, increasing the extent of interparticle contact area and enabling sufficient electron transport; therefore, the Cu film becomes highly conductive.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support by the National Science Council of Taiwan (NSC 102-2221-E-007-023-MY3, NSC 102-2221-E-007-090-MY2, NSC 101-2623-E-007-013-IT, and NSC 102-2633-M-007-002), the Ministry of Economic Affairs, Taiwan (101-EC-17-A-09-S1-198), National Tsing Hua University (102N2051E1, 102N2061E1), and the assistance from Center for Energy and Environmental Research, National Tsing-Hua University.

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CONCLUSIONS In summary, monodisperse Cu nanocubes have been synthesized with well-defined shape and good uniformity from a hot organic solution. The TOP and ODA ligands play an important role in selectively forming single seed crystals stabilizing specific crystal planes, both of which are critical to achieving monodisperse Cu nanocubes. The nanocubes tend to self-assemble into square arrays and RS supercrystals; their assembly is herein confirmed via TEM, SEM, and SAXS. The XRD patterns suggest that the preferential orientation of the Cu nanocubes is with their {100} planes parallel to the substrate. These monodisperse Cu nanocubes can be dispersed in an organic solvent as a conductive Cu ink, which provides a convenient and inexpensive method to build dense Cu nanocube films over a large area. After annealing at 350 °C, films exhibited electrical resistivity of 1.33 × 10−7 Ω m, one order higher than that of a bulk Cu film but several orders of magnitude lower than conducting polymers.62 The high conductivity of the Cu nanocube films is compatible with their use in various electronic devices. The successful synthesis of monodisperse Cu nanocubes is an important step toward studying and exploiting the properties and self-assembly of Cu nanomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

SAED pattern, TEM and SEM images of Cu nanocubes. Photography image of the immersion process to make large area Cu nanocube superlattice. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(H.-Y.T.) Tel.: + 886-3-572-3661. Fax: + 886-3-571-5408. Email: [email protected]. 1792

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