Control of interparticle separation and exchange coupling of NPs

One of the outstanding challenges in the fields of nanophotonics and metamaterials is the ability to achieve precise control of the structural characteristics of macroscopic NP superlattices, such as interparticle distance and proximate NP coordination numbers, to enhance efficiency of energy (or charge) coupling or transfer for applications in sensing, solid-state lighting, solar-energy conversion, and electrical-energy storage.^{Reference Wang, Brandl, Nordlander and Halas14–Reference Choi and Alivisatos17} In particular, three-dimensional (3D) metal nanostructure arrays with strong and tunable surface plasmonic resonance (SPR), resulting from local NP interactions, will allow for the development of advanced optical materials.^{Reference Valentine, Zhang, Zentgraf, Ulin-Avila, Genov, Bartal and Zhang18–Reference Atwater and Polman20}

Controlling interparticle separation is critical in tuning interparticle exchange coupling for collective chemical and physical properties. Both top-down processes such as electron-beam (e-beam) and ion-beam lithography,^{Reference Liu, Guo, Fu, Kaiser, Schweizer and Giessen19,Reference Verellen, Sonnefraud, Sobhani, Hao, Moshchalkov, Dorpe, Nordlander and Maier21,Reference Jain and El-Sayed22} and bottom-up methods such as self-assembly of colloidal NPs^{Reference Murray, Kagan and Bawendi3,Reference Alivisatos6,Reference Hu, Chen, Li, Au, Hartland, Li, Marquez and Xia23,Reference Burda, Chen, Narayanan and El-Sayed24} have been vigorously pursued to achieve this goal. While these fabrication methods have had some successes, they also appear to have some fundamental limitations. The top-down processes are limited to spatial resolutions of ∼10 nm by the e-beam or ion-beam size, and also, by their inability to fabricate complicated and tunable 3D nanostructures for new and strong SPR coupling. As for bottom-up synthesis methods, NP (or colloid) self-assembly has been extensively explored and form well-defined NP superstructures through balanced interparticle interactions (Figure 1).

Figure 1.

Interparticle separation and interactions through balanced forces.

Within the self-assembled arrays, balanced forces lead to intrinsic interparticle separations that are determined by the organic ligands bound to the particle surfaces. Bottom-up methods have been developed to change the organic ligand length to control interparticle separation in order to achieve exchange coupling between particles. For example, near-field coupling has been demonstrated through alkanethiolate-stabilized gold NP superlattices by changing the length of the organic ligands.^{Reference Lin, Chen and Gwo16} In this work, alkanethiolate-stabilized gold NPs were synthesized and self-assembled through solvent evaporation into ordered two-dimensional (2D) arrays, where the lattice constants were controlled by the alkyl chain lengths, ranging from C12 (1-dodecanethiolate), C14 (1-tetradecanethiolate), C16 (1-hexadecanethiolate) to C18 (1-octadecanethiolate) (Figure 2).^{Reference Lin, Chen and Gwo16} Due to the different lengths of the ligands, the interparticle separation was tuned from 2.2 nm to 3.4 nm. Tunable plasmonic response was observed to shift from visible to near infrared wavelengths.

Figure 2.

(a) Optical extinction spectra of four kinds of thiolate-stabilized nanoparticle superlattices with the same metal core size, but different alkyl chain lengths (C12 to C18). (b) Exponential dependence of the resonance peak position on the interparticle gap. The right inset shows the relationship (both solid and dotted lines) between interparticle gap and thiolate chain. The left inset shows a schematic of gold nanoparticles (red) with the organic ligands in blue. Modified with permission from Reference 16. © 2010 American Chemical Society.

In addition to solvent evaporation,^{Reference Murray, Kagan and Bawendi3,Reference Fan, Yang, Boye, Sigmon, Malloy, Xu, Lopez and Brinker5,Reference Fan, Chen, Brinker, Clawson and Alam25–Reference Wright, Gabaldon, Burckel, Jiang, Tian, Liu, Brinker and Fan31} Langmuir–Blodgett (LB)^{Reference Tao, Sinsermsuksakul and Yang32,Reference Heath, Knobler and Leff33} and layer-by-layer assembly^{Reference Lin, Chen and Gwo16} have also been extensively investigated. Despite these extensive efforts, the NP arrays formed using these methods have been limited to ∼2-nm interparticle distances by the intrinsic organic monolayer on the particle surface (Figure 1). It has been predicted that physical phenomena will arise at extremely small interparticle separations (<2 nm), although these have yet to be experimentally achieved.^{Reference Jain, Huang and El-Sayed34} Material configurations have been mainly limited to multiple plasmonic oligomeric or 2D arrays. Although 3D ordered nanostructures can be formed, questions remain about the practicality of self-assembly techniques to systematically manipulate NP structures for tunable optical and electronic properties. Overall, methods to develop 3D NP material systems with precisely and systematically controlled structural parameters to achieve strong and tunable collective optical and electronic properties are not available, and their generation remains a significant challenge.

Pressure-directed assembly

Despite progress in NP self-assembly, the ability to engineer assembled structures of NPs with control over the particle separation remains challenging. We discuss a novel external pressure-directed engineering pathway to systematically and precisely control interparticle separations, to investigate structure–property correlations, and to synthesize new nanostructures. We recently invented a new method we call pressure-directed assembly (PDA) for assembling NP arrays with new architectures.^{Reference Wu, Wang and Fan35–Reference Wang, Schliehe, Wang, Nagaoka, Cao, Bassett, Wu, Fan and Weller40}

Starting with films of ordered spherical gold NP arrays with [110]-oriented, face-centered cubic (fcc) mesophases, the films are compressed under a high-pressure field inside a symmetric diamond anvil cell (DAC). Under high hydrostatic or uniaxial pressure, the unit-cell dimensions of the 3D ordered NP arrays (i.e., inter-NP spacings) were found to reversibly shrink and spring back with the application and release of external pressure, respectively. The process allows precise and systematic tuning of interparticle separation distance, which, with control of the applied pressure, can be manipulated reversibly at the angstrom scale until the particles make contact and consolidation occurs. The pressure-regulated fine-tuning of interparticle separations overcomes current limitations imposed by the intrinsic organic monolayers and is ideal for controlled investigation of distance-dependent collective optical and electrical phenomena. Moreover, beyond a certain threshold pressure, NPs are forced to come in contact with their neighbors and sinter, forming new classes of chemically and mechanically stable 1D–3D nanostructures that cannot be manufactured by other existing top-down or bottom-up methods.

The PDA method was inspired by embossing and imprinting manufacturing processes; these are accomplished by applying pressure on a planar surface or substrate to fabricate new patterns or structures. These are rapid, cost-effective processes that exhibit high throughput and high fidelity.

The platform that enables in situ optical and structural analysis during processing is a DAC.^{Reference Mao and Bell41} The DAC has two flat diamond anvils, which allow samples to be quantitatively compressed between the flat parallel faces of the anvils with flat culets. Samples are sealed by a metal gasket with a small hole and are compressed between the two parallel anvils. Ruby crystal chips are used for determining the pressure inside of the DAC chamber by measuring the ruby’s pressure-dependent fluorescence peak position. The pressure fields in the sample chamber can be controlled to be hydrostatic or uniaxial, depending on whether pressure-transmitting media is used. The transparent nature of the diamond anvils allows for a variety of optical investigations of materials under pressure such as ultraviolet-visible (UV-vis) absorption/transmission, photoluminescence, Raman, and Fourier transform infrared spectroscopy. More importantly, x-ray sources can be used to explore structural or phase evolution in situ under pressure. The DAC remains an efficient tool for exploration and identification of new high-pressure phases, synthesis of new materials, and in situ studies of structure and properties.^{Reference Jacobs and Alivisatos42–Reference Grünwald, Lutker, Alivisatos, Rabani and Geissler44}

It should be noted that the LB method has also been used to create 2D uniaxial pressure fields to the direct assembly of a monolayer of NP arrays and tune NP separation.^{Reference Tao, Sinsermsuksakul and Yang32,Reference Heath, Knobler and Leff33} Because of the uniaxial nature of the LB method, pressure over a certain threshold often causes spatial-phase collapse of the monolayers. Consequently, this 2D method is restricted to a very small range of pressures to avoid the collapse of NP monolayers, limiting the tunability of interparticle separations.

Upon loading ordered NP films into the DAC, thin films of NP arrays were compressed continuously to a range of desired high pressures and then released to ambient pressure to analyze the assembly dynamics. Gold NP arrays with fcc mesophases and [110] orientation were used as a model material system for demonstration of the PDA method. In our studies, spherical gold NPs with an average diameter of 5.2 nm and a standard deviation of 4.2% were synthesized.^{Reference Zheng, Fan and Stucky45} Ordered gold NP-polymer films were fabricated through solvent evaporation on silicon wafers.

Representative scanning electron microscope (SEM) images taken from the surface and cross section of gold NP films (Figure 3a–b, respectively) reveal that the film consists of 3D ordered arrays. Analysis of corresponding electron diffraction patterns (insets in Figure 3a–b) indicated that the gold NPs are organized in a periodic, ordered fcc mesophase with a preferred orientation along the [110] direction. Up to a compression of 9 GPa, the films were found to maintain their order and orientation, as confirmed by synchrotron-based small-angle synchrotron x-ray scattering (SAXS).^{Reference Wu, Bai, Sun, Haddad, Boye, Wang, Huang and Fan36,Reference Wu, Bai, Sun, Haddad, Boye, Wang and Fan37} 2D SAXS patterns and the integrated 2D spectrum (Figure 3c–d, respectively) collected at ambient pressure confirm the fcc mesophase with Fm $\bar 3$m space group. The NP assembly exhibited preferred packing orientation with {110} planes parallel to the substrate and vertical to the x-ray beam. The calculated unit-cell parameter a was 104 Å. The NP films were continuously compressed in the DAC up to 13 GPa and gradually released back to ambient pressure at room temperature.

Figure 3.

Structure of ordered spherical gold nanoparticle (NP) films. (a) Scanning electron microscope (SEM) image of [110] orientation taken from a plan view of the film. The insets display the size (D) distribution of the NPs and the electron diffraction pattern of the periodic array film. (b) SEM cross-sectional view of the film. The inset shows the electron diffraction pattern. (c) Typical small-angle x-ray scattering (SAXS) pattern (shown in different colors) of the arrays loaded in a diamond anvil cell at ambient pressure. (d) SAXS spectrum integrated from (c) with fitted peaks. The theoretical fitting curve (red-dashed line) confirms the fcc symmetry. Solid-blue curves present Bragg reflections of each diffraction plane specified by Miller indices (hkl) that are characteristic of an fcc mesophase with space group Fm $\bar 3$m. The calculated unit-cell parameter a is 104 Å. The inserted table shows a comparison between experimental (d _e) and theoretical (d _t) spacings of different crystallographic planes. d _t values were calculated using a lattice constant of a = 104 Å in space group Fm $\bar 3$m.

Figure 4 shows representative electron microscope images of the samples after pressure release. The ordered fcc spherical NP superlattice was converted to an ordered nanowire array (Figure 4a). The gold nanowire array consisted of individual gold nanowires with hexagonal close packing. The SAXS pattern shown in Figure 4e–f is indexed to represent a 2D hexagonal mesophase with p6mm symmetry. The primary peaks are assigned as 10, 11, 20, 21, and 30 reflections.

Figure 4.

Structure of gold nanowires and their ordered arrays. (a) Representative scanning electron microscope (SEM) image of the arrays after pressure is released from 13 GPa to 0 GPa. (b) Plan-view SEM image of the arrays along the nanowire c-axial direction. The upper inset is the fast Fourier transform pattern of the arrays. The measured center-to-center distance (also the hexagonal unit-cell constant a _hex) between nanowires is 81.4 Å. (c) Cross-sectional SEM image of the arrays, taken perpendicular to the nanowire c-axis. The distance between nanowires is ∼71 Å, consistent with the calculated hexagonal (spacing between (110) planes) d ₁₁₀ = ${{\sqrt 3 a_{{\rm{hex}}} } \mathord{\left/ {\vphantom {{\sqrt 3 a_{{\rm{hex}}} } 2}} \right. \kern-\nulldelimiterspace} 2}$ = 70.5 Å. (d) Transmission electron microscope (TEM) image of the arrays reassembled from dispersed nanowires. The inset shows a high-resolution TEM image. (e) Small-angle x-ray scattering (SAXS) pattern (shown in different colors) collected from a nanowire sample after pressure was released. (f) SAXS spectrum integrated from (e) with fitted peaks, consistent with a two-dimensional hexagonal mesophase with p6mm symmetry. The inset compares experimental planar spacing (d _e) and theoretical planar spacing (d _t) of crystal planes. d _t values were calculated based on lattice constant a = 81.4 Å in the space group p6mm. Modified with permission from Reference 37. © 2010 Wiley.

Figure 4b–c shows representative SEM images of [100]- and [110]-oriented nanowire mesophases along with their corresponding electron diffraction patterns. These SEM images and SAXS patterns revealed a unit cell with lattice parameter a _hex = 81.4 Å and a uniform, minimum center-to-center spacing between nanowires of 8.1 nm. Based on the packing geometry, this corresponds to an ultrahigh nanowire density of ∼1.7 × 10¹² nanowires per square centimeter.^{Reference Beckman, Johnston-Halperin, Luo, Green and Heath46} The neat nanowire bundles confirm that the nanowires have a uniform length (Figure 4a). The nanowire bundles can be dispersed in organic solvents such as toluene to better resolve individual nanowires.

The transmission electron microscope (TEM) image in Figure 4d shows that the nanowires appear uniform in diameter and length, with an average diameter of 6.1 nm and a standard deviation of 4.6%. The nanowire length ranges from 400 nm to 1.5 µm, depending on the film thickness of the starting ordered NP arrays. The inset high-resolution TEM image in Figure 4d reveals that the gold nanowires are not single crystalline. Along the c-axis, each nanowire consists of crystalline NP domains with a size close to that of the original spherical gold NPs. Thus, the formation of gold nanowires occurs through consolidation of spherical gold NPs. These individual nanowires can form stable colloidal dispersions and reassemble into ordered arrays upon solvent evaporation (Figure 4d). Overall, PDA provides more efficient control over the uniformity of both the diameter and length of the nanowires as well as the formation of ordered, nanowire arrays of ultrahigh density compared to chemistry-based methods.^{Reference Huo, Tsung, Huang, Zhang and Yang47–Reference Wang, Hu, Lieber and Sun49}

PDA and phase transformation

To investigate the PDA pathway and the aforementioned nanowire formation process, in situ high-pressure SAXS (HP-SAXS) experiments were performed to track the structural evolution of the NP films during the compression and release process. The HP-SAXS results (Figure 5) suggest the formation of gold nanowire arrays via phase transformation from a 3D fcc mesophase to a 2D hexagonal nanowire mesophase. Before formation of the gold nanowires, the spherical NP mesophase exhibits isotropic unit-cell dimension shrinkage. During this reversible process, the interparticle separation can be precisely controlled by external pressure.

Figure 5.

Structural evolution of gold nanoparticle (NP) assemblies during compression and release. (a) Representative high-pressure small-angle x-ray scattering (HP-SAXS) patterns (shown in different colors) of gold NP assemblies collected at 0, 7.7, 9.6, and 13 GPa. (b) Integrated data from HP-SAXS images of assemblies at varied pressures (P) during compression and release (release pressures are denoted by green curves). Up to 7.7 GPa, the HP-SAXS patterns are consistent with the fcc mesophase. After releasing the pressure to below 8 GPa, the HP-SAXS patterns are consistent with 2D hexagonal mesophase. The vertical dotted lines represent the (111) and (311) peak positions. (c) The d-spacings of the first Bragg reflection along the pressure course (solid-red arrowed lines/curves). (d) d-spacing ratios (R) at different pressures. When the pressure is lower than 9 GPa, the NP assembly has the fcc structure and R= d ₃₁₁/d ₁₁₁, where R is almost constant and is very close to the theoretical value of $\sqrt {3/11} &#x003D; 0.522$ for the fcc structure. At the intermediate stage, R _inter varied between 0.522 and 0.5. After lowering the pressure below 7.7 GPa, R _hex= d ₂₀₀/d ₁₀₀ becomes ∼0.5, the theoretical ratio for a 2D hexagonal mesophase, consistent with HP-SAXS indexing. Note: d, interplanar distance. Modified with permission from Reference 37. © 2010 Wiley.

Figure 5a–b shows representative HP-SAXS patterns collected at varied compression and release pressures. At ambient pressure, the gold NP film exhibits an fcc mesophase. In the pressure range from 0 GPa to ∼9 GPa, HP-SAXS results confirmed that the gold NP assembly remained fcc in structure with a [110] orientation. Upon increasing the pressure to 9 GPa, all of the HP-SAXS peaks shifted slightly to higher 2θ (x-ray scattering angle) values (lower d-spacings where d is defined as the distance between planes), indicating unit-cell shrinkage. When the pressure was gradually released, all peaks returned to their original positions. Thus, the change in the unit-cell dimension is reversible between ambient pressure and 9 GPa. This reversible behavior is further verified by the d-spacing changes shown in Figure 5b–c, and the constancy of the d-spacing ratio R at varying pressures (Figure 5d). Below 9 GPa, R remained constant and close to the theoretical value of R = d ₃₁₁/d ₁₁₁ = $\sqrt {3/11} &#x003D; 0.522$ for an fcc mesophase. The lattice constant shrunk from 104 Å at ambient pressure to 90 Å at 9 GPa and returned to 103 Å when the pressure was released. Consequently, the interparticle separation distances along the [110] direction varies continuously and reversibly.

The NP assembly exhibited a mixed phase or structure that could not be assigned to either the fcc or 2D hexagonal phase over the high-pressure range 9 GPa ↔13 GPa. The unit-cell dimension changes are irreversible during this pressure course. Our HP-SAXS data^{Reference Wu, Bai, Sun, Haddad, Boye, Wang and Fan37} showed that all the peaks shifted to lower 2θ (higher d-spacing) rather than a continuous shift to lower d-spacing. This suggests that a structure transformation has taken place. More importantly, the d-spacing ratio R moves away from the theoretical value for fcc (0.522) down to ∼0.51, further confirming a structural transformation. When the pressure was released to 8 GPa, all of the HP-SAXS peaks continued to shift to lower 2θ values instead of reversibly returning to their original positions. Below 8 GPa, the d-spacing ratio further decreased down to ∼0.5, the theoretical d ₂₀₀/d ₁₀₀ ratio for a 2D hexagonal mesophase. Additionally, the HP-SAXS patterns are consistent with a 2D hexagonal mesophase.

TEM results further confirmed that the gold nanowires formed through the consolidation of spherical NPs. We observed the formation of gold NP dimers, trimers, and short nanorods (Figure 6a). The formation of 1D nanostructures along the c-axis distinguishes them from previously reported 1D NP nanostructures in which the NPs are connected through organic ligand binding.^{Reference DeVries, Brunnbauer, Hu, Jackson, Long, Neltner, Uzun, Wunsch and Stellacci1,Reference Tang, Kotov and Giersig9,Reference Worden, Dai and Huo50} Compression also slightly increased the diameter of the final gold nanowires in comparison with the original gold NPs.

Figure 6.

A proposed mechanism for the formation of one-dimensional nanostructures. (a) Representative transmission electron microscope images of the sintered gold nanoparticles (NPs). Red-dotted ovals show the dimers. (b) Schematic model showing oriented sintering of gold NPs into gold nanowire arrays along the [110] direction under compression. The arrows show the pressure direction. Modified with permission from Reference 37. © 2010 Wiley.

Based on these experimental results, the formation of well-ordered gold nanowires is illustrated in Figure 6b and can be summarized as follows: below 9 GPa, the sample is compressed under a hydrostatic pressure field. The isotropic pressure, which is uniformly applied to the gold NP assembly in all directions, causes the fcc unit-cell dimension to shrink uniformly. The fcc mesophase and its [110] orientation are retained. For pressures above 9 GPa, a nonhydrostatic stress is generated and applied perpendicular to the NP film. Due to the [110] orientation of the NP mesophase, gold NPs along the [110] direction experience greater stress than other crystallographic directions. Thus, NP consolidation preferentially occurs along the [110] direction. Continuous compression drives the spherical NPs to touch and finally consolidate into gold nanowires along the [110] direction. Ultimately, bundles (or arrays) of well-oriented nanowires are formed with the c-axis along the former fcc [110] direction.

We have so far produced 1D nanorods,^{Reference Wu, Bai, Sun, Haddad, Boye, Wang and Fan37,Reference Li, Wen, Li, Wang, Clem and Fan39} nanowires,^{Reference Wu, Bai, Sun, Haddad, Boye, Wang and Fan37,Reference Li, Wen, Li, Wang, Clem and Fan39} nanosheets,^{Reference Wang, Schliehe, Wang, Nagaoka, Cao, Bassett, Wu, Fan and Weller40} and 3D interconnected architectures^{Reference Wu, Bai, Sun, Haddad, Boye, Wang, Huang and Fan36} via the PDA method. TEM studies indicated that scale parameters in the final nanostructures are highly related (but not identical) to those of the initial NP arrays. For example, the diameter of the nanowires is close to that of the NPs in the starting materials. Controlling well-defined structures in the starting NP arrays is crucial for understanding the PDA process. Importantly, our preliminary results demonstrate that we can control the final structure by altering the structure (mesophase, orientation) of the starting NP arrays. These nanostructures are expected to possess new individual and collective physical properties (e.g., SPR).

Recently, the PDA method was successfully extended to silver NP arrays to tune particle separation and synthesize silver nanowires.^{Reference Li, Wen, Li, Wang, Clem and Fan39} Silver NPs were synthesized through a thermal decomposition method using myristic acid as the surface ligand. The Ag NPs were fairly monodisperse with an average diameter of 5.5 nm and a narrow size distribution. Through a solvent evaporation process, the NPs self-assembled into highly ordered fcc superlattices with a lattice constant of ∼12 nm and a [110] orientation. Figure 7a–b shows representative TEM images of the 1D Ag nanostructures formed by compression. The resulting Ag nanowire array consists of individual Ag nanowires with hexagonal close packing. These nanowire arrays were collected from the DAC and then dissolved in organic solvents such as toluene to form stable colloidal dispersions. As in the case of the gold NPs, the Ag nanowires were uniform in diameter and length, with an average diameter of 7.6 nm and standard deviation of 10%. High-resolution (HR)TEM imaging (Figure 7b, c, and f) revealed that the Ag nanowires are polycrystalline with apparent twinning faults. Along the c-axis, each nanowire consists of crystalline nanodomains whose sizes are close to those of the original spherical Ag NPs. This indicates that the Ag nanowires form by direct consolidation of spherical Ag NPs to form a nanowire along the c-axis. The HRTEM studies confirmed that the Ag NPs are consolidated through specific {111} crystal surface interactions (Figure 7c and f). Due to such an oriented connection, the final Ag nanowires form a zigzag surface morphology, as proposed in Figure 7e.

Figure 7.

Representative transmission electron microscope (TEM) images of Ag nanowires synthesized under high-pressure-induced mechanical consolidation of spherical Ag nanoparticle (NP) arrays. (a) A bundle of Ag nanowires. (b) High-resolution TEM (HRTEM) image of reassembled nanowires after solvent evaporation. (c) HRTEM image of a single Ag nanowire showing the zigzag morphology and twinning microstructure. Red circles are used for guidance in viewing individual crystal domains composed of Ag NPs. Solid circles show the images close to the reader, while dashed circles show those farther away from the reader. (d) Ag nanorods. (e) Schematic of fivefold twinning microstructures and oriented sintering through (111) attachment evident in the HRTEM image in (f). (f) HRTEM image of a single Ag nanowire showing oriented crystal sintering. The arrows point to the individual NPs and interface. Modified with permission from Reference 39. © 2014 Macmillan Publishers Limited.

Pressure-directed tuning of interparticle separation and optical coupling

As previously discussed, high-pressure compression allows reversible manipulation of structural parameters of NP superlattices such as interparticle distance (Figure 5) during compression and release. This provides unique robustness for in situ and reversible interrogation of both chemical and physical coupling interactions that depend on interparticle separation in NP assemblies, which has not normally been feasible for top-down and bottom-up fabrication methods.^{Reference Lu, Liu and Lee51} We observed tunable SPR of Ag NPs and nanowires depending upon the external compression and release of pressure (Figure 8).^{Reference Li, Wen, Li, Wang, Clem and Fan39} At ambient pressure, the UV-vis spectrum shows a single peak at 518 nm corresponding to the Ag NP assemblies. Significant redshifts ranging from 518 nm to 548 nm were observed upon increasing pressure. Based on the TEM studies, the Ag NPs are not consolidated below 8 GPa. Therefore, we infer that the redshift is due to particle interactions (or SPR) as the interparticle distance decreases upon increasing the external pressure (Figure 8b). We observed that below 8 GPa, SPR spectra shifted back to the original peak position when the pressure was completely released. Thus, the SPR can be reversibly tuned through application of external pressure. The fact that the sizes of the NPs and the organic chains, and the dielectric environment, all remain unchanged throughout the course of these PDA experiments, together with the apparent reversibility of the shifts, indicates that the large observed surface plasmon band shifts are a consequence of strong collective behavior.

Figure 8.

Pressure-induced optical coupling: structure–property correlation of Ag NPs (nanoparticles) superlattice during compression (black) and release (blue). (a) In situ ultraviolet-visible spectra of thin films of ordered Ag NPs superlattice. (b) Schematic of NP assembly and consolidation under high pressure: (1) Below 8 GPa, NP superlattices shrink and spring back reversibly; (2) at the threshold pressure of 8 GPa, NPs contact, driven by high pressure, and start to consolidate; (3) above 8 GPa, NPs consolidate and form nanorods and nanowires depending on the applied pressure. (c) Transmission electron microscope images of corresponding nanostructures at different stages of Ag NP aggregation in (b) and optical signature in (a). Note: P, pressure. Modified with permission from Reference 39. © 2014 Macmillan Publishers Limited.

Above 8 GPa, we observed a new SPR peak corresponding to the formation of nanorods and nanowires and their interactions. According to the TEM and HP-SAXS results, Ag NPs start to contact each other and consolidate into nanorods and nanowires at pressures above 8 GPa. The corresponding SPR spectra displayed a gradual blueshift from 548 nm to 523 nm, which is indicative of the gradual formation of nanorods and their growth to nanowires from 8 GPa to 15 GPa. When the pressure was released, the SPR peak redshifted to 556 nm, which we infer is consistent with the deaggregation of nanowires at decreased pressures.