Ag nanoparticle array before mechanical compression

Ag nanoparticles were synthesized through a thermal decomposition method using myristic acid as surface ligand6. Figure 1 shows a representative TEM image and size distribution of the Ag nanoparticles used in this study. The Ag nanoparticles are fairly monodisperse with an average diameter of 5.5 nm and a narrow size distribution (Fig. 1a, inset). The HR-TEM image in Fig. 1b shows that these nanoparticles are crystalline with faceted surfaces. The Ag nanoparticles display five-fold twinning morphology with surface termination of (111) facets.

Figure 1: Representative electron microscopy images of monodispersive silver nanoparticles used in the stress-induced sintering processes. (a) TEM image of the monodispersive silver nanoparticles. Inset shows the size distribution of the silver nanoparticles. (b) A representative HR-TEM image of the silver nanoparticles with [110] orientation. (c) A typical HP-SAXS image of the ordered Ag nanoparticle arrays loaded in DAC at ambient pressure. (d) The SAXS pattern integrated from c with fitted peaks. The theoretical fitting curve (red-dashed line) further confirms fcc symmetry. The solid blue curves present the Bragg reflections of each diffraction plane specified by Miller indices (hkl) that are characteristic of an fcc mesophase with space group of Fm m (the calculated unit cell parameter a=120.5 Å). The relative weak intensity of (220) peak is due to the preferred orientation along [110] direction. (e) Representative plan-view SEM image of an ordered Ag nanoparticle array. Inset shows the Fast-Fourier transform from the image in (e). The scale bars in (a), (b) and (e) represent 50, 5 and 100 nm, respectively. Full size image

The measured lattice fringe 0.234 nm corresponds to a (111) spacing. Through a solvent evaporation process, described in the Methods section, these nanoparticles self-assembled into highly ordered superlattices. Figure 1c,d show the SAXS patterns of ordered nanoparticle arrays loaded in a DAC without pressurization. The SAXS image and integrated pattern collected at ambient pressure indicate a superlattice structure specific to a face-centred-cubic (fcc) mesophase with space group Fm m. The nanoparticle assembly exhibits a preferred orientation with (110) planes parallel to the substrate, which is normal to incident X-rays after mounting the sample-loaded DAC on the stage. The unit cell parameter a is calculated to be 120.5 Å. Figure 1e shows a representative plan-view scanning electron microscopy (SEM) image, confirming the [110] orientation. Fast-Fourier transform (Fig. 1e, inset) analyses confirm that Ag nanoparticles are organized in a periodically ordered fcc mesophase with a preferred [110] orientation.

Stress-induced formation of Ag nanowires

The pressure in DAC was monitored using the pressure-dependent ruby fluorescent method (ruby R 1 line)17. HP-SAXS and HP-WAXS measurements, in combination with TEM studies, were carried out on Ag nanoparticle arrays over a pressure range of 0–15 GPa. Figure 2a,b show the representative TEM images of 1D Ag nanostructures after pressure was released to ambient (amb) conditions. The resulting Ag nanowire array consists of individual Ag nanowires with a 2D hexagonal (hex) packing. These nanowire arrays were collected from the DAC and then dissolved in organic solvents such as toluene to form stable colloidal suspensions. The wires produced were uniform in diameter and length, and had an average diameter of 7.6 nm with an s.d. of 10%. HR-TEM imaging (Fig. 2b,c,f) reveals that the Ag nanowires are polycrystalline with an apparent twinning feature. Along the c axis, each nanowire consists of crystalline nanodomains that have domain sizes close to that of the original spherical Ag nanoparticles. This suggests that the Ag nanowires were developed through direct sintering of spherical Ag nanoparticles along the c axis. Detailed HR-TEM studies confirm that Ag nanoparticles are sintered through specific (111) to (111) facet surface interactions (Fig. 2c,f). Owing to such oriented sintering, the final Ag nanowires display a typical zigzag surface morphology, as proposed in Fig. 2e. We believe the increase of the nanowire diameter (7.6 nm) in comparison with the original nanoparticle (5.5 nm) is likely to be caused by uniaxial compression along the nanowire c axis, as proposed in the subsequent discussion section.

Figure 2: Representative TEM images of Ag nanostructures synthesized under high-pressure-induced mechanical sintering of spherical Ag nanoparticle arrays. (a) A bundle of Ag nanowires. (b) HR-TEM image of reassembled nanowires after solvent evaporation. (c) HR-TEM image of a single Ag nanowire showing the zigzag morphology and twinning microstructure. Red circles are used for guidance of viewing individual crystal domains made up of Ag nanoparticles. Solid circles show the images close to the view direction, while dash circles show those further away from the view direction. (d) Ag nanorods. (e) Schematic of five-fold twinning microstructures and oriented sintering through (111) attachment evident in the HR-TEM image in f. (f) HR-TEM image of a single Ag nanowire showing oriented crystal sintering. Scale bars, 100, 20, 5, 20 and 5 nm (a–f, respectively). Full size image

Stress-induced assembly of Ag nanoparticles and nanowires

Integrated HP-SAXS and HP-WAXS patterns of Ag nanoparticle arrays provide direct experimental evidence about how increasing pressure governs nanoparticle assembly towards an ultimate formation of Ag nanowires. Figure 3 shows the complex structural evolution of nanoparticle arrays during high-pressure-induced assembly and sintering processes. We observed two pressure-directed phase processes at distinct length scales, separated by a threshold pressure of ~

8 GPa. Below 8 GPa, we observed a mesoscale pressure-directed assembly of Ag nanoparticle arrays. From ambient pressure to 8 GPa, the HP-SAXS patterns are consistent with the indexed fcc mesophase. TEM studies revealed that Ag nanoparticles were not sintered and still maintained the original spherical shape. Gradual shift of all the SAXS peaks to higher 2θ (or lower d-spacing) (Fig. 3a,b) suggests a pressure-induced shrinkage of the nanoparticle superlattice. When pressure was gradually released from 8 GPa to ambient pressure, all the HP-SAXS peaks shifted back to their original positions, which indicates that the change of Ag superlattice dimension is reversible between atmospheric pressure and 8 GPa. This reversible behaviour was further verified by the d-spacing changes (Fig. 3c) and the constant d-spacing ratio R at varied pressure (Fig. 3d). Below 8 GPa, R remained the same and close to the theoretical value R (for example, the ratio of d-spacing of the second to the first major peak)= for an fcc superlattice. The lattice constant a reduced from 120.5 Å at ambient pressure to 112.3 Å at 7 GPa and returned back to 120.5 Å when pressure was completely released. In this case, the interparticle separation distance along the [110] direction can be reversibly tuned between 85.2 and 79.4 Å. This reversible behaviour allows in situ investigation of pressure-tuned, reversible Ag nanoparticle SPR behaviour versus interparticle spacing.

Figure 3: Structural evolution of Ag nanoparticle arrays during high-pressure-induced assembly and sintering processes. (a) Representative HP-SAXS images of Ag nanoparticle arrays collected at ambient pressure (Amb or 0 GPa), 7 GPa, 9.12 GPa and 14.58 GPa. (b) The integrated spectra from the HP-SAXS images at varied pressures. (c) The d-spacing changes of the first Bragg reflection along with the pressure in each HP-SAXS spectrum in b. (d) d-spacing ratio R at different pressures. Full size image

Stress-induced structural rearrangement

A second phase, structural rearrangement process, which starts at 8 GPa and finishes at 15 GPa, is consistent with a phase transformation of the nanoparticle array and resulting uniaxial particle consolidation. In the intermediate stage from 8 to 15 GPa, the overall particle lattice was distorted and could not be simply indexed in either an fcc or a 2D hexagonal mesophase. R varied between 0.522 and 0.5. After release of the applied pressure, the HP-SAXS patterns were consistent with a 2D hexagonal mesophase, in which R=(d 20 /d 10 ) becomes ~

0.5, which is both equal to the theoretical ratio d 20 /d 10 for a 2D hexagonal mesophase, and consistent with the HP-SAXS indexing and TEM results.

During the second phase transformation, we observed a reversible atomic structural distortion within the Ag nanoparticles. At ambient pressure, the atomic lattice of Ag nanoparticles exhibited a cubic crystal microstructure. On increase of pressure to 14.28 GPa, all WAXS peaks shifted to higher angle. This pressure-induced decrease of d-spacing indicates an apparent shrinkage of lattice dimension (Fig. 4a). After release of pressure back to ambient pressure, all the HP-WAXS peaks shifted back to the original positions. Thus, the overall phase transformation process was reversible. This was also confirmed by the fact that both the increase and decrease of d 111 spacing in the pressure circle follow the same linear pathway (Fig. 4b). It is interesting to note that after compression, the WAXS patterns became much more intense (Fig. 4c), indicative of increased number of atomic planes or crystalline coherence. This observation suggests that high-pressure compression may refine the Ag microstructure.

Figure 4: Microstructural evolution of Ag nanoparticles (atomic lattice) during high-pressure-induced assembly and sintering processes. (a) Representative HP-WAXS patterns collected at various pressures. (b) The change of d 111 spacing as a function of pressure. (c) Integrated HP-WAXS patterns of Ag nanoparticles before (black) and after (red) the pressure was released to ambient condition. Full size image

Mechanical property

As discussed here and in previous work12,13,14,15,16,18, high-pressure stress can effectively induce Ag microstructure phase transformations and potentially manipulate the mechanical stability of Ag nanoparticles. Gu et al.12 found an unexpected high stiffness of Ag and gold (Au) nanoparticles under high pressure. In our studies, we found that Ag nanoparticles exhibit a threshold pressure of 8 GPa. We used the Vinet (equation (1)) and the third-order Birch–Murnaghan (B–M) equation of states (equation (2)) to extract the effective bulk moduli for Ag nanoparticles. In the equations, B 0 is the ambient pressure bulk modulus, B 0 ′ is the pressure derivative of the bulk modulus (B 0 ′=dB 0 /dP), V is the unit cell volume under pressure and V 0 is the initial unit cell volume at ambient pressure.

Based on the experimental compression curves (volume versus pressure in Fig. 5), the fitted bulk modulus of Ag is 57.1 GPa from the Vinet equation of state (equation (1)) and 44.2 GPa from the third-order B–M equation of state (equation (2)).

Figure 5: Calculation of bulk modulus. Experimental compression curve for Ag (volume as a function of pressure) and the calculated bulk modules from fitting of Birch–Murnaghan and Vinet’s equation of state. Full size image

Stress-induced optical coupling

As discussed above, the high-pressure compression allows reversible manipulation of structural parameters of nanoparticle superlattices such as interparticle distance (Fig. 3) 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 distance in nanoparticle assemblies, which has not normally been feasible for top-down and bottom-up fabrication methods19. We observed a tunable SPR of Ag nanoparticles and nanowires depending on external compression and release of pressure (Fig. 6). At ambient pressure, the ultraviolet–visible spectrum shows a single peak at 518 nm corresponding to the Ag nanoparticle assemblies. Significant red shifts from 518 to 548 nm were observed on increasing pressure. Based on the TEM and HP-SAXS results (Fig. 3c,d), the Ag nanoparticles are not sintered before 8 GPa. Thus, we infer the red shift is due to inter-nanoparticle interactions (or SPR) as the interparticle distance becomes smaller on increase of the external pressure (Fig. 3c). We observed that below 8 GPa, SPR spectra shifted back to the original peak position when pressure was completely released. Thus, the SPR appears to be reversibly tuned by pressure. 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 nanoparticles start to contact each other and sinter into nanorods and nanowires at pressures above 8 GPa. The corresponding SPR spectra displayed a gradual blue shift from 548 to 523 nm, indicative of the gradual formation of nanorods and their growth to nanowires from 8 to 15 GPa. When the pressure was released, the SPR peak red shifted to 556 nm, which we infer is consistent with the de-aggregation of nanowires with decreased pressure.