Optical-Force-Dominated Directional Reshaping of Au Nanodisks in Al

Sep 5, 2018 - The asymmetric reshaping of the heterodimer is observable structurally, through electron microscopic imaging, and through changes in the...
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Optical Force-Dominated Directional Reshaping of Au Nanodisks in Al-Au Heterodimers Chao Zhang, Thejaswi Tumkur, Jian Yang, minhan lou, liangliang dong, Linan Zhou, Peter Nordlander, and Naomi J. Halas Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03033 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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Optical Force-Dominated Directional Reshaping of Au Nanodisks in Al-Au Heterodimers Chao Zhang 1, 4 //, Thejaswi Tumkur 1, 4//, Jian Yang 2, 4, Minhan Lou 1, 4, Liangliang Dong 3, 4, Linan Zhou 3, 4, Peter Nordlander 1, 2, 4, 5, and Naomi J. Halas 1, 2, 3, 4, 5 *

1 Department of Electrical and Computer Engineering, 2 Department of Physics and Astronomy, 3 Department of Chemistry, 4 Laboratory for Nanophotonics, Smalley-Curl Institute, and 5 Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States

*Corresponding author: e-mail: [email protected] // C.Z and T.T contributed equally to this work Abstract: The optical reshaping of metallic nanostructures typically requires intense laser pulses to first approach or achieve melting, followed by surface-tension-dominated reshaping, transforming the original nanostructures into more spherical morphologies. Here we report the directional optical reshaping of the Au nanodisk of an Al-Au heterodimer in the illuminated junction of an atomic force microscope (AFM). Both a heightening and repositioning of the Au nanodisk component is induced, reducing the gap between the two nanodisks. There are three contributors to this process: a photothermal softening of the Au lattice, the optical force applied to the Au nanodisk by the Al nanodisk, and also the optical force from the nearby AFM tip. The asymmetric reshaping of the heterodimer is observable structurally, through electron microscopic imaging, and through changes in the heterodimer optical response. This optical force-directed shape manipulation may have potential applications in nanofabrication, optically-induced nanomanufacturing, sensing, and quality control. Keywords: optical force, reshaping, plasmonics, dimer, heterodimer

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Light-matter interactions at the nanoscale have led to a rich abundance of applications in areas as wide-ranging as optoelectronics, sensing, communications, data storage and energy harvesting.1-4 It is well known that the size, morphology and arrangement of the individual components of a nanophotonic system determines its overall optical response. Consequently, a significant number of studies have reported efforts towards fabricating or actively modifying the morphology of nanostructures using controlled illumination.5,

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The light-induced reconfiguration of

nanostructures has been increasingly utilized to achieve reversible morphological changes, such as thermal and photo-induced phase transitions in transition metal oxides under femtosecond pulse illumination,7 and irreversible tuning through photopolymerization,8 photothermal reshaping,9-12 welding13, 14 and fragmentation.15, 16 Photothermal processes usually require high input pulse intensities

12, 16, 17

(> 1 mJ/cm2) to melt and subsequently reshape nanostructures – a process

governed by strong Van der Waals forces between nanoparticle and substrate, as well as additional factors such as surface tension. As a result, nanostructures typically tend to reshape into more spherical morphologies in order to minimize their surface energy, limiting the extent of optical control over shape, directionality and degree of reshaping.

Optical forces in photoexcited nanoscale systems can be used to control and manipulate nanoparticles, with demonstrations of optical trapping,18 optomechanical actuation,19 and terahertz-induced optical migration.20 Plasmonic systems, in particular those consisting of strongly coupled metallic nanoparticles, can give rise to very large local electromagnetic field enhancements upon illumination, resulting in strong local optical forces.21-25 When the individual nanostructures of a coupled plasmonic system are asymmetric in structure or composition, their plasmon mode hybridization and consequently their optical force distribution becomes more 2 ACS Paragon Plus Environment

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complex.26-28

We recently utilized photoinduced force microscopy (PIFM) to image the

photoinduced forces between an atomic force microscope (AFM) tip and plasmonic nanorods, homodimers, and heterodimers, where strong correlations between local electromagnetic field enhancements and optical forces are observed.29, 30 These studies have shown that the optical forces between a plasmonic nanostructure and a tip can be very strong. Previous studies have shown that the optical forces between individual nanoparticles in a plasmonic dimer also can be very strong, 21-24

and both can exceed interparticle Van der Waals forces. Optical forces of this magnitude

should make reconfiguration or reshaping of plasmonic nanostructures possible.

Here we report the laser-induced directional reshaping of Au nanodisks that are components of bimetallic Al-Au nanodisk heterodimers (Fig. 1). Al-Au nanodisk heterodimers are chosen for this experiment because Al and Au nanodisks are different not only optically, but also thermally and mechanically, allowing us to investigate how the reshaping depends on the wavelength and polarization of the excitation, and the materials of the nanodisks. A conceptual schematic of the reshaping that occurs is shown in Figure 1a. When the entire heterodimer-tip system is illuminated, the Au nanodisk is both reshaped and repositioned closer to the Al nanodisk, shrinking the interparticle distance. Under illumination by laser pulses of moderate intensities (30 ~ 234 µJ/cm2), a very noticeable reshaping of the Au nanodisks in the heterodimer is observed, even though the temperature induced by photothermal heating is well below the melting point of Au.

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Fig. 1. (a) Schematic of laser-induced reshaping of Al-Au nanodisk heterodimers. (b) Calculated absorption cross section of a Au nanodisk in the heterodimer when excited by longitudinally (solid red line) or transversely (dotted red line) polarized light. Calculated absorption cross section of a Au nanodisk monomer of the same size (black line). (c, d) Scanning electron microscope (SEM) images of Al-Au nanodisk heterodimers before (left) and after (right) illumination with (c) 500 nm (178 µJ/cm 2) and (d) 600 nm (153 µJ/cm2) laser pulses with longitudinal polarization. The Al nanodisks are on the left and Au nanodisks on the right. (e) SEM images of Al-Au nanodisk heterodimers before (upper) and after (lower) illumination by 600 nm laser pulses with longitudinal polarization of a higher intensity (234 µJ/cm2). Most of the Au nanodisks become completely deformed, and merge entirely on top of the Al nanodisks. Scale bar: 100 nm for all SEM images.

Al-Au nanodisk heterodimers (Al nanodisk diameter = 99 ± 8.9 nm, Au nanodisk diameter = 65 ± 9.6 nm, gap = 8.4 ± 6.5 nm) were fabricated on a glass coverslip using hole-mask colloidal lithography.26, 30, 31 The sizes of the nanodisks were chosen to have resonances accessible by our light source. Such an asymmetric heterodimer structure can be conceptualized as a simple “forced plasmon” system, where two dissimilar plasmonic nanoparticles couple to each other through nearfield interactions. The interaction is highly dispersive: the higher energy Al plasmon will force an out-of-phase higher energy plasmon oscillation in the Au disk and the lower energy dipolar mode of the Au will drive an in-phase dipolar oscillation in the Al disk.29, 30 Here we extend these studies by examining how the interaction between the Al and the Au nanodisk change the optical 4 ACS Paragon Plus Environment

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absorption in the Au nanodisk (Fig. 1b). The optical absorption in the Au nanodisk is enhanced not only at its own resonance wavelength (~600 nm, in-phase bonding mode), but also at the resonance wavelength range of the Al nanodisk (~400-500 nm, out-of-phase anti-bonding mode) (Fig. 1b, red solid lines). This is a clear example of the optical properties of the antenna-reactor effect, recently demonstrated for plasmonic photocatalysts,

26, 32

where the antenna nanoparticle

(Al) induces an additional optical absorption feature in a directly adjacent, reactor nanoparticle (Au). This modified absorption spectrum of the Au nanodisk is strongest when the heterodimer is excited with light polarized along the dimer axis, due to the strong interparticle coupling induced in this manner and much weaker for transverse incident polarized light. Due to the large cross section of the Al disk, for transverse polarization the absorption in the Au nanodisk in the heterodimer (Fig. 1b, red dotted line) is smaller than that of an individual Au monomer (Fig. 1b, black line).

Laser reshaping of the Al-Au heterodimers was performed using a customized AFM (Vistascope Molecular Vista, Inc.). An AFM cantilever with a silicon tip (radius ~10 nm, TAP300GD-G, Budget Sensors) was placed on top of the heterodimer sample with an average tip-sample distance of 20 nm. Laser illumination was achieved from the backside of the glass substrate through an oil immersion objective (NA = 1.4, PlanApo, Olympus). Incident light was focused to a diffractionlimited spot on the top surface of the substrate right underneath the AFM tip in a confocal configuration. Unpolarized, broadband output of a supercontinuum laser (SuperK Extreme, NKT Photonics, pulse duration: 10 ps, repetition rate: 78 MHz) was used as the light source and the excitation wavelength was selected using a variable bandpass filter with a bandwidth of 10 nm. A linear polarizer was used to control the incident polarization prior to entering the objective. For 5 ACS Paragon Plus Environment

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each illumination condition, a 5-by-5 µm2 area containing ~ 50 heterodimers was raster scanned through the light spot under the AFM tip at a speed of 1.8 µm/sec.

After illuminating the heterodimers with laser pulses of moderate intensities at both 500 nm and 600 nm wavelengths (178 µJ/cm2 for 500 nm and 153 µJ/cm2 for 600 nm, respectively) with longitudinal polarization, we observe a very noticeable reshaping of the Au nanodisks (Fig. 1c1d). Scanning electron microscope (SEM) images of the Al-Au nanodisk heterodimers were taken from top and at high angle (75o from surface normal) before and after illumination to characterize the reshaping. As shown in the representative SEM images in Fig. 1c and 1d, prior to illumination, the pristine Al and Au nanodisks have truncated cone shapes with flat top surfaces and inclined sidewalls. After illumination, the Au nanodisks are reshaped to taller nanocones with smaller base diameters (see Fig. S1 and Fig. S2 for full-scale SEM images). The Au nanodisks also move towards the adjacent Al nanodisks, narrowing the dimer gaps (Fig. S2). When illuminating the AlAu nanodisk heterodimers with 600 nm laser pulses of higher intensity (234 µJ/cm2), the reshaping of the Au nanodisks becomes even more dramatic: of the Au nanodisks become completely deformed, and most merge entirely with the Al nanodisks, sitting on top the originally adjacent nanostructures (Fig. 1e, Fig. S3). In stark contrast, the size and shape of the Al nanodisks remain virtually unchanged for illumination in this intensity range. As a control experiment, we illuminated the Al-Au nanodisk heterodimers using 500 and 600 nm laser pulses at transverse polarization. The reshaping was negligible compared with the longitudinal polarization case (Fig. S4). Neither did off-resonance excitation at λ = 760 nm (169 µJ/cm2, longitudinal polarization) induce any reshaping (Fig. S5). We also performed the same experiments on Al-Al nanodisk dimers and Au-Au nanodisk dimers. The Al-Al dimers did not show any reshaping under 520 nm 6 ACS Paragon Plus Environment

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illumination, while both Au nanodisks reshaped in the Au-Au dimers under 600 nm illumination, both with longitudinal polarization (Fig. S6).

Figure 2. (a) The calculated wavelength-dependent temperature increase of a Au nanodisk (red) and an Al nanodisk (blue) in an Al-Au heterodimer (Al nanodisk base diameter = 99 nm, Au nanodisk base diameter = 65 nm, gap at base = 8.4 nm. For both nanodisks, height = 35 nm, side wall angle = 75o) on a glass substrate, when excited by a laser pulse of intensity 167 µJ/cm2, polarized along the interparticle axis. The vertical dashed lines indicate the excitation wavelengths of 500, 600, and 760 nm. (b) Optical force distribution on the surface of the Au nanodisk component of an Al-Au heterodimer, when illuminated at 500, 600, and 760 nm with longitudinal polarization, in an illuminated AFM junction geometry consisting of a Si tip (tip radius = 10 nm) positioned 10 nm above the Au nanodisk. The force vectors are depicted by arrows, with length proportional to the optical force vector magnitude. In all cases the largest optical force is directed toward the Al nanodisk; the strongest optical force occurs for photoexcitation at 600 nm wavelength. (c) Calculated deformation of the Au nanodisk in an Al-Au heterodimer (arbitrary scale) when illuminated at 500 nm, 600 nm, and 760 nm for longitudinal polarization. Only part of the Al nanodisks (gray) are shown in (b) and (c) for simplicity. A 3-nm bottom layer of the Au nanodisk was fixed in this calculation, to account for adhesion of the Au nanodisk to the substrate.

Numerical simulations of Al-Au heterodimers in an AFM junction geometry were performed to provide theoretical insight into the mechanism of the observed reshaping (Fig. 2). To calculate the temperature increase, a longitudinally polarized Gaussian beam was used as the excitation source for consistency with the experiment. The pulse energy was held constant at 167 µJ/cm2 for all

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wavelengths, corresponding to experimental pulse intensity values (Fig. 1c, d). The temperature increase of the Al nanodisk and the Au nanodisk induced by a single laser pulse was evaluated as the energy absorbed by the respective nanodisk divided by its heat capacity.33 The temperature increase for the Al nanodisk in the heterodimer is very small, below 30 K, because Al nanodisk has a small absorption cross section and large heat capacity. Also the Al nanodisk is covered with a rigid native oxide shell which prohibits its reshaping. For the Au nanodisk in the heterodimer, the peak temperature was determined to be nominally 300 oC when excited at 600 nm and 140 oC when excited at 500 nm (Fig. 2a, red line). These temperatures are well below the bulk melting temperature of Au (1064oC), however, the higher temperature begins to approach the depressed melting temperature range observed for multicrystalline Au nanostructures.34, 35 It is therefore quite plausible that the Au nanodisks were only slightly softened at these photothermally achieved temperatures, unlikely to undergo thermally-driven reshaping.

However, to further test the

possibility of thermal reshaping, the thermal stability of Al-Au heterodimers were tested by heating samples of the nanostructures to 400 oC and 700 oC in a furnace under dark conditions. At each temperature the heterodimers were heated for 1.5 min, corresponding to the total illumination time for one heterodimer in the laser reshaping experiment. The Au nanodisks showed no significant morphological changes when heated to 400 oC, but reshaped into hemispheres at 700 oC – a reshaping induced by surface tension due to the surface melting of Au at 700 oC (Fig. S7).36

Since the temperature of the Au nanodisks under illumination remains below the threshold for thermal reshaping, the optical forces induced by the nanodisk junction and the AFM tip junction geometry appear to play the predominant roles in the reshaping process. The optical force distribution on the surface of the Au nanodisk within the experimental geometry, under 8 ACS Paragon Plus Environment

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longitudinally polarized illumination, was calculated (Fig. 2b). The induced charge and currents on the Au surface were first calculated based on Maxwell equations using the Finite Element Method (FEM), then the optical forces were determined by the Lorentz force equation. A Si AFM tip with a radius of 10 nm was placed above the Au nanodisk. The tip-sample distance in the simulations was assumed to be 10 nm, smaller than the experimental average distance of 20 nm, since the tip is vertically oscillated in the experiment and positioned statically in the theoretical model. To be consistent with the use of a large NA objective in the experiment, the optical force distribution was calculated using different excitation directions. The contributions of all incident angles within the entire excitation cone were integrated to obtain the total optical force distribution. The strongest in-plane optical force was observed at the excitation wavelength of 600 nm, and is due to the resonant near-field coupling between the two nanodisks. When the heterodimer resonates in its in-phase bonding mode, the transient charge densities of the two nanodisks near the gap are always of opposite signs within one optical cycle,30 giving rise to this strong attractive force. The upward optical forces on the top surface of the Au nanodisk are mostly induced by the coupling between the Au nanodisk and the AFM tip. The upward and in-plane attractive optical forces experienced by the Au nanodisk are on the order of 50 pN. It was previously suggested that optical forces of similar or smaller magnitude can overcome surface tension forces to bend Au nanorods37 or reshape Au nanospheres9. Although these experiments were performed on melted or nearly melted Au nanoparticles, it was also suggested that the surface tension of solid Au is only slightly larger than that of liquid gold38. Therefore we believe that the optical forces in our experiment are capable of reshaping the Au nanodisks. The optical force field was then imported into a mechanical model based on FEM to simulate the reshaping. For each excitation wavelength, the static elastic deformation of the Au nanodisks was calculated so that the restoring force

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balanced the optical force applied at the surface, assuming continuous illumination. A 3-nm layer at the bottom of the Au nanodisk was fixed in the calculation to account for adhesion of the nanodisk to the substrate. For all three excitation wavelengths, an upward reshaping of the Au nanodisk is observed, with the most drastic reshaping induced by λ = 600 nm illumination. This is qualitatively consistent with our experiments. We note that the same Young’s modulus of Au was used in the simulations for all three excitation wavelengths. However, since Young’s modulus depends on temperature, we expect the magnitude of the reshaping of the Au nanodisks to be stronger for 600 nm excitation than for 500 nm and much stronger than for 760 nm excitation.

To quantitatively analyze the reshaping, we measured the base diameter and height of the Al and Au nanodisks in the SEM images obtained before and after illumination with 500, 600, and 760 nm laser pulses. Approximately 20 heterodimers were analyzed for each excitation wavelength. The correlated morphological changes of each nanodisk, plotted as the change in height (Δ Height) against the change in diameter (Δ Diameter) induced by illumination, are shown in Fig. 3a-c. For 500 and 600 nm excitation, most Au disks (red squares) showed positive Δ Height and negative Δ Diameter, consistent with our qualitative observation. On average, the reshaping for  = 600 nm excitation was observed to be more dramatic than that induced by  = 500 nm, most likely due to the combination of higher photothermally-induced nanodisk temperature and stronger optical forces applied by the adjacent Al nanodisk at 600 nm. For  = 760 nm photoexcitation, no obvious reshaping was observed. Although the magnitude of the optical force at  = 760 nm appears to be comparable with that at  = 500 nm (Fig. 2b), the temperature increase of the Au nanodisk for λ = 760 nm illumination is negligible (< 10 K): from this, we infer that optical forces of this magnitude applied to a rigid nanodisk are insufficient to induce noticeable reshaping. For all excitation 10 ACS Paragon Plus Environment

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wavelengths the Al nanodisks (Fig. 3a-3c, blue circles) remained unchanged. This is most likely due to the negligible temperature increase of the Al nanodisks at any of the photoexcitation wavelengths, and the presence of a rigid aluminum oxide layer around the surface.

Figure 3. (a-c) Correlated change in diameter and height for Au disks (red squares) and Al disks (blue circles) in Al-Au heterodimers, induced by laser illumination at (a) 500 nm, (b) 600 nm, and (c) 760 nm. (d) Degree of reshaping (DoR) of Au nanodisks in Al-Au heterodimers with different initial gap sizes, illuminated by 500 nm (178 µJ/cm2, green diamonds), 600 nm (153 µJ/cm2, orange squares), and 760 nm (169 µJ/cm2, red circles) laser pulses. Solid lines are linear fits of the experimental data for each excitation wavelength.

At longitudinal polarization, the strength of the near field coupling depends critically on the interparticle gap size. 21, 23 The small variations in gap size of the nanodisk heterodimers fabricated for this experiment enabled us to investigate how the initial gap size affects the reshaping process. We define the Degree of Reshaping of the Au disk as DoR = √(∆𝐻𝑒𝑖𝑔ℎ𝑡)2 + (∆𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟)2 to provide a convenient metric for analyzing this contribution. For each excitation wavelength, the 11 ACS Paragon Plus Environment

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average DoRs of the heterodimers with the same initial gap size were calculated (Fig. 3d). For both 500 and 600 nm excitation, the average DoR is larger for smaller initial gap size, in other words, the heterodimers with smaller initial gap widths showed greater reshaping. This is in agreement with the forced plasmon picture of the Al-Au heterodimer: as the gap size of the heterodimer is decreased, the near field coupling between the two disks increases. As a result, the induced optical absorption, the photothermal temperature increase, and the amplitude of the optical force experienced by the Au nanodisk increases with decreasing gap size, resulting in larger DoRs.

Figure 4. Dark-field scattering spectra of Al-Au nanodisk heterodimers after illumination. (a) Experimental ensemble darkfield scattering spectra of Al-Au heterodimers before illumination (gray line) and after illuminated by: 500 nm (green lines) or 600 nm (orange lines) laser pulses at longitudinal (solid lines) or transverse (dashed lines) polarizations. (b) High-angle SEM images of Al-Au nanodisk heterodimers after illumination, corresponding to the spectra in (a). (c) Calculated scattering cross sections of heterodimers with dimensions obtained from (b). Dashed vertical lines in (a) and (c) correspond to the center of resonance peak of pristine Al-Au heterodimers. The laser intensities for each illumination were adjusted individually so that the Au nanodisks in each group had a similar temperature increase of ~ 50 K. Scale bar: 100 nm for all SEM images.

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Dark-field scattering spectroscopy has been used extensively to probe the optical properties of plasmonic dimers, due to their high sensitivity to nanometer-scale structural changes, particularly in geometries with nanometer-scale gap widths.39-42 We measured the scattering spectra of Al-Au heterodimers before and after reshaping using a dark-field microscope coupled to a spectrometer. Four groups of Al-Au heterodimers with similar initial geometries were illuminated with  = 500 nm and 600 nm laser pulses at longitudinal and transverse polarization, respectively. For each group, the laser intensity was carefully adjusted to be inversely proportional to the absorption cross section of the Au nanodisk of the heterodimer. Therefore, the light energy absorbed by the Au nanodisk and thus the peak temperature were the same for all four heterodimer groups studied (the average temperature increase was ≈ 50 K). In this way, the role of optical forces could be isolated from the photothermal contributions that otherwise could affect the nanoparticle reshaping process. The scattering spectrum of pristine Al-Au heterodimers (Fig. 4b, black box) obtained using incident light with longitudinal polarization, shows a peak at 600 nm, corresponding to the Aunanodisk-dominated bonding mode (Fig. 4a, black line). For heterodimers illuminated with λ = 500 and 600 nm laser pulses at longitudinal polarization, the Au nanodisks became taller in height and smaller in base diameter, consistent with our earlier observations (Fig. 4b, Fig. S8, green and orange solid boxes). As a result, the scattering peak blueshifted after reshaping (Fig. 4a, green and orange solid lines). For transverse excitations, no noticeable shift was observed in the scattering peaks compared to the pristine heterodimers (Fig. 4a, dashed lines) and no reshaping was observed in the SEM images (Fig. 4b, Fig. S8, dashed boxes). Although the Au nanodisks were heated to the same temperature for all four groups, near-field coupling between the two nanodisks at longitudinal polarization induced much stronger optical forces, resulting in Au nanodisk reshaping.

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The scattering spectra of Al-Au heterodimers calculated by FDTD simulations with geometries obtained from the respective SEM images agree very well with the experimental findings (Fig. 4c).

To isolate the contribution of the AFM tip to the reshaping process we performed control experiments by illuminating the Al-Au heterodimer structures while maintaining the AFM tip significantly far (~ 500 µm) away from the heterostructure. For optical excitation at  = 600 nm, reshaping of the Au nanodisk was still observed, but to a lesser degree than the tip-engaged case (Fig. S9). Numerical simulations of the optical force distribution within the nanodisk heterostructure without the presence of the AFM tip shows a much weaker upward force on the top surface of the Au nanodisk (Fig. S10). This suggests that the near-field interaction between the sample and the tip provides an additional out-of-plane contribution to the optical force that enhances the reshaping of the Au nanodisk in the upward direction. To examine whether the directionality of reshaping of the Au nanodisk in the heterodimer was affected by the scanning direction of the AFM cantilever, the direction of raster scanning was rotated to be perpendicular to the heterodimer axis. The reshaping of the Au disk was similar to the case when the tip was scanned parallel to the heterodimer axis, suggesting that the reshaping process is essentially unaffected by tip scanning direction (Fig. S11). Because a pulsed laser was used as the excitation source, the directional reshaping only occurred during the laser illumination. The movement of the AFM tip within the actual pulse duration is negligible when compared to the dimensions of the heterodimers, consistent with our observations that the reshaping process is independent of scanning direction.

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In conclusion, we observe that under optical illumination, nanodisk heterodimers can undergo a highly unusual asymmetric reshaping process. In the case of Al-Au nanodisk heterodimers, only the Au nanodisk becomes both substantially reshaped and repositioned, narrowing the gap size of the heterodimer. This material- and direction-specific reshaping is due to several factors. An increased optical absorption in the Au nanodisk by near-field coupling to the Al nanodisk results in a sizeable difference in laser-induced heating of the Au nanostructure, contributing in part to its preferential reshaping. However the primary contribution is due to the optical forces induced by the nanometer scale gap between the Al and the Au nanodisks. Other aspects, such as the presence of an AFM tip in a photoinduced force microscope geometry, as well as the specific gap dimensions, also contribute to the magnitude of the reshaping and repositioning process. The combined effects of localized photothermal heating and strong optical forces shown to contribute to this effect could also be used in nanofabrication processes more generally, as a contact-free method for reshaping nanostructures. Since this nanoscale reshaping also narrows the interparticle gaps which results in larger junction fields, it is an effect that could be exploited in developing nanoscale sensors for chemical and biological sensing. Supporting Information Available: Details of the fabrication process, full-scale SEM images of the heterodimers analyzed in the paper, method of analyzing heterodimer SEM images, calculations of the absorption cross sections, optical forces, and darkfield scattering spectra are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org/. ACKNOWLEDGMENT: We gratefully acknowledge support from the Robert A. Welch Foundation, C-1220 (N.J.H.) and C-1222 (P.N.), the National Science Foundation Grant ECCS1610229, and the Air Force Office of Scientific Research Multidisciplinary Research Program of 15 ACS Paragon Plus Environment

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the University Research Initiative (AFOSR MURI FA9550-15-1-0022). T. T. acknowledges support from the Smalley Institute at Rice University through the J. Evans Attwell-Welch postdoctoral fellowship. J.Y acknowledges Dr. Alessandro Alabastri for helpful discussions regarding numerical simulations. C.Z acknowledges Professor Kevin F. Kelly for helpful discussions regarding processing heterodimer SEM images, and Professor Stephan Link for helpful discussion regarding the reshaping mechanism.

Notes The authors declare no competing financial interest.

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