Direct Visualization of Photomorphic Reaction Dynamics of Plasmonic

Letter pubs.acs.org/JPCL

Cite This: J. Phys. Chem. Lett. 2018, 9, 4045−4052

Direct Visualization of Photomorphic Reaction Dynamics of Plasmonic Nanoparticles in Liquid by Four-Dimensional Electron Microscopy Xuewen Fu,*,† Bin Chen,† Caizhen Li,‡ Heng Li,† Zhi-Min Liao,*,‡ Dapeng Yu,§ and Ahmed H. Zewail†,∥

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Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, United States ‡ State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China § Institute for Quantum Science and Technology and Department of Physics, South University of Science and Technology of China (SUSTech), Shenzhen 518055, China S Supporting Information *

ABSTRACT: Liquid-cell electron microscopy (LC-EM) provides a unique approach for in situ imaging of morphology changes of nanocrystals in liquids under electron beam irradiation. However, nanoscale real-time imaging of chemical and physical reaction processes in liquids under optical stimulus is still challenging. Here, we report direct observation of photomorphic reaction dynamics of gold nanoparticles (AuNPs) in water by liquid-cell four-dimensional electron microscopy (4D-EM) with high spatiotemporal resolution. The photoinduced agglomeration, coalescence, and fusion dynamics of AuNPs at different temperatures are studied. At low laser fluences, the AuNPs show a continuous aggregation in several seconds, and the aggregate size decreases with increasing fluence. At higher fluences close to the melting threshold of AuNPs, the aggregates further coalesced into nanoplates. While at fluences far above the melting threshold, the aggregates fully fuse into bigger NPs, which is completed within tens of nanoseconds. This liquid-cell 4D-EM would also permit study of other numerical physical and chemical reaction processes in their native environments.

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and absorption spectroscopies, provided only indirect information on the NPs’ morphology including aggregation size and fractal dimension, which have limited spatiotemporal resolution.14,15 Electron microscopy imaging could provide the morphologies of the NPs through drying the solution before and after the light irradiation on a substrate; however, the dynamics of the photomorphic process of the colloidal NPs is still elusive because of the lack of real-time images of this process.16−18 Liquid-cell electron microscopy (LC-EM) provides a powerful approach to in situ image NPs in liquids, such as colloidal NP motion,19−21 NP autoaggregation,22,23 and nanocrystal growth as well as shape evolution,24−26 at the nanometer scale with the time resolution being tens of milliseconds. The imaging and control of chemical and physical processes in solution under external stimulus of electron irradiation are important for such a technique to provide a deep understanding of related underlying mechanisms. However, real-time imaging with external stimulus of light illumination is inaccessible with conventional LC-EM.

wing to the unique optical and electronic properties that are not available in either molecules or bulk counterparts, noble metallic nanoparticles (NPs), for example gold NPs (AuNPs), have attracted great interest and found vast applications in a wide range of fields such as solar cells,1,2 photocatalysis,3,4 biosensors,5,6 and drug delivery.7,8 In general, metallic NPs exhibit a surface-enhanced plasmon band as a result of intraband transitions through the photoexcitation of free conduction electrons on their surface. The surface plasmonic properties (e.g., peak position, width, and intensity) of the NPs are determined by their size and shape.9 It has been widely demonstrated that the morphology and size of the metal colloidal NPs can be significantly modified by light or laser irradiation. Because of subsequent aggregation, coalescence, and fusion of NPs, other nanostructures, including nanorods, nanobipyramids, nanoprisms, and nanodisks, can form, where the structures of the products strongly depend on the light wavelength, light intensity, and capping ligands.10−15 The control of the morphology and size of the metallic NPs by light not only provides a unique avenue of synthesis but also allows one to modulate their surface plasmon properties in a highly dynamic fashion.15 In previous studies, however, the prevailing methods for detecting the morphological change of NPs under light irradiation, e.g., time-resolved or static optical scattering © XXXX American Chemical Society

Received: April 29, 2018 Accepted: July 5, 2018 Published: July 5, 2018 4045

DOI: 10.1021/acs.jpclett.8b01360 J. Phys. Chem. Lett. 2018, 9, 4045−4052

Letter

The Journal of Physical Chemistry Letters

Figure 1. Schematic illustration of real-time imaging of photomorphic reaction dynamics of AuNPs in liquid by 4D-EM. (a) Schematic of operation mechanism of liquid-cell 4D-EM. (b) Schematic of 4D imaging of photomorphic reaction dynamics of AuNPs capped with citrate ligands in aqueous solution. For the real-time imaging of the dynamical process at long times, a sequence of fs-laser pulses (λ = 520 nm, 350 fs fwhm, 1 kHz) are used to irradiate the citrate-capped AuNPs in a liquid cell; meanwhile, a continuous e-beam with a low electron dose is utilized to image the particles. Alternatively, for the single-pulse imaging at nanosecond time scale, the AuNPs are excited using a single fs-laser pulse and a precisely timed ns-electron pulse is used to image the AuNPs at specific delay times. (c) Schematic of fs-laser pulse-induced photomorphic reaction of AuNPs in liquid with the presence of citrate ions that act as a surfactant and photoreducing agent. I: Agglomeration of AuNPs at a low laser fluence due to photodeionization of the particles. II: Pathway of coalescence, where the adjacent AuNPs coalesced with each other at a laser fluence close to the threshold fluence of AuNPs melting through a surface melting effect. III: Pathway of fusion, where the AuNPs fused into a bigger particle if the laser fluence is above the melting threshold of AuNPs. The real geometries of the coalesced nanoplates and the fused particles in experiment are shown in Figures 3d and 4a.

Liquid-cell four-dimensional electron microscopy (4D-EM) recently developed in our group27 permits the ability to in situ visualize the liquid-phase dynamical processes stimulated by laser at nanometer scale with the time resolution down to nanosecond (ns) or even picosecond (ps).28−32 Here, we report the direct visualization and analysis of the photomorphic reaction dynamics of plasmonic AuNPs in aqueous solution by the liquid-cell 4D-EM recently developed at Caltech.27 The photomorphic processes of the citratecapped AuNPs at different temperatures induced by femtosecond (fs)-laser-pulse excitation have been investigated. At low fs-laser fluences, the AuNPs show a high affinity to aggregate with their neighbors, and the final aggregate size decreases with increasing laser fluence. At high fluences close to the melting threshold of gold, the AuNP aggregates exhibit a subsequent coalescence through a surface melting effect and transform into nanoplates. At a fluence far above the melting threshold of gold, the aggregates fully fuse into bigger NPs and the fusion process completes within tens of nanoseconds. The underlying mechanisms for these reaction dynamical processes are further discussed, combined with the measurement of localized surface plasmon oscillation around the AuNPs by photon-induced near-field electron microscopy (PINEM). Our work demonstrates the real-time and real-space visualization of the photomorphic reaction of plasmonic NPs in liquid by liquid-cell 4D EM and reveals the underlying dynamical process, which is of fundamental interest for both the growing applications of plasmonic NPs and the in situ dynamical imaging techniques. We believe the unique liquid-cell 4D-EM technique in this work will also find broad applications in the study of other numerical physical and chemical reaction dynamical processes in their native environments.

The experiments were performed in our liquid-cell 4D-EM instrument, as schematically illustrated in Figure 1a. The citrate (negatively charged) capped spherical AuNPs (diameter of ∼60 nm) in aqueous solution were sandwiched between two electron-transparent, ∼20 nm thick silicon nitride membranes with the liquid thickness of several hundred nanometers (Figure 1b). The detailed protocol of the liquid-cell preparation is described in the Supporting Information and is also available elsewhere.27,33 The liquid-cell sample was then integrated to 4D-EM for measurement (Figure 1a). Generally, green fs-laser pulses (520 nm, 350 fs full-width half-maximum (fwhm) pulse duration) were utilized to trigger the photomorphic reaction of the AuNPs in the liquid-cell, and the subsequent dynamical process was imaged by precisely timed ns-electron pulses, which were generated by ns-laser ultraviolet (UV) pulses (266 nm, 10 ns fwhm pulse duration) excitation on the photocathode inside the transmission electron microscopy (TEM) instrument. For the photomorphic reaction of the AuNPs at long time scale under repetitive fslaser pulse excitation, continuous electron-beam (e-beam) imaging mode (regular TEM mode) was used, where a sequence of 1 kHz fs-laser pulses were used to irradiate the AuNPs while a continuous e-beam with a very low electron dose (lower than that of the conventional TEM mode by at least 1 order of magnitude) was presented to trace the realtime evolution of their morphology. The typical photomorphic reaction processes of the AuNPs in the presence of the citrate ions (capping ligand) occur at different temperatures, such as agglomeration, coalescence, and fusion processes, are schematically illustrated in Figure 1c. To image those dynamics at ultrafast time scale, single-pulse electron imaging mode was used to capture the transient morphologies of the AuNPs induced by a single intensive fs-laser pulse excitation. In the 4046


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Figure 2. Real-time imaging of the photoinduced agglomeration of AuNPs (diameter of ∼60 nm) in aqueous solution. (a−e) Typical snapshots of the AuNPs agglomeration under 1 kHz fs-laser pulses (fluence of 3.3 mJ/cm2) irradiation at different times (also see Movie S1). (f) Time dependence of the relative size of the particle aggregate along the x and y directions denoted in panel a.

Figure 3. Photoinduced agglomeration and coalescence of AuNPs in aqueous solution with increasing laser fluence. (a) The stabilized AuNPs in the water liquid cell before the fs-laser pulse irradiation for reference. (b−d) Images of the final stable aggregates after the agglomeration process ended under 1 kHz fs-laser pulse excitation with increasing fluence from 2.0 to 7.7 mJ/cm2 (see also Movies S2−S4). (e) Laser fluence dependence of the relative size of the particle aggregate along the x and y directions as denoted in panel a.

single-pulse imaging mode, the fs-laser pump pulse and the nselectron probe pulse were synchronized by a digital delay generator to control the time delay between them. First, we studied the morphological evolution of the AuNPs under 1 kHz fs-laser pulse irradiation with a low fluence of 3.3 mJ/cm2 (far below the melting threshold of the AuNPs) by continuous e-beam imaging mode. The reaction dynamical process was recorded by video mode of the camera, where the frame rate is 10 frames per second; that is, the exposure time for each image is 0.1 s. A set of typical snapshots of the intermediate morphologies of the AuNPs at different illumination times are presented in Figure 2a−e (see also Movie S1). Before the fs-laser pulse excitation, the AuNPs inside the liquid cell were stabilized by the Coulombic repulsion between the negatively charged citrate capping ligand and weakly bound near the substrate surface by the residual electrostatic interaction (Figure 2a).20,34 Upon the fslaser pulses irradiation, the AuNPs were activated to move and gradually aggregate with their neighboring particles (Figure 2b−e), resulting in a stable NP aggregate. In this final

aggregate, the AuNPs attached with their neighbors (Figure 2e), which is similar to the previously reported aggregation of AuNPs induced by capillary forces and solvent fluctuations in conventional LC-EM.23 In order to estimate the time scale of this process, we analyzed the size of the aggregate along two orthogonal directions (x and y noted in Figure 2a). A set of snapshots with a time interval of 1 s were extracted for Movie S1. The dimensional sizes of the AuNP aggregate along both x and y directions in each snapshot were measured and individually normalized by their initial values before laser pulse excitation. As shown in Figure 2f, upon the fs-laser pulse irradiation, the AuNPs aggregated rapidly in the first 4 s, followed by a slow shrinking process in both directions, and the entire aggregation process completed in ∼7 s. This observation is much faster than the aggregation process (several minutes) of citrate-capped AgNPs induced by continuous-wave laser with the same fluence,15 which is probably due to the different optical peak intensities and absorption efficiencies between the fs-pulsed and continuouswave lasers. 4047


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Figure 4. Photomorphic reaction dynamics of AuNPs in aqueous solution induced by a single intensive fs-laser pulse. (a) Continuous-pulse imaging of AuNPs agglomeration and fusion process under intensive laser pulse (fluence of 33 mJ/cm2) excitation shot by shot (see also Movie S5). The dashed colored circles show the particles of interest. The last column is the difference image obtained by subtracting the initial image from the final image after two laser shots, where the red and blue colored dots denote the particles before and after laser shots, respectively. (b) and (c) Single-pulse electron imaging results of fusion dynamics at nanosecond time scale for two- and three-AuNP clusters, respectively. (First column of panels b and c) Images of the AuNPs before exposure to the laser pulse, shown as a reference. (Second and third columns of panels b and c) Singlepulse images of the AuNPs before and at specific time delays after the fs-laser pulse excitation, respectively. (Last column of panels b and c) Difference images obtained by subtracting the single-pulse image before the fs-laser pulse excitation from that at the short delay. Negative and positive contrasts are indicated by blue and red, respectively.

The laser fluence dependence of the photomorphic reaction of the AuNPs was further investigated under 1 kHz fs-laser pulse irradiation with the fluence ranging from 2.0 to 7.7 mJ/ cm2 (Movies S2−S4). For these experiments, after each dynamical process completed at a lower fluence (the AuNPs formed a stable aggregate and exhibits no more change even with more irradiation time at the same fluence), the laser fluence was then increased to a higher value to trigger the resulted AuNP aggregates for observation of their further aggregation. The images of the initial AuNPs and their final aggregates at different fluences are presented in panels a and b−d of Figure 3, respectively. Before exposure to the laser pulse, the AuNPs stabilized and dispersed in the liquid cell because of the static electronic forces between the surface ligands outside the AuNPs (Figure 3a). After fs-laser pulse irradiation at 2.0 mJ/cm2 fluence, the dispersed AuNPs began to form several NP aggregates and stabilized (Figure 3b). In each aggregate, there was some space between the adjacent particles. When the fluence was increased to 4.9 mJ/cm2 (still far below the melting threshold of gold), these NP aggregates further shrank and transformed into smaller aggregates with most of the particles attaching to each other (Figure 3b), similar to the result in Figure 2e. Interestingly, when the fluence was further increased to 7.7 mJ/cm2 (close to the melting threshold), the adjacent particles in these aggregates coalesced (partially fused) with each other, and finally the aggregates transformed into nanoplates (Figure 3d). At this fluence, the estimated transient nonequilibrium temperature of the AuNPs is 750−800 K (Supporting Information),27 which is below the melting point of gold.36 Therefore, the transformation of AuNP aggregates into nanoplates occurs through

the NP surface coalescence, which will be discussed later. For a better understanding of the fluence dependence, the change of relative size of the particle aggregate along two orthogonal directions (x and y noted in Figure 3a) with increasing laser fluence is plotted in Figure 3e. Note that without laser excitation, the morphology of the AuNPs dispersed in the liquid cell shows no obvious change under the continuous ebeam irradiation in our experiment because of the very low electron dose, which was set lower than that of the conventional TEM mode by at least 1 order of magnitude. Thus, the observed aggregation and coalescence of the AuNPs are induced by the fs-laser pulse excitation. The liquid environment is important for the observed dynamical process as the suspended AuNPs in liquid could move easily under laser pulse excitation. Actually, for the dried AuNPs dispersed on silicon nitrite substrate without contacting with each other, no aggregation and coalescence can be observed under the similar laser pulse excitation in our experiment (images are not shown) because the particles were firmly bound on the substrate after the liquid was evaporated. However, if the dried AuNPs initially contact with each other, they could coalesce or fuse under laser pulse excitation with an appropriate fluence. To explore the coalescence and fusion dynamics of the AuNPs at higher temperatures above the melting point (∼1337 K) of AuNPs, we further studied the behavior of the AuNPs under fs-laser pulse excitation shot-by-shot at a high fluence of 33 mJ/cm2 by recording a movie (10 frames/s) using continuous e-beam mode (Figure 4a; see also Movie S5). This fluence is far above the melting threshold of the AuNPs.27 Prior to the fs-laser pulse, the AuNPs initially dispersed and stabilized in the liquid cell (Figure 4a, left most panel). After 4048


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Figure 5. Laser fluence dependence of localized surface plasmon field of AuNPs in aqueous solution determined by PINEM. (a) Time-dependent PINEM electron energy spectrum of AuNPs in liquid under fs-laser pulse excitation with a fluence of 2.8 mJ/cm2. (b) PINEM electron energy spectrum at t = 0 fs, which consists of discrete peaks on both the higher- and lower-energy sides of the ZLP separated by multiple photon energy quanta (∼2.4 eV). (c) Laser fluence dependence of PINEM signal of the AuNPs in aqueous solution (plot by the relative intensity of the first peak at the gain side). The relative PINEM intensity of the AuNPs linearly increases with the laser fluence in the used fluence range (see the linear fit), which implies that the localized surface plasmon field of the AuNPs linearly increases with incident laser fluence. The inset schematically shows the distribution of photon-induced oscillation charges and near electric fields near the AuNP surface, where the incident fs-laser pulse is normal to the plane, and a typical electron energy filtered PINEM image of an AuNP by the first gain peak. The green arrow indicates the electric field polarization of the incident laser.

the first fs-laser shot, some particles, e.g., the two AuNPs marked by the pink and white circles, agglomerated and coalesced (partially fused) between their surface to form nanorods; simultaneously, three other AuNPs (marked by the green circle) also aggregated and completely fused into a bigger, irregular particle. With the second fs-laser pulse, the third AuNP marked by the pink circle also agglomerated and coalesced, resulting in a longer nanorod. In addition, both the nanorod (white circles in Figure 4a) and the irregular particle (green circles in Figure 4a) further fused and reshaped into ellipsoidal and spherical particles, respectively. However, the other surrounding AuNPs did not agglomerate after those two fs-laser pulses, which may suggest the dependence of attraction on the surrounding environment of these particles, as also discussed by El-Sayed, Link, and others.35 These coalesced nanorods through the surface fusion are usually crystals containing certain twins within the fusion boundary, which has been evidenced in previous high-resolution in situ LC-EM observation of nanocrystal growth through particle attachment.36 Besides the agglomeration and fusion, the NPs also exhibited apparent displacements, as shown by the difference image in the last column of Figure 4a, which was probably due to the driving force arising from the invisible steams around the AuNPs induced by the intensive fs-laser pulse excitation.27,38 Single-pulse imaging of the “high-temperature” fusion process above the melting point was further performed in our experiment to reveal the related dynamics at the nanosecond time scale. A set of typical results of fusion dynamics of two- and three-AuNP clusters in aqueous solution are displayed in panels b and c of Figure 4, respectively. The first column shows the continuous-pulse image of initial AuNPs without laser excitation for a reference. The singlepulse images of the AuNPs before laser exposure (the second column) exhibit the same morphology as that of their reference images. The third column presents the single-pulse images at specific delays after the fs-laser pulse excitation (fluence of 33 mJ/cm2). As shown clearly in the third column, after the intensive laser pulse excitation, both the two- and three-AuNP clusters melted and fused into a bigger spherical particle, and

their fusion process was already completed within the measurement time delays, 30 and 45 ns, respectively. The similar fusion process is also observed by single-pulse imaging measurement on more AuNP clusters (Figure S1). The singlepulse imaging results illustrate that the high-temperature fusion process of the AuNPs in liquid is faster than tens of nanoseconds. In addition, the fused bigger particle also showed an apparent displacement due to the strong local photothermal effect, as shown by their difference images in the last column. In all the experiments mentioned above, because of the localized surface plasmon enhanced optical absorption of AuNPs happening under fs-laser pulse irradiation at 520 nm, the AuNPs are heated to a substantial nonequilibrium temperature within several picoseconds by electron−phonon coupling.37 Subsequently, the thermal energy in the AuNPs dissipates to the surroundings including the citrate capping ligand, surrounding liquid molecules, and low-frequency van der Waals modes between the particles and the substrate. This strong localized plasmonic heating effect of plasmonic NPs has also been evidenced by previous Raman spectroscopy38 and cathodoluminescence spectroscopy39 investigations. Moreover, the presence of fs-pulse excitation would induce the continuous photon-oxidation of the negatively charged citrate capping ligands, resulting in the photodeionization and even peeling of the stabilizing ligands on the surface of AuNPs.15,16 Both of them result in the destabilization of the AuNPs dispersed in the liquid cell. Meanwhile, complex interactions including the dipolar interaction among the particles (see the inset in Figure 5c, which will be discussed more later) due to the photon-induced localized surface plasmon oscillation (acting like dipoles) and the van der Waals attraction between the particles10,40 would result in the subsequent agglomeration of such destabilized AuNPs (see schematic process I in Figure 1c and Figure 2). The optical trapping force may contribute to this process if the size of the initial ensemble of the AuNPs is comparable to the laser beam waist,41,42 which can be neglected in this work because of the big fs-laser spot. In addition, the radiometric force arising from the nonuniform photothermal effect on the particles may also have some minor effect on the aggregation.43 4049


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the adjacent AuNPs, namely, the plasmon “hot point”, would also facilitate this surface coalescence process. Note that the fslaser pulse-induced ionization of the capping ligands plays an important role in the initial aggregation of the stabilized AuNPs in liquid medium but plays a minor role in this hightemperature fusion process. When the fs-laser fluence was far above the melting threshold of the AuNPs, the particles in the aggregate would fully melt into hot liquid, rapidly fuse together, and reshape into a bigger spherical particle because of the liquid surface tension and lower surface energy (see schematic process III in Figure 1c and experimental result in Figure 4a). This process also explains the observed fusion of the nanorods with additional fs-laser pulse excitation in the experiment (Figure 4a). Such fusion and shape transformation dynamics of the AuNPs under an intensive fs-laser pulse excitation occur on picosecond or nanosecond scale because of the ultrafast electron−phonon and phonon−phonon couplings among the particles and surroundings37 and have already ended within tens of nanoseconds as observed by our singlepulse imaging measurement (Figure 4b,c). Different from the previous in situ LC-EM observation of NP nucleation and growth in solution resulting from electron beam21,24,51 or laser52 irradiation-induced decomposition of a chemical precursor, the observed photomorphic processes among the plasmonic AuNPs in our liquid-cell 4D-EM, including aggregation, coalescence, and fusion, are mainly due to the plasmonic excitation and strong photothermal effect of the NPs under fs-laser pulse excitation. As shown in our experiment, the photomorphic process of colloidal gold NPs under fs-laser excitation depends on the laser pulse sequence, fluence, and even duration (determining the quench rate) through the transient surface plasmon and photothermal effects, which could provide a simple, fast, controllable, and scalable route toward modification of gold NPs and reshaping of gold nanorods for exceptional optical response.53 In conclusion, we realized the direct visualization of the photomorphic dynamics of plasmonic NPs in liquid by liquidcell 4D-EM at nanometer−nanosecond time scale. The morphology evolution of the citrate-capped AuNPs dispersed in aqueous solution during the fs-laser-induced photomorphic reaction at different temperatures has been imaged in real time and space. At relatively low laser fluences, the AuNPs exhibit a fluence-dependent agglomeration and further coalescence when the fluence approaches the melting threshold of gold. When the laser fluence is far above the melting threshold of gold, the AuNP aggregates further fuse and transform into bigger particles within tens of nanoseconds. Our results provide a further insight into the photomorphic reaction dynamics of AuNPs in liquid under laser irradiation, which has potential to advance their plasmonic applications with tunable NP morphology and size. In the future, liquid-phase 4D imaging with a spherical-aberration corrector-improved 4DEM may even allow the imaging of the photomorphic reaction process of the NPs, especially the NP alignment for oriented attachment and the specific boundary features of the coalesced NPs36 at atomic resolution by using the ultrafast imaging electron pulse. Moreover, the continually improved temporal resolution of liquid-cell 4D-EM would also enable deeper insights into photoinduced reaction dynamics of thermodynamic nanomaterials, including their nucleation, growth, and interaction processes. Because of the unique high spatiotemporal resolution of this novel liquid-cell 4D-EM imaging technique, it would also permit the ability to explore other

It has been widely demonstrated that the photon-induced localized surface plasmon of AuNP is due to the collective oscillations of the free electrons on the particle surface,38,44,45 as schematically shown in the inset of Figure 5c. Because of such optically driven induced charge density oscillation, the AuNP behaves as an oscillating electric dipole, where the orientation of the electric dipole is determined by the electricfield polarization of the linear polarized incident laser. For the AuNP assembly in the liquid cell under fs-laser pulse excitation, all the AuNP dipoles have the same orientation and attract to each other to form NP aggregates. This localized surface plasmon near-field distribution due to the photoninduced charge density oscillation on the AuNP surface can be directly visualized by PINEM.46,47 Details of the PINEM measurement in our experiment are described in the Supporting Information and are also available elsewhere.46−48 Basically, the fs-laser pulse-induced transient surface plasmon field around the gold NPs mediates the interaction between the incident fs-laser pulse and the imaging fs-electron pulse, resulting in the quantum modulation of the imaging electron energy. Figure 5a presents the time-dependent electron energy spectrum obtained from AuNPs in liquid with an excitation laser fluence of 2.8 mJ/cm2, while the corresponding electron energy spectra at time zero (t = 0 fs) is shown in Figure 5b. Apparently, the electron energy spectrum consists of discrete peaks at integer multiples of 2.4 eV incident photons on both sides of the zero loss peak (ZLP) due to the gain/loss of photon energy through photon-electron coupling, and these peaks occur only in 1−2 ps (Figure 5a), indicating the transient behavior of the surface plasmon field. The inset of Figure 5c shows a typical energy-filtered image (PINEM image at time zero) of a AuNP by the first gain peak (fluence of 2.8 mJ/cm2), where the localized surface plasmon near field (oscillating electric dipole) can be seen clearly around the particle surface along the laser polarization direction. We further focus on the gain side, i.e., the PINEM signal at the left of the ZLP, and measure the PINEM intensity (normalized intensity of the first gain peak) under different laser fluences. As shown in Figure 5c, the PINEM intensity increases linearly with the laser fluence. As demonstrated by Park et al.,49 the PINEM intensity is given by the square modulus of the near field, namely, IPINEM ∝ |F⃗ |2. Therefore, the localized surface plasmon and the electric dipole charge density of the AuNPs also increase linearly with the laser fluence. Thus, the dipolar interaction between the AuNPs will be enhanced under higher laser fluence, resulting in the stronger attraction between the particles and further shrinkage of the aggregates (see the experimental result in Figure 3b,c), as well as higher aggregation speed. When the laser fluence was further increased to 7.7 mJ/cm2, the nonequilibrium temperature of the AuNPs is 750−800 K (Supporting Information),27 which is still below the melting point of gold.36 It has been theoretically demonstrated that at nanoscale, a 1−2 nm gold liquid layer will be formed on the AuNP surface through a surface melting effect at a temperature closely below the gold melting point.50 Therefore, the nanoplates observed in our experiment at the fluence of 7.7 mJ/cm2 is formed through the nanoscale surface melting effect, where the adjacent particles in the aggregates coalesce during the solidification of the surface liquid and form nanoplates (see schematic process II in Figure 1c and experiment result in Figure 3d). In addition, the laser pulse-induced surface plasmon resonance enhancement at the small gap between 4050


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(3) Beydoun, D.; Amal, R.; Low, G.; McEvoy, S. Role of nanoparticles in photocatalysis. J. Nanopart. Res. 1999, 1 (4), 439− 458. (4) Chen, X.; Zheng, Z.; Ke, X.; Jaatinen, E.; Xie, T.; Wang, D.; Guo, C.; Zhao, J.; Zhu, H. Supported silver nanoparticles as photocatalysts under ultraviolet and visible light irradiation. Green Chem. 2010, 12 (3), 414−419. (5) Haes, A. J.; Van Duyne, R. P. A nanoscale optical biosensor: sensitivity and selectivity of an approach based on the localized surface plasmon resonance spectroscopy of triangular silver nanoparticles. J. Am. Chem. Soc. 2002, 124 (35), 10596−10604. (6) Beeram, S. R.; Zamborini, F. P. Purification of gold nanoplates grown directly on surfaces for enhanced localized surface plasmon resonance biosensing. ACS Nano 2010, 4 (7), 3633−3646. (7) Ghosh, P.; Han, G.; De, M.; Kim, C. K.; Rotello, V. M. Gold nanoparticles in delivery applications. Adv. Drug Delivery Rev. 2008, 60 (11), 1307−1315. (8) Paciotti, G. F.; Myer, L.; Weinreich, D.; Goia, D.; Pavel, N.; McLaughlin, R. E.; Tamarkin, L. Colloidal gold: a novel nanoparticle vector for tumor directed drug delivery. Drug Delivery 2004, 11 (3), 169−183. (9) Link, S.; El-Sayed, M. A. Size and temperature dependence of the plasmon absorption of colloidal gold nanoparticles. J. Phys. Chem. B 1999, 103 (21), 4212−4217. (10) Satoh, N.; Hasegawa, H.; Tsujii, K.; Kimura, K. Photoinduced coagulation of Au nanocolloids. J. Phys. Chem. 1994, 98 (8), 2143− 2147. (11) Fujiwara, H.; Yanagida, S.; Kamat, P. V. Visible laser induced fusion and fragmentation of thionicotinamide-capped gold nanoparticles. J. Phys. Chem. B 1999, 103 (14), 2589−2591. (12) Takahashi, H.; Niidome, Y.; Sato, T.; Yamada, S. Effects of capping thiols on the laser-induced fusion of gold nanoparticles and deposition onto glass substrates in cyclohexane. Colloids Surf., A 2004, 247 (1), 105−113. (13) Inasawa, S.; Sugiyama, M.; Yamaguchi, Y. Bimodal size distribution of gold nanoparticles under picosecond laser pulses. J. Phys. Chem. B 2005, 109 (19), 9404−9410. (14) Pocoví-Martínez, S.; Parreno-Romero, M.; Agouram, S.; PérezPrieto, J. Controlled UV− C Light-Induced Fusion of ThiolPassivated Gold Nanoparticles⊥. Langmuir 2011, 27 (9), 5234−5241. (15) Lee, G. P.; Shi, Y.; Lavoie, E.; Daeneke, T.; Reineck, P.; Cappel, U. B.; Huang, D. M.; Bach, U. Light-driven transformation processes of anisotropic silver nanoparticles. ACS Nano 2013, 7 (7), 5911− 5921. (16) Lee, G. P.; Bignell, L. J.; Romeo, T. C.; Razal, J. M.; Shepherd, R. L.; Chen, J.; Minett, A. I.; Innis, P. C.; Wallace, G. G. The citratemediated shape evolution of transforming photomorphic silver nanoparticles. Chem. Commun. 2010, 46 (41), 7807−7809. (17) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K.; Schatz, G. C.; Zheng, J. Photoinduced conversion of silver nanospheres to nanoprisms. Science 2001, 294 (5548), 1901−1903. (18) Wu, X.; Redmond, P. L.; Liu, H.; Chen, Y.; Steigerwald, M.; Brus, L. Photovoltage mechanism for room light conversion of citrate stabilized silver nanocrystal seeds to large nanoprisms. J. Am. Chem. Soc. 2008, 130 (29), 9500−9506. (19) Powers, A. S.; Liao, H.-G.; Raja, S. N.; Bronstein, N. D.; Alivisatos, A. P.; Zheng, H. Tracking Nanoparticle Diffusion and Interaction during Self-Assembly in a Liquid Cell. Nano Lett. 2017, 17, 15. (20) Zheng, H.; Claridge, S. A.; Minor, A. M.; Alivisatos, A. P.; Dahmen, U. Nanocrystal diffusion in a liquid thin film observed by in situ transmission electron microscopy. Nano Lett. 2009, 9 (6), 2460− 2465. (21) Zheng, H.; Smith, R. K.; Jun, Y.-w.; Kisielowski, C.; Dahmen, U.; Alivisatos, A. P. Observation of single colloidal platinum nanocrystal growth trajectories. Science 2009, 324 (5932), 1309− 1312. (22) Liu, J.; Wang, Z.; Sheng, A.; Liu, F.; Qin, F.; Wang, Z. L. In Situ Observation of Hematite Nanoparticle Aggregates by Using Liquid

photoassisted chemical, physical, and biological dynamical processes in their native environments.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b01360. Materials and Methods, Supporting Figure S1 (PDF) Movie S1: 1 kHz fs-laser pulse-induced continuous aggregation of AuNPs in liquid cell at a fluence of 3.3 mJ/cm2 (AVI) Movie S2: 1 kHz fs-laser pulse-induced continuous aggregation of AuNPs in liquid cell at a fluence of 2.0 mJ/cm2 (AVI) Movie S3: 1 kHz fs-laser pulse-induced continuous aggregation of AuNPs in liquid cell at a fluence of 4.9 mJ/cm2 (AVI) Movie S4: 1 kHz fs-laser pulse-induced continuous aggregation of AuNPs in liquid cell at a fluence of 7.7 mJ/cm2 (AVI) Movie S5: Continuous electron imaging of AuNP agglomeration and fusion process under intensive laser pulse excitation shot-by-shot at a fluence of 33 mJ/cm2 (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xuewen Fu: 0000-0001-6116-4680 Zhi-Min Liao: 0000-0001-6361-9626 Author Contributions

A.H.Z. and X.F. proposed and designed the project; X.F., B.C., and C.L. performed the experiment; X.F. and B.C. analyzed the experimental data; all authors contributed to the discussion and writing of the manuscript. Notes

The authors declare no competing financial interest. ∥ Deceased.

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ACKNOWLEDGMENTS This work was partially supported by the Air Force Office of Scientific Research Grant FA9550-11-1-0055S for research conducted in The Gordon and Betty Moore Center for Physical Biology at California Institute of Technology, and the National Key Research and Development Program of China (No. 2016YFA0300802) and NSFC (No. 11234001). We would like to thank J. S. Baskin and Mohammed Th. Hassan for help on setting the femtosecond laser system. We thank Mohammed Th. Hassan for help on PINEM measurement. We also wish to thank J. Tang for fruitful discussion.

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REFERENCES

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Direct Visualization of Photomorphic Reaction Dynamics of Plasmonic

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