Watching Plasmonic Crystals Move to the Acoustic Beat

initial cycles, discharge capacities of 179, 160, 142, 122, 109, 97, and 89 mAh/g were observed at 0.5, 1.0, 2.0, 4.0, 6.0, 8.0, and 10.0 A/g, respectively. Looking at the stability, at a high current density of 10 A/g, the capacity was retained at 90 mAh/g for over 50,000 cycles, being far more stable than hollow titania nanospheres or commercial P25 titania nanoparticles. Clearly, these observations show that the highly accessible mesoporosity in the single layer significantly enhances electrochemical performance due to facile electrolyte access and short diffusion pathways for electronic and sodium-ion transport, in addition to accommodation of volume expansion upon lithium intercalation. Detailed ex situ TEM and other experiments led to mechanistic insights regarding the micellar assembly. Specifically, it was found that the Pluronic F127/titania monomicelles surrounded by glycerol displayed remarkable stability, preventing monomicelle self-assembly at room temperature and providing the ability to precisely tune the assembly behavior. Hence, glycerol

was suggested to act as a viscous structure-directing and confining agent for titania monomicelle assembly. Summarizing, Zhao and colleagues have developed a powerful novel assembly scheme for the generation of precisely controlled single-layer titania mesopore coatings on a surprisingly diverse collection of substrate compositions and shapes. It will be of great interest to see if this assembly scheme can be extended to other metal oxides to create highly controlled pore architectures with outstanding access for mass transport in combination with numerous other intriguing physical properties. 1. Lan, K., Yuan, X., Wang, R., Zhao, Z., Zhang, W., Zhang, X., Elzatahry, A., and Zhao, D. (2019). Confined Interfacial Monomicelle Assembly for Precisely Controlled Coating of Single-Layered Titania Mesopores. Matter 1, this issue, 527–538. 2. Chen, W., Glackin, C.A., Horwitz, M.A., and Zink, J.I. (2019). Nanomachines and Other Caps on Mesoporous Silica Nanoparticles for Drug Delivery. Acc. Chem. Res. 52, 1531– 1542. 3. Argyo, C., Weiss, V., Brauchle, C., and Bein, T. (2014). Multifunctional Mesoporous Silica Nanoparticles as a Universal Platform

for Drug Delivery. Chem. Mater. 26, 435–451. 4. Ashley, C.E., Carnes, E.C., Phillips, G.K., Padilla, D., Durfee, P.N., Brown, P.A., Hanna, T.N., Liu, J., Phillips, B., Carter, M.B., et al. (2011). The targeted delivery of multicomponent cargos to cancer cells by nanoporous particle-supported lipid bilayers. Nat. Mater. 10, 389–397. 5. Prieto, G., Tu¨ysu¨z, H., Duyckaerts, N., Knossalla, J., Wang, G.H., and Schu¨th, F. (2016). Hollow Nano- and Microstructures as Catalysts. Chem. Rev. 116, 14056–14119. 6. Furukawa, S., Reboul, J., Diring, S., Sumida, K., and Kitagawa, S. (2014). Structuring of metal-organic frameworks at the mesoscopic/macroscopic scale. Chem. Soc. Rev. 43, 5700–5734. 7. Tian, B., Zheng, X., Kempa, T.J., Fang, Y., Yu, N., Yu, G., Huang, J., and Lieber, C.M. (2007). Coaxial silicon nanowires as solar cells and nanoelectronic power sources. Nature 449, 885–889. 8. Cabral, H., Miyata, K., Osada, K., and Kataoka, K. (2018). Block Copolymer Micelles in Nanomedicine Applications. Chem. Rev. 118, 6844–6892. 9. Richardson, J.J., Cui, J., Bjo¨rnmalm, M., Braunger, J.A., Ejima, H., and Caruso, F. (2016). Innovation in Layer-by-Layer Assembly. Chem. Rev. 116, 14828–14867. 10. Yue, Q., Li, J., Zhang, Y., Cheng, X., Chen, X., Pan, P., Su, J., Elzatahry, A.A., Alghamdi, A., Deng, Y., and Zhao, D. (2017). Plasmolysis inspired nanoengineering of functional yolkshell microspheres with magnetic core and mesoporous silica shell. J. Am. Chem. Soc. 139, 15486–15493.

Preview

Watching Plasmonic Crystals Move to the Acoustic Beat David J. Flannigan1,* Femtosecond photoexcitation produces coherent, picosecond structural oscillations in plasmonic nanocrystals. Because the motions are tiny and fleeting, elucidating the influence of non-uniform interactions is challenging. Now, researchers have directly imaged such angstrom-scale single-particle dynamics using a highly sensitive ultrafast electron microscope. The combination of partially filled conduction-band states and nanoscale geometries leads to enhanced electric-field intensity at the surface of opti-

308

cally irradiated metallic nanocrystals. This is especially the case for a particular type of resonant excitation, wherein optical wavelength is tuned to

Matter 1, 302–311, August 7, 2019 ª 2019 Elsevier Inc.

a shape- and size-dependent coherent resonance of the electrons, initiating what is known as a localized surface plasmon resonance (LSPR). Owing to the sensitivity of the optical response to shape, size, and local environment, LSPRs have been explored for a diverse set of applications; including sensing, photocatalysis, and photothermal therapy. Importantly, optical excitation of such materials also leads to the

1Department

of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, MN 55455, USA *Correspondence: [email protected] https://doi.org/10.1016/j.matt.2019.07.010

generation of coherent structural vibrations, the intrinsic behaviors of which also depend sensitively upon particle size and geometric shape.1 While systems consisting of uniform boundary conditions can be straightforward to understand, the structural dynamics can become complex when non-uniform particle-particle and particle-substrate interactions are present. Complexity is further increased for particles that are rich with internal defects and surface imperfections. Because transient optical properties are intimately linked to structural dynamics, elucidation of intra-particle motions provides a richer view of the overall behaviors and offers a route to tunable optical properties via manipulation of shape, morphology, and composition. Now, a team from UNIST and KAIST in Korea have directly imaged the angstrom-scale structural responses of plasmonic metallic nanocrystals and determined the intra-particle effects of substrate interactions using a highly sensitive ultrafast transmission electron microscope.2 The authors’ study represents an important advance in experimental capability, enabling direct imaging of nanocrystal structural dynamics at combined spatiotemporal resolutions not yet achieved with this approach. Before describing the authors’ work, other approaches to studying metallic nanocrystal structural dynamics are briefly described to provide context. The photoinduced acoustic vibrational responses of plasmonic metallic nanocrystals have been extensively studied using ultrafast pump-probe spectroscopic techniques.1 Such measurements generally involve tracking in time the change in optical absorption caused by photoinduced structural oscillations. For excitation of coherent vibrational modes, a corresponding coherent oscillation of the transient absorption signal is observed, with the typical picosecond periods dictated by particle size and shape. Combined

with the transient optical response, knowledge of the particle dimensions, composition, and elastic constants enables determination of the symmetries and the behaviors of the excited vibrational modes. For spectroscopic measurements of large clusters of particles, signal is averaged over the entire ensemble, and the effects of variability in structure, morphology, and interfacial contacts are not resolved. Typical approaches to addressing this include conducting ensemble-averaged studies on clusters of particles having the narrowestpossible distribution of shapes and sizes—or better yet, conducting experiments on isolated single particles.3 Indeed, spectroscopic studies on single particles are especially attractive for elucidating the influence of structure, morphology, and boundary conditions on the observed optical and mechanical responses, as complexities introduced by particle-particle coupling of electric fields and vibrational modes are absent. Spectroscopic studies of the roles of inhomogeneities and interfacial contacts on single-particle structural responses are inherently indirect; dynamics may be either inferred through modeling the optical response or via separate microscopic characterization and subsequent correlation. While such approaches can produce accurate results, challenges arise for complex geometric structures, highly defective particles with subtle angstrom-scale morphological variations, and non-uniform particle-substrate interfacial contacts. Accordingly, high spatiotemporal resolution ultrafast scattering methods that enable direct probing of structural responses potentially provide a means to access such information. The mostfrequently used methods to date include ultrafast electron diffraction (UED) and femtosecond (fs) X-ray scattering. Unlike all-optical methods, the probe part of the pump-probe scheme in UED and fs X-ray scattering

is a discrete packet of electrons or X-ray photons, respectively. Such approaches enable access to structural dynamics typically via monitoring time-varying changes in diffracted beams (e.g., intensity, scattering angle, Bragg-spot q-range, etc.). A wealth of information is contained in these timevarying coherently scattered beams; including changes in atomic-plane spacing, changes in atomic vibration amplitude and structural order/disorder, and the general manner in which particles move mechanically. Indeed, these approaches have been used to elucidate a variety of structural phenomena in metal nanocrystals, including the spatiotemporal mapping of photoinduced intra-particle stress fields and the elucidation of ensemble-averaged photomechanical responses.4,5 As with all-optical approaches, ultrafast scattering methods are not without challenges. For instance, for ultrafast diffraction, reciprocal-space information is typically gathered from an ensemble of particles, thus hindering elucidation of spatially localized, angstrom-scale single-particle structural dynamics (though ultrafast convergent-beam approaches may be used to overcome this limitation, and diffractive imaging offers additional attractive opportunities6,7). Further, discrete and localized structural disorder and interfacial non-uniformities cannot be spatially resolved but instead contribute to the spatially averaged diffuse-scattering background signal or the Bragg-spot q-range. In this respect, the somewhat recent advent of fs transmission electron microscopy (TEM) (also dubbed ultrafast electron microscopy, UEM) provides a viable approach; in essence, UEM extends the diffraction and, more importantly, the imaging capabilities of TEM to the fs timescale.8 The critical difference when compared to dedicated diffraction approaches is UEM provides access to the image plane of an objective

Matter 1, 302–311, August 7, 2019

309

lens, such that structural inhomogeneities can be directly visualized and probed in real space. Indeed, fs bright-field imaging with UEM has been used to study the picosecond acousto-plasmonic dynamics of single, isolated particles and few-particle clusters of randomly-oriented Au nanorods.9 In that study, the authors used the extreme sensitivity of diffraction contrast to resolve the excitation and the evolution of intra- and inter-particle coherent structural vibrational modes and the formation of intra-particle localized vibrational hotspots. Such an approach is sensitive to angstrom-scale variations in particle dimension and orientation, but mainly specific to the incident electron direction; no dimensional changes perpendicular to the probing direction (i.e., in the plane of the substrate) were resolved. The advance reported by Kwon, Han, and co-workers exactly addresses limits in UEM lateral spatial resolution on the ultrafast timescale. The key development was the implementation of a highly sensitive direct electron detector. Such detectors do not employ a scintillator coated on the detector active element in order to ultimately collect signal. Instead, signal is generated via direct electron detection, thus improving the point spread function and the detective quantum efficiency. Further, because the active element is extremely sensitive and essentially devoid of noise, acquisition times can be significantly reduced for reaching a given signal-to-noise ratio. For UEM, this is a very important capability, as beam currents are typically orders of magnitude lower than in conventional TEM, and image acquisition times are thus much longer. Because of this, spatial resolution is ultimately limited by instabilities inherent in the instrument and the laboratory environment. Kwon, Han, and co-workers show that by employing a direct electron detector, they are able to image picosecond lateral displacements on the order of

310

Matter 1, 302–311, August 7, 2019

one to two angstroms in individual gold nanoparticles on graphene substrates. This demonstration is an exciting development for at least two immediate reasons: 1. It advances the current spatiotemporal imaging resolution limits of UEM to the combined subnanometer and picosecond regime. 2. It opens the way to conducting realspace imaging studies of the effects of ever-present materials imperfections and interfacial interactions at combined angstrom-fs levels. The authors were also able to resolve the angstrom-scale structural response of individual gold nanocrystals due to apparent preferential excitation of a transverse acoustic vibrational mode. Using a combination of contrast mechanisms, excitation and evolution of a coherent, angstrom-scale transverse structural breathing mode was directly imaged. This manifested in the images as a coherent expansion and contraction of the contrast associated with the particle diameter, mainly arising from mass-thickness contrast. By tracking the spatial extent of the image contrast with time, the authors were able to determine the periodicity of the oscillation and demonstrate that the vibrational dynamics match a single, specific acoustic mode. Further, the sensitivity of the measurements revealed that only certain particles in the field of view appeared to oscillate structurally, and no particles displayed measurable longitudinal vibrations despite the use of linearly-polarized excitation and the statistical probability of at least some particles being properly oriented for such excitations to occur. The authors explained this by noting that nonuniform particle-substrate interactions were likely at work (and were potentially dominating the overall dynamics), arising from a broad distribution of adhesion strengths and particle-substrate structural commensurability. Indeed, in one case, relatively largeamplitude oscillations at the particle ends relative to the central region were

observed, suggesting the presence of non-uniform interfacial interactions. In addition to the demonstrated improvement in UEM spatiotemporal imaging resolution and the spatially resolved distribution of specific vibrational responses, the authors observed temporal manifestation of subtle particle-substrate interfacial effects, thus supporting the idea of a broad distribution of adhesion strengths. This was accomplished by combining the angstrom-scale structural response with the picosecond temporal evolution of real-space dynamics, specifically with respect to the precise moment of photoexcitation (i.e., true time-zero, determined here using the photon-induced near-field effect10). Specifically, a significant delay was observed in the onset of coherent structural oscillations following fs photoexcitation, in some cases on the order of hundreds of picoseconds. As pointed out by the authors, such large lag times are not observed in particles suspended in liquids (i.e., in systems devoid of particle-substrate interactions). Further, a range of lag times was observed for individual particles within a common field of view, indicating the observation was not the result of a shift in time-zero position from one measurement to the next. Taken altogether, these observations support the hypothesis that non-uniform particle-substrate interactions were not only influencing but were potentially dominating the observed dynamics. The measurements reported by the authors are impressive and exciting, and one can imagine a host of experiments that could be performed to probe the acousto-plasmonic structural dynamics of various plasmonic systems, architectures, and structures. In addition, the work represents an important advance in the field of UEM development, and this researcher for one is inspired by the work reported by the authors

and is also excited to see what comes next.

ACKNOWLEDGMENTS The author thanks Prof. Vivian Ferry for reading the manuscript and providing comments and feedback. 1. Hartland, G.V. (2006). Coherent excitation of vibrational modes in metallic nanoparticles. Annu. Rev. Phys. Chem. 57, 403–430. 2. Kim, Y.-J., Jung, H., Han, S.W., and Kwon, O.-H. (2019). Ultrafast electron microscopy visualizes acoustic vibrations of plasmonic nanorods at the interfaces. Matter 1, this issue, 481–495.

3. Olson, J., Dominguez-Medina, S., Hoggard, A., Wang, L.-Y., Chang, W.-S., and Link, S. (2015). Optical characterization of single plasmonic nanoparticles. Chem. Soc. Rev. 44, 40–57. 4. Clark, J.N., Beitra, L., Xiong, G., Higginbotham, A., Fritz, D.M., Lemke, H.T., Zhu, D., Chollet, M., Williams, G.J., Messerschmidt, M., et al. (2013). Ultrafast three-dimensional imaging of lattice dynamics in individual gold nanocrystals. Science 341, 56–59. 5. Ruan, C.-Y., Murooka, Y., Raman, R.K., and Murdick, R.A. (2007). Dynamics of sizeselected gold nanoparticles studied by ultrafast electron nanocrystallography. Nano Lett. 7, 1290–1296. 6. Feist, A., Rubiano da Silva, N., Liang, W., Ropers, C., and Scha¨fer, S. (2018). Nanoscale

diffractive probing of strain dynamics in ultrafast transmission electron microscopy. Struct. Dyn. 5, 014302. 7. Miao, J., Ishikawa, T., Robinson, I.K., and Murnane, M.M. (2015). Beyond crystallography: diffractive imaging using coherent x-ray light sources. Science 348, 530–535. 8. Zewail, A.H. (2010). Four-dimensional electron microscopy. Science 328, 187–193. 9. Valley, D.T., Ferry, V.E., and Flannigan, D.J. (2016). Imaging intra- and interparticle acousto-plasmonic vibrational dynamics with ultrafast electron microscopy. Nano Lett. 16, 7302–7308. 10. Barwick, B., Flannigan, D.J., and Zewail, A.H. (2009). Photon-induced near-field electron microscopy. Nature 462, 902–906.

Matter 1, 302–311, August 7, 2019

311