Perspective Cite This: ACS Photonics 2018, 5, 4711−4723
pubs.acs.org/journal/apchd5
Plasmonic Hot Carriers Imaging: Promise and Outlook Yu Jin Jang,†,‡,⊥ Kyungwha Chung,⊥,§ June Sang Lee,⊥,∥ Chi Hun Choi,∥ Ju Won Lim,∥ and Dong Ha Kim*,∥ †
Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Suwon 16419, Republic of Korea Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea § Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea ∥ Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Republic of Korea
ACS Photonics 2018.5:4711-4723. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 12/21/18. For personal use only.
‡
ABSTRACT: Extraordinary light−matter interaction on the surface of metallic nanostructures can excite surface plasmons (SPs), followed by generation of charge carriers with high energy, that is, “hot electrons and holes”, via nonradiative decay. Such plasmonic hot carriers are potentially useful for photocatalysis, electrocatalysis, photovoltaics, optoelectronics, and theragnosis since hot carrier transfer to the desired substrate can accelerate specific redox reactions or facilitate electrical benefits on devices. In this regard, there is a growing interest in the detection and visualization of hot carriers at the location where plasmonic hot carriers are practically generated and transferred by means of conventional or newly developed procedures, as summarized in Table 1 of the main paper. Although direct imaging of plasmonic hot carriers or pathways are still challenging due to ultrafast dynamics of plasmonic hot carriers, state-of-the-art microscopic approaches have successfully demonstrated the mapping of the localized surface plasmons (LSPs) and plasmonic hot carriers. In addition, more accessible and facile approaches by mediation of chemical probes have also been emerged in recent years for the same purpose. The aim of this Perspective is to provide an idea of how spatial information on the generation and transfer of plasmonic hot carriers can be associated with the future design of plasmonic nanomaterials or nanocomposites to increase the output of hot carrier-driven processes. Along with a comprehensive overview of surface plasmon decay into plasmonic hot carriers and the necessity of plasmonic hot carrier imaging, we will highlight some recent advances in plasmonic hot carrier imaging techniques and provide remarks on future prospects of these techniques. KEYWORDS: plasmonics, hot carriers, near-field, nanophotonics, imaging
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for plasmonic hot carriers. Perspectives of plasmonic hot carrier science for the future research are also provided. On a planar metal film, excited SPs can propagate along the interface until energy decays to the ground state. This phenomenon is known as surface plasmon polaritons (SPPs). Propagation length of SPPs can vary in the range of hundreds of nanometers (nm) to millimeters (mm) depending on the material and the excitation wavelength.16 In order to excite SPPs, momentum mismatch between light and SPPs has to be resolved through coupling with a high refractive index glass prism,17 metal grating structures,18 nanoantenna,19 or nanotips.20 Meanwhile, for metallic nanostructures with subwavelength dimension, collective electron clouds will oscillate on the surface. This nonpropagating charge oscillation is called localized surface plasmon resonance (LSPR). In case of LSPR, the resonance occurs with light having a restricted range of frequency due to their finite dimensions. The resonance
urface plasmons (SPs) are collective oscillations of free electrons in the conduction band of metal at the metal− dielectric interface due to electric restoring force. Surface plasmon resonance (SPR), the coupling between SPs and the incident electromagnetic wave, leads to the generation of an enhanced electric field confined to the surface. This light− matter interaction results in several compelling consequences that could expand their applications in optical sensing,1,2 light emission,3,4 solar energy harvesting,5,6 photodetector,7−10 waveguiding,11 data storage,12 and so on. Meanwhile, direct observation of SPs has become a challenging task and critical issue in research. Imaging of plasmons could be obtained at a resolution of tens of nanometers by sophisticated spectroscopic techniques such as photoemission electron microscopy (PEEM),13 electron energy loss spectroscopy (EELS),14 and Kelvin probe force microscopy (KPFM).15 However, more convoluted designs of imaging techniques are demanded to directly observe hot carrier dynamics in high resolution. Thus, the aim of this Perspective is to provide a short overview of fundamental aspects of plasmonics and plasmonic hot carrier generation, state-of-the-art imaging techniques of SPs and hot carriers, and nanochemistry as an emerging imaging method © 2018 American Chemical Society
Received: Revised: Accepted: Published: 4711
July 25, 2018 November 9, 2018 November 11, 2018 November 12, 2018 DOI: 10.1021/acsphotonics.8b01021 ACS Photonics 2018, 5, 4711−4723
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Figure 1. Fundamental mechanism of plasmon excitation and decay. (a, b) Absorption processes of Au through intra- and interband transitions, respectively. Reproduced with permission from ref 22. Copyright 2015 Nature Publishing Group. (c−f) Plasmonic excitation and dynamics of nonradiative decay. (g) Redirected flow of photon by plasmon resonance. (c−g) Reproduced with permission from ref 23. Copyright 2015 Nature Publishing Group. (h) Charge separation pathways between metal and semiconductor. (i) Illustration of Au-tipped CdSe NR and their electronic structure and transfer by PICTT mechanism. (h, i) Reproduced with permission from ref 40. Copyright 2015 American Association for the Advancement of Science.
time scale from 100 fs to 1 ps with continuous distribution in energy (Figure 1e). Electron−phonon collisions then take place within the time scale from 1 to 10 ps, while phonon− phonon collision takes place from 10 ps to 10 ns, radiating heat to the environment (Figure 1f). Nonradiative decay has been considered as a deteriorating factor and often expressed by “lossy” and “parasitic” in highefficiency silicon solar cells.25−27 On the contrary, it has attracted attention from researchers in the field of physics,23,24,28,29 photocatalysis,30,31 and optoelectronics32−34 because it can generate hot carriers. However, some limitations hinder the practical application of hot carriers for energy conversion, including ultrafast decay of hot carriers and broad distribution of carrier energy. A Schottky barrier that can be found in most photovoltaic and photocatalytic systems inhibits effective charge transfer by blocking hot electrons having lower energy than a Schottky barrier from being injected to the conduction band of semiconductor.35 In this context, under-
wavelength can be observed as a strong absorption band, which heavily relies on the composition, size, and shape of metal nanostructures and refractive index of the surrounding medium.1 After SPs are excited by incident light, plasmons decay either radiatively (scattering) or nonradiatively (absorption) to relax plasmonic energy.21 The nonradiative decay creates an electric field by which charge carriers with high energy (hot electrons and hot holes) are generated via intraband transition of electrons in metal (Figure 1a),22 inducing a higher energy than that obtained by thermal excitation.23 In case of interband transition from d-band to s-band, it creates short-lived holes in d-band and electrons in s-band that are closely located to the Fermi level (Figure 1b).22,24 For a more detailed explanation, the time scale of the nonradiative decay process is illustrated in Figure 1c−f. Once Landau damping occurs within a time scale of a femtosecond (1−100 fs), hot carriers can be relaxed close to the thermal energy via electron−electron scattering on a 4712
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Figure 2. Catalytic advantages of plasmonic hot carriers. (a) Schematic description of plasmonic antenna/catalytic reactor heterojunction. Reproduced with permission from ref 43. Copyright 2016 National Academy of Science. (b) Illustrated representation of hot electron transfer to the conduction band of ce-MoS2 for plasmon-mediated electrocatalytic hydrogen evolution. Reproduced with permission from ref 51. Copyright 2015 American Chemical Society. (c) Generation of high valence Ni sites on the surface of Ni(OH)2 catalyst via hot hole-induced oxidation for oxygen evolution reaction. Reproduced with permission from ref 52. Copyright 2016 American Chemical Society.
plasmonic characteristics is also expected.42 Thus, careful design of nanostructure with optimized surface modification is necessary to achieve efficient application of plasmonics.
standing the hot carrier generation process in terms of time and space is essential for optimizing the design of plasmonic hot carrier-based devices. It has been theoretically and experimentally confirmed that the intensified electromagnetic field at the hot spot region can enhance the rate of hot carrier generation.29,36−38 Dombi et al. have observed acceleration of electrons at hot spots and explained this phenomenon as rescattering of electrons initially pushed away from the surface of plasmonic nanostructures and scattered back to the surface due to the evanescent field.37 Besteiro et al. have also described a theory depicting the enhanced hot carrier generation rate by field enhancement and by the effect of nonconservation of electron momentum at the hot spot.29 Thus, configuration of metal nanostructures with hot spots is an important factor to be considered for the design of devices. It is also significant to envisage hot carrier transfer at the metal−semiconductor (or adsorbate atoms) interface to improve charge collection efficiency. Conventional hot electron transfer in most metal−semiconductor composites can be divided into three steps: (i) excitation of plasmons by light illumination, (ii) generation of hot carriers by Landau damping, and (iii) transfer to the conduction band of semiconductor (Figure 1h, left). Hot carrier transfer should take place within 100 fs, before thermal relaxation. The schematic diagram shown in Figure 1h, middle, demonstrates a direct interfacial charge transfer transition (DICTT) by optical excitation of an electron. This can be an efficient pathway for fast extraction of hot electrons. Such transition was observed in metal−adsorbate systems. For examples, strong chemisorption between the surface of Pt metal and adsorbate CO atoms can induce formation of hybridized electronic states.39 In addition, Wu et al. have proposed a plasmon-induced interfacial charge-transfer transition (PICTT) pathway in CdSe nanorods with Au nanoparticle at the tip as a more efficient plasmon-induced hotelectron transfer mechanism in metal−semiconductor junction (Figure 1h, right, Figure 1i).40 Similarly, Boerighter et al. have also reported the evidence of LSPR-induced direct charge transfer in a methylene blue (MB)-covered Ag nanocube system.41 Plasmon-induced direct transfer exhibited relatively high quantum yield and fast rate, suggesting that it is a solution to overcome ultimate limitation of hot carriers. DICTT and PICTT are both involved in chemical interface damping (CID) process that can be realized by interfacial strong coupling between metal and semiconductor, inducing modification of electronic structures. Efficient charge transfer can be obtained by CID process. Meanwhile, damping of
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ADVANTAGES AND POTENTIAL APPLICATIONS OF PLASMONIC HOT CARRIERS Catalysis is one of the major fields that requires hot carrier imaging techniques to more intuitively understand how hot carriers influence and change catalytic dynamics, activity, and product selectivity. Plasmon-mediated (photo)electrocatalytic systems have been widely investigated due to their strong light harvesting and utilization capabilities in the past decade. Plasmonic materials are recognized as “nanoantenna”43 to increase optical absorption of catalyst in a wide range of solar spectrum to boost responsivity under illumination. Strategic construction of antenna-reactor geometry is also possible where plasmonic materials can donate hot electrons to highly catalytic materials to increase electron density or plasmonic hot holes that act as electron scavenger to fully utilize holes remaining in the host catalyst for oxidation reaction (Figure 2a).44−49 Despite these significant research accomplishments, the exact mechanism of plasmon-mediated (photo)electrocatalysts is somewhat ambiguous because catalytic efficiencies heavily depend on active sites present in catalytic materials.6,50−53 One must carefully investigate whether hot carriers can actually modulate the active sites of catalysts rather than simply assuming that the increase in catalytic activities is always in direct relationship with the generation of hot carriers. To do so, direct imaging techniques should be considered to locate the presence of hot carriers and determine how they affect the overall catalytic performance. In reductive conversion, one of the most intuitive works for a plasmon-mediated catalytic system has been demonstrated by Shi et al. for hydrogen evolution reactions (HER) employing chemically exfoliated MoS2 nanosheets (ce-MoS2) with Au nanorods as the antenna-reactor-type electrocatalyst.51 These introduced Au nanorods function as a nanoantenna to convert 808 nm laser light source into hot carriers as a means of LSPR decay. The generated hot electrons are then injected into the conduction band of ce-MoS2 to increase the electron density of host catalyst, resulting in 3-fold increase in current density and decreased overpotential of 300 to 120 mV with the help of a hole scavenger (Figure 2b). The aforementioned work demonstrates that hot electrons indeed possess potential to drive complicated and important reactions such as renewable energy production. However, the exact location of the catalyst where hot carriers are donated is currently unclear. 4713
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Figure 3. Direct visualizations of LSPR modes. (a) A series of EELS-STEM spectra on Ag nanoprisms and three corresponding EELS intensity mapping images after Gaussian fitting for resonance modes at 1.75 (top), 2.70 (middle), and 3.70 eV (bottom), respectively. Reproduced with permission from ref 14. Copyright 2007 Springer Nature. (b) LSPR spectral components of an Ag nanocube with five distinct excitation modes labeled α, β, γ, δ, and ε, obtained from applying non-negative matrix factorization (NMF) to unprocessed EELS spectra (blue-dots). All spectra were measured at the position of large blue marker in an inset image (top). 3D visualizations of EELS-LSPR with combining five distinct LSPR modes (bottom). Reproduced with permission from ref 65. Copyright 2013 Springer Nature. (c) 2D spatial mapping images of CL spectra along an Au nanowire at distinct resonance wavelengths (592, 640, and 730 nm). Reproduced with permission from ref 67. Copyright 2007 American Chemical Society. (d) 2D imaging of EELS (top) and CL spectra (bottom) for an Au triangular nanoprism. Reproduced with permission from ref 75. Copyright 2015 American Chemical Society. (e) Images of surface potential for Ag nanoparticles on a GaN layer under dark (top) and UV illumination conditions (bottom) measured by KPFM. Reproduced with permission from ref 15. Copyright 2017 Springer Nature.
incorporation of Au nanoparticles. In their study, excited hot electrons were extracted to the glassy carbon electrode while the remaining hot holes efficiently oxidized Ni species to the high valence state. This phenomenon induced a significant increase in the catalytic ability of host catalyst to oxidize adsorbed water molecules into oxygen. The authors supported their finding with electron paramagnetic resonance (EPR) spectroscopy to detect the formation of NiIII active sites on the surface of the host catalyst. The catalyst showed a substantial decrease in overpotential from 340 to 270 mV at 10 mA·cm−2 as well. It should be emphasized that the induction of high valence state for transition metals is also observed for other electrocatalytic materials such as Fe(OOH) and CoO. Thus, the strategic utilization of hot holes can be used as an alternative and effective method to induce catalytically active centers for oxidative reactions (Figure 2c).52 However, the authors also faced obstacles because locating exact positions of active centers generated by hot holes was impossible. Such information would provide more fundamental understanding of the catalytic dynamics between host catalyst and plasmonic
This limitation is crucial for optimizing catalytic performances because catalytic materials have certain site-specific active centers that can promote desired reactions. For MoS2-based HER, the active sites are located at the edge of the material while the surface of MoS2 are rather inactive toward HER.54,55 However, there has not been enough research effort or hot carrier visualization techniques to investigate the exact location where hot electrons are donated to pinpoint the contribution of the hot electron to increase the overall catalytic performance. This limitation calls for more intuitive visualization techniques and mechanistic studies to further emphasize the role of hot electrons in reductive conversion reactions. For oxidative conversion, hot holes can react either with hole scavengers to promote the separation of generated hot electrons from hot holes (to prevent the recombination between hot electrons and holes) in metallic antenna or with electrons from the reactor to create high valence catalytic centers to induce oxidative reaction in materials. Liu et al. have demonstrated that the highly valence state of Ni (either III or IV) in the Ni(OH)2 electrocatalyst is achieved through 4714
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materials so that optimization of similar systems could be more intuitive. So far, researchers have relied on density functional theory (DFT) calculations or finite-difference time-domain (FDTD) method to show the role of plasmonic effects. However, it should be emphasized that these calculated results cannot fully support catalytic mechanisms. Direct evidence through hot carrier visualization techniques is needed to provide the fundamental understandings to further optimize plasmon-mediated catalytic systems and promote future research opportunities in this field.
By exploiting the fact that SPPs can be excited by incident electron beam via electromagnetic interaction71 and it generates far-field emitted photons (i.e., cathodoluminescence (CL)), the visualization of LSPRs can be achieved through CL imaging spectroscopy coupled with electron excitation system such as scanning electron microscope (SEM)72,73 or transmission electron microscope (TEM).64,74 As shown in Figure 3c, 2D CL images of Au nanowires are demonstrated for different resonance wavelengths, highlighting its dispersion behavior as a plasmonic waveguide with distinct eigenmodes.67 For CL imaging, key distinctions from EELS system are elucidated in Figure 3d, by comparing their respective mapping images over Au nanoprisms.75 The imaging mechanism of CL is enabled by the scattering of evanescent EM field to far-field, whereas that of EELS corresponds to optical extinction including an inelastic energy interaction with materials. Therefore, both aforementioned imaging techniques are accessible to dipolar modes that are manifested due to radiative losses, whereas higher-order modes are only captured by EELS, underscoring their ability to detect nonradiative modes (Figure 3d). It is also in qualitative agreement with observations of dark-mode plasmons which are mostly probed by EELS, not by CL.76 Additionally, in contrast with SEM, PEEM has been suggested as a powerful technique to capture a LSPR near-field intensity map. By employing multiphoton PEEM system with ultrashort-pulsed laser, it enabled timeresolved photoemitted electron measurement, which visualized the dynamics of LSPRs for different Au nanostructures within spatial resolution of sub-10 nm.13 Alternative approaches of plasmon imaging with nanoscale resolution include the use of tip-scanning methods based on an AFM setup, where a nanoscale metal tip provides topographic information on nanostructure in high spatial resolution. By utilizing scanning near field optical microscope (SNOM)60,61 or KPFM technique,13,77,78 the scanning tip can measure nearfield enhancement or work function differences arisen from charge distribution of SPPs bound to the surface of materials. Hence, direct imaging of plasmonic modes over a variety of nanoplasmonic structures including metal−insulator−metal (MIM) devices can be done by characterizing via KPFM system with spatial resolution of 2 nm.78 Such dedicated mapping techniques make it possible to probe variations of highly localized field in vicinity of Ag nanoparticles on GaN surfaces under dark and UV illuminated conditions.15 From Figure 3e, one can observe reduction of surface potential for Ag nanoparticles after UV illumination, which is a consequence of increased surface work function due to excess accumulation of electrons from SP-induced localized field. Based on data obtained, the contribution of SP enhancement to GaN ultraviolet detectors could be effectively analyzed.
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DIRECT OBSERVATION OF LOCALIZED SURFACE PLASMONS THROUGH HIGH-RESOLUTION IMAGING TECHNIQUES Visualization of plasmons confined in the nanoscale space can provide in-depth and intuitive demonstration of localization and propagation of surface plasmons over nanomaterials. Highly resolved real-space imaging of plasmonic properties enables investigation of interplay between nanostructures and excitation of LSPR as well as a clear illustration of nanoplasmonics and quantum properties in subwavelength scales. Due to the fact that LSPR is highly dependent on the size, shape, composition and surroundings of materials in nanoscale dimensions, such imaging techniques would need to satisfy prerequisites of abilities to spectrally and spatially resolve nanometric variations for direct visualization of plasmons at extreme scales. However, it is often challenging to obtain nanoscale optical characteristics over real-space domain at subwavelength spatial volumes due to confinement of plasmons over surface and limited spatial resolution of optical imaging imposed by aperture sizes. Therefore, a complex characterization technique is needed to observe distinct plasmonic properties over spatial dimensions by probing features such as evanescent electric field,56 SPR angle shifts,57,58 near-field intensity,59−61 photoemission electrons,13,62−64 electron energy-loss,14,65 or far-field photon emissions66−68 with coupling mapping-systems such as atomic force microscope (AFM)69 and scanning microscope.70 Using electron microscopy system, low-loss region of EELS spectra has been found to be associated with the stopping of fast-moving electrons by SPs-induced local electric field. Nelayah et al. have achieved spectral and spatial distribution of SPs of Ag nanoprisms by mapping distinct plasmon resonance modes in a nanoscale precision via an EELS technique incorporating scanning transmission electron microscope (STEM).14 By capturing EELS spectrum at every scanned points of STEM, plasmonic characteristics of Ag nanoprism were imaged in two-dimensional (2D) plane with highly precise spatial resolution (∼18 nm). As depicted in Figure 3a, one can see different resonant peaks over energyloss (1.75, 2.70, and 3.20 eV) from EELS spectra corresponding to different LSPR modes. By mapping them over 2D space for each LSPR modes, 3-fold plasmonic characteristics of triangular nanoprism could be clearly observed by depicting relative differences of EELS intensities and plasmons localizations for each LSPR mode. Similarly, three-dimensional (3D) imaging of LSPR has been acquired by probing Ag nanocubes in different angles/orientations and reconstructing the image via “compressed sensing electron tomography” (Figure 3b), hence, obtaining an intuitive visualization of plasmon hybridization modes excited by corners, edges, and faces of nanocubes.65
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CURRENT STATUS OF HOT CARRIER IMAGING TECHNIQUES Nonetheless, these above observations are mostly limited by characterizing “near-field/electric field” caused by LSPR excitation. Alternatively, investigation of imaging hot-carrier generation for decay of SPs should suggest alternative pathways for exploring plasmon mapping, not only to understand dynamics of hot electrons through a highly resolved real-space visualization but also to manipulate the generation of hot electrons as an ultrasensitive imaging/ sensing platform. However, direct visualization of hot electrons/hot spots has been difficult due to their ultrashort 4715
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Figure 4. Hot carrier imaging techniques. (a) Illustration of nanoscale mapping setup with SNoiM on GaAs/AlGaAs quantum well. (b) SEM image of GaAs/AlGaAs quantum well. (c) Hot electron distribution mapping and (d) corresponding electrostatic potential energy of the conduction band edge. (a−d) Reproduced with permission from ref 59. Copyright 2018 American Association for the Advancement of Science. (e) Schematic illustration of circuit for hot-electron nanoscopy (TIA: transimpedance amplifier). (f) Diagram of electronic band of metal tip and semiconductor (Egap: band gap energy of semiconductor, Φm and χs are the work function of metal and semiconductor, respectively; r is the radius of depletion region). (g−h) Topography and plasmonic hot electron mapping image of locally oxidized GaAs surface. (e−h) Reproduced with permission from ref 81. Copyright 2013 Springer Nature.
of hot electrons traveling along the surface within femtosecond dynamics through a diffusive transport model. Ultrafast dynamics of nonlocal hot electrons can also be obtained through a scattering type scanning noise microscope (s-SNOM), also called scanning noise microscope (SNoiM).59 As shown in Figure 4a−d, hot electrons in the nonequilibrium state of GaAs/AlGaAs quantum well within nanoconstricted quasi-2D electron gas (2DEG) can generate local current fluctuation (i.e., shot noise), which is subjected to an electromagnetic (EM) evanescent field. A contact-free tungsten tip at a few nanometers above the surface is scanned through a nanodevice with an electrons-constricted system and scatters EM evanescent field to be captured by a highly sensitive confocal microscope with a spatial resolution of ∼50 nm. By mapping images of shot-noise, dynamic behaviors and
lifetime (∼100 fs) to overcome the decay of excited energy via thermal and phonon dissipation. Lock et al. have reported a nonlocal atomic manipulation of target adsorbate molecules on Si(111) surfaces caused by dynamics of hot-electrons, through pump−probe measurements by coupling a scanning tunneling microscope (STM) system.79 STM technique provides imaging performance with atomic resolution and ultrafast response (∼ps) when it is coupled with terahertz pulse.80 Such design induces charge injection from the STM tip and the charges propagate along the surface with manipulating electrons on adsorbate molecules. The nonlocal behavior underscores dynamic mechanisms of hot electrons. Such characteristics depending on temperature and bias-voltage provide quantitative analysis 4716
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Figure 5. (a) Hot electron conversion of 4-NTP to 4-ATP. (b) Schematic illustration of Ag BT structure after resonant light irradiation and (c) reaction with carboxylic acid functionalized Au NPs in HEPES buffer containing 1-ethyl-3-(3-(dimethylamino)propyl)-carbodiimide/Nhydroxysuccinimide (EDC/NHS) coupler. Tips marked with red color on the Ag BT in (b) indicate the region where the transition of 4-NTP to 4ATP has mainly occurred. The formation of amide bond linked the Au NPs to the Ag BT as depicted on the left side of (c). (d) Near-field distribution map of Ag BT calculated by FDTD simulation at 633 nm for parallel polarization. (e) SEM image of Ag BT decorated with Au NPs after the plasmonic excitation and incubation in Au NP dispersion. The scale bar represents 100 nm. (f) Histogram summarized the location and the frequency of Au NP appearance after plasmon-driven chemical reaction. It maps the density/energy of plasmonic hot carriers and the most reactive sites in Ag BT. Reprinted with permission from ref 38. Copyright 2017 Springer Nature.
regard, a few simple and effective experimental concepts have been recently reported.38,82−88 They harness a specific chemical reaction or engineering mechanism driven by plasmon-induced hot carriers to not only monitor the formation and spatial confinement but also trace the quantity and energy of plasmonic hot carriers. They can be considered as indirect imaging methods because the probe system is based on analyzing differences in structural or optical properties of the plasmonic nanostructure before and after chemical reactions, instead of directly detecting immediate temporal changes on the nanostructure after plasmonic excitation. However, it can be a facile and powerful strategy for indepth study of plasmonic hot carriers as long as we understand the mechanism behind the nanochemistry because we can intuitively and easily obtain information via widely used characterization tools such as SEM, UV-vis-NIR spectrometer, photoluminescence (PL) spectroscopy, surface-enhanced Raman scattering (SERS) spectroscopy, and so on. Xie et al. have designed a core−satellite Ag−Ag superstructure that was covalent assembly of Ag NPs (dsatellite ∼ 25 nm) onto a large Ag NP template (dcore ∼ 100 nm).82 Sure enough, the superstructure exhibited intense local electric field at the nanogap between Ag satellite NPs, suggesting that Ag superstructure could be a good platform for generating plasmonic hot carriers. At resonant excitation, the appearance of SERS signal at ∼1590 cm−1 indicated that electron acceptor molecules deposited onto Ag NPs, 4-nitrothiophenol (4-NTP) in this system, were reduced to 4-aminothiophenol (4-ATP; Figure 5a) without conventional chemical reducing agents. This result shows that hot electrons generated from Ag superstructure was the only electron source for the sixelectron-mediated reduction reaction. Cortés et al. have exploited the aforementioned transition from 4-NTP to 4-ATP induced by hot electrons in order to decorate Ag bow ties (BTs; l ∼ 200 nm with a tip separation of ∼20 nm) with reporter Au NPs (d ∼ 15 nm) at a specific region where the plasmonic hot carriers are spatially and
energy dissipation of hot carriers can be revealed through a nanothermometric approach. Furthermore, hot electrons can be manipulated to perform a scanning probe microscopy, also called “hot-electron nanoscopy” (Figure 4e−h).81 Through adiabatic focusing of SPs by Au tapered tip, it allows SPPs generation with high plasmonto-hot-electron conversion efficiency (∼30%) at distinct wavelengths and high spatial resolution (below 50 nm). Due to high sensitivity of Schottky barrier height in energy level between metal nanotip and GaAs interface (Figure 4f), SPPsgenerated hot electrons can be utilized for a new nanoscopic imaging technique, which enables imaging of localized chemical sensitivity. Therefore, 3D imaging of locally oxidized or ion-implanted GaAs surface is manifested, underscoring the high chemical sensitivity and high spatial resolution of nanoscopic hot-electrons imaging technique. It is noteworthy that only this work has demonstrated direct imaging of hot carriers induced by SP excitation. Except SNoiM, the above-mentioned imaging techniques utilized, in fact, Schottky barrier or adsorbate molecules for selective detection of hot electrons. This implies that direct mapping of hot electrons needs to overcome several disturbances originated from the nature of hot electrons as well as imaging methods. Moreover, nanoprobe-based microscopes can provide the spatial resolution of 50 nm, which is still moderate for investigating various materials in nanoscale.
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NANOCHEMISTRY AS AN EMERGING TECHNIQUE FOR IMAGING PLASMONIC HOT CARRIERS Fast dissipation of energetic hot carriers on femto- to picosecond time scale and the necessity of sophisticated machines, devices, or methods for higher spatial resolution limit empirical observation of plasmonic hot carrier generation and localization. They also impede understanding of relevant physics and kinetics on plasmonic hot carriers. These shortcomings have encouraged researchers to find new approaches to replace or complement these conventional imaging techniques. In this 4717
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Figure 6. (a) Schematic illustration of hot electron-induced polymerization on Au nanoparticle-on-mirror (NPoM) structure. (b) SEM images of Au NPoM (top) and Au NPoM with PNIPAM layer in the middle (bottom). All scale bars are 100 nm. Reproduced with permission from ref 84. Copyright 2017 American Chemical Society. (c) Schematic illustration and (d) PAINT images of Au antenna array with different composition of thiol molecules depending on the orientation of Au NR and light polarization. Reproduced with permission from ref 87. Copyright 2018 American Chemical Society.
Figure 7. (a) Annular dark-field scanning transmission electron microscopy (ADF-STEM) images (top) and corresponding EELS maps of Au nanoprisms (bottom). (b) SEM images of Au nanoprisms before (left) and after (right) irradiation. Dashed lines in the right image represent Au nanoprisms in the left image. Energies of incident light (Einc) are indicated in these images. All scale bars are 100 nm. (c) Ion-induced SEM image of Au nanoprisms (left) and corresponding NanoSIMS images to map elemental distributions of 197Au− (middle) and 12C14N− specifically adsorbed onto PVP (right). Reproduced with permission from ref 83. Copyright 2016 Springer Nature.
strongly localized.38 The surface of Ag BT structures at the starting point was uniformly covered by self-assembled monolayer (SAM) of 4-NTP and the binding process of Au NPs via the formation of amide bonds between terminal amino groups in 4-ATP on Ag BTs and carboxylic acid groups in Au NPs was initiated right after irradiation of Ag BTs at 633 nm for 2 min (Figure 5b,c). FDTD simulations revealed that the distribution of plasmonic energy was associated with the geometry of plasmonic nanostructures comprising hot spots at the tip of Ag BTs (Figure 5d). This implies that different magnitudes of plasmonic response along edges of Ag BTs
could result in a spatial gradient of 4-ATP concentration, rendering the location of Au NPs site-specific. In agreement with this prediction, most Au NPs were found to be preferentially bound at the tip of Ag BTs in between the gap, followed by the corners and edges (Figure 5e,f). This clearly demonstrates the geometrical prerequisite of efficient plasmon energy conversion. In other words, experimental detection of hot spots can be realized by tracking plasmondriven chemistry. Ding et al. have placed Au NPs (d ∼ 80 nm) on Au substrate with a spacing of 0.6 nm thiophenol SAM to concentrate hot 4718
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38, 82, 84, 85, 87, 88, 106
59, 81
electron microscope chemistry
physics
observing the change in the general photophysical properties in a given simultaneous detection of the presence of plasmonic hot limited number of characterization tools due to material carriers and their effect on the physical properties the temporary change under illumination investigating the change in the photophysical properties in a given material by high resolution imaging in spectral and spatial distribution high complexity of characterization tools microscopic excitation and detection tracking the change in the structural information, optical properties or chemical wide variety of redox reactions/easily accessible difficulty in real-time monitoring compositions after the plasmon-driven chemical reactions characterization tools
6, 30, 44, 51−53,102, 103 104, 105 difficulty in real space monitoring
difficulty in real space monitoring
well-developed model system to confirm the electrical gain from plasmonic hot carriers easy detection with an ample amount of photo- and electrocatalysts catalysis
mechanisms
measuring the plasmon-enhanced photocurrent, photoresponse or photoconductivity in photovoltaic or optoelectronic devices measuring the plasmon-enhanced photocurrent or photoresponse in photoand electrocatalytic systems
CONCLUSION AND OUTLOOK As described in Table 1, a variety of imaging methods validated the generation of plasmon induced hot carriers. However, the real space mapping of the change in photocurrent level with nanoscale precision hardly achieved in plasmonic photovoltaic or optoelectronic devices. The changes in some physical properties are only transient under light illumination and thus analytical tools to figure out the contribution of hot carriers are
device
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methodology
Table 1. Methods to Visualize the Generation and Distribution of Plasmonic Hot Carriers
advantages
disadvantages
ref
electrons at the gap under resonant condition.84 Since hot electrons can participate in the formation of Au−C bond and thus successive polymerization reactions in monomer solutions of divinylbenzene (DVB) and N-isopropylacrylamide (NIPAM), the observation of a shell-like structure of polydivinylbenzene (PDVB) and poly(N-isopropylacrylamide) (PNIPAM) layers around Au NPs, as shown in the top-view images of SEM in Figure 6b, reveals the presence of hot electrons at the anticipated region (Figure 6a). On the contrary, Simoncelli et al. have exploited a chemical bond breaking to visualize the location of hot electron generation and transfer.87 As shown in Figure 6c, Au nanorod (NR) arrays were designed to consist of one part excited by a linear polarization of the light source at resonant wavelength and the other insensitive to the polarization. The surface of Au NRs was covered with SAM of thiol molecules coupled with fluorescent labels (F1). Femtosecond laser irradiation at 950 nm produced hot electrons selectively in the reactive Au NR along the longitudinal direction. It induced Au−S desorption, leaving the site open to a new fluorophore (F2) with thiol functional group at the other end. Point accumulation for imaging in nanoscale topography (PAINT) maps showed the transition from Au NRs with F1, the combination of bare Au NR and Au NRs with F1 to Au NRs with both F1 and F2 by detecting fluorescence signals from Au NR arrays before and after the irradiation (Figure 6d). These results provide us useful information on exactly where to generate and detect plasmonic hot carriers even with a more complex geometry. Another strong point of nanochemistry is that chemical probes often show better feasibility and flexibility than microscopic characterizations for tracing pathways of plasmonic hot carriers. According to experimental results observed by Zhai et al., anisotropic growth of Au nanoprisms (l ∼ 500 nm and t ∼ 22 nm) from spherical Au NP nanoseeds (d ∼ 7 nm) is originated by the fact that polyvinylpyrrolidone (PVP), one of the most common surfactants used in the synthesis of NPs,89 also mediates growth reaction by accumulating hot electrons along the perimeter of Au nanoprisms.83 This indicates that spatial mapping of the adsorption site of PVP could obtain the information on the generation site of plasmonic hot carriers in real-space. Indeed, comparison between Figure 7b and c obtained from SEM and nanoscale secondary-ion mass spectrometry (NanoSIMS), respectively, indicates that the position of selective decoration of PVP on Au nanoseeds is in accordance with the growth direction of Au nanoprisms. It is noteworthy to stress that the spatial distribution of plasmonic hot carriers figured out by chemical approach is in contrast with that obtained from EELS maps of Au nanoprisms at resonant wavelength which apparently reveal hot spots in the center (Figure 7a). Such discrepancy highlights the advantages of using chemical probes as an imaging tool for plasmonic hot carriers under complex catalytic or synthetic reaction conditions involving multiple constituent elements, reactive sites, and paths.
98−101
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mostly limited to a few optical mapping techniques. In this respect, we focused on the approaches with electron microscopes and nanochemistry in this Perspective, which enable the real space imaging/mapping of the presence, distribution, and transfer of hot carriers in high precision. In spite of advances that have been made, there is still room for further investigation, modifications, and improvement in terms of strategies, targets, precision, or performance of the plasmon mapping because micro/nanoscopic imaging techniques aforementioned have been exploited for a very limited number of material systems, and only proof-of-concept studies have been performed with the nanochemistry techniques. It is worth noting that the information gained from spatial (the specific region where to observe plasmonic hot carriers) and quantitative (the amount of available plasmonic hot carriers) studies of plasmonic hot carriers is generally based on numerical predictions. Thus, we can predict that a combination of plasmonic hot carrier imaging techniques summarized in this Perspective and numerical simulations can give us an idea of composition or geometry of plasmonic structures which empirically and efficiently convert absorption event into generation of plasmonic hot carriers. In the application point of view, it will eventually provide a more direct and practical clue to the design of plasmonic structures that can significantly enhance photocatalytic, electrocatalytic, photovoltaic, and optoelectronic properties. Other materials exhibiting physical and optical properties akin to those of general noble metals (a large number of free electrons on the surface interacting at a specific wavelength of incident light, followed by the evolution of SPR band-like optical signals) can also be applied to the aforementioned techniques to reveal their fundamental plasmonic characteristics.90 Specifically, non-noble metals (Al and Cu),43,45,91 copper chalcogenides (Cu2−xX; X: S, Se or Te),92 doped-metal oxides (Al- or Ga-doped ZnO, Nb-doped TiO2, P-doped Si, etc.),93 metal oxides with oxygen vacancy94 and two-dimensional electrides (Ca2N, Y2C, etc.)95−97 are promising candidates. Compared with noble metals, they show distinctly different absorption and scattering characteristics in terms of wavelength range and band alignment at the heterojunction in devices. Therefore, simple demonstrations of the abovementioned plasmonic hot carrier imaging techniques would be interesting to understand fundamental properties of energetic charge carriers generated by new plasmonic materials. Finally, mapping temporal change of plasmonic hot carriers (hot carrier dynamics) in real-space is also attractive since it can provide a deeper insight into the functional principle and the specificity of plasmonic hot carriers.28 By tracing simple redox reactions (described in the nanochemistry section) at different time scales, we can elucidate reaction pathways or rate-determining steps in plasmon-mediated catalytic or synthetic processes. Comparison of a certain redox reaction driven by different sources, that is, conventional reducing agents or plasmonic hot carriers, is also needed if we can visualize systems at the very early stage.
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Dong Ha Kim: 0000-0003-0444-0479 Author Contributions ⊥
Y.J.J., K.C., and J.S.L. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Mid-career Researcher Program through the National Research Foundation of Korea (NRF) grant funded by the MEST (2017R1A2A1A05022387) and by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (2018M3D1A1058536). K.C. acknowledges the financial support from National Research Foundation of Korea for the Research Fellow Program (NRF2018R1A6A3A11044025).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chi Hun Choi: 0000-0001-8203-0788 4720
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DOI: 10.1021/acsphotonics.8b01021 ACS Photonics 2018, 5, 4711−4723
