Plasmonic Hot Carriers Imaging: Promise and Outlook - ACS

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Perspective

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 ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01021 • Publication Date (Web): 12 Nov 2018 Downloaded from http://pubs.acs.org on November 12, 2018

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ACS Photonics

Plasmonic Hot Carriers Imaging: Promise and Outlook

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Yu Jin Jang,1,† Kyungwha Chung,2,† June Sang Lee,3,† Chi Hun Choi,3 Ju Won Lim,3 and

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Dong Ha Kim3,*

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1Center

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Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea

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2Department

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of Korea

for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS),

of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic

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3Department

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Republic of Korea

of Chemistry and Nano Science, Ewha Womans University, Seoul 03760,

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ACS Paragon Plus Environment

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ABSTRACT

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Extraordinary light-matter interaction on the surface of metallic nanostructures can excite

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surface plasmons (SPs) followed by generation of charge carriers with high energy, i.e., “hot

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electrons and holes”, via non-radiative decay. Such plasmonic hot carriers are potentially

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useful for photocatalysis, electrocatalysis, photovoltaics, optoelectronics and theragnosis

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since hot carrier transfer to the desired substrate can accelerate specific redox reactions or

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facilitate electrical benefits on devices. In this regard, there is a growing interest in the

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detection and visualization of hot carriers at the location where plasmonic hot carriers are

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practically generated and transferred by means of conventional or newly developed

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procedures as summarized in Table 1. Although direct imaging of plasmonic hot carriers or

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pathways are still challenging due to ultrafast dynamics of plasmonic hot carriers, state-of-

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the-art microscopic approaches have successfully demonstrated the mapping of the localized

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surface plasmons (LSPs) and plasmonic hot carriers. In addition, more accessible and facile

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approaches by mediation of chemical probes have also been emerged in recent years for the

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same purpose. The aim of this Perspective is to provide an idea of how spatial information on

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the generation and transfer of plasmonic hot carriers can be associated with future design of

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plasmonic nanomaterials or nanocomposites to increase the output of hot carrier-driven

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processes. Along with a comprehensive overview of surface plasmon decay into plasmonic

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hot carriers and the necessity of plasmonic hot carrier imaging, we will highlight some recent

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advances in plasmonic hot carrier imaging techniques and provide remarks on future

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prospects of these techniques.

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ACS Photonics

TOC

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KEYWORDS: plasmonics, hot carriers, near-field, nanophotonics, imaging

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Introduction

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Surface plasmons (SPs) are collective oscillations of free electrons in the conduction band of

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metal at the metal-dielectric interface due to electric restoring force. Surface plasmon

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resonance (SPR), the coupling between SPs and the incident electromagnetic wave, leads to

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the generation of enhanced electric field confined to the surface. This light-matter interaction

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results in several compelling consequences that could expand their applications in optical

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sensing,1-2 light emission,3-4 solar energy harvesting,5-6 photodetector,7-10 waveguiding,11 and

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data storage,12 and so on. Meanwhile, direct observation of SPs has become a challenging

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task and critical issue in researches. Imaging of plasmons could be obtained at resolution of

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tens of nanometers by sophisticated spectroscopic techniques such as photoemission electron

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microscopy (PEEM),13 electron energy loss spectroscopy (EELS)14 and Kelvin probe force

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microscopy (KPFM).15 However, more convoluted designs of imaging techniques are

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demanded to directly observe hot carrier dynamics in high resolution. Thus, the aim of this

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Perspective is to provide a short overview of fundamental aspects of plasmonics and

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plasmonic hot carrier generation, state-of-the-art imaging techniques of SPs and hot carriers,

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and nanochemistry as an emerging imaging method for plasmonic hot carriers. Perspectives

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of plasmonic hot carrier science for the future research are also provided.

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On a planar metal film, excited SPs can propagate along the interface until energy

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decays to the ground state. This phenomenon is known as surface plasmon polaritons (SPPs).

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Propagation length of SPPs can vary in the range of hundreds of nanometers (nm) to

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millimeters (mm) depending on the material and the excitation wavelength.16 In order to

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excite SPPs, momentum mismatch between light and SPPs has to be resolved through

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coupling with a high refractive index glass prism,17 metal grating structures,18 nanoantenna,19 4


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or nanotips.20 Meanwhile, for metallic nanostructures with subwavelength dimension,

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collective electron clouds will oscillate on the surface. This non-propagating charge

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oscillation is called localized surface plasmon resonance (LSPR). In case of LSPR, the

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resonance occurs with light having a restricted range of frequency due to their finite

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dimensions. The resonance wavelength can be observed as a strong absorption band which

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heavily relies on the composition, size, and shape of metal nanostructures and refractive

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index of the surrounding medium.1

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After SPs are excited by incident light, plasmons decay either radiatively (scattering)

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or non-radiatively (absorption) to relax plasmonic energy.21 The non-radiative decay creates

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an electric field by which charge carriers with high energy (hot electrons and hot holes) are

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generated via intra-band transition of electrons in metal (Figure 1(a)),22 inducing a higher

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energy than that obtained by thermal excitation.23 In case of inter-band transition from d-band

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to s-band, it creates short-lived holes in d-band and electrons in s-band that are closely

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located to Fermi level (Figure 1(b)).22, 24 For a more detailed explanation, the timescale of

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the non-radiative decay process is illustrated in Figure 1(c)-(f). Once Landau damping occurs

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within a timescale of a femtosecond (1 to100 fs), hot carriers can be relaxed close to the

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thermal energy via electron-electron scattering on a timescale from 100 fs to 1 ps with

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continuous distribution in energy (Figure 1(e)). Electron-phonon collisions then take place

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within timescale from 1 to 10 ps while phonon-phonon collision takes place from 10 ps to 10

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ns, radiating heat to the environment (Figure 1(f)).

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Non-radiative decay has been considered as a deteriorating factor and often

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expressed by “lossy” and “parasitic” in high-efficiency silicon solar cells.25-27 On the

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contrary, it has attracted attention from researchers in the field of physics,23-24, 5


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photocatalysis30-31 and optoelectronics32-34 because it can generate hot carriers. However,

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some limitations hinder the practical application of hot carriers for energy conversion,

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including ultrafast decay of hot carriers and broad distribution of carrier energy. A Schottky

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barrier that can be found in most photovoltaic and photocatalytic systems inhibits effective

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charge transfer by blocking hot electrons having lower energy than a Schottky barrier from

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being injected to the conduction band of semiconductor.35 In this context, understanding the

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hot carrier generation process in terms of time and space is essential for optimizing the design

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of plasmonic hot carrier-based devices. It has been theoretically and experimentally

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confirmed that the intensified electromagnetic field at the hot spot region can enhance the rate

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of hot carrier generation.29, 36-38 Dombi et al. have observed acceleration of electrons at hot

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spots and explained this phenomenon as re-scattering of electrons initially pushed away from

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the surface of plasmonic nanostructures and scattered back to the surface due to the

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evanescent field.37 Besteiro et al. have also described a theory depicting the enhanced hot

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carrier generation rate by field enhancement and by the effect of nonconservation of electron

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momentum at hot spot.29 Thus, configuration of metal nanostructures with hot spots is an

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important factor to be considered for the design of devices. It is also significant to envisage

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hot carrier transfer at the metal-semiconductor (or adsorbate atoms) interface to improve

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charge collection efficiency.

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Conventional hot electron transfer in most metal-semiconductor composites can be

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divided into three steps: i) excitation of plasmons by light illumination, ii) generation of hot

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carriers by Landau damping, and iii) transfer to the conduction band of semiconductor

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(Figure 1(h) left). Hot carrier transfer should take place within 100 fs, before thermal

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relaxation. The schematic diagram shown in Figure 1(h) middle demonstrates a direct 6


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interfacial charge transfer transition (DICTT) by optical excitation of an electron. This can be

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an efficient pathway for fast extraction of hot electrons. Such transition was observed in

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metal-adsorbate systems. For examples, strong chemisorption between the surface of Pt metal

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and adsorbate CO atoms can induce formation of hybridized electronic states.39 In addition,

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Wu et al. have proposed plasmon-induced interfacial charge-transfer transition (PICTT)

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pathway in CdSe nanorods with Au nanoparticle at the tip as a more efficient plasmon-

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induced hot-electron transfer mechanism in metal-semiconductor junction (Figure 1(h) right,

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Figure 1(i)).40 Similarly, Boerighter et al. have also reported the evidence of LSPR-induced

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direct charge transfer in methylene blue (MB)-covered Ag nanocube system.41 Plasmon-

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induced direct transfer exhibited relatively high quantum yield and fast rate, suggesting that it

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is a solution to overcome ultimate limitation of hot carriers. DICTT and PICTT are both

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involved in chemical interface damping (CID) process that can be realized by interfacial

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strong coupling between metal and semiconductor, inducing modification of electronic

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structures. Efficient charge transfer can be obtained by CID process. Meanwhile, damping of

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plasmonic characteristics are also expected.42 Thus, careful design of nanostructure with

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optimized surface modification is necessary to achieve efficient application of plasmonics.

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Advantages and potential applications of plasmonic hot carriers

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Catalysis is one of the major fields that require hot carrier imaging techniques to more

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intuitively understand how hot carriers influence and change catalytic dynamics, activity, and

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product selectivity. Plasmon-mediated (photo)electrocatalytic systems have been widely

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investigated due to their strong light harvesting and utilization capabilities in the past decade.

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Plasmonic materials are recognized as “nanoantenna”43 to increase optical absorption of

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catalyst in a wide range of solar spectrum to boost responsivity under illumination. Strategic 7


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construction of antenna-reactor geometry is also possible where plasmonic materials can

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donate hot electrons to highly catalytic materials to increase electron density or plasmonic hot

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holes that act as electron scavenger to fully utilize holes remaining in the host catalyst for

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oxidation reaction (Figure 2(a)).44-49 Despite these significant research accomplishments, the

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exact mechanism of plasmon-mediated (photo)electrocatalysts is somewhat ambiguous

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because catalytic efficiencies heavily depend on active sites present in catalytic materials.6, 50-

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53

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of catalysts rather than simply assuming that the increase in catalytic activities is always in

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direct relationship with generation of hot carriers. To do so, direct imaging techniques should

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be considered to locate the presence of hot carriers and determine how they affect the overall

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catalytic performance.

One must carefully investigate whether hot carriers can actually modulate the active sites

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In reductive conversion, one of the most intuitive works for plasmon-mediated

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catalytic system has been demonstrated by Shi et al. for hydrogen evolution reactions (HER)

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employing chemically exfoliated MoS2 nanosheets (ce-MoS2) with Au nanorods as the

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antenna-reactor type electrocatalyst.51 These introduced Au nanorods function as

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nanoantenna to convert 808 nm laser light source into hot carriers as a means of LSPR decay.

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The generated hot electrons are then injected into the conduction band of ce-MoS2 to increase

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the electron density of host catalyst, resulting in three-fold increase in current density and

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decreased overpotential of 300 mV to 120 mV with help of hole scavenger (Figure 2(b)).

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The aforementioned work demonstrates that hot electrons indeed possess potential to drive

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complicated and important reactions such as renewable energy production. However, the

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exact location of the catalyst where hot carriers are donated is currently unclear. This

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limitation is crucial for optimizing catalytic performances because catalytic materials have 8


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certain site-specific active centers that can promote desired reactions. For MoS2-based HER,

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the active sites are located at the edge of the material while the surface of MoS2 are rather

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inactive toward HER.54-55 However, there has not been enough research effort or hot carrier

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visualization techniques to investigate the exact location where hot electrons are donated to

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pinpoint the contribution of the hot electron to increase the overall catalytic performance.

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This limitation calls for more intuitive visualization techniques and mechanistic studies to

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further emphasize the role of hot electrons in reductive conversion reactions.

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For oxidative conversion, hot holes can react either with hole scavengers to promote

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the separation of generated hot electrons from hot holes (to prevent the recombination

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between hot electrons and holes) in metallic antenna or with electrons from the reactor to

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create high valence catalytic centers to induce oxidative reaction in materials. Liu et al. have

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demonstratedthat the highly valence state of Ni (either III or IV) in Ni(OH)2 electrocatalyst is

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achieved through incorporation of Au nanoparticles. In their study, excited hot electrons were

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extracted to the glassy carbon electrode while the remaining hot holes efficiently oxidized Ni

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species to the high valence state. This phenomenon induced a significant increase in the

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catalytic ability of host catalyst to oxidize adsorbed water molecules into oxygen. The authors

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supported their finding with electron paramagnetic resonance (EPR) spectroscopy to detect

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the formation of NiIII active sites on the surface of the host catalyst. The catalyst showed a

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substantial decrease in overpotential from 340 to 270 mV at 10 mA·cm-2 as well. It should be

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emphasized that the induction of high valence state for transition metals is also observed for

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other electrocatalytic materials such as Fe(OOH) and CoO. Thus the strategic utilization of

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hot holes can be used as an alternative and effective method to induce catalytically active

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centers for oxidative reactions (Figure 2(c)).52 However, the authors also faced obstacle 9


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because locating exact positions of active centers generated by hot holes was impossible.

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Such information would provide more fundamental understanding of the catalytic dynamics

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between host catalyst and plasmonic materials so that optimization of similar systems could

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be more intuitive. So far, researchers have relied on density functional theory (DFT)

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calculations or finite-difference time-domain (FDTD) method to show the role of plasmonic

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effects. However, it should be emphasized that these calculated results cannot fully support

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catalytic mechanisms. Direct evidence through hot carrier visualization techniques is needed

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to provide the fundamental understandings to further optimize plasmon-mediated catalytic

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systems and promote future research opportunities in this field.

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Direct observation of localized surface plasmons through high-resolution imaging

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techniques

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Visualization of plasmons confined in the nanoscale space can provide in-depth and intuitive

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demonstration of localization and propagation of surface plasmons over nanomaterials.

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Highly-resolved real-space imaging of plasmonic properties enables investigation of interplay

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between nanostructures and excitation of LSPR as well as a clear illustration of

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nanoplasmonics and quantum properties in subwavelength scales. Due to the fact that LSPR

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is highly dependent on the size, shape, composition and surroundings of materials in

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nanoscale dimensions, such imaging techniques would need to satisfy prerequisites of

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abilities to spectrally and spatially resolve nanometric variations for direct visualization of

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plasmons at extreme scales. However, it is often challenging to obtain nanoscale optical

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characteristics over real-space domain at subwavelength spatial volumes due to confinement

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of plasmons over surface and limited spatial resolution of optical imaging imposed by

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aperture sizes. Therefore, a complex characterization technique is needed to observe distinct 10


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plasmonic properties over spatial dimensions by probing features such as evanescent electric

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field,56 SPR angle shifts,57-58 near-field intensity,59-61 photoemission electrons,13, 62-64 electron

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energy-loss,14,

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atomic force microscope (AFM)69 and scanning microscope.70

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or far-field photon emissions66-68 with coupling mapping-systems such as

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Using electron microscopy system, low-loss region of EELS spectra has been found

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to be associated with the stopping of fast-moving electrons by SPs-induced local electric

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field. Nelayah et al. have achieved spectral and spatial distribution of SPs of Ag nanoprisms

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by mapping distinct plasmon resonance modes in a nanoscale precision via an EELS

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technique incorporating scanning transmission electron microscope (STEM).14 By capturing

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EELS spectrum at every scanned points of STEM, plasmonic characteristics of Ag nanoprism

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were imaged in two-dimensional (2D) plane with highly precise spatial resolution (~18 nm).

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As depicted in Figure 3(a), one can see different resonant peaks over energy-loss (1.75, 2.70,

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and 3.20 eV) from EELS spectra corresponding to different LSPR modes. By mapping them

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over 2D space for each LSPR modes, threefold plasmonic characteristics of triangular

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nanoprism could be clearly observed by depicting relative differences of EELS intensities and

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plasmons localizations for each LSPR mode. Similarly, three-dimensional (3D) imaging of

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LSPR has been acquired by probing Ag nanocubes in different angles/orientations and

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reconstructing the image via ‘compressed sensing electron tomography’ (Figure 3(b)), hence

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obtaining an intuitive visualization of plasmon hybridization modes excited by corners,

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edges, and faces of nanocubes.65

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By exploiting the fact that SPPs can be excited by incident electron beam via

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electromagnetic

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cathodoluminescence (CL)), the visualization of LSPRs can be achieved through CL imaging

interaction71

and

it

generates

far-field

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emitted

photons

(i.e.,

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spectroscopy coupled with electron excitation system such as scanning electron microscope

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(SEM)72-73 or transmission electron microscope (TEM).64, 74 As shown in Figure 3(c), 2D CL

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images of Au nanowires are demonstrated for different resonance wavelengths, highlighting

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its dispersion behavior as a plasmonic waveguide with distinct eigenmodes.67 For CL

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imaging, key distinctions from EELS system are elucidated in Figure 3(d), by comparing

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their respective mapping images over Au nanoprisms.75 The imaging mechanism of CL is

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enabled by the scattering of evanescent EM field to far-field, whereas that of EELS

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corresponds to optical extinction including an inelastic energy interaction with materials.

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Therefore, both aforementioned imaging techniques are accessible to dipolar modes that are

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manifested due to radiative losses, whereas higher-order modes are only captured by EELS,

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underscoring their ability to detect nonradiative modes (Figure 3(d)). It is also in qualitative

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agreement with observations of dark-mode plasmons which are mostly probed by EELS, not

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by CL.76 Additionally, in contrast with SEM, PEEM has been suggested as a powerful

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technique to capture a LSPR near-field intensity map. By employing multiphoton PEEM

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system with ultrashort-pulsed laser, it enabled time-resolved photo-emitted electron

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measurement, which visualized the dynamics of LSPRs for different Au nanostructures

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within spatial resolution of sub-10 nm.13

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Alternative approaches of plasmon imaging with nanoscale resolution include the use

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of tip-scanning methods based on an AFM set-up, where a nanoscale metal tip provides

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topographic information of nanostructure in high spatial resolution. By utilizing scanning

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near field optical microscope (SNOM)60-61 or KPFM technique,13, 77-78 the scanning tip can

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measure near-field enhancement or work function differences arisen from charge distribution

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of SPPs bound to the surface of materials. Hence, direct imaging of plasmonic modes over a 12


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variety of nanoplasmonic structures including metal-insulator-metal (MIM) devices can be

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done by characterizing via KPFM system with spatial resolution of 2 nm.78 Such dedicated

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mapping techniques make it possible to probe variations of highly localized field in vicinity

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of Ag nanoparticles on GaN surfaces under dark and UV illuminated conditions.15 From

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Figure 3(e), one can observe reduction of surface potential for Ag nanoparticles after UV

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illumination, which is a consequence of increased surface work function due to excess

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accumulation of electrons from SP-induced localized field. Based on data obtained, the

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contribution of SP enhancement to GaN ultraviolet detectors could be effectively analyzed.

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Current status of hot carrier imaging techniques

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Nonetheless, these above observations are mostly limited by characterizing ‘near-

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field/electric field’ caused by LSPR excitation. Alternatively, investigation of imaging hot-

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carrier generation for decay of SPs should suggest alternative pathways for exploring

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plasmon mapping, not only to understand dynamics of hot electrons through a highly

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resolved real-space visualization, but also to manipulate the generation of hot electrons as an

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ultrasensitive imaging/sensing platform. However, direct visualization of hot electrons/hot

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spots has been difficult due to their ultrashort lifetime (~100 fs) to overcome the decay of

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excited energy via thermal and phonon dissipation.

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Lock et al. have reported a nonlocal atomic manipulation of target adsorbate

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molecules on Si(111) surfaces caused by dynamics of hot-electrons, through pump-probe

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measurements by coupling a scanning tunneling microscope (STM) system.79 STM technique

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provides imaging performance with atomic resolution and ultrafast response (~ps) when it is

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coupled with terahertz pulse.80 Such design induces charge injection from the STM tip and

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the charges propagate along the surface with manipulating electrons on adsorbate molecules. 13


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The nonlocal behavior underscores dynamic mechanisms of hot electrons.

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characteristics depending on temperature and bias-voltage provide quantitative analysis of hot

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electrons travelling along the surface within femtosecond dynamics through a diffusive

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transport model.

Such

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Ultrafast dynamics of nonlocal hot electrons can also be obtained through a

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scattering type scanning noise microscope (s-SNOM), also called scanning noise microscope

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(SNoiM).59 As shown in Figure 4 (a-d), hot-electrons in non-equilibrium state of

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GaAs/AlGaAs quantum well within nano-constricted quasi-2D electron gas (2DEG) can

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generate local current fluctuation (i.e., shot noise) which is subjected to electro-magnetic

10

(EM) evanescent field. A contact-free tungsten tip at a few nanometers above the surface is

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scanned through a nanodevice with an electrons-constricted system and scatters EM

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evanescent field to be captured by a highly sensitive confocal microscope with a spatial

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resolution of ~50 nm. By mapping images of shot-noise, dynamic behaviors and energy

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dissipation of hot carriers can be revealed through a nanothermometric approach.

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Furthermore, hot electrons can be manipulated to perform a scanning probe

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microscopy, also called ‘hot-electron nanoscopy (Figure 4(e-h)).81 Through adiabatic

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focusing of SPs by Au tapered tip, it allows SPPs generation with high plasmon-to-hot-

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electron conversion efficiency (~30%) at distinct wavelengths and high spatial resolution

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(below 50 nm). Due to high sensitivity of Schottky barrier height in energy level between

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metal nano-tip and GaAs interface (Figure 4(f)), SPPs-generated hot electrons can be utilized

21

for a new nanoscopic imaging technique, which enables imaging of localized chemical

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sensitivity. Therefore, 3D imaging of locally oxidized or ion-implanted GaAs surface is

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manifested, underscoring the high chemical sensitivity and high spatial resolution of 14


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nanoscopic hot-electrons imaging technique. It is noteworthy that only this work has

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demonstrated direct imaging of hot carriers induced by SP excitation.

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Except SNoiM, the above-mentioned imaging techniques utilized, in fact, Schottky

4

barrier or adsorbate molecules for selective detection of hot electrons. This implies that direct

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mapping of hot electrons needs to overcome several disturbances originated from the nature

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of hot electrons as well as imaging methods. Moreover, nanoprobe-based microscopes can

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provide the spatial resolution of 50 nm, which is still moderate for investigating various

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materials in nanoscale.

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Nanochemistry as an emerging technique for imaging plasmonic hot carriers

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Fast dissipation of energetic hot carriers on femto- to pico-second timescale and the necessity

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of sophisticated machines, devices or methods for higher spatial resolution limit empirical

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observation of plasmonic hot carrier generation and localization. They also impede

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understanding of relevant physics and kinetics on plasmonic hot carriers. These shortcomings

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have encouraged researchers to find new approaches to replace or complement these

15

conventional imaging techniques. In this regard, a few simple and effective experimental

16

concepts have been recently reported.38,

17

engineering mechanism driven by plasmon-induced hot carriers to not only monitor the

18

formation and spatial confinement but also trace the quantity and energy of plasmonic hot

19

carriers. They can be considered as indirect imaging methods because the probe system is

20

based on analyzing differences in structural or optical properties of the plasmonic

21

nanostructure before and after chemical reactions, instead of directly detecting immediate

22

temporal changes on the nanostructure after plasmonic excitation. However, it can be a facile

23

and powerful strategy for in-depth study of plasmonic hot carriers as long as we understand

82-88

They harness a specific chemical reaction or

15


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Page 16 of 40

1

the mechanism behind the nanochemistry because we can intuitively and easily obtain

2

information via widely used characterization tools such as SEM, optical microscopy,

3

absorption,

4

spectroscopy, and so on.

photoluminescence

(PL),

surface-enhanced

Raman

scattering

(SERS)

5

Xie et al. have designed a core-satellite Ag-Ag superstructure which was covalent

6

assembly of Ag NPs (dsatellite ~25 nm) onto a large Ag NP template (dcore ~100 nm).82 Sure

7

enough, the superstructure exhibited intense local electric field at the nanogap between Ag

8

satellite NPs, suggesting that Ag superstructure could be a good platform for generating

9

plasmonic hot carriers. At resonant excitation, the appearance of SERS signal at ~1590 cm-1

10

indicated that electron acceptor molecules deposited onto Ag NPs, 4-nitrothiophenol (4-NTP)

11

in this system, were reduced to 4-aminothiophenol (4-ATP) (Figure 5(a)) without

12

conventional chemical reducing agents. This result shows that hot electrons generated from

13

Ag superstructure was the only electron source for the six-electron-mediated reduction

14

reaction.

15

Cortés et al. have exploited the aforementioned transition from 4-NTP to 4-ATP

16

induced by hot electrons in order to decorate Ag bow ties (BTs; l ~200 nm with a tip

17

separation of ~20 nm) with reporter Au NPs (d ~15 nm) at a specific region where the

18

plasmonic hot carriers are spatially and strongly localized.38 The surface of Ag BT structures

19

at the starting point was uniformly covered by self-assembled monolayer (SAM) of 4-NTP

20

and the binding process of Au NPs via the formation of amide bonds between terminal amino

21

groups in 4-ATP on Ag BTs and carboxylic acid groups in Au NPs was initiated right after

22

irradiation of Ag BTs at 633 nm for 2 min (Figure 5(b-c)). FDTD simulations revealed that

23

the distribution of plasmonic energy was associated with the geometry of plasmonic 16


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1

nanostructures comprising hot spots at the tip of Ag BTs (Figure 5(d)). This implies that

2

different magnitudes of plasmonic response along edges of Ag BTs could result in a spatial

3

gradient of 4-ATP concentration, rendering the location of Au NPs site-specific. In agreement

4

with this prediction, most Au NPs were found to be preferentially bound at the tip of Ag BTs

5

in between the gap, followed by the corners and edges (Figure 5(e-f)). This clearly

6

demonstrates the geometrical prerequisite of efficient plasmon energy conversion. In other

7

words, experimental detection of hot spots can be realized by tracking plasmon-driven

8

chemistry.

9

Ding et al. have placed Au NPs (d ~80 nm) on Au substrate with a spacing of 0.6 nm

10

thiophenol SAM to concentrate hot electrons at the gap under resonant condition.84 Since hot

11

electrons can participate in the formation of Au-C bond and thus successive polymerization

12

reactions in monomer solutions of divinylbenzene (DVB) and N-isopropylacrylamide

13

(NIPAM), the observation of shell-like structure of polydivinylbenzene (PDVB) and poly(N-

14

isopropylacrylamide) (PNIPAM) layers around Au NPs as shown in the top-view images of

15

SEM in Figure 6(b) reveals the presence of hot electrons at the anticipated region (Figure

16

6(a)).

17

On the contrary, Simoncelli et al. have exploited a chemical bond breaking to

18

visualize the location of hot electron generation and transfer.87 As shown in Figure 6(c), Au

19

nanorod (NR) arrays were designed to consist of one part excited by a linear polarization of

20

the light source at resonant wavelength and the other insensitive to the polarization. The

21

surface of Au NRs was covered with SAM of thiol molecules coupled with fluorescent labels

22

(F1). Femtosecond laser irradiation at 950 nm produced hot electrons selectively in the

23

reactive Au NR along the longitudinal direction. It induced Au-S desorption, leaving the site 17


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1

open to a new fluorophore (F2) with thiol functional group at the other end. Point

2

accumulation for imaging in nanoscale topography (PAINT) maps showed the transition from

3

Au NRs with F1, the combination of bare Au NR and Au NRs with F1 to Au NRs with both

4

F1 and F2 by detecting fluorescence signals from Au NR arrays before and after the

5

irradiation (Figure 6(d)). These results provide us useful information on exactly where to

6

generate and detect plasmonic hot carriers even with a more complex geometry.

7

Another strong point of nanochemistry is that chemical probes often show better

8

feasibility and flexibility than microscopic characterizations for tracing pathways of

9

plasmonic hot carriers. According to experimental results observed by Zhai et al., anisotropic

10

growth of Au nanoprisms (l ~500 nm and t ~22 nm) from spherical Au NP nanoseeds (d ~7

11

nm) is originated by the fact that polyvinylpyrrolidone (PVP), one of the most common

12

surfactants used in the synthesis of NPs,89 also mediates growth reaction by accumulating hot

13

electrons along the perimeter of Au nanoprisms.83 This indicates that spatial mapping of the

14

adsorption site of PVP could obtain the information on the generation site of plasmonic hot

15

carriers in real-space. Indeed, comparison between Figure 7(b) and (c) obtained from

16

nanoscale secondary-ion mass spectrometry (NanoSIMS) and SEM, respectively, indicates

17

that the position of selective decoration of PVP on Au nanoseeds is in accordance with the

18

growth direction of Au nanoprisms. It is noteworthy to stress that the spatial distribution of

19

plasmonic hot carriers figured out by chemical approach is in contrast with that obtained from

20

EELS maps of Au nanoprisms at resonant wavelength which apparently reveal hot spots in

21

the center (Figure 7(a)). Such discrepancy highlights the advantages of using chemical

22

probes as an imaging tool for plasmonic hot carriers under complex catalytic or synthetic

23

reaction conditions involving multiple constituent elements, reactive sites, and paths. 18


Page 18 of 40

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1

Conclusion and outlook

2

As described in Table 1, a variety of imaging methods validated the generation of plasmon

3

induced hot carriers. However, the real space mapping of the change in photocurrent level

4

with nanoscale precision hardly achieved in plasmonic photovoltaic or optoelectronic

5

devices. The changes in some physical properties are only transient under light illumination

6

and thus analytical tools to figure out the contribution of hot carriers are mostly limited to a

7

few optical mapping techniques. In this respect, we focused on the approaches with electron

8

microscopes and nanochemistry in this Perspective, which enable the real space

9

imaging/mapping of the presence, distribution and transfer of hot carriers in high precision.

10

In spite of advances that have been made, there is still room for further investigation,

11

modifications and improvement in terms of strategies, targets, precision or performance of

12

the plasmon mapping because micro-/nanoscopic imaging techniques aforementioned have

13

been exploited for a very limited number of material systems and only proof-of-concept

14

studies have been performed with the nanochemistry techniques.

15

It is worth noting that the information gained from spatial (the specific region where

16

to observe plasmonic hot carriers) and quantitative (the amount of available plasmonic hot

17

carriers) studies of plasmonic hot carriers is generally based on numerical predictions. Thus,

18

we can predict that a combination of plasmonic hot carrier imaging techniques summarized in

19

this Perspective and numerical simulations can give us an idea of composition or geometry of

20

plasmonic structures which empirically and efficiently convert absorption event into

21

generation of plasmonic hot carriers. In the application point of view, it will eventually

22

provide a more direct and practical clue to the design of plasmonic structures that can

23

significantly enhance photocatalytic, electrocatalytic, photovoltaic, and optoelectronic 19


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1

Page 20 of 40

properties.

2

Other materials exhibiting physical and optical properties akin to those of general

3

noble metals (a large number of free electrons on the surface interacting at a specific

4

wavelength of incident light, followed by the evolution of SPR band-like optical signals) can

5

also be applied to the aforementioned techniques to reveal their fundamental plasmonic

6

characteristics.90 Specifically, non-noble metals (Al and Cu),43,

7

(Cu2-xX; X: S, Se or Te),92 doped-metal oxides (Al- or Ga-doped ZnO, Nb-doped TiO2, P-

8

doped Si, etc.),93 metal oxides with oxygen vacancy94 and two dimensional electrides (Ca2N,

9

Y2C, etc)95-97 are promising candidates. Compared with noble metals, they show distinctly

10

different absorption and scattering characteristics in terms of wavelength range and band

11

alignment at the heterojunction in devices. Therefore, simple demonstrations of the above-

12

mentioned plasmonic hot carrier imaging techniques would be interesting to understand

13

fundamental properties of energetic charge carriers generated by new plasmonic materials.

45, 91

copper chalcogenides

14

Finally, mapping temporal change of plasmonic hot carriers (hot carrier dynamics) in

15

real-space is also attractive since it can provide a deeper insight into the functional principle

16

and the specificity of plasmonic hot carriers.28 By tracing simple redox reactions (described

17

in the nanochemistry section) at different time scales, we can elucidate reaction pathways or

18

rate-determining steps in plasmon-mediated catalytic or synthetic processes. Comparison of a

19

certain redox reaction driven by different sources, i.e., conventional reducing agents or

20

plasmonic hot carriers, is also needed if we can visualize systems at the very early stage.

21 22

20


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1

ACS Photonics

FIGURES

2 3

Figure 1. Fundamental mechanism of plasmon excitation and decay. (a)-(b) Absorption

4

processes of Au through intra- and inter-band transitions, respectively. Reproduced with 21


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Page 22 of 40

1

permission from ref 22. Copyright 2015 Nature Publishing Group. (c) -(f) Plasmonic exciation

2

and dynamics of non-radiative decay. (g) Redirected flow of photon by plasmon resonance.

3

(c)-(g): Reproduced with permission from ref

4

(h) Charge separation pathways between metal and semiconductor. (i) Illustration of Au-

5

tipped CdSe NR and their electronic structure and transfer by PICTT mechanism. (h)-(i):

6

Reproduced with permission from ref

7

Advancement

40.

23.

Copyright 2015 Nature Publishing Group.

Copyright 2015, American Association for the of

22


Science.

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1 2

Figure 2. Catalytic advantages of plasmonic hot carriers. (a) A schematic description of

3

plasmonic antenna/catalytic reactor heterojunction. Reproduced with permission from ref 43.

4

Copyright 2016 National Academy of Science. (b) An illustrated representation of hot

5

electron transfer to the conduction band of ce-MoS2 for plasmon-mediated electrocatalytic

6

hydrogen evolution. Reproduced with permission from ref

7

Chemical Society. (c) The generation of high valence Ni sites on the surface of Ni(OH)2

8

catalyst via hot hole-induced oxidation for oxygen evolution reaction. Reproduced with

9

permission from ref 52. Copyright 2016 American Chemical Society.

10 11

23


51.

Copyright 2015 American

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Page 24 of 40

1 2

Figure 3. Direct visualizations of LSPR modes. Direct visualizations of LSPR modes (a) A

3

series of EELS-STEM spectra on Ag nanoprisms and three corresponding EELS intensity

4

mapping images after Gaussian fitting for resonance modes at 1.75 (top), 2.70 (middle), and

5

3.70 eV (bottom), respectively. Reproduced with permission from ref

6

Springer Nature. (b) LSPR spectral components of an Ag nanocube with five distinct

7

excitation modes labelled α, β, γ, δ and ε, obtained from applying non-negative matrix

8

factorization (NMF) to unprocessed EELS spectra (blue-dots). All spectra were measured at

9

the position of large blue marker in an inset image (top).; 3D visualizations of EELS-LSPR

10

with combining five distinct LSPR modes (bottom). Reproduced with permission from ref 65.

11

Copyright 2013 Springer Nature. (c) 2D spatial mapping images of CL spectra along an Au

12

nanowire at distinct resonance wavelengths (592, 640 and 730 nm). Reproduced with 24


14.

Copyright 2007

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1

permission from ref 67. Copyright 2007 American Chemical Society. (d) 2D imaging of EELS

2

(top) and CL spectra (bottom) for an Au triangular nanoprism. Reproduced with permission

3

from ref 75. Copyright 2015 American Chemical Society. (e) Images of surface potential for

4

Ag nanoparticles on a GaN layer under dark (top) and UV illumination conditions (bottom)

5

measured by KPFM. Reproduced with permission from ref

6

Nature.

7

25


15.

Copyright 2017 Springer

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Page 26 of 40

1 2

Figure 4. Hot carrier imaging techniques. (a) Illustration of nanoscale mapping setup with

3

SNoiM on GaAs/AlGaAs quantum well. (b) SEM image of GaAs/AlGaAs quantum well. (c)

4

Hot electron distribution mapping and (d) corresponding electrostatic potential energy of the

5

conduction band edge. (a)-(d): Reproduced with permission from ref

6

American Association for the Advancement of Science. (e) Schematic illustration of circuit

7

for hot-electron nanoscopy (TIA: transimpedance amplifier). (f) Diagram of electronic band

8

of metal tip and semiconductor (Egap: band gap energy of semiconductor, Φm and χs are the

9

work function of metal and semiconductor, respectively; r is the radius of depletion region).

10

(g)-(h) Topography and plasmonic hot electron mapping image of locally oxidized GaAs 26


59.

Copyright 2018

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ACS Photonics

surface. (e)-(h): Reproduced with permission from ref 81. Copyright 2013 Springer Nature.

2 3

Figure 5. (a) Hot electron conversion of 4-NTP to 4-ATP. (b) Schematic illustration of Ag

4

BT structure after resonant light irradiation and (c) reaction with carboxylic acid

5

functionalized Au NPs in HEPES buffer containing 1-Ethyl-3-(3-dimethylaminopropyl)-

6

carbodiimide/N-hydroxysuccinimide (EDC/NHS) coupler. Tips marked with red color on the

7

Ag BT in (b) indicate the region where the transition of 4-NTP to 4-ATP has mainly

8

occurred. The formation of amide bond linked the Au NPs to the Ag BT as depicted on the

9

left side of (c). (d) Near-field distribution map of Ag BT calculated by FDTD simulation at

10

633 nm for parallel polarization. (e) SEM image of Ag BT decorated with Au NPs after the

11

plasmonic excitation and incubation in Au NP dispersion. (f) The histogram summarized the

12

location and the frequency of Au NP appearance after plasmon-driven chemical reaction. It

13

maps the density/energy of plasmonic hot carriers and the most reactive sites in Ag BT.

14

Reprinted with permission from 38. Copyright 2017 Springer Nature.

15

27


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Page 28 of 40

1 2

Figure 6. (a) Schematic illustration of hot electron-induced polymerization on Au

3

nanoparticle-on-mirror (NPoM) structure. (b) SEM images of Au NPoM (top) and Au NPoM

4

with PNIPAM layer in the middle (bottom). All scale bars are 100 nm.84 Copyright 2017

5

American Chemical Society. (c) Schematic illustration and (d) PAINT images of Au antenna

6

array with different composition of thiol molecules depending on the orientation of Au NR

7

and light polarization. Reproduced with permission from ref

8

Chemical Society.

9 10

28


87.

Copyright 2018 American

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1 2

Figure 7. (a) Annular dark-field scanning transmission electron microscopy (ADF-STEM)

3

images (top) and corresponding EELS maps of Au nanoprisms (bottom). (b) SEM images of

4

Au nanoprisms before (left) and after (right) irradiation. Dashed lines in the right image

5

represent Au nanoprisms in the left image. Energies of incident light (Einc) are indicated in

6

these images. All scale bars are 100 nm. (c) Ion-induced SEM image of Au nanoprisms (left)

7

and corresponding NanoSIMS images to map elemental distributions of

8

12C14N-

9

Copyright 2016 Springer Nature.

197Au-

(middle) and

specifically adsorbed onto PVP (right). Reproduced with permission from ref

29


83.

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Page 30 of 40

Table 1. Methods to visualize the generation and distribution of plasmonic hot carriers. Methodology

Mechanisms

Advantages

Disadvantages

Ref.

Device

Measuring the plasmon-enhanced photocurrent, photoresponse or photoconductivity in photovoltaic or optoelectronic devices

Well-developed model system to confirm the electrical gain from plasmonic hot carriers

Difficulty in real space monitoring

98-101

Catalysis

Measuring the plasmon-enhanced photocurrent or photoresponse in photoand/or electrocatalytic systems

Easy detection with an ample amount of photoand/or electrocatalysts

Difficulty in real space monitoring

Physics

Observing the change in the general photophysical properties in a given material

Simultaneous detection of the presence of plasmonic hot carriers and their effect on the physical properties

Limited number of characterization tools due to the temporary change under illumination

104-105

Electron microscope

Investigating the change in the photophysical properties in a given material by microscopic excitation and detection

High resolution imaging in spectral and spatial distribution

High complexity of characterization tools

59, 81

Chemistry

Tracking the change in the structural information, optical properties or chemical compositions after the plasmon-driven chemical reactions

A wide variety of redox reactions / Easily accessible characterization tools

Difficulty in real time monitoring

30


6, 30, 44, 5153, 102-103

38, 82, 84-85, 87-88, 106

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ACS Photonics

AUTHOR INFORMATION Corresponding author *E-mail: [email protected] ORCID Yu Jin Jang: [email protected] / 0000-0001-8116-3618 Kyungwha Chung: [email protected] / 0000-0002-6774-4720 June Sang Lee: [email protected] / 0000-0001-8766-3269 Chi Hun Choi: [email protected] / 0000-0001-8203-0788 Ju Won Lim: [email protected] / 0000-0001-7021-3173 Dong Ha Kim: [email protected] / 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.

ACKNOWLEDGEMENTS This

work

was

supported

by

grant

(2017R1A2A1A05022387

and

NRF-

2018R1A6A3A11044025) of National Research Foundation of Korea funded by the Korean Government

31


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