Ultrafast Plasmon-Enhanced Hot Electron Generation at Ag Nanocluster/Graphite Heterojunctions

Localized surface plasmon resonances of size-selected large silver nanoclusters (n = 70-100) soft-landed on a C60 organic substrate

Silver nanoclusters (Agn NCs) exhibit a remarkable optical property known as localized surface plasmon resonance (LSPR) in the visible to ultraviolet wavelengths. In this study, we address the size gap in LSPR responses between small NCs and nano-islands by synthesizing large Agn NCs with a countable number of atoms (n = 70-100) using a magnetron sputtering method, which were precisely size-selected and soft-landed onto substrates. The monodispersed Agn NCs were immobilized on a pre-decorated substrate with fullerene (C60) molecules, and their LSPR behaviors were characterized using two-photon photoemission (2PPE) spectroscopy. Due to the distinct polarization selectivity of incident light associated with LSPR, the intensity ratio between p- and s-polarized lights (Ip/Is) in 2PPE spectroscopy serves as a reliable indicator of LSPR and its structural correlations. From n = 70 to 100, the Ip/Is value gradually decreases as the cluster size increases. This decrease is attributed to the enhancement of s-polarized light (Is), indicating that large Agn NCs on a C60 substrate undergo a deformation from spherical to flattened geometries, particularly above approximately n = 55.

Site-Selective Excitation of Ti3+ Ions in Rutile TiO2 via Anisotropic Intra-Atomic 3d → 3d Transition

Site-selective excitation (SSE), which is usually realized by tuning the wavelength of absorbed light, is an ideal way to study bond-selective chemistry, analyze the crystal structure, investigate protein conformation, etc., eventually leading to active manipulation of desired processes. Herein, SSE has been explored in (110)-, (100)-, and (011)-faced rutile TiO2, a prototypical material in both surface science and photocatalysis fields. Using ultraviolet photoelectron spectroscopy and photon energy-, substrate orientation-, and laser polarization-dependent two-photon photoemission spectroscopy (2PPE), intra-atomic 3d → 3d transition from the split Ti3+ 3d orbitals, i.e., band gap states and excited states at ∼1.00 eV below and ∼2.40 eV above the Fermi level, respectively, has been proven for all of the samples, suggesting that it is a common property of this material. The distinct structure of rutile TiO2 results in the anisotropic 3d → 3d transitions with the transition dipole moment along the long axes ([110] and [11̅0]) of TiO6 blocking units. This anisotropy facilitates the selective excitation of Ti3+ ions in the two types of TiO6, which cannot be realized by conventional wavelength tuning, via polarization alignment of the excitation source. Discovery in this work builds the foundation for future investigation of site-selective photophysical and photochemical processes and eventually possible active manipulation in this material at the atomic level.

The construction of lattice-matched CdS-Ag2S heterojunction photocatalysts: High-intensity built-in electric field effectively boosts bulk-charge separation efficiency

The built-in electric field of heterojunction can effectively promote carrier separation and transfer. While, its interface orientation is often random, leading to lattice mismatch and high resistance, thus limiting the efficiency of interfacial charge transfer. Herein, the lattice-matched heterojunction (CdS-Ag₂S) was constructed by ion-exchange epitaxial growth. The results of surface photovoltage spectroscopy (SPV), transient photovoltage spectroscopy (TPV), and time-resolved photoluminescence (TRPL) show that the lattice-matched heterojunction has higher charge separation efficiency and longer photogenerated carrier lifetime than that of lattice-mismatched one. The lattice-matched CdS-Ag₂S has a high built-in electric field (BIEF) value of 103.42 and a bulk-charge separation (BCS) efficiency of 68.71%, which is about three times higher than that of the lattice-mismatched heterojunction (CdS-Ag₂S-M). In addition, the photodegradation efficiency of CdS-Ag₂S towards norfloxacin (NOR) was also 3.4 times higher than that of CdS-Ag₂S-M. The above results and density functional theory (DFT) calculations indicate that improving the lattice matching at the heterojunction is beneficial for establishing a high-intensity built-in electric field and effectively promoting bulk-charge separation efficiency, thus achieving excellent photocatalytic performance. This work provides an essential reference for the research of high-performance heterojunction photocatalysts.

Epitaxial growth of highly symmetrical branched noble metal-semiconductor heterostructures with efficient plasmon-induced hot-electron transfer

Epitaxial growth is one of the most commonly used strategies to precisely tailor heterostructures with well-defined compositions, morphologies, crystal phases, and interfaces for various applications. However, as epitaxial growth requires a small interfacial lattice mismatch between the components, it remains a challenge for the epitaxial synthesis of heterostructures constructed by materials with large lattice mismatch and/or different chemical bonding, especially the noble metal-semiconductor heterostructures. Here, we develop a noble metal-seeded epitaxial growth strategy to prepare highly symmetrical noble metal-semiconductor branched heterostructures with desired spatial configurations, i.e., twenty CdS (or CdSe) nanorods epitaxially grown on twenty exposed (111) facets of Ag icosahedral nanocrystal, albeit a large lattice mismatch (more than 40%). Importantly, a high quantum yield (QY) of plasmon-induced hot-electron transferred from Ag to CdS was observed in epitaxial Ag-CdS icosapods (18.1%). This work demonstrates that epitaxial growth can be achieved in heterostructures composed of materials with large lattice mismatches. The constructed epitaxial noble metal-semiconductor interfaces could be an ideal platform for investigating the role of interfaces in various physicochemical processes.

Two-Photon Photoemission Spectroscopy and Microscopy for Electronic and Plasmonic Characterizations of Molecularly Designed Organic Surfaces

Functional surfaces decorated with organic molecules and/or nanoclusters (NCs) composed of several tens of atoms are promising for use in future photoelectronic substrates, whose functionalities are governed by molecular local electronic/plasmonic excitations at the interfaces. Here, we combine two-photon photoemission spectroscopy (2P-PES) and microscopy (2P-PEEM) to investigate the local excited-state dynamics at organic surfaces functionalized with NCs. The 2P-PES and 2P-PEEM for organic fullerene (C₆₀) layers on graphite and Au substrates demonstrated photophysical characterization of electronic and plasmonic properties, including propagating surface plasmon polaritons (SPPs). The SPP propagation at the Au interface buried by overlayered C₆₀ can be visualized by Ag_n NC deposition, which enhances plasmon-induced hot electrons, where the threshold number of Ag atoms (n ≥ 9) for the plasmonic response is revealed by the size dependence of 2P-PES for Ag_n NCs on C₆₀ layers.

Silver nanoparticle enhanced metal-organic matrix with interface-engineering for efficient photocatalytic hydrogen evolution

Integrating plasmonic nanoparticles into the photoactive metal-organic matrix is highly desirable due to the plasmonic near field enhancement, complementary light absorption, and accelerated separation of photogenerated charge carriers at the junction interface. The construction of a well-defined, intimate interface is vital for efficient charge carrier separation, however, it remains a challenge in synthesis. Here we synthesize a junction bearing intimate interface, composed of plasmonic Ag nanoparticles and matrix with silver node via a facile one-step approach. The plasmonic effect of Ag nanoparticles on the matrix is visualized through electron energy loss mapping. Moreover, charge carrier transfer from the plasmonic nanoparticles to the matrix is verified through ultrafast transient absorption spectroscopy and in-situ photoelectron spectroscopy. The system delivers highly efficient visible-light photocatalytic H₂ generation, surpassing most reported metal-organic framework-based photocatalytic systems. This work sheds light on effective electronic and energy bridging between plasmonic nanoparticles and organic semiconductors.

In Situ Raman Probing of Hot-Electron Transfer at Gold-Graphene Interfaces with Atomic Layer Accuracy

Plasmonic metals under photoexcitation can generate energetic hot electrons to directly induce chemical reactions. However, the capability and fundamental insights of the transportation of these hot electrons at plasmonic metal-2D material interfaces remain unclear. Herein, hot-electron transfer at Au-graphene interfaces has been in situ studied using surface-enhanced Raman spectroscopy (SERS) with atomic layer accuracy. Combining in situ SERS studies with density functional theory calculations, it is proved that hot electrons can be injected from plasmonic Au nanoparticles to graphene and directly penetrate graphene to trigger photocatalytic reactions. With increasing graphene layers, the transportation of hot electrons decays rapidly and would be completely blocked after five layers of graphene. Moreover, the transfer of hot electrons can be modulated by applying an external electric field, and the hot-electron transfer efficiency under electrochemical conditions is improved by over three times in the presence of a monolayer of graphene.

Confined Hot Electron Relaxation at the Molecular Heterointerface of the Size-Selected Plasmonic Noble Metal Nanocluster and Layered C60

The plasmonic response of metallic nanostructures plays a key role in amplifying photocatalytic and photoelectric conversion. Since the plasmonic behavior of noble metal nanoparticles is known to generate energetic charge carriers such as hot electrons, it is expected that the hot electrons can enhance conversion efficiency if they are transferred into a neighboring molecule or semiconductor. However, the method of transferring the energized charge carriers from the plasmonically generated hot electrons to the neighboring species remains controversial. Herein, we fabricated a molecularly well-defined heterointerface between the size-selected plasmonic noble-metal nanoclusters (NCs) of Ag_n (n = 3-55)/Au_n (n = 21) and the organic C₆₀ film to investigate hot electron generation and relaxation dynamics using time-resolved two-photon photoemission (2PPE) spectroscopy. By tuning the NC size and the polarization of the femtosecond excitation photons, the plasmonic behavior is characterized by 2PPE intensity enhancement by 10-100 times magnitude, which emerge at n ≥ 9 for Ag_n NCs. The 2PPE spectra exhibit contributions from low-energy electrons forming coherent plasmonic currents and hot electrons with an excitation energy up to photon energy owing to two-photon excitation of an occupied state of the Ag_n NC below the Fermi level. The time-resolved pump-probe measurements demonstrate that plasmon dephasing generates hot electrons which undergo electron-electron scattering. However, no photoemission occurs via the charge transfer state forming Ag_n⁺C₆₀^- located in the vicinity of the Fermi level. Thus, this study reveals the mechanism of ultrafast confined hot electron relaxation within plasmonic Ag_n NCs at the molecular heterointerface.

Hot-Carrier Generation in Plasmonic Nanoparticles: The Importance of Atomic Structure

Metal nanoparticles are attractive for plasmon-enhanced generation of hot carriers, which may be harnessed in photochemical reactions. In this work, we analyze the coherent femtosecond dynamics of photon absorption, plasmon formation, and subsequent hot-carrier generation through plasmon dephasing using first-principles simulations. We predict the energetic and spatial hot-carrier distributions in small metal nanoparticles and show that the distribution of hot electrons is very sensitive to the local structure. Our results show that surface sites exhibit enhanced hot-electron generation in comparison to the bulk of the nanoparticle. Although the details of the distribution depend on particle size and shape, as a general trend, lower-coordinated surface sites such as corners, edges, and {100} facets exhibit a higher proportion of hot electrons than higher-coordinated surface sites such as {111} facets or the core sites. The present results thereby demonstrate how hot carriers could be tailored by careful design of atomic-scale structures in nanoscale systems.

Ultrafast Photoemission Electron Microscopy: Imaging Plasmons in Space and Time

Plasmonics is a rapidly growing field spanning research and applications across chemistry, physics, optics, energy harvesting, and medicine. Ultrafast photoemission electron microscopy (PEEM) has demonstrated unprecedented power in the characterization of surface plasmons and other electronic excitations, as it uniquely combines the requisite spatial and temporal resolution, making it ideally suited for 3D space and time coherent imaging of the dynamical plasmonic phenomena on the nanofemto scale. The ability to visualize plasmonic fields evolving at the local speed of light on subwavelength scale with optical phase resolution illuminates old phenomena and opens new directions for growth of plasmonics research. In this review, we guide the reader thorough experimental description of PEEM as a characterization tool for both surface plasmon polaritons and localized plasmons and summarize the exciting progress it has opened by the ultrafast imaging of plasmonic phenomena on the nanofemto scale.

Infrared driven hot electron generation and transfer from non-noble metal plasmonic nanocrystals

Non-noble metal plasmonic materials, e.g. doped semiconductor nanocrystals, compared to their noble metal counterparts, have shown unique advantages, including broadly tunable plasmon frequency (from visible to infrared) and rich surface chemistry. However, the fate and harvesting of hot electrons from these non-noble metal plasmons have been much less explored. Here we report plasmon driven hot electron generation and transfer from plasmonic metal oxide nanocrystals to surface adsorbed molecules by ultrafast transient absorption spectroscopy. We show unambiguously that under infrared light excitation, hot electron transfers in ultrafast timescale (<50 fs) with an efficiency of 1.4%. The excitation wavelength and fluence dependent study indicates that hot electron transfers right after Landau damping before electron thermalization. We revealed the efficiency-limiting factors and provided improvement strategies. This study paves the way for designing efficient infrared light absorption and photochemical conversion applications based on non-noble metal plasmonic materials.

Hybrid quadrupole plasmon induced spectrally pure ultraviolet emission from a single AgNPs@ZnO:Ga microwire based heterojunction diode

Ultraviolet light-emitting materials and devices with high-efficiency are required for many applications. One promising way to enhance the ultraviolet luminescence efficiency is by incorporating plasmonic nanostructures. However, a large energy mismatch between the plasmons and the light emitters greatly limits the direct realization of light enhancement. In this work, a single Ga-doped ZnO microwire prepared with large-sized Ag nanoparticle (the diameter d ∼ 200 nm) deposition (AgNPs@ZnO:Ga MW) was utilized to construct a high-performance heterojunction diode, with p-GaN serving as the hole injection layer. In addition to enhanced optical output, the emission spectra also revealed that typical near-band-edge (NBE) emission with higher wavelength stability centered around 378.0 nm was achieved, accompanied by narrowing of the spectral linewidth to around 10 nm. Thus, the interfacial and p-GaN emissions were successfully suppressed. The spectral profile of the emission spectra of the heterojunction diodes precisely matched the photoluminescence spectrum of the single ZnO:Ga MW, which indicates that the single ZnO:Ga MW can act as the active region for the radiative recombination of electrons and holes in the diode operation. In the emission mechanism, hybrid quadrupole plasmons induce the generation of hot electrons, which are then injected into the conduction band of the neighboring ZnO:Ga and are responsible for the NBE-type emission of the single MW based heterojunction diode. This novel emission enhancement and modulation principle can aid in the design and development of new types of luminescent materials and devices with high-efficiency, spectral stability and spectral purity.

Visualization of Surface Plasmons Propagating at the Buried Organic/Metal Interface with Silver Nanocluster Sensitizers

Visualization of surface plasmon polariton (SPP) propagation at dielectric/metal interfaces is indispensable in providing opportunities for the precise designing and controlling of the functionalities of future plasmonic nanodevices. Here, we report the visualization of SPPs propagating along the buried organic/metal interface of fullerene (C₆₀)/Au(111), through dual-colored two-photon photoemission electron microscopy (2P-PEEM) which precisely visualizes the SPP propagation of plasmonic metal nanostructures. Although SPPs excited by near-infrared photons at the few monolayer C₆₀/Au(111) interface are clearly visualized as interference beat patterns between the SPPs and incident light, faithfully reflecting SPP properties modulated by the overlayer, photoemission signals are suppressed for thicker C₆₀ films, due to less valence electrons participating in 2P-photoemission processes. With the use of silver (Ag_n (n = 21 and 55)) nanoclusters, which exhibit enhancement of overall photoemission intensities due to localized surface plasmons functioning as SPP sensitizers, it is revealed that the 2P-PEEM is applicable to the imaging of SPPs for thick C₆₀/Au(111) interfaces, where SPP properties are hardly modulated by the added small amount (∼0.1 monolayer) of Ag_n sensitizers.

Optical field tuning of localized plasmon modes in Ag microcrystals at the nanofemto scale

Nanoscale plasmonic field enhancement at sub-wavelength metallic particles is crucial for surface sensitive spectroscopy, ultrafast microscopy, and nanoscale energy transduction. Here, we demonstrate control of the spatial distribution of localized surface plasmon modes at sub-optical-wavelength crystalline silver (Ag) micropyramids grown on a Si(001) surface. We employ multiphoton photoemission electron microscopy (mP-PEEM) to image how the plasmonic field distributions vary with the photon energy, light polarization, and phase in coherent two-pulse excitation. For photon energy hυ > 2.0 eV, the mP-PEEM images show single photoemission locus, which splits into a dipolar pattern that straddles the Ag crystal at a lower energy. We attribute the variation to the migration of plasmon resonances from the Ag/vacuum to the Ag/Si interfaces by choice of the photon energy. Furthermore, the dipolar response of the Ag/Si interface follows the polarization state of light: for linearly polarized excitations, the plasmon dipole follows the in-plane electric field vector, while for circularly polarized excitations, it tilts in the direction of the handedness due to the conversion of spin angular momentum of light into orbital angular momentum of the plasmons excited in the sample. Finally, we show the coherent control of the spatial plasmon distribution by exciting the sample with two identical circularly polarized light pulses with delay defined with attosecond precision. The near field distribution wobbles at the pyramid base as the pump-probe delay is advanced due to interferences among the contributing fields. We illustrate how the frequency, polarization, and pulse structure can be used to design and control plasmon fields on the nanofemto scale for applications in chemistry and physics.

Plasmon catalytic PATP coupling reaction on Ag-NPs/graphite studied via in situ electrochemical surface-enhanced Raman spectroscopy

Plasmon-induced hot holes and electrons play different roles in the PATP coupling reaction, resulting in two different catalytic reaction paths.

Space- and time-resolved UV-to-NIR surface spectroscopy and 2D nanoscopy at 1 MHz repetition rate

We describe a setup for time-resolved photoemission electron microscopy with aberration correction enabling 3 nm spatial resolution and sub-20 fs temporal resolution. The latter is realized by our development of a widely tunable (215-970 nm) noncollinear optical parametric amplifier (NOPA) at 1 MHz repetition rate. We discuss several exemplary applications. Efficient photoemission from plasmonic Au nanoresonators is investigated with phase-coherent pulse pairs from an actively stabilized interferometer. More complex excitation fields are created with a liquid-crystal-based pulse shaper enabling amplitude and phase shaping of NOPA pulses with spectral components from 600 to 800 nm. With this system we demonstrate spectroscopy within a single plasmonic nanoslit resonator by spectral amplitude shaping and investigate the local field dynamics with coherent two-dimensional (2D) spectroscopy at the nanometer length scale ("2D nanoscopy"). We show that the local response varies across a distance as small as 33 nm in our sample. Further, we report two-color pump-probe experiments using two independent NOPA beamlines. We extract local variations of the excited-state dynamics of a monolayered 2D material (WSe₂) that we correlate with low-energy electron microscopy (LEEM) and reflectivity measurements. Finally, we demonstrate the in situ sample preparation capabilities for organic thin films and their characterization via spatially resolved electron diffraction and dark-field LEEM.

Single-Plasmon Thermo-Optical Switching in Graphene

While plasmons in noble metal nanostructures enable strong light-matter interactions on commensurate length scales, the overabundance of free electrons in these systems inhibits their tunability by weak external stimuli. Countering this limitation, the linear electronic dispersion in graphene endows the two-dimensional material with both an enhanced sensitivity to doping electron density, enabling active tunability of its highly confined plasmon resonances, and a very low electronic heat capacity that renders its thermo-optical response extraordinarily large. Here we show that these properties combined enables a substantial optical modulation in graphene nanostructures from the energy associated with just one of their supported plasmons. We base our analysis on realistic, complementary classical and quantum-mechanical simulations, which reveal that the energy of a single plasmon, absorbed in a small, moderately doped graphene nanoisland, can sufficiently modify its electronic temperature and chemical potential to produce unity-order modulation of the optical response within subpicosecond time scales, effectively shifting or damping the original plasmon absorption peak and thereby blockading subsequent excitation of a second plasmon. The proposed thermo-optical single-plasmon blockade consists in a viable ultralow power all-optical switching mechanism for doped graphene nanoislands, while their combination with quantum emitters could yield applications in biological sensing and quantum nano-optics.

Plasmon-Induced Direct Hot-Carrier Transfer at Metal-Acceptor Interfaces

Plasmon-induced hot-carrier transfer from a metal nanostructure to an acceptor is known to occur via two key mechanisms: (i) indirect transfer, where the hot carriers are produced in the metal nanostructure and subsequently transferred to the acceptor, and (ii) direct transfer, where the plasmons decay by directly exciting carriers from the metal to the acceptor. Unfortunately, an atomic-level understanding of the direct-transfer process, especially with regard to its quantification, remains elusive even though it is estimated to be more efficient compared to the indirect-transfer process. This is due to experimental challenges in separating direct from indirect transfer as both processes occur simultaneously at femtosecond time scales. Here, we employ time-dependent density-functional theory simulations to isolate and study the direct-transfer process at a model metal-acceptor (Ag₁₄₇-Cd₃₃Se₃₃) interface. Our simulations show that, for a 10 fs Gaussian laser pulse tuned to the plasmon frequency, the plasmon formed in the Ag₁₄₇-Cd₃₃Se₃₃ system decays within 10 fs and induces the direct transfer with a probability of about 40%. We decompose the direct-transfer process further and demonstrate that the direct injection of both electrons and holes into the acceptor, termed direct hot-electron transfer (DHET) and direct hot-hole transfer (DHHT), takes place with similar probabilities of about 20% each. Finally, effective strategies to control and tune the probabilities of DHET and DHHT processes are proposed. We envision our work to provide guidelines toward the design of metal-acceptor interfaces that enable more efficient plasmonic hot-carrier devices.

Correlated Materials Characterization via Multimodal Chemical and Functional Imaging

Multimodal chemical imaging simultaneously offers high-resolution chemical and physical information with nanoscale and, in select cases, atomic resolution. By coupling modalities that collect physical and chemical information, we can address scientific problems in biological systems, battery and fuel cell research, catalysis, pharmaceuticals, photovoltaics, medicine, and many others. The combined systems enable the local correlation of material properties with chemical makeup, making fundamental questions of how chemistry and structure drive functionality approachable. In this Review, we present recent progress and offer a perspective for chemical imaging used to characterize a variety of samples by a number of platforms. Specifically, we present cases of infrared and Raman spectroscopies combined with scanning probe microscopy; optical microscopy and mass spectrometry; nonlinear optical microscopy; and, finally, ion, electron, and probe microscopies with mass spectrometry. We also discuss the challenges associated with the use of data originated by the combinatorial hardware, analysis, and machine learning as well as processing tools necessary for the interpretation of multidimensional data acquired from multimodal studies.

Surface enhanced perfect absorption in metamaterials with periodic dielectric nanostrips on silver film

Integrated dielectric metamaterials with plasmonic structures can cause drastic optical resonances and strengthen the capacity of light absorption. Here, we describe the optical properties of silicon nanoarrays on a thin silver film for extreme light confinement at subwavelength nanoscales. We attain the nearly total absorption in silicon nanostrips, which support magnetic quadruple Mie-type resonances in the visible regions. The Mie resonant field of the dielectric nanostrip engages the screening response of the silver film, resulting in plasmon resonance configuration and thus achieving perfect light absorption in the dielectric nanostrip. Moreover, we can attain similar results in other nanostructures, such as silicon cylinder and rhombus column arrays. Because it can sustain hybridized plasmon modes and magnetic modes, the combined system will benefit the application of solar energy accumulation.

Tuning the Intrinsic Properties of Carbon Nitride for High Quantum Yield Photocatalytic Hydrogen Production

The low quantum yield of photocatalytic hydrogen production in carbon nitride (CN) has been improved upon via the modulation of both the extrinsic and intrinsic properties of the material. Although the modification of extrinsic properties has been widely investigated in the past, recently there has been growing interest in the alteration of intrinsic properties. Refining the intrinsic properties of CN provides flexibility in controlling the charge transport and selectivity in photoredox reactions, and therefore makes available a pathway toward superior photocatalytic performance. An analysis of recent progress in tuning the intrinsic photophysical properties of CN facilitates an assessment of the goals, achievements, and gaps. This article is intended to serve this purpose. Therefore, selected techniques and mechanisms of the tuning of intrinsic properties of CN are critically discussed here. This article concludes with a recommendation of the issues that need to be considered for the further enhancement in the quantum efficiency of CN photocatalysts.

Tunable Nonthermal Distribution of Hot Electrons in a Semiconductor Injected from a Plasmonic Gold Nanostructure

For semiconductors photosensitized with organic dyes or quantum dots, transferred electrons are usually considered thermalized at the conduction band edge. This study suggests that the electrons injected from a plasmonic metal into a thin semiconductor shell can be nonthermal with energy up to the plasmon frequency. In other words, the electrons injected into the semiconductor are still hot carriers. Photomodulated X-ray absorption measurements of the Ti L_2,3 edge are compared before and after excitation of the plasmon in Au@TiO₂ core-shell nanoparticles. Comparison with theoretical predictions of the X-ray absorption, which include the heating and state-filling effects from injected hot carriers, suggests that the electrons transferred from the plasmon remain nonthermal in the ∼10 nm TiO₂ shell, due in part to a slow trapping in defect states. By repeating the measurements for spherical, rod-like, and star-like metal nanoparticles, the magnitude of the nonthermal distribution, peak energy, and number of injected hot electrons are confirmed to be tuned by the plasmon frequency and the sharp corners of the plasmonic nanostructure. The results suggest that plasmonic photosensitizers can not only extend the sunlight absorption spectral range of semiconductor-based devices but could also result in increased open circuit voltages and elevated thermodynamic driving forces for solar fuel generation in photoelectrochemical cells.

Sculpting Extreme Electromagnetic Field Enhancement in Free Space for Molecule Sensing

A strongly confined and enhanced electromagnetic (EM) field due to gap-plasmon resonance offers a promising pathway for ultrasensitive molecular detections. However, the maximum enhanced portion of the EM field is commonly concentrated within the dielectric gap medium that is inaccessible to external substances, making it extremely challenging for achieving single-molecular level detection sensitivity. Here, a new family of plasmonic nanostructure created through a unique process using nanoimprint lithography is introduced, which enables the precise tailoring of the gap plasmons to realize the enhanced field spilling to free space. The nanostructure features arrays of physically contacted nanofinger-pairs with a 2 nm tetrahedral amorphous carbon (ta-C) film as an ultrasmall dielectric gap. The high tunneling barrier offered by ta-C film due to its low electron affinity makes an ultranarrow gap and high enhancement factor possible at the same time. Additionally, its high electric permittivity leads to field redistribution and an abrupt increase across the ta-C/air boundary and thus extensive spill-out of the coupled EM field from the gap region with field enhancement in free space of over 103 . The multitude of benefits deriving from the unique nanostructure hence allows extremely high detection sensitivity at the single-molecular level to be realized as demonstrated through bianalyte surface-enhanced Raman scattering measurement.

Coherent Electron Transfer at the Ag/Graphite Heterojunction Interface

Charge transfer in transduction of light to electrical or chemical energy at heterojunctions of metals with semiconductors or semimetals is believed to occur by photogenerated hot electrons in metal undergoing incoherent internal photoemission through the heterojunction interface. Charge transfer, however, can also occur coherently by dipole coupling of electronic bands at the heterojunction interface. Microscopic physical insights into how transfer occurs can be elucidated by following the coherent polarization of the donor and acceptor states on the time scale of electronic dephasing. By time-resolved multiphoton photoemission spectroscopy (MPP), we investigate the coherent electron transfer from an interface state that forms upon chemisorption of Ag nanoclusters onto graphite to a σ symmetry interlayer band of graphite. Multidimensional MPP spectroscopy reveals a resonant two-photon transition, which dephases within 10 fs completing the coherent transfer.

Elucidating ultrafast electron dynamics at surfaces using extreme ultraviolet (XUV) reflection–absorption spectroscopy

Time-resolved XUV reflection–absorption spectroscopy probes core-to-valence transitions to reveal state-specific electron dynamics at surfaces.

Dependence of hot electron transfer on surface coverage and adsorbate species at semiconductor–molecule interfaces

Developing a molecular-level understanding of how a hot electron transfer process can be enhanced at semiconductor–molecule interfaces is central to advancing various future technologies.