Near-Field Manipulation in a Scanning Tunneling Microscope Junction with Plasmonic Fabry-Pérot Tips

Upgrade of a variable temperature scanning tunneling microscope for nanometer-scale spectromicroscopy

Tip-enhanced Raman spectroscopy (TERS), tip-enhanced photoluminescence (TEPL), and scanning tunneling microscope-induced luminescence (STML) combine the high spatial resolution of probe microscopies with the spectroscopic capabilities of optical techniques. Here, we describe the upgrade of an ultrahigh vacuum (UHV) variable-temperature scanning probe microscope (VT-SPM) to perform tip-enhanced spectromicroscopy experiments at cryogenic temperatures. The home-made design includes a portable focusing lens (NA=0.45) that allows the simultaneous collection and injection of light from the tip-sample junction while assuring easy tip and sample transfers. We demonstrate the capabilities of our upgrade to resolve electroluminescence (EL), Raman, and TERS spectra using plasmonically active probes (Ag and Au tips) on various surfaces. We are able to observe the vibrational levels of C60 deposited on Ag(111) with a lateral resolution of ∼2 nanometers. Moreover, we use the tunability of the gap plasmon distribution to observe intense anti-Stokes signals of C60, highlighting the spectral sensitivity of the system. This upgrade opens new possibilities for studying surface chemistry, catalysis, and molecular electronics at state-of-the-art spatial and spectral resolutions using accessible SPM systems.•Portable lens holder•In-situ adjustable position•Nanometer-scale vibrational spectroscopy.

Quantitative comparison of local field enhancement from tip-apex and plasmonic nanofocusing excitation via plasmon-assisted field emission resonances

Plasmonic nanofocusing via off-site excitation offers a promising approach to minimize background light scattering and enhance excitation efficiency in tip-enhanced optical spectroscopy. However, a comprehensive understanding of its effectiveness compared to direct tip-apex excitation remains limited. Here we introduce plasmon-assisted field emission resonances as a practicable approach for quantitative evaluation of the local field enhancement in a scanning-tunneling-microscope junction via off-site excitation compared to direct tip-apex excitation. By using single-groove pyramidal tips suitable for multi-wavelength excitations, we find that the near-field intensity is approximately 3.8 and 1.7 times higher for off-site excitation than for direct apex excitation at excitation wavelengths of 780 nm and 633 nm, respectively. These results are further supported by numerical electromagnetic field simulations. Our findings demonstrate the effective implementation of plasmonic nanofocusing in low-temperature scanning tunneling microscopy, paving the way for more precise and background-free tip-enhanced optical spectroscopy at the atomic scale.

Nanoscale chemical characterization of materials and interfaces by tip-enhanced Raman spectroscopy

Materials and their interfaces are the core for the development of a large variety of fields, including catalysis, energy storage and conversion. In this case, tip-enhanced Raman spectroscopy (TERS), which combines scanning probe microscopy with plasmon-enhanced Raman spectroscopy, is a powerful technique that can simultaneously obtain the morphological information and chemical fingerprint of target samples at nanometer spatial resolution. It is an ideal tool for the nanoscale chemical characterization of materials and interfaces, correlating their structures with chemical performances. In this review, we begin with a brief introduction to the nanoscale characterization of materials and interfaces, followed by a detailed discussion on the recent theoretical understanding and technical improvements of TERS, including the origin of enhancement, TERS instruments, TERS tips and the application of algorithms in TERS. Subsequently, we list the key experimental issues that need to be addressed to conduct successful TERS measurements. Next, we focus on the recent progress of TERS in the study of various materials, especially the novel low-dimensional materials, and the progresses of TERS in studying different interfaces, including both solid-gas and solid-liquid interfaces. Finally, we provide an outlook on the future developments of TERS in the study of materials and interfaces.

Broadband Tip-Enhanced Nonlinear Optical Response in a Plasmonic Nanocavity

We report a significantly broad nonlinear optical response enhanced in a tip-substrate plasmonic nanocavity. Focusing on the near-field second harmonics of the wavelength-tunable femtosecond laser, we demonstrate that the tip-enhancement of nonlinear optical effects efficiently works over the broad wavelength range through the visible to infrared region. We also found that this broadband nonlinear optical property is directly affected not only by the nanometer-scale sharpness of the tip apexes but also by the micrometer-scale surface geometry of the tip shafts. While spatially nonlocal plasmonic modes excited throughout the micrometer-scale tip shafts enhance near-to-mid-infrared incoming light, the radiation of visible-to-near-infrared second harmonics is boosted by localized plasmons at the nanogap. These two plasmonic modes simultaneously affect the excitation and emission processes, realizing the strong and broad enhancement of second harmonic generation. Our results provide a new basis for the physical understanding and fine manipulation of nonlinear optical phenomena enhanced in plasmonic nanocavities.

Inelastic Light Scattering in the Vicinity of a Single-Atom Quantum Point Contact in a Plasmonic Picocavity

Electromagnetic fields can be confined in the presence of metal nanostructures. Recently, subnanometer scale confinement has been demonstrated to occur at atomic protrusions on plasmonic nanostructures. Such an extreme field may dominate atomic-scale light-matter interactions in "picocavities". However, it remains to be elucidated how atomic-level structures and electron transport affect plasmonic properties of a picocavity. Here, using low-temperature optical scanning tunneling microscopy (STM), we investigate inelastic light scattering (ILS) in the vicinity of a single-atom quantum point contact (QPC). A vibration mode localized at the single Ag adatom on the Ag(111) surface is resolved in the ILS spectrum, resulting from tip-enhanced Raman scattering (TERS) by the atomically confined plasmonic field in the STM junction. Furthermore, we trace how TERS from the single adatom evolves as a function of the gap distance. The exceptional stability of the low-temperature STM allows to examine distinctly different electron transport regimes of the picocavity, namely, in the tunneling and QPC regimes. This measurement shows that the vibration mode localized at the adatom and its TERS intensity exhibits a sharp change upon the QPC formation, indicating that the atomic-level structure has a crucial impact on the plasmonic properties. To gain microscopic insights into picocavity optomechanics, we scrutinize the structure and plasmonic field in the STM junction using time-dependent density functional theory. The simulations reveal that atomic-scale structural relaxation at the single-atom QPC results in a discrete change of the plasmonic field strength, volume, and distribution as well as the vibration mode localized at the single atom. These findings give a qualitative explanation for the experimental observations. Furthermore, we demonstrate that strong ILS is a characteristic feature of QPC by continuously forming, breaking, and reforming the atomic contact and how the plasmonic resonance evolves throughout the nontunneling, tunneling, and QPC regimes.

Waveguide-Integrated Light-Emitting Metal-Insulator-Graphene Tunnel Junctions

Ultrafast interfacing of electrical and optical signals at the nanoscale is highly desired for on-chip applications including optical interconnects and data processing devices. Here, we report electrically driven nanoscale optical sources based on metal-insulator-graphene tunnel junctions (MIG-TJs), featuring waveguided output with broadband spectral characteristics. Electrically driven inelastic tunneling in a MIG-TJ, realized by integrating a silver nanowire with graphene, provides broadband excitation of plasmonic modes in the junction with propagation lengths of several micrometers (∼10 times larger than that for metal-insulator-metal junctions), which therefore propagate toward the junction edge with low loss and couple to the nanowire waveguide with an efficiency of ∼70% (∼1000 times higher than that for metal-insulator-metal junctions). Alternatively, lateral coupling of the MIG-TJ to a semiconductor nanowire provides a platform for efficient outcoupling of electrically driven plasmonic signals to low-loss photonic waveguides, showing potential for applications at various integration levels.

SPP standing waves within plasmonic nanocavities

Surface plasmons usually take two forms: surface plasmon polaritons (SPP) and localized surface plasmons (LSP). Recent experiments demonstrate an interesting plasmon mode within plasmonic gaps, showing distinct characters from the two usual forms. In this investigation, by introducing a fundamental concept of SPP standing wave and an analytical model, we reveal the nature of the recently reported plasmon modes. The analytical model includes SPP propagating and SPP reflection within a metal-insulator-metal (MIM) cavity, which is rechecked and supplemented by numerical simulations. We systematically analyze SPP standing waves within various nanocavities. During the discussion, some unusual phenomena have been explained. For example, the hot spot of a nanodimer could be off-tip, depending on the order of standing wave mode; and that a nanocube on metal film can be viewed as a nanocube dimer with the same separation. And many other interesting phenomena have been discussed, such as dark mode of SPP standing wave and extraordinary optical transmission. The study gives a comprehensive understanding of SPP standing waves, and may promote the applications of cavity plasmons in ultrasensitive bio-sensings.

Joule Heating in Single-Molecule Point Contacts Studied by Tip-Enhanced Raman Spectroscopy

Heating and cooling in current-carrying molecular junctions is a crucial issue in molecular electronics. The microscopic mechanism involves complex factors such as energy inputs, molecular properties, electrode materials, and molecule-electrode coupling. To gain an in-depth understanding, it is a desired experiment to assess vibrational population that represents the energy distribution stored within the molecule. Here, we demonstrate the direct observation of vibrational heating in a single C₆₀ molecule by means of tip-enhanced Raman spectroscopy (TERS). The heating of respective vibrational modes is monitored by anti-Stokes Raman scattering in the TERS spectra. The precise control of the gap distance in the single-molecule junction allows us to reveal a qualitatively different heating mechanism in distinct electron transport regimes, namely, the tunneling and single-molecule point contact (SMPC) regimes. Strong Joule heating via inelastic electron-vibration scattering occurs in the SMPC regime, whereas optical heating is predominant in the tunneling regime. The strong Joule heating at the SMPC also leads to a pronounced red shift of the Raman peak position and line width broadening. Furthermore, by examining the SMPC with several types of contact surfaces, we show that the heating efficiency is related to the current density at the SMPC and the vibrational dissipation channels into the electrode.

Nanoscale coherent phonon spectroscopy

Coherent phonon spectroscopy can provide microscopic insight into ultrafast lattice dynamics and its coupling to other degrees of freedom under nonequilibrium conditions. Ultrafast optical spectroscopy is a well-established method to study coherent phonons, but the diffraction limit has hampered observing their local dynamics directly. Here, we demonstrate nanoscale coherent phonon spectroscopy using ultrafast laser-induced scanning tunneling microscopy in a plasmonic junction. Coherent phonons are locally excited in ultrathin zinc oxide films by the tightly confined plasmonic field and are probed via the photoinduced tunneling current through an electronic resonance of the zinc oxide film. Concurrently performed tip-enhanced Raman spectroscopy allows us to identify the involved phonon modes. In contrast to the Raman spectra, the phonon dynamics observed in coherent phonon spectroscopy exhibit strong nanoscale spatial variations that are correlated with the distribution of the electronic local density of states resolved by scanning tunneling spectroscopy.

Plasmonic phenomena in molecular junctions: principles and applications

Molecular junctions are building blocks for constructing future nanoelectronic devices that enable the investigation of a broad range of electronic transport properties within nanoscale regions. Crossing both the nanoscopic and mesoscopic length scales, plasmonics lies at the intersection of the macroscopic photonics and nanoelectronics, owing to their capability of confining light to dimensions far below the diffraction limit. Research activities on plasmonic phenomena in molecular electronics started around 2010, and feedback between plasmons and molecular junctions has increased over the past years. These efforts can provide new insights into the near-field interaction and the corresponding tunability in properties, as well as resultant plasmon-based molecular devices. This Review presents the latest advancements of plasmonic resonances in molecular junctions and details the progress in plasmon excitation and plasmon coupling. We also highlight emerging experimental approaches to unravel the mechanisms behind the various types of light-matter interactions at molecular length scales, where quantum effects come into play. Finally, we discuss the potential of these plasmonic-electronic hybrid systems across various future applications, including sensing, photocatalysis, molecular trapping and active control of molecular switches.

Nanoscale Heating of an Ultrathin Oxide Film Studied by Tip-Enhanced Raman Spectroscopy

We report on the nanoscale heating mechanism of an ultrathin ZnO film using low-temperature tip-enhanced Raman spectroscopy. Under the resonance condition, intense Stokes and anti-Stokes Raman scattering can be observed for the phonon modes of a two-monolayer (ML) ZnO on an Ag(111) surface, enabling us to monitor local heating at the nanoscale. It is revealed that the local heating originates mainly from inelastic electron tunneling through the electronic resonance when the bias voltage exceeds the conduction band edge of the 2-ML ZnO. When the bias voltage is lower than the conduction band edge, the local heating arises from two different contributions, namely direct optical excitation between the interface state and the conduction band of 2-ML ZnO or injection of photoexcited electrons from an Ag tip into the conduction band. These optical heating processes are promoted by localized surface plasmon excitation. Simultaneous mapping of tip-enhanced Raman spectroscopy and scanning tunneling spectroscopy for 2-ML ZnO including an atomic-scale defect demonstrates visualizing a correlation between the heating efficiency and the local density of states, which further allows us to analyze the local electron-phonon coupling strength with ∼2  nm spatial resolution.

Remote Dual-Cavity Enhanced Second Harmonic Generation in a Hybrid Plasmonic Waveguide

On-chip nanoscale optical platforms capable of efficient second harmonic generation (SHG) are highly desired for optical sensing, subwavelength coherent sources, and quantum photonic devices. Here, we develop a remotely excited dual cavity resonance scheme to achieve significantly enhanced SHG in a CdSe nanobelt on Au film hybrid waveguide system. The SHG emission with superior efficiency originates from counter-propagating plasmonic modes interference in a horizontal Fabry-Pérot (FP) cavity enabled by remote excitation of propagating surface plasmons, which is further enhanced through a vertical FP cavity. With this effective cooperation of hybrid plasmon modes and FP cavity modes, 2 orders of magnitude enhancement of the conversion efficiency (3.5 × 10^-4 W^-1) is achieved compared to the off-resonance case. Our design provides new insight into the development of a multifunctional hybrid plasmonic device toward on-chip nonlinear nanophotonic applications.

Atomic-Scale Structural Fluctuations of a Plasmonic Cavity

Optical spectromicroscopies, which can reach atomic resolution due to plasmonic enhancement, are perturbed by spontaneous intensity modifications. Here, we study such fluctuations in plasmonic electroluminescence at the single-atom limit profiting from the precision of a low-temperature scanning tunneling microscope. First, we investigate the influence of a controlled single-atom transfer from the tip to the sample on the plasmonic properties of the junction. Next, we form a well-defined atomic contact of several quanta of conductance. In contact, we observe changes of the electroluminescence intensity that can be assigned to spontaneous modifications of electronic conductance, plasmonic excitation, and optical antenna properties all originating from minute atomic rearrangements at or near the contact. Our observations are relevant for the understanding of processes leading to spontaneous intensity variations in plasmon-enhanced atomic-scale spectroscopies such as intensity blinking in picocavities.

The Expanding Frontiers of Tip-Enhanced Raman Spectroscopy

Fundamental understanding of chemistry and physical properties at the nanoscale enables the rational design of interface-based systems. Surface interactions underlie numerous technologies ranging from catalysis to organic thin films to biological systems. Since surface environments are especially prone to heterogeneity, it becomes crucial to characterize these systems with spatial resolution sufficient to localize individual active sites or defects. Spectroscopy presents as a powerful means to understand these interactions, but typical light-based techniques lack sufficient spatial resolution. This review describes the growing number of applications for the nanoscale spectroscopic technique, tip-enhanced Raman spectroscopy (TERS), with a focus on developments in areas that involve measurements in new environmental conditions, such as liquid, electrochemical, and ultrahigh vacuum. The expansion into unique environments enables the ability to spectroscopically define chemistry at the spatial limit. Through the confinement and enhancement of light at the apex of a plasmonic scanning probe microscopy tip, TERS is able to yield vibrational fingerprint information of molecules and materials with nanoscale resolution, providing insight into highly localized chemical effects.

Optical scanning tunneling microscopy based chemical imaging and spectroscopy

Through coupling optical processes with the scanning tunneling microscope (STM), single-molecule chemistry and physics have been investigated at the ultimate spatial and temporal limit. Electrons and photons can be used to drive interactions and reactions in chemical systems and simultaneously probe their characteristics and consequences. In this review we introduce and review methods to couple optical imaging and spectroscopy with scanning tunneling microscopy. The integration of the STM and optical spectroscopy provides new insights into individual molecular adsorbates, surface-supported molecular assemblies, and two-dimensional materials with subnanoscale resolution, enabling the fundamental study of chemistry at the spatial and temporal limit. The inelastic scattering of photons by molecules and materials, that results in unique and sensitive vibrational fingerprints, will be considered with tip-enhanced Raman spectroscopy. STM-induced luminescence examines the intrinsic luminescence of organic adsorbates and their energy transfer and charge transfer processes with their surroundings. We also provide a survey of recent efforts to probe the dynamics of optical excitation at the molecular level with scanning tunneling microscopy in the context of light-induced photophysical and photochemical transformations.

Dramatic Enhancement of Tip-Enhanced Raman Scattering Mediated by Atomic Point Contact Formation

Tip-enhanced Raman scattering (TERS) in ångström-scale plasmonic cavities has drawn increasing attention. However, Raman scattering at vanishing cavity distances remains unexplored. Here, we show the evolution of TERS in transition from the tunneling regime to atomic point contact (APC). A stable APC is reversibly formed in the junction between an Ag tip and ultrathin ZnO or NaCl films on the Ag(111) surface at 10 K. An abrupt increase of the TERS intensity occurs upon APC formation for ZnO, but not for NaCl. This remarkable observation is rationalized by a difference in hybridization between the Ag tip and these films, which determines the contribution of charge transfer enhancement in the fused plasmonic junction. The strong hybridization between the Ag tip and ZnO is corroborated by the appearance of a new vibrational mode upon APC formation, whereas it is not observed for the chemically inert NaCl.

Characteristics of a bidirectional multifunction focusing and plasmon-launching lens with multiple periscope-like waveguides

A device incorporating a series of periscope-like waveguides to achieve bidirectional focusing and plasmon launching is proposed. Optimizing the number, positions, and dimensions of the waveguides and tuning the waveguide optical paths both produce the required phase shifts to shape wavefronts and achieve constructive interference at the desired points. Due to the symmetry and reversibility of the structure, the lens can focus the light incident on both sides. Energy redistribution to a specific multi-focus can also be achieved by applying appropriate phase shifts. This simple and high performance structure makes the bidirectional plasmonic launcher easy to implement in various application situations.

Atomic-Scale Dynamics Probed by Photon Correlations

Light absorption and emission have their origins in fast atomic-scale phenomena. To characterize these basic steps (e.g., in photosynthesis, luminescence, and quantum optics), it is necessary to access picosecond temporal and picometer spatial scales simultaneously. In this Perspective, we describe how state-of-the-art picosecond photon correlation spectroscopy combined with luminescence induced at the atomic scale with a scanning tunneling microscope (STM) enables such studies. We outline recent STM-induced luminescence work on single-photon emitters and the dynamics of excitons, charges, molecules, and atoms as well as several prospective experiments concerning light-matter interactions at the nanoscale. We also describe future strategies for measuring and rationalizing ultrafast phenomena at the nanoscale.

Single Photon Emission from a Plasmonic Light Source Driven by a Local Field-Induced Coulomb Blockade

A hallmark of quantum control is the ability to manipulate quantum emission at the nanoscale. Through scanning tunneling microscopy-induced luminescence (STML), we are able to generate plasmonic light originating from inelastic tunneling processes that occur in the vacuum between a tip and a few-nanometer-thick molecular film of C₆₀ deposited on Ag(111). Single photon emission, not of molecular excitonic origin, occurs with a 1/e recovery time of a tenth of a nanosecond or less, as shown through Hanbury Brown and Twiss photon intensity interferometry. Tight-binding calculations of the electronic structure for the combined tip and Ag-C₆₀ system results in good agreement with experiment. The tunneling happens through electric-field-induced split-off states below the C₆₀ LUMO band, which leads to a Coulomb blockade effect and single photon emission. The use of split-off states is shown to be a general technique that has special relevance for narrowband materials with a large bandgap.

Tip-Enhanced Raman Excitation Spectroscopy (TERES): Direct Spectral Characterization of the Gap-Mode Plasmon

The plasmonic properties of tip-substrate composite systems are of vital importance to near-field optical spectroscopy, in particular tip-enhanced Raman spectroscopy (TERS), which enables operando studies of nanoscale chemistry at a single molecule level. The nanocavities formed in the tip-substrate junction also offer a highly tunable platform for studying field-matter interactions at the nanoscale. While the coupled nanoparticle dimer model offers a correct qualitative description of gap-mode plasmon effects, it ignores the full spectrum of multipolar tip plasmon modes and their interaction with surface plasmon polariton (SPP) excitation in the substrate. Herein, we perform the first tip-enhanced Raman excitation spectroscopy (TERES) experiment and use the results, both in ambient and aqueous media, in combination with electrodynamics simulations, to explore the plasmonic response of a Au tip-Au substrate composite system. The gap-mode plasmon features a wide spectral window corresponding to a host of tip plasmon modes interacting with the plasmonic substrate. Simulations of the electric field confinement demonstrate that optimal spatial resolution is achieved when a hybrid plasmon mode that combines a multipolar tip plasmon and a substrate SPP is excited. Nevertheless, a wide spectral window over 1000 nm is available for exciting the tip plasmon with high spatial resolution, which enables the simultaneous resonant detection of different molecular species. This window is robust as a function of tip-substrate distance and tip radius of curvature, indicating that many choices of tips will work, but it is restricted to wavelengths longer than ∼600 nm for the Au tip-Au substrate combination. Other combinations, such as Ag tip-Ag substrate, can access wavelengths as low as 350 nm.

Resolving the Correlation between Tip-Enhanced Resonance Raman Scattering and Local Electronic States with 1 nm Resolution

Low-temperature tip-enhanced Raman spectroscopy (TERS) enables chemical identification with single-molecule sensitivity and extremely high spatial resolution even down to the atomic scale. The large enhancement of Raman scattering obtained in TERS can originate from physical and/or chemical enhancement mechanisms. Whereas physical enhancement requires a strong near-field through excitation of localized surface plasmons, chemical enhancement is governed by resonance in the electronic structure of the sample, which is also known as resonance Raman spectroscopy. Here we report on tip-enhanced resonance Raman spectroscopy (TERRS) of ultrathin ZnO layers epitaxially grown on a Ag(111) surface, where both enhancement mechanisms are operative. In combination with scanning tunneling spectroscopy (STS), it is demonstrated that the TERRS intensity strongly depends on the local electronic resonance of the ZnO/Ag(111) interface. We also reveal that the spatial resolution of TERRS is dependent on the tip-surface distance and reaches nearly 1 nm in the tunneling regime, which can be rationalized by strong-field confinement resulting from an atomic-scale protrusion on the tip apex. Comparison of STS and TERRS mapping clearly shows a correlation between resonantly enhanced Raman scattering and the local electronic states at near-atomic resolution. Our results suggest that TERRS is a new approach for the atomic-scale optical characterization of local electronic states.