In this talk, I will discuss the most recent findings reported by our and other research groups that shed light on the nanoscale properties of mono- and bimetallic nanostructures. This information was revealed by tip-enhanced Raman spectroscopy (TERS), a modern analytical technique that has single-molecule sensitivity and subnanometer spatial resolution. TERS findings have shown that plasmonic reactivity and the selectivity of bimetallic nanostructures are governed by the nature of the catalytic metal and the strength of the rectified electric field on their surfaces. TERS has also revealed that the catalytic properties of bimetallic nanostructures directly depend on the interplay between the catalytic and plasmonic metals. We anticipate that these findings will be used to tailor synthetic approaches that are used to fabricate novel nanostructures with desired catalytic properties. The experimental and theoretical results discussed in my talk will facilitate a better understanding of TERS and explain artifacts that could be encountered upon TERS imaging of a large variety of samples. Consequently, plasmon-driven chemistry should be considered as an essential part of near-field microscopy.

Gradually varying thickness thin-film waveguide was applied to tip-enhanced Raman spectroscopy of molecular films and dielectric substrate. The waveguide as a scanning probe with nanometer-resolution showed that high-durability, a long lifetime, good reproducibility, and an electric field enhancement up to ~10^7. Finite element analysis indicated that the waveguide works in 3.8-octave spanning with high stability, tolerance, and design freedom.

Designing, synthesizing and controlling plasmonic metal nanostructures with a superhigh precision (nm or sub-nm precision) are of paramount importance for the reliable and widespread use of plasmonic nanostructures in optics, nanoscience, chemistry, materials science, energy and biotechnology. In particular, synthesizing plasmonic nanostructures, often with a nanogap, that can generate ultrastrong, controllable and quantifiable optical signals is the key to the practical use of plasmonic enhancement-based spectroscopies such as surface-enhanced Raman scattering (SERS), but has been highly challenging. Here, I will introduce the design, synthetic strategies and characterization of molecularly tunable and structurally reproducible plasmonically coupled and enhanced nanostructures (e.g., plasmonic nanogap structures) with strong, controllable and quantifiable plasmonic signals including SERS signals. I will then show their potential in addressing some of important challenges in plasmonics, biosensing, bioimaging and biocomputing including quantitative SERS and scalable DNA computing, and discuss how these new plasmonic materials and platforms can lead us to new breakthroughs in next-generation disease diagnostics including the liquid biopsies for early-stage cancers and infectious diseases.

Plasmon-enhanced biosensing has attracted considerable attention due to its capability of highly sensitive biomolecular detection and analysis. Here, we demonstrate several types of gap-mode plasmon-enhanced spectroscopies for detection of biomolecular complexes. The first one is demonstrated with a nanometric gap between a silver tip and gold substrate, enabling to detect gap-enhanced Raman scattering from an inorganic-binding peptide and to spectroscopically elucidate the binding dynamics and mechanism. The other demonstration is based on gap-mode SPR sensing of coronavirus (SARS-CoV-2) in which antibody-functionalized gold nanoparticle with relatively large diameter played crucial plasmonic roles in detecting SARS-CoV-2 nucleocapsid proteins at femtomolar lever.

Plasmon nanofocusing has been gaining much attention as a tool to create a nanoscale light source over a broad wavelength range. The nanoscale light source is generated by plasmons propagating on a tapered plasmonic structure toward its apex, which eventually induces a strong light source at the apex in a nanometric volume. It has recently been recognized as a broadband plasmon phenomenon because it is just based on plasmon propagation. We have recently reported the generation of white nanolight source and its application for broadband scattering spectral near-field optical imaging. We will talk more about recent advances that we made using the broadband property of plasmon nanofocusing.

Almost all local deviations from the perfect crystalline structure will express themselves through a change of symmetry in the proximity of the perturbation as a consequence of which the Raman spectrum is expected to change. These changes typically occur on length scales between a few unit cells and tens of nanometers. Tip-enhanced Raman spectroscopy offers both the spatial resolution and signal enhancement to detect these areas. Knowing that regions with technically different crystallographic phases exist naturally and may also be engineered, the capability of imaging these regions opens opportunities for targeted surface engineering of functional surfaces.

In this communication, we will present the concept of Electrochemical Tip Surface-Enhanced Raman Spectroscopy (EC-Tip-SERS) on a redox- and Raman- active model molecule, the Nile blue, in which a temporal resolution in the order of millisecond can be attained. Then, we will consider the use of Tip-SERS in the characterization of complex rotaxane-based giant assemblies containing porphyrins, and in probing their molecular motion on gold surfaces . Finally, we will demonstrate the use of EC-Tip-SERS in scrutinizing electrocatalytic transient mechanisms, such as those associated to the oxygen reduction reaction using methyl viologen SAMs as electrocatalyts.

In scanning near-field optical microscope, the optical resolution is determined by the actual size of the optical nano-antenna at the apex of the tip. In this talk, I will introduce our recent improvements in fabricating the ‘Campanile’ near-field Probe with the gap in a sub-20nm scale size. The near-field optical performance of the probe will be demonstrated through the polarization-resolved transmission measurement and the nano-photoluminescence mapping of the WSe2 monolayer.

SERS have been attracted as a bioanalytical tool by utilizing plasmonic nanomaterials to enhance the intensity of Raman scattering. Two different analytes such as nucleic acid target or natural biomolecules inside single cell have been investigated in our research group. For nucleic acid target, it is inevitable for the use of surrogate to determine the presence of specific nucleic acid target. Our recent study showed that SERS is superior to assay performance of fluorescence-based assay (Sensors and Actuators B: Chemical Volume 329, 15 February 2021, 129134, ACS Appl. Mater. Interfaces 2022, 14, 1, 138–149). Instead obtaining the Raman signals from surrogate, obtaining molecular information from the cell is also interesting and meaningful applications. For this purpose, we investigated the molecular signals of mitochondria with mitochondria targeting AuNPs and Raman analysis for single live cell. Diverse changes of molecular signals could be obtained depends on the type of stimulus such as chemical or physical. In this talk, I will focus on the recent progress studied in this area (ACS omega 4 (5), 8188-8195, Nanoscale Advances 3 (12), 3470-3480).

SERS is widely recognized as a powerful tool for in-situ observation of electrified interfaces such as electrode-electrolyte interfaces because of the high chemical sensitivity and high surface selectivity. It is, however, known that SERS spectra often suffer from generation of spectral background; the background continuum dominates especially in lower-frequency region. Recently, the origin of the background continuum has been ascribed to plasmon-enhanced inelastic light scattering by free electrons in the conduction band of solid surface. In this talk, we will demonstrate that both electronic SERS, as a spectral background, and low-frequency vibrational SERS are useful for studying electrified interfaces.

The growing need for higher power and energy density batteries requires a fundamental understanding of the solid electrode-liquid electrolyte interface (SLI). Details governing the salt solvation and desolvation mechanisms are critical for the interfacial charge transfer process across the solid-liquid interface. I will show in this talk that gap-mode SERS is ideal to probe salt solvation structure in the immediate vicinity (< 20 nm) of the solid-liquid interface. SERS lacks the nanoscale resolution in the sample plane. To compensate for this, TERS is used to probe the decomposition product or solid-electrolyte interphase (SEI) at 10 nm nanoscale resolution.

Ultrafast vibrational and electronic spectroscopy unravels the dynamics underlying the functionality of quantum, energy, and biological materials. To understand and ultimately control the processes that underly the emergent function in these materials requires imaging the elementary excitations on their natural time and length scales. To achieve this goal, we have developed scanning probe microscopy with ultrafast and shaped laser pulse excitation for multiscale spatio-temporal optical nano-imaging. However, mostly limited to short-lived transient states, the contrast obtained has remained insufficient to probe important weak and long-lived excitations such as low-conductivity carriers, molecular vibrations, lattice phonons, and their couplings. Here, we demonstrate ultrafast heterodyne pump-probe (HPP) nano-imaging to simultaneously resolve quantum dynamics in space, time, and frequency. In ultrafast infrared nano-imaging based on excitation modulation and sideband detection we isolate and selectively characterize excited-state electron and vibration dynamics from femto- do micro-second times scales. In exemplary applications I will show ultrafast movies to resolve the fundamental quantum dynamics from the few-femtosecond coherent to the thermal transport regime. Specifically, in quantum materials, in ultrafast nano-imaging of photoinduced carrier and phase behavior we identify distinct transient nano-domain behavior revealing competing electronic and lattice degrees of freedom. Further in lead halide perovskites from transient vibrational nano-FTIR imaging we resolve excited state polaron dynamics and polaron-cation coupling underlying their photovoltaic response. Lastly, by simultaneous probing of carrier dynamics and interfacial phonon softening we establish nano-thermometry to imagine interfacial thermal transport dynamics in 2D semiconductor/insulator heterostructures. These examples show how HPP nano-imaging opens the door to elementary processes and interactions in functional materials with full spatio-temporal-spectral resolution.

We discuss about ultra-sensitive infrared spectroscopic techniques enhanced by metamaterials. To suppress unwanted background and noises in IR spectroscopy, metamaterial absorber with three-dimensional vertical-oriented metal-insulator-metal (v-MIM) structure was introduced. Owing to small footprint of v-MIM, the density of hot spots was dramatically increased resulting in strong signal enhancement and efficient suppression of background light. Using this device, 20 ppm concentration of carbon dioxide and butane molecules were detected. In addition, metamaterial reflector made of all dielectric structure were also investigated. The dielectric metasurfaces consisting of silicon microdisk arrays were designed and fabricated, realizing perfect reflection at certain mid-infrared wavelengths.

This work describes extreme light localization for intracellular molecular imaging and sensing with a high signal-to-noise ratio and precision. We explore localization techniques by which achievable resolution may be customized for subcellular dynamics of molecular complexes. We have also conducted plasmon-enhanced fluorescence correlation spectroscopy of cellular organelles with improved precision. The approach was extended to switching-based light localization to circumvent the diffraction limit and to use random disordered composite metallic islands for improved structured light microscopy. Extreme light localization also proves useful for enhancing Raman microscopy. Localization-based super-resolved Raman microscopy and techniques in combination with structured illumination will be discussed.

We report the first observation of optical near-field coupling to an ultrafast wavepacket of free, low-energy electrons. Transient optical near-fields, highly spatially confined around a nanometer-sized Yagi-Uda-antenna are probed in a point-projection-microscope with 30 -fs

Label-free chemical volumetric imaging is highly desired for biomedical applications and material science. Here, we present bond-selective intensity diffraction tomography (BS-IDT) based on the integration of mid-infrared photothermal microscopy with the computational imaging technique. BS-IDT demonstrates high-speed (up to ~6Hz) and large field of view (~100µmx100µm) chemical 3D imaging and enables the mid-infrared fingerprint spectra extraction with high fidelity. The superior performance of BS-IDT is validated by volumetric hyperspectral chemical imaging on cancer cells and Caenorhabditis elegans.

Two-dimensional (2D) materials are very promising materials for many applications. Stacking order of the 2D materials has a strong influence in their optical and electronic properties, e. g. the semiconducting bandgap and spin properties of 2-layer and 3-layer MoS2 show strong dependence on its stacking order. While conventional semiconductor devices are fabricated using different materials, i.e. hetero-structures, 2D material-based can be homo-structured utilizing the stacking dependent property. Such devices are made by using the same material, but differ in thickness or in stacking sequence, to perform various functions, e.g. in pn junction, transistors, solar cells and LEDs.

Two-dimensional (2D) hybrid organic-inorganic halide perovskites exhibit enhanced environmental stability and efficient excitonic emission. Altering soft organic and rigid inorganic layers produce a complex response to high pressure, resulting in various phenomena such as tuning of band gap and excitonic energy, phase transitions, and PL enhancement. Understanding the structure-properties relations is crucial to achieving similar improved properties under ambient conditions. In this talk, several indicative examples of the high-pressure evolution of 2D perovskites with different organic components are presented. Discussion of the structural and optical properties is based on in-situ Raman and PL microscopy and imaging.