Ilya Milekhin

Local optical properties of monolayer MoS₂ on rough plasmonic surfaces are governed by coupled effects of morphology, mechanical strain, defects, and electromagnetic enhancement. Here, the optical methods of gap-mode tip-enhanced Raman scattering (TERS) and tip-enhanced photoluminescence (TEPL), supported by direct structural methods of scanning electron microscopy (SEM) and atomic force microscopy (AFM), as well as by finite-element electromagnetic modeling are used to investigate a monolayer MoS₂ flake conformably covering a rough Au nanodisk approximately 50 nm in height. Under 638.2 nm excitation, the TEPL response is strongly heterogeneous and the emission maximum varies from approximately 655 to 670 nm, whereas the TERS phonon response is too weak for reliable nanoscale strain quantification. Excitation at 785.3 nm, which is below the optical band gap of monolayer MoS2 and therefore does not produce a detectable PL signal, yields a substantially stronger and more spatially confined TERS response at high-curvature asperities due to resonant plasmonic enhancement in the coupled tip–asperity junction. The calculations of 3D electric field distribution in the vicinity of a single Au nanodisk containing hemispherical protrusions covered by a single MoS2 layer show that the MoS₂ coating reduces the electric field in the vicinity of the nanostructure and shifts the coupled tip–protrusion plasmonic response toward longer wavelengths relative to the bare Au nanodisk. Moreover, the calculated Raman-related enhancement factor (|E|/|E₀|)⁴ becomes markedly stronger for 785.3 nm compared to 638.2 nm in agreement with our observations in the TERS experiments. The combined results separate wavelength-dependent plasmonic enhancement from morphology-related spectral heterogeneity and identify high-curvature regions as candidates for strain- and defect-localized emission, potentially relevant to single-photon sources.

Surface-enhanced infrared absorption (SEIRA) spectroscopy based on plasmonic nanoantennas is a powerful tool for ultrasensitive molecular detection; however, further enhancement of its performance remains a key challenge. In this work, we demonstrate arrays of gold nanoantennas embedded into a silicon substrate using metal-assisted chemical etching (MACE). This approach enables precise control of the nanoantenna embedment depth and formation of nanoscale capillary gaps. We show that both the embedment depth and the capillary gap critically influence the localized surface plasmon resonance (LSPR) energy and near-field distribution. Full-wave electromagnetic simulations reveal that the capillary gap acts as a key parameter governing field confinement. By comparing experimental and simulated IR transmission spectra, the gap size is determined to be 6±1 nm. The embedded geometry enhances the effective interaction volume between the analyte and the plasmonic field, resulting in improved SEIRA sensitivity by factor 3. This approach provides a robust platform for designing highly sensitive mid-IR plasmonic sensors.

We investigate structural and optical properties of single GaP nanowires (NWs) using atomic force microscopy and tip-enhanced Raman spectroscopy (TERS). The nanowires, with varying diameters and a Ga catalyst droplet at the tip, were grown on Si(111) substrates via a self-catalyzed vapor-liquid-solid mechanism. We perform TERS study on single NWs mechanically transferred on Si substrates with a thin Au layer utilizing TERS tips with Au and Ag coverage. It is shown that Ag-coated tip provides the maximal enhancement of TERS by optical phonons while probing GaP NWs with incident light at a wavelength of 532 nm. We detect background emission induced by gap plasmon inherent for joint system composed of Au cluster on the tip apex and Ga droplet of the NW using Au-coated TERS tip. TERS imaging enables us to track evolution of GaP phonon modes intensity along the NW with a spatial resolution of about 20 nm and to probe both the enhancement and quenching of TERS signal. Moreover, we manage to utilize phonon mode of Ag₂S originated from the Ag-coated TERS tip, to scan the NW and construct TERS map which reproduces the peculiarities of the TERS image obtained for GaP TO phonon intensity.

We study the dependence of the optical response of square lattices of plasmonic nanoparticles on the mutual orientation of the lattice and the plane of incidence of illuminating light. It is shown that the lattice sums for two orientations of the plane of incidence—along the rows of the squares and along their diagonal—are different. The presented micro-ellipsometric spectra confirm that different lattice sums and different conditions for the excitation of surface plasmons in the substrate at different orientations result in a distinct dispersion of the observed modes.

The structural properties of lattice-matched InAlAs/InP layers grown by molecular beam epitaxy have been studied using atomic force microscopy, scanning electron microscopy and micro-photoluminescence spectroscopy. The formation of the surface pits with lateral sizes in the micron range and a depth of about 2 ÷ 10 nm has been detected. The InP substrate annealing temperature and value of InAlAs alloy composition deviation from the lattice-matched In_xAl_1−xAs/InP case (x = 0.52) control the density of pits ranging from 5 × 10⁵ cm⁻² ÷ 10⁸ cm⁻². The pit sizes are controlled by the InAlAs layer thickness and growth temperature. The correlation between the surface pits and threading dislocations has been detected. Moreover, the InAlAs surface is characterized by composition inhomogeneity with a magnitude of 0.7% with the cluster lateral sizes and density close to these parameters for surface pits. The experimental data allow us to suggest a model where the formation of surface pits and composition clusters is caused by the influence of a local strain field in the threading dislocation core vicinity on In adatoms incorporating kinetic.

Monolayer triangular WS₂ and MoS₂ islands grown by chemical vapor deposition was investigated by near-field photoluminescence (nano-PL) enhanced by the metallized atomic force microscope (AFM) tip. To achieve maximum near-field response from WS₂ and MoS₂ materials fabricated Au and Ag metallized AFM tips were used. Maximum nano-PL responds from the islands is observed under the resonant conditions when the energy of the localized surface plasmon of the metallized probe coincides with the energy of the exciton luminescence of the WS₂ and MoS₂ materials. Nano-PL mapping of the exciton response allows visualizing structural defects and determine the local thickness changes of monolayer islands with nanometer spatial resolution.

We demonstrate local phonon analysis of single AlN nanocrystals by two complementary imaging spectroscopic techniques: tip-enhanced Raman scattering (TERS) and nano-Fourier transform infrared (nano-FTIR) spectroscopy. Strong surface optical (SO) phonon modes appear in the TERS spectra with their intensities revealing a weak polarization dependence. The local electric field enhancement stemming from the plasmon mode of the TERS tip modifies the phonon response of the sample, making the SO mode dominate over other phonon modes. The TERS imaging allows the spatial localization of the SO mode to be visualized. We were able to probe the angle anisotropy on the SO phonon modes in AlN nanocrystals with nanoscale spatial resolution. The excitation geometry and the local nanostructure surface profile determine the frequency position of SO modes in nano-FTIR spectra. An analytical calculation explains the behaviour of SO mode frequencies vs. tip position with respect to the sample.

Hexagonal boron nitride (hBN), sometimes referred to as white graphene, receives growing interest in the scientific community, especially when combined into van der Waals (vdW) homo- and heterostacks, in which novel and interesting phenomena may arise. hBN is also commonly used in combination with two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs). The realization of hBN-encapsulated TMDC homo- and heterostacks can indeed offer opportunities to investigate and compare TMDC excitonic properties in various stacking configurations. In this work, we investigate the optical response at the micrometric scale of mono- and homo-bilayer WS₂ grown by chemical vapor deposition and encapsulated between two single layers of hBN. Imaging spectroscopic ellipsometry is exploited to extract the local dielectric functions across one single WS₂ flake and detect the evolution of excitonic spectral features from monolayer to bilayer regions. Exciton energies undergo a redshift by passing from hBN-encapsulated single layer to homo-bilayer WS₂, as also confirmed by photoluminescence spectra. Our results can provide a reference for the study of the dielectric properties of more complex systems where hBN is combined with other 2D vdW materials into heterostructures and are stimulating towards the investigation of the optical response of other technologically-relevant heterostacks.