Transition metal dichalcogenides (TMDCs) have emerged as highly promising materials for nanophotonics and optoelectronics due to their exceptionally high refractive indices, strong excitonic photoluminescence (PL) in monolayer configurations, and the versatility to engineer van der Waals (vdW) heterostructures. In this work, we exploit the intense excitonic PL of a MoSe2 monolayer combined with the high refractive index of bulk WS2 to fabricate microdisk cavities with tunable light emission characteristics. These microdisks are created from a 50-nm-thick WS2/MoSe2/WS2 double heterostructure using frictional mechanical scanning probe lithography. The resulting cavities achieve a 4-10-fold enhancement in excitonic PL from the MoSe2 monolayer at wavelengths near 800 nm. The excitonic PL peak is modulated by sharp spectral features, which correspond to whispering gallery modes (WGMs) supported by the cavity. A microdisk with a diameter of 2.35 mu m demonstrates WGMs with a quality factor of up to 700, significantly surpassing theoretical predictions and suggesting strong potential for lasing applications. The spectral positions of the WGMs can be finely tuned by adjusting the microdisk's diameter and thickness, as confirmed by theoretical calculations. This approach offers a novel route for developing ultra-compact, all-TMDC double heterostructure light sources with record-small size.

Strong local plasmon enhancement by a metallised (Ag) atomic force microscopy (AFM) tip modifies the phonon response of AlN nanostructures, making the surface optical (SO) phonon modes dominant over other phonon modes in tip-enhanced Raman scattering (TERS). Here, we demonstrate polarization-dependent TERS by SO phonon modes in a single AlN nanorod, with the TERS spectra recorded for incident light polarization directed along or perpendicular to the TERS tip as well as perpendicular to or along the nanorod axis. TERS images recorded for two orthogonal polarizations reveal different distributions of the Raman intensity enhancement. In the conventional TERS geometry (light polarization along the TERS tip and perpendicular the nanorod), intense TERS signals stem from the nanorod centre, while a polarization rotation by 90 degrees with respect to the tip axis modifies the TERS image, leading to Raman intensity enhancement at the nanorod edges. This phenomenon is explained by the different local electric field distribution over the nanorod calculated in the TERS tip vicinity for two polarizations.

Phonon occupation is temperature (T) dependent; it may participate in scattering with electronic states of topological insulators (TIs). Longitudinal optical (LO) phonons A1g1$$ {\mathrm{A}}_{1g}<^>1 $$ and A1g2$$ {\mathrm{A}}_{1g}<^>2 $$ in Bi2 - xSbxTe3 - ySey (BSTS) was resonantly excited by a photon energy (Ep) 2.33 eV due to electronic transition of unoccupied conduction band. The intensity of these modes was enhanced in Bi2Te3 film at Ep 1.87 eV, which is close to the electronic transition of unoccupied Dirac states. Fr & ouml;hlich coupling strength was the main mechanism for higher intensity of A1g1$$ {\mathrm{A}}_{1g}<^>1 $$ and A1g2$$ {\mathrm{A}}_{1g}<^>2 $$ modes. At 300 K, the intensity of the A1g2$$ {\mathrm{A}}_{1g}<^>2 $$ mode was significantly decayed in both the BSTS and Bi2Te3 at Ep 2.33 and 1.87 eV due to the anharmonic coupling. However, at similar value of T, spectral profile of A1g1$$ {\mathrm{A}}_{1g}<^>1 $$ and Eg2$$ {\mathrm{E}}_g<^>2 $$ modes was not affected because of the lower probability of decay rate of these phonons. In resonant condition, well-resolved Raman forbidden surface modes were observed at T of 50 K. At 300 K, more asymmetric Fano profile of surface phonon was observed due to the anharmonic coupling. The study indicated that at high T, A1g1$$ {\mathrm{A}}_{1g}<^>1 $$ mode and anharmonic coupling may become primary cause for scattering with electronic states of the TIs. However, at low T, primarily, both the LO phonons participated in the scattering.

The continuous scaling of electronic devices leads to the necessity of controlling the structure of low-dimensional materials. Surface-enhanced Raman spectroscopy (SERS) is the technique of choice for studying the crystallinity, composition, and mechanical properties of nanomaterials. Surface-enhanced Raman scattering (SERS) utilizes the electromagnetic field generated by localized surface plasmon resonances (LSPR) in metal nanostructures to enhance the optical response from the sample. Previously the SERS activity of graphene placed on a substrate with an array of gold nanodisks was demonstrated and the enhancement of the main vibrational modes up to 50 times was achieved. Taking into account these results an approach for further improvement of the technique is suggested. It is known that SERS enhancement factor (EF) is dependent on the gap between a metal particle and a sample. In the present study, we exploited the magnetic properties of $\mathrm{Fe3O4} / \mathrm{Au}$ nanoparticle aggregates to make closer contact between the graphene film and the gold nanostructures on the Si substrate in the presence of an external magnetic field. Such an approach resulted in increasing the intensity of graphene G mode with the enhancement factor of 5. The first findings and further perspectives on the study are discussed.

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.

Monolayer triangular WS 2 and MoS 2 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 WS 2 and MoS 2 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 WS 2 and MoS 2 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.

Using in situ reflection electron microscopy we have presented latest studies of Si(111) and Bi2Se3(0001) surface processes during sublimation, homo- and heteroepitaxial growth of layered metal chalcogenides. A structural kinetic diagram of interaction between a selenium molecular beam and Si(111) surface has been demonstrated. We have shown congruent sublimation caused by annealing of Bi2Se3(0001) substrates in a selenium flux, visualized and described layer by layer homoepitaxial Bi2Se3 growth. We have presented techniques of layered SnSe2 and In2Se3 growth on Si(111) and Bi2Se3(0001) surfaces and shown that heteroepitaxial growth starts with the 2D islands formation. The nucleation and growth of an impurity-induced phase in the form of 0.4 nm high 2D islands during high-temperature submonolayer indium deposition on Bi2Se3(0001) under exposure to Se molecular beam has been demonstrated.

Experimental data on studying the phonon and optical properties of GaAs nanowires with orientation (111) located on a gold substrate with the help of Raman scattering spectroscopy (RSS) and photoluminescence (PL) are presented. Structural parameters of nanowires are determined by the atomic-force microscopy (AFM) and scanning electron microscopy (SEM) methods. In the micro-RSS and micro-PL spectra of a single GaAs nanowire, the modes of optical phonons of GaAs and their overtones up to the third order and an exciton luminescence band are observed. In the micro-PL spectra, anisotropy of the PL intensity is observed; the maximum/minimum signal is observed at the polarization-vector direction along/across the wire. Mapping of nano-PL of a single GaAs nanowire is performed with a spatial resolution of 20 nm, which is significantly smaller than the diffraction limit. When passing to the nanometer scale, a plasmon amplification of the signal of the near-field exciton nano-PL conditioned by the metallized AFM-needle is revealed.

Nanoscale deformations and corrugations occur in graphene-like two-dimensional materials during their incorporation into hybrid structures and real devices, such as sensors based on surface-enhanced Raman scattering (SERS-based sensors).

We have studied the structural and morphological features of SnSe films grown on Si(111) and BiSe(0001) surfaces in an reflection electron microscope. On both substrates, the SnSe growth started at 100 degrees C as an amorphous layer, and when thickness reached 1 nm, crystallized by raising the growth temperature to 250 degrees C without interruption of Sn and Se fluxes. The introduction of this growth-initiating stage has decreased the concentration of screw dislocations on films' surfaces to similar to 18 and similar to 2 mu m for the Si(111) and BiSe(0001) substrates, respectively. High-resolution transmission electron microscopy investigation has shown that the layered SnSe film has a hexagonal lattice structure corresponding to the space group (no. 164) with lattice parameters = 0.38 nm and = 0.62 nm. Raman spectroscopy has shown vibrational modes corresponding to the 1T-SnSe phase. We have shown that the decrease in Se:Sn flux ratio switches growth mode from Frank-van der Merwe type SnSe epitaxy to Volmer-Weber type nucleation of SnSe 3D islands.

Layered SnSe _2 films of nearly 50 and 30 nm in thickness were grown on Si(111) and Bi _2 Se _3 (0001) substrates, respectively, with the use of in situ reflection electron microscopy. In both cases, the growth of films occurred by a multilayer mechanism with the formation of pronounced hills. The height of atomic steps was measured by atomic force microscopy (AFM) as 0.6 nm, which corresponded to the SnSe _2 layer thickness. The surface ex situ AFM image of SnSe _2 grown on a Si(111) substrate demonstrated a high concentration of screw dislocations in the film ( ∼ 12 μ m ^-2 ) and the existence of domains with a triangular faceting of steps, which had three types of orientation with respect to the substrate. The growth of a SnSe _2 film on single crystal Bi _2 Se _3 surface(0001) was revealed to occur with the formation of hills, which had a hexangular faceting and were identically oriented with respect to the substrate. The hills were formed by the multilayer mechanism both in the regions, where screw dislocations cropped out, and due to the periodic nucleation of 2D islands on the highest terraces, which attained 1 μ m in size. Using Raman scattering, the films on both substrates were shown to have the spectra typical for the 1T-SnSe _2 phase.

In this work, the phonon spectra of atomically thin ZnSenanoplatelets(NPLs) were studied using Raman and infrared spectroscopies. Atomicallythin ZnSe NPLs were grown on the Si substrate covered with a 100 nmthick gold layer by the colloidal method in the temperature rangeof 100-170 & DEG;C. The triangular and rectangular NPLs with2.5- and 4-monolayer thicknesses, respectively, were synthesized,as determined by atomic force microscopy (AFM) measurements. Longitudinaloptical (LO) and transverse optical (TO) phonon modes in ZnSe NPLswere found at 198 and 252 cm(-1), respectively, andthe surface optical phonon mode was observed at about 216 cm(-1). The LO and TO phonon modes revealed the opposite frequency shiftswith increasing growth temperature. The effect was induced by thephonon confinement, and it confirmed the formation of NPLs of differentthicknesses. Comparing the frequency positions of longitudinal andTO phonon modes confined in ZnSe NPLs with the data on the dispersionof optical phonons in bulk ZnSe, the thickness of the NPLs formedat a low temperature was derived as 0.567 nm, correlating well withthe AFM results.

This work is devoted to the study of plasmon-enhanced Raman scattering by the fundamental vibrational modes of multilayered graphene films. The film thickness was similar to 3.5 nm, which corresponds to similar to 10 monolayers. Multilayered graphene films were placed on a plasmonic substrate consisting of arrays of gold nanodisks (50-250 nm in diameter). Surface-enhanced Raman scattering by the main vibrational modes of multilayered graphene film placed on an array of Au nanodisks of various sizes was implemented. The measurements were performed at excitation wavelengths of 532, 638, and 785 nm. A resonant SERS enhancement of the main vibrational modes of multilayered graphene by a factor of 25 was achieved for nanodisks with a diameter of 103 nm upon excitation at 638 nm. A stronger local enhancement of Raman scattering in multilayered graphene (by a factor of 50) placed on Au nanodisk array is achieved using gap-mode tip-enhanced Raman scattering (gap-mode TERS). Nanofolds in the graphene film appeared due to the corrugated surface of the plasmonic substrate were visualized with nanometer spatial resolution. It is shown that the frequency positions of G and 2D modes of nanofolds decrease with respect to the corresponding values in flat multilayered graphene manifesting mechanical stresses in the nanofolds up to 0.7%. The results obtained shed light on the effects of the interaction of multilayered graphene with metal nanostructures and are important in creating hybrid metal/graphene plasmonic substrates.

Owing to the extraordinary physical and chemical properties, and the potential to couple with nanoplasmonic structures, two-dimensional (2D) transition metal dichalcogenides are promising materials for next-generation (opto-)electronic devices. Targeting the application stage, it is essential to understand the mechanisms of photoluminescence (PL) quenching and enhancement at the nanoscale. In this work, using monolayer MoSe2/hBN heterostructure on Au nanotriangles (NTs) as an example, we report on the local PL quenching and enhancement in the quantum tunneling regime at MoSe2/hBN/plasmonic nanostructure interfaces. By exploiting tip-enhanced photoluminescence spectroscopy, we were able to resolve and image the nanostructures locally. Moreover, by studying the different near-field emission behavior of MoSe2/SiO2, MoSe2/hBN, MoSe2/NT, and MoSe2/hBN/NT, we investigate the localized surface plasmon resonance, electron tunneling, and highly localized strain as the three competing mechanisms of local PL quenching and enhancement in the quantum tunneling regime at the nanoscale.

The paper presents the results of investigation of the optical and structural properties for tungsten disulfide (WS2) islands using atomic force microscopy (AFM), Raman scattering, and photoluminescence (PL). The morphology and local composition as well as oxidation induced changes in the WS2 islands were investigated. Micro- Raman and micro-PL spectra reveal the fundamental vibrational end exciton modes in WS2 islands, respectively. The intensity distribution maps of the phonon and exciton modes are discussed. The frequency dependences for the fundamental vibrational modes (A 1g and E2g) in WS2 islands on their thickness is obtained. A low-frequency shift of the E2g mode and a high-frequency shift of the A 1g mode with increasing the number of layers are reported. A shift in the PL peak energy position and changes in intensity induced by defects in the WS2 islands are observed. It is suggested that the shift of the PL energy, apparently, can be associated with the sample oxidation.

SnSe2 films of nearly 50 and 30 nm in thickness were grown on Si(111) and Bi2Se3(0001) substrates, respectively, with the use of in situ reflection electron microscopy. In both cases, the growth of films occurred by a multilayer mechanism with the formation of pronounced hills. The height of atomic steps was measured by atomic force microscopy (AFM) as 0.6 nm, which corresponded to the SnSe2 layer thickness. The surface ex situ AFM image of SnSe2 grown on a Si(111) substrate demonstrated a high concentration of screw dislocations in the film (similar to 12 mu m(-2)) and the existence of domains with a triangular faceting of steps, which had three types of orientation with respect to the substrate. The growth of a SnSe2 film on single crystal Bi2Se3 surface(0001) was revealed to occur with the formation of hills, which had a hexangular faceting and were identically oriented with respect to the substrate. The hills were formed by the multilayer mechanism both in the regions, where screw dislocations cropped out, and due to the periodic nucleation of 2D islands on the highest terraces, which attained 1 mu m in size. Using Raman scattering, the films on both substrates were shown to have the spectra typical for the 1T-SnSe2 phase.

A comprehensive study of molybdenum disulfide monolayers formed on a silicon substrate is carried out using Raman scattering (RS), photoluminescence (PL), and by comparison with the data of atomic force microscopy (AFM). Maps of the intensity distribution of exciton PL and Raman scattering by optical phonons from monolayer MoS $${}_{2}$$ films are obtained. The dependences of the frequencies of the fundamental vibrational modes of MoS $${}_{2}$$ (A $${}_{\textrm{1g}}$$ and E $${}_{\textrm{2g}}$$ ) on the thickness of monolayer coatings are obtained. An enhancement of the Raman mode of the optical phonon of silicon by a bilayer of molybdenum disulfide is found. A hypothesis on the interference enhancement of Raman scattering of light by phonon modes of silicon is proposed.

This work presents an overview of the latest results and new data on the optical response from spherical CdSe nanocrystals (NCs) obtained using surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS). SERS is based on the enhancement of the phonon response from nanoobjects such as molecules or inorganic nanostructures placed on metal nanostructured substrates with a localized surface plasmon resonance (LSPR). A drastic SERS enhancement for optical phonons in semiconductor nanostructures can be achieved by a proper choice of the plasmonic substrate, for which the LSPR energy coincides with the laser excitation energy. The resonant enhancement of the optical response makes it possible to detect mono- and submonolayer coatings of CdSe NCs. The combination of Raman scattering with atomic force microscopy (AFM) using a metallized probe represents the basis of TERS from semiconductor nanostructures and makes it possible to investigate their phonon properties with nanoscale spatial resolution. Gap-mode TERS provides further enhancement of Raman scattering by optical phonon modes of CdSe NCs with nanometer spatial resolution due to the highly localized electric field in the gap between the metal AFM tip and a plasmonic substrate and opens new pathways for the optical characterization of single semiconductor nanostructures and for revealing details of their phonon spectrum at the nanometer scale.