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Niobium chloride (Nb3Cl8) is a layered 2D semiconducting material with many exotic properties including a breathing kagome lattice, a topological flat band in its band structure, and a crystal structure that undergoes a structural and magnetic phase transition at temperatures below 90 K. Despite being a remarkable material with fascinating new physics, the understanding of its phononic properties is at its infancy. In this study, we investigate the phonon dynamics of Nb3Cl8 in bulk and few layer flakes using polarized Raman spectroscopy and density-functional theory (DFT) analysis to determine the material's vibrational modes, as well as their symmetrical representations and atomic displacements. We experimentally resolved 12 phonon modes, 5 of which are A1g modes the remaining 7 are Eg modes, which is in strong agreement with our DFT calculation. Layer-dependent results suggest that the Raman peak positions are mostly insensitive to changes in layer thickness, while peak intensity and FWHM are affected. Raman measurements as a function of excitation wavelength (473, 532, 633, 785 nm) show a significant increase of the peak intensities when using a 473 nm excitation source, suggesting a near resonant condition. Low-temperature Raman measurements carried out at 7.6 K did not show any changes in the phonon modes and their symmetries, suggesting that the observed Raman modes may not be sensitive to the structural phase transition. Magneto-Raman measurements carried out at 140 and 2 K between -2 to 2 T show that Raman modes are not magnetically coupled. Overall, the study presented here significantly advances the fundamental understanding of the layered material Nb3Cl8 which can be further exploited for future applications., 16 pages, 8 figures, 1 table

Two-dimensional monolayer transition metal dichalcogenides (TMDs) have unique optical and electronic properties for applications pertaining to field effect transistors, light emitting diodes, photodetectors, and solar cells. Vertical interfacing of WS2 and MoS2 layered materials in combination with other families of 2D materials were previously reported. On the other hand, lateral heterostructures are particularly promising for the spatial confinement of charged carriers, excitons and phonons within an atomically-thin layer. In the lateral geometry, the quality of the interface in terms of the crystallinity and optical properties is of paramount importance. Using plasmonic near-field tip-enhanced technology, we investigated the detailed nanoscale photoluminescence (nano-PL) characteristics of the hetero-interface in a monolayer WS2-MoS2 lateral heterostructure. Focusing the laser excitation spot at the apex of a plasmonic tip improved the PL spatial resolution by an order of magnitude compared to the conventional far-field PL. Nano-PL spatial line profiles were found to be more pronounced and enhanced at the interfaces. By analyzing the spectral signals of the heterojunctions, we obtained a better understanding of these direct band gap layered semiconductors, which may help to design next-generation smart optoelectronic devices.

We present recent developments in Raman probe of confined optical and acoustic phonons in nonpolar semiconducting nanowires, with emphasis on Si and Ge. First, a review of the theoretical spatial correlation phenomenological model widely used to explain the downshift and asymmetric broadening to lower energies observed in the Raman profile is given. Second, we discuss the influence of local inhomogeneous laser heating and its interplay with phonon confinement on Si and Ge Raman line shape. Finally, acoustic phonon confinement, its effect on thermal conductivity, and factors that lead to phonon damping are discussed in light of their broad implications on nanodevice fabrication.

We report the observation of the superconducting proximity effect in nanoribbons of a candidate topological insulator (Bi2Se3) which is interfaced with superconducting (tungsten) contacts. We observe a supercurrent and multiple Andreev reflections for channel lengths that are much longer than the inelastic and diffusive thermal lengths deduced from normal state transport. This suggests that the proximity effect couples preferentially to a ballistic surface transport channel, even in the presence of a coexisting diffusive bulk channel. When a magnetic field is applied perpendicular to the plane of the nanoribbon, we observe magnetoresistance oscillations that are periodic in magnetic field. Quantitative comparison with a model of vortex blockade relates the occurrence of these oscillations to the formation of Pearl vortices in the region of proximity induced superconductivity., Submitted to Physical Review B (regular article)