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Hybrid heterostructures based on bulk GaN and two-dimensional (2D) materials offer novel paths toward nanoelectronic devices with engineered features. Here, we study the electronic properties of a mixed-dimensional heterostructure composed of intrinsic n-doped MoS2 flakes transferred on p-doped GaN(0001) layers. Based on angle-resolved photoemission spectroscopy (ARPES) and high resolution x-ray photoemission spectroscopy (HR-XPS), we investigate the electronic structure modification induced by the interlayer interactions in MoS2/GaN heterostructure. In particular, a shift of the valence band with respect to the Fermi level for MoS2/GaN heterostructure is observed, which is the signature of a charge transfer from the 2D monolayer MoS2 to GaN. The ARPES and HR-XPS revealed an interface dipole associated with local charge transfer from the GaN layer to the MoS2 monolayer. Valence and conduction band offsets between MoS2 and GaN are determined to be 0.77 and −0.51eV, respectively. Based on the measured work functions and band bendings, we establish the formation of an interface dipole between GaN and MoS2 of 0.2 eV.

We probe the room temperature photoluminescence of $N$-layer molybdenum ditelluride $({\mathrm{MoTe}}_{2})$ in the continuous wave (cw) regime. The photoluminescence quantum yield of monolayer ${\mathrm{MoTe}}_{2}$ is three times larger than in bilayer ${\mathrm{MoTe}}_{2}$ and 40 times greater than in the bulk limit. Mono- and bilayer ${\mathrm{MoTe}}_{2}$ display almost symmetric emission lines at 1.10 and 1.07 eV, respectively, which predominantly arise from direct radiative recombination of the A exciton. In contrast, $N\ensuremath{\ge}3\ensuremath{-}\mathrm{layer}\phantom{\rule{4pt}{0ex}}{\mathrm{MoTe}}_{2}$ exhibits a much reduced photoluminescence quantum yield and a broader, redshifted, and seemingly bimodal photoluminescence spectrum. The low- and high-energy contributions are attributed to emission from the indirect and direct optical band gaps, respectively. Bulk ${\mathrm{MoTe}}_{2}$ displays a broad emission line with a dominant contribution at 0.94 eV that is assigned to emission from the indirect optical band gap. As compared to related systems (such as ${\mathrm{MoS}}_{2},\phantom{\rule{0.16em}{0ex}}{\mathrm{MoSe}}_{2},\phantom{\rule{0.16em}{0ex}}{\mathrm{WS}}_{2}$, and ${\mathrm{WSe}}_{2}$), the smaller energy difference between the monolayer direct optical band gap and the bulk indirect optical band gap leads to a smoother increase of the photoluminescence quantum yield as $N$ decreases. In addition, we study the evolution of the photoluminescence intensity in monolayer ${\mathrm{MoTe}}_{2}$ as a function of the exciton formation rate ${W}_{\mathrm{abs}}$ up to $3.6\ifmmode\times\else\texttimes\fi{}{10}^{22}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. The line shape of the photoluminescence spectrum remains largely independent of ${W}_{\mathrm{abs}}$, whereas the photoluminescence intensity grows sublinearly above ${W}_{\mathrm{abs}}\ensuremath{\sim}{10}^{21}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. This behavior is assigned to exciton-exciton annihilation and is well captured by an elementary rate equation model.

We report on the temperature dependence of the anharmonic decay rate of zone-center (G mode) optical phonons in both single-walled carbon nanotubes and graphite. The measurements are performed using a pump-probe Raman scattering scheme with femtosecond laser pulses. For nanotubes, measured over a temperature range of 6 K-700 K, we observe little temperature dependence of the decay rate below room temperature. Above 300 K, the decay rate increases from 0.8 to 1.7 ps-1. The decay rates observed for graphite range from 0.5 to 0.8 ps-1 for temperatures from 300 K-700 K. We compare the behavior observed in carbon nanotubes and graphite and discuss the implications of our results for the mechanism of the anharmonic decay of optical phonons in both systems., 21 pages, 3 figures

We examine the excitonic nature of high-lying optical transitions in single-walled carbon nanotubes by means of Rayleigh scattering spectroscopy. A careful analysis of the principal transitions of individual semiconducting and metallic nanotubes reveals that in both cases the line shape is consistent with an excitonic model, but not one of free carriers. For semiconducting species, sidebands are observed at $\ensuremath{\sim}200\text{ }\text{meV}$ above the third and fourth optical transitions. These features are ascribed to exciton-phonon bound states. Such sidebands are not apparent for metallic nanotubes, as expected from the reduced strength of excitonic interactions in these systems.