Among the most studied semiconducting transition metal dichalcogenides (TMDCs), WS2 showed several advantages in comparison to their counterparts, such as a higher quantum yield, which is an ...important feature for quantum emission and lasing purposes. We studied transferred monolayers of WS2 on a drilled Si3N4 substrate in order to have insights about on how such heterostructure behaves from the Raman and photoluminescence (PL) measurements point of view. Our experimental findings showed that the Si3N4 substrate influences the optical properties of single-layer WS2. Beyond that, seeking to shed light on the causes of the PL quenching observed experimentally, we developed density functional theory (DFT) based calculations to study the thermodynamic stability of the heterojunction through quantum molecular dynamics (QMD) simulations as well as the electronic alignment of the energy levels in both materials. Our analysis showed that along with strain, a charge transfer mechanism plays an important role for the PL decrease.
Understanding the thermodynamic properties of materials is a fundamental issue in physics, and its knowledge is crucial for targeting a specific material for possible applications. In this work, we ...report a temperature‐ and pressure‐dependent Raman study of bulk GaSe0.5Te0.5 alloy, besides their relevant thermodynamic parameters. Our results show a nonlinear redshift for the
A1g and
E2g vibrational modes as the temperature increases in the temperature range from 10 to 748 K. Such behavior is well described by considering both thermal expansion and phonon–phonon coupling contributions. By combining density functional theory (DFT) calculations and Raman spectroscopy experiments, the anharmonic constants relative to the three‐ and four‐phonon decay processes, mode‐Grüneisen parameters, Debye temperature, thermal expansion coefficient, and bulk modulus were estimated for GaSe0.5Te0.5 alloy. Furthermore, the high‐pressure measurements and DFT calculations, performed in the pressure range from 0 to 26.4 GPa, show a quadratic trend for the
ωA1g and
ωE2g modes as a function of pressure, with the
A1g modes being more compressible than
E2g one, that is,
∂ωA1g∂P>∂ωE2g∂P. No structural phase transition is observed until the maximum pressure reached in the experiment. This study took a step forward in the understanding of mechanical and thermal properties related to GaSe0.5Te0.5 alloy, whose determined parameters are important for designing new applications.
We present a temperature‐ and pressure‐dependent Raman spectroscopy study of bulk GaSe0.5Te0.5 in order to access its thermal and mechanical properties. By combining DFT calculations and Raman spectroscopy experiments, the anharmonic constants relative to the three‐ and four‐phonon decay processes, mode‐Grüneisen parameters, Debye temperature, thermal expansion coefficient, and bulk modulus were estimated.
Phonons play a fundamental role in the electronic and thermal transport of 2D materials which is crucial for device applications. In this work, we investigate the temperature-dependence of A 1g1 and ...A 1g2 Raman modes of suspended and supported mechanically exfoliated few-layer gallium sulfide (GaS), accessing their relevant thermodynamic Grüneisen parameters and anharmonicity. The Raman frequencies of these two phonons soften with increasing temperature with different θ=∂ω/∂T temperature coefficients. The first-order temperature coefficients θ of A 1g2 mode is ∼ −0.016 cm−1/K, independent of the number of layers and the support. In contrast, the θ of A 1g1 mode is smaller for two-layer GaS and constant for thicker samples (∼ −0.006 2 cm−1 K−1). Furthermore, for two-layer GaS, the θ value is ∼ −0.004 4 cm−1 K−1 for the supported sample, while it is even smaller for the suspended one (∼ −0.002 9 cm−1 K−1). The higher θ value for supported and thicker samples was attributed to the increase in phonon anharmonicity induced by the substrate surface roughness and Umklapp phonon scattering. Our results shed new light on the influence of the substrate and number of layers on the thermal properties of few-layer GaS, which are fundamental for developing atomically-thin GaS electronic devices.
The inner region of the Milky Way halo harbors a large amount of dark matter (DM). Given its proximity, it is one of the most promising targets to look for DM. We report on a search for the ...annihilations of DM particles using γ-ray observations towards the inner 300 pc of the Milky Way, with the H.E.S.S. array of ground-based Cherenkov telescopes. The analysis is based on a 2D maximum likelihood method using Galactic Center (GC) data accumulated by H.E.S.S. over the last 10 years (2004-2014), and does not show any significant γ-ray signal above background. Assuming Einasto and Navarro-Frenk-White DM density profiles at the GC, we derive upper limits on the annihilation cross section ⟨σv⟩. These constraints are the strongest obtained so far in the TeV DM mass range and improve upon previous limits by a factor 5. For the Einasto profile, the constraints reach ⟨σv⟩ values of 6×10^{-26} cm^{3} s^{-1} in the W^{+}W^{-} channel for a DM particle mass of 1.5 TeV, and 2×10^{-26} cm^{3} s^{-1} in the τ^{+}τ^{-} channel for a 1 TeV mass. For the first time, ground-based γ-ray observations have reached sufficient sensitivity to probe ⟨σv⟩ values expected from the thermal relic density for TeV DM particles.
Polystyrene is a thermoplastic that is widely used in industry to manufacture of various materials. In this work, the objective was to investigate the effect of the Crystal Violet (CV) dye on ...polystyrene films under visible light. The films were produced by the casting method. Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and viscosimetry were used to monitor the effect of radiation and
1
H Nuclear Magnetic Resonance (
1
H-NMR) was employed to evaluate the interaction between the polymer and the dye. Carbonyl Index confirmed that oxygenated species were formed during exposure to light and photo-oxidation is the major mechanism involved in CV-sensitized PS degradation. The variations in relaxation times indicated that electrostatic attraction is the main form of interaction between the materials involved and molecular dynamics calculations confirm the
1
H-NMR results. The CV dye could initiate degradation reactions and acted as an efficient photosensitizer in PS matrices.
Spectral lines are among the most powerful signatures for dark matter (DM) annihilation searches in very-high-energy γ rays. The central region of the Milky Way halo is one of the most promising ...targets given its large amount of DM and proximity to Earth. We report on a search for a monoenergetic spectral line from self-annihilations of DM particles in the energy range from 300 GeV to 70 TeV using a two-dimensional maximum likelihood method taking advantage of both the spectral and spatial features of the signal versus background. The analysis makes use of Galactic center observations accumulated over ten years (2004-2014) with the H.E.S.S. array of ground-based Cherenkov telescopes. No significant γ-ray excess above the background is found. We derive upper limits on the annihilation cross section ⟨σv⟩ for monoenergetic DM lines at the level of 4×10^{-28} cm^{3} s^{-1} at 1 TeV, assuming an Einasto DM profile for the Milky Way halo. For a DM mass of 1 TeV, they improve over the previous ones by a factor of 6. The present constraints are the strongest obtained so far for DM particles in the mass range 300 GeV-70 TeV. Ground-based γ-ray observations have reached sufficient sensitivity to explore relevant velocity-averaged cross sections for DM annihilation into two γ-ray photons at the level expected from the thermal relic density for TeV DM particles.
Abstract
This work, the photoluminescence (PL) of a suspended monolayer of WSe
2
was investigated to avoid the substrate effect, and its evolution with temperature and excitation energy. Raman ...spectroscopy and PL demonstrated the monolayer characteristic of the sample due to the absence of the B
1
2
g
peak along with a symmetric PL peak centered at approximately 1.65 eV. The PL spectrum exhibited two main emission bands attributed to a neutral exciton (X
0
) and a trion (X
T
), where X
0
showed a redshift with temperature variation from 10 K to 310 K, and the intensity ratio between the bands varied from 2.68 to 3.96 eV. The energy evolution of the bands as a function of temperature was analyzed using the modified Varshni equation, yielding an electron–phonon coupling constant with a value of 1.54 (2.11) for X
0
(X
T
). The intensity behavior was studied using the multi-level model, which provided activation energies of 58.0 meV for X
0
and 94.6 meV for X
T
. The redshift of the mentioned bands is explained by the Urbach formalism, attributing this shift to an increase in phonon density in the material. Computational calculations demonstrated an increase in the material’s lattice parameter with rising temperature, resulting in an increase in the W–Se bond length and a decrease in the distance between Se atoms from different sublattices, causing the direct transition to decrease with temperature. In conclusion, this study showed that the observed evolution in the PL spectrum of the suspended WSe
2
monolayer could be related to the increase of phonon density in the material which is important for the future applications of 2D materials in optoelectronics.
Dwarf spheroidal galaxies of the Local Group are close satellites of the Milky Way characterized by a large mass-to-light ratio and are not expected to be the site of nonthermal high-energy gamma-ray ...emission or intense star formation. Therefore they are among the most promising candidates for indirect dark matter searches. During the last years the High Energy Stereoscopic System (H.E.S.S.) of imaging atmospheric Cherenkov telescopes observed five of these dwarf galaxies for more than 140 hours in total, searching for TeV gamma-ray emission from annihilation of dark matter particles. The new results of the deep exposure of the Sagittarius dwarf spheroidal galaxy, the first observations of the Coma Berenices and Fornax dwarves and the reanalysis of two more dwarf spheroidal galaxies already published by the H.E.S.S. Collaboration, Carina and Sculptor, are presented. In the absence of a significant signal new constraints on the annihilation cross section applicable to weakly interacting massive particles (WIMPs) are derived by combining the observations of the five dwarf galaxies. The combined exclusion limit depends on the WIMP mass and the best constraint is reached at 1-2 TeV masses with a cross-section upper bound of ~ 3.9 x 10 super(-24) cm super(3) s super(-1) at a 95% confidence level.
PDF
