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Ternary layered single crystals of HfGeTe4 have been synthesized and systematically investigated to study their electronic and vibrational properties using angle-resolved photoemission spectroscopy and Raman scattering (RS) spectroscopy. The experimental results were supported through density functional theory by calculating the electronic band structures, phonons dispersion, lattice thermal conductivity (κL), and phonon lifetimes. The measured Fermi surfaces represent integrated intensities at different binding energies along the opposite symmetry directions, revealing the anisotropic Fermi surfaces. Their band structure shows different band curvatures in the high symmetry direction with anisotropic band dispersions. Moreover, the change in magnetoresistance under different orientations reveals anisotropic electrical conductivity, which further supports our results of 2D anisotropic crystals. HfGeTe4 shows a strong in-plane anisotropic response under polarized RS with phonon intensities changing with the rotation angle, manifesting twofold and fourfold symmetry orientations. Furthermore, the Raman scattering spectra show phonon softening observed with increasing temperature, which could be attributed to the phonon anharmonicity estimated from both the frequency shift and linewidth. The results were further supported by lattice-dynamics calculations. We have found very low lattice thermal conductivity (∼0.31 Wm−1 K−1 at 300 K), which decays as the temperature is increased due to enhanced phonon–phonon scattering, as confirmed by the decreased phonon lifetimes at higher temperatures. The unique structure of this material coupled with the anisotropy and strong phonon anharmonicity depicts significant potential in 2D nanoelectronic applications. [ABSTRACT FROM AUTHOR]

The temperature evolution of the resonant Raman scattering from high-quality bilayer 2H-MoS 2 encapsulated in hexagonal BN flakes is presented. The observed resonant Raman scattering spectrum as initiated by the laser energy of 1.96 eV, close to the A excitonic resonance, shows rich and distinct vibrational features that are otherwise not observed in non-resonant scattering. The appearance of 1st and 2nd order phonon modes is unambiguously observed in a broad range of temperatures from 5 to 320 K. The spectrum includes the Raman-active modes, i.e. E 1g 2 (Γ ) and A 1g (Γ ) along with their Davydov-split counterparts, i.e. E 1u (Γ ) and B 1u (Γ ). The temperature evolution of the Raman scattering spectrum brings forward key observations, as the integrated intensity profiles of different phonon modes show diverse trends. The Raman-active A 1g (Γ ) mode, which dominates the Raman scattering spectrum at T = 5 K quenches with increasing temperature. Surprisingly, at room temperature the B 1u (Γ ) mode, which is infrared-active in the bilayer, is substantially stronger than its nominally Raman-active A 1g (Γ ) counterpart. [ABSTRACT FROM AUTHOR]

Temperature-dependent (5 K-300 K) Raman scattering study of A1g/A'1 phonon modes in mono-layer (1L), bilayer (2L), trilayer (3L), and tetralayer (4L) MoTe2 is reported. The temperature evolution of the modes' intensity critically depends on the flake thickness. In particular with λ = 632.8-nm light excitation, a strongly non-monotonic dependence of the A1g mode intensity is observed in 2L MoTe2. The intensity decreases with decreasing temperature down to 220 K, and the A1g mode almost completely vanishes from the Stokes scattering spectrum in the temperature range between 160 K and 220 K. The peak recovers at lower temperatures, and at T = 5 K, it becomes three times more intense that at room temperature. Similar non-monotonic intensity evolution is observed for the out-of-plane mode in 3L MoTe2 in which tellurium atoms in all three layers vibrate in-phase. The intensity of the other out-of-plane Raman-active mode (with vibrations of tellurium atoms in the central layer shifted by 180° with respect to the vibrations in outer layers) only weakly depends on temperature. The observed quenching of the Raman scattering in 2L and 3L MoTe2 is attributed to a destructive interference between the resonant and non-resonant contributions to the Raman scattering amplitude. The observed "antiresonance" is related to the electronic excitation at the M point of the Brillouin zone in few-layer MoTe2. [ABSTRACT FROM AUTHOR]

The optical response of bulk germanium sulfide (GeS) is investigated systematically using different polarization-resolved experimental techniques, such as photoluminescence (PL), reflectance contrast (RC), and Raman scattering (RS). It is shown that while the low-temperature (T = 5 K) optical band-gap absorption is governed by a single resonance related to the neutral exciton, the corresponding emission is dominated by the disorder/impurity- and/or phonon-assisted recombination processes. Both the RC and PL spectra are found to be linearly polarized along the armchair direction. The measured RS spectra over a broad range from 5 to 300 K consist of six Raman peaks identified with the help of Density Functional Theory (DFT) calculations: A g 1 , A g 2 , A g 3 , A g 4 , B 1 g 1 , and B 1 g 2 , which polarization properties are studied under four different excitation energies. We found that the polarization orientations of the A g 2 and A g 4 modes under specific excitation energy can be useful tools to determine the GeS crystallographic directions: armchair and zigzag. [ABSTRACT FROM AUTHOR]

The temperature evolution of the resonant Raman scattering from high-quality bilayer 2H-MoS 2 encapsulated in hexagonal BN flakes is presented. The observed resonant Raman scattering spectrum as initiated by the laser energy of 1.96 eV, close to the A excitonic resonance, shows rich and distinct vibrational features that are otherwise not observed in non-resonant scattering. The appearance of 1st and 2nd order phonon modes is unambiguously observed in a broad range of temperatures from 5 to 320 K. The spectrum includes the Raman-active modes, i.e. E 1g 2 (Γ ) and A 1g (Γ ) along with their Davydov-split counterparts, i.e. E 1u (Γ ) and B 1u (Γ ). The temperature evolution of the Raman scattering spectrum brings forward key observations, as the integrated intensity profiles of different phonon modes show diverse trends. The Raman-active A 1g (Γ ) mode, which dominates the Raman scattering spectrum at T = 5 K quenches with increasing temperature. Surprisingly, at room temperature the B 1u (Γ ) mode, which is infrared-active in the bilayer, is substantially stronger than its nominally Raman-active A 1g (Γ ) counterpart. [ABSTRACT FROM AUTHOR]