Atomic thin transition metal dichalcogenides (TMDCs) are a large class of low dimensional materials, having many unique properties such as indirect-direct band gap transition, strong spin-orbit coupling, valence band maximum (VBM) and conductance band minimum (CBM) at the corners of the Brillouin zone for monolayer TMDCs, and conservation of spatial symmetry or broken symmetry depending on the number of layers. The photoluminescence (PL) and Raman spectra can be used to effectively detect the small variations in the atomic and energy band structure of monolayer and multilayer TMDCs. This study thoroughly investigates the PL and Raman spectra of atomic thin MoS₂ and WS₂.

Many second-order Raman vibrational modes assisted by defects have been observed in the Raman spectrum, unde...[ Read more ]

Atomic thin transition metal dichalcogenides (TMDCs) are a large class of low dimensional materials, having many unique properties such as indirect-direct band gap transition, strong spin-orbit coupling, valence band maximum (VBM) and conductance band minimum (CBM) at the corners of the Brillouin zone for monolayer TMDCs, and conservation of spatial symmetry or broken symmetry depending on the number of layers. The photoluminescence (PL) and Raman spectra can be used to effectively detect the small variations in the atomic and energy band structure of monolayer and multilayer TMDCs. This study thoroughly investigates the PL and Raman spectra of atomic thin MoS₂ and WS₂.

Many second-order Raman vibrational modes assisted by defects have been observed in the Raman spectrum, under 514.5 nm (corresponding to 2.40 eV) excitation. The frequencies of the 2LA(M), A_1g(M) + LA(M) and 2LA(M) ‒ 2E_2g¹(M) modes are 352, 582 and 297cm^-1, respectively. LA(M) refers to the longitudinal acoustic mode around point M in the Brillouin zone.

Monolayer TMDCs such as WS₂ are direct band gap semiconductors. Exciton is formed by the hole and electron at the VBM and CBM, respectively. The recombination energy of A exciton in monolayer WS₂ varies from 2.053 eV at 77 K to 2.01eV at room temperature. The direct band gap at point K decreases with increasing temperature due to electron-phonon coupling. The chemical bonding weakens with enhanced vibration of lattice, which increases the energy band gap. The relation between band gap and temperature is fitted and the energy band gap at absolute temperature is 2.057 eV, which is consistent with the density functional theory (DFT) calculation.

Bilayer TMDCs such as WS₂ and MoS₂ are indirect band gap semiconductors. The indirect band gap in bilayer MoS₂ is 1.50 eV, whereas that in the hexagonal boron nitride(hBN)-encapsulated bilayer is 1.52 eV. The variation in indirect band gap is due to the sensitivity of the VBM at the Γ point. The indirect band gap of trilayer TMDCs is 1.47 eV.

The emerging indirect energy gap in a heterostructure with monolayer WS₂ and MoS₂ is caused by the hybridization of the Γ point in the two layers. The extent of hybridization is determined by the interlayer distance, which can be varied with the time of annealing at 393K. The interlayer distance varies from 1.6 to 0.8 nm after 12 h of annealing. The indirect band gap emerges at 1.79 eV after 6 h of annealing and shifts down to 1.4 eV after 12 h of annealing.

The biaxial tensile strain in the hBN-encapsulated multilayer MoS₂ induced by annealing at 600 K can be detected by the redshift of E_2g¹ in the Raman spectra, which is consistent with the DFT calculation. The redshift of the in-plane mode, E_2g¹, is 1.57 cm^-1 and the blueshift of the out-of-plane mode, A_1g, is 0.28cm^-1. For the hBN-encapsulated trilayer TMDCs, the redshift of E_2g¹ is only 0.156 cm^-1.The competition between weak Van der Waals interaction from hBN and tensile strain effect determines the shift of frequency for two modes.