Low-Temperature Raman Investigations and Photoresponse of a Detector Based on High-Quality WSe2 Crystals

The WSe2 single crystals were grown by a high-yield vapor-phase synthesis technique. The grown crystals have 2H-hexagonal lattice structure with a P63/mmc space group. Excitonic resonances A′ and B′ are observed due to splitting of the ground and excited states of A and B transitions, respectively, resulting from inter- and intralayer perturbation of the d-electron band of the W atom by p orbitals of the Se atom. Raman spectroscopy over the temperature range of 77 to 298 K shows that the A1g and E2g modes exhibit a linear dependence on temperature. The shifting of frequency from 251.46 to 253.71 cm–1 follows the first-order temperature coefficient of 9.8 × 10–3 cm–1 K–1. The width of the Raman peaks at half maxima also exhibits a systematic linear dependence on temperature. On the basis of the semiquantitative model, it is stated that the thermal expansion contribution mainly dominates in WSe2 single crystals. Subsequently, the photodetector was analyzed in the temperature range of 200 to 298 K under periodic exposure of 670 nm light of 5 mW/cm2 power intensity. The device exhibits a photoresponsivity of 43.92 mA/W and a response time of 0.85 s.