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.