Broadband photodetection in wide temperature range: Layer-by-layer exfoliation monitoring of WS2 bulk using microscopy and spectroscopy

Abstract

Exfoliation of bulk WS2 powder has been conducted by ultrasonication, and its layer by layer peeling is monitored by HRTEM, XRD, UV-Vis, and Raman spectroscopy. HRTEM confirms the peeling process and selected area electron diffraction pattern shows the crystallinity of nanosheets which is in accordance with the X-ray diffraction results. Raman and UV-visible spectroscopies further confirm the exfoliation of WS2 to nanosheets having a few layers. A photodetector developed from a few-layered WS2 film is used for photodetection over wide wavelength and temperature ranges. The detector exhibits the highest sensitivity at 635 nm with a value of 382% at 77 K against a value of 138% at 300 K, while the response and the recovery time are ∼78 ms and ∼40 ms, respectively. Temperature, excitation wavelength, and laser power-dependent studies show the threshold limit of its faithful operation. Temperature-dependent photoresponsivity and sensitivity are explained in terms of the carrier–phonon scattering dependent transport mechanism. The scattering cross-section analysis using the density functional theory model reveals that it is the in-plane and out-of-plane acoustic modes that play a key role in carrier transport, rather than the optical phonons. The influence of atmospheric constituents (N2, O2) on detector performance is checked by a successive exposure to dry N2 and ambient air flow. A 14% reduction in photoresponsivity under O2 exposure is observed and suitable analysis is given in terms of O2 induced enhancement of the electron–hole recombination process, leading to a loss of photogenerated carriers. The sensor's cycling performance remains unaffected even after a prolonged exposure in ambient conditions (55% RH), showing its chemical stability even for long-term use.