Layered, 2D MnPS3: UV Photodetectors and Humidity Sensors

Abstract

Two dimensional, layered metal thiophosphates with general formula, MPX3 (M is metal and X is a chalcogen atom) have been emerging as a new class of tunable band gap semiconductors. In this family, manganese phosphosulphide (MnPS3) is a p-type semiconductor with a large band gap of 3.0 eV and hence is a good candidate for UV light detection. In the present study, highly oriented, large-sized crystals have been grown by chemical vapour transport technique with iodine as the transport agent. Field effect transistors based on bulk and few-layer MnPS3 have been fabricated by exfoliating the crystals on Si/SiO2 substrates. Electrical characteristics reveal p-type behaviour with current on-off ratio of ~103 under ambient conditions. The transistor devices are responsive to ultraviolet light with photoresponstivity of 288AW-1 at a wavelength of 365 nm. Fairly high photo gain value of 860 is achieved. Density functional theory (DFT) calculations reveal various parameters that are responsible for the observed p-type conduction observed in electrical transport studies. The MnPS3 semiconductors show very good humidity sensing behaviour. Liquid exfoliation technique has been used to exfoliate the bulk crystals into few-layer 2D MnPS3. Very stable, few layer colloids of MnPS3 is vacuum filtered and the membrane is transferred on to interdigitated electrodes to form the sensing device. The results revealed that the conductivity of the films increases five to six orders of magnitude with a change in relative humidity (RH) from 0% to 98%. A fast response and recovery time of ~1-2 s and ~ 4 s are observed respectively. AC impedance spectroscopy and Raman spectroscopy techniques are utilized to understand the sensing mechanism. Further, the sensor has been shown to monitor human breath and water evaporation from skin without any physical contact.