Optimization of Photodetection Analysis for MXene Thin Films
Authors:   , Loh Kristine Q. , Baxter Jason B. 
Published: July 2021

Two-dimensional (2D) materials have unique optical, electronic, and thermal properties compared to their bulk counterparts due to their atomically thin layers. The discovery and utilization of 2D materials have opened an emerging field of interest that pushes the boundaries of technology through material system understanding. MXenes have garnered increasing attention for various photovoltaic and optoelectronic applications due to their tunable properties. MXenes are a novel class of 2D materials with the general formula of M_[n+1]X_nT_x where M is an early transition metal, X is either carbon or nitrogen, T represents surface termination such as O and F, and n is between one and four. The response of MXenes to various stimuli such as light, humidity, and storage condition is under investigation, even for the canonical material Ti₃C₂T_x. The aim of this work is to develop and optimize the analysis procedure for probing the effects of light, electrical contact, and environment on optoelectronic properties of Ti₃C₂T_x and Mo₂TiC₂T_x MXenes for photodetection applications. By determining best practices for testing, we can then move toward understanding the mechanisms of optoelectronic behavior of MXenes while eliminating potential confounding variables. The best configuration for optoelectronic characterization involves placing a mask over the inactive area on the substrate, using a transparent conducting oxide with an etched channel as an electrical contact method, and maintaining an inert environment throughout the characterization. Allowing illumination on the inactive area as well as the electrodes suppresses the photoresponse of MXene. In testing various contact methods for analyzing optoelectronic behavior, using silver paste as an electrical contact influenced the photoresponse of the material, as it suppressed, heightened, or even changed the sign of the photoresponse. Both materials are also sensitive to ambient environment; they destabilize in room air, and conductance sharply decreases. However, in an inert environment, both materials remain stable over extended periods of time. As a result, an airtight containment unit designed to pack MXene samples in an inert environment allowed measurement of conductivity changes upon illumination. By comparing the photoresponse of a metallic MXene, Ti₃C₂T_x, with that of a predicted semiconductor-like Mo₂TiC₂T_x MXene, the mechanisms behind photodetection were investigated. Removing silver paste as a confounding variable revealed Ti₃C₂T_x consistently displayed negative photoconductivity, where the conductivity of the material decreased upon illumination. Using silver paint as an electrical contact method for Ti₃C₂T_x caused films less than 3.5 nm thick to exhibit a positive photoconductivity, where the conductivity of the material increased upon illumination. Mo₂TiC₂T_x deposited on patterned FTO did not suffer from large changes in photoresponse due to Ag paint application, but changing the substrate from pristine glass slides to patterned FTO led to the development of a switch from negative to positive photoconductivity at an initial resistance on the order of 105 [omega]. Disentanglement of the effects of contact method, substrate, and temperature on photoresponse will further our fundamental understanding of this material's optoelectronic behavior and will open a multitude of opportunities for this rising class of 2D materials. Keywords: Materials Science, MXenes, Nanostructured Materials, Nanotechnology, Photodetector, Two-dimensional Materials