Thickness dependence of optical and electronic properties of FeCl2 films under high pressure

In this study, we propose an unconventional approach to investigate the pressure-induced changes in crystal structure, electrical transport, and optical bandgap by examining bulk FeCl2 and FeCl2 films with a thickness of 600 nm. Our analysis involved Raman spectroscopy, electrical transport measurements, and Ultra-violet-visible (UV-vis) absorption spectroscopy. Analyzing the Raman spectroscopy data, we discovered subtle distinctions in the sequence of structural phase transitions between bulk FeCl2 and the 600 nm thick film of FeCl2 under varying pressures. Specifically, while bulk FeCl2 underwent an insulating-to-metal transition at 52 GPa, the film of FeCl2 maintained its semiconductor behavior even at pressures as high as 60 GPa. Furthermore, by studying the optical absorption spectra, we observed a consistent decrease in the direct band gap of bulk FeCl2 with increasing pressure. In contrast, the film of FeCl2 exhibited an increase in the optical band gap beyond 34.1 GPa. These remarkable findings offer valuable insights into the manipulation of the mechanical, electrical, and optical properties of layered nanomaterials.