Probing the Anisotropic Bandstructure of Titanium Trisulfide Nanoribbons via Photocurrent Spectroscopy*

As an emerging 2D semiconductor, titanium trisulfide (TiS₃) has a strong anisotropic photoresponse and a bandgap in the near-infrared regime, which render it a promising candidate for polarized and infrared optoelectronics. However, due to its quasi-1D atomic structure, TiS₃ often exists in the form of nanoribbon, and the absorption measurement of the individual ribbon is challenging. In this work, by performing low-temperature polarization-dependent photocurrent spectroscopy, we characterize the polarization-resolved absorption spectra of both thick and thin TiS₃ flakes. We find that the absorption edge is 0.90 eV for a 200-nm-thick flake and 0.96 eV for a 15-nm-thick ribbon, indicating a modification of the bandgap by the thickness. While the thick TiS₃ flake possess two highly anisotropic resonance peaks at 1.23 eV and 1.41 eV, the thin ribbon only exhibit one resonance at 1.34 eV. The bandstructure and the absorption spectrum obtained from first principle calculations are in excellent agreement with our experimental results. Our findings have advanced the understanding of the optical properties of TiS₃ nanoflakes, which will inspire future optoelectronics applications.