Titanium trisulfide (TiS3): a 2D semiconductor with quasi-1D optical and electronic properties

Abstract

We present characterizations of few-layer titanium trisulfide (TiS3) flakes which, due to their reduced in-plane structural symmetry, display strong anisotropy in their electrical and optical properties. Exfoliated few-layer flakes show marked anisotropy of their in-plane mobilities reaching ratios as high as 7.6 at low temperatures. Based on the preferential growth axis of TiS3 nanoribbons, we develop a simple method to identify the in-plane crystalline axes of exfoliated few-layer flakes through angle resolved polarization Raman spectroscopy. Optical transmission measurements show that TiS3 flakes display strong linear dichroism with a magnitude (transmission ratios up to 30) much greater than that observed for other anisotropic two-dimensional (2D) materials. Finally, we calculate the absorption and transmittance spectra of TiS3 in the random-phase-approximation (RPA) and find that the calculations are in qualitative agreement with the observed experimental optical transmittance.

Figure 1. Crystal structure of TiS₃.

The bond lengths between the titanium and sulphur are shorter along the b-axis than along the a-axis. This results in highly conducting chains which lead to strong anisotropic electrical and optical properties. Structure models are produced using VESTA.

Figure 2

(a) Optical image of an exfoliated nanosheet. (b) AFM scan of the same nanosheet after patterning 12 Au/Ti electrodes. (c) Transfer curves measured at room temperature between 5 pairs of electrodes where 0° is designated as the high conductance (b-axis). (d) Polar plot of the room temperature conductance (μS) measured for all 12 pairs of electrodes at back gate voltages of −40 V, 0 V, and 40 V. (e) Transfer curves for the same devices at a temperature of 25 K. (f) Polar plot of the conductance (μS) at a temperature of 25 K and gate voltages of 40 V (outer curve) and 20 V (inner curve).

Figure 3

(a) Raman spectra of a TiS₃ ribbon with horizontal excitation and detection polarization (see the arrows in the insets). In the top (bottom) panel the ribbon has been aligned almost perpendicular (parallel) to the excitation/detection polarization. The insets show the position of the TiS₃ ribbon with respect to the illumination polarization. The peak around 370 cm⁻¹ shows the most noticeable change with the change of ribbon alignment. (b) Intensity of the 370 cm⁻¹ Raman peak of a 3 nm thick TiS₃ flake (3–4 layers) as a function of the excitation polarization angle (the detection polarization is fixed along the horizontal axis). The minimum intensity occurs when the excitation polarization is parallel to the b-axis of the flake. (Bottom panels) optical and atomic force microscopy images of the studied flake. The determined b-axis is in good agreement with the straight edges of the TiS₃ flake.

Figure 4

(a) Transmittance of the red, green, and blue channels as a function of the excitation polarization angle. (b) Calculated absorption spectra when the field is aligned parallel to the b-axis (dashed line) and a-axis (solid line). The inset shows the transmittance in the a–b plane for energies red (1.9 eV), green (2.4 eV), and blue (2.72 eV) excitations.