Abstract
Monolayer TiS2 is the lightest member of the transition metal dichalcogenide family with promising applications in energy storage and conversion systems. The use of TiS2 has been limited by the lack of rapid characterization of layer numbers via Raman spectroscopy and its easy oxidation in wet environment. Here, we demonstrate the layer-number-dependent Raman modes for TiS2. 1T TiS2 presents two characteristics of the Raman active modes, A1g (out-of-plane) and Eg (in-plane). We identified a characteristic peak frequency shift of the Eg mode with the layer number and an unexplored Raman mode at 372 cm–1 whose intensity changes relative to the A1g mode with the thickness of the TiS2 sheets. These two characteristic features of Raman spectra allow the determination of layer numbers between 1 and 5 in exfoliated TiS2. Further, we develop a method to produce oxidation-resistant inks of micron-sized mono- and few-layered TiS2 nanosheets at concentrations up to 1 mg/mL. These TiS2 inks can be deposited to form thin films with controllable thickness and nanosheet density over square centimeter areas. This opens up pathways for a wider utilization of exfoliated TiS2 toward a range of applications.
Highlights
Transition metal dichalcogenides (TMDs) are layered materials, which have recently attracted great interest because of their tunable electronic structure with the layer number.[1−3]Titanium disulfide (TiS2) belongs to this family and in its layered bulk form has attracted interest since the early 1980s as an electrode material for lithium-ion batteries.[4−7] TiS2 crystallizes in an octahedral (1T) phase, which is energetically more stable compared to its hexagonal (2H) phase.[8]
A single triatomic sheet of TiS2 is formed by a Ti atom layer sandwiched between two layers of S atoms covalently bonded to the Ti atoms
In its bulk form, it exhibits a semimetallic behavior, and theoretical calculations for monolayer TiS2 suggest a similar semimetal behavior with a small overlap of valence band maximum and conduction band minimum.[9]. This behavior is strengthened under compressive strain when the band overlap increases, whereas a small indirect band gap starts to emerge under tensile strain.[9]
Summary
Transition metal dichalcogenides (TMDs) are layered materials, which have recently attracted great interest because of their tunable electronic structure with the layer number.[1−3]. In its bulk form, it exhibits a semimetallic behavior, and theoretical calculations for monolayer TiS2 suggest a similar semimetal behavior with a small overlap of valence band maximum and conduction band minimum.[9] This behavior is strengthened under compressive strain when the band overlap increases, whereas a small indirect band gap starts to emerge under tensile strain.[9] A further increase of the tensile strain leads to a transition from indirect to direct band gap,[9] with the direct band gap ranging between 0.383 and 0.389 eV.[9]. Experimental measurements have repeatedly demonstrated the semimetallic behavior of bulk TiS2, the monolayer materials have exhibited a semiconducting behavior under standard nonstrain conditions. Liquid exfoliation of bulk layered materials is being extensively used for large-scale production of inks of nanosheets with high concentration (up to 40 g/L) in organic solvents or water with surfactants.[24−26] The results of these
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