Ultraviolet (UV) detectors are a new detection technology developed after infrared and laser detection technologies. Compared with infrared detectors, UV detectors have a higher signal-to-noise ratio because they are not affected by environmental heat sources. Thus, UV detectors have potential value and development prospects in many fields, including optical communication, medical treatment, electronics, and fire warning. Two-dimensional materials have been widely studied in the field of photodetectors in recent years owing to the absence of dangling bonds on the surface, interlayers linked by van der Waals forces, and no lattice mismatch when constructing heterojunctions. However, research on UV photodetectors based on two-dimensional materials is faced with the problems of few wide-bandgap two-dimensional materials and poor detector performance, particularly slow response speed. This work reports the energy band structure and optical absorption characteristics of a novel wide-bandgap two-dimensional semiconductor, TlGaS₂. A UV photodetector based on a TlGaS₂ nanosheet is fabricated. The detector exhibits a clear response from the UV to solar-blind UV spectral regions, lower dark currents, and faster response speeds. We hope that our research can broaden the wide-bandgap two-dimensional material system, and provide new ideas and methods for the realization of high-performance two-dimensional material UV photodetectors.

In this study, tapes are used to mechanically exfoliate TlGaS₂ single crystals into nanosheets, which are dry-transferred onto SiO₂/Si substrates using polydimethylsiloxane (PDMS). The structure and chemical composition of the nanosheets are determined via Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The morphology and thickness of the nanosheets are characterized using atomic force microscopy, and the spectral absorption range of the material is determined using an absorption spectrum test. The energy band structure and density of states of the TlGaS₂ material are calculated by first principles to determine its energy band structure, which uses density functional theory (DFT) theory with projector augmented-wave (PAW) pseudopotentials and a hybrid functional. The obtained TlGaS₂ nanosheets are fabricated into devices via a lift-off process using electron beam lithography and electron beam evaporation to obtain patterned two-terminal electrodes. The optoelectronic properties of the fabricated detector devices are tested using a semiconductor tester under laser irradiation or a mercury lamp with an added filter.

According to the first-principle calculation [Fig. 2(a)], TlGaS2 is an indirect bandgap semiconductor, and the valence band maximum is located at the Γ point. The bandgap width is 2.59 eV, and there is also a direct bandgap of 2.76 eV at the Γ point. The calculated results are consistent with the absorption spectrum test results for the material [Fig. 2(c)]. The TlGaS2 nanosheets absorb energy near 480 nm (2.6 eV) through an indirect transition, and there is a transition from indirect to direct near 430 nm (4.85 eV), which shows a steep absorption edge. At the same time, the absorption spectra show that TlGaS2 has high absorption in the UV region, even in the solar-blind UV region. The UV photodetector based on mechanically exfoliated TlGaS2 nanosheets forms a good ohmic contact with the Ti/Au electrode. The photoelectric test (Fig. 3) shows that the detector s dark current is on the order of 10-13 A. It has a certain response to optical signals smaller than 480 nm, and the steep increase in the photocurrent from 430 nm is consistent with the change trend of the absorption spectrum of the material. The detector exhibits an obvious photoresponse in the entire UV band. The best performance is achieved at 360 nm, with a responsivity of 57 mA/W and a detection rate of 2.69×1010 cm·Hz1/2·W-1. A transient test shows that the detector has a relatively fast response speed. After testing [Fig. 4(a)], the TlGaS2 detector shows the on-state response time of 51.8 μs and the off-state response time of 45.1 μs. In addition, the detector exhibits a good low-frequency noise performance [Fig. 4(b)] and air stability [Fig. 4(c)]. Finally, a photocurrent-mapping test reveals the working principle of the detector. The material is excited by UV light to separate electrons and holes. They are affected by the potential difference between the two ends of the detector and move to both ends of the electrode to form the photocurrent, thus realizing conversion from an optical signal to an electrical signal.

In this study, the crystal structure and chemical composition of mechanically exfoliated TlGaS₂ nanosheets are examined, and Tl and Ga are found to exist in the form of +1 and +3 valences, respectively. The energy band structure and density of states of TlGaS₂ are calculated using DFT, and the calculated bandgap value is almost consistent with the optical bandgap measured by the absorption spectrum experiment. Meanwhile, the absorption spectrum test shows that the TlGaS₂ material has high absorption in UV, even solar-blind UV, and a transition occurs from indirect bandgap to direct bandgap at 440 nm. Photodetectors based on TlGaS₂ nanosheets are successfully fabricated, and the working principle is that the photogenerated carriers generated by the photoconductive effect form the photocurrent under an external electric field. The working wavelength range of the detector is consistent with that of the absorption spectrum, which is less than 480 nm. In particular, for UV and solar-blind UV signals, the detector exhibits a distinct response. The detector has the best responsivity and detection rate under the 360 nm signal, and at the same time shows the very low dark current (~10^-13 A) and the very fast response time (51.8 μs for on-state and 45.1 μs for off-state), which is better than most wide-bandgap 2D material photodetectors. This work provides a new material choice for studying high response speed two-dimensional material UV photodetectors or for suppressing the dark current of the detectors.