Ultrathin SnSe2 flakes: a new member in two-dimensional materials for high-performance photodetector

Abstract

Two-dimensional (2D) VIA group compounds, such as transition metal dichalcogenides (TMDs), IIIA–VIA and IVA–VIA group compounds, as a new set of layered materials rising in recent years, have an obvious advantage over the extended studied monolayer graphene in optoelectronics due to their inherent bandgaps [1, 2]. Similar as graphene, the initial fabrication of 2D VIA group compounds depends mainly on mechanical exfoliation method, although such a method is lack of control in morphology and yield of product. Based on the exfoliated samples, the optical, electronic, and optoelectronic properties of 2D materials have been systematically studied [1–3]. Meanwhile, the efforts in exploration of new member of 2D materials are never ceased. SnSe2 is a layered material belonging to IVA–VIA group, which is fabricated and studied on the basis of exfoliated thin samples in past decades [4]. It is wondering that whether the indirect-to-direct bandgap transition would occur in SnSe2 when the material decreases from bulk down to few layers, just like in many other reported TMDs [1]. However, the fabrication of ultrathin SnSe2 is a challenge then. Chemical vapor deposition (CVD) method has been widely used as an effect way to grow thin TMDs [5]. Recently, the fabrication of SnSe2 through CVD method has also been attempted. Uniform hexagonal SnSe2 flakes have been achieved, but the thicknesses of samples are still unsatisfied [6]. In a recent work published in Advanced Materials by Zhai’s group [7], they have demonstrated for the first time the synthesis of ultrathin SnSe2 by CVD method, which achieved large-sized triangle shape flakes of SnSe2 with a thinnest thickness down to two layers, as shown in Fig. 1a, b. The success of Zhai’s group might be attributed to their novel strategy in the growth, in which a new SnI2 source was adopted. SnI2 has a low melting point and thus can easily form a relative uniform growth environment that is supposed to be beneficial to the formation of ultrathin SnSe2 flakes. The author further studied the optical properties of SnSe2 based on assynthesized ultrathin flakes by Raman spectroscopy and time-resolved photoluminescence (PL) spectroscopy (Fig. 1c, d). Their results evidenced the high uniformity and high quality of as-synthesized SnSe2 flakes and an indirect bandgap of *1.73 eV. More interestingly, the PL intensity of samples showed a negligible difference even when the thickness of sample decreases to two layers, suggesting the absence of indirect-to-direct bandgap transition in SnSe2 under current condition. However, whether the case would be different as the thickness of SnSe2 decreases to monolayer is still unknown. To explore the optoelectronic properties of ultrathin SnSe2 flake, the author fabricated devices on the as-synthesized sample (Fig. 1e) and explored it under 530-nm light with intensity of 6.38 mW/cm. The device exhibited excellent properties in photodetection, including a high responsivity of 1.1 9 10 A/W, a high external quantum efficiency (EQE) of 2.61 9 10 %, and a fast response time of 14.5 ms for rise and 8.1 ms for decay, which are superior to most other TMD-based photodetectors [8]. The high performance of ultrathin SnSe2 flake-based photodetector is attributed to the high quality of sample and the Schottky contacts in device [9]. The success in fabrication of ultrathin SnSe2 richens the member of 2D materials family, providing us more Y. Ma (&) State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China e-mail: yingma@hust.edu.cn