Graphene has raised a huge wave in 2D materials field, breeding lots of graphene analogs with applications in optical and electrical devices, energy conversion and storage, biology, etc.^{[1, 2]}. Graphene presents superior carrier mobility, while its zero-bandgap restricts its transistor application. To make up this shortcoming, new 2D materials with certain bandgaps and high carrier mobility are being developed. Two typical materials are transition metal dichalcogenides (TMDs) and black phosphorus, which exhibit layered structure, layer-dependent band structure and strong quantum constraints^[3-5]. As a congener of phosphorus, arsenic can complement the bandgap of existed 2D materials^[6-18]. Arsenic has several allotropes, i.e. semimetal gray arsenic, semiconductive black arsenic and insulating yellow arsenic. 2D arsenic, i.e. arsenenes, were developed.

In 2015, arsenene came into sights after a number of reports on theoretical predictions^[6-8]. Monolayer arsenenes with buckled and puckered honeycomb structures were fabricated, and corresponding phonon spectra and electrical band structures were calculated based on density functional theory (DFT) (Fig. 1(a) and 1(b))^{[6, 7]}. Both puckered and buckled arsenenes are dynamically stable with an indirect bandgap (Fig. 1(a) and 1(b))^{[6, 7]}. Zeng et al. calculated the band structures of monolayer, bilayer and trilayer arsenenes, revealing metal-to-semiconductor transition of buckled arsenene (Fig. 1(c))^[7]. The bandgap of arsenene can be adjusted by applying external strains or external electric fields, leading to a transition from indirect bandgap to direct bandgap^{[6, 7]}. The calculated room-temperature carrier mobilities for puckered and buckled arsenenes suggested that they can compete with polysilicon, single-layer TMDs and graphene^[8]. Buckled arsenene was calculated to have a high mobility up to 10⁴ cm²/(V·s)^[8]. The metal–oxide–semiconductor field effect transistor (MOSFET) was simulated based on monolayer arsenene (Fig. 1(d))^[9], presenting channel lengths below 10 nm. Then, Lu et al. investigated the electrical contacts, many-body effect, carrier mobility, and device performance of monolayer arsenene^{[10, 11]}. By introducing 2D graphene between arsenene and metal, the Fermi level pinning effect can be effectively suppressed^[10]. Besides, arsenene FETs with ultra-short channel (<10 nm) can satisfy the low-power and high-performance requirements in International Technology Roadmap for Semiconductors^[11].

Scientists have been trying to prepare high quality arsenic crystal by using “top-down” exfoliation or to develop techniques to prepare arsenenes^{[12-14, 17, 18]}. Tsai et al. first reported multilayer arsenene nanoribbons on InAs substrate by using a plasma assisted processing (Fig. 2(a))^[12]. The arsenene nanoribbons showed photoluminescence (PL)^[9]. After that, Chen et al. and Zhong et al. discovered natural minerals of arsenic (orthorhombic crystal), which can be exfoliated into 2D arsenene nanoflakes (Fig. 2(b))^{[13, 14]}. Black arsenene shows anisotropy and semiconductive features (Fig. 2(c))^{[13, 14]}. However, obtaining gray arsenene by using mechanical exfoliation is difficult because of strong interlayer force. Qi et al. studied the interaction between arsenene and 14 solvents to find the optimal solvent for exfoliation (Fig. 2(d))^[15]. Wang et al. exfoliated bulk arsenic crystals into nanoflakes by using DMSO as the solvent^[16]. The arsenene nanosheets showed therapeutic effect to acute promyelocytic leukemia (APL)^[16]. Hu et al. prepared gray arsenene single-crystal nanoflakes on mica substrate (Fig. 2(e))^[17]. Gray arsenene nanoflakes were transformed into vitreous arsenene via a simple wet chemistry method^[18]. The completely vitrified arsenene nanoflakes keep hexagonal shape, showing amorphous characteristic^[18]. The two-terminal device based on completely vitrified arsenene presented electrical field-controlled drain current^[18].

In short, arsenene is an interesting material with anisotropy, considerable carrier mobility and obvious PL. It may find applications in functional devices.

This work was supported by the National Key Research and Development Program of China (2017YFA0208200), the Fundamental Research Funds for the Central Universities of China (0205-14380266), the National Natural Science Foundation of China (22022505 and 21872069), the Natural Science Foundation of Jiangsu Province (BK20180008), and the Shenzhen Fundamental Research Program of Science, Technology and Innovation (JCYJ20180307155007589). L. Ding thanks the National Key Research and Development Program of China (2017YFA0206600) and the National Natural Science Foundation of China (51773045, 21772030, 51922032, 21961160720) for financial support.