Abstract2D layered materials have emerged in recent years as a new platform to host novel electronic, optical, or excitonic physics and develop unprecedented nanoelectronic and energy applications. By definition, these materials are strongly anisotropic between the basal plane and cross the plane. The structural and property anisotropies inside their basal plane, however, are much less investigated. Black phosphorus, for example, is a 2D material that has such in‐plane anisotropy. Here, a rare chemical form of arsenic, called black‐arsenic (b‐As), is reported as a cousin of black phosphorus, as an extremely anisotropic layered semiconductor. Systematic characterization of the structural, electronic, thermal, and electrical properties of b‐As single crystals is performed, with particular focus on its anisotropies along two in‐plane principle axes, armchair (AC) and zigzag (ZZ). The analysis shows that b‐As exhibits higher or comparable electronic, thermal, and electric transport anisotropies between the AC and ZZ directions than any other known 2D crystals. Such extreme in‐plane anisotropies can potentially implement novel ideas for scientific research and device applications.

2D elemental layered crystals, such as graphene and black phosphorus (B-P), have received tremendous attentions due to their rich physical and chemical properties. In the applications of nanoelectronic devices, graphene shows super high electronic mobility, but it lacks bandgap which impedes development in logical devices. As an alternative, B-P shows high mobility of up to about 1000 cm 2 V −1 s −1 . However, B-P is very unstable and degrades rapidly in ambient conditions. Orthorhombic arsenic (black arsenic; b-As) is the "cousin" of B-P; theoretical prediction shows that b-As also has excellent physical and chemical properties, but there is almost no experimental report on b-As. Herein, it is reported on the unique transport characteristics and stability of monolayer and few-layer b-As crystals which are exfoliated from the natural mineral. The properties of field-effect transistors (FETs) strongly depend on the thickness of crystals. In the monolayer limit, the performance shows relatively high carrier mobilities and large on/off ratios. Moreover, the b-As crystals exhibit a relatively good ambient stability. The few-layer arsenic based FET still function after exposure to air for about one month. Therefore, b-As is expected to be a promising 2D material candidate in nanoelectronic devices.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201802581.properties. [9][10][11] It is considered to be a more promising material than graphene in opto/ nanoelectronic devices. [1,5,6,8,9] However, the deterioration of B-P under atmospheric conditions definitely hinders its applications in practical devices. [12,13] To improve its performance, many attempts have been made. Studies have found that alloying of B-P is a good choice, such as the black arsenic-phosphorus alloy shows tunable bandgap, excellent optical properties, and good ambient stability. [14,15] Another way is to find the new alternative materials.B-As (named arsenene), as a cousin of B-P, has the similar structure configuration with B-P, which is expected to have excellent physical and chemical properties. [16][17][18] Just like most TMDCs and B-P, theoretical studies have verified that the band structures of b-As also have layer dependence. [16] Bulk layered b-As is a direct semiconductor with the bandgap of about 0.3 eV, whereas the monolayer b-As is an indirect bandgap semiconductor with the gap value of about 1-1.5 eV. [16][17][18][19][20] This characteristic leads it to a possibility for applications in optoelectronic and logical devices. Moreover, few-layer b-As is predicted to have high carrier mobility (several thousand square centimeters per volt-second). [20,21] These excellent physical properties make it a good candidate for applications in electronic devices. So far, the experimental synthesis of black arsenic crystals still faces great challenges. [22,23] A recent paper reported that black arsenic is metastable and often stabilized by impurities; and it is very difficult to synt...