Chemical Vapor Transport Growth of Selenene and its Heterostructures with TMDs

2-dimensional (2D) materials have attracted a lot of scientific attention recently beyond graphene, due to their distinct physicochemical features. Elemental 2D crystals have emerged as promising materials for advanced electronics and optoelectronics applications.^[1] In particular, group-VI elemental 2D semiconductors, including selenene and tellurene, have attracted significant attention for their simple composition and excellent properties, such as higher carrier mobility, better environmental stability, and high photoconductivity, which triggered intense activities on their fundamental and application-oriented research.^[2] 2D heterostructures based on various 2D layered materials with distinct properties have been demonstrated to exhibit novel physicochemical properties for their potential application in electronics, optoelectronics, and catalysis, owing to their tunable band alignment and sharp interfaces.^[3] Forming new structures by stacking transition metal dichalcogenide (TMD) layers has been explored by several groups; however, the mechanical transfer method is time-consuming and suffers from having reasonable control on the layer stacking. Single-step and two-step chemical vapor deposition (CVD) growth of lateral and vertical TMD heterostructures have been considered to be a promising approach to realize the potential applications of these heterostructures.^[4]. Achieving controllable growth of high-quality, ultra-thin flakes of elemental 2D materials remains a challenge. Herein, we demonstrate a seed-assisted chemical vapor transport (CVT) growth of ultra-thin triangular flakes of highly crystalline trigonal selenium (t-Se) oriented in (0001) direction, with lateral size >30 μm.^[5,6] To study their promising photo-electrocatalytic properties and further realize the device applications, we employed a single-step CVT approach to grow selenene/WSe₂ heterostructures. Vertical heterostructures of bi-layer selenene and monolayer WSe₂ domains obtained via the CVT method are characterized by optical microscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The Se/WSe₂ heterostructure exhibit improvements in the HER activity as evidenced by the overpotential which is dramatically decreased from 163 mV to 121 mV for a current density of 10 mA cm^-2and the Tafel slope also reduces from 109 mV dec^-1to 75 mV dec^-1upon shining the light. The method could be extended to synthesize different 2D elemental and TMD heterostructures.