1. Introduction
From the successful fabrication of atomically thin carbon films by Andre Geim and Konstantin Novoselov through mechanical exfoliation in 2004[1], graphene not only verified the possibility for stable existence of two-dimensional materials but also exhibited novel and outstanding physical properties including ultra-high carrier mobility[2], ultra-high thermal conductivity[3] and superior mechanical properties[4], which was awarded the Nobel prize in physics 2010. Subsequently, investigations on exploring new 2D materials and their applications have become the frontier fields in condensed physics, such as graphyne[5, 6], two-dimensional transition metal dichalcogenides (TMDs)[7-12], MXenes[13], hexagonal boron nitride (h-BN)[14, 15], black phosphorene (BP)[16, 17], black arsenic[18, 19]. By breaking the weak van der Waals interaction between layers as a “top-down” method like exfoliation or aggregating atoms together as a “bottom-up” method like chemical vapor deposition, researchers have developed variable methods to produce versatile two-dimensional materials and fabricated diverse devices like a field-effect transistor, photodetector and PN diode based on their suitable electronic structures and atomic size along the out-of-plane direction[20-23].