A novel hydrogen plasma treatment to convert an amorphous phosphorus film deposited on silicon substrates into a thin crystalline layer is successfully developed. The amorphous phosphorus layer is deposited on desired substrates as silicon using vacuum evaporation in a direct-current plasma reactor in a controllable fashion. The formation of 2D phosphorene layers is based on the phase transition of a previously deposited amorphous film into the crystalline black phosphorus flakes. Direct transformation from red phosphorus to black can be achieved at a temperature of 300 C on desired substrates as silicon, although mica and glass can also be used. This allotrope transformation can be achieved without any high pressure or high temperature. Apart from hydrogen plasma treatment, ultraviolet (UV) exposure to see any possible improvement in the 2D layer formation is also used herein. Various characterization techniques including scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and Raman spectroscopy are used to study the crystallinity and morphology of the samples. In addition, the plasma-treated flakes are used to realize photodetectors which show excellent response to illuminating light. While the hydrogen plasma treatment leads to crystalline phosphorene sheets, the UV treatment results in granular and partially crystalline nanostructures.The world of 2D materials has witnessed a growing path to apply this appealing family in various applications including electronics, optoelectronics, bioelectronics, and sensing and energy devices. [1][2][3][4][5] Owing to its unique properties, phosphorene (few layers or monolayers of black phosphorus (BP)) has grasped the researchers' attention around the globe. [6,7] High-pressure and high-temperature procedures, sonication-based methods, and even phase transition using the presence of SnI 4 /Sn are the most common techniques used to realize few-or monolayer phosphorene through a complete phase transition from red phosphorus (RP) to its BP allotrope. Mechanical exfoliation of phosphorene flakes and their transfer to desired substrates is a main line of research for the fabrication of phosphorenebased electronic and optical devices. Owing to an unstable and sensitive nature of phosphorene, its successful transfer onto the target substrate is a challenge and requires extreme precautions to avoid extended exposure to air and humidity. [8][9][10][11][12][13] Direct formation of the crystalline phosphorene flakes on desired substrates has always been a technological challenge. Recently various bottom-up techniques have been demonstrated to realize 2D materials including phosphorene flakes. As reported, there is no successful fabrication of phosphorene by means of the chemical vapor deposition (CVD) technique or wet chemical methods. The most successful bottom-up method which has been introduced recently is pulsed laser deposition (PLD) which requires a bulk target. [14] Accordingly, the evolution of a bottom-up growth method, which does not nee...