HfS 2 and HfSe 2 , have rarely been studied but can possess many extraordinary physical properties. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] For instance, the calculated room-temperature mobility is able to reach as high as 1800 and 3500 cm 2 V −1 s −1 for HfS 2 and HfSe 2 monolayers, respectively, which are much higher than that of widely studied MoS 2 (≈340 cm 2 V −1 s −1 ). [8] Additionally, theoretical calculations also show that HfSe 2 -based field-effect transistors (FETs) can have sheet current densities of up to 5000 µA µm −1 , ≈650 times higher than that of MoS 2 . [9] These outstanding properties make layered HfSe 2 have large potential for low-power electronic devices and optoelectronic devices.In spite of the superiority in theoretical prediction, comparatively limited experimental results are available probably due to the difficulty in synthesizing 2D HfSe 2 layers. Recently, back-gated FETs were fabricated from exfoliated multilayer HfSe 2 , which exhibit an n-type semiconductor behavior with a high on/off current ratio of 10 6 and modest mobilities lower than 10 cm 2 V −1 s −1 . [16][17][18] Moreover, the photo transistors utilizing exfoliated few-layered HfSe 2 as the channel material were also fabricated, [17,18] which demonstrate a reasonable photoresponsivity from the visible to the near-infrared region of light. Nevertheless, all of the studies mentioned above utilized mechanically exfoliated HfSe 2 layers for device fabrication, which totally cannot control the thickness and enlarge the lateral dimension, hindering the study and practical applications of 2D HfSe 2 . In recent years, molecular beam epitaxy (MBE) [19,20] and ion beam-assisted process [21] have been developed to prepare HfSe 2 layers. Unfortunately, the MBE technique requires complicated and expensive ultrahigh vacuum system and the production rate and yield are quite low, while the crystal quality of HfSe 2 films prepared by ion beam-assisted process is insufficient for device applications since the sputtered Hf film could not be uniformly selenized owing to the diffusion limited reaction. Thus, the synthesis of large-area high-quality 2D HfSe 2 layers is still challenging. Chemical vapor deposition (CVD) is one of the most common and convenient methods for synthesizing various 2D materials such as MoSe 2 , [24] WSe 2, [25] and HfS 2 . [26,27] Nevertheless, the synthesis of high-quality 2D HfSe 2 layers using a CVD method has not yet been reported.Alloying 2D TMDs have been demonstrated as an effective approach to realize the bandgap engineering, which is crucial Alloying transition metal dichalcogenides (TMDs) with different compositions is demonstrated as an effective way to acquire 2D semiconductors with widely tunable bandgaps. Herein, for the first time, the large-area synthesis of layered HfS 2(1−x) Se 2x alloys with fully tunable chemical compositions on sapphire by chemical vapor deposition is reported, greatly expanding and enriching the family of 2D TMDs semiconductors. The configuration an...
