Electron-Doping-Enhanced Trion Formation in Monolayer Molybdenum Disulfide Functionalized with Cesium Carbonate

We report effective and stable electron doping of monolayer molybdenum disulfide (MoS₂) by cesium carbonate (Cs₂CO₃) surface functionalization. The electron charge carrier concentration in exfoliated monolayer MoS₂ can be increased by about 9 times after Cs₂CO₃ functionalization. The n-type doping effect was evaluated by <i>in situ</i> transport measurements of MoS₂ field-effect transistors (FETs) and further corroborated by <i>in situ</i> ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and Raman scattering measurements. The electron doping enhances the formation of negative trions (<i>i.e.</i>, a quasiparticle comprising two electrons and one hole) in monolayer MoS₂ under light irradiation and significantly reduces the charge recombination of photoexcited electron–hole pairs. This results in large photoluminescence suppression and an obvious photocurrent enhancement in monolayer MoS₂ FETs

Direct Observation of Semiconductor–Metal Phase Transition in Bilayer Tungsten Diselenide Induced by Potassium Surface Functionalization

Structures determine properties of materials, and controllable phase transitions are, therefore, highly desirable for exploring exotic physics and fabricating devices. We report a direct observation of a controllable semiconductor–metal phase transition in bilayer tungsten diselenide (WSe₂) with potassium (K) surface functionalization. Through the integration of <i>in situ</i> field-effect transistors, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy measurements, and first-principles calculations, we identify that the electron doping from K adatoms drives bilayer WSe₂ from a 2H phase semiconductor to a 1T′ phase metal. The phase transition mechanism is satisfactorily explained by the electronic structures and energy alignment of the 2H and 1T′ phases. This explanation can be generally applied to understand doping-induced phase transitions in two-dimensional (2D) structures. Finally, the associated dramatic changes of electronic structures and electrical conductance make this controllable semiconductor–metal phase transition of interest for 2D semiconductor-based electronic and optoelectronic devices

Colossal Ultraviolet Photoresponsivity of Few-Layer Black Phosphorus

Black phosphorus has an orthorhombic layered structure with a layer-dependent direct band gap from monolayer to bulk, making this material an emerging material for photodetection. Inspired by this and the recent excitement over this material, we studied the optoelectronics characteristics of high-quality, few-layer black phosphorus-based photodetectors over a wide spectrum ranging from near-ultraviolet (UV) to near-infrared (NIR). It is demonstrated for the first time that black phosphorus can be configured as an excellent UV photodetector with a specific detectivity ∼3 × 10¹³ Jones. More critically, we found that the UV photoresponsivity can be significantly enhanced to ∼9 × 10⁴ A W^–1 by applying a source-drain bias (<i>V</i>_SD) of 3 V, which is the highest ever measured in any 2D material and 10⁷ times higher than the previously reported value for black phosphorus. We attribute such a colossal UV photoresponsivity to the resonant-interband transition between two specially nested valence and conduction bands. These nested bands provide an unusually high density of states for highly efficient UV absorption due to the singularity of their nature

Surface Functionalization of Black Phosphorus via Potassium toward High-Performance Complementary Devices

Two-dimensional black phosphorus configured field-effect transistor devices generally show a hole-dominated ambipolar transport characteristic, thereby limiting its applications in complementary electronics. Herein, we demonstrate an effective surface functionalization scheme on few-layer black phosphorus, through in situ surface modification with potassium, with a view toward high performance complementary device applications. Potassium induces a giant electron doping effect on black phosphorus along with a clear bandgap reduction, which is further corroborated by in situ photoelectron spectroscopy characterizations. The electron mobility of black phosphorus is significantly enhanced to 262 (377) cm² V^–1 s^–1 by over 1 order of magnitude after potassium modification for two-terminal (four-terminal) measurements. Using lithography technique, a spatially controlled potassium doping technique is developed to establish high-performance complementary devices on a single black phosphorus nanosheet, for example, the p–n homojunction-based diode achieves a near-unity ideality factor of 1.007 with an on/off ratio of ∼10⁴. Our findings coupled with the tunable nature of in situ modification scheme enable black phosphorus as a promising candidate for further complementary electronics