Optically Pumped Polaritons in Perovskite Light-Emitting Diodes

Lead halide perovskites have achieved significant progress in light-emitting diodes (LEDs) with high efficiency in the past few decades. They are also ideal candidates for reaching the strong exciton–photon coupling regime due to their large exciton binding energy and oscillator strength. The generation and control of exotic phenomena in perovskite electroluminescent microcavities, such as electrically pumped polariton lasing and polariton LEDs, operating in the strong coupling regime at room temperature, is still a great challenge. Here, we demonstrate room-temperature strong coupling in a perovskite LED structure. The best device shows a current efficiency of 4.5 cd/A and an external quantum efficiency of 1.4% while exhibiting anticrossing behavior via optical pumping. Our approach represents a new strategy to explore ultrafast LEDs as well as electrically pumped perovskite lasing

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

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