UC Berkeley

With the innovation of technologies that will impact our daily life such as the internet of things (IoT), wearable health care systems, digitally extended reality, and autonomous vehicles, the development of next-generation optoelectronic devices that possess advanced functionality and high performance has emerged as a substantial component for the evolution of future technology. Two-dimensional (2D) semiconductors have been extensively studied in the exploration of new phenomena and properties that are not seen in conventional bulk semiconductors, especially for optoelectronic applications such as light-emitting diodes (LEDs) and photodetectors. While the majority of existing technology relies on bulk crystalline semiconductors, unprecedented possibilities can be opened up for developing next-generation optoelectronics owing to the unique properties and the tuning capabilities that 2D semiconductors offer.Chapter 2 presents the material processing approaches that are applied in atomically-thin semiconductor monolayers to enhance their luminescence efficiency. A combination of optimized doping and growth conditions enables extraordinarily bright luminescence in 2D transition-metal dichalcogenide (TMDC) monolayers with large scale and high stability. Chapter 3 describes the first critical step towards highly efficient LEDs at all brightness using monolayer semiconductors. The fundamental limitations of degraded luminescence efficiency in 2D semiconductors at high photocarrier densities have been identified and overcome by applying small mechanical strain to the materials, leading to the complete suppression of nonradiatvie recombination at all exciton densities. Chapter 4 demonstrates the room-temperature infrared optoelectronic devices whose operating wavelengths can be widely modulated by utilizing strain-tunable bandgap in black phosphorus. Mid-wavelength infrared (MWIR) LEDs with large spectrum tunability are developed and applied in non-dispersive infrared gas sensing technology. Highly-responsive photodetectors exhibit their detectivity exceeding those of state-of-the-art photodetectors with their detection wavelengths spanning from short-wavelength infrared (SWIR) to MWIR range.