Two-Dimensional Semiconductor Optoelectronics

A fundamental figure of merit for high performance optoelectronic devices is their photoluminescence (PL) quantum yield (QY). This value, which accounts for the fraction of absorbed photons are re-emitted by the material governs the ultimate performance of optoelectronic devices such as light emitting diodes, photodetectors/photovoltaics, and solar cells. Two-dimensional (2D) semiconductors have emerged as a promising material system for high-performance optoelectronic and electronic applications. This class of materials possess out-of-plane van der Waals bonding, and as a result has naturally terminated surfaces, even when the material thickness is scaled to the monolayer limit. As a result, surface recombination, a dominant non-radiate pathway in most materials, can be mediated due to the naturally terminated surfaces of van der Waals materials. This offers a potentially pathway to enable near-unity PL QY at the monolayer limit./nDespite this, the room-temperature PL QY for most monolayer 2D semiconductors is extremely poor. The prototypical 2D material, MoS2 has a maximum QY of 0.6% which indicates a considerable defect density. In the first section of this thesis we will address approaches of obtaining 2D materials with high PL QY, primarily focusing on an air-stable solution-based chemical treatment by an organic superacid which uniformly enhances the photoluminescence and minority carrier lifetime of MoS2 monolayers by over two orders of magnitude. The treatment defect-mediated non-radiative recombination pathways, thus resulting in a final QY of over 95% with a longest observed lifetime of 10.8±0.6 nanoseconds. This treatment methodology was also extended to other 2D semiconductors such as WS2 as well as scaled to large area films prepared by chemical vapor deposition./nLeveraging 2D materials with high brightness, the second section of this work will then discuss the development of light emitting devices. Firstly, we discuss the development of a transient electroluminescent device structure for use with monolayer 2D materials. One of the primary challenges for 2D materials scaled down to the monolayer limit has been the efficient simultaneous injection of electrons and holes. To overcome this crucial limitation, I will discuss the development of a device which uses a single metal-semiconductor contact and a gate. Utilizing an AC potential applied to the gate, this device achieves electron and hole injection via tunneling. This results in excess electrons and holes present during the AC transient, which subsequently recombine to emit light. This device is then combined with the passivation methodologies presented in the first section of this thesis. Next, I will demonstrate light emitting diodes based on black phosphorous (bP). Unlike the majority of 2D semiconductors which are direct gap only at the monolayer limit, bP maintains a direct gap over all thickness. Moreover, this bandgap is thickness tunable and can be varied from 1.8 eV for a monolayer down to 0.3 eV for thick films. This property is directly leveraged to fabricate devices with an emission wavelength which can tuned over range of 960 µm to 4.1 µm with external quantum efficiencies as high as 4.4% at room temperature. Notably, bP LEDs offer high performance for emission at infrared wavelengths where the quantum efficiency traditional III-V and II-VI LEDs are limited by auger recombination. The final section of this work I will discuss the development of infrared detectors based on 2D semiconductors.

Advisor: Javey, Ali