Prof Michael Johnston

Abstract:

Terahertz radiation encompasses a wide band of the electromagnetic spectrum, spanning from microwaves to infrared light, and is a particularly powerful tool for both fundamental scientific research and applications such as security screening, communications, quality control, and medical imaging. Considerable information can be conveyed by the full polarization state of terahertz light, yet to date, most time-domain terahertz detectors are sensitive to just one polarization component. Here we demonstrate a nanotechnology-based semiconductor detector using cross-nanowire networks that records the full polarization state of terahertz pulses. The monolithic device allows simultaneous measurements of the orthogonal components of the terahertz electric field vector without cross-talk. Furthermore, we demonstrate the capabilities of the detector for the study of metamaterials.

Abstract:

Understanding and controlling grain growth in metal halide perovskite polycrystalline thin films is an important step in improving the performance of perovskite solar cells. We demonstrate accurate control of crystallite size in CH3NH3PbI3 thin films by regulating substrate temperature during vacuum co-deposition of inorganic (PbI2) and organic (CH3NH3I) precursors. Films co-deposited onto a cold (−2 °C) substrate exhibited large, micrometer-sized crystal grains, while films that formed at room temperature (23 °C) only produced grains of 100 nm extent. We isolated the effects of substrate temperature on crystal growth by developing a new method to control sublimation of the organic precursor, and CH3NH3PbI3 solar cells deposited in this way yielded a power conversion efficiency of up to 18.2%. Furthermore, we found substrate temperature directly affects the adsorption rate of CH3NH3I, thus impacting crystal formation and hence solar cell device performance via changes to the conversion rate of PbI2 to CH3NH3PbI3 and stoichiometry. These findings offer new routes to developing efficient solar cells through reproducible control of crystal morphology and composition.

Abstract:

AbstractInorganic cesium‐based metal halide perovskite (MHP) semiconductors have great potential as active layers in optoelectronic devices, such as perovskite light‐emitting diodes (PeLEDs) and perovskite lasers. However, precise control of crystal type, quality, and thickness is required to create high‐performance and reproducible devices. Vapor‐phase vacuum deposition enables fabrication of MHP thin films and devices with excellent uniformity and control over layer thickness, although a full understanding of crystal growth mechanisms and products has proved elusive. Here, conditions of vapor co‐deposition of CsBr and PbBr are related with the optical performance and atomic microstructure of resulting CsPbBr3 thin films. It is found that the structure is predominantly photoactive γ‐CsPbBr3 over a wide range of conditions, but the presence of impurity phases and Ruddlesden–Popper (RP) planar defects both degrade optical performance as quantified through measured amplified spontaneous emission (ASE) thresholds. Furthermore, the atomic structure of the dominant impurity phases is resolved: CsPb2Br5 and Cs4PbBr6. It is revealed that a small nominal excess of CsBr‐precursor flux during co‐evaporation can significantly enhance the nucleation of thin films, resulting in well‐defined grains greater than 500 nm in size and the relative suppression of RP planar defects. Such films exhibit intensified photoluminescence (PL) emission and a reduced ASE threshold of 30.9 µJ cm−2.