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:

Two-dimensional perovskites have emerged as promising materials for optoelectronic applications owing to their excellent environmental stability and tunable quantum confinement. Such 2D perovskites can incorporate a particularly versatile range of organic cations of different size, chemical nature, and optoelectronic character. However, understanding and controlling thin-film transport for this vast family of materials remains a key challenge to their successful application in devices. Here, we systematically investigate odd-even effects in thin films of Ruddlesden-Popper-type (RP) lead-iodide 2D perovskites based on nonconjugated alkylammonium spacer cations with chain lengths ranging from three to eight carbon atoms. A pronounced odd-even dependence on the carbon number is observed in both optical and transport properties, including absorption coefficients, photoluminescence energies and lifetimes, and excitation diffusion dynamics. Notably, the coefficients for charge-carrier diffusion out of the film plane─extracted via a dynamic photon reabsorption approach─display an opposite odd-even trend to the in-plane charge-carrier mobility obtained from optical pump-terahertz probe measurements, causing a pronounced odd-even modulation of the thin-film mobility anisotropy. Grazing-incidence wide-angle X-ray scattering measurements reveal that this behavior is related to cation-controlled nanostructural orientation: even-numbered alkyl spacer cations induce lead-iodide planes lying highly oriented within the film plane, while odd-numbered ones cause more disordered stacking. Furthermore, the observed 1/d²-dependence on interplane distance d in ordered films demonstrates that Förster resonance energy transfer underpins diffusion of excitations between lead-iodide layers. Our findings establish a direct structure-transport correlation in 2D perovskite films and provide valuable guidelines for the design of optoelectronic devices.