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Representation of THz spectroscopy of a metamaterial with a Nanowire THz sensor

Representation of THz spectroscopy of a metamaterial with a Nanowire THz sensor

Credit: Rendering by Dimitars Jevtics

Prof Michael Johnston

Professor of Physics

Research theme

  • Photovoltaics and nanoscience

Sub department

  • Condensed Matter Physics

Research groups

  • Terahertz photonics
  • Advanced Device Concepts for Next-Generation Photovoltaics
michael.johnston@physics.ox.ac.uk
Johnston Group Website
  • About
  • Publications

Three-dimensional cross-nanowire networks recover full terahertz state

Science American Association for the Advancement of Science 368:6490 (2020) 510-513

Authors:

Kun Peng, Dimitars Jevtics, Fanlu Zhang, Sabrina Sterzl, Djamshid Damry, Mathias Rothmann, Benoit Guilhabert, Michael J Strain, Hark H Tan, Laura M Herz, Lan Fu, Martin D Dawson, Antonio Hurtado, Chennupati Jagadish, 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.
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Control over crystal size in vapor deposited metal-halide perovskite films

ACS Energy Letters American Chemical Society (ACS) 5 (2020) 0c00183

Authors:

Kilian B Lohmann, Jay B Patel, Mathias Uller Rothmann, Chelsea Q Xia, Robert DJ Oliver, Laura M Herz, Henry J Snaith, Michael B Johnston

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.
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An ultrafast switchable terahertz polarization modulator based on III--V semiconductor nanowires

Nano Letters: a journal dedicated to nanoscience and nanotechnology American Chemical Society (2017)

Authors:

MB Johnston, JL Boland, D Damry
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Efficient planar heterojunction perovskite solar cells by vapour deposition

Nature Springer Science and Business Media LLC 501:7467 (2013) 395-398

Authors:

Mingzhen Liu, Michael B Johnston, Henry J Snaith
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Ruddlesden-Popper Defects Act as a Free Surface: Role in Formation and Photophysical Properties of CsPbI<sub>3</sub>.

Advanced materials (Deerfield Beach, Fla.) (2025) e2501788

Authors:

Weilun Li, Qimu Yuan, Yinan Chen, Joshua RS Lilly, Marina R Filip, Laura M Herz, Michael B Johnston, Joanne Etheridge

Abstract:

The perovskite semiconductor, CsPbI3, holds excellent promise for solar cell applications due to its suitable bandgap. However, achieving phase-stable CsPbI3 solar cells with high power conversion efficiency remains a major challenge. Ruddlesden-Popper (RP) defects have been identified in a range of perovskite semiconductors, including CsPbI3. However, there is limited understanding as to why they form or their impact on stability and photophysical properties. Here, the prevalence of RP defects is increased with increased Cs-excess in vapor-deposited CsPbI3 thin films while  superior structural stability but inferior photophysical properties are observed. Significantly, using electron microscopy, it is found that the atomic positions at the planar defect are comparable to those of a free surface, revealing their role in phase stabilization. Density functional theory (DFT) calculations reveal the RP planes are electronically benign, however, antisites observed at RP turning points are likely to be malign. Therefore it is proposed that increasing RP planes while reducing RP turning points offers a breakthrough for improving both phase stability and photophysical performance. The formation mechanism revealed here can apply more generally to RP structures in other perovskite systems.
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