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Ultrafast & Terahertz Photonics Group

Recent publications from the group [all | THz | perovskites | nano | biomedical]

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Metal composition influences optoelectronic quality in mixed-metal lead-tin triiodide perovskite solar absorbers

M. T. Klug, R. L. Milot, J.B. Patel, T. Green, H. C. Sansom, M. D. Farrar, A. J. Ramadan, S. Martani, Z. Wang, B. Wenger, J. M. Ball, L. Langshaw, A. Petrozza, M. B. Johnston, L. M. Herz and H. J. Snaith
Energy & Environmental Science (May 2020) [ pdf ] [ ref ]

Klug 2020Current designs for all-perovskite multi-junction solar cells require mixed-metal Pb–Sn compositions to achieve narrower band gaps than are possible with their neat Pb counterparts. The lower band gap range achievable with mixed-metal Pb–Sn perovskites also encompasses the 1.3 to 1.4 eV range that is theoretically ideal for maximising the efficiency of single-junction devices. Here we examine the optoelectronic quality and photovoltaic performance of the ((HC(NH2)2)0.83Cs0.17)(Pb1−ySny)I3 family of perovskite materials across the full range of achievable band gaps by substituting between 0.001% and 70% of the Pb content with Sn. We reveal that a compositional range of “defectiveness” exists when Sn comprises between 0.5% and 20% of the metal content, but that the optoelectronic quality is restored for Sn content between 30–50%. When only 1% of Pb content is replaced by Sn, we find that photoconductivity, photoluminescence lifetime, and photoluminescence quantum efficiency are reduced by at least an order of magnitude, which reveals that a small concentration of Sn incorporation produces trap sites that promote non-radiative recombination in the material and limit photovoltaic performance. While these observations suggest that band gaps between 1.35 and 1.5 eV are unlikely to be useful for optoelectronic applications without countermeasures to improve material quality, highly efficient narrower band gap absorber materials are possible at or below 1.33 eV. Through optimising single-junction photovoltaic devices with Sn compositions of 30% and 50%, we respectively demonstrate a 17.6% efficient solar cell with an ideal single-junction band gap of 1.33 eV and an 18.1% efficient low band gap device suitable for the bottom absorber in all-perovskite multi-junction cells.

Wed 07 Oct 2020, 13:03 | Tags: THz spectroscopy, photoluminescence, Milot, perovskites, 2020

Evaluation of transdermal drug delivery using terahertz pulsed imaging

J. Wang, H. Lindley-Hatcher, K. Liu, E. Pickwell-MacPherson
Biomedical Optics Express 11 4484 (August 2020) [ pdf ] [ ref ]

DiagramTransdermal drug delivery (TDD) is widely used for painless dosing due to its minimally invasive nature compared to hypodermic needle injection and its avoidance of the gastrointestinal tract. However, the stratum corneum obstructs the permeation of drugs into skin. Microneedle and nanoneedle patches are ways to enhance this permeation. In this work, terahertz (THz) imaging is utilized to compare the efficacy of different TDD methods including topical application and via a needle patch. Our work shows the feasibility and potential of using THz imaging to quantify and evaluate different transdermal application methods.

Thu 24 Sep 2020, 10:01 | Tags: MacPherson, THz imaging, biomedical, 2020

An Ultrafast Shakedown Reveals the Energy Landscape, Relaxation Dynamics and Concentration of the N3VH0 Defect in Diamond

D.J.L. Coxon, M. Staniforth, B.G. Breeze, S.E. Greenough, J.P. Goss, M. Monti, J. Lloyd-Hughes, V.G. Stavros, and M.E. Newton
J. Phys. Chem. Lett. 11, 6677 (July 2020) [ pdf (with SI) ] [ ref ]

Ultrafast shakedownAtomic-scale defects can control the exploitable optoelectronic performance of crystalline materials, and several point defects in diamond are emerging functional components for a range of quantum technologies. Nitrogen and hydrogen are common impurities incorporated into diamond, and there is a family of defects that includes both. The N3VH0 defect is a lattice vacancy where three nearest neighbor carbon atoms are replaced with nitrogen atoms and a hydrogen is bonded to the remaining carbon. It is regularly observed in natural and high-temperature annealed synthetic diamond, and gives rise to prominent absorption features in the mid-infrared. Here, we combine time- and spectrally-resolved infrared absorption spectroscopy to yield unprecedented insight into the N3VH0 defect’s vibrational dynamics following infrared excitation of the C–H stretch. In doing so, we gain fundamental information about the energies of quantized vibrational states, and corroborate our results with theory. We map out, for the first time, energy relaxation pathways, which include multiphonon relaxation processes and anharmonic coupling to the C–H bend mode. These advances provide new routes to quantify and probe atomic-scale defects.

Fri 17 Jul 2020, 22:00 | Tags: nanomaterials, Lloyd-Hughes, 2020

Emergent Antipolar Phase in BiFeO3-La0.7Sr0.3MnO3 Superlattice

W. Dong, J.J.P. Peters, D. Rusu, M. Staniforth, A. Brunier, J. Lloyd-Hughes, A.M. Sanchez and M. Alexe
Nano Lett. 20 X, XXX (July 2020) [ pdf ] [ ref ]

Emergent antipolar phaseFerroelectric–paraelectric superlattices show emerging new states, such as polar vortices, through the interplay and different energy scales of various thermodynamic constraints. By introducing magnetic coupling at BiFeO3–La0.7Sr0.3MnO3 interfaces epitaxially grown on SrTiO3 substrate, we find, for the first time in thin films, a sub-nanometer thick lamella-like BiFeO3. The emergent phase is characterized by an arrangement of a two unit cell thick lamella-like structure featuring antiparallel polarization, resulting an antiferroelectric-like structure typically associated with a morphotropic phase transition. The antipolar phase is embedded within a nominal R3c structure and is independent of the BiFeO3 thickness (4–30 unit cells). Moreover, the superlattice structure with the morphotropic phase demonstrates azimuth-independent second harmonic generation responses, indicating a change of overall symmetry mediated by a delicate spatial distribution of the emergent phase. This work enriches the understanding of a metastable state manipulated by thermodynamic constraints by lattice strain and magnetic coupling.

Tue 14 Jul 2020, 21:05 | Tags: nanomaterials, Lloyd-Hughes, 2020

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