Mark Lockrey joined the Microstructural Analysis Unit (MAU) at the University of Technology Sydney (UTS) in 2018 as a Scientific Officer. Mark received his PhD in Physics at the University of Technology, Sydney in 2015, where his studies focused on cathodoluminescence (CL) analysis of III-nitride materials, then worked at the Australian National University (ANU) in 2015 as a Micronalysis Research Officer and SEM Process Engineer for the ANFF ACT Node, where he operated the FESEM-CL system. Mark Currently works at UTS where he operates the cathodoluminescence (CL), XRD and TEM systems. His research include CL analysis of plasmonic materials, wide bandgap semiconductors and organic semiconductors as well as the development new CL analysis techniques.
Cathodoluminescence (CL) analysis of nano-materials, wide bandgap semiconductors and organic semiconductors.
Effects of electron beam irradiation on semiconductors.
Inorganic peroviskite solarcells.
Development of new Cathodoluminescence (CL) measurment techniques
Fusco, Z, Rahmani, M, Bo, R, Tran-Phu, T, Lockrey, M, Motta, N, Neshev, D & Tricoli, A 2019, 'High-Temperature Large-Scale Self-Assembly of Highly Faceted Monocrystalline Au Metasurfaces', Advanced Functional Materials, vol. 29, no. 2.View/Download from: UTS OPUS or Publisher's site
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Localized surface plasmon resonance (LSPR) devices based on resonant metallic metasurfaces have shown disruptive potential for many applications including biosensing and photocatalysis. Despite significant progress, highly performing Au plasmonic nanotextures often suffer of suboptimal electric field enhancement, due to damping effects in multicrystalline domains. Fabricating well-defined Au nanocrystals over large surfaces is very challenging, and usually requires time-intensive multi-step processes. Here, presented are first insights on the large-scale self-assembly of monocrystalline Au nano-islands with tunable size and separation, and their application as efficient LSPR surfaces. Highly homogeneous centimeter-sized Au metasurfaces are fabricated by one-step deposition and in situ coalescence of hot nanoparticle aerosols into a discontinuous monolayer of highly faceted monocrystals. First insights on the mechanisms driving the high-temperature synthesis of these highly faceted Au nanotextures are obtained by molecular dynamic and detailed experimental investigation of their growth kinetics. Notably, these metasurfaces demonstrat high-quality and tunable LSPR, enabling the fabrication of highly performing optical gas molecule sensors detecting down to 3 × 106 variations in refractive index at room temperature. It is believed that these findings provide a rapid, low-cost nanofabrication tool for the engineering of highly homogenous Au metasurfaces for large-scale LSPR devices with application ranging from ultrasensitive optical gas sensors to photocatalytic macroreactors.
Riesen, N, Lockrey, M, Badek, K & Riesen, H 2019, 'On the origins of the green luminescence in the "zero-dimensional perovskite" Cs4PbBr6: conclusive results from cathodoluminescence imaging.', Nanoscale, vol. 11, no. 9, pp. 3925-3932.View/Download from: Publisher's site
There is great interest in the use of highly-efficient all-inorganic halide perovskites CsnPbBr2+n for optoelectronic applications. There however remains considerable debate as to the origins of the green luminescence in the zero-dimensional phase of the perovskite Cs4PbBr6, with theories suggesting it originates either from defects in the Cs4PbBr6 lattice or CsPbBr3 impurities/inclusions. The confusion has arisen due to the two phases being miscible and typically co-existing. Moreover, low impurity levels of CsPbBr3 in Cs4PbBr6 are difficult to detect by XRD measurements, yet have much stronger photoluminescence than bulk CsPbBr3 that exhibits quenching, further contributing to the confusion as to the origins of the green photoluminescence. With the rise of significant debate and misconceptions, we provide conclusive evidence that the green emission from Cs4PbBr6 is indeed due to nanocrystalline CsPbBr3 impurities. This is demonstrated by undertaking cathodoluminescence and EDX measurements on samples prepared mechanochemically by ball-milling. Cathodoluminescence imaging clearly shows the presence of small crystals embedded in/or between larger crystallites of Cs4PbBr6 and they emit around 520 nm. EDX shows that the smaller crystal inclusions have a Pb:Br ratio that is approximately 2 times higher, confirming the CsPbBr3 phase, which has the expected size-dependent shift to shorter wavelengths (about 528 to 515 nm). These studies make significant inroads into understanding these lead halide perovskites for their use in a variety of optoelectronic and photovoltaic applications.
Zhao, B, Lockrey, MN, Caroff, P, Wang, N, Li, L, Wong-Leung, J, Tan, HH & Jagadish, C 2018, 'The effect of nitridation on the polarity and optical properties of GaN self-assembled nanorods.', Nanoscale, vol. 10, no. 23, pp. 11205-11210.View/Download from: UTS OPUS or Publisher's site
We report on the effect of nitridation on GaN self-assembled nanorods grown on the c-plane sapphire by metalorganic chemical vapour deposition (MOCVD). Nitridation conditions are found to critically influence the nanorod morphology and optical properties. The nanorod polarity was determined through a direct observation of atomic dumbbell pairs. While purely N-polar wires are obtained under optimised nitridation, incomplete or missing nitridation leads to mixed polarity. By comparing the morphology and the crystal structure with spatially resolved cathodoluminescence results, our study unambiguously establishes a link between appropriate nitridation duration and a homogeneous improvement in optical quality.
Li, Z, Yang, I, Li, L, Gao, Q, Chong, JS, Li, Z, Lockrey, MN, Tan, HH, Jagadish, C & Fu, L 2018, 'Reducing Zn diffusion in single axial junction InP nanowire solar cells for improved performance', Progress in Natural Science: Materials International, vol. 28, no. 2, pp. 178-182.View/Download from: UTS OPUS or Publisher's site
© 2018 Elsevier B.V. In this work axial n-i-p junction InP nanowires were grown by selective-area metal organic vapor phase epitaxy (SA-MOVPE) technique with the growth sequence starting from n-segment. The optical properties and carrier lifetimes of the n-, i- and p-type segments were studied and compared using time-resolved photoluminescence (PL) and cathodoluminescence (CL) measurements. We demonstrate for the first time that CL is capable of resolving the electrical profile of the nanowires, namely the varied lengths of the n-, i- and p-segments, providing a simple and effective approach for nanowire growth calibration and optimization. The CL result was further confirmed by electron beam induced current (EBIC) and photocurrent mapping measurements performed from the fabricated single nanowire solar cell devices. It is revealed that despite a non-optimized device structure (very long n-region and short i-region), the n-i-p nanowire solar cells show improved power conversion efficiency (PCE) than the previously reported p-i-n (growth starts with p-segment) single nanowire solar cells due to reduced p-type dopant (Zn) diffusion during the growth of n-i-p solar cell structure.
Yang, I, Zhang, X, Zheng, C, Gao, Q, Li, Z, Li, L, Lockrey, MN, Nguyen, H, Caroff, P, Etheridge, J, Tan, HH, Jagadish, C, Wong-Leung, J & Fu, L 2018, 'Radial Growth Evolution of InGaAs/InP Multi-Quantum-Well Nanowires Grown by Selective-Area Metal Organic Vapor-Phase Epitaxy.', ACS nano, vol. 12, no. 10, pp. 10374-10382.View/Download from: UTS OPUS or Publisher's site
III-V semiconductor multi-quantum-well nanowires (MQW NWs) via selective-area epitaxy (SAE) is of great importance for the development of nanoscale light-emitting devices for applications such as optical communication, silicon photonics, and quantum computing. To achieve highly efficient light-emitting devices, not only the high-quality materials but also a deep understanding of their growth mechanisms and material properties (structural, optical, and electrical) are extremely critical. In particular, the three-dimensional growth mechanism of MQWs embedded in a NW structure by SAE is expected to be different from that of those grown in a planar structure or with a catalyst and has not yet been thoroughly investigated. In this work, we reveal a distinctive radial growth evolution of InGaAs/InP MQW NWs grown by the SAE metal organic vapor-phase epitaxy (MOVPE) technique. We observe the formation of zinc blende (ZB) QW discs induced by the axial InGaAs QW growth on the wurtzite (WZ) base-InP NW and propose it as the key factor driving the overall structure of radial growth. The role of the ZB-to-WZ change in the driving of the overall growth evolution is supported by a growth formalism, taking into account the formation-energy difference between different facets. Despite a polytypic crystal structure with mixed ZB and WZ phases across the MQW region, the NWs exhibit high uniformity and desirable QW spatial layout with bright room-temperature photoluminescence at an optical communication wavelength of 1.3 m, which is promising for the future development of high-efficiency light-emitting devices.
Duong, T, Peng, J, Walter, D, Xiang, J, Shen, H, Chugh, D, Lockrey, M, Zhong, D, Li, J, Weber, K, White, TP & Catchpole, KR 2018, 'Perovskite Solar Cells Employing Copper Phthalocyanine Hole-Transport Material with an Efficiency over 20% and Excellent Thermal Stability', ACS Energy Letters, vol. 3, no. 10, pp. 2441-2448.View/Download from: UTS OPUS or Publisher's site
© Copyright 2018 American Chemical Society. We investigate the properties of an inexpensive hole-transporting material (HTM), copper phthalocyanine (CuPc), deposited by a solution-processing method in perovskite solar cells (PSCs). Cracks are found to be abundant on the as-deposited CuPc films, which lead to serious shunts and interface recombination. Surprisingly, shunts and interface recombination are significantly reduced and cell performance is greatly improved after heat treatment at 85 °C. We find that the enhancement is due to heat-induced migration of Au particles away from the cracks. Furthermore, Au is found to dope the CuPc film, and the doping effect is greatly enhanced by the heat treatment. Using CuPc and quadruple-cation perovskite, an efficiency of over 20% and negligible hysteresis is achieved after the heat treatment, which is the highest value reported for this structure. Additionally, PSCs employing CuPc and dual-cation perovskite show excellent thermal stability after >2000 h at 85 °C and good light stability at 25 °C.
Duong, T, Wu, YL, Shen, H, Peng, J, Zhao, S, Wu, N, Lockrey, M, White, T, Weber, K & Catchpole, K 2018, 'Light and elevated temperature induced degradation (LeTID) in perovskite solar cells and development of stable semi-transparent cells', Solar Energy Materials and Solar Cells, vol. 188, pp. 27-36.View/Download from: UTS OPUS or Publisher's site
© 2018 Elsevier B.V. The stability of perovskite solar cells (PSCs) is one of the major challenges to their commercialization. In this work, the stability of state-of-the-art mesoporous PSCs employing multi-cation mixed-halide perovskite and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) as the hole-transport-layer (HTL) is studied under illumination at an elevated temperature in N2environment. The Rb in the quadruple-cation perovskite is found to segregate when the film undergoes aging under simultaneous light and heat exposure. More importantly, the PTAA layer prevents diffusion of gold into the perovskite layer when aged at 85 °C in the dark, but not under light. As a result, the PSCs thermally aged under light degrade much more severely than cells thermally aged in the dark. Therefore, the term Light and elevated Temperature Induced Degradation (LeTID) is introduced for PSCs. The effect is also evident for PCSs employing a copper(II) 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (CuPC) HTL. This effect seriously impacts the operational stability of PSCs and it might not be detected with the current stress tests defined in the IEC-61646 standard. PSCs with a transparent contact and robust perovskite composition are developed with improved stability against LeTID. The cells retain more than 90% of the initial efficiency after > 160 h operating under illumination at 85 °C in N2environment.
Zhu, L, Lockrey, M, Phillips, MR & Cuong, T-T 2018, 'Spatial Distribution of Defect Luminescence in ZnO Nanorods: An Investigation by Spectral Cathodoluminescence Imaging', PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, vol. 215, no. 19.View/Download from: UTS OPUS or Publisher's site
Berg, A, Caroff, P, Shahid, N, Lockrey, MN, Yuan, X, Borgstrom, MT, Tan, HH & Jagadish, C 2017, 'Growth and optical properties of In (x) Ga1-x P nanowires synthesized by selective-area epitaxy', NANO RESEARCH, vol. 10, no. 2, pp. 672-682.View/Download from: Publisher's site
Peng, J, Wu, Y, Ye, W, Jacobs, DA, Shen, H, Fu, X, Wan, Y, Duong, T, Wu, N, Barugkin, C, Nguyen, HT, Zhong, D, Li, J, Lu, T, Liu, Y, Lockrey, MN, Weber, KJ, Catchpolea, KR & White, TP 2017, 'Interface passivation using ultrathin polymer-fullerene films for high-efficiency perovskite solar cells with negligible hysteresis', ENERGY & ENVIRONMENTAL SCIENCE, vol. 10, no. 8, pp. 1792-1800.View/Download from: Publisher's site
Ma, Q, Huang, S, Chen, S, Zhang, M, Lau, CFJ, Lockrey, MN, Mulmudi, HK, Shan, Y, Yao, J, Zheng, J, Deng, X, Catchpole, K, Green, MA & Ho-Baillie, AWY 2017, 'The Effect of Stoichiometry on the Stability of Inorganic Cesium Lead Mixed-Halide Perovskites Solar Cells', Journal of Physical Chemistry C, vol. 121, no. 36, pp. 19642-19649.View/Download from: Publisher's site
© 2017 American Chemical Society. Metal halide perovskite solar cells that use the inorganic cation Cs have been shown to have better thermal stability than the organic cation containing counterparts, and CsPbI 2 Br has a more suitable (lower) band gap than CsPbIBr 2 as a photovoltaic energy harvesting material. However, increase in iodine content reduces structural stability due to the preference toward the non-perovskite orthorhombic phase when the film is exposed to air. In this work, the effect of varying stoichiometry of CsPbI 2 Br perovskite on film quality such as the grain size, presence of impurities and nature of impurity grains, photoluminescence, morphology, and elemental distribution are studied. Details on how to vary the stoichiometry during the dual source thermal evaporation process are reported. It is found that the air stability of CsPbI 2 Br film correlates with the CsBr-to-PbI 2 deposition rate ratio, in which the CsBr-rich CsPbI 2 Br is the most stable upon air exposure, while the stoichiometrically balanced CsPbI 2 Br perovskite film gives the best photovoltaic performance. The encapsulated device maintains 90% of the initial performance after 240 h damp and heat test at 85 °C and 85% relative humidity.
Ma, Q, Huang, S, Chen, S, Zhang, M, Lau, CFJ, Lockrey, MN, Mulmudi, HK, Shan, Y, Yao, J, Zheng, J, Deng, X, Catchpole, K, Green, MA & Ho-Baillie, AWY 2017, 'The Effect of Stoichiometry on the Stability of Inorganic Cesium Lead Mixed-Halide Perovskites Solar Cells', JOURNAL OF PHYSICAL CHEMISTRY C, vol. 121, no. 36, pp. 19642-19649.View/Download from: Publisher's site
Duong, T, Mulmudi, HK, Wu, Y, Fu, X, Shen, H, Peng, J, Wu, N, Nguyen, HT, Macdonald, D, Lockrey, M, White, TP, Weber, K & Catchpole, K 2017, 'Light and Electrically Induced Phase Segregation and Its Impact on the Stability of Quadruple Cation High Bandgap Perovskite Solar Cells', ACS Applied Materials and Interfaces, vol. 9, no. 32, pp. 26859-26866.View/Download from: Publisher's site
© 2017 American Chemical Society. Perovskite material with a bandgap of 1.7-1.8 eV is highly desirable for the top cell in a tandem configuration with a lower bandgap bottom cell, such as a silicon cell. This can be achieved by alloying iodide and bromide anions, but light-induced phase-segregation phenomena are often observed in perovskite films of this kind, with implications for solar cell efficiency. Here, we investigate light-induced phase segregation inside quadruple-cation perovskite material in a complete cell structure and find that the magnitude of this phenomenon is dependent on the operating condition of the solar cell. Under short-circuit and even maximum power point conditions, phase segregation is found to be negligible compared to the magnitude of segregation under open-circuit conditions. In accordance with the finding, perovskite cells based on quadruple-cation perovskite with 1.73 eV bandgap retain 94% of the original efficiency after 12 h operation at the maximum power point, while the cell only retains 82% of the original efficiency after 12 h operation at the open-circuit condition. This result highlights the need to have standard methods including light/dark and bias condition for testing the stability of perovskite solar cells. Additionally, phase segregation is observed when the cell was forward biased at 1.2 V in the dark, which indicates that photoexcitation is not required to induce phase segregation.
Yuan, X, Saxena, D, Caroff, P, Wang, F, Lockrey, M, Mokkapati, S, Tan, HH & Jagadish, C 2017, 'Strong Amplified Spontaneous Emission from High Quality GaAs1-xSbxSingle Quantum Well Nanowires', Journal of Physical Chemistry C, vol. 121, no. 15, pp. 8636-8644.View/Download from: UTS OPUS or Publisher's site
© 2017 American Chemical Society. Quantum confinement in semiconductor nanowires is of contemporary interest. Enhancing the quantum efficiency of quantum wells in nanowires and minimizing intrinsic absorption are necessary for reducing the threshold of nanowire lasers and are promising for wavelength tunable emitters and detectors. Here, we report on growth and optimization of GaAs 1-x Sb x /Al 1-y Ga y As quantum well heterostructures formed radially around pure zinc blende GaAs core nanowires. The emitted photon energy from GaAs 0.89 Sb 0.11 quantum well (1.371 eV) is smaller than the GaAs core, thus showing advantages over GaAs/Al 1-y Ga y As quantum well nanowires in photon emission. The high optical quality quantum well (internal quantum efficiency reaches as high as 90%) is carefully positioned so that the quantum well coincides with the maximum of the transverse electric (TE01) mode intensity profile. The obtained superior optical performance combined with the supported Fabry-Perot (F-P) cavity in the nanowire leads to the strong amplified spontaneous emission (ASE). Detailed studies of the amplified cavity mode are carried out by spatial-spectral photoluminescence (PL) imaging, where emission from nanowire is resolved both spatially and spectrally. Resonant emission is generated at nanowire ends and is polarized perpendicular to the nanowire, in agreement with the simulated polarization characteristics of the TE01 mode in the nanowire. The observation of strong ASE for single QW nanowire at room temperature shows the potential application of GaAs 1-x Sb x QW nanowires as low threshold infrared nanowire lasers.
Harikesh, PC, Mulmudi, HK, Ghosh, B, Goh, TW, Teng, YT, Thirumal, K, Lockrey, M, Weber, K, Koh, TM, Li, S, Mhaisalkar, S & Mathews, N 2016, 'Rb as an Alternative Cation for Templating Inorganic Lead-Free Perovskites for Solution Processed Photovoltaics', CHEMISTRY OF MATERIALS, vol. 28, no. 20, pp. 7496-7504.View/Download from: Publisher's site
Duong, T, Mulmudi, HK, Shen, H, Wu, Y, Barugkin, C, Mayon, YO, Nguyen, HT, Macdonald, D, Peng, J, Lockrey, M, Li, W, Cheng, Y-B, White, TP, Weber, K & Catchpole, K 2016, 'Structural engineering using rubidium iodide as a dopant under excess lead iodide conditions for high efficiency and stable perovskites', NANO ENERGY, vol. 30, pp. 330-340.View/Download from: Publisher's site
Ton-That, C, Zhu, L, Lockrey, MN, Phillips, MR, Cowie, BCC, Tadich, A, Thomsen, L, Khachadorian, S, Schlichting, S, Jankowski, N & Hoffmann, A 2015, 'Molecular nitrogen acceptors in ZnO nanowires induced by nitrogen plasma annealing', PHYSICAL REVIEW B, vol. 92, no. 2.View/Download from: UTS OPUS or Publisher's site
Lockrey, MN & Phillips, MR 2012, 'Stability Of InGaN/GaN MQW Luminescence Under Prolonged High Current Injection', Microscopy and Microanalysis, vol. 18, no. S2, pp. 1876-1877.View/Download from: Publisher's site
Sprouster, DJ, Ruffell, S, Bradby, JE, Williams, JS, Lockrey, MN, Phillips, M, Major, RC & Warren, OL 2011, 'Structural characterization of B-doped diamond nanoindentation tips', Journal of Materials Research, vol. 26, no. 24, pp. 3051-3057.View/Download from: UTS OPUS or Publisher's site
We report on the electrical and structural properties of boron-doped diamond tips commonly used for in-situ electromechanical testing during nanoindentation. The boron dopant environment, as evidenced by cathodoluminescence (CL) microscopy, revealed significantly different boron states within each tip. Characteristic emission bands of both electrically activated and nonelectrically activated boron centers were identified in all boron-doped tips. Surface CL mapping also revealed vastly different surface properties, confirming a high amount of nonelectrically activated boron clusters at the tip surface. Raman microspectroscopy analysis showed that structural characteristics at the atomic scale for boron-doped tips also differ significantly when compared to an undoped diamond tip. Furthermore, the active boron concentration, as inferred via the Raman analysis, varied greatly from tip-to-tip. It was found that tips (or tip areas) with low overall boron concentration have a higher number of electrically inactive boron, and thus non-Ohmic contacts were made when these tips contacted metallic substrates. Conversely, tips that have higher boron concentrations and a higher number of electrically active boron centers display Ohmic-like contacts. Our results demonstrate the necessity to understand and fully characterize the boron environments, boron concentrations, and atomic structure of the tips prior to performing in situ electromechanical experiments, particularly if quantitative electrical data are required.
Lockrey, MN & Phillips, M 2011, 'Characterisation Of The Optical Properties Of InGaN MQW Structures Using A Combined Sem And Cl Spectral Mapping System', Journal of Semiconductors, vol. 32, no. 1, pp. 0-0.View/Download from: UTS OPUS or Publisher's site
We demonstrate the ability of a combined scanning electron microscope and cathodoluminescence (CL) spectral mapping system to provide important spatially resolved information. The degree of inhomogeneity in spectral output across a multi-quantum well sample is measured using the SEM-CL system as well as measuring the efficiency roll-off with increasing carrier concentration. The effects of low energy electron beam modification on the InGaN/GaN multi quantum wells have also been characterized.
Phillips, M, Manning, TJ, Nenstiel, C, Lockrey, MN, Ton-That, C & Hoffmann, AV 2011, 'High Temperature In-Situ Cathodoluminescence Studies of the Thermal Stability of Hydrogen in p-type Magnesium Doped Gallium Nitride', Microscopy and Microanalysis, Vol 17, Supplement 2, High Temperature In-Situ Cathodoluminescence Studies of the Thermal Stability of Hydrogen in p-type, Nashville, pp. 1-2.