Reimers, JR, Sajid, A, Kobayashi, R & Ford, MJ 2018, 'Understanding and Calibrating Density-Functional-Theory Calculations Describing the Energy and Spectroscopy of Defect Sites in Hexagonal Boron Nitride.', Journal of chemical theory and computation, vol. 14, no. 3, pp. 1602-1613.View/Download from: UTS OPUS or Publisher's site
Defect states in 2-D materials present many possible uses but both experimental and computational characterization of their spectroscopic properties is difficult. We provide and compare results from 13 DFT and ab initio computational methods for up to 25 excited states of a paradigm system, the VNCB defect in hexagonal boron nitride (h-BN). Studied include: (i) potentially catastrophic effects for computational methods arising from the multireference nature of the closed-shell and open-shell states of the defect, which intrinsically involves broken chemical bonds, (ii) differing results from DFT and time-dependent DFT (TDDFT) calculations, (iii) comparison of cluster models to periodic-slab models of the defect, (iv) the starkly differing effects of nuclear relaxation on the various electronic states that control the widths of photoabsorption and photoemission spectra as broken bonds try to heal, (v) the effect of zero-point energy and entropy on free-energy differences, (vi) defect-localized and conduction/valence-band transition natures, and (vii) strategies needed to ensure that the lowest-energy state of a defect can be computationally identified. Averaged state-energy differences of 0.3 eV are found between CCSD(T) and MRCI energies, with thermal effects on free energies sometimes also being of this order. However, DFT-based methods can perform very poorly. Simple generalized-gradient functionals like PBE fail at the most basic level and should never be applied to defect states. Hybrid functionals like HSE06 work very well for excitations within the triplet manifold of the defect, with an accuracy equivalent to or perhaps exceeding the accuracy of the ab initio methods used. However, HSE06 underestimates triplet-state energies by on average of 0.7 eV compared to closed-shell singlet states, while open-shell singlet states are predicted to be too low in energy by 1.0 eV. This leads to misassignment of the ground state of the VNCB defect. Long-range corrected ...
Sajid, A, Reimers, JR & Ford, MJ 2018, 'Defect states in hexagonal boron nitride: Assignments of observed properties and prediction of properties relevant to quantum computation', Physical Review B, vol. 97, no. 6.View/Download from: UTS OPUS or Publisher's site
© 2018 American Physical Society. Key properties of nine possible defect sites in hexagonal boron nitride (h-BN), VN,VN-1,CN,VNO2B,VNNB,VNCB,VBCN,VBCNSiN, and VNCBSiB, are predicted using density-functional theory and are corrected by applying results from high-level ab initio calculations. Observed h-BN electron-paramagnetic resonance signals at 22.4, 20.83, and 352.70 MHz are assigned to VN,CN, and VNO2B, respectively, while the observed photoemission at 1.95 eV is assigned to VNCB. Detailed consideration of the available excited states, allowed spin-orbit couplings, zero-field splitting, and optical transitions is made for the two related defects VNCB and VBCN. VNCB is proposed for realizing long-lived quantum memory in h-BN. VBCN is predicted to have a triplet ground state, implying that spin initialization by optical means is feasible and suitable optical excitations are identified, making this defect of interest for possible quantum-qubit operations.
Xu, Z-Q, Elbadawi, C, Tran, TT, Kianinia, M, Li, X, Liu, D, Hoffman, TB, Nguyen, M, Kim, S, Edgar, JH, Wu, X, Song, L, Ali, S, Ford, M, Toth, M & Aharonovich, I 2018, 'Single photon emission from plasma treated 2D hexagonal boron nitride.', Nanoscale, vol. 10, no. 17, pp. 7957-7965.View/Download from: UTS OPUS or Publisher's site
Artificial atomic systems in solids are becoming increasingly important building blocks in quantum information processing and scalable quantum nanophotonic networks. Amongst numerous candidates, 2D hexagonal boron nitride has recently emerged as a promising platform hosting single photon emitters. Here, we report a number of robust plasma and thermal annealing methods for fabrication of emitters in tape-exfoliated hexagonal boron nitride (hBN) crystals. A two-step process comprising Ar plasma etching and subsequent annealing in Ar is highly robust, and yields an eight-fold increase in the concentration of emitters in hBN. The initial plasma-etching step generates emitters that suffer from blinking and bleaching, whereas the two-step process yields emitters that are photostable at room temperature with emission wavelengths greater than ∼700 nm. Density functional theory modeling suggests that the emitters might be associated with defect complexes that contain oxygen. This is further confirmed by generating the emitters via annealing hBN in air. Our findings advance the present understanding of the structure of quantum emitters in hBN and enhance the nanofabrication toolkit needed to realize integrated quantum nanophotonic circuits.
Munir, T, Munir, HS, Kashif, M, Fakhar-E-Alam, M, Shahzad, A, Amin, N, Sajid, A & Umair, M 2017, 'Synthesis and characterization of Copper Oxide nanoparticles by solution evaporation method', JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, vol. 19, no. 5-6, pp. 417-423.View/Download from: UTS OPUS
Zhu, L, Lem, LLC, Nguyen, TP, Fair, K, Ali, S, Ford, MJ, Phillips, MR & Ton-That, C 2017, 'Indirect excitons in hydrogen-doped ZnO', Journal of Physics D: Applied Physics, vol. 50, no. 11.View/Download from: UTS OPUS or Publisher's site
© 2017 IOP Publishing Ltd. We present a correlative experimental and theoretical study of bound excitons in hydrogen-doped ZnO, with a particular focus on the dynamics of their metastable state confined in the sub-surface region, using a combination of surface-sensitive characterisation techniques and density functional theory calculations. A metastable sub-surface emission at 3.31 eV found in H-doped ZnO is attributed to the radiative recombination of indirect excitons localised at basal plane stacking faults (BSFs) where the excitonic transition involves electrons bound to bond-centre hydrogen donors in the potential well of the BSF. Additionally, our work shows the electrical transport of ZnO Schottky junctions is dominated by electrons confined at BSFs in the near-surface region.
Grosso, G, Moon, H, Lienhard, B, Ali, S, Efetov, DK, Furchi, MM, Jarillo-Herrero, P, Ford, MJ, Aharonovich, I & Englund, D 2017, 'Tunable and high-purity room temperature single-photon emission from atomic defects in hexagonal boron nitride', NATURE COMMUNICATIONS, vol. 8.View/Download from: UTS OPUS or Publisher's site
Tawfik, SA, Ali, S, Fronzi, M, Kianinia, M, Tran, TT, Stampfl, C, Aharonovich, I, Toth, M & Ford, MJ 2017, 'First-principles investigation of quantum emission from hBN defects.', Nanoscale, vol. 9, no. 36, pp. 13575-13582.View/Download from: UTS OPUS or Publisher's site
Hexagonal boron nitride (hBN) has recently emerged as a fascinating platform for room-temperature quantum photonics due to the discovery of robust visible light single-photon emitters. In order to utilize these emitters, it is necessary to have a clear understanding of their atomic structure and the associated excitation processes that give rise to this single photon emission. Here, we performed density-functional theory (DFT) and constrained DFT calculations for a range of hBN point defects in order to identify potential emission candidates. By applying a number of criteria on the electronic structure of the ground state and the atomic structure of the excited states of the considered defects, and then calculating the Huang-Rhys (HR) factor, we found that the CBVN defect, in which a carbon atom substitutes a boron atom and the opposite nitrogen atom is removed, is a potential emission source with a HR factor of 1.66, in good agreement with the experimental HR factor. We calculated the photoluminescence (PL) line shape for this defect and found that it reproduces a number of key features in the experimental PL lineshape.
Murtaza, G, Sajid, A, Rizwan, M, Takagiwa, Y, Khachai, H, Jibran, M, Khenata, R & Bin Omran, S 2015, 'First principles study of Mg2X (X=Si, Ge, Sn, Pb): Elastic, optoelectronic and thermoelectric properties', MATERIALS SCIENCE IN SEMICONDUCTOR PROCESSING, vol. 40, pp. 429-435.View/Download from: Publisher's site
Sajid, A, Sibghat-Ullah, Murtaza, G, Kbenata, R, Manzar, A & Bin Omran, S 2014, 'Electronic structure and optical properties of chalcopyrite CuYZ(2) (Y=Al, Ga, In; Z=S, Se): an ab initio study', JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, vol. 16, no. 1-2, pp. 76-81.
Sajid, A, Murtaza, G & Reshak, AH 2013, 'SHIFT OF BAND GAP FROM DIRECT TO INDIRECT AND OPTICAL RESPONSE OF LiF UNDER PRESSURE', MODERN PHYSICS LETTERS B, vol. 27, no. 9.View/Download from: Publisher's site
Alay-e-Abbas, SM & Sajid, A 2013, 'FP-LAPW+lo Study of Structural, Electronic, and Optical Properties of Mg1-xSrxTe Alloys', CHINESE JOURNAL OF PHYSICS, vol. 51, no. 4, pp. 790-801.View/Download from: Publisher's site
Sajid, A, Afaq, A & Murtaza, G 2013, 'First Principles Study of Electronic and Optical Properties of Magnesium based Chalcogenides', CHINESE JOURNAL OF PHYSICS, vol. 51, no. 2, pp. 316-326.View/Download from: Publisher's site
Sajid, A, Alay-E-Abbas, SM, Afaq, A & Shaukat, A 2012, 'STRUCTURAL, ELECTRONIC AND OPTICAL PROPERTIES OF MgSxSe1-x, MgSxTe1-x AND MgSexTe1-x (0 <= x <= 1) ALLOYS FROM FIRST PRINCIPLES', INTERNATIONAL JOURNAL OF MODERN PHYSICS B, vol. 26, no. 17.View/Download from: Publisher's site
Grosso, G, Moon, H, Lienhard, B, Ali, S, Furchi, MM, Walsh, M, Efetov, DK, Jarillo-Herrero, P, Ford, MJ, Aharonovich, I & Englund, D 2017, 'Tunable quantum emission from atomic defects in hexagonal boron nitride', 2017 Conference on Lasers and Electro-Optics, CLEO 2017 - Proceedings, Conference on Lasers and Electro-Optics, IEEE, San Jose, CA, USA, pp. 1-2.View/Download from: UTS OPUS or Publisher's site
© 2016 Optical Society of America. We demonstrate that strain control of exfoliated hexagonal boron nitride allows spectral tuning of single photon emitters over 6 meV. We propose a material processing that sharply improves the single-photon purity with g(2)(0) = 0.077, and brightness with emission rate exceeding 107counts/sec at saturation.