Materials Science and Engineering

Biography

Biography: Sajid Ali

Abstract

Hexagonal boron nitride (h-BN) is a wide band gap (~6 eV) 2-dimensional (2-D) material with the potential to host many such colour centres [1-8] that are promising candidates for quantum applications. Defects can dramatically alter the electronic and magnetic properties of the host Semiconductor. While both nitrogen (N_v) and boron (B_v) vacancies can act as paramagnetic centres in h-BN [9], electron paramagnetic resonance (EPR) studies indicate that N_v are more important [10-15].  Two types of paramagnetic centres have been identified: (i) three-boron centres (TBC) in which an unpaired electron interacts with three equivalent boron (B¹¹) nuclei, producing 10-line EPR spectra, and (ii) one-boron centres (OBC) in which an unpaired electron interact with only a single B¹¹, producing 4-line EPR spectra.

A-priori calculations using density-functional theory (DFT) can provide useful tools for the interpretation of EPR by comparing the experimental and calculated hyperfine constants [16-20]. We consider detailed models of the TBC and OBC defects in h-BN, as well as proposing many new defect centres, particularly defects involving introduced carbon impurity atoms one’s which are thought of being responsible for single photon emission in hBN. We do Group-theoretical analysis to suggest new directions for experimental studies. Key properties of 9 possible defect sites in hexagonal boronitride (h-BN), V_N, V_N^-1, C_N, V_NO_2B, V_NN_B, V_NC_B, V_BC_N, V_BC_NSi_N, and V_NC_BSi_B, are predicted using density-functional theory (DFT) that are corrected by applying results from high-level ab initio calculations. Detailed consideration of the available excited states, allowed spin-orbit couplings, zero-field splitting, and optical transitions is made for the two related defects V_NC_B(Fig.1.) and V_BC_N. We propose that ground-state spin polarization and long-lived quantum memory in h-BN can be achieved for V_BC_N and V_NC_B respectively.

FIG.1. (a) Key DFT orbitals from the (1)¹A₁ closed-shell ground-state electronic structure of V_NC_B. The states are labelled according to the symmetry of Irreducible representation as per C_2v point group. x(y,z)-axis are perpendicular(in the) to the plane of defect.  (b) HSE06 adiabatic energies of low lying states of V_NC_B  as calculated by DFT, with, in (), these energies corrected according to ab initio CCSD(T), EOMCCSD, and CASPT2 calculations for a model compound [23].  Allowed transition polarizations d, spin-orbit couplings l driving non-radiative transitions and zero-field splittings are also indicated.

Recent Publications

1. A. Sajid, J. R. Reimers, and M. J. Ford, Physical Review B 97, 064101 (2018).

2. J. Reimers, A. Sajid, R. Kobayashi, and M. Ford, Journal of chemical theory and computation  (2018).

3. (A.Sajid, S. A. Tawfik)^{Equal Authors}, M. Fronzi, M. Kianinia, T. T. Tran, C. Stampfl, I. Aharonovich, M. Toth, and M. J. Ford, Nanoscale  (2017).

4. G. Grosso, H. Moon, B. Lienhard, A. Sajid, D. K. Efetov, M. M. Furchi, P. Jarillo-Herrero, M. J. Ford, I. Aharonovich, and D. Englund, Nature Communications 8, 705 (2017).

5. i. Zhu, L. L. C Lem, T. Nguyen, K. Fair, A. Sajid, M. J Ford¹, M. R. Phillips¹ and C. Ton-That