Excited State Properties of Point Defects in Semiconductors and Insulators Investigated with Time-Dependent Density Functional Theory

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We present an efficient particle−particle random phase approximation (ppRPA) approach that predicts accurate excitation energies of point defects, including the nitrogen-vacancy (NV − ) and silicon-vacancy (SiV 0 ) centers in diamond and the divacancy center (VV 0 ) in 4H silicon carbide, with errors of ±0.2 eV compared with experimental values. Starting from the (N + 2)electron ground state calculated with density functional theory (DFT), the ppRPA excitation energies of the N-electron system are calculated as the differences between the two-electron removal energies of the (N + 2)-electron system. We demonstrate that the ppRPA excitation energies converge rapidly with a few hundred canonical active-space orbitals. We also show that active-space ppRPA has weak DFT starting-point dependence and is significantly cheaper than the corresponding ground-state DFT calculation. This work establishes ppRPA as an accurate and low-cost tool for investigating excited-state properties of point defects and opens up new opportunities for applications of ppRPA to periodic bulk materials.

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confidence: 70%

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confidence: 99%

Accurate Excitation Energies of Point Defects from Fast Particle–Particle Random Phase Approximation Calculations

Li,

Jin,

et al. 2024

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“…Previous studies have shown that the wave function of the 3 E state can be represented by a single Slater determinant obtained with a neutral excitation of an electron from the to e̅ level . Neglecting electron–phonon interactions, researchers have previously computed vertical excitation energies at the 3 A 2 ground-state geometry by constrained occupation DFT (CDFT), ,, TDDFT, ,, and quantum embedding theories − or QMC . As expected, many-body theories predict vertical excitations different from those of single-particle energy (Kohn–Sham eigenvalues) differences.…”

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confidence: 88%

“…We performed TDDFT/DDH calculations with a C 214 N supercell for the 3 E state for the same 200 configurations used to compute the single-particle renormalizations. We used the TDDFT implementation of the WEST code , after obtaining the ground-state wave functions with the Quantum Espresso package. , Due to the DJT effect, the 3 E state splits into 3 E low and 3 E high branches. These two branches can be considered to originate from the following single-particle electronic excitations: i → f with i = a̅ 1 , f = e̅ normall normalo normalw , and e̅ normalh normali normalg normalh , respectively.…”

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confidence: 99%

“…Instead, the VEEs for the low-lying singlet states ( 1 E , 1 A 1 ), which are multiconfigurational, may not be obtained from single-particle renormalization differences, as we discuss below. We used the DDH functional and the spin-flip TDDFT implementation of the WEST package, , which was shown to be adequate for reproducing the VEEs of the low-lying singlet states of the NV – center . We carried out spin-flip TDDFT calculations for the 200 and 400 configurations obtained from TL sampling at 300 K in the C 214 N and C 62 N supercells, respectively.…”

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confidence: 99%

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Quantum Vibronic Effects on the Excitation Energies of the Nitrogen-Vacancy Center in Diamond

Kundu,

Galli

2024

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We investigated the impact of quantum vibronic coupling on the electronic properties of solid-state spin defects using stochastic methods and first-principles molecular dynamics with a quantum thermostat. Focusing on the negatively charged nitrogen-vacancy center in diamond as an exemplary case, we found a significant dynamic Jahn–Teller splitting of the doubly degenerate single-particle levels within the diamond’s band gap, even at 0 K, with a magnitude exceeding 180 meV. This pronounced splitting leads to substantial renormalizations of these levels and, subsequently, of the vertical excitation energies of the doubly degenerate singlet and triplet excited states. Our findings underscore the pressing need to incorporate quantum vibronic effects into first-principles calculations, particularly when comparing computed vertical excitation energies with experimental data. Our study also reveals the efficiency of stochastic thermal line sampling for studying phonon renormalizations of solid-state spin defects.

Self-Trapped Excitons in Metal-Halide Perovskites Investigated by Time-Dependent Density Functional Theory

Jin,

Rusishvili,

Govoni

et al. 2024

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We present a theoretical study of the formation of self-trapped excitons (STEs) and the associated broadband emission in metal−halide perovskites Cs 4 SnBr 6 and Cs 2 AgInCl 6 , using time-dependent density functional theory (TDDFT) with the dielectric-dependent hybrid (DDH) functional. Our approach allows for an accurate description of the excitonic effect and geometry relaxation in the electronic excited states and yields optical gap, STE emission energy, and emission spectra in reasonable agreement with experiments. We point out the significance of considering geometry relaxations in the electronic excited state by showing that the exciton−phonon coupling computed in the ground-state atomic geometry is insufficient to describe the physical properties of STEs. Overall, we find that TDDFT with the DDH hybrid functional is a suitable approach for the study of the formation of STEs in perovskite and provides insights for designing metal−halide perovskites with tailored emission properties.