Giant shift upon strain on the fluorescence spectrum of VNNB color centers in h-BN
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-09-25 |
| Journal | npj Quantum Information |
| Authors | Song Li, Jyh-Pin Chou, Alice Hu, Martin B. Plenio, PĂ©ter Udvarhelyi |
| Institutions | UniversitÀt Ulm, Budapest University of Technology and Economics |
| Citations | 37 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis analysis focuses on the giant shift in the Zero-Phonon Line (ZPL) emission of the neutral Nitrogen Antisite-Vacancy pair (VNNB) color center in hexagonal Boron Nitride (h-BN) when subjected to mechanical strain.
- Giant Strain Sensitivity: The VNNB defect exhibits an exceptionally large ZPL-strain coupling parameter calculated at 12 eV/strain.
- Wavelength Variation: This high sensitivity results in a massive ZPL wavelength scatter of approximately 100 nm for typical h-BN lattice strains (±1%).
- Microscopic Mechanism: The giant shift is fundamentally driven by strong electron-phonon coupling inherent to the h-BN system, which is 2.5 times larger than that observed in diamond NV centers.
- Optical Activation: The strong coupling induces a static Pseudo Jahn-Teller (PJT) effect, causing the ground state to distort from C2v to Cs symmetry. This distortion activates an otherwise forbidden optical transition, enabling photoluminescence.
- Experimental Explanation: The results provide a plausible, quantitative explanation for the experimental observation of h-BN quantum emitters that share similar optical properties but possess widely scattered ZPL wavelengths.
- Engineering Implication: The extreme sensitivity to strain positions VNNB as a prime candidate for use in nanoscale stress detectors and for active spectral tuning in quantum circuitry.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| ZPL-Strain Coupling Parameter | 12 | eV/strain | Axial uniaxial strain (parallel or perpendicular) |
| Calculated Unstrained ZPL Energy | 1.90 | eV | B2(Cs) â A1(C2v) optical transition |
| Calculated ZPL Wavelength | ~653 | nm | Corresponds to 1.90 eV (unstrained) |
| ZPL Wavelength Shift Range | ~100 | nm | Resulting from ±1% strain in h-BN lattice |
| Jahn-Teller Energy (EJT) | 95 | meV | Energy difference between C2v and Cs configurations |
| Electron-Phonon Coupling Strength (F) | 178 | meV | Dimensionless generalized coordinate system |
| Effective Phonon Mode Energy (ħÏ) | 23 | meV | Membrane phonon mode energy |
| Ground State Symmetry | Cs | N/A | Distorted due to static Pseudo Jahn-Teller effect |
| Excited State Symmetry | C2v | N/A | Remains stable under C2v configuration |
| DFT Hybrid Functional Mixing Parameter | 0.32 | N/A | Used in HSE functional to match experimental band gap |
| Bulk Interlayer Distance | 3.37 | A | Optimized using DFT-D3 method |
Key Methodologies
Section titled âKey MethodologiesâThe study combined group theory analysis with advanced ab initio Density Functional Theory (DFT) calculations to model the VNNB defect properties under strain.
- DFT Implementation: Calculations were performed using the Vienna ab initio Simulation Package (VASP) with Projector Augmented Wave (PAW) pseudopotentials and a plane-wave basis set cutoff of 450 eV.
- Electronic Structure: The Heyd, Scuseria, and Ernzerhof (HSE) screened hybrid density functional was employed, utilizing a mixing parameter of 0.32 to accurately reproduce the experimental h-BN band gap (5.9 eV).
- Defect Modeling: The VNNB defect was modeled in a large 9x5â3 supercell containing 160 atoms to ensure negligible defect-defect interaction.
- Strain Application: Uniaxial strain (up to ±2%) was simulated by changing the lattice constant of the supercell, transforming the basis to an orthorhombic structure to model parallel (||) and perpendicular (â„) strain directions relative to the C2 axis.
- Geometry Optimization: Atomic coordinates were relaxed until the maximum force on any atom was less than 0.01 eV/A.
- Excited State Calculation: The ZPL energy was calculated as the total energy difference between the ground state and the excited state using the ASCF method.
- Electron-Phonon Analysis: The Adiabatic Potential Energy Surface (APES) was calculated to quantify the Pseudo Jahn-Teller (PJT) parameters, including the coupling strength (F = 178 meV) and the effective phonon mode energy (Ä§Ï = 23 meV).
Commercial Applications
Section titled âCommercial ApplicationsâThe extreme strain sensitivity and 2D nature of the VNNB color center in h-BN offer significant advantages for emerging quantum and mechanical technologies.
- Quantum Sensing:
- Nanoscale Stress Detection: The 12 eV/strain coupling parameter enables the use of VNNB SPEs as highly sensitive, localized strain or stress detectors at the nanoscale.
- Spin-Electro-Mechanical Systems: The strong electron-strain coupling provides a mechanism for controlling mechanical resonators via spin-motion coupling, foundational for advanced quantum control systems.
- Quantum Information Processing (QIP):
- Tunable Quantum Emitters: Strain can be utilized as an external knob to precisely tune the ZPL wavelength over a wide range (~100 nm), allowing for spectral matching or multiplexing of quantum emitters on a single chip.
- Integrated Photonics: As a 2D material emitter, h-BN facilitates easier integration with nanophotonic waveguides and circuitry, offering higher light collection efficiency compared to bulk 3D materials (e.g., diamond).
- Advanced Materials Engineering:
- Defect Engineering: Understanding the role of local strain in scattering ZPL energies allows engineers to better control and stabilize the optical properties of h-BN quantum emitters during synthesis (e.g., CVD or exfoliation).
- 2D Material Devices: The findings support the development of active quantum components embedded directly within flexible or layered 2D electronic devices.
View Original Abstract
Abstract We study the effect of strain on the physical properties of the nitrogen antisite-vacancy pair in hexagonal boron nitride ( h -BN), a color center that may be employed as a quantum bit in a two-dimensional material. With group theory and ab initio analysis we show that strong electron-phonon coupling plays a key role in the optical activation of this color center. We find a giant shift on the zero-phonon-line (ZPL) emission of the nitrogen antisite-vacancy pair defect upon applying strain that is typical of h -BN samples. Our results provide a plausible explanation for the experimental observation of quantum emitters with similar optical properties but widely scattered ZPL wavelengths and the experimentally observed dependence of the ZPL on the strain.