Ensemble Negatively-Charged Nitrogen-Vacancy Centers in Type-Ib Diamond Created by High Fluence Electron Beam Irradiation

At a Glance

Metadata	Details
Publication Date	2021-12-30
Journal	Quantum Beam Science
Authors	Shuya Ishii, Seiichi Saiki, Shinobu Onoda, Y. MASUYAMA, Hiroshi Abe
Institutions	Takasaki Advanced Radiation Research Institute, National Institutes for Quantum Science and Technology
Citations	17
Analysis	Full AI Review Included

Executive Summary

This research investigates the high-fluence electron beam irradiation method for creating high concentrations of negatively-charged nitrogen-vacancy (NV^-) centers in Type-Ib diamond, optimizing material preparation for quantum sensing applications.

Core Value Proposition: Confirmed that 2 MeV electron irradiation followed by 1000 °C annealing is an effective procedure for creating high-concentration ensemble NV^- centers, essential for highly sensitive quantum sensors.
Achieved Concentration: NV^- concentrations reached approximately 10 ppm in the highest initial P1 concentration sample (Ib-80) after 8.0 x 10¹⁸ e/cm² fluence.
Conversion Efficiency: High conversion efficiencies from initial substitutional nitrogen (P1 centers) to NV^- centers were achieved, ranging from 12% to 19% (highest in the Ib-52 sample).
Spin Coherence Time (T₂): Measured T₂ values ranged from 1.3 µs to 2.7 µs. The T₂ values were confirmed to be primarily limited by the total nitrogen concentration, consistent with established models.
Mechanism Insight: Analysis showed that not all consumed P1 centers convert into NV^- centers, suggesting that residual defects containing nitrogen or trapped charges are created due to high irradiation damage accumulation.
Charge State Control: The process successfully maintained the majority of NV centers in the desired NV^- charge state, with minimal NV⁰ formation observed even at high fluence.

Technical Specifications

Parameter	Value	Unit	Context
Electron Beam Energy	2	MeV	Irradiation source
Maximum Irradiation Fluence	8.0 x 10¹⁸	e/cm²	Maximum dose applied
Fluence Rate	1.0 x 10¹⁷	e/cm²/h	Irradiation speed
Annealing Temperature	1000	°C	Post-irradiation thermal treatment
Annealing Time	2	h	Duration of thermal treatment
Annealing Vacuum Pressure	~1.0 x 10^-4	Pa	Vacuum level during annealing
Initial P1 Concentration ([P1]_initial) Range	46 to 80	ppm	Type-Ib diamond starting material
Maximum NV^- Concentration ([NV^-])	~10	ppm	Achieved in Ib-80 sample
Conversion Efficiency (P1 to NV^-)	12 to 19	%	Ratio of [NV^-] to [P1]_initial
Maximum Spin Coherence Time (T₂)	2.7 ± 0.96	µs	Measured for Ib-52 sample
Minimum Spin Coherence Time (T₂)	1.3 ± 0.48	µs	Measured for Ib-80 sample
NV^- Zero-Phonon Line (ZPL)	638	nm	PL measurement wavelength
NV⁰ Zero-Phonon Line (ZPL)	575	nm	PL measurement wavelength
Excitation Laser Wavelength	532	nm	PL and ODMR excitation source

Key Methodologies

The creation and characterization of ensemble NV^- centers followed a precise sequence of material processing and advanced spectroscopy:

Material Preparation:
- Used commercially available High-Pressure High-Temperature (HPHT) synthesized Type-Ib diamonds.
- Initial P1 concentrations ([P1]_initial) were measured by ESR and categorized (Ib-80, Ib-72, Ib-52, Ib-46).
- Samples were cleaned using a mixture of sulfuric acid and nitric acid at approximately 200 °C to remove surface contamination.
Electron Beam Irradiation:
- Irradiation was performed using a 2 MeV electron beam in atmosphere.
- Fluence was varied up to 8.0 x 10¹⁸ electrons/cm².
- Samples were kept on a water-cooled copper plate to mitigate heating effects during the high-fluence process.
Thermal Annealing:
- After irradiation, samples were annealed in a furnace at 1000 °C for 2 hours.
- Annealing was conducted under high vacuum (~1.0 x 10^-4 Pa) to facilitate vacancy diffusion and trapping by nitrogen atoms, forming NV centers.
Defect Quantification (ESR):
- Electron Spin Resonance (ESR) spectroscopy (X-band, RT) was used to monitor the consumption of P1 centers (unpaired electron, S = 1/2) and the creation of NV^- centers (triplet ground state).
- Concentrations were determined by comparing double-integrated ESR intensity to a calibrated reference sample.
Charge State Analysis (PL):
- Photoluminescence (PL) spectra (532 nm excitation) were acquired to analyze the charge state ratio.
- The ratio of NV^- (638 nm ZPL) to NV⁰ (575 nm ZPL) was determined by fitting the superposition of typical spectra, confirming successful charge conversion to NV^-.
Coherence Time Measurement (ODMR/Hahn-Echo):
- Optically Detected Magnetic Resonance (ODMR) was used to identify the resonance frequency of the NV^- ensemble.
- Rabi oscillations determined the state flip (π pulse) time.
- Hahn-echo pulse sequences were performed to measure the spin coherence time (T₂), a critical parameter for sensor sensitivity.

Commercial Applications

The creation of high-concentration, high-quality ensemble NV^- centers in bulk diamond is foundational for next-generation quantum technologies, particularly in sensing and metrology.

Quantum Sensing and Metrology:
- High-Sensitivity Magnetometry: Ensemble NV^- centers are used to detect weak magnetic fields with high spatial resolution, crucial for applications like non-destructive testing and geological surveys.
- AC Magnetometry: Longer T₂ times (up to 2.7 µs) directly improve the sensitivity of AC magnetic field detection.
Biological and Medical Imaging:
- Nanoscale Thermometry: NV^- centers function as robust, room-temperature thermometers for measuring temperature gradients inside living cells and biological systems.
- pH Nanosensing: Used for monitoring local chemical environments within biological samples.
- Tracking Dynamics: Used to track 3D rotational dynamics in nanoscopic biological systems.
Quantum Information Processing (QIP):
- NV^- centers serve as solid-state qubits, though ensemble creation is typically optimized for sensing rather than single-qubit operations.
Material Science Research:
- The ability to precisely control defect concentration and charge state allows for the fabrication of standardized diamond materials for fundamental studies of spin physics and decoherence mechanisms.

View Original Abstract

Electron beam irradiation into type-Ib diamond is known as a good method for the creation of high concentration negatively-charged nitrogen-vacancy (NV−) centers by which highly sensitive quantum sensors can be fabricated. In order to understand the creation mechanism of NV− centers, we study the behavior of substitutional isolated nitrogen (P1 centers) and NV− centers in type-Ib diamond, with an initial P1 concentration of 40-80 ppm by electron beam irradiation up to 8.0 × 1018 electrons/cm2. P1 concentration and NV− concentration were measured using electron spin resonance and photoluminescence measurements. P1 center count decreases with increasing irradiation fluence up to 8.0 × 1018 electrons/cm2. The rate of decrease in P1 is slightly lower at irradiation fluence above 4.0 × 1018 electrons/cm2 especially for samples of low initial P1 concentration. Comparing concentration of P1 centers with that of NV− centers, it suggests that a part of P1 centers plays a role in the formation of other defects. The usefulness of electron beam irradiation to type-Ib diamonds was confirmed by the resultant conversion efficiency from P1 to NV− center around 12-19%.

Tech Support

Original Source

DOI: https://doi.org/10.3390/qubs6010002

References

2011 - Dynamic Jahn-Teller effect in the NV− center in diamond [Crossref]
2019 - On the models for the investigation of charged defects in solids: The case of the VN- defect in diamond [Crossref]
2020 - Sensitivity optimization for NV− diamond magnetometry [Crossref]
2012 - High spatial and temporal resolution wide-field imaging of neuron activity using quantum NV− diamond [Crossref]
2013 - Nanometre-scale thermometry in a living cell [Crossref]
2019 - pH nanosensor using electronic spins in diamond [Crossref]
2020 - Tracking the 3D rotational dynamics in nanoscopic biological systems [Crossref]
2000 - Radiation damage of diamond by electron and gamma irradiation [Crossref]
2009 - Diamonds with a high density of nitrogen-vacancy centers for magnetometry applications [Crossref]