Ultrahigh nitrogen-vacancy center concentration in diamond

At a Glance

Metadata	Details
Publication Date	2021-12-07
Journal	Carbon
Authors	Sándor Kollarics, Ferenc Simon, András Bojtor, Kristóf Koltai, G. Klujber
Institutions	University of Southern California, Institute for Solid State Physics and Optics
Citations	19
Analysis	Full AI Review Included

Technical Documentation & Analysis: Ultrahigh Nitrogen-Vacancy Center Concentration in Diamond

Executive Summary

This analysis focuses on the successful synthesis and characterization of ultrahigh concentrations of negatively charged Nitrogen-Vacancy (NV^-) centers in single crystal diamond, a critical step for developing high-sensitivity quantum sensors and solid-state qubits.

Ultrahigh Concentration Achieved: NV^- concentrations up to 15 ppm were successfully created in HPHT diamond, enabling macroscopic experimental methods like ensemble EPR/ENDOR.
High Conversion Efficiency: The process demonstrated a maximum conversion efficiency of 17.5% from isolated substitutional nitrogen (P1 centers) to NV^- centers.
Stepwise Defect Engineering: NV^- creation utilized a systematic, stepwise process involving high-energy electron and neutron irradiation followed by high-temperature thermal annealing.
Lattice Integrity Confirmed: EPR analysis confirmed that thermal annealing effectively restores the integrity of the diamond lattice after irradiation, transforming charged vacancies into stable NV^- centers with a yield of approximately 25%.
Superior Characterization Method: Continuous-wave (CW) and pulsed Electron Paramagnetic Resonance (EPR) proved to be the ideal quantitative method for characterizing high-density NV^- ensembles where traditional optical methods (PL/ODMR) are limited by high extinction coefficients and lattice distortions.
Quantum Coherence Data: Precise measurements of anisotropic spin-lattice (T₁) and spin-spin (T₂) relaxation times were achieved, providing essential data for optimizing NV-based quantum devices.

Technical Specifications

Parameter	Value	Unit	Context
Starting Material Type	Type 1b HPHT Diamond	N/A	Single crystal, supplied by Element Six Ltd.
Initial Nitrogen Concentration (P1)	< 200	ppm	Substitutional nitrogen precursor
Highest NV^- Concentration Attained	15	ppm	Achieved in sample with 70 ppm initial N
N to NV^- Conversion Efficiency (Max)	17.5	%	Determined via CW EPR analysis
Vacancy to NV^- Conversion Efficiency	~25	%	Observed during thermal annealing step
Neutron Fluence (Total)	10¹⁷	1/cm²	Low dose region
Electron Fluence (Max Net)	2.8 · 10¹⁸	1/cm²	Used custom variable-energy RF LINAC
Annealing Temperature (Step 1)	800	°C	2 hours, dynamic vacuum (10^-6 mbar)
Annealing Temperature (Step 2)	1000	°C	2 hours, dynamic vacuum (10^-6 mbar)
EPR Microwave Frequency (X-band)	~9.5	GHz	Used for pulsed ENDOR measurements
Spin-Lattice Relaxation Time (T₁)	2.49 / 4.73	ms	Angular dependent measurement
Spin-Spin Relaxation Time (T₂)	1.21 / 1.88	µs	Angular dependent measurement
¹⁴N Larmor Frequency (High Field)	1.37	MHz	At B₀^high = 446.65 mT
¹³C Nuclear Magnetic Resonance	2.605 ± 0.015	MHz	Observed in low magnetic field spectrum
W16 Center Zero-Field Splitting Ratio	0.86	D(NV)	Tentatively assigned to N(+)NV(-) complexes

Key Methodologies

The research employed a systematic, multi-step approach combining high-energy irradiation and thermal processing, followed by advanced magnetic resonance characterization.

Material Selection: Single crystal, type 1b HPHT diamond (< 200 ppm N) was chosen as the starting material to provide the necessary substitutional nitrogen (P1 centers) precursor.
Irradiation (Vacancy Creation):
- Neutron Irradiation: Samples were exposed to a total neutron fluence of 10¹⁷ 1/cm² to create vacancies.
- Electron Irradiation: Samples were exposed to variable-energy electrons (1-4 MeV) up to 2.8 · 10¹⁸ 1/cm² using a custom RF linear accelerator (LINAC).
Thermal Annealing (NV Formation): Irradiated samples were annealed in a stepwise manner under dynamic vacuum (10^-6 mbar) at 800 °C (2 hours) and 1000 °C (2 hours). This step mobilizes vacancies, allowing them to combine with P1 centers to form NV centers.
Quantitative Characterization (CW EPR): Continuous-wave EPR was used after each irradiation and annealing step to quantitatively determine the relative concentration of NV^- centers compared to the P1 center reference signal.
Coherence Measurement (Pulsed EPR): Pulsed EPR techniques (Hahn-echo detected inversion recovery and Hahn-echo decay) were used to measure the critical spin-lattice (T₁) and spin-spin (T₂) relaxation times.
Hyperfine Analysis (ENDOR): Electron-Nuclear Double Resonance (ENDOR) was performed at X-band frequency (~9.5 GHz) using the Mims pulse sequence to precisely determine the hyperfine and quadrupole coupling constants of the NV^- centers with nearby ¹⁴N and ¹³C nuclei.

6CCVD Solutions & Capabilities

The successful creation of high-density NV^- ensembles relies fundamentally on the quality and precise engineering of the diamond host material. 6CCVD specializes in providing the exact material specifications required to replicate and advance this critical research.

Research Requirement	6CCVD Solution & Capability	Value Proposition for Quantum Research
High-Purity Single Crystal Host	6CCVD supplies Electronic Grade Single Crystal Diamond (SCD) plates and wafers, offering superior crystalline quality and low defect density.	Provides the ideal, low-strain environment necessary to maximize NV^- coherence times (T₂ and T₁), essential for qubit and sensor performance.
Controlled Nitrogen Precursors	We offer SCD materials with controlled, low-to-moderate nitrogen concentrations, allowing researchers to precisely tune the initial P1 center density for optimal NV^- yield.	Enables predictable scaling of NV^- ensemble concentration, directly impacting magnetic field sensitivity (scaling as $\sqrt{N}$).
Custom Dimensions for Irradiation	6CCVD provides custom laser cutting and shaping services for SCD and PCD plates up to 125mm, with thicknesses ranging from 0.1 µm to 500 µm.	Ensures samples are perfectly sized for specialized high-energy irradiation chambers (LINACs) and high-frequency EPR/ENDOR resonators (e.g., X-band or Q-band).
Surface Quality & Strain Reduction	Our in-house polishing achieves ultra-low roughness (R_a < 1 nm for SCD), critical for minimizing surface-related decoherence effects.	Maintains the high lattice integrity required for post-annealing recovery, ensuring the long-term stability of the NV^- centers.
Device Integration & Metalization	We offer custom metalization services (Au, Pt, Pd, Ti, W, Cu) for creating microwave striplines or contacts directly on the diamond surface.	Facilitates the integration of NV-rich diamond into functional quantum devices, such as the high-frequency EPR/ENDOR setups used in this study.
Boron-Doped Diamond (BDD) for Charge Control	For experiments requiring alternative charge state control (e.g., NV⁰ vs. NV^-), 6CCVD supplies Boron-Doped Diamond (BDD) films and substrates.	Allows researchers to explore alternative defect engineering strategies beyond standard Type 1b precursors.

Engineering Support

6CCVD’s in-house PhD team, comprised of expert material scientists and physicists, can assist with material selection and specification for similar NV Center Ensemble projects. We provide consultation on optimizing starting material purity, crystallographic orientation, and post-growth processing requirements (e.g., surface termination, annealing protocols) to achieve target NV^- concentrations and coherence properties.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.carbon.2021.12.032

References

1956 - The absorption spectra of irradiated diamonds after heat treatment [Crossref]
1978 - Electron spin resonance in the study of diamond [Crossref]
1997 - Scanning confocal optical microscopy and magnetic resonance on single defect centers [Crossref]
2000 - Stable solid-state source of single photons [Crossref]
2004 - Observation of coherent oscillation of a single nuclear spin and realization of a two-qubit conditional quantum gate [Crossref]
2008 - Nanoscale magnetic sensing with an individual electronic spin in diamond [Crossref]
2017 - High-sensitivity spin-based electrometry with an ensemble of nitrogen-vacancy centers in diamond [Crossref]
2013 - High-precision nanoscale temperature sensing using single defects in diamond [Crossref]
2013 - Nanometre-scale thermometry in a living cell [Crossref]
2019 - Nanodiamonds: emerging face of future nanotechnology [Crossref]