Efficient conversion of nitrogen to nitrogen-vacancy centers in diamond particles with high-temperature electron irradiation

At a Glance

Metadata	Details
Publication Date	2020-08-19
Journal	Carbon
Authors	Yuliya Mindarava, Rémi Blinder, Christian Laube, Wolfgang Knolle, Bernd Abel
Institutions	Helmholtz-Zentrum Berlin für Materialien und Energie, Leibniz Institute of Surface Engineering
Citations	41
Analysis	Full AI Review Included

Executive Summary

Core Value Proposition: The research demonstrates an optimized method—High-Temperature (HT) electron irradiation—for efficiently converting substitutional nitrogen (P1 centers) into negatively-charged Nitrogen-Vacancy (NV^-) centers in commercial diamond micro- and nanodiamond powders.
Maximum Efficiency: A P1 to NV^- conversion yield of up to 25% ± 3% was achieved in 2 µm diamond particles using a high irradiation dose (9 x 10¹⁸ cm^-2).
Optimized Process: The HT technique involves simultaneous 10 MeV electron irradiation and annealing at 800 °C, which is hypothesized to reduce the formation of undesirable vacancy clusters compared to conventional Room Temperature (RT) irradiation followed by separate annealing.
Nanodiamond Advantage: HT irradiation proved superior for 25 nm nanodiamonds, yielding significantly higher NV^- concentrations than RT methods, likely because simultaneous annealing allows vacancies to diffuse out of the small crystals rather than aggregating.
Spin Quality Maintained: Despite high defect concentrations, the NV^- centers exhibited long spin coherence (T₂ ~2 µs) and spin-lattice relaxation (T₁ ~2 ms) times, confirming the process does not cause severe crystal damage.
Secondary Defects: Increasing irradiation dose led to the concomitant creation of other spin-1 defects (W16 and W33 centers), which must be accounted for in high-dose applications.

Technical Specifications

Parameter	Value	Unit	Context
Electron Irradiation Energy	10	MeV	Used for vacancy creation
High-Temperature (HT) Annealing	800	°C	Performed simultaneously with irradiation
Final Air Oxidation Temperature	620	°C	Used to remove graphitic surface residues
Maximum Irradiation Dose (2 µm)	9 x 10¹⁸	cm^-2	Highest dose tested for conversion
Maximum Conversion Yield (2 µm)	25 ± 3	%	P1 to NV^- conversion efficiency
Conversion Yield (100 nm, max)	13 ± 2.6	%	P1 to NV^- conversion efficiency
NV^- T₂ (2 µm, typical range)	2.1 - 2.6	µs	Coherence time (Hahn echo)
NV^- T₁ (2 µm, typical range)	2.3 - 2.6	ms	Spin-lattice relaxation time
Initial P1 Concentration (2 µm batch 1)	74 ± 12	ppm	Starting material (Type Ib HPHT)
Initial P1 Concentration (100 nm)	27 ± 5	ppm	Starting material (Type Ib HPHT)
Vacancy Diffusion Coefficient (800 °C)	4.07 x 10^-16	cm² s^-1	Calculated diffusion rate
Root Mean Square Vacancy Displacement (2 x 10¹⁸ cm^-2 dose)	63.8	nm	Vacancy travel distance during irradiation
HT Irradiation Dose Rate	2 x 10¹³	cm^-2 s^-1	Rate of electron delivery

Key Methodologies

The study compared Room Temperature (RT) and High Temperature (HT) electron irradiation methods on Type Ib HPHT diamond powders of varying sizes (2 µm, 100 nm, 25 nm).

Starting Materials: Commercial diamond powders (Microdiamant MSY series) were used, characterized by Continuous Wave Electron Paramagnetic Resonance (CW EPR) to determine initial P1 (substitutional nitrogen) concentrations.
HT Irradiation Setup: Samples were placed in a quartz furnace under permanent argon flow (150 ml/min) and subjected to 10 MeV electron irradiation while simultaneously maintaining the temperature at 800 °C.
RT Irradiation Setup: Samples were irradiated at room temperature (below 300 °C) using 10 MeV electrons, followed by a separate annealing step at 800 °C for 5 hours in an argon atmosphere.
Dose Variation: For optimization, 2 µm and 100 nm samples were subjected to varying HT irradiation doses, ranging up to 9 x 10¹⁸ cm^-2.
Surface Treatment: All irradiated and annealed samples underwent a final air oxidation step at 620 °C for 5 hours to remove surface graphitization.
Defect Quantification (EPR): CW EPR was the primary tool for absolute quantification of P1 and NV^- concentrations, using spin-counting techniques adapted for powder samples (half-field transition integration for NV^- in larger particles).
Defect Quantification (Optical): For 25 nm particles, NV^- concentration was cross-validated using a combined Atomic Force Microscope (AFM) and confocal microscope setup, deducing defect numbers from photoluminescence (PL) intensity.
Spin Dynamics Measurement: Pulsed EPR (Hahn echo sequence) was used to measure the coherence time (NV T₂), and Inversion/Saturation Recovery sequences were used for spin-lattice relaxation time (NV T₁). Instantaneous Diffusion (ID) analysis was used to locally estimate spin density.

Commercial Applications

The ability to synthesize fluorescent nanodiamonds (FNDs) with tailored, high NV^- concentrations and preserved spin coherence is critical for several high-tech sectors:

Quantum Sensing and Metrology:
- Nanoscale Magnetometry: High-density NV^- ensembles increase the signal-to-noise ratio, improving the sensitivity of diamond-based magnetic sensors used for measuring fields in biological systems or integrated circuits.
- Temperature Sensing: NV^- centers are highly effective quantum thermometers.
Biomedical and Pharmaceutical Industries:
- Fluorescence Biomarkers: FNDs are biocompatible, photostable, and non-toxic, making them superior alternatives to traditional organic dyes for long-term cellular imaging and tracking.
- Drug Delivery Systems: Nanodiamonds can be functionalized for targeted drug delivery, with the NV^- centers providing tracking capabilities.
Magnetic Resonance Imaging (MRI):
- Hyperpolarized MRI Tracers: High NV^- density is essential for efficient ¹³C nuclear spin hyperpolarization, creating highly sensitive MRI contrast agents for enhanced medical diagnostics.
Quantum Information Technology:
- Quantum Registers: The controlled creation of NV^- defects allows for the fabrication of solid-state quantum bits (qubits) and components for quantum memory and computing architectures.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.carbon.2020.07.077

References

2013 - The nitrogen-vacancy colour centre in diamond [Crossref]
2012 - The biocompatibility of nanodiamonds and their application in drug delivery systems [Crossref]
2009 - Functionalized fluorescent nanodiamonds for biomedical applications [Crossref]
2017 - Biomedical applications of nanodiamond (review) [Crossref]
2016 - Diamond quantum devices in biology [Crossref]
2017 - Phase-encoded hyperpolarized nanodiamond for magnetic resonance imaging
2019 - Controlling the fluorescence properties of nitrogen vacancy centers in nanodiamonds [Crossref]
2010 - Efficient production of NV colour centres in nanodiamonds using high-energy electron irradiation [Crossref]