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	University of Tsukuba, Leibniz Institute of Surface Engineering
Citations	41
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Yield NV Center Conversion via HT Electron Irradiation

Executive Summary

This research successfully demonstrates an optimized method for creating high-density, negatively-charged Nitrogen-Vacancy (NV⁻) centers in diamond particles using simultaneous High-Temperature (HT) electron irradiation and annealing. This technique is critical for advancing quantum sensing and biomedical applications.

Optimized Defect Creation: The HT irradiation method (10 MeV electrons, 800 °C annealing) significantly improved NV⁻ conversion yield compared to traditional Room Temperature (RT) methods, particularly in smaller nanodiamonds (25 nm).
Record Conversion Efficiency: Achieved a maximum P1 (substitutional nitrogen) to NV⁻ conversion yield of 25 ± 3% in 2 µm microdiamonds, a figure crucial for maximizing quantum signal strength.
Material Quality Maintained: Despite high irradiation doses (up to 9 x 10¹⁸ cm^-2), the resulting NV⁻ centers maintained long spin coherence times (T₂ up to 2.7 µs) and spin-lattice relaxation times (T₁ up to 2.6 ms), confirming minimal severe lattice damage.
Scalability for Bulk Applications: The use of high-energy (10 MeV) electrons ensures a homogeneous vacancy distribution over large depths (>1 cm), making this technique highly scalable for processing large quantities of diamond material, including 6CCVD’s SCD and PCD wafers.
Application Focus: The resulting high-density NV⁻ materials are ideal for next-generation applications in nanoscale optical imaging, magnetic sensing, and ¹³C nuclear spin hyperpolarization.

Technical Specifications

Parameter	Value	Unit	Context
Irradiation Energy	10	MeV	High-energy electron beam source
Annealing/Irradiation Temp	800	°C	Simultaneous High-Temperature (HT) process
Maximum Irradiation Dose	9 x 10¹⁸	cm^-2	Highest dose tested on 2 µm particles
Maximum Conversion Yield	25 ± 3	%	P1 to NV⁻ conversion (2 µm particles)
Particle Sizes Tested	25, 100, 2	nm, µm	Commercial HPHT Type Ib diamond powder
Max NV⁻ Concentration	13.5	ppm	Achieved in 2 µm sample (9MSY2) via CW EPR
NV⁻ Coherence Time (T₂)	1.9 - 3.2	µs	Measured via Hahn echo (dependent on particle size/dose)
NV⁻ Relaxation Time (T₁)	1.6 - 2.6	ms	Measured via Pulsed EPR (dependent on particle size/dose)
Vacancy Diffusion Length (l)	~63.8	nm	Calculated for 2 x 10¹⁸ cm^-2 dose at 800 °C
Secondary Defects	W16, W33	-	Spin-1 defects increasing linearly with irradiation dose

Key Methodologies

The core innovation lies in the High-Temperature (HT) irradiation technique, which couples vacancy creation and migration into a single, highly efficient step.

Starting Material Selection: Commercial Type Ib HPHT diamond powder was used, characterized by high initial substitutional nitrogen (P1) concentrations ranging from 5.2 ppm (25 nm) to 74 ppm (2 µm).
HT Irradiation Setup: Samples were placed in a quartz furnace under permanent argon flow (1 bar) and simultaneously irradiated with 10 MeV electrons. The temperature was actively regulated to 800 °C throughout the process.
Dose Optimization: Irradiation doses were systematically varied (0.5 x 10¹⁸ cm^-2 to 9 x 10¹⁸ cm^-2) to map the conversion efficiency dependence, demonstrating that higher doses yield higher NV⁻ concentration until saturation effects begin.
Surface Cleaning: Post-irradiation, all samples underwent air oxidation at 620 °C for 5 hours to remove surface graphitic residues, a critical step for maintaining high spin coherence properties.
Defect Quantification: Nitrogen (P1) and NV⁻ concentrations were precisely quantified using Continuous Wave Electron Paramagnetic Resonance (CW EPR) and Pulsed EPR techniques, complemented by AFM-confocal microscopy for nanodiamonds.
Spin Property Measurement: NV⁻ spin coherence (T₂) and spin-lattice relaxation (T₁) times were measured using Hahn echo and Inversion/Saturation-Recovery sequences via Pulsed EPR to assess material quality and decoherence sources.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-quality, customized diamond materials required to replicate and advance this high-yield NV⁻ creation research, transitioning it from powder experiments to scalable quantum devices.

Applicable Materials

The research highlights the need for precise control over initial nitrogen concentration (P1 centers) to maximize NV⁻ conversion yield. 6CCVD offers superior control over doping and crystal quality compared to commercial HPHT powders.

Research Requirement	6CCVD Material Solution	Technical Advantage
High Initial P1 Concentration	High-Nitrogen MPCVD Polycrystalline Diamond (PCD)	Provides a Type Ib equivalent starting material with high, uniform nitrogen content necessary for high NV⁻ density. Available in large formats (up to 125mm diameter).
High Purity, Low Defects	Optical Grade Single Crystal Diamond (SCD)	For applications requiring maximum coherence (e.g., bulk quantum sensing), 6CCVD can supply SCD with controlled nitrogen doping (Type Ib or Type Ia) for targeted NV⁻ creation while minimizing secondary defects (W16, W33).
Surface-Sensitive Applications	Polished SCD or PCD Wafers	The paper noted surface effects dominate T₂ in 25 nm particles. 6CCVD offers ultra-smooth polishing (Ra < 1nm for SCD, < 5nm for PCD) essential for minimizing surface-induced decoherence in thin films or bulk substrates used for sensing.

Customization Potential

The scalability demonstrated by the 10 MeV electron irradiation process is perfectly matched by 6CCVD’s large-area MPCVD growth capabilities.

Custom Dimensions: While the paper used powders, scaling this research to bulk quantum devices requires large, high-quality substrates. 6CCVD provides PCD plates up to 125mm and SCD substrates up to 10mm thick, ready for large-scale electron irradiation processing.
Tailored Thickness: We offer SCD and PCD layers from 0.1 µm (for thin-film sensing) up to 500 µm, allowing researchers to precisely match the required thickness to the penetration depth of the irradiation source (10 MeV electrons penetrate >1 cm).
Advanced Metalization: For integrating NV⁻-rich diamond into functional devices (e.g., microwave delivery structures for EPR/ODMR), 6CCVD provides in-house custom metalization services, including Au, Pt, Pd, Ti, W, and Cu deposition, directly onto the diamond surface.

Engineering Support

The optimization of NV⁻ creation yield (25% achieved here) is highly dependent on the starting material quality and the precise irradiation/annealing recipe. 6CCVD’s in-house PhD team specializes in defect engineering and material optimization for quantum applications.

We offer consultation on:

Selecting the optimal nitrogen concentration in the starting MPCVD material to balance high NV⁻ yield against residual P1 decoherence.
Designing custom diamond geometries (plates, wafers, or specific microstructures) suitable for high-energy electron irradiation and subsequent device fabrication.
Analyzing the impact of surface preparation and polishing on NV⁻ coherence times for similar Quantum Sensing and Hyperpolarization projects.

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.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]