High Temperature Treatment of Diamond Particles Toward Enhancement of Their Quantum Properties

At a Glance

Metadata	Details
Publication Date	2020-06-10
Journal	Frontiers in Physics
Authors	Marco D. Torelli, Nicholas Nunn, Zachary R. Jones, Thea Vedelaar, Sandeep K. Padamati
Institutions	College of Staten Island, University of Wisconsin–Madison
Citations	15
Analysis	Full AI Review Included

Executive Summary

This analysis summarizes the impact of Rapid Thermal Annealing (RTA) on the quantum properties of fluorescent diamond particles, primarily focusing on the negatively charged nitrogen-vacancy (NV^-) center.

Core Achievement: RTA significantly enhances the magnetic modulation contrast of NV^- fluorescence in 20 µm HPHT diamond particles compared to standard annealing methods.
Performance Gain: The maximum achievable magnetic modulation contrast increased by approximately 4x, rising from ~5% (standard 850 °C/2 h) to ~20% (optimized 1740 °C/8 min RTA) under 532 nm excitation.
Defect Reduction: EPR data confirm that RTA effectively eliminates parasitic paramagnetic defects, including negatively charged vacancies (V^-) and unwanted triplet centers (W16-18), especially at temperatures >1700 °C.
Lattice Healing: RTA treatment improves the diamond lattice quality, evidenced by the elongation of spin-lattice relaxation times (T_SL) for P1, V^-, and NV^- centers.
Imaging Utility: Modulation was demonstrated under 420 nm excitation, leveraging the non-modulating H3 center peak (~520 nm) as an internal reference for self-calibration in environments with rapidly changing fluorescent backgrounds.
Nanodiamond Results: RTA treatments (up to 1700 °C) did not yield a statistically significant improvement in the NV^- T₁ relaxation time for 140 nm nanodiamonds, suggesting higher temperatures or longer dwell times may be required for smaller particles due to surface effects.

Technical Specifications

Parameter	Value	Unit	Context
Starting Material	Type Ib HPHT Diamond	N/A	Contains substitutional N (P1 centers)
Particle Sizes Tested	~140 nm, 20 µm	N/A	Used for T₁ relaxometry and magnetic modulation, respectively
Initial Nitrogen Content	~110	ppm	Substitutional nitrogen (P1)
Irradiation Fluence (20 µm)	1.5 x 10¹⁹	e/cm²	3 MeV electron beam
Irradiation Fluence (140 nm)	1 x 10¹⁹	e/cm²	3 MeV electron beam
Standard Annealing	850 °C / 2 h	N/A	Control sample treatment
Optimal RTA Condition	1740 °C / 8 min	N/A	Produced highest magnetic modulation
Max Modulation Contrast	~20	%	1740 °C/8 min sample, 532 nm excitation
Standard Modulation Contrast	~5	%	850 °C/2 h sample, 532 nm excitation
Modulation Contrast (420 nm)	~6	%	1740 °C/8 min sample
Applied Magnetic Field	~150	mT	Static field, above saturation regime
NV^- ZPL Separation	~2.87	GHz	Triplet spin state separation
NV^- Content (Standard)	7.7	ppm	850 °C/2 h sample (EPR data)
NV^- Content (Highest)	9.8	ppm	1500 °C/5 min RTA sample (EPR data)
V^- Content (RTA 1900 °C)	0	ppm	Negatively charged vacancies quenched
H3 Center ZPL	~520	nm	Non-modulating reference peak

Key Methodologies

The study utilized high-energy electron irradiation followed by Rapid Thermal Annealing (RTA) to control the formation and quality of NV^- centers in HPHT diamond particles.

Material Preparation: Type Ib HPHT diamond particles (140 nm or 20 µm) containing ~110 ppm substitutional nitrogen were selected.
Irradiation: Particles were irradiated using a 3 MeV electron beam to fluences of 1.5 x 10¹⁹ e/cm² (20 µm) or 1 x 10¹⁹ e/cm² (140 nm) to create vacancies.
Rapid Thermal Annealing (RTA): Irradiated particles were rapidly annealed at high temperatures (1500 °C to 1900 °C) for short durations (1 to 8 minutes) to mobilize vacancies and form NV centers.
- Tested RTA Conditions (20 µm): 1500 °C/5 min, 1700 °C/3 min, 1900 °C/1 min, and 1740 °C/8 min.
- Control Condition: 850 °C/2 h (standard annealing).
Purification: Following RTA, particles were oxidized (e.g., 850 °C in air or 500 °C in air, followed by acid reflux for 140 nm samples) to remove graphitic carbon formed during the high-temperature treatment.
Characterization (20 µm): Magnetic modulation of fluorescence was measured using static magnetic fields (~150 mT) under 514 nm, 532 nm, and 420 nm excitation. EPR spectroscopy (X-band, 50 K) was used to quantify paramagnetic defects (P1, V^-, Ni^-_S, and NV^- triplets) and measure spin-lattice relaxation times (T_SL).
Characterization (140 nm): Optical T₁ relaxometry was performed to assess the NV^- spin-lattice relaxation time in nanoscale particles.

Commercial Applications

The enhanced quantum properties resulting from RTA processing are critical for advancing diamond-based technologies in several high-tech sectors:

Quantum Sensing and Metrology:
- Development of highly sensitive nanoscale sensors for magnetic fields, temperature, and strain, leveraging the improved NV^- coherence and modulation contrast.
Biomedical Imaging and Diagnostics:
- Background-Free Imaging: Utilizing magnetic modulation for high-fidelity detection of nanodiamonds in complex biological environments (cells, tissue) with high autofluorescence and light scattering.
- Intracellular Probes: Creating robust probes for measuring local events (temperature, pH, radical species) inside living cells.
Nuclear Magnetic Resonance (NMR) Enhancement:
- Hyperpolarization Agents: Using RTA-treated particles as dynamic nuclear polarization (DNP) agents to significantly enhance the sensitivity of NMR and Magnetic Resonance Imaging (MRI) at the cellular level.
Quantum Information Processing:
- Providing high-quality diamond material with reduced parasitic defects and improved lattice order, which is foundational for developing solid-state quantum registers and networks.

View Original Abstract

Fluorescence of the negatively charged nitrogen-vacancy (NV-) center of diamond is sensitive to external electromagnetic fields, lattice strain, and temperature due to the unique triplet configuration of its spin states. Their use in particulate diamond allows for the possibility of localized sensing and magnetic-contrast-based differential imaging in complex environments with high fluorescent background. However, current methods of NV- production in diamond particles are accompanied by the formation of a large number of parasitic defects and lattice distortions resulting in deterioration of the NV- performance. Therefore, there are significant efforts to improve the quantum properties of diamond particles to advance the field. Recently it was shown that rapid thermal annealing (RTA) at temperatures much exceeding the standard temperatures used for NV- production can efficiently eliminate parasitic paramagnetic impurities and, as a result, by an order of magnitude improve the degree of hyperpolarization of 13C via polarization transfer from optically polarized NV- centers in micron-sized particles. Here, we demonstrate that RTA also improves the maximum achievable magnetic modulation of NV- fluorescence in micron-sized diamond by about 4x over conventionally produced diamond particles endowed with NV-. This advancement can continue to bridge the pathway toward developing nano-sized diamond with improved qualities for quantum sensing and imaging.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphy.2020.00205

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