Identification of NV Centers in Synthetic Fluorescent Nanodiamonds and Control of Defectiveness of Crystallites Using Electron Paramagnetic Resonance
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2022-01-01 |
| Journal | Оптика и спектроскопия |
| Authors | V. Yu. Osipov, Bogdanov K.V., François Treussart, Arfaan Rampersaud, А. В. Баранов |
| Institutions | Centre National de la Recherche Scientifique, Columbus NanoWorks (United States) |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”- Core Achievement: Successful identification and quality control of high-concentration Nitrogen Vacancy (NV-) centers in 100 nm synthetic fluorescent nanodiamonds (FNDs) using Electron Paramagnetic Resonance (EPR).
- High Concentration: The optimized irradiation and annealing process yielded high NV- concentrations, reaching up to 5.5 ppm (FND-3 sample), correlating strongly with high luminescence intensity (600-820 nm).
- Quality Assessment Method: The crystalline quality of the local environment surrounding the NV- centers was characterized by analyzing the saturation curves of the EPR signal intensity (Ipp) versus microwave power (PMW).
- Defect Sensitivity: The allowed EPR transition (x, y Δms = 1) was found to be highly sensitive to changes in particle size and the presence of near-surface defects (5-10 nm deep) induced by mechanical grinding.
- Material Quality: EPR and fluorescence results confirm that the crystalline quality of the 100 nm FNDs is comparable to bulk diamond, making them suitable for bright, superbright luminescence applications.
- EPR Diagnostics: The EPR method provides a non-optical diagnostic tool to independently estimate NV- concentration and assess the crystal lattice quality, crucial for selecting optimal diamond powders.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Nanodiamond Average Size | 104 | nm | Particle distribution width ~76 nm |
| Impurity Nitrogen Concentration (Original HPHT) | 150 ± 10 | ppm | Atomic nitrogen in microcrystals |
| Maximum NV- Concentration (FND-3) | 5.5 | ppm | Estimated via EPR integral intensity |
| Electron Beam Energy | 5 | MeV | High-energy irradiation source |
| Electron Current Density | 32 | µA/cm2 | During irradiation |
| Annealing Temperature | 800 | °C | Performed in inert atmosphere |
| Minimum Exposure Dose | 7 x 1018 | cm-2 | Corresponding to 16h exposure |
| EPR Operating Frequency | 9.434-9.444 | GHz | Q-band spectrometer |
| NV- Ground State Fine Structure Parameter (D) | 954 x 10-4 | cm-1 | Zero magnetic field splitting |
| NV- Zero Phonon Line (ZPL) | 637 | nm | Peak of photoluminescence spectrum |
| EPR Forbidden Transition g-factor (g1) | 4.27 | - | Used for concentration estimation |
| Near-Surface Defect Layer Depth | 5-10 | nm | Layer affected by mechanical grinding |
| Critical Saturation Power (FND-2) | ~0.4 | mW | PMW where Ipp (g=4.27) begins to saturate |
Key Methodologies
Section titled “Key Methodologies”- Starting Material Selection: High-Pressure/High-Temperature (HPHT) Ib synthetic diamond microcrystals (up to 150 µm) with high atomic nitrogen content (150 ± 10 ppm) were chosen.
- Mechanical Size Reduction: The microcrystals were subjected to intensive crushing and grinding, followed by centrifugation in an aqueous medium to isolate the 100 nm submicron fraction.
- Vacancy Induction: The powdered material was irradiated with a high-energy electron beam (5 MeV, 32 µA/cm2) at controlled temperatures (using a special cooling device) to create vacancies in the lattice.
- NV Center Formation: Samples were subsequently annealed at 800 °C in an inert atmosphere, allowing vacancies to migrate and be captured by isolated nitrogen impurities, forming NV- centers.
- Chemical Purification: Intensive purification in boiling acids was performed post-annealing to remove metallic impurities, particularly iron-containing complexes, which can quench fluorescence.
- EPR Spectroscopy: EPR spectra were recorded at room temperature (9.5 GHz) to detect the characteristic forbidden (Δms = 2, g = 4.27) and allowed (Δms = 1) transitions of the NV- center.
- Crystal Quality Analysis: Saturation curves (Ipp vs. √PMW) were generated for the g = 4.27 signal. The shape of these curves qualitatively characterizes the spin relaxation times and the crystalline quality of the local environment around the NV- centers.
- Luminescence Measurement: Luminescence spectra (excited by 532 nm laser) were measured to confirm NV- presence (ZPL at 637 nm) and quantify fluorescence intensity, validating the concentration estimates derived from EPR integral intensity.
Commercial Applications
Section titled “Commercial Applications”- Quantum Computing and Communication: The high-quality NV- centers serve as robust solid-state qubits and single-photon emitters, critical components for developing quantum processors and secure optical communication channels.
- Nanoscale Sensing and Metrology: FNDs are utilized in high-sensitivity sensors for measuring magnetic fields, temperature, and strain at the cellular level, leveraging the long spin coherence times achieved in these high-quality crystallites.
- Biomedical Imaging and Diagnostics: The bright, photostable fluorescence of the 100 nm FNDs makes them ideal for biological applications, including tracking endosomal dynamics, organelle contouring, and in vivo imaging (e.g., injected under the skin).
- Advanced Material Quality Control: The EPR saturation curve technique offers a reliable, non-optical method for quality assurance in the manufacturing of fluorescent nanodiamonds, ensuring that powders meet strict crystalline quality requirements for high-performance applications.
- Micro-Optical Devices: The studied FNDs, due to their high fluorescence intensity and bulk-like crystalline quality, are suitable for integration into micro-optical devices and fiber systems.
View Original Abstract
A 100 nm synthetic diamond particle with a large (> 4 ppm) amount of nitrogen vacancy (NV) centers has been studied. The latter exhibit lines associated with forbidden Delta m_s = 2 and allowed Delta m_s = 1 transitions in the electron paramagnetic resonance (EPR) spectra of the ground state of the NV (-) center. The luminescence intensity of particles in the range 550-800 nm increases with an increase in the irradiation dose of 5 MeV electrons and correlates with the integrated intensity of the peak EPR line with a g-factor g = 4.27. This value is used to estimate the concentration of NV (-) centers and to select diamond powders with the highest fluorescence intensity. The dependence of the EPR signal intensity of the Delta m_s = 2 transition of the NV (-) center on the microwave power that increases before decaying rather well characterizes the crystal quality of the local environment of the centers under study in these particles. The intensity of the x, y Delta m_s = 1 transition (at ~ 281.2 mT, 9.444 GHz) turns out to be sensitive to changes in particle size in the submicron range and the appearance of near-surface defects obtained during mechanical processing. Keywords: luminescence, nitrogen vacancy centers, synthetic diamond, nanocrystals, electron paramagnetic resonance.