Long spin coherence times of nitrogen vacancy centers in milled nanodiamonds

At a Glance

Metadata	Details
Publication Date	2022-05-02
Journal	Physical review. B./Physical review. B
Authors	Benjamin D. Wood, G. A. Stimpson, J. E. March, Yashna Lekhai, Colin Stephen
Institutions	Cardiff University, University of Warwick
Citations	48
Analysis	Full AI Review Included

Executive Summary

This research demonstrates a significant advancement in the fabrication of high-quality nanodiamonds (NDs) containing negatively charged Nitrogen Vacancy (NV^-) centers, achieving long electron spin coherence times (T2) using a scalable milling process.

Record Coherence Time for Milled NDs: An electron spin coherence time (T2) exceeding 400 µs (specifically 462 ± 130 µs using XY8-4 dynamical decoupling) was achieved at room temperature in a single NV^- center within a milled nanodiamond.
Scalable Fabrication Method: The NDs were produced via Si₃N₄ ball milling of Chemical Vapor Deposition (CVD) bulk diamond, allowing for high-volume conversion of the entire bulk material, unlike less scalable etching techniques.
Material Purity: The source material was high-purity CVD diamond with a natural ¹³C abundance and a low single substitutional nitrogen concentration (121 ppb).
Verification Technique: A novel technique combining Confocal Fluorescence Microscopy (CFM) with etched silicon grid mapping and Scanning Electron Microscopy (SEM) was used to image the exact nanodiamonds for which T2 measurements were taken, providing crucial size and location context.
Engineering Relevance: The achieved T2 times are comparable to or longer than those previously reported for etched nanodiamonds, making this high-volume milling technique viable for advanced quantum sensing and levitation applications.

Technical Specifications

Parameter	Value	Unit	Context
Longest T2 (ND1, XY8-4)	462 ± 130	µs	Room temperature, dynamical decoupling
Longest T2 (ND1, XY8-1)	323 ± 21	µs	Room temperature, dynamical decoupling
Longest T2 (ND1, Hahn Echo)	177 ± 24	µs	Room temperature
Average T2 (Hahn Echo, 7 NDs)	51	µs	Room temperature
Single Substitutional Nitrogen	121	ppb	Bulk CVD diamond source material
Expected Final NV Concentration	~1	ppb	After irradiation and annealing
Atomic Density of Diamond	1.77 x 10²³	cm^-3	Used for size estimation
Estimated ND1 Size (R_max)	106 ± 2	nm	Maximum distance NV^- could be from surface
Average ND Size (7 NDs, R_max)	83	nm	Estimated maximum distance from surface
Single NV Confirmation (ND1)	0.39 ± 0.02	g⁽²⁾(0)	Hanbury Brown-Twiss measurement (ideal is 0)
Electron Irradiation Energy	4.5	MeV	Used for vacancy creation
External Magnetic Field Range	26 to 50	mT	Applied during ODMR and T2 measurements
Centrifugation Force	40 x 10³	g	Used for particle separation

Key Methodologies

The nanodiamonds were fabricated and characterized using a multi-step process focused on achieving high purity and verifying the properties of individual particles.

CVD Diamond Preparation:
- Source material was single-crystal CVD diamond (Element Six) with natural ¹³C abundance and a low nitrogen concentration (121 ppb).
- Irradiation: The bulk diamond was irradiated with 4.5 MeV electrons for one minute to create vacancies.
- Annealing: A three-stage annealing process was used to mobilize vacancies and form NV^- centers: 400°C (3 hours), 800°C (4 hours), and 1200°C (2 hours).
Nanodiamond Fabrication (Milling):
- The bulk diamond was processed using Si₃N₄ ball milling to produce nanodiamonds, avoiding magnetic contamination associated with steel milling.
- Cleaning: Samples were acid cleaned in H₃PO₄, then cleaned in NaOH to remove Si₃N₄ contaminants.
- Surface Treatment: Nanodiamonds were annealed in air, resulting in surfaces consisting primarily of C-Si, COOH, C=O, C-O, C=C, and C-C bonds.
Sample Deposition and Mapping:
- Nanodiamonds were suspended in methanol (1 mg ml^-1 density) and sprayed onto n-type silicon wafers.
- The silicon wafers were plasma etched using photolithography to create a grid system, allowing for precise location and re-identification of individual nanodiamonds across different instruments (CFM and SEM).
Optical and Spin Characterization:
- Single NV Identification: Confocal Fluorescence Microscopy (CFM) was used to locate fluorescent spots. Hanbury Brown-Twiss (HBT) measurements (g⁽²⁾(0) less than 0.5) confirmed the presence of single NV^- centers.
- Magnetic Alignment: An external magnetic field (26 to 50 mT) was aligned precisely to the NV^- axis by monitoring the fluorescent count rate.
- Coherence Measurement: Spin-echo decay experiments were performed at room temperature using three sequences: Hahn echo (THE), XY8-1, and XY8-4 dynamical decoupling to determine the T2 time.
- Size Verification: Scanning Electron Microscopy (SEM) was used to image the exact nanodiamonds measured for T2, providing an estimate of the maximum distance (R_max) the NV^- center could be from the surface.

Commercial Applications

The successful fabrication of high-coherence NV^- nanodiamonds via a scalable milling process opens doors for several high-value commercial and research applications.

Application Area	Specific Use Case	Technical Requirement Met
Quantum Sensing (Magnetometry)	AC magnetometry for biological systems (e.g., local sensing within living cells).	T2 times > 400 µs enable high sensitivity (on the order of 100 nT Hz^-1/2).
Biomedical Imaging & Sensing	Cellular biomarkers, nanoscale thermometry, electrometry, and magnetic imaging of ferritins in single cells.	Small size (sub-100 nm) allows cellular uptake; long T2 ensures high measurement fidelity.
Quantum Information Technologies	Solid-state spin registers and quantum memory elements.	Long T2 coherence time is fundamental for maintaining quantum states.
Macroscopic Quantum Physics	Nanodiamond levitation experiments to probe macroscopic spatial superpositions and the quantum nature of gravity.	Requires large quantities of high-quality NDs (enabled by milling) with long T2 for spin control in vacuum.
High-Volume Manufacturing	Production of large quantities of uniform, high-coherence NDs for industrial sensing applications.	Milling processes the full 3D volume of bulk diamond, offering superior scalability compared to etching pillars.

View Original Abstract

Nanodiamonds containing negatively charged nitrogen vacancy centres\n(${\text{NV}}^{-}$) have applications as localized sensors in biological\nmaterial and have been proposed as a platform to probe the macroscopic limits\nof spatial superposition and the quantum nature of gravity. A key requirement\nfor these applications is to obtain nanodiamonds containing ${\text{NV}}^{-}$\nwith long spin coherence times. Using milling to fabricate nanodiamonds\nprocesses the full 3D volume of the bulk material at once, unlike etching, but\nhas, up to now, limited ${\text{NV}}^{-}$ spin coherence times. Here, we use\nnatural isotopic abundance nanodiamonds produced by\n${\text{Si}}{3}{\text{N}}{4}$ ball milling of bulk diamond grown by chemical\nvapour deposition with an average single substitutional nitrogen concentration\nof $121 ~\text{ppb}$. We show that the electron spin coherence times of\n${\text{NV}}^{-}$ centres in these nanodiamonds can exceed $400 ~\mu\text{s}$\nat room temperature with dynamical decoupling. Scanning electron microscopy\nprovides images of the specific nanodiamonds containing ${\text{NV}}^{-}$ for\nwhich a spin coherence time was measured.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevb.105.205401