Reduction of surface spin-induced electron spin relaxations in nanodiamonds

At a Glance

Metadata	Details
Publication Date	2020-08-03
Journal	Journal of Applied Physics
Authors	Zai-Li Peng, Jax Dallas, Susumu Takahashi, Zai-Li Peng, Jax Dallas
Institutions	University of Southern California
Citations	11
Analysis	Full AI Review Included

Executive Summary

This study investigates and successfully mitigates the primary source of spin relaxation (T₁ and T₂) in nanodiamonds (NDs)—magnetic field fluctuations caused by surface spins—critical for high-sensitivity quantum sensing applications.

Core Problem Addressed: Surface defects (dangling bonds) in NDs significantly shorten the electron spin relaxation times (T₁ and T₂) of nitrogen-vacancy (NV) and single-substitutional nitrogen (P1) centers, limiting quantum sensor performance.
Solution Implemented: Air annealing at 550 °C was used to efficiently etch the diamond surface, removing graphite layers and surface dangling bonds.
Quantitative T₁ Improvement: The contribution of surface spins (Γ_S) to the longitudinal relaxation time (T₁) was suppressed by a factor of 7.5 ± 5.4 after 7 hours of annealing.
Quantitative T₂ Improvement: The transverse relaxation time (T₂) was improved by a factor of 1.2 ± 0.2 after 7 hours of annealing, comparable to previous reports on graphite removal.
Surface Spin Density Reduction: Modeling based on the T₁ dependence suggests that the surface spin density (p_S) in annealed NDs is approximately 100 times smaller than in non-annealed samples.
Mechanism Verification: High-frequency (HF) Electron Paramagnetic Resonance (EPR) spectroscopy confirmed the reduction of surface spin signals, and pulsed EPR successfully isolated the surface spin contribution (Γ_S) from the intrinsic spin-lattice relaxation (CT⁵).
Etching Performance: Dynamic Light Scattering (DLS) confirmed a uniform and linear etching rate of 3.5 nm/hour at 550 °C.

Technical Specifications

Parameter	Value	Unit	Context
Initial ND Diameter (Mean)	50 ± 20	nm	Smallest ND sample studied
Annealing Temperature	550	°C	Standard temperature for air annealing
Annealing Duration (Max)	9	hours	Maximum duration tested for etching
ND Etching Rate	3.5	nm/hour	Determined by DLS analysis
ND Weight Reduction Rate	0.12	hour^-1	Normalized weight reduction rate
Initial Surface Spin Ratio (I_S/I_P1)	61	N/A	50-nm NDs, non-annealed
Final Surface Spin Ratio (I_S/I_P1)	5	N/A	50-nm NDs, after 9 hours annealing
T₁ Surface Spin Contribution (Γ_S) Reduction	7.5 ± 5.4	Factor	Suppression achieved after 7 hours annealing
T₂ Improvement Factor	1.2 ± 0.2	Factor	Improvement achieved after 7 hours annealing
Surface Spin Density (p_S) Reduction	~100	Factor	Estimated reduction based on Γ_S model (p_S/d⁴)
HF EPR Frequency (CW)	230	GHz	Used for high-resolution spectral analysis
HF EPR Frequency (Pulsed)	115	GHz	Used for T₁/T₂ measurements (higher output power)
EPR Magnetic Field	12.1	Tesla	Superconducting magnet field strength
T₁ (50 nm ND, 200 K)	0.382 ± 0.080	ms	Longitudinal relaxation time (P1 center)
T₂ (50 nm ND, 200 K)	0.413 ± 0.007	µs	Transverse relaxation time (P1 center)
Bulk Diamond T₁ Coefficient (C)	(2.96 ± 0.52) x 10^-10	s^-1K^-5	Coefficient for spin-orbit induced tunneling (CT⁵)

Key Methodologies

The study utilized a multi-step process involving material preparation, surface treatment, and advanced spectroscopic characterization:

Nanodiamond Selection: Five sizes of Type-Ib diamond powders (10 µm down to 50 nm mean diameter) were used, containing nitrogen impurities in the range of 10 to 100 ppm.
Air Annealing (Etching):
- Preparation: ND samples (~30 mg) were dispersed in acetone via ultrasound sonication (10 min) and then dried overnight in a fume hood.
- Process: Annealing was performed in a quartz tube furnace stabilized at 550 °C.
- Homogeneity Control: Samples were periodically mixed using a lab spatula (30 seconds every 10 minutes) to ensure uniform etching across the powder.
Etching Characterization (DLS): Dynamic Light Scattering (DLS) was used to measure particle size reduction and confirm uniform etching, yielding a linear etching rate of 3.5 nm/hour.
Surface Spin Characterization (CW HF EPR):
- Continuous-wave (CW) EPR at 230 GHz was used to resolve and quantify the signals from P1 centers (narrow signal) and surface spins (broad signal).
- The ratio I_S/I_P1 was calculated to determine the relative population of surface spins before and after annealing.
Spin Relaxation Measurement (Pulsed HF EPR):
- Pulsed EPR at 115 GHz was employed for T₁ (inversion recovery sequence) and T₂ (spin echo sequence) measurements due to the higher output power available at this frequency.
- Measurements were performed across a temperature range (100 K to 300 K).
Data Analysis: T₁ data was fitted using the model 1/T₁ = CT⁵ + Γ_S, allowing the extraction of the surface spin contribution (Γ_S) and quantifying its reduction after annealing.

Commercial Applications

This research directly supports the development and optimization of diamond-based quantum technologies, particularly those relying on NV centers near the surface or in nanodiamonds.

Quantum Sensing and Magnetometry:
- High-Sensitivity Sensors: Longer T₁ and T₂ times are critical for improving the sensitivity of NV-based AC and DC magnetic field sensors, enabling detection of smaller magnetic fields (e.g., proportional to 1/√T₂).
- Nanoscale Imaging: NDs with improved spin properties are ideal for nanoscale magnetic imaging (e.g., detecting small ensembles of Gd³⁺ spins or single proton spins).
Electron Paramagnetic Resonance (EPR) Spectroscopy:
- NV-Detected EPR: Improved coherence times enhance the sensitivity of NV-detected EPR, allowing detection of external spins within several nanometers of the NV center.
Nanomaterial Engineering:
- Surface Control: The air annealing method provides a reliable, scalable technique for controlling the diamond surface termination and removing decoherence sources (dangling bonds, graphite layers).
- General Spin/Optical Property Improvement: The methodology is potentially applicable to improving the spin and optical properties of other nanomaterials where surface defects dominate relaxation processes.

View Original Abstract

Nanodiamonds (NDs) hosting nitrogen-vacancy (NV) centers are promising for applications of quantum sensing. Long spin relaxation times (T1 and T2) are critical for high sensitivity in quantum applications. It has been shown that fluctuations of magnetic fields due to surface spins strongly influence T1 and T2 in NDs. However, their relaxation mechanisms have yet to be fully understood. In this paper, we investigate the relation between surface spins and T1 and T2 of single-substitutional nitrogen impurity (P1) centers in NDs. The P1 centers located typically in the vicinity of NV centers are a great model system to study the spin relaxation processes of the NV centers. By employing high-frequency electron paramagnetic resonance spectroscopy, we verify that air annealing removes surface spins efficiently and significantly reduces their contribution to T1.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0007599

References

1997 - Scanning confocal optical microscopy and magnetic resonance on single defect centers [Crossref]
2006 - Processing quantum information in diamond [Crossref]
2005 - Anisotropic interactions of a single spin and dark-spin spectroscopy in diamond [Crossref]
2006 - Room-temperature coherent coupling of single spins in diamond [Crossref]
2006 - Coherent dynamics of coupled electron and nuclear spin qubits in diamond [Crossref]
2008 - Quenching spin decoherence in diamond through spin bath polarization [Crossref]
2008 - Scanning magnetic field microscope with a diamond single-spin sensor [Crossref]
2008 - Nanoscale magnetic sensing with an individual electronic spin in diamond [Crossref]
2008 - Nanoscale imaging magnetometry with diamond spins under ambient conditions [Crossref]
2009 - Sensing of fluctuating nanoscale magnetic fields using nitrogen-vacancy centers in diamond [Crossref]