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

Technical Analysis: Reduction of Surface Spin-Induced Electron Spin Relaxations in Nanodiamonds

This technical documentation analyzes the research concerning the mitigation of surface-induced decoherence in nanodiamonds (NDs) for quantum sensing applications. The findings directly inform the material requirements for high-performance Nitrogen-Vacancy (NV) and P1 center platforms, aligning perfectly with 6CCVD’s expertise in high-purity MPCVD diamond.

Executive Summary

The following points summarize the core findings regarding spin relaxation control in nanodiamonds:

Quantum Sensing Limitation: Long spin relaxation times (T₁ and T₂) are critical for high-sensitivity NV-based quantum sensing, but surface spins (dangling bonds) in nanodiamonds significantly reduce these times.
Target System: The study used single-substitutional nitrogen impurity (P1) centers as a model system, as their T₁ and T₂ relaxation mechanisms are similar to those of nearby NV centers.
Decoherence Mitigation: Air annealing (oxidative etching) was employed to efficiently remove the surface shell and associated surface spins.
Process Validation: Dynamic Light Scattering (DLS) confirmed uniform etching with a linear rate of 3.5 nm/hour at 550 °C.
T₁ Improvement: High-Frequency (HF) EPR spectroscopy confirmed that air annealing significantly suppressed the surface spin contribution (Γ_s) to T₁ by a factor of 7.5 ± 5.4 after 7 hours.
T₂ Improvement: The spin coherence time (T₂) was observed to improve by a factor of 1.2 ± 0.2 under the same annealing conditions.
Material Implication: The results establish a crucial methodology for surface engineering, necessary to suppress spin relaxation processes in NDs and other diamond-based quantum platforms.

Technical Specifications

Parameter	Value	Unit	Context
Annealing Temperature	550	°C	Air annealing process
Etching Rate (Linear)	3.5	nm/hour	Determined via DLS characterization
Initial ND Diameter Range	50 to 10	nm to µm	Range of Type-Ib diamond powders studied
Nitrogen Impurity Concentration	10 to 100	ppm	In Type-Ib diamond powders
Surface Spin Shell Thickness (Pre-Annealing)	~9	nm	Based on previous HF EPR study
T₁ Surface Contribution (Γ_s) Suppression	7.5 ± 5.4	Factor	After 7 hours annealing at 550 °C
T₂ Improvement Factor	1.2 ± 0.2	Factor	After 7 hours annealing at 550 °C
HF EPR Frequencies	230 and 115	GHz	Used for cw and pulsed EPR spectroscopy
T₁ (50 nm ND, 200 K, Non-Annealed)	0.382 ± 0.080	ms	P1 center spin relaxation time
T₂ (50 nm ND, 200 K, Non-Annealed)	0.413 ± 0.007	µs	P1 center spin coherence time

Key Methodologies

The experimental success relied on precise material handling and advanced spectroscopic analysis:

Material Selection: Used commercially available Type-Ib diamond powders (mechanically milled/ground) with mean diameters ranging from 50 nm to 10 µm.
Sample Preparation: Nanodiamond samples (approx. 30 mg) were uniformly dispersed in acetone via 10 minutes of ultrasound sonication, followed by solvent evaporation.
Air Annealing (Oxidative Etching): Samples were placed in a quartz tube furnace stabilized at 550 °C. Periodic mixing (30 seconds every 10 minutes) was performed to ensure homogeneous application of the oxidative etching across the powder volume.
Size and Etching Confirmation: Dynamic Light Scattering (DLS) was used to characterize particle size reduction, confirming a uniform etching process and a linear rate of 3.5 nm/hour.
Spin Quantification (cw EPR): Continuous-wave (cw) High-Frequency (HF) EPR spectroscopy (230 GHz) was used at room temperature to distinguish and quantify the EPR signals from P1 centers and surface spins (dangling bonds).
Relaxation Time Measurement (Pulsed EPR): Pulsed HF EPR (115 GHz) was employed to measure the spin relaxation times:
- T₁ (longitudinal relaxation) was measured using the inversion recovery sequence.
- T₂ (transverse relaxation/coherence time) was measured using the spin echo sequence.

6CCVD Solutions & Capabilities

The research highlights the critical need for materials with controlled surface properties and high internal purity to maximize spin coherence times (T₁ and T₂) for quantum applications. 6CCVD’s MPCVD diamond materials and processing capabilities are ideally suited to meet and exceed the requirements demonstrated in this study.

Applicable Materials for Quantum Sensing

The Type-Ib material used in the paper contains native nitrogen impurities (P1 centers). For next-generation quantum sensing, researchers require materials with superior purity and controlled defect creation.

Application Requirement	6CCVD Recommended Material	Technical Rationale
High-Coherence NV Platforms	Optical Grade Single Crystal Diamond (SCD)	Ultra-low native nitrogen content (< 1 ppm) ensures minimal background spin bath noise, maximizing bulk T₁ and T₂ coherence times before surface processing.
Shallow NV Creation & Sensing	High-Purity SCD Plates (0.1 µm - 500 µm)	Provides a clean, stable substrate necessary for precise, shallow NV creation via ion implantation, allowing researchers to replicate and improve upon the surface spin reduction techniques demonstrated.
High-Power EPR Substrates	Polycrystalline Diamond (PCD) Wafers	Offers high thermal conductivity and low dielectric loss, ideal for substrates supporting high-frequency (115 GHz, 230 GHz) EPR experiments requiring high microwave power.

Customization Potential for Advanced Research

6CCVD offers comprehensive material engineering services to support the replication and extension of this research:

Custom Dimensions: While the paper used ND powder, 6CCVD supplies large-area diamond platforms. We offer PCD wafers up to 125 mm in diameter and SCD plates up to 10 mm thick, enabling the scaling of quantum devices and large-area sensor arrays.
Precision Surface Engineering: We provide ultra-low roughness polishing (Ra < 1 nm for SCD, Ra < 5 nm for inch-size PCD). This smooth surface is crucial for achieving uniform etching rates (like the 3.5 nm/hour oxidative etching) and minimizing surface defects prior to annealing.
Integrated Device Fabrication: For researchers integrating EPR components or microwave circuitry, 6CCVD offers in-house custom metalization using materials including Au, Pt, Pd, Ti, W, and Cu, deposited directly onto the diamond surface.

Engineering Support & Global Logistics

6CCVD’s in-house PhD team specializes in material selection and optimization for quantum defects (NV, P1, SiV). We can assist researchers in selecting the optimal SCD or PCD grade to minimize bulk decoherence and maximize the effectiveness of surface treatments, such as the air annealing method used to improve T₁ and T₂ in NV-based sensing projects.

We offer reliable global shipping (DDU default, DDP available) to ensure your critical materials arrive safely and promptly, regardless of location.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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]