Impact of Surface Functionalization on the Quantum Coherence of Nitrogen-Vacancy Centers in Nanodiamonds

At a Glance

Metadata	Details
Publication Date	2018-03-20
Journal	ACS Applied Materials & Interfaces
Authors	Robert G. Ryan, Alastair Stacey, Kane M. O’Donnell, Takeshi Ohshima, Brett C. Johnson
Institutions	Centre for Quantum Computation and Communication Technology, Curtin University
Citations	51
Analysis	Full AI Review Included

Diamond Quantum Coherence Enhancement via Surface Engineering: Analysis for 6CCVD

This technical analysis reviews research demonstrating a robust method for significantly improving the quantum coherence ($T_1$) of Nitrogen Vacancy (NV) centers in nanodiamonds through targeted surface functionalization, directly addressing a critical limitation in nanoscale quantum sensing.

Executive Summary

Core Achievement: Borane reduction of thermally oxidized nanodiamond surfaces resulted in an average doubling (110% increase) of the NV center spin lattice relaxation time ($T_1$).
Mechanism Verified: The $T_1$ increase is directly correlated with the spectroscopic reduction of surface $sp^2$ carbon species (C=O, C=C) and their replacement by stable $sp^3$ bonded C-O and C-H groups.
Spin Noise Mitigation: The findings implicate double-bonded carbon species as a dominant source of high-frequency surface magnetic spin noise responsible for limiting quantum coherence in near-surface NV centers.
Material Preparation: NV centers were generated in 50 nm Type Ib nanodiamonds via 2 MeV electron irradiation followed by high-temperature annealing (1000 °C in vacuum).
Technological Impact: This systematic surface engineering approach validates the pathway toward achieving ‘bulk-like’ quantum properties in diamond probes smaller than 50 nm, crucial for advancing high-sensitivity nanoscale magnetometry and quantum computing applications.
Surface Sensitivity: The study confirms that for nanoscale diamond (<50 nm), quantum properties and surface chemistry cannot be treated independently, requiring precise, tailored surface termination.

Technical Specifications

Extracted physical and performance parameters from the study:

Parameter	Value	Unit	Context
Material Source	Type Ib HPHT	N/A	Nanodiamond (ND) precursors
ND Size Range	50 ± 10	nm	Used for $T_1$ measurements
Electron Irradiation Energy	2	MeV	To induce NV formation
Electron Irradiation Fluence	1 x 10¹⁸	electrons/cm²	NV generation dosage
Annealing Temperature	1000	°C	In 10^-6 Torr vacuum
Thermal Oxidation Temp./Duration	475 / 2	°C / hours	Starting material preparation
Borane Reduction Temp./Duration	67 / 24	°C / hours	Surface functionalization step
Average $T_1$ Enhancement	110	%	After borane reduction vs. oxidized state
NV Polarization Wavelength	532	nm	Green laser excitation
Carbonyl Reduction Indicator	Greatly Reduced	N/A	C=O peak in Oxygen K-edge NEXAFS (532 eV)
Surface Analysis Depth	~1	nm	NEXAFS Partial Electron Yield (PEY) detection depth

Key Methodologies

The experimental workflow combines material preparation, controlled surface functionalization, and advanced spectroscopic and quantum characterization:

NV Center Generation:
- HPHT Type Ib nanodiamonds were irradiated with high-energy electrons (2 MeV).
- Vacuum annealing was performed at 1000 °C for 2 hours to mobilize vacancies and form the negatively charged NV centers.
Initial Surface Termination (Oxidation):
- The annealed NDs were thermally oxidized in air at 475 °C for 2 hours, resulting in a surface dominated by carboxyls (COOH), hydroxyls (R-OH), ketones, and $sp^2$ carbon species.
Deposition for Imaging:
- NDs were deposited onto a polyallylamine hydrochloride (PAH) polymer layer on marked glass coverslips to ensure uniform distribution and prevent agglomeration for confocal imaging.
Borane Reduction Functionalization:
- The oxidized NDs (both powder and coverslip samples) were refluxed in 1 M Borane Tetrahydrofuran (BH₃.THF) solution at 67 °C for 24 hours under a N₂ atmosphere.
- The reaction was quenched with 2 M HCl, converting C=O groups to C-O and C=C species to C-H bonds.
Quantum Coherence Measurement:
- The spin lattice relaxation time ($T_1$) was measured on single NV centers at room temperature using a custom confocal microscope setup (532 nm excitation, 100x 1.3NA objective) via the $T_1$ sensing protocol (laser polarization, dark evolution time $\tau$, readout pulse).
Spectroscopic Characterization:
- FTIR Spectroscopy: Used to analyze bulk functional groups, confirming the removal of C=C (1630 cm^-1) and C=O groups (1780 cm^-1) and the increase in C-H species (2962 cm^-1).
- NEXAFS Spectroscopy (Carbon & Oxygen K-edges): Provided highly surface-specific (top ~1 nm) verification of the borane reduction efficacy, showing a clear reduction in the sp² C=O peak (532 eV, 286.5 eV).

6CCVD Solutions & Capabilities

This research highlights the critical dependence of quantum coherence on precise surface engineering and starting material purity. 6CCVD specializes in providing the foundational materials and advanced processing required to replicate and scale this cutting-edge quantum research from nanodiamonds to bulk-like systems.

Applicable Materials

To achieve and surpass the $T_1$ coherence times demonstrated, high-purity, defect-controlled diamond material is essential.

Material Recommendation	Material Grade	Application Context	6CCVD Advantage
High Purity Single Crystal Diamond (SCD)	Electronic/Optical Grade	Ideal for growing NV-active layers, particularly for deep NV centers or studies requiring minimized background noise and maximum coherence. Achieves ‘bulk-like’ properties required for long $T_1$.	SCD up to 500 µm thickness, Ra < 1nm polishing ensures optimal surface for subsequent functionalization chemistry.
Polycrystalline Diamond (PCD)	High-Density, Optical	Suitable for large-area sensor arrays and integration into macro-scale devices, offering robust platforms for scaled magnetometry.	Wafers up to 125mm diameter, Polishing Ra < 5nm for inch-size plates.
Metalized SCD/PCD	Custom Metalization	If the borane reduction or subsequent functionalization required electrochemical initiation or integration into microwave circuitry (for $T_2$ measurements, not $T_1$), metal contacts are necessary.	In-house capability for Au, Pt, Pd, Ti, W, Cu metal stacks ensures compatibility with high-purity diamond.

Customization Potential

The experimental workflow (irradiation, high-temperature annealing, surface modification) requires substrates that can withstand extreme processing conditions and geometric precision for device integration.

Precision Geometry: While this paper focused on ND powder, scaling this technology to on-chip solid-state quantum devices requires patterned structures. 6CCVD offers custom laser cutting and shaping services to create high-precision geometries, slots, or cavities in SCD/PCD plates required for optimizing microwave delivery or optical collection efficiency.
Surface Preparation for Chemistry: Achieving the desired reduction outcome relies heavily on the starting surface quality. 6CCVD provides superior polishing (Ra < 1 nm for SCD), ensuring the surface is chemically homogeneous and ready for controlled thermal oxidation or chemical functionalization steps like the borane reduction described.
Chemical Adaptability: 6CCVD provides substrates with specific as-grown surface terminations (e.g., hydrogen-terminated or oxygen-terminated) upon request, allowing researchers to bypass certain preliminary steps and jump directly to specialized functionalization protocols.

Engineering Support

6CCVD’s in-house PhD team provides unparalleled expertise in synthesizing and tailoring MPCVD diamond for quantum applications.

NV Engineering Consultation: Our experts can assist research groups in optimizing NV center preparation protocols (high-energy irradiation/annealing sequences) in large SCD wafers to achieve maximum density and coherence characteristics that approach the ‘bulk-like’ standard sought by this research.
Material Selection for Quantum Projects: If your goal is high magnetic sensitivity for nanoscale magnetometry (as referenced in the paper), our team can advise on the optimal diamond thickness, nitrogen concentration, and surface finish to maximize $T_1$ and $T_2$ performance.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. Global shipping (DDU default, DDP available) ensures prompt delivery worldwide.

View Original Abstract

Nanoscale quantum probes such as the nitrogen-vacancy (NV) center in diamonds have demonstrated remarkable sensing capabilities over the past decade as control over fabrication and manipulation of these systems has evolved. The biocompatibility and rich surface chemistry of diamonds has added to the utility of these probes but, as the size of these nanoscale systems is reduced, the surface chemistry of diamond begins to impact the quantum properties of the NV center. In this work, we systematically study the effect of the diamond surface chemistry on the quantum coherence of the NV center in nanodiamonds (NDs) 50 nm in size. Our results show that a borane-reduced diamond surface can on average double the spin relaxation time of individual NV centers in nanodiamonds when compared to thermally oxidized surfaces. Using a combination of infrared and X-ray absorption spectroscopy techniques, we correlate the changes in quantum relaxation rates with the conversion of sp<sup>2</sup> carbon to C-O and C-H bonds on the diamond surface. These findings implicate double-bonded carbon species as a dominant source of spin noise for near surface NV centers. The link between the surface chemistry and quantum coherence indicates that through tailored engineering of the surface, the quantum properties and magnetic sensitivity of these nanoscale systems may approach that observed in bulk diamond.

Tech Support

Original Source

DOI: https://doi.org/10.1021/acsami.7b19238