NMR technique for determining the depth of shallow nitrogen-vacancy centers in diamond

At a Glance

Metadata	Details
Publication Date	2016-01-25
Journal	Physical review. B./Physical review. B
Authors	Linh Pham, Stephen J. DeVience, Francesco Casola, Igor Lovchinsky, Alexander O. Sushkov
Institutions	Harvard University, Massachusetts Institute of Technology
Citations	147
Analysis	Full AI Review Included

6CCVD Technical Documentation: NV Center Depth Determination via Nanoscale NMR

Source Analysis: Pham, L. M., et al. (2015). NMR Technique for Determining the Depth of Shallow Nitrogen-Vacancy Centers in Diamond. arXiv:1508.04191v1 [quant-ph].

Executive Summary

This paper validates a robust, non-destructive Nuclear Magnetic Resonance (NMR) technique, utilizing the Nitrogen-Vacancy (NV) center in diamond as a nanoscale magnetic sensor, to determine the depth of shallow individual NV centers with ultra-high precision. This methodology is critical for advancing quantum sensing, imaging, and computing applications.

Core Achievement: Demonstrated NV depth determination with an industry-leading uncertainty of approximately 1 nm.
Methodology: Utilized a scanning confocal microscope combined with an XY8k dynamical decoupling pulse sequence to measure the NV spin coherence perturbation caused by statistically-polarized proton spins in standard immersion oil placed on the diamond surface.
Depth Range Verified: Successfully measured NV depths ranging from 4.6 nm up to 15.3 nm.
Material Input: Experiments relied on high-quality Single Crystal Diamond (SCD) substrates prepared using low-energy ion implantation (e.g., 3.0-keV ¹⁵N ions) followed by thermal annealing.
Implication for Engineering: Provides a crucial validation tool, confirming that SRIM simulations can underestimate true NV depth by a factor up to two, underscoring the necessity of in-situ experimental depth profiling for critical quantum applications.
6CCVD Relevance: Requires ultra-high purity, highly polished SCD substrates—a core specialization of 6CCVD’s MPCVD manufacturing capabilities.

Technical Specifications

Extracted quantitative data points from the research detailing experimental inputs and achieved results.

Parameter	Value	Unit	Context
NV Depth Uncertainty	~1	nm	Precision of depth determination method
Mean NV Depth (Sample A)	10.5 ± 2.8	nm	Implantation: 3.0-keV ¹⁵N ions
Mean NV Depth (Sample C)	8.5 ± 2.8	nm	Implantation: 2.5-keV ¹⁴N ions
Shallowest Measured NV Depth	4.6	nm	NV Center C111 (Table I)
Deepest Measured NV Depth	15.3	nm	NV Center A008 (Table I)
Static Magnetic Field (B₀)	150 - 1609	G	Range used for experimental verification
T_2n* (Nuclear Dephasing Time)	~60	µs	Expected value for protons in immersion oil
Implantation Density	~8 × 10⁷	cm^-2	NV density targeted for single-center isolation
NV Electronic Spin Coherence (T₂)	≥ 100	µs	Typical coherence time required for sensing
Optical Pumping Source	532	nm	Laser wavelength used for initialization

Key Methodologies

The following ordered list summarizes the crucial material preparation steps and the NMR detection recipe employed to achieve single NV depth determination.

Substrate Preparation: High-purity diamond substrates were subjected to low-energy, low-dosage ion implantation (e.g., 3.0-keV ¹⁵N or 2.5-keV ¹⁴N) to introduce nitrogen ions near the surface.
NV Center Formation: Subsequent high-temperature annealing was performed to mobilize vacancies, allowing them to pair with implanted nitrogen atoms to form the negatively-charged NV centers.
Confocal Interrogation: Individual, isolated NV centers were addressed using a custom scanning confocal microscope setup under ambient conditions.
Spin Bath Application: Standard microscope immersion oil (rich in protons) was applied directly to the diamond surface to serve as the statistically-polarized nuclear spin sample.
Dynamic Decoupling Sequence: An XY8k dynamical decoupling pulse sequence (N MW π-pulses) was applied to the NV electronic spin to selectively couple to specific Fourier components of the fluctuating magnetic field produced by the nuclear spin bath.
Phase Measurement: The accumulated NV spin phase variance (Δφ²(τ)) was measured as a function of the free evolution time (τ). Maximum contrast dip was observed when τ was approximately half the proton Larmor period (τ ≈ π/ω_L).
Depth Extraction: The shape and amplitude of the NMR-induced contrast dip were fitted to a complex theoretical model relating the NV spin interaction with the surface proton bath. The variance B²_RMS, which scales inversely with d³_NV, directly yielded the individual NV center depth (d_NV).

6CCVD Solutions & Capabilities

The research demonstrates the essential link between ultra-precise material engineering and advanced quantum measurement techniques. Replicating and extending this high-precision NV depth determination requires starting materials with stringent quality controls that 6CCVD is uniquely positioned to supply.

Applicable Materials

To achieve the isolated, shallow NV centers required for single-center NMR sensing, researchers require substrates with extremely low native nitrogen content and superior surface quality.

6CCVD Material Solution	Relevance to NMR Sensing	Customization Capabilities
Optical Grade SCD (Ultra-Low N)	Essential for ensuring that implanted NVs are isolated for single-center spectroscopy (low background noise).	Wafers up to 125 mm diameter, custom shapes via laser cutting.
High Coherence SCD	Optimized MPCVD growth for long electronic T₂ times (T₂ ≥ 100 µs), maximizing the sensitivity and duration of the NMR measurement sequence.	Thickness control from 0.1 µm up to 500 µm.
Highly Polished Substrates	Critical requirement for shallow NV creation (4 nm to 15 nm depths). 6CCVD guarantees surface roughness Ra < 1 nm (SCD), minimizing surface charge noise and ensuring stable NV properties.	Polishing services available for SCD and inch-size PCD (Ra < 5 nm).

Customization Potential

The experimental success hinges on controlling the material environment and device geometry. 6CCVD offers specialized post-processing services to support advanced NV center fabrication:

Precision Diamond Substrates: 6CCVD supplies SCD materials optimized for subsequent ion implantation, ensuring low strain and minimal defect density prior to processing.
Custom Dimensions: While the paper does not specify dimensions, 6CCVD routinely provides custom-sized plates and wafers, crucial for fitting into specialized confocal or cryo-NMR setups.
Metalization Services: Although not explicitly used in this specific measurement setup, future applications requiring on-chip microwave delivery or local static field generation (e.g., integrated micro-striplines) would necessitate thin-film coatings. 6CCVD offers in-house metalization with common stacks including Au, Pt, Pd, Ti, W, and Cu.

Engineering Support

NV depth determination is fundamental to applications in nanoscale Magnetic Resonance Imaging (MRI) and quantum gyroscopes. The stability of shallow NVs (as discussed on Page 5) is a major challenge.

6CCVD’s in-house PhD-level material scientists and technical engineers are available to consult on projects requiring:

Optimizing substrate selection for specific ion implantation recipes (e.g., controlling ¹⁴N vs. ¹⁵N purity).
Determining the necessary surface preparation (polishing, cleaning) to stabilize very shallow NV centers (< 5 nm depth) for robust sensitive spin measurements.
Integrating diamond substrates into complex experimental setups, providing material specifications for thermal, mechanical, and optical interfacing.

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

View Original Abstract

We demonstrate a robust experimental method for determining the depth of\nindividual shallow Nitrogen-Vacancy (NV) centers in diamond with $\sim1$ nm\nuncertainty. We use a confocal microscope to observe single NV centers and\ndetect the proton nuclear magnetic resonance (NMR) signal produced by objective\nimmersion oil, which has well understood nuclear spin properties, on the\ndiamond surface. We determine the NV center depth by analyzing the NV NMR data\nusing a model that describes the interaction of a single NV center with the\nstatistically-polarized proton spin bath. We repeat this procedure for a large\nnumber of individual, shallow NV centers and compare the resulting NV depths to\nthe mean value expected from simulations of the ion implantation process used\nto create the NV centers, with reasonable agreement.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevb.93.045425