Nanoscale solid-state nuclear quadrupole resonance spectroscopy using depth-optimized nitrogen-vacancy ensembles in diamond

At a Glance

Metadata	Details
Publication Date	2022-04-25
Journal	Applied Physics Letters
Authors	Jacob Henshaw, Pauli Kehayias, Maziar Saleh Ziabari, Michael Titze, Erin Morissette
Institutions	Sandia National Laboratories, Brown University
Citations	22
Analysis	Full AI Review Included

Technical Documentation & Analysis: Depth-Optimized NV Ensembles for Nanoscale QRS Sensing

Executive Summary

This research paper successfully identifies the optimal depth for shallow Nitrogen-Vacancy (NV) ensembles in single-crystal diamond (SCD) to achieve maximal sensitivity for Nuclear Quadrupole Resonance Spectroscopy (NQR) and Nuclear Magnetic Resonance (NMR) of statistically-polarized spins on the surface.

Key findings and commercial value proposition:

Optimal Depth Determined: Minimal measurement time (t(SNR=3)) was achieved at an NV ensemble depth of 5.4 nm, corresponding to a 2 keV ¹⁵N ion implantation energy. This precise depth calibration is crucial for high-sensitivity nanoscale magnetometry.
Enhanced Performance: The optimized 5.4 nm NV ensemble demonstrated superior performance compared to previous single-NV detection methods, achieving shorter measurement times for sensing 2D materials like hexagonal boron nitride (hBN).
Material Foundation: The work relies on high-quality, electronic-grade single-crystal diamond (SCD) with controlled nitrogen concentration (100 ppm peak) and specific [100] orientation, a core product offering of 6CCVD.
Advanced Processing Requirements: Achieving optimal results necessitates precise ion implantation, ultra-high vacuum annealing (up to 1100 °C), and critical oxygen surface termination—services and expertise 6CCVD supports through material specification and consultation.
Target Application: Validates NV ensembles as leading solid-state quantum sensors for characterizing magnetic phenomena and spin dynamics in next-generation 2D materials (e.g., hBN, $\text{CrBr}_{3}$), directly supporting quantum technology development.

Technical Specifications

Parameter	Value	Unit	Context
Optimal NV Ensemble Depth	5.4	nm	Minimum t(SNR=3) for ${}^{19}\text{F}$ sensing (Figure of Merit)
Implantation Energy (Optimal)	2	keV	Used to achieve the 5.4 nm NV depth
Implantation Species	${}^{15}\text{N}$	N/A	Used to create the NV centers
Peak Nitrogen Concentration	100	ppm	Target concentration achieved via fluence adjustment
Minimal Measurement Time, t(SNR=3)	$\approx 2 \times 10^{-4}$	s	Achieved at the 5.4 nm NV depth (Figure 3f)
Excitation Diameter (Ensemble)	40	µm	Diameter of 532 nm laser beam used for NV excitation
NV Center Zero Field Splitting (Delta)	2.87	GHz	Intrinsic property of the NV system
NV Coherence Time (T₂)	$30 - 50$	µs	Measured for XY8-256 sequence (depth dependent)
Diamond Crystal Orientation	[100]	N/A	Surface orientation of the SCD substrates used
Annealing Temperature (Maximum)	1100	°C	Used for high-vacuum activation of NV centers
hBN Flake Thickness (Target Material)	$\approx 100$	nm	Used for ${}^{11}\text{B}$ NQR spectroscopy demonstration

Key Methodologies

The study involved specialized material preparation and advanced dynamic decoupling sequences for quantum sensing.

Diamond Preparation (SCD Substrates): Electronic-grade, natural abundance SCD plates ($2 \times 2 \times 0.5 \text{ mm}^3$, [100] orientation) were sourced (Element Six).
Nitrogen Implantation: A series of seven diamonds were implanted with ${}^{15}\text{N}$ ions at different energies (1 keV to 7 keV) at an $8^{\circ}$ tilt. Fluences (ranging from $5 \times 10^{12}$ to $2 \times 10^{13} \text{ ions}/\text{cm}^{2}$) were carefully selected via SRIM simulations to maintain a consistent peak nitrogen concentration of 100 ppm.
NV Activation Annealing: Samples underwent ultra-high vacuum (UHV, < $1 \times 10^{-8}$ Torr) annealing, ramping in stages up to $1100^{\circ}\text{C}$ for 2 hours, followed by a cooldown period of 12 hours.
Acid and Surface Treatment: Post-annealing, samples were rigorously cleaned in a mixture of sulfuric, nitric, and perchloric acids ($250^{\circ}\text{C}$), followed by UV/Ozone treatment and a final oxygen surface termination anneal ($450^{\circ}\text{C}$ in oxygen atmosphere) to stabilize the NV charge state.
Depth Characterization via NMR: Fomblin oil (containing ${}^{19}\text{F}$ spins) was applied to the surface. XY8-N dynamic decoupling sequences were performed to measure the magnetic field variance ($\text{B}_{\text{RMS}}$) and fluorescence contrast, allowing the precise determination of the effective NV ensemble depth (d).
NQR Sensing Demonstration: The optimized 5.4 nm NV diamond was used with exfoliated 100 nm thick hBN flakes. XY8-256 noise spectroscopy detected the ${}^{11}\text{B}$ NQR spectrum at a bias field $\text{B}_{0} \approx 2.95 \text{ mT}$.

6CCVD Solutions & Capabilities

This research highlights the paramount need for ultra-high-quality, highly controlled diamond materials for next-generation quantum sensing applications. 6CCVD is uniquely positioned to supply and engineer the necessary foundation materials.

Applicable Materials

To replicate or extend this depth optimization research and subsequent nanoscale sensing, researchers require:

Electronic-Grade Single Crystal Diamond (SCD): Required for maximizing NV coherence time ($\text{T}{2}$) and achieving low background decoherence ($\text{T}{2,\text{bg}}$). 6CCVD specializes in high-purity MPCVD SCD wafers and plates.
Low Nitrogen (Low $\text{N}_{\text{s}}$) SCD Substrates: Essential for controlled NV formation via external implantation, as used in this paper. We supply substrates with inherent low nitrogen content ($< 1 \text{ ppb}$).
Custom Orientation Plates: The paper uses [100] orientation. 6CCVD routinely supplies SCD plates in custom orientations (e.g., [100], [111]) up to 500 µm thickness, ensuring optimal alignment for $\text{NV}$ axes relative to surface electric fields and magnetic gradients.

Customization Potential

The experimental success hinges on precise material dimensions and post-processing steps that 6CCVD supports:

Requirement from Paper	6CCVD Capability	Engineering Advantage
$2 \times 2 \times 0.5 \text{ mm}^{3}$ Plates	Custom Dimensions: Plates/wafers up to 125 mm. Substrates up to 10 mm thick.	Provides flexibility for miniaturized experimental setups or scaling up to larger arrays.
High Precision Polishing	Ultra-Low Roughness SCD: Ra < 1 nm (SCD).	Crucial for nanoscale proximity sensing and minimizing surface spin noise which degrades T₂ and sensitivity.
Precise Doping/Concentration	Custom Doping: We offer in-situ $\text{N}$ doping (SCD) or $\text{B}$ doping (BDD) via MPCVD, or consultative support for specific external ion implantation requirements (like ¹⁵N 2 keV implantation).	Ensures researchers hit the optimal $\text{N}$ density (0.1-1 ppm bulk, 5 ppm surface) to minimize $\text{T}_{2}$ degradation.
Metalization for MW Control	Custom Metalization: $\text{Au}, \text{Pt}, \text{Pd}, \text{Ti}, \text{W}, \text{Cu}$ internal capability.	Enables integrated fabrication of on-chip copper loops or waveguides (as used in the paper for microwave spin control) directly onto the diamond substrate.

Engineering Support

This study demonstrates that the true figure of merit for nanoscale magnetometry depends critically on the interplay between NV depth, preparation methods (annealing, surface termination), and the resulting coherence time ($\text{T}_{2}$).

6CCVD’s in-house PhD team provides consultative support for similar Nanoscale Quantum Sensing projects. We assist clients in designing material specifications that:

Optimize SCD substrate properties ($\text{T}_{2,\text{bg}}$) based on the target NV density.
Specify the required polish (Ra) and surface termination (e.g., oxygen, as used here) to stabilize the NV charge state and minimize surface noise.
Ensure compatibility between substrate dimension, orientation, and desired on-chip integration (e.g., microwave circuits, metal contacts).

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

View Original Abstract

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spectroscopy of bulk quantum materials have provided insight into phenomena, such as quantum phase criticality, magnetism, and superconductivity. With the emergence of nanoscale 2D materials with magnetic phenomena, inductively detected NMR and NQR spectroscopy are not sensitive enough to detect the smaller number of spins in nanomaterials. The nitrogen-vacancy (NV) center in diamond has shown promise in bringing the analytic power of NMR and NQR spectroscopy to the nanoscale. However, due to depth-dependent formation efficiency of the defect centers, noise from surface spins, band bending effects, and the depth dependence of the nuclear magnetic field, there is ambiguity regarding the ideal NV depth for surface NMR of statistically polarized spins. In this work, we prepared a range of shallow NV ensemble layer depths and determined the ideal NV depth by performing NMR spectroscopy on statistically polarized 19F in Fomblin oil on the diamond surface. We found that the measurement time needed to achieve a signal-to-noise ratio of 3 using XY8-N noise spectroscopy has a minimum at an NV ensemble depth of 5.5 ± 1.5 nm for ensembles activated from 100 ppm nitrogen concentration. To demonstrate the sensing capabilities of NV ensembles, we perform NQR spectroscopy on the 11B of hexagonal boron nitride flakes. We compare our best diamond to previous work with a single NV and find that this ensemble provides a shorter measurement time with excitation diameters as small as 4 μm. This analysis provides ideal conditions for further experiments involving NMR/NQR spectroscopy of 2D materials with magnetic properties.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0083774

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