Skip to content
6CCVD Research

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

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2022-04-25
JournalApplied Physics Letters
AuthorsJacob Henshaw, Pauli Kehayias, Maziar Saleh Ziabari, Michael Titze, Erin Morissette
InstitutionsBrown University, Center for Integrated Nanotechnologies
Citations22
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This research focuses on optimizing Nitrogen-Vacancy (NV) ensemble depth in diamond to maximize sensitivity for nanoscale Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) spectroscopy, particularly for 2D materials.

  • Optimal Depth Determination: The ideal effective NV ensemble depth for sensing statistically-polarized surface spins was experimentally determined to be 5.4 nm.
  • Figure of Merit: This 5.4 nm depth (achieved via 2 keV 15N implantation) provided the minimum measurement time (t(SNR=3)) required to achieve a signal-to-noise ratio of 3 for 19F NMR in Fomblin oil.
  • Implantation Parameters: The optimal NV layer was created using 2 keV 15N ions at a fluence of 7.5 x 1012 ions/cm2, targeting a 100 ppm peak nitrogen concentration.
  • Sensing Demonstration: The optimized ensembles were successfully used to perform NQR spectroscopy on 11B nuclei within hexagonal boron nitride (hBN) flakes, establishing a solid-state NQR standard.
  • Performance Advantage: The optimized NV ensemble significantly outperforms previous single-NV NQR detection methods when the excitation diameter (D) is greater than 4 ”m (for bulk hBN) or 5 ”m (for monolayer hBN), due to the increased photon count rate (IPL).
  • Methodology: NV depth was calibrated by fitting the fluorescence contrast decay observed during XY8-N dynamic decoupling measurements of 19F Larmor precession.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Optimal 15N Implant Energy2keVYields minimum t(SNR=3)
Optimal Effective NV Depth (d)5.4nmMeasured using 19F NMR fitting
Optimal Implant Fluence (2 keV)7.5 x 1012ions/cm2Target 100 ppm N concentration
Target Nitrogen Concentration100ppmPeak concentration via SRIM simulation
Diamond MaterialElectronic-grade, [100]OrientationNatural 1.1% 13C abundance
NV Zero Field Splitting (Δ)2.87GHzIntrinsic spin-1 system splitting
NV Coherence Time (T2)~40”sFor 5.4 nm ensemble (XY8-256 measurement)
Minimum Measurement Time (t(SNR=3))~10-5sFigure of merit for 19F sensing
Laser Excitation Wavelength532nmNV initialization and readout
Laser Power (CW)285mWUsed for NV illumination
Excitation Area Diameter40”mIllumination spot size
19F Larmor Frequency (Îłn)2π x 40.08MHz/TUsed for depth calibration
11B NQR Frequency (vQ)1.4599 ± 0.0004MHzMeasured in 100 nm thick hBN flake
Bias Magnetic Field (B0)~32mTUsed for 19F NMR measurements

Key Methodologies

Section titled “Key Methodologies”

The NV ensembles were prepared and characterized using a multi-step process involving ion implantation, high-temperature annealing, and advanced quantum sensing sequences.

Diamond Preparation and Activation

Section titled “Diamond Preparation and Activation”
  1. Material: 2x2x0.5 mm3 electronic-grade single-crystal diamonds with [100] surface orientation.
  2. Implantation: 15N ions implanted at 8° tilt, with energies ranging from 1 keV to 7 keV. Fluences were adjusted (5x1012 to 2x1013 ions/cm2) to maintain a peak nitrogen concentration of 100 ppm.
  3. Annealing (NV Activation): Samples underwent ultra-high vacuum (< 1 x 10-8 Torr) annealing:
    • Ramp to 400 °C (2 hours), pause (2 hours).
    • Ramp to 550 °C (2 hours), pause (2 hours).
    • Ramp to 800 °C (2 hours), pause (4 hours).
    • Ramp to 1100 °C (2 hours), pause (2 hours), followed by 12-hour cooldown.
  4. Cleaning and Surface Termination:
    • Acid cleaning: 1:1:1 mixture of sulfuric, nitric, and perchloric acids at 250 °C (1 hour).
    • UV/Ozone treatment: 90 minutes.
    • Oxygen termination: Anneal in oxygen atmosphere at 450 °C (4 hours).
    • Final clean: Piranha solution (3:1 H2SO4/H2O2) heated to 100 °C.

NV Sensing and Depth Calibration

Section titled “NV Sensing and Depth Calibration”
  1. Optical Setup: Room-temperature epifluorescence microscope (NA = 0.8, 100x objective). 532 nm laser excitation (40 ”m spot). NV fluorescence (> 650 nm) detected by an Avalanche Photodiode (APD).
  2. Spin Control: Microwave (mw) field driven by a copper loop, aligned with the NV axis using permanent magnets (B0 ≈ 32 mT).
  3. NMR/NQR Sequence: XY8-N dynamic decoupling sequence used for noise spectroscopy. This sequence involves initialization, a superposition state, 8k refocusing pulses, and final readout.
  4. Depth Determination: Effective NV depth (d) was determined by applying Fomblin oil (source of statistically-polarized 19F spins) to the diamond surface. The NV fluorescence contrast was fitted using the magnetic field variance (BRMS) model, treating d as a free parameter.
  5. NQR Measurement: 11B NQR was performed on 100 nm thick hBN flakes exfoliated onto the diamond surface, using XY8-256 noise spectroscopy near the anticipated NQR frequency (1.461 MHz).

Commercial Applications

Section titled “Commercial Applications”

This depth-optimized NV ensemble technology provides a critical tool for characterizing materials at the nanoscale, enabling advances in quantum technology and fundamental condensed matter physics.

  • Quantum Computing and Information Processing:
    • Characterization and quality control of solid-state qubits and quantum memories, particularly those relying on surface proximity.
    • Sensing and mapping of noise sources (e.g., surface spins, charge noise) that limit qubit coherence.
  • Nanoscale Materials Science and 2D Materials:
    • Directly probing magnetic ordering, phase transitions, and spin dynamics in novel 2D materials (e.g., hBN, magnetic insulators) using NQR/NMR, overcoming the sensitivity limitations of inductive methods.
    • Imaging and analysis of magnetic domains and domain wall pinning in atomically thin films.
  • High-Resolution Spectroscopy:
    • Performing NMR/NQR spectroscopy on extremely small sample volumes (nm3 to ”m3), relevant for analyzing scarce biological samples or complex synthesized nanomaterials.
  • Semiconductor and Device Physics:
    • Using shallow NV ensembles as in situ sensors to map critical electrical properties, such as band bending and charge dynamics, in operating semiconductor devices.
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 Όm. This analysis provides ideal conditions for further experiments involving NMR/NQR spectroscopy of 2D materials with magnetic properties.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2007 - Magnetic resonance and superconductivity: Some history, ancient and in the making [Crossref]
  2. 2019 - Near room temperature antiferromagnetic ordering with a potential low-dimensional magnetism in AlMn2B2 [Crossref]
  3. 1996 - NMR study of the ferromagnetic phases of SmMn2Ge2 as a function of temperature and pressure [Crossref]
  4. 2015 - Evolution of hyperfine parameters across a quantum critical point in CeRhIn5 [Crossref]
  5. 2020 - Emerging intrinsic magnetism in two-dimensional materials: Theory and applications [Crossref]
  6. 2018 - Magnetism in two-dimensional van der Waals materials [Crossref]
  7. **** - Recent advances in two-dimensional magnets: Physics and devices towards spintronic applications [Crossref]
  8. 2019 - Magnetism in two-dimensional materials beyond graphene [Crossref]
  9. 1999 - High-resolution NMR spectroscopy of sample volumes from 1 nL to 10 ÎŒL [Crossref]
  10. 2018 - Probing condensed matter physics with magnetometry based on nitrogen-vacancy centres in diamond [Crossref]

Reads Similar to This

A Three-Dimensional Finite Element Analysis Model of SAW Torque Sensor with Multilayer Structure Enhanced Optical 13 C Hyperpolarization in Diamond Treated by High‐Temperature Rapid Thermal Annealing Moving arc electrical discharge machining of polycrystalline diamond