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6CCVD Research

Ultrahigh nitrogen-vacancy center concentration in diamond

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2021-12-07
JournalCarbon
AuthorsSĂĄndor Kollarics, Ferenc Simon, AndrĂĄs Bojtor, KristĂłf Koltai, G. Klujber
InstitutionsUniversity of Southern California, University of California, Los Angeles
Citations19
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This research details the successful, stepwise synthesis of ultrahigh concentrations of negatively charged nitrogen-vacancy (NV-) centers in HPHT diamond using combined electron and neutron irradiation followed by thermal annealing.

  • High Concentration Achieved: A maximum NV- concentration of 15 ppm was attained, derived from starting material containing approximately 100 ppm substitutional nitrogen.
  • High Conversion Efficiency: The conversion efficiency from isolated substitutional nitrogen (P1 centers) to NV- centers reached 17.5%.
  • Process Quantification: Continuous-wave Electron Paramagnetic Resonance (EPR) was established as the ideal quantitative tool, showing that approximately 25% of the irradiation-induced vacancies successfully form NV- centers during annealing.
  • Lattice Integrity: The stepwise annealing process proved effective, restoring the integrity of the diamond lattice by eliminating the EPR signal associated with charged vacancies (V-).
  • Anisotropic Relaxation: Pulsed EPR measurements revealed a clear orientation dependence for both spin-lattice (T1) and spin-spin (T2) relaxation times, crucial for sensor optimization.
  • Precise Spectroscopy: The first X-band Electron-Nuclear Double Resonance (ENDOR) results on high-concentration NV- ensembles were presented, allowing for high-precision determination of 14N hyperfine and quadrupole coupling constants.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Highest NV- Concentration15ppmAchieved from 70 ppm N starting material.
N to NV- Conversion Efficiency17.5%Maximum yield determined by CW EPR.
Vacancy to NV- Conversion Yield~25%Efficiency during thermal annealing step.
Starting Nitrogen Concentrationless than 200ppmType 1b HPHT single crystal diamond.
Electron Irradiation Energy Range1 to 4MeVVariable-energy RF linear accelerator (LINAC).
Electron Irradiation Fluence (Net)up to 2.8 · 10181/cm2Total fluence applied.
Neutron Irradiation Fluence (Total)10171/cm2Low dose region (100 eV - 1 MeV range).
Annealing Temperature (Standard)800°C2 hours, dynamic vacuum (10-6 mbar).
Annealing Temperature (High)1000°C2 hours (used for electron irradiated samples).
T1 Relaxation Time (Parallel NV axis)4.73msMagnetic field along the NV axis.
T2 Relaxation Time (Parallel NV axis)1.88”sHahn-echo decay measurement.
T2 Relaxation Time (109.47° NV axis)1.21”sShorter due to perpendicular fluctuating fields.
14N Quadrupole Coupling (P)-4.8MHzDetermined via X-band ENDOR.
14N Hyperfine Coupling (A)-2.2MHzDetermined via X-band ENDOR.
W16 Center Zero-Field Splitting (D)0.86 D(NV)N/AS=1 triplet spin state, tentatively N(+)NV(-) complex.

Key Methodologies

Section titled “Key Methodologies”

The NV- centers were produced using a systematic, stepwise process involving irradiation and subsequent thermal annealing, monitored quantitatively by EPR.

  1. Starting Material Selection: Single crystal, type 1b HPHT diamond containing less than 200 ppm substitutional nitrogen (P1 centers) was used.
  2. Neutron Irradiation: Three samples were exposed to neutron irradiation (1017 1/cm2 fluence) in a training reactor.
  3. Electron Irradiation: Six samples were exposed to electron irradiation (1-4 MeV, up to 2.8 · 1018 1/cm2 fluence) using a custom variable-energy RF linear accelerator (LINAC).
    • The LINAC uses a fixed 2.5 MeV structure plus a variable phase/amplitude second structure to achieve the energy range.
  4. Thermal Annealing (Stepwise): Irradiated samples were annealed under dynamic vacuum (10-6 mbar).
    • Neutron irradiated samples: 800 °C for 8 hours.
    • Electron irradiated samples: 800 °C for 2 hours, followed by 1000 °C for 2 hours.
  5. Quantitative Characterization (CW EPR): CW EPR was used after each irradiation/annealing step to determine the relative concentration of NV- centers compared to the P1 centers (substitutional nitrogen).
    • The spin susceptibility difference between P1 (S=1/2) and NV- (S=1) was accounted for by normalizing the NV- integral intensity by a factor of 8/3.
  6. Defect Analysis: EPR confirmed the presence of charged vacancies (S=1/2, g=2 signal) exclusively in electron-irradiated samples, which disappeared upon annealing as they converted to NV- centers.
  7. Coherence Measurement (Pulsed EPR): Hahn-echo detected inversion recovery (for T1) and Hahn-echo decay (for T2) were used to measure relaxation times, focusing on orientation dependence.
  8. Hyperfine Analysis (ENDOR): Pulsed ENDOR measurements (X-band, 9.5 GHz) using the Mims pulse sequence were performed to precisely determine the 14N hyperfine and quadrupole coupling constants.

Commercial Applications

Section titled “Commercial Applications”

The production of high-density, well-characterized NV- ensembles is critical for advancing diamond-based quantum technologies, particularly in sensing and computing.

  • Quantum Sensing and Metrology:

    • Enhanced Sensitivity: Using ensembles improves magnetic field sensitivity by a factor of √N (where N is the number of NV- centers), enabling detection limits in the fT/√Hz range.
    • Nanoscale Thermometry/Electrometry: NV-based thermometers are appealing for life-science applications due to diamond’s biocompatibility and nanometer-scale resolution.
    • Vector Field Sensing: Precise ENDOR data allows for the determination of hyperfine and quadrupole coupling constants, which are necessary for sensing the vector components of magnetic fields.

  • Solid-State Quantum Computing:

    • Qubit Realization: NV- centers serve as solid-state spin quantum bits (qubits).
    • Coherence Optimization: Accurate measurement of T1 (spin-lattice) and T2 (spin-decoherence) relaxation times is crucial, as these parameters define the longest timescale over which quantum information can be stored coherently.
    • Nuclear Spin Control: The combination of the electron spin with nearby nuclei (characterized by ENDOR) is a prerequisite for nucleus-based quantum computing architectures.

  • Microwave Sources:

    • High-density NV ensembles can be used to realize continuous-wave room-temperature diamond masers.

  • Material Characterization:

    • EPR-based quantitative characterization is superior to optical methods (fluorescence/absorption) for high-density NV samples, providing a reliable technique for quality control in commercial diamond production.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 1956 - The absorption spectra of irradiated diamonds after heat treatment [Crossref]
  2. 1978 - Electron spin resonance in the study of diamond [Crossref]
  3. 1997 - Scanning confocal optical microscopy and magnetic resonance on single defect centers [Crossref]
  4. 2000 - Stable solid-state source of single photons [Crossref]
  5. 2004 - Observation of coherent oscillation of a single nuclear spin and realization of a two-qubit conditional quantum gate [Crossref]
  6. 2008 - Nanoscale magnetic sensing with an individual electronic spin in diamond [Crossref]
  7. 2017 - High-sensitivity spin-based electrometry with an ensemble of nitrogen-vacancy centers in diamond [Crossref]
  8. 2013 - High-precision nanoscale temperature sensing using single defects in diamond [Crossref]
  9. 2013 - Nanometre-scale thermometry in a living cell [Crossref]
  10. 2019 - Nanodiamonds: emerging face of future nanotechnology [Crossref]

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