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6CCVD Research

Anomalous Formation of Irradiation-Induced Nitrogen-Vacancy Centers in 5 nm-Sized Detonation Nanodiamonds

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2022-03-14
JournalThe Journal of Physical Chemistry C
AuthorsFrederick T.-K. So, Alexander I. Shames, Daiki Terada, Takuya Genjo, Hiroki Morishita
InstitutionsETH Zurich, Kyoto University
Citations14
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”
  • Anomalous NV- Formation: 5 nm Detonation Nanodiamonds (DNDs) exhibit unique Nitrogen-Vacancy (NV-) center formation during electron irradiation (max 100 °C) without requiring the conventional 800 °C high-temperature annealing (HTA).
  • “Self-Annealing” Mechanism: This effect is termed “self-annealing,” driven by a combination of small particle size (low volumetric heat capacity and low thermal conductivity) and extremely high substitutional nitrogen concentration [Ns].
  • High Nitrogen Compensation: Monte Carlo simulations confirm that the DNDs’ high [Ns] (850-1100 ppm) compensates for the high vacancy loss rate inherent to small particles (due to large surface-to-volume ratio).
  • Superior Yield for Small Sizes: The NV- concentration in 5 nm DNDs (1.3 ppm after irradiation) surpasses that of 10 nm and 20 nm HPHT nanodiamonds, breaking the expected size-dependent trend for HPHT NDs.
  • Enrichment and Saturation: DNDs showed a 12.5-fold increase in NV- concentration upon irradiation (up to 1.5×1019 e-/cm2) with no sign of saturation, confirming a deficiency of vacancies relative to nitrogen impurities in the DND core.
  • Reliable Quantification: Continuous-wave Electron Paramagnetic Resonance (EPR) spectroscopy, specifically using the half-field (HF) transition (geff = 4.23), was established as the reliable method for quantifying NV- centers in nanodiamond powders down to 5 nm.
  • Crystal Integrity: TEM and EELS confirmed that the high-fluence electron irradiation did not cause significant structural damage or graphitization (sp2 carbon formation) in the DND lattice.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Nanodiamond Type/Size5nmDetonation Nanodiamonds (DNDs)
Comparison ND Sizes (HPHT)10, 20, 30, 50, 100nmHigh-Pressure High-Temperature (HPHT) NDs
Electron Irradiation Energy1 and 2MeVUsed to create vacancies
Maximum Electron Fluence1.5×1019e-/cm2Highest fluence tested on DNDs
Irradiation Temperature (Max)100°CTemperature during electron irradiation
Conventional Annealing Temp800°CStandard high-temperature annealing (HTA)
Pristine DND NV- Content0.11ppm (atomic ratio)NV- content before irradiation
Irradiated DND NV- Content (Max)1.3ppm (atomic ratio)After 1.5×1019 e-/cm2, without HTA
Irradiated/Annealed 100 nm HPHT NV- Content (Max)3.5ppm (atomic ratio)After 1.5×1019 e-/cm2 and 800 °C HTA
Substitutional Nitrogen [Ns] (DNDs)850-1100ppmEstimated P1 center concentration
Substitutional Nitrogen [Ns] (HPHT NDs)70-100ppmEstimated P1 center concentration
Vacancy Migration Activation Energy (V0)2.1 - 2.3eVDiffusion barrier for neutral vacancy (V0)
EPR Measurement Frequency9.87GHzX-band continuous-wave EPR
NV- HF EPR geff4.23DimensionlessEffective g-factor for half-field transition
Diamond Thermal Conductivity (Bulk)>2000W/(m¡K)Reference value
Diamond Thermal Conductivity (2.5 nm NDs)10-28W/(m¡K)Calculated value (Ref. 53)

Key Methodologies

Section titled “Key Methodologies”

The study employed a “bottom-up” approach, creating nano-sized particles first, followed by defect creation and characterization.

  1. Nanodiamond Sourcing and Pre-treatment:

    • DNDs: 5 nm detonation nanodiamonds (DNDs) were obtained as colloidal dispersions and freeze-dried.
    • HPHT NDs: Commercial HPHT NDs (10-100 nm) were sourced and freeze-dried.
    • Boiling Acid Treatment: All samples were treated at 130 °C for 3 days in a 1:3 nitric acid/sulfuric acid mixture to remove paramagnetic Fe3+ impurities that interfere with EPR signals.

  2. Electron Irradiation (Vacancy Creation):

    • Conditions: Dry ND powders were irradiated on a water-flow cooled copper plate, maintaining the temperature around 80 °C (maximum 100 °C).
    • Energy/Fluence: Irradiation was performed using 2 MeV and 1 MeV electrons across a fluence range up to 1.5×1019 e-/cm2.

  3. High-Temperature Annealing (HTA):

    • Standard HTA: Performed at 800 °C for 2 h under high vacuum (pressure < 10-6 mbar).
    • DND Temperature Scan: DNDs were also annealed at temperatures ranging from 400 °C to 800 °C (for 2 h) to verify the “self-annealing” hypothesis.

  4. NV- Quantification (EPR Spectroscopy):

    • Technique: Continuous-wave Electron Paramagnetic Resonance (EPR) spectroscopy (X-band, ~9.87 GHz).
    • Signal Used: The half-field (HF) transition signal at geff = 4.23 was measured and quantified via double integration to determine the NV- concentration (in ppm atomic ratio).

  5. Structural Integrity Check:

    • Methods: Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy (EELS) were used on highly irradiated samples.
    • Result: Confirmed that the π* transition peak for sp2 carbon (graphitization) did not increase, indicating the diamond lattice remained intact.

  6. Modeling:

    • Method: Monte Carlo annealing simulations were performed to model NV formation probability as a function of particle diameter and substitutional nitrogen concentration [Ns].
    • Model Basis: Simulating a single vacancy performing a random walk in a spherical crystal until it either forms an NV center (next to Ns) or is lost to the surface.

Commercial Applications

Section titled “Commercial Applications”

The findings regarding efficient, low-temperature defect creation in ultra-small nanodiamonds are highly relevant for advanced quantum technologies and biomedical engineering:

  • Quantum Sensing and Metrology:

    • Room-Temperature Sensors: DNDs containing NV- centers are leading candidates for room-temperature quantum sensors (e.g., magnetic field, temperature, strain).
    • Simplified Manufacturing: Eliminating the need for high-temperature annealing (800 °C) simplifies the fabrication process, reducing costs and energy consumption for producing fluorescent nanodiamonds.

  • Biomedical and Cellular Applications:

    • Intracellular Sensing: 5 nm DNDs are small enough for efficient uptake and operation inside living cells, enabling high-resolution measurements of biologically relevant quantities (e.g., pH, temperature, 3D orientation tracking).
    • Spin-Enhanced Diagnostics: Used for ultrasensitive diagnostics where small, stable, and bright fluorescent probes are required.

  • Defect Engineering in Nanocrystals:

    • General Semiconductor Defect Creation: The observed “self-annealing” mechanism provides a pathway for creating stable point defects (color centers) in other very small semiconductor nanocrystals (e.g., SiC, ZnO) at low processing temperatures, which is critical for maintaining surface functionality or integration with temperature-sensitive materials.

  • Quantum Information Science:

    • Solid-State Qubits: Efficient, high-density creation of NV- centers in small crystals is fundamental for developing scalable solid-state quantum computing and memory architectures.
View Original Abstract

Nanodiamonds containing negatively charged nitrogen-vacancy (NV$^-$) centers\nare versatile room-temperature quantum sensors in a growing field of research.\nYet, knowledge regarding the NV-formation mechanism in very small particles is\nstill limited. This study focuses on the formation of the smallest\nNV$^-$-containing diamonds, 5 nm detonation nanodiamonds (DNDs). As a reliable\nmethod to quantify NV$^-$ centers in nanodiamonds, half-field signals in\nelectron paramagnetic resonance (EPR) spectroscopy are recorded. By comparing\nthe NV$^-$ concentration with a series of nanodiamonds from high-pressure\nhigh-temperature (HPHT) synthesis (10 - 100 nm), it is shown that the formation\nprocess in 5 nm DNDs is unique in several aspects. NV$^-$ centers in DNDs are\nalready formed at the stage of electron irradiation, without the need for\nhigh-temperature annealing. The effect is explained in terms of\n”self-annealing”, where size and type dependent effects enable vacancy\nmigration close to room temperature. Although our experiments show that NV$^-$\nconcentration generally increases with particle size, remarkably, the NV$^-$\nconcentration in 5 nm DNDs surpasses that of 20 nm-sized nanodiamonds. Using\nMonte Carlo simulations, we show that the ten times higher substitutional\nnitrogen concentration in DNDs compensates the vacancy loss induced by the\nlarge relative particle surface. Upon electron irradiation at a fluence of $1.5\n\times 10 ^{19}$ e$^-$/cm$^2$, DNDs show a 12.5-fold increment in the NV$^-$\nconcentration with no sign of saturation. These findings can be of interest for\nthe creation of defects in other very small semiconductor nanoparticles beyond\nNV-nanodiamonds as quantum sensors.\n

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2001 - Optical Properties of Diamond [Crossref]

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