NV-centers in nanodiamonds - How good they are

At a Glance

Metadata	Details
Publication Date	2017-12-10
Journal	Diamond and Related Materials
Authors	Taras Plakhotnik, Haroon Aman
Institutions	The University of Queensland
Citations	49
Analysis	Full AI Review Included

NV-Centers in Nanodiamonds: Quantifying Yield and Brightness for Quantum Applications

Executive Summary

This paper presents a critical analysis of the photophysical properties of Nitrogen-Vacancy (NV) centers, focusing on commercially sourced nanodiamonds (NDs), and concludes that commonly cited quantum yield (QY) figures are substantially overstated.

Revised Quantum Yield (QY): The estimated luminescence quantum yield ($\Phi_{\text{NV}}$) is significantly lower than the frequently cited 100%. Bulk diamond QY is estimated at $\sim$0.5, while NDs in water are typically $\leq$ 0.2, dropping to as low as 5% in certain geometries/environments.
Radiative Rate Discrepancy: The radiative decay rate ($k_r$) in NDs (as low as 3 MHz in air/vacuum) is significantly slower compared to the revised estimate for bulk diamond (44 $\pm$ 11 MHz), directly impacting overall brightness.
Inhomogeneity and Size: The wide variability in observed brightness (up to a factor of 1000) is primarily attributed to the large variance in crystal volume/size distribution, not uneven NV concentration.
Geometry Dependence: NV absorption cross-section ($\sigma$) and radiative rates ($k_r$) are highly dependent on the crystal’s shape (aspect ratio/ellipsoid parameters) and the refractive index ($n$) of the surrounding medium, a critical factor for ND sensing/imaging applications.
Absorption Cross-Section: The absorption cross-section in nanocrystals is estimated to be smaller than in bulk diamond, further suggesting reduced efficiency in ND systems.
Implication for Quantum Sensing: High-end quantum applications requiring high photon count rates and stability benefit strongly from NV centers embedded in high-quality, homogeneous bulk Single Crystal Diamond (SCD), rather than heterogeneous nanodiamond solutions.

Technical Specifications

A summary of key experimental and derived photophysical parameters discussed in the research paper, highlighting the contrast between nanodiamonds (ND) and bulk diamond (SCD).

Parameter	Value (ND, 70nm)	Value (Bulk/SCD)	Unit	Context/Reference
Excitation Wavelength ($\lambda$)	532	532	nm	Coherent Verdi-V5 Laser
Max Power Density	3.5	N/A	kW/cm²	Excitation light beam on slide
Average Saturation Intensity ($I_s$)	70 $\pm$ 15	N/A	kW/cm²	For tested nanodiamonds
Specific Brightness ($\beta$)	1.5 $\times 10^3$	N/A	photon/nm³	Derived absolute specific brightness
Max Emission Rate per NV	2.5	N/A	MHz	For 100 nm crystal, 300 NV centers advertised
Radiative Decay Rate ($k_{r[b]}$)	N/A	44 $\pm$ 11	MHz	Estimated theoretical value for bulk SCD
Radiative Decay Rate ($k_{r[nano]}$)	8 $\pm$ 4	N/A	MHz	NDs immersed in water
Radiative Decay Rate ($k_{r[nano]}$)	5 $\pm$ 3	N/A	MHz	NDs deposited on glass substrate
Bulk Absorption Cross-section ($\sigma_{[b]}$)	N/A	(3.1 $\pm$ 0.8) $\times 10^{-17}$	cm²	At 532 nm, low temperature ZPL measurement [21]
Nanocrystal Absorption Cross-section ($\sigma_{[nc]}$)	(1.3 $\pm$ 0.3) $\times 10^{-17}$	N/A	cm²	Estimated value on glass substrate
Quantum Yield ($\Phi_{\text{NV}}$)	0.09 - 0.58	$\sim$0.5	N/A	Revised estimate based on new data and theory
Collection Efficiency ($\Phi_{\text{opt}}$)	0.14	0.023	N/A	Standard NA=0.9 objective on glass substrate

Key Methodologies

The core of the research involves analyzing the size-dependent photophysics of NV centers in nanodiamonds using optical spectroscopy and advanced theoretical modeling of electromagnetic interactions.

Sample Preparation and Excitation:
- Commercially available 70-nm fluorescent nanodiamonds (1 mg/mL concentration) were diluted (2000x) and deposited onto pre-cleaned glass microscope slides using controlled evaporation techniques (water droplets) to ensure homogeneous distribution over a 1.2 mm spot.
- Continuous excitation was performed using a 532 nm laser at 50 mW power (max density 3.5 kW/cm²).
Luminescence Measurement and Saturation Analysis:
- Photon count rate ($k^{(\text{det})}$) was measured for individual crystals using a microscope objective (NA=0.9) and a photon counting CCD.
- Count rate was fitted to a saturation curve (Equation 1) to determine the maximum detectable photon count rate ($R^{(\text{det})}$) and the saturation power ($P_s$).
Size Distribution Determination:
- The total diamond volume deposited was calculated using mass concentration and density ($\rho \approx 3.5$ g cm^-3).
- The diameter ($d_j$) of individual detected crystals was estimated using the measured $R^{(\text{det})}_j$ values combined with the total diamond volume (Equation 3).
- Results were validated against Dynamic Light Scattering (DLS) data.
Theoretical Modeling of Local Fields and Rates:
- A significant portion of the work involved rigorous electrodynamic analysis to calculate local field shielding factors ($\eta$) and depolarizing factors ($\delta$) for NV centers based on crystal shape (modeled as ellipsoids with varying aspect ratios $a, b, c$).
- This modeling quantified how the nanocrystal environment—including surrounding media (air, water) and substrate interactions—affects the absorption cross-section ($\sigma$) and spontaneous emission rates ($k_r$), leading to revised estimates of $\Phi_{\text{NV}}$.
ODMR and Population Dynamics:
- A reduced four-level rate equation model was used to describe NV population dynamics (Figure 2), incorporating intersystem crossing rates ($k_{\text{TS}}$, $k_{\text{ST}}$) and spin sub-level populations ($\alpha$).
- This dynamic model was combined with Optically Detected Magnetic Resonance (ODMR) contrast data to refine the crucial internal parameter $\alpha$ (relative population of $m=\pm 1$ sublevels), allowing for the determination of absolute $\Phi_{\text{NV}}$.

6CCVD Solutions & Capabilities

The findings of this research underscore the limitations of using highly inhomogeneous, synthetically crashed nanodiamonds (HTHP source material) for applications demanding high and stable NV performance. 6CCVD, specializing in high-purity MPCVD diamond, offers materials that inherently overcome the size-related and environmental drawbacks detailed in this paper.

Applicable Materials for High-Performance NV Centers

The paper confirms that NV performance metrics (specifically $k_r$ and $\Phi_{\text{NV}}$) are maximized in bulk diamond where geometry and environment effects are minimized.

Application Requirement	NV-Center Finding	Recommended 6CCVD Solution
Maximum Quantum Yield ($\Phi_{\text{NV}}$)	Bulk diamond QY ($\sim$0.5) is superior to NDs ($\leq$ 0.2).	Optical Grade Single Crystal Diamond (SCD): Provides the highest material purity and crystal homogeneity, eliminating the surface-enhanced non-radiative decay pathways prevalent in NDs.
Controlled Geometry & Thickness	Radiative rates are highly dependent on crystal shape and interface distance.	Custom SCD Plates/Wafers (0.1 µm - 500 µm): Allows engineers to use precisely defined, low-aspect-ratio SCD films for integration into photonic devices, minimizing field interaction losses and maximizing photon collection ($\Phi_{\text{opt}}$).
High Density Sensing/Thermal Management	Bulk diamond’s high thermal conductivity prevents localized heating in nano-structures.	High-Purity SCD Substrates (up to 10mm): Ideal for NV-based temperature/magnetic sensing arrays, leveraging diamond’s exceptional thermal properties ignored in ND research.
Refractive Index Modification	NV performance depends critically on surrounding $n$ (e.g., water vs. air).	Custom Polycrystalline Diamond (PCD) Wafers: Scalable, inch-sized (up to 125mm) substrates for large-area quantum device fabrication where stable $n$ interfaces are paramount.

Customization Potential

6CCVD provides the necessary fabrication precision to transition research findings on NV-center environments into functional quantum devices.

Precision Polishing: To replicate or improve upon the substrate effects analyzed in the paper, 6CCVD offers ultra-low surface roughness: Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD. This is crucial for minimizing scattering losses and ensuring reliable coupling at dielectric interfaces.
Custom Dimensions: Unlike the heterogeneous 70 nm NDs studied, 6CCVD supplies single-crystal wafers up to 125mm, enabling large-scale, reproducible device fabrication.
Integrated Metalization: To enhance photon collection (as attempted by utilizing bullseye gratings in referenced work [33]), 6CCVD provides in-house, custom thin-film deposition of Au, Pt, Pd, Ti, W, and Cu for creating integrated photonic structures or electrodes.

Engineering Support

The complexity of NV-center photophysics, particularly the interplay between local fields, crystal shape, and radiative lifetime, requires specialized knowledge. 6CCVD’s in-house PhD team provides authoritative support for developing NV-based systems.

We offer detailed consultation on material selection, NV defect formation methods (e.g., implantation followed by annealing), and optimizing diamond thickness and surface preparation for projects involving:

Optically Detected Magnetic Resonance (ODMR) Sensing
Single Photon Sources (SPS)
Quantum Information Processing (QIP) architecture integration

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

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.diamond.2017.12.004

References

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2017 - Diamonds for quantum nano sensing [Crossref]
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1975 - Dielectric sphere-sphere and sphere-plane problems in electrostatics [Crossref]