Role of High Nitrogen‐Vacancy Concentration on the Photoluminescence and Raman Spectra of Diamond
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2022-08-03 |
| Journal | physica status solidi (a) |
| Authors | Mona Jani, Mariusz Mrózek, Anna Maria Nowakowska, Patrycja Leszczenko, Wojciech Gawlik |
| Institutions | Jagiellonian University |
| Citations | 11 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This study investigates the competition between Photoluminescence (PL) from Nitrogen-Vacancy (NV) centers and the characteristic Raman scattering signal in high-density diamond samples, proposing a critical methodological refinement for material characterization.
- Problem Identification: Under standard green laser excitation (532 nm), the intense PL from high concentrations of neutral NV centers (NV0) completely overwhelms and obscures the fundamental diamond Raman peak (1332 cm-1).
- Contradiction to Paradigm: This observation contradicts the common assumption that visible Raman spectroscopy provides a universal, complete characterization of diamond crystalline quality, especially in NV-rich materials (>1 ppm).
- Mechanism Confirmation: Experimental data from bulk monocrystalline diamonds (MCDs) with NV gradients demonstrated that the intrinsic diamond Raman peak amplitude remains stable but is simply buried by the overwhelming NV0 PL intensity.
- NIR Solution: Near-Infrared (NIR) excitation (1064 nm) is highly effective, as NV center fluorescence is minimal in this region. This technique successfully isolates the undistorted 1332 cm-1 Raman signal, enabling accurate assessment of diamond crystallinity regardless of NV density.
- New Diagnostic Tool: The findings pave the way for a new quantitative method: calibrating the ratio of the NV0 Zero-Phonon Line (ZPL) amplitude to the diamond Raman peak amplitude to characterize high NV densities (0.1-10 ppm) in dense ensemble samples.
- Nanodiamond Limitations: For very small fluorescent nanodiamonds (FNDs, 140 nm and smaller), the Raman signals are significantly weaker, making characterization challenging even with NIR excitation.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Characteristic Diamond Raman Shift | 1332 | cm-1 | Monocrystalline Diamond (MCD) |
| NV0 Zero-Phonon Line (ZPL) | 575 (1406) | nm (cm-1) | Visible excitation (2.156 eV) |
| NV- Zero-Phonon Line (ZPL) | 637 (3090) | nm (cm-1) | Visible excitation (1.945 eV) |
| NV Concentration Range Studied | 4 to 36 | ppm | MCD-1 (Volume irradiated, gradient) |
| Initial Nitrogen Concentration [Ni] | ~380 | ppm | MCD-1 (Type Ib HPHT) |
| Ultra-Pure Diamond [NV] | < 0.03 | ppb | MCD-3 (Type IIa CVD) |
| Proton Implantation Energy | 1.8 | MeV | MCD-2 (Surface irradiation) |
| NV Layer Thickness (MCD-2) | ~20 | µm | Proton stopping range |
| Visible Excitation Wavelengths | 532, 633 | nm | Confocal Raman Microscopy |
| NIR Excitation Wavelength | 1064 | nm | FT-Raman Spectroscopy |
| 532 nm Laser Power (MCDs) | 0.185 | mW | Reduced power for bulk samples |
| NIR Laser Power | 300 | mW | Diode-pumped Nd:YAG |
| Visible Raman Spectral Resolution | 3 | cm-1 | WITec Alpha 300 setup |
| NIR Raman Spectral Resolution | 4 | cm-1 | Bruker MultiRAM setup |
| HPHT Nanodiamond Size | ~167 | nm | Pristine ND powder |
| Fluorescent Nanodiamond Sizes | 1 µm, 140 | µm, nm | Carboxylated FND slurries |
Key Methodologies
Section titled “Key Methodologies”The study utilized a combination of irradiation, annealing, and advanced spectroscopy to create and characterize diamond samples with controlled NV concentration gradients.
Sample Preparation (NV Center Creation)
Section titled “Sample Preparation (NV Center Creation)”- Pristine Nanodiamonds (NDs): HPHT ND powders (~167 nm) were suspended in water, sonicated, and dried on glass microslides.
- Fluorescent Nanodiamonds (FNDs): Carboxylated FND slurries (1 µm and 140 nm, 3 ppm NV) were drop-dried onto CaF2 substrates.
- Monocrystalline Diamond 1 (MCD-1, Volume NV Gradient):
- Material: Type Ib HPHT diamond ([Ni] ~ 380 ppm).
- Irradiation: Non-spatially-uniform 3 MeV electron beam (vacancies created uniformly through the depth).
- Annealing: 4 hours in vacuum at ~750 °C, resulting in an NV concentration gradient (4 to 36 ppm) across the lateral surface.
- Monocrystalline Diamond 2 (MCD-2, Surface NV Layer):
- Material: Type Ib HPHT diamond ([Ni] ~ 50 ppm).
- Irradiation: Focused 1.8 MeV proton beam implantation, creating vacancies in a top layer of ~20 µm.
- Annealing: 2 hours in vacuum at ~900 °C, creating a high-density NV layer close to the surface.
- Monocrystalline Diamond 3 (MCD-3, Ultra-Pure): Type IIa electronic-grade CVD diamond, used as a pristine reference.
Spectroscopic Characterization
Section titled “Spectroscopic Characterization”- Visible Raman and PL:
- Instrument: WITec Alpha 300 confocal Raman microscope.
- Excitation: 532 nm (green) and 633 nm (red) lasers.
- Detection: CCD detector.
- Procedure: Spectra collected with 10 accumulations and 0.5 s acquisition time per spot. Laser power was reduced to 0.185 mW for MCDs to prevent damage/heating.
- Near-Infrared (NIR) Raman:
- Instrument: Bruker MultiRAM FT-Raman spectrometer.
- Excitation: 1064 nm (diode-pumped Nd:YAG laser).
- Power: 300 mW.
- Detection: Germanium detector cooled with liquid nitrogen.
- Procedure: Spectra collected over 32 scans per sample, focusing on the 4000-400 cm-1 range.
Commercial Applications
Section titled “Commercial Applications”The findings directly impact the manufacturing, characterization, and quality control of diamond materials used in high-technology sectors relying on NV centers.
- Quantum Sensing and Metrology:
- Application: Fabrication of high-sensitivity magnetometers and thermometers based on NV ensembles.
- Value: NIR Raman provides essential quality control by verifying the integrity and crystallinity of the diamond lattice, which is critical for maintaining long spin coherence times (T2) in high-NV density sensor materials.
- Quantum Information Processing (QIP):
- Application: Development of scalable solid-state qubits and quantum registers using dense NV ensembles.
- Value: The proposed NV0 ZPL/Raman ratio calibration method offers a non-destructive, quantitative tool for characterizing the actual NV density (0.1-10 ppm range) achieved after irradiation and annealing processes.
- Nanomedicine and Bio-imaging:
- Application: Use of Fluorescent Nanodiamonds (FNDs) for intracellular tracking, bio-particle detection, and drug delivery.
- Value: Ensures that FNDs (especially those >140 nm) maintain high crystalline quality, which is necessary for stable and bright PL signals required for in vivo applications.
- Advanced Materials Quality Control (QC):
- Application: Characterization of bulk diamond substrates (HPHT and CVD) used in high-power electronics, optical windows, and heat sinks.
- Value: NIR Raman eliminates spectral interference from color centers, allowing engineers to accurately assess phase purity and detect non-diamond carbon components (D-band, G-band) that affect material performance.
View Original Abstract
A photoluminescence (PL) and Raman spectroscopy study of various diamond samples that have high concentrations of nitrogen‐vacancy (NV) color centers up to multiple parts per million (ppm) is presented. With green, red, and near‐infrared (NIR) light excitation, it is demonstrated that while for samples with a low density of NV centers the signals are primarily dominated by Raman scattering from the diamond lattice, for higher density of NVs, a combination of Raman scattering from the diamond lattice and fluorescence from the NV centers is observed, while for the highest NV densities the Raman signals from diamond are completely overwhelmed by the intense NV’s fluorescence. However, under NIR excitation, Raman diamond signatures can be observed for some diamonds. These observations reveal different roles of two mechanisms of light emission and contradict the naïve belief that Raman scattering enables the complete characterization of a diamond crystalline sample.