Effects of Thermal Oxidation and Proton Irradiation on Optically Detected Magnetic Resonance Sensitivity in Sub-100 nm Nanodiamonds
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-03-30 |
| Journal | ACS Applied Materials & Interfaces |
| Authors | Pietro AprĂ , G. Zanelli, Elena Losero, Nour-Hanne Amine, Greta Andrini |
| Institutions | Torino e-district, Fondazione Bruno Kessler |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study details a successful protocol for creating ultra-small nanodiamonds (NDs) suitable for high-performance quantum sensing, overcoming the typical trade-off between size reduction and sensitivity loss.
- Core Achievement: Produced NDs with an average size of 18.5 nm (max height), achieving a shot-noise-limited ODMR temperature sensitivity of approximately 10 K/âHz.
- Size vs. Performance: Maintained the ODMR sensitivity of the initial, larger NDs (median ~55 nm) despite an approximate 8-fold volume reduction, enabling nanoscale spatial resolution.
- Methodology: Combined thermal oxidation (for purification and surface chemistry control) with 2 MeV proton irradiation (for nitrogen-vacancy (NV) center density enhancement).
- Surface Optimization: Prolonged thermal oxidation effectively removed disordered sp2 carbon phases and introduced oxygen-containing surface groups, stabilizing the desired negatively charged NV- state (NV-/NV0 ratio up to ~3.5).
- Fluorescence Enhancement: High-dose proton irradiation (4 x 1016 cm-2) resulted in a 3-fold increase in average maximum photoluminescence (PL) intensity from isolated NDs compared to oxidized-only samples.
- Biomedical Relevance: The resulting sub-20 nm NDs offer enhanced biocompatibility, higher spatial resolution, and improved drug delivery potential for intracellular sensing applications.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Initial ND Median Size | ~55 | nm | HPHT commercial NDs (Pureon MSY 0-0.1) |
| Final ND Average Size (IrrhighND) | 18.5 ± 0.4 | nm | Maximum height via AFM analysis |
| ODMR Temperature Sensitivity | ~10 | K/âHz | Shot-noise-limited sensitivity for IrrhighND |
| Proton Irradiation Energy | 2 | MeV | H+ ion beam used for vacancy creation |
| High Irradiation Fluence (Fhigh) | 4 x 1016 | cm-2 | Corresponds to ~200 ppm vacancy density |
| Low Irradiation Fluence (Flow) | 1 x 1015 | cm-2 | Corresponds to ~5 ppm vacancy density |
| PL Intensity Enhancement (IrrhighND vs OxhighND) | 14 | times | Measured on thick, compacted ND layers |
| NV-/NV0 Ratio (OxlowND) | ~3.5 | Ratio | Optimal ratio achieved after 3 h oxidation |
| NV-/NV0 Ratio (IrrlowND) | ~2.5 | Ratio | Higher ratio than IrrhighND (due to lower fluence) |
| Diamond Raman Peak | ~1332 | cm-1 | First-order Raman diamond peak |
| NV- Zero-Phonon Line (ZPL) | 638 | nm | Characteristic emission wavelength |
| NV0 Zero-Phonon Line (ZPL) | 576 | nm | Characteristic emission wavelength |
Key Methodologies
Section titled âKey MethodologiesâThe optimization protocol involved a sequence of thermal and irradiation treatments to control ND size, surface chemistry, and NV center density:
-
Initial Annealing (AnnND):
- Conditions: 800 °C for 2 h in N2 flow.
- Purpose: Convert amorphous sp2 carbon phases covering the ND surface into graphite for subsequent removal.
-
Thermal Oxidation (OxlowND / OxhighND):
- Conditions: 500 °C in air environment. Durations were 3 h (low) or 36 h (high).
- Purpose: Selective etching of graphitic phases (purification) and introduction of oxygen-containing surface terminations to stabilize the NV- charge state.
-
Proton Irradiation:
- Conditions: 2 MeV H+ ion beam applied to a 30 ± 10 ”m thick ND layer deposited on a Si substrate.
- Fluences: 1 x 1015 cm-2 (IrrlowND) and 4 x 1016 cm-2 (IrrhighND).
- Purpose: Create lattice vacancies within the diamond core.
-
Post-Irradiation Annealing:
- Conditions: 800 °C for 4 h in N2 flow.
- Purpose: Promote the diffusion and coupling of generated vacancies with native nitrogen impurities to form additional NV centers.
-
Final Oxidation (IrrND):
- Conditions: 500 °C in air environment for 12 h.
- Purpose: Remove surface graphitization caused by the 800 °C annealing and achieve the final, reduced ND size and stable oxygen surface termination.
-
Characterization:
- Surface Chemistry: Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy.
- Size/Morphology: Atomic Force Microscopy (AFM) for size distribution and Dynamic Light Scattering (DLS) for aggregation/dispersibility.
- Optical/Structural: Raman and Photoluminescence (PL) spectroscopy.
- Sensing Performance: Optically Detected Magnetic Resonance (ODMR) measurements to determine shot-noise-limited temperature sensitivity.
Commercial Applications
Section titled âCommercial ApplicationsâThe optimized sub-20 nm nanodiamonds are critical enablers for next-generation quantum sensing technologies, particularly those requiring high spatial resolution and biocompatibility.
- Biomedical Quantum Sensing:
- Intracellular thermometry and magnetic field sensing in living cells and in vivo environments.
- Monitoring thermal effects induced by chemical stimuli or metabolic activity alteration.
- Mapping temperature gradients within single cells (e.g., related to neuron activity).
- Drug Delivery and Theranostics:
- The small size (<20 nm) improves drug loading capacity (higher surface area) and facilitates traversal of biological barriers for targeted delivery.
- Nanoscale Metrology:
- High-precision measurement of physical quantities (temperature, weak magnetic fields) at the nanometric level, crucial for characterizing cellular membrane channels (tens of nm dimensions).
- Quantum Technology Research:
- Potential use in advanced ODMR protocols (pulsed-ODMR) for enhanced sensitivity.
- Exploration in fundamental physics, such as quantum gravity effects detection, where small, stable quantum emitters are required.
View Original Abstract
In recent decades, nanodiamonds (NDs) have emerged as innovative nanotools for weak magnetic fields and small temperature variation sensing, especially in biological systems. At the basis of the use of NDs as quantum sensors are nitrogen-vacancy center lattice defects, whose electronic structures are influenced by the surrounding environment and can be probed by the optically detected magnetic resonance technique. Ideally, limiting the NDsâ size as much as possible is important to ensure higher biocompatibility and provide higher spatial resolution. However, size reduction typically worsens the NDsâ sensing properties. This study endeavors to obtain sub-100 nm NDs suitable to be used as quantum sensors. Thermal processing and surface oxidations were performed to purify NDs and control their surface chemistry and size. Ion irradiation techniques were also employed to increase the concentration of the nitrogen-vacancy centers. The impact of these processes was explored in terms of surface chemistry (diffuse reflectance infrared Fourier transform spectroscopy), structural and optical properties (Raman and photoluminescence spectroscopy), dimension variation (atomic force microscopy measurements), and optically detected magnetic resonance temperature sensitivity. Our results demonstrate how surface optimization and defect density enhancement can reduce the detrimental impact of size reduction, opening to the possibility of minimally invasive high-performance sensing of physical quantities in biological environments with nanoscale spatial resolution.