Laser-Synthesis of NV-Centers-Enriched Nanodiamonds - Effect of Different Nitrogen Sources

At a Glance

Metadata	Details
Publication Date	2020-06-09
Journal	Micromachines
Authors	Luca Basso, Mirko Sacco, Nicola Bazzanella, M. Cazzanelli, Alessandro Barge
Institutions	Center for Neuroscience and Cognitive Systems, University of Turin
Citations	7
Analysis	Full AI Review Included

Executive Summary

The research successfully demonstrates a single-step, scalable synthesis route for Nitrogen-Vacancy (NV) center-enriched Nanodiamonds (NDs) using Pulsed Laser Ablation (PLA) of N-doped graphite.

Method Validation: Optically Detected Magnetic Resonance (ODMR) confirmed the formation of NV^- centers, proving the NV origin of the strong photoluminescence (PL) observed in the synthesized NDs.
Optimal Medium Identified: Ablation in Liquid Nitrogen (LN₂) yielded the highest NV-ND production efficiency, showing a PL intensity 22 ± 4 times greater than the untreated target.
Thermodynamic Mechanism: The high efficiency in LN₂ is attributed to extreme thermodynamic conditions within the ablation plume, specifically high pressure (~3.5 GPa) and ultra-rapid quenching (10¹¹-10¹² K/s), which favor diamond phase nucleation.
Nitrogen Source Dominance: When ablating in LN₂, the medium itself is the dominant source of nitrogen for NV formation, overshadowing the nitrogen content initially doped into the graphite target.
Tunable Doping Potential: Ablation in water (a non-nitrogen-contributing medium) demonstrated that varying the target’s initial N-doping level successfully controls the resulting NV concentration in the NDs, opening a path toward tunable NV density synthesis.

Technical Specifications

Parameter	Value	Unit	Context
Laser Type	KrF Excimer	N/A	Pulsed Laser Ablation (PLA)
Laser Wavelength	248	nm	PLA setup
Pulse Duration	20	ns	PLA setup
Repetition Rate	10	Hz	PLA setup
Target N Concentration (High N)	23.2	mg/g	N-doped graphite target
Target N Concentration (Low N)	5.8	mg/g	N-doped graphite target
Nanodiamond Size	<100	nm	Clustered nanoparticles (SEM)
Diamond Raman Peak	1335	cm^-1	Compressively-strained NDs
NV^- ODMR Frequency	~2870	MHz	Zero-field splitting (ZFS) confirmation
PL Efficiency (LN₂ vs. Target)	22 ± 4	Ratio	Highest NV-ND production yield
Ablation Pressure (LN₂)	~3.5	GPa	Estimated internal plume pressure
Cooling Rate (LN₂)	10¹¹-10¹²	K/s	Rapid quenching condition in liquid medium
Synthesis Efficiency (Water)	6-7	%	Conversion efficiency of graphite to fluorescent NDs

Key Methodologies

The synthesis and characterization process involved precise target preparation followed by comparative pulsed laser ablation in three distinct media.

N-Doped Graphite Target Preparation:
- Graphite powder was functionalized via 1,3-dipolar cycloaddition using an in situ-generated azomethine ylide (from glycine/histidine and paraformaldehyde) in DMF at 130 °C.
- This process introduced nitrogen atoms via a pyrrolidine ring adduct.
- The resulting powder was quantified using Thermogravimetric Analysis (TGA) to determine nitrogen concentration (e.g., 23.2 mg/g for the “High N” target).
- Powder (200 mg) was pressed at 50 bar to form a 1 cm diameter target pellet.
Pulsed Laser Ablation (PLA) Setup:
- A KrF excimer laser (248 nm, 20 ns pulse duration) was used at 10 Hz repetition rate.
- Single-pulse energy was set to ~500 mJ, totaling 3000 pulses per experiment.
- The laser beam was focused to a spot size of ~1 mm² on the target surface.
Confining Media Comparison:
- Water (W): Target was covered by a ~5 mm liquid layer in a glass vial.
- Liquid Nitrogen (LN₂): Target was immersed in LN₂, contained within a polystyrene box to minimize evaporation.
- Nitrogen Atmosphere (N₂): Ablation occurred in a vacuum chamber filled with N₂ gas at a partial pressure of 1 Pa. Substrate temperature was maintained at ~100 °C during deposition.
Post-Synthesis and Characterization:
- Liquid-ablated samples were dried in an oven. N₂-ablated films were annealed at 300 °C for 1 hour.
- Morphology and size (<100 nm) were confirmed by Field Emission Scanning Electron Microscopy (SEM).
- Crystal structure was verified by Raman spectroscopy (diamond peak at 1335 cm^-1).
- Optical properties and NV concentration were assessed via wide-field Photoluminescence (PL) imaging.
- NV center formation was unequivocally confirmed using Optically Detected Magnetic Resonance (ODMR) spectroscopy, monitoring PL decrease at ~2870 MHz.

Commercial Applications

The ability to synthesize highly dense, fluorescent NV-NDs in a single, scalable step is crucial for advancing quantum technologies and biomedical applications.

Quantum Sensing and Metrology:
- Nanoscale sensing of magnetic fields, temperature, and electric fields.
- Detection of paramagnetic species and changes in pH/redox potential within biological environments.
Biomedical and Bioimaging:
- Long-term, photostable cell tracking due to ND biocompatibility and robust fluorescence.
- Nanoscale thermometry for localized temperature sensing inside cells.
Quantum Computing and Information:
- Development of NV centers as robust solid-state qubits for quantum information protocols.
- Realization of hybrid quantum systems, coupling NV spins to mechanical resonators for advanced strain imaging.
Materials Science:
- Production of high-density NV ensembles (where sensitivity scales as N^-1/2) required for high-efficiency bulk sensing applications.
- Controlled synthesis of NDs with tunable NV concentration by adjusting target doping levels.

View Original Abstract

Due to the large number of possible applications in quantum technology fields—especially regarding quantum sensing—of nitrogen-vacancy (NV) centers in nanodiamonds (NDs), research on a cheap, scalable and effective NDs synthesis technique has acquired an increasing interest. Standard production methods, such as detonation and grinding, require multistep post-synthesis processes and do not allow precise control in the size and fluorescence intensity of NDs. For this reason, a different approach consisting of pulsed laser ablation of carbon precursors has recently been proposed. In this work, we demonstrate the synthesis of NV-fluorescent NDs through pulsed laser ablation of an N-doped graphite target. The obtained NDs are fully characterized in the morphological and optical properties, in particular with optically detected magnetic resonance spectroscopy to unequivocally prove the NV origin of the NDs photoluminescence. Moreover, to compare the different fluorescent NDs laser-ablation-based synthesis techniques recently developed, we report an analysis of the effect of the medium in which laser ablation of graphite is performed. Along with it, thermodynamic aspects of the physical processes occurring during laser irradiation are analyzed. Finally, we show that the use of properly N-doped graphite as a target for laser ablation can lead to precise control in the number of NV centers in the produced NDs.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi11060579

References

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