Low-Power Laser Graphitization of High Pressure—High Temperature Nanodiamond Films

At a Glance

Metadata	Details
Publication Date	2020-05-11
Journal	Applied Sciences
Authors	Konstantin G. Mikheev, T. N. Mogileva, Arseniy E. Fateev, Nicholas Nunn, Olga Shenderova
Institutions	Adámas Nano (United States), North Carolina State University
Citations	14
Analysis	Full AI Review Included

Executive Summary

This research demonstrates a novel, low-power method for the structural modification and graphitization of High-Pressure High-Temperature (HP-HT) nanodiamond (ND) films.

Core Achievement: Successful laser-induced graphitization (sp³ to sp² carbon conversion) of 150 nm HP-HT ND films using a continuous-wave (CW) 633 nm He-Ne laser, requiring less than 10 mW of optical power.
Threshold Intensity: Graphitization initiates sharply when the focused laser intensity exceeds a threshold of approximately 33 kW/cm².
Mechanism: The process is driven by a two-step sequential resonance absorption mechanism involving Ni- and Ni-N-related impurity centers, leading to photoionization and the generation of free electrons.
Observation: The structural change is accompanied by transient green up-conversion luminescence (461-535 nm region), confirming the activation and ionization of these impurity centers.
Spatial Selectivity: Modification occurs primarily when the film is moved relative to the laser spot, indicating a non-uniform distribution of the active impurity centers within the ND particles.
Structural Result: The modified area exhibits a significant change in morphology, characterized by a surface notch (150 nm deep) and surrounding parallel ridges (100 nm high).
Implication: This technique provides a method for the selective destruction of impurity-rich ND particles, opening possibilities for laser-assisted purification of HP-HT nanodiamonds.

Technical Specifications

Parameter	Value	Unit	Context
Nanodiamond Source Material	HP-HT monocrystals	N/A	Average particle size 150 nm.
Film Thickness (Modification)	20	µm	Formed on quartz substrate.
Laser Type	CW He-Ne	N/A	Continuous wave excitation source.
Laser Wavelength	633	nm	Used for excitation and modification.
Maximum Laser Power	8.4	mW	Output using 100x objective.
Graphitization Threshold Intensity	~33	kW/cm²	Minimum intensity required for sp³ to sp² conversion.
Maximum Intensity Used	65	kW/cm²	Used for stationary irradiation tests.
Laser Beam Waist Diameter (100x)	4.1	µm	1/e² level.
Laser Written Line Width	~5	µm	Transverse width of the blackened segment.
Unmodified Diamond Raman Shift	1331	cm^-1	Characteristic HP-HT ND peak.
sp² Carbon Raman Shifts	1320 and 1580	cm^-1	Typical broad lines for graphitic carbon.
AFM Notch Depth (Blackened Area)	~150	nm	Depth of the laser-induced trench.
AFM Ridge Height (Blackened Area)	~100	nm	Height of material raised adjacent to the notch.
Extinction Spectrum Maximum	208	nm	Corresponds to 5.96 eV (diamond bandgap).
Up-Conversion Luminescence Peaks	485 and 528-529	nm	Associated with Ni- and Ni-N-related centers (blue-green emission).
Substitutional Nitrogen Concentration	100-300	ppm	Impurity level in HP-HT ND.

Key Methodologies

Sample Preparation: HP-HT nanodiamond particles (150 nm average size) were dispersed in deionized water. Films (20 µm thick) were formed by drop-casting the aqueous suspension onto quartz substrates and drying at room temperature.
Irradiation Setup: Experiments utilized a Horiba HR800 Raman spectrometer setup with a CW He-Ne laser (633 nm) as the excitation source. The beam was focused using 10x, 50x, or 100x objectives.
Laser Modification: Films were moved relative to the focused laser spot using a coordinate table to “write” patterns (lines, squares, triangles). Modification was tested in ambient air and Ar atmosphere.
In-Situ Observation: Green up-conversion luminescence accompanying the graphitization event was visually observed and recorded using a photo camera, confirming the activation of Ni-related photoluminescent centers.
Structural Analysis (XRD): X-ray diffraction confirmed the high-quality diamond structure of the starting material (peaks corresponding to {111}, {220}, {311} planes).
Spectroscopic Analysis (Raman): Raman spectra were used to confirm the structural transformation: the diamond peak (1331 cm^-1) disappeared in the blackened region, replaced by broad sp² carbon peaks (1320 and 1580 cm^-1).
Morphological Analysis (AFM/SEM): Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were used to characterize the surface morphology before and after laser treatment, revealing the granular structure and the formation of laser-induced notches and ridges.

Commercial Applications

The ability to selectively modify nanodiamonds based on their impurity content using low-power visible lasers has implications for several advanced materials and manufacturing sectors:

Nanodiamond Purification: The targeted destruction (graphitization) of nanodiamonds containing Ni- and N-related defects enables a laser-assisted extraction process to yield higher purity HP-HT nanodiamonds, critical for sensitive applications.
Micro-Optics and Patterning: Localized sp³ to sp² conversion allows for the direct writing of conductive or absorptive micro-patterns on insulating ND films, useful for fabricating micro-scale optical structures, diffractive elements, or embedded electrical contacts.
Quantum Sensing Materials: While the goal here is destruction, the detailed understanding of the resonant absorption and ionization dynamics of Ni- and N-related centers (which often act as precursors to quantum defects like NV centers) is valuable for controlling defect creation and stability in quantum sensing applications.
Advanced Composites: Creating localized conductive pathways (sp² carbon) within bulk insulating ND matrices could be used to engineer novel thermal or electrical properties in composite materials.

View Original Abstract

Laser-induced graphitization of 100 nm monocrystals of diamond particles synthesized by high-pressure high-temperature (HP-HT) methods is not typically observed. The current study demonstrates the graphitization of 150 nm HP-HT nanodiamond particles in ca. 20-μm-thick thin films formed on a glass substrate when the intensity of a focused 633 nm He-Ne laser exceeds a threshold of ~ 33 kW/cm2. Graphitization is accompanied by green luminescence. The structure and morphology of the samples were investigated before and after laser excitation while using X-ray diffraction (XRD), Raman spectroscopy, atomic force (AFM), and scanning electron microscopy (SEM). These observations are explained by photoionization of [Ni-N]- and [N]-centers, leading to the excitation of electrons to the conduction band of the HP-HT nanodiamond films and an increase of the local temperature of the sample, causing the transformation of sp3 HP-HT nanodiamonds to sp2-carbon.

Tech Support

Original Source

DOI: https://doi.org/10.3390/app10093329

References

2012 - The properties and applications of nanodiamonds [Crossref]
2019 - Optical limiting properties of surface functionalized nanodiamonds probed by the Z-scan method [Crossref]
2019 - Colloids of detonation nanodiamond particles for advanced applications [Crossref]
2019 - Immobilization of Single Particles of Detonation Nanodiamonds in Langmuir-Blodgett Films Using Octadecylamine [Crossref]
2001 - Detonation synthesis ultradispersed diamonds: Properties and applications [Crossref]
2014 - Surface Modifications of Detonation Nanodiamonds Probed by Multiwavelength Raman Spectroscopy [Crossref]
2015 - Science and engineering of nanodiamond particle surfaces for biological applications (Review) [Crossref]
1955 - Man-Made Diamonds [Crossref]
1979 - Optical absorption and luminescence in diamond [Crossref]
1997 - Scanning Confocal Optical Microscopy and Magnetic Resonance on Single Defect Centers [Crossref]