Laser-Induced Graphitization of Diamond Under 30 fs Laser Pulse Irradiation
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-03-18 |
| Journal | The Journal of Physical Chemistry Letters |
| Authors | Bakhtiar Ali, Han Xu, D. Chetty, R. T. Sang, I. V. Litvinyuk |
| Institutions | Griffith University, Quantum (Australia) |
| Citations | 21 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Non-Thermal Graphitization: Ultra-short 30 fs laser pulses successfully converted sp3 diamond to sp2 graphitic carbon in single-crystal CVD diamond, achieving highly localized structural transformation with minimal to no Heat Affected Zone (HAZ) or thermal cracking.
- Quality Control via Fluence: A critical fluence threshold of 3.9 J/cm2 was identified. Irradiation below this threshold produced primarily highly crystalline sp2-aromatic graphitic fraction, while irradiation above this threshold resulted predominantly in amorphous carbon formation.
- Enhanced Processing Efficiency: A fractional increase in fluence (~20%, from 3.3 J/cm2 to 3.9 J/cm2) near the ablation threshold resulted in a substantial three-fold increase in the depth of the sp2 graphitized layer, demonstrating highly non-linear processing efficiency.
- Reduced Ablation Threshold: The use of sub-50 fs pulses (30 fs) significantly reduced the CVD ablation threshold by approximately 20-30% compared to longer femtosecond pulses (60-100 fs).
- Clean Carbon Structure: Raman analysis confirmed that the graphitized regions contained only sp2 aromatic rings, being completely devoid of sp2 olefinic chains.
- Surface Chemistry: C=O carbonyl groups were observed below the critical fluence, with gradual C=O cleavage occurring as irradiation fluence increased.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Laser Pulse Duration | 30 | fs | Ultra-short pulsed laser irradiation |
| Laser Wavelength | 800 | nm | Ti3+:sapphire system |
| Repetition Rate | 1 | kHz | Laser system operation |
| Material | n-type <100> CVD Diamond | N/A | Nitrogen content ~200 ppm |
| Fluence Range Tested | 2.2 to 6.8 | J/cm2 | Peak fluence range |
| Intensity Range Tested | 0.73 x 1014 to 2.3 x 1014 | W/cm2 | Peak intensity range |
| Critical Fluence Threshold | 3.9 | J/cm2 | Transition from crystalline graphite to amorphous carbon |
| Nominal Ablation Threshold (CVD) | 3.0 - 4.0 | J/cm2 | Reported for longer fs pulses |
| Focal Spot Size (1/e max intensity) | 10 | ”m | Beam focusing on sample surface |
| Scanning Speed | 15 | mm/s | Irradiation speed |
| Pulses per Site | 667 | N/A | Irradiation dose |
| Graphitization Depth (Low Fluence) | ca. 0.025 | ”m | At 2.2 J/cm2 |
| Graphitization Depth (High Fluence) | 0.2 | ”m | At 6.8 J/cm2 |
| Effective Crystallite Size (La) Range | 10 to 19 | nm | Decreased with increasing fluence |
| Hydrogen Content (H) | 5 to 15 | at.% | Increased above 3.9 J/cm2 fluence |
| Diamond Absorption Coefficient (α) | 1.16 x 105 | cm-1 | Used for depth calculation of sp2 layer |
Key Methodologies
Section titled âKey MethodologiesâThe study employed ultra-short pulsed laser irradiation combined with advanced spectroscopic and microscopic analysis to characterize the phase transformation in diamond.
- Laser Irradiation Setup: A Ti3+:sapphire laser system (800 nm, 30 fs, 1 kHz) was used. The linearly polarized Gaussian beam was focused to a 10 ”m spot size (M2 = 1.3) onto the CVD diamond sample surface.
- Fluence Control: Pulse energy was attenuated to achieve four distinct peak fluences (2.2, 3.3, 3.9, and 6.8 J/cm2).
- Processing Parameters: Samples were irradiated normal to the beam at a scanning speed of 15 mm/s, resulting in a dose of 667 pulses per irradiation site.
- Raman Spectroscopy: Micro-Raman spectra were obtained using an unpolarized 514 nm Ar+ ion laser at 293 K. Low power (0.1 mW) and extended acquisition time were used to prevent thermal damage during measurement.
- Spectral Deconvolution: Raman bands (sp3 diamond mode at ~1332 cm-1, D and G sp2 modes, and C=O mode at 1710 cm-1) were reconstituted using fully symmetric Gaussians after linear photoluminescence background subtraction.
- Crystallite Size Determination: The effective crystallite size (La) of the sp2 fraction was calculated using the ratio of the D to G peak intensities (I(D)/I(G)) based on the Tuinstra and Koenig relation, adapted for 514 nm excitation.
- Graphitized Depth Calculation: The depth (d) of the sp2 layer was estimated by applying the Beer-Lambert law to the attenuation of the 1332 cm-1 sp3 diamond core mode intensity.
- Microscopy: Optical microscopy and Scanning Electron Microscopy (SEM) were used to measure the width (w) of the graphitized tracks and observe surface topography and ablation characteristics.
Commercial Applications
Section titled âCommercial ApplicationsâThe ability to create highly localized, high-quality graphitic structures within diamond substrates using ultra-short pulses opens avenues for advanced carbon-based device manufacturing.
- Opto-Photonic Devices: Direct, single-step fabrication of robust ultra-wide bandgap âall carbonâ opto-electronic devices, including âgraphene-on-diamondâ heterostructures, without requiring high-temperature annealing or metal catalysts.
- Radiation Detection and Sensing: Development of miniaturized broad-beam light detectors and X-ray pixel detectors utilizing embedded graphitic electrodes for enhanced performance in radiotherapy and synchrotron applications.
- Energy Technology: Manufacturing of miniaturized thermionic solar cells using graphite distributed electrodes for photon-enhanced energy conversion.
- Infrared (IR) Optics: Production of robust broad-beam polarization filters and other IR optical components by leveraging the unique absorption properties of the laser-induced graphitic tracks.
- Precision Machining: Enhanced femtosecond laser processing of diamond and its derivatives (natural, nano-, micro-diamonds) for fabrication of systems with previously unattainable composition and properties, benefiting from the reduced ablation threshold and absence of HAZ/cracking.
View Original Abstract
The degree of laser-induced graphitization from a sp<sup>3</sup>-bonded to a sp<sup>2</sup>-bonded carbon fraction in a single crystal chemical vapor deposited (CVD) diamond under varying fluence of an ultrashort pulsed laser (30 fs, 800 nm, 1 kHz) irradiation has been studied. The tetrahedral CVD sp<sup>3</sup> phase is found to transition to primarily an sp<sup>2</sup> aromatic crystalline graphitic fraction below the critical fluence of 3.9 J/cm<sup>2</sup>, above which predominantly an amorphous carbon is formed. A fractional increase of fluence from 3.3 to 3.9 J/cm<sup>2</sup> (âŒ20%) results in a substantially (âŒ3-fold) increased depth of the sp<sup>2</sup> graphitized areas owing to the nonlinear interactions associated with a fs laser irradiation. Additionally, formation of a CâO carbonyl group is observed below the critical threshold fluence; the CâO cleavage occurrs gradually with the increase of irradiation fluence of 30 fs laser light. The implications for these findings on enhancement of fs driven processing of diamonds are discussed.