Chemometric Raman Spectral Analysis of Diamond-like Carbon Films Deposited by Plasma Discharge
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-06-25 |
| Journal | Journal of Photopolymer Science and Technology |
| Authors | Yoshihisa Osano, Hiroyuki Fukue, Susumu Takabayashi, Shinsuke Kunitsugu, Yuichi Imai |
| Institutions | Okayama University of Science, Engineering Systems (United States) |
| Citations | 2 |
| Analysis | Full AI Review Included |
Chemometric Raman Spectral Analysis of Diamond-like Carbon Films
Section titled âChemometric Raman Spectral Analysis of Diamond-like Carbon FilmsâExecutive Summary
Section titled âExecutive SummaryâThis study introduces a chemometric approach using Nonlinear Least Squares (NLS) fitting combined with a pseudo-Voigt function to automate and standardize the five-peak separation analysis of Diamond-like Carbon (DLC) Raman spectra.
- Core Problem Solved: Conventional two-peak fitting is structurally limited, and manual five-peak fitting is slow, requires high expertise, and lacks reproducibility.
- Methodology: Automated five-peak separation analysis using the Levenberg-Marquardt NLS algorithm and a modified pseudo-Voigt function.
- Computational Efficiency: The pseudo-Voigt function successfully replaced the computationally complex Voigt function (R2 = 0.99996), reducing execution time by over 80% (0.110 s vs. 0.575 s).
- Accuracy Improvement: Automated NLS fitting consistently achieved higher fitting accuracy (higher R2 and lower chi-squared, X2) compared to conventional manual fitting for both nonhydrogenated (HF-HiPIMS) and hydrogenated (AC-HV-CVD) DLC films.
- Engineering Impact: The automated process performs over 100 calculations in less than 3 seconds, providing highly precise, reproducible, and fast structural analysis results independent of the analystâs skill level.
- Structural Insight: The method facilitates consistent interpretation of the five active Raman bands (N, D, G-, G+, Dâ), which correlate strongly with the sp3 C-C/(sp3 C-C + sp2 C=C) ratio.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Deposition Method 1 | HF-HiPIMS | N/A | Nonhydrogenated DLC |
| Target Material | Graphite (3-inch diameter) | N/A | HF-HiPIMS |
| Operating Pressure | 0.5 | Pa | HF-HiPIMS |
| Applied Voltage (Negative) | -810 | V | HF-HiPIMS |
| Waveform Frequency | 200 | Hz | HF-HiPIMS |
| Deposition Method 2 | AC-HV-CVD | N/A | Hydrogenated DLC |
| AC Voltage | 5 | kV | AC-HV-CVD |
| Offset Voltage | 2 | kV | AC-HV-CVD |
| CH4 Gas Flow Rate | 96 | sccm | AC-HV-CVD |
| Operating Pressure | 39 | Pa | AC-HV-CVD |
| Raman Analysis | N/A | N/A | Spectrometer (Raman-11) |
| Laser Wavelength | 532 | nm | Excitation source |
| Output Power | 0.5 | mW | Laser power |
| Spot Diameter | 2.55 | ”m | Measurement area |
| Fitting Accuracy (HF-HiPIMS) | |||
| Conventional Manual R2 | 0.98874 | N/A | Voigt function |
| Automated NLS R2 | 0.99486 | N/A | Pseudo-Voigt function |
| Fitting Accuracy (AC-HV-CVD) | |||
| Conventional Manual R2 | 0.97583 | N/A | Voigt function |
| Automated NLS R2 | 0.97624 | N/A | Pseudo-Voigt function |
| Execution Time Comparison | |||
| Voigt Function (Avg) | 0.575 | s | Conventional method |
| Pseudo-Voigt Function (Avg) | 0.110 | s | New automated method |
Key Methodologies
Section titled âKey MethodologiesâThe study utilized two distinct plasma deposition methods to create DLC films and applied a chemometric approach for spectral analysis.
1. DLC Film Deposition
Section titled â1. DLC Film Depositionâ- Nonhydrogenated DLC (HF-HiPIMS):
- Used a 3-inch graphite target with Ar sputtering gas (5 sccm).
- Substrate rotation: 5 rpm.
- High-frequency inclusion high-power impulse magnetron sputtering (HF-HiPIMS) applied a negative voltage of -810 V at 200 Hz.
- Deposition occurred at 0.5 Pa operating pressure for 2 hours.
- Hydrogenated DLC (AC-HV-CVD):
- Used alternating current high voltage burst plasma chemical vapor deposition (AC-HV-CVD).
- Process gas: CH4 (96 sccm).
- Electrical parameters: 5 kV AC voltage, 2 kV offset voltage, 10 kHz frequency.
- Deposition occurred at 39 Pa operating pressure for 1 hour.
2. Chemometric Raman Spectral Analysis
Section titled â2. Chemometric Raman Spectral AnalysisâThe analysis followed a standardized, automated procedure using proprietary Python code:
- Data Preparation: Load spectral data, define the analysis range, and apply the differential spectrum method to identify initial peak positions.
- Preprocessing: Perform baseline removal and normalization of the spectra.
- Fitting Algorithm: Execute Nonlinear Least Squares (NLS) fitting using the Levenberg-Marquardt algorithm.
- Function Selection (Pseudo-Voigt): The complex Voigt function (Vj), which involves a convolution integral, was replaced by a modified pseudo-Voigt function (Pj).
- Pj is a linear combination of Gaussian (Gj) and Lorentzian (Lj) functions.
- The Breit-Wigner-Fano (BWF) function (B3) was applied exclusively to the G band (j=3) to account for asymmetry, though it simplifies to Lorentzian for insulating DLC films (where the asymmetry parameter q is infinite).
- Five-Peak Separation: The NLS method automatically optimized parameters for the five active Raman bands (N, D, G-, G+, Dâ) derived from structural symmetry operations.
- Validation: Fitting accuracy was confirmed by checking the coefficient of determination (R2) and chi-squared (X2) values, demonstrating superior performance compared to conventional manual fitting.
Commercial Applications
Section titled âCommercial ApplicationsâThe ability to rapidly and accurately characterize the sp3/sp2 ratio and structural properties of DLC films is critical for quality control and material development in several high-performance industries:
- Automotive Industry: DLC films are used for engine components and moving parts due to their low coefficient of friction and high hardness, improving fuel efficiency and lifespan.
- Medical Devices: DLCâs biocompatibility makes it suitable for coatings on implants, surgical tools, and other medical devices.
- Packaging Industry: DLC films provide excellent gas barrier properties, particularly for polyethylene terephthalate (PET) bottles, extending the shelf life of beverages.
- Industrial Coatings: Applications requiring high wear resistance, such as tools, molds, and precision components.
- Materials Research and Development (R&D): The automated, reproducible analysis method accelerates the development cycle for new DLC formulations by providing consistent, high-precision structural feedback.
View Original Abstract
In this study, a chemometric approach for five-peak separation analysis of the Raman spectra of diamond-like carbon (DLC) films was investigated. DLC films were deposited by high-frequency inclusion high-power impulse magnetron sputtering and alternating current high voltage burst plasma chemical vapor deposition. We used the pseudo-Voigt function as an alternative to the conventional Voigt function and applied the nonlinear least squares method. The results not only facilitate automated analysis but also guarantee highly accurate results regardless of the analystâs level of expertise. This approach is expected to lead to consistent interpretation of Raman spectral analysis of DLC films and further research and understanding of the properties of DLC films.