Investigating the Possible Origin of Raman Bands in Defective sp2/sp3 Carbons below 900 cm−1 - Phonon Density of States or Double Resonance Mechanism at Play?
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2019-11-29 |
| Journal | C – Journal of Carbon Research |
| Authors | C. Pardanaud, Gilles Cartry, Luc Lajaunie, Raúl Arenal, Josephus G. Buijnsters |
| Institutions | Universidad de Zaragoza, Centre National de la Recherche Scientifique |
| Citations | 29 |
| Analysis | Full AI Review Included |
6CCVD Technical Analysis: Low-Frequency Raman for $sp^{2}/sp^{3}$ Defect Quantification
Section titled “6CCVD Technical Analysis: Low-Frequency Raman for $sp^{2}/sp^{3}$ Defect Quantification”This technical documentation analyzes the findings of the research paper, “Investigating the Possible Origin of Raman Bands in Defective $sp^{2}/sp^{3}$ Carbons below 900 cm-1,” to highlight 6CCVD’s capability in providing highly customized diamond materials essential for replicating and advancing this characterization methodology.
Executive Summary
Section titled “Executive Summary”This study successfully leverages multiwavelength Raman spectroscopy to explore the low-frequency spectral region (300-900 cm-1) in defective carbon materials, yielding novel methods for quantifying $sp^{2}$ content in diamond films.
- Novel Characterization Range: The research confirms that the low-frequency region (300-900 cm-1), often attributed solely to the Phonon Density of States (PDOS), provides critical structural information about defects in CVD diamond and amorphous carbon.
- $sp^{2}$ Content Correlation: A strong inverse correlation was established between the film’s $sp^{2}$ content (as determined by EELS) and the relative height of the broad Raman band at 400 cm-1 ($H_{400}$) compared to the G band height ($H_G$).
- Standardized $sp^{3}$ Estimation: A major finding is the correlation between the $H_{400}/H_G$ ratio (measured using the accessible 633 nm laser) and the $H_{diamond}/H_G$ ratio (which typically requires a less common UV 325 nm laser). This offers a potential standard tool for rapidly estimating $sp^{3}$ content in defective diamond films.
- Microstructure Differentiation: The study analyzed three key diamond microstructures generated by varying $CH_4/H_2$ concentrations, confirming that $sp^{2}$ content increases with higher methane flow.
- Defect Mechanism Differentiation: The low-frequency spectral response for amorphous carbon (PDOS mechanism) was shown to be non-dispersive, contrasting sharply with the highly dispersive response observed in implanted graphite (Double Resonance mechanism).
Technical Specifications
Section titled “Technical Specifications”The following parameters and data points, extracted from the analysis of CVD diamond film growth and characterization, define the operational boundaries of this research.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Analyzed Spectral Range | 300 to 900 | cm-1 | Phonon Density of States (PDOS) Region |
| Laser Excitation Wavelengths | 325, 514, 633 | nm | Multiwavelength Raman analysis |
| CVD Diamond Thickness | ~3.5 | µm | Nominal film thickness |
| CVD Substrate Temperature | ~800 | °C | Consistent deposition parameter |
| CVD Filament Temperature | 2200 | °C | HFCVD growth parameter |
| CVD System Pressure | 15 | mbar | HFCVD growth parameter |
| Methane Concentration | 1.0 to 3.0 | vol.% | Controlled parameter defining $sp^{2}$ content |
| Diamond Band Position | 1332 | cm-1 | Characteristic SCD signal |
| G Band Position (Graphite) | 1582 | cm-1 | Characteristic $sp^{2}$ signal in pure materials |
| PDOS Feature Position | 400 | cm-1 | Broad band used for $H_{400}/H_G$ correlation |
| High $sp^{3}$ Sample | D (1%) | - | Methane: 1.0 vol.% (lowest $sp^{2}$ content) |
| Low $sp^{3}$ Sample | D (3%) | - | Methane: 3.0 vol.% (highest $sp^{2}$ content) |
Key Methodologies
Section titled “Key Methodologies”The experiment required meticulous control over material properties, particularly the $sp^{2}/sp^{3}$ ratio, achieved through precise material synthesis and advanced multi-spectroscopic characterization.
- CVD Diamond Synthesis (HFCVD):
- Precursor Gas: Methane ($CH_4$) and Hydrogen ($H_2$) mixtures.
- Composition Control: Methane concentration was systematically varied (1.0, 2.0, and 3.0 vol.%) to produce diamond films ranging from high-purity microcrystalline diamond (MCD) to highly defective ‘cauliflower’ diamond.
- Seeding: Mirror-polished p-type silicon (100) substrates were seeded using a suspension of diamond micropowder (1-2.5 µm) in an ultrasonic bath.
- Hydrogenated Amorphous Carbon (a-C:H) Preparation:
- PECVD deposition on Si wafer using pure methane (13.56 MHz RF plasma, 2 Pa, -200 V DC self-bias).
- Post-Heating: Samples were heated under 1.5 bar Argon atmosphere at 500 °C for varying times (15, 120, 1000 minutes) to modify hydrogen content and microstructure.
- Graphite Defect Induction:
- Highly Oriented Pyrolytic Graphite (HOPG) was exposed to RF deuterium plasma (0.2 Pa, 100W) to induce controlled in-plane and out-of-plane defects via ion bombardment (250 eV/D and 400 eV/D impact energy).
- $sp^{2}/sp^{3}$ Quantification (EELS):
- Electron Energy Loss Spectroscopy (EELS) was performed in cross-section geometry (TEM preparation required mechanical thinning and ion-milling).
- The $sp^{2}$ fraction was quantified using the $R$ ratio (based on the $\pi^*$ peak) referenced against HOPG.
- Multiwavelength Raman Spectroscopy:
- Spectra were recorded using 633 nm, 514 nm, and 325 nm lasers.
- Laser power was carefully adjusted (maximum ~1 mW/µm2) to prevent sample evolution during analysis.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”The findings in this paper confirm the critical link between precise material synthesis (specifically $sp^{2}$ content control) and advanced spectroscopic characterization. 6CCVD is uniquely positioned to supply the requisite diamond materials and customization services needed to replicate or extend this research into commercial applications.
Applicable Materials
Section titled “Applicable Materials”To replicate the controlled $sp^{2}$ variation and defect analysis presented in this study, 6CCVD recommends materials spanning the spectrum of diamond quality:
- Optical Grade Single Crystal Diamond (SCD): Required for applications mirroring the D (1%) sample (1.0 vol.% $CH_4/H_2$ equivalent) where maximizing $sp^{3}$ content and minimizing defects (low $sp^{2}$ incorporation) is essential for accurate Raman and EELS analysis or high-purity device fabrication.
- 6CCVD Capability: SCD wafers with thicknesses from 0.1 µm up to 500 µm, offering superior material purity control.
- Controlled Polycrystalline Diamond (PCD) Films: Necessary for the higher defect density samples (D (2%) and D (3%) equivalents) where specific, reproducible levels of $sp^{2}$ incorporation are needed for defect correlation studies or wear-resistant coatings.
- 6CCVD Capability: PCD growth allows precise tuning of grain size and defect density through adjusted gas mixture ratios, ensuring reproducible $sp^{2}$ content control, directly supporting the methodology described.
- Boron-Doped Diamond (BDD) Substrates: While not the focus of this paper, BDD’s electrical properties often depend on its microstructure and defect incorporation, which can be analyzed using the low-frequency PDOS approach demonstrated here.
Customization Potential
Section titled “Customization Potential”The methodology relies on standardized samples (e.g., films grown on Si). 6CCVD offers the flexibility necessary for scaling research results or integrating them into complex device architectures:
| Service | 6CCVD Offering | Relevance to Research Needs |
|---|---|---|
| Custom Dimensions | Plates/wafers up to 125mm (PCD). | Enables large-scale characterization mapping or fabrication of high-power optical/electronic components. |
| Metalization Layers | Au, Pt, Pd, Ti, W, Cu (Internal capability). | Allows for direct integration of diamond materials (SCD, PCD, BDD) into devices requiring ohmic contacts or reflective/protective coatings (e.g., Ti/Pt/Au stack often used in carbon studies). |
| Precision Polishing | Ra < 1nm (SCD), Ra < 5nm (Inch-size PCD). | Crucial for cross-section analysis (TEM/EELS) and subsequent analysis that requires highly uniform surface morphology. |
| Thickness Control | SCD/PCD from 0.1µm to 500µm. | Essential for optimizing the depth of penetration for various Raman laser wavelengths (325 nm vs. 633 nm) used in this study. |
Engineering Support
Section titled “Engineering Support”This paper’s innovative use of the 633 nm laser to quantify $sp^{3}$ content offers a faster quality control pathway for diamond manufacturers and consumers. 6CCVD’s in-house PhD team can assist researchers in standardizing this new characterization technique for MPCVD Diamond Quality Control projects.
We provide consultation on material selection, growth recipe optimization (to achieve specific $sp^{2}/sp^{3}$ ratios), and custom processing to ensure the diamond material meets the exact specifications required for cutting-edge spectroscopic analysis and subsequent device integration.
Call to Action: For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. 6CCVD offers global shipping (DDU default, DDP available) for your advanced material needs.
View Original Abstract
Multiwavelength Raman spectroscopy (325, 514, 633 nm) was used to analyze three different kinds of samples containing sp2 and sp3 carbons: chemical vapor deposited diamond films of varying microstructure, a plasma-enhanced chemical vapor deposited hydrogenated amorphous carbon film heated at 500 °C and highly oriented pyrolytic graphite exposed to a radio-frequent deuterium plasma. We found evidence that the lower part of the phonon density of states (PDOS) spectral region (300-900 cm−1) that rises when defects are introduced in crystals can give more information on the structure than expected. For example, the height of the PDOS, taken at 400 cm−1 and compared to the height of the G band, depends on the sp2 content, estimated by electron energy-loss spectroscopy. This ratio measured with 633 nm laser is more intense than with 514 nm laser. It is also correlated for diamond to the relative intensity ratio between the diamond band at 1332 cm−1 and the G band at ≈1500-1600 cm−1 when using 325 nm laser. Moreover, it is found that the shape of the PDOS of the exposed graphite samples is different when changing the wavelength of the laser used, giving evidence of a double resonance mechanism origin with the rise of the associated D3, D4 and D5 bands, which is not the case for a-C:H samples.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
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