Graphene Induced Diamond Nucleation on Tungsten
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-01-01 |
| Journal | IEEE Open Journal of Nanotechnology |
| Authors | Yonhua Tzeng, C. S. Chang |
| Institutions | National Cheng Kung University |
| Citations | 5 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research introduces a novel, non-conventional method for achieving high-density, uniform diamond nucleation on non-diamond substrates, specifically tungsten (W), leveraging the unique properties of monolayer graphene edges.
- Novel Nucleation Mechanism: Diamond nucleation is chemically induced along the etched edges of monolayer graphene covering a tungsten substrate, eliminating the need for traditional nanodiamond seeding or Bias Enhanced Nucleation (BEN).
- High Density and Uniformity: The process yields a high nucleation density, estimated up to 1010 cm-2, resulting in continuous, pinhole-free polycrystalline diamond films.
- Role of Tungsten: The underlying tungsten is critical; it reacts with carbon atoms at the graphene edges to form stabilizing sp3 Carbon-Tungsten (C-W) bonds, which act as precursors for stable diamond nuclei formation.
- Scalable Edge Creation: Hydrogen-rich plasma etches the graphene, creating a network of micro-ribbons with abundant edges. Nucleation density increases proportionally with the number of randomly stacked graphene layers, maximizing the total reactive edge length.
- Substrate Protection: The remaining graphene layer protects the W substrate from excessive etching and promotes the formation of tungsten carbide (WC) primarily within the etched holes.
- Process Simplification: This technique offers a complementary route for diamond deposition, particularly advantageous for complex or electrically insulating substrates where BEN is difficult to apply uniformly.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Nucleation Density (Estimated) | 1010 | cm-2 | Based on 0.1 ”m spacing between nuclei (Fig. 2C). |
| Nucleation Density (Continuous Film) | > 108 | cm-2 | Achieved for continuous film with grain size < 1 ”m. |
| Substrate Temperature | 900 | °C | Standard Plasma Enhanced CVD (PECVD) growth temperature. |
| Gas Pressure | 50 | Torr | Standard PECVD operating pressure. |
| Methane Concentration (Nucleation) | 1 | % | Diluted in 99% H2 for initial nucleation phase. |
| Methane Concentration (Growth) | 3 | % | Diluted in 97% H2 for continuous film growth (4 hr duration). |
| Plasma Frequency | 915 | MHz | Microwave Plasma CVD system frequency. |
| Diamond Raman Peak Shift | 2 | cm-1 | Shifted from standard 1332 cm-1 to 1334 cm-1 due to internal stress. |
| Graphene Layers Tested | 1, 2, 3, 4 | layers | Nucleation density increased with layer count due to increased edge length. |
| Diamond Grain Size (4 layers) | < 1 | ”m | Achieved in continuous film grown with 4 stacked graphene layers. |
Key Methodologies
Section titled âKey MethodologiesâThe diamond films were grown using a microwave Plasma Enhanced Chemical Vapor Deposition (PECVD) system on silicon dioxide (SiO2) substrates coated with tungsten (W).
- Substrate Preparation: Silicon dioxide (SiO2) wafers were coated with a thin film of tungsten (W) using RF magnetron sputtering.
- Graphene Transfer: Monolayer graphene, initially grown on copper foil via CVD, was transferred onto the W/SiO2 substrate using a wet transfer technique.
- Layer Stacking: For density optimization, individual monolayer graphene sheets were randomly stacked (1, 2, 3, or 4 layers) to maximize the total length of exposed graphene edges.
- PECVD Nucleation Phase: The substrates were placed in the PECVD reactor and exposed to a plasma generated at 50 Torr pressure and 900 °C substrate temperature. The gas mixture consisted of 1% CH4 diluted in 99% H2.
- Edge Activation: The hydrogen-rich plasma preferentially etches the graphene, particularly at defects and domain boundaries, creating a network of graphene micro-ribbons and highly reactive edges.
- Chemical Nucleation: Carbon atoms at the newly exposed graphene edges react with the underlying W to form sp3 C-W bonds, which serve as stable sites for subsequent insertion of carbon radicals (like CH3) from the plasma, leading to diamond nucleus formation.
- PECVD Growth Phase: Continuous diamond films were grown using a 3% CH4 in 97% H2 mixture for 4 hours under the same temperature and pressure conditions.
- Characterization: Nucleation density, film quality, and phase composition were verified using SEM (for morphology and density), Raman spectroscopy (confirming diamond peak at 1334 cm-1 and graphene G-band), and XRD (confirming diamond and tungsten carbide formation).
Commercial Applications
Section titled âCommercial ApplicationsâThis graphene-induced nucleation method is highly relevant for applications requiring uniform, strongly adhering diamond films on non-diamond or complex substrates, bypassing the limitations of BEN and seeding.
- High-Power Electronics: Deposition of diamond (an excellent thermal conductor) onto refractory metal heat sinks (W, Mo) for thermal management in high-frequency and high-power devices (e.g., GaN/AlGaN HEMTs).
- Protective Coatings: Creating highly durable, chemically inert, and wear-resistant diamond coatings on complex geometries or large-area substrates where uniform ion bombardment (BEN) is impractical.
- Micro-Electro-Mechanical Systems (MEMS): Fabrication of diamond-based MEMS components, leveraging the ability to deposit uniform films without the need for conductive substrates or complex seeding steps.
- Optical Components: Manufacturing robust, large-area diamond optical windows for IR and millimeter-wave applications, capitalizing on the pinhole-free nature of the resulting films.
- Biomedical Implants: Producing uniform, biocompatible diamond coatings for dental implants or artificial retinas, where strong adhesion and chemical inertness are critical for long-term safety.
View Original Abstract
Chemical vapor deposition (CVD) of a diamond film on a non-diamond substrate begins with the insertion of diamond seeds or the formation of diamond nuclei on the substrate. For the deposition of a smooth, large-area and pin-hole free diamond film that adheres well to the substrate, diamond seeds or nuclei need to be of high density, uniformly distributed and adhere well to the substrate. Diamond seeding is not a diamond nucleation process. Bias enhanced nucleation (BEN) is the most effective means of heterogeneous nucleation of diamond for CVD diamond. It is based on a negative biasing voltage between the substrate and the diamond CVD plasma to accelerate positive ions from the plasma to bombard the substrate. Both direct diamond seeding and BEN have technical barriers in practical applications. New diamond nucleation techniques are desired. This paper reports novel heterogenous diamond nucleation along edge line of graphene on tungsten leading to the deposition of continuous diamond films. Based on experimental observation, a diamond nucleation mechanism assisted by sp3 C-W bonds at graphene edge is proposed. It is wished that scientists will become interested in revealing the precise diamond nucleation mechanism. With that, further optimization of this invention may lead to a new, complementary diamond nucleation process for practical deposition of diamond films.