研究目的
To systematically investigate the graphene deposition on refractory metals (e.g., Ta and Re) at temperatures much higher than 1000 °C to understand if higher temperatures improve graphene quality and the underlying growth mechanisms.
研究成果
Elevated temperatures (up to ~3000 °C) are beneficial for improving graphene quality on refractory metals by enhancing surface diffusion and reducing defects. On Ta, monolayer graphene is achieved due to carbide formation suppressing carbon segregation, but residual bonds degrade quality. On Re, optimal growth is at 1400 °C to avoid multilayer formation from carbon segregation. However, graphene quality is comparable or worse than that on Cu or Ni, and practical applications are limited by handling difficulties and high costs. This study provides fundamental insights into high-temperature CVD graphene growth on refractory metals.
研究不足
The graphene quality on refractory metals is not as high as expected due to residual bonds with substrates (e.g., on Ta) and carbon segregation issues (e.g., on Re at high temperatures). Refractory metals become brittle after high-temperature processing and are hard to handle or reuse. Transfer techniques are less efficient compared to Cu or Ni, with lower success rates. The catalytic ability of refractory metals may not surpass that of Cu or Ni at the same temperatures.
1：Experimental Design and Method Selection:
Graphene thin films were grown using chemical vapor deposition (CVD) on refractory metals (Ta and Re) in commercial cold wall nanocarbon CVD systems. The study focused on varying growth temperatures (up to ~3000 °C), gas compositions (e.g., CH4, C2H2, Ar, H2), and flow rates to optimize graphene quality. Mechanisms such as carbon absorption, carbide formation, and carbon segregation were analyzed.
2：Sample Selection and Data Sources:
Commercial Ta (99.9%, 20–50 μm thick) and Re (99.99%, 50 μm thick) foils were purchased from Goodfellow and Advent. In-house evaporated 200 nm thick Ta films on Si substrates with 400 nm oxide were also used. Samples were cleaned organically (5 min in isopropanol and 5 min in acetone) before deposition.
3：9%, 20–50 μm thick) and Re (99%, 50 μm thick) foils were purchased from Goodfellow and Advent. In-house evaporated 200 nm thick Ta films on Si substrates with 400 nm oxide were also used. Samples were cleaned organically (5 min in isopropanol and 5 min in acetone) before deposition. List of Experimental Equipment and Materials:
3. List of Experimental Equipment and Materials: CVD systems (Aixtron, Black Magic), thermal couple or infrared pyrometer for temperature monitoring, mass flow controllers for gases (H2, Ar, C2H2, CH4), PMMA polymer for transfer support, etching solutions (HF:HNO3:H2O = 1:1:6 for Ta, 0.25 M NaOH for electrochemical bubbling, H2O2 for Re), optical microscopy, SEM, TEM, Raman spectroscopy, XRD, electrical measurement setups for field-effect transistors.
4：25 M NaOH for electrochemical bubbling, H2O2 for Re), optical microscopy, SEM, TEM, Raman spectroscopy, XRD, electrical measurement setups for field-effect transistors. Experimental Procedures and Operational Workflow:
4. Experimental Procedures and Operational Workflow: After cleaning, substrates were annealed in Ar and H2 atmosphere, followed by introduction of carbon precursors (C2H2 or CH4) for graphene growth. Growth times varied (e.g., 5-17 min). Quench cooling was applied. Graphene transfer involved wet transfer using PMMA support: etching for Ta (HF:HNO3:H2O solution) and bubbling or H2O2 etching for Re. Characterizations included optical imaging, Raman spectroscopy, XRD, SEM, TEM, and electrical measurements.
5：Data Analysis Methods:
Raman spectra were analyzed for I2D/IG and ID/IG ratios to assess graphene quality. XRD patterns were analyzed using Jade software. SEM and TEM images were used to observe morphology and defects. Electrical properties were measured to calculate field-effect mobility.
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CVD system
Black Magic
Aixtron
Used for chemical vapor deposition of graphene on refractory metals at high temperatures.
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Mass flow controller
Controls the flow rates of gases (H2, Ar, C2H2, CH4) during CVD growth.
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Thermal couple
Monitors substrate temperature during CVD growth.
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Infrared pyrometer
Monitors substrate temperature during CVD growth.
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PMMA polymer
Used as mechanical support for graphene transfer processes.
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Optical microscope
Used for imaging graphene samples after transfer.
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SEM
Used for scanning electron microscopy to observe graphene morphology.
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TEM
Used for transmission electron microscopy to analyze graphene structure and defects.
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Raman spectrometer
Used for Raman spectroscopy to assess graphene quality via I2D/IG and ID/IG ratios.
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XRD equipment
Used for X-ray diffraction to characterize carbide formation in substrates.
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Electrical measurement setup
Used for fabricating and measuring field-effect transistors to assess graphene electrical properties.
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