Synthesis and Mechanism Study of Carbon Nanowires, Carbon Nanotubes, and Carbon Pompons on Single-Crystal Diamonds

At a Glance

Metadata	Details
Publication Date	2024-05-21
Journal	Crystals
Authors	Shuai Wu, Qiang Wang, Kesheng Guo, Lei Liu, Jie Bai
Institutions	Ji Hua Laboratory, Chongqing Jiaotong University
Citations	3
Analysis	Full AI Review Included

Executive Summary

This study successfully demonstrates the selective synthesis of three distinct carbon nanomaterials—Carbon Pompons (CPs), Carbon Nanowires (CNWs), and Carbon Nanotubes (CNTs)—on single-crystal diamond substrates using high-power Microwave Plasma Chemical Vapor Deposition (MPCVD).

Selective Deposition Control: The morphology of the deposited carbon was precisely controlled by adjusting the substrate surface condition (rectangular pits vs. flat surface) and the nitrogen (N₂) flow rate.
Heterostructure Integration: This work establishes a novel approach for integrating diamond with carbon nanomaterials, producing robust heterostructures under high-power conditions.
Role of Substrate Morphology: CPs (20 µm diameter) were selectively grown inside rectangular diamond pits, where plasma-enhanced etching resulted in a localized temperature increase (55 K higher than flat surfaces).
Role of N₂ Flow Rate: N₂ concentration was identified as the critical factor differentiating CNW and CNT growth on Mo catalyst films. Low N₂ flow (3 sccm) yielded solid CNWs (80 nm diameter), while high N₂ flow (9 sccm) yielded hollow CNTs (400 nm diameter).
Catalytic Mechanism: Optical Emission Spectroscopy (OES) confirmed that the introduction of N₂ promoted the formation of CN groups, which, along with C₂ groups, played vital catalytic roles in guiding carbon atom precipitation at the Mo/diamond interface.
Structural Confirmation: Raman spectroscopy confirmed the presence of stress-induced blue shifts in CNTs and CNWs, and defect peaks associated with the formation of CPs.

Technical Specifications

The following table summarizes key operational parameters and resulting material characteristics derived from the four primary experiments.

Parameter	Value	Unit	Context
Substrate Type	CVD (100) Type IIb	N/A	Single-crystal diamond
Microwave Frequency	2450	MHz	MPCVD System
Max Operating Pressure	300	Torr	MPCVD System capability
CP Average Diameter	20	µm	Deposited in rectangular pit (Expt 1)
CNP Average Diameter	10	µm	Deposited on flat surface (Expt 2)
CNW Average Diameter	80	nm	Mo-catalyzed (Expt 3)
CNT Average Diameter	400	nm	Mo-catalyzed (Expt 4)
Mo Catalyst Thickness	150	nm	Sputtered film for CNW/CNT growth
Temperature Difference (Pit vs. Flat)	55	K	Difference between CP (1258 K) and CNP (1203 K) sites
CNT Raman Shift (Diamond Peak)	1337	cm^-1	Blue-shifted due to internal stresses
CNW Raman Shift (Diamond Peak)	1330	cm^-1	Accompanied by a broad graphite peak

Key Methodologies

The synthesis utilized a butterfly-shaped resonant cavity MPCVD system capable of high-power operation (>10 kW).

Substrate Preparation:
- Substrates were 3x3 mm CVD (100) single-crystal diamonds.
- Wet chemical cleaning involved heating in H₂SO₄/HNO₃ (1:1) at 473 K, followed by ultrasonic treatment in deionized water, ethanol, and acetone.
- For CNW/CNT growth (Expts 3 & 4), a 150 nm Mo film was deposited via magnetron sputtering to serve as the catalyst, forming MoC during growth.
CP/CNP Deposition (Expts 1 & 2, No N₂):
- Gas Mixture: 300 sccm H₂, 45 sccm CH₄.
- Power/Pressure: 5 kW, 100 Torr.
- Temperature Control: CP growth occurred in the rectangular pit (1258 K); CNP growth occurred on the flat surface (1203 K).
- Outcome: CP formation was driven by high temperature and plasma-enhanced etching within the pit region.
CNW Deposition (Expt 3, Low N₂):
- Substrate: Mo-coated diamond.
- Gas Mixture: 200 sccm H₂, 45 sccm CH₄, 3 sccm N₂.
- Power/Pressure: 4 kW, 80 Torr.
- Temperature: 973 K.
- Outcome: Solid Carbon Nanowires (CNWs) were formed, attributed to the lower N₂ flow rate.
CNT Deposition (Expt 4, High N₂):
- Substrate: Mo-coated diamond.
- Gas Mixture: 200 sccm H₂, 45 sccm CH₄, 9 sccm N₂.
- Power/Pressure: 4.5 kW, 80 Torr.
- Temperature: 993 K.
- Outcome: Hollow Carbon Nanotubes (CNTs) were formed, facilitated by the higher N₂ flow rate promoting CN group formation and methane decomposition.

Commercial Applications

The ability to integrate highly stable carbon nanomaterials directly onto single-crystal diamond substrates opens pathways for advanced composite materials in several high-performance sectors.

Industry/Application	Relevance to Diamond-Carbon Heterostructures
Advanced Microelectronics	Diamond’s high carrier mobility and low dielectric constant, combined with the conductive network provided by CNWs/CNTs, are ideal for fabricating high-frequency, high-power electronic devices and vacuum nanoelectronics emitters.
Thermal Management Systems	Utilizing the superior thermal conductivity of diamond (24 W/cm·K) as a substrate, with integrated CNTs/CNWs acting as efficient thermal pathways, for cooling high-density integrated circuits and power modules.
Mechanical and Tribological Coatings	Creation of super-hard, low-friction coatings for cutting tools and mechanical parts. Covalent bonding between the carbon nanostructures and the diamond substrate ensures high adhesion and reduced wear (up to 20% reduction reported in related studies).
Electrochemical Sensing and Catalysis	CNTs grown on diamond (BDD) substrates exhibit enhanced effective electrode area and heightened sensitivity, making them suitable for advanced biosensors and electrochemical reactors.
Diffractive Optics and Photonics	Potential for creating novel composite optical elements where the carbon nanostructures are used to tailor surface properties or enhance light interaction on the diamond surface.

View Original Abstract

Carbon nanomaterials are in high demand owing to their exceptional physical and chemical properties. This study employed a mixture of CH4, H2, and N2 to create carbon nanostructures on a single-crystal diamond using microwave plasma chemical vapor deposition (MPCVD) under high-power conditions. By controlling the substrate surface and nitrogen flow rate, carbon nanowires, carbon nanotubes, and carbon pompons could be selectively deposited. The results obtained from OES, SEM, TEM, and Raman spectroscopy revealed that the nitrogen flow rate and substrate surface conditions were crucial for the growth of carbon nanostructures. The changes in the plasma shape enhanced the etching effect, promoting the growth of carbon pompons. The CN and C2 groups play vital catalytic roles in the formation of carbon nanotubes and nanowires, guiding the precipitation and composite growth of carbon atoms at the interface between the Mo metal catalysts and diamond. This study demonstrated that heterostructures of diamond-carbon nanomaterials could be produced under high-power conditions, offering a new approach to integrating diamond and carbon nanomaterials.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst14060481

References

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2024 - On the vibrational behavior of the conventional and hetero-junction carbon nanotubes [Crossref]
2024 - High-performance carbon nanofiber conductive films induced by titanium carbide [Crossref]
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2021 - Nanoengineering of Advanced Carbon Materials for Sodium-Ion Batteries [Crossref]
2019 - Preparation and characterisation of carbon spheres for carbon dioxide capture [Crossref]