Physicochemical and Mechanical Performance of Freestanding Boron-Doped Diamond Nanosheets Coated with C -H -N -O Plasma Polymer

At a Glance

Metadata	Details
Publication Date	2020-04-15
Journal	Materials
Authors	Michał Rycewicz, Łukasz Macewicz, Jiří Kratochvíl, Alicja Stanisławska, Mateusz Ficek
Institutions	Institute of Fluid Flow-Machinery, Polish Academy of Sciences
Citations	2
Analysis	Full AI Review Included

Executive Summary

This research details the successful fabrication and characterization of a novel, mechanically stable, and optically transparent composite material consisting of a freestanding Boron-Doped Diamond (BDD) nanosheet coated with a nylon-like (C:H:N:O) plasma polymer film.

Core Achievement: Creation of flexible, freestanding, conductive diamond nanosheets encapsulated in a protective, transparent polymer layer, overcoming the inherent brittleness of pristine diamond foils.
Mechanical Stability: The nylon coating significantly enhances mechanical stability, allowing the composite to be handled and transferred, a critical step for flexible device integration.
Optical Performance: The nylon film exhibits excellent transparency, with an extinction coefficient near zero across the visible and near-infrared (500-1690 nm) spectrum, making it suitable for spectroelectrochemical applications.
Fabrication Method: BDD nanosheets were grown via Microwave Plasma-Assisted Chemical Vapor Deposition (MWPACVD) and subsequently coated using magnetron sputtering of a Nylon 6.6 target.
Interface Properties: A transition zone of approximately 1 µm thickness, consisting of a nylon-diamond mixture, formed during deposition, providing strong adhesion and intermediate mechanical properties between the soft polymer and the hard diamond.
Application Potential: The resulting flexible stack is ideal for developing robust electronic sensors, flexible biosensing devices, and thermal heat spreaders operating stably in aqueous environments.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Nanosheet Thickness	4.2	µm	MWPACVD growth (720 min)
Nylon Coating Thickness (Tested)	500 and 2000	nm	Nanoindentation tests
Nylon Film Roughness (on Si)	6.75	nm	Spectroscopic ellipsometry (489 nm thick film)
Diamond Growth Temperature	500	°C	MWPACVD process
Diamond Growth Pressure	50	Torr	MWPACVD process
Boron/Carbon ([B]/[C]) Ratio	10,000	ppm	Dopant concentration
BDD Charge Carrier Density	6.2 x 10¹⁹	cm^-3	Hall effect measurement
BDD Resistivity (Uncoated)	0.11	Ωcm	Freestanding nanosheet
BDD Hall Mobility	9	cm² V^-1 s^-1	Hall effect measurement
Nylon Refractive Index (n_p)	1.62	(unitless)	At 589 nm
Nylon Extinction Coefficient (k)	~0	(unitless)	Range 500 nm to 1690 nm (VIS/NIR)
Nylon Deposition Speed	8.4 ± 0.8	nm·min^-1	Spectroscopic ellipsometry
Nylon Sputtering Pressure	3	Pa	Working pressure (Argon)
Nylon Sputtering Power	50	W	RF power (13.56 MHz)
Nylon-Diamond Transition Zone	~1000	nm	Cross-section SEM analysis
Nylon Surface Energy (Nylon 6,6)	46	mJ/m²	Previous study

Key Methodologies

The composite structure was fabricated in a multi-step process involving diamond growth, mechanical delamination, and polymer coating.

1. Boron-Doped Diamond (BDD) Growth (MWPACVD)

Substrate Preparation: Polished Tantalum foil (1 cm x 1 cm) was seeded for 30 minutes using a colloid of nanoscale diamond particles in water.
CVD Setup: Growth was performed in a SEKI Technotron AX5400S MW PACVD system operating at 2.45 GHz.
Growth Parameters: Substrate temperature was maintained at 500 °C, and microwave power was 1100 W.
Gas Mixture: Methane concentration was kept below 2%, with a total flow rate of 300 sccm. Diborane (B₂H₆) was used as the dopant precursor, achieving a [B]/[C] ratio of 10,000 ppm.
Result: After 720 minutes of growth at 50 Torr, a polycrystalline BDD foil of 4.2 µm thickness was obtained.

2. Polymeric C:H:N:O (Nylon-like) Film Deposition (Magnetron Sputtering)

Setup: A 3-inch balanced magnetron equipped with a Nylon 6.6 target was used.
Vacuum and Gas Flow: The chamber was pumped to a base pressure less than 5 x 10^-4 Pa. Argon gas was introduced at 20 SCCM, setting the working pressure to 3 Pa.
Sputtering: 50 W of RF power (13.56 MHz) was applied to the dielectric target. Substrate temperature was kept below 50 °C.
Stabilization: After deposition, samples were held under vacuum for 15 minutes to allow the plasma polymer to relax and improve film stability.

3. Nanosheet Transfer and Characterization

Delamination and Transfer: Due to low adhesion, the BDD nanosheets were mechanically removed (delaminated) from the tantalum substrate using tweezers.
Mounting: The freestanding nanosheet was transferred onto a p-type silicon substrate using silver paste (EPO-TEK H20E).
Curing: The sample was cured for 3 hours in a vacuum oven at 80 °C.
Analysis:
- Morphology: Scanning Electron Microscopy (SEM) confirmed full surface encapsulation, though isotropic cracks were observed due to the rough nanodiamond surface topography.
- Composition: Raman spectroscopy confirmed the presence of characteristic nylon chemical bonds and indicated a decrease in crystallinity compared to bulk nylon.
- Mechanical Testing: Nanoindentation tests (NanoTest™ Vantage, Berkovich indenter) were performed in multiload mode to determine hardness (H) and reduced Young’s modulus (E) profiles, revealing the properties of the nylon, the diamond, and the intermediate transition zone.

Commercial Applications

The development of flexible, transparent, conductive, and mechanically robust diamond-polymer composites opens doors for several high-value engineering applications:

Flexible Biosensing and Electrochemistry:
- Application: Flexible chemical multielectrode sensors, neural probes, and wearable electrochemical devices.
- Benefit: BDD provides a wide potential window and low background current, while the nylon provides mechanical flexibility and stability in aqueous (water-based) biological environments.
Optoelectronics and Transparent Conductors:
- Application: Optically transparent conductive probes and flexible display components.
- Benefit: The nylon coating is highly transparent in the visible and NIR range, and the BDD layer provides conductivity, enabling spectroelectrochemical sensing with large anisotropy.
Thermal Management:
- Application: Flexible thermal heat spreaders for electronics.
- Benefit: Diamond’s superior thermal conductivity, combined with the flexible polymer base, allows for robust heat dissipation in non-planar or dynamic electronic systems.
High-Performance Composites:
- Application: Functionally-engineered thermoplastics and structural components requiring high strength and flexibility.
- Benefit: The diamond reinforcement significantly increases the hardness (two-fold) and Young’s Modulus (four-fold) of the polymer composite compared to pure nylon.

View Original Abstract

The physicochemical and mechanical properties of thin and freestanding heavy boron-doped diamond (BDD) nanosheets coated with a thin C:H:N:O plasma polymer were studied. First, diamond nanosheets were grown and doped with boron on a Ta substrate using the microwave plasma-enhanced chemical vapor deposition technique (MPECVD). Next, the BDD/Ta samples were covered with nylon 6.6 to improve their stability in harsh environments and flexibility during elastic deformations. Plasma polymer films with a thickness of the 500-1000 nm were obtained by magnetron sputtering of a bulk target of nylon 6.6. Hydrophilic nitrogen-rich C:H:N:O was prepared by the sputtering of nylon 6.6. C:H:N:O as a film with high surface energy improves adhesion in ambient conditions. The nylon-diamond interface was perfectly formed, and hence, the adhesion behavior could be attributed to the dissipation of viscoelastic energy originating from irreversible energy loss in soft polymer structure. Diamond surface heterogeneities have been shown to pin the contact edge, indicating that the retraction process causes instantaneous fluctuations on the surface in specified microscale regions. The observed Raman bands at 390, 275, and 220 cm−1 were weak; therefore, the obtained films exhibited a low level of nylon 6 polymerization and short-distance arrangement, indicating crystal symmetry and interchain interactions. The mechanical properties of the nylon-on-diamond were determined by a nanoindentation test in multiload mode. Increasing the maximum load during the nanoindentation test resulted in a decreased hardness of the fabricated structure. The integration of freestanding diamond nanosheets will make it possible to design flexible chemical multielectrode sensors.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma13081861

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