Controlled seeding density of nanodiamonds on silicon and its influence on diamond film adhesion

At a Glance

Metadata	Details
Publication Date	2025-03-03
Journal	Functional Diamond
Authors	Zhixue Xing, Stephan Handschuh‐Wang, Tao Wang, Peigang Han, Bin He
Institutions	Shenzhen Institutes of Advanced Technology, Materials Technology (United Kingdom)
Citations	3
Analysis	Full AI Review Included

Technical Documentation & Analysis: Controlled Nanodiamond Seeding for Enhanced Adhesion

Reference: Xing et al. (2025). Controlled seeding density of nanodiamonds on silicon and its influence on diamond film adhesion. Functional Diamond, 5:1, 2472623.

Executive Summary

This research provides critical insights into optimizing the adhesion of microcrystalline diamond (MCD) coatings on silicon substrates, a fundamental challenge for high-performance diamond applications.

Adhesion Mechanism: The study successfully utilized an electrostatic self-assembly strategy to control nanodiamond (ND) seeding density, demonstrating that adhesion is highly sensitive to the homogeneity of the initial nucleation layer.
Optimal Density Identified: Superior adhesion (minimal cracking, no delamination) was achieved at a precise seeding density of 1.81 x 10¹¹ cm^-2, corresponding to a 0.005 wt% ND colloidal solution.
Colloid Stability: The ND colloid was meticulously stabilized, maintaining a constant hydrodynamic diameter (ca. 30 nm) and positive Zeta potential (ca. +32 mV) across varying concentrations, isolating seeding density as the sole variable.
Failure Mode Analysis: Higher seeding densities (≥ 0.01 wt%) resulted in ND aggregation, which led to inhomogeneous bonding, increased crack lengths (up to 180 µm), and poor sample-to-sample reliability.
Stress Independence: Residual compressive stress (ranging from -0.96 GPa to -1.59 GPa) was found to be constant regardless of seeding density, confirming that adhesion variability stemmed purely from the quality of the ND nucleation layer.
Application Relevance: These findings are crucial for commercial applications requiring high-adhesion diamond coatings, such as wear-resistant machining tools (e.g., WC-Co) and advanced functional electrodes.

Technical Specifications

Parameter	Value	Unit	Context
Optimal Seeding Density	1.81 x 10¹¹	cm^-2	Yielded shortest crack length (0.005 wt% ND solution).
Diamond Film Thickness	3.16 ± 0.11	µm	Average thickness of Microcrystalline Diamond (MCD) coating.
Shortest Average Crack Length	107 ± 8	µm	Measured via 50 N Vickers Indentation (Optimal Adhesion).
ND Hydrodynamic Diameter	ca. 30	nm	Constant size maintained for colloidal stability.
ND Zeta Potential	ca. +32	mV	Positive charge conducive to electrostatic adsorption on oxidized Si.
Residual Stress (Surface)	ca. -0.96	GPa	Compressive stress at the diamond-air interface.
Residual Stress (Subsurface)	ca. -1.59	GPa	Higher compressive stress near the Si interface.
CVD Growth Pressure	1000	Pa	HFCVD process parameter.
Substrate Material	Single Crystal Silicon	N/A	<100>/<110><110> orientation.

Key Methodologies

The experiment focused on controlling the nucleation layer quality using a highly stable nanodiamond colloid and Hot Filament Chemical Vapor Deposition (HFCVD) for film growth.

Substrate Pre-treatment: Polished single crystal silicon wafers were cleaned (DI water/propanol) and oxidized using UV-Ozone for 15 minutes to create a hydrophilic, negatively charged surface conducive to electrostatic adsorption.
Colloid Stabilization: Nanodiamond (ND) particles (5 nm, positive Zeta potential) were stabilized using the surfactant TMAEMC (5 x 10^-6 mol/L) and the pH was precisely adjusted to 3 to ensure long-term colloidal stability and prevent aggregation.
Controlled Seeding: Substrates were immersed in ND colloid solutions (ranging from 0.00033 to 0.1 wt% ND) and ultrasonicated for 15 minutes, achieving seeding densities between 4 x 10⁸ cm^-2 and 1.95 x 10¹¹ cm^-2.
HFCVD Growth Parameters: Microcrystalline diamond (MCD) films were grown in a commercial HFCVD chamber using eight tantalum filaments.
- Gases: H₂ (500 sccm) and CH₄ (25 sccm).
- Power: 10.5 kW.
- Duration: 270 minutes (achieving ca. 3.1 µm thickness).
Characterization: Seeding density was measured by Atomic Force Microscopy (AFM). Film adhesion was quantified using 50 N Vickers Indentation, measuring the average crack length orthogonal to the crater.

6CCVD Solutions & Capabilities

The research highlights the critical need for precise material control, from the nucleation layer to the final film properties, to ensure reliable diamond coating adhesion. 6CCVD is uniquely positioned to supply the high-quality CVD diamond materials and engineering support necessary to replicate and advance this research, particularly for industrial wear-resistant and functional applications.

Applicable Materials

The study utilized Microcrystalline Diamond (MCD) films grown on silicon. 6CCVD offers superior alternatives and complementary materials for advanced adhesion studies and commercial deployment:

Polycrystalline Diamond (PCD): 6CCVD provides high-purity PCD wafers up to 125 mm in diameter and thicknesses from 0.1 µm to 500 µm. Our PCD is ideal for large-scale coating applications, such as the wear-resistant coatings for machining tools mentioned in the paper’s introduction.
Single Crystal Diamond (SCD): For applications requiring ultimate thermal conductivity or optical transparency (e.g., optical coatings, high-power electronics), 6CCVD supplies SCD plates with thicknesses up to 500 µm.
Boron-Doped Diamond (BDD): For electrochemical applications (e.g., water purification, supercapacitors) where high surface area and conductivity are paramount, 6CCVD offers custom BDD films, which benefit directly from optimized seeding protocols to maximize active surface area and adhesion.

Customization Potential

The paper demonstrated that film thickness (ca. 3.1 µm) and substrate roughness are crucial variables. 6CCVD’s manufacturing capabilities allow for precise control over these parameters:

Requirement from Research	6CCVD Capability	Benefit to Customer
Precise Thickness Control	SCD/PCD films available from 0.1 µm up to 500 µm. Substrates up to 10 mm.	Enables replication of ultrathin films (≤ 100 nm) for optical sensors or thick films for cutting tools.
Substrate Roughness	Ultra-low roughness polishing: Ra < 1 nm (SCD), Ra < 5 nm (Inch-size PCD).	Essential for high-performance optical coatings and minimizing biofouling (as noted in the paper’s introduction).
Interface Engineering	In-house metalization services (Au, Pt, Pd, Ti, W, Cu) and laser cutting.	Allows researchers to integrate custom interlayers (e.g., TiB₂ or SiC, as discussed in related literature) or create complex electrode geometries for functional diamond projects.

Engineering Support

The core challenge identified—avoiding ND aggregation during seeding—is a complex surface chemistry problem. 6CCVD’s in-house team of PhD material scientists specializes in advanced CVD nucleation and growth protocols.

Nucleation Optimization: We offer consultation on advanced seeding techniques (e.g., Bias Enhanced Nucleation (BEN) or optimized colloidal chemistry) to ensure homogeneous, high-density nucleation layers, overcoming the aggregation issues observed in this study.
Adhesion on Challenging Substrates: The paper noted the difficulty of coating hard metals like WC-Co. Our engineering team can assist clients in selecting the appropriate diamond material and interface strategy to achieve robust adhesion on thermal-expansion mismatched substrates.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Several parameters are known to influence the adhesion of diamond coatings to non-diamond substrates. In the current work, we investigate the effect of seeding density on the adhesion of microcrystalline diamond coatings on silicon substrates. To this end, controlled seeding densities on silicon substrates were established by an electrostatic self-assembly seeding strategy. The seeding density was altered by changing the wt% of the ND colloidal seed solution while maintaining the pH and surfactant concentration. This resulted in colloidally stable ND particles with virtually constant hydrodynamic diameter (ca. 30 nm) and Zeta potential (ca. +32 mV) while the wt% of the ND in the colloid was altered between 0.00033 and 0.1 wt%. With these diluted solutions the seeding density was controlled between 4 × 108 cm−2 and 1.95 × 1011 cm−2. Subsequently, microcrystalline diamond coatings with a thickness of 3.1 ± 0.1 µm were grown. The adhesion of the diamond coating to the silicon substrate was evaluated by indentation. Best adhesion was found for a seeding density of 1.81 × 1011 cm−2, featuring no delamination and low sample to sample variation. Counterintuitively, further increase in seeding density resulted in an increase of crack length and sample to sample variation. This decline in adhesion was attributed to ND aggregates formed during the seeding step, which either desorb or form areas with poor diamond-silicon bonding upon diamond growth. Therefore, this result is of import for diamond film adhesion studies and commercial diamond coated cutting tools using high seeding densities being prone to aggregation.

Tech Support

Original Source

DOI: https://doi.org/10.1080/26941112.2025.2472623

References

2024 - Anomalously strong size effect on thermal conductivity of diamond microparticles [Crossref]
2024 - Applications of diamond films: a review [Crossref]
2020 - Robust biomimetic hierarchical diamond architecture with a self-cleaning, antibacterial, and antibiofouling surface [Crossref]
2019 - Biocompatibility and functionalization of diamond for neural applications [Crossref]
2019 - Quantum nanophotonics with group IV defects in diamond [Crossref]
2016 - Diamond electrochemistry at the nanoscale: a review [Crossref]
2018 - Boron doped diamond: a designer electrode material for the twenty-first century [Crossref]
2021 - Ultrathin diamond nanofilms—development, challenges, and applications [Crossref]
2000 - Diamond thin films: a 21st-century material [Crossref]
2023 - Bottom-up strategy of multi-level structured boron-doped diamond for the durable electrode in water purification [Crossref]