Impact of Methanol Concentration on Properties of Ultra-Nanocrystalline Diamond Films Grown by Hot-Filament Chemical Vapour Deposition

At a Glance

Metadata	Details
Publication Date	2021-12-21
Journal	Materials
Authors	Lidia Mosińska, Robert Szczęsny, Marek Trzciński, M.K. Naparty
Institutions	Bydgoszcz University of Science and Technology, Institute of Mathematics
Analysis	Full AI Review Included

Technical Documentation & Analysis: Ultra-Nanocrystalline Diamond for Advanced Sensors

Executive Summary

This research demonstrates a critical pathway for tuning the properties of Ultra-Nanocrystalline Diamond (UNCD) films—specifically grain size, surface roughness, and electrical resistivity—solely by adjusting the methanol precursor concentration during Hot-Filament Chemical Vapour Deposition (HF-CVD).

Core Achievement: A slight increase in methanol concentration (4% to 7% vol%) resulted in a 13-fold increase in surface resistance and a reduction in grain size from 24 nm to 13 nm.
Mechanism: Tuning the precursor ratio modifies the sp³/sp² phase content and controls the concentration of surface functional groups (-H, -OH, =O), which are crucial for electrochemical sensor performance.
Application Relevance: The ability to precisely control resistivity and surface chemistry makes UNCD highly suitable for use as active layers in electrochemical transducers and biosensors (e.g., drinking water quality monitoring).
6CCVD Value Proposition: 6CCVD specializes in high-quality MPCVD Polycrystalline Diamond (PCD/UNCD) and offers superior scalability, uniformity, and precise control over material properties (including Boron Doping for stable conductivity control) necessary to transition this research from lab-scale (5x5 mm²) to commercial production (up to 125 mm wafers).
Material Control: We provide custom polishing (Ra < 5 nm for inch-size PCD) and metalization services (Ti/Pt/Au, etc.) essential for integrating these UNCD films into functional micro-electrochemical devices.

Technical Specifications

The following hard data points were extracted from the analysis of UNCD films grown under varying methanol concentrations:

Parameter	Value Range	Unit	Context
Methanol Concentration	4 to 7	vol%	Working gas (CH₃OH/H₂)
Total Pressure	50	mbar	HF-CVD process condition
Substrate Temperature	~1000	K	Growth temperature
Gas Flow Rate	100	sccm	Standard working gas flow
Deposition Time	6	h	Standard growth time
SEM Grain Size (UNCD-4)	24 ± 2	nm	Lowest methanol concentration
SEM Grain Size (UNCD-7)	13 ± 2	nm	Highest methanol concentration
XRD Grain Size ((111) plane)	8-3 ± 2	nm	Confirmed ultranano-sized crystallites
Surface Resistance Increase	13-fold	N/A	Increase attributed to decreasing grain size
Mean Surface Roughness (Ra, UNCD-4)	6.56	nm	Highest roughness (4% CH₃OH)
Mean Surface Roughness (Ra, UNCD-7)	1.43	nm	Lowest roughness (7% CH₃OH)
RMS Surface Roughness (Rq, UNCD-7)	1.89	nm	Lowest roughness (7% CH₃OH)
C=C sp² Bonding (XPS)	284.5 ± 0.1	eV	Correlated with G-band intensity (Raman)
C-C, C-H sp³ Bonding (XPS)	285.4 ± 0.1	eV	Tetrahedral configuration

Key Methodologies

The Ultra-Nanocrystalline Diamond (UNCD) films were synthesized using a home-made Hot-Filament Chemical Vapour Deposition (HF-CVD) reactor.

Substrate Preparation: Quartz plates (5 mm x 5 mm²) were used as substrates.
Seeding: Substrates were ultrasonically treated sequentially in:
- Chloroform (4 min).
- Diamond powder suspension in methanol (6 min).
- Pure methanol (6 min).
Filament: Tungsten wire (0.5 mm cross-section) coiled into a 5 mm diameter spring.
Working Gas Mixture: Methanol vapour (CH₃OH) and Hydrogen (H₂).
Process Parameters:
- Total Pressure: 50 mbar.
- Substrate Temperature: ~1000 K.
- Gas Flow Rate: 100 sccm.
- Methanol Concentration: Varied between 4 vol% and 7 vol%.
Characterization: Layers were analyzed using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), Raman Spectroscopy (532 nm laser), and X-ray Photoelectron Spectroscopy (XPS).

6CCVD Solutions & Capabilities

The research successfully demonstrated that UNCD properties can be finely tuned without traditional doping, relying instead on precise control of precursor concentration. 6CCVD provides the high-quality materials and engineering control necessary to scale this technology for commercial applications, offering superior uniformity and larger dimensions than typical HF-CVD systems.

Applicable Materials

To replicate and extend this research, 6CCVD recommends the following materials, optimized for electrochemical and sensor applications:

6CCVD Material	Relevance to Research	Customization & Advantage
Polycrystalline Diamond (PCD)	Directly matches the UNCD structure required for the active sensor layer.	Available in plates/wafers up to 125 mm diameter, enabling large-scale sensor array fabrication.
Boron-Doped Diamond (BDD)	While the paper avoided doping, BDD offers stable, highly controllable conductivity (metallic or semiconducting).	Recommended for high-performance electrochemical sensors where stable, low resistivity is required, offering superior performance compared to tuning resistivity solely via sp² content.
Optical Grade SCD	Not directly used, but available for high-power optical windows or substrates requiring extreme purity and low defect density.	SCD thicknesses from 0.1 µm to 500 µm available.

Customization Potential

The research utilized small (5x5 mm²) quartz substrates. Scaling this technology requires larger, highly uniform diamond films and precise integration features.

Research Requirement	6CCVD Capability	Technical Advantage
Small Substrate Size	Custom Dimensions up to 125 mm	Enables transition from laboratory samples to commercial, inch-size wafers for high-volume manufacturing (HVM) of sensors.
Surface Roughness (Ra ~1.43 nm)	Advanced Polishing Services	We guarantee Ra < 5 nm for inch-size PCD wafers, ensuring the smooth, uniform surface topography critical for reproducible sensor performance and low background current.
Electrode Integration	Custom Metalization	We offer in-house deposition of standard electrode materials (Au, Pt, Pd, Ti, W, Cu) tailored to specific sensor designs, eliminating the need for external processing steps.
Surface Functionalization	Custom Termination Services	While the paper used methanol to control -OH/=O groups, 6CCVD offers controlled hydrogen or oxygen termination post-growth to optimize the surface chemistry for specific analytes (e.g., for drinking water sensors).

Engineering Support

The ability to precisely control the sp³/sp² ratio and surface functionalization is paramount for high-sensitivity diamond sensors.

6CCVD’s in-house PhD team specializes in optimizing MPCVD growth recipes to achieve specific material properties, including controlled grain size distribution and defect density, crucial for electrochemical transducer projects like the one described.
We offer consultation on selecting the optimal material (e.g., comparing the stability and performance of highly tuned UNCD vs. stable, conductive BDD) based on the required electrochemical window and sensitivity targets.
We ensure global delivery (DDU default, DDP available) of high-purity diamond materials, minimizing supply chain complexity for international research and development teams.

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

View Original Abstract

Diamond is a very interesting material with a wide range of properties, making it highly applicable, for example, in power electronics, chemo- and biosensors, tools’ coatings, and heaters. Due to the high demand for this innovative material based on the properties it is already expected to have, it is important to obtain homogeneous diamond layers for specific applications. Doping is often chosen to modify the properties of layers. However, there is an alternative way to achieve this goal and it is shown in this publication. The presented research results reveal that the change in methanol content during the Hot Filament Chemical Vapour Deposition (HF CVD) process is a sufficient factor to tune the properties of deposited layers. This was confirmed by analysing the properties of the obtained layers, which were determined using Raman spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and an atomic force microscope (AFM), and the results were correlated with those of X-ray photoelectron spectroscopy (XPS). The results showed that the increasing of the concentration of methanol resulted in a slight decrease in the sp3 phase content. At the same time, the concentration of the -H, -OH, and =O groups increased with the increasing of the methanol concentration. This affirmed that by changing the content of methanol, it is possible to obtain layers with different properties.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma15010005

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