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	Institute of Mathematics, Bydgoszcz University of Science and Technology
Analysis	Full AI Review Included

Executive Summary

This research demonstrates a novel, non-doping strategy for tuning the structural, electrical, and chemical properties of Ultra-Nanocrystalline Diamond (UNCD) films grown via Hot Filament Chemical Vapour Deposition (HF CVD).

Parameter Control: Properties were successfully modified solely by adjusting the methanol (CH₃OH) concentration in the H₂ working gas mixture, ranging from 4 vol% to 7 vol%.
Structural Modification: Increasing methanol concentration resulted in a significant reduction in UNCD grain size, decreasing from 24 ± 2 nm (4% CH₃OH) to 13 ± 2 nm (7% CH₃OH), confirmed by SEM and AFM.
Electrical Impact: The surface resistance of the films increased dramatically by more than one order of magnitude (13-fold increase) across the tested concentration range, attributed to the increased specific surface area resulting from smaller grains.
Phase and Chemistry: A slight decrease in the desirable tetrahedral (sp³) phase content was observed with increasing methanol. Simultaneously, the concentration of surface functionalization groups (-H, -OH, and =O) increased.
Application Relevance: This fine-tuning capability allows for the fabrication of UNCD layers with tailored properties, making them suitable for use as active transducer layers in electrochemical sensors without introducing external dopants.

Technical Specifications

Parameter	Value	Unit	Context
Methanol Concentration Range	4 to 7	vol%	Working gas (CH₃OH/H₂)
Total Pressure	50	mbar	HF CVD process condition
Substrate Temperature	~1000	K	HF CVD process condition
Working Gas Flow Rate	100	sccm	Duration for all samples
Deposition Time	6	h	Duration for all samples
SEM Grain Size (4% CH₃OH)	24 ± 2	nm	Estimated from high-resolution SEM
SEM Grain Size (7% CH₃OH)	13 ± 2	nm	Estimated from high-resolution SEM
XRD Crystallite Size (All)	8-3 ± 2	nm	Based on Scherrer’s equation, (111) diamond reflex
Surface Roughness (Ra, 4% CH₃OH)	6.56	nm	Atomic Force Microscopy (AFM)
Surface Roughness (Ra, 7% CH₃OH)	1.43	nm	Atomic Force Microscopy (AFM)
Resistance Increase	13	fold	Surface resistance change (4% vs 7% methanol)
Diamond Peak (Raman)	1332	cm^-1	Weak sharp sp³ diamond feature
C-C, C-H sp³ Bonding (XPS)	285.4 ± 0.1	eV	Binding energy of tetrahedral carbon
C=C sp² Bonding (XPS)	284.5 ± 0.1	eV	Binding energy of graphitic carbon
C-O Bonding (XPS)	286.8 ± 0.1	eV	Binding energy of hydroxyl/ether groups
C=O Bonding (XPS)	288.9 ± 0.1	eV	Binding energy of carbonyl groups

Key Methodologies

The UNCD films were synthesized using a home-made Hot Filament Chemical Vapour Deposition (HF CVD) reactor, followed by comprehensive material characterization:

Substrate Preparation: Quartz plates were ultrasonically cleaned sequentially in chloroform (4 min), a suspension of diamond powder in methanol (6 min), and finally in pure methanol (6 min).
HF CVD Growth:
- Tungsten filament (0.5 mm cross-section) was used.
- The working gas was a mixture of methanol vapor and hydrogen (H₂).
- Methanol concentration was varied systematically between 4 vol% (UNCD-4) and 7 vol% (UNCD-7).
- Deposition was carried out for 6 hours at 50 mbar total pressure and a substrate temperature of ~1000 K.
Structural and Morphological Analysis:
- Scanning Electron Microscopy (SEM) was used to visualize homogeneous nanocrystals and estimate grain size.
- X-ray Diffraction (XRD) confirmed the presence of ultra-nano-sized crystallites (8-3 nm range) using the Scherrer’s equation on the (111) diamond reflex.
- Atomic Force Microscopy (AFM) measured 2D topography and quantified mean surface roughness (Ra).
Chemical and Phase Analysis:
- Raman Spectroscopy (532 nm laser) identified the sp³ diamond peak (1332 cm^-1) and non-diamond phases (D₁, D₂, D₄, G modes), correlating non-diamond components with C-H/sp³ and C-OH/sp³ defects.
- X-ray Photoelectron Spectroscopy (XPS) quantified the carbon bonding states (C=C sp², C-C/C-H sp³, C-O, C=O) to confirm surface functionalization changes.
Electrical Characterization:
- Surface resistance was measured using a 2-point probe configuration with tungsten needles spaced 1 mm apart.

Commercial Applications

The ability to precisely control UNCD film properties (grain size, resistivity, and surface functionalization) through a single process parameter (methanol concentration) makes this technology highly relevant for:

Electrochemical Sensors: Tailoring the high surface area and specific surface chemistry for use as active layers in transducers, particularly for high-sensitivity detection (e.g., monitoring drinking water quality).
Biosensors and Chemo-sensors: Utilizing the high electrochemical stability and tunable surface groups (-OH, =O) for selective surface functionalization and enhanced sensing performance.
Micro Electrochemical Devices: Providing stable, protective coatings in harsh or aggressive environments.
Advanced Energy Storage: Application in supercapacitors and fuel cells, where high surface area and controlled conductivity are critical for performance.
Power Electronics: Leveraging the intrinsic properties of diamond (wide band gap, high breakdown voltage) for high-power, high-frequency applications.

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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