Effect of Boron Doping Concentration on the Wettability and Surface Free Energy of Polycrystalline Boron-Doped Diamond Film

At a Glance

Metadata	Details
Publication Date	2023-01-29
Journal	Coatings
Authors	Peng Wang, Qiyuan Yu, Xiaoxi Yuan, Zheng Cui, Yaofeng Liu
Institutions	Jilin University, Jilin Engineering Normal University
Citations	1
Analysis	Full AI Review Included

Executive Summary

This study investigated the relationship between Boron doping concentration and the wettability/Surface Free Energy (SFE) of Polycrystalline Boron-Doped Diamond (PBDD) films, providing crucial data for engineering applications.

Core Finding: The Surface Free Energy (SFE) of PBDD films consistently increases as the Boron doping concentration rises.
SFE Range: Reliable SFE values ranged from 42.89 mJ/m² (lowest doping, B-01) up to 52.26 mJ/m² (highest doping, B-50), demonstrating tunability via the deposition recipe.
Morphology Independence: Despite varying doping levels, all PBDD films exhibited similar surface morphology and roughness (Root-Mean-Square Roughness, Rq, remained stable around 130 nm).
Methodology Validation: The Owens-Wendt-Kaelble (OWK) and the Lifshitz-van der Waals/acid-base (van Oss) approaches were confirmed as suitable and reliable methods for estimating PBDD SFE.
Unsuitable Methods: The Fowkes approach, Berthelot’s (geometric mean) combining rule, and Antonow’s rule failed to provide reliable, constant SFE values for a specific solid.
Process Control: PBDD films were successfully deposited on silicon wafers using Microwave Plasma Chemical Vapor Deposition (MPCVD) under controlled temperature (800 °C) and pressure (8.5 KPa).

Technical Specifications

Parameter	Value	Unit	Context
Deposition Method	MPCVD	N/A	Microwave Plasma Chemical Vapor Deposition
Microwave Frequency	2.45	GHz	N/A
Deposition Temperature	800	°C	Reaction chamber condition
Deposition Pressure	8.5	KPa	Reaction chamber condition
Deposition Time	11	hours	N/A
Film Thickness	~7	µm	Final PBDD film thickness
Roughness (Rq) Range	128.89 to 131.13	nm	Root-Mean-Square Roughness across all doping levels
Lowest SFE (B-01)	42.89	mJ/m²	Estimated by Lifshitz-van der Waals/acid-base approach
Highest SFE (B-50)	52.26	mJ/m²	Estimated by Lifshitz-van der Waals/acid-base approach
Standard Contact Angle (B-30)	72.7	°	Mean CA using 3.5 µL DI water drop volume
Gas Ratio Range (H₂/CH₄/Gas-B)	200/4/0.1 to 200/4/5	sccm	Boron doping concentration control (B-01 to B-50)
Diamond sp³ Peak (B-01)	1332	cm^-1	Characteristic Raman peak position

Key Methodologies

Substrate Preparation: Silicon wafers were mechanically abraded using ~300 nm diamond powder to increase roughness, followed by ultrasonic treatment in an alcohol/diamond powder mixture for 60 minutes to maximize diamond nucleation sites.
PBDD Deposition: Films were grown using Microwave Plasma Chemical Vapor Deposition (MPCVD) operating at 2.45 GHz. The reaction chamber was maintained at 800 °C and 8.5 KPa for 11 hours.
Doping Control: Boron doping concentration was controlled by adjusting the flow ratio of the mixture gases (Hydrogen (H₂), Methane (CH₄), and Gaseous Boron (Gas-B)). Gas-B was introduced via hydrogen passing through a solution of trimethyl borate.
Morphological and Structural Analysis: Films were characterized using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) for surface morphology and roughness (Rq), X-ray Diffraction (XRD) for crystal structure, and Raman spectroscopy for doping confirmation (Fano effect and sp³ peak shift).
Contact Angle Measurement: Contact angles (CA) were measured using three probe liquids: DI water (polar), diiodomethane (nonpolar), and glycerol (polar). A standardized drop volume of 3.5 µL was used based on line tension effect analysis.
Surface Free Energy (SFE) Calculation: SFE was reliably estimated using two primary multi-liquid approaches:
- Owens-Wendt-Kaelble (OWK): A “two-liquids” approach.
- Lifshitz-van der Waals/acid-base (van Oss): A “three-liquids” approach.

Commercial Applications

The ability to precisely tune the SFE of PBDD films via boron doping, while maintaining consistent morphology and high conductivity, is critical for several advanced engineering fields:

Electrochemistry and Sensing:
- Wastewater Treatment: PBDD electrodes are utilized for high-efficiency electrochemical oxidation processes, where controlled SFE optimizes the wetting of the electrode surface by the electrolyte, enhancing reaction kinetics.
- Biosensors: Used in sensing devices for medical applications (e.g., selective detection of dopamine in human serum), where surface energy dictates molecular adsorption and selectivity.
Adhesion and Coatings:
- Protective Coatings: By tuning SFE, PBDD films can be optimized for specific adhesion requirements on various substrates, improving the lifespan and anti-corrosion properties of the coating.
- Antifouling Surfaces: While high SFE was observed here, the fundamental understanding of SFE control allows for future modifications to create low SFE, non-sticking surfaces if required.
Energy Storage:
- Supercapacitors: PBDD is used in high-performance aqueous symmetric supercapacitors. SFE control is vital for ensuring optimal infiltration of the electrolyte into the porous diamond structure, maximizing capacitance and charge/discharge efficiency.
Semiconductor Technology:
- High Power Devices: PBDD is used in metal semiconductor field effect transistors (FETs) and other semiconductor devices, where interface energy plays a role in device stability and breakdown voltage.
Micro-Electro-Mechanical Systems (MEMS):
- PBDD films are employed in MEMS packaging and fabrication, where precise control over surface energy is necessary to manage capillary forces and interface interactions during manufacturing and operation.

View Original Abstract

The wettability and surface free energy of diamonds are crucial for their applications. In this study, polycrystalline boron-doped diamond (PBDD) films with different boron doping concentrations were prepared, and the effect of the boron doping concentration on the wettability and surface free energy (SFE) of the film was investigated. The SFEs of the PBDD films were investigated by employing the surface tension component approach and the equation-of-state approach. The investigation suggested that the alternative formulation of Berthelot’s rule, the Lifshitz-van der Waals/acid-base (van Oss) approach, and the Owens-Wendt-Kaelble approach were suitable for estimating the SFEs of PBDD films, whereas the Fowkes approach, Berthelot’s (geometric mean) combining rule, and Antonow’s rule could not provide reliable results. Results showed that the SFEs of PBDD films increased with increasing boron doping concentration, and the SFEs were 43.26-49.66 mJ/m2 (Owens-Wendt-Kaelble approach), 42.89-52.26 mJ/m2 (Lifshitz-van der Waals/acid-base), and 44.38-48.73 mJ/m2 (alternative formulation of Berthelot’s rule). This study also provides a reference for the application of empirical and physics-based semi-empirical approaches to SFE estimation.

Tech Support

Original Source

DOI: https://doi.org/10.3390/coatings13020305

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