Templated Synthesis of Diamond Nanopillar Arrays Using Porous Anodic Aluminium Oxide (AAO) Membranes

At a Glance

Metadata	Details
Publication Date	2023-02-27
Journal	Nanomaterials
Authors	Chenghao Zhang, Zhichao Liu, Chun Li, Jian Cao, Josephus G. Buijnsters
Institutions	Harbin Institute of Technology, Delft University of Technology
Citations	9
Analysis	Full AI Review Included

Technical Documentation & Analysis: Templated Synthesis of Diamond Nanopillar Arrays

Executive Summary

This document analyzes the research on the template-assisted synthesis of ordered diamond nanopillar arrays, highlighting the technical achievements and connecting them directly to 6CCVD’s advanced MPCVD diamond capabilities for replication and scale-up.

Novel Synthesis Method: A low-cost, bottom-up Hot-Filament Chemical Vapor Deposition (HFCVD) method was successfully demonstrated for growing highly ordered Nanocrystalline Diamond (NCD) nanopillar arrays.
Template and Substrate: The process utilized sacrificial ultra-thin Anodic Aluminum Oxide (AAO) membranes transferred onto the ultra-smooth nucleation side (S_a ~0.8 nm) of pre-grown Boron-Doped Diamond (BDD) sheets.
Controlled Geometry: Two distinct, vertically aligned pillar geometries were achieved: submicron pillars (~325 nm diameter) and nanoscale pillars (~85 nm diameter), both exhibiting an aspect ratio of approximately 2.5.
Stress Management: Significant tensile residual stress (up to 3.2 GPa) was observed in the as-grown, AAO-confined pillars due to the Coefficient of Thermal Expansion (CTE) mismatch between the AAO and NCD, which was fully released upon template removal.
High-Value Applications: The resulting ordered diamond nanostructures are highly relevant for next-generation applications in electrochemical sensing, skeletal tissue engineering, and advanced optics (photonic bandgap crystals).
6CCVD Value Proposition: 6CCVD provides the necessary large-area BDD substrates, custom thickness control (0.1 µm to 500 µm), and post-processing services (polishing, metalization) required to scale this template-assisted fabrication technique to commercial wafer sizes.

Technical Specifications

The following hard data points were extracted from the research paper detailing the synthesis and resulting material properties.

Parameter	Value	Unit	Context
Substrate Material	Polycrystalline Boron-Doped Diamond (BDD)	Film Thickness	4 µm
CVD Method	Hot-Filament CVD (HFCVD)	N/A	Template-assisted growth
Substrate Temperature	~725	°C	During HFCVD growth cycle
Gas Flow (CH₄:H₂)	6:300	SCCM	Reactive gas mixture ratio
Gas Pressure	10	mbar	HFCVD chamber conditions
Large Pillar Diameter (Nominal)	~325	nm	Derived from AAO template (~300 nm pore)
Small Pillar Diameter (Nominal)	~85	nm	Derived from AAO template (~70 nm pore)
Large Pillar Height	~650	nm	Aspect ratio ~2.5
Small Pillar Height	~200	nm	Aspect ratio ~2.5
Residual Tensile Stress (Confined)	3.2 ± 0.1	GPa	Calculated from Raman shift (1324.5 cm^-1)
Flat NCD Diamond Peak Position	1333.5 ± 0.3	cm^-1	Stress-free reference
Substrate Surface Roughness (S_a)	~0.8	nm	Nucleation side of BDD sheet

Key Methodologies

The template-assisted bottom-up synthesis involved three critical stages: substrate preparation and template transfer, HFCVD growth, and sacrificial template removal.

Substrate Preparation:
- 4 µm thick Polycrystalline Boron-Doped Diamond (BDD) films were grown on silicon wafers.
- The Si substrate was completely removed by etching in boiling 10 M KOH aqueous solution for 3 h, releasing the free-standing BDD sheet.
- The smooth nucleation side (S_a ~0.8 nm) of the BDD sheet was used as the target substrate for template adhesion.
AAO Template Transfer:
- Ultra-thin AAO membranes (1.6 µm or 0.2 µm thickness, corresponding to ~300 nm or ~70 nm pores) were transferred onto the BDD nucleation side in an acetone liquid environment.
HFCVD Growth:
- The BDD/AAO stack was loaded into an HFCVD chamber.
- Growth parameters were fixed: Substrate temperature ~725 °C, gas pressure 10 mbar, and gas flow ratio CH₄:H₂ at 6:300 SCCM.
- Growth time was varied (0.5 h or 3 h) to control pillar height, resulting in vertically aligned Nanocrystalline Diamond (NCD) pillars confined within the AAO pores.
Template Removal:
- The AAO template was chemically removed by immersion in concentrated H₃PO₄ acid at 200 °C for 2 h.
- This step released the ordered diamond nanopillar arrays and simultaneously relieved the significant residual tensile stress built up during the cooling phase of the CVD process.

6CCVD Solutions & Capabilities

The successful replication and commercial scaling of this templated synthesis technique require high-quality, customizable diamond materials and advanced post-processing capabilities—all core competencies of 6CCVD.

Applicable Materials

The foundation of this research is the use of a highly conductive, smooth diamond substrate. 6CCVD provides the ideal starting material:

Heavy Boron-Doped Polycrystalline Diamond (BDD-PCD): The paper utilized BDD films for their conductivity and smooth nucleation side. 6CCVD specializes in high-quality BDD-PCD, offering precise control over boron concentration (tunable electrical conductivity) essential for electrochemical sensing applications (as cited in the paper).
Nanocrystalline Diamond (NCD): The resulting nanopillars are NCD. 6CCVD’s MPCVD systems are optimized to control the methane concentration and growth conditions necessary to tune the NCD grain size, which is critical for optimizing the performance of photonic and electrochemical structures.

Customization Potential for Scale-Up

The research used small, 10 × 10 mm² samples. 6CCVD enables the transition to industrial scale and advanced device integration.

Research Requirement	6CCVD Capability	Value Proposition
Substrate Size	Plates/wafers up to 125 mm (PCD/BDD)	Enables large-area pattern transfer (AAO-on-diamond) for commercial production and wafer-scale integration.
Film Thickness	SCD/PCD films from 0.1 µm to 500 µm	Provides flexibility for optimizing the initial BDD substrate thickness, crucial for mechanical stability during the lift-off and etching steps.
Surface Quality	Polishing services achieving Ra < 5 nm (PCD)	Guarantees the ultra-smooth nucleation surface (S_a ~0.8 nm used in the paper) required for reliable, uniform AAO template adhesion via van der Waals forces.
Device Integration	Custom Metalization (Au, Pt, Pd, Ti, W, Cu)	Essential for fabricating functional Microelectrode Arrays (MEAs) and electrochemical sensors, allowing direct electrical contact to the conductive BDD pillars.
Geometry Control	Laser Cutting and shaping services	Allows precise cutting of the final nanopillar arrays into custom shapes required for specific biomedical or optical device footprints.

Engineering Support

The successful synthesis of these nanostructures relies heavily on controlling the CVD environment to manage residual stress and material quality.

Stress Management Expertise: The paper highlights significant residual stress (3.2 GPa) due to CTE mismatch. 6CCVD’s in-house PhD team can assist researchers in optimizing CVD parameters (temperature ramps, gas mixtures) to minimize intrinsic stress and maximize the structural integrity of the resulting high-aspect-ratio diamond nanostructures.
Application Optimization: 6CCVD provides consultation on material selection (e.g., specific boron doping levels, NCD vs. Ultrananocrystalline Diamond) to maximize efficiency for similar projects in electrochemical sensing, quantum sensing, and biomedical engineering.

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

View Original Abstract

Diamond nanostructures are mostly produced from bulk diamond (single- or polycrystalline) by using time-consuming and/or costly subtractive manufacturing methods. In this study, we report the bottom-up synthesis of ordered diamond nanopillar arrays by using porous anodic aluminium oxide (AAO). Commercial ultrathin AAO membranes were adopted as the growth template in a straightforward, three-step fabrication process involving chemical vapor deposition (CVD) and the transfer and removal of the alumina foils. Two types of AAO membranes with distinct nominal pore size were employed and transferred onto the nucleation side of CVD diamond sheets. Subsequently, diamond nanopillars were grown directly on these sheets. After removal of the AAO template by chemical etching, ordered arrays of submicron and nanoscale diamond pillars with ~325 nm and ~85 nm diameters were successfully released.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano13050888

References

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