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

Executive Summary

This research details a novel, bottom-up approach for synthesizing highly ordered diamond nanopillar arrays using sacrificial Anodic Aluminum Oxide (AAO) membranes and Hot-Filament Chemical Vapor Deposition (HFCVD).

Core Achievement: Successful fabrication of vertically aligned nanocrystalline diamond (NCD) pillar arrays in two distinct sizes: submicron (~325 nm diameter, 650 nm height) and nanoscale (~85 nm diameter, 200 nm height).
Methodology Advantage: The template-assisted growth utilizes the smooth nucleation side of pre-grown boron-doped diamond (BDD) sheets, eliminating the complex and often irreproducible seeding step involving diamond nanoparticles.
Template Utilization: Commercial ultrathin AAO membranes served as the growth template, providing 2D confinement for selective diamond growth within the nanopores.
Stress Analysis: Raman spectroscopy and Finite Element Modeling (FEM) revealed significant tensile residual stress (up to 3.2 GPa) in the confined pillars, caused by the mismatch in the Coefficient of Thermal Expansion (CTE) between the AAO and diamond during cooling.
Stress Release: This residual stress was fully released upon chemical removal of the AAO template using hot phosphoric acid (H₃PO₄).
Versatility: The technique is demonstrated to be viable for feature sizes well below 100 nm, offering a low-cost, high-throughput alternative to traditional top-down lithography and etching methods.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	Polycrystalline BDD film	N/A	Thickness 4 µm, nucleation side used.
Substrate Roughness (S_a)	~0.8	nm	Arithmetic mean surface roughness of BDD nucleation side.
AAO Template 1 (Nominal Pore Size)	~300	nm	Used for submicron pillars (Thickness 1.6 µm).
AAO Template 2 (Nominal Pore Size)	~70	nm	Used for nanoscale pillars (Thickness 0.2 µm).
Submicron Pillar Diameter (Released)	Average ~325	nm	Range: 275 to 375 nm (300 nm AAO template).
Nanoscale Pillar Diameter (Released)	Average ~85	nm	Grown using 70 nm AAO template.
CVD Substrate Temperature	~725	°C	HFCVD growth condition.
Gas Pressure	10	mbar	HFCVD growth condition.
CH₄ Flow Rate	6	SCCM	Reactive gas flow rate.
H₂ Flow Rate	300	SCCM	Carrier gas flow rate.
Diamond Peak Position (Stress-Free)	1333.5 ± 0.3	cm^-1	Typical diamond one-phonon line (Released NCD pillars).
Diamond Peak Position (Confined, 325 nm)	1324.5 ± 0.3	cm^-1	Red-shifted due to residual stress.
Calculated Tensile Residual Stress	3.2	GPa	Stress in AAO-confined 325 nm pillars (modeled).
AAO Removal Etchant	Concentrated H₃PO₄	N/A	Etching temperature 200 °C, time 2 h.

Key Methodologies

The fabrication process is a straightforward, three-step sequence:

Substrate Preparation:
- A 4 µm thick polycrystalline BDD film was separated from its Si substrate by etching in boiling 10 M KOH solution for 3 h.
- The smooth nucleation side (S_a ~0.8 nm) of the BDD sheet was selected as the target substrate for AAO transfer.
AAO Template Transfer (Step I):
- Ultrathin AAO membranes (5 x 5 mm²) were transferred onto the BDD sheet’s nucleation side in an acetone liquid environment.
- Two membrane types were used: 1.6 µm thick (~300 nm pores) for submicron pillars, and 0.2 µm thick (~70 nm pores) for nanoscale pillars.
Diamond Growth (Step II - HFCVD):
- The AAO/BDD stack was loaded into the HFCVD chamber.
- Conditions: Substrate temperature was maintained at ~725 °C, and gas pressure was 10 mbar.
- Gas Mixture: CH₄ (6 SCCM) and H₂ (300 SCCM).
- Growth Duration: 3 h for 325 nm pillars; 0.5 h for 85 nm pillars. Diamond grew selectively within the AAO nanopores, confined in 2D.
Template Removal (Step III):
- The AAO template was completely removed by chemical etching via immersion in concentrated H₃PO₄ acid at 200 °C for 2 h, successfully releasing the ordered diamond nanopillar arrays.
Stress Analysis:
- Raman spectroscopy measured a red-shift of the diamond peak (e.g., 1324.5 cm^-1 for 325 nm pillars) in the confined state compared to the stress-free state (1333.5 cm^-1).
- FEM modeling confirmed that thermal contraction mismatch between AAO and diamond during cooling generated tensile stress, primarily concentrated in the top part of the pillars.

Commercial Applications

The resulting highly ordered, high-aspect-ratio diamond nanopillar arrays are valuable for applications requiring structured diamond surfaces, leveraging diamond’s extreme properties (hardness, chemical stability, biocompatibility).

Biomedical and Bioelectronics:
- Skeletal Tissue Engineering: Structured substrates for guided cell growth and enhanced cell adhesion (Ref [51]).
- Microelectrode Arrays (MEAs): Tunable electrode shapes and sizes for neural interfaces, retinal implants, and cell signal reception, especially when using heavily boron-doped diamond (BDD) pillars.
Optics and Photonics:
- Photonic Bandgap Crystals: Highly ordered arrays for controlling light propagation (Ref [38]).
- Quantum Emitters: Potential platforms for solid-state quantum emitters, benefiting from the controlled geometry and high material quality.
Electrochemistry and Sensing:
- Ultrasensitive Electrochemical Sensors: High surface area electrodes for enhanced detection sensitivity (Ref [53]).
- Matrix-Free Mass Spectrometry: Diamond nanowires/nanopillars serve as novel platforms for ionization.
Advanced Materials and Devices:
- Field Emission Devices: High-aspect-ratio structures provide excellent geometric enhancement factors for electron emission.
- Thermal Management: Structured diamond surfaces for high-efficiency heat dissipation components.

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

2017 - Diamond thin films: Giving biomedical applications a new shine [Crossref]
2013 - Efficient coupling of a single diamond color center to propagating plasmonic gap modes [Crossref]
2022 - On the electrooxidation of kraft black liquor on boron-doped diamond [Crossref]
2021 - Fabrication and tribological properties of textured diamond coatings on WC-Co cemented carbide surfaces [Crossref]
2022 - GaN-based heterostructures with CVD diamond heat sinks: A new fabrication approach towards efficient electronic devices [Crossref]
2013 - Conductivity of boron-doped polycrystalline diamond films: Influence of specific boron defects [Crossref]
2007 - Ultrananocrystalline diamond film as an optimal cell interface for biomedical applications [Crossref]
2015 - Single-crystal diamond nanowire tips for ultrasensitive force microscopy [Crossref]
2008 - Fabrication of diamond nanopillars and their arrays [Crossref]
2020 - Influence of helium ion irradiation on the structure and strength of diamond [Crossref]