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Precision Chemical Nanoengineering: Integrating Top-Down and Bottom-Up Methodologies for the Fabrication of 3-D Adaptive Nanostructured Architectures

Final Report Summary - NANO3D (Precision Chemical Nanoengineering: Integrating Top-Down and Bottom-Up Methodologies for the Fabrication of 3-D Adaptive Nanostructured Architectures)

The overall objective of 'Precision chemical nanoengineering: integrating top-down and bottom-up methodologies for the fabrication of 3-D adaptive nanostructured architectures' (NANO3D) was to integrate top-down lithographic techniques which enable precise spatial patterning of surfaces from micron- to the nanoscale, with the controlled stepwise self-assembly and self-organisation of nanometre scale chemical and biochemical entities to these surfaces, to fabricate three dimensional adaptive nanostructured architectures. The challenging project objectives have largely been met. Key research highlights are summarised below:

- Nanoscale patterning of self-assembled C60 adducts and citrate passivated gold nanoparticle structures .
- Micro-scale patterning of hairpin oligonucleotides bearing photolabile groups using UV-photolitography, and the specific deposition of gold nanoparticles on surfaces mediated by DNA. This is one of the first demonstrations of the use of DNA-bearing photolabile groups to obtain patterns on silicon oxide surfaces.
- A polymer nanowire photodetector with potential for applications in nanoscale building blocks for future hybrid nanophotonic devices and systems.
- An optically pumped single nanowire laser.
- Formation of, and substrate-adaptive behaviour in integrated micron-scale 'wall and roof' structures comprising blends of hydrogels and nanocrystals.
- Ink-jet deposition of wall and roof microstructures on silicon oxide substrates.
- Microfluidic scale-up route for formation of wall structures based on the process route of sequential deposition of nanoparticle- and polyelectrolyte layers.
- Strong dissemination of results throughout the project (both published articles and presentations). Published work included three journal cover articles, indicative of the high quality of the research output.

Certain aspects of the project will require further development, especially the more structurally complex and in particular some aspects of the adaptive nature of such structures. The project investigated many aspects of such nanotechnology architecture, including:

Molecular foundations

There have been a number of materials developed in the category of molecular foundations, including self-assembled monolayer structures, gold nanostructures of various types, polymer nanowires and silane derivatives for silica surface functionalisation.

Building blocks

Functional oligonucleotides: researchers synthesised nucleic acid derivatives with a hairpin structure having two functional groups:

(i) a classical anchoring group such as amino or thiol group as well as azido or alkynyl groups and
(ii) a photolabile group, a group with a bond capable of being broken by light irradiation).

Polymer nanowire photodetector: reseachers developed photoconductive nanowires, synthesised from poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)] (F8T2) via a solution-assisted template wetting method. Individual F8T2 nanowires were electrically interfaced using either bottom or top-contact geometries.

Single nanowire laser: researchers also demonstrated photonic functionality in polyfluorene (PFO)-based nanowires processed using solution- and melt-assisted template wetting routes. They show a range of optical functions, from polarised emission to microcavity effects and single nanowire lasers.

Adaptice architectures

Wall and roof structures were prepared by casting copolymer solutions in photoresist structures followed by photocrosslinking of the copolymer. This addresses the objective of a three dimensional adaptive wall whose properties respond to a change in the environment.

Scale-up

Two approaches were adopted for scale-up in the project, ink jet printing and microfluidics. The aim of this part of the project was to investigate the potential of these techniques to deliver materials and carry out processing at the wafer scale, rather than over the limited areas that most nanoscale processing has achieved to date. Both ink jet printing and microfluidics show promise for enabling the transfer of techniques developed in the project to larger scales, although there are difficulties inherent in both techniques. The project focussed on the design and fabrication of a microfluidic cell and the analysis of gold structures made using the cell.

Gold nanoparticle structure deposition and layer build-up

A process developed in the 'Building blocks' work package was used to create structures on the silicon wafer surface. This involved the deposition of thiol-stabilised gold nanoparticles. The layers were deposited by spin-casting and were then exposed to ultraviolet light from a frequency-doubled argon ion laser, wavelength 244 nm.

The results show that it is possible to build up structures using microfluidics but that retaining any surface preparation, such as patterning, before the cell is fabricated is difficult. This is due to the harsh conditions encountered during preparation of the cell, notably the temperature required for satisfactory bonding, but also some of the commonly used cleaning and surface preparation methods are very aggressive.

Nevertheless the results do show that microfluidics could have a place in wafer-scale processing at the nanoscale for some materials, as there are other advantages such as the very small quantities of material employed and the ability to deliver materials precisely to multiple sites on a wafer simultaneously.

The future

This project has not only demonstrated the feasibility of integrating top-down and bottom-up lithographic process, but also shown how the self-assembly of materials to these patterned surfaces can be automated via incorporation into ink-jet printing and microfluidic technologies. This capability is likely to be of significant relevance for the next generation nanotechnologies whereby conventional top-down approaches are not going to be scalable.

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