Conclusions

The aim of this PhD project is to investigate the potential of colloidal lithography and explore its application for the creation of highly accurate periodic nanostructures. The project is furthermore aimed to achieve high-throughput and novel nanostructures by developing facile and precise fabrication approaches suitable for mass production. The current research intends to produce ordered arrays of nanopatterns on surfaces of photoresist and PDMS. The research has involved modelling and simulation to study the behaviour of electromagnetic waves for both monolayer and bilayer submicron spheres; fabrication of single and dual patterned nanostructures using polystyrene spheres as micro-lenses; preparation of PDMS soft moulds using self assembled colloidal spheres as a template; structural characterisation of PDMS soft moulds using SEM stereoscopic and FIB milling techniques; fabrication of 2D and 3D periodic nanostructures through PDMS/PDMS replication and monolayer reassembly processes, and finally patterning of ordered arrays of metallic nanopillars using an electroforming process.

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The significant contributions of the research presented in this thesis in modified colloidal lithography as a versatile tool for fabrication of multi-material nanostructures can be summarised as follows:

A. Proposed a novel bilayer sphere fabrication method for creating uniform

arrays of dual nanopatterns on photoresist surface

A bilayer of closely packed microspheres has been self assembled on a thin layer of AZ 5214-E resist for the fabrication of uniform arrays of nanoholes. Two sets of well ordered nanoholes with different sizes have been successfully created on the AZ 5214-E resist after exposing the sample by UV light. This is the first report of creating the arrays of binary nanopatterns through one colloidal lithography process.

B. Characterisation of PDMS soft mould using SEM stereoscopic and FIB

milling techniques

Both an SEM stereoscopic technique and a FIB milling method have been used to characterise the morphology of PDMS soft moulds. The surface of a PDMS microbowl structure has been successfully reconstructed through the SEM stereoscopic technique and the geometrical analysis has been conducted using Alicona Imaging software. The measurement results have been validated with FIB milling characterization results.

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C. Developed a PDMS/PDMS replication process for producing 2D periodic

structure at nanoscale

A nanofabrication method has been presented for producing hybrid nanostructured arrays based on a novel PDMS/PDMS replication approach. A nanopatterned PDMS replica has been successfully constructed through a direct casting of PDMS slurry onto a surface treated nanopatterned PDMS template. Both the surfaces of the nanopatterned PDMS template and the nanopatterned PDMS replica remained undamaged after the demoulding process and can be employed for further fabrication processes.

D. Proposed a novel patterning technique for fabrication of 3D nanostructures

with ordered arrays

A novel nanopatterning technique has been introduced on the basis of monolayer self assembly of PS spheres, PDMS nano-replication, reassembly of monolayer PS spheres and metal coating. Triangular gold nanopatterns have been successfully created over uniformly distributed pillars of a PDMS template.

E. Developed electroforming process for manufacturing uniform arrays of

metallic nanopatterns

Metallic nanostructures have been produced using a nickel electroforming process. Uniform arrays of single and double patterned metallic nanopillars have been successfully fabricated using the aforementioned nanostructures as master templates.

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All the project objectives have been achieved through systematic research in the project involving repeated nanofabrication processes, simulation and characterisation. The following conclusions can be drawn from this research.

1. It has been observed from the computer simulation that polystyrene microspheres can act as near field micro-lenses due to their appropriate values of optical index. It has been realised that the FWHM of the hot spot at the bottom of microspheres tends to be bigger as the wavelengths get bigger.

2. It has been proven that AZ 5214-E resist is a suitable choice to satisfy the fabrication requirements in terms of good assembly of PS spheres as the initial step of MCL and BCL techniques.

3. The proposed BCL method has been successfully developed for producing two sets of nanopatterns in one CL process. It has been found that the sizes of the resulting nanopatterns, created by either the MCL method or BCL method, are precisely adjustable by using different exposure doses.

4. The proposed BCL technique has been successfully extended for the production of other types of periodic nanostructure. Apart from fabricating uniform arrays of binary nanoholes using the BCL method, this method has revealed its potential for the manufacturing of triangular-shaped nanopillars.

5. A PDMS/PDMS replication process has been developed as an effective method for the production of periodic nanostructures. The surface wettability nature of a

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PDMS template and the peeling off ability of PDMS pairs has been fully inspected. The experimental results confirmed that the contact angles of PDMS slurry were decreased from 31º to 8º and 12º when a gold film and platinum film were used as a releasing layer. It has been proven that the thickness of at least a 20 nm gold layer or a 40 nm platinum layer works best as the releasing film.

6. The usage of oxygen plasma for treating the surface of pre-patterned PDMS confirmed an extensive improvement on the hydrophobicity nature of the PDMS template and subsequently assisted reassembly process. Dimensions of PDMS nanopillars can be tuned by changing the spheres’ size, while the thickness of gold nanopatterns can be adjusted by controlling the thickness of the metal deposition.

7. Nickel electroforming has been developed as a compatible and precise method for the production of metallic structures at nanoscale. The usages of SDS surfactant and a vacuum have been found as effective approaches to improve the quality of electroformed patterns at nanoscale. The resultant metallic nanopatterns confirmed that the proposed method is able to fabricate even small nano-features with high dimensional accuracy.

Based on the experimental and analysis works presented in this thesis, the proposed nanofabrication techniques using modified colloidal lithography prove to be repeatable, facile and effective. It is furthermore proven that high quality periodic nanostructures can be yielded through the proposed nanofabrication techniques without the need of either expensive or sophisticated patterning instruments.

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Therefore, the research aims have been successfully reached. The methodology and analysis adopted in this project have led to the achievement of the project’s aims. The proposed nanofabrication methods which have been studied through this research are qualified for those applications in which volume nanopatterning are required. In addition, this project opens up a new route for fabrication of periodic nanostructures with different materials. The presented approaches can be further extended to create other nanostructures, and thus have wider applications.