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Laser-Initiated Liquid-Assisted Colloidal Lithography

Final Report Summary - LILAC (Laser-Initiated Liquid-Assisted Colloidal Lithography)

There is ever growing interest in engineering well-defined micro- and nano-scaled topographies on a wide range of surfaces, e.g. semiconductors, biomedical metals and polymeric materials, as a means to controlling their optical, tribological, wettability and/or biological properties. Many different top-down and bottom-up nanofabrication technologies (e.g. self-assembly processes, chemical etching, nano-imprinting, photolithography, electron beam and nanosphere lithographies) have been demonstrated in the quest to engineer such surfaces. This project explored some of the particular opportunities offered by laser based lithographies. Use of laser radiation enables non-contact surface structuring with high temporal and spatial resolution; the energies and intensities provided by ultrashort laser pulses can lead to very rapid localized heating, high thermal gradients, and fast subsequent cooling and resolidification rates. On a gross scale, changes in incident wavelength (e.g. from infrared to ultraviolet) typically result in different absorption efficiencies, and different modes of light-matter interaction; on a finer scale, near field effects in the interaction of laser light with transparent particles on a substrate have been examined experimentally and theoretically. The strong enhancement of the electromagnetic field in the vicinity of the particle offers a route to nanoprocessing structures with lateral size well below /2, the diffraction limit. The near field – laser ablation (NF-LA) method allows visualization of the near-field distribution via local ablation of the substrate in the shape of a crater (that corresponds to the peak in the field distribution). Patterning larger substrate areas, still with a single laser shot, can be achieved using a monolayer array of colloidal particles as a mask.
Modern applications continue to challenge our ability to generate sophisticated and well-defined surface nano- and micro-structures with controlled density, distribution and topography. The overall purpose of this research project was to develop and apply a novel laser-initiated liquid-assisted colloidal lithography (LILAC) method for controllable nanostructuring of a wide range of surfaces.
More complex and user controllable 2-D near-field intensity distributions beneath a colloidal particle have been demonstrated in this project by immersing the surface-supported colloidal particles in a range of different liquids. Single pulse LILAC processing yielded complex 3-D patterning of both silicon and gallium arsenide substrates. The detailed topographies are shown to depend on whether the index of refraction of the colloidal particle (ncolloid) is greater or smaller than that of the liquid nliquid (i.e. whether the incident light converges or diverges upon interaction with the particle). The observed surface patterns derive from concentric rings that originate from beneath the centre of each particle in the irradiated array, but the detailed form of these rings and their mutual interference is shown to be sensitively dependent on the sign and the magnitude of (ncolloid - nliquid) and the particle size.
The LILAC technique involves a complex interplay of laser light scattering by the colloidal particles, by the liquid medium and by the substrate – all of which can be varied and potentially controlled by the user. These characteristic fingerprints open the way to achieving sub-diffraction limited patterning (‘carving’) of the surfaces of a wide range of materials.
Use of 3-D finite-difference time-domain methods has provided a conceptual framework for envisioning both the ability to generate complex surface patterns and the potential of a highly localized, one shot laser ablation source with application into the semiconductor industry.
Many aspects of this project will have direct benefits for academic research in wide areas of science, engineering and other areas beyond. The new opportunities afforded by creating a new, low cost nanoscale lithographical concept and method will have broad impacts in many academic communities – including, but nor restricted to: laser-material processing, near field optics (the distinctive near field optical effects based on the superposition of waves scattered and focused by colloids will advance studies in this area), liquid manipulation, optical trapping, colloidal particles (new shape and material designs), soft (bio) science materials, nano-patterning and imprinting, etc. Moreover, the fundamental scientific components of the research, which probe the role of both the liquid and the colloid particle in the patterning of solid surfaces, will also provide valuable information which will be made accessible to the commercial and public sectors.