Interferometric lithography (IL) has been used for many years in semiconductor nanofabrication , but has been little used for molecular nanopatterning. We have found that in combination with self-assembled monolayers as resists, it provides a very simple and rapid means to fabricate nanostructured metals, oxides, polymers and biomolecules over areas as large as 1 sq. cm. Anna Tsargorodska provides an introduction to the methodology that we use in this video:
We use a simple Lloyd's mirror interferometer which consists of a sample and mirror set at an angle 2 theta relative to each other. A laser beam is directed at the assembly; half the beam falls on the sample and half falls on the mirror, from where it is directed onto the sample to interfere with the first half of the beam. The result is a series of bands of alternating constructive and destructive interference, with a sinusoidal intensity cross-section. The period may be varied by changing the angle between sample and mirror, and the width of the resulting structures may be controlled by selection of the appropriate exposure.
When a self-assembled monolayer (SAM) of alkylthiolates on gold is exposed to light from a frequency doubled argon ion laser (244 nm), the adsorbates are photo-oxidised to yield weakly bound sulfonate species that are readily displaced from the surface . Immersion of the sample in a solution of a suitable etchant (mercaptoethylamine in ethanol, with a drop of ammonia added, is effective for gold ), the oxidation products are displaced by the etch solution and the underlying gold is dissolved (see (a) above). When the sample is exposed in an interferometer, bands of photo-oxidation are caused (see (b) above) where the sample is exposed to bands of constructive interference in the interferogram; in the regions between, the monolayer is little modified. Etching of such a sample will yield wires at a period determined by the angle set between the mirror and sample. Alternatively, if the sample is rotated and exposed a second time, a pattern of intersecting bands of exposure will result ((c) above), enabling the fabrication of a wide variety of other structures . The image below illustrates a selection of gold nanostructures formed simply by varying the angle of rotation between the two exposures for gold films coated with a monolayer resist.
A wide variety of methods appears to be compatible with this simple exposure scheme:
- fabrication of gold nanostructures by using self-assembled monolayers of alkylthiolates as resists ;
- fabrication of re-useable, self-cleaning titania nanostructures as templates for protein patterning using self-assembled monolayers of alkylphosponates as resists [5,6];
- nnopatterning of proteins by interferometric photodegradation of protein-resistant silane films ;
- fabrication of nanostructured polymers by photolysis of brominated surfaces, followed by atom-transfer radical polymerisation ;
- protein nanopatterning by interferometric exposure of protein-resistant poly(cyesteine methacrylate) brushes ;
- protein nanopatterning by interferometric exposure of self-assembled monolayers of alkylthiolates on gold .
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- S. Alang Ahmad, A. Hucknall, A. Chilkoti and G. J. Leggett, “Micro- and Nano Structured Poly(oligo(ethylene glycol)methacrylate) Brushes Grown from Photopatterned Halogen Initiators by Atom Transfer Radical Polymerization”, Biointerphases 2011, 6, 8
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- S. Patole, C. Vasilev, O. El-Zubir, L. Wang, M. P. Johnson, A. J. Cadby, G. J. Leggett and C. N. Hunter, "Interference lithographic nanopatterning of plant and bacterial light-harvesting complexes on gold substrates", Interface Focus 2015, 5, 20150005. http://rsfs.royalsocietypublishing.org/content/5/4/20150005