UoSThe Nanoscale Analytical Science Group

Studying the chemistry of molecular-scale interfaces

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Nanofabrication

The integration of top-down (lithographic) with bottom-up (synthetic chemical) methodologies remains a major challenge in molecular nanoscience. There is a critical length range, between ca. 100 nm and the dimensions of a single biomacromolecule, in which there are few established methods for the execution of chemically specific molecular transformations. Our work on nanofabrication revolves around the use of photochemical methods to execute selective molecular transformations in nanometre-scale regions at surfaces. Photochemistry is an attractive tool, because organic chemistry furnishes us with a wide choice of photochemical strategies, and because photolithography remains the go-to fabrication tool many years after its eventual demise was predicted. The challenge is to find ways to execute photochemical transformations on nanometre length scales. We have found that near-field methods yield exquisite control at length-scales down to a few tens of nm, and interferometric lithography offers remarkable performance over macroscopic areas via fast, inexpensive, simple approaches (see below and the easyNanofab page for a "how-to-do-it" movie).

Scanning near-field photolithography

For a recent review of our work in this area see Nanoscale 2012, 4, 1840.

Conventional optical techniques, based on propagating electromagnetic fields, are subject to diffraction effects, imposing a limit on resolution, to a first approximation, of ca. half the wavelength of the light being used. However, close to a sub-wavelength aperture, there is a near- or evanescent-field, that decays rapidly with distance from the aperture (hence it typically goes undetected under normal conditions) but is not subject to diffraction effects. For the past decade we have been developing techniques for the control of surface composition and reactivity at nanometre-scale resolution using scanning near-field photolithography (SNP). Rather than using a mask as is done in conventional photolithography, we use a scanning near-field optical microscope coupled to a UV laser. We use two types of near-field microscope: a home-built shear force system built in collaboration with Jamie Hobbs (Department of Physics and Astronomy), that employs an etched optical fibre attached to a tuning fork as the probe, and a commercial cantilever-based system (WiTec) that employs AFM-style diving board probes with hollow pyramidal tips that have apertures at their apices.

While near-field microscopy is regarded as a complex technique, near-field lithography is comparatively straightforward: the main practical problems associated with near-field microscopy are connected with signal detection, and the translation of a near-field probe across a surface is readily accomplished using either shear-force or optical deflection methods.

Our early work focused on self-assembled monolayers (SAMs) of alkylthiolates adsorbed on gold surfaces [1,2]. On exposure to UV light with a wavelength of ca. 250 nm, the strongly bound alkylthiolate adsorbate complexes are photooxidised to yield weakly bound sulfonates that may be displaced by rinsing or in a simple sulution-phase replacement process, leading to the adsorption of a second thiolate in regions exposed to UV light. Exposure through a mask enables the fabrication of micrometre-scale patterns, like the hydrophilic squres used to template the formation of the fluorescent vesicles in the image below by spontaneous dewetting from fluorinated regions. Exposure using a near-field probe leads to the formation of features that may be as small as 9 nm using 244 nm excitation [3], to our knowledge the best resolution achieved to date in a photolithographic process.

A wide range of methodologies, based on monolayers of thiols, silanes and phosphonic acids, and thin films of nanoparticles and polymers, have been developed for use on metal and oxide surfaces, enabling the fabrication of metal nanowires, nanostructured polymers and nanopatterned oligonucleotides and proteins. Some of the structures fabricated this way are illustrated below.

(a) Concentric rings of carboxylic acid groups fabricated on silicon surfaces by photochemical dehalogenation of a benzylchloride functionalised silane. [4]

(b) 100 nm trenches fabricated in aluminium by the use of scanning near-field photolithography of alkylphosphonate SAMs on aluminium oxide, followed by wet etching through the SAM resist. [5]

(c) Dragon structure formed by photochemical modification of a SAM of aryl azide functionalised phosphonic acids on aluminium oxide. [6]

(d) 60 nm gold nanowires formed by near-field sintering of trilayers of thiol-stabilised gold nanoparticles on a silicon substrate. [7]

Strategies based upon the use of nitrophenyl-based photocleavable protecting groups have enabled the introduction of synthetic chemical methodology into nanofabrication [8]. Deprotection of a nitrophenyl protected amine by localised exposure to a near field enables subsequent functionalisation of the surface at nanometre resolution. Much of our work is focussed on biological problems, including photosynthetic membrane proteins [9] and other optically active biomolecules [10]. The use of nitrophenyl protecting groups with oligo(ethylene glycol) substituents enables the selective introduction of binding sites to an otherwise protein-resistant surface [11]. Using this approach, micrometre- and nanometre-scale patterns of amine groups were formed and functionalised first with biotin, and then subsequently with dye-labelled NeutrAvidin or streptavidin. In the panel below, the protein nanolines are ca. 50 micrometres long but only 200 nm wide - a very high aspect ratio. Despite this, the lines are clearly visible at low magnification and the surrounding surfce exhibites very dark contrast, indicating high resistance to protein adsorption in unmodified regions.

Large Area Fabrication

A major criticism that may be directed at scanning probe lithography techniques is that they rely upon serial processes (ie a sequence of steps executed one after the other), making them too slow for the patterning of large areas. We have explored two contrasting solutions to this problem, the use of arrays of near-field probes operating in parallel, and the use of interferometric exposure methods to facilitate rapid large-area exposure.

Parallel Near-Field Lithography: The Snomipede

Researchers at IBM developed the "Millipede" to facilitate local probe lithography over macroscopic areas. Consisting of an array of over a thousand cantilevers, all separately actuated, it was based on the premise that a large number of probes executing serial processes in parallel is equivalent to a parallel process. As part of a large consortium from four universities (for full details see www.snomipede.org), we sought to fuse this concept of parallel local probe lithography with the use of near-field excitation, and built a "Snomipede" consisting of an array of cantilever-type near-field probes capable of being controlled independently. [12] The image below shows part of a 16-probe array, together with electron micrographs of one of the pyramidal tips and, at higher magnification, the screen and 100 nm aperture (formed by electron beam lithography) at its apex.

Two alternative methods of excitation of probes were developed. One (see figure below) involved the use of a liquid crystal spatial light modulator to direct an array of individually steerable light beams through an objective into the probe array. The figure below shows part of an array of patterns fabricated in a photosensitive monolyer over an area over 1 mm wide using this instrument. Near-field exposure causes photodeprotection, yielding a small change in the surface adhesiveness that can be imaged by friction force microscopy. The line width achieved was 125 nm, four orders of magnitude smaller than the region over which the pattern was fabricated.

A second instrument was constructed that utilised a digital mirror device in conjunction with a zone plate lens array to steer light beams into probe arrays. Using this instrument, features as small as 70 nm were fabricated in parallel in photoresist films with the sample and probe array completey submerged under water. Parallel near-field techniques appear to be rugged and versatile, and to facilitate fabrication at very high resolution over macroscopic regions.

For further information, visit www.snomipede.org.

Interferometric Lithography

The requisites for achieving the very high resolution realised by near-field lithography are (i) the excitation of a specific photochemical pathway in a group distributed with monolayer coverage at a surface (in the extreme, a monatomic resist in the case of thiol photochemistry where the excitation occurs in the adsorbate sulfur); and (ii) a suitable means to confine the excitation. In collaboration with Gabriel Lopez (Duke) and Steve Brueck (New Mexico) we explored the possibility of using interferometric exposure for nanopatterning SAMs. Interferometric lithography (IL) has been used for many years in semiconductor processing but was under-explored for patterning organic thin films. We found that it provides a highly effective way to modify the chemistry of a SAM at nanometre scale resolution. Exposure of alkylphosphonate SAMs on titania surfaces at 244 nm using a Lloyd’s mirror interferometer (in which half a laser beam is incident on the sample, and the other half of the beam is reflected off a mirror onto the sample, where it interferes with the first half of the beam) caused the spatially periodic photocatalytic degradation of the adsorbates, yielding nanopatterns that extended over square centimetre areas. Nanopatterned monolayers were also employed as resists for etching of the metal film, and enabled the fabrication of lines of Ti as narrow as 46 nm and dots as small as 35 nm. [13] We have also used IL to pattern alkylthiolate monolayers on gold, enabling the fabrication of molecular patternms and gold nanostructures, and to pattern monolayers of protein-resistant silanes on glass, enabling the rapid fabrication of protein nanostructures over square centimetre areas. [14]

 

References

  1. S. Sun, K.S. L. Chong and G. J. Leggett, "Scanning Near-field Optical Lithography of Self-assembled Monolayers", J. Am. Chem. Soc. 2002, 124, 2414-2415. http://pubs.acs.org/doi/abs/10.1021/ja017673h
  2. S. Sun and G. J. Leggett, "Matching the resolution of Electron Beam Lithography using Scanning Near-field Photolithography", Nano Lett. 2004, 4, 1381-1384. http://pubs.acs.org/doi/abs/10.1021/nl049540a
  3. M. T. Montague, R. E. Ducker, K. S. L. Chong, R. J. Manning, F. J. M. Rutten, M. C. Davies and G. J. Leggett, "Fabrication of Biomolecular Nanostructures by Scanning Near-Field Photolithography of Oligo(ethylene glycol) Terminated Self-Assembled Monolayers", Langmuir 2007, 23, 7328-7337. http://pubs.acs.org/doi/abs/10.1021/la070196h
  4. S. Sun, M. Montague, K. Critchley, M.-S. Chen, W. J. Dressick, S. D. Evans and G. J. Leggett, "Fabrication of Biological Nanostructures by Scanning Near-field Photolithography of Chloromethylphenylsiloxane Monolayers", Nano Lett. 2006, 6, 29-33. http://pubs.acs.org/doi/abs/10.1021/nl051804l
  5. S. Sun and G. J. Leggett, "Micrometer and Nanometer Scale Photopatterning of Self-assembled Monolayers of Phosphonic Acids on Aluminum Oxide", Nano Lett. 2007, 7, 3753-3758. http://pubs.acs.org/doi/abs/10.1021/nl072181%2B
  6. O. El Zubir, I. Barlow, E. ul Haq, H, Tajuddin, N. H. Williams and G. J. Leggett, Langmuir 2013, 29, 1083. http://pubs.acs.org/doi/abs/10.1021/la303746e
  7. S. Sun, P. Mendes, K. Critchley, S. Diegoli, M. Hanwell, S. D. Evans, G. J. Leggett, J. A Preece and T. A. Richardson, "Fabrication of Gold Micro- and Nanostructures by Photolithographic Exposure of Thiol-Stabilized Gold Nanoparticles", Nano Lett. 2006, 6, 345-349. http://pubs.acs.org/doi/abs/10.1021/nl052130h
  8. S. A. Alang-Ahmad, L.-S. Wong, E. U. Haq, J. K. Hobbs, G. J. Leggettand J. Micklefield, “Micrometer- and Nanometer Scale Photopatterning using  2-Nitrophenylpropyloxycarbonyl (NPPOC) Protected Aminosiloxane Monolayers”, J. Am. Chem. Soc., 2009, 131 1513-1522. http://pubs.acs.org/doi/abs/10.1021/ja807612y
  9. N. P. Reynolds, S. J. Janusz, M. Escalante-Marun, J. Timney, R. E. Ducker, J. D. Olsen, C. Otto, V. Subramanian, G. J. Leggett and C. N. Hunter, "Directed Formation of Micro- and Nanoscale Patterns of Functional Light Harvesting LH2 Complexes", J. Am. Chem. Soc. 2007, 129, 14625-14631. http://pubs.acs.org/doi/abs/10.1021/ja073658m
  10. N. P. Reynolds, J. D. Tucker, P. A. Davison, J. Timney, C. N. Hunter and G. J. Leggett, “Site-specific immobilization and micrometer and nanometer scale photopatterning of yellow fluorescent protein on glass surfaces”, J. Am. Chem. Soc. 2009,131, 896-897. http://pubs.acs.org/doi/abs/10.1021/ja8079252
  11. S. A. Alang Ahmad, L. S. Wong, E. ul Haq, J. K. Hobbs, G. J. Leggett and J. Micklefield, “Protein micro- and nano-patterning using aminosilanes with protein-resistant photolabile protecting groups”, J. Am. Chem. Soc. 2011, 133, 2749-2759. http://pubs.acs.org/doi/abs/10.1021/ja1103662
  12. E. ul Haq, Z. Liu, Y. Zhang, S. Alang Ahmad, L. S. Wong, S. P. Armes, J. K. Hobbs, G. J. Leggett, J. Micklefield, C. J. Roberts and J. M. R. Weaver, “Parallel Scanning Near-Field Photolithography: The Snomipede”, Nano Lett. 2010, 10, 4375-4380. http://pubs.acs.org/doi/abs/10.1021/nl1018782
  13. G. Tizazu, O. El-Zubir, S. R. J. Brueck, G. J. Leggett and G. P. Lopez, “Large Area Nanopatterning of Alkylphosphonate Self-Assembled Monolayers on Titanium Oxide Surfaces by Interferometric Lithography”, Nanoscale 2011, 3, 2511 - 2516. http://pubs.rsc.org/en/Content/ArticleLanding/2011/NR/c0nr00994f
  14. G. Tizazu, O. El-Zubir, S. Patole, A. McLaren, C. Vasilev, D. J. Mothersole, Al. Adawi, C. N. Hunter, D. G. Lidzey, G. P. Lopez and G. J. Leggett, “Micrometer and Nanometer Scale Photopatterning of Proteins on Glass Surfaces by Photo-degradation of Films formed from Oligo(ethylene glycol) Terminated Silanes”, Biointerphases 2012, 7, 54. http://link.springer.com/article/10.1007/s13758-012-0054-6

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