The continuous miniaturization of electronic and photonic devices over the past few decades has demanded the development of adaptable, high-throughput, nanoscale fabrication techniques. Consequently, fabricating well-defined nanostructures is at the forefront of nanomanufacturing challenges. The ability to control the growth of metal and metal-oxide structures at the nanoscale has enabled many interesting observations and applications in the areas of photonics, sensors, photovoltaics, plasmonics, and spectroscopy. Numerous techniques are available for constructing nanoscale patterns, each with its own advantages and limitations. Most fabrication methods, however, involve multiple steps or sophisticated instrumentation. Therefore, the development of simple, cost-effective nanopatterning techniques that are adaptable to a variety of materials (including polymers, metals, and nonmetals) is of paramount importance for sustained innovation and growth.
A simple approach to create large-area (a few square centimeters) nanopatterns is by irradiating optical interference patterns on photosensitive polymers and organometallic compounds. We and other groups have shown that with this technique one- and two-dimensional submicron structures can be readily fabricated and the structures' periodicity can be easily tuned. Additionally, a polymer structure made this way can be used as a template to fabricate an ordered array of metals or inorganic oxides and also as a master or mold for soft lithography.
Figure 1 shows the schematic of a typical experimental setup used for photofabricating metallic structures or polymer surface relief gratings (SRGs), in which the surface becomes corrugated. An argon ion laser beam with appropriate polarization is spatially filtered and collimated. A mirror reflects half of the beam, recombining it with the other half to form an interference pattern at the sample plane (orthogonal to the mirror). Exposing the thin film of metal precursor or photoresist to the interference pattern produces surface structures either by photodecomposition or by photoisomerization (a change in the molecule's shape), which in turn leads to migration of polymer chains or cross-linking.
Figure 1. Schematic of a simple laser interference setup. A mirror perpendicular to the sample reflects half of the laser beam so that it interferes with the other half at the sample surface.
Atomic force microscopy images of one-dimensional (a) and two-dimensional (b) surface relief gratings (SRGs) made by light interference on azopolymer film.1
A commonly used class of polymers are the azobenzene functionalized polymers, in which an azobenzene group is either incorporated in the polymer's main chain or attached as a side chain. We have used such azopolymers to create large-area one- and two-dimensional SRGs (see Figure 2).1 When irradiated with light of the correct wavelength and polarization, the azobenzene changes shape (technically, it undergoes trans-cis isomerization), softening the azopolymer. The resulting chain migration in the presence of the light produces the large sinusoidal modulation of the azopolymer film's surface. The formation of the SRG (for instance, the amplitude of the surface modulation) depends on the polarization of light used.
An azopolymer SRG can serve as a template for fabricating one- and two-dimensional metal and metal-oxide patterns.2, 3 Metal or metal-oxide nanoparticles or precursors are deposited on the template's surface and subsequently coalesced by sintering in air and burning off the polymer scaffold. Figure 3 shows an example of patterned gold lines that we produced in this way.
Patterned gold lines made by depositing the metal on an azopolymer SRG.2
(a) Scanning electron microscopy (SEM) image. (b) Energy dispersion spectroscopy image. Scale bar is 1μm.
SEM images of metal nanopatterns fabricated by photodecomposition of organometallic precursors. (a) Continuous silver lines made from silver glutamate.4
(b) Silver nanoparticle pattern made from a silver-amine complex.5
Interfering light can also be used for direct fabrication of periodic metal nanostructures by photodecomposition of organometallic compounds and complexes. Figure 4 shows images of two such patterns: silver lines generated using silver glutamate4 and a pattern of silver nanoparticles (about 100nm in width) made using a silver-amine complex.5 This simple fabrication of metallic patterns without needing pre-patterned templates and sophisticated instruments is of considerable technological interest. The silver nanoparticle array, for example, could be used to generate excitations called surface plasmons on a chip.
We conclude that light interference is a versatile tool for creating large-area nanoscale patterns of various materials on a broad range of substrates in a single step. Many different one- and two-dimensional periodic structures can be fabricated, with their properties tuned by altering the experimental conditions. We are currently working on photofabricating structures on flexible plastic substrates for organic photovoltaic devices. We will also be exploring the use of nanopatterned gold and silver for sensing chemical and biochemical analytes with surface-enhanced Raman scattering. More generally, photofabrication of nanoscale structures is expected to lead to interesting applications in plasmonics, photonics, diffractive optics, and nanotechnology.
Abhishek Kumar, Akshay Kokil, Ramaswamy Nagarajan, Jayant Kumar
University of Massachusetts
Abhishek Kumar is a graduate student in physics.
Akshay Kokil is a postdoctoral researcher at the Center for Advanced Materials.
Ramaswamy Nagarajan is a professor of plastics engineering.
Jayant Kumar is a professor of physics and director of the Center for Advanced Materials.
Lian Li, Lynne A. Samuelson
US Army Natick Soldier Research, Development, and Engineering Center (NSRDEC)
Lian Li is a scientist at the NSRDEC.
Lynne A. Samuelson is a scientist at the NSRDEC.
1. N. K. Viswanathan, D. Y. Kim, S. Bian, J. Williams, W. Liu, L. Li, L. Samuelson, J. Kumar, S. K. Tripathy, Surface relief structures on azo polymer films, J. Mater. Chem. 9,
no. 9, pp. 1941-1955, 1999. doi:10.1039/A902424G
2. L. Li, F. Yan, M. Cazeca, L. Samuelson, J. Kumar, Fabrication of gold nano-structures with azopolymer templates, J. Macromol. Sci., Part A 44, no. 12, pp. 1299-1303, 2007.
3. M. Kim, B. Kang, S. Yang, C. Drew, L. A. Samuelson, J. Kumar, Facile patterning of periodic arrays of metal oxides, Adv. Mater. 18,
no. 12, pp. 1622-1626, 2006. doi:10.1002/adma.200502690
4. A. Kumar, S. Nagarajan, K. Yang, R. Anandakathir, J. Singh, R. Nagarajan, A. Jain, J. Kumar, A simple technique for submicron scale patterning of silver using visible light interference, J. Macromol. Sci., Part A 45,
no. 11, pp. 963-966, 2008. doi:10.1080/10601320802380273
5. A. Kokil, A. Kumar, S. Balasubramaniam, R. Nagarajan, J. Kumar, Facile synthesis and patterning of silver nanoparticles for surface plasmon generation, J. Nanophoton.
5, pp. 053515, 2011. doi:10.1117/1.3614008