Structural Transformation by Electrodeposition on Patterned Substrates (STEPS) - A New Versatile Nanofabrication Method
Original Entry: Peter Foster, AP 225, Fall 2011 In Progress...
Authors: Philseok Kim, Alexander K Epstein, Mughees Khan, Lauren D. Zarzar, Darren J. Lipomi, George M. Whitesides, and Joanna Aizenberg
Publication:Kim et al. Structural Transformation by Electrodeposition on Patterned Substrates (STEPS): A New Versatile Nanofabrication Method. Nano Lett
Keywords: Nanofabrication, high-aspect-ratio nanostructure, electrodeposition, replica molding, gradient structure, three- dimensional patterning
The main idea in this paper is to present a method to make a versatile array of high aspect ratio (HAR) structures from a single template. A common way to make periodic arrays of these structures is to use deep reactive ion etching to make a master model of your array out of silicon. PDMS can then be used to make a mold of the master and then another array can be made using the PDMS mold. The problem with the approach is that generally one has to make a new master if they want to change some property of the array. This paper presents an approach where a variety of structures can be made from the same master.
The method is called Structural Transformation by Electrodeposition on Patterned Substrates (STEPS). The general idea is that gold can be deposited on the HAR array to serve as an electrode. The array is immersed in a solution containing a conductive polymer (sodium dodecylbenzenesulfonate (NaDBS)-doped polypyrrole in this case). When a voltage is applied between the solution and the deposited gold, the conductive polymer will deposit on the array. Depending on how the gold is deposited on the array and how long the voltage is applied for, one can create many different structures.
Figure 1 shows the three main ways this technique is used. STEPS I (at the top of the figure) uses sputter coating to coat the structures in gold. This leads to a uniform covering of the entire surface. When a voltage is applied, the polymer uniformly coats the surface, increasing the cylinder's radius uniformly. STEPS II (in the middle of figure 1) uses electron beam evaporation for the gold coating, with the gold coming from a direction parallel to the long axis of the cylinder (i.e. from above). There is a slight subtlety here having to do with how the original array is made. It is made by deep reactive ion etching, which leads to a scalloping of the sides of the resulting cylinders. Thus, when the gold is evaporated on, there are certain areas on the cylinder's side that become coated in gold and some that are not. This leads to a series of gold rings. When a voltage is applied, the conductive polymer initially coats the bottom of thus substrate. As the thickness of this polymer layer increases, eventually electrical contact is made with the first ring and polymer begins to deposit there. This continues to more and more rings as time goes on. Since the polymer is deposited at the bottom of the structure for a longer amount of time than near the top, the resulting structure is a nanocone. STEPS III (at the bottom of figure 1) is much like STEPS II, but with the evaporated gold coming down at an angle instead of coming from the surface normal.This anisotropic distribution of gold is preserved once the conducting polymer begins to deposit, leading to curved structures. Since the STEPS process is solution based, one can slowly remove the substrate from the solution with deposition is taking place. If this is done continuously, you end up with an array with a gradient of cylinder sizes. If this is done in small steps, you can make several distinct regions where each region has a uniform radius that's different from the other regions.
Figures 2 and 3 show what one can do with these HAR structures. Figure 2 shows how they made concentric gold rings (ring resonators) which can be used for optical sensing. First gold was sputter coated onto the initial array of nanopillars, leading to a uniform coating. STEPS was used to deposit polymer layers of different thicknesses, and a second round of gold sputter coating was performed. The entire array was embedded in epoxy and an oxygen plasma was used to remove the polymer layer as well as the initial nanopillars. An ultramicrotome was then used to slice thin sections of the resulting structures, leading to thin slices of epoxy containing concentric gold rings. Figure 3 shows fluorescently labeled bacteria on a periodic array of nanopillars. In one direction of the array there is a gradient in pillar radius and in the orthogonal direction, there is a gradient in the cylinder spacing. Figure 3(a) shows the bacteria when the spacing is lage and figure 3(b) shows the bacteria when the spacing is lowered. It is evident that at some point there is a spontaneous patterning of the bacteria on the substrate. This phenomenon isn't detailed very well in this paper, but the authors claim to be looking into it and say it will be published elsewhere.
What's neat about this paper is the simplicity of the method used. Using this relatively method, a plethora of different structures can be made from a single master. There's lots of interesting things that people can do with these arrays, but this method can also be as a highly controllable method to make nanorods (simply remove the nanopillars from the substrate). Nanorods can be used in everything from measuring nanoscale distances, to gene delivery . Because of how easy it is to make a gradient of diameters, etc. it becomes much easier to quickly make a variety of nanorods in order to find what's best for a given application.
Kim et al. Structural Transformation by Electrodeposition on Patterned Substrates (STEPS): A New Versatile Nanofabrication Method. Nano Lett
Liu et al. Three-Dimensional Plasmon Rulers. Science (2011)
Salem et al. Multifunctional nanorods for gene delivery. Nat Mater (2003) vol. 2 (10) pp. 668-671