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Simple Nanotechnology
"Nanofabrication" is the process of making functional structures with arbitrary patterns having minimum dimensions currently defined (more-or-less arbitrarily) to be ~100 nm. Microelectronic devices and information technologies have improved, and will continue to improve, as a result of large-scale, commercial implementation of nanofabrication. The motivation for these improvements is to increase the density of components, to lower their cost, and to increase their performance per device, and per integrated circuit. Methods used to generate nanoscale structures and nanostructured materials are commonly characterized as "top-down" and "bottom-up". The conventional top-down techniques include photolithography and scanning beam (or maskless) lithography (e.g., electron beam and focused ion beam lithography). The limitations of these conventional approaches when applied to innovative problems - high capital and operating costs, the difficulty in accessing the facilities necessary to use them, and their restricted applicability to many important classes of problems - motivate our exploration and development of new, or "unconventional" nanofabrication techniques. Unconventional techniques have the potential to be the ultimate, low-cost method for certain types of nanomanufacturing; approaches based on reel-to-reel processing are particularly attractive for low-cost processes. Unconventional approaches are also operationally much simpler to use than are conventional techniques, and thus help to open nanoscience and nanotechnology to exploration by a wide range of disciplines, especially those historically only weakly connected to electrical engineering and applied physics.
Nanofabrication by Molding
The Whitesides group has developed four unique methods for fabricating nanostructures by molding (Figures 1, 2): (1) Replica Molding (RM) consists of three steps: i) creating a topographically patterned master (usually by conventional techniques; see, for example, ii) transferring the pattern of this master into PDMS by replica molding; and iii) fabricating a replica of the original master by solidifying a liquid precursor against the PDMS mold. (2) Solvent-Assisted Micromolding (SAMIM) uses an elastomeric mold and an appropriate solvent to emboss polymer films. (3) Micromolding In Capillaries (MIMIC) uses capillarity to fill a series of channels in a topographically patterned PDMS stamp with a fluid, low-viscosity polymer or ceramic precursor. (4) Microtransfer Molding (µTM), prepolymer fills the recessed regions of the mold, and excess prepolymer is removed from the top surface using a flat edge. After placing the mold in contact with a rigid substrate, the prepolymer is cured by appropriate means.
Nanofabrication by Stamping
We have developed two methods for patterning molecules on surfaces with high resolution (Figure 3). In microcontact printing (µCP), molecules are transfered from a patterned PDMS stamp to a substrate by the formation of covalent bonds. In electrical microcontact printing (e-µCP), a flexible electrode is used to pattern a thin film of electret-based material (i.e., that accepts and maintains an electrostatic potential), probably by injecting and trapping charges.
Edge Lithography
We are exploring several methods for creating nanostructures from using the topographical changes in the edges of patterns. One approach is to pattern nanostructures by selective removal or deposition of material at the edges of lithographically-defined topographic features, such as SAMs (Figure 4).
A second approach (Controlled Undercutting), patterns arrays of nanostructured trenches can be fabricated by the controlled undercutting of topographic features using isotropic wet etching, followed by deposition of a thin film (Figure 5).
A third approach is Phase-Shifting Photolithography (Figure 6). In this technique, the vertical edges of a transparent, topographically patterned substrate can induce changes in the phase of incident, collimated light to create narrow regions of constructive and destructive interference. Phase-shifting photolithography uses this phenomenon to project "dark or "bright" spots of incident light onto the surface of a photoresist.
We and others have discovered that exposing the edge of a thin film can lead to the formation of nanostructure (Figure 7). This method of edge lithography takes advantage of the numerous methods that can grow thin films over large areas with a thickness between 1 and 50 nm. Converting these films - which are thin in the vertical direction - into structures that are thin in the lateral direction is an approach to fabricating nanostructures.
Approaching Zero Through Crystalline Fracture (Cracking)
We have demonstrated a convenient method to generate steps in a planar, surface with vertical dimension ranging from the microscale to the atomic ( less than 0.5 nm) scale (Figure 8). The process involves introducing a crack halfway into a wafer of single-crystal silicon. These cracks have the following attributes: i) they are continuous steps of smoothly decreasing height, which run in straight lines along crystal planes; ii) the step edges of the cracks are typically ~10 µm in height at edge of the wafer (where they initiate) and decrease to 0 nm at the "tip" of the crack (where they disappear into the atomically smooth surface of the silicon wafer; hence "approaching zero"); and iii) these steps are continuous and linear, thereby making them easy to find and characterize. We demonstrate the use of crystal fracture for metrology in nanoscience, by probing the limits of polymeric replication with 0.4 nm resolution (Figure 9).
Functional, Dispersable, Nanostructures from Templates
Metallic half-shells with submicron diameters: We have demonstrated the use of spherical silica colloids on substrate as template on which metallic half-shells are formed. Dissolution of the template releases hollow metallic (Au, Pt, Pd) hemispheres with nanometric-scale dimensions (Figure 10).
Metallic rods with submicron diameters: We use the method of Martin to perform sequential electrodeposition of multiple components with a porous template and to generate multi-functional nanostructures. For example, it is possible to generate nanorods with alternating sections of gold and nickel (Figure 11). The gold provides a surface that can be functionalized with thiol chemistry, while the nickel allows the nanorods to be manipulated with an external magnetic field. The rods naturally self-assemble into hexagonal bundles through magnetic interactions. The magnetic forces polarize the disk-like section within the individual rods, perpendicular to the physical (long) axis of the rods and promote lateral interactions that direct the self-assembly of the rods. Free-standing metallic pyramidal shells: We fabricate metallic shells with a pyramidal structure where the tips have a radius of curvature of ~50 nm (Figure 12). The templates are formed by anisotropic etching of Si. The metal shells are formed by electrodeposition. The uniformity of the templates fabricated by photolithography or soft lithography ensures the uniformity in shape and size of the pyramidal shells.
Select Publications
1. Xia, Y. and Whitesides, G. M. Angew. Chem. 1998, 37, 550.
2. Xia, Y. et al. Chem. Rev. 1999, 99, 1823.
3. Gates, B. D. et al. Annu. Rev. Mater. Res. 2004, 34, 339.
4. Gates, B. D. et al. Chem. Rev. 2005, in press.
5. Kim, E., Xia, Y. and Whitesides, G. M. Nature 1995, 376, 581.
6. Zhao, X., Xia, Y. and Whitesides, G. M. Adv. Mat. 1996, 8, 837.
7. Xia, Y. et al. Science 1996, 273, 347.
8. Odom, T. W. et al. Langmuir 2002, 18, 5314.
9. Gates, B. D. and Whitesides, G. M. JACS 2003, 125, 14986.
10. Xu, Q. et al. JACS, 2005, 127, 854-855.
11. Kumar, A., Biebuyck, H. A. and Whitesides, G. M. Langmuir 1994, 10, 1498.
12. Love, J. C. et al. JACS 2002, 124, 1576.
13. Jacobs, H. O. and Whitesides, G. M. Science 2001, 291, 1763.
14. Aizenberg, J., Black, A. J. and Whitesides, G. M. Nature 1999, 398, 495.
15. Odom, T. W. et al. JACS 2002, 124, 12112.
16. Love, J. C., Paul, K. E. and Whitesides, G. M. Adv. Mater. 2001, 13, 604.
17. Xu, Q., Gates, B. and Whitesides, G. M. JACS 2004, 126, 1332.
18. Gates, B. D. et al. Angew. Chem. Int. Ed. 2004, 43, 2780.
19. Xu, Q. et al. JACS, 2005, 127, 854-855.
20. Love, J. C. et al. Nano.Lett. 2002, 2, 891.
21. Love, J. C. et al. JACS. 2003, 125, 12696.
22. Qiaobing, X. et al. Nano.Lett. 2004, 4, 2509.
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