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Dynamic Patterning of Nanostructures by Combined Electrokinetic Forces

October 14, 2009
NanoPen represents a versatile approach to patterning a range of nanostructured materials, including nanorods and nanotubes.


Reviewed by Jeff Morse, PhD, National Nanomanufacturing Network

The ability to pattern various types of nanostructures on a surface has implications for a range of developing technologies including photovoltaic’s, nanoelectronics, sensors, advanced nanofabrication, optoelectronics, and medical diagnostics. Various methods for nanostructure patterning on a substrate have been explored such as self-assembly, dip-pen lithography, or contact printing. These techniques typically require complicated instrumentation, and have demonstrated limited versatility. Optical techniques, such as creating concentrated current densities in a photoconductive layer, or using optical tweezers to generate localized heating provide for convective flow patterns, have demonstrated the ability to manipulate and immobilize nanostructures, yet require very high optical power densities or suffer from slow collection rates.

Jashmidi Figure 1
The NanoPen mechanism: a.) Configuration, b.) Simulation illustrating electrokinetic force vectors, c.) direct writing of nanoparticle lines, and d.) Patterning of gold nanoparticles as a function of time.
Recently, Jamshidi et. al. investigated the extension of optoelectronic tweezer (OET) techniques as a means for direct writing of nanoparticle patterns, and studied the impact of combined electrokinetic forces on nanoparticle manipulation and immobilization. Combining optically induced dielectrophoresis (DEP) with light-induced AC electroosmosis (LACE), and electrothermal (ET) flow, the authors demonstrate a new tool called NanoPen for the collection and permanent immobilization of various nanoparticles and structures.


The NanoPen configuration consists of a top optically transparent indium tin oxide (ITO) electrode in parallel with a bottom ITO electrode coated with a 1 µm layer of hydrogenated amorphous silicon (a-Si:H). Between the electrodes is a liquid buffer of KCl/DI (deionized) water proportioned to achieve conductivity in the range 1-10 mS/m in which the nanoparticles are suspended. An AC voltage is applied across the electrodes, and the conductivity of the a-Si:H layer is locally modulated by illuminating with an optical beam or pattern. The change in conductivity of the illuminated a-Si:H modifies the local AC electric field profile, effectively transferring the field to the liquid in the area of the optical spot. This provides sufficient DEP forces in the areas of high electric field intensity to attract or repel nanoparticles from the buffer solution, thereby providing the combined electrokinetic forces to collect and immobilize the nanoparticles into a pattern defined by the optical spot. The optical illumination brings about two other major electrokinetic forces. LACE results from the interaction of the lateral electric field components with the electrical double layer present on the a-Si:H surface, mainly for frequencies <50 kHz. Electrothermal forces result from localized heating by the incident optical pattern, which in turn modulates the permittivity and conductance of the liquid buffer, thereby enhancing the DEP forces on the liquid suspension creating a vortex flow pattern around the illuminated area.

By exploiting the combined forces available within the NanoPen configuration, the authors were able to demonstrate rapid patterning of nanoparticles using AC voltages in the 10-20 Vpp, 10-100 kHz range. Pattern size is controlled by optical spot size and intensity, illumination time, and nanoparticle concentration in the buffer solution. NanoPen represents a versatile approach to patterning a range of nanostructured materials, including nanorods and nanotubes. Ultimately the pattern dimensions will be dictated by the optical spot size, thus enabling a means for controlled collection and manipulation of nanostructures from solution. Further investigations may explore approaches for patterning multiple nanostructure types incorporating advanced microfluidic platforms, thereby extending the versatility of this technique.

Last updated: October 15, 2009
 

DOI: 10.4053/er299-091014

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Tags: Advanced Processes + Tools, Nanopatterning/Lithography

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