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Nanopillar formation mechanism

Scientists at the California Institute of Technology (Caltech) have uncovered the physical mechanism by which arrays of nanoscale pillars can be grown on polymer films with very high precision, in potentially limitless patterns. This nanofluidic process can replace conventional lithographic patterning techniques now used to build three-dimensional nano- and micro scale structures for use in optical, photonic, and biofluidic devices. The fabrication of high-resolution, large-area nanoarrays relies heavily on conventional photolithographic patterning techniques, which involve treatments using ultraviolet light and harsh chemicals that alternately dissolve and etch silicon wafers and other materials. For example, photolithography is used to fabricate integrated circuits and micro electromechanical devices. However, the repeated cycles of dissolution and etching cause a significant amount of surface roughness in the nanostructures, ultimately limiting their performance. But this process is inherently two-dimensional and thus three-dimensional structures must be patterned layer by layer.In an effort to reduce cost, processing time, and roughness, researchers have been exploring alternative techniques whereby molten films can be patterned and solidified in situ, and in a single step.

Formation hypothesis

While using techniques involving thermal gradients, when molten polymer nanofilms were inserted within a slender gap separating two silicon wafers that were held at different temperatures, arrays of nanoscale pillars spontaneously developed. These protrusions grew until they reached the top wafer; the resulting pillars were typically several hundred nanometers high and several microns apart. These pillars sometimes merged, forming patterns that looked like bicycle chains when viewed from above; in other films, and the pillars grew in evenly spaced, honeycomb-like arrays. Once the system was brought back down to room temperature, the structures solidified in place to produce self-organized features.

Germany researchers who had observed this phenomenon hypothesized that the pillars arise from infinitesimal, but very real pressure fluctuations along the surface of an otherwise quiescent flat film. They proposed that the differences in surface pressure were caused by equally tiny variations in the way individual packets (or quanta) of vibrational energy, known as phonons, reflect from the film interfaces.

Researchers hypothesized that the difference in acoustic impedance between the air and polymer is believed to generate an imbalance in phonon flux that causes a radiation pressure that destabilizes the film, allowing pillar formation. Their mechanism is the acoustic analogue of the Casimir force, which is quite familiar to physicists working at the nanoscale.

Later it was hypothesized that a self-organizing instability was able to reproduce the strange formations and that nanopillars, in fact, form not via pressure fluctuations but through a simple physical process known as thermo capillary flow. In most liquids, cooler regions will have a higher surface tension than warmer ones, and this imbalance can cause the liquid to flow from warmer- to cooler-temperature regions, a process known as thermo capillary flow.

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