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Nanopillar basics

Researchers use the term nanopillar with a loose definition. Previous work in ‘nanopillars’ includes structures with diameters as large as 2 μm, randomly grown vertical pillar structures using the vapor-liquid-solid technique and etched structures using a randomly distributed mask material. In the vertical direction, the nanopillar may consist of bulk, quantum well or super lattice materials. But a nanopillar can be considered as semi conducting material patterned, by using electron beam lithography and dry etching, into a three-dimensional cylinder with diameter less than 100 nm. In general, epitaxially grown super lattice or multiple quantum well structures are used as the ‘substrate’ material for these nanopillars. Using electron beam lithography and reactive ion etching techniques, low dimensional nanopillar structures have been formed in GaSb, InAs/GaSb, GaInAs and GaInP, GaN, InGaN and AlGaN based materials. Etched pillar diameters of 20 nm have been achieved with aspect ratios over 10:1. GaInAs/InP nanopillars exhibit a strong photoluminescence peak wavelength blue-shift compared to the as-grown quantum well material, confirming the expected quantum size effect confinement in such nanostructures. In addition, top and bottom metal contacts have been successfully realized using a polyimide planarization and etch back procedure. Optical measurements indicate photoconductive response in selected nanopillar arrays.
This is a ‘top-down’ fabrication method for pseudo-quantum dots in contrast to ‘bottom-up’ growth or assembly techniques such as Stranski-Krastanov three-dimensional growth mode. The diameter of the etched nanostructure is important due to the onset of quantum size effects. With high aspect ratio and fine control over the period and size, the optical and electronic properties of the semiconductor material nanopillar structures can be controlled.
Nanopillars for light-emitting diodes
Light detectors and light-emitting diodes (LEDs) are examples of optoelectronic devices that can take advantage of the use of nanopillars. Ultra-high sensitivities have been reported in nanowire avalanche photodiodes and quantum confinement effects have been reported in photo detectors with embedded nanopillars at low temperatures. Nanopatterned samples exhibit enhanced luminescence due to the extraction of guided modes from nanopillar arrays.
Nanopillar fabrication
Nanopillar is an elongated structure with sidewalls defined by e-beam lithography and dry etching using top-down approach, by direct epitaxial growth with low lateral diffusivity, or by self-assembling techniques using bottom-up approach. The properties vary depending on roughness, electronic states or relaxation degree and fabrication tool. The use of external coatings and thermal/chemical treatments help to control these properties, but light-matter interaction and carrier transport are unavoidably affected by them. For nanopillar fabrication, samples are coated with polymethylmethacrylate (PMMA) and then soft baked. Lithography is performed using an electron beam system. A dot field is patterned in the PMMA and the focus of the e-beam is varied to adjust the dot size to build structures in the submicron range without quantum confinement effects. High defocus levels distort the dot field after resist development. After e-beam lithography, a metal mask consisting of a Ni layer is deposited, followed by a lift-off procedure. The pattern is then transferred into the GaN layer to form nanopillars via dry etching in an ECR-RIE system using a SiCl4: Ar plasma etch process.
Nanopillar for flexible solar panel
Researchers of University of California, Berkeley have made solar panel putting small 'nanopillars' on aluminum foil with a new method which is thrice more efficient than the available efficient nanowires solar cells. The panels could also be made flexible by embedding the cells in transparent polymer so that rolls of flexible panels on thin aluminum foil can be made cutting manufacturing cost to one tenth of conventional silicon and thin-film technologies. The nanopillars allow the researchers to use cheaper, lower-quality materials. The solar cells are made of uniform 500-nanometer-high pillars of cadmium sulfide embedded in a thin film of cadmium telluride. Both materials are semiconductors used in thin-film solar cells. So far the nanopillar solar cells have been shown to have an efficiency of about 6%, but the researchers say that further improvements are possible. They are aiming for a doubling of the efficiency by using indium oxide, a transparent material, for the top copper-gold contacts surface, letting more light through offering a versatile path to the manufacture of low-cost solar modules.
P3HT Nanopillar Arrays for solar cells
Researchers have developed a fabrication process to manufacture solar cell using Poly (3-hexylthiophene) (P3HT) nanopillar array structures on ITO (indium tin oxide) substrates. The nanostructure had an average nanopillar height of 300 nm and an average distance between adjacent nanopillars of 50 nm. Nanoporous anodic alumina templates are used as pattern for molding via replication the P3HT nanostructure. The geometric features were controlled by adjusting the anodization parameters (anodization voltage, temperature, kind of acid electrolyte and concentration, etc.). The applied infiltration technique consists of a combination between the spin-coating and the melt-assisted template wetting methods. As result, P3HT nanopillar arrays with quasi-hexagonal arrangement remain fixed on the ITO substrate after remove the nanoporous anodic alumina template.
A low cost process to fabricate nanopillars
Researchers at the University of Nebraska-Lincoln and the Lawrence Livermore National Laboratory have developed a highly versatile, low cost strategy to fabricate nanometer and submicron scale pillar structures. This strategy can be applied to silicon and III-Vs semiconductor substrates and many other materials suited well for fabrication of self-aligned devices that can tolerate defective patterns. The researchers fabricated forests of silicon pillars of sub-500 nm diameter and with an aspect ratio up to 10 using a combination of the nanosphere lithography and deep reactive ion etching techniques. The nanosphere etches mask coated silicon substrates were etched using oxygen plasma and a time-multiplexed ‘Bosch’ process to produce nanopillars of different length, diameter and separation. This fabrication method combines nanosphere lithography and reactive ion etching (both standard and deep) to produce hexagonally close packed semiconductor nanopillars. A single layer of patterned features of only hexagonally close packed pillar patterns of limited domain size can be generated on a substrate at a time. The present demonstrated scheme provides an alternative inexpensive method for nanoscale, self-aligned pillar fabrication.
Nanopillar chip
UCLA researchers have developed a new method that helps improve diagnosis, prognosis and treatment monitoring tumor cells. Circulating tumor cells provide critical information for examining and diagnosing cancer metastasis, determining patient prognosis, and monitoring the effectiveness of therapies. Several methods have been developed to track tumor cells, but the UCLA researchers have developed a faster and cheaper method.
The researchers developed a silicon chip that is covered with densely packed nanopillars. For the cell-capture, researchers incubated the nanopillar chip in a culture medium and coated with anti-EpCAM, an antibody protein that can help recognize and capture tumor cells. The nanopillar chip captured more than 10 times the amount of cells captured by the currently used flat structure.

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