Self-assembled Polystyrene Spheres Research Articles

Large-Area Patterning of Carbon Nanotube Ring Arrays

Single-walled carbon nanotubes were assembled into large-area arrays of nanoscale rings on silicon via direct patterning with a self-assembled colloidal polystyrene sphere mask. Nanotubes from liquid suspension gathered at the base of each sphere in the mask to form rings, and the resulting arrays consisted of well-ordered nanotube rings with diameters of 203+/-21 nm and 97+/-14 nm for rings formed with 780 and 450 nm colloidal spheres, respectively. Ring heights were found to be 4.7+/-1.8 nm and 5.9+/-1.4 nm for 780 and 450 nm sphere masks, respectively. A first-order geometric model was proposed to account for the observed ring diameters. The approach presented demonstrates an efficient and straightforward path for patterning carbon nanotubes into well-defined surface distributions on various substrates with highly controlled and tunable dimensions.

Organically modified mixed-oxide sol–gel films with complex compositions and pore structures

Films with polymodal porosity (macro-, meso- and micropores) as well as complex materials compositions (different metals, different organic groups) were prepared. First, self-assembled polystyrene spheres on glass slides were coated with a sol prepared from tetraethoxysilane, phenyl- or methyltriethoxysilane and ethyl acetoacetate-stabilized zirconium propoxide or titanium iso-propoxide in the presence of a non-ionic surfactant. Macropores were created by dissolving the polystyrene porogen. Subsequent calcination at different temperatures resulted in the sequential formation of meso- and micropores. Post-calcination treatment with phosphonic acids allowed the introduction of a second type of organic group.

Fabrication and phase transition of long-range-ordered, high-density GST nanoparticlearrays

We use a monolayer of self-assembled polystyrene (PS) spheres (220 nm indiameter) as a reactive ion etching (RIE) mask to fabricate large-scale (more than1 cm2), long-range-ordered,high-density Ge1Sb2Te4 (GST) phase-change material nanoparticle arrays. Atomic force microscopy (AFM) andscanning electron microscopy (SEM) images show that periodic and isolated GST nanoparticlearrays 30–80 nm high and 150–200 nm bottom diameter were formed. Phase transitions ofthe GST films and nanoparticle arrays were studied by subjecting the samples tohigh-resolution transmission electron microscopy (HRTEM) electron-beam radiation.Nanocrystals between 2 and 5 nm in diameter were observed for GST nanoparticle arrays dueto the large surface-to-volume ratio. The density of the GST arrays can reach as high as109 cm−2, which can lead to the realization of future high-density phase-change random accessmemory (PCRAM).

Optical response from lenslike semiconductor nipple arrays

The authors reported the use of recessive size reduction in self-assembled polystyrene sphere mask with anisotropic etching to form lenslike nipple arrays onto the surface of silicon and gallium nitride. These devices are shown to exhibit a filling factor near to an ideal close-packed condition and paraboloidlike etch profile with slope increased proportionally to the device aspect ratio. Specular reflectivity of less than 3% was observed over the visible spectral range for the 0.35-μm-period nipple-lens arrays. Using two-dimensional rigorous coupled-wave analysis, the latter phenomenon can be ascribed to a gradual index matching mechanism accessed by a high surface-coverage semiconductor nipple array structure.

Self-assembly structure formation on patterned InP surfaces

Self-assembly of polystyrene spheres guided by patterned n-type InP substrates has been investigated. InP surfaces were patterned using a variety of methods including wet chemical etching, sputter coating, thermal evaporation, and photo lithography. The self-assembly of polystyrene spheres depended on the appearance of patterns and was affected by the deposition techniques (sputter coating and thermal evaporation) of Au micro-squares. SEM and AFM were used to characterize the surface morphologies.

Large-Area Well-Ordered Nanodot Array Pattern Fabricated With Self-Assembled Nanosphere Template

This paper has demonstrated a simple method to fabricate a large-area (~1 cm2) well-ordered gold nanodot array pattern with high throughput at low cost. At first, self-assembled polystyrene (PS) spheres are closely packed on a water surface, and then transferred from the water surface to a solid substrate by dip coating. This results in a large-area defect-free close-packed PS sphere monolayer template on the solid substrate. It is worthy noting that the large-area close-packed PS sphere template can be transferred to the surfaces of various solids including silicon, oxides and metals, offering great flexibility. Furthermore, the close-packed PS monolayer can be tailored to a loose-packed sphere array template by reactive ion etching (RIE) technique. The sphere size is reduced upon etching but the spacing between spheres retains unchanged. By utilizing the PS sphere template, gold nanodot arrays with feature size down to 30 nm have been fabricated.

Fabrication and optical properties of 2D ordered arrays of CdS nanocrystals by nanosphere lithography

We realized the fabrication of 2D ordered arrays of CdS nanocrystals by nanosphere lithography by use of a monolayer of self-assembled polystyrene spheres 220nm in diameter as a mask. We used scanning electron microscope (SEM) to characterize the structure of samples. The results indicate that the 2D arrays of CdS nanocrystals produced by nanosphere lithography have highly ordered structure which has the same period of the original PS mask. According to the transmission spectrum，it is estimated that the forbidden band width of the CdS nanocrystals is around 2.60eV，which shows a size-dependence of the CdS nanocrystals. This structure has the promising application in the light filter and single-photon emitter.

FABRICATION OF SILICON NANOARRAYS BY DIRECT NANOSPHERE LITHOGRAPHY

We present the fabrication of large-scale two-dimensional periodic silicon nanoarrays using nanosphere lithography. The techniques start from a monolayer of self-assembled polystyrene (PS) spheres of 220 nm in diameter on water surface, which works as a mask to fabricate large-scale periodic silicon nanoarrays by direct plasmatherm reactive ion (RIE) etching. AFM images of the nanoarrays show that the patterns of PS templates are well transferred to the Si surface. The tips stand 50–80 nm high and the lateral size is around 150 nm. The optimum fabrication conditions can be chosen via the analysis of the experimental data.

Magnetization reversal in exchange biased nanocap arrays**This paper was presented at the Materials Research Society Fall 2006 Meeting, 27 November–1 December 2006, as part of Symposium P: Nanoscale Magnets–Synthesis, Self-assembly, Properties and Applications, organized by J Fassbender, J Chapman and C A Ross.

Arrays of self-assembled polystyrene spheres with various particle sizes have been used as a substrate to study the exchange bias effect along the out-of-plane direction of Pt/Co multilayers capped with IrMn layers. The evolution of the reversal process of the resulting magnetic nanocaps was investigated by magnetic force microscopy (MFM) and magnetic transmission x-ray microscopy (M-TXM). Tip–sample interaction-induced irreversible and reversible switching events have been observed during multiple scanning cycles in MFM imaging which are ascribed to the so-called training effect. During M-TXM imaging a drastic change in morphology has been found due to the x-ray exposure, leading to the formation of much larger spherical particles. Interestingly, these merged particles reveal again an exchange coupled single-domain magnetic cap with magnetic behaviour similar to magnetic films deposited directly on spheres of similar size.

Shape-induced anisotropy in antidot arrays from self-assembled templates

Using self-assembly of polystyrene spheres, well-ordered templates have been prepared on glass and silicon substrates. Strong guiding of self-assembly is obtained on photolithographically structured silicon substrates. Magnetic antidot arrays with three-dimensional architecture have been prepared by electrodeposition in the pores of these templates. The shape anisotropy demonstrates a crucial impact on magnetization reversal processes.

Large-Size Liftable Inverted-Nanobowl Sheets as Reusable Masks for Nanolithiography

A low-cost procedure is introduced for fabricating large-area, liftable, ordered TiO2 nanobowl sheets. The sheet is made using the template of self-assembled polystyrene spheres, followed by atomic layer deposition (ALD), ion milling, and etching. By introducing a thin organic layer between the nanobowls and the substrate, the whole sheet can be lifted-off in full size. The dimension of the holes at the bottom of the nanobowls is controlled by additional ALD; thus, the sheet has been applied as a reusable mask for producing nanodot patterns with designed sizes. This technique demonstrates a simple and economic nanolithiography approach for producing various designed patterns without using a clean room, and it has a great potential for scale-up, mass production, and commercial applications.

Band structure and optical properties of opal photonic crystals

A theoretical approach for the interpretation of reflectance spectra of opal photonic crystals with fcc structure and (111) surface orientation is presented. It is based on the calculation of photonic bands and density of states corresponding to a specified angle of incidence in air. The results yield a clear distinction between diffraction in the direction of light propagation by (111) family planes (leading to the formation of a stop band) and diffraction in other directions by higher-order planes (corresponding to the excitation of photonic modes in the crystal). Reflectance measurements on artificial opals made of self-assembled polystyrene spheres are analyzed according to the theoretical scheme and give evidence of diffraction by higher-order crystalline planes in the photonic structure.

Periodicity and alignment of large-scale carbon nanotubes arrays

Intensive studies have been carried out on controlling the periodicity and alignment of large-scale periodic arrays of carbon nanotubes (CNTs) using plasma-enhanced chemical vapor deposition. Catalytic dots are first prepared by self-assembly of polystyrene spheres on chromium-coated silicon substrates. Preparation parameters for CNTs growth including temperature, thickness of catalytic dots, plasma current intensity, and pregrowth plasma etching time are fine tuned and analyzed to achieve optimal combinations. High-quality aligned CNTs arrays with long-range periodicity and controlled diameters have been achieved. The good periodicity and alignment are critical for their applications such as photonic crystals, negative index of refraction, etc.

Growth of large periodic arrays of carbon nanotubes

Large periodic arrays of carbon nanotubes have been grown by plasma-enhanced hot filament chemical vapor deposition on periodic arrays of nickel dots that were prepared by polystyrene nanosphere lithography. A single layer of self-assembled polystyrene spheres was first uniformly deposited on a silicon wafer as a mask, and then electron beam vaporization was used to deposit a nickel layer through the mask. The size of and spacing between the nickel dots are tunable by varying the diameter of the polystyrene spheres, which consequently determines the diameter and site density of carbon nanotubes. The technique can be scaled up at much lower cost than electron beam lithography.