Square Array Of Pillars Research Articles

Self-Assembly of a Two-Dimensional Au-Nanocluster Superlattice and Its Photoluminescence Spectra

A solution of dodecanethiol-capped Au nanoparticles (diameter approximately 2 nm), prepared through reduction of hydrogen tetrachloroaurate, was coated on a patterned cross-linked polydimethylsiloxane (PDMS) film (thickness > or =1 microm) comprising array of square pillars, by either spin- or dip coating. Drying in ambient conditions for about 5 days resulted in the self-assembly of a superlattice (300 nm x 300 nm x 90 nm) of monodisperse nanoparticle clusters for the dip-coated samples, the clusters occupying the interstitial locations between four adjacent pillars on the polymer surface. Initial patterning of the PDMS is found to give rise to new photoluminescence bands in the near ultraviolet and visible regions, which the nanoclusters quench.

Fabrication of high-aspect ratio Si pillars for atom probe ‘lift-out’ and field ionization tips

A process for fabricating high-aspect ratio (∼1:20), micron-sized Si [0 0 1] pillars using mechanical and chemical size reduction is presented. A dicing saw was used for mechanically patterning an array of square pillars with side lengths of >20 μm. These pillars were then reduced in size using an aqueous NaOH and KOH solution heated to 100 °C. The chemical etch reduces the pillar size within the time range amenable for focus ion beam milling and/or attachment for atom probe ‘lift-out’ specimens. The pillars can be formed with either a flat top surface or into <100 nm tip points for direct field ionization.

Pillars as antipinning centers in superconducting films

Using the nonlinear Ginzburg-Landau theory we study vortex configurations in a superconducting thin film with a square array of pillars in the presence of a uniform applied magnetic field. The presence of the pillars changes the vortex structures in the superconducting film considerably: a transition between triangular and square vortex lattices takes place with increasing size and/or height of the pillars and vortex lines and vortex clusters can be obtained for particular applied magnetic fields. All of these findings are summarized into an equilibrium vortex phase diagram, which shows the transition between different ground-state vortex configurations as a function of the radius and periodicity of pillars. For larger radius of the pillars vortices start to penetrate the pillars and order in vortex shell structures both inside and around the pillars. The theoretical results are complemented by an experimental study of the vortex configurations in single-crystal based Nb pillars with radius $R\ensuremath{\sim}0.5--2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Using Bitter decoration, concentric shells of vortices are revealed inside the pillars and shell-like structures are found at the interstitial sites, in accordance with the theoretical results. In addition, the observed distortion of the vortex shell structures by weak pinning centers present in our Nb samples is investigated by molecular dynamics simulations. We also show that the transition between predicted vortex states can be obtained from the temperature dependence of local magnetization measurements.

Control of morphology in pattern directed dewetting of thin polymer films

We investigate the creation of large area mesoscale structures by controlling the dewetting pathways of thin polymer films on physically heterogeneous substrates comprising a two-dimensional (2-D) array of square pillars. Depending on the initial configuration and thickness of the film, dewetting produces a variety of both ordered and disordered structures. The substrate pattern strongly influences the dewetting pathways as well as the organization and size of the polymer structures. The key findings are: (i) the lateral confinement imposed by the substrate pattern can reduce the length-scale of the dewetted structure by one to two orders of magnitude as compared to dewetting on the same homogeneous substrate. (ii) When the polymer film is thin ( 40 nm), dewetting progresses by nucleation of large holes, uncorrelated to the substrate pattern. The dewetted pattern in this case forms on multiple length-scales consisting of ∼10 μm droplets resulting from the coalescence of the holes and small (<1 μm) droplets at the interstitial locations due to stick–slip of the retracting contact line.

Superwetting of Structured Surfaces

Superwetting of structured surfaces, sometimes referred to as hemi-wicking, was studied both experimentally and theoretically. Structured substrates with regular arrays of square pillars or frustra were machined from graphite blocks and then treated to render them lyophilic. Liquids spread over these surfaces to produce noncircular wetting areas. If the channels between the features were made shallower or narrower, liquids wicked more and spread over a larger area. The inherent wettability of the graphite was relatively unimportant; large differences in the contact angles had little influence on the spreading. Practically, this means that, to achieve extensive coverage, near-zero contact angles are not required. A combination of the appropriate surface structure and moderate inherent wettability can effectively flatten liquids, spreading them over very large areas.