Substrate Pattern Research Articles

Imprinting substrate structures onto a nematic liquid crystal

By means of Monte Carlo simulations in the grand canonical ensemble we study the morphology of the nematic phase of a simple model liquid crystal interacting with an alternating sequence of chemically different stripes. The stripes anchor molecules such that their orientation is either parallel or perpendicular with the substrate plane. The different molecular orientations are realized through anchoring functions that cause an energetic penalty for molecules oriented in an undesired fashion. We consider combinations of monostable and degenerate anchoring fields. The nature of the nematic phase is characterized through both the local nematic order parameter and the associated local director field. We observe states of uniaxial or biaxial symmetry depending on the ratio of stripe widths and the range of fluid-substrate attraction. In some cases the specific substrate pattern causes regions of biaxial symmetry to coexist with a bulk-like regime sufficiently far away from the substrates in which the local director field indicates a (homogeneous) bent state of the nematic liquid crystal.

Biomimetic Wet Adhesion of Viscoelastic Liquid Films Anchored on Micropatterned Elastic Substrates

Inspired by the natural adhesives in the toe pads of arthropods and some other animals, we explore the effectiveness and peel failure of a thin viscoelastic liquid film anchored on a micropatterned elastic surface. In particular, we focus on the role of the substrate pattern in adhesion energy of the liquid layer and in allowing its clean separation without cohesive failure. Peel tests on the microfabricated wet adhesives showed two distinct modes of adhesive (interfacial) and cohesive (liquid bulk) failures depending on the pattern dimensions. The adhesion energy of a viscoelastic liquid layer on an optimized micropatterned elastic substrate is ~3.5 times higher than that of a control flat bilayer and ~26 times higher than that of a viscoelastic film on a rigid substrate. Adhesive liquid layers anchored by narrow microchannels undergo clean, reversible adhesive failure rather than the cohesive failure seen on flat substrates. An increase in the channel width engenders cohesive failure in which droplets of the wet adhesive remain on the peeled surface.

Purification and Characterization of Peroxidase from Sweet Gourd (Cucurbita moschata Lam. Poiret)

Peroxidase (EC 1.11.1.7; donor: hydrogen peroxide oxidoreductase) is an oxidoreductase enzyme found in many fruits and vegetables. This enzyme was purified from sweet gourd (Cucurbita moschata Lam. Poiret) by ammonium sulphate precipitation and CM-Sephadex ion-exchange chromatography. Furthermore, optimum pH, optimum temperature, optimum ionic strength, stable pH, and stable temperature conditions were determined as 7.2, 50°C, 0.4 M, 8.0, and 40°C, respectively. The molecular weight (MW) of the enzyme was estimated to be 85 kDa by SDS-PAGE method. The values of Km and Vmax were calculated from the Lineweaver-Burk graph for guaiacol/H2O2 substrate patterns.

An unusual climbing dune, Big Hellfire Pass, Stewart Island, New Zealand: exploration through environment, vegetation and trait patterns

Big Hellfire Dune on the western coast of Stewart Island/Rakiura, New Zealand, is an unusual dune climbing 220 m in altitude on the Ruggedy Range, to dissipate on the leeward side of Hellfire Pass. To record vegetation and environmental characters we ran six transects parallel to the coast, at 50-m altitudinal intervals up the dune. Topography reflected substrate and sand deposition patterns. Wind speed was higher further up the dune, and conductivity and water content were lower, while organic matter and pH did not change. The dune has a rich native flora, with very few exotic species. Analysis defined four plant communities, reflecting openness, and cover of Ficinia spiralis, extent of marginal shrubland of Brachyglottis rotundifolia, and presence of some forest species. The adaptive ability of dune plants was investigated in relation to altitude, using leaf traits. The most widespread species B. rotundifolia and F. spiralis adapt with thicker, shorter, wider leaves, shorter petioles and lower specific leaf areas closer to the coast, and again at the Pass. Vegetation patterns appear to be driven by rainfall and the stability of the fine, even-sized sand, not by altitude. Anthropogenic disturbances from invading weeds and trampling from exotic mammals threaten Big Hellfire Dune, requiring monitoring to protect this unusual climbing dune.

Probing the Adhesion of Hepatocellular Carcinoma HepG2 and SK‐Hep‐1 Cells

AbstractCarcinoma cell differentiation stage is an important indicator of cell behavior. For example, cell mobility is much higher for poorly‐differentiated hepatocellular carcinoma SK‐Hep‐1 cells than for well‐differentiated HepG2 cells. In this study, we have cultured HepG2 and SK‐Hep‐1 cells on chemically patterned polydimethylsiloxane (PDMS) substrates to observe differences in the adhesion properties and cell structure that occur due to the patterns. Both cell lines showed a preference for the hydrophobic regions on the patterned PDMS surface with SK‐Hep‐1 cells achieving a higher density than HepG2 for the same cell‐count solutions. Further, SK‐Hep‐1 cells adopted the square or hexagonal shape of the surface patterns while HepG2 cells maintained their more rounded shape. AFM force measurement arrays were also performed on the cell surfaces to measure and map adhesion values between the tip and cell surface membrane. These results demonstrate that, in addition to cell shape and size, adhesion expression in hepatocellular carcinoma cells is differentiation stage dependent. Further, the ability of the SK‐Hep‐1 cells to adopt the shape of the substrate pattern indicates they are more structurally labile, which may contribute to their higher mobility.

Patterned paper and alternative materials as substrates for low-cost microfluidic diagnostics

In recent years a strong need has been realised for the creation of low-cost diagnostics for use in impoverished regions of the world. The use of paper has been championed for this purpose by many research groups globally over the past 5 years, leading to the formation of a large body of knowledge. This work reviews and summarises the many techniques developed to date for the patterning of paper substrates to create channels for the flow of liquids. The work also explores methods for increasing the functionality of paper-based microfluidics, the detection mechanisms employed so far and some of the interesting applications addressed. The review will also investigate the recent use of some alternative materials—both as primary substrates and in hybrids—which will become progressively more important in the future.

Influence of Substrate Wettability on the Morphology of Thin Polymer Films Spin-Coated on Topographically Patterned Substrates

We show that the morphology of a thin polymer film spin coated directly on to a topographically patterned substrate is strongly influenced by the wettability of the substrate, in addition to other well-known parameters such as concentration of the polymer solution (c(n)), spin speed (RPM), and spin duration. Similar to spin coating on a flat surface, (1, 2) on a topographically patterned substrate as well, a continuous film forms only above a critical polymer solution concentration (c(t)*), for a specific RPM and dispensed drop volume. It is believed that for c(n) > c(t)*, the resulting continuous film on a topographically patterned substrate has an undulating top surface, where the surface undulations are in phase with the underlying substrate patterns. (3) On the basis of experiments involving spin coating of polymer thin films on topographically patterned grating substrates, we show that the surface undulations on the film are in phase with the substrate patterns only when the substrate is completely wetted (CW) by the solvent. In contrast, when the substrate is partially wetted (PW) by the solvent, then the undulations are 180° out of phase with respect to the substrate patterns. We further show that for c(n) < c(t)*, a variety of ordered and disordered structures, like array of aligned droplets, isolated strips of polymers, etc., result on both CW and PW substrates, depending on c(n).

Fabrication of crack‐free thick AlN film on a‐plane sapphire by low‐pressure HVPE

AbstractThe periodic trench patterning of sapphire substrates is useful for the growth of crack‐free AlN thick films. However, the in‐plane epitaxial relationship between AlN and a‐plane sapphire depends on the initial growth temperature. Therefore, two trench directions, the &lt;0001&gt;a‐plane sapphire and &lt;1‐100&gt;a‐plane sapphire directions are investigated in this study. We investigated the control of in‐plane epitaxial relationship between AlN and a‐plane sapphire by low‐pressure HVPE and the effect of the trench direction of AlN, which results in two different in‐plane epitaxial relationships on a‐plane sapphire (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Surface Patterning by Microcontact Chemistry

In this Feature Article we describe recent progress in covalent surface patterning by microcontact chemistry. Microcontact chemistry is a variation of microcontact printing based on the transfer of reactive "ink" molecules from a microstructured, elastomeric stamp onto surfaces modified with complementary reactive groups, leading to a chemical reaction in the area of contact. In comparison with other lithographic methods, microcontact chemistry has a number of advantageous properties including very short patterning times, low consumption of ink molecules, high resolution and large area patterning. During the past 5 years we and many others have investigated a set of different reactions that allow the modification of flat and also spherical surfaces in an effective way. Especially click-type reactions were found to be versatile for substrate patterning by microcontact chemistry and were applied for chemical modification of reactive self-assembled monolayers and polymer surfaces. Microcontact chemistry has already found broad application for the production of functional surfaces and was also used for the preparation of DNA, RNA, and carbohydrate microarrays, for the immobilization of proteins and cells and for the development of sensors.

Homogeneity of Ge-rich nanostructures as characterized by chemical etching and transmission electron microscopy

The extension of SiGe technology towards new electronic and optoelectronic applications on the Si platform requires that Ge-rich nanostructures be obtained in a well-controlled manner. Ge deposition on Si substrates usually creates SiGe nanostructures with relatively low and inhomogeneous Ge content. We have realized SiGe nanostructures with a very high (up to 90%) Ge content. Using substrate patterning, a regular array of nanostructures is obtained. We report that electron microscopy reveals an abrupt change in Ge content of about 20% between the filled pit and the island, which has not been observed in other Ge island systems. Dislocations are mainly found within the filled pit and only rarely in the island. Selective chemical etching and electron energy-loss spectroscopy reveal that the island itself is homogeneous. These Ge-rich islands are possible candidates for electronic applications requiring locally induced stress, and optoelectronic applications which exploit the Ge-like band structure of Ge-rich SiGe.

From finite-amplitude equilibrium structures to dewetting in thin polymer films on chemically patterned substrates

We study templating and patterning strategies of an ultrathin (<100 nm) polystyrene (PS) film on substrates with varying lateral confinement produced by linear chemical patterns. The patterned surfaces are fabricated by coating self-assembled monolayers (SAMs) of alternating less (–CH3) and more wettable (–COOH) micro-stripes on gold-coated silicon surfaces. The width of the less wettable stripe was changed from 1 to 15 μm to uncover the influence of the lateral confinement on the dewetted patterns while keeping constant the film thickness (65 nm) and the width of the more wettable stripes (3 μm). When the less wettable stripes are highly confined (<6 μm), both experiments and simulations show a finite amplitude periodic deformation of the PS free surface that replicates the substrate pattern without film rupture. For moderately confined less wettable stripes, ideal templating of the substrate patterns on the polymer surface was observed by film rupture. Combined ridge and drop morphologies were observed when the less wettable stripes have higher widths (low confinement). Shallow ridges are found to form over the more wettable stripes at the initial stages of dewetting whereas larger droplets are found to form over the less wettable stripes at the later stages of dewetting. Interestingly, the arrangement of the droplets does not randomize even when the lateral confinement of the less wettable patterns is rather low, reflecting a long-term memory of boundary effects. A parametric kinetic study uncovered the following morphologies: (i) small aspect ratio pattern replication when the less wettable stripes are highly confined; (ii) larger aspect ratio for the ridges on a substrate with moderately confined less wettable stripes; (iii) near equilibrium morphology when the ridges and droplets coexist on a less wettable stripe with very low confinement. In the latter case, although the droplets are much larger than the ridges, they have similar aspect ratios.

Dewetting-driven hierarchical self-assembly of small semiconducting molecules

We describe the self-organization of PCBM and a cyanine dye on chemically patterned surfaces during spin coating from solution. On homogeneous surfaces, a transient bilayer forms, which in a later stage decomposes into PCBM droplets in a matrix of the cyanine dye. On the patterned surface also a PCBM droplet phase develops, but the final film structure is greatly determined by contact line pinning of the PCBM domains to the substrate pattern. Three characteristic morphology regimes separated by wetting transitions were observed for different ratios between the natural domain dimensions and the underlying pattern periodicity. We demonstrate that contact line pinning can be an important means to control the film morphology in systems where films are coated from solution. This process can be exploited as a general and versatile method for patterning small semiconducting molecules into 1D and 2D photonic crystals.

Directed Assembly of Block Copolymers by Sparsely Patterned Substrates

Self-consistent field theory and dissipative particle dynamics simulations are used to demonstrate the controlled positioning and alignment of cylindrical domains by chemical surface patterns in an asymmetric slit. Self-assembly of the copolymer melt is simulated near dense and sparse surface patterns, both with one and two characteristic length scales. In particular, we compare the effect of a hexagonal pattern, sparse rectangular pattern, and triangle pattern. The parallel alignment of microdomains between preferentially attractive homogeneous surfaces is shown to transform into the stable perpendicular hexagonal phase in the case of the substrate patterns, which commensurate with the hexagonal morphology in the bulk. For the sparse rectangular pattern, when two different length scales of interactions are involved, and for the sparse hexagonal pattern, we observe that the self-assembly of diblocks multiplies the density of the surface patterns by a factor of 2 and 4, respectively. A 2-fold resolution enhancement was found for sparse triangle pattern with two characteristic length scales. The analysis of the circularly averaged structure factor allowed us to distinguish the structure orientation in a film. Additional peak in the region of small lateral wave numbers is attributed to undulational deformations of cylinder domains under preferable film boundaries. This theoretical work serves to rationalize modern nanolithographic fabrication of high-spatial-frequency arrays using lower-spatial-frequency templates.

Thermal simulation of pulsed direct laser interference patterning of metallic substrates using the smoothed particle hydrodynamics approach

This work presents a new approach to the thermal modelling of direct laser interference patterning (DLIP). The spatial and temporal evolution of the temperature distribution within metallic substrates, which are irradiated by nanosecond pulses during the DLIP process, is computed by means of a smoothed particle hydrodynamics (SPH) method. The developed model considers the conversion of laser energy into heat within a very thin surface layer, heat conduction into the bulk material and the effect of latent heat during involved phase transformations. The importance of proper determination of characteristic SPH parameters and adequate spatial resolution of the computational domain on the accuracy of the numerical solution is discussed in detail. The computed temperature distributions are in good agreement with the results of a previously developed FEM model and correspond very well to simultaneously performed experimental investigations.

Static substrate deposition: Toward longer time scale deposition simulations

We report on the development of a new method for simulating the deposition of atoms onto surfaces, which allows for longer time scales and/or larger system sizes. The method involves simulating only the deposited material on a purely static substrate and corrects for the missing thermal fluctuations and energy exchange effects that surface atoms experience as a result of a static substrate. This method may be viewed as an intermediate approximation between the discrete kinetic Monte Carlo and the full molecular dynamics (MD) methods, because it retains the full trajectory dynamics for the deposited atoms. The method can achieve speed-up times of approximately two orders of magnitude for submonolayer depositions on large simulation substrates. We first apply the method to an ideal prototypical system involving the deposition of Pt material on Pt substrates with two-dimensional Ag monolayer substrate patterns, where we show that it produces nanostructures and small cluster diffusion behavior almost identical to that observed during full MD simulations. As an ideal second application of the method, we use it to deposit Pt and Au onto a reconstructed Au(111) surface representative of a herringbone reconstruction and comment on the findings. Finally, we comment on the limitations and possible future improvements.

Surface infusion micropatterning of elastomeric substrates

Surface infusion micropatterning (SIM) is a novel microfabrication process for simultaneous topographical and chemical patterning of elastomeric substrates. The SIM process involves three steps: (1) infusion of a monomer into the substrate, (2) photopolymerization through a patterned contact mask, and (3) drying. For the first time, SIM is demonstrated to create wells and channels (typical depth 5–22 μm, width 20–200 μm) in two substrate materials, a crosslinked polydimethylsiloxane elastomer and a thermoplastic polyurethane elastomer. High-resolution surface features produced include a “checkerboard” well pattern and a microfluidic channel system. The surface micropatterns have been characterized by scanning electron microscopy, optical microscopy, and optical profilometry to quantify channel depth and shape. Because of wall curvature effects, SIM is most suitable for producing shallow (aspect ratio <0.5) microfluidic channels in soft elastomeric materials. Due to the different chemical composition of the interpenetrating polymer network formed in the exposed regions, SIM also produces surface chemical patterning, as illustrated by selective dye-staining experiments. The potential for SIM to impact emerging technologies is discussed in the light of process advantages and limitations.

Stormwater control measure (SCM) design standards to limit stream erosion for Piedmont North Carolina

Summary This study evaluated the potential impacts of sub-bankfull flows produced by stormwater control measures (SCMs) on stream geomorphic stability. In part, design standards for SCMs include peak flow attenuation to maintain pre-development flow conditions to those of undeveloped watersheds or return urbanized, developed watersheds back to the pre-developed state. Most SCMs target lower frequency storms, usually the 2-and/or 10-year discharge events, but leave peak flows resulting from higher frequency storms uncontrolled. SCMs are possibly subjecting streams to longer and more frequent periods of erosion, increasing stream channel instability. The d 65 substrate size, pattern, profile, and dimension of 33 reference stream cross-sections in Piedmont North Carolina were modeled using the continuous simulations program, SWMM, to develop (1) a unit critical discharge metric in L/s/ha, Q c = 0.0035( d 65 ) 1.5048 , (2) allowable annual erosional hour standard, Log(AAEH) = −1.26Log( d 65 ) + 1.21, and (3) allowable volume of eroded bedload standard, Log(AV) = −0.64( Q c ) − 1.52, for watersheds containing SCMs discharging into surface waters. The unit critical discharge represents a threshold that, once exceeded, incipient motion of the d 65 particle can occur. These standards represented benchmarks of stable, naturally eroding reference streams. Ninety-four percent of the unit critical discharges were less than the 2-year 24-h event, indicating the necessity of controlling higher frequency sub-bankfull flows. The standards were applied to an urbanized watershed (one sub-catchment containing a structural SCM and two sub-catchments without) in Raleigh, North Carolina. The unit critical discharge metric appeared to adequately represent urbanized stream geomorphic processes for the sub-catchment undergoing urbanization (4.5% difference) but not for the mature urbanized sub-catchments (47.5% and 68.8% difference). Depending on the long-term management goal of the unstable stream, this metric is not applicable for all urbanized watersheds due to the discrepancy between the field and calculated unit critical discharges. Standards developed from urbanized reference streams could possibly better represent SCMs in urbanized watersheds. All three sub-catchments failed to meet the erosional standards demonstrating the ability of the standards to predict unstable geomorphic processes in streams. The addition of a detention SCM (wet pond), in the urbanized sub-catchment extended the duration of erosive flows from 37 to 87 h/ha/yr, but reduced the estimated volume of eroded bedload from 1.81 to 0.99 m 3 /m/ha/yr when compared to uncontrolled urbanization (no wet pond). Alterations to the design of the wet pond, increased volume size and change in orifice diameter, were explored to see if erosional standards could be better met. This study demonstrated the effect of current SCM design standards on stream stability and why geomorphic processes of stream channels should be incorporated in SCM design standards.

Hippocampal networks on reliable patterned substrates

Toward the goal of reproducible live neuronal networks, we investigated the influence of substrate patterns on neuron compliance and network activity. We optimized process parameters of micro-contact printing for reproducible geometric patterns of 10 μm wide lines of polylysine with 4, 6, or 8 connections at a constant square array of nodes overlying the recording electrodes of a multielectrode array (MEA). We hypothesized that an increase in node connections would give the network more inputs resulting in higher neuronal outputs as network spike rates. We also chronically stimulated these networks during development and added astroglia to enhance network activity. Our results show that despite frequent localization of neuron somata over the electrodes, the number of spontaneously active electrodes was reduced 3-fold compared to random networks, independent of pattern complexity. Of the electrodes active, the overall spike rate was independent of pattern complexity, consistent with homeostasis of activity. Lower mean burst rates were seen with higher levels of pattern complexity; however, burst durations increased 1.6-fold with pattern complexity ( n = 6027 bursts, p < 0.001). Inter-burst interval and percentage of active electrodes displaying bursts also increased with pattern complexity. The extra-burst (non-burst or isolated) spike rate increased 4-fold with pattern complexity, but this relationship was reversed with either chronic stimulation or astroglia addition. These studies suggest for the first time that patterns which limit the distribution of branches and inputs are deleterious to activity in a hippocampal network, but that higher levels of pattern complexity promote non-burst activity and favor longer lasting, but fewer bursts.

Modifying surface properties of diamond-like carbon films via nanotexturing

Diamond-like amorphous carbon (DLC) films have been grown by pulsed-dc plasma-enhanced chemical vapour deposition on silicon wafers, which were previously patterned by means of colloidal lithography. The substrate conditioning comprised two steps: first, deposition of a self-assembled monolayer of silica sub-micrometre spheres (∼300 nm) on monocrystalline silicon (∼5 cm2) by Langmuir–Blodgett technique, which acted as lithography template; second, substrate patterning via ion beam etching (argon) of the colloid samples (550 eV) at different incidence angles. The plasma deposition of a DLC thin film on the nanotextured substrates resulted in hard coatings with distinctly different surface properties compared with planar DLC. Also, in-plane anisotropy was generated depending on the etching angle. The samples were morphologically characterized by scanning electron microscopy and atomic force microscopy. The anisotropy introduced by the texture was evidenced in the surface properties, as shown by the directional dependences of wettability (water contact angle) and friction coefficient. The latter was measured using a nanotribometer and a lateral force microscope. These two techniques showed how the nanopatterns influenced the tribological properties at different scales of load and contact area. This fabrication technique finds applications in the industry of microelectromechanical systems, anisotropic tribological coatings, nanoimprint lithography, microfluidics, photonic crystals, and patterned surfaces for biomedicine.

Controlled van der Waals Heteroepitaxy of InAs Nanowires on Carbon Honeycomb Lattices

We report on unconventional, noncovalent heteroepitaxy of vertical indium arsenide (InAs) nanowires on thin graphitic films in terms of van der Waals (VDW) interactions. Nearly coherent in-plane lattice matching (misfit of 0.49%) between InAs and the graphitic surface plays a critical role in the epitaxial formation of vertical InAs nanowires on graphitic substrates. Otherwise, gallium arsenide (misfit of -6.22%) is grown to be island morphologies. Cross-sectional transmission electron microscopy analyses show that 1-2 monomolecular layer ledges or kinks facilitate heterogeneous nucleation of InAs on nonwetting graphitic surfaces, forming the nuclei and promoting the subsequent nanowire growth with strong VDW interactions at the heterojunction. We further demonstrate the controlled VDW epitaxy method for high-yield and uniform InAs nanowire arrays on honeycomb carbon surface utilizing substrate surface etching and patterning techniques. Our work opens a new platform for the III-arsenide/graphene hybrid junction electronics and optoelectronics.