Periodic ZnO nanorod arrays defined by polystyrene microsphere self-assembled monolayers

Microsphere-Toward Future of Optical Microscopes

Optical microscope is one of the most widely used imaging tools for its great flexibility, reliable design, and low cost. Optical microsphere nanoscope (OMN) is invented as a method that can greatly enhance the observation power of conventional optical microscopes. In this perspective, the promising outlook for this approach is briefly discussed. There exists a great freedom to apply this method in various applications. OMN has been successfully commercialized. Our past experience and strategies are summarized in this perspective, which serves as a good reference for the future technology entrepreneurs. Based on our story and model, the factors for success are listed. It can be used to evaluate other commercialization projects and find out the directions that require further improvement.

Patterned Well-Aligned ZnO Nanorods Assisted with Polystyrene Monolayer by Oxygen Plasma Treatment

Zinc oxide is known as a promising material for sensing devices due to its piezoelectric properties. In particular, the alignment of ZnO nanostructures into ordered nanoarrays is expected to improve the device sensitivity due to the large surface area which can be utilized to capture significant quantities of gas particles. However, ZnO nanorods are difficult to grow on the quartz substrate with well-ordered shape. So, we investigated nanostructures by adjusting the interval distance of the arranged ZnO nanorods using polystyrene (PS) spheres of various sizes (800 nm, 1300 nm and 1600 nm). In addition, oxygen plasma treatment was used to specify the nucleation site of round, patterned ZnO nanorod growth. Therefore, ZnO nanorods were grown on a quartz substrate with a patterned polystyrene monolayer by the hydrothermal method after oxygen plasma treatment. The obtained ZnO nanostructures were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscope (FE-SEM).

Fermentation based carbon nanotube multifunctional bionic composites

The exploitation of the processes used by microorganisms to digest nutrients for their growth can be a viable method for the formation of a wide range of so called biogenic materials that have unique properties that are not produced by abiotic processes. Here we produced living hybrid materials by giving to unicellular organisms the nutrient to grow. Based on bread fermentation, a bionic composite made of carbon nanotubes (CNTs) and a single-cell fungi, the Saccharomyces cerevisiae yeast extract, was prepared by fermentation of such microorganisms at room temperature. Scanning electron microscopy analysis suggests that the CNTs were internalized by the cell after fermentation bridging the cells. Tensile tests on dried composite films have been rationalized in terms of a CNT cell bridging mechanism where the strongly enhanced strength of the composite is governed by the adhesion energy between the bridging carbon nanotubes and the matrix. The addition of CNTs also significantly improved the electrical conductivity along with a higher photoconductive activity. The proposed process could lead to the development of more complex and interactive structures programmed to self-assemble into specific patterns, such as those on strain or light sensors that could sense damage or convert light stimulus in an electrical signal.

Ordered Monolayer Gold Nano-urchin Structures and Their Size Induced Control for High Gas Sensing Performance

The synthesis of ordered monolayers of gold nano-urchin (Au-NU) nanostructures with controlled size, directly on thin films using a simple electrochemical method is reported in this study. In order to demonstrate one of the vast potential applications, the developed Au-NUs were formed on the electrodes of transducers (QCM) to selectively detect low concentrations of elemental mercury (Hg(0)) vapor. It was found that the sensitivity and selectivity of the sensor device is enhanced by increasing the size of the nanospikes on the Au-NUs. The Au-NU-12 min QCM (Au-NUs with nanospikes grown on it for a period of 12 min) had the best performance in terms of transducer based Hg(0) vapor detection. The sensor had 98% accuracy, 92% recovery, 96% precision (repeatability) and significantly, showed the highest sensitivity reported to date, resulting in a limit of detection (LoD) of only 32 μg/m3 at 75 °C. When compared to the control counterpart, the accuracy and sensitivity of the Au-NU-12 min was enhanced by ~2 and ~5 times, respectively. The results demonstrate the excellent activity of the developed materials which can be applied to a range of applications due to their long range order, tunable size and ability to form directly on thin-films.

Three Dimensional Sculpturing of Vertical Nanowire Arrays by Conventional Photolithography

Ordered nanoarchitectures have attracted an intense research interest recently because of their promising device applications. They are always fabricated by self-assembling building blocks such as nanowires, nanodots. This kind of bottom up approaches is limited in poor control over height, lateral resolution, aspect ratio, and patterning. Here, we break these limits and realize 3D sculpturing of vertical ZnO nanowire arrays (NAs) based on the conventional photolithography approach. These are achieved by immersing nanowire NAs in thick photoresist (PR) layers, which enable the cutting and patterning of ZnO NAs as well as the tailoring of NAs. Our strategy of 3D sculpturing of NAs promisingly paves the way for designing novel NAs-based nanoarchitectures.

Efficient surface enhanced Raman scattering on confeito-like gold nanoparticle-adsorbed self-assembled monolayers

Confeito-like gold nanoparticles adsorbed on the self-assembled monolayer hierarchy of dendrimer/polystyrene spheres can strongly enhance Raman scattering, being superior to those on the self-assembled monolayer of 3-aminopropyltriethoxysilane.

Influence of ZnO nanowire array morphology on field emission characteristics

In this work the growth and field emission properties of vertically aligned and spatially ordered and unordered ZnO nanowires are studied. Spatially ordered nanowire arrays of controlled array density are synthesized by both chemical bath deposition and vapour phase transport using an inverse nanosphere lithography technique, while spatially unordered arrays are synthesized by vapour phase transport without lithography. The field emission characteristics of arrays with 0.5, 1.0, and 1.5 μm inter-wire distances, as well as unordered arrays, are examined, revealing that, within the range of values examined, field emission properties are mainly determined by variations in nanowire height, and show no correlation with nanowire array density. Related to this, we find that a significant variation in nanowire height in an array also leads to a reduction in catastrophic damage observed on samples during field emission because arrays with highly uniform heights are found to suffer significant arcing damage. We discuss these results in light of recent computational studies of comparable nanostructure arrays and find strong qualitative agreement between our results and the computational predictions. Hence the results presented in this work should be useful in informing the design of ZnO nanowire arrays in order to optimize their field emission characteristics generally.

Facile fabrication of ordered nanostructures from protruding nanoballs to recessional nanosuckers via solvent treatment on covered nanosphere assembled monolayers

We present a facile lithographic nanosphere production process by laminating a nanosphere monolayer with a UV resin and applying various gentle solvent treatments to produce "bottom-up" and "bottom-down" nonclose-packed patterns and investigate their practical applications in nanolenses for optical display and nanosuckers for adhesion. The solvents effect depending on its solubility parameter and solubility tendency toward the interior polystyrene nanospheres was discussed. The polymer-based nanosucker pattern displays shear adhesion force as high as 75.2 N/cm(2).

Controllable synthesis of ZnO nanostructures on the Si substrate by a hydrothermal route

In this paper, controllable synthesis of various ZnO nanostructures was achieved via a simple and cost-effective hydrothermal process on the Si substrate. The morphology evolution of the ZnO nanostructures was well monitored by tuning hydrothermal growth parameters, such as the seed layer, solution concentration, reaction temperature, and surfactant. X-ray diffraction and photoluminescence measurements reveal that crystal quality and optical properties crucially depend on the morphology of the ZnO nanostructures. The ease of synthesis and convenience to tune morphology and optical properties bring this approach great potential for nanoscale applications.

An invisible template method toward gold regular arrays of nanoflowers by electrodeposition

A new approach, an invisible template method that is realized through controlling the interface electroconductivity of an electrode surface, is presented to synthesize gold regular arrays of nanoflowers with variable separations through further electrochemical deposition. Using polystyrene monolayer colloidal crystals as the first template, a hexagonally packed 1-hexadecanethiol pattern was self-assembled and used as an invisible template to control the interface electroconductivity. Further electrochemical deposition under appropriate conditions can easily lead to gold regular arrays of nanoflowers. This new approach demonstrates a simple route to the fabrication of novel gold micro/nanostructured arrays that may find applications as SERS active substrates, superhydrophobic materials, and so forth.

Fabrication of single gold particle arrays with pattern directed electrochemical deposition

A simple and efficient method for fabricating gold nanoparticle (AuNP) arrays is developed. With this method, the AuNP arrays are fabricated by taking an electrochemical deposition (ECD) process on the ITO substrate, which is initially patterned with nanoimprint lithography (NIL). The stamp for NIL is fabricated by the cost-efficient nanosphere lithography (NSL). The size of the AuNPs can be adjusted by varying the potential and duration of ECD. In this work, the diameters of AuNPs are varied from 130 to 420 nm. The AuNP arrays can be readily extended to other conductive substrates, which may be applied for detecting and sensing.

Electroplated porous polypyrrole nanostructures patterned by colloidal lithography for drug-delivery applications

Porous nanostructures of polypyrrole (Ppy) were fabricated using colloidal lithography and electrochemical techniques for potential applications in drug delivery. A sequential fabrication method was developed and optimized to maximize the coverage of the Ppy nanostructures and to obtain a homogeneous layer over the substrate. This was realized by masking with electrophoretically-assembled polystyrene (PS) nanospheres and then electroplating. Drug/biomolecule adsorption and the release characteristics for the porous nanostructures of Ppy were investigated using rhodamine B (Rh-B). Rh-B is an easily detectable small hydrophobic molecule that is used as a model for many drugs or biological substances. The porous Ppy nanostructures with an enhanced surface area exhibited higher Rh-B loading capacity than bulk planar films of Ppy. Moreover, tunability of surface morphology for further applications (e.g., sensing, cell adhesion) was demonstrated.

Precisely-controlled fabrication of single ZnO nanoemitter arrays and their possible application in low energy parallel electron beam exposure

Precisely-controlled fabrication of single ZnO nanoemitter arrays and their possible application in low energy parallel electron beam exposure are reported. A well defined polymethyl methacrylate (PMMA) nanohole template was employed for local solution-phase growth of single ZnO nanoemitter arrays. Chlorine plasma etching for surface smoothing and pulsed-laser illumination in nitrogen for nitrogen doping were performed, which can significantly enhance the electron emission and improve the emitter-to-emitter uniformity in performance. Mechanisms responsible for the field emission enhancing effect are proposed. Low voltage (368 V) e-beam exposure was performed by using a ZnO nanoemitter array and a periodical hole pattern (0.72-1.26 μm in diameter) was produced on a thin (25 nm) PMMA. The work demonstrates the feasibility of utilizing single ZnO nano-field emitter arrays for low voltage parallel electron beam lithography.

Nanorainforest solar cells based on multi-junction hierarchical p-Si/n-CdS/n-ZnO nanoheterostructures

Solar cells based on one-dimensional nanostructures have recently emerged as one of the most promising candidates to achieve high-efficiency solar energy conversion due to their reduced optical reflection, enhanced light absorption, and enhanced carrier collection. In nature, the rainforest, consisting of several stereo layers of vegetation, is the highest solar-energy-using ecosystem. Herein, we gave an imitation of the rainforest configuration in nanostructure-based solar cell design. Novel multi-layer nanorainforest solar cells based on p-Si nanopillar array/n-CdS nanoparticles/n-ZnO nanowire array heterostructures were achieved via a highly accessible, reproducible and controllable fabrication process. By choosing materials with appropriate bandgaps, an efficient light absorption and enhanced light harvesting were achieved due to the wide range of the solar spectrum covered. Si nanopillar arrays were introduced as direct conduction pathways for photon-generated charges' efficient collection and transport. The unique strategy using PMMA as a void-filling material to obtain a continuous, uniform and low resistance front electrode has significantly improved the overall light conversion efficiency by two orders of magnitude. These results demonstrate that nanorainforest solar cells, along with wafer-scale, low-cost and easily controlled processing, open up substantial opportunities for nanostructure photovoltaic devices.

Controllable growth of highly ordered ZnO nanorod arrays via inverted self-assembled monolayer template

This article presents a facile and effective approach to the controllable growth of highly ordered and vertically aligned ZnO nanorod arrays on the GaN substrate via a hydrothermal route by using the TiO(2) ring template deriving from the polystyrene microsphere self-assembled monolayer. The size of TiO(2) ring template can be flexibly tuned from 50 to 400 nm for the 500 nm polystyrene microspheres by varying the time of reactive ion etching and the concentration of TiO(2) sol. As a result, the diameter of the individual ZnO nanorods can be potentially tuned over a wide range. The combination of several characterization techniques has demonstrated that the ordered ZnO nanorods are highly uniform in diameter and height with perfect alignment and are epitaxially grown along [0001] direction. This work provides a novel and accessible route to prepare oriented and aligned ZnO nanorod arrays with high crystalline quality.

High performance field-effect transistors fabricated with laterally grown ZnO nanorods in solution

We have exploited a method for the lateral growth of multiple ZnO nanorods between electrodes in solution without the use of a metal catalyst to fabricate high performance field-effect transistors (FETs). This method enables us to directly align overlapped or overlap-free nanowires between electrodes by eliminating the vertical growth components and complex structural networks. The overlap-free ZnO nanorod FETs showed better performance with a mobility of ∼ 8.5 cm(2) V( - 1) s( - 1) and an on/off ratio of ∼ 4 × 10(5) than the overlapped ZnO nanorod FETs having a mobility of ∼ 5.3 cm(2) V( - 1) s( - 1) and an on/off ratio of ∼ 3 × 10(4). All the FETs fabricated in this work showed much better performance than the previously reported solution-based ZnO FETs.

A catalyst-free and facile route to periodically ordered and c-axis aligned ZnO nanorod arrays on diverse substrates

In this work we present a method for the deposition of periodically ordered, c-axis aligned ZnO nanorod arrays. By using chemical bath deposited films in conjunction with silica templating through nanosphere monolayers, masks suitable for high temperature deposition are created. A vapour phase transport technique is then used to deposit ordered arrays, quickly and inexpensively in a manner ideal for low cost, scalable and reproducible growth on a diverse range of substrates.

Site-specific multi-stage CVD of large-scale arrays of ultrafine ZnO nanorods

Multi-stage growth of ZnO nanorod arrays has been carried out by Au-assisted chemical vapor deposition (CVD) in order to better understand and more precisely control the growth behaviors. It is evidenced that Au-catalyzed vapor-liquid-solid (VLS) growth only dominates the initial site-specific nucleation of the nanorods, while the subsequent growth is governed by a vapor-solid (VS) epitaxy mechanism. The sequential VLS and VS behaviors permit the fabrication of large-scale highly ordered arrays of ZnO nanorods with precisely tunable diameters and embedded junctions by controlling reactant concentration and nanorod top morphology. Based on the above results, two routes to fabricate ultrafine ZnO nanorod arrays are proposed and stepwise nanorod arrays with ultrafine top segment (~10 nm in diameter) have been achieved. Temperature-dependent photoluminescence (PL) and spatial resolved PL were carried out on the nanorod arrays and on individual nanorods, indicating high quality optical properties and tunable light emission along the length of the stepwise nanorods.

Nonvolatile resistive switching in single crystalline ZnO nanowires

We demonstrate nonvolatile resistive switching in single crystalline ZnO nanowires with high ON/OFF ratios and low threshold voltages. Unlike the mechanism of continuous metal filament formation along grain boundaries in polycrystalline films, the resistive switching in single crystalline ZnO nanowires is speculated to be induced by the formation of a metal island chain on the nanowire surface. Resistive memories based on bottom-up semiconductor nanowires hold potential for next generation ultra-dense nonvolatile memories.

Selective patterning of ZnO nanorods on silicon substrates using nanoimprint lithography

In this research, nanoimprint lithography (NIL) was used for patterning crystalline zinc oxide (ZnO) nanorods on the silicon substrate. To fabricate nano-patterned ZnO nanorods, patterning of an n-octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) on SiO2 substrate was prepared by the polymer mask using NI. The ZnO seed layer was selectively coated only on the hydrophilic SiO2 surface, not on the hydrophobic OTS SAMs surface. The substrate patterned with the ZnO seed layer was treated with the oxygen plasma to oxidize the silicon surface. It was found that the nucleation and initial growth of the crystalline ZnO were proceeded only on the ZnO seed layer, not on the silicon oxide surface. ZnO photoluminescence spectra showed that ZnO nanorods grown from the seed layer treated with plasma showed lower intensity than those untreated with plasma at 378 nm, but higher intensity at 605 nm. It is indicated that the seed layer treated with plasma produced ZnO nanorods that had a more oxygen vacancy than those grown from seed layer untreated with plasma. Since the oxygen vacancies on ZnO nanorods serve as strong binding sites for absorption of various organic and inorganic molecules. Consequently, a nano-patterning of the crystalline ZnO nanorods grown from the seed layer treated with plasma may give the versatile applications for the electronics devices.

Anisotropic nanomaterials: structure, growth, assembly, and functions

Comprehensive knowledge over the shape of nanomaterials is a critical factor in designing devices with desired functions. Due to this reason, systematic efforts have been made to synthesize materials of diverse shape in the nanoscale regime. Anisotropic nanomaterials are a class of materials in which their properties are direction-dependent and more than one structural parameter is needed to describe them. Their unique and fine-tuned physical and chemical properties make them ideal candidates for devising new applications. In addition, the assembly of ordered one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) arrays of anisotropic nanoparticles brings novel properties into the resulting system, which would be entirely different from the properties of individual nanoparticles. This review presents an overview of current research in the area of anisotropic nanomaterials in general and noble metal nanoparticles in particular. We begin with an introduction to the advancements in this area followed by general aspects of the growth of anisotropic nanoparticles. Then we describe several important synthetic protocols for making anisotropic nanomaterials, followed by a summary of their assemblies, and conclude with major applications.

Aerosol assisted fabrication of two dimensional ZnO island arrays and honeycomb patterns with identical lattice structures

Two dimensional island arrays and honeycomb patterns consisting of ZnO nanocrystal clusters were fabricated on predefined TiO(2) seed patterns prepared by vacuum free, aerosol assisted wet-chemical synthesis. The TiO(2) seed patterns were prepared by applying an aerosol of a water soluble titanium complex on hexagonally close-packed polystyrene bead arrays for different lengths of time. Scanning electron microscopy revealed that a dot array grows into a honeycomb shape as increasing amounts of the precursor were deposited. ZnO nucleation on substrates with a dot array and honeycomb patterns resulted in the formation of two discrete patterns with contrasting fill fractions of the materials.

Colloidal self-assembly meets nanofabrication: from two-dimensional colloidal crystals to nanostructure arrays

Self-assembly of colloidal microspheres or nanospheres is an effective strategy for fabrication of ordered nanostructures. By combination of colloidal self-assembly with nanofabrication techniques, two-dimensional (2D) colloidal crystals have been employed as masks or templates for evaporation, deposition, etching, and imprinting, etc. These methods are defined as "colloidal lithography", which is now recognized as a facile, inexpensive, and repeatable nanofabrication technique. This paper presents an overview of 2D colloidal crystals and nanostructure arrays fabricated by colloidal lithography. First, different methods for fabricating self-assembled 2D colloidal crystals and complex 2D colloidal crystal structures are summarized. After that, according to the nanofabrication strategy employed in colloidal lithography, related works are reviewed as colloidal-crystal-assisted evaporation, deposition, etching, imprinting, and dewetting, respectively.

Exploiting zinc oxide re-emission to fabricate periodic arrays

The synthesis of hexagonal ring-shaped structures of zinc oxide using nanosphere lithography and metal/metal oxide sputtering is demonstrated. This synthesis exploits the surface re-emission of zinc oxide to deposit material in regions lying out of the line-of-sight of the sputtering source. These rings can nucleate the hydrothermal growth of zinc oxide crystals. Control over the growth could be exercised by varying growth solution concentration or temperature or by applying an external potential.

Aminosilane micropatterns on hydroxyl-terminated substrates: fabrication and applications

The technique to pattern aminosilanes on hydroxyl-terminated substrates will open up extensive applications in many fields. There are some existing methods to pattern aminosilanes, in particular, (3-aminopropyl)triethoxysilane (APTES) on SiO(2) and glass substrates through indirect routes. However, few reports focus on the direct patterning of APTES by microcontact printing (microCP), due to the volatility of "inks" which consist of APTES and organic solvents. This report shows that high-quality APTES patterns on hydroxyl-terminated substrates can be directly obtained by microCP using an APTES aqueous solution as "ink". Gold nanoparticles (Au NPs) have been used to verify the presence and quality of APTES patterns on which they are selectively adsorbed. Thus-obtained Au NP patterns can serve as templates for the growth of ZnO nanostructures. Lectins are also successfully immobilized on the APTES patterns, with glutaraldehyde as linker. We believe that our method will serve as a general approach and find a wide range of applications in the fabrication of patterns and devices.

Generation of dual patterns of metal oxide nanomaterials based on seed-mediated selective growth

A facile route for the generation of the dual patterns of metal oxide nanomaterials, for example, ZnO and CuO, has been developed by printing the oxide seeds through a combination of microcontact printing (microCP) and microfluidic (microF) techniques, followed by the simultaneous growth of the two metal oxide nanomaterials in a one-step solution reaction based on hydrothermal, seed-mediated selective growth. The obtained dual patterns of ZnO nanorods and CuO nanoneedles show a sharp boundary between them, indicating well-defined dual-pattern generation. Also, the simultaneous growth of metal oxide nanomaterials is highly material-selective for the specific seeds prepatterned on substrates, resulting in the selective growth of ZnO nanorods and CuO nanoneedles on the ZnO and CuO seeds, respectively. Moreover, the generation of high-quality dual patterns has been similarly realized on a flexible poly(ethylene terephthalate) (PET) wafer. This study demonstrates the well-controlled hydrothermal growth of different metal oxide nanomaterials in the same reaction solution on the preprinted oxide seeds on the target substrates. It opens up an avenue to develop multifunctional devices of different metal oxides with the combination of microCP and microF techniques.

Colloidal-crystal-assisted patterning of crystalline materials

Colloidal crystals have shown great potential as versatile templates for the fabrication of patterned micro- and nanostructures with complex architectures and novel properties. The patterning of functional crystalline materials in two and three dimensions is essential to the realization of their applications in many technologically important fields. This article highlights some recent progress in the fabrication of 2D and 3D patterned crystalline materials with the assistance of colloidal crystals. By combining a bioinspired synthetic strategy based on a transient amorphous phase with a colloidal-crystal templating method, unique 3D ordered macroporous (3DOM) calcite single crystals can be created. Moreover, patterned arrays of regular ZnO nanopillars with controlled size, shape, and orientation can be fabricated via a facile wet chemical approach by using masks derived from monolayer colloidal crystals (MCC).

Seedless Pattern Growth of Quasi-Aligned ZnO Nanorod Arrays on Cover Glass Substrates in Solution

A hybrid technique for the selective growth of ZnO nanorod arrays on wanted areas of thin cover glass substrates was developed without the use of seed layer of ZnO. This method utilizes electron-beam lithography for pattern transfer on seedless substrate, followed by solution method for the bottom-up growth of ZnO nanorod arrays on the patterned substrates. The arrays of highly crystalline ZnO nanorods having diameter of 60 ± 10 nm and length of 750 ± 50 nm were selectively grown on different shape patterns and exhibited a remarkable uniformity in terms of diameter, length, and density. The room temperature cathodluminescence measurements showed a strong ultraviolet emission at 381 nm and broad visible emission at 585-610 nm were observed in the spectrum.

Lateral arrays of vertical ZnO nanowalls on a periodically polarity-inverted ZnO template

Well aligned ZnO nanowall arrays with submicron pitch were grown on a periodically polarity-inverted ZnO template using a carbothermal reduction process. Under the conditions of a highly dense Au catalyst for increasing nucleation sites, ZnO nanowalls with a thickness of 126 +/- 10 nm, an average height of 3.4 microm, and a length of about 10 mm were formed on the template. The nanowalls were only grown on a Zn-polar surface due to a different growth mode with an O-polar surface. The results of x-ray diffraction and photoluminescence (PL) measurements revealed a single crystalline, vertical alignment on the template, and a large surface to volume ratio of the ZnO nanowalls.

In situ comprehensive characterization of optoelectronic nanomaterials for device purposes

We have combined optical fiber probe and nanoprobe techniques in a scanning electron microscope, which enables in situ optical, electrical and structural characterization of optoelectronic nanomaterials and nanodevices. The nanoprobe technique, employing sharp metal tips, is used for in situ nano-manipulation, contact and electrical measurement. The fiber probe, coupled to a spectrometer or a laser and controlled by a nano-manipulator, allows local optical detection or excitation. We show in situ assembly of a light emitter and photodetector based on individual nanostructures, demonstrating the potential application of the above technique in building prototype optoelectronic devices and selecting suitable nanostructures for device purposes. In addition, the angular resolving power of the fiber probe detection is demonstrated to be useful for studying nanoscale waveguides.

Contact-line speed and morphology in vertical deposition of diluted colloids

We report experimental results on the speed of a receding contact line, in a vertical deposition configuration, during the deposition of polystyrene colloidal particles. We study the effects of the initial concentration of the suspension and compare the measured speeds with the corresponding dried deposit. It is observed that multilayer structures are associated with high speeds. This result is explained through a region in the bulk of the suspension where the concentration of particles increases until multilayer is formed depending on the porosity of the previously deposited structures.