Monte Carlo Simulation Modeling of Nanoparticle-Polymer Cosuspensions.

Creation of high-quality and novel polymer-particle nanocomposites to a large extent depends on understanding the behaviors of individual polymer chains and particles, especially at the mixing state in a liquid solvent. Simulations can help identify critical parameters and equations that govern the suspension behaviors. This study is the first attempt to understand the agglomeration processes of ZnO nanoparticle and poly(methyl methacrylate) polymer cosuspensions through a constant number Monte Carlo simulation. A modified Derjaguin-Landau-Verwey-Overbeek theory is used to describe the particle-particle interactions that lead to agglomeration. The average agglomerate size and number are measured as a function of suspension resting time, particle to polymer volume ratio, polymer chain length, and suspension drying. The agglomerate size increases persistently with the resting time and particle content increase, ranging from 1.2 µm for the 1 vol % particle content suspension to 4.6 µm for the 20 vol % particle content suspension after 30 min of suspension resting. The agglomerate size distribution for all of the particle contents follows a lognormal distribution. As the polymer chain length increases, agglomeration also becomes more severe. If drying is accounted for and thus the solids loading continually increases, the suspension becomes much more stable because of increases in viscosity and depletion stabilization.

Patterning of ZnO Quantum Dot and PMMA Hybrids with a Solvent-Assisted Technique.

Imprinting of nanoparticle-polymer hybrids has been a challenging task due to the agglomeration of nanoparticles, especially for metal oxides because of their highly hydrophilic and polar surfaces. We hereby report an effective submicron patterning process of ZnO quantum dot/poly(methyl methacrylate) hybrids with a solvent-assisted lithographic technique. Feature sizes down to 250 nm have been achieved with a ZnO content up to 50 vol %, about 10 times higher than the literature-reported inorganic contents. With higher ZnO contents, particles show a tendency to aggregate, and the samples have less flexibility as demonstrated by larger bending radii before failure. The higher ZnO content samples also produce stronger photoluminescence responses. This family of materials has a great potential to be used in flexible optical devices.

Suspension-based imprint lithography of ZnO-PMMA hybrids.

Imprint lithography has been explored as a method to transfer arrays of patterned features onto pure polymers and polymer/metallic nanoparticle composites. Despite the success of this method for those materials, it has never been achieved on the sub-micron scale with polymer-oxide particle hybrids. This study patterns ZnO-PMMA (poly(methyl methacrylate)) hybrids via imprint lithography from co-suspensions of PMMA and ZnO nanoparticles in anisole from 1 vol% to 20 vol% ZnO solids loading. ZnO nanoparticles are functionalized with nonanoic acid to disperse the nanoparticles in anisole with dissolved PMMA. The feature fidelity of the patterned arrays decreases with increasing ZnO content, indicating an increase in particle agglomeration as the ZnO particle content increases. Feature size, ZnO content, and ZnO nanoparticle agglomeration are critical factors influencing the photoluminescence (PL) intensity. The ZnO solids loading at a 500 nm feature size needs to be 10 vol% or higher for the enhanced PL response. When the ridge size increases to 1 µm, ZnO solids loading as low as 1 vol% is feasible. This method of lithographic patterning of nanoparticle-polymer suspensions can be applied to a wide variety of hybrid devices and has the potential to open many applications including optical devices and biomedical screening.

Experimental and Modeling Study of Solvent Diffusion in PDMS for Nanoparticle-Polymer Cosuspension Imprint Lithography.

This study is the first that focuses on solvent migration in a polydimethylsiloxane (PDMS) stamp during the imprint lithography of ZnO-poly(methyl methacrylate) (PMMA) hybrid suspensions. Using suspensions with varying solids loading levels and ZnO/PMMA ratios, the uptake of the anisole solvent in the stamp is evaluated as a function of time. Laser confocal microscopy is employed as a unique technique to measure the penetration depth of the solvent into the stamp. The suspension solids loading affects the anisole saturation depth in the PDMS stamp. For the suspensions with low solids loading, the experimental data agree with the model for non-Fickian diffusion through a rubbery-elastic polymer. For the suspensions with high solids loading, the data agree more with a sigmoidal diffusion curve, reflecting the rubbery-viscous behavior of a swelling polymer. This difference is due to the degree of swelling in the PDMS. Higher solids loadings induce more swelling because the rate of anisole diffusing into the stamp is increased, likely due to the less dense buildup of the solids as the suspension dries.

Formation mechanism of TiO2 nanotubes and their applications in photoelectrochemical water splitting and supercapacitors.

Structural observations of the transition of TiO2 nanopores into nanotubes by increasing the OH(-) concentration in the electrolyte challenge the validity of existing formation mechanisms of anodic TiO2 nanotubes. In this study, dehydration of titanium hydroxide in the cell wall is proposed as the mechanism that leads to the separation of neighboring nanotubes. Based on this understanding, bamboo-type TiO2 nanotubes with large surface area and excellent interconnectivity are achieved by cycling high and low applied potentials. After thermal treatment in a H2 atmosphere, the bamboo-type TiO2 nanotubes show large photoelectrochemical water splitting efficiency and supercapacitors performace.

Selective focused-ion-beam sculpting of TiO2 nanotubes and mechanism understanding.

Anodic TiO(2) nanotubes with different structures, doping agents, and decorations have been studied in order to improve energy conversion and storage efficiencies such as in dye sensitized solar cells, solar fuels, and electrochemical supercapacitors. However, the top surface modification of TiO(2) nanotubes has never been addressed. In this study, anodic TiO(2) nanotubes have been selectively closed by high energy focused ion beams and re-opened by low energy focused ion beams. Under a 30 kV Ga(+) beam, TiO(2) nanotubes are closed with a 65 nm shield layer covering the top entrance when the ion dose is larger than 1.2 × 10(17) ions per cm(2); under a 5 kV Ga(+) beam, the shield layer is removed and the closed tubes are re-opened. An ion-induced viscous flow model has been proposed to explain the influence of Ga(+) ion beam flux, substrate temperature, initial tube diameter, ion beam dwell time, and the incidence angle of the ion beam.

Hierarchically branched titania nanotubes with tailored diameters and branch numbers.

Over the past decade, electrochemical anodization of self-organized TiO(2) nanotubes has been studied intensively with the main focus being on uniform diameters along the TiO(2) nanotube depth direction. In the present work, hierarchically branched TiO(2) nanotubes with tailored diameters and branch numbers are successfully achieved by adjusting the anodization voltage. Reducing the applied voltage by a factor of 1/√n causes a one trunk nanotube to diverge into n-branched TiO(2) nanotubes, whose diameters are 1/√n of the trunk nanotube diameter (n is an integer). Multiple layers of branched TiO(2) nanotubes are also obtained by further dividing the branched nanotubes when the applied voltage is further reduced step-by-step with a 1/√n factor. Enlargement and termination of TiO(2) nanotubes occur when the anodization voltage increases by √n times. Alternating increase and decrease in the applied voltage lead to a more sophisticated hierarchical structure of TiO(2) nanotubes. The fundamental understanding of these processes is discussed.

Fabrication of porous nanocomposites with controllable specific surface area and strength via suspension infiltration.

Porous ceramics are promising candidates for a variety of applications, including separation membranes, catalyst supports, tissue engineering scaffolds, energy storage devices, and microelectronics. We describe a novel method for creating porous ceramics with controllable specific surface area and high strength. The fabrication procedure involves infiltrating aqueous suspensions of silica nanoparticles into a porous ceramic scaffold. The samples are then freeze-dried to maintain a homogeneous distribution of nanoparticles, followed by partial sintering to bond the infiltrated nanoparticles into place. By repeating this infiltration process multiple times, the specific surface area of the composite can be varied from less than one to well over 100 m(2)/g. It is also found that this infiltration increases the mechanical strength of the composite. Water flux experiments demonstrate the potential use of these materials as liquid membranes, with no detectable damage to the structure observed after these flux tests. While this initial work focused on silica nanoparticles and ceramic scaffolds, the basic approach would to applicable to a wide variety of other materials, meaning that the method described here would be generally applicable for creating porous materials with precisely controllable properties.

Moiré pattern nanopore and nanorod arrays by focused ion beam guided anodization and nanoimprint molding.

Nanoporous anodic aluminum oxide has been intensively studied as templates in the fabrication of various nanomaterials. Most of the research focuses on highly ordered hexagonal nanopore arrangement with homogeneous area-specific pore density. Anodic aluminum oxide with alternating area-specific nanopore densities has seldom been addressed. In this study, focused ion beam patterned concave arrays created by overlapping two periodic patterns show the exceptional ability of guiding the subsequent anodization and fabricating porous anodic aluminum oxide with Moiré patterns, which have a wide range of interpore distances and area-specific pore densities. The periodicity of the Moiré patterns can be predicted by the interpore distance of the initial patterns and the rotation angle. The depth of the nanopores of these Moiré patterns is around 1 µm. Vertically aligned and high aspect ratio h-PDMS nanorod arrays with Moiré pattern arrangements have been successfully synthesized using the patterned porous anodic aluminum oxide as templates.

Influence of patterned concave depth and surface curvature on anodization of titania nanotubes and alumina nanopores.

Vertically aligned TiO(2) nanotube and Al(2)O(3) nanopore arrays have been obtained by pattern guided anodization with uniform concave depths. There are some studies about the effect of surface curvature on the growth of Al(2)O(3) nanopores. However, the surface curvature influence on the development of TiO(2) nanotubes is seldom studied. Moreover, there is no research about the effect of heterogeneous concave depths of the guiding patterns on the anodized TiO(2) nanotube and Al(2)O(3) nanopore characteristics, such as diameter, growth direction, and termination/bifurcation. In this study, focused ion beam lithography is used to create concave patterns with heterogeneous depths on flat surfaces and with uniform depths on curved surfaces. For the former, bending and bifurcation of nanotubes/nanopores are observed after the anodization. For the latter, bifurcation of a large tube into two smaller tubes occurs on concave surfaces, while termination of existing tubes occurs on convex surfaces. The growth direction of all TiO(2) nanotubes is perpendicular to the local surface and thus is different on different facets of the same Ti foil. At the edge of the Ti foil where two facets meet, the nanotube growth direction is bent, resulting in a large stress release that causes the formation of cracks.

Novel patterns by focused ion beam guided anodization.

Focused ion beam patterning is a powerful technique for guiding the growth of ordered hexagonal porous anodic alumina. This study shows that, with the guidance of the focused ion beam patterning, hexagonal porous anodic alumina with interpore distances from 200 to 425 nm can be fabricated at 140 V in 0.3 M phosphoric acid. When the interpore distance is increased to 500 nm, alternating diameter nanopore arrays are synthesized with the creation and growth of new small pores at the junctions of three large neighboring pores. Moreover, alternating diameter nanopore arrays in hexagonal arrangement are fabricated by focused ion beam patterning guided anodization. Interpore distance is an important parameter affecting the arrangement of alternating diameter nanopore arrays. Different types of novel patterns are obtained by designing different focused ion beam concave arrays. The fundamental understanding of the process is discussed.

Unique nanopore pattern formation by focused ion beam guided anodization.

The fabrication of ordered porous templates in large areas with sophisticated patterns beyond mono-sized pores in hexagonal packing remains a great technical challenge. Conventional anodization cannot overcome this limitation and a new approach needs to be sought. This study is focused on designing pore patterns in square and hexagonal arrangements via focused ion beam lithography with varying interpore distances in order to form organized pore arrays with more sophisticated patterns. The results demonstrate that the small pores from the anodization can be meshed with the larger pores from the focused ion beam lithography and unique, complicated nanopore patterns can be created. Large pore patterns that cross the grain boundaries are also made possible with the guidance of the pores from focused ion beam lithography. A fundamental pore pattern formation mechanism is proposed.

Focused ion beam lithography and anodization combined nanopore patterning.

In this study, focused ion beam lithography and anodization are combined to create different nanopore patterns. Uniform-, alternating-, and gradient-sized shallow nanopore arrays are first made on high purity aluminum by focused ion beam lithography. These shallow pore arrays are then used as pore initiation sites during anodization by different electrolytes. Depending on the nature of the anodization electrolyte, the nanopore patterns by focused ion beam lithography play different roles in further pore development during anodization. The pore-to-pore distance by focused ion beam lithography should match with that by anodization for guided pore development to be effective. Ordered and heterogeneous nanopore arrays are obtained by the focused ion beam lithography and anodization combined approach.

Photothermal self-healing of gold nanoparticle-polystyrene hybrids.

This study focused on the processing and photothermal healing of gold nanoparticle (Au NP) and polystyrene (PS) hybrid films. Effects of Au NP contents were investigated using hybrid films with the NP content from 0 to 1 wt% via a solvent-assisted approach. The as-synthesized Au NPs showed an average diameter of 4-5 nm with a face-centered cubic structure. The Au NP agglomeration deteriorated as the content increased and the interparticle distance decreased. The film transparency and flexibility also decreased with the NP content. The Au-PS films demonstrated desirable photothermal healing behaviors, which required more energy with the defect size increase. The simulated temperature distribution on the hybrid films during the photo-induced healing showed good agreement with the experimental results, with particle agglomeration degrading the healing properties. The developed hybrid films can be used in functional devices and coatings with high flexibility and healed using photon energy sources.

Hierarchical and nanosized pattern formation using dual beam focused ion beam microscope.

Direct and controlled nanopatterning is a promising new area to study in nanofabrication. Dual ion and electron beam microscopy offers such nanofabrication potentials with simultaneous high resolution electron beam imaging and ion beam patterning. This work investigates the unexplored potentials of ion and electron dual beam microscope to create different feature shape and hierarchical sized nanopatterns. Nanotrack and nanopore patterns with 100 nm distance were created with focused ion beam on high purity aluminum substrate. The track width is less than 32 nm and the pore size is less than 50 nm. Different size nanopores have been precisely aligned to form hierarchical pore patterns. Also, the nanopatterns have been selectively modified to change the local nanofeature dimensions by sweeping low dose ions across a defined area. This work shows that dual beam focused ion beam technique is capable of overcoming some basic limitations of current nanofabrication processes and creating innovative nanopatterns for different applications.

Multiwall carbon nanotube and TiO2 sol assembly.

Carbon nanotube and TiO2 assemblies (CNT/TiO2) have the great promise of combining the advantages of CNTs and TiO2 as photocatalytic and energy conversion materials. In this study, nanoscale TiO2 sol was assembled onto multiwall carbon nanotubes (MWCNTs). The CNT/TiO2 sol assemblies were characterized by transmission electron microscopy, energy dispersive spectroscopy, and Fourier transform infrared spectroscopy. The study shows that the acidic functional group concentration on CNT surfaces increases with surface modification temperature, up to 1.179 mmol/g. Higher concentration of acidic functional group greatly improves the amount of TiO2 sol assembled onto MWCNTs. Higher TiO2 sol concentration also improves the amount of TiO2 sol assembled onto MWCNT surfaces. The assembly mechanism is mainly by chemical reaction between the -COOH groups of MWCNTs and the -OH groups of TiO2 sol by esterification.

Co3O4/Sm-Doped CeO2/Co3O4 Trilayer Coating on AISI 441 Interconnect for Solid Oxide Fuel Cells.

In this work, a novel Co/Sm-doped CeO2 (SDC)/Co trilayer of ∼6 µm is deposited by alternating electrodeposition and electrophoresis and oxidized to a Co3O4/SDC/Co3O4 trilayer structure. This coating is unique and effective in the following aspects: (1) The area specific resistance of the coated interconnect is more stable and lower than that of the uncoated interconnect after thermal treatment at 800 °C for 400 h. (2) The Co3O4/SDC/Co3O4 coating layer can effectively inhibit Cr diffusion and evaporation and significantly slow the oxidation rate of the interconnect. (3) The Sm0.5Sr0.5Co0.2Fe0.8O3 cathode in the electrolyte/cathode/interconnect half-cell retains its initial stoichiometry after 100 h of the thermal treatment. Subsequently, the ohmic resistance RΩ, high frequency polarization resistance RH, and low frequency polarization resistance RL of the half-cell with the Co3O4/SDC/Co3O4 coated interconnect are all smaller than those of the half-cell with the bare interconnect. The Co3O4/SDC/Co3O4 coating layer has great advantages to be used as a protective layer for the metallic interconnect in solid oxide fuel cells to improve cell performance, stability, and durability.

Highly ordered titania nanotube arrays with square, triangular, and sunflower structures.

Focused ion beam guided anodization enables the fabrication of TiO(2) nanotubes in a square arrangement with square cell shapes and in a graphite lattice arrangement with triangular cell shapes, which is impossible through self-organized anodization. TiO(2) nanotubes in sunflower patterns are also obtained, which demonstrates the great potential of guided anodization in fabricating asymmetrical nanotubes.