Jianyu Liang

Periodic array of uniform ZnO nanorods by second-order self-assembly

A nonlithographic second-order self-assembly process for synthesizing uniform and ordered arrays of nanorods and nanodots is presented and applied to the fabrication of ZnO nanorod arrays. Nucleation sites were defined by patterning Au nanodot catalysts with a self-organized array of nanopores formed in anodized aluminum oxide (AAO). The self-assembled vertically aligned ZnO nanorods grown on GaN exhibit hexagonal facets, and have a uniform diameter of 60 nm and a mean length of 400 nm. The growth technique is simple, robust, and offers a direct control over array and single nanorod configurations. The growth temperature is significantly lower than normal, and yet, the resultant defect level is much lower than normal.

The specific heat and effective thermal conductivity of composites containing single-wall and multi-wall carbon nanotubes.

We present a study of the specific heat and effective thermal conductivity in anisotropic and randomly oriented multi-wall carbon nanotube (MWCNT) and randomly oriented single-wall carbon nanotube (SWCNT) composites from 300 to 400 K. Measurements on randomly oriented MWCNTs and SWCNTs were made by depositing a thin film of CNTs within a calorimetric cell. Anisotropic measurements were made on MWCNTs grown inside the highly ordered, densely packed nanochannels of anodic aluminum oxide. The specific heat of randomly oriented MWCNTs and SWCNTs showed similar behavior to the specific heat of bulk graphite powder. However, the specific heat of aligned MWCNTs is smaller and has weaker temperature dependence than that of the bulk above room temperature. The effective thermal conductivity of randomly oriented MWCNTs and SWCNTs is similar to that of powder graphite, exhibiting a maximum value near 364 K indicating the onset of phonon-phonon scattering. The effective thermal conductivity of the anisotropic MWCNTs increased smoothly with increasing temperature and is indicative of the one-dimensional nature of the heat flow.

Nanoheteroepitaxy of GaN on a nanopore array Si surface

We report the growth by molecular beam epitaxy and the optical characterization of GaN films nucleated on a Si(111) surface that has been patterned by dry etching an ordered array of nanometer-scale pores prior to the growth. The etching is performed using an anodized aluminum oxide membrane as a mask. The nanopore array surface with the pore diameter of 60 nm and periodicity of 110 nm exhibits significant effects on emissivity and the optical properties of the resulting film. Room-temperature photoluminescence intensity increases by a factor of 5 for GaN grown on nanoporous Si. Peak shifts in photoluminescence and Raman spectroscopy suggest that the material grown on nanopores may be more relaxed than films grown on flat substrates. The effects of nanopore topography on the nucleation of GaN films offer a potential path to significant improvement of III-nitride heteroepitaxy for device applications.

Nonlithographic fabrication of lateral superlattices for nanometric electromagnetic-optic applications

A nonlithographic technique that utilizes highly ordered anodized aluminum oxide (AAO) porous membrane as a template is employed as a general fabrication means for the formation of an array of vastly different two-dimensional lateral superlattice nanostructures. The fact that material systems as different as metals, semiconductors, and carbon nanotubes can be treated with the same ease attest to the generality of this nanofabrication approach. The original AAO membranes determine the uniformity, packing density, and size of the nanostructures. The flexibility of using a variety of materials, the accurate control over fabrication process, and the command over AAO template attributes, gives us the freedom of engineering various physical properties determined by the shape, size, composition, and doping of the nanostructures. The novel nanomaterial platform realized by this unique technique is powerfully enabling for a broad range of applications.

A growth pathway for highly ordered quantum dot arrays

To realize the desired zero-dimensional behavior of a quantum dot ensemble, the ability to fabricate quantum dots with a high packing density and a high degree of size, shape, and spacing uniformity is crucial. Here we report highly ordered InAs nanodot arrays grown by molecular-beam epitaxy on nonlithographically nanopatterned GaAs. Approximately 20 billion dots are grown in a 1cm2 area with the smallest size dispersion ever reported and forming a lateral superlattice in hexagonal dense packing form. These techniques presage a pathway to controlled growth of periodic quantum dot superstructures, which offer macroscopic spatial coherence in the interaction of quantum dots with radiation.

Nanoheteroepitaxial growth of GaN on Si nanopillar arrays

Nanoheteroepitaxial growth of GaN by metal-organic chemical-vapor deposition on dense arrays of (111) Si nanopillars has been investigated. Scanning electron microscopy, cross-sectional transmission electron microscopy, and electron-diffraction analysis of 0.15-μm-thick GaN layers indicate single-crystal films. Most of the mismatch defects were in-plane stacking faults and the threading dislocation concentration was <108cm−2 at the interface and decreased away from the interface. High-resolution transmission electron microscopy indicated that grain-boundary defects could heal and were followed by high quality, single-crystal GaN. Facetted voids were also present at the GaN∕Si interface and are believed to be an additional strain-energy reduction mechanism. The unusual defect behavior in these samples appears to be related to the high compliance of the nanopillar silicon substrate.

Role of AAO template filling process parameters in controlling the structure of one-dimensional polymer nanoparticles.

Solution template wetting is a common technique used to fabricate elongated polymer nanostructures; however, the parameters controlling the resulting morphology remain unclear. The purpose of this investigation was to elucidate the effects of process variables on the types of nanostructures obtained and to understand the physical mechanisms associated with structure development. 1 wt% polystyrene-THF solutions were infiltrated into commercial and homemade anodized aluminum oxide (AAO) templates. The wetting interaction between the AAO template and the polymer solution was examined through contact angle measurements. In general, for moderate dipping times (<18 h), the morphology of the nanopolymer was rod-like at low molecular weights, while tubes were observed at high molecular weight, even at this low concentration. Nanorods were obtained for all molecular weights for extended dipping times. The data suggest that phase separated layers may grow sequentially from the pore walls and yield nanotubes if the growth is interrupted or produce nanorods for unhindered deposition over long periods.

Fabrication of Protein Nanotubes Using Template-Assisted Electrostatic Layer-by-Layer Methods

One-dimensional protein nanostructures offer many advantages for biomedical applications. Rather than fabricate primary nanostructures with inorganic materials and then functionalize with proteins, it is desirable to develop a fabrication method to make nanostructures that are entirely protein. Fabrication of protein and polymer nanostructures is possible by layer-by-layer assembly within nanoporous templates. Typically these structures are composites of two or more materials. Few studies have demonstrated the fabrication of single component protein nanostructures using this method. In this paper, we report our effort toward the fabrication of single-component avidin nanotubes using a layer-by-layer electrostatic assembly method adapted from the literature. We investigated the use of two different template pretreatment methods to strengthen the attraction between the initial protein layer and our template. During our investigation, we revealed a significant flaw with the published works upon which our fabrication method was based which seriously compromised the legitimacy of the approach. As a result, we modified our initial method, and we are able to demonstrate the fabrication of glucose oxidase/avidin nanostructures using an electrostatic layer-by-layer assembly in conjunction with one of the template pretreatment methods we investigated.

Core―shell polymer nanorods by a two-step template wetting process

One-dimensional core-shell polymer nanowires offer many advantages and great potential for many different applications. In this paper we introduce a highly versatile two-step template wetting process to fabricate two-component core-shell polymer nanowires with controllable shell thickness. PLLA and PMMA were chosen as model polymers to demonstrate the feasibility of this process. Solution wetting with different concentrations of polymer solutions was used to fabricate the shell layer and melt wetting was used to fill the shell with the core polymer. The shell thickness was analyzed as a function of the polymer solution concentration and viscosity, and the core-shell morphology was observed with TEM. This paper demonstrates the feasibility of fabricating polymer core-shell nanostructures using our two-step template wetting process and opens the arena for optimization and future experiments with polymers that are desirable for specific applications.

Fabrication of free-standing Cu nanorod arrays on Cu disc by template-assisted electrodeposition

Free-standing Cu nanorod arrays on Cu foil have been fabricated by a template-assisted method. Cu nanorods were potentiostatically deposited on mechanically polished Cu foil using anodized aluminum oxide templates as the deposition mask. Three electrolyte systems were compared, including two acid copper sulfate based solutions and one alkaline solution. The most uniform nanorods were achieved in the alkaline electrolyte. The weight gain per unit area after electrodeposition has been used as a direct measure of average length of deposited Cu nanorods. It was found that our control over the uniformity in nanorod length across the array is important in reaching the maximized aspect ratio without aggregation. Through controlling the weight change it was possible to control the aspect ratio of nanorods and to avoid aggregation of nanorods. Our capability to fabricate free-standing Cu nanorod arrays of uniform height with maximized aspect ratio on Cu foil is especially important in applying this nanostructured Cu as a current collector in Li ion batteries.