Candice Pelligra

Directed self-assembly of hybrid oxide/polymer core/shell nanowires with transport optimized morphology for photovoltaics

Hybrid organic-inorganic solar cells are promising for the development of next generation low-cost, high efficiency photovoltaics (PVs). They combine the facile solution processability, large optical extinction coefficients, and good hole mobility of conjugated polymers with the high electron affinity and electron mobility of inorganic nanoparticles.[1] The most common configuration for effectively generating photocurrent with these materials is the bulk heterojunction (BHJ) device where intimate mixing of the polymer with inorganic nanoparticles generates a random bicontinuous morphology with nanometerscale dimensions. The morphology and ultimately the performance of this photoactive layer depend on a complex interplay of materials parameters and processing conditions such as temperature, polymer-solvent and particle-solvent interactions, solvent evaporation rate, solution composition (polymer volume fraction), and post-deposition treatments.[2] In particular, the role of polymer–particle miscibility has been well highlighted by recent reports.[3] Although there has been steady progress in improving device efficiency, the inherently disordered and kinetically-dictated structure of BHJ devices is suboptimal, particularly in terms of charge transport, but also in exciton utilization.[1a,4] An ideal hybrid device is one in which donor and acceptor materials are arranged in a densely packed vertical array. Nanometer-scale periodicity would minimize radiative decay of excitons, and the vertical alignment of the materials ensures a non-tortuous path for charge transport.[1a,4a,5] Such an ordered BHJ motif (OBHJ) has thus been the focus of recent interest,[6] but its realization, particularly in a manner compatible with low-cost fabrication of devices, remains elusive. A further refinement of the OBHJ entails atomic and molecularscale control of the donor and acceptor materials such that their

Liquid Crystalline Order and Magnetocrystalline Anisotropy in Magnetically Doped Semiconducting ZnO Nanowires

Controlled alignment of nanomaterials over large length scales (>1 cm) presents a challenge in the utilization of low-cost solution processing techniques in emerging nanotechnologies. Here, we report on the lyotropic liquid crystalline behavior of transition-metal-doped zinc oxide nanowires and their facile alignment over large length scales under external fields. High aspect ratio Co- and Mn-doped ZnO nanowires were prepared by solvothermal synthesis with uniform incorporation of dopant ions into the ZnO wurtzite crystal lattice. The resulting nanowires exhibited characteristic paramagnetic behavior. Suspensions of surface-functionalized doped nanowires spontaneously formed stable homogeneous nematic liquid crystalline phases in organic solvent above a critical concentration. Large-area uniaxially aligned thin films of doped nanowires were obtained from the lyotropic phase by applying mechanical shear and, in the case of Co-doped nanowires, magnetic fields. Application of shear produced thin films in which the nanowire long axes were aligned parallel to the flow direction. Conversely, the nanowires were found to orient perpendicular to the direction of the applied magnetic fields. This indicates that the doped ZnO possesses magnetocrystalline anisotropy sufficient in magnitude to overcome the parallel alignment which would be predicted based solely on the anisotropic demagnetizing field associated with the high aspect ratio of the nanowires. We use a combination of magnetic property measurements and basic magnetostatics to provide a lower-bound estimate for the magnetocrystalline anisotropy.

Large area vertical alignment of ZnO nanowires in semiconducting polymer thin films directed by magnetic fields

We demonstrate the use of magnetic fields for the directed assembly of ZnO nanowires in semiconducting polymer films suitable for ordered bulk heterojunction photovoltaics. Using rotational field annealing, Co-doped ZnO nanowires with negative paramagnetic anisotropy were successfully aligned out-of-plane with respect to the substrate and polymer film.

Scalable High‐fidelity Growth of Semiconductor Nanorod Arrays with Controlled Geometry for Photovoltaic Devices Using Block Copolymers

Controlled density semiconducting oxide arrays are highly desirable for matching nanometer length scales specific to emerging applications. This work demonstrates a facile one-step method for templating hydrothermal growth which provides arrays with high-fidelity tuning of nanorod spacing and diameter. This solution-based method leverages the selective swelling of block copolymer micelle templates, which can be rationally designed by tuning molecular weight and volume fraction.

Directed Assembly of Hybrid Nanomaterials and Nanocomposites

Hybrid nanomaterials are molecular or colloidal-level combinations of organic and inorganic materials, or otherwise strongly dissimilar materials. They are often, though not exclusively, anisotropic in shape. A canonical example is an inorganic nanorod or nanosheet sheathed in, or decorated by, a polymeric or other organic material, where both the inorganic and organic components are important for the properties of the system. Hybrid nanomaterials and nanocomposites have generated strong interest for a broad range of applications due to their functional properties. Generating macroscopic assemblies of hybrid nanomaterials and nanomaterials in nanocomposites with controlled orientation and placement by directed assembly is important for realizing such applications. Here, a survey of critical issues and themes in directed assembly of hybrid nanomaterials and nanocomposites is provided, highlighting recent efforts in this field with particular emphasis on scalable methods.

Multiscale patterning of a metallic glass using sacrificial imprint lithography

Bulk metallic glasses (BMGs) have been developed as a means to achieve durable multiscale, nanotextured surfaces with desirable properties dictated by topography for a multitude of applications. One barrier to this achievement is the lack of a bridging technique between macroscale thermoplastic forming and nanoimprint lithography, which arises from the difficulty and cost of generating controlled nanostructures on complex geometries using conventional top-down approaches. This difficulty is compounded by the necessary destruction of any resulting reentrant structures during rigid demolding. We have developed a generalized method to overcome this limitation by sacrificial template imprinting using zinc oxide (ZnO) nanostructures. It is established that such structures can be grown inexpensively and quickly with tunable morphologies on a wide variety of substrates out of solution, which we exploit to generate the nanoscale portion of the multiscale pattern through this bottom-up approach. In this way, we achieve metallic structures that simultaneously demonstrate features from the macroscale down to the nanoscale, requiring only the top-down fabrication of macro/microstructured molds. Upon detachment of the formed part from the multiscale molds, the ZnO remains embedded in the surface and can be removed by etching in mild conditions to both regenerate the mold and render the surface of the BMGs nanoporous. The ability to pattern metallic surfaces in a single step on length scales from centimeters down to nanometers is a critical step toward fabricating devices with complex shapes that rely on multiscale topography for their intended functions, such as biomedical and electrochemical applications. Biomedical and optical devices stand to benefit from a multiscale patterning technique developed by researchers in the USA. Bulk metallic glasses (BMGs) are extremely strong and corrosion-resistant alloys that can be thermoplastically molded with features spanning centimeter to nanometer dimensions. The generation of multiscale molds with conventional lithography, however, requires several costly steps. To simplify BMG patterning from the bottom-up, Chinedum Osuji from Yale University and co-workers from Yale and Rutgers grew tunably packed nanowires and nanosheets of zinc oxide (ZnO) directly onto mold surfaces. When a BMG is formed then detached from this modified mold, the nanostructures embed themselves into the metallic surface. A subsequent mild etching procedure removes the ZnO and gives the BMG a nanoporous surface—an economical route to biomimetic textures for applications ranging from biomanipulation to fuel cells.

Use of the Gabor Filter for Edge Detection in the Analysis of Zinc Oxide Nanowire Images

Semi-automated processing of microscopy images can significantly enhance the capacity for objective nanostructure characterization. However, background complexity, intricate object features and overlapping of objects can make this a difficult task. Here we describe use of the Gabor filter [1, 2] to facilitate the identification and outlines of the top surfaces of nanowires of zinc oxide (ZnO), imaged with scanning electron microscopy (SEM). The Gabor filter is a sinusoidal function multiplied by a Gaussian envelope. The sinusoidal shape makes the filter sensitive to spatial frequencies and the Gaussian envelope limits the frequency sensitivity to localized areas of the image. In particular, the Gabor filter enhances lines and edges in a direction perpendicular to the direction of the sinusoid. This sensitivity to linear parts of an image make it particularly well suited to analysis of the top surfaces of the ZnO nanowires which have a regular hexagonal shape and bright edges in the SEM images.