Self-assembly of photovoltaic nanomaterials

Nanostructured materials are attracting a lot of attention due to their promising potential applications in areas as diverse as catalysis, adsorption, separation, photovoltaics and thermoelectrics, et cetera. In the areas of photovoltaics and thermoelectrics, especially strong theoretical arguments exist which suggest that the use of nanostructured materials in these areas will allow construction of devices that are more efficient than the current devices that use bulk materials. For example, the quantum confinement effects seen at length scales smaller than 10 nm result in highly efficient carrier multiplication and enhanced thermoelectric figure-of-merit in semiconductor quantum dots and quantum wires, respectively. For realizing practical applications of nanostructured materials in abovementioned areas, it is necessary to establish synthesis methods that are scalable, easily reproducible and inexpensive. Further, there is need to characterize the resultant nanomaterials for their structural, electronic and optical properties and establish a mechanistic understanding of the synthesis process. In this dissertation, we report on the synthesis of thin films of a variety of nanomaterials including metal oxides, metals and compound semiconductors having different nanostructures such as orthorhombic, rhombohedral and double-gyroid. Surfactant templating coupled with evaporation induced self-assembly was employed to synthesize starting metal oxide thin films. Particular attention was focused on the double-gyroid nanostructure, which has 3-dimensionally continuous interpenetrating networks of inorganic walls and nanopores. By optimizing synthesis conditions, highly ordered and phase-pure double-gyroid silica films were synthesized and the synthesis process thoroughly characterized using 29Si NMR and liquid-phase small angle X-ray scattering. A detailed mechanistic picture of the synthesis was constructed from this information. The nanopores in double-gyroid silica films were found to be highly open and accessible, unlike most other nanostructures. Taking advantage of the accessibility of the pore networks, a number of metals and compound semiconductors were electrodeposited in double-gyroid silica films to create their 3-dimensional nanowire arrays. Silica films could be etched away to create self-supporting nanowire thin films of these materials. The nanowire arrays of semiconductors such as PbSe, CdSe and CuInSe2 are of special interest for photovoltaic applications. The nanowire arrays we have synthesized are 3-D analogs of semiconductor quantum dots and quantum rods that have been studied extensively in the literature. We also demonstrate that the double-gyroid semiconductor thin films do display quantum confinement effects. Double-gyroid PbSe films, for example, display well-resolved excitonic peaks in their reflectance spectra resulting from the discrete nature of electronic energy levels, while double-gyroid CdSe films display fluorescence in the visible spectrum that is absent in their bulk counterparts. The range of structures and compositions synthesized in this work and the easily scalable nature of the solution-phase syntheses employed opens up an array of opportunities to explore their potential in areas mentioned above

Synthesis of thermally stable highly ordered nanoporous tin oxide thin films with a 3D face-centered orthorhombic nanostructure

Thin films of nanoporous tin oxide with a 3D face-centered orthorhombic nanostructure have been synthesized by self-assembly that is controlled by post-coating thermal treatment under controlled humidity. In contrast to the conventional evaporation-induced self-assembly (EISA), the films here have no ordered nanostructure after dip-coating. However, the initial coatings are formed under conditions that inhibit significant hydrolysis and condensation for extended periods. This allows the use of postsynthesis thermal vapor treatments to completely control the formation of the nanostructure. With EO106-PO70-EO106 (Pluronic F127) triblock copolymer as the template, highly ordered nanostructures were generated by exposing the disordered films to a stream of water vapor at elevated temperature, which rehydrates the films and allows the formation of the thermodynamically favored phase. Further exposure to water vapor drives the condensation reaction through the elimination of HCl. The X-ray diffraction pattern from the nanostructure was indexed in the space group Fmmm as determined by analysis of 2D small-angle X-ray scattering patterns at various angles of incidence. The nanostructure is then stabilized and made nanoporous by extended controlled thermal treatments. After self-assembly and template removal, the films are thermally stable up to 600 degrees C and retain an ordered, face-centered orthorhombic nanostructure

Controlling interfacial curvature in nanoporous silica films formed by evaporation-induced self-assembly from nonionic surfactants. I. Evolution of nanoscale structures in coating solutions

The double-gyroid phase of nanoporous silica films formed by evaporation-induced self-assembly (EISA) has been shown to possess facile mass-transport properties and may be used as a robust template for the nanofabrication of metal and semiconductor nanostructures. Recently, we developed a new synthesis of double-gyroid nanoporous silica films where the aging time of the coating solution prior to EISA was the key parameter required to control the interfacial curvature that results upon self-assembly of the film. Here, we use Si-29 nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS) to investigate the nanoscale structure of the coating solutions used to obtain double-gyroid nanoporous silica films. NMR and SAXS were carried out on the water, ethanol, silica, and poly(ethylene oxide)-b-poly(propylene oxide)-b-alkyl (EO17-PO12-C-14) surfactant coating solutions as well as similar solutions that excluded either the silica or the surfactant. NMR data reveal that the silica monomers in the coating solution condense very rapidly to form rings and connected ring species. After 1 day of aging, all monomers and dimers have disappeared, and the distribution is dominated by Q(2) and Q(3) species, where the superscript in Q(n) describes the number of silicon atoms in the second coordination shell of the central silicon. Over the course of the next 9 days, the Q(3) population slowly rises at the expense of the Q(2) and Q(3t) populations. Absolute intensity SAXS measurements reveal that the size of the silica clusters increases steadily during this aging period, reaching an average radius of gyration of 9.0 A after 9 days of aging. Longer aging results in the continued growth of clusters with a mass fractal dimension of 1.8. Absolute intensity SAXS data also reveals that micelles are not present in the coating solution. At 9% volume fraction of surfactant, the coating solution is far above the aqueous critical micellar concentration. However, even a small amount of ethanol inhibits micellization. SAXS data also shows that when surfactant is present the radius of gyration is larger but increases more slowly. This indicates that there are weak associative interactions between the silica clusters and surfactant in solution that reduce the cluster-cluster growth rate. In part II of this work, we use the results discovered here to interpret the effects of aging on interfacial curvature in the nanostructured films that self-assemble from these solutions

Controlling interfacial curvature in nanoporous silica films formed by evaporation-induced self-assembly from nonionic surfactants. II. Effect of processing parameters on film structure

The double-gyroid phase of nanoporous silica films has been shown to possess facile mass-transport properties and may be used as a mold to fabricate a variety of highly ordered inverse double-gyroid metal and semiconductor films. This phase exists only over a very small region of the binary phase diagram for most surfactants, and it has been very difficult to synthesize metal oxide films with this structure by evaporation-induced self-assembly (EISA). Here, we show the interplay of the key parameters needed to synthesize these structures reproducibly and show that the interfacial curvature may be systematically controlled. Grazing angle of incidence small-angle X-ray scattering (GISAXS) is used to determine the structure and orientation of nanostructured silica films formed by EISA from dilute silica/(poly(ethylene oxide)-b-poly(propylene oxide)-b-alkyl) surfactant solutions. Four different highly ordered film structures are observed by changing only the concentration of the surfactant, the relative humidity during dip-coating, and the aging time of the solution prior to coating. The highly ordered films progress from rhombohedral (R (3) over barm) to 2D rectangular (c2m) to double-gyroid (distorted Ia (3) over bard) to lamellar systematically as interfacial curvature decreases. Under all experimental conditions investigated, increasing the aging time of the coating solution was found to decrease the interfacial curvature. In particular, this parameter was critical to being able to synthesize highly ordered, pure-phase double-gyroid films. The key role of the aging time is shown via processing diagrams that map out the interplay between the aging time, composition, and relative humidity. Si-29 nuclear magnetic resonance (NMR) spectroscopy and solution-phase small-angle X-ray scattering (SAXS) of the aged coating solutions presented in part I of this series are then used to interpret the effects of aging prior to dip-coating. Specifically, it was found that a predictive model based on volume fractions and the silica cluster stoichiometry obtained from Si-29 NMR qualitatively explains the trends observed with composition and aging. However, apart from the effects of relative humidity, a quantitative comparison of the predicted phase with the experimental processing diagram suggests that less-condensed silica clusters are more effective at swelling the EO blocks at early aging times. This enhanced swelling decreases with aging time and results in lower-curvature nanostructures such as the double-gyroid. The decrease in swelling is attributed to the decreased thermodynamic driving force for the more-condensed silica clusters to mix with the EO block of the surfactant

Nanofabrication of double-gyroid thin films

Nanoporous silica films with the double-gyroid structure offer tremendous technological potential for sensors and separations because of their high surface area and potentially facile transport properties. Further, metals and semiconductors with similar structure open up new opportunities for high-surface-area electrodes, photoelectrochemical devices, photovoltaics, and thermoelectrics. Here, we report a new robust synthesis of highly ordered nanoporous silica films with the double-gyroid structure by evaporation-induced self-assembly (EISA) at room temperature and laboratory humidity using a commercially available EO17-PO12-C-14 surfactant. The continuous nanoporous films are synthesized on conducting electrodes. Electrochemical impedance spectroscopy is then used to quantitatively measure the accessible surface area of the underlying electrode via transport through the pore system. It is found that the double-gyroid-structure silica films expose a much higher fraction of the electrode than other commonly synthesized nanostructures such as 2D centered rectangular or 3D rhombohedral nanostructures. The double-gyroid nanoporous-film-coated electrodes are then used to fabricate inverse double-gyroid platinum nanostructures by electrodeposition, followed by etching to remove the silica. The structure of both the nanoporous silica films and the nanoporous platinum films (after etching) have been elucidated using high-resolution field-emission scanning electron microscopy (FESEM), comparing measured and simulated 2D grazing angle-of-incidence small-angle X-ray scattering (GISAXS) patterns, and comparing observed and simulated transmission electron microscopy (TEM) images. Both films are highly (211) oriented and described by a cubic Ia (3) over bard space group that has undergone uniaxial contraction perpendicular to the substrate. Upon this contraction, Ia (3) over bard symmetry is broken, but the films retain the double-gyroid topology. The nanoporous silica and the platinum nanowires have a characteristic wall or wire thicknesses of approximately 3 nm. This nanofabrication process opens up a facile general route for fabrication of ordered structures on the sub-5 nm length scale

Simulation and interpretation of 2D diffraction patterns from self-assembled nanostructured films at arbitrary angles of incidence: From grazing incidence (above the critical angle) to transmission perpendicular to the substrate

A method to calculate the location of all Bragg diffraction peaks from nanostructured thin films for arbitrary angles of incidence from just above the critical angle to transmission perpendicular to the film is reported. At grazing angles, the positions are calculated using the distorted wave Born approximation (DWBA), whereas for larger angles where the diffracted beams are transmitted though the substrate, the Born approximation ( BA) is used. This method has been incorporated into simulation code ( called NANOCELL) and may be used to overlay simulated spot patterns directly onto two-dimensional (2D) grazing angle of incidence small-angle X-ray scattering (GISAXS) patterns and 2D SAXS patterns. The GISAXS simulations are limited to the case where the angle of incidence is greater than the critical angle (alpha(i) \u3e alpha(c)) and the diffraction occurs above the critical angle (alpha(f) \u3e alpha(c)). For cases of surfactant self-assembled films, the limitations are not restrictive because, typically, the critical angle is around 0.2 degrees but the largest d spacings occur around 0.8 degrees 2 theta. For these materials, one finds that the DWBA predicts that the spot positions from the transmitted main beam deviate only slightly from the BA and only for diffraction peaks close the critical angle. Additional diffraction peaks from the reflected main beam are observed in GISAXS geometry but are much less intense. Using these simulations, 2D spot patterns may be used to identify space group, identify the orientation, and quantitatively fit the lattice constants for SAXS data from any angle of incidence. Characteristic patterns for 2D GISAXS and 2D low-angle transmission SAXS patterns are generated for the most common thin film structures, and as a result, GISAXS and SAXS patterns that were previously difficult to interpret are now relatively straightforward. The simulation code (NANOCELL) is written in Mathematica and is available from the author upon request