I would like to thank Shannon Boettcher, Bryce Sadtler, Ron Grimm, Rob Coridan, and Fan Yang. Finally, I would like to thank my wife Laura, who has been my constant companion and support throughout the past ten years in college and graduate school.
Solar Energy
Solution-Phase Growth
Solution-based syntheses have been explored to produce a wide range of useful materials from metal oxides to elemental semiconductors. These syntheses are particularly promising for the growth of metal oxides that could serve as photoanodes for the oxidation of water to oxygen.
Self-Assembly
Magnetic Field Directed Assembly
The degree of vertical alignment and the minimum field strength required for alignment were evaluated as a function of wire length, coating thickness, magnetic history, and substrate surface properties. The degree of vertical alignment and the minimum field strength required for alignment were evaluated as a function of the length, coating thickness, and magnetic field history of the Ni-coated Si microwires, as well as substrate surface properties.
Results
Panel F presents the degree of alignment for each sample under a 2.5 kilogauss (kG) magnetic field, as a function of the thickness of the Ni film. Panel C demonstrates the impact of the substrate surface energy on the alignment of the Ni-coated Si microwires.
Discussion
Panel A shows that the alignment process occurred at lower field strengths as the length of the wire increased due to the greater available magnetic torque. This distance can be adjusted by varying the original packing fraction of the horizontally deposited film (Figure 3).
Conclusions
Experimental
The magnetic properties of the microwire arrays were characterized using a Quantum Design superconducting quantum interference device. The average distance between particles was calculated from the coordinates of the wires in the vertical image.
Supporting Information
This behavior allowed control over the density of the final vertical array of microwires by controlling the initial packing density of the microwires. 4 Investigation of the force balance in the vertical magnetic alignment of microwire ensembles. A variable alignment response has been observed by tuning the surface tension of the solvent used, as well as the surface chemistry of the alignment substrate and the microwire itself.
By using empirically derived values for magnetic and surface energetic parameters it is possible to use this model predictively to determine the necessary applied magnetic field to align nano- or microwires with arbitrary geometries. Furthermore, some epitaxial growth processes are not suitable for removing and manipulating the fabricated elements from the substrate.
Theory
The subscript term in the Gibbs free energy refers to the three components of the system: the microwire surface (1), the substrate surface (2) and the liquid medium in which they come into contact (3). Finally, magnetic attraction is dependent on the magnetization constant of the microwire (MW) at the given value of applied field. The magnetic torque (TM), represented in Equation 7, is a function of the microwire magnetization constant and is considered an upward force in the case of a microwire confined to a sample surface.
The magnetic torque also depends on the applied field (H), as well as on the magnetically responsive volume of the microwire. Finally, the magnetic torque depends on the angle (Θ) between the applied field vector and the axial direction of the microwire.
Results
Force curves were obtained using both a hydrophobic Si-H surface and a hydrophilic Si-OH surface. In this plot, the microwire exhibits less adhesion with the lower energy Si-H surface than with the Si-H surface. Force curves were performed with a test Ni-coated Si microwire on Si-H and Si-OH substrates in the presence of both water and isopropanol to study the combined effect of substrate and solvent interactions on adhesion.
Higher adhesion energies were observed from the force curves directed to the Si-H substrate than the Si-OH substrate, with the highest value observed from the Si-H/H2O system. Traces are shown for combinations of different surface tension solvents (water and isopropanol) along with different surface energy substrates (Si–H and Si–OH).
Discussion
This is below the pKa of the carboxyl group where it is in the acidic form. At low pH values, the native oxide of Si will tend to strip, leaving a hydride-terminated surface with low surface energy. At high pH values, a hydroxyl layer will form at the surface of Si, giving a higher surface energy, and ultimately a strong repulsion to the negatively charged carboxylate groups when the Si-OH surface groups become deprotonated.
Without the secondary effect of the surface chemistry of the Si substrate, the difference in adhesion strength between the two pH ranges is not as great as would be expected based on the relative differences in surface energy of the two systems at different pH values. The graph in Figure 17 shows the dependence of the magnetic field required for alignment on the microwire radius, where larger radius microwires were easier to align.
Conclusions
Experimental
[53.54] A submicron tungsten microprobe tip (Micromanipulator Inc.) was used to transfer a dot of epoxy to the tip of a cantilever under an optical microscope. For non-magnetic microwires, capillary forces were used for transfer, i.e., the tip of the microprobe was pressed into the target microwire until it adhered. For magnetic-coated microwires, a microprobe tip with a magnetized sputter-deposited Ni coating was used to achieve transfer to the cantilever.
A Keithley source meter controlled by LabTracer software was used to power the magnetic field sweep. Previous work has shown that microwire array devices can be used to fabricate high efficiency devices such as photoelectrochemical [6] and photovoltaics.
Densification of Magnetically Aligned Arrays
This can be seen in Figure 20, where some wires were horizontal in panel A, but all were aligned by the image in panel B. A second method was explored for increasing the density of a microwire array, involving an aligned deposition of the wires was involved. microwires as opposed to post-deposition alignment. A magnetic field is applied so that the microwires remain parallel to the axis of the pores.
This means that although the production of the template itself is energy intensive, the overall cost is low if it can be reused indefinitely. The microwire arrays fabricated here can be released from the template using a polymer fill-in process, in which the array is peeled off after polymer curing, resulting in a flexible, free-standing array.
Photoelectrochemical Device Fabrication
An intermediate PDMS layer is cast in the F plate, which isolates the conductive polymer layer from the solution and thus prevents shunting currents. This would assume that the sputtered Ag is able to make a proper ohmic contact through the bases of the wires, rather than the entire bottom being in contact with the circuit as originally suggested. An island of PEDOT:PSS Nafion mixture was deposited on one of the Ni contacts.
Ni–PEDOT:PSS–Ag device where the first Ni–Si junction replaces the Si liquid contact in the final proposed microwire device. The results of the electrical test of the device in Figure 25 are shown in Figure 26.
Conclusions
Graphene layers have potential for the conformal protection of Si microwire arrays against corrosion in solution. A single layer of graphene has been shown to protect planar Si surfaces from photocorrosion in aqueous, but typical graphene synthetic methods are only applicable to planar substrates. A layer of graphene is a layer of resonance-stabilized carbon that is relatively resistant to chemical attack and is able to protect an underlying substrate from oxidation by water or polar solvents.
It has been shown that, although bare n-Si electrodes are only stable for about 30 s in the light under aqueous conditions, that by protecting the same electrode with a layer of graphene, it can support current densities of more than 10 mA cm-2 for > 1000 s. The second method uses a layer of Ni as a catalyst to seed bilayer graphene growth on the Si surface, after which the Ni etching exposes the graphene.
Characterization Methods
The small peak at 1345 cm-1 is known as the D peak and corresponds to the defect density of the graphene layer. Additionally, the G and 2D peaks should be sharp, as both peak broadening and peak shift can carry information about the quality of graphene. Photoelectrochemical cells can be used to test the duration of water stability provided to the underlying silicon by a graphene layer.
The wire is passed through a 1/4" glass tube, which is sealed to the sample with epoxy (Hysol 9460). The epoxy is applied to cover the back and edges of the specimen, leaving no contact with the solution beyond the face of the specimen.
Growth of Graphene on Alkylated Silicon Through a Silicon Carbide
In Figure 29, the peak at about 284.8 eV corresponds to aliphatic carbon species, in this case a combina-. The peak at about 283.65 eV corresponds to the Si–C bond of the octyl chain with the Si substrate. The peak at about 282.75 eV corresponds to the carbon incorporated into the desired SiC phase.
At about. At 600◦C this effect can be seen as the SiC peak increases in intensity and is the only major signal remaining, indicating a largely SiC surface within the top several nm of the sample. This sample is now suitable for further annealing and conversion to graphene via either conventional heating [71, 72] or microwave-induced heating.
Nickel-Catalyzed Bilayer Growth of Graphene on Silicon
This process is conformal and has the distinct advantage of being easily incorporated into the magnetic alignment process described in previous sections, utilizing the ferromagnetic alignment handle as the graphene growth catalyst. A Raman spectrum of the above process performed on Si with a 300 nm oxide is shown in Figure 32. All attempts to grow graphene under these conditions using thin oxide buffer layers (2 nm SiO2 and 10 nm SiO2 grown via dry oxidation, and 2 nm Al2O3 grown via atomic layer deposition and evaporation) was unsuccessful and resulted in the formation of bulk nickel silicide.
A 400 nm Ni layer was used as a catalyst and removed after growth using Marble's reagent for 10 min. This process will need to be tuned to maintain the lower temperatures required to prevent silicide formation while increasing the quality of the graphene film produced to that of bulk SiO2.
Direct Growth of Graphitic Carbon on Silicon
Graphite films were grown on flat Si wafers and Si microwires using methane as a carbon precursor during a slow annealing process at 850 ◦C in a 50:100 sccm H2:Ar mixture. Photoelectrochemical stability measurements were performed in the form of repetitive current-voltage scans as shown in Figure 36. The graphite-coated Si sample has a lower initial current but is relatively stable at about 0.1 mA cm-2.
These samples show stability only when passing low current levels, giving no significant advantage over bare Si. However, with further tuning of the graphitization process, it may be possible to deposit a thinner, more conductive layer with a lower defect density in order to chemically protect the Si photoelectrodes.
Conclusions
To make the fabrication of any such array where the preferred orientation of an ensemble of nano or micro crystals is desired, it is more scalable desirable to move towards the solution phase growth of the micro crystal components. Using this information, test unit manufacturing procedures have been developed to enable investigation of the impact of various fitting conditions on unit performance specifications. Self-assembly of microscopic chiplets at a liquid-liquid-solid interface forming a flexible segmented monocrystalline solar cell.
Electric field-directed convective assembly of ellipsoidal colloidal particles to create optically and mechanically anisotropic thin films. Comparison of the electrical properties and chemical stability of crystalline silicon(111) surfaces alkylated with Grignard reagents or olefins with Lewis acid catalysts.
