Patchy particles are patterned particles with at least one well-defined patch that can have highly directional and strongly anisotropic interactions with other particles or surfaces. Multiple theoretical studies point to interesting self-assembly of these particles into superstructures and, as a result, a multitude of possible applications. However, reliable synthetic methods for patchy particles, especially at the sub-micron level, are still a challenge and an active area of research. This work presents a novel synthesis route for making patchy particles at the sub-micron level that involves the use of capillary condensation. Colloidal silica particles of various sizes were synthesized and ordered into closely-packed arrays via evaporative self-assembly. Various chemical agents were capillary condensed into the voids of this assembly which, due to the face-centered cubic structure of the crystallized colloidal silica, produced distinct \patches" on the particle surface. The patches on these particles were successfully functionalized with gold nanoparticles. This method was shown to provide control over the patch size by modifying the silica particle radius, which thermodynamically changes the amount of capillary condensation. The patchy nature of the resultant particles was confirmed using infrared spectroscopy, scanning electron and optical microscopies, energy dispersive x-ray analysis and zeta potential measurements.

Carbon nanotubes (CNTs) are nanometer-scale structures that have attracted broad interest due to their exceptional thermal, electronic, and mechanical properties. As a result, there has been a large effort to develop applications of these materials in various fields including nanoelectronics and nanophotonics, energy storage, and biomedical fields. However, controlled production and manufacturing of CNTs still remains a challenge. In this work we demonstrate a method for controlling the placement and distribution of carbon nanotubes on surfaces using colloidal lithography. CNTs in ring-like geometries display interesting properties due to their nanoscale curved structure. Although several methods have been introduced for the fabrication of these structures, large scale fabrication of CNT rings with controllable diameter in a practical manner has thus far been elusive. Here, we use colloidal lithography to assemble nanotubes from solution into rings with tunable diameter and controllable placement in large-area periodic arrays. Several parameters and conditions such as the mask size, concentration and type of solvent for the CNT solutions are tested, and nanotubes with different quality and purity are used. Characterization of the CNT ring arrays using scanning electron microscopy (SEM) and atomic force microscopy (AFM) are performed. These results demonstrate large periodic areas of rings with good uniformity throughout the arrays. The arrays consist of rings with diameters between 180–220 nm when using 780 nm diameter sphere colloidal masks. Analysis of ring thickness for these rings indicated their cross-sections are composed of approximately 10-15 individual tubes. Rings made with 450 nm spheres had diameters between 100-150 nm, showing the tunability of the ring diameter enabled by our method. In some cases, mesh-like structures in the form of periodic interconnected carbon nanotubes were also observed. Our results demonstrate an efficient and straightforward approach for patterning carbon nanotubes into well-defined surface distributions with controlled and tunable dimensions.

Nanoscale protein arrays have shown promise for biological and biomedical applications. Compared to traditional protein arrays, nanoarrays have the potential for higher throughput, better sensitivity, and require less sample volumes. In this thesis, protein nanoarrays were fabricated using a simple and inexpensive "natural lithography" approach. This method allows the fabrication of large-area ordered nanoparticle arrays consisting of metallic dots with tunable diameters down to 10 nm or less. The nanoparticle arrays are formed by depositing metal through the openings of colloidal monolayer polystyrene sphere masks. After removing the masks, nanoarrays remain and are exposed to further processing. COOH-terminated self-assembled monolayers (SAM) and N-hydroxysuccinimide (NHS) chemistry is used for surface functionalization. These surface modifications covalently attach proteins onto the nanoparticles. A single monolayer of immunoglobulin G (IgG) molecules is successfully attached on the functionalized surfaces and the bioactivity of the protein arrays is tested by attaching anti-IgG molecules, as a standard immunological assay. Results of fabrication trials and efforts to control nanoparticle size, spacing, and surface adhesion are described. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) images of hexagonal gold nanoarrays consisting of approximately 150 nm particles and 3.5x108-1.5x109 per cm2 array density is shown, depending on the size of colloidal spheres. An increased height of approximately 6 nm characterized via scanning probe methods shows the attachment of a single monolayer of protein molecules to the nanoparticles. This was confirmed with SEM. A similar height increase was detected via AFM showing the attachment of anti-IgG molecules onto IgG functionalized particles. Potential applications of the protein nanoarrays and future work are discussed.

Single-walled carbon nanotubes (SWNTs), with their superior combination of electrical and mechanical properties, have drawn attention from many researchers for potential applications in electronics. Many SWNT-based electronic device prototypes have been developed including transistors, interconnects and flexible electronics. In this thesis, a fabrication method for patterned SWNT networks and devices based on colloidal lithography is presented. Patterned SWNT networks are for the first time formed via solution deposition on a heterogeneous surface. This method demonstrates a simple and straight-forward way to fabricate SWNT networks in a controllable manner. Colloidal sphere monolayers were obtained by drop-casting from solution onto clean substrates. The colloidal monolayer was utilized as a mask for the fabrication of patterned SWNT networks. SWNT networks were shown to be patterned either by depositing SWNT solutions on top of a colloidal monolayer or by depositing a mixed SWNT-colloidal sphere aqueous suspension on the substrates. Colloidal monolayers were examined by optical microscopy and it was found that the monolayer quality can be affected by the concentration of colloids in solution. Polystyrene colloidal solution with concentration of 0.02 wt% ~ 0.04 wt % was found optimal for maximum coverage of colloidal monolayers on SiO2 substrates. After removing the colloidal spheres, the topology of the patterned SWNT networks was characterized by atomic force microscopy and scanning electron v microscopy. Two-dimensional ordered arrays of SWNT rings and SWNTs interconnecting the SWNT rings were observed in the resulting network structure. The height of the rings was about 4-10 nm and the diameter was about 400 nm. In some samples, mesh-like patterned SWNT networks are also observed. It is hypothesized that the capillary forces induced by Van der Waals interaction at liquid/air/solid interfaces play an important role during the formation of the patterned SWNT networks. Raman spectroscopy was also employed to identify the chirality and diameter of the SWNTs in the networks. Both metallic and semiconducting SWNTs were found in the networks and the diameter of the SWNTs was about 1 to 2 nm. The electrical properties of SWNT networks, including random SWNT networks, partially patterned SWNT networks and fully patterned SWNT networks were characterized by a probe station and a Keithley 4200 semiconductor measurement system. The random SWNT networks had two-terminal resistance varying between several MΩ to several hundred MΩ. Field effect behavior was observed in some devices with relatively high resistance and nonlinear I-V curves. Those devices had on/off ratio of less than 100. There was significant leakage current in the ―off‖ state likely due to metallic tube pathways in the networks. The partially patterned SWNT networks had resistance that varied from 20 KΩ to 10 MΩ, but did not display field effect behavior in our studies. The resistance of the patterned SWNT networks was about 10 MΩ - 100 MΩ. The electrical characteristics of the patterned SWNT networks as thin film transistors were investigated, and the on/off ratio of the devices varied from 3 to 105. The upper limit of mobility in the devices was about ~ 0.71 – 5 cm2/V·s. The subthreshold slope of patterned SWNT network FETs can be as low as 210 meV/dec. / Graduate / 0544

Multijunction solar cells made from a combination of CQDs of differing sizes and thus bandgaps are a promising means by which to increase the energy harvested from the Sun’s broad spectrum. In this dissertation, we first report the systematic engineering of 1.6 eV PbS CQD solar cells, optimal as the front cell responsible for visible wavelength harvesting in tandem photovoltaics. We rationally optimize each of the device’s collecting electrodes—the heterointerface with electron accepting TiO2 and the deep-work-function hole-collecting MoO3 for ohmic contact—for maximum efficiency. Room-temperature processing enables flexible substrates, and permits tandem solar cells that integrate a small-bandgap back cell atop a low thermal-budget larger-bandgap front cell. We report an electrode strategy that enables a depleted heterojunction CQD PV device to be fabricated entirely at room temperature. We develop a two-layer donor-supply electrode (DSE) in which a highly doped, shallow work function layer supplies a high density of free electrons to an ultrathin TiO2 layer via charge-transfer doping. Using the DSE we build all-room-temperature-processed small-bandgap (1 eV) colloidal quantum dot solar cells suitable for use as the back junction in tandem solar cells. We further report in this work the first efficient CQD tandem solar cells. We use a graded recombination layer (GRL) to provide a progression of work functions from the hole-accepting electrode in the bottom cell to the electron-accepting electrode in the top cell. The recombination layers must allow the hole current from one cell to recombine, with high efficiency and low voltage loss, with the electron current from the next cell. We conclude our dissertation by presenting the generalized conditions for design of efficient graded recombination layer solar devices. We demonstrate a family of new GRL designs experimentally and highlight the benefits of the progression of dopings and work functions in the interlayers.

Thesis (M.S.)--West Virginia University, 2000. / Title from document title page. Document formatted into pages; contains xi, 81 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

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Made available in DSpace on 2014-06-11T19:23:29Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-04-16Bitstream added on 2014-06-13T19:29:33Z : No. of bitstreams: 1 ravaro_lp_me_bauru.pdf: 967737 bytes, checksum: 4467c85af230546c2464c3465069b809 (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Dióxido de estanho (Sn'O IND. 2') é um semicondutor do tipo n, que é transparente na região do ultravioleta/visível. Possui muitas aplicações como, por exemplo, eletrodos transparentes, sensores de gás, coletores solares e dispositivos eletro-ópticos. Quando dopado com íons terras-raras, Sn'O IND. 2' pode ser utilizado na confecção de dispositivos para comunicação óptica, principalmente na forma de filmes finos. Os íons terras-raras têm grande relevância devido às transições eletrônicas, que vão do ultravioleta ao infravermelho próximo. Outro aspecto importante está ligado às características físico-químicas da suspensão coloidal de onde são depositados os filmes. Filmes obtidos com pH elevado da suspensão apresentam alta resistividade elétrica e baixa cristalinidade em relação aos filmes obtidos com pHs das suspensões ácidas. O nível mais profundo de energia dos defeitos em Sn'O IND. 2' foi alterado da energia de 140eV até 67eV para variação do pHs 11, até 6, de acordo com a avaliação da energia de ativação. Os difratogramas destes filmes indicam aumento de cristalinidade com a diminuição do pH. Xerogéis de Sn'O IND. 2':Er2% com alteração do pH em relação a suspensão neutra apresentaram espectro de emissão mais intenso na região infravermelha para a amostra com pH7 e um pequeno alargamento dos picos de emissão para a amostra com pH4 e mais acentuado para a amostra com pH11, em bom acordo com medidas de Raman. Relatamos também a emissão na região visível de filme fino de Sn'O IND. 2' dopado com 'Er POT. 3+', que é um formato adequado da amostra para confecção de dispositivos. / Tin dioxide (Sn'O IND. 2') is an n-type oxide semicondutor, which is transparent in the ultraviolet/visible range. It presents many types of applications, such as transparent electrodes, gas sensors, solar collectors and eletro-optical devices. When doped with rare-earth ions, Sn'O IND. 2' may be used to make optical communication devies, in the thin film configuration. Rare-earth ions have great relevance due to their electronic transitions, covering from ultraviolet to near infrared. Another important characteristic is related to the physical-chemical properties of the starting colloidal suspension to deposit the films. Films obtained with high pH of the suspension presents high electric resistivity and low crystallinity compared to films obtained with acid pH. The deepest energy level of the defects in Sn'O IND. 2' has been changed from the energy of 140 eV to 67 e V when the pH changes from 11 to 6. Diffractograms of these films show increase in the crystallinity with pH decrease. Sn'O IND. 2':Er2% xerogels with modified pH show more intense emission spectra in the infrared for sample with pH 7 and a low broadening of emission peaks for the sample with pH 4 and moer intense for pH 11, suggesting an ideal pH for higher emission samples, in good agreement with Raman shift spectra. We also report emission in the visible range from of an 'Er POT. 3+' -doped thin film, which is a very convenient form for devices fabrication.

A particulate suspension under shear is a classic example of a system driven out of equilibrium. While it is possible to predict the equilibrium phase behaviour of a quiescent suspension, linking microscopic details to bulk properties under flow remains an open challenge. Our current understanding of sheared suspensions is restricted to two disparate regimes, the colloidal regime, for particle sizes d < 1 μm and the granular regime, for d > 50 μm. The physics of the industrially-relevant intermediate size regime, 1 μm ≲ d ≲ 50 μm, is unclear and has not been explored previously. In this thesis, we use conventional rheometry on a range of model spheres to develop the foundations of a predictive understanding of suspension flow across the entire size spectrum. In the first part of the thesis, we show that in repulsive particulate systems the rheology is characterised by two viscosity "branches" diverging at different volume fractions φRCP and φm, which represent states of flow with lubricated (frictionless) and frictional interactions between particles. In the intermediate size regime, there is a transition between these two branches above a critical onset stress σ* which manifests as shear thickening. This σ* is related to a barrier (invariably due to the charge or steric stabilisation) keeping particle surfaces apart. Our data are quantitatively fit by the Wyart and Cates theory for frictional thickening [1] if we assume that probability distribution of forces in the system is similar to in dry granular media. The onset stress for shear thickening is found to decrease with the inverse square of the particle size σ* / d ̄ 2 for diverse systems. We show that it is the competition between the scaling of σ*(d) and the size dependence of the entropic stress scale (~ d ̄ 3) that controls the crossover from colloidal to granular rheology with increasing size. Granular systems are "always shear thickened" under typical experimental conditions, while colloidal systems are always in a frictionless state. In the second part of the thesis, we explore the validity of the frictional framework for shear thickening. Although it quantitatively predicts our steady-state rheology, the frictional framework contradicts traditional fluid-mechanical thinking and has yet to be rigorously tested experimentally. In fact, there is a large body of literature that attributes thickening to purely hydrodynamic effects. Using dimensional analysis and simple physical arguments we examine possible physical origins for thickening and show that previously-proposed mechanisms can be subdivided into three types: two-particle hydrodynamic thickening, many-particle hydrodynamic thickening ("hydroclusters") and frictional-contact driven thickening. Many of these mechanisms can are inconsistent with the experimental two-branch phenomenology and can be disregarded. We further narrow down possible causes of thickening using the technique of flow reversal, which disentangles the relative contributions of contact and hydrodynamic forces to the viscosity. Consistent with recent simulations [2] and theory [1], we find that in each case thickening is dominated by the formation of frictional contacts and that hydrodynamic thickening, if present, is subdominant.