Browsing by Subject "Surface chemistry"
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Item A study of interfacial tension in ternary systems(Texas Tech University, 1966-05) Prochaska, Frank OttoInterfacial tensions were determined for the five ternary systems water-isopropyl alcohol-benzene, water-isopropyl alcohol-toluene, water-tertiary butyl alcohbl-benzene, water-normal propyl alcohol-toluene, and water-dlethylamlne-toluene in their regions of two liquid phases. Also determined were surface tensions, densities, and refractive indices of the phases. A Fisher Surface Tensiometer employing the ring method was used to make the surface and interfacial tension determinations. A Fisher- Davidson Gravltometer was used to make the density measurements and refractive indices were determined by means of a Bausch and Lomb Abbe-3L Refractometer. A comparison was made of the values determined for the binary and pure liquid phases of the above ternary systems with the accepted values found in the literature. A maximum deviation of 2.81 was indicated. As the other measurements were made under the same conditions all values determined were considered to be in error by no more than 2.8%. A relative immiscibility scale was established for each system by means of the differences of the refractive indices of the conjugate phases. Interfacial tensions of each of the systems were correlated against the corresponding immicibilities established for the phases. A straight line function was indicated for plots of the immiscibility values against the log of the log of one hundred times the interfacial tensions. Correlation coefficients determined for the curves showed a 951 certainty of this type of correlation for one of the systems, a 981 certainty for another, and 991 certainties for the remaining three systems.Item Adhesion of particles on indoor flooring materials(2007-12) Lohaus, James Harold, 1968-; Siegel, Jeffrey A.This dissertation involved a theoretical and experimental investigation of the adhesive forces between spherical particles of four different diameters and two selected flooring materials under different air velocities. Previous theoretical work and experiments described in the literature tended to be conducted with idealized surfaces, and therefore have limited applicability to indoor environments. Controlled experiments were designed, constructed and executed to measure the air velocity required to overcome adhesion forces. The diameters of the particles investigated were 0.5, 3.0, 5.0 and 9.9 [mu]m, and the flooring materials were linoleum and wooden flooring. The critical velocity, the flow at which 50% of the particles detached, is presented as a function of particle diameter for each surface. The measured values were then compared to empirical and theoretical models as well as to a scaling analysis that considers component forces that act on a particle-surface system. The results suggest that critical velocity decreases with increasing particle diameter and that existing models have limited applicability to resuspension from flooring materials.Item Adsorbate interactions at organic/metal interfaces(2005) Scharff, Robert Jason; Campion, AlanThe vibrational and electronic structure of 2-12˚A thick films of pyromellitic dianhydride (PMDA) adsorbed on clean and H2O pre-covered Cu(111) surfaces have been measured at 110K and 295K using high resolution electron energy loss spectroscopy (HREELS) and surface Raman spectroscopy. On both surfaces at low temperature, the adsorption geometry favors a flatlying orientation. Adsorption at 295K results in an upright orientation that is consistent with previous accounts in the literature. The adsorption at 110K leads to the now well documented ring-opening reaction of one of the anhydride groups producing a carboxylate and carbonyl species, whereas, at room temperature CO is lost to the gas phase. The presence of pre-adsorbed H2O at 110K results in a hydrogen bonded complex with the carboxylate group, dramatically altering the electronic structure of the PMDA film. Surface Raman scattering experiments of the H2O-PMDA co-adsorbate complex exhibit resonant scattering behavior that is attributed to the so-called “first layer” or “chemical” SERS effect due to dynamical charge transfer excitations on “atomically smooth” Cu(111) at 110KItem Adsorption, reaction and interfacial electronic structures of aromatic molecules on single crystal surfaces(2005) Wei, Wei; White, John M.Electron transfer at organic/metal interfaces is fundamental to a large number of problems in surface science. Electronic interactions at such an interface are responsible for charge injection from an electrode to the molecular film. The efficiency or rate of charge injection is determined by the energetic alignment of molecular orbitals to the metal Fermi level and the electronic coupling strength (wavefunction mixing) between molecular orbitals and metal bands. Two experimental investigations were performed with two-photon photoemission spectroscopy (2PPE). First, the energetic alignments of naphthalene/Cu(111) were probed. Three transitions involving unoccupied orbitals were found and identified as having π* molecular orbital character—the first lying 0.4 eV above the vacuum level vii (π* b1u), the second 0.3 eV below the vacuum level (π* b3g), and the third 1.1 eV below the vacuum level (π* b2g). In the second experiment, the interfacial electronic structures of chemisorbed styrene on Cu(111) were successfully investigated. We observed unoccupied states 3.5 eV above the Fermi level and occupied states 2.0 eV below the Fermi level. Polarization results reveal that the occupied and unoccupied states arise from bonding and antibonding orbitals formed by hybridization of copper (surface state and d-band orbitals) and styrene (π1* and π2* orbitals). For the first time, two-photon photoemission spectroscopy was employed to explore a surface chemical reaction: epoxidation of styrene on Cu(111). With 100 L oxygen on a Cu(111) surface, the atomic oxygen occupies three-fold hollow HCP sites rather than FCC sites. Its 2p states hybridize strongly with the dz2 states of the Cu atoms in the second layer. After styrene is adsorbed on Cu metal sites of this oxygen-covered surface, it undergoes efficient epoxidation to styrene oxide. The 2PPE results show that the change in the electronic structures of the adsorbed reactant is consistent with the surface reaction: the oxygen-induced feature from the Cu-O bonding disappears and a new state appears. However, 1000 L oxygen-covered Cu(111) is catalytically inert for styrene epoxidation: as styrene is added, no new features appear in 2PPE, and there is no evidence for chemical reaction in thermal desorption. This study could open up a new area of solid state and surface catalytic chemistry.Item Controlling seal formation and improving seedling emergence using polyacrylamide polymers(Texas Tech University, 1997-05) Perkins, Clinton ToddSoils from arid and semiairid regions are often structurally unstable and seal. This surface sealing phenomenon negatively effects many components of the soil-plant system, thereby limiting crop production. Delayed or erratic plant emergence are direct effects of the surface sealing and subsequent crusting phase of the soU. Increased water runoff loss, decreased water infiltration, and enhanced wind erosion are all negative indirect effects of surface seals. Since the 1980's, there has been renewed interest in the use of water soluble polymers for enhancing soil physical properties. Several factors contributing to this use are more efficient application strategies, availability of inexpensive and, more effective polymers. Better understanding of the beneficial effects of using polymers on surface sealing (increased infiltration and reduced surface strength and hardening) and on the emergence and development of various crops will make crop growing more economical and profitable in soils susceptible to seal formation. Subsequently, farmers (especially from the Southern High Plains of Texas) will benefit from a wider range of crops that they could grow and thus improve productivity and profits. This project investigated polymer addition to the soil surface to (i) decrease soil sealing and (ii) enhance seedling emergence. Polymers of differing charges and molecular weight were sprayed on the soil surface at several rates.Item Diamond-like carbon films made by sputtering and PECVD(Texas Tech University, 1997-08) Zheng, ZhongqiangThis thesis describes the work on a-C:H thin films, including their preparation and characterization. It begins with introducing seme basic concepts such as sp1, sp2, sp3, their electronic structure, diamond-like carbon, and their applications to antifuse devices. Then, in Chapter II, film deposition techniques and characterization methods are presented in detail. The techniques of deposition consist of radio-frequency sputtering (r.f sputtering) and plasma-enhanced chemical vapor deposition (PECVD) systems. The methods of characterization are composed of optical absorption used to determine the band gap Eg, and infrared spectroscopy used to study C-C and CH bonding structures. Hew to measure self-biases and how to apply biases on anode are also contained in this chapter. In Chapter HI, the results of a-C:H films prepared by both r.f sputtering and PECVD are listed and discussed. Their growth rate, band gap and concentration of hydrogen are calculated and studied. They change with pressure and power in the same way as we expect according to theory. For instance, the concentration of hydrogen decreases with increasing power since higher power can cause mere removal of hydrogen from the film. Finally, conclusions are made, and future research directions are mentioned. Based en our good and believable results, a-C:H is considered to be a premising material to be used in wide applications.Item Dynamic behavior of ultra-thin polymer films deposited on surface acoustic wave(SAW) devices: A feasibility study of saw applications(1996-08) Ahuja, Ashish; Narayan, Raghu S.; James, Darryl; Tock, Richard W.The main objective of this research effort was the investigation of relevant applications that a cutting edge technology afforded. While the work did not attempt to delve into mathematical interpretation of SAW device responses, it did seek to evaluate the usefulness of the SAW technology from an engineering perspective.Item Fundamental surface science investigations of systems designed to address technological issues(2003) Yan, Xiaoming; White, John M.Organometallic chemical vapor deposition of (MeCp)Ir(COD) onto Rh is simulated experimentally with and without co-reactant oxygen via isothermal reaction mass spectrometry. Auger electron spectroscopy (AES) is used to analyze the resulting film purity. Without oxygen, continuous film deposition occurs above 750 K. A large amount of carbon incorporates, and a final composition of C4Ir is inferred. At the steady state of film growth, acetylene is the only volatile product. Before reaching steady state, various hydrocarbon species are observed. With enough oxygen, the precursor combusts and pure Ir is deposited above 600 K. At steady state, the main by-products are CO and H2O. The thermal decomposition of tert-butoxy (TBO) with co-adsorbed O and NO is studied using temperature programmed desorption and AES on Rh foil and Cu(111). On Cu(111) with NO(a), some TBO decomposes below 240 K to form H2O, CO, CO2, C2H2, proposed imide and acetate, and others disproportionate to tert-butyl alcohol, isobutene and adsorbed oxygen at 610 K. On Rh with NO(a), two oxygen-containing fragments—TBO and a stabilized oxametallacycle—coexist. The proposed oxametallacycle decomposes at 350 K to acetone, while TBO, relatively stable in the presence of N and NO, decomposes to isobutene at 500 K. On Rh with O(a), TBO is stable only up to 380 K where, assisted by O, it decomposes to acetone and butene via a transient form of the oxametallacycle. Thermally evaporated Ag is deposited onto a thin solid water layer on clean hafnia, titania and functionized titiania surfaces. After thermal desorption of water, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) reveals Ag particles on these surfaces. On HfO2, particles have lateral dimensions between 5 and 20 nm and, in many cases, with heights exceeding the thickness of the original water layer. More interesting, particles form 1D bead-like strings spontaneously on 18 L ice. However, on trimethyl acetic acid (TMAAH) pre-saturated TiO2(110), only a few huge particles form. The difference is attributed to the different surface hydrophilicities, which affect both the initial ice layer growth and the competition between dewetting and desorption of adsorbed water.Item Growth, structure, and chemistry of 1B metal nanoclusters supported on TiO₂(110)(2006) Pillay, Devina; Hwang, Gyeong S.Cu, Ag, and Au nanoclusters dispersed on TiO2(110) surfaces are utilized in a wide variety of applications ranging from microelectronics to heterogeneous catalysis. The unique chemical reactivity of these clusters is largely dependent on their size, shape, spatial distribution, and interfacial interaction with the oxide support. This implies that atomic level control of these properties can offer great opportunities in the development of novel devices based on supported metal nanoclusters. It is therefore necessary to understand how formation and restructuring of these clusters alter their geometric and electronic characteristics. This thesis involves the development of a theoretical foundation for studying the growth, structure, and chemistry of Cu, Ag, and Au on TiO2(110) surfaces. Using density functional theory calculations, we have identified factors that control the chemical reactivity of these supported metal nanoclusters. First we investigated the electronic and geometric structures of the stoichiometric and reduced rutile TiO2(110) surfaces. Then we examined the surface chemistry of TiO2 towards gaseous CO and O2, as well as the structure and growth of 1B metal nanoclusters on TiO2(110). We also examined how the electronic and geometric properties of mixed metal nanoclusters, CuAun(n≤ 3), differ versus their single metal counterparts, Cum and Aum (m ≤ 4). Finally, we considered CO oxidation reactions on TiO2(110)-supported small Au clusters. While current experimental techniques are limited to providing complementary atomic-level real space information, first principles-based atomic level simulations greatly contribute to elucidating the fundamental behavior and properties of Cu, Ag, and Au nanoclusters on TiO2(110). First principles modeling has paved the way for new catalyst development by investigating how the geometric, electronic, and chemical properties of TiO2-supported 1B metal nanoclusters vary with surface defects, adsorbates, and metal dopants before valuable time and manpower is invested in experimental synthesis and characterization.Item Molecular-dynamics study of the Staebler-Wronski Effect in hydrogenated amorphous silicon using large supercells(Texas Tech University, 1997-05) Ha, ByeonqchulHydrogenated amorphous silicon is a semiconductor which has many practical device applications. However, it is known that in this material, there is a "Staebler-Wronski (SW) Effect," which is a light-induced degradation. The effort to understand the source of the SW Effect has continued for two decades. Numerous theories and experiments have been published about the SW Effect, but its exact cause is not known. A fundamental problem is to understand the role hydrogen plays in the SW Effect. In this thesis, the molecular dynamics technique is used to study the bond breaking model of the SW Effect and to explore the role hydrogen plays in this effect. Large (224 and 231 atom) supercells, prepared by an ab-initio technique, are used. The molecular dynamics calculations are performed using a semi-empirical total energy functional. This work is an extension of the work of Park and Myles to large supercells.Item Phage display technology for surface functionalization of a synthetic biomaterial(2005) Sanghvi, Archit Bharat; Schmidt, Christine E.Item Quantum state-resolved studies of sticking and elastic scattering of H₂ from Cu(100)(2006) Kim, Jonghyuk; Sitz, Greg OrmanItem Removal of formaldehyde from indoor air : enhancing surface-mediated reactions on activated carbon(2013-08) Carter, Ellison Milne; Katz, Lynn E.; Speitel, Gerald E.Formaldehyde is a ubiquitous and hazardous indoor air pollutant and reducing concentrations in indoor environments is a public health priority. The goals of this doctoral work were to advance analytical methods for continuous monitoring of formaldehyde at very low concentrations (sub-20 ppb[subscript v]) and to improve fundamental, mechanistic understanding of how structural and chemical properties of activated carbon influence removal of formaldehyde from indoor environments. To achieve these goals, emerging sensor-based technology was evaluated for its ability to detect and quantify ppb[subscript v]-level formaldehyde concentrations on a continuous basis at relative humidity levels characteristic of residential indoor environments. Also, a combination of spectroscopic and selective titration techniques was employed to characterize molecular-level structural and chemical properties of traditional and chemically treated granular activated carbon (GAC). In addition to selecting two different commercially available GACs for study, design and preparation of a laboratory-prepared, chemically treated GAC was pursued to create nitrogen-doped GAC with desirable surface chemical properties. Performance of all GACs was evaluated with respect to formaldehyde removal through a series of packed bed column studies. With respect to continuous formaldehyde monitoring, a method detection limit for emerging sensor technology was determined to be approximately 2 ppb[subscript v], and for relative humidity levels characteristic of indoor environments (> 40%), quantitative, continuous formaldehyde measurements less than 10 ppb[subscript v] were robust. The two commercially available GACs tested were both capable of removing formaldehyde; however, the GAC with greater density of basic surface functional groups and greater electron-donating potential (Centaur) removed twice as much formaldehyde (on a GAC mass basis) as the less basic GAC (BPL). A laboratory-prepared GAC (BPL-N) was successfully created to contain pyridinic and pyrrolic nitrogen, which was associated with increased surface density of basic functional groups, as well as with increased electron-donating potential. BPL-N exhibited better removal capacity for formaldehyde than BPL and Centaur. Furthermore, packed bed column studies of BPL-N and BPL formaldehyde removal performance yielded evidence to support the hypothesis that electron-donating potential, especially nitrogen functional groups at the BPL-N surface, promote catalytic removal of gas-phase formaldehyde via oxidation.Item Semiconductor nanowires : from a nanoscale system to a macroscopic material(2011-12) Holmberg, Vincent Carl; Korgel, Brian Allan, 1969-Semiconductor nanowires are one-dimensional nanoscale systems that exhibit many unique properties. Their nanoscale size can lead to defect densities and impurity populations different than bulk materials, resulting in altered diffusion behavior and mechanical properties. Synthetic methods now support the large-scale production of semiconductor nanowires, enabling a new class of materials and devices that use macroscopic quantities of nanowires. These advances have created an opportunity to fabricate bulk structures which exhibit the unique physical properties of semiconductor nanowires, bridging the properties of a nanoscale system with macroscopic materials. High aspect ratio germanium nanowires were synthesized in supercritical organic solvents using colloidal gold nanocrystal seeds. The nanowires were chemically passivated inside the reactor system using in situ thermal hydrogermylation and thiolation. The chemical stability of the passivated nanowires was studied by exposure to highly corrosive and oxidative environments. Chemical surface functionalization of germanium nanowires was investigated by covalently tethering carboxylic acid groups to the surface, as a general platform for the further functionalization of nanowire surfaces with molecules such as polyethylene glycol. Surface functionalization with dopant-containing molecules was also explored as a potential route for doping nanowires. In addition, static charging was exploited in the development of an electrostatic deposition method for semiconductor nanowires. In situ transmission electron microscopy experiments were conducted on gold-seeded germanium nanowires encapsulated within a volume-restricting carbon shell. A depressed eutectic melting temperature was observed, along with strong capillary effects, and the solid-state diffusion of gold into the crystalline stem of the germanium nanowire, occurring at rates orders of magnitude slower than in the bulk. Copper, nickel, and gold diffusion in silicon nanowires were also investigated. The rate of gold diffusion was found to be a strong function of the amount of gold available to the system. Finally, germanium nanowires were found to exhibit exceptional mechanical properties, with bending strengths approaching that of an ideal, defect-free, perfect crystal, and strength-to-weight ratios greater than either Kevlar or carbon fiber. Macroscopic quantities of nanowires were used to fabricate large sheets of free-standing semiconductor nanowire fabric, and the physical, morphological, and optical properties of the material were investigated.Item Surface and interfacial chemistry of high-k dielectric and interconnect materials on silicon(2001-08) Kirsch, Paul Daniel; Ekerdt, John G.Surfaces and interfaces play a critical role in the manufacture and function of silicon based integrated circuits. It is therefore reasonable to study the chemistries at these surfaces and interfaces to improve existing processes and to develop new ones. Model barium strontium titanate high-k dielectric systems have been deposited on ultrathin silicon oxynitride in ultrahigh vacuum. The resulting nanostructures are characterized with secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS). An interfacial reaction between Ba and Sr atoms and SiOxNy was found to create silicates, BaSixOy or SrSixOy. Inclusion of N in the interfacial oxide decreased silicate formation in both Ba and Sr systems. Furthermore, inclusion of N in the interfacial oxide decreased the penetration of Ba and Sr containing species, such as silicides and silicates. Sputter deposited HfO2 was studied on nitrided and unnitrided Si(100) surfaces. XPS and SIMS were used to verify the presence of interfacial HfSixOy and estimate its relative amount on both nitrided and unnitrided samples. More HfSixOy formed without the SiNx interfacial layer. These interfacial chemistry results are then used to explain the electrical measurements obtained from metal oxide semiconductor (MOS) capacitors. MOS capacitors with interfacial SiNx exhibit reduced leakage current and increased capacitance. Lastly, surface science techniques were used to develop a processing technique for reducing thin films of copper (II) and copper (I) oxide to copper. Deuterium atoms (D*) and methyl radicals (CH3*) were shown to reduce Cu2+ and/or Cu1+ to Cu0 within 30 min at a surface temperature of 400 K under a flux of 1×1015 atoms/cm2 ·s. Temperature programmed desorption experiments suggest that oxygen leaves the surface as D2O and CO2 for the D* and CH3* treated surfaces, respectively.Item Surface chemistry and directed assembly of nanostructures on dielectric surfaces(2006) Stanley, Scott Kendyl; Ekerdt, John G.Item Surface chemistry of FeHx with dielectric surfaces : towards directed nanocrystal growth(2008-08) Winkenwerder, Wyatt August, 1981-; Ekerdt, John G.The surface chemistry of GeH[subscript x] with dielectric surfaces is relevant to the application of germanium (Ge) nanocrystals for nanocrystal flash memory devices. GeH[subscript x] surface chemistry was first explored for thermally-grown SiO₂ revealing that GeH[subscript x] undergoes two temperature dependent reactions that remove Ge from the SiO₂ surface as GeH₄ and Ge, respectively. Ge only accumulates due to reactions between GeH[subscript x] species that form stable Ge clusters on the SiO₂ surface. Next, a Si-etched SiO₂ surface is probed by GeH[subscript x] revealing that the Si-etching defect activates the surface toward Ge deposition. The activation involves two separate reactions involving, first, the capture of GeH[subscript x] by the defect and second, a reaction between the captured Ge and remaining GeH[subscript x] species leading to the formation of Ge clusters. Reacting the defect with diborane, deactivates it toward GeH[subscript x] and also deactivates intrinsic hydroxyl groups toward GeH[subscript x] adsorption. A structure is proposed for the Si-etching defect. The surface chemistry of GeHx with HfO₂ is studied showing that the hafnium germinate that forms beneath the Ge nanocrystals exists as islands and not a continuous film. Annealing the hafnium germinate under a silane atmosphere will reduce it to Ge while leading to the deposition of hafnium silicate (HfSiO[subscript x]) and silicon (Si). Treating the HfO₂ with silane prior to Ge nanocrystal growth yields a surface with hafnium silicate islands on which Si also deposits. Ge deposition on this surface leads to the suppression of hafnium germinate formation. Electrical testing of capacitors made from Ge nanocrystals and HfO₂ shows that Ge nanocrystals encapsulated in Si/HfSiO[subscript x] layers have greatly improved retention characteristics.Item Surface evolution and self assembly of epitaxial thin films: nonlinear and anisotropic effects(2007-12) Pang, Yaoyu, 1979-; Huang, Rui, doctor of civil and environmental engineeringA strained epitaxial film can undergo surface instability and self assemble into discrete islands. The unique physical features of these islands make self-assembly an enabling technique for advanced device technology while control of the island size, shape, and alignment is critical. During the process of self-assembly, the stress field and the interface interaction have profound effects on the dynamics of surface evolution. In this dissertation, a continuum model is developed to study the nonlinear dynamics of surface pattern evolution and self assembly in epitaxial thin films. Within the framework of non-equilibrium thermodynamics, a nonlinear evolution equation is developed, and a spectral method is implemented for numerical simulations. The effects of stress and wetting are examined. It is found that, without wetting, the nonlinear stress field induces a “blow-up” instability. With wetting, the thin film self assembles into an array of discrete islands lying on a thin wetting layer. The dynamics of island formation and coarsening over a long time and a large area is well captured by the interplay of the nonlinear stress field and the wetting effect in the present model. For single-crystal epitaxy, the anisotropic material properties in the bulk and surface play important roles in the process of self assembly and pattern formation. In particular, this study investigates the effects of anisotropic mismatch stress and generally anisotropic elasticity. First, under an anisotropic mismatch stress, a bifurcation of surface pattern is predicted. The effect of anisotropic elasticity on pattern evolution is then investigated for two specific systems, one for SiGe films on Si substrates with different surface orientations, and the other for hexagonal silicides on Si substrates. It is shown that the consideration of elastic anisotropy reveals a much richer dynamics of surface pattern evolution as opposed to isotropic models. Based on the theoretical and numerical results from the present study, experimental approaches may be developed to control the size and organization of self assembled surface patterns in epitaxial systems.Item Surface functionalization and self-assembly of ligand-stabilized silicon nanocrystals(2015-05) Yu, Yixuan; Korgel, Brian Allan, 1969-; Ekerdt, John; Milliron, Delia; Mullins, Charles; Downer, MikeSilicon nanocrystals or quantum dots combine the abundance and nontoxicity of silicon with size-tunable energy band structure of quantum dots to form a new type of functional material that has applications in biomedical fluorescence imaging, photodynamic therapy, light-emitting devices, and solar cells. The surface is the major concern for using silicon nanocrystals in bio-related applications. Room temperature hydrosilylation is introduced to functionalize silicon nanocrystals in the dark to minimize temperature/photon-induced side reactions that can potentially damage the nanocrystal surface and capping ligands. As a proof of concept, silicon nanocrystals are passivated with styrene at room temperature, without showing styrene polymerization. Silicon nanocrystals are also conjugated to iron oxide nanocrystals through room temperature hydrosilylation to generate fluorescent/magnetic cell labeling probes. Thermally-induced thiolation is used to generate silicon nanocrystals passivated with silicon-sulfur bond that is metastable and can turn to silicon-carbon bond through a ligand exchange. The band gap and emission color of silicon nanocrystals depend on size. Monodisperse silicon nanocrystals and their self-assembly are of great importance for the applications in light-emitting devices and solar cells. Silicon nanocrystals are size-selected through a modified size-selective precipitation. Face-centered cubic superlattices are formed with monodisperse silicon nanocrystals, and characterized by using grazing incidence small angle X-ray scattering. The structure of silicon nanocrystal superlattice is stable at temperatures up to 375oC, due to the covalent Si-C bond on the nanocrystal surface. Silicon and gold nanocrystals are assembled to a simple hexagonal AlB2 binary superlattice that shows interesting thermal behavior. Finally, superlattices made with alkane thiol-capped sub-2 nm gold nanocrystals are used as model systems to study the superlattice phase transitions. Halide ions are found to be critical for order-to-order structural rearrangements in dodecanethiol-capped 1.9 nm gold nanocrystals superlattices at 190oC. Reversible amorphous-to-crystalline transition upon heating is discovered for octadecanethiol capped 1.66 nm gold nanocrystal superlattices, which is attributed to the ligand melting transition.