Browsing by Subject "Nanoscience"
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Item A sustainable nanosciences research facility for Lubbock, Texas(Texas Tech University, 2003-12) May, Heath AllenNoneItem Colloidal nanocrystal assemblies : self-organization, properties, and applications in photovoltaics(2011-12) Goodfellow, Brian William; Korgel, Brian Allan, 1969-; Chelikowsky, James R; Dodabalapur, Ananth; Ekerdt, John G; Vanden Bout, David AColloidal nanocrystal assemblies offer an attractive opportunity for designer metamaterials. The ability to permute chemical composition, size, shape, and arrangement of nanocrystals leads to an astounding number of unique materials properties that find use in an extensive array of applications---ranging from solar cells to medicine. However, to take full advantage of these materials in useful applications, the nature of their assembly and their behavior under external stimuli must be well understood. Additionally, the assembly of colloidal nanocrystals into thin films provides a promising pathway to the solution-processing of inorganic materials that are prohibitively too expensive and/or difficult to deposit by conventional methods. Nanocrystal superlattices (NCSLs) of sterically stabilized nanocrystals were assembled by slow evaporation of colloidal dispersions on various substrates. Detailed analysis of the NCSL structures was carried out using transmission and scanning electron microscopy (TEM and SEM) and small-angle x-ray scattering (SAXS). Body-centered cubic (bcc) NCSLs, in particular, were studied in detail and ligand packing frustration was proposed as a significant driving force for their assembly. The behavior of NCSLs was also studied by SAXS under mild heating and solvent vapor exposure revealing several remarkable order-order, order-disorder, and amorphous-crystalline structural transitions. Colloidal Cu(In [subscript 1-x] Ga [subscript x])Se₂ (CIGS) nanocrystals were synthesized by arrested precipitation and formulated into inks. These inks were spray deposited into thin films under ambient conditions to serve as the active light absorbing material in printed low-cost photovoltaic (PV) devices. These devices, which were fabricated without the need for high temperature processes, have achieved power conversion efficiencies above 3 % under AM1.5 illumination. While the efficiencies of these devices are still too low for commercial viability, this work does provide a proof of concept that reasonable efficient solar cells can be created with a low-cost printable process using nanocrystal inks. Since high temperatures are not used to form the light-absorbing layer, nanocrystal-based solar cells were built on flexible light weight plastic substrates. The main obstacle to achieving high power conversation efficiencies was found to be the ability to extract the photo induced charge carriers. Nanocrystal films suffer from poor transport that leads to high recombination rates in thicker films. To date, the best efficiencies have been achieved with thin light absorber layers that only absorb a fraction of the incident light.Item Colloidal nanocrystals with near-infrared optical properties : synthesis, characterization, and applications(2011-12) Panthani, Matthew George; Korgel, Brian Allan, 1969-; Dodabalapur, Ananth; Chelikowsky, James; Mullins, C. Buddie; Manthiram, ArumugamColloidal nanocrystals with optical properties in the near-infrared (NIR) are of interest for many applications such as photovoltaic (PV) energy conversion, bioimaging, and therapeutics. For PVs and other electronic devices, challenges in using colloidal nanomaterials often deal with the surfaces. Because of the high surface-to-volume ratio of small nanocrystals, surfaces and interfaces play an enhanced role in the properties of nanocrystal films and devices. Organic ligand-capped CuInSe2 (CIS) and Cu(InXGa1-X)Se2 (CIGS) nanocrystals were synthesized and used as the absorber layer in prototype solar cells. By fabricating devices from spray-coated CuInSe nanocrystals under ambient conditions, solar-to-electric power conversion efficiencies as high as 3.1% were achieved. Many treatments of the nanocrystal films were explored. Although some treatments increased the conductivity of the nanocrystal films, the best devices were from untreated CIS films. By modifying the reaction chemistry, quantum-confined CuInSeXS2-X (CISS) nanocrystals were produced. The potential of the CISS nanocrystals for targeted bioimaging was demonstrated via oral delivery to mice and imaging of nanocrystal fluorescence. The size-dependent photoluminescence of Si nanocrystals was measured. Si nanocrystals supported on graphene were characterized by conventional transmission electron microscopy and spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM). Enhanced imaging contrast and resolution was achieved by using Cs-corrected STEM with a graphene support. In addition, clear imaging of defects and the organic-inorganic interface was enabled by utilizing this technique.Item Controlling infrared radiation with subwavelength metamaterials and silicon carbide(2011-05) Neuner, Burton Hamilton; Shvets, G.; Fink, Manfred; Florin, Ernst-Ludwig; Yao, Zhen; Zhang, XiaojingThe control and manipulation of infrared (IR) radiation beyond the capabilities of natural materials using silicon carbide (SiC), metamaterials, or a combination thereof, is presented. Control is first demonstrated using SiC, a polar crystal that exhibits a dielectric permittivity less than zero in the mid-IR range, through the excitation of tightly confined surface phonon-polaritons (SPPs), thus enabling a multitude of applications not possible with conventional dielectrics. Optimal, or critical coupling to SPPs is explored in SiC films through Otto-configuration attenuated total reflection. One practical application based on Otto-coupled SPPs is presented: IR refractive index sensing is shown for three pL-scale fluid analytes. It is then demonstrated that when two SiC films are brought to a few-micron separation, IR radiation can excite surface modes that possess phase velocities near the speed of light, a property required for efficient table-top particle accelerators. Metamaterials are engineered with subwavelength structure and possess optical properties not found in nature. Two such metamaterials will be introduced: metal films perforated with arrays of rectangular holes display the ability to control IR light polarization through spoof surface plasmon excitation, and metal/dielectric multilayers patterned with subwavelength-pitch corrugations display frequency-tunable, wide-angle, perfect IR absorption. Two experiments, which have implications in polarization control and thermal emission, combine the benefits of SiC with those of metamaterials: extraordinary optical transmission and absorption are observed in SiC hole arrays, and the design of individual SiC antennas permits the control of the bulk metamaterial responses of impedance and absorption/emission. Finally, a new optical beamline based on Fourier transform IR spectroscopy was designed, built, characterized, and implemented, serving as the major experimental objective of this dissertation. The novel beamline, which confines radiation to a 200-micron diameter and enables angle-dependent IR spectroscopy, was verified using multiple metamaterial structures.Item Experimental demonstration of new optical properties in hybrid nanostructures(2015-12) Hartsfield, Thomas Murray; Li, Elaine; Bengtson, Roger; Shih, Chih-Kang; Sitz, Greg; Wang, ZhengIn this dissertation, I present experimental investigation of the optical properties of nanoscale systems composed of both metallic and semiconductor components. Metallic nanostructures may act as resonant cavities for conduction electrons, allowing drastic electromagnetic field enhancement and the concentration of these surface plasmon field modes into tiny volumes. Semiconductor quantum dot emitters demonstrate desirable and broadly tunable optical properties due to the quantized nature of their internal electron states. When paired together, the absorption, emission, optical gain, and internal energy decay pathways of the quantum dot as well as the scattering of the cavity may be strongly modified. This work focuses on the optical properties of two such model hybrid nanostructure systems. Of the many studies of plasmonic cavities, relatively few investigate the influence of a quantum dot on the scattering of the plasmonic cavity itself. The main experimental challenge lies in the difficulty of placing an absorber or emitter at the desired position: the very virtue of the small mode volume of a plasmonic cavity demands precise spatial emitter placement. We will study the simplest plasmonic cavity, a single metal nanoparticle and a single quantum dot. We assembled a hybrid nanostructure using a nanomanipulation “nano-golfing” technique and demonstrated for the first time that the state of a single quantum dot can resonantly control the scattering of a vastly larger plasmonic cavity, manifested as a Fano resonance. A device of this design could potentially be used as a photon source capable of outputting photons of classical or quantum statistics on demand. We then turn to the optical properties of the emitter element of a hybrid nanostructure. We measured the ability of an atomically smooth Ag film to influence the optical properties of a quantum dot. This novel system has been shown to produce more uniform emitter-plasmon coupling and a greater product of excitation and radiative decay rates than possible with traditional systems relying upon rough metal films. Applications utilizing coupling between metallic films and quantum emitters could see benefit from high quality atomically smooth films as demonstrated by our studies.Item Nanoparticles produced via laser ablation of microparticles(2001-12) Henneke, Dale Edwin; Mullins, C. B.; Brock, J. R. (James Rush), 1930-Several different nanoparticle synthesis techniques exist. One synthesis method uses the laser ablation of microparticles (LAM) entrained in an aerosol technique to produce charged nanoparticles. Here, we discuss nanoparticles made using the LAM process. Since nanoparticles made in this fashion are charged in the ablation process, they are not overly susceptible to agglomeration. The synthesized particles appear to have diameters that are dependant on background pressure, but are relatively insensitive to gas type. Once the particles have been made, they must be collected in a capping solution to prevent agglomeration from occurring. Two distinct methods of collection have been devised. The first collection scheme uses an impactor to remove any large (>300 [nm]) unablated material. The charged nanoparticle aerosol is then supersonically impacted directly into a surfactant capping solution. The suspension is then gathered for subsequent processing. In the second collection method, the charged nature of the nanoparticles is utilized to deflect the aerosol in an electric field. The electric field drives the particles onto a surfactant laden electrode. Size distributions for silver were determined using a transmission electron microscope; the mean particle diameter was found to be 5 [nm]. The collected nanoparticles were found to be flocculated; this can be explained by inadequate coverage of the capping molecule. For this reason, an annealing step to provide better surface coverage of the cap is performed. As the suspensions are heated, the size of the flocculents decreases. The decrease in flocculent size indicates that nanoparticles are leaving the main group and going into suspension.