Browsing by Subject "Quantum dot"
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Item The application of light trapping structures and of InGaAs/GaAs quantum wells and quantum dots to improving the performance of single-junction GaAs solar cells(2012-05) McPheeters, Claiborne Ott; Yu, Edward T.; Alu, Andrea; Bank, Seth R.; Chen, Ray T.; Zhang, John X.High efficiency photovoltaic solar cells are expected to continue to be important for a variety of terrestrial and space power applications. Solar cells made of optically thick materials often cannot meet the cost, efficiency, or physical requirements for specialized applications and, increasingly, for traditional applications. This dissertation investigates improving the performance of single-junction GaAs solar cells by incorporating InGaAs/GaAs quantum wells and quantum dots to increase their spectral response bandwidth, and by incorporating structures that confine light in the devices to improve their absorption of it. InGaAs/GaAs quantum dots-in-wells extend the response of GaAs homojunction devices to wavelengths >1200 nm. Nanoparticles that are randomly deposited on the top of optically thick devices scatter light into waveguide modes of the device structures, increasing their absorption of electromagnetic energy and improving their short-circuit current by up to 16%. Multiply periodic diffractive structures have been optimized using rigorous software algorithms and fabricated on the back sides of thin film quantum dot-in-well solar cells, improving their spectral response at wavelengths 850 nm to 1200 nm, where only the quantum dot-in-well structures absorb light, by factors of up to 10. The improvement results from coupling of diffracted light to waveguide modes of the thin film device structure, and from Fabry-Perot interference effects. Simulations of absorption in these device structures corroborate the measured results and indicate that quantum well solar cells of ~2 µm in thickness, and which are equipped with optimized backside gratings, can achieve 1 Sun Airmass 0 short-circuit current densities of up to ~5 mA/cm2 (15%) greater than GaAs homojunction devices, and of up to >2 mA/cm2 (7%) greater than quantum well devices, with planar back reflectors. A combination of Fabry-Perot interference and diffraction into waveguide modes of the thin devices is shown to dominate the simulated device response spectra. Simulations also demonstrate the importance of low-loss metals for realizing optimal light trapping structures. Such device geometries are promising for reducing the cost of high efficiency solar cells that may be suitable for a variety of traditional and emerging applications.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 Manipulating fluorescence dynamics in semiconductor quantum dots and metal nanostructures(2011-12) Ratchford, Daniel Cole; Li, Elaine; Chelikowsky, James; Florin, Ernst-Ludwid; Shvets, Gennady; Vanden Bout, DavidRecent scientific progress has resulted in the development of sophisticated hybrid nanostructures composed of semiconductor and metal nanoparticles. These hybrid structures promise to produce a new generation of nanoscale optoelectronic devices that combine the best attributes of each component material. The optical response of metal nanostructures is dominated by surface plasmon resonances which create large local electromagnetic field enhancements. When coupled to surrounding semiconductor components, the enhanced local fields result in strong absorption/emission, optical gain, and nonlinear effects. Although hybrid nanostructures are poised to be utilized in a variety of applications, serious hurdles for the design of new devices remain. These difficulties largely result from a poor understanding of how the structural components interact at the nanoscale. The interactions strongly depend on the exact composition and geometry of the structure, and therefore, a quantitative comparison between theory and experiment is often difficult to achieve. Colloidal semiconductor quantum dots are strong candidates for integration with metal nanostructures because they have a variety of desirable optical properties, such as tunable emission and long term photostability. However, one potential drawback of colloidal quantum dots is the intermittency in their fluorescence (commonly referred to as “blinking”). Blinking was first observed over a decade ago, yet there is still no complete theory to explain why it occurs. In spite of the lack of a full theoretical explanation, multiple methods have been used to reduce blinking behavior, including modifying quantum dot interfaces and coupling quantum dots with metal nanostructures. This thesis focuses on studying the coupling between colloidal quantum dots and metal nanoparticles in simple model systems. Atomic force microscopy nanomanipulation is used to assemble the hybrid structures with a controlled geometry. The experimental studies report for the first time the modified fluorescence decay, emission intensity, and blinking of a single quantum dot coupled to a single Au nanoparticle. Since the geometry of the structure is known, these studies provide reliable information on the interparticle coupling, and quantitative experimental results are shown to be consistent with classical electrodynamic theories.Item Micro-patterning colloidal quantum dots based light sources for cellular array imaging(2014-08) Bhave, Gauri Suresh; Zhang, John X. J.; Dunn, Andrew Kenneth, 1970-Lab-on-chip systems have been developed for various applications like point of care diagnostics and compact imaging systems. Compact, on-chip imaging systems face a challenge in the integration of multicolor light sources on-chip. This is because of the unavailability of compact, individually addressable, multicolor light sources on a single planar substrate. Colloidal Quantum Dot based Light Emitting Diodes (QDLEDs), which have found wide appeal, due to their unique properties like their tunable and narrow emission bandwidth and easy fabrication, are ideal for lab-on-chip integration. Among different types of QDLED structures implemented, inorganic QDLEDs have shown great promise. We have demonstrated designs and fabrication strategies for creating QDLEDs with enhanced performance. In particular: (I) We introduce a sandwich structure with a spin coated inorganic hole transporting layer of nickel oxide underlying the QD layer and with a spin coated zinc oxide electron transporting layer, with patterning of anode and cathode on the substrate. Compared to the use of sputtered thin films, solution processed charge transporting layers (CTLs) improve robustness of the device, as crystalline ZnO shows low CB and VB edge energy levels, efficiently suppressing hole leakage current resulting in LEDs with longer lifetimes. We also use Atomic Layer Deposition to deposit an additional hole injecting layer to protect the QDs from direct contact with the anode. With this device design, we demonstrate a working lifetime of more than 12 hours and a shelf-life of more than 240 days for the devices. Our solution based process is applicable to micro-contact printed and also spin-coated QD films. QDLEDs with spin-coated CTLs show a lifetime increase of more than three orders of magnitude compared to devices made using sputtered CTLs. (II) We implement strategies of the enhancement of light extraction from the fabricated QDLEDs. We discuss the integration of a two dimensional grating structure based on a metal-dielectric-metal plasmonic waveguide with the metal electrode of a QDLED, with the aim of enhancing the light intensity by resonant suppression of transmitted light. The grating structure reflects the light coupled with the metal electrode in the QDLED and we found an increase of 34.72% in the electroluminescence intensity from the area of the pattern and an increase of 32.63% from photoluminescence of QDs deposited on a metal surface. (III) We demonstrate the capability of our fabricated devices as a light source by measuring intensity across stained cells with QDLEDs of two different wavelengths and show the correlation as expected with the absorption profile of the fluorescent dye. We measure the absorption from the biological samples using QDLEDs fabricated with various design modifications, as a quantification of the improvements in device performance, directly affecting to our target application.Item Two photon luminescence from quantum dots using broad and narrowband ultrafast laser pulses(2009-05-15) Balasubramanian, HaribhaskarNonlinear optical microscopy (NLOM) offers many advantages when imaging intact biological samples. By using ultrafast lasers in the near infrared and two photon excitation (TPE), signal production is limited to the focal volume and provides an excellent means for rendering thin, microscopic images from within the sample. Exogenous fluorophores/lumiphores may be used as efficient contrast agents to tag specific targets and provide enhanced signal. The efficiency of the TPE process in these contrast agents is broadly assumed to vary inversely with the laser pulsewidth, ?. In this work, we investigate the TPE efficiency of transform limited broadband (~133nm, ~10fs) and narrowband (~11nm, ~170fs) pulses in the generation of twophoton luminescence from semiconductor nanocrystals or quantum dots (QD?s) both theoretically and experimentally. Compared to standard organic dyes, QD?s possess a relatively broad, uniform spectral response that enables better use of the full bandwidth from the broadband laser. Theoretical calculations including both degenerate and non-degenerate TPE indicate a rolloff from the 1/? behavior as the pulses? spectral bandwidth becomes broader than the absorption spectra of the QD?s. Experimentally measured enhancement in luminescence intensity while using a broadband pulse is compared with the simulated enhancement in two-photon luminescence. A combination of increased understanding of the excitation processes in NLOM and proper selection of contrast agents will help in advancing the role of broadband ultrafast lasers in NLOM.