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Non-Poissonian statistics, aging and "blinking'" quantum dots.

Description: This dissertation addresses the delicate problem of aging in complex systems characterized by non-Poissonian statistics. With reference to a generic two-states system interacting with a bath it is shown that to properly describe the evolution of such a system within the formalism of the continuous time random walk (CTRW), it has to be taken into account that, if the system is prepared at time t=0 and the observation of the system starts at a later time ta>0, the distribution of the first sojourn times in each of the two states depends on ta, the age of the system. It is shown that this aging property in the fractional derivative formalism forces to introduce a fractional index depending on time. It is shown also that, when a stationary condition exists, the Onsager regression principle is fulfilled only if the system is aged and consequently if an infinitely aged distribution for the first sojourn times is adopted in the CTRW formalism used to describe the system itself. This dissertation, as final result, shows how to extend to the non-Poisson case the Kubo Anderson (KA) lineshape theory, so as to turn it into a theoretical tool adequate to describe the time evolution of the absorption and emission spectra of CdSe quantum dots. The fluorescence emission of these single nanocrystals exhibits interesting intermittent behavior, namely, a sequence of "light on" and "light off" states, departing from Poisson statistics. Taking aging into account an exact analytical treatment is derived to calculate the spectrum. In the regime fitting experimental data this final result implies that the spectrum of the "blinking" quantum dots must age forever.
Date: August 2004
Creator: Aquino, Gerardo
Partner: UNT Libraries

First principle thousand atom quantum dot calculations

Description: A charge patching method and an idealized surface passivation are used to calculate the single electronic states of IV-IV, III-V, II-VI semiconductor quantum dots up to a thousand atoms. This approach scales linearly and has a 1000 fold speed-up compared to direct first principle methods with a cost of eigen energy error of about 20 meV. The calculated quantum dot band gaps are parametrized for future references.
Date: March 30, 2004
Creator: Wang, Lin-Wang & Li, Jingbo
Partner: UNT Libraries Government Documents Department

Towards bulk based preconditioning for quantum dotcomputations

Description: This article describes how to accelerate the convergence of Preconditioned Conjugate Gradient (PCG) type eigensolvers for the computation of several states around the band gap of colloidal quantum dots. Our new approach uses the Hamiltonian from the bulk materials constituent for the quantum dot to design an efficient preconditioner for the folded spectrum PCG method. The technique described shows promising results when applied to CdSe quantum dot model problems. We show a decrease in the number of iteration steps by at least a factor of 4 compared to the previously used diagonal preconditioner.
Date: May 25, 2006
Creator: Dongarra, Jack; Langou, Julien; Tomov, Stanimire; Channing,Andrew; Marques, Osni; Vomel, Christof et al.
Partner: UNT Libraries Government Documents Department

The use of bulk states to accelerate the band edge statecalculation of a semiconductor quantum dot

Description: We present a new technique to accelerate the convergence of the folded spectrum method in empirical pseudopotential band edge state calculations for colloidal quantum dots. We use bulk band states of the materials constituent of the quantum dot to construct initial vectors and a preconditioner. We apply these to accelerate the convergence of the folded spectrum method for the interior states at the top of the valence and the bottom of the conduction band. For large CdSe quantum dots, the number of iteration steps until convergence decreases by about a factor of 4 compared to previous calculations.
Date: May 10, 2006
Creator: Vomel, Christof; Tomov, Stanimire Z.; Wang, Lin-Wang; Marques,Osni A. & Dongarra, Jack J.
Partner: UNT Libraries Government Documents Department

Optical transitions and nature of Stokes shift in spherical CdSquantum dots

Description: We study the structure of the energy spectra along with the character of the states participating in optical transitions in colloidal CdS quantum dots (QDs) using the ab initio accuracy charge patching method combined with the folded spectrum calculations of electronic structure of thousand-atom nanostructures. In particular, attention is paid to the nature of the large resonant Stokes shift observed in CdS quantum dots. We find that the top of the valence band state is bright, in contrast with the results of numerous k {center_dot} p calculations, and determine the limits of applicability of the k {center_dot} p approach. The calculated electron-hole exchange splitting suggests the spin-forbidden valence state may explain the nature of the ''dark exciton'' in CdS quantum dots.
Date: December 16, 2005
Creator: Demchenko, Denis O. & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

On the design of reversible QDCA systems.

Description: This work is the first to describe how to go about designing a reversible QDCA system. The design space is substantial, and there are many questions that a designer needs to answer before beginning to design. This document begins to explicate the tradeoffs and assumptions that need to be made and offers a range of approaches as starting points and examples. This design guide is an effective tool for aiding designers in creating the best quality QDCA implementation for a system.
Date: October 1, 2006
Creator: DeBenedictis, Erik P.; Frank, Michael P. (Florida State University, Tallahassee, FL); Ottavi, Marco & Frost-Murphy, Sarah E. (University of Notre Dame, Notre Dame, IN)
Partner: UNT Libraries Government Documents Department

Multifunctional Organic-Inorganic Hybrid Nanophotonic Devices

Description: The emergence of optical applications, such as lasers, fiber optics, and semiconductor based sources and detectors, has created a drive for smaller and more specialized devices. Nanophotonics is an emerging field of study that encompasses the disciplines of physics, engineering, chemistry, biology, applied sciences and biomedical technology. In particular, nanophotonics explores optical processes on a nanoscale. This dissertation presents nanophotonic applications that incorporate various forms of the organic polymer N-isopropylacrylamide (NIPA) with inorganic semiconductors. This includes the material characterization of NIPA, with such techniques as ellipsometry and dynamic light scattering. Two devices were constructed incorporating the NIPA hydrogel with semiconductors. The first device comprises a PNIPAM-CdTe hybrid material. The PNIPAM is a means for the control of distances between CdTe quantum dots encapsulated within the hydrogel. Controlling the distance between the quantum dots allows for the control of resonant energy transfer between neighboring quantum dots. Whereby, providing a means for controlling the temperature dependent red-shifts in photoluminescent peaks and FWHM. Further, enhancement of photoluminescent due to increased scattering in the medium is shown as a function of temperature. The second device incorporates NIPA into a 2D photonic crystal patterned on GaAs. The refractive index change of the NIPA hydrogel as it undergoes its phase change creates a controllable mechanism for adjusting the transmittance of light frequencies through a linear defect in a photonic crystal. The NIPA infiltrated photonic crystal shows greater shifts in the bandwidth per ÂșC than any liquid crystal methods. This dissertation demonstrates the versatile uses of hydrogel, as a means of control in nanophotonic devices, and will likely lead to development of other hybrid applications. The development of smaller light based applications will facilitate the need to augment the devices with control mechanism and will play an increasing important role in the future.
Date: May 2008
Creator: Garner, Brett William
Partner: UNT Libraries

Final Technical and Scientific Report

Description: The objective of this project is to study the ultrafast carrier dynamics of in two types of semiconductor quantum dots: self-assembled quantum dots (SAQDs) and patterned quantum dots (PQDs) and to correlate these dynamics with the shape, size and material composition of the dots, thereby obtaining a fundamental scientific understanding of these nanoscale systems.
Date: July 11, 2007
Creator: Krishna, Sanjay & Hufftaker, Diana
Partner: UNT Libraries Government Documents Department

Metal-inducd assembly of a semiconductor-island lattice: Getruncated pyramids on Au-patterned Si

Description: We report the two-dimensional alignment of semiconductor islands using rudimentary metal patterning to control nucleation and growth. In the Ge on Si system, a square array of sub-micron Au dots on the Si (001) surface induces the assembly of deposited Ge adatoms into an extensive island lattice. Remarkably, these highly ordered Ge islands form between the patterned Au dots and are characterized by a unique truncated pyramidal shape. A model based on patterned diffusion barriers explains the observed ordering and establishes general criteria for the broader applicability of such a directed assembly process to quantum dot ordering.
Date: August 28, 2005
Creator: Robinson, J.T.; Liddle, J.A.; Minor, A.; Radmilovic, V.; Yi,D.O.; Greaney, P.A. et al.
Partner: UNT Libraries Government Documents Department

Deformation potentials of CdSe quantum dots

Description: The size dependent deformation potentials of CdSe quantum dots are studied by first principle and semi-empirical pseudopotentials calculations. They find that the amplitude of the quantum dot deformation potential is only slightly larger than the bulk value, and this increase is mostly caused by the off {Lambda} point deformation potentials in the bulk, which are larger in amplitude than the {Lambda} point deformation potential.
Date: June 2, 2004
Creator: Li, Jingbo & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Final Report for DE-FG36-08GO18007 "All-Inorganic, Efficient Photovoltaic Solid State Devices Utilizing Semiconducting Colloidal Nanocrystal Quantum Dots"

Description: We demonstrated robust colloidal quantum dot (QD) photovoltaics with high internal quantum efficiencies. In our structures, device durability is derived from use of all-inorganic atmospherically-stable semiconducting metal-oxide films together with QD photoreceptors. We have shown that both QD and metal-oxide semiconducting films and contacts are amenable to room temperature processing under minimal vacuum conditions, enabling large area processing of PV structures of high internal efficiency. We generated the state of the art devices with power conversion efficiency of more than 4%, and have shown that efficiencies as high as 9% are achievable in the near-term, and as high as 17% in the long-term.
Date: September 30, 2011
Creator: Bawendi, Vladimir Bulovic and Moungi
Partner: UNT Libraries Government Documents Department

Efficient multi-exciton emission from quantum dots.

Description: The fundamental spontaneous emission rate an emitter can be modified by its photonic environment. By enhancing the spontaneous emission rate, there is a possibility of extracting multi-exciton energies through radiative decay. In this report, we explore using high Q and small volume cavities to enhance the spontaneous emission rate. We observed greater than 50 folds enhancement in the spontaneous emission from photonic crystal waveguide or microcavity using close-packed monolayer of PbS quantum dot emitters.
Date: September 1, 2010
Creator: Luk, Ting Shan
Partner: UNT Libraries Government Documents Department

Lighting Up Enzymes for Solar Hydrogen Production (Fact Sheet)

Description: Scientists at the National Renewable Energy Laboratory (NREL) have combined quantum dots, which are spherical nanoparticles that possess unique size-tunable photophysical properties, with the high substrate selectivity and fast turnover of hydrogenase enzymes to achieve light-driven hydrogen (H2) production. They found that quantum dots of cadmium telluride coated in carboxylic acids easily formed highly stable complexes with the hydrogenase and that these hybrid assemblies functioned to catalyze H2 production using the energy of sunlight.
Date: February 1, 2011
Partner: UNT Libraries Government Documents Department

Fluorescent CdSe/ZnS nanocrystal-peptide conjugates for long-term, nontoxic imaging and nuclear targeting in living cells

Description: One of the biggest challenges in cell biology is the imaging of living cells. For this purpose, the most commonly used visualization tool is fluorescent markers. However, conventional labels, such as organic fluorescent dyes or green fluorescent proteins (GFP), lack the photostability to allow the tracking of cellular events that happen over minutes to days. In addition, they are either toxic to cells (dyes), or difficult to construct and manipulate (GFP). We report here the use of a new class of fluorescent labels, silanized CdSe/ZnS nanocrystal-peptide conjugates, for imaging the nuclei of living cells. CdSe/ZnS nanocrystals, or so called quantum dots (qdots), are extremely photostable, and have been used extensively in cellular imaging of fixed cells. However, most of the studies about living cells so far have been concerned only with particle entry into the cytoplasm or the localization of receptors on the cell membrane. Specific targeting of qdots to the nucleus of living cells ha s not been reported in previous studies, due to the lack of a targeting mechanism and proper particle size. Here we demonstrate for the first time the construction of a CdSe/ZnS nanocrystal-peptide conjugate that carries the SV40 large T antigen nuclear localization signal (NLS), and the transfection of the complex into living cells. By a novel adaptation of commonly used cell transfection techniques for qdots, we were able to introduce and retain the NLS-qdots conjugate in living cells for up to a week without detectable negative cellular effects. Moreover, we can visualize the movement of the CdSe/ZnS nanocrystal-peptide conjugates from cytoplasm to the nucleus, and the accumulation of the complex in the cell nucleus, over a long observation time period. This report opens the door for using qdots to visualize long-term biological events that happen in the cell nucleus, and provides a new nontoxic, ...
Date: June 14, 2004
Creator: Chen, Fanqing & Gerion, Daniele
Partner: UNT Libraries Government Documents Department

Conference Paper NFO-7:7th International Conference on Near-Field Optics and Related Technologies

Description: The seventh conference in the NFO conference series, held here in Rochester, provided to be the principal forum for advances in sub-wavelength optics, near-field optical microscopy, local field enhancement, instrumental developments and the ever-increasing range of applications. This conference brought together the diverse scientific communities working on the theory and application of near-field optics (NFO) and related techniques.
Date: October 18, 2004
Creator: Novotny, Prof.Dr. Lukas
Partner: UNT Libraries Government Documents Department

Final LDRD report : infrared detection and power generation using self-assembled quantum dots.

Description: Alternative solutions are desired for mid-wavelength and long-wavelength infrared radiation detection and imaging arrays. We have investigated quantum dot infrared photodetectors (QDIPs) as a possible solution for long-wavelength infrared (8 to 12 {mu}m) radiation sensing. This document provides a summary for work done under the LDRD 'Infrared Detection and Power Generation Using Self-Assembled Quantum Dots'. Under this LDRD, we have developed QDIP sensors and made efforts to improve these devices. While the sensors fabricated show good responsivity at 80 K, their detectivity is limited by high noise current. Following efforts concentrated on how to reduce or eliminate this problem, but with no clear path was identified to the desired performance improvements.
Date: February 1, 2008
Creator: Cederberg, Jeffrey George; Ellis, Robert & Shaner, Eric Arthur
Partner: UNT Libraries Government Documents Department

Patterning quantum dot arrays using DNA replication principles.

Description: The convergence of nanoscience and biotechnology has opened the door to the integration of a wide range of biological molecules and processes with synthetic materials and devices. A primary biomolecule of interest has been DNA based upon its role as information storage in living systems, as well as its ability to withstand a wide range of environmental conditions. DNA also offers unique chemistries and interacts with a range of biomolecules, making it an ideal component in biological sensor applications. The primary goal of this project was to develop methods that utilize in vitro DNA synthesis to provide spatial localization of nanocrystal quantum dots (nQDs). To accomplish this goal, three specific technical objectives were addressed: (1) attachment of nQDs to DNA nucleotides, (2) demonstrating the synthesis of nQD-DNA strands in bulk solution, and (3) optimizing the ratio of unlabeled to nQD-labeled nucleotides. DNA nucleotides were successfully attached to nQDs using the biotin-streptavidin linkage. Synthesis of 450-nm long, nQD-coated DNA strands was demonstrated using a DNA template and the polymerase chain reaction (PCR)-based method of DNA amplification. Modifications in the synthesis process and conditions were subsequently used to synthesize 2-{micro}m long linear nQD-DNA assemblies. In the case of the 2-{micro}m structures, both the ratio of streptavidin-coated nQDs to biotinylated dCTP, and streptavidin-coated nQD-dCTPs to unlabeled dCTPs affected the ability to synthesize the nQD-DNA assemblies. Overall, these proof-of-principles experiments demonstrated the successful synthesis of nQD-DNA using DNA templates and in vitro replication technologies. Continued development of this technology may enable rapid, spatial patterning of semiconductor nanoparticles with Angstrom-level resolution, as well as optically active probes for DNA and other biomolecular analyses.
Date: November 1, 2004
Creator: Crown, Kevin K. & Bachand, George David
Partner: UNT Libraries Government Documents Department

Comparison of quantum confinement effects between quantum wires and dots

Description: Dimensionality is an important factor to govern the electronic structures of semiconductor nanocrystals. The quantum confinement energies in one-dimensional quantum wires and zero-dimensional quantum dots are quite different. Using large-scale first-principles calculations, we systematically study the electronic structures of semiconductor (including group IV, III-V, and II-VI) surface-passivated quantum wires and dots. The band-gap energies of quantum wires and dots have the same scaling with diameter for a given material. The ratio of band-gap-increases between quantum wires and dots is material-dependent, and slightly deviates from 0.586 predicted by effective-mass approximation. Highly linear polarization of photoluminescence in quantum wires is found. The degree of polarization decreases with the increasing temperature and size.
Date: March 30, 2004
Creator: Li, Jingbo & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department