Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2008. / Wartell, Roger, Committee Member ; Bunz, Uwe, Committee Member ; Doyle, Donald, Committee Member ; Fahrni, Christoph, Committee Member ; Schuster, Gary, Committee Chair.

Techn. Hochsch., Diss., 2002--Aachen.

University, Diss., 2004--Frankfurt (Main).

Master of Science / Department of Chemistry / Emily McLaurin / Quantum dots (QDs) are novel semiconductors of interest for applications because of their special tunable properties. Among the many types of QDs, InP QDs attract attention because they do not have toxic-heavy-metal elements such as Cd or Pb. Charge-transfer (CT) is important in applications of InP QDs. CT consists of two or more molecules and some of them donate electrons and others accept those electrons. An understanding of CT between QDs with tetracyanoquinodimethane (TCNQ) is important for applications of QDs in photovoltaic and photocatalytic materials. TCNQ is an organic electron acceptor and CT complexes of TCNQ exhibit metallic electric conductivity. Previous research about CT between QDs and TCNQ examined PbS and CdSe QDs, but toxic-heavy-metals limit future application of these materials. So, it is important to research CT between InP QDs and TCNQ. This thesis examines how the amount of InP QDs (QD:TCNQ ratio) and diameters of InP QDs affect the CT between InP QDs and TCNQ. In this thesis, InP QDs are synthesized by a microwave-assisted ionic liquid (MAIL) method and InP QDs of different sizes are isolated using size-selective precipitation. Then, TCNQ-InP QD solutions are prepared with different ratios, with and without light, and with InP QDs of different sizes. These InP QDs and InP QDs-TCNQ samples are characterized using UV-Vis-NIR absorption, photoluminescence (PL), time-correlated single photon counting (TCSPC), and FT-IR spectroscopies. In Chapter 2, the details of synthesizing InP QDs, size selection, and preparation of different TNCQ-InP QD solutions are presented. Then, factors that affect the interaction between InP QDs and TCNQ and possible reasons for these factors are discussed. Based on calculations and experimental results, the carbon atom with the biggest amount of positive charge in TCNQ and phosphorous in InP QDs are likely the acceptor and donor, respectively. CT is affected by the amount of InP QDs in solution, and more InP QDs will reduce more TCNQ. The CT is also affected by the size of the InP QDs and enhanced by light.

No description available.

No description available.

Despite the poor understanding of charge transfer in conducting polymers, conducting-polymer-based devices have achieved considerable commercial success. This success is based largely on the reproducibility of the measurable properties. It is the purpose of this study to further clarify charge transfer characteristics of conducting polymers under varying conditions. We studied a conducting polymer called polyaniline. Polyaniline was available in its doped conducting form called emeraldine salt and in its undoped form called emeraldine base. Three types of polyaniline were studies, electrosynthesized (doped by HCI), chemically synthesized (doped by HCI) and commercial polyaniline obtained from Adlrich Company and doped by camphor sulfonic acid. Initially we investigated whether charge transfer was ionic or electronic by observing the change in resistance with time for a fixed applied current. It was concluded that condition in this material is electronic. Electrical measurements were obtained using the four-point probe and the Montgomery methods. The samples investigated were in pellet and film forms. We investigated charge transfer over the temperature regime 30-450 K by applying the following methods: scanning electron microscopy to investigate the surfaces of pellet and film samples, Fourier transform infrared spectroscopy and Raman spectroscopy to investigate the effect of annealing on the molecular structure of the polymer and thermal analysis to investigate the loss of substances from the polymer as a result of the annealing process. The conductivity of the material was also analyzed over the entire temperature regime. The following were observed: • Conductivity in polyaniline is governed my monomer units. • The decrease in conductivity with increase annealing temperature is related to moisture loss, loss of dopant ions and polymer degradation. • The variable range hopping model in three dimensions, satisfactorily describes charge transfer mechanism in polyaniline. • Conductivity in polyaniline is temperature activated. • Conductivity varies with position on the sample surface. • The effect of pellet pressing pressure to conductivity is negligible. • Current-voltage characteristics for polyaniline exhibit non-ohmic behavior at high current values, (>0.2 mA for T <80 K), applied between the current probes of a four-point probe measuring instrument. / Dissertation (MSc (Physics))--University of Pretoria, 2006. / Physics / unrestricted

There is a need for monolithic devices capable of spatial resolution in imaging and ionizing radiation detection. In this thesis, a GaAs acoustic charge transport device (ACT) was studied for this purpose. A new method of charge injection has been demonstrated for the ACT. Using near-infrared optical pulses incident through thin semi-transparent chromium windows, electron-hole-pairs were separated by the electric field in a depleted n-type channel region of the device. For light penetration less than the depth of the electron potential minimum, and for small injection levels, calculations indicated that electrons and holes were separated at their saturation velocities. Holes moving toward the surface of the substrate could recombine with electrons at an evaporated Schottky metal plate. Electrons moving toward the channel centre were bunched and transported by the electric field coupled to a <110> propagating surface acoustic wave (SAW) on (100) cut GaAs. Quantum efficiency, defined as the number of electrons collected at the output per incident photon on the GaAs surface, was greater than 9% at an optical wavelength of 730 nm. When compensation was made for the loss and reflection due to the chromium windows, the quantum efficiency was in excess of 24%. Charge transfer efficiency was greater than 0.992 with the ACT clocked at 360 MHz. The demonstrated optical injection technique may be of use in future ACT imaging devices. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Ce travail porte sur la neutralization résonnante d’ions Li+ sur des métaux et agrégatsmétalliques supportés. La neutralization sur Pd(100) a été étudié, pour différentsparamètres. La neutralisation s’avère très efficace sur cette surface avec un travail desortie grand et est en contradiction avec ce qu’on attend dans le cadre des modèlestraditionnels. Il est proposé qu’une description du processus RCT modifié, issued’études précédentes sur des métaux nobles, doit être utilisé. Dans celle ci desprocessus de neutralisation survenant à des distances atome- surface faible ont lieu etaussi un comportement adiabatique du système aux faible énergies de collisions mèneà une neutralization efficace.Les résultats de l’étude des processus de transfert d’électrons lors de l’interactiond’ions de Li+ avec des agrégats d’or supporté sur HOPG avec Al2O3 sont présentés etdiscuté. L’imagerie STM pour les agrégats d’or supporté sur un substrat HOPG viergeet aussi un substrat de HOPG bombardé par des ions de AR sont présentés. Lesobservations révèlent que agrégats d’or forment préférentiellement des chaines 1D lelong des marches sur HOPG vierge. Dans le cas de HOPG bombardé, la taille et lahauteur des agrégats sont dépendants des défauts de surface.Nous avons trouvé que la neutralisation est très efficace sur les petits agrégats et engénéral est beaucoup plus grande que sur des surfaces de cristaux d'or. Nous montronsdes effets liés à la nature du substrat, comme dans le cas de l’alumine ou le cas desdifférences observées sur des chaines d’agrégats sur HOPG vierge et les agrégatsformés sur des défauts / The present work investigates the neutralization of Li+ ions on metals and supportedmetal clusters. Neutralization on a transition metal surface Pd (100) for differentparameters was studied. Highly efficient neutralization on this surface with a highwork functions was observed and contradicts our traditional views on resonant chargetransfer (RCT) mechanism. A modified RCT picture involving new neutralizationprocesses occurring at a short atom-surface distance and an adiabatic behaviourleading to efficient neutralization at large distances, that has emerged from previousstudies on noble metal surfaces appears in qualitative agreement with our data.The experimental results on the dependence of the Li neutralization on the Auclusters supported on different substrates are reported and discussed. As acomplement to this, a STM study into the morphology of Au nanoparticles on apristine HOPG substrate as well as Ar+ ions sputtered HOPG substrate has beenperformed. The observations reveal that Au clusters preferentially form onedimensional chains along steps on pristine HOPG. In the case of Ar+ ions sputteredHOPG substrate, the size and height of cluster are dependent on surface defects.We found that neutralization is very efficient on small clusters and in general muchlarger than on surfaces of gold crystals. We demonstrate existence of strongdifferences as a function of cluster support type as in case of alumina supports orcluster chains on HOPG and clusters on defects on HOPG terraces.

A fundamental understanding of interfacial charge transfer at donor-acceptor interfaces is very crucial as it is considered among the most important dynamical processes for optimizing performance in many light harvesting systems, including photovoltaics and photo-catalysis. In general, the photo-generated singlet excitons in photoactive materials exhibit very short lifetimes because of their dipole-allowed spin radiative decay and short diffusion lengths. In contrast, the radiative decay of triplet excitons is dipole forbidden; therefore, their lifetimes are considerably longer. The discussion in this thesis primarily focuses on the relevant parameters that are involved in charge separation (CS), charge transfer (CT), intersystem crossing (ISC) rate, triplet state lifetime, and carrier recombination (CR) at silver nanocluster (NCs) molecular-acceptors interfaces. A combination of steady-state and femto- and nanosecond broadband transient absorption spectroscopies were used to investigate the charge carrier dynamics in various donor-acceptor systems. Additionally, this thesis was prolonged to investigate some important factors that influence the charge carrier dynamics in Ag29 silver NCs donor-acceptor systems, such as the metal doping and chemical structure of the nanocluster and molecular acceptors. Interestingly, clear correlations between the steady-state measurements and timeresolved spectroscopy results are found. In the first study, we have investigated the interfacial charge transfer dynamics in positively charged meso units of 5, 10, 15, 20-tetra (1- methyl-4-pyridino)-porphyrin tetra (p-toluene sulfonate) (TMPyP) and neutral charged 5, 10, 15, 20-tetra (4-pyridyl)-porphyrin (TPyP), with negatively charged undoped and gold (Au)- doped silver Ag29 NCs. Moreover, this study showed the impact of Au doping on the charge carrier dynamics of the system. In the second study, we have investigated the interfacial charge transfer dynamics in [Pt2 Ag23 Cl7 (PPh3)10] silver NCs doped with platinum (Pt), with neutral charged 5, 10, 15, 20-tetra (4-pyridyl)-porphyrin (TPyP). Here, we evaluated the effects of Pt (II) doping on the interfacial charge-transfer dynamics between TPyP and silver NCs.