The ultrafast charge dynamics in a number of nanostructured materials relevant to the production of renewable energy are investigated using ultrafast transient absorption spectroscopy. The materials include mercury telluride and cadmium mercury telluride quantum dots, and gold nanoparticles loaded on titanium dioxide colloidal spheres. The analysis of the resultant pump-induced transmittance change spectra and transients allow the determination of charge relaxation routes including multiple exciton generation, trion formation and direct-surface trapping. The investigation of HgTe QDs passivated with thioglycerol, mercaptopropionic acid and dodecanethiol ligands suggests that mercaptopropionic acid ligand results in better passivation of HgTe QDs due to its carboxylic acid group. It allows more electron density donation to the QD surface to passivate the traps related with unsaturated Hg bonds and hence supresses the associated non-radiative processes. The decay lifetimes of the thioglycerol/dodecanethiol-capped QDs in addition to the photo-induced absorption feature in their spectra, are found to be consistent with surface charge trapping observed in CdSe QDs. In comparison, the transients obtained for mercaptopropionic acid passivated QDs coupled with the pump-induced transmittance change spectrum show no sign of any surface-related processes. Therefore, our analyses allow the determination of multiple exciton generation for the first time in these QDs with a quantum yield of 1.36 ± 0.04 when photo-exciting with photons of energy 3.1 times the band gap. Such result should turn researchers’ attention to those ligands which could improve the QD solar cell field. The study of exciton dynamics in CdxHg(1-x)Te alloy QDs is also presented here. Their pump-induced transmittance change spectrum show two bleaches: at the shoulder position of the steady state absorption and at the PL peak. The exciton dynamics of these materials are studied using four different wavelengths, two of them are above the MEG threshold. The resultant transmittance transients and the pump-induced transmittance change spectrum are free of any photo-induced absorption or long-lived surface trapping. Hence, the decay of the transients obtained above the MEG threshold for well-stirred samples at low pump fluences is attributed to biexciton recombination. The assessment of multiple exciton generation reveals a quantum yield value of 1.12 ± 0.01 when photo-exciting with 2.6 times the band gap. Finally, the investigation of the recovery of the plasmon bleach in TiO2 colloidal spheres decorated with different sizes of Au NPs is presented in this thesis. The pump-induced transmittance change spectra obtained for two different wavelengths show bleaches at the plasmon band maximum superimposed with two wings of absorption features at shorter and longer wavelengths. The resultant transmittance transients for these samples are well-described by bi-exponential decay with a very quick decline of a few ps associated with electron–phonon scattering, followed by a slower decay over a few 10s of ps associated with heat dissipation. Only the heat dissipation rate is found to be dependent on the size of the Au NPs as it rises from 49 ± 3 ps to 128 ± 6 ps when the diameter of the Au NPs is increased from 12.2 ± 2.2 nm to 24.5 ± 2.8 nm, respectively.

Quantum dots (QDs) have the potential to produce more than one exciton per incident photon, if the photon energy is greater than twice the band gap energy. This process of multiple exciton generation (MEG) has the potential to lead to a step change in the efficiency of solar panels, by utilising energy commonly wasted as heat in conventional solar cells. A wide range of CdSe/CdTe and CdTe/CdSe quantum dots with and without a CdS shell were synthesised with varying core sizes and shell thicknesses. The excited state dynamics of these samples were studied with transient absorption and photoluminescence studies, with their MEG efficiencies obtained. Record MEG efficiencies were obtained with values reaching 142 ± 9 % achieved. The charge separation afforded by the type-II electronic configuration, allowed the first attractive biexciton interaction for a type-II QD system, with the potential for reducing the creation energy for a second exciton this affords. Efficient surface passivation of QDs was achieved through the reaction of CdCl2 with CdTe QDs, with near unity photoluminescence quantum yields (PLQYs) achieved. The suppression of surface trap states resulted in mono-exponential photoluminescence decay traces, with a resultant increase in exciton lifetime. Further CdCl2 treatment was carried out on CdSe/CdTe quasi-type-II QDs with alternating ‘Cd rich’ and ‘Te rich’ surfaces to elucidate the processes involved in surface treatment. It is shown that Te surface atoms are preferentially etched upon treatment, with the reaction being more aggressive when ‘Te rich’ surfaces are treated. The importance of surface composition is studied with trap states associated with chalcogen dangling bonds more prevalent and hence the increased requirement for their passivation is outlined. Control of the core/shell interface is also shown to be important in reducing trap states and ultimately increasing PLQYs, which is desirable for many optoelectronic applications.

The work presented in this thesis was submitted to The University of Manchester for the degree of Doctor of Philosophy in June 2010 by Stuart K Stubbs and is entitled “Photo-physics and applications of colloidal quantum dots”. In this thesis the results of spectroscopic studies on various colloidal quantum dots, particularly related to the measurement and characterisation of multiple exciton generation are presented. Research conducted with Nanoco Technologies Ltd. that involved the design and development of hybrid quantum dot organic light emitting diodes for use in flat panel display technology is also presented. Cadmium selenide (CdSe), indium phosphide (InP), and lead sulphide (PbS) type I and cadmium selenide/cadmium telluride type II colloidal quantum dots were characterised using steady state photoluminescence and absorption spectroscopy. The fluorescence lifetimes of the decay of single excitons was measured in these quantum dots using time correlated single photon counting. An ultrafast transient absorption spectrometer was designed, built, and used to observe the picosecond dynamics of the decay of multiexcitons. These absorption transients were analysed in order to extract the quantum efficiency of producing multiple excitons per absorbed photon. The characteristic signature for multiple exciton generation was first found in CdSe using a time correlated single photon counting set-up. Results from the transient absorption spectrometer demonstrated efficient multiple exciton generation in InP for the first time as well as in PbS, where the efficiency was found to agree with values obtained by other research groups. The absorption transients taken for the type II CdSe/CdTe type II quantum dots demonstrated some novel decay dynamics that could not be attributed to the generation of multiple excitons. Quantum dot organic light emitting diodes were fabricated using Nanoco Technologies high quality cadmium based quantum dots and were shown to demonstrate bright, colour saturated emission originating from the quantum dot layer only. Using quantum dots of different sizes and structures red, green and blue devices were made and shown to be appropriate both in terms of brightness and chromaticity for the use as the red, green and blue pixels of a flat panel display. Because heavy metals like cadmium are restricted or banned from commercial products in many countries, Nanoco Technologies heavy metal free quantum dots, made from InP, were also incorporated in devices. Devices are demonstrated that emit from the quantum dot layer only, albeit at a lower luminance and efficiency than that found in the cadmium containing devices. This was the first demonstration of a heavy metal free, hybrid quantum dot organic light emitting diode.

Semiconductor nanostructures are currently an active area of research, especially in the field of photovoltaics as they will play a major role in next generation solar devices that break the current theoretical limit for light-to-current conversion. For instance, the efficiency of the nanostructure-based solar cells can be increased due to carrier multiplication, or multiple exciton generation (MEG) process, where absorption of a single energetic photon results in the generation of several charge carriers. In order to design nanostructures with the desired properties, a detailed theoretical approach for studying photoexcited state processes is necessary. The approach developed in this work is based on many-body perturbation theory (MBPT) and the Boltzmann transport equation (BE) in combination with density functional theory (DFT) in order to compute quantum efficiency (QE). Conclusions about QE are made after studying all the major relaxation channels in a photoexcited system, such as exciton-to-biexciton decays, biexciton recombination and phonon-mediated exciton relaxation. In all calculations, excitonic effects have been included by solving the Bethe-Salpeter equation (BSE). Then, by including excitons in the MBPT calculations, the exciton-to- biexciton rates R1→2 as well as the biexciton-to-exciton rates R2→1 are computed by taking into account the singlet fission (SF) process. The methods developed here have been applied to various semiconductor nanostructures such as pristine chiral (6,2), (6,5) and (10,5) and functionalized (6,2) SWCNTs. We predict efficient MEG in the (6,2) and (6,5) SWCNTs within the solar spectrum range starting at the 2Eg energy threshold and with QE reaching ~ 1:6 at about 3Eg, where Eg is the electronic gap. Also, methods for MEG rates calculations have been improved by taking into account exciton-exciton interactions in the intermediate biexciton state, where results show a small (~ 40 meV) red-shift in the biexciton density of states. Finally, the MEG-BE technique is applied in studying charge transfer. Charge transfer has been studied in a doped silicon quantum dot (QD) - functionalized SWCNT system where it was found that an initial excitation localized on either the QD or CNT evolves into a transient CT state. / National Science Foundation (NSF CHE-1413614)

Covalent SP3-hybridization defects in single-walled carbon nanotubes (CNTs) have been prevalent in recent experimental and theoretical studies for their interesting photophysical properties. These systems are able to act as excellent sources of single, infrared photons, even at room temperature, making them marketable for applications to sensing, telecommunications, and quantum information. This work was motivated by recent experimental studies on controllable defect placement and concentration as well as investigating carrier multiplication (CM) using DFT-based many-body perturbation theory (MBPT) methods to describe excitonic relaxation processes. We find that pristine CNTs do not yield appreciable MEG at the minimum threshold of twice the optical gap 2Eg, but covalent functionalization allows for improved MEG at the threshold. Finally, we see that defect-defect interactions within CNT systems can be modeled simply as HJ-aggregates in an effective Hamiltonian model, which is shown to be valid for certain, highly-redshifted defect configurations at low defect-defect separation lengths.

This work describes an experimental investigation of methods of synthesis, determination of structural and physical properties, and analysis and correlation of the properties to the structures of semiconductor quantum dots and quantum dot-polymer hybrid structures. These structures are investigated for applications in flexible solar cell devices. The main synthesis process used in the work was a Laser-Assisted Spray (LAS) process that was developed in our laboratory to deposit surfactant-free PbSe quantum dot (QD) films directly on a substrate. The QD films formed by this technique are in close contact with each other forming a percolation path for charge transport. Analytical instruments that include Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) were used for structural characterization while optical absorption spectroscopy and photoluminescence were used for determining the quantum confinement of charge carriers in PbSe QDs. In addition, charge transport across lithographically patterned paths was used to determine the transport characteristics and generation of photocurrent in the fabricated structures. Absorption spectroscopy confirmed the quantum confinement of PbSe QDs deposited by LAS deposition. Room temperature current-voltage measurements across a 2 micrometer tunnel junction formed by the QDs produced a power-law dependence of the form I ∝ V2.19 that describes a percolation path of dimensionality slightly above two-dimensional. Absence of surfactants in LAS deposited films improved the conductivity by more than three orders of magnitude. Temperature dependent conductance studies showed thermally activated transport at high temperatures and temperature independent tunneling followed by previously unobserved metallic conduction at low temperatures. The LAS system was successfully modified by incorporating two spray nozzles to transport aerosols of two different precursors, one containing the QDs and the other containing the polymer. This new co-deposition system was successfully used to deposit QDs/Polymer hybrid structures. The TEM and XRD studies of LAS co-deposited films were shown to be uniformly distributed and crystalline. The photo-current experiments of QD/polymer hybrid composites showed clear evidence of enhanced carrier generation and transport as a result of intimate contact between quantum dots (QDs).

xx, 134 p. : ill. (some col.) / Solar energy is a green alternative to fossil fuels but solar technologies to date have been plagued by low conversion efficiencies and high input costs making solar power inaccessible to much of the developing world. Semiconductor nanoparticles (NPs) may provide a route to efficient, economical solar devices through a phenomenon called multiple exciton generation (MEG). Through MEG, semiconductor NPs use a high-energy input photon to create more than one exciton (electron-hole pair) per photon absorbed, thereby exhibiting large photoconversion efficiencies. While MEG has been studied in many NP systems, and we understand some of the factors that affect MEG, a rigorous analysis of the NP-ligand interface with respect to MEG is missing. This dissertation describes how the NP ligand shell directly affects MEG and subsequent charge carrier recombination. Chapter I describes the motivation for studying MEG with respect to NP surface chemistry. Chapter II provides an in-depth overview of the transient absorption experiment used to measure MEG in the NP samples. Chapter III highlights the effect of oleic acid and sodium 2, 3-dimercaptopropane sulfonate on MEG in PbS NPs. The differences in carrier recombination were accounted for by two differences between these ligands: the coordinating atom and/or the secondary structure of the ligand. Because of these hypotheses, experiments were designed to elucidate the origin of these effects by controlling the NP ligand shell. Chapter IV details a viable synthetic route to thiol and amine-capped PbS NPs using sodium 3-mercaptopropane sulfonate as an intermediate ligand. With the versatile ligand exchange described in Chapter IV, the MEG yield and carrier recombination was investigated for ligands with varying headgroups but the same secondary structure. The correlation of ligand donor atom to MEG is outlined in Chapter V. Finally, Chapter VI discusses the conclusions and future outlook of the research reported in this dissertation. This dissertation includes previously published and unpublished co-authored material. / Committee in charge: Dr. Geraldine L. Richmond, Chairperson; Dr. David R. Tyler, Advisor; Dr. Mark C. Lonergan, Member; Dr. Catherine J. Page, Member; Dr. Hailin Wang, Outside Member

In this thesis ultrafast exciton dynamics of several colloidal quantum dots have been studied using visible transient absorption spectroscopy. The resultant transient decays and differential transmission spectra were analysed to determine the ultrafast relaxation channels, multiple exciton generation (MEG) efficiency and multi-exciton interactions in the observed materials. All QDs were preliminarily optically characterized using steady state absorption and photoluminescence spectroscopies. In addition, a high repetition infrared femtosecond pump probe experiment was designed and built to detect the picosecond intraband carrier relaxations in quantum dots. Picosecond carrier dynamics of type-II ZnTe/ZnSe and of CuInSe2 and CuInS2 type-I quantum dots were investigated. The common feature of these materials is that they are eco-friendly materials, being alternatives to the toxic Cd- and Pb- based materials. It was found that surface trapping occurred in both cases for electrons in the hot states, and in the minimum of the conduction band for ZnTe/ZnSe core/shell materials. Trion formation was observed in ZnTe/ZnSe core/shell dots at high power and unstirred conditions. The hot and cold electron trapping processes in type-II dots and CuInS2 and CuInSe2 dots shifted, distorted and moderately cancelled the bleach features. In addition, intra-gap hole trapping was observed in CuInS2 and CuInSe2 dots which results in a long decay feature in the recorded transients. MEG competes with Auger cooling, surface mediated relaxation and phonon emission. To enhance the MEG quantum yield, the rival mechanisms were suppressed in well-engineered CdSe/CdTe/CdS and CdTe/CdSe/CdS core/shell/shell and CdTe/CdS core/shell type-II quantum dots. The MEG slope efficiency and threshold for a range of different core size and shell thickness were found to be (142±9)%/Eg and (2.59±0.16)Eg, respectively. The observed threshold was consistent with the literature, whereas, the obtained slope efficiency was about three times higher than the previously reported values. The biexciton interaction energy of the dots stated in the previous paragraph was also studied. To date, time-resolved photoluminescence (TRPL) has been employed to study exciton interactions in type-II quantum dots and large repulsive biexciton interaction energy values between 50-100 meV have been reported. However, unlike the TRPL method, the TA experiment ensures that only two excitons remain in the band edge of the dot. Using this method, large attractive biexciton interaction energies up to ~-60 meV was observed. These results have promising implications regarding enhancing the MEG quantum yield.

No / The objective of this review is to provide an overview concerning what the authors believe to be the most important photoelectrochemical techniques for the study of semiconductor nanoparticles. After a short historical background and a brief introduction to the area of photoelectrochemistry, the working principles and experimental setups of the various static and dynamic techniques are presented. Experimental details which are of crucial importance for their correct execution are emphasized, and applications of the techniques as found in the recent research literature as applied to semiconductor nanoparticles are illustrated.