Using two different model systems, this thesis considers the old, but fascinating question: how do atoms or particles possessing a particular set of individual characteristics combine to form assemblies with quite distinct, ensemble characteristics, and how do those characteristics evolve as a function of the size of the assembly? For the last thirty years, numerous experiments studying the emergence of collective material properties have focused on a class of semiconducting, colloidal nanocrystals commonly known as quantum dots, which are notable for the size-dependence of their optical properties. Despite years of effort, even the most uniform quantum dot samples possess some heterogeneity in size, shape, and composition, which has prevented complete structure determination and hindered understanding of structure-property relationships. Chapter 1 of this thesis presents an approach to overcoming this challenge and reports the synthesis of a set of four, new, atomically precise cadmium selenide nanocrystal samples, which we call CdSe(350 nm), CdSe(380 nm), CdSe(408 nm), and CdSe(435 nm) after their lowest energy absorption features. We determine their structures and formulas through a combination of single crystal and powder X-ray diffraction measurements, elemental analysis, and spectroscopy. We also describe the optical properties of these samples and their sensitivity to ligand coverage, compare them to other previously reported cadmium selenide nanomaterials, and discuss ongoing experiments. Because CdSe(350 nm), CdSe(380 nm), CdSe(408 nm), and CdSe(435 nm) are atomically precise, they allow us to correlate specific structural features with material properties, which is the focus Chapter 2. Here we present a series of Raman scattering experiments designed to probe the evolution of vibrational structure with size. We find that the Cd-Se stretching region of the Raman spectra exhibits two peaks, which are assigned to primarily surface-derived and interior-derived atomic motions using density functional theory calculations. By performing variable temperature measurements, we discover that the smallest sample, CdSe(350 nm), exhibits behavior that can be well-described using a model developed for small molecules while the vibrations of the largest measured cluster, CdSe(408 nm), are better described by a model developed for bulk materials. This observation is evidence that the transition to a more bulk-like vibrational structure occurs relatively rapidly when cadmium selenide materials are approximately 2 nm in size. The emergence of collective material properties is also the subject of Chapter 3, but the topic is approached from a different perspective. Instead of focusing on a series of atomically precise clusters that differ in size, Chapter 3 presents a series of molecules composed of atomically precise clusters. We prepare octahedral hexaruthenium carbonyl clusters, [Ru₆C(CO)₁₆]²⁻, and use them as building blocks to assemble oligomers linked by single metal atom bridges. We synthesize and structurally characterize a set of compounds varying in length (from monomer to trimer) and linker atom identity (cadmium and mercury) and study the effect on electronic structure using infrared and UV-Visible absorption spectroscopies and density functional theory calculations. With increasing oligomer length, the UV-Vis absorption profile changes and shifts to lower energy, which we attribute in part to the development of coupling between neighboring clusters. Our calculations show that the infinite polymer composed of [Ru₆C(CO)₁₆]²⁻ linked by Hg²⁺ would be a one-dimensional semiconductor with a 1.5 eV direct band-gap. More detailed abstracts can be found at the beginning of each chapter.

CdSe solar cells with ZnTe as the window layer deposited by CSS process have shown Voc's around 630mV. However the currents were very low and also the voltages were not meeting the desired objectives. To improve the performance the contact energy at the ZnTe/Cu interface should be minimized by doping the window layer. Thermal evaporation was used to deposit ZnTe to have more control over the composition of the film. Initial experiments were done by depositing Cu doped ZnTe films on plain glass by co-evaporating both ZnTe and Cu. The conductivity was in the order of 10e3 which shows copper present in the film in the order of 1e22 S/cm³. This accomplishes a tunneling contact with the top electrode. Using the ZnTe:Cu contacts in complete devices resulted in disappointing voltages and currents. Efforts were made to deal with the poor performance of the cells. Devices were made on different types of TCO coated glass substrates but, were resulting in the same numbers which shows the type of TCO has an insignificant effect on the performance. The Cu doping has been helping in raising the Vocs but at the same time marred the currents whose effect has been unexplainable. Further experiments have been made changing the ZnTe thickness and concentration of Cu doping. Experiments were done increasing the substrate temperature as high as 5000C during ZnTe deposition and a Se flux has been introduced so as to compensate the loss of Se from CdSe at such high substrate temperature. But these experiments resulted in dismal performance indicating the domination of defects in the undoped ZnTe layer.

A new strategy was developed to switch the luminescence of semiconductor quantum dots with chemical stimulations. It is based on the photoinduced transfer of either energy from CdSe-ZnS core-shell quantum dots to [1,3]oxazine ligands or electrons from the organic to the inorganic components. Upon addition of base or acid, energy or electron transfer pathways respectively become operative, leading to changes in the luminescence of the nanoparticles. These changes are fully reversible and can be exploited to probe the pH of aqueous solutions from 3 up to 11 and this design can lead to the development of pH-sensitive luminescent probes for biomedical applications based on the semiconductor quantum dots. Secondly, an operating principle to transduce the supramolecular association of complementary receptor-substrate pairs into an enhancement in the luminescence of sensitive quantum dots was identified. This system is based on the electrostatic adsorption of cationic quenchers on the surface of anionic quantum dots. The adsorbed quenchers efficiently suppress the emission character of the associated nanoparticles on the basis of photoinduced electron transfer. In the presence of target receptors able to bind the quenchers and prevent electron transfer, however, the luminescence of the quantum dots is restored. Thus, complementary receptor-substrate pairs can be identified with luminescence measurements relying on this system and this protocol can be adapted to signal protein-ligand interactions. Thirdly, a photochromic spiropyran with dithiolane appendage to adsorb on the surface of cadmium sulfide system was designed. The properties of the resulting photochrome-nanoparticle assemblies vary significantly with the experimental conditions selected for the preparation of the inorganic component. Finally, photochromic materials based on the photoinduced transfer of electrons from CdSe-ZnS core-shell quantum dots to bipyridinium dications were developed.

Thesis (Ph. D.)--University of Massachusetts Amherst, 2009. / Includes bibliographical references (p. 153-161). Print copy also available.

Research into renewable energy sources is crucially increasing to counteract the ever more concerning impact of non-renewable sources. Theoretically, Quantum Dot Solar Cells (QDSCs) can achieve much greater efficiencies than current, commercial solar cells, but its expansion is still in its very early stages of scientific study and development. In this project TiO2, one of the most efficient and cost-effective photocatalysts, is coupled with Cadmium Selenide (CdSe) Quantum Dots (QD) in a study of interfacial charge transfers. Thus far, in other studies, CdSe QDs have shown some of the most promising results of QDSCs. EPR spectroscopy has been used here to study charge transfer processes in CdSe quantum dot (QD) sensitised titania. Visible light excitation of QDs directly adsorbed onto titania surfaces causes electron transfer to the titania, producing characteristic EPR signals of trapped electrons in the titania. Under ultraviolet excitation the trapped electron signals seen in titania alone are suppressed in the presence of directly adsorbed quantum dots, as is the formation of superoxide in the presence of oxygen. These observations suggest that reverse electron transfer from the titania to the QDs can also occur. No visible light excited electron transfer occurs in the case of QDs attached to the titania surface via bi-linker molecules, but under ultraviolet excitation a similar suppression of electron trapping in the titania phase is seen. These results show that the nature of the interface between the QDs and the titania phase is crucially important in the electron transfer processes in both directions. The study also looks at the pitfalls of synthesis techniques used for making the CdSe QDs as well as the method of attaching it to the TiO2. Ionic deposition, which generally resulted in the best photocurrents in other studies, was discovered early on this project produced very impure samples. Direct Adsorption produces low titania surface coverage, which can potentially be improved. Whereas the lack of discussion in literature of clear purification methods in synthesis techniques for attaching QDs via a bi-linker molecule, through ligand exchange, causes a significant drawback in the study of such systems.

abstract: The energy band gap of a semiconductor material critically influences the operating wavelength of an optoelectronic device. Realization of any desired band gap, or even spatially graded band gaps, is important for applications such as lasers, light-emitting diodes (LEDs), solar cells, and detectors. Compared to thin films, nanowires offer greater flexibility for achieving a variety of alloy compositions. Furthermore, the nanowire geometry permits simultaneous incorporation of a wide range of compositions on a single substrate. Such controllable alloy composition variation can be realized either within an individual nanowire or between distinct nanowires across a substrate. This dissertation explores the control of spatial composition variation in ternary alloy nanowires. Nanowires were grown by the vapor-liquid-solid (VLS) mechanism using chemical vapor deposition (CVD). The gas-phase supersaturation was considered in order to optimize the deposition morphology. Composition and structure were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD). Optical properties were investigated through photoluminescence (PL) measurements. The chalcogenides selected as alloy endpoints were lead sulfide (PbS), cadmium sulfide (CdS), and cadmium selenide (CdSe). Three growth modes of PbS were identified, which included contributions from spontaneously generated catalyst. The resulting wires were found capable of lasing with wavelengths over 4000 nm, representing the longest known wavelength from a sub-wavelength wire. For CdxPb1-xS nanowires, it was established that the cooling process significantly affects the alloy composition and structure. Quenching was critical to retain metastable alloys with x up to 0.14, representing a new composition in nanowire form. Alternatively, gradual cooling caused phase segregation, which created heterostructures with light emission in both the visible and mid-infrared regimes. The CdSSe alloy system was fully explored for spatial composition variation. CdSxSe1-x nanowires were grown with composition variation across the substrate. Subsequent contact printing preserved the designed composition gradient and led to the demonstration of a variable wavelength photodetector device. CdSSe axial heterostructure nanowires were also achieved. The growth process involved many variables, including a deliberate and controllable change in substrate temperature. As a result, both red and green light emission was detected from single nanowires. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2012

We report on the fundamental properties and device applications of semiconductor nanoparticles. ZnO nanowires and CdSe quantum dots were used, prepared, characterized, and assembled into novel light-emitting diodes and solar cells. ZnO nanowire films were grown electrochemically using aqueous soluble chloride-based electrolytes as precursors at temperatures below 90° C. Dopants were added to the electrolyte in the form of chloride compounds, which are AlCl3, CoCl2, CuCl2, and MnCl2. The optical, magnetic, and structural properties of undoped and transition-metal-ion doped ZnO nanowires were explored. Our results indicate that the as-grown nanowire structures have considerable internal strain, resulting in clearly visible lattice distortions in bright and dark-field transmission electron micrographs. Photo and electroluminescence studies indicate that the strain-induced defects strongly dominate any dopant-related effects. However, annealing at moderate temperature as well as laser annealing induces strain relaxation and leads to dopant activation. Hence, the optical and electrical properties of the nanowires significantly improve, allowing these nanowires to become feasible for use in the fabrication of solar cell and LED devices. In addition, the magnetic impurities incorporated into our ZnO nanowires show superparamagnetic behavior at room-temperature, while Al-doped and undoped ZnO nanowires show no magnetic behavior. The electroluminescence (EL) is achieved from a vertical hybrid p-n junction LED arrangement consisting of a hole-conducting polymer and n-type ZnO nanowires, our group was the first to report this vertical nanowire-based LED in Könenkamp et al., 2004 [12]. The observed EL spectra show an ultraviolet excitonic emission peak and a broad defect-related emission band in the visible range. After annealing at 380° C, the defect related EL peak exhibits a characteristic shift to higher wavelengths, where the magnitude of the shift is dependent on the dopant type. Aluminum incorporation exhibited the most improved exciton related-emission, leading to the emergence of a narrow excitonic luminescence peak around 390 nm, which is close to the bandgap of ZnO. The comparison of spectra obtained from temperature-dependent photoluminescence (PL) measurements, before and after thermal annealing, also indicates that the optical activity of impurities changes noticeably upon annealing. The internal quantum efficiency for PL is measured to be as high as 16 percent for Al-doped samples annealed at 380° C. The PL measurements also show that the excitonic luminescence is preferentially guided, while the defect related emission is more isotropically emitted. The nanostructured heterojunction solar cell is designed such that thin CdSe quantum dot films are embedded between a ZnO nanowire film and a hole-conducting polymer layer. This arrangement allows for enhanced light absorption and an efficient collection of photogenerated carriers. Here, we present a detailed analysis of the pyridine solution and 1,2- ethanedithiol ligand exchange processes of the quantum dots, deposition processes of this quantum dot layer, the conformality of this layer on deeply nanostructured samples, and the effect of a surfactant-aided thermal annealing process. Annealing creates a structural conversion of the quantum dot layers into an extremely thin continuous poly-crystalline film, with typical grain diameters of 30-50 nm. This transition is accompanied by a loss of quantum confinement and a significant improvement of the charge transport in the CdSe layer. The combination of the solution and ligand exchange of CdSe quantum dots, as well as the deposition and optimized annealing processes of this quantum dot layer, resulted in solar cells with an open-circuit voltage up to 0.6 V, a short circuit current of ~15 mA/cm2, an external quantum efficiency of 70 percent, and an energy conversion efficiency of 3.4 percent. This 3.4 percent efficiency is presently one of the best efficiencies obtained for this type of device.

Quantum dots are semiconductive particles whose properties are highly influenced by the presence of at least one electron. Cadmium selenide quantum dots were synthesized via colloidal synthesis. Contrary to previous preparations, more focus was placed on the temperature rather than the duration of time at which they form. A series of colored solutions were obtained because the excited quantum dots of various sizes emitted specific wavelengths of light. The emission spectra showed that the temperature-dependent quantum dots were successfully synthesized. The quantum dots were also immobilized on various surfaces, and the luminescence properties were examined. The quantum dots that were immobilized in sol-gels through chemiluminescence (CL) analyses were found to be stable and were able to maintain their luminescence properties with extensive uses and long-term storage. Linear calibration curves were obtained for concentrations of hydrogen peroxide from 1.75 x 10-4 M to 1.75 x 10-2 M in TCPO-CL.

Quantum dots show great potential for use as spectral converters in solar cells, lighting applications, and biological imaging. These applications require precise control of quantum dot size to maximize performance. The quality, size, and fluorescence of quantum dots depend on parameters that are difficult to control using traditional batch synthesis processes. An alternative, high precision process was developed for the synthesis of cadmium-selenide quantum dots using a microfluidic reactor and fluorescence flow cell. The process required creating separate cadmium and selenium precursors that were then mixed in a nitrogen environment at 17°C. Using an NE-300® syringe pump, the solution was pumped through a microfluidic reactor submerged in a 240°C oil bath. The reactor then fed through a water quench bath at 25°C to terminate the nucleation and growth reaction. The fluorescence profiles of the quantum dot solutions were then characterized with an in-line fluorescence flow cell used in conjunction with an Ocean Optics® USB4000® spectrometer and a ThorLabs® LED UV light source. Flow rates through the reactor were varied from 0.05ml/min to 2ml/min. A central peak wavelength was registered in the fluorescence profiles for each flow rate. Monodisperse Cd-Se quantum dot solutions were synthesized across a broad spectrum of wavelengths ranging from 490nm to 620nm. An empirical relationship between flow rate and center wavelength was determined.

This dissertation focuses on the solution-state polymer assembly of conjugated polymers with specific attention to nano- and molecular-scale morphology. Understanding how to control these structures holds potential for applications in polymer-based electronics. Optimization of conjugated polymer morphology was performed with three objectives: 1) segregation of donor and acceptor materials on the nanometer length-scale, 2) achieving molecular-scale ordering in terms of crystallinity within distinct domains, and 3) maximizing the number and quality of well-defined donor/acceptor interfaces. Chapter 1 introduces the development of a mixed solvent method to create crystalline poly(3-hexyl thiophene) (P3HT) fibrils in solution. Chapter 2 describes fibril purification and approaches to robust and functional fibrils, while chapters 3 and 4 demonstrate the formation of hybrid nanocomposite wires of P3HT and cadmium selenide (CdSe) nanoparticles by two methods: 1) co-crystallization of free and P3HT-grafted CdSe for composite nanowires and 2) direct attachment of CdSe nanoparticles at fibril edges to give superhighway structures. These composite structures show great potential in the application of optoelectronic devices, such as the active layer of solar cells. Finally, ultrafast photophysical characterization of these polymers, using time-resolved photoluminescence and transient absorption, was performed to determine the aggregation types present in suspended fibrils and monitor the formation and decay of charged species in fibrils and donor-acceptor systems