In the past decade, there has been increasing interest in the potential of new thin-film solar technologies. Research in this field has focused largely on organic photovoltaics (OPVs) utilizing a bulk heterojunction (BHJ) design, consisting of blended films of conjugated polymer electron donors and fullerene electron acceptors.Use of semiconductor nanocrystals or “quantum dots” as a replacement acceptor material for the fullerene has been proposed as an alternative. In this research, charge transfer between lead sulfide nanocrystals and a conducting polymer, "poly(2,3-didecyl-quinoxaline-5,8-diyl-alt-N-octyldithieno[3,2-b:2′,3′-d]pyrrole) " or PDTPQx is investigated by time resolved spectroscopy. Previous work has found that incident light exciting the polymer results in transfer of an electron from the polymer to lead sulfide nanocrystals present in the blend. This electron transfer leaves a net positive charge on the polymer, which is accompanied by a characteristic spectroscopic signature. It is found in this research that selective excitation of the lead sulfide nanocrystals using an infrared laser also results in observable positive charge carriers on PDTPQx. This indicates that light absorption at the lead sulfide nanocrystals results in transfer of positive charges from the nanocrystals to the polymer. Confirmation of this additional charge transfer process has important implications for the potential this system in photovoltaics.

Organic photovoltaics (OPVs) are a promising solar energy technology due to the possibility of cost-competitive large-scale manufacture. Polymer based devices typically have a bulk heterojunction (BHJ) architecture. However, BHJ materials degrade rapidly under typical operating conditions, contributing to limited device lifetimes. The degradation due to photooxidation of a model OPV system of poly[9,9’-dioctylfluorene-alt-(bis(N,N’-(4-butylphenyl))-bis(N, N’-phenyl-1,4-phenylenediamine)] (PFB) and poly[9,9’-dioctylfluorene-alt-1,4-benzothaidiazole] (F8BT) was studied by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Photooxidation products as a function of increasing photon dose at 405 nm were identified in PFB:F8BT, PFB-only, and F8BT-only films. Degradation kinetics on these films were determined by fitting Gaussian peaks to the IR spectra and measuring changes in peak area as a function of photon dose. Preliminary results suggest that multiple carbonyl species are formed in both polymers under ambient conditions. Analysis of the data collected in this experiment will be used to potentially evaluate the chemical kinetics of the degradation process and the formation of undesired chemical species in order to find a way to extend the lifetime of the device. Gaining insight into what limits the lifetime of current solar cell materials will help to improve the durability of plastic solar cells, and thereby solar energy as a widespread technology.

Solar cells, through their ability to convert energy from light directly into electricity, pose a promising alternative for supplying our world’s growing demand for clean affordable energy. Many factors governing the efficiency of these photovoltaic devices are still being explored and improved upon through methods aimed at boosting photo-voltages, improving energy-conversion efficiency, and finding materials accessible for large-scale production and implementation. Here, I investigate the potential of a new type of solar cell that utilizes electron-impurity Auger de-excitation. I will discuss the effect of Auger de-excitation observed in Mn doped CdS quantum dots and explore the potential for boosting the photo-voltage via the harvesting of hot electrons. I will explain the processes involved in both synthesizing Mn:CdS quantum dots and fabricating quantum dot-based solid-state devices. I will show preliminary results from photoluminescence quenching measurements used to quantify Auger de-excitation in fabricated solid-state devices as well as device properties and characterization measurements. In the future, full-cell PV devices will be constructed and tested against the efficiencies of today’s leading solar cells.

Colloidal semiconductor nanocrystals (NCs), often referred to as quantum dots, are a class of materials that exhibit size dependent photo physical properties. This class of materials has great potential for application in a variety of fields such as photovoltaics, light emitting diodes and spintronics. Doping NCs with transition metals provides additional tunability of the optical and physical properties has been synthesized. By tuning the energy gap of these NCs within thermal reach of the Mn2+ 4T1 excited state a highly temperature sensitive dual emission is acheived. The synthetic route to these dual emitting nanocrystals required the synthesis of a Mn2+:ZnSe core followed by the sequential growth of three shell layers. Doping Mn2+ into a Zn1-xCdxSe allow however, could prove to be a more efficient route to dual emitting nanocrystals with higher stability. Successful incorporation of Mn2+ into Zn1-xCdxSe alloys has resulted in dual emitting NCs.

Fe₃O₄ nanoparticles have been extensively studied for biomedical applications including magnetic resonance imaging, chemotherapeutic delivery and gene delivery due to their advantageous properties of superparamagnetism, biocompatibility and biodegradability. However, ultrafine, monodisperse and sized controlled nanoparticles synthesized by thermal decomposition are commonly encapsulated by hydrophobic layers and can’t be dispersed in aqueous solutions. Besides, during the surface modification of nanoparticles, it is still difficult to obtain a highly stable and dense polymeric surface layer. To this end, a robust multi-step polyethylene glycol(PEG)ylation approach is presented to produce biocompatible multilayer surface coated magnetic iron oxide nanoparticles that are hydrophilic and display long-term stability in biological media. Ultrafine, monodisperse and 12nm hydrophobic nanoparticles were first synthesized by elevated temperature decomposition of nontoxic iron oleate complex in the presence of 1-octadecene and oleic acid. Subsequently, the oleic acid layer of nanoparticles was replaced by 3-(triethoxysilyl)propyl succinic anhydride(SSA) through ligand exchange to form SSA layer coated nanoparticles. Then, during the first step of PEGylation, to prevent the aggregation of nanoparticles, adequate amount of methyl PEG-NH₂ was attached to the part of SSA layer through the coupling reaction of functional groups. And in the second step of PEGylation, NH₂-PEG-NH₂ was attached to SSA to achieve a SSA/PEG dense multilayer surface with an adequate number of functional groups for further conjugation of therapeutics or targeting agents. The tools such as transmission electron microscopy (TEM), dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR) were utilized to characterize the structure and properties of surface modified nanoparticles. The excellent dispensability and stability of surface modified nanoparticles in biological media was confirmed by no significant size change after at least 30 day’s incubation in Dulbecco’s phosphate buffered saline (PBS) and dulbecco’s modified Eagle’s medium (DMEM) with 10% of fetal bovine serum (FBS), demonstrating a huge potential for biomedical applications.

A growing number of applications seek to use electrostatic effects in non-polar media. However, the science behind particle charging in such systems is not fully understood. Previous work has suggested that the presence of surface hydroxyl groups, when combined with certain surfactants, can induce charging on particles. These surfactants also serve to prevent particle aggregation through steric stabilization. However, dispersing these particles typically requires extended sonication or some other dispersive technique. One possible solution for decreasing the dispersion time is to first hydrophobically surface-treat the particles. Unfortunately, this process has the potential to remove the hydroxyl groups necessary for particle charging. Therefore, our group has investigated the effects of surface modification on particle hydrophobicity and chargeability. Silica particles on the micron and submicron scale were surface treated with tri-methoxy(octyl)silane and methoxy(dimethyl)octylsilane in order to create hydrophobic silica particles with varying amounts of hydroxide groups on their surfaces. The resulting surface energy and acid/base characteristics of these particles were then tested using Inverse Gas Chromatography, with the intent of comparing these properties to the particles' chargeability. This was assessed by measuring their electrophoretic mobilities using a Zeta Potential Analyzer. Our results indicated that particles could both be made hydrophobic and chargeable after surface treatments. However, further investigation is required in order to better understand the mechanism behind this charging. If the surface modification of these particles can preserve their chargeability while adding a steric barrier, this process could be used to reduce dispersion time for charged particles in non-polar media.

Dr. Gerald Pollack and lab associates have discovered that when a polymer or gel is placed in a chamber with an aqueous suspension of polystyrene microspheres, the microspheres migrate away from the polymer or gel surface to about ~200-500 micrometers, leaving a microsphere free zone. This region is known as an “exclusion zone” (EZ) and is characterized by crystalline water. We tested the effects of changing the solution temperature on the size of the exclusion zone. A critical aspect of this experiment is what role thermal energy plays, if any, on the exclusion zone. In this experiment, we placed the polymer (Nafion) in an aluminum chamber, which was then filled with microsphere solution (500:1 deionized water to polystyrene microspheres). We then placed this chamber into a larger chamber filled with water. This larger chamber was connected to a water circulator with which we could control the temperature of the water surrounding the aluminum chamber. In our experiments, we varied the temperature of the circulating water at 15 randomly chosen temperatures between 5ËšC and 75ËšC, and measured the size of the EZ. We found that EZ size increases as temperature diminishes. This finding has fundamental implications for understanding basic properties of water, and particularly the EZ phase of water.