The McLaurin Research Laboratory
Microwave-Assisted Synthesis of Materials
We seek to evaluate and improve the scientific value of microwave-assisted syntheses of colloidal nanoparticles by developing new methodologies and detailing aspects of the syntheses that explain observed differences from conventional syntheses. Then we can determine and explain observed differences in microwave-assisted syntheses of nanoparticles and use this understanding to develop improved pathways to materials. We can use effects due to rapid heating that alters normal thermodynamic processes but is not connected to an increase in the bulk reaction temperature to synthesize NCs.
The largest, disputed variable in most microwave-assisted syntheses is temperature. It is difficult to measure temperature during microwave reactions as local heating can occur, creating hot-spots. External IR sensors measure the temperature of the reaction vial, not the temperature in solution. Fiber optic temperature probes can measure temperature within the reaction vessel, but can’t give information about thermal gradients. Reaction temperature must be taken into account especially to assess how it affects the reaction products. Some tools currently available for this include a ruby optical thermometer and a SiC microwave reaction vial. The ruby thermometer provides an alternative mechanism for temperature measurement and the SiC vessel acts as a “microwave control” as it should absorb the microwaves and heat the reaction solution as in conventional syntheses.
In non-absorbing solvents like decane or octadecene temperature measurements from the IR sensor generally overestimate the solution reaction temperature as the glass vial is heated more than its contents. Alternatively when the vial contents are strongly absorbing, the IR sensor will tend to underestimate the reaction temperature, measuring the cooler outside glass temperature as opposed to that in solution. Based on obtained values, the ability of the reactants to absorb microwaves can be better assesed and used to inform design and interpretation of microwave-assisted reaction pathways.
Optical sensing with Doped Semiconductor Nanocrystals
It is estimated diabetes now affects 9.8% of the world population. The amount of glucose in blood is used as a clinical indicator of diabetes. Although prevention is key to reduction of this global health issue, detection and management will be increasingly important in the coming years. Electrochemical glucose sensors are commercially available, but require an invasive electrode for detection. Minimally invasive sensors for selective, continuous monitoring of glucose are needed to meet future sensing demands.
Non-invasive optical glucose sensors employ a variety of sensing methods and strategies. Many sensors are designed to report a signal (usually emission of light) related to glucose-binding. Unfortunately, their sensing ranges are small and they are often not selective for glucose. Alternatively, enzymes and coenzymes, such as glucose oxidase (GOx) form or consume metabolites such as oxygen (O2), hydrogen peroxide (H2O2), and protons (H+) in the presence of glucose. These metabolites are easier to detect so I propose detecting glucose using the oxidizing power of H2O2. Oxidation of a luminescent indicator by H2O2 will produce an optical signal change that can be easily monitored.
Semiconductor NCs are popular for optical sensing because of their large, broad one- and two-photon absorptions, tunable absorptions and emissions, and photostability. In addition, many of these nanomaterials have been doped with transition metal (TM) impurities, expanding the range of optical and electronic applications for such materials. Issues with semiconductor dopants often involve the presence of the dopant in an inhomogeneous environment. When a dopant ion is present on the NC surface, its coordination environment differs from that of an interstitial dopant. Conflicting signals often result making surface dopants undesirable and hence they must be prevented or removed. Instead of trying to suppress the presence of surface dopants, I propose using them for redox sensing. Changes in the NC photoluminescence (PL) due to redox reactions of Cu2+/+-doped II-VI semiconductor NCs will optically signal the presence of glucose.
Recent mechanisms hypothesize the color of the Cu luminescence differs with oxidation state (1+ vs 2+).Cu+-doped NCs luminesce through the dopant whereas Cu2+-doped NC luminescence is analogous to that of undoped-NCs. A luminescence color change related to the oxidation or reduction of Cu in doped NCs is an enticing opportunity for glucose sensing. A redox sensing scheme is shown in Figure 1. At the top, GOx oxidizes G and is regenerated by O2,forming H2O2.This oxidizing product then reacts with a Cu+ ion at the NC surface forming Cu2+, resulting in a change in PL color. Thus, in addition to being interesting materials on a fundamental level, Cu-doped NCs offer a sensing mechanism based on changes in the Cu oxidation state.
Donor-Acceptor Processes in Near-Infrared Semiconducting Nanocrystal/Polymer Composites
Rising costs of carbon-based fuels and concern over their long-term environmental effects has spurred research into alternative fuels. Solar energy promises a renewable, accessible source, yet costs remain too high for large scale integration into the energy economy. Photovoltaics, dominated by silicon technologies, have reached efficiencies of 44.7% by capturing solar spectrum light from the ultraviolet to the visible to the infrared. Careful engineering of semiconductor bandgap energies and lattice parameters through alloying was required. Translation of such technologies to semiconductor nanocrystals can incorporate new benefits, namely their solution processability, tunable absorptions, and large surface-to-volume ratios, resulting in a new class of photovoltaics.
Hybrid photovoltaics composed of conducting polymers and semiconductor nanocrystals offer a pathway to the production of solar modules at ultra-low costs. These devices combine the benefits of light absorbing, conducting polymers with stable inorganic, conducting nanocrystals, however, viable solar conversion efficiencies are yet to be realized. This is ascribed to poor conductivity between the polymer and nanocrystal due to the presence of insulating ligands on the nanocrystal surface. These native ligands prevent aggregation during synthesis of the particles, but in turn prevent efficient charge transfer between the nanocrystals and surrounding materials. Strategies to improve device performance to-date generally rely on empirical evidence of the manipulation of the polymer-nanocrystal interaction through film treatment post-assembly and addition of molecular “linkers”. The exact role of these molecular additives are unknown.
Careful control of the polymer-nanocrystal interaction, the interface between the two charge carrier layers, is a logical next step in improving device performance. Never-the-less, only small advances have been made in this area. The results paint a much more complex picture of the polymer-nanocrystal interface than initially anticipated. In addition, the small advances in these hybrid systems are largely in CdSe-based systems, with a few reports of PbS/Se-based hybrid systems. Lessons learned from these systems may not translate well to the III-V materials responsible for the current leading solar cell efficiencies.
III-V semiconductor nanocrystals are notoriously difficult to synthesize. Advances in their synthesis in terms of reduction of size-distribution, increase of quantum yield, and control over bandgap tunability pale in comparison to CdSe-based syntheses. Recent advances in novel synthetic methodologies, namely microwave synthesis, yielded comparable quality nanocrystals in fractions of the time required and with significantly less harsh reaction conditions than the dominant synthetic methods for InP, GaP, and InGaP nanocrystals. Expansion of this synthetic method to other III-V materials, such as InAs, and control over bandgap tunability will provide new synthetic methodologies for obtaining these materials.
Examination and optimization of energy and charge transfer processes affecting these organic-semiconductor systems can improve efficiencies, but the large variation in polymer-nanocrystal interactions makes them difficult to study. We propose new donor-acceptor architectures composed of individual nanocrystals with polymers and their molecular components as a platform to study and optimize energy and charge transfer between the nanocrystal and donor on a molecular level.
Heterostructured Nanomaterials for Photocatalysis
Exploration of small anisotropic heterostructures with well-defined composition and structure has been driven by their unique, tailorable properties related to light absorption, charge separation, and surface chemistry. These properties have implications for applications ranging from photocatalysis and energy storage to sensing and biomimetics. Key to advancement of anisotropic heterostructures is improving their synthesis as it is difficult to tune reaction conditions to form homogenous mixtures of the desired product. I propose synthesis and development of anisotropic heterostructures to better understand how their interfaces (core/arm, semiconductor-metal) affect their electronic properties.
Tuning of bandgap energies and band offsets in semiconductors has resulted in nanostructures with specific electronic properties. A type II band offset yields charge separation that results in a longer lived excited state as compared to a single semiconductor material. In addition, the availability of new morphologies allows separation of carriers and catalytic sites within the same nanostructure. In tetrapods a semiconductor core is surrounded by four arms in a tetrahedral fashion. This asymmetry has been shown to be desirable for longer, more efficient charge separation vs symmetric nanostructures. Combination of type II band offsets with anisotropic structures improves charge separation further.
Addition of metal particles to the ends of anisotropic structures also aids in charge separation and suppresses recombination while presenting a catalytic surface for redox reactions. This structure then acts as a photocatalyst for H2 production in the presence of a sacrificial h+ scavenger. Adding a metal-oxide catalyst to the arms of the tetrapod presents areas for O2 production. Use of different domains within the same nanostructure for photocatalysis takes advantage of the electronic properties of these heterostructures. However, the interfaces between the metal and semiconductor, as well as core and arms, play key roles in catalyst efficiency.
A critical problem hindering use of complex heterostructures are the interfaces between different domains. Although the type II architecture decreases recombination, defects present at the core/arm interfaces as well as semiconductor-metal interfaces enhance recombination. Understanding the charge transfer processes at these interfaces through spectroscopy and microscopy of the semiconductor material is critical for obtaining efficient photocatalysis, as well as enhancing other properties of the material.