"On-Wire Lithography" (OWL) utilizes anodized aluminum oxide templates to create 1-D structures with both positive and negative features made from metallic, semiconducting, or polymeric materials. OWL can be used to synthesize multi-component structures, where one component can absorb light and non-linearly transfer energy to an adjacent nanostructure, not capably of directly absorbing the light. These unique structures, which can be easily fabricated by OWL, are opening up a new area of energy research based upon plasmon-enhanced catalysis. These catalyzed reactions can be used to create chemical fuels or function as components of fuel cell or water- splitting devices.

By utilizing anisotropic nanoparticles, different crystal structures can be assembled, where the most stable structures are those that maximize parallel face-to-face interactions between particles. Nanoprisms, nanorods, rhombic dodecahedra, and octahedra have been assembled into ordered 1-, 2-, and 3-D crystallographic arrangements, respectively. Moreover, the interplay between the DNA length and nanoparticle anisotropy provides a means to control the phase behavior of the colloidal superlattices.

DNA-programmed nanoparticle crystallization processes are governed by predictable and mathematically definable relationships between particle size and DNA length. These relationships allow us to program the lattice parameters of DNA-NP crystals using both DNA length and NP diameter. Furthermore, there is a relationship between NP size, DNA length, and the ability to form ordered crystal structures. Small angle X-ray scattering (SAXS) experiments show strong correlation between experimentally- formed FCC crystals and a perfect FCC model.

This patterning methodology utilizes PPL to transfer block copolymer inks to an underlying substrate; these templated structures can then be used to form kinetically stabilized individual nanoparticles of defined sizes over a large areas. This in turn enables the synthesis of a library of particle structures that can be used as a test-bed for the optimization of catalytic and plasmonic properties.

Our group very recently discovered a novel method to spontaneously form hexagonally packed nanoscale filament arrays from relatively dilute aqueous solutions of self-assembling organic molecules. The goal of the current project is to entrap three-dimensional hybrid assemblies with the expectation that some of them will have architectures of interest for photovoltaics or thermoelectric behavior.

The use of light to alter the conductance of CdSe quantum dot (QD) films will allow the production of devices which consist of layers of QDs whose mode of operation switches from thermoelectric operation (generate a current based on a change in temperature) to photovoltaic operation (generate a current from light irradiation) in response to IR or UV light, respectively. These devices can generate electricity during the day and night, increasing the power output of the films, thus lowering their cost and improving their applicability. As a preliminary investigation into this phenomenon, an adaptable diarylethene photoswitch — a molecular entity capable of reversibly switching between a low conductance state and a high conductance state when irradiated with different wavelengths of light — has been synthesized and used to connect a network of CdSe QDs. The current density of the cross-linked films can be enhanced by a factor of 22 when the ligand is switched from its open, non-conductive conformation to its closed, conductive geometry. The ligand is opened by illumination with visible light (500-650 nm) and closed with UV light (300-400 nm).  The cross-linked films are robust and show consistent switching through six on/off cycles.

Ferroelectric materials are an important class of advanced functional materials that are widely recognized for their ability to retain two distinct states of polarization — dipoles per unit volume — that can be reversibly switched through the application of an external electric field. With this distinctive bistability, ferroelectric capacitors are currently utilized in a wide range of unique applications including RAM memory architectures, sensors in ultrasound imaging, and detectors in infrared cameras. By moving away from hard-to-process ceramic ferroelectrics and turning to solution processable organic molecules, low cost and energy efficient ferroelectric technology can be realized. A general design strategy for organic molecules has been developed for the rapid growth of large multi-component ferroelectric crystals. In order to meet this challenge, molecules were tailored with flexible appendages or ‘arms’. These arms are capable of forming hydrogen bonds which lock smaller neighboring molecules into place within the crystal lattice. Several of these co-crystals have been shown to exhibit polarization hysteresis — at ambient temperatures and pressures — a distinct indicator of ferroelectric bistability.  Ferroelectricity in multi-component organic systems is quite rare and we have shown that this platform is successful at producing functional materials, like ferroelectrics, that are quite valuable for applications in emerging technologies.

The goals of this project are to create complexes between semiconductor quantum dots (QDs) and organic ligands where the ligands strongly perturb the electronic and optical properties of the quantum dot. In this case, the ligand – phenyldithiocarbamate (PTC) – decreased the energy of the excited state of the quantum dot by increasing the space in which the electrons can delocalize. This delocalization only occurs upon photoexcitation, so it is a non-equilibrium property of the system. In a film of QDs, this delocalization increases the electronic coupling between the QDs, and therefore increases the electrical conductivity of the film in its photoexcited state. Plots of the change in absorption energy of the QDs (of a range of radii) when coordinated by PTC vs. the physical radius of the QD (black), and the corresponding change in apparent excitonic radius (red). Note that the delocalization radius (shown in pink in the schematic on the right) does not depend on QD size.

The goal of this project is to create an array of semiconductor quantum dots (QDs) with photoconductivity that is switchable by tuning the wavelength of light to which the material is exposed. We accomplish this by using a cross-linking ligands that has a wavelength-sensitive conformation – one conformation is conjugated and conductive, and the other is non-conjugated and non-conductive.The sensitivity of the conformation of the DAE ligand to the wavelength of light (left) imparts a photo- switchability to the photoconductivity of QD arrays, where the QDs are crosslinked by the ligand (right).

Measurement of equilibrium constants for binding between ligands, especially weakly binding ligands, and small (<10 nm) colloidal particles is notoriously difficult. We have accomplished this with systems of semiconductor quantum dots (~3 nm in diameter) and aniline molecules using NMR spectroscopy. We have found that the subsitutent on the aniline does not affect its binding strength with the QDs, and we use our measured equilibrium constants to predict the effect of these molecules on the fluorescence of the QDs as a function of their surface coverage.

This project is concerned with utilizing gold nanorods that are fabricated with electrochemical methods for solar energy applications.  The fabrication of these rods can be controlled such that assemblies of the rods can form preordained curved structures (see figure) that can be used to position the rods to capture sunlight.  In addition, the optical properties of these structures allow for unique energy transport capabilities of relevance to solar fuels.

Gold nanorods are of great interest for plasmon-driven photochemistry applications as the plasmon resonance wavelength can be controlled as one wishes by varying aspect ratio and rod diameter. In this project we have used the OWL technique to make a wide range of nanorod structures, and then a combined theory/experiment study has determined the variation of plasmon wavelength and width.  Of particular interest is that the plasmon width narrows as plasmon is red-shifted, indicating that plasmon-enhanced chemistry can be developed to take advantage of often wasted red and near-IR wavelengths of solar radiation.

Wet chemistry methods provide important opportunities to synthesize nanoparticles that are tailored for plasmon-enhanced photochemistry as the synthesis can usually be scaled to bulk quantities. However control of nanoparticle shape and size is a challenge. In this work we demonstrate how to use a Ag-assisted, seed-mediated growth method that makes gold bipyramids and rhombic dodecahedra with excellent shape and size control.   Theory and experiment are in excellent agreement for the optical properties.

We used the photocatalytic reduction of Ag salt to Ag NPs to test the enhanced photocatalytical activity of our particles. The green curve is the UV-Vis spectrum of formed Ag NPs upon using hybrid Fe₂O₃-Pd NP as catalysts and the blue curve is the spectrum upon using pure Fe₂O₃ as catalysts. The peak at about 450 nm is the surface plasmon band of Ag NPs. The higher the peak, the more the Ag NPs are produced, which means the higher activity of the catalyst used. We can see the hybrid Fe₂O₃-Pd NPs show much higher activity than Fe₂O₃ NPs without Pd domain. To quantify the enhanced photocatalytic activity, we monitored the increase of the photo-generated Ag NPs with the increase of irradiation time in presence of hybrid Fe₂O₃-Pd NPs as shown by this green line or Fe₂O₃ NPs shown by the blue line. The photocatalytic activity of hybrid Fe₂O₃-Pd NPs was enhanced by about 6 times compared with Fe₂O₃ NPs in this case. We also did the recycle of the catalysts. This plot shows that our hybrid Fe₂O₃-Pd NP photocatalysts are very stable and robust. The photocatalytical activity did not decrease much for many circles.

This is the project of Ru containing MOF photocatalysts. We expect that Ru containing MOFs can solve the multiple electron issue of CO2 reduction because lots of Ru(bpy)₃ complex are hold closely inside MOF crystal matrix. The synthesis of Ru-MOF is shown in the image on the left. We put Ru containing MOFs in acetonitrile solution in presence of methyl viologen cation  and electic sacrifical reagent TEOA. When shining visible light, the solution color quickly changed from colorless to dark blue indicating the formation of MV^+• because of the electron transfer from excited Ru complex to MV²⁺. This is a promising initial result and more experiments are on their way.