Welcome to the Zhu research group

Zhu lab & Center for Materials Chemistry:
Electron transfer, Photovoltaics, and Interfacial Science

| Group members | Publications | Collaborators | Funding | Presentations |

Research Programs - we are interested in the physics and chemistry of material interfaces

Solar energy conversion: organic donor-acceptor interfaces Due to the low dielectric constant of organic materials, the formation of a coulombically bound electron-hole pair across an interface, i.e., a charge-transfer (CT) exciton, is a necessary step towards charge separation. We use femtosecond time-resolved two-photon photoemission (2PPE) spectroscopy & crystalline organic semiconductor (e.g., pentacene) thin films as model system to probe CT excitons. We discover a series of CT excitons with binding energies < 0.5 eV. These CT excitons are essentially solutions to the atomic-H like Schrödinger equation with cylindrical symmetry. We conclude that charge separation at organic donor-acceptor interfaces must involve hot CT excitons. Read more about this research project in publications 135 & 140.
Solar energy conversion: quantum dot interfaces Semiconductor quantum dots (QDs) are emerging as key materials for light harvesting in next generation photovoltaics. QDs allow tunable optical absorption, solution processability, & potentially high efficiency. We are investigating how interfacial electronic coupling between quantum dots or between an electron conductor (e.g., ZnO or TiO2) and QDs control charge separation using a variety of tools, such as absorption, emission, and nonlinear optical spectroscopies. In the example shown here, we use monolayer PbSe QD on TiO2(110). We change capping molecules to systematically control interfacial electronic coupling. We provide the first experimental evidence of hot electron transfer from QDs to TiO2. Read more about this research project in publications 144 (& more to come!).
Peeking inside organic electronic devices Organic semiconductor materials are central to a number of emerging technologies, such as organic thin film transistors (OTFTs), orgnic photovoltaics (OPV), and organic light emitting devices (OLEDs). We use in situ spectroscopy to directly probe operating devices built on optical waveguides. The example shows a polythiophene transistor gated with a polymer electrolyte dielectric. In situ spectroscopy clearly shows polarons in the polymer channel under gate bias. This spectroscopic insight allows us to determine the nature of electrostatic and electrochemical doping, the Mott insulator-to-metal transition, and the origin of charge traps and bipolarons in organic devices. Read more about this research project in publications 126 & 136.
Surface chemistry for biochips and microarrays Interfacing manmade materials to biological systems is a common challenge to a number of important fields, including biomaterials, biosensors, microarrays, and nanomedicine. We are designing surface chemistry to rationally control such interfaces and probing these soft interfaces using physical tools. Recent highlights include the development of surface chemistry to optimize the activity of immobilized proteins and allows the “digital” switching of this activity. Another design has led to the successful fabrication of a fluidic and air-stable supported lipid bilayer; this can be the basis for cell mimicking microarrays in high throughput studies, e.g., the screening of nanomedicine targeting cell surface receptors. Read more about this research project in publications 131 & 149.