The primary goal of the Chemical and Nanoscale Science Group, within NREL's Chemical and Materials Science Center, is to understand photoconversion processes in nanoscale, excitonic photoconversion systems, such as semiconductor quantum dots, molecular dyes, conjugated molecules and polymers, nanostructured oxides, and carbon nanotubes. Closely associated with this goal are efforts to gain an understanding of how to use chemistry and physical tools to control and maximize the photoconversion process. The innovative chemistry and physics that evolve from these fundamental studies are used on a number of applied projects, maximizing the benefits from these discoveries.
Our funding is primarily from the DOE Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences and through the Center for Advanced Solar Photophysics—an Energy Frontier Research Center co-led by Los Alamos National Laboratory and NREL. Additional funding is available through NREL's National Center for Photovoltaics, as well as external agencies such as the Defense Advanced Research Projects Agency (DARPA).
We discuss details of our research under the following topics:
- Unique Tools
- Multiple Charge Pairs from a Single Photon
- Molecular Organic Semiconductors
- Nanostructured Oxides and Chalcogenides
- Carbon Nanostructures.
The Chemical Science team uses many state-of-the-art, investigative tools that employ high time- and energy-resolution, high spatial resolution, or all of these combined to study the conversion of light to charges or chemical species. Examples of these are transient absorption spectroscopy, transient photocurrent or photovoltage, and plasmon-resonance imaging. We also collaborate at the Office of Science user facilities at Brookhaven National Laboratory, Stanford Linear Accelerator Center, Argonne National Laboratory, Oak Ridge National Laboratory, and other scientific centers.
At NREL, two major investigative tools in use are transient microwave conductivity (TRMC) and terahertz spectroscopy (THz).
Time-Resolved Microwave Conductivity. TRMC uses the interaction of mobile charge carriers with microwave radiation to probe their number and mobility as a function of time after photogeneration. This technique facilitates the study of processes such as carrier trapping and recombination, exciton annihilation and quenching, and more. The technique is applicable to a variety of the low-mobility material systems studied in the NREL Chemical and Nanoscale Science Group: conjugated polymers, small organic molecules (e.g., molecular light-harvesting chromophores and fullerene derivatives), single-walled carbon nanotubes, and semiconductor nanocrystals. A tutorial presentation was developed to provide information about the measurement technique, data analysis, and wealth of information that can be extracted using TRMC. In addition, several TRMC case studies conducted at NREL are provided to delve further into this investigative tool.
Terahertz Spectroscopy. THz employs terahertz radiation instead of microwaves to get information similar to that obtained by TRMC, but much faster. Whereas TRMC is limited to nanosecond resolution, THz goes to femptosecond timescales. THz also gives phase information for understanding the carrier transport mechanism in detail—something that is very important, for example, in single-wall carbon nanotubes.
Multiple Charge Pairs from a Single Photon
Our research strategy addresses a wide range of scientific disciplines, including molecular synthesis as a tool for controlling the physical properties of the systems studied, as well as computational chemistry to predict energetic and electronic properties. The Chemical Science team has world-class expertise in the synthesis of II-VI, III-V, and IV-VI colloidal quantum dots, quantum rods, and other composite nanostructures. These systems are studied either by themselves in solution, coupled in larger arrays, or combined with systems such as conjugated polymers. Carrier dynamics, impact ionization, and multi-exciton generation (MEG) are studied using time-resolved spectroscopic techniques such as transient absorption of light and microwaves and terahertz spectroscopy. See the Chemical Reviews article Semiconductor Quantum Dots and Quantum Dot Arrays and Applications of Multiple Exciton Generation to Third-Generation Photovoltaic Solar Cells, and these articles on Nano Letters: Comparing Multiple Exciton Generation in Quantum Dots To Impact Ionization in Bulk Semiconductors: Implications for Enhancement of Solar Energy Conversion and Variations in the Quantum Efficiency of Multiple Exciton Generation for a Series of Chemically Treated PbSe Nanocrystal Films
Colloidal quantum dots are also coupled with: (1) self-assembling proteins to help understand inter-quantum dot communication, and (2) single-wall carbon nanotubes and conjugated polymers to investigate exciton and photoinduced electron-transfer mechanisms. We also study singlet fission in organic systems, which is analogous to multiple-exciton generation in quantum dots. See the Journal of Physical Chemistry article Toward Designed Singlet Fission: Electronic States and Photophysics of 1,3-Diphenylisobenzofuran for more.
Molecular Organic Semiconductors
Molecules such as perylenes, porphyrins, and phthalocyanines are synthesized with substituents to control the electronic structure that can promote and control charge-carrier transport in thin films. The goal is to develop a new class of efficient, excitonic solar cells. A major goal in this effort is to understand and control doping in organic systems, as well as to understand the consequences of doping to solar energy conversion using organic semiconductors.
Nanostructured Oxides and Chalcogenides
We are investigating aspects of charge-carrier generation, mobility, and transport in nanostructured metal oxides with a combination of both experiment and theory using Monte-Carlo simulations and time-of-flight techniques. Such nanostructured oxides are used in dye-sensitized solar cells, as well as in charge-storage applications such as supercapacitors and batteries. They can also be used as scaffolding for water-splitting catalysts. A major goal of our group is to create ordered oxide nanostructures and to study the impact of such order on charge transport and recombination to redox species in the surrounding electrolyte or solid-state conductor. See the Journal of Physical Chemistry article In2S3 Atomic Layer Deposition and Its Application as a Sensitizer on TiO2 Nanotube Arrays for Solar Energy Conversion.
Research into the electronic structure of single-wall carbon nanotubes and graphene using photoluminescence spectroscopy is combined with studies on how they interact with molecules, semiconducting polymers, and colloidal quantum dots using quantum chemical calculations and newly developing approaches such as time-resolved microwave conductivity. Our team has world-renowned capabilities to synthesize carbon nanotubes at high purity and to type-sort the tubes based on chirality. We also study the impacts of doping of these carbon nanosystems.
For staff profiles, publications, and contact information, see the Chemical and Materials Science staff page.