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Chemical and Nanoscale Science

Learn about our research staff including staff profiles, publications, and contact information.

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

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 LettersComparing 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

Illustration of gray cube on left labeled as quantum dot related to a diagram to the right that has five lines (energy levels) in a top set and five lines (energy levels) in a bottom set, with the two sets separated by a gap labeled as Egap. Three small red solid circles (electrons) on second energy level up (labeled first electron energy level) in top set and three small green open circles (holes) on second energy level down (labeled first hole energy level) in bottom set. Arrows from two holes to two electrons; arrows are labeled as multiple exciton generation. Electron in fifth level up in top set has arrow down to third electron. Hole in fifth energy level down in bottom set has arrow up to third hole. Wavy line with arrow points from lowermost hole to uppermost electron and is labeled hv. Another label says One photon yields thee electron-hole pairs.Graph of quantum yield on y-axis (from 1.0 to 4.0 electrons/photon) and E/Eg on x-axis (from 2 to 6). Six data types are plotted: TCE, untreated, by CH3CN, EtOH, meEtOH, and EDT CH3CH. Points of three data types are connected by lines that show trend curving upward from lower left toward upper right. A stair-step line is also drawn on the plot, with straight-line segments between the following: (2,1) to (2.2), (2,2) to (3,2), (3,2) to (3,3), (3,3) to (4,3), (4,3) to (4,4), and (4,4) to (5,4).

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.

 Illustration showing on left two stacked ball-and-stick molecular models offset horizontally slightly, and same set of models on right. On left models, the lower molecule has gray arrows pointing up and down, and top molecule has red arrows, with one up and one down. On right models, the lower molecule has red arrows, both pointing up, and top molecule has two red arrows, both pointing down. Curving arrow above points from left set to right set. Left end is labeled |S0S and right end is labeled |T1T1Graph with ΔOD x 103 on y-axis (from -5 to 10) and time delay on x-axis (from 0 to 200 ps). Red curve grows logarithmically from left to right, blue curve jumps up to y=5 near left then curves down toward y=0 line, black line jumps down to y=-5 near left then gradually curves down slightly to right. Inset graph shows bleach fraction versus time delay (ps). The one plotted line curves smoothly from from about (0,-1) to about (200, -1.8).

Multiple-exciton generation by singlet fission in a novel molecule was shown to lead to a doubling of the number of generated excitons per absorbed photon. This can potentially lead to radically improved solar conversion efficiency of cells based on this idea.

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.

Graph with binding energy on y-axis (-0.3 to 0 eV) and carrier separation distance on x-axis (-150 to 150 angstroms). A trumpet-bell-shaped curve pointing upwards is labeled XSC, epsilon=4. A narrower but similar-shaped curve is nested in the other curve and is labeled CSC, epsilon=15. A flattened oval near the top of the graph is yellow on the left and orange on the right. A much smaller flattened oval with same color pattern is near the bottom of the graph; both are labeled electron wavefunctions Illustration showing two molecular diagrams: the center of each has a cluster of seven hexagonal rings with zigzag tails extending to the right and left. The top diagram is labeled PPEEB and the lower diagram is labeled n-type dopant. The diagrams are similar, except the lower one has a circled negative sign in the center of the center hexagonal ring and a circled plus sign on the right end

Intentional doping of a designed perylene shows that a large density of adventitious dopants control the conductivity of normal perylenes.

Nanostructured Oxides and Chalcogenides

Two photos. Top and bottom photos are labeled a and b, respectively. Both have a 200 nanometer scale bar and show roughly circular cross-sections of clusters of tubes, with tens of tubes per each cluster.

Designed arrays of TiO2nanorods are coated by atomic-layer deposition with an In2S3 light-absorbing layer. This system can be used to create efficient semiconductor-sensitized solar cells and shows interesting charge-transport behavior.

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.

Carbon Nanostructures

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.