Department of Chemistry

Physical and Theoretical Chemistry

Research ImageThe Department of Chemistry at the University of North Carolina at Chapel Hill, offers a wide range of research opportunities in theoretical and experimental physical chemistry. Our program has broadened from its traditional areas of excellence in molecular chemical physics to include research activities in biophysical and surface chemistry, and materials and environmental sciences. Experimental efforts within these areas utilize state-of-the-art instrumentation, such as high-resolution and ultra-fast laser systems, molecular beam techniques, mass spectrometry, ion-scattering, scanning probe microscopy, and magnetic resonance spectrometry.

Research in theoretical chemistry involves developing computational models of chromatin, the structure of complex fluids, and polymer dynamics. Students at UNC have access to high-performance computer workstations, as well as RENCI/UNC Research Computing, which is home to one of the best computing facilities in the world, including a 4160-processor Dell Linux cluster.

 

Recent Research Highlights

Ultrafast Electron Transfer

Kinetic models based on Fermi's Golden Rule are commonly employed to understand photoinduced electron transfer dynamics at molecule-semiconductor interfaces. Implicit in such second-order perturbative descriptions is the assumption that nuclear relaxation of the photoexcited electron donor is fast compared to electron injection into the semiconductor. This approximation breaks down in systems where electron transfer transitions occur on 100-fs time scale. In an article published in the Journal of Chemical Physics, researchers in the Moran and Kanai Groups present a fourth-order perturbative model that captures the interplay between time-coincident electron transfer and nuclear relaxation processes initiated by light absorption.

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The model consists of a fairly small number of parameters, which can be derived from standard spectroscopic measurements, for example linear absorbance or fluorescence, and/or first-principles electronic structure calculations. Insights provided by the model are illustrated for a two-level donor molecule coupled to both (i) a single acceptor level and (ii) a density of states, DOS, calculated for TiO2 using a first-principles electronic structure theory. These numerical calculations show that second-order kinetic theories fail to capture basic physical effects when the DOS exhibits narrow maxima near the energy of the molecular excited state. Overall, the team concludes that the present fourth-order rate formula constitutes a rigorous and intuitive framework for understanding photoinduced electron transfer dynamics that occur on the 100-fs time scale.

 

Imaging Charge Separation in Nanowires

Silicon nanowires incorporating p-type/n-type (p n) junctions have been introduced as basic building blocks for future nanoscale electronic components. Controlling charge flow through these doped nanostructures is central to their function, yet our understanding of this process is inferred from measurements that average over entire structures or integrate over long times.

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Published in Nano Letters, researchers from the Cahoon and Papanikolas Groups describe how they used femtosecond pump-probe microscopy to directly image the dynamics of photogenerated charge carriers in silicon nanowires encoded with p-n junctions along the growth axis. Initially, motion is dictated by carrier-carrier interactions, resulting in diffusive spreading of the neutral electron-hole cloud. Charge separation occurs at longer times as the carrier distribution reaches the edges of the depletion region, leading to a persistent electron population in the n type region. Time-resolved visualization of the carrier dynamics yields clear, direct information on fundamental drift, diffusion, and recombination processes in these systems, providing a powerful tool for understanding and improving materials for nanotechnology.

 

Representative Publications

Hierarchically-Structured NiO Nanoplatelets as Mesoscale p-Type Photocathodes for Dye-Sensitized Solar Cells. Cory J. Flynn, EunBi E. Oh, Shannon M. McCullough, Robert W. Call, Carrie L. Donley, Rene Lopez, and James F. Cahoon. J. Phys. Chem. C, 2014, 118 (26), pp 14177–14184.

Modeling Time-Coincident Ultrafast Electron Transfer and Solvation Processes at Molecule-Semiconductor Interfaces. Lesheng Li, Paul G. Giokas, Yosuke Kanai, and Andrew M. Moran. J. Chem. Phys. 140, 234109 (2014).

Imaging Charge Separation and Carrier Recombination in Nanowire p-i-n Junctions Using Ultrafast Microscopy. Michelle M Gabriel , Erik Grumstrup , Justin R. Kirschbrown , Christopher W. Pinion , Joseph D Christesen , David F. Zigler , Emma M Cating , James F. Cahoon , and John Michael Papanikolas. Nano Lett., Just Accepted Manuscript, DOI: 10.1021/nl5012118, Publication Date (Web): May 27, 2014.

Ultrafast Carrier Dynamics of Silicon Nanowire Ensembles: The Impact of Geometrical Heterogeneity on Charge Carrier Lifetime. Erik M. Grumstrup , Emma M. Cating , Michelle M. Gabriel , Christopher W. Pinion , Joseph D. Christesen , Justin R. Kirschbrown , Ernest L. Vallorz III, James F. Cahoon, and John M. Papanikolas. J. Phys. Chem. C, 2014, 118 (16), pp 8626–8633.

Ultrafast Carrier Dynamics in Individual Silicon Nanowires: Characterization of Diameter-Dependent Carrier Lifetime and Surface Recombination with Pump-Probe Microscopy. Erik M. Grumstrup , Michelle M. Gabriel , Emma M. Cating , Christopher W. Pinion , Joseph D. Christesen , Justin R. Kirschbrown , Ernest L. Vallorz III, James F. Cahoon, and John M. Papanikolas. J. Phys. Chem. C, 2014, 118 (16), pp 8634–8640.

Free Energy Barrier for Melittin Reorientation from a Membrane-Bound State to a Transmembrane State. Sheeba J. Irudayam, Tobias Pobandt, and Max L. Berkowitz. J. Phys. Chem. B, 2013, 117 (43), pp 13457–13463.

Interplay between Vibrational Energy Transfer and Excited State Deactivation in DNA Components. Brantley A. West, Jordan M. Womick, and Andrew M. Moran. J. Phys. Chem. A, 2013, 117 (29), pp 5865–5874.

Uncovering Molecular Relaxation Processes with Nonlinear Spectroscopies in the Deep UV. Brantley A. West, Brian P. Molesky, Paul G. Giokas, Andrew M. Moran. Chemical Physics, Volume 423, 23 September 2013, Pages 92-104.

Gas Phase Acidity Measurement of Local Acidic Groups in Multifunctional Species: Controlling the Binding Sites in Hydroxycinnamic Acids. Andres Guerrero, Tomas Baer, Antonio Chana, Javier González, and Juan Z. Dávalos. J. Am. Chem. Soc., 2013, 135 (26), pp 9681–9690.

Melittin Creates Transient Pores in a Lipid Bilayer: Results from Computer Simulations. Kolattukudy P. Santo , Sheeba J. Irudayam , and Max L. Berkowitz. J. Phys. Chem. B, 2013, 117 (17), pp 5031–5042.