The 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.
Scientists in the Cahoon and Papanikolas groups, as published in the Journal of Physical Chemistry C, have studied ultrafast carrier dynamics in silicon nanowires with average diameters of 40, 50, 60, and 100 nm using transient absorption spectroscopy. After 388 nm photoexcitation near the direct band gap of silicon, broadband spectra from 400 to 800 nm were collected between 200 fs and 1.3 ns. The transient spectra exhibited both absorptive and bleach features that evolved on multiple time scales, reflecting contributions from carrier thermalization and recombination as well as transient shifts of the ground-state absorption spectrum. The initially formed “hot” carriers relaxed to the band edge within the first 300 fs, followed by recombination over several hundreds of picoseconds.
The charge carrier lifetime progressively decreased with decreasing diameter, a result consistent with a surface-mediated recombination process. Recombination dynamics were quantitatively modeled using the diameter distribution measured from each sample, and this analysis yielded a consistent surface recombination velocity of 2 × 104 cm/s across all samples. The results indicate that transient absorption spectroscopy, which interrogates thousands of individual nanostructures simultaneously, can be an accurate probe of material parameters in inhomogeneous semiconductor samples when geometrical differences within the ensemble are properly analyzed.
In a collaboration between the Cahoon and Papanikolas groups, published in the Journal of Physical Chemistry C, ultrafast charge carrier dynamics in silicon nanowires (NWs) grown by a vapor–liquid–solid mechanism were interrogated with optical pump–probe microscopy. The high time and spatial resolutions achieved by the experiments provide insight into the charge carrier dynamics of single nanostructures. Individual NWs were excited by a femtosecond pump pulse focused to a diffraction-limited spot, producing photogenerated carriers (electrons and holes) in a localized region of the structure. Photoexcited carriers undergo both electron–hole recombination and diffusional migration away from the excitation spot on similar time scales. The evolution of the carrier population is monitored by a delayed probe pulse that is also focused to a diffraction-limited spot. When the pump and probe are spatially overlapped, the transient signal reflects both recombination and carrier migration. Diffusional motion is directly observed by spatially separating the pump and probe beams, enabling carriers to be generated in one location and detected in another.
Quantitative analysis of the signals yields a statistical distribution of carrier lifetimes from a large number of individual NWs. On average, the lifetime was found to be linearly proportional to the diameter, consistent with a surface-mediated recombination mechanism. These results highlight the capability of pump–probe microscopy to quantitatively evaluate key recombination characteristics in semiconductor nanostructures, which are important for their implementation in nanotechnologies.
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 13457â€“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 5865â€“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 GonzÃ¡lez, and Juan Z. DÃ¡valos. J. Am. Chem. Soc., 2013, 135 (26), pp 9681â€“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 5031â€“5042.