Dr. Ken L Knappenberger, Jr.Associate Professor
Ph.D(2005) The Pennsylvania State University Postdoc (2005-2008) University of California, Berkeley and Lawrence Berkeley National Lab
Research InterestDescribing electron- and energy-transfer processes in nanoscale systems is critical not only for understanding fundamental energy redistribution mechanisms in nanoscopic media, but also for developing next-generation devices based on these technologies.
Research in the Knappenberger group involves understanding these processes in nanoscale assemblies through development and implementation of novel optical spectroscopy techniques, including single-molecule and time-resolved methods. The resulting detailed chemical information will address critical issues in nanoscale chemical physics and single-molecule analytical chemistry and direct the assembly of carefully designed nanoscale architectures.
Energy transfer though coupled nanocrystals
For applications such as improved solar energy conversion, electronic energy must be transferred non-radiatively from a photoexcited species to a single catalytic charge-separation site. To approach this, the possibility of electronic energy transfer between multiple semiconducting nanocrystals (quantum dots) of differing bandgaps is being explored. Coupling multiple quantum dots of different sizes yields a photonic device with an intrinsic “optical gradient” that is capable of mimicking the first steps that occur in light-harvesting photosynthetic proteins. Visible photons will be absorbed, but the electronic energy can be funneled by carefully arranging quantum nanostructures with descending bandgap separations. This is advantageous because energy can be efficiently delivered to specific charge-separation sites while decreasing undesirable losses to trap states.
The efficiency of electronic energy transfer is directly reflected in the radiative decay, or fluorescence lifetime, of the donating nanostructure, as well as the signal amplitudes of the donor and acceptor emission. These properties will be measured coincidently in multiple emission-wavelength channels allowing their correlation. Fluorescence lifetime imaging (FLIM), which is capable of time-resolved detection of single-molecule emission, is especially suited to study energy flow on nanometer-length scales. This technique has the inherent advantage of revealing the influence of local heterogeneity on the dynamics and, thereby, providing information on the distribution of active sites, which would be lost in an ensemble average.
Metal-enhanced light harvesting
Nanoparticles often portray strikingly different chemical and physical properties than their bulk counterparts, and, perhaps more intriguingly, these vary widely with particle size and shape. One example of this behavior in metal nanoclusters is the surface plasmon resonance (SPR) phenomena, which provides selective enhancement of molecular spectroscopies such as surface-enhanced Raman spectroscopy (SERS) and metal-enhanced fluorescence (MEF). In addition, surface plasmons can deliver polarized fluorescence in an electron wave over distances spanning tens to hundreds of nanometers. This effect may enhance electron and electronic energy transfer efficiency in artificial light-harvesting arrays, thereby leading to more efficient photovoltaic devices. The influence of nanoparticle-molecule spatial orientation, distance and spectral overlap on energy transfer will be studied with femtosecond transient absorption and single-molecule fluorescence techniques.
Fluorescence lifetime measurements performed at the single-molecule level are very powerful because they can reveal coupling with the environment. However, these measurements are generally limited to sub-picosecond timescales. Energy transfer in coupled nanoparticles can be very efficient and occur in fewer than tens of picoseconds, meaning that many of the defining processes may go undetected. Thus, part of the proposed research will involve efforts to improve the temporal resolution of single-molecule methods. One especially promising method is the use of stimulated emission-depletion fluorescence microscopy, in which two temporally-delayed pulses interact with the chromophore prior to emission. As a result, the time-domain information is limited only by the temporal duration of the laser pulse.
|Dowgiallo AM, Knappenberger KL. Ultrafast electron-phonon coupling in hollow gold nanospheres. Phys Chem Chem Phys. 2011, Nov 4. [Epub ahead of print]|
|Dowgiallo AM, Schwartzberg AM, Knappenberger KL. Structure-dependent coherent acoustic vibrations of hollow gold nanospheres. Nano Lett. 2011, Aug 10;11(8):3258-62|
|Attar AR, Blumling DE, Knappenberger KL Jr. Photodissociation of thioglycolic acid studied by femtosecond time-resolved transient absorption spectroscopy. J Chem Phys. 2011, Jan 14;134(2):024514|
|Chandra M, Dowgiallo AM, Knappenberger KL Jr. Controlled plasmon resonance properties of hollow gold nanosphere aggregates. J Am Chem Soc. 2010, Nov 10;132(44):15782-9|
|Priyam A, Blumling DE, Knappenberger KL Jr. Synthesis, characterization, and self-organization of dendrimer-encapsulated HgTe quantum dots. Langmuir. 2010, 2010 Jul 6;26(13):10636-44|