- Aug 13, 2017
- Active Faculty
Condensed Matter Physics - Experimental
Biomedical-Physical Sciences Bldg.
567 Wilson Rd., Room 4214
B110 Biomedical-Physical Sciences Bldg.
J.A. McGuire*, M. Sykora*, I. Robel, L.A. Padilha, J. Joo, J.M. Pietryga, and V.I. Klimov, "Spectroscopic Signatures of Photocharging due to Hot-Carrier Transfer in Solutions of Semiconductor Nanocrystals under Low-Intensity Ultraviolet Excitation," ACS Nano 4, 6087 (2010). (*equal contributors)
M. Mueller, X. Yan, J.A. McGuire, L.S. Li, "Triplet States and Electronic Relaxation in Photo-Excited Graphene Quantum Dots," Nano Letters 10, 2679 (2010).
J.A. McGuire, J. Joo, J.M. Pietryga, R.D. Schaller, and V.I. Klimov, "New Aspects of Carrier Multiplication in Semiconductor Nanocrystals," Accounts of Chemical Research 41, 1810 (2008).
V.I. Klimov, S.A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J.A. McGuire, and A. Piryatinski, "Single-Exciton Optical Gain in Nanocrystals," Nature 447, 441 (2007).
J.A. McGuire and Y.R. Shen, "Ultrafast Vibrational Dynamics at Water Interfaces," Science 313, 1945 (2006).
Professional Activities & Interests / Biographical Information
nanoscience, surface science, nonlinear and ultrafast optics
Our research is focused on the optical properties of reduced dimensional and nanoscale systems, particularly the effects of confinement on the interactions between different degrees of freedom (charge, spin, vibrational) as manifested in electronic and vibrational dynamics. We typically address these problems via ultrafast nonlinear optical techniques. Coherent nonlinear optical processes are especially sensitive to the symmetries of a system, while optical techniques are among the few ways to directly access dynamics on the sub-picosecond timescales that characterize many dynamic processes in condensed phases.
We study a variety of systems. One part of our research is focused on nanoscale systems exhibiting strong electronic confinement. Systems of interest include doped and heterostructured semiconductor nanocrystal quantum dots. Our most recent work has focused on studying the electronic properties of colloidal graphene quantum dots, nanoscale particles of sp2-hybridized carbon with narrow size dispersion, in which confinement opens a large gap from the otherwise gapless extended graphene band structure (Figure 1).
We are also interested in dynamic processes at surfaces. In particular, we are working on extending our earlier studies of the relaxation dynamics of vibrational excitations at water interfaces, where the hydrogen bond network is essential to numerous physical, chemical, and biological phenomena and leads to many unusual properties of water including sub-picosecond relaxation dynamics (Figure 2). Other systems of interest include oxides showing complex phase diagrams and couplings between multiple degrees of freedom (e.g., spin and charge degrees of freedom as in multiferroic materials).
Figure 1: Colloidal graphene quantum dot (A) consisting of 132 conjugated carbon atoms (illustrated in blue) and ligands (black) that undergoes internal conversion (IC) and intersystem crossing (ISC) (illustrated in the Jablonski diagram in B) after optical excitation. S0 represents the ground electronic state and Sn and Tn (n>0) represent singlet and triplet excited states. Relaxation is followed by fluorescence (Fl., panel C) and phosphorescence (Phos., panel D). (From Mueller et al., Nano Lett. 10, 2679 (2010).)
The optical techniques that we use vary according to the system and problem. For surface studies, second-order nonlinear optical techniques are particularly powerful. In cases where the bulk system is centrosymmetric and the dipole approximation holds, even-order nonlinear optical processes (i.e., those involving an even number of interactions with the input fields) are surface specific. An example of such a process is infrared-visible sum-frequency generation in which one IR photon and one visible photon are destroyed and a photon at the sum frequency (?SF=?vis+?IR) is created. For understanding dynamics at surfaces we must extend these processes at least to fourth order. Similarly, we can use single- and two-photon optical transitions to probe the low-energy electronic structure of nanoscale systems and determine the symmetries of different excitonic states.
Figure 2: Measurement of vibrational dynamics at water surfaces. A: Schematic of one possible input and output beam geometry in a pump-probe measurement of surface vibrational dynamics using a sum-frequency (SF) probe. B: Illustration of the corresponding photon energies and delays. g and e indicate electronic ground and excited states, and the dashed horizontal line indicates a virtual state. C: False-color diagram of ultrafast vibrational relaxation of the OH-stretch vibration at the fused-silica/H2O interface. The panel on the left shows the experimental sum-frequency signal generated by mixing a 100 fs infrared (IR) probe beam with a 100 fs pulse of 800 nm light after excitation with a separate 100 fs pulse of 3400 cm-1 IR. The signal is normalized by the SF response in the absence of the IR pump. Excitation at 3400 cm-1 creates a broadband excitation of the OH-stretch modes reflected in the appearance of the red "hole" in the SFG spectrum. The excited population subsequently decays on a ~300 fs timescale followed by thermal weakening of the hydrogen-bond network on a ~700 fs timescale resulting in a blue shift of the OH-stretch spectrum reflected in the long-lived (red) "hole" on the low-energy side of the spectrum and the (blue) increase of the SFG signal on the high-energy side of the spectrum. The panel on the right shows the results of a fit to such a two-timescale model. (Modified from J.A. McGuire and Y.R. Shen, Science 313, 1945 (2006).)