Doctor of Philosophy, University of California Berkeley (2016)
Bachelor of Arts, Carleton College (2011)
Nonequilibrium Thermodynamics of Colloidal Gold Nanocrystals Monitored by Ultrafast Electron Diffraction and Optical Scattering Microscopy.
Metal nanocrystals exhibit important optoelectronic and photocatalytic functionalities in response to light. These dynamic energy conversion processes have been commonly studied by transient optical probes to date, but an understanding of the atomistic response following photoexcitation has remained elusive. Here, we use femtosecond resolution electron diffraction to investigate transient lattice responses in optically excited colloidal gold nanocrystals, revealing the effects of nanocrystal size and surface ligands on the electron-phonon coupling and thermal relaxation dynamics. First, we uncover a strong size effect on the electron-phonon coupling, which arises from reduced dielectric screening at the nanocrystal surfaces and prevails independent of the optical excitation mechanism (i.e., inter- and intraband). Second, we find that surface ligands act as a tuning parameter for hot carrier cooling. Particularly, gold nanocrystals with thiol-based ligands show significantly slower carrier cooling as compared to amine-based ligands under intraband optical excitation due to electronic coupling at the nanocrystal/ligand interfaces. Finally, we spatiotemporally resolve thermal transport and heat dissipation in photoexcited nanocrystal films by combining electron diffraction with stroboscopic elastic scattering microscopy. Taken together, we resolve the distinct thermal relaxation time scales ranging from 1 ps to 100 ns associated with the multiple interfaces through which heat flows at the nanoscale. Our findings provide insights into optimization of gold nanocrystals and their thin films for photocatalysis and thermoelectric applications.
View details for DOI 10.1021/acsnano.0c00673
View details for PubMedID 32208676
Resolving and Controlling Photoinduced Ultrafast Solvation in the Solid State
JOURNAL OF PHYSICAL CHEMISTRY LETTERS
2017; 8 (17): 4183–90
Solid-state solvation (SSS) is a solid-state analogue of solvent-solute interactions in the liquid state. Although it could enable exceptionally fine control over the energetic properties of solid-state devices, its molecular mechanisms have remained largely unexplored. We use ultrafast transient absorption and optical Kerr effect spectroscopies to independently track and correlate both the excited-state dynamics of an organic emitter and the polarization anisotropy relaxation of a small polar dopant embedded in an amorphous polystyrene matrix. The results demonstrate that the dopants are able to rotationally reorient on ultrafast time scales following light-induced changes in the electronic configuration of the emitter, minimizing the system energy. The solid-state dopant-emitter dynamics are intrinsically analogous to liquid-state solvent-solute interactions. In addition, tuning the dopant/polymer pore ratio offers control over solvation dynamics by exploiting molecular-scale confinement of the dopants by the polymer matrix. Our findings will enable refined strategies for tuning optoelectronic material properties using SSS and offer new strategies to investigate mobility and disorder in heterogeneous solid and glassy materials.
View details for DOI 10.1021/acs.jpclett.7b01689
View details for Web of Science ID 000410600600035
View details for PubMedID 28829138
- Tuning Thermally Activated Delayed Fluorescence Emitter Photophysics through Solvation in the Solid State ACS ENERGY LETTERS 2017; 2 (7): 1526–33
- Uncovering Single-Molecule Photophysical Heterogeneity of Bright, Thermally Activated Delayed Fluorescence Emitters Dispersed in Glassy Hosts JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2016; 138 (41): 13551–60
- Discerning Variable Extents of Interdomain Orientational and Structural Heterogeneity in Solution-Cast Polycrystalline Organic Semiconducting Thin Films JOURNAL OF PHYSICAL CHEMISTRY LETTERS 2015; 6 (16): 3155–62
- Relating the Physical Structure and Optoelectronic Function of Crystalline TIPS-Pentacene ADVANCED FUNCTIONAL MATERIALS 2015; 25 (13): 2038–46
Exciton dynamics reveal aggregates with intermolecular order at hidden interfaces in solution-cast organic semiconducting films
2015; 6: 5946
Large-scale organic electronics manufacturing requires solution processing. For small-molecule organic semiconductors, solution processing results in crystalline domains with high charge mobility, but the interfaces between these domains impede charge transport, degrading device performance. Although understanding these interfaces is essential to improve device performance, their intermolecular and electronic structure is unknown: they are smaller than the diffraction limit, are hidden from surface probe techniques, and their nanoscale heterogeneity is not typically resolved using X-ray methods. Here we use transient absorption microscopy to isolate a unique signature of a hidden interface in a TIPS-pentacene thin film, exposing its exciton dynamics and intermolecular structure. Surprisingly, instead of finding an abrupt grain boundary, we reveal that the interface can be composed of nanoscale crystallites interleaved by a web of interfaces that compound decreases in charge mobility. Our novel approach provides critical missing information on interface morphology necessary to correlate solution-processing methods to optimal device performance.
View details for DOI 10.1038/ncomms6946
View details for Web of Science ID 000348810700002
View details for PubMedID 25581561
- Revealing Exciton Dynamics in a Small-Molecule Organic Semiconducting Film with Subdomain Transient Absorption Microscopy JOURNAL OF PHYSICAL CHEMISTRY C 2013; 117 (42): 22111–22