Structure-Function Relationships in Nanomaterials
A bent silicon nanowire experiences compressive strain at the inner edge of the bend and tensile strain at the outer edge. The associated changes in the lattice spacing cause profound changes to the electronic structure in those regions.
The electronic structure of semiconductor nanomaterials is strongly coupled to both physical structure and chemical composition. In nanowires, like the one shown at left, the lifetime of photoexcited electrons and holes are sensitively dependent on the diameter, surface quality, and doping level of each individual wire. In addition to these traits, secondary structural features like bending can strongly affect the electron-hole lifetime. These effects arise from the lattice compression and tension that result from bends in the wire.
With sub-micron spatial resolution and ultrafast temporal resolution, pump-probe microscopy can provide unparalleled insight into how these structural effects change the electronic structure within a single nanostructure. We are currently investigating how bending affects carrier mobilities in inorganic semiconductor nanostructures to better understand how we might use strain to engineer the functionality of nano-devices.
At right are recent microscopy measurements performed on reversibly stretched silicon nanowires. The NWs were deposited on stretched PDMS substrates and the electron-hole recombination kinetics were measured a the same location as the NW was initially straight (1), bent (2), and re-straightened (3). The recovery of the slower recombination kinetics in (3) show the strain-induced lifetime decrease is reversible, and likely due to modulation of the band structure.
Our research efforts are also focused on self-assembled organic nanostructures that exhibit high long range crystal order and may be strong candidates for low-cost printable electronics that are flexible and have high charge mobility.
Ultrafast Super-Resolution Microscopy
Super-resolution pump-probe images: The diffraction limited image at t = 0 ps gives a ~ 220 nm linewidth, whereas the super-resolution is much narrower (134 nm).
We are actively developing new methods to enable ultrafast spectroscopy with sub-diffraction limited spatial resolution. With these super-resolution microscopies, we can interrogate materials on smaller length scales and provide greater insight into how structural and chemical variation at interfaces affect overall material properties.
Using pulse shaping techniques and Fourier optics, we are working to collect transient kinetics and spectra from sample areas smaller than ~ 100 nm x 100 nm - more than a factor of two below the diffraction limit.
Solution-Processed Photovoltaic Materials
Top: brightfield image of perovskite grains. The dark areas are perovskite. Bottom: Pump probe image of same region at t = 0 ps. The bright green areas are perovskite and the dark areas are substrate.
We have ongoing projects studying the dynamics of charge and exciton transport in disordered semiconductors that may one day form the foundation for highly efficient, low-cost, and spectrally-tunable solar cells. These materials are processed from solution to produce polycrystalline films, where each single crystal domain is separated by a grain boundary. We are interested in fundamental questions about how the variation in structure and composition that is intrinsic to these materials affect their performance - both as light absorbers and charge transporters.
As part of our efforts studying charge transport, we photoexcite the sample with 70 femtosecond laser pulses and directly image the motion of charge carriers as they are transported through the material either through diffusion or electric field-induced drift. The result are "movies" of charge carrier motion that tell us about electron and hole mobility, trapping, and relaxation processes - all on sub-micron length scales.
Charge carrier diffusion through a single perovskite domain. The pump-probe delay spans 0 - 1.6 ns.