Papanikolas Group

Ultrafast Spectroscopy and Ultrafast Microscopy

Papanikolas Group

Ultrafast Spectroscopy and Ultrafast Microscopy

Ultrafast Microscopy:
Visualization of Charge Carrier Dynamics in Nanostructures

Recently Published

Group Members

Carrier Dynamics in Si Nanowires

We are applying the ultrafast microscopy methods developed in our laboratory to the characterize the carrier dynamics (e.g. electron-hole recombination, charge separation, diffusion and drift) in Si NWs. There is intense interest in nanowires, particularly Si NWs, for use in a wide variety of nanotechnologies. Ultrafast microscopy provides a unique view of the nanowire behavior that is difficult (or impossible) to obtain using conventional experimental methods. These projects are being pursued in partnership with Cahoon group, who are experts in the growth, characterization and manipulation of nanowire materials.

Research Highlights

James Cahoon

Comparison of  Microscopy and Transient Absorption

Two back-to-back papers that appeared in J. Phys. Chem. C showed that transient absorption methods tend to overestimate the electron-hole recombination rate due to averaging over bent structures in the sample. The observation motivated our investigation of strain effects in nanowire dynamics.

<Read>                  <Read>

Watching Electrons Move in Nanowires

The movement of electrons through objects is central to the function of all electronic devices.  Using a microscope that is capable of exciting a single object in one place, and probing it in another, we have imaged carrier motion (animation, right).  Electrons and holes are created at the location of the red circle by photoexcitation with a femtosecond laser pulse that has been focused to a diffraction limited spot by a microscope objective. As time increases, the carriers recombine and the intensity of the spot decreases (note the scaling factor in lower right corner). As this happens, the  spot also elongates along the nanowire reflecting the motion of the charge cloud. By analyzing the rate of growth of the charge cloud we can determine the carrier diffusion constant at a specific point in a single nanostructure.  The blue spot at longer times is the result of local heating by the pump pulse. <Read>

Spatially Resolved Charge Carrier Recombination in Nanowires Strained by Bending Deformation

The combined spatial and temporal resolution of pump-probe microscopy offers a unique opportunity to study nanostructures that are strained through mechanical deformation. We have recently investigated the effect of inhomogeneous strain on the electron-hole recombination dynamics in bent Silicon nanowires. The movie at the left shows the free carrier population in a nanowire that was bent upon transfer to the substrate. The carrier population decays more rapidly in the bent regions of the structure.  Experiments performed on flexible substrates (i.e. PDMS) reveal that the recombination rate returns to values commensurate with the straight regions when the NW is straightened. This reversible behavior indicates that the faster recombination is not simply due to the introduction of defects into the lattice, but rather the result of intrinsic changes to the nanowire band structure in the vicinity of the  bend. <Read>

Imaging Charge Separation in a

Nanowire p-n Junction


Papanikolas Group
Department of Chemistry
University of North Carolina at Chapel Hill