Visualization of photocurrents in nanostructures


Photoemission currents in Ag nanowires

An attractive alternative for achieving nanometer spatial and femtosecond temporal resolution using electrons as probes is femtosecond point-projection microscopy (fs-PPM). The combination of a nanotip electron source and lens-less imaging allows for a temporal resolution comparable to ultrafast optical microscopy and a spatial resolution that can, in principle, reach the single nm level. Due to the use of low-energy electrons, fs-PPM is highly sensitive to local electric fields, which makes it especially well suited to visualizing charge carrier separation and dynamics.

In this benchmark experiment we visualizes dynamics that are considerably faster than what has been shown so far with this technique, illustrating the potential of fs-PPM in combining nanoscale spatial and femtosecond temporal resolution in imaging charge carrier dynamics. The multiphoton ionization and subsequent space-charge driven dynamics of photoelectrons emitted from silver nanowires is captured on timescales as low as 33 fs.

 

 

Further reading:
Faruk Krecinic, Jannik Malter, Alexander Paarmann, Melanie Müller and Ralph Ernstorfer
Point-projection microscopy of nano-localized photoemission currents at sub-40 femtosecond time scales
arXiv: 2181602

InP nanowires

Semiconductor nanowires (NWs) and, in particular, heterostructured NWs are promising candidates for future nanoscale electronic and optoelectronic devices, as well as ideal model systems for exploring fundamental semiconductor physics on nanometer length scales. Within the last years there has been vast progress in controlling the doping level in both radial and axial direction during NW growth. In order to understand the physics of carrier transport in these structures, it is of major importance to study the dynamics of charge carriers upon photoexcitation. So far, typical studies such as time-resolved photoluminescence and photoemission electron microscopy provide either spatially- or time-averaged information, respectively. In this regard, it is most appealing to combine nanometer spatial with femtosecond temporal resolution to directly measure the spatio-temporal evolution of photoexcited charge distributions in such nanoobjects.

We investigate ultrafast photocurrents in heterostructured InP-NWs with femtosecond temporal and nanometer spatial resolution, employing our newly developed technique of femtosecond low-energy electron point-projection microscopy (fsPPM). Low-energy electrons are very sensitive to detect transient fields in the near-surface region of nanoobjects generated by ultrafast photocurrents on nanometer dimensions. Figure 1 shows the layout of the experiment.

Experimental configuration of femtosecond low-energy electron point projection microscopy. The trajectories of low-energy electrons are very sensitive to electric fields in the vicinity of nanoobjects (NW: nanowire). Ultrafast carrier propagation in nanoobjects result in transient changes of the electric potential. The visualization of changes of the electric fields therefore reveals information of the photocurrents inside the nanoobject.

Figure 1: Experimental configuration of femtosecond low-energy electron point projection microscopy. The trajectories of low-energy electrons are very sensitive to electric fields in the vicinity of nanoobjects (NW: nanowire). Ultrafast carrier propagation in nanoobjects result in transient changes of the electric potential. The visualization of changes of the electric fields therefore reveals information of the photocurrents inside the nanoobject.

Further reading:

  • M. Müller, A. Paarmann, and R. Ernstorfer:
    Femtosecond electrons probing currents and atomic structure in nanomaterials.
    Nature Communications 5, 5292 (2014), [doi: 10.1038/ncomms6292].
    open access: arXiv:1405.4992