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Vertical arrays of semiconductor nanowires show great potential for material-efficient and high-performance solar cells. The characterization and correlation between material structure and properties of the individual nanowires are crucial for the continued performance improvement of such devices. In this work, we developed a method with a scanning tunneling microscope (STM) probe inside a scanning electron microscope (SEM) to enable the studies of single photovoltaic nanowires. The STM probe is used to contact individual nanowires in ensembles. We combine the STM-SEM with an in situ light emitting diode (LED) illumination source to study both the electrical and photovoltaic properties of vertical GaAs nanowires with radial p-i-n junctions. We also illustrate that the local charge separation ability within the nanowires can be studied by electron beam induced current (EBIC) measurements. The in situ SEM setup allows the correlation between properties and nanowire structure. The data show that the quality of the electrical contact to the semiconductor nanowire is crucial to be able to investigate the inherent properties of the nanowires. We have established a procedure to make high-quality ohmic contacts to the nanowires with the STM probe. We also show that the effect of mechanical strain on the electrical properties can be investigated by the STM-SEM setup.

The tunable electrical properties of reduced graphene oxide (rGO) make it an ideal candidate for many applications including energy storage. However, in order to utilize the material for applications it is essential to understand the behavior of the material on the nanoscale, especially how naturally occurring phenomena like wrinkling affect the electronic transport. Here, we use a transmission electron microscope (TEM) with electrical probe in-situ holder to perform localized electrical measurements on wrinkled, supported rGO flakes. The TEM allows for observation of the local wrinkled structure of the rGO and simultaneously an electrical probe is used to perform localized resistance measurements. For these measurements, there is no correlation between the electrode distance and the measured resistance indicating that contact resistance varies and dominates the measurements. There is, however, a correlation between increasing number of wrinkles underneath the probe and decreasing resistance, indicating that the wrinkles can provide surface area for contact with the probe and thus lower the resistance. The overall resistance is on the order of single kΩ, if the contact between the probe and the rGO is optimized. These measurements give evidence that rGO with wrinkling can compete as a leading type of graphene for certain applications.

While it is well established that elevated temperatures can induce surface roughening of metal surfaces, the effect of a high electric field on the atomic structure at ambient temperature has not been investigated in detail. Here we show with atomic resolution using in situ transmission electron microscopy how intense electric fields induce reversible switching between perfect crystalline and disordered phases of gold surfaces at room temperature. Ab initio molecular dynamics simulations reveal that the mechanism behind the structural change can be attributed to a vanishing energy cost in forming surface defects in high electric fields. Our results demonstrate how surface processes can be directly controlled at the atomic scale by an externally applied electric field, which promotes an effective decoupling of the topmost surface layers from the underlying bulk. This opens up opportunities for development of active nanodevices in e.g. nanophotonics and field-effect transistor technology as well as fundamental research in materials characterization and of yet unexplored dynamically-controlled low-dimensional phases of matter., Comment: The manuscript is 10 pages long and the supplemental material is 15 pages. There are six supplemental movies that can be provided upon request to LDK

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