When small objects are imaged with electrons, they are exposed to charge. If the charge cannot drain off, it accumulates on the objects and influences the path of subsequent electrons. In electron microscopy of insulating materials, this leads to distortions of the electron trajectories that can obstruct the formation of an image. In contrast, when grounded conductors are exposed to electrons, the electrostatic potential in the object equilibrates and electron micrographs of conductors exhibit almost no signs of charging, which allows perfect imaging. However, charge-induced distortions of electron trajectories contain information about the electrostatic potential and the charge transport. If charge-induced distortions could be visualized and quantitatively analyzed, they would reveal the electrical conductivity of the object.
We investigated the charging of zinc oxide (ZnO) nanowires (NWs) while they are imaged with a low-energy electron point source (LEEPS) microscope. A LEEPS microscope operates below $200 eV where electrons are very slow and thus sensitive to charging. NWs are considered as building blocks for novel electronic devices and sensors, and NW electronics is a rapidly growing field. However, a standard method for the fast and reliable electrical characterization of a single NW [1,2] is still missing and thus of very high interest. In current protocols, NWs are deposited on isolating substrates, contacted via lithography and then the resistance is determined by transport or field-effect measurements. [3] Such procedures are complex and time-consuming. Other methods employ scanning probe techniques for contacting nanotubes and nanobelts. [4,5] In all of these methods the presence of the substrate may influence the charge transport through the nanoscale objects. Recently, freestanding NWs were contacted with movable metal electrodes under visual control in electron microscopes. The resistance of DNA molecules, [6] carbon nanotubes [7] as well as silicon [8] and zinc oxide [9,10] NWs has been measured with LEEPS and with scanning and transmission electron microscopy (SEM and TEM, respectively). It is, however, difficult to extract the intrinsic wire resistance from simple two-point