Investigation of single-walled carbon nanotubes with a low-energy electron point projection microscope

There has been controversy about the interpretation of the interference patterns recorded with low-energy electron point projection microscopy. With a highly coherent single-atom electron source, we have used a point projection microscope (PPM) to image a suspended and isolated single-walled carbon nanotube at different tip-sample separations. The nanotube and the surrounding structure are also imaged with a transmission electron microscope. Through numerical simulations, we can fit well the interference patterns of the nanotube recorded by the PPM at different separations. Our simulation results indicate that the interference patterns can be considered as electron holograms at large tip-sample separations (or small magnifications). However, at small tip-sample separations, the interference patterns are dominated by the biprism effect due to significant charge density induced on the nanotube, and thus, the interference patterns contain little information about the internal structure of the object. The results provide a reason why the images obtained by point projection microscopy so far have never achieved a resolution smaller than 2 nm. New research directions for achieving high-resolution imaging of biological molecules with low-energy electrons are also discussed.