Reducing the accelerating voltage of TEM/STEM is becoming essential when one aims to image any light element matters and/or beam sensitive objects. Lower acceleration voltage is beneficial to enhance the image/EELS contrast as well as minimise the knock-on effect which heavily destructs the atomic structures. In order to compensate the poor spatial resolution due to the low acceleration voltage, more sophisticated electron optics are definitely required to reduce the residual geometric/chromatic aberrations.
Under a JST triple C project, Sawada et al. designed a new type of Cs corrector with triple dodecapole elements (the DELTA system) to reduce the six-fold astigmatism, which is very advantageous for the TEM/STEM operated at low accelerating voltages around 15 to 60 kV. A JEOL 2100F equipped with this corrector shows a world best performance in terms of the spatial resolution normalized by the wave length, which was proved by the graphene 106 pm reflection corresponding to the 15.3 times of the used wave length (7 pm at 30kV) [1]. This is a clear proof that the atomic resolution can be achievable in STEM-ADF mode even at 30kV. We have also employed a double Wien-filter monochromator for low-voltage operations.
In this presentation, I summarise our recent progresses of low-voltage TEM/STEM within the scheme of triple C project. The current status of our monochromator and aberration corrector developments will be presented. The examples of in situ microscopy/spectroscopy at atomic scale such as graphene growth [2], oxidation [3], defect formation and phase transition in dichalcogenides [4,5] will be shown. Also the exiton behaviour at semiconductor interface examined at nanometer-scale will be also presented [6]. Eventually the chemical analysis of 1D atomic chains involving the light element detection will be discussed [7, 8].