Scanning transmission electron microscopy under controlled low-pressure atmospheres

Abstract

Transmission electron microscopy (TEM) is carried out in vacuum to minimize the interaction of the imaging electrons with gas molecules while passing through the microscope column. Nevertheless, in typical devices, the pressure remains at 10 −7 mbar or above, providing a large number of gas molecules for the electron beam to crack, which can lead to structural changes in the sample. Here, we describe experiments carried out in a modified scanning TEM (STEM) instrument, based on the Nion UltraSTEM 100. In this instrument, the base pressure at the sample is around 2 × 10 −10 mbar, and can be varied up to 10 −6 mbar through introduction of gases directly into the objective area while maintaining atomic resolution imaging conditions. We show that air leaked into the microscope column during the experiment is efficient in cleaning graphene samples from contamination, but ineffective in damaging the pristine lattice. Our experiments also show that exposure to O 2 and H 2 O lead to a similar result, oxygen providing an etching effect nearly twice as efficient as water, presumably due to the two O atoms per molecule. H 2 and N 2 environments have no influence on etching. These results show that the residual gas environment in typical TEM instruments can have a large influence on the observations, and show that chemical etching of carbon-based structures can be effectively carried out with oxygen. arXiv:1811.04266v3 [cond-mat.mes-hall]

Process Pathway Controlled Evolution of Phase and Van‐der‐Waals Epitaxy in In/In2O3 on Graphene Heterostructures

“…Importantly, we study the In/In 2 O 3 deposition in a "quasi-in-situ" manner where indium is first thermally evaporated in situ onto the graphene in ultra-high-vacuum (UHV, ≈10 −9 mbar base pressure) conditions and then directly transferred without vacuum break into the aberration corrected STEM instrument. [69,70] This allows capturing the intrinsic state of In clusters on graphene in virtually oxygen-and waterfree conditions. Subsequently we employ controlled ambient air exposure at variable temperatures and times, followed by reloading into the STEM to stepwise monitor the oxidation of specific In particles toward In 2 O 3 on graphene.…”

Contamination of TEM Holders Quantified and Mitigated With the Open-Hardware, High-Vacuum Bakeout System

“…Oxygen and water molecules absorbed onto specimen surfaces create highly reactive radicals when irradiated with electrons. These radicals can cause lattice damage by chemically etching the specimen—a process sometimes confused with knock-on damage (Leuthner et al, 2019). In cryo-TEM, water surrounding the sample is required to be in a vitreous state.…”

Open-hardware, High-vacuum Storage for TEM Holders Remedies and Quantifies Hydrocarbon Contamination

“…Due to the large inelastic cross-section of carbon, even small quantities can hamper accurate atomic species detection in electron energy loss spectroscopy (EELS) [7]. Oxygen and water molecules pose problems of lattice damage by chemically etching the specimen during imaging [8]. These constraints on high-resolution and spectroscopic chemical imaging demand clean, high-vacuum microscopes with dry pumps.…”