  Surface impurities and contamination often seriously degrade the properties
of two-dimensional materials such as graphene. To remove contamination, thermal
annealing is commonly used. We present a comparative analysis of annealing
treatments in air and in vacuum, both ex situ and "pre-situ", where an
ultra-high vacuum treatment chamber is directly connected to an
aberration-corrected scanning transmission electron microscope. While ex situ
treatments do remove contamination, it is challenging to obtain atomically
clean surfaces after ambient transfer. However, pre-situ cleaning with
radiative or laser heating appears reliable and well suited to clean graphene
without undue damage to its structure.
