A two-step process for synthesizing stable Cs 2 SnI 6 perovskite thin films is reported in this letter. The two-step process includes the co-evaporation of two precursors SnI 2 and CsI onto a glass substrate, followed by a post thermal annealing process in iodine vapor. Using this technique, pure Cs 2 SnI 6 perovskite thin films were successfully synthesized without any wet process. These perovskite thin films are found to be stable under ambient conditions. They also show an electron mobility up to 509 cm 2 V −1 s −1 , which is higher than the mobilities of films prepared by solution processes reported in the literature. IMPACT STATEMENTA novel two-step dry process to synthesize phase-pure, air-stable Cs 2 SnI 6 perovskite thin film with higher electron mobility than that of the films prepared by the solution process.

Knowledge on the symmetry and perfection of a 2D material deposited or transferred to a surface is very important and valuable. We demonstrate a method to map the reciprocal space structure of 2D materials using reflection high energy diffraction (RHEED). RHEED from a 2D material gives rise to 'streaks' patterns. It is shown that from these streaks patterns at different azimuthal rotation angles that the reciprocal space intensity distribution can be constructed as a function of momentum transfer parallel to the surface. To illustrate the principle, we experimentally constructed the reciprocal space structure of a commercial graphene/SiO/Si sample in which the graphene layer was transferred to the SiO/Si substrate after it was deposited on a Cu foil by chemical vapor deposition. The result reveals a 12-fold symmetry of the graphene layer which is a result of two dominant orientation domains with 30° rotation relative to each other. We show that the graphene can serve as a template to grow other materials such as a SnS film that follows the symmetry of graphene.

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