  The chemical behaviour of single metal atoms largely depends on the local
coordination environment, including interactions with the substrate and with
the surrounding gas or liquid. However, the key instrumentation for studying
such systems at the atomic scale generally requires high vacuum conditions,
limiting the degree to which the aforementioned environmental parameters can be
investigated. Here we develop a new platform for transmission electron
microscopy investigation of single metal atoms in liquids and study the dynamic
behaviour of individual platinum atoms on the surface of a single layer MoS2
crystal in water. To achieve the record single atom resolution, we introduce a
double liquid cell based on a 2D material heterostructure, which allows us to
submerge an atomically thin membrane with liquid on both sides while
maintaining the total specimen thickness of only ~ 70 nm. By comparison with an
identical specimen imaged under high vacuum conditions, we reveal drastic
differences in the single atom resting sites and atomic hopping behaviour,
demonstrating that in situ imaging conditions are essential to gain complete
understanding of the chemical activity of individual atoms. These findings pave
the way for in situ liquid imaging of chemical processes with single atom
precision.
