  (Li0.8Fe0.2)OHFeSe is a newly-discovered intercalated iron-selenide
superconductor with a Tc above 40 K, which is much higher than the Tc of bulk
FeSe (8 K). Here we report a systematic study of (Li0.8Fe0.2)OHFeSe by low
temperature scanning tunneling microscopy (STM). We observed two kinds of
surface terminations, namely FeSe and (Li0.8Fe0.2)OH surfaces. On the FeSe
surface, the superconducting state is fully gapped with double coherence peaks,
and a vortex core state with split peaks near EF is observed. Through
quasi-particle interference (QPI) measurements, we clearly observed intra- and
inter-pocket scatterings in between the electron pockets at the M point, as
well as some evidence of scattering that connects gamma and M points. Upon
applying magnetic field, the QPI intensity of all the scattering channels are
found to behave similarly. Furthermore, we studied impurity effects on the
superconductivity by investigating intentionally introduced impurities and
intrinsic defects. We observed that magnetic impurities such as Cr adatoms can
induce in-gap states and suppress superconductivity. However, nonmagnetic
impurities such as Zn adatoms do not induce visible in-gap states. Meanwhile,
we show that Zn adatoms can induce in-gap states in thick FeSe films, which is
believed to have an (s+-)wave pairing symmetry. Our experimental results
suggest it is likely that (Li0.8Fe0.2)OHFeSe is a plain s-wave superconductor,
whose order parameter has the same sign on all Fermi surface sections.
