  We apply high resolution scanning tunneling microscopy to study intrinsic
defect states of bulk FeSe. Four types of intrinsic defects including the type
I dumbbell, type II dumbbell, top-layer Se vacancy and inner-layer Se-site
defect are extensively analyzed by scanning tunneling spectroscopy. From
characterized depression and enhancement of density of states measured in a
large energy range, the type I dumbbell and type II dumbbell are determined to
be the Fe vacancy and Se$_\mathrm{Fe}$ defect, respectively. The top-layer Se
vacancy and possible inner-layer Se-site vacancy are also determined by
spectroscopy analysis. The determination of defects are compared and largely
confirmed in the annular dark-field scanning transmission electron microscopy
measurement of the exfoliated FeSe. The detailed mapping of defect states in
our experiment lays the foundation for a comparison with complex theoretical
calculations in the future.
