  Using low-temperature scanning tunneling microscopy (STM), we studied the
vortex states of single-layer FeSe film on SrTiO3 (100) substrate, and the
local behaviors of superconductivity at sample boundaries. We clearly observed
multiple discrete Caroli-de Gennes-Matricon (CdGM) states in the vortex core,
and quantitative analysis shows their energies well follow the formula: E =
{\mu}{\Delta}^2/E_F, where {\mu} is a half integer and {\Delta} is the mean
superconducting gap over the Fermi surface. Meanwhile, a fully gapped spectrum
without states near zero bias is observed at [110](Fe) oriented boundary of 1
ML and 2 ML FeSe films, and atomic step edge of 1 ML FeSe. Accompanied with
theoretical calculations, our results indicate a s-wave pairing without
sign-change in the high-TC FeSe_SrTiO3 superconductor.
