  We report the growth process, structure, and electronic states of 1 unit
layer (u.l.) of a Ce-Pt intermetallic compound on Pt(111) using scanning
tunneling microscopy/spectroscopy (STM/STS) and low-energy electron
diffraction. An ordered $(2 \times 2)$ structure was observed in the form of
films or nanoislands depending on Ce dose by annealing at around 700 K. A
structural model constructed from atomically resolved STM images and
quasiparticle interference (QPI) patterns indicates the formation of a new
surface compound 1 u.l. CePt$_6$ on Pt(111) terminated by a Pt layer. A lateral
lattice constant of the 1 u.l. CePt$_6$ on Pt(111) is expanded from the value
of a bulk CePt$_5$ crystal to match the Pt(111) substrate. By measuring
d$I$/d$V$ spectra and QPI, we observed an onset energy of the surface state
found on Pt(111) above Fermi energy ($E_F$) shifts below $E_F$ on the Pt layer
of the 1 u.l. CePt$_6$ due to charge transfer from the underneath CePt$_2$
layer. We discuss a possible two-dimensional coherent Kondo effect with the
observed spectra on the 1 u.l. CePt$_6$.
