  Using low energy electron diffraction (LEED), Auger electron spectroscopy
(AES), scanning tunnelling microscopy (STM) and high resolution photo-electron
spectroscopy (HR-PES) techniques we have studied the annealing effect of one
silicon monolayer deposited at room temperature onto a Ni (111) substrate. The
variations of the Si surface concentration, recorded by AES at 300{\deg}C and
400{\deg}C, show at the beginning a rapid Si decreasing followed by a slowing
down up to a plateau equivalent to about 1/3 silicon monolayer. STM images and
LEED patterns, both recorded at room temperature just after annealing, reveal
the formation of an ordered hexagonal superstructure(rot3xrot3)R30{\deg}-type.
From these observations and from a quantitative analysis of HR-PES data,
recorded before and after annealing, we propose that the (rot3 x
rot3)R30{\deg}superstructure corresponds to a two dimensional (2D) Ni2Si
surface silicide.
