  Charge density wave (CDW) resulted from a small distortion in the lattice is
able to create new orders beyond the original lattice. In 2H-NbSe2, one of the
layered transition metal dichalcogenides (TMD), the 3x3 charge order appears in
two-dimensional (2D) layers. Although CDW is usually described by a sine wave,
the spatial distribution within a 2D layer has never been systematically
visualized. Here by using scanning tunneling microscopy (STM) and density
functional theory (DFT), we have monitored the evolution of 3x3 CDW along
c-axis and realized a nearly tomography scan of CDW of the topmost layer. The
results show that the strength of 3x3 charge order varies while increasing the
tunneling current. The 3x3 charge order is relatively strong at the outermost
Se level and decreases while probing in between Se and Nb levels.
Interestingly, the 3x3 charge order gets strong again as reaching Nb level but
along with a phase shift. We further calculated the orbital charge
distributions and found that both CDW intensity modulation and phase shift are
strongly correlated with the distribution of Se p orbitals and Nb d orbitals.
