Direct imaging of surface states hidden in the third layer of Si (111)-7 × 7 surface by p_z-wave tip

The direct visualization of the surface states of dimers in Si (111)-7 × 7 is still challenging although the DAS model has been proposed 33 years ago. In this letter, based on the partial density of states of the atoms in the reconstructed layers examined by the first principles calculations, scanning tunneling spectroscopy images with a p_z-wave tip were carried out, which show a skull pattern at ∼0.7 eV, and 9 bright short lines at ∼1.00 eV in each unit cell, exhibiting obviously the electronic states from the third-layer atoms and dimers. The improved sensitivity (intensity and resolution) of scanning tunneling spectroscopy was ascribed to the enhanced tunneling matrix elements for p_z-wave tip compared with s-wave tip. This discovery provides us a platform to explore the quantum states hidden in the deep of the materials surface.