Bonding at silicon surfaces

1. Bonding at silicon surfaces   Si=Si dimers -bonded chain graphitic silicon Si(100)2x1 Si(111)2x1 Si(111)5x2-Au (cleaved) Conventional wisdom tells us that carbon forms -bonds while silicon does not. Silicon surfaces break the rule, since the broken bonds are desperately seeking a bonding partner.They rearrange themselves to form in-plane -bonds.Sucha rearrangement of surface atoms is called surface reconstruction.

2.   Si(100)2x1 By varying the sample voltage in a scanning tunneling microscope one can detect either the occupied, bonding  orbitals, or the unoccupied, antibonding * orbitals. As for the H2 molecule, the bonding orbitals accumulate charge between two atoms to form a covalent bond, while the antibonding orbitals have a zero (= node).

3. Si(111)7x7 Hexagonal fcc (diamond) (eclipsed) (staggered) Most stable silicon surface Adatom Rest Atom >100 atoms rearrange themselves to minimize broken bonds.

4. Two mechanisms for the stability of Si(111)7x7 Bumps and holes Adatoms trade three broken bonds for one. They form the bumps in the STM image. Broken bonds disappear when holes are created at a stacking fault.

5. Surface structures and nxm superlattices 2 1 = centered 2x2

6. Ionic crystal surfaces need to be neutral Otherwise an infinite charge sheet builds up at the surface with infinite Coulomb energy. NaCl(100) surface (with the approximate size of the ions)