DAS model (rus. модель поверхности Si(111)7×7) — a surface model with dimers, adatoms and stacking faults.


The model was proposed by Kunio Takayanagi in 1985 to describe the reconstruction of an atomically clean surface of Si(111)7×7. A unit cell with 7x7 reconstruction consists of an angle cavity and two triangular subcells separated by dimer chains; each subcell contains 6 atoms (adatoms) adsorbed on the crystal surface but not yet associated with the crystalline lattice; the atomic layer below the adatom layer in one of the subcells has a stacking fault orientation. The adequacy of this model was confirmed by multiple tests.


Atomically clean surface of Si(111)7×7. STM images of (а) filled and unfilled (b) electron states of surface; c — schematic representation of surface (plan and side views) in accordance with Takayanagi DAS model (dimer-adatom-stacking fault). Yellow circles represent Si atoms, red circles - dimerizated Si atoms, and blue - second layer Si rest-atoms. Elementary cell 7x7 is highlighted with a diamond. Half of the elementary cell with package defect is marked as FH (faulted half); the half with no package defect is marked as UH (unfaulted half). You may see that the half of the cell with package defect depicted in the STM image of filled states (a) appears brighter than the other half. Maximums in the STM image correspond to adatoms.


  • Zotov Andrey V.
  • Saranin Alexander A.


  1. Takayanagi K., Tanishiro Y., Takahashi S., Takahashi M. // Surf. Sci. 1985. V. 164. P. 367.

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