2OverlayersAdsorbed species frequently form well-defined overlayer structures. Each particular structure may only exist over a limited coverage range of the adsorbate, and in some adsorbate/substrate systems a whole progression of adsorbate structures are formed as the surface coverage is gradually increased.
3Surface unit cellThe primitive unit cell is the simplest periodically repeating unit which can be identified in an ordered arrayThe fcc(100) surface
4Unit cell definition fcc(110) surface Note : the length of the vectors is related to the bulk unit cell parameter, a , by |a1| = |a2| = a / √ 2fcc(111) surface
5Wood's NotationWood's notation involves specifying the lengths of the overlayer vectors, b1 & b2 , in terms of a1 & a2 respectively - this is then written in the format :( |b1|/|a1| x |b2|/|a2| )i.e. a ( 2 x 2 ) structure has |b1| = 2|a1| and |b2| = 2|a2| .Substrate : fcc(100) Substrate unit cell Adsorbate unit cell
6More overlayersSubstrate : fcc(100) Substrate unit cell Adsorbate unit cellSubstrate : fcc(110) Substrate unit cell Adsorbate unit cell
7And more Substrate : fcc(111) Substrate unit cell Adsorbate unit cell Substrate : fcc(100) c(2 x 2)(√2 x √2)R45