2 OverlayersAdsorbed 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.
3 Surface unit cellThe primitive unit cell is the simplest periodically repeating unit which can be identified in an ordered arrayThe fcc(100) surface
4 Unit 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
5 Wood'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
6 More overlayersSubstrate : fcc(100) Substrate unit cell Adsorbate unit cellSubstrate : fcc(110) Substrate unit cell Adsorbate unit cell
7 And more Substrate : fcc(111) Substrate unit cell Adsorbate unit cell Substrate : fcc(100) c(2 x 2)(√2 x √2)R45
10 Matrix Notationmore general system which can be applied to all ordered overlayersrelates the vectors b1 & b2 to the substrate vectors a1 & a2 using a simple matrix
11 For the (2 x 2) structure we have : MatrixSubstrate : fcc (100) (2 x 2) overlayerFor the (2 x 2) structure we have :
12 More MatrixSubstrate : fcc (100) c(2 x 2) overlayer
13 Surface CoverageOverlayer coverage can be determined simply by counting atoms within a given area of surface, but the area chosen must be representative of the surface as a whole.Structure?Coverage?