1. Classification of Overlayer Structures

2. Overlayers
Adsorbed 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 cell
The primitive unit cell is the simplest periodically repeating unit which can be identified in an ordered array
The 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 / √ 2
fcc(111) surface

5. Wood's Notation
Wood'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 overlayers
Substrate : fcc(100) Substrate unit cell Adsorbate unit cell
Substrate : 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

8. Non-Wood’s!
Substrate : fcc(110) c(2 x 2)

9. Common overlayer

10. Matrix Notation
more general system which can be applied to all ordered overlayers
relates the vectors b1 & b2 to the substrate vectors a1 & a2 using a simple matrix

11. For the (2 x 2) structure we have :
Matrix
Substrate : fcc (100) (2 x 2) overlayer
For the (2 x 2) structure we have :     

12. More Matrix
Substrate : fcc (100) c(2 x 2) overlayer

13. Surface Coverage
Overlayer 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?

14. Test

15. Some systems Geometry of Pt(111)+c(4x2)-2CO
Geometry of Fe(110)+p(2x2)-S
Geometry of Ni(111)+(2x2)-C2H2
Geometry of Ni(110)+p(2x1)-2CO