Classification of Overlayer Structures

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Classification of Overlayer Structures

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.

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

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

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

More overlayers Substrate : fcc(100) Substrate unit cell Adsorbate unit cell Substrate : fcc(110) Substrate unit cell Adsorbate unit cell

And more Substrate : fcc(111) Substrate unit cell Adsorbate unit cell
Substrate : fcc(100) c(2 x 2) (√2 x √2)R45

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

Common overlayer

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

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

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

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?

Test

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