Presentation on theme: "Classification of Overlayer Structures. Overlayers Adsorbed species frequently form well- defined overlayer structures. Each particular structure may."— Presentation transcript:
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 Note : the length of the vectors is related to the bulk unit cell parameter, a, by |a 1 | = |a 2 | = a / √ 2 fcc(110) surface fcc(111) surface
Wood's Notation Wood's notation involves specifying the lengths of the overlayer vectors, b 1 & b 2, in terms of a 1 & a 2 respectively - this is then written in the format : ( |b 1 |/|a 1 | x |b 2 |/|a 2 | ) i.e. a ( 2 x 2 ) structure has |b 1 | = 2|a 1 | and |b 2 | = 2|a 2 |. 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)
Matrix Notation more general system which can be applied to all ordered overlayers relates the vectors b 1 & b 2 to the substrate vectors a 1 & a 2 using a simple matrix
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?
Some systems Geometry of Fe(110)+p(2x2)-S Geometry of Pt(111)+c(4x2)-2CO Geometry of Ni(111)+(2x2)-C2H2 Geometry of Ni(110)+p(2x1)-2CO