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Classification of Overlayer Structures. Overlayers Adsorbed species frequently form well- defined overlayer structures. Each particular structure may.

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Presentation on theme: "Classification of Overlayer Structures. Overlayers Adsorbed species frequently form well- defined overlayer structures. Each particular structure may."— Presentation transcript:

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 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

5 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

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 b 1 & b 2 to the substrate vectors a 1 & a 2 using a simple matrix

11 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 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


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