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**Classification of Overlayer Structures**

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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.

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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

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**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

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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

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

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**And more Substrate : fcc(111) Substrate unit cell Adsorbate unit cell**

Substrate : fcc(100) c(2 x 2) (√2 x √2)R45

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

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Common overlayer

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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

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**For the (2 x 2) structure we have : **

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

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

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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?

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Test

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**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

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2D Crystallography Selvage (or selvedge (it. cimosa)): Region in the solid in the vicinity of the mathematical surface Surface = Substrate (3D periodicity)

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