Transcript Substrate unit cell Adsorbate unit cell

Classification of Overlayer
Structures
Overlayers
Adsorbed species frequently form welldefined 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
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