Overlayer

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An overlayer is a layer of adatoms adsorbed onto a surface, for instance onto the surface of a single crystal.^[1]

On single crystals[]

Adsorbed species on single crystal surfaces are frequently found to exhibit long-range ordering; that is to say that the adsorbed species form a well-defined overlayer structure. 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 structure are formed as the surface coverage is gradually increased.^[2]

The periodicity of the overlayer (which often is larger than that of the substrate unit cell) can be determined by low-energy electron diffraction (LEED), because there will be additional diffraction beams associated with the overlayer.^[3]

Types[]

There are two types of overlayers: commensurate and incommensurate. In the former the substrate-adsorbate interaction tends to dominate over any lateral adsorbate-adsorbate interaction, while in the latter the adsorbate-adsorbate interactions are of similar magnitude to those between adsorbate and substrate.^[4]

Notation[]

An overlayer on a substrate can be notated in either Wood's notation or matrix notation.^[5]

Wood's notation[]

Wood's notation takes the form

${\displaystyle {\text{M}}(hk\ell )-\left({\tfrac {|{\textbf {a}}_{0}|}{|{\textbf {a}}_{s}|}}\times {\tfrac {|{\textbf {b}}_{0}|}{|{\textbf {b}}_{s}|}}\right)-{\text{R}}\alpha ^{\circ }-{\text{A}}}$

where M is the chemical symbol of the substrate, A is the chemical symbol of the overlayer, ${\displaystyle (hk\ell )}$ are the Miller indices of the surface plane, R and ${\displaystyle \alpha }$ correspond to the rotational difference between the substrate and overlayer vectors, and the vector magnitudes shown are those of the substrate (${\displaystyle s}$ subscripts) and of the overlayer (${\displaystyle 0}$ subscripts). This notation can only describe commensurate overlayers however, while matrix notation can describe both.

Matrix notation[]

Matrix notation differs from Wood's notation in the second term, which is replaced by the ${\displaystyle G}$ matrix that describes the overlayer primitive vectors in terms of the substrate primitive vectors:

${\displaystyle {\begin{pmatrix}{\textbf {a}}_{0}\\{\textbf {b}}_{0}\end{pmatrix}}=G{\begin{pmatrix}{\textbf {a}}_{s}\\{\textbf {b}}_{s}\end{pmatrix}}}$, where ${\displaystyle G={\begin{pmatrix}G_{11}&G_{12}\\G_{21}&G_{22}\\\end{pmatrix}}}$

and so hence matrix notation has the form

${\displaystyle {\text{M}}(hk\ell )-G-{\text{R}}\alpha ^{\circ }-{\text{A}}}$

See also[]

• Surface reconstruction
• Superstructure
• LEED#Superstructures

Citations[]

References[]

Textbooks[]

• Kolasinski, Kurt W. (2012). Surface Science: Foundations of Catalysis and Nanoscience (3 ed.). Wiley. ISBN 978-1119990352.
• Attard, Gary; Barnes, Colin (1998). Surfaces. Oxford Chemistry Primers. ISBN 978-0198556862.

Websites[]

• "6.1: Classification of Overlayer Structures". chem.libretexts.org. Retrieved 2019-03-12.

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