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The behavior of molecules and materials
is determined in many instances not by what happens in their
interior, but by what happens on their surfaces. Chemical
reactions often take place at surfaces - and surfaces
can promote chemical reactions, as in the field of
heterogeneous catalysis which contributes so much to the
chemical industry and hence to our lives. Friction, the
overcoming of which consumes huge amounts of energy,
takes place between surfaces sliding over each other. The
growth of crystals takes place at surfaces; and if we
want to prevent it, we need to modify the surface.
Computers are guiding and illuminating our knowledge of
surface structure and behavior.
A sodium chloride
(NaCl) surface. There is a slight rearrangement of the
surface: sodium ions move into the surface and chloride
ions relax away from the surface.
We will look first at our ability to model the structure
and energies of the surfaces of crystals, which we will
show leads into modeling their shape, or morphology. We
will then consider the reactions at surfaces and the
catalysis, before looking at the intriguing problem of
the interfaces between different solid materials.
Atomic Arrangement at
Surfaces
What do the structures of
surfaces look like at the atomic level? This might seem
an easy question to answer. Can't we simply consider a surface as being
made by slicing a crystal in two? In which case,
is not the structure of the surface just like that in a
crystal? In many cases, this type of model seems to work
well. For example, the surfaces of many metals are, to a
good approximation, terminations of the structure of the
crystal, as illustrated.
But the approximation is in many cases inaccurate and in
some cases completely wrong. We
will start with the undramatic but significant changes in
surface structure manifested by that simple, classical
crystal structure, sodium chloride. In understanding the
subtle changes that occur on the surface of the material,
the computer can again come to our aid. The image above
shows a simulated structure of the surface of NaCl
(obtained using energy minimization methods of the type
described in elsewhere on this site).
Note that both the Na and Cl ions are slightly displaced
perpendicularly to the surface:
the Na+ ions move
slightly into the surface and the Cl-
ions move slightly away from the surface. Clearly the atoms on
the surface layer are subjected to different forces from
those in the bulk of the crystal; and they respond by
making these small but significant displacements.
Here are further examples where there are far more
substantial rearrangements:
Rearrangement in the
surface of Al2O3.
The surface of silicon
exhibits rings of silicon atoms.
In the first corundum, Al2O3, we see that there are very substantial
changes in the separation between the layers of atoms at
the surface compared with corresponding ones in the bulk
crystal.
Perhaps the most dramatically
'different' surface structures are found with the
reconstructed surfaces of semiconductors such as silicon.
The bond breaking which accompanies the formation of the
surface is followed by an extensive re-bonding process
leading to fascinating new, complex surface structures,
one of which is shown.
Once again computational methods have proved
of great value both in helping to elucidate the details of these structures
and in giving guidance as to the factors controlling their stability.
Steps and kinks on a metal surface.
The perfect surface structures shown here
are idealized. Real surfaces contain irregularities,
and some of the commonest are shown above, where we see 'steps'
separating regions of 'perfect' surface (known as terraces).
These steps
themselves contain irregularity - the corners and kinks shown on the figure.
In addition, we can have defects and irregularities at surfaces.
Such sites may play important roles in chemical reaction at surfaces as
discussed later.
Image of a hard tip interacting with a surface.
Our ability to learn about the structures
of real surfaces has been revolutionized in recent years by the discovery
of new types of microscopy which are all based on a very simple idea, which
is that if you want to learn about the structure of an object, one strategy
is to poke it with something sharp. The image above illustrates the
basic mode of operation of these increasingly widely used techniques
for imaging surfaces.
An afm tip scanning a surface.
Here an atomically sharp tip scans over
the surface. And in the version of the technique we will now consider,
known as Atomic Force Microscopy, as it scans the surface it is constrained
to have a constant interaction force with the surface. To do this, the
tip must move up and down; and these motions are in response to the structure
of the surface, and can be translated into an image of the surface as shown.
But if we are to interpret these images,
we need to be able to model the interaction between the surface and the
tip. Recent work of Alexander Shluger, and colleagues has shown how we
can simulate tip-surface interaction. The image below shows a
tip model of
magnesium oxide scanning over a rock salt (sodium chloride) surface maintaining
a constant tip-surface force. The tip moves up and down and we can use
this to produce the simulated image shown.
A model MgO AFM tip interacting with a NaCl surface.
A model MgO AFT tip interacting with a NaCl surface with a step.
These simulations show, however, that sometimes
the tip can drastically perturb the surface. Here we see what happens when
the tip scans over an edge. There is now major rearrangement as the tip
drives into the edge. Clearly this kind of information is
vital if we are to plan and interpret these experiments properly.
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