Self-assembly of 8CB on grafite
"In the nanoworld things are possible, which are impossible, and even inunimaginable in the normal world. When you throw a box of bricks higgledy-piggledy on the floor and you wait till a wall or a house builds itself then you can wait for years or centuries but nothing happens. If you think that some extra energy might help then try to warm the stones with a hairdryer: Yet nothing happens! Even if the hairdryer is applied for years, still no walls are erected.
This example looks quite silly but in the nanoworld it is reality. When you put a little bit of liquid crystal, like 8CB (a vaseline-like liquid), on a substrate like graphite and you heat it with a hairdryer the molecules start to arrange themselves nicely in an ordered fashion on the substrate during cool down. The molecules lie down with their heads together in groups of eight. The ideal instrument to observe this behaviour is the STM because the STM can visualize the individual molecules."
The above text has been written (by Former van der Ploeg and Rens Vingerhoets) during one of the master classes for high school students at the University of Nijmegen. It descibes very well how different the nanoworld is from the normal everyday world.
Self-assembly plays an important role in nature where it is often a very complex process. By considering more simple model systems we can get more insight in the self-assembling process and we can perform experiments with it.
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| The model for a self-assembled layer of 8CB |
Self-assembly often comes about by a subtle balance of mutual attracting and repulsive forces between molecules, the substrate, and the environment.
In the case of 8CB the CN heads of the molecules have an electric dipole charge, which leads to a preferential configuration with the heads against each other. This way the charges are counterbalanced. The tails of the molecules adsorb easely on the graphite subtrate and also have mutual interactions favoring positioning against each other. Also other parts of the molecules, the phenyl rings, have mutual attractive interactions. The balance of all these forces determines the obtained structure.
Animations of the model
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self-assembly of 8CB
measured with a STM.
489kbytes .gif
A click of the mouse starts the animation! |
The figure shows that in this case similar parts of the molecule settle down besides each other.
The model is not always correct
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| Sometimes another structure.(75*75 nm) |
Sometimes, but rarely, an other structure shows up, even when the sample preparation is apparently the same. We do not know why but it shows nicely how a probable small effect can disturb the subtle balance of forces which governs the selfassembling process
Other model systems
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| Isophtaal-acid.(26*26 nm) |
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| candle wax(35*35 nm) |
There are many model sysems we can study. Some examples are isophtaal-acid on graphite and stearine-acid (candle wax) on graphite
Jan Gerritsen.
latest update: 27 Jan 2004
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