By studying oligomers (molecules built out of a smallnumber of identical units) with particular end groupsbut varying molecular length, many researchers haveconfirmed that off-resonant conduction is proportionalto exp( -BL), where L is the molecular length and B isthe decay constant, as expected from the picture of aneffective tunnelling barrier. Systematic studies of dif.ferent types of molecules have examined what, physi-cally, sets the decay constant. Unsurprisingly, it turnsout that the chemistry of the molecules matters greatly.Molecules that we tend to think of as insulating ("sat-urated" molecules, such as waxes) have large decayconstants, while molecules with big, puffy, extendedorbitals ("conjugated" molecules, the extreme limitbeing graphite) have small decay constants. By fixingthe molecule type and varying the end groups, research-ers have also examined the effect of molecule-metalcontacts on both conductance and junction stability.
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