No abstract

The calcite and vaterite modifications of CaCO 3 possess threefold symmetry axes in their crystal structures, while the aragonite modification does not. Since many self-assembled monolayers (SAMs) of simple thiols on gold organise in a hexagonal close-packed manner, patterns for the templated crystallisation of the first two polymorphs on SAMs are simple to achieve. This is not true in the case of aragonite. A substituted disulphide derived from anthracene-2-carboxylic acid, which has been previously established as forming centred rectangular lattices when assembled on a gold (111) surface, has therefore been employed to form substrates for the crystallisation of CaCO 3 with the specific intention of preferentially inducing the growth of aragonite. For comparison, we have also performed such crystallisations on a SAM substrate derived from hexadecane thiol. While the crystallisation of the CaCO 3 polymorphs is invariably accompanied by the problem of competing phases, SrCO 3 crystallises from solution uniquely in the strontianite modification, which is isostructural with aragonite. We have thus performed the crystallisation of SrCO 3 as well and present the results.

The self-assembly of monolayers of thiols on gold(111) surfaces yields substrates that are able to template in a controlled manner, the nucleation and growth of crystals of calcium carbonate from solution. In the absence of additives, various factors such as the nature of the thiol, the temperature, and the pH are now established as influencing the nature and relative amounts of the different CaCO3 phases (calcite, vaterite, and aragonite). Recently, we have been able to extend the use of thiol/gold self-assembled monolayers as templates for the growth of inorganic crystals by utilizing protected gold colloids instead of flat gold surfaces. The thiol monolayers that protect the colloids provide heterogeneous interfaces for the initial nucleation of the inorganic crystal. The utility of such a designer seeding is demonstrated through the crystallization of CaCO3 in the calcite modification and SrCO3 in the strontianite modification.

In order to find factors which determine the two-dimensional structure of self-assembled monolayers (SAMs), several classes of thiols and disulfides on gold (111) have been investigated by atomic force microscopy (AFM). SAMs were formed from a series of symmetrical and asymmetrical diethylalkanoate disulfides, ω-hydroxyand ω-carboxyalkanethiols, diacetylene disulfides, and different anthracene terminated thiols and disulfides. In all the cases, two-dimensional crystalline structures could be resolved; even for an asymmetrical diethylalkanoate disulfide that had a chain length difference of five methylene units. The lattices were analyzed quantitatively. Two distinctly different types of crystalline structures were observed, namely, a hexagonal and a centered rectangular lattice. For the diethylalkanoate disulfides with short alkyl chains (n e 10) both structural phases were observed, domains with a hexagonal lattice existing simultaneously with centered rectangular domains. The length of the alkyl chain determined the probability of finding disulfides in the hexagonal structure. This dependence on the shape of the molecules as well as the clear contrast of SAMs of asymmetric disulfides suggest that the AFM tip penetrates into the SAMs and probes, at least partially, the interior of the layers. With the atomic force microscope no difference was observed between SAMs formed from thiols and those from disulfides.