Carbon as a fundamental building-block or framework element has dominated theoretical discussions of molecular nanotechnology (MNT). I have previously suggested that silicates (not silicon) provide an interesting alternative. Such structures consist of SiO4 tetrahedra sharing bridging oxygens to form 2- and 3-D frameworks. A number of other elements, however, are also capable of forming highly polymeric oxide frameworks, in particular certain transition metals such as Ti, V, Mn, Nb, Mo, Ta and especially W. Some can form frameworks in combination with main-group elements such as Si and P (e.g., Ti and W phosphates), whereas others can form frameworks alone; e.g., WO3 has several polymorphs consisting of open structures formed by corner-sharing octahedra. Such framework-forming elements have stable high oxidation states that are nonetheless capable of sharing bridging oxygens.
For nanotechnology the potential advantage of such transition-metal oxide frameworks (TMOFs) is that their bulk electronic and optical properties can be varied drastically by reduction or oxidation of some of the framework atoms, due to the multiple stable oxidation states of a transition element. The same framework can thus be shifted from (say) an insulator to a metal merely by changing the redox potential, in contrast to frameworks based on main-group elements.
Tungsten bronzes, for example, which are hard, brittle crystals with metallic luster and conductivity, consist of a WO3 framework in which some of the open sites are occupied by monopositive ions (e.g., Li+, Na+). Charge balance is preserved by reduction of some of the framework atoms from W6+ to W5+, with the result that electrons become delocalized to yield typical metallic behavior. In contrast, stoichiometric WO3 itself is a wide-bandgap insulator.
Reversible bronze formation, driven by an electric potential, is the basis of so-called "electrochromic" systems: application of a potential reduces some W6+ to W5+, with charge balance maintained by intercalation of small ions such as H+ or Li+. The material thus changes from transparent to reflective. These materials have been proposed for "smart" windows and for information storage. In "photochromic" systems, the redox change is driven instead by photon absorption, a particularly attractive feature for (say) a self-shading window. Conducting TMOFs, being "metals" whose work functions are sensitive to their chemical environment, are also finding application in chemical sensors.
The intercalation of small cations to maintain charge balance that accompanies framework reduction is also of obvious interest for solid electrolytes, e.g., for fuel cells. Switchable intercalation of ions out of an adjacent solution is also promising for a molecular separator, with obvious applications in pollution control and resource extraction. For example, reversible intercalation of Li+ into _-MnO2 has been suggested for selective extraction of lithium from brines.
Many TMOF-forming elements also form oligomeric oxyanions, so-called polyoxometallates, which are stable or metastable in aqueous solution over wide pH ranges. These are potentially attractive molecular building blocks. Thus, as with silicates, TMOFs could possibly be assembled molecularly directly from aqueous solution.