New Semiconducting Phases through Inorganic/Organic Co-Assembly

     New semiconducting mesostructured composites have been formed through an Inorganic/Organic Co-assembly. Another continuing area of research in the group is the synthesis and characterization of new semiconducting inorganic phases through co-assembly of Zintl clusters reduced, soluble main group clusters) with organic surfactants. While the work described above nicely shows that optical or magnetic properties can be introduced into insulating silica phases using host/guest chemistry, the range of materials possibilities is much larger if the framework itself is a semiconductor. In our recent work in this area, we have shown that nanoperiodic versions of semiconductors ranging from pure group IV materials (Ge, SiGe, SnGe, etc) to metal-bridged group IV calcogenides (Pt coupled SnTe₄^4-, Rh coupled Ge₄Se₁₀^4-, etc) can be synthesized by solution phase self-organization.^1,2,3,4 By varying the elemental composition of the material, composites can be produced with optical band gaps that span the visible. Moreover, by changing variables as subtle as the oxidation state of the bridging ion, semiconductors can be produced with systematically varying band gaps and conductivities.

      In an exciting new area for us, we have also begun using ultraviolet photoemission spectroscopy (UPS) to determine absolute energy levels for these materials.⁵ We find that in addition to tuning band gaps, we can also tune absolute valence and conduction band energies by changing the composition of the composites. We are currently in the process of examining periodic trends in absolute energy levels to understand how elemental composition systematically controls band energies.

      While the Zintl systems are optimal for producing moderate and narrow band gap amorphous semiconductors, one oxide system also shows promise as a wide band gap semiconductor: titania. Mesoporous amorphous titania can be readily synthesized using surfactant or polymer templating. Unfortunately, the mobility of amorphous titania (which is a hydrated phase) is very low and so the walls must be carefully crystallized to produce a mesoporous crystalline anatase material. Once crystalline grain growth begins, however, it frequently proceeds unchecked and results in destruction of the nanometer-scale periodicity. Random trial-and-error synthesis has shown that under some circumstances, the titania framework can be crystallized without destroying the nanoporous architecture, but there has been no systematic understanding of the process. In recent experiments, we used in-situ X-ray diffraction to determine activation energies for both titania crystallization and for destruction of the nanoscale pores in order to determine the types of thermal treatments that should most effectively crystallize the titania framework without destruction of the nanoscale pores.⁶ The results both help provide a route to periodic nanocrystalline, nanoporous semiconductors, and provide fundamental insight into the links between atomic and nanometer scale restructuring.

      Now that a broad range of periodic semiconducting frameworks have been produced, we are working to combine our host/guest chemistry with these new materials. For example, incorporation of a semiconducting polymer into the pores of a semiconducting framework will produce a semiconductor-semiconductor heterojunction. If the energy levels of the two semiconductors are appropriately aligned, this junction can be used to separate optically-produced electron hole pairs, thus forming the basis for a photovoltaic. Experiments involving semiconducting polymers incorporated into both Zintl and titania-based frameworks show that photoinduced charge separation is possible and that the composites can show a strong photoresponse.