High Nuclearity Clusters and 3-Dimensional Frameworks of Lanthanides
We are interested in developing the fascinating chemistry of high nuclearity lanthanide complexes because of their potential applications in biology, medicine and materials science. In addition, two and three dimensional metal-ligand framework structures which contain pores or cavities which are capable of entrapping small organic molecules are of interest for their applications in catalysis and separation science. Specific design of these kinds of materials allows for the control over useful and/or interesting properties (e.g. magnetic, electronic, optic, host guest etc). Recent studies have focused on the near infra-red (NIR) properties of Yb(III), Nd(III) and Er(III) for use in bioassays and laser systems. Since f-f transitions are parity forbidden suitable chromophores can be used which act as antennas or sensitizers, transferring energy indirectly to the lanthanide ion, and resulting in high absorption coefficients.
In general, control over the stoichiometries and structures of high nuclearity lanthanide complexes is challenging due to the difficulty in controlling the variable coordination environment of the ions. Despite this we have successfully investigated a number of interesting systems which exhibit interesting architectures and photophysical properties. For example, although “salen” type Schiff base complexes have been known for may years, there have been very few definitive structural studies of these materials. We recently reported the synthesis and X-ray structure of the first structurally characterized neutral, binary lanthanide complex Tb4L6 (L= salen). This compound adopts an interesting stacked multidecker structure which appears to be common for complexes of this type. In fact we have structurally characterized a variety of multidecker stacked complexes which contain up to as many as 6 Ln(III) ions and 9 L ligands ( as shown below).
Thin Films of Amorphous Metallic Alloys
The overall goals of this research are to develop a detailed chemical description of the CVD growth processes and precursor decomposition pathways that lead to films of novel amorphous metal alloys. Film growth and materials characterization data provide information relevant to computational studies and both of these provide synergistic feedback to guide the rational design of organometallic precursors. Our initial studies came from a multidisciplinary project with the research groups of Professors John Ekerdt and Gyeong Hwang (Department of Chemical Engineering, UT Austin) aimed at studies of CVD and atomic layer deposition (ALD) film growth of thin films of crystalline metals.
Certain types of amorphous materials, in which there is no long range order at the atomic level, are well known. For example, optically clear glasses based on oxides of silicon, metalloids and metals have been known for thousands of years. In the microelectronics industry amorphous silicon is used in solar cell technologies and amorphous films of Si and Si/Ge alloys have applications as semiconductors. More recently amorphous thin films of transition metal nitrides such as WN and TaN have been investigated for barrier layer applications and diffusion barriers. The amorphous nature of SiO2, among other characteristics, has proven central in its role as the gate dielectric in microelectronics. Interest in thin films of amorphous metallic alloys stems from their potential use as catalysts, in ultra large scale microelectronics devices (ULSIs) and as diffusion barriers.
Thin film growth by chemical vapor deposition (CVD) offers several important advantages over physical growth methods including economical scale-up, ease of operation, mild growth conditions, and the ability to achieve conformal coverage on features with high aspect ratios. There are very few examples of thin film growth of amorphous metallic alloys using CVD. This may be due to the fact that the chemical nature of the precursor is critically important to the outcome of the CVD process and suitable precursors for these materials have simply not been developed.
Initial Discoveries and Future Work
Since ligand selection can often have a significant effect on the outcome of a CVD process we explored the use of cis-H2Ru(PMe3)4 (1), which contains Ru-H and Ru-P bonds. The Ru-H bond was selected to facilitate dissociative adsorption of 1, and the PMe3 ligands were selected as potentially stable, volatile leaving groups. Surprisingly, under the growth conditions employed, highly conformal, smooth films of amorphous RuP alloys (P 15-20 %)
Our future studies are aimed at the development of suitable molecular precursors designed for the CVD of thin films of amorphous metallic alloys. Examples of several compounds currently under investigation are shown below.