Cluster Self-Assembled Thin Films
The field of study of preparation and properties of clusters of molecules and atoms has moved from the margins of academic curiosity to mainstream materials design activity. In the broad field of nanotechnology bounded by the 1–100 nm length scales, science of molecular clusters is at the interface between sub-atomic and macroscopic science, at the left end of the spectrum. Physical principles governing cluster(s) properties are not generally predictable and new discoveries are being made on a regular basis that tend to suggest that cluster science will become a dominant scientific discipline of the 21st century. Well-engineered cluster deposition systems available today from equipment manufacturers have boosted activity in the field of cluster-assembled solids.
So far, most researchers have focused on inorganic clusters, primarily due to their size-dependent properties, e.g., optical and electrical. One of the goals for cluster(s) research is to be able to custom build solids that use them as basic building blocks in which they retain their special properties or collectively exhibit new phenomena. This capability would, in principle, lead to creation of a wide range of new materials in the optical, electronic, and bio fields, which would not be possible otherwise.
Our research interests are distinguished from other major efforts in the field by focusing on clusters based on organic, organo-metallic and bio-materials. These have not received the same level of attention as the inorganic materials because it is generally believed that the weak electronic coupling in the absence of delocalized electrons in molecular-based materials would preclude new phenomena at small sizes. Emerging interest and importance of this field is actually an accidental by-product of the display and flexible electronics industries’ need to invent an atmospheric (non-vacuum) web coating process capable of producing device quality, thin films of organic materials. One of the issues associated with such low/non-vacuum processes is inherent tendency to create clusters of atoms or molecules which are generally considered to be defect (pinning) sites for electron/hole transport processes in thin films devices, leading to deteriorated performance.
Our group embraced the inherent tendencies of atmospheric processes to form clusters to build cluster-assembled thin films. One concern was that such clusters of molecules would be held together by weak van der waals forces and hence would be meta-stable. Serendipitously, the custom designed, patented process which was chosen for precipitation of molecular clusters, namely, Rapid Expansion of Supercritical Solvents (RESS) at constant pressure and temperature, generated cluster structures which were found to be stable for years. The momentary presence of extreme precipitation conditions such as turbulent, high shear flows at extremely cold temperatures induced by Joule-Thomson cooling in the supersonic CO2 jets appeared to have created “kinetically locked-in” (meta-structures) cluster structures. It was believed that these structures, which were, thermodynamically “forbidden” under standard processing conditions, once formed, became very stable because, by reverse logic, they would experience energy barriers (i.e., for dissociation) similar to those which prevented their formation under standard conditions.
It has been speculated that these non-equilibrium clusters have intermolecular distances and configurations that would be uncommon, according to standard thermodynamics rules. These clusters do not behave like normal solids and do not undergo agglomeration or Oswald ripening to minimize excess surface free energy. Substantial experimental evidence has been generated to confirm their tendency to spontaneously organize into stable, 3-D super-lattice structures with a high degree of alignment. The absence of any measurable signal in the XRD measurements at wide angles implied lack of order at the molecular length scales.
Many of these organic clusters were found to be liquid-like at room temperature and had process dependent photoluminescence spectra. Metal surfaces coated with thin cluster films retained their super-hydrophobicity even when subsequently covered by a coating of 20 nm thin gold-palladium metal. Functional OLED devices built with cluster-assembled films efficiently transported holes and electrons through them, which is important for device applications.
The constant pressure and temperature RESS process, as a nano-materials fabrication platform has resulted in 24 US patents. Many of these results are reported in previous publications.