Thin Film Oxides and Interfacial Materials
15 May 2009
The central motivation underpinning Thin Film Oxides and Interfacial Materials research is that when dissimilar materials are stacked against each other with atomic level proximity and forced to interact with each other under constrained conditions, the imposed geometrical constraint yields a composite system with novel materials properties that are not exhibited by the constituent materials. Essentially, the stacking of dissimilar materials creates an artificial physical boundary, commonly referred to as an “interface”. By innovative manipulation of the atomic nature of the interface, we are able to create new generation materials systems. The above motivation was exploited in 2009 to demonstrate two particular successful examples:
Our research team demonstrated a highly innovative method to achieve superior piezoelectric properties in nanolayered ferroelectric oxide films on technologically relevant Silicon wafers. This work, published in Advanced Materials (Varatharajan et al 2009), provided the first practical, inexpensive method to achieve attractive piezoelectric properties, at the nanoscale, using industrially viable synthesis and fabrication platforms. This simple yet pioneering solution involved coating a mechanically pliable yet chemically stable layer between the piezoelectric ferroelectric material and an underlying silicon substrate. This coating was achieved by a facile spin-on process and as such employs an extremely economical fabrication method. By manipulating the mechanical interactions between the layers, we were able to create hierarchical microstructures (see Figure 1), typically observed only in bulk samples. This is a critical finding, as it not only led to dramatically enhanced piezoelectric response but also reduced the driving voltages (by almost 500%) required to achieve the superior piezoelectric response, thereby improving energy-efficiency. These observations have also been explained with a comprehensive non-linear thermodynamic model that explores in complete detail the energy landscape for ferroelectric bilayers and their response to external stimuli.
A second focus is the development of new Pb-free ferroelectrics. In this program we use atomic scale intermixing to create what is called “compositional gradients” in a single sample. Using specific rare-earth dopants, the group demonstrated novel morphotropic phase behavior in Pb-free films. Particularly we showed that the phase microstructure at the morphotropic phase boundary is extremely complex. (Cheng et al , Phys. Rev B 2009) Instead of a single-phase material, the morphotropic phase boundary demonstrates an assemblage of 4 different lattice orderings (Figure 2), and thus creating in the process what we now call as “nanodomain” hierarchical structure.