Controlling Interface Properties for Advanced Energy Applications
Internal interfaces in materials play an important role in the performance of many devices used in energy applications including solar cells, LEDs, passive electronics, and fuel cells. Efficiencies in energy and power consumption may be realized by optimizing and often miniaturizing these devices. Our studies show that internal boundaries and biomaterial interfaces cause local property variations. These effects will dominate device performance as the systems become smaller. A fundamental understanding of the effect of atomic structure on local properties is a prerequisite to device optimization. Developing this understanding requires new probes that access local properties, controlled interface structure, atomic resolution electron microscopy and first principles calculations of geometric and electronic structure.
New Probes of Local Properties
The two new multiple modulation techniques, developed at Penn, have been shown to successfully overcome barriers to quantifying local electrical behavior.
Scanning Impedance Microscopy SIM
The modulating electric signal is applied laterally across the surface rather than to the tip, allowing R, C, and trap state time constants to be accessed.
Nanoimpedance Microscopy NIM
Monitoring the frequency dependence over 6 orders of magnitude yields the real and imaginary contributions to impedance.
Structure and Properties of Internal Interfaces
The charge at SrTiO3 grain boundaries is due to periodic under coordinated Ti in the boundary so the amount at any boundary depends on the atomic structure, i.e. there is one electron associated with each Ti polyhedron. This interface charge causes local dielectric constant suppression adjacent to the interface. In addition the charge induces ferroelectric dipole formation and alignment at a mid temperature phase transition in SrTiO3.
Extension to Interfaces of Nanocontacts
Characterizing the properties of a metal-semiconductor nano-contact is crucial to the emergence of nano-electronic devices. We have shown that contact potential can be size dependent. Discovering the mode of conduction through the contact and the effects of surface states on the interface barrier are two of the most important aspects of understanding the nature of the contact. By combining SIM with STM and spectroscopy, the fundamental properties governing the interaction of metals and semiconductors can be determined.