Group Research in detail
In our group we are interested in the properties of a material at an interface. In fact, often the system of interest is so small that the surface area dominates over the volume – we called this surface and interface mediated phenomena. We study interface morphology, chemistry and structure and its effect ultimately on the properties.
Group Research Interests
Development of domain wall nanoelectronicsIn 2009 it was demonstrated that certain types of domain walls in an otherwise insulating material are able to conduct current. We have developed devices based on domain wall nanoelectronics, a new avenue to design functional materials by using nanofabrication to artificially engineer domain walls in ferroelectrics. This program funded by the ARC led to a breakthrough paper published in Science Advances and coverage by a number of media outputs. A follow-up paper, showing that the charge state of the wall can be used to access multi-level data storage, was recently published in Advanced Functional Materials.
Using specially designed nanofabricated electrodes and scanning probe techniques, we demonstrate a prototype non-volatile ferroelectric domain wall memory, scalable to below 100 nm, whose binary state is defined by the existence or absence of conductive walls. The device can be read out non-destructively at moderate voltages (<3 V), exhibits relatively high OFF-ON ratios (~103) with excellent endurance and retention characteristics, and has multilevel data storage capacity. Our work thus constitutes an important step toward integrated nanoscale ferroelectric domain wall memory devices.
--Science Advances 2017, 3: e1700512
Topological transitions in ultrathin ferroelectric filmsThe recent discovery of complex domain arrangements in nanoscale ferroelectrics has triggered an explosion in the search for exotic topologies that do not exist in the parent bulk materials. In 2017, our group first demonstrated the nanoscale bubble domains in an ultrathin structured ferroelectric film by tuning its depolarization field via an STO spacer layer. It is found that the bubbles exhibit polarization rotation with mixed Néel-Bloch character to a point where distinguishing between a domain wall and domain becomes nearly impossible. The induced rotation must break the tetragonal symmetry of the parent PZT matrix thereby representing an entirely different ferroic phase.
Manipulation of physical interactions across nanostructured polar oxide thin film interfacesAs semiconductor devices are shrunk towards the atomic-scale regime, quantum mechanical effects move from becoming a small correction to dominating device performance – and offering entirely new functionalities. Using complex oxide heterostructures we show new ways to exploit these. Prof. Nagy’s group has made critical breakthroughs using multiferroic oxide nanoelectronic devices that rely on the fundamental laws of quantum mechanics exploiting electron tunnelling and/or spin transport writing of topological defects via SPM tip bias , SPM based nanoscale control of phase variants in bismuth ferrite and the link between domain wall geometry and transport in ferroelectrics.
Interface engineering of ferroelectric domains in thin films with colossal piezoelectric propertiesNanofabrication and interface engineering of the electrical and mechanical constraints can lead to thin films and nanomaterials systems with superior electromechanical properties, suitable for sensors and actuators. This concept has led to significant outcomes, such as realization of colossal piezoelectric coefficients in ferroelectric thin, origins of non-linear electric field Rayleigh behaviour, thermodynamic theory of inter-domain and inter-layer couplings, giant piezoelectricity in ferroelectric nanowires and ferroic domain motion in thin film multilayers. In 2007, Prof. Nagy co-authored the first report of direct measurement of displacement charge at a polar oxide interface. [Nat. Mater. 6 64 2007] His paper [Appl. Phys. Lett. 84, 5225, 2004] holds the record for the smallest direct measurement of polarization in a ferroelectric.
Effect of atomic scale inhomogeneity on polar interfacesCrucial to all modern nanoelectronic devices, interfaces are present wherever dissimilar materials meet. Due to the aggressive trend for miniaturization, these interfaces have become an unavoidable and dominating factor that influence performance. Even the presence of single atomic –level defect can adversely affect the nanoscale device performance. We used highly sophisticated aberration-corrected microscopy and atomic-scale spectroscopy techniques to not only investigate the consequences but also see how to exploit defects associated with materials lattice. This has led to the ideology of “designer defect engineering”. At interfaces (sometimes buried in the interior of solid samples) unique materials behaviour can emerge and underpin a plethora of fundamentally new properties. Our group demonstrated that when dissimilar materials are stacked against each other with atomic level proximity, the imposed proximity yields a nanoscale composite system with fascinating materials properties not exhibited by either of the constituent materials. [Phys. Rev. B 2014, Nature Comms 2012, ACS Nano 2013] This philosophy underpins strain engineering methodologies explored in this proposal. At such tailored interfaces, the atomic structures induce strong coupling between the mechanical, electrical, and magnetic properties, providing the ability to use electricity to affect mechanical properties or magnetics to affect electrical properties. We have successfully exploited this coupling to yield a new class of interface materials, several orders faster and with lower energy consumption, for technologies such as piezoelectric sensors and semiconductor logic transducers; paving the way for next-generation nanoelectronic devices.
Chemical solution deposition derived ferroelectric thin film with heterostructureThe chemical solution deposition (CSD) technique is of particular industrial interest due to it being low-cost and offering accurate control of the precursor composition, as well as enabling processing ease for large-area wafers. We are the first group in the world to achieve high quality epitaxial bismuth ferrite thin films using CSD method. Recently, we’ve also successfully prepared homogenous mixed-phase bismuth ferrite epitaxial thin films on LaAlO3 substrate by CSD, which shows enhanced electromechanical properties.