Modifications of the chemical and electronic ferroelectric surface structure under water adsorption – Surf-FER

Ferroelectrics (FE) are of prime importance in many applications, from FE based memories, through capacitive components to nanoscale resistive switching. Since the polarisation may be switched by applying an electric field, FE may provide a low power consumption alternative in electronics. However, the charge state of the surface is crucial in determining the interaction of the FE with its environment. Most FE will function under near ambient conditions; it is likely that relaxation and adsorption phenomena must be integrated into the description of performance.
A second major domain of potential applications is the field of FE-assisted photocatalysis. Typical FE can be assimilated to wide band gap semiconductors. The charged surface state can be a source of electron-hole pairs renewable under UV illumination, enhancing photochemical reactions such as water dissociation to provide a new and efficient mode of H2 production.
In FE, the surface charge creates a depolarizing electric field opposite to the field responsible for the FE polarization. The surface charge may be partially or fully screened by free charge carriers or defects in the bulk or by charges from adsorbates. Thus, a FE is a classical example of how bulk properties may be determined by surface physics. In nanoscale films, the charge state of a FE surface can even impose ultimate limits on polarization and thickness.
Stability of FE phases and states is determined by a balance between bulk thermodynamics and screening mechanisms for polarization, which may include formation of stripe domains, in-plane closure domains, domains branching, surface charge compensation due to band bending, oxygen vacancy formation, or chemisorption of charged species. Through the water adsorption and dissociation, H2O molecules and OH groups can interact with the polarization of the FE to form particular electrical boundary conditions. The presence of hydrogen in FE also leads to imprint effects, stabilizing one particular direction of the polarization.
Experimental data on surface FE states is dominated either by area averaged diffraction, or by near field microscopy. Neither furnishes full spectroscopic information directly on the electronic structure. Thus, non-destructive surface science techniques, allowing high resolution analysis of the atomic, chemical and electronic structure of FE surfaces is required.
Surf-FER will study how the polarization affects molecules adsorption on a FE surface and how adsorption may in return affect the FE properties.
The second aim will be to understand how the adsorbed molecule might be dissociated under illumination, with a view to using FE materials to promote water photolysis by favouring electron-hole separation and surface reactivity.
Surf-FER is a systematic study of the polarization dependent atomic, chemical and electronic structure of FE domains before and after exposure to water. The consortium unites expertise in the required experimental and theoretical tools, making it unique and scientifically powerful.
Epitaxially strained films will be grown using Pulsed Laser Deposition and Molecular Beam Epitaxy. The use of nanometric films will make the results pertinent to future applications in nanoelectronics and clean H2 production. The combination of near field tunneling and far field spatially resolved electron spectroscopy will allow for the first time access to full spectroscopic information on sub-micron sized FE domains.
The chemistry of adsorption/dissociation will be directly correlated with atomistic structural modifications and electronic structure changes induced by a modification of the surface charge and polarization. State of the art DFT calculations will be developed to compute the externally unscreened surface as produced after deposition in UHV.
The results obtained within this project will go well beyond the photocatalysis aspect since it concerns the general behaviour of FE films and of charged surfaces.