"We used a modelling technique based on quantum mechanics, called density functional theory, to study the properties of model ferroelectric thin films and nanoparticles.
1st objective: role of surface and interface in ferroelectric thin films.
In this section of the project we looked at two different ferroelectrics: lead titanate (PbTiO3) and bismuth ferrite (BiFeO3). The reason for studying these two materials is that they present different properties which could affect the surface and interfacial behavior.
We first examined PbTiO3. We used density functional theory to disentangle the role played by the surface, interface and electrostatic properties in determining the direction and strength of the polarization in thin film. We observed that the direction of the polarization depends only on electrostatic properties and not on the nature of the chemical bonding at the surface and interface. We also show that the structure of the interface and surface can be tailored toward a specific polarization direction and strength, and that great control in the engineering of ferroelectrics thin films can be achieved. This can be done by engineering specific defects or adsorbates at the surface or at the interface. An example of this is an O adatom (with a -2 formal charge) screening the surface charge of a positively charged surface (occurring if the polarization is pointing towards the surface). Conversely, depositing an O adatom (with a -2 formal charge) on a negatively charged surface (occurring if the polarization is pointing away from the surface) would lead to the opposite effect: the large negative surface charge would be unfavorable and the polarization would flip direction. This work has been recently published [1,2].
In BiFeO3 we instead studied the interaction between the ferroelectric polarization and the charged layers. Indeed, In the (001) direction bismuth ferrite has a structure of layers with alternating positive and negative charge. The surface charge due to the ferroelectric polarization, and the amount of charge needed to screen the charged layers is the same in magnitude in bismuth ferrite. The sign instead depends to the direction of the ferroelectric polarization and the surface termination. Thus a system where the two contributions cancel out can be constructed, and they have non-charged surfaces. On the contrary, when the sign of the two contributions is the same, heavily charged surfaces occur. We showed that such charged systems are unstable, and in our simulations the polarization direction spontaneously reverses, leading to the system with uncharged surfaces. We also showed that, as in the case of PbTiO3, defects can effectively screen this surface charge and lead to a stabilization of this unfavorable system. This work is under review, and a preprint is available [3].
2nd objective: water splitting using ferroelectric materials.
We investigated how the difference in surface charge in the self-compensating (""happy"") and charged (""unhappy"") surfaces of bismuth ferrite (001) affects the behavior of adsorbed water. We observed that water adsorbs molecularly on happy surfaces and dissociatively on unhappy ones. Moreover, we found that the dissociation products of water can stabilize the unhappy surface, H+ ions compensating the charges on the negative surface and OH- ones those on the positive one. We used these results to build a water splitting cycle which goes as follows (see image Summary.jpg):
1) Water adsorbs molecularly on a happy system;
2) If the ferroelectric polarization is externally switched using, for example an electric field, the system becomes unhappy and water dissociates
3) Only the dissociation products which stabilize the surface charges are left on the surface
4) By switching the polarization a second time, the surfaces are happy again. More dissociation products desorb and molelcular water takes their place on the surface.
This work is under review, and a preprint is available [3].
References:
1. Chiara Gattinoni, Nives Strkalj, Rea Hardi, Manfred Fiebig, Morgan Trassin, and Nicola A. Spaldin,, Proc. Nat. Acad. Sci., vol 117 (2020)
2 Nives Strkalj, Chiara Gattinoni, Alexander Vogel, Marco Campanini, Rea Haerdi, AntonellaRossi, Marta D. Rossell, Nicola A. Spaldin, Manfred Fiebig and Morgan Trassin, Nat. Comm. accepted (2020)
3 Ipek Efe, Nicola A Spaldin, Chiara Gattinoni,
https://arxiv.org/abs/2010.14895(odnośnik otworzy się w nowym oknie)"
