The surfaces of ferroelectrics are particularly interesting in catalysis studies because the material’s polarizability offers one additional parameter that can be tuned (and potentially varied in time) to influence catalytic reactions. Thus, scientists are trying to understand better these materials’ surfaces at the atomic scale. In a new study just published online in Nature Communications, scientists from TACO subprojects P02 and P07 and their international collaborators target the perovskite crystal KTaO3 that forms a prototypical polar surface.
When cleaved along the (001) surface, KTaO3† forms KO and TaO2 terraces. Because of the polar nature of the surface, intrinsic excess charge (i.e., electrons) accumulates at the TaO2 termination. To date, one question that has remained unanswered was how this extra charge arranges at the surface. Does it disperse to form a two-dimensional electron gas (2DEG), as is commonly assumed, or does it localize to form polarons and other charge inhomogeneities?
The scientists combined atomically resolved scanning tunneling microscopy (STM) of cleaved KTaO3(001) samples with detailed density functional theory (DFT) calculations to tackle this question. Surprisingly, the STM images revealed solid evidence that the excess charge does not disperse homogeneously like a 2DEG but instead localizes in various forms on the surface. These include charge density waves (CDW), polarons, and bound pairs of polarons called bipolarons. Furthermore, these structures form ordered patterns on the TaO2 terraces.
As usual, these experimental results require underpinning by theoretical calculations. The team of scientists performed DFT simulations of the polar TaO2 termination to gain further insights into the processes. These calculations were done on the Vienna Scientific Cluster using the renowned VASP package. It turns out that, indeed, CDWs, polarons, and bipolarons (and combinations of them) are energetically favorable to the 2DEG. They can even coexist, which is in agreement with experimental results. The simulations confirm the rich electronic physics on this polar surface, where electronic states compete with each other.
The novel physical effect could open the possibility of controlling surface ferroelectric polarization by charge trapping. It could also be important for particular applications in catalysis. Therefore, in a second forthcoming study, the team of authors looks into details of the chemistry on the KTaO3(001) surface. This study is expected to be published in late August and will be announced here.
†Incidentally, the chemical formula could also be written as TaKO3, which we do like for obvious reasons.
In this video, first author Michele Reticcioli talks about the paper:
Abstract:
Excess charge on polar surfaces of ionic compounds is commonly described by the two-dimensional electron gas (2DEG) model, a homogeneous distribution of charge, spatially-confined in a few atomic layers. Here, by combining scanning probe microscopy with density functional theory calculations, we show that excess charge on the polar TaO2 termination of KTaO3(001) forms more complex electronic states with different degrees of spatial and electronic localization: charge density waves (CDW) coexist with strongly-localized electron polarons and bipolarons. These surface electronic reconstructions, originating from the combined action of electron-lattice interaction and electronic correlation, are energetically more favorable than the 2DEG solution. They exhibit distinct spectroscopy signals and impact on the surface properties, as manifested by a local suppression of ferroelectric distortions.
