Influence of the ferroelectric polarization on the electronic structure of BaTiO3 thin films

Enhanced piezoresponse in van der Waals 2D CuCrInP2S6 through nanoscale phase segregation

van der Waals metal chalcogen thiophosphates have drawn elevated interest for diverse applications, including energy harvesting, electronics and optoelectronics. Despite this progress, the role of nanoscale ion migration in complex intermediary thiophosphate compounds has not been well understood, resulting in their structure-property characteristics remaining elusive. Herein, we focus on copper-deficient CuCrInP2S6 as a prototypic layered thiophosphate compound to address this shortcoming. Piezo force microscopy reveals that this material exhibits unusual cage-like domain networks with an enhanced piezo response at the domain boundaries. The associated piezoelectric coefficient d33 is found to be among the highest across reported van der Waals multi-layered materials. These results are further complemented with Kelvin probe microscopy and second harmonic generation spectroscopy that disclose significantly elevated non-linear optical emission along these domain boundaries. Ab initio calculations performed in conjunction with nudge elastic theory provide a deeper insight into the diffusion processes responsible for these observed phenomena. These findings shed new light into intermediary thiophosphate based 2D compounds, highlighting future prospects towards their use in emergent piezoelectric based technological applications.

Effect of Average Grain Size on the Uniformity and Ferroelectricity of BTO/PSX Thin Films Processed by Low-Temperature Solution Method

In this study, we utilized 50 nm BaTiO3 (BTO) nanoparticles and polysiloxane (PSX) with a higher concentration of methyl and silica groups to fabricate insulating layers at a low curing temperature of 100 °C using a solution-based method. This approach aims to enhance film uniformity while retaining the ferroelectric properties. Consequently, we maintained a minimal leakage current in thin-film transistors (TFTs) while achieving transfer characteristics characterized by a distinct hysteresis. Moreover, we verified the presence of ferroelectricity in 50 nm BTO nanoparticles. Compared with prior research, we confirm that decreasing nanoparticle size effectively reduces film roughness but also leads to a reduction in polarization intensity due to smaller diameter BTO nanoparticles. Additionally, a higher proportion of methyl and silica groups effectively lowers the curing temperature of PSX. At the same time, the hydrogen ions released in the polycondensation reaction can also effectively suppress the oxygen vacancies at the interface between dielectric and channel layers, improving the TFT electrical characteristics.

inserexs: reflection choice software for resonant elastic X-ray scattering

This paper presents inserexs, an open-source computer program that aims to pre-evaluate the different reflections for resonant elastic X-ray scattering (REXS) diffraction experiments. REXS is an extremely versatile technique that can provide positional and occupational information about the atoms present in a crystal. inserexs was conceived to help REXS experimentalists know beforehand which reflections to choose to determine a parameter of interest. Prior work has already proven this to be useful in the determination of atomic positions in oxide thin films. inserexs allows generalization to any given system and aims to popularize resonant diffraction as an alternative technique to improve the resolution of crystalline structures.

Investigation of local distortion effects on X-ray absorption of ferroelectric perovskites from first principles simulations

Understanding the role of ferroelectric polarization in modulating the electronic and structural properties of crystals is critical for advancing these materials for overcoming various technological and scientific challenges. However, due to difficulties in performing experimental methods with the required resolution, or in interpreting the results of methods therein, the nanoscale morphology and response of these surfaces to external electric fields has not been properly elaborated. In this work we investigate the effect of ferroelectric polarization and local distortions in a BaTiO₃ perovskite, using two widely used computational approaches which treat the many-body nature of X-ray excitations using different philosophies, namely the many-body, delta-self-consistent-field determinant (mb-ΔSCF) and the Bethe-Salpeter equation (BSE) approaches. We show that in agreement with our experiments, both approaches consistently predict higher excitations of the main peak in the O-K edge for the surface with upward polarization. However, the mb-ΔSCF approach mostly fails to capture the L_2,3 separations at the Ti-L edge, due to the absence of spin-orbit coupling in Kohn-Sham density functional theory (KS-DFT) at the generalized gradient approximation level. On the other hand, and most promising, we show that application of the GW/BSE approach successfully reproduces the experimental XAS, both the relative peak intensities as well as the L_2,3 separations at the Ti-L edges upon ferroelectric switching. Thus simulated XAS is shown to be a powerful method for capturing the nanoscale structure of complex materials, and we underscore the need for many-body perturbation approaches, with explicit consideration of core-hole and multiplet effects, for capturing the essential physics in these systems.

Thickness-dependent polarization of strained BiFeO3 films with constant tetragonality

We measure the ferroelectric polarization of BiFeO3 films down to 3.6 nm using low energy electron and photoelectron emission microscopy. The measured polarization decays strongly below a critical thickness of 5-7 nm predicted by continuous medium theory whereas the tetragonal distortion does not change. We resolve this apparent contradiction using first-principles-based effective Hamiltonian calculations. In ultrathin films, the energetics of near open circuit electrical boundary conditions, i.e., an unscreened depolarizing field, drive the system through a phase transition from single out-of-plane polarization to nanoscale stripe domains. It gives rise to an average polarization close to zero as measured by the electron microscopy while maintaining the relatively large tetragonal distortion imposed by the nonzero polarization state of each individual domain.