Direct observation of strong surface reconstruction in partially reduced nickelate films

Surface and interfacial polarity in complex oxide thin films significantly impacts their physical and chemical properties, influencing ferroelectricity, superconductivity, magnetism, and catalysis. Controlling crystal plane orientation and termination during film growth allows for the creation of polar surfaces and interfaces, where structural distortions arise from the interplay of short-range covalency and long-range electrostatic effects. The abrupt reduction in coordination at surfaces alters lattice and electronic structures, potentially affecting the overall properties of the oxide material. For instance, in LaNiO₃, polar distortion and octahedral rotations at the polar surface weaken Ni 3d and O 2p orbital hybridization, decreasing metallicity and leading to thickness-dependent transport behavior. The excess charges on polar surfaces can give rise to charge-density waves, localized electron polarons, and two-dimensional electron gases (2DEG). Surface polarization also enhances electro- and photocatalytic performance by tuning adsorption intensity and charge separation. Controlling surface polarity is crucial for engineering functionalities in electronic devices and surface catalysts. However, achieving precise control over single atomic layer terminated crystal film surfaces and probing their local atomic and electronic structures presents a significant challenge. Theoretical studies suggest that electronic orbitals can be modified by polar surface distortions; for example, the NiO₂-terminated negatively charged surface in LaNiO₃ exhibits large orbital polarization due to the eliminated Ni-O bond in the out-of-plane direction. Surface termination can also influence polar distortions and electronic conduction in LaNiO₃. Other studies have shown polarization-controlled surface reconstruction and competing electronic states at polar surfaces. Infinite-layer nickelates, which become superconducting upon doping, are synthesized through oxygen deintercalation via topochemical reduction, which can modify polarity at interfaces and surfaces due to the removal of apical oxygen ions. While theoretical calculations predict high Ni *e_g* orbital polarization due to polar distortion, experimental studies are limited due to challenges in sample synthesis and characterization. The present study aims to control oxygen deintercalation to modify surface polarity in a Pr_0.8Sr_0.2NiO_2+x film, using atomic-resolution electrostatic-field imaging via 4D-STEM and STEM-EELS to image atomic and electronic structure variations at the surface layer.

Extensive research has explored the impact of surface polarity on the properties of complex oxide thin films. Studies on LaNiO₃ have demonstrated the influence of polar distortion and octahedral rotations on metallicity and transport behavior. The role of surface termination in modifying polar distortions and electronic conduction has also been investigated. Theoretical calculations have predicted significant orbital polarization in certain surface terminations. The formation of two-dimensional electron gases (2DEGs) at polar interfaces has been a significant area of research, particularly in systems like LaAlO₃/SrTiO₃. The importance of surface reconstruction and the interplay between competing electronic states at polar surfaces has been highlighted in various studies. The synthesis of infinite-layer nickelates via topochemical reduction and its effect on surface polarity have been theoretically explored, with predictions regarding Ni *e_g* orbital polarization. However, experimental verification and detailed atomic-scale characterization of these effects have remained limited due to the difficulty in sample preparation and characterization.

A pristine Pr_0.8Sr_0.2NiO₃ film was grown on a NdGaO₃ (110) substrate using ozone-assisted layer-by-layer molecular beam epitaxy (MBE). The sample was then cut into pieces, and two were subjected to topochemical reduction at 230 °C for 6 and 18 h with CaH₂ powder in a sealed tube. TEM samples were prepared using focused ion beam and further cleaned by a Fischione NanoMill system. STEM imaging and EELS spectrum imaging were performed using a probe-corrected electron microscope (JEOL ARM200F) at 200 kV. STEM-ABF and HAADF images were acquired with specific collection semi-angles. A Gatan K2 camera was used for EELS with an energy resolution of ~1 eV. 4D-STEM experiments were conducted in 1-bit mode with continuous reading and writing. Electrostatic field maps were calculated using a simplified quantum mechanical model, relating the projected electric field to the change in momentum transfer of the electron beam and sample thickness. EELS fine-structure analysis of Ni-L_2,3 and O-K edges was performed to study the electronic structure, with spectra extracted layer by layer. Elemental distribution was analyzed from layer-by-layer resolved O-K EELS maps, with cation atom columns identified using Ga-L_2,3, Nd-M_4,5, Pr-M_4,5, and Ni-L_2,3 edges. Atomic-resolution STEM-EDX was employed to assess Sr distribution. The oxygen content was determined by integrating O K edges and normalizing with Ni-L_2,3 edges.

High-angle annular dark-field (HAADF) STEM images revealed a Ni displacement of ~0.27 Å at the surface of the pristine Pr_0.8Sr_0.2NiO₃ film, decreasing to zero within ~3 unit cells. In the reduced Pr_0.8Sr_0.2NiO_2+x film, this displacement increased to ~0.37 Å (6 h reduction) and ~0.45 Å (18 h reduction). Annular bright-field (ABF) images showed the coexistence of oxygen octahedra rotation and polar distortion in the pristine sample, with the polar distortion dominating the surface region. The reduced sample exhibited a stronger polar distortion (~0.56 Å displacement) without nonpolar rotation. 4D-STEM revealed that the magnitude and symmetry of the electric fields surrounding Ni atom columns changed near the surface of the reduced sample. Line profiles showed increased field strength near the surface, which was attributed to valence changes or charge redistribution rather than structural changes, as confirmed by simulations. EELS analysis showed a gradual decrease in the maximum intensity ratio of peaks A and B of the O-K edge in the reduced sample, indicating a decrease in Ni 3d and O 2p hybridization or oxygen vacancy formation. A shift of the Ni-L₃ edge to lower energies suggested a decrease in Ni valence from the inner layer to the surface in the reduced sample, while the pristine sample showed nearly constant Ni³⁺ valence except for a slight decrease at the surface. EELS mapping revealed a significant decrease in O signal intensity in the reduced sample, with an estimated O/Ni ratio of ~2.5 at the surface, consistent with the change in Ni valence. The gradual decrease in oxygen concentration indicated thickness-dependent oxygen deintercalation.

The results demonstrate that oxygen deintercalation during topochemical reduction leads to a stronger polar distortion at the surface of Pr_0.8Sr_0.2NiO_2+x films. The observed strong polar distortion is coupled with a decrease in Ni valence and the formation of oxygen vacancies near the surface. The screening length of the depolarization field is about three unit cells in both pristine and reduced samples. The increase in the electric field strength surrounding Ni atoms near the surface, observed by 4D-STEM, is directly related to the change in Ni valence, as confirmed by EELS analysis. This study provides direct experimental evidence of the connection between oxygen vacancy formation and enhanced surface polarity. The combined use of STEM-ABF, 4D-STEM, and EELS allows simultaneous probing of local structural and charge information, offering a powerful approach for understanding surface polarity in nickelates. The findings have implications for engineering surface polarity in functional materials and modifying surface catalysts.

This study provides real-space imaging of structural distortion and electronic reconstruction at negatively charged polar surfaces in Pr_0.8Sr_0.2NiO_2+x films. The surface reconstruction differs significantly between pristine and partially reduced samples, with stronger polar distortion and oxygen vacancies in the latter. Atomic electric field mapping directly images electronic structure evolution, revealing changes in Ni valence consistent with EELS analysis. The combined techniques provide valuable insights into atomic-scale surface polarity and offer potential for manipulating surface properties in functional materials and catalysis.

The study focuses on a specific nickelate composition and substrate. The generalization of findings to other nickelate systems or substrates requires further investigation. The influence of potential surface contamination or other factors affecting surface polarity is not fully explored. The relatively small size of the analyzed area may not fully capture the heterogeneity of the surface reconstruction.