Probing La-based nickelates with Ni 1s core-level photoelectron spectroscopy

摘要

本文通过Ni 2p与Ni 1s核心层光电子能谱对比研究了La₃Ni₂O₇、Nd₃Ni₂O₇和LaNiO₃的电子结构。由于La 3d与Ni 2p能级严重重叠且存在La高能伴峰，传统Ni 2p谱难以可靠提取La基镍酸盐的本征信号。利用硬X射线光电子能谱测量深的Ni 1s核心能级，其无自旋-轨道耦合且多极相互作用可忽略，提供了纯净的电荷转移激发视角。结果表明，Ni 1s谱能清晰区分钙钛矿LaNiO₃与双层Ruddlesden-Popper相的材料差异，并揭示La₃Ni₂O₇相比Nd₃Ni₂O₇主峰展宽、强度降低而伴峰增强。结合DFT+DMFT计算，这些谱学变化可归因于电荷转移能与杂化强度的改变，其中La₃Ni₂O₇因拉伸应变导致Ni与配体杂化减弱。该方法展示了Ni 1s核心层能谱对电子结构细微变化的敏感性，为系统表征不同应变、掺杂或层数的镍酸盐提供了有效途径。

材料

• La₃Ni₂O₇
• Nd₃Ni₂O₇
• LaNiO₃
• La₂CuO₄

方法

• hard X-ray photoelectron spectroscopy (HAXPES)
• Ni 1s core-level spectroscopy
• Ni 2p core-level spectroscopy
• density functional theory plus dynamical mean-field theory (DFT+DMFT)

关键词

• charge transfer excitations
• hybridization strength
• la 3d overlap
• spin orbit coupling free
• epitaxial strain
• ruddlesden popper nickelates
• satellite peaks

亮点

• Ni 1s core-level spectroscopy provides a clean probe free of interference from La 3d levels, enabling reliable comparison of nickelate electronic structures.
• The technique reveals that tensile strain in La₃Ni₂O₇ weakens Ni–ligand hybridization, altering the charge-transfer satellite intensity relative to the compressive-strained Nd₃Ni₂O₇.
• This approach offers a systematic way to characterize nickelates with different strains, doping levels, or layer numbers in the Ruddlesden-Popper series.

结论

• Reliable extraction of the intrinsic Ni 2p line shape is not feasible for La-based nickelates due to overlap with La 3d core levels and the presence of La-derived satellites.
• Ni 1s core-level photoelectron spectroscopy is a suitable alternative, free of spin-orbit coupling and multiplet interactions, and can resolve subtle differences in electronic structure among nickelates.
• Spectral differences between La₃Ni₂O₇ and Nd₃Ni₂O₇, including a broadened main peak and enhanced satellite, are attributed to changes in charge-transfer energy and hybridization strength due to tensile strain.
• DFT+DMFT calculations confirm that reduced hybridization broadens the main line and enhances the satellite, consistent with experimental observations.

主要论断

• Ni 2p core-level photoemission spectra of La-based nickelates are severely compromised by overlap with La 3d core levels and intrinsic high-energy La satellites, preventing reliable analysis.
  • 证据: Figure 1 shows Ni 2p and La 3d spectra of La₃Ni₂O₇ and LaNiO₃ with overlapping features; La 3d of Ni-free La₂CuO₄ reveals high-energy satellites in the same energy range as Ni 2p.
• Deep Ni 1s core-level spectroscopy provides a clean, overlap-free view of charge-transfer excitations, allowing clear distinction between different nickelates (e.g., perovskite vs. bilayer, La₃Ni₂O₇ vs. Nd₃Ni₂O₇).
  • 证据: Figure 2 and Figure 3 show Ni 1s spectra with clearly resolved main and satellite peaks, free from La interference; LaNiO₃ displays a markedly different line shape from the bilayer compounds.
• The broader main peak and enhanced satellite in La₃Ni₂O₇ relative to Nd₃Ni₂O₇ result from weakened Ni–ligand hybridization due to tensile strain, as confirmed by DFT+DMFT simulations that reproduce the spectral changes when hybridization is reduced.
  • 证据: Figure 4 shows DFT+DMFT simulations where reduced hybridization yields a broader main peak and enhanced satellite, matching the experimental La₃Ni₂O₇ spectrum.

研究流程

• Synthesis of nickelate samples — High-quality samples of La₃Ni₂O₇, Nd₃Ni₂O₇, LaNiO₃, and reference La₂CuO₄ were synthesized for comparative core-level spectroscopy.
  • 材料: La₃Ni₂O₇ thin film; Nd₃Ni₂O₇ thin film; LaNiO₃ single crystal; La₂CuO₄ single crystal (reference); LaAlO₃ substrate; NdGaO₃ substrate
  • 方法: Reactive ozone-assisted molecular beam epitaxy (thin films); Floating zone technique under 125 bar O₂ (LaNiO₃); Floating-solvent traveling-zone method (La₂CuO₄)
  • 观察: Thin films and single crystals produced
• HAXPES measurements — HAXPES provided bulk-sensitive Ni 1s and Ni 2p spectra of the nickelates.
  • 材料: Synthesized nickelate samples; HAXPES endstation at BL₁₂XU (SPring-8); MB Scientific A-1 HE analyzer
  • 方法: Hard x-ray photoelectron spectroscopy (HAXPES) at 8 keV and 10 keV photon energies
  • 观察: Ni 1s, Ni 2p, and La 3d core-level spectra recorded for all samples
• Comparative spectral analysis — Conventional Ni 2p spectroscopy is unreliable for La-based nickelates; Ni 1s provides an overlap-free, clean probe of intrinsic electronic excitations.
  • 材料: Ni 2p and Ni 1s spectra of Nd₃Ni₂O₇, La₃Ni₂O₇, LaNiO₃; La 3d spectra of La₂CuO₄ and La-based nickelates
  • 方法: Line-shape comparison; Examination of La 3d features using Ni-free La₂CuO₄ reference; Assessment of spectral overlaps and satellites
  • 观察: Ni 2p strongly overlaps with La 3d; La 3d shows high-energy satellites extending into Ni 2p region; La 3d core-level shapes differ between materials, preventing simple subtraction; Ni 1s spectra are free from La overlap, show a clear main peak and charge-transfer satellite; Ni 1s clearly distinguishes perovskite LaNiO₃ from bilayer Ruddlesden–Popper phases; La₃Ni₂O₇ shows a broader main peak and enhanced satellite compared to Nd₃Ni₂O₇
• DFT+DMFT simulation and interpretation — Tensile strain in La₃Ni₂O₇ weakens Ni–ligand hybridization, causing the observed broader main peak and stronger satellite; Ni 1s is sensitive to such electronic structure changes.
  • 材料: DFT+DMFT electronic structure of Nd₃Ni₂O₇; Artificial hybridization densities
  • 方法: DFT+DMFT calculations of core-level spectra; Systematic variation of charge-transfer energy Δ and hybridization strength
  • 观察: Reducing hybridization broadens the main peak and enhances the satellite; Reducing Δ narrows the main peak; Simulated trend for reduced hybridization matches experimental La₃Ni₂O₇ spectra