Nb掺杂SrTiO$_3$ (100)、(110)和(111)单晶中的表面增强共价性与Madelung势
Surface enhanced covalency and Madelung potentials in Nb doped SrTiO$_3$ (100), (110) and (111) single crystals
AI总结：
利用角分辨光电子能谱研究了Nb掺杂SrTiO3不同晶面表面增强共价性对Madelung势的影响，定量确定了表面电子占据数并发现Madelung势偏移随表面取向变化。
摘要：
利用角分辨光电子能谱，实验研究了在630 °C下于UHV中退火后(100)、(110)和(111) SrTiO$_3$表面的表面增强共价性对Madelung势的影响。对芯能级谱（O 1s、Sr 3d和Ti 2p）的解卷积区分了体相和表面成分，并通过表面增强共价性（SEC）进行了解释。通过将实验测量的结合能与在Localized-Hole Point-Ion Model框架内开发的理论计算进行比较，我们定量确定了体相和表面Sr与Ti位点的有效电子占据数。结果证实了体相STO中Sr-O键的本质离子性和Ti-O键的部分共价性。三种表面的阳离子Ti和Sr电子占据数均大于体相。表面共价性使表面的Madelung势偏移了ΔEM。ΔEM在(111)表面最小，经(100)递增，在(110)达到最大。角分辨价带谱和功函数值也证实了这一趋势。结果与表面主要由d-d电荷涨落主导、而体相中金属-配体电荷转移在能量上更有利相一致。
英文摘要：
The influence of surface enhanced covalency on the Madelung potential is experimentally investigated using angle-resolved photoemission for (100), (110) and (111) SrTiO$_3$ surfaces after annealing in UHV at 630 °C. Deconvolution of the core level spectra (O 1s, Sr 3d and Ti 2p) distinguishes bulk and surface components, which are interpreted in terms of surface enhanced covalency (SEC). By comparing the experimentally measured binding energies with theoretical calculations developed in the framework of the Localized-Hole Point-Ion Model, we quantitatively determine the effective electron occupancy at bulk and surface Sr and Ti sites. Our results confirm the essentially ionic character of Sr$-$O bond and the partially covalent character of Ti$-$O bond in bulk STO. The cation Ti and Sr electron occupation is greater for all the three surfaces than in the bulk. Surface covalency shifts the Madelung potential at the surface by ΔEM. ΔEM is a minimum for the (111) surface, and increases through (100), attaining a maximum for (110). The angle-resolved valence band spectra and the work function values also confirm this trend. The results are consistent with d$-$d charge fluctuations dominating at the surface, whereas metal-ligand charge transfers are more energetically favourable in the bulk.
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