Abstract
We combined photoelectron spectroscopy with first-principles calculations to investigate electronic properties of ${\mathrm{SrTiO}}_{3}$ doped with Ni impurities. High-quality epitaxial pristine ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$:${\mathrm{Ni}}_{x}$ films with $x=0.06$ and 0.12 were prepared by pulsed laser deposition. Electronic band structure calculations for the ground state, as well as one-step model photoemission calculations, which were obtained by means of the Korringa-Kohn-Rostoker Green's function method, predict the formation of localized $3d$-impurity bands in the band gap of ${\mathrm{SrTiO}}_{3}$ close to the valence band maxima. The measured valence bands at the resonance Ni $2p$ excitation and band dispersion confirm these findings.
Highlights
Theoretical and experimental studies have pointed out that these lattice defects from different types of atoms and doping sites are able to adjust the absorption edge and able to modify in different ways the electronic structure and electronic conductivity of these materials: In the context of photocatalysis, transition metal (TM) doping of perovskite oxides is one of the most effective strategies to absorb visible light due to the 3d bands of the dopant, which can cause a shift of the valence band and/or the conduction band and form new energy levels in the band gap [17,18,19,20,21]
A comparison of the Ni 2p spectra is presented in Fig. 2(d): In the case of strontium titanate (STO):Ni0.06 and STO:Ni0.12, the binding energy positions of the Ni 2p doublet are found at 855.6 eV (Ni 2p3/2) and 873.2 eV (Ni 2p1/2) separated by E = 17.6 eV
A variety of spectroscopy techniques such as x-ray photoemission spectroscopy (XPS), X-ray absorption (XAS), resonant photoemission (resPES), and angle-resolved photoemission spectroscopy (ARPES) were applied to achieve an understanding of the complicated manybody physics related to the Ni impurities, of atomiclike character, in STO films
Summary
Transition metal oxides with perovskite structure are receiving a large amount of attention from scientists because of their outstanding properties, such as high-temperature superconductivity [1,2,3], large negative magnetoresistance [4], multiferroicity [5], and the formation of a two-dimensional electron gas (2DEG) at oxide surfaces and interfaces [6,7,8,9]. Theoretical and experimental studies have pointed out that these lattice defects from different types of atoms and doping sites are able to adjust the absorption edge and able to modify in different ways the electronic structure and electronic conductivity of these materials: In the context of photocatalysis, transition metal (TM) doping of perovskite oxides is one of the most effective strategies to absorb visible light due to the 3d bands of the dopant, which can cause a shift of the valence band and/or the conduction band and form new energy levels in the band gap [17,18,19,20,21]. Experimental results are compared with density functional theory (DFT) calculations based on the Korringa-Kohn-Rostoker (KKR) Green’s function method [33,34]
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