Concentration of Vacancies at Metal-Oxide Surfaces: Case Study of MgO(100)

Richter, Norina A.; Sicolo, Sabrina; Levchenko, Sergey V.; Sauer, Joachim; Scheffler, Matthias

doi:10.1103/physrevlett.111.045502

Cited by 113 publications

(163 citation statements)

References 44 publications

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“…Oxygen vacancies, typically positively charged in similar oxides 39,40 , lead to electrostatic interactions that alter the energetics of vacancies nearby. Furthermore, the formation of equilibrium space-charge regions 7,39,41,42 at the interface can lead to vacancy-rich dislocation cores and vacancy-depleted spacecharge zones, which would further influence the oxygen vacancy energy and defect concentration. In addition, either oxygen-rich or oxygen-poor conditions 43 encountered during deposition would have a bearing on the oxygen vacancy energies.…”

Section: Resultsmentioning

confidence: 99%

Termination chemistry-driven dislocation structure at SrTiO3/MgO heterointerfaces

et al. 2014

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Exploiting the promise of nanocomposite oxides necessitates a detailed understanding of the dislocation structure at the interfaces, which governs diverse and technologically relevant properties. Here we report atomistic simulations demonstrating a strong dependence of the dislocation structure on the termination chemistry at the SrTiO 3 /MgO heterointerface. The SrO-and TiO 2 -terminated interfaces exhibit distinct nearest neighbour arrangements between cations and anions, leading to variations in local electrostatic interactions across the interface that ultimately dictate the dislocation structure. Networks of dislocations with different Burgers vectors and dislocation spacing characterize the two interfaces. These networks in turn influence the overall stability of and the behaviour of oxygen vacancies at the heterointerface, which will dictate vital properties such as mass transport at the interface. To date, the observed correlation between the dislocation structure and the termination chemistry at the interface has not been recognized, and offers novel avenues for fine-tuning oxide nanocomposites with enhanced functionalities.

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Section: Resultsmentioning

confidence: 99%

Termination chemistry-driven dislocation structure at SrTiO3/MgO heterointerfaces

et al. 2014

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“…Within the quantum-mechanically treated substrate, we employ the virtual crystal approximation to model the free charge carriers. [30,35,52] Here, the oxygen atoms are replaced by an electrically neutral pseudo-atom that contains a core with a charge of 8 + δ and the same number of electrons. The excess electrons go to the bottom of the conduction band.…”

Section: Methodsmentioning

confidence: 99%

Band Bending Engineering at Organic/Inorganic Interfaces Using Organic Self‐Assembled Monolayers

Hofmann

Rinke

2017

Adv Elect Materials

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The electron acceptors receive charge from the semiconductor until their acceptor levels are energetically in resonance with the Fermi energy. The resulting ground-state charge transfer across the interface gives rise to the formation of a space-charge region and associated band bending. This space-charge region can affect the charge-transport properties across the interface and significantly weakens the binding between substrate and adsorbate. [8] The spatial extent and the magnitude of the band bending depend on the amount of charge-transfer from the bulk to the adsorbate as well as on the bulk doping concentration and profile. Experimentally, those parameters are often challenging to control and not always well known. Moreover, they are subject to change during device operation, e.g., due to migration of charged defects, impeding the long-term stability of (opto)electronic devices.When the electron affinity of the adsorbate is larger than the substrate's work function, i.e., when the lowest unoccupied molecular orbital (LUMO) is below the Fermi-energy (E F ) as schematically shown in Figure 1a, the bulk crystal donates electrons to the adsorbate. The ensuing dipole moment (ΔΦ) shifts the now partially occupied LUMO upward in energy until it is in resonance with the Fermi-energy. [9,10] The overall interface dipole, ΔΦ, is often divided into a band bending contribution ΔΦ BB (i.e., the long-range, quasi-parabolic potential within the substrate), and a surface-dipole contribution ΔΦ SD (the approximately linear potential change in the "empty" space between substrate and adsorbate), as indicated in Figure 1b. The relative contribution of ΔΦ BB and ΔΦ SD depends strongly on the doping concentration of the substrate. [8] In principle, for low doping concentrations and strong electron acceptors, band bending could be as large as the total band gap of the substrate (≈3.5 eV in ZnO). In practice, however, ΔΦ BB is almost always limited by the presence of defect states at or near the surface, such as oxygen vacancies, [11][12][13] provided they are present in sufficient concentrations.Since the type and concentration of these defects is difficult to control, band bending is typically hard to engineer. In this article, we use these defect states as an analogy to demonstrate a new concept that uses properly designed, covalently attached self-assembled monolayers (SAMs) to act like surface defects that limit band bending at desired values. In order to do so, the highest occupied molecular orbitals (HOMOs) of these SAMs need to lie within the band gap of the inorganic substrate. This Adsorbing strong electron donors or acceptors on semiconducting surfaces induces band bending, whose extent and magnitude are strongly dependent on the doping concentration of the semiconductor. This study applies hybrid density-functional theory calculations together with the recently developed charge reservoir electrostatic sheet technique to account for charge transfer from the bulk of the semiconductor to the interface. This study further...

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“…[9] Defects will definitely fulfill a key function with respect to the activation of either methane or oxygen by changing the electronic structure of the wide band gap magnesium oxide particularly with regard to facilitate the transfer of electrons between the solid surface and the adsorbed molecules, which undergo a redox reaction. [10][11][12][13][14][15] The present work addresses relations between the nature and abundance of morphological surface defects such as steps and corners on pure magnesium oxide and its reactivity in the oxidative coupling of methane.…”

Section: Li-doped Mgo Was Discovered By Lunsford Et Al As An Active mentioning

confidence: 99%

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: I. Kinetic study

Schwach

Hamilton

Thum

et al. 2015

Journal of Catalysis

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The role of surface structure and defects in the oxidative coupling of methane (OCM) was studied over magnesium oxide as a model catalyst. Pure, nano-structured MgO catalysts with varying primary particle size, shape and specific surface area were prepared by sol–gel synthesis, oxidation of metallic magnesium, and hydrothermal post treatments. The initial activity of MgO in the OCM reaction is clearly structure-sensitive. Kinetic studies reveal the occurrence of two parallel reaction mechanisms and a change in the contribution of these pathways to the overall performance of the catalysts with time on stream. The initial performance of freshly calcined MgO is governed by a surface-mediated coupling mechanism involving direct electron transfer between methane and oxygen. The two molecules are weakly adsorbed at structural defects (steps) on the surface of MgO. The proposed mechanism is consistent with high methane conversion, a correlation between methane and oxygen consumption rates, and high C₂H₄ selectivity after short times on stream. The water formed in the OCM reaction causes sintering of the MgO particles and loss of active sites by degradation of structural defects, which is reflected in decreasing activity of MgO with time on stream. At the same time, gas-phase chemistry becomes more important, which includes the formation of ethane by coupling of methyl radicals formed at the surface and the partial oxidation of C₂H₆. The mechanistic concepts proposed in this work (Part I) will be substantiated in Part II by spectroscopic characterization of the catalysts (Schwach et al. [1])

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Concentration of Vacancies at Metal-Oxide Surfaces: Case Study of MgO(100)

Cited by 113 publications

References 44 publications

Termination chemistry-driven dislocation structure at SrTiO3/MgO heterointerfaces

Termination chemistry-driven dislocation structure at SrTiO3/MgO heterointerfaces

Band Bending Engineering at Organic/Inorganic Interfaces Using Organic Self‐Assembled Monolayers

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: I. Kinetic study

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