Geometric Characterization of a Singly Charged Oxygen Vacancy on a Single-Crystalline MgO(001) Film by Electron Paramagnetic Resonance Spectroscopy

Sterrer, Martin; Fischbach, Esther; Risse, Thomas; Freund, Hans‐Joachim

doi:10.1103/physrevlett.94.186101

Cited by 189 publications

(236 citation statements)

References 18 publications

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“…STM images of similarly grown MgO(100) thin films show a high density of such sites. 45,84 Our calculations (see Figure 6) show that a Ca monomer binds to a straight step edge site with an adsorption heat of 2.1 eV (205 kJ/mol) and a charge transfer of 0.6 e. Most of this charge (68%) is transferred to the under-coordinated O atoms in the step and does not contribute to the dipole moment perpendicular to the surface. An integration of the charge difference shows this dipole perpendicular to the surface to be 2.3 D, causing an initial work function drop with slope -9.7 eV/ML, much closer to the experiment (-7 eV/ML) than Ca binding to the terrace (-14 eV/ML).…”

Section: Density Functional Theory Calculationsmentioning

confidence: 88%

“…Later studies on grown MgO surfaces concluded that singly charged (F+) oxygen vacancies were not present because there was no significant EPR signal detected on the pristine surface. 45 To determine if deposited Ca atoms are similarly trapped at such point defects and nucleate clusters, we calculated the binding energy of Ca at both neutral (F) and charged (F+) oxygen vacancies. Figure 5 parts c and d show that Ca binds more weakly to these defects than to the terrace.…”

Section: Density Functional Theory Calculationsmentioning

confidence: 99%

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Calcium Adsorption on MgO(100): Energetics, Structure, and Role of Defects

et al. 2008

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Abstract:The adsorption of Ca on the MgO(100) surface at 300 K has been studied using microcalorimetry, in combination with LEED, AES, ISS, work function, sticking probability measurements, and density functional theory (DFT) calculations. The MgO(100) thin films (∼4 nm thick) were grown epitaxially on a 1 µm thick Mo(100) single-crystal. The sticking probability of Ca on MgO(100) at 300 K is unity. On the basis of AES and ISS measurements, it was determined that Ca grows mainly as 3D particles on the MgO(100) surface with a density of ∼1 × 10 12 islands/cm 2 . Ca adsorbs initially at defect sites with a very high heat of adsorption (∼410 kJ/mol). DFT calculations attribute this high initial heat to Ca binding to kink sites (376 kJ/mol), step sites (205 kJ/mol), and lower concentrations of stronger binding sites. The heat of adsorption decreases rapidly with coverage, reaching a minimum of 162 kJ/mol at ∼0.3 ML, where Ca is mainly adding to small 3D Ca clusters. Afterward, it increases to the value of bulk Ca heat of sublimation (178 kJ/mol) at ∼1.2 ML, attributed to the increase in stability with increasing Ca particle size. A 1.0 eV decrease of the work function with Ca coverage from 0 to 0.3 ML indicates that Ca adsorbed at defects is cationic, in agreement with calculations showing that Ca donates electron density to the MgO. Light ion sputtering of the MgO-(100) surface generates point defects, but these do not change the heat of adsorption versus coverage, implying that they do not nucleate Ca particles. Oxygen vacancies are a likely candidate; DFT calculations show that F and F+ center vacancies bind Ca more weakly than terrace sites. More extensive sputtering creates extended defects (such as steps and kinks) that adsorb Ca with heats of adsorption up to ∼400 kJ/mol, similar to that at the intrinsic defect sites.

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Section: Density Functional Theory Calculationsmentioning

confidence: 88%

Section: Density Functional Theory Calculationsmentioning

confidence: 99%

Calcium Adsorption on MgO(100): Energetics, Structure, and Role of Defects

et al. 2008

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“…However, already this value is a factor of ten larger than the highest number of electrons that can be stored in paramagnetic point defects. 5 Distributing the trapped charges along the circumference of all dislocation lines, as estimated from the STM images, yields a density of 3-5 electrons per nm line defect. It should be noted that at such high carrier densities electron-electron interactions start to affect the EPR spectra, giving rise to a broadening of the resonance.…”

Section: Resultsmentioning

confidence: 99%

“…3 For example, oxygen vacancies created in an MgO surface by electron bombardment can be filled with one ͑F + ͒ or two extra electrons ͑F 0 center͒. 4,5 Electron trapping also occurs at morphological peculiarities, such as kinks and reverse corners. 6 The presence of trapped electrons strongly affects the chemical properties of oxide surfaces.…”

Section: Introductionmentioning

confidence: 99%

Electron trapping in misfit dislocations of MgO thin films

et al. 2010

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Misfit dislocations in a thin MgO/Mo͑001͒ film have been investigated by conductance and light-emission spectroscopy using scanning tunneling microscopy and electron-paramagnetic resonance ͑EPR͒ spectroscopy. The line defects exhibit a higher work function than the pristine MgO, being explained by their ability to trap electrons. The electron traps are associated with a nonstoichiometric defect composition in thin oxide films and attractive pockets in the Madelung potential in thicker ones. The latter traps can be reproducibly filled by the adsorption of atomic hydrogen, which gives rise to a free-electronlike signal in EPR spectroscopy.

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“…2͑a͔͒ of these reduced TiO 2 nanobelts shows a single sharp symmetrical signal ͑g = 2.003͒ which can be attributed to singleelectron trapped on oxygen vacant sites, also known as singly occupied F + -centers. 12,13 Studies carried out on Codoped titanate nanotubes have shown the importance of this type of oxygen vacancies in mediating room temperature ferromagnetism, 2-5 with the widely accepted view of F-centers induced ferromagnetic coupling between Co 2+ via a donor impurity band exchange model. 18,19 The enlarged plot of the same graph ͑inset͒ also reveals the presence of active species trapped on Ti 3+ sites ͑g = 1.933͒ but the amount of these species is significantly smaller compared to that of the former.…”

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confidence: 99%

Interesting magnetic behavior from reduced titanium dioxide nanobelts

Chong

Kadowaki

Xia³

et al. 2008

Applied Physics Letters

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The magnetic behavior of titanium dioxide nanobelts has been investigated with and without Co doping. Room temperature ferromagnetism was observed when the Co-doped anatase TiO 2 nanobelts were prepared via vacuum annealing of 2.5 at. % Co-doped titanate nanobelts, while annealing them in air resulted in paramagnetic ordering. Interestingly, by vacuum annealing the undoped titanate nanobelts under the same conditions, superparamagnetic ordering was observed in the resulting anatase TiO 2 nanobelts. The electron paramagnetic resonance of this latter sample shows a strong symmetrical signal at g = 2.003 suggesting some sort of exchange interactions among the localized electrons' spin moments from single electron trapped in oxygen vacancies.

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Geometric Characterization of a Singly Charged Oxygen Vacancy on a Single-Crystalline MgO(001) Film by Electron Paramagnetic Resonance Spectroscopy

Cited by 189 publications

References 18 publications

Calcium Adsorption on MgO(100): Energetics, Structure, and Role of Defects

Calcium Adsorption on MgO(100): Energetics, Structure, and Role of Defects

Electron trapping in misfit dislocations of MgO thin films

Interesting magnetic behavior from reduced titanium dioxide nanobelts

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