Formation of surface color centers at differently coordinated sites: MgO/Ag(1,1,19)

Abstract

In order to find criteria for discrimination between surface color centers at terraces and steps, and bulk color centers, the characteristic losses of these centers have been investigated by electron energy loss spectroscopy on an epitaxial MgO film grown on a Ag ͑1,1,19͒ surface. This film contains a significantly higher step density compared with a film grown on Ag͑100͒. The generation of four distinct losses at 2.0, 2.4, 2.8, and 3.4 eV and a broad loss centered at 5.5 eV have been observed that are induced by electron bombardment of the MgO surface. The latter loss is attributed to bulk color centers (F b centers͒. By comparing the measured loss energies with experiments performed on MgO/Ag͑100͒ and with theoretical literature data, the observed losses at 2.0 and 2.8 eV can be consistently assigned to transitions of color centers located on step sites, whereas those at 2.4 and 3.4 eV are attributed to terrace sites. The kinetics of color center formation during electron bombardment, as well as the annihilation of F centers by simple exposure to O 2 gas at room temperature, was determined and compared for differently coordinated color centers. While all surface color centers follow the same kinetics, we show that the F b centers can only decay as long as the existence of surface color centers provides active sites for dissociating oxygen molecules. This corroborates the assignment of F b centers.

Defect complexes in Li-doped MgO

“…Lower electron energies did not result in a measurable emission response of the oxide film, in contrast to conventional metal surfaces where intense plasmon-mediated light can be detected already at 5-V sample bias. Note that the impact of energetic electrons causes the MgO surface to degrade with time due to electron-stimulated O desorption [7]. We have tried to minimize this effect by choosing fresh sample areas for every experimental run and by keeping accumulation times for spectral acquisition below 60 s.…”

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

“…Here, the pristine film was exposed to electrons with an energy of 100 eV. The production of color centers in such a way was shown recently by specific losses observed in the electron energy loss spectrum of MgO films grown on Ag(001) [11,12]. The defect generation by electron bombardment was explained by electron stimulated desorption of oxygen atoms or ions via a multielectron Auger decay resulting from excitation of Mg 2s and 2p electrons [11].…”

Properties of oxide thin films and their adsorption behavior studied by scanning tunneling microscopy and conductance spectroscopy

“…Hereby, optical spectroscopy 131,132,133 , XPS and EELS 247,248 are ideally suited to detect defect-induced electronic states in the band gap, while electronparamagnetic resonance spectroscopy (EPR) is sensitive to oxide defects carrying an unpaired electron. 249 Thermal desorption and infrared-absorption spectroscopy, on the other hand, enable an investigation of defect-mediated adsorption processes.…”