Surface Color Centers Research Articles

Properties of compacts of barium fluoride nanopowder produced by pulsed electron beam evaporation

Abstract Mesoporous crystalline BaF 2 nanopowder with a specific surface area of up to 34.8 m 2 /g was produced by a pulsed electron beam through the evaporation of targets of micro-sized BaF 2 powder with optical purity ( S red = 0.38 m 2 /g). The texture and thermal features of the BaF 2 nanopowder (before and after annealing in air at a temperature of 200 °C) and some properties of compacts made of the BaF 2 nanopowder by magnetic-pulse compaction and static pressing were studied. Glassy, opaque, and black compacts, which are promising materials for use as scintillators and dosimeters, were produced by magnetic-pulse compaction in a vacuum at a temperature of about 425 °C. It was found that heating up to 425 °C is sufficient for removing defects in nanopowders. The effect of pressing methods on the pulsed cathodoluminescence and photoluminescence spectra of the compacts was studied. Luminescent properties were found to depend on pressing technology. It was also determined that surface color centers, the number of which decreases during heating, are responsible for the pulsed cathodoluminescence band near ∼560 nm.

Adsorption of PTCDA on NaCl Surfaces with Color Centers: Charge Transfer and Formation of Radical Ions

Defect centers in crystalline surfaces significantly influence the local electronic structure and thus the reactivity. We investigated the interaction between surface color centers in the NaCl(100) surface with adsorbed 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) using dielectric-dependent global hybrid density functionals. A strong interaction between the unpaired electron in the color center and PTCDA is observed leading to a single electron transfer and the formation of a PTCDA radical anion. This leads to a stronger interaction between the molecule and the surface, resulting in a narrowing of the electronic gap, higher adsorption energy, and stronger distortion of the molecule. The effect has a significant influence on the IR spectrum of PTCDA.

Band Bending in Mg-Colored and O2-Activated Ultrathin MgO(001) Films

Ultrathin MgO films grown on Ag(001) have been investigated using X-ray and ultraviolet photoemission spectroscopies for oxide films successively exposed to Mg and O2 flux. Studying work functions and layer-resolved Auger shifts allows us to keep track of band profiles from the oxide surface to the interface and reveal the charge-transfer mechanisms underlying the controlled creation of Mg-induced surface color centers and the catalytic enhancement of O2 activation. Our results demonstrate that one can intimately probe the catalytic properties of metal-supported ultrathin oxide films by studying the electronic band alignment at interfaces.

Effect of Pre-Irradiation Annealing and Laser Modification on the Formation of Radiation-Induced Surface Color Centers in Lithium Fluoride

It is shown that surface color centers of the same type are formed in the surface layer and in regions with damaged crystal structure inside crystalline lithium fluoride after γ-irradiation. Results are presented from a study of the effect of pre-irradiation annealing on the efficiency with which surface centers are formed in lithium fluoride nanocrystals. Raising the temperature for pre-irradiation annealing from room temperature to 250°C leads to a substantial reduction in the efficiency with which these centers are created. Surface color centers are not detected after γ-irradiation for pre-irradiation annealing temperatures of 300°C and above. Adsorption of atmospheric gases on the crystal surface cannot be regarded as a necessary condition for the formation of radiation-induced surface centers.

Formation and optical properties of ZnSe and ZnS nanocrystals in glasses

ZnSe and ZnS nanocrystals (NCs) were precipitated in glasses through heat-treatment. High resolution transmission electron microscope image confirmed that ZnSe NCs were formed in the silicate glasses, and the X-ray diffraction patterns confirmed that ZnS NCs were precipitated in the borosilicate glasses. Absorption shoulders of the ZnSe NCs and ZnS NCs appeared at ~2.76eV and ~3.78eV, corresponding to the bandgap energies of bulk ZnSe and ZnS crystals, respectively. Instead of the intrinsic emissions, broad-band visible photoluminescence from the surface defects or color centers in the glasses was observed from these ZnSe or ZnS NCs.

One-pot preparation of crystalline-amorphous double-layer structured WO3 films and their electrochromic properties

WO3 films with crystalline-amorphous double-layer structure were prepared via one-pot electrochemical synthesis for the first time. According to scanning electron microscopy images combined with X-Ray diffraction patterns, the crystalline layer and the amorphous layer can be distinguished. Results of electrochromic measurements show that the as-synthesized films have shorter response time than crystalline films and better durability than amorphous films. The films were then modified by electrochemical lithiation/delithiation to produce a porous surface and electrochromic behaviors (response time, coloration efficiency, etc.) were studied by controlling the current and cycle times during electrochemical lithiation/delithiation. Response time is shortened and coloration efficiency is improved after electrochemical lithiation/delithiation. The formation of color centers was also analyzed. Increasing the concentration of H+ ions is beneficial to the formation of surface color centers, while increasing the applied current benefits the formation of body color centers.

Additive coloring of thin, single crystalline MgO(001) films

Electron bombardment of single-crystalline MgO(001) films has been shown to result in the formation of surface color centers. In this report we present an alternative way to produce surface color centers on single-crystalline MgO(001) films using additive Mg coloring. Electron paramagnetic resonance (EPR) spectroscopy was used to prove the formation of surface paramagnetic color centers upon deposition of small amounts of Mg on MgO(001) films at 50 K. With increasing coverage the number of Mg induced color centers rises up to a coverage of about 0.03 monolayers and thereafter decreases with increasing Mg coverage. At high coverage and low temperatures metallic Mg clusters nucleate on the surface of the MgO(001) film as shown by infrared (IR) spectroscopy using CO as a probe molecule.

Electron Paramagnetic Resonance and Scanning Tunneling Microscopy Investigations on the Formation of F+ and F0 Color Centers on the Surface of Thin MgO(001) Films

The formation of surface color centers (F(S) centers) by electron bombardment of thin MgO(001) films is investigated using electron paramagnetic resonance and low-temperature scanning tunneling microscopy. At low electron doses both techniques indicate the formation of singly occupied color centers (F(S)(+)), whereas at high electron doses the doubly occupied type (F(S)(0)) is dominant. It is suggested that with increasing electron dose F(S)(+) centers are transformed into F(S)(0). Tunneling spectra of individual F(S)(0) centers reveal a large distribution of energetic positions of occupied and unoccupied states, which is caused by local variations of the coordination number of the defects and explains the broad signals usually detected with integrating spectroscopic techniques.

Density-functional model cluster studies of EPR g tensors of Fs+ centers on the surface of MgO

We report g tensors of surface color centers, so-called F(s) (+) centers, of MgO calculated with two density-functional approaches using accurately embedded cluster models. In line with recent UHV measurements on single-crystalline MgO film, we determined only small g-tensor anisotropies and negative shifts Deltag identical with g-g(e) for all F(s) (+) sites considered, namely, (001)-terrace, step, edge, and corner sites. The g values are very sensitive to the local structure of the defect: relaxation reverses the sign of Deltag. However, accounting for the spin-orbit interaction either self-consistently or perturbatively yields very similar results. In addition to the values of the tensor components, their direction with respect to the surface was determined. In contrast to edges, significant deviations from ideal C(2v) symmetry were found for F(s) (+) centers at steps. Recent data on single-crystalline thin films are reevaluated in the light of these results.

Point defects as the centers of titanium dioxide sensitization in the visible spectral range

The formation of surface point defects in the initial stage of TiO2 reduction (x ≤ 1012 cm−2) has been studied by mass spectrometry and ultraviolet photoelectron spectroscopy (UPS). Heating to 720 K or UV illumination in ultrahigh vacuum creates surface color centers in TiO2 with an energy spectrum extending from the Fermi level to the valence band top. The continuous photoelectron spectrum exhibits a peak at 2.7 eV, which varies in a manner correlated with the behavior of the optical absorption bands at 2.55 and 2.81 eV assigned to oxygen vacancies in the TiO2 crystal structure. The interaction of the surface point defects with molecular oxygen has been studied and a special form of the photoadsorbed oxygen (with Edes = 1.37 eV) is found. It is shown that the surface color centers may serve as centers of TiO2 sensitization in the visible spectral range.

Surface damage and color centers generated by femtosecond pulses in borosilicate glass and silica

Color center generation by femtosecond laser pulses (30 fs) is observed in a fluence range below the damage threshold in an alkali-free barium borosilicate (BBS) glass, and in a thin layer of SiO2 on a fused silica substrate. The color centers are characterized spectroscopically. The optical density of the color centers in BBS glass is by two orders of magnitude higher than that in silica. A “healing” process with a time constant of about 30 h can be found.

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

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 (1,1,19) surface. This film contains a significantly higher step density compared with a film grown on Ag(100). 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 ${(F}_{b}$ centers). By comparing the measured loss energies with experiments performed on MgO/Ag(100) 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 ${\mathrm{O}}_{2}$ 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.

First evidence of a single-ion electron trap at the surface of an ionic oxide.

A single electron can be trapped by a triply coordinated single cation at a magnesium oxide surface (see picture). The resulting center, denoted MgOcorner(H+/e−), is a member of a new family of surface color centers and represents a significant advance for the concept of electron-trapping sites in solids.

Effects of electron irradiation on the structure and morphology of CaF 2/Si(111)

With the use of X-ray Standing Waves (XSW) and Atomic Force Microscopy (AFM), we have studied the structure and morphology of CaF 2 films (thickness range 3–15 Å) grown on Si(111) before and after modification by electron irradiation. The Ca monolayer of the as-grown CaF 2 monolayer relaxes towards the Si substrate. The F bilayer embedding the Ca monolayer has bulk spacing. Low Energy (80–100 eV) Electron-Stimulated Desorption (ESD) of fluorine reduces mainly the occupation of the top F layer due to generation of surface color centers. The Ca layer is affected only marginally by the electron bombardment. For 15 Å, CaF 2 films the color centers nucleate and form Ca clusters embedded in the film. These clusters are removed under ambient conditions (pin hole formation). Inspection of the holes shows that the Ca clusters penetrate the entire CaF 2 film.

Surface color centers on epitaxial NaCl films

The defect formation in epitaxial NaCl films (up to 5.6 nm thick) during low-energy (50--1500 eV) electron bombardment has been studied by a combination of low-energy electron diffraction and energy loss spectroscopy. At temperatures below 200 K two types of color center are generated at the surface, showing distinct electronic transition energies and different kinetics of formation. A rate equation analysis provides evidence for a random distribution of stable, immobile $F$ and $M$ centers on the surface. Their saturation density increases with the film thickness, indicating an indirect generation mechanism involving the whole film, presumably via self-trapping of excitons. The angular distribution of the inelastic electron scattering of excitons reveals their surface polariton character, providing the energy transport from film bulk to surface and within the surface. At room temperature the aggregation of Na clusters is observed. Patterns of high and low surface $F$- and $M$-center densities are potentially invaluable as templates for nanostructure devices on ionic crystal surfaces like NaCl(001), because of the chemical reactivity of surface color centers.

Photoreactions of small molecules at the surface of alkali metal halides

The photostimulated reactions of small molecules, i.e., O 2 photoadsorption and photodesorption, H 2 and CO photooxidation, and CO 2 photodecomposition on alkali halides (alkali halides) surface are discussed in this work. Most of 19 alkali halides show the activity in the reactions mentioned above. Three spectral regions of alkali halides stimulation by light are distinguished in relation to kinetics of oxygen photoadsorption and reactivity of adsorbed oxygen species. They are in correspondence with: (i) bulk exciton absorption bands; (ii) extrinsic and intrinsic bulk and surface absorption bands and (iii) color centers absorption bands. In the latter case the activity of alkali halides arises after pre-excitation in exciton bands, which leads to sensitization of alkali halides to visible light. The H 2 and CO photooxidation on alkali halides is two-step photoprocess involving some excited states of adsorbed oxygen. The latter is quenching by gaseous oxygen as it is shown for hydrogen oxidation over KBr. The photodecomposition of carbon dioxide is treated as CO 2 dissociative adsorption with CO evolution into the gas phase.

Vacancies and Electron Deficient Surface Anions on the Surface of MgO Nanoparticles

In the course of surface defect characterization on MgO nanoparticles three different types of O- species were observed by electron paramagnetic resonance (EPR) spectroscopy. They are a result of UV excitation under high-vacuum conditions/in O2 atmosphere or of surface color center bleaching by N2O, respectively. On the other hand, the O- species induce homolytic H2 splitting which was evidenced for the first time by infrared (IR) spectroscopy. The resulting products are a so far unknown IR active OH group and an uncharged surface mobile H atom. The latter one acts as a reducing agent for either one of two types of anion vacancies which are transformed into EPR active surface color centers FS+. They are fundamentally different from the classical FS+(H) center obtained in predominant abundance by UV irradiation of MgO in H2 atmosphere. One of the above-mentioned O- species is created by bleaching of FS+(H) by N2O. The respective EPR signal is exactly the same as that obtained by thermal decomposition of a ...

MSINDO study of the adsorption of water molecules at defective MgO(100) surfaces

The adsorption of water molecules at defective MgO(100) surfaces is studied with the semiempirical molecular orbital method MSINDO, a recently developed modification of the SINDO1 method. A cluster simulation is used for the MgO surfaces. Defects are generated by removing surface atoms from these clusters. The defects investigated are a monoatomic step and the surface color centers F s and M s. Molecular and dissociative adsorption structures and their corresponding adsorption energies are calculated. The results are compared with calculations of the defect-free surface and experimental data.

Surface color centers as novel hydrogen bond acceptors

In recent combined electron paramagnetic resonance and infrared spectroscopic studies a very specific interaction between surface OH groups and surface color centers was observed on nanometer-sized MgO particles. The positions of the respective OH stretching bands point to hydrogen bond-like interactions. On the other hand, the bandwidths indicate that the color center electron as proton acceptor is strongly localized.

Generation and EPR characterization of color centers at the surface of ionic oxides

The activation of the surface of high surface area alkali-earth oxides to form surface color centers is described in detail. Two methods (the radiative coloring and the additive coloring by means of vapors of low ionization energy metals) have been adopted to form surface electron excess centers of various complexity. The EPR technique is essential to describe the nature and the structural features of the paramagnetic centers. The electron rich surface quickly reacts with a variety of adsorbed molecules forming adsorbed anions and radical anions. Two examples of such reactivity are illustrated in the final part of the paper.