Medium energy ion scattering investigation of methylthiolate-induced modification of the Au(111) surface

100 keV H⁺ scattering has been used to investigate the structure of the methylthiolate/Au(111) interface in the Au(111)(√3 × √3)R30° phase. Adsorption of the thiolate onto the clean Au(111) surface leads to a large drop in the scattered ion yield due to the lifting of the clean surface ‘herring-bone’ reconstruction, but the thiolate-covered surface shows an ion yield higher than that of an unreconstructed Au(111) surface, providing direct evidence of a significant number of Au atoms that are displaced from their bulk-terminated positions at the buried interface. Simulations for two different Au adatoms models at the interface, namely, the Au-adatom-monothiolate (AAM) and Au-adatom-dithiolate (AAD) models, show significant sensitivity to the exact values of interlayer spacings and atomic vibrational amplitudes, but the comparison with experimental results appears to favour the AAD model with 0.17 ML Au adatoms in bridging sites at the interface.     

X-ray photoelectron spectroscopy of fast-frozen hematite colloids in aqueous solutions. 4. Coexistence of alkali metal (Na+, K+, Rb+, Cs+) and chloride ions

Colloidal suspensions of hematite in contact with aqueous solutions of 50 mM alkali metal chloride electrolytes (NaCl, KCl, RbCl, CsCl) were investigated by cryogenic X-ray photoelectron spectroscopy (XPS) and electrophoretic mobility. Suspension pH values were varied from 2 to 11 in order to evaluate effects of positively- and negatively-charged hematite surfaces. XPS revealed coexisting cations and chloride ions both below and above the point of zero charge. Concentration profiles of adsorbed cations point to a Hofmeister series in the order of Na⁺ > K⁺ > Rb⁺ ≈ Cs⁺. Binding energies of photoelectrons emitted from electrolyte ions increased with pH at roughly 0.04 eV per pH unit. This shift was attributed to variations in the surface electric potential of hematite. This effect, compounded by rises in aliphatic carbon signals with pH, called for referencing of all spectra to the 530.0 eV oxide component of the hematite O1s spectrum. This departure from the traditional use of the external C 1s 285.0 eV peak is hereby proposed for cryogenic XPS studies of interfacial reactions involving hematite.     

Ion beam induced reaction of carbon films on Si(1 0 0)

The irradiation effects of 2 MeV He⁺ and Ar⁺ ions on the film structure of the C–Si system were investigated with RHEED and XPS. The formation of SiC phase and/or the growth of epitaxial SiC were possible by He⁺ irradiation for the carbon films up to 0.7 nm in thickness, which was thinner than that by Ar⁺ irradiation. The He⁺ irradiation could not grow the turbostratic graphite which could be grown by Ar⁺ irradiation. The mechanism of the formation and the epitaxial growth of SiC by ion irradiation was discussed from the view point of the energy transfer from the irradiated ions.     

Optical and physical characteristics of In-doped ZnO nanorods

In this paper the effect of indium dopants on structure, optical, electrical and mechanical properties of ZnO nanorods are studied. The average surface potentials and the surface currents of ZnO:In nanorods were 0.25–0.84 mV and 2.2–200 MΩ-cm, respectively. The turn-on threshold field for the vertical ZnO nanorods was around 2–16 V μm⁻¹. Emission current densities of 3.3–911.4 mA cm⁻² were obtained for an electrical field of 60–160 V μm⁻¹. The photoluminescence (PL) spectrum measured at 15–300 K showed that the intensity of the peak at 2.06 eV increased with decreasing temperature, while the peak at 2.06 eV further red shifted and the peak at 3.39 eV blue shifted.     

Electronic excitations and luminescence of SrAlF5 crystals doped with Ce3+ ions

This paper reports the results of a time-resolved photoluminescence and energy transfer processes study in Ce³⁺ doped SrAlF₅ single crystals. Several Ce³⁺ centers emitting near 4 eV due to 5d-4f transitions of Ce³⁺ ions substituting for Sr²⁺ in non-equivalent lattice sites were identified. The lifetime of these transitions is in the range of 25–35 ns under intra-center excitation in the energy region of 4–7 eV at T = 10 K. An effective energy transfer from lattice defects to dopant ions was revealed in the – 7–11 eV energy range. Both direct and indirect excitation channels are efficient at room temperature. Excitons bound to dopants are revealed at T = 10 K under excitation in the fundamental absorption region above 11 eV, as well as radiative decay of self-trapped excitons resulting in luminescence near 3 eV.     

Radioluminescence of synthetic quartz related to alkali ions

The radioluminescence (RL) of synthetic quartzes (GEMMA Quartz & Crystal Company) has been measured at room temperature. Some samples were treated by electrodiffusion (“sweeping”) in order to change the concentrations of alkali ions, mainly Li⁺ and Na⁺, which in quartz are known to be linked to Al ions, substitutional for Si ions.The RL emission spectra show evidence of a role of alkali ions in affecting some specific emissions. All the spectra could be analysed as composed of four bands in the blue and UV region. Specifically, the well known blue emission at around 470 nm was seen to be composed by two bands at 430 nm (2.86 eV) and at 485 nm (2.53 eV). Effects of irradiation, during the RL measurements, were clearly seen only in the “Li swept in” sample, namely an increase in the 485 nm band intensity and a decrease in the 430 nm band one. The previously reported UV emission was detected at 355 nm (3.44 eV) in all the samples, being the most intense band in the “swept out” sample. A further UV emission was detected at 315 nm (3.94 eV), more intense in untreated samples.Possible assignments of the detected emission bands are discussed in relation to the defects of quartz, specifically focusing on the Al centres that are most affected by sweeping procedures.     

Super-protonic phase transition and fast ionic conductivity of Li+ in [Li0.2(NH4)0.8]2TeCl6

The differential scanning calorimetry diagram of [Li_0.2(NH₄)_0.8]₂TeCl₆ showed one anomaly at 526 K accompanied with a shoulder at 505 K.The conductivity plot exhibits two anomalies at 496 and 526 K, which characterize the beginning and the end of the crossing to superionic conductor state. The low temperature conduction is ensured essentially by Li⁺. A sudden jump confirms the presence of a superionic protonic transition related to the fast motion of Li⁺ and H⁺ ions. Above 526 K, the high temperature phase is characterized by high electrical conductivity (10^− 3 Ω^− 1 m^− 1) and low activation energy (E_a < 0.3 eV).The dielectric constant evolution as a function of frequency and temperature revealed the same anomaly.Transport properties in this material appear to be due to Li⁺ and H⁺ ions' hopping mechanism.     

Relaxation and thermal vibrations at the NaF(100) surface

The relaxation and the thermal vibrations of the NaF(100) surface are investigated in the temperature range between 25 K and 230 K by means of low-energy electron diffraction (LEED) and a subsequent I(V) structure analysis based on the tensor LEED approach (TLEED). According to the experiments, the NaF(100) surface is not significantly relaxed and has the ideal truncated bulk structure. The thermal vibrational amplitudes of the ions in the topmost layer are significantly enhanced compared to the bulk by a factor of 1.35 ± 0.15 and are equal within the error-bars for Na⁺ and F^? ions. Moreover, the relaxation and the dynamics of the NaF(100) surface are investigated using periodic density functional theory (DFT) calculations using pseudopotentials. In agreement with the experimental findings, the calculated relaxation of the NaF(100) surface is weak with static shifts of the ions of 0.01 Å to 0.02 Å. In the topmost layer, the Na⁺ ions are predicted to be slightly inward shifted, whereas the F^? ions are outward shifted, in accordance to predictions of previous shell-model calculations. A Born Oppenheimer molecular dynamics (BO-MD) simulation of the dynamics at the NaF(100) surface leads to a smaller enhancement of thermal motions of the ions at the surface compared to the experiment.     

Photolysis of SF6 adsorbed on Si(111)-7 × 7 by monochromatic soft X-ray

Continuous-time photoelectron spectroscopy (PES) and photon-exposure-dependent photon-stimulated desorption (PSD) were employed to investigate the monochromatic soft X-ray-induced dissociation of SF₆ molecules adsorbed on Si(111)-7 × 7 at 30 K (SF₆ dose = 3.4 × 10¹³ molecules/cm², ～ 0.5 monolayer). The photon-induced evolution of adsorbed SF₆ was monitored at photon energies of 98 and 120 eV [near the Si(2p) edge], and sequential valence-level PES spectra made it possible to deduce the photolysis cross section as a function of energy. It was found that the photolysis cross sections for 98 and 120 eV photons are ～ 2.7 × 10^? 17 and ～ 3.7 × 10^?17 cm², respectively. The changes in the F^? and F⁺ PSD ion yields were also measured during irradiation of 120 eV photons. The photon-exposure dependencies of the F^? and F⁺ ion yields show the characteristics: (a) the dissociation of adsorbed SF₆ molecules is ascribable to the substrate-mediated dissociations [dissociative attachment (DA) and dipolar dissociation (DD) induced by the photoelectrons emitting from the silicon substrate]; (b) at early stages of photolysis, the F^? yield is mainly due to DA and DD of the adsorbed SF₆ molecules, while at high photon exposure the F^? formation by electron capture of the F⁺ ion is likely to be the dominant mechanism; (c) the F⁺ ion desorption is associated with the bond breaking of the surface SiF species; (d) the surface SiF is formed by reaction of the surface Si atom with the fluorine atom or F^? ion produced by scission of S–F bond of SF_n (n = 1–6) species.     

Dielectric relaxations in undoped,Ce-doped and Ce,Zr-codoped Lu3Al5O12 single crystals

Dielectric spectroscopy was performed on single crystals of pure, Ce-doped or Ce,Zr-codoped Lu₃Al₅O₁₂, before and after UV- or X-irradaiation, at various frequencies within the range 100 Hz–1 MHz as the temperature was scanned from 110 to 353 K. All samples previously subjected to ionising radiations gave spectra showing loss peaks with Arrhenius characteristics of permanent dipoles relaxation. We attribute the dipoles to defect-stabilised pairs of anion–anion vacancies (oxygen ions and oxygen vacancies) that have captured holes and photo-electrons separately, thus forming O^?- and F⁺-like centers. The dielectric relaxation peaks disappeared in undoped or doped samples annealed at 573 K, suggesting that charge carrier traps are relatively deep. UV–visible absorption spectra have also been measured, which tend to support our proposed interpretation. Further evidence for deep traps has come from thermally stimulated luminescence experiments.     

Nanostructures on GaAs surfaces due to 60 keV Ar+-ion beam sputtering

The effect of 60 keV Ar⁺-ion beam sputtering on the surface topography of p-type GaAs(1 0 0) was investigated by varying angle of incidence of the ion (0–60°) with respect to substrate normal and the ion fluence (2 × 10¹⁷–3 × 10¹⁸ ions/cm²) at an ion flux of 3.75 × 10¹³ ions/cm²-s. For normal incidence and at a fluence of 2 × 10¹⁷ ions/cm², holes and islands are observed with the former having an average size and density of 31 nm and 4.9 × 10⁹ holes/cm², respectively. For 30° and 45° off-normal incidence, in general, a smooth surface appears which is unaffected by increase of fluence. At 60° off-normal incidence dots are observed while for the highest fluence of 3 × 10¹⁸ ions/cm² early stage of ripple formation along with dots is observed with amplitude of 4 nm. The applicability and limitations of the existing theories of ion induced pattern formation to account for the observed surface topographies are discussed.     

Defect related luminescence in Hg0.2Cd0.8Te nano- and micro-crystals grown by the solvothermal method

The photoluminescence (PL) properties of nano- and micro-crystalline Hg_1?xCd_xTe (x≈0.8) grown by the solvothermal method have been studied over the temperature range 10–300 K. The emission spectra of the samples excited with 514.5 nm Ar⁺ laser consist of five prominent bands around 0.56, 0.60, 0.69, 0.78 and 0.92 eV. The entire PL band in this NIR region is attributed to the luminescence from defect centers. The features like temperature independent peak energy and quite sensitive PL intensity, which has a maximum around 50 K is illustrated by the configuration coordinate model. After 50 K, the luminescence shows a thermal quenching behavior that is usually exhibited by amorphous semiconductors, indicating that the defects are related to the compositional disorder.     

Structure determination of the Cu(0 0 1)–c(4 × 4)-Sn surface by low-energy electron diffraction

Low-energy electron diffraction (LEED) have been used to determine the Cu(0 0 1)–c(4 × 4)-Sn structure formed at 300 K. It is demonstrated that a structural model suggested by scanning tunneling microscopy observations is correct: The model consists of one substitutional Sn atom and four Sn adatoms in the unit cell. Optimum parameters of the determined c(4 × 4) structure reveal that Sn adatoms laterally are displaced by 0.30 Å away from ideal fourfold-hollow sites along the 〈100〉 directions. It is proposed that such displacements of the Sn adatoms cause the formation of a network of octagonal rings on Cu(0 0 1). The substitutional Sn atom is located at each center of the octagonal rings. The formation conditions of the network are discussed.     

Emission of Cu-related complexes in ZnO:Cu nanocrystals

Photoluminescence (PL), its temperature dependence, scanning electronic microscopy (SEM) and X ray diffraction (XRD) have been applied for the comparative study of varying the emission, morphology and crystal structure of ZnO and ZnO:Cu nanocrystals (NCs) versus technological routines, as well as the dependence of ZnO:Cu NC parameters on the Cu concentration. A set of ZnO and ZnO Cu NCs was prepared by the electrochemical (anodization) method at a permanent voltage and different etching durations with follows thermal annealing at 400 °C for 2 h in ambient air. The size of ZnO NCs decreases from 300 nm×540 nm down to 200 nm×320 nm with etching duration increasing. XRD study has confirmed that thermal annealing stimulates the ZnO oxidation and crystallization with the formation of wurtzite ZnO crystal lattice. XRD method has been used for monitoring the lattice parameters and for confirming the Cu doping of ZnO Cu NCs. In ZnO Cu NCs four defect related PL bands are detected with the PL peaks at 1.95–2.00 eV (A), 2.15-2.23  eV (B), 2.43–2.50 eV (C) and 2.61–2.69 eV (D). Highest PL intensities of orange, yellow and green emissions have been obtained in ZnO Cu NCs with the Cu concentration of 2.28 at%. At Cu concentration increasing (≥2.28 at%) the PL intensities of the bands A, B, C decrease and the new PL band peaked at 2.61–2.69 eV at 10 K appears in the PL spectrum. The variation of PL intensities for all PL bands versus temperature has been studied and the corresponding activation energies of PL thermal decay have been estimated. The type of Cu-related complexes is discussed using the correlation between the PL spectrum transformation and the variation of XRD parameters in ZnO Cu NCs.     

Franck–Hertz effect in cathodo- and photoluminescence of wide-gap materials

A brief overview of previously obtained and novel data on the manifestations of an analogue of Franck–Hertz effect in photo- and cathodoluminescence of wide-gap inorganic materials is presented. On the example of NaCl:Tl⁺ and MgO:Cr³⁺ single crystals, the excitation processes of the luminescence of 6s² Tl⁺ ions and 3d³ Cr³⁺ ions by 5–15 keV electrons or 5–20 eV photons at 6–420 K have been studied. The rapid processes of the direct energy transfer to Tl⁺ by hot conduction electrons or to Cr³⁺ centers by hot electrons and/or hot valence holes have been separated from rapid excitonic and more inertial electron–hole processes.     

Influence of Cu doping on the microstructure,optical properties and photoluminescence features of Cd0.9Zn0.1S nanoparticles

Cd_0.9−xZn_0.1Cu_xS (0≤x≤0.06) nanoparticles were successfully synthesized by a conventional chemical co-precipitation method at room temperature. Crystalline phases and optical absorption of the nanoparticles have been studied by X-ray diffraction (XRD) and UV–visible spectrophotometer. XRD confirms the phase singularity of the synthesized material, which also confirmed the formation of Cd–Zn–Cu–S alloy nanocrystals rather than separate nucleation or phase formation. Elemental composition was examined by the energy dispersive X-ray analysis and the microstructure was examined by scanning electron microscope. The blue shift of absorption edge below Cu=2% is responsible for dominance of Cu⁺ while at higher Cu concentration dominated Cu²⁺, d–d transition may exist. It is suggested that the addition of third metal ion (Cu²⁺/Cu⁺) is an effective way to improve the optical property and stability of the Cd_0.9Zn_0.1S solid solutions. When Cu is introduced, stretching of Cd–Zn–Cu–S bond is shifted lower wave number side from 678 cm⁻¹ (Cu=0%) to 671 cm⁻¹ (Cu=6%) due to the presence of Cu in Cd–Zn–S lattice and also the size effect. The variation in blue band emission peak from 456 nm (∼2.72 eV) to 482 nm (∼2.58 eV) by Cu-doping is corresponding to the inter-band radiation combination of photo-generated electrons and holes. Intensity of red band emission centered at 656 nm significantly increased up to Cu=4%; beyond 4% it is decreased due to the quenching of Cu concentration.     

Fabrication and luminescent properties of Al2O3:Cr3 + microspheres via a microwave solvothermal route followed by heat treatment

AlOOH:Cr^3 + powders were synthesized via a microwave solvothermal route at 433 K for 30 min and were used as the precursor and template for the preparation of γ-Al₂O₃:Cr^3 + by thermal transformation at 773 K for 2 h in air. The obtained γ-Al₂O₃ based powders were microspheres with an average diameter about 1.9 μm. Photoluminescence (PL) spectra showed that the Al₂O₃:Cr^3 + particles presented a symmetric broad R band at 696 nm without appreciable splitting when excited at 462 nm. It is shown that the 0.04 mol% of doping concentration of Cr^3 + ions in γ-Al₂O₃:Cr^3 + is optimum. According to Dexter's theory, the critical distance between Cr^3 + ions for energy transfer was determined to be 47.54 Å. Based on the corresponding PL spectrum, full width at half maximum (FWHM) of Al₂O₃:Cr^3 + (0.04 mol%) was calculated to be 3.35 nm.     

Electrical and dielectric properties of β-BaB2O4 (BBO) and CsLiB6O10 (CLBO) crystals

In barium borate (BBO) crystals, sodium and potassium ions, inherited due to the preparation technique, are dominant charge carriers. The conductivity between layers is higher; the conductivity activation energy and the conductivity at 350 °C being equal to 1.01±0.05 eV and (1.3±0.2)×10⁻⁸ S/cm, respectively. The conductivity activation energy and the conductivity at 350 °C along the channels are equal to 1.13±0.05 eV and to (4±0.2)×10⁻⁹ S/cm, respectively. Relative static permittivity is almost isotropic, and equal to 7.65±0.05. Upon storing of cesium–lithium borate (CLBO) crystals, pre-heating to 600 °C eliminates the influence of surface humidity. At 500 K, the ionic conductivity ranges from 4×10⁻¹² to 2×10⁻¹⁰ S/cm; the conductivity activation energy ranges from 1.01 to 1.17 eV. Relative static permittivity is equal to 7.4±0.3.     

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2 eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21 eV) measured at 7 K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5 eV were detected and assigned to charge-transfer absorption from O^2? to Fe³⁺ ions; the intrinsic PL band at 3.28 eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54 eV were separated; the strong PL band at 1.72 eV was revealed and attributed to a radiative transition in Fe³⁺ ion.     

Structural,ferroelectric, optical properties of A-site-modified Bi0.5(Na0.78K0.22)0.5Ti0.97Zr0.03O3 lead-free piezoceramics

We reported the role of A-site modification on the structural, ferroelectric, optical and electrical field-induced strain properties of Bi_0.5(Na_0.78K_0.22)_0.5Ti_0.97Zr_0.03O₃ lead-free piezoceramics. The Li⁺ ions with concentration from 0 to 5 mol% were used to substitute at A-site. There was no phase transition when Li⁺ ions was added up to 5 mol%. The electric field-induced strain (S_max/E_max) values increased from 600 to 643 pm/V for 2 mol% Li⁺-added which results from distortion both rhombohedral and tetragonal phase structures. The band gap reduced from 2.88 to 2.68 eV and the saturation polarization decreased from 46.2 to 26.1 μC/cm² when Li⁺ ions concentration increased from 0 to 5 mol% respectively. We expect that this work could be helpful for further understanding the role of A-site dopants in comparison with B-site modification in lead-free Bi_0.5(Na,K)_0.5TiO₃-based ceramics.