Conduction in magnesium phthalocyanine thin films with aluminium electrodes

The various electrical properties and the nature of conduction mechanisms of magnesium phthalocyanine thin film devices with top and bottom aluminium electrodes have been investigated. The conduction mechanism was identified as injection limited essentially due to the electrode material. Even with the same electrode materials, the device showed asymmetric conduction behavior in the forward and reverse bias. In general the conduction was interpreted as a Schottky emission with barrier height Φ_s=1.07 eV for the forward bias and Φ_s=1.09 eV in the reverse bias. The effect of oxygen on the conductivity of the device has also been investigated. In the oxygen doped samples the conductivity is decreased which may be attributed to an interfacial layer between the electrode and the organic layer. Further in the oxygen doped sample while a Schottky emission is observed at lower voltages Poole-Frenkel conductivity was identified in the higher voltage region.     

Enhanced reactivity and selectivity in oxidation of Cu(1 0 0) and α-Cu-Al(5 at.%)(1 0 0) surfaces studied by electron and ion spectroscopies

In this study we investigate the influence of alloying on the reactivity and bonding of oxygen on α-Cu-Al(5 at.%)(1 0 0) oriented single crystal surfaces by X-ray photoelectron spectroscopy (XPS), ultra-violet spectroscopy (UPS) and low energy ion scattering (LEIS) spectroscopy, at room temperature. It was found that alloying results in an enhanced reactivity of both Cu and Al sites in comparison with the pure metals. According to adsorption curves calculated from XPS, saturation of the alloy surface occurs for exposures of ∼15 L. At saturation the total amount of adsorbed oxygen is similar for the alloy and pure copper surfaces. It was determined that first mostly Al sites are oxidized, followed by simultaneous oxidation of Cu and Al sites. At saturation the amount of oxygen bonded to Cu sites is ∼1.7 larger then that bonded to Al sites. From a comparison of the XPS and LEIS data analysis as a function of oxygen exposure it was found that oxidation of α-Cu-Al(5 at.%)(1 0 0) alloy is a multi-stage process with fast and slow stages. These stages involve an interplay of chemisorption, sub-surface diffusion of oxygen and Al segregation. UPS measurements show an increase in the work function of the alloy surface with oxygen adsorption. This is a contrast to pure Cu surfaces where the work function decreases at the initial stages of oxidation followed by an increase with oxygen exposure. Annealing to 400 °C drives the oxidized alloy surface into its thermodynamic state resulting in the formation of an aluminum oxide layer. Possible mechanisms to explain the enhanced reactivity of the alloy surface compared to that of pure copper are suggested and discussed.     

Photoemission study of oxygen chemisorption on tin

Photoemission measurements have been made at photon energies from 5–12 eV and at 21.2 eV on evaporated Sn films and the same films with varying room temperature exposure to oxygen. For hν ? 9 eV the quantum yield for Sn with exposures of as much as 4000 L oxygen (1 L = 1 Langmuir = 10^?6 Torr sec) differs only slightly from the clean metal. For hν ? 9 eV no change in yield is observed with oxygen exposure. The energy distribution of photoemitted electron (EDC's) from Sn with increasing exposure to oxygen above ? 20 L are characterized by the growth of two peaks which were not present in the EDC's for the clean metal, located 2.9 ± 0.1 eV and 4.8 ± 0.1 eV below the Fermi level. We associate this structure with the presence of SnO₂. No sharp resonance which could be associated with adsorbed oxygen was seen. Uniformly reduced emission from metallic Sn states and a Fermi level as sharp as for the clean metal is observed in the EDC's at all oxygen exposures. In addition, no change in work function with oxygen exposure was detected. The effects of oxygen saturate for exposures ? 4000 L. We suggest that under the conditions used in this experiment, the oxygen penetrates beneath the surface forming SnO₂ and leaving metallic Sn on the surface.     

Ar+ excited aluminium Auger electron emission from Cu-Al alloys

The aluminium Auger electron emission from Cu-Al alloys in solid solution bombarded with 2–16 keV Ar⁺ ions is studied as a function of the Al concentration. A linear law is observed for the intensity of the high energy Auger peak at 76 eV which originates only from the primary asymmetric collisions Ar → Al where two vacancies are created in the 2p level of the Al atom. On the contrary, a parabolic law is found for the intensity of the principal Auger peak at 63.5 eV (one Al 2p vacancy) which originates from the asymmetric collisions Ar → Al and from the symmetric collisions Al → Al together. The proportion of asymmetric collisions among collisions effective for the principal Auger emission from pure aluminium can be deduced from these results. It appears as an increasing function of the bombardment energy: its value is nearly equal to 6% at 10 keV and 18% at 15 keV.     

Attenuation length and escape depth of excited electrons in solids

Escape probability and mean escape depth λ_e of emitted electrons are determined as a function of attenuation length λ_a of excited electrons, their initial energy E_n, and the height of the surface barrier χ. A variable energy loss parameter permits to apply the proposed model to different kinds of electron emission, including the energy loss free Auger and ESCA electrons. An analytical expression was found correlating the internal energy distribution of excited electrons and the external energy distribution of emitted electrons. By means of this excitation energies of secondary electrons and exoelectrons were determined.     

Understanding the mechanism of aluminium nanoparticle oxidation

Aluminium nanoparticles have gained importance in the last decade because of their increased reactivity as compared with traditional micron-sized particle. The physics of burning of aluminium nanoparticle is expected to be different than that of micron-sized particles, and the current article is motivated by these differences. We have previously measured the size resolved reactivity of nanoaluminium by single-particle mass spectrometry, to which we now add transmission electron microscope (TEM) and an on-line density measurement. The latter two studies revealed the presence of hollow particles following oxidation of nanoaluminium and indicating the significance of diffusion of aluminium in the overall process. Based on experimental evidence, we believe that aluminium nanoparticle oxidation occurs in two regimes. Prior to melting of aluminium slow oxidation occurs through the diffusion of oxygen through the aluminium oxide shell. Above the melting point, we transition to a fast oxidation regime whereby both aluminium and oxygen diffuse through the oxide shell to enhance the oxidation rate.

We also develop a phenomenological model for nanoaluminium oxidation that accounts for the experimentally observed rates, the fact that both fuel and oxidizer are diffusing, and a new effect related to internal pressure gradients. The latter phenomen is based on molecular dynamic simulations suggesting that there are large pressure gradients present inside these particles, with the aluminium core under a positive pressure and the aluminium oxide shell under a negative pressure. We have considered the effect of these pressure gradients on the oxidation process. A power law relation was obtained (t ∝ r ^{1.6± 0.1}) between the time required for oxidation and particle radius.     

Study of adsorption phenomena on aluminium by interatomic Auger transition spectroscopy: I. Initial oxidation of Al

The initial oxidation process of aluminium is studied by using the combined molecular beam evaporation and Auger electron spectroscopy under ultra-high vacuum. At the initial stage of oxygen exposure to the clean aluminium at room temperature, the chemisorption of the oxygen atoms on the surface of aluminium is turned out to be dominant from the behaviour of the energy shifted Auger signal of aluminium. Then, the formation of the alumina (Al₂O₃) type bonding is followed, which is concluded from the interatomic Auger signals.     

Sonochemical synthesis of aluminium and aluminium hybrids for remediation of toxic metals

Spherical shaped nano-size aluminium oxide and its hybrids with indole and indole derivatives have been synthesized using sol–gel and post grafting methods coupled with sonication (Branson Digital SonifierS-250D; 20 kHz; 40%) for the remediation of toxic metals (lead and mercury). Different spectroscopic techniques (FTIR, SEM, BET, XRD, and XPS) have been applied to assess the properties of synthesized aluminium oxide and its hybrids. FTIR spectra showed the absorption bands of aluminium oxide (Al-O-Al) and aluminium hybrids (Al-O-C) at 800–400 cm⁻¹ and 1650–1100 cm⁻¹ region, respectively. SEM showed spherical shaped clusters of aluminium oxide which changed into the net-shape structure after the hybrid synthesis. It is worth noting that sonication energy increases the total surface area of aluminium oxide when it gets hybridized with indole and its derivatives from 82 m²/g to 167 m²/g; it also improved the product yield from 68% to 78%. Simultaneously, FTIR, SEM and BET analysis of non-sonicated aluminium oxide and its hybrids were also recorded for comparison. While XRD and XPS analysis were only conducted for sonicated aluminium oxide and its hybrids to manifest the structural and compositional properties. XRD patterns indexed as the cubic crystal system with an average 41 nm crystallite size of sonicated aluminium oxide which remains unaffected after hybrid synthesis. A survey scan under XPS confirmed the presence of all expected elements (aluminium, oxygen, carbon, nitrogen) and deconvolution of each recorded peak showed binding of element with its neighboring elements. The performance of aluminium oxide and its hybrids synthesize with and without sonication are also evaluated using a time-dependent batch adsorption protocol optimize for one hour. The maximum adsorption of lead (37%) and mercury (40%) are found onto sonicated aluminium oxide. The sonicated aluminium hybrids showed 43–63% of lead and 55–67% of mercury at pH 7. The fitness of experimental data using adsorption kinetics and isotherms revealed that adsorption follows Pseudo-second-order kinetic, Langmuir, and Freundlich isotherms.     

Adsorption of oxygen on low-index surfaces of the TiAl<Subscript>3</Subscript> alloy

Method of the projector augmented waves in the plane-wave basis within the generalized-gradient approximation for the exchange-correlation functional has been used to study oxygen adsorption on (001), (100), and (110) low-index surfaces of the TiAl₃ alloy. It has been established that the sites that are most energetically preferred for the adsorption of oxygen are hollow (H) positions on the (001) surface and bridge (B) positions on the (110) and (100) surfaces. Structural and electronic factors that define their energy preference have been discussed. Changes in the atomic and electronic structure of subsurface layers that occur as the oxygen concentration increases to three monolayers have been analyzed. It has been shown that the formation of chemical bonds of oxygen with both components of the alloy leads to the appearance of states that are split-off from the bottoms of their valence bands, which is accompanied by the formation of a forbidden gap at the Fermi level and by a weakening of the Ti–Al metallic bonds in the alloy. On the Al-terminated (001) and (110) surfaces, the oxidation of aluminum dominates over that of titanium. On the whole, the binding energy of oxygen on the low-index surfaces with a mixed termination is higher than that at the aluminum-terminated surface. The calculation of the diffusion of oxygen in the TiAl₃ alloy has shown that the lowest barriers correspond to the diffusion between tetrahedral positions in the (001) plane; the diffusion of oxygen in the [001] direction occurs through octahedral and tetrahedral positions. An increase in the concentration of aluminum in the alloy favors a reduction in the height of the energy barriers as compared to the corresponding barriers in the γ-TiAl alloy.     

Electron-stimulated desorption of potassium and cesium atoms from oxidized molybdenum surfaces

The electron-stimulated desorption (ESD) yields and energy distributions for potassium (K) and cesium (Cs) atoms have been measured from K and Cs layers adsorbed at 300 K on oxidized molybdenum surfaces with various degrees of oxidation. The measurements were carried out using a time-of-flight method and surface ionization detector. The ESD appearance threshold for K and Cs atoms is independent of the molybdenum oxidation state and is close to the oxygen 2s level ionization energy of 25 eV. Additional thresholds for both K and Cs atoms are observed at about 40 and 70 eV in ESD from layers adsorbed on an oxygen monolayer-covered molybdenum surface; they are associated with resonance processes involving Mo 4p and 4s excitations. The ESD energy distributions for K and Cs atoms consist of single peaks. The most probable kinetic energy of atoms decreases in going from cesium to potassium and with increasing adsorbed metal concentration; it lies in the energy range around 0.35 eV. The K and Cs atom ESD energy distributions from adlayers on an oxygen monolayer-covered molybdenum surface are extended toward very low kinetic energies. The data can be interpreted by means of the Auger stimulated desorption model, in which neutralization of adsorbed alkali-metal ions occurs after filling of holes created by incident electrons in the O 2s, Mo 4s or Mo 4p levels.     

Electronic relaxations of radiative defects of the anion sublattice in caesium bromide crystals and exoemission of electrons

The thermostimulated exoelectron emission (TSEE) is applied for investigation of the processes of radiative defects recombination in the nearsurface layer. Results of TSEE studies of radiatively excited CsBr crystals are presented. Dose dependences of the decay kinetics, TSEE spectrum structure and exosums were studied. Concentration of exoemission-active centres (EAC) and TSEE kinetics parameters have been calculated on the base of the bulk thermoactivated anion sublattice defect recombination. The attained result correlate with the electron centres concentration for irradiated crystals. In the framework of the Auger-like anion defects recombinational bulk model of exoemission from irradiated wide-band-gap crystals, the energy spectra of the exoelectrons excited on the F-centres are attained.     

Broadening of the energy distribution of thermal-field emitted electrons from carbon fibres

The energy distribution of electrons thermal-field emitted from carbon fibres and post-accelerated up to 25 keV has been measured. At low total emission currents (of the order of nano-amperes) the energy distribution of the emitted electrons agrees, at room temperature, with the theoretical prediction for the free-electron model, and at elevated temperatures (up to 1520 K) the energy spread is about 0.1 eV higher than predicted. This difference may be explained by assuming that the emission occurs from sharp microfibrils. With increasing current the energy distribution becomes broadened. It is shown that this broadening is independent of the tip temperature and most probably due to Coulomb interaction of the beam electrons in regions of high current density within the electron optical system.     

Photon and electron emission during halogen adsorption on sodium

The adsorption of chlorine, bromine and iodine on UHV evaporated sodium films is accompanied by the emission of photons and electrons. Although the emission probabilities are small (～10^?7 and ～10^?5 per adsorbed molecule respectively), the effect provides a sensitive means of monitoring the adsorption process. For moderate exposures, the photon and electron yields are proportional to the pressure. At high exposures, due to the formation of a halide film on the surface, the photon yield decreases rapidly, eventually becoming proportional to the square root of the pressure. The high sensitivity allows use of the light signal to measure diffusion through the halide layer. The spectral distributions of the observed photons are characterized by wide (～1 eV) emission bands in the range 4000–8000 A. The spectra are influenced by surface composition, as demonstrated most clearly in the Na + I₂ spectrum, where a peak occurs on the high energy side of the main band when the NaI film starts to form. The origin of the spectra, their comparison with gas phase chemiluminesccnce and a model of the reaction kinetics are discussed.     

The reactions of oxygen and water with the rare-earth metals terbium to lutetium studied by x-ray photoelectron spectroscopy

The X-ray excited oxygen 1s photoelectron spectra of water adsorbed on clean evaporated films of the heavy rare-earth metals, terbium to lutetium, is characterised by two peaks: (1) at 531.0 ± 0.5 eV binding energy, and (2) at 533.0 ± 0.5 eV binding energy, assigned respectively to oxide and hydroxide species. Variation of the relative intensities of these peaks with exposure to water leads to the postulate that the oxidation mechanism is island growth with a layer of hydroxide at the surface of the oxide island. At low temperatures, adsorption of water gives two additional peaks: (3) at 534.5 ± 0.5 eV, and (4) at 535.7 ± 0.2 eV, assigned respectively to chemisorbed and condensed water. On adsorption of small amounts of dry oxygen the O 1s spectra exhibit solely peak (1), whose intensity increases with further oxygen treatment to reach a steady value after ～40 L exposure. The kinetics of reaction with oxygen follow a logarithmic relation, once correction is made for the effect of escape-depth on peak intensity. However, ytterbium, with a closed 4f shell in the metallic state, exhibits oxidation characteristics different from the other rare-earth metals; its oxygen 1s intensity increases linearly with exposure, and the steady plateau level is reached sharply rather than asymptotically. Island growth with a limited number of nucleation sites may explain this behaviour.     

Exoelectron Emission from Pyroceramic

The nonstationary electron emission (exoelectron emission) and the emission of negative charges from the surface of finely-crystalline multicomponent dielectric materials subjected to light and thermal stimulation, electron injection, and x-irradiation are examined. It is demonstrated that the exoelectron emission of dielectric materials without preliminary irradiation is associated with adsorption layers located on the sample surface. After x-irradiation and electron injection, maxima appear in the temperature dependence of the intensity of exoelectron emission caused by various defects in subsurface regions of the examined pyroceramic samples.     

The influence of surface microchemistry in protective film formation on multi-phase magnesium alloys

The high strength:weight ratio of magnesium alloys makes them an ideal metal for automotive and aerospace applications where weight reduction is of significant concern. Unfortunately, magnesium alloys are highly susceptible to corrosion particularly in salt-spray conditions. This has limited their use in the automotive and aerospace industries, where exposure to harsh service conditions is unavoidable. The simplest way to avoid corrosion is to coat the magnesium-based substrate by a process such as electroless plating, which is a low-cost, non line of sight process.Magnesium is classified as a difficult to plate metal due to its high reactivity. This means that in the presence of air magnesium very quickly forms a passive oxide layer that must be removed prior to plating. Furthermore, high aluminium content alloys are especially difficult to plate due to the formation of intermetallic species at the grain boundaries, resulting in a non-uniform surface potential across the substrate and thereby further complicating the plating process.The objective of this study is to understand how the magnesium alloy microstructure influences the surface chemistry of the alloy during both pretreatment and immersion copper coating of the substrate.A combination of scanning electron microscopy, energy dispersive spectroscopy and scanning Auger microscopy has been used to study the surface chemistry at the various stages of the coating process. Our results indicate that the surface chemistry of the alloy is different on the aluminum rich β phase of the material compared to the magnesium matrix which leads to preferential deposition of the metal on the aluminum rich phase of the alloy.     

Nb-Ti-Al合金高温氧化机理电子理论研究

采用递归法计算了Nb合金的电子态密度、原子镶嵌能、亲和能和团簇能等电子结构参数,研究Nb合金高温氧化机理.研究表明,氧在Nb合金表面的吸附能较低,易在合金表面吸附,并逐渐扩散到Nb合金的基体中.氧在合金基体中镶嵌能为负值,氧的态密度和Nb相似,在Nb中具有很高的溶解度.Ti,Al在合金晶内的镶嵌能均高于各自在合金表面的镶嵌能,Ti,Al从合金内部向合金表面扩散,最终在Nb合金表面偏聚,形成富Ti,Al的表层.团簇能计算结果表明Nb合金表面的Ti,Al原子各自均有聚集倾向,分别形成Ti和Al原子团.氧与合金     

Au_7团簇吸附O_2和CO及其对CO的催化反应机理研究

本文采用密度泛函理论,研究了Au_7团簇催化CO的氧化反应机理.研究发现,二维平面结构的Au_7团簇更容易吸附CO和O_2分子. Au_7团簇吸附一个O_2分子的吸附能为0.64 eV,但在吸附多个O_2分子时,平均吸附能有了明显的下降,表明Au_7团簇进行多吸附O_2分子的可能性不大. Au_7团簇吸附一个CO分子的吸附能为1.26 eV,且在吸附多个CO分子时,平均吸附能虽有减少,但减小的幅度不大,说明Au_7团簇有可能吸附多个CO分子.此外,在Au_7团簇催化CO的氧化反应过程中,整个反应克服的最高势垒仅为0.34 eV,说明Au_7团簇有望成为良好的CO氧化催化剂.     

铌合金电子结构及其高温氧化行为

为了从电子层面揭示Nb合金高温氧化的物理本质,采用递归法计算了Nb合金的电子态密度、原子镶嵌能、亲和能等电子结构参数,探索Nb合金高温氧化机理.研究表明:氧在Nb中具备较高的扩散速率和溶解度,且氧与Nb较易发生反应,生成氧化物,这使Nb的抗高温氧化性较差.原子镶嵌能的计算结果表明,合金元素Ti,Si,Cr在基体中稳定性较低,易向Nb合金表面扩散,形成富Ti,Si,Cr的表层.合金表层中氧与Nb,Ti,Si,Cr间具有较大亲和性,可以生成相应的氧化物,形成对合金具有保护作用的氧化膜. 关键词： 递归法 高温氧化 Nb合金     

NiTi(110)表面氧原子吸附的第一性原理研究

为了研究给定的NiTi的表面氧化过程,在保持体系中Ni和Ti原子总数相等的条件下,构建了一系列Ti原子在表面反位的c(2×2)-NiTi(110)缺陷体系,并利用第一性原理计算研究了氧原子在各种NiTi(110)反位缺陷体系的吸附行为以及表面形成能.计算结果表明:吸附氧原子的稳定性与表面Ti原子的富集程度有很大的关联性,体系表面Ti原子富集程度越高,氧原子吸附的稳定性越高;当覆盖度较高时,由于氧原子的吸附,可使Ni和Ti原子在表面出现反位.在富氧条件(μ_o≥-9.35 eV)下,氧原子在表面第1层中的全部Ni原子与第3层全部Ti换位的反位缺陷体系上的吸附最稳定,此时随着氧原子的吸附,表面上的Ti原子升高,导致向上膨胀生长形成二氧化钛层,且在其下方形成富Ni层,由此可合理地解释实验上发现NiTi合金氧化形成二氧化钛层的可能原因.