Evidence for oxygen abstraction from NO<Subscript>2</Subscript> upon thermal scattering from an Al(111) surface

Evidence is presented for the existence of a non-adiabatic reaction channel, namely the abstraction of an oxygen atom from a nitrogen dioxide molecule upon scattering from an aluminium(111) surface. This reaction channel was studied by exposing the sample to an NO₂ molecular beam and subsequently analysing the scattered flux using REMPI spectroscopy. In these experiments, a considerable amount of NO emitted from the surface was detected. The emitted NO has a rotational temperature of ca. 260 K that increases only slowly with surface temperature. In summary, these results provide first evidence for the abstraction reaction NO_2(g)NO_(g)+O_(a) upon NO₂/Al(111) scattering, which may arise when two electrons are rapidly transferred to the incoming molecule while it is unfavourably oriented for concurrent adsorption of both fragments. PACS 82.65.+r; 68.43.-h     

Growth of AgInSe<Subscript>2</Subscript> on Si(100) substrate by thermal evaporation technique

AgInSe₂ films were prepared by a thermal evaporation technique onto Si(100) substrates at a pressure of 10⁻⁵ mbar. Structural and optical properties of films deposited at 300 and 473 K have been investigated. The film composition was studied by energy dispersive analysis through X-rays. X-ray diffraction patterns indicate that AgInSe₂ films have chalcopyrite structure with strong preferred orientation in the (112) direction. Average vertical crystallite size of 25 nm was observed. The optical energy gaps of 1.20 and 1.90 eV were obtained due to the fundamental absorption edge and a transition originating from crystal field splitting, respectively. Field emission scanning electron microscopy shows loosely packed grains of spherical symmetry with some facets.     

Effect of atomic passivation at Ni-MoS2 interfaces on contact behaviors

Recently, spintronics devices using MoS₂ and ferromagnetic electrode have been in the spotlight. However, strong Fermi level pinning (FLP) is known to occur between MoS₂ and ferromagnetic electrode, resulting in a large Schottky barrier height (SBH), which makes it difficult to inject electron from ferromagnetic electrode to semiconductor. To resolve this problem, we study the reduction of FLP occurring between two materials by investigating the effect of atomic passivation at Ni-MoS₂ interfaces on contact behaviors. Such atomic passivation can reduce the FLP and magnetic moments induced at S atoms of MoS₂. Especially, the largest decrease in the FLP occurs in the case of H atom passivation. Besides, the N, O, and F atom passivation confirms the possibility of ohmic contact, indicated from small SBHs (~0.2 eV). As a result, the SBH and thus the efficiency of the device can be controlled by atomic passivation at metal-semiconductor interfaces.     

Metal/TiO<Subscript>2</Subscript> interfaces for memristive switches

The interfaces between metal electrodes and the oxide in TiO₂-based memristive switches play a key role in the switching as well as in the I–V characteristics of the devices in different resistance states. We demonstrate here that the work function of the metal electrode has a surprisingly minor effect in determining the electronic barrier at the interface. In contrast, Ti oxides can be readily reduced by most electrode metals. The amount of oxygen vacancies created by these chemical reactions essentially determines the electronic barrier at the device interfaces.     

Ab initio calculations of the atomic and electronic structure of MgF<Subscript>2</Subscript> (011) and (111) surfaces

The results of ab initio slab calculations of surface relaxations, rumplings and charge distribution for the different terminations of the MgF₂ (011) and (111) polar surfaces are presented and discussed. We have employed the computer code CRYSTAL with the Gaussian basis set and the hybrid B3PW exchange-correlation functional. Despite the ionic nature of the chemical bonding at both surfaces, a considerable decrease of the optical band gap is predicted (1.3 eV or 10%) for the (111) surface as compared to the bulk.     

Deposition of Ni<Subscript> 13</Subscript> and Cu<Subscript> 13</Subscript> clusters on Ni(111) and Cu(111) surfaces

The soft deposition of Ni₁₃ and Cu₁₃ clusters on Ni(111) and Cu(111) surfaces is studied by means of constant-energy molecular-dynamics simulations. The atomic interactions are described by the Embedded Atom Method. It is shown that the shape of the nickel clusters deposited on Cu(111) surfaces remains rather intact, while the copper clusters impacting on Ni(111) surfaces collapse forming double and triple layered products. Furthermore, it is found that for an impact energy of 0.5 eV/atom the structures of all investigated clusters show the lowest similarity to the original structures, except for the case of nickel clusters deposited on a Cu(111) surface. Finally, it is demonstrated that when cluster and substrate are of different materials, it is possible to control whether the deposition results in largely intact clusters on the substrate or in a spreading of the clusters. This separation into hard and soft clusters can be related to the relative cohesive energy of the crystalline materials.     

Acoustic emission and thermal expansion of Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> and Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> crystals

The temperature dependence of the elongation per unit length for Pb(Mg_1/3Nb_2/3)O₃ crystals unannealed after growth and mechanical treatment is investigated in the course of thermocycling. It is revealed that this dependence deviates from linear behavior at temperatures below 350°C. The observed deviation is characteristic of relaxors, is very small in the first cycle, increases with increasing number n of thermocycles, and reaches saturation at n≥3. In the first cycle, a narrow maximum of the acoustic emission activity is observed in the vicinity of 350°C. In the course of thermocycling, the intensity of this maximum decreases and becomes zero at n>3. For (1?x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ crystals, the dependence of the temperature of this acoustic emission maximum on x exhibits a minimum. It is assumed that the phenomena observed are associated with the phase strain hardening due to local phase transitions occurring in compositionally ordered and polar nanoregions.     

The transport and thermal properties of glassy LiPO<Subscript>3</Subscript>/crystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript> (ZrO<Subscript>2</Subscript>) composite electrolytes

Glassy LiPO₃/crystalline Al₂O₃ and glassy LiPO₃/crystalline ZrO₂ (0–12.5 vol.% of oxide fillers) composite solid electrolytes have been prepared by glass matrix softening. Their thermal and transport properties have been investigated by differential scanning calorimetry (DSC) and impedance spectroscopy methods. The addition of ZrO₂ leads to a decrease in the crystallization temperature of LiPO₃ glass. It was found that the conductivity behavior depends on the nature of the dispersed addition. In the case of the Al₂O₃ addition, the increase in the electrical conductivity is observed. The ionic conductivity of the LiPO₃/10% Al₂O₃ composite reaches 5.8 × 10^?8 S/cm at room temperature. In contrast, the conductivity in the LiPO₃/ZrO₂ composite system decreases.     

Sulfide passivation of InAs(100) substrates in Na<Subscript>2</Subscript>S solutions

Treatment of the InAs(100) surface with a 1M aqueous solution of sodium sulfide (Na₂S) is found to result in the removal of a natural oxide layer from this surface with the formation of a continuous chemisorbed passivating layer of sulfur atoms that are coherently bonded to indium atoms of the crystal surface. No etching of the InAs surface in the sulfide solution occurs. The passivated InAs samples are characterized by a multiple increase in the photoluminescence intensity. The sulfide layer is desorbed from the InAs surface at temperatures of ～400°C. This leads to the formation of a clean In-stabilized (100) surface with a (4 × 2) reconstruction. A simple technique is developed using sulfide passivation for preparing atomically smooth (2 × 4) growth surfaces of the InAs(100) substrates that are suitable for molecular-beam epitaxy of highly perfect layers of compounds based on CdSe.     

Effect of thermal treatment on the electrotransport properties of thin-film In<Subscript>2</Subscript>O<Subscript>3</Subscript>, ZnO materials and the multilayer (In<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZnO)<Subscript>83</Subscript> heterostructure

The effect heat treatment has on the electrotransport mechanisms in films of ZnO and In₂O₃, and in a multilayer (In₂O₃/ZnO)₈₃ structure obtained via ion-beam sputtering, is studied. It is shown that there is a mechanism of weak electron localization in the In₂O₃ and (In₂O₃/ZnO)₈₃ samples. The relaxation processes that occur during the heat treatment of In₂O₃ films are found to increase the length of elastic electron scattering, but to reduce this parameter in multilayer heterostructures.     

Enhanced electrochemical properties and thermal stability of LiNi<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> by surface modification with Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>

Layered LiNi_1/3Co_1/3Mn_1/3O₂ cathode material is synthesized via a sol-gel method and subsequently surface-modified with Eu₂O₃ layer by a wet chemical process. The effect of Eu₂O₃ coating on the electrochemical performances and thermal stability of LiNi_1/3Co_1/3Mn_1/3O₂@Eu₂O₃ cells is investigated systematically by the charge/discharge testing, cyclic voltammograms, AC impedance spectroscopy, and DSC measurements, respectively. In comparison, the Eu₂O₃-coated sample demonstrates better electrochemical performances and thermal stability than that of the pristine one. After 100 cycles at 1C, the Eu₂O₃-coated LiNi_1/3Co_1/3Mn_1/3O₂ cathode demonstrates stable cyclability with capacity retention of 92.9 %, which is higher than that (75.5 %) of the pristine one in voltage range 3.0–4.6 V. Analysis from the electrochemical measurements reveals that the remarkably improved performances of the surface-modified composites are mainly ascribed to the presence of Eu₂O₃-coating layer, which could efficiently suppress the undesirable side reaction and increasing impedance, and enhance the structural stability of active material.     

Effect of deuteration on the thermal properties and structural parameters of the (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>WO<Subscript>2</Subscript>F<Subscript>4</Subscript> oxyfluoride

The thermal properties and structure of (ND₄)₂WO₂F₄ crystals are investigated. It is established that deuteration does not lead to a change in the symmetry of the initial phase Cmcm but considerably decreases the extent of its disordering, which, in turn, brings about a substantial decrease in the phase transition entropy. Apart from the anomalies associated with phase transitions characteristic of the protonic compound, the heat capacity exhibits two additional anomalies. Analysis of the phase diagram of the deuterated crystal reveals a triple point at a pressure p = 0.18 GPa, which is predicted for (NH₄)₂WO₂F₄ at about 0.7 GPa.     

Microscopic bonding and macroscopic strain relaxations at Si-SiO<Subscript>2</Subscript> interfaces

Optical second harmonic generation studies of the temperature dependence of the phase angle differences from contributions of terrace and edge bonding at silicon–silicon dioxide interfaces prepared on vicinal Si(111) wafers have revealed an interface relaxation process at an annealing temperature between 850 and 900 °C. Complementary studies by synchrotron soft X-ray photoelectron spectroscopy have established that this relaxation is associated with changes in the concentration and composition of local suboxide bonding environments in an ultrathin, interfacial transition region that is approximately one molecular layer thick. This relaxation occurs at a significantly lower temperature than an approximately 990±10 °C relaxation of macroscopic compressive strain in the bulk of the SiO₂ film. This paper establishes an analogy between i) the Si–SiO₂ interface in which there is a transition from a rigid substrate, Si, to an ideal continuous random covalent network, SiO₂, in which the average number of bonding constraints/atom matches the network dimensionality, and ii) a concentration dependent transition between under- and over-constrained local bonding in non-crystalline glass alloys such as Ge_xSe_1-x. The interfacial suboxide transition region is demonstrated to have properties in common with a regime of alloy compositions in which self-organization reduces bond constraint induced strain, thereby stabilizing these compositions against aging as for example in time-dependent changes in the glass transition temperature. These comparisons provide important new insights into defect formation at Si–SiO₂ interfaces, as well as interfaces between Si and alternative high-k dielectrics being considered for advanced Si devices including Al₂O₃ and transition metal silicate alloys, e.g., (ZrO₂)_x(SiO₂)_1-x. This new perspective is also extended to interfaces between GaN and SiO₂, where the interfacial transition region is a suboxide of Ga, GaO_x, with x<1.5. PACS 85.30.Tv; 42.65.-k; 79.40.+z; 73.20.-r; 77.55.+f     

Collision dynamics of He@C<Subscript>60</Subscript>+He@C<Subscript>60</Subscript> at low energies

A semi-empirical molecular dynamics model is developed. The central collisions of C₆₀+C₆₀ and He@C₆₀+He@C₆₀ at different incident energies are investigated based on this model. It is found that the dimer structures have been produced at proper incident energies and these fullerene dimers could be formed by a self-assembly of C₆₀ fullerene and He@C₆₀. The He atom has a significant effect at higher incident energy and this embedded He atom can enhance the stability of the dimer structure.     

Evolution in shapes of a series of (111)-based In<Subscript>2</Subscript>O<Subscript>3</Subscript> particles

In₂O₃ particles with different morphology were controllably synthesized on silicon substrates by thermal evaporation of In grains at 900 °C. The structure and morphology of the In₂O₃ particles were evaluated using X-ray diffraction, and scanning and transmission electron microscopies. The evolution in shapes as the ratio of {100} relative to {111} increases is clearly observed. The photoluminescence spectrum of the obtained In₂O₃ structures exhibits UV emission centered at about 378 nm and wide-band emission covering the green and orange regions with three peaks around 525, 572, and 604 nm. PACS 81.05.Hd; 81.07.Bc; 81.16.-c; 61.46.-w; 81.40.Gh     

Effect of thermal annealing on the structure of ZnSe/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposite films

The ZnSe/Al₂O₃ nanocomposite films synthesized by laser evaporation followed by heat treatment are studied. X-ray diffraction and electron-microscopic investigations of the as-deposited films demonstrate the presence of ZnSe crystallites in an Al₂O₃ amorphous matrix. Annealing changes the structures of ZnSe and Al₂O₃, increases the ZnSe crystallite size, and causes the appearance of the ZnSeO₄ phase. The presence of aluminum oxide layers decreases the phase transformation temperature of zinc selenide.     

Electronic structure and thermoelectric properties of (Mg<Subscript>2</Subscript>X)<Subscript>2</Subscript> / (Mg<Subscript>2</Subscript>Y)<Subscript>2</Subscript> (X,Y = Si,Ge, Sn) superlattices from first-principle calculations

To identify thermoelectric materials containing abundant, low-cost and non-toxicelements, we have studied the electronic structures and thermoelectric properties of(Mg₂X)₂/(Mg₂Y)₂ (X, Y = Si, Ge, Sn) superlattices withstate-of-the-art first-principles calculations using a modified Becke and Johnson (mBJ)exchange potential. Our results show that (Mg₂Ge)₂/ (Mg₂Sn)₂ and(Mg₂Si)₂/(Mg₂Sn)₂ are semi-metals using mBJ plusspin-orbit coupling (mBJ +SOC), while (Mg₂Si)₂/ (Mg₂Ge)₂ ispredicted to be a direct-gap semiconductor with a mBJ gap value of 0.46 eV andmBJ + SOC gap valueof 0.44 eV. Thermoelectric properties are predicted by through solving the Boltzmanntransport equations within the constant scattering time approximation. It is found that(Mg₂Si)₂/(Mg₂Ge)₂ has a larger Seebeck coefficient andpower factor than (Mg₂Ge)₂/ (Mg₂Sn)₂ and(Mg₂Si)₂/(Mg₂Sn)₂ for both p-type and n-type doping. Thedetrimental influence of SOC on the power factor of p-type (Mg₂X)₂/(Mg₂Y)₂ (X, Y = Si, Ge, Sn) is analyzed as afunction of the carrier concentration, but there is a negligible SOC effect for n-type.These results can be explained by the influence of SOC on their valence and conductionbands near the Fermi level.     

Investigations of dielectric enhancement in (Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>)<Subscript>1-x</Subscript>(TiO<Subscript>2</Subscript>)<Subscript>x</Subscript> ceramics prepared by laser-sintering technique

The maximum dielectric permittivity of Ti-doped Ta₂O₅ ceramics may reach 450 by a laser-sintering technique. The aim of this study is to investigate the mechanisms of the dielectric enhancement based on the unique structural and morphological properties of the laser-sintered ceramics. The reason for the dielectric enhancement is due to the crystal structure distortion in the high-temperature phase, the oriented grain growth taking place in a direction deviating from [001] in the laser-sintered ceramics. The concurrent nature of quenching effects, a sharp temperature gradient and mass transfer in liquid phase originated from laser high energy irradiation with strict directivity leads to the structural and morphological properties. PACS 81.40.Tv; 61.80.Ba; 77.22.-d; 77.22.Ch     

Vibrational dynamics of the organometallic compound triarylorganoantimony (V) SbPh<Subscript>3</Subscript>[O<Subscript>2</Subscript>CC(OH)Ph<Subscript>2</Subscript>]<Subscript>2</Subscript>

A complete normal coordinate analysis was performed for five-coordinate non-rigid triarylantimony diester SbPh₃(O₂CR)₂, known to be a bioactive molecule, using Wilson G-F matrix method and Urey Bradley force field. The study of vibrational dynamics was performed using the concept of group frequencies and band intensities.