Substrate-induced freezing of alkane monolayers adsorbed on Au(1 1 1) dependant on the alkane/gold misfit

The molecular structure of the interfaces between pure C_nH_2n+2 (n=10, 12, 14, 16) and Au(1 1 1) surface has been studied by scanning tunneling microscopy. At 291 K, self-organized monolayers with a lamella structure are formed with these alkanes. The ordered monolayers are melting at temperature higher by 46, 28, 15, 5 K than the melting point of bulk C_nH_2n+2 crystals with n=10, 12, 14, 16, respectively. Two kinds of melting process were observed: (i) a direct solid/liquid phase transition within the monolayer for C₁₀H₂₂, C₁₂H₂₆ and C₁₄H₃₀ molecules, (ii) an intermediate phase for C₁₆H₃₄ molecules. This mesophase corresponds to a two-dimensional liquid crystal formed by molecules moving along their axis and along Au1 1 0. These results agree well with calculations using a geometric model taking in account the misfit between the CH₂–CH₂–CH₂ period along alkyl chain and the gold lattice along 1 1 0 direction.     

Atomistic modeling of femtosecond laser-induced melting and atomic mixing in Au film - Cu substrate system

The mechanisms of femtosecond laser-induced transient melting and atomic mixing in a target composed of a 30 nm Au film deposited on a bulk Cu substrate are investigated in a series of atomistic simulations. The relative strength and the electron temperature dependence of the electron-phonon coupling of the metals composing the layered target are identified as major factors affecting the initial energy redistribution and the location of the region(s) undergoing transient melting and resolidification. The higher strength of the electron-phonon coupling in Cu, as compared to Au, results in a preferential sub-surface heating and melting of the Cu substrate, while the overlaying Au film largely retains its original crystalline structure. The large difference in the atomic mobility in the transiently melted and crystalline regions of the target makes it possible to connect the final distributions of the components in the resolidified targets to the history of the laser-induced melting process, thus allowing for experimental verification of the computational predictions.     

Finite element analysis of three-dimensional laser-induced transient thermal grating in diamond/ZnSe system

This paper uses finite element method to obtain the three-dimensional temperature field of laser-induced transient thermal grating （TTG） for two-layered structure of diamond film on ZnSe substrate. The numerical results indicate that unique two-times heating process is gradually experienced in the area between two adjacent grating stripes. However, there is a little change for the temperature field along the depth direction for the diamond film due to its great thermal conductivity. It further finds that the thickness of the diamond film has a significant influence on the temperature field in diamond/ZnSe system. The results are useful for the application of laser-induced TTG technique in film/substrate system.     

Analysis of methanol extraction from aqueous solution by n-hexane: Equilibrium diagrams as a function of temperature

This paper contains the results of a new experimental study of liquid–liquid equilibrium (LLE) as a function of temperature for the ternary mixture n-hexane + methanol + water under atmospheric conditions. The computed coexistence curves were very asymmetrical with respect to n-alkane + water composition at low operation temperature. A comparative analysis was performed by application of group contribution methods to predict the experimental equilibria behaviour of this ternary mixture. The experimental tie lines data were correlated to test consistency with the Othmer–Tobias equation. The obtained results point out the strong interaction by hydrogen bonds forming the so-called iceberg clusters of hydroxylated molecules at low concentration of n-hexane, a slight influence of temperature being observed. As a result of this intense interaction, sharp separations of n-hexane + methanol mixtures by extraction are possible at low temperature and small quantities of solvent. Extraction process simulation confirms n-hexane as an adequate medium for diluted methanol concentration.     

A study of the thermal-induced nonlinearity of Au and Ag colloids prepared by the chemical reaction method

The nonlinear optical properties of metal Au and Ag colloidal solutions prepared by the chemical reaction method were investigated by a single beam Z-scan technique. Under CW 633 nm excitation, the Au and Ag colloidal solutions exhibited a large thermal-induced refractive index n₂. At the same time, the thermo-optic coefficients dn/dT of the Au and Ag colloidal solutions were obtained. The mechanism responsible for the process of nonlinear refraction was discussed in term of laser heating effect.     

Adsorption of oxygen on Au(111) by exposure to ozone

Atomic oxygen coverages of up to 1.2 ML may be cleanly adsorbed on the Au(111) surface by exposure to O₃ at 300 K. We have studied the adsorbed oxygen layer by AES, XPS, HREELS, LEED, work function measurements and TPD. A plot of the O(519 eV)/Au(239 eV) AES ratio versus coverage is nearly linear, but a small change in slope occurs at Θ_O=0.9 ML. LEED observations show no ordered superlattice for the oxygen overlayer for any coverage studied. One-dimensional ordering of the adlayer occurs at low coverages, and disordering of the substrate occurs at higher coverages. Adsorption of 1.0 ML of oxygen on Au(111) increases the work function by +0.80 eV, indicating electron transfer from the Au substrate into an oxygen adlayer. The O(1s) peak in XPS has a binding energy of 530.1 eV, showing only a small (0.3 eV) shift to a higher binding energy with increasing oxygen coverage. No shift was detected for the Au 4f_7/2 peak due to adsorption. All oxygen is removed by thermal desorption of O₂ to leave a clean Au(111) surface after heating to 600 K. TPD spectra initially show an O₂ desorption peak at 520 K at low Θ_O, and the peak shifts to higher temperatures for increasing oxygen coverages up to Θ_O=0.22 ML. Above this coverage, the peak shifts very slightly to higher temperatures, resulting in a peak at 550 K at Θ_O=1.2 ML. Analysis of the TPD data indicates that the desorption of O₂ from Au(111) can be described by first-order kinetics with an activation energy for O₂ desorption of 30 kcal mol⁻¹ near saturation coverage. We estimate a value for the Au–O bond dissociation energy D(Au–O) to be 56 kcal mol⁻¹.     

金属纳米薄膜在石墨基底表面的动力学演化

采用分子动力学方法研究了金属Au和Pt纳米薄膜在石墨(烯)基底表面的动力学演化过程,探讨了金属薄膜和石墨(烯)基底间的相互作用对金属纳米薄膜在固态基底表面的去湿以及脱附的动力学演化的影响.研究结果表明,在高温下,相同层数的Au和Pt纳米薄膜在单层石墨基底表面上存在不同的去湿现象,主要表现为厚度较小的Pt纳米薄膜在去湿过程中有纳米空洞形成,而同样厚度的Au薄膜在去湿过程中没有形成空洞.Au和Pt两种金属薄膜在高温下都去湿形成纳米液滴,这些液滴最终都以一定的速度脱离基底.在模拟的薄膜厚度范围内(0.2—2.3 nm),Au和Pt纳米液滴脱离基底的速度随厚度增加表现出不同的变化规律.Pt纳米液滴的脱离速度随薄膜初始厚度的增加先增加后减少,而Au脱离速度随厚度的增加先减少,达到一个临界厚度后脱离速度突然迅速增加.利用薄膜与基底间相互作用的不同导致去湿过程中的黏滞耗散不同,定性分析了这种变化规律的原因.此外,进一步研究还发现金属液滴的脱离时间与薄膜厚度和模拟温度的依赖关系,发现脱离时间随薄膜厚度的增加而增加,随模拟温度的升高而减小.这些研究结果可以为金属镀膜、浮选、表面清洁、器件表面去湿等工业生产过程提供理论指导.     

Superconducting properties of ZnO-doped (Bi, Pb)-2223 thick film on Ni and NiO substrates prepared by spray deposition technique

The superconductivity of ZnO-doped (Bi, Pb)-2223 thick film on the Ni and NiO substrates, which was prepared by the spray deposition technique with cold forging, was investigated by characterizing the critical current density (J_c), the critical temperature (T_c), the orientation factor (f), and the microstructure of the film. The thickness of the thick film prepared by the spray deposition method was approximately 10 μm. The maximum J_c value of (Bi, Pb)-2223 film on NiO substrate was approximately 2200 A/cm² (I_c = 110 mA) when the film was sintered at 865 °C for 1 h with a cooling rate of 0.5 °C/min from 865 °C to 650 °C; in the case of Ni substrate, a maximum J_c value of approximately 2000 A/cm² (I_c = 100 mA) was obtained for the (Bi, Pb)-2223 thick film when a cooling rate was 3 °C/min. Such a difference in the J_c values of (Bi, Pb)-2223 thick film on Ni and NiO substrates is attributed to the presence of reaction layer at the (Bi, Pb)-2223 and substrate interface. In addition, the variations in the orientation factor of (Bi, Pb)-2223 thick film on NiO substrate related to those of J_c values. The J_c values of (Bi, Pb)-2223 film on NiO substrate with ZnO doping extremely depended on the amount of ZnO doping and the 0.5 wt% ZnO-doped (Bi, Pb)-2223 thick film deposited on NiO substrate, which was sintered at 835 °C for 1 h in air with a cooling rate of 1 °C/min, showed a J_c value of approximately 1200 A/cm² (I_c = 60 mA). Thus, it is considered that a small amount of ZnO doping was effective in lowering the sintering temperature of (Bi, Pb)-2223 thick film, resulting the improvement in the intragranular weak bonding or Josephson junction.     

Adsorption of NO on Au atoms and dimers supported on MgO(1 0 0): DFT studies

The adsorption of NO on single gold atoms and Au₂ dimers deposited on regular O²⁻ sites and neutral oxygen vacancies (F_s sites) of the MgO(1 0 0) surface have been studied by means of DFT calculations. For Au₁/MgO the adsorption of NO is stronger when the Au atom is supported on an anionic site than when it is on a F_s site, with adsorption binding energies of 1.1 and 0.5 eV, respectively. In the first case the spin density is mainly concentrated on the metal atom and protruding from the surface. In such a way, an active site against radicals such as NO is generated. On the F_s site, the presence of the vacancy delocalizes the spin into the substrate, weakening its coupling with NO. For Au₂/MgO, as this system has a closed-shell configuration, the NO molecules bonds weakly with Au₂. Regarding the N–O stretching frequencies, a very strong shift of 340–400 cm⁻¹ to lower frequencies is observed for Au₁/MgO in comparison with free NO.     

Formation of Mo and MoSx nanoparticles on Au(1 1 1) from Mo(CO)6 and S2 precursors: electronic and chemical properties

Mo(CO)₆ can be useful as a precursor for the preparation of Mo and MoS_x nanoparticles on a Au(1 1 1) substrate. On this surface the carbonyl adsorbs intact at 100 K and desorbs at temperatures lower than 300 K. Under these conditions, the dissociation of the Mo(CO)₆ molecule is negligible and a desorption channel clearly dominates. An efficient dissociation channel was found after dosing Mo(CO)₆ at high temperatures (>400 K). The decomposition of Mo(CO)₆ yields the small coverages of pure Mo that are necessary for the formation of Mo nanoclusters on the Au(1 1 1) substrate. At large coverages of Mo (>0.15 ML), the dissociation of Mo(CO)₆ produces also C and O adatoms. Mo nanoclusters bonded to Au(1 1 1) exhibit a surprising low reactivity towards CO. Mo/Au(1 1 1) surfaces with Mo coverages below 0.1 ML adsorb the CO molecule weakly (desorption temperature<400 K) and do not induce C–O bond cleavage. These systems, however, are able to induce the dissociation of thiophene at temperatures below 300 K and react with sulfur probably to form MoS_x nanoparticles. The formed MoS_x species are more reactive towards thiophene than extended MoS₂(0 0 0 2) surfaces, MoS_x films or MoS_x/Al₂O₃ catalysts. This could be a consequence of special adsorption sites and/or distinctive electronic properties that favor bonding interactions with sulfur-containing molecules.     

Evaluation of different models for the dissociation of silane on the Si(1 1 1)7 × 7 surface using the extended Brenner empirical potential

Several models have been proposed in the literature for the initial stages of the dissociative chemisorption of silane (SiH₄) on the Si(1 1 1)7 × 7 surface. In this paper, geometry optimisation calculations using the extended Brenner empirical potential have been performed to determine which of these models yields the minimum energy structure. The lowest energy configurations are found to correspond to the dissociation of silane into SiH₂ and two hydrogen atoms. The minimum energy structure involves the adsorption of the two hydrogen atoms onto the dangling bonds of an adjacent adatom and rest atom, and the insertion of the remaining SiH₂ fragment into one of the adatom backbonds. These results are discussed in the light of the existing experimental data.     

Structures of sulfur on TiO2(1 1 0) determined by scanning tunneling microscopy, X-ray photoelectron spectroscopy and low-energy electron diffraction

The temperature dependent adsorption of sulfur on TiO₂(1 1 0) has been studied with X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). Sulfur adsorbs dissociatively at room temperature and binds to fivefold coordinated Ti atoms. Upon heating to 120°C, 80% of the sulfur desorbs and the S 2p peak position changes from 164.3±0.1 to 162.5±0.1 eV. This peak shift corresponds to a change of the adsorption site to the position of the bridging oxygen atoms of TiO₂(1 1 0). Further heating causes little change in S coverage and XPS binding energies, up to a temperature of 430°C where most of the S desorbs and the S 2p peak shifts back to higher binding energy. Sulfur adsorption at 150°C, 200°C, and 300°C leads to a rich variety of structures and adsorption sites as observed with LEED and STM. At low coverages, sulfur occupies the position of the bridging oxygen atoms. At 200°C these S atoms arrange in a (3×1) superstructure. For adsorption between 300°C and 400°C a (3×3) and (4×1) LEED pattern is observed for intermediate and saturation coverage, respectively. Adsorption at elevated temperature reduces the substrate as indicated by a strong Ti³⁺ shoulder in the XPS Ti 2p_3/2 peak, with up to 15.6% of the total peak area for the (4×1) structure. STM of different coverages adsorbed at 400°C indicates structural features consisting of two single S atoms placed next to each other along the [0 0 1] direction at the position of the in-plane oxygen atoms. The (3×3) and the (4×1) structure are formed by different arrangements of these S pairs.     

Au聚体的吸附位点对杀草强分子表面增强拉曼光谱的影响研究

杀草强是一种白色结晶粉末状的化学除草剂,对环境有极强的破坏性,大量使用会造成农残污染,对生物体具有致癌作用。目前利用密度泛函理论探究杀草强分子的拉曼增强机理的相关研究相对较少,开展了Au聚体吸附位点对杀草强分子表面增强拉曼光谱的影响研究。采用Multiwfn软件结合VMD软件探究了杀草强分子表面静电势分布,得出N₁, N₄和N₆是杀草强分子与Au原子配位的最佳位置。基于密度泛函理论,运用GaussView5.0和Gaussian09软件,在B3LYP/6-31++G(d, p)基组水平上对杀草强分子进行几何构型优化,并对C, H, N原子使用6-31++G(d, p)基组,Au原子使用LANL2DZ赝式基组,计算了杀草强分子的常规拉曼散射光谱和杀草强分子与Au₄聚体以及Au₆聚体吸附的表面增强拉曼散射光谱,并进行特征峰指认和比较。结果发现在Au与N₁配位形成的复合物中,在1 064, 1 200, 1 392和1 592 cm^-1处杀...     

Effect of metal catalyst on the mechanism of hydrogen spillover in three-dimensional covalent-organic frameworks

Hydrogen spillover mechanism of metal-supported covalent-organic frameworks COF-105 is investigated by means of the density functional theory, and the effects of metal catalysts M_4(Pt_4, Pd_4, and Ni_4) on the whole spillover process are systematically analyzed. These three metal catalysts exhibit several similar phenomena:(i) they prefer to deposit on the tetra(_4-dihydroxyborylphenyl) silane(TBPS) cluster with surface-contacted configuration;(ii) only the H atoms at the bridge site can migrate to 2,3,6,7,10,11-hexahydroxy triphenylene(HHTP) and TBPS surfaces, and the migration process is an endothermic reaction and not stable;(iii) the introduction of M_4 catalyst can greatly reduce the diffusion energy barrier of H atoms, which makes it easier for the H atoms to diffuse on the substrate surface. Differently, all of the H2 molecules spontaneously dissociate into H atoms onto Pt_4 and Pd_4clusters. However, the adsorbed H2 molecules on Ni_4 cluster show two types of adsorption states: one activated state with stretched H–H bond length of 0.88 ?A via the Kubas interaction and five dissociated states with separated hydrogen atoms. Among all the M_4 catalysts, the orders of the binding energy of M_4 deposited on the substrate and average chemisorption energy per H2 molecule are Pt_4Ni_4Pd_4. On the contrary, the orders of the migration and diffusion barriers of H atoms are Pt_4Ni_4Pd_4, which indicates that Pt_4 is the most promising catalyst for the hydrogen spillover with the lowest migration and diffusion energy barriers. However, the migration of H atoms from Pt_4 toward the substrate is still endothermic. Thus direct migration of H atom from metal catalyst toward the substrate is thermodynamically unfavorable.     

Annealing effects on structure and laser-induced damage threshold of Ta2O5/SiO2 dielectric mirrors

The effects of annealing on structure and laser-induced damage threshold (LIDT) of Ta₂O₅/SiO₂ dielectric mirrors were investigated. Ta₂O₅/SiO₂ multilayer was prepared by ion beam sputtering (IBS), then annealed in air under the temperature from 100 to 400 °C. Microstructure of the samples was characterized by X-ray diffraction (XRD). Absorption of the multilayer was measured by surface thermal lensing (STL) technique. The laser-induced damage threshold was assessed using 1064 nm free pulsed laser at a pulse length of 220 μs.

It was found that the center wavelength shifted to long wavelength gradually as the annealing temperature increased, and kept its non-crystalline structure even after annealing. The absorbance of the reflectors decreased after annealing. A remarkable increase of the laser-induced damage threshold was found when the annealing temperature was above 250 °C.     

Investigation of the role of oxygen in NO reduction by C2H4 on the surface of stepped Pt(3 3 2)

The influence of pre-dosed oxygen on NO–C₂H₄ interactions on the surface of stepped Pt(3 3 2) has been investigated using Fourier transform infrared reflection–absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). The presence of oxygen significantly suppresses the adsorption of NO on the steps of Pt(3 3 2), leading to a very specific adsorption state for NO molecules when oxygen–NO co-adlayers are annealed to 350 K (assigned as atop NO on step edges). An oxygen-exchange reaction also takes place between these two kinds of adsorbed molecules, but there appears to be no other chemical reaction, which can result in the formation of higher-valence NOx.

C₂H₄ molecules which are post-dosed at 250 K to adlayers consisting of ¹⁸O and NO do not have strong interactions with either the NO or the ¹⁸O atoms. In particular, interactions which may result in the formation of new surface species that are intermediates for N₂ production appear to be absent. However, C₂H₄ is oxidized to C¹⁸O₂ by ¹⁸O atoms at higher annealing temperature. This reaction scavenges surface ¹⁸O atoms quickly, and the adsorption of NO molecules on step sites is therefore quickly restored. As a consequence, NO dissociation on steps proceeds very effectively, giving rise to N₂ desorption which closely resembles that following only NO exposure on a clean Pt(3 3 2), both in peak intensity and desorption temperature. It is concluded that the presence of ¹⁸O₂ in the selective catalytic reduction (SCR) of NO with C₂H₄ on the surface of Pt(3 3 2) does not play a role of activating reactants.     

Location of hydrogen in intercalation compounds of layered transition metal disulfides, A H NMR study

¹H NMR spectra of the hydrogen bronzes H_xNbS₂, H_xTaS₂ (2H modification), H_xTiS₂ and H_xVS₂ (1T-modif ication) have been measured in order to study location and mobility of hydrogen in layered chalcogenide intercalation compounds. In those materials which crystallize with 2H-modification the large majority of hydrogen atoms is immobile at ambient temperature and appears to be located in intralayer positions with symmetrical hydrogen metal four center bonding. In H_xTiS₂ and H_xVS₂ hydrogen occurs clustered at adjacent tetrahedral sites in the van der Waals gap and diffusion takes place between adjacent tetrahedral sites via S-H…S type hydrogen bonds.     

Spin-polarized ion scattering spectroscopy of CCl4 adsorption on Fe(0 0 1) surfaces

The interaction of CCl₄ molecules with Fe(0 0 1) surfaces was investigated by spin-polarized ion scattering spectroscopy (SP-ISS). It was observed that CCl₄ molecules adsorb dissociatively on the surface at ambient temperature (290 K), and consequently, iron and chlorine were major surface constituents. It was found that the chlorine adatoms are located atop of iron atoms of the second surface layer (hollow sites of the surface). It is indicated that the spin state of iron atoms at the surface is not affected by exposure to a CCl₄ atmosphere, while almost no spin is induced in the chlorine adatoms. Similar behavior is observed in the spin states of iron and chlorine on an oxygen preadsorbed-Fe(0 0 1) surface. The difference in the spin states of iron and chlorine clarifies the local property of the incidence ion neutralization and element selectivity of SP-ISS in this CCl₄/Fe system.     

Electrical conductivity, optical and metal–semiconductor contact properties of organic semiconductor based on MEH-PPV/fullerene blend

The electrical conductivity, optical and metal–semiconductor contact properties of the MEH-PPV:C₇₀ organic semiconductor have been investigated. The electrical conductivity results show that the MEH-PPV:C₇₀ film is an organic semiconductor. The optical band gap of the film was found to be 2.06 eV and the fundamental absorption edge in the film is formed by the direct allowed transitions. The refractive index dispersion curve of the film obeys the single oscillator model and E_d and E_o dispersion parameters were found to be 10.61 and 3.89 eV, respectively. The electrical characterization of the ITO/MEH-PPV:C₇₀ diode have been investigated by current–voltage characteristics. ITO/MEH-PPV:C₇₀ diode indicates a non-ideal current–voltage behavior with ideality factor n (2.50) and barrier height φ_B (0.90 eV) values.     

高次谐波体声波谐振器的机械品质因数

机械品质因数Q_M是高次谐波体声波谐振器(High-overtone Bulk Acoustic Resonator,HBAR)一个关键的特性参数。首次较系统地研究了Q_M随构成HBAR的3个组成部分(基底、压电薄膜和电极)的结构参数(厚度)和性能参数(特性阻抗与机械衰减因子)的变化规律。在谐振频率附近,将HBAR的分布参数等效电路简化为集总参数等效电路,首次用解析表达式给出它们的变化规律,分析了Q_M在给定频率最近谐振点的变化情况。结果表明,固定压电层厚度,Q_M随基底厚度的连续增加略呈振荡(非单调)上升,当基底厚度很大时趋于基底材料的机械品质因数;固定基底厚度,Q_M随压电层厚度的连续增加呈波浪式下降;选择低损耗的蓝宝石或YAG作为基底可以获得较大的Q_M值;电极的损耗必须考虑,它会降低Q_M值;与Au电极相比,具有较低损耗的A1电极选择适当厚度可以获得较高的Q_M值;此外,Q_M随频率的增加呈下降趋势。上述的结果为HBAR的优化设计提供了相应的理论依据。根据我们对K_eff2的研究,Q_M与K_eff2的变化规律往往是相悖的,因此在设计HBAR时要在这两者作适当的权衡。