Theoretical study of the photodesorption mechanism of nitric oxide on a Ag(111) surface: a nonequilibrium Green's function approach to hot-electron tunneling

The photoinduced desorption of NO molecules on a Ag surface was studied theoretically using a recently developed method based on the nonequilibrium Green's function approach combined with the density functional theory. Geometry optimizations for the stable NO dimer phase were carried out, and two structures of adsorbed dimers were identified. We calculated the reaction probabilities as a function of incident photon energy for each of the dimers and compared them with experimental action spectra. The two main features of the action spectra, (i) a long tail to the long wavelength (approximately 600 nm) and (ii) a rapid increase at approximately 350 nm, were well reproduced. By theoretical analysis, we found the importance of quantum interference for the interfacial charge transfer between the metal substrate and the adsorbate, as well as the contribution of secondary electrons. Our calculations suggest that the photoactive species is dimeric and that the resonant level is single for the photodesorption of NO.     

State-resolved investigation of the photodesorption dynamics of NO from (NO)2 on Ag nanoparticles of various sizes in comparison with Ag(111)

The translational and internal state energy distributions of NO desorbed by laser light (2.3, 3.5, and 4.7 eV) from adsorbed (NO)(2) on Ag nanoparticles (NPs) (mean diameters, D = 4, 8, and 11 nm) have been investigated by the (1 + 1) resonance enhanced multiphoton ionization technique. For comparison, the same experiments have also been carried out on Ag(111). Detected NO molecules are hyperthermally fast and both rotationally and vibrationally hot, with temperatures well above the sample temperature. The translational and rotational excitations are positively correlated, while the vibrational excitation is decoupled from the other two degrees of freedom. Most of the energy content of the desorbing NO is contained in its translation. The translational and internal energy distributions of NO molecules photodesorbed by 2.3, 3.5, and in part also 4.7 eV light are approximately constant as a function of Ag NPs sizes, and they are the same on Ag(111). This suggests that for these excitations a common mechanism is operative on the bulk single crystal and on NPs, independent of the size regime. Notably, despite the strongly enhanced cross section seen on NP at 3.5 eV excitation energy in p-polarization, i.e., in resonance with the plasmon excitation, the mechanism is also unchanged. At 4.7 eV and for small particles, however, an additional desorption channel is observed which results in desorbates with higher energies in all degrees of freedom. The results are well compatible with our earlier measurements of size-dependent translational energy distributions. We suggest that the broadly constant mechanism over most of the investigated range runs via a transient negative ion state, while at high excitation energy and for small particles the transient state is suggested to be a positive ion.     

Ceramic Films via Organometallic Complex as Single Source Precursor

Fe2(CO)6(μ-S2) was used as a single source precursor in attempt to produce FeS film via MOCVD. Pyrolysis of Fe2(CO)6(μ-S2) at temperature below 500℃ produced Fe1-xS or Fe7S8 powder as indicated by its powder X-ray spectra. At 750 ℃, polycrystalline FeS powder was obtained. In film deposition, polycrystalline Fe1-xS or Fe7Ss films were obtained on Si(100) and Ag/Si(100) substrates below 500 ℃. SEM micrographs showed the film on Si(100) substrate containing whisker like grains. However, pillar like grains were obtained on Ag/Si(100) substrate.Deposition rates are also different for different substrates as evaluated by the thickness of the films, which were obtained by SEM micrographs of the cross section of the films. At 750℃, similar polycrystalline Fe1-xS or Fe7S8 film was obtained.     

Initial formation of contact layers on Ni/SiC samples studied by XPS

This study deals with the quantitative assessment of the coverage and thickness of Ni silicide films formed during annealing of SiC substrates with sputtered thin films of Ni. The analytical approach involves the use of XPS and depth profiling by means of successive ion etchings and XPS analyses. For either 3 or 6 nm initial Ni film thickness, a 10 nm Ni₂Si product is formed. On top of this product, the C released is accumulated in a very thin (1–2 nm) film. In neither case, the Ni₂Si covers the whole surface, although the coverage is almost complete (～90%) in the latter case. For the greater initial Ni‐film thickness of 17 nm, the thickness of the Ni₂Si product corresponds well to the value of 25 nm expected from the Ni/Ni₂Si stoichiometric relationship. This thickness is significantly greater than a critical level and the film covers the whole surface. Carbon is similarly accumulated in a very thin layer on the top surface, although the major part of C (～70%) is found inside the main reaction product layer. Copyright © 2008 John Wiley & Sons, Ltd.     

热对银电极阳极膜光电流测定的影响

利用光电流 ,光热信息和热电流方法研究了热对Ag在 1mol/LNaOH溶液中表面阳极膜光电流的影响 .所用光源为 4 88nm和 5 32nm的激光 .结果表明Ag2 O层具有半导体光电流特性 ,在Ag ,Ag2 O和AgO的转变过程中 ,光照下电极表面的温升不同 .对照循环伏安曲线 ,及光电流和热电流曲线 ,可以证明在电位朝负方向扫描过程中于 0 .37V出现的阳极光电流峰和 0 .1V出现的阴极光电流峰主要是由光照下温度升高引起的热电流所造成的 .光电流和热电流随着光强度的增强而线性增强     

Epitaxy‐like orientation of nanoscale Ag islands grown on air‐oxidized Si(110)‐(5 × 1) surfaces

Growth of Ag islands under ultra‐high vacuum condition on air‐oxidized Si(110)‐(5 × 1) surfaces has been investigated by in situ reflection high energy electron diffraction and ex situ scanning electron microscopy and cross‐sectional transmission electron microscopy. A thin oxide is formed on Si via exposure of the clean Si(110)‐(5 × 1) surface to air. The oxide layer has a short range order. Deposition of Ag at different thicknesses and at different substrate temperatures reveal that the crystalline qualities of the Ag film are almost independent of the thickness of the Ag layer and depend only on the substrate temperature. Ag deposition at room temperature leads to the growth of randomly oriented Ag islands while preferred orientation evolves when Ag is deposited at higher temperatures. For deposition at 550 °C sharp spots in the reflection high energy electron diffraction pattern corresponding to an epitaxial orientation with the underlying Si substrate are observed. The presence of a short range order on the oxidized surface apparently influences the crystallographic orientation of the Ag islands. Copyright © 2011 John Wiley & Sons, Ltd.     

Internal field switching in CdSe quantum dot films on Si

If a thin film (tens of nm) of CdSe quantum dots (4 nm diameter) is deposited by chemical bath deposition onto various substrates, the films, although essentially intrinsic, behave as if they were n-type with respect to charge separation. However, films deposited under certain deposition conditions on Si (both n(+)- and p(+)-type) behave as if they were p-type. In this case, we show that it is possible to switch this p-type photoresponse by either light illumination intensity or injection of electrons from an external filament. Using both surface photovoltage spectroscopy and a novel adaptation of X-ray photoelectron spectroscopy, we show how this behavior results from a Cd(OH)(2) layer adsorbed at the Si surface at the beginning of the deposition. This response is explained by a competition between a high concentration of relatively shallow hole traps in the CdSe and a lower concentration of deeper electron traps in the Cd(OH)(2). The relative occupancies of these traps determine the fields in the film and their response to external parameters.     

Preparation and Characterization of Ag2 O/Organicsilicone Self-assemblied Composite Film

The composite film of nanometer AgO2/silane coupling reagent aminopropyltriethoxy-silane (CH3O)3Si(CH2)3NH2was prepared on single-crystal silicon by the self-assembly of silane on the hydroxylated substrate followed with the deposition of nanometer AgO2 on the silane SAMs from an aqueous Ag2O gel. The resultant composite film was characterized by means of X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The contact angles of distilled water on the silane SAMs and the composite film were measured to compare the surface states. The experiment shows that the nanometer Ag2O can be easily incorporated in the silane SAMs and lead to changed surface state of the composite film. Nanometer Ag2O crystallites in a size of about 20 nm distribute quite uniformly in the composite film. It was anticipated that the composite film might find application to the protection of single-crystal Si substrate in MEMS devices and also propose a novel single electron device structure based on nanoscale Ag2O colloidal particles.     

Theoretical Investigation on the Adsorption of Ag^＋ and Hydrated Ag^＋ Cations on Clean Si（111） Surface

In this paper, the adsorption of Ag^＋ and hydrated Ag^＋ cations on clean Si（111） surface were investigated by using cluster （Gaussian 03） and periodic （DMol^3） ab initio calculations. Si（111） surface was described with cluster models （Si14H17 and Si22H21） and a four-silicon layer slab with periodic boundary conditions. The effect of basis set superposition error （BSSE） was taken into account by applying the counterpoise correction. The calculated results indicated that the binding energies between hydrated Ag^＋ cations and clean Si（111） surface are large, suggesting a strong interaction between hydrated Ag^＋ cations and the semiconductor surface. With the increase of number, water molecules form hydrogen bond network with one another and only one water molecule binds directly to the Ag^＋ cation. The Ag^＋ cation in aqueous solution will safely attach to the clean Si（111） surface.     

Flexible Near‐Infrared Photovoltaic Devices Based on Plasmonic Hot‐Electron Injection into Silicon Nanowire Arrays

The development of flexible near‐infrared (NIR) photovoltaic (PV) devices containing silicon meets the strong demands for solar utilization, portability, and sustainable manufacture; however, improvements in the NIR light absorption and conversion efficiencies in ultrathin crystalline Si are required. We have developed an approach to improve the quantum efficiency of flexible PV devices in the NIR spectral region by integrating Si nanowire arrays with plasmonic Ag nanoplates. The Ag nanoplates can directly harvest and convert NIR light into plasmonic hot electrons for injection into Si, while the Si nanowire arrays offer light trapping. Taking the wavelength of 800 nm as an example, the external quantum efficiency has been improved by 59 % by the integration Ag nanoplates. This work provides an alternative strategy for the design and fabrication of flexible NIR PVs.     

Quasiclassical trajectory calculations of the OH+NO2 association reaction on a global potential energy surface

We report a full-dimensional potential energy surface (PES) for the OH+NO(2) reaction based on fitting more than 55,000 energies obtained with density functional theory-B3LYP6-311G(d,p) calculations. The PES is invariant with respect to permutation of like nuclei and describes all isomers of HOONO, HONO(2), and the fragments OH+NO(2) and HO(2)+NO. Detailed comparison of the structures, energies, and harmonic frequencies of various stationary points on the PES are made with previous and present high-level ab initio calculations. Two hydrogen-bond complexes are found on the PES and confirmed by new ab initio CASPT2 calculations. Quasiclassical trajectory calculations of the cross sections for ground rovibrational OH+NO(2) association reactions to form HOONO and HONO(2) are done using this PES. The cross section to form HOONO is larger than the one to form HONO(2) at low collision energies but the reverse is found at higher energies. The enhancement of the HOONO complex at low collision energies is shown to be due, in large part, to the transient formation of a H-bond complex, which decays preferentially to HOONO. The association cross sections are used to obtain rate constants for formation of HOONO and HONO(2) for the ground rovibrational states in the high-pressure limit.     

A 20-A-thick interwoven sheet consisting of nanotubes

Slow diffusion of AgNO3 with Me2Si(4-Py)2 yields an interwoven 2-nm-thick sheet consisting of the building block [Ag3(Me2Si(4-Py)2)4](NO3)3.H2O. The interweaving induces unique nanotunnels with a 16 x 18 A2 cross section with a 7 x 8 A2 square pore. The anion NO3- of the skeletal sheet can be reversibly exchanged with appropriately sized anions without the collapse of its structural integrity. The compound has a melting point (140 degrees C) and exists as the first 2D ionic liquid up to 193 degrees C.     

Molecular depth profiling with cluster ion beams

Peptide-doped trehalose thin films have been characterized by bombardment with energetic cluster ion beams of C60+ and Aux+ (x = 1, 2, 3). The aim of these studies is to acquire information about the molecular sputtering process of the peptide and trehalose by measurement of secondary ion mass spectra during erosion. This system is important since uniform thin films of approximately 300 nm thickness can be reproducibly prepared on a Si substrate, allowing detailed characterization of the resulting depth profile with different projectiles. The basic form of the molecular ion intensity as a function of ion dose is described by a simple analytical model. The model includes parameters such as the molecular sputtering yield, the damage cross section of the trehalose or the peptide, and the thickness of a surface layer altered by the projectile. The results show that favorable conditions for successful molecular depth profiling are achieved when the total sputtering yield is high and the altered layer thickness is low. Successful molecular depth profiles are achieved with all of the cluster projectiles, although the degree of chemical damage accumulation was slightly lower with C60. With C60 bombardment, the altered layer thickness of about 20 nm and the damage cross section of about 5 nm2 are physically consistent with predictions of molecular dynamics calculations available for similar chemical systems. In general, the model presented should provide guidance in optimizing experimental parameters for maximizing the information content of molecular depth profiling experiments with complex molecular thin film substrates.     

Preparation, structure, and optical properties of nanoporous gold thin films

Thin nanoporous gold (np-Au) films, ranging in thickness from approximately 40 to 1600 nm, have been prepared by selective chemical etching of Ag from Ag/Au alloy films supported on planar substrates. A combination of scanning electron microscopy (SEM) imaging, synchrotron grazing incidence small angle X-ray scattering, and N2 adsorption surface area measurements shows the films to exhibit a porous structure with intertwined gold fibrils exhibiting a spectrum of feature sizes and spacings ranging from several to hundreds of nanometers. Spectroscopic ellipsometry measurements (300-800 nm) reveal the onset of surface plasmon types of features with increase of film thicknesses into the approximately 200 nm film thickness range. Raman scattering measurements for films functionalized with a self-assembled monolayer formed from 4-fluorobenzenethiol show significant enhancements which vary sharply with film thickness and etching times. The maximum enhancement factors reach approximately 10(4) for 632.8 nm excitation, peak sharply in the approximately 200 nm thickness range for films prepared at optimum etching times, and show high spot to spot reproducibility with approximately 1 microm laser spot sizes, an indication that these films could be useful as durable, highly reproducible surface-enhanced Raman substrates.     

Change in the interfacial reaction of Hf-silicate film as a function of thickness and stoichiometry

Medium energy ion scattering and high-resolution transmission electron microscopy are used to investigate the depth of the interfacial reaction of Hf-silicate film. The interfacial reaction is critically affected by the film thickness and the mole fraction of HfO(2) in silicate film. The interfacial compressive strain generated at the surface of the Si substrate is dependent on the film thickness during the postannealing process in film with a thickness of approximately 4 nm. Finally, the phase separation phenomenon demonstrates critically different behaviors at different film thicknesses and stoichiometries because the diffusion of Si from interface to surface is dependent on these factors. Moreover, the oxidation by oxygen impurity in the inert ambient causes SiO(2) top formation.     

Film thicknesses determined by X-ray photoelectron spectroscopy

The knowledge of the reduced thickness of overlayers in the thickness range up to 5 nm is a valuable contribution to quantitative analysis in XPS. A new method for its determination is described which can be applied to most of the commercially available instruments. The method considers influences like countrate statistics, surface roughness, thickness distribution, elastic electron scattering, divergence of the photoelectrons detected by the spectrometer and energy dependence of the inelastic mean free path of the electrons in matter. The reliability of the results is verified by XPS-measurements performed on SiO₂-layers on Si.Besides, the XPS-results obtained from Si₃N₄-layers on Si measured under different take-off angles demonstrate the applicability of this method for the investigation of film growth. A further aspect is given by the recognition of elastic electron scattering by a simplified expression no longer asking for Monte Carlo calculations.     

Photocalorimetric measurement of the heat flow during optically and thermally induced solid state reaction between Ag and As33S67 thin films

Thin films of amorphous chalcogenide with composition of As₃₃S₆₇ (thickness d = 300 nm) and silver film (thickness d = 30 nm) on top of chalcogenide film were deposited by vacuum thermal evaporation. Prepared bilayer Ag/As₃₃S₆₇ was illuminated and photo-induced dissolution and diffusion (OIDD) process of silver in chalcogenide film studied by means of photocalorimetry. The heat flow connected with OIDD process during light exposure as a function of light energy, light intensity and temperature has been studied by means of photocalorimetry. The enthalpies of OIDD process were obtained and attributed to the bond energies of the formed bonds.     

溶解氧及波长对光助阳极沉积CeO2薄膜的影响

本文主要研究在含有0.05 mol·L^-1硝酸铈、0.1 mol·L^-1乙酸铵和70%（体积比）乙醇的镀液中通过阳极电沉积法在316 L不锈钢电极表面上制备CeO₂薄膜,并讨论了三种单色光波长以及镀液中溶解氧对阳极沉积CeO₂的影响. 采用计时安培曲线、椭圆偏振光谱、扫描电子显微镜、掠角X射线衍射和拉曼光谱方法研究了薄膜的电镀行为、表面形貌及其结构. 结果表明,本研究中的有效光照射波长为365 nm和254 nm,415 nm的波长不足以将电子从导带激发到价带. 随着入射光波长从254 nm增加到415 nm,薄膜的厚度与结晶度均逐渐减小,波长的变化对CeO₂薄膜表面形貌的影响很小. 少量的氧气对光助阳极沉积CeO₂薄膜有积极作用,但会生成较多的铈的氧化物颗粒吸附在电极表面. 在光电化学系统中,O₂常用作电子捕获剂. 随着溶解氧含量的增加,O₂将捕获对沉积有积极作用的光生电子,进而抑制阳极沉积的反应速率.     

Synthesis, characterization, and 3D-FDTD simulation of Ag@SiO2 nanoparticles for shell-isolated nanoparticle-enhanced Raman spectroscopy

Au-seed Ag-growth nanoparticles of controllable diameter (50-100 nm), and having an ultrathin SiO(2) shell of controllable thickness (2-3 nm), were prepared for shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Their morphological, optical, and material properties were characterized; and their potential for use as a versatile Raman signal amplifier was investigated experimentally using pyridine as a probe molecule and theoretically by the three-dimensional finite-difference time-domain (3D-FDTD) method. We show that a SiO(2) shell as thin as 2 nm can be synthesized pinhole-free on the Ag surface of a nanoparticle, which then becomes the core. The dielectric SiO(2) shell serves to isolate the Raman-signal enhancing core and prevent it from interfering with the system under study. The SiO(2) shell also hinders oxidation of the Ag surface and nanoparticle aggregation. It significantly improves the stability and reproducibility of surface-enhanced Raman scattering (SERS) signal intensity, which is essential for SERS applications. Our 3D-FDTD simulations show that Ag-core SHINERS nanoparticles yield at least 2 orders of magnitude greater enhancement than Au-core ones when excited with green light on a smooth Ag surface, and thus add to the versatility of our SHINERS method.     

Cross sections for electron trapping by DNA and its component subunits I: Condensed tetrahydrofuran deposited on Kr

We report cross sections for electron capture processes occurring in condensed tetrahydrofuran (THF) for incident electron energies in the range of 0-9 eV. The charge trapping cross section for 6-9 eV electrons is very small, and an upper limit of 4 x 10(-19) cm2 is estimated from our results. This latter is thus also an upper bound for the cross section for dissociative electron attachment process that is known to occur at these energies for condensed THF. At energies close to zero eV electron trapping proceeds via intermolecular stabilization. The cross section for this process is strongly dependent on the quantity of deposited THF. Since THF may model the furyl ring found in deoxyribose, these measurements indicate that this ring likely plays little role in either initiating or enhancing strand break damage via the attachment of the low energy secondary electrons produced when DNA is exposed to ionizing radiation.