Surfactant-induced surface freezing at the alkane-water interface

Long-chain alkanes exhibit surface freezing at the alkane-air but not the alkane-water interface. Ellipsometry and surface tensiometry are used to show that a simple cationic surfactant, hexadecyltrimethylammonium bromide (CTAB), can induce surface freezing at the tetradecane-water interface even when present in mole fractions as low as 0.1. The surface-freezing temperature T(s) is a linear function of the interfacial excess of CTAB. The excess surface entropy below T(s), S(sigma)=-0.76+/-0.02 mJ K-1 m(-2), is consistent with a rotator phase. Ellipsometry provides strong evidence for a frozen monolayer in which the chains are oriented near the surface normal.     

Kinetics and energetics of oligomer desorption from surfaces

The dynamics of oligomer desorption from surfaces has been studied by measuring the desorption kinetics of a set of straight chain alkanes [ H(CH2)(n)H, with n = 5 to 60] from the surface of single crystalline graphite. Desorption is observed to be a first-order process and the preexponent of the desorption rate constant has a value nu = 10(19.6+/-0.5) sec(-1) and is independent of the oligomer chain length. More interestingly, we find that the barrier to desorption has a nonlinear dependence on chain length and takes the form DeltaE(double dagger)(des) = A+Bn(alpha), with the exponent alpha = 0.50+/-0.01.     

Self-consistent interpretation of the 2D structure of the liquid Au82Si18 surface: bending rigidity and the Debye-Waller effect

The structural and mechanical properties of 2D crystalline surface phases that form at the surface of liquid eutectic Au82Si18 are studied using synchrotron x-ray scattering over a large temperature range. In the vicinity of the eutectic temperature the surface consists of a 2D atomic bilayer crystalline phase that transforms into a 2D monolayer crystalline phase during heating. The latter phase eventually melts into a liquidlike surface on further heating. We demonstrate that the short wavelength capillary wave fluctuations are suppressed due to the bending rigidity of 2D crystalline phases. The corresponding reduction in the Debye-Waller factor allows for measured reflectivity to be explained in terms of an electron density profile that is consistent with the 2D surface crystals.     

Self-assembly of zwitterionic molecules on a Au(23 23 21) surface at low temperature

The adsorption of zwitterionic (E) 4-methoxy-4′-(3-n-sulfonatopropyl)stilbazolium (MSPS) molecules on the stepped vicinal Au(23 23 21) surface was studied by scanning tunneling microscopy (STM) at low temperature as a function of the molecular coverage. At the sub-half monolayer, a supramolecular network based on MSPS is preferentially adsorbed onto face-centered cubic stacking regions. At the monolayer, a complete periodic supramolecular film is obtained.     

Molecule length-induced reentrant self-organization of alkanes in monolayers adsorbed on Au(111)

We report the observation by scanning tunneling microscopy of the reentrant self-organization of n-alkanes (C(n)H(2n+2)) in monolayers adsorbed on Au(111) induced by a variation of the chain length. In the investigated range of lengths ( 10相似文献   

Liquid crystal deposition on poled, single crystalline lithium niobate

For the purpose of elucidating the mechanisms for molecular organization at poled ferroelectric surfaces, single crystalline lithium niobate (LN), ‘Z-cut’ along the (0 0 0 1) plane, has been prepared and characterized and subsequently exposed to liquid crystal molecules. As a model system we chose to study the anchoring of 4-n-octyl-4′-cyanobiphenyl (8CB) to LN. Liquid crystalline films are of interest because of their useful electronic and optical properties as well as chemical sensing attributes. Low-energy electron diffraction (LEED), atomic force microscopy (AFM), surface contact angle measurements (CA), and X-ray photoelectron spectroscopy (XPS) were used to characterize the surface of lithium niobate as well as the nature of 8CB films grown on the surface. Atomically flat LN surfaces were prepared as a support for monolayer thick, 8CB molecular domains. 8CB liquid crystal molecules were deposited by an ambient vaporization technique and the films were analyzed using XPS and CA. Understanding electrostatic anchoring mechanisms and thin film organization for this molecule on uniformly poled surfaces allows for a fuller appreciation of how molecular deposition of other polarizable molecules on periodically poled and patterned poled lithium niobate surfaces would occur.     

LCAO studies of hydrogen chemisorption on nickel: II. Cluster models for adsorption sites

The adsorption of single hydrogen atoms, investigated by means of cluster calculations, has been compared with the adsorption of hydrogen monolayers on periodic crystals (paper I). From the similarity of the adsorption energy curves we conclude that the (direct and indirect) interactions between adsorbed hydrogen atoms are relatively small up to monolayer coverage. For adsorption on different sites of ideal low index surfaces the stability decreases in the order Atop > Bridge > Centred. For Atop adsorption it increases with a decreasing number of nearest neighbours to the nickel atom in the NiH “surface molecule”, thus leading to especially strong adsorption sites at the edges of a stepped surface and to low stability in the notches. In general, we find that the Ni_nH “surface molecule” with n = 1, 2, 3 or 4 determines the equilibrium positions for H adsorption; the inclusion of one shell of neighbours to the nickel atoms is sufficient to explain the differences in adsorption energy. The Extended Hückel method is not well suited to study dissociative chemisorption of H₂, although some qualitative trends are correct.     

Crystalline ice growth on PT(111): observation of a hydrophobic water monolayer

The growth of crystalline water films on Pt(111) is investigated using rare gas physisorption. The water monolayer wets Pt(111) at all temperatures investigated (20-155 K). At low temperatures (T< or =120 K), additional water layers kinetically wet the monolayer surface. However, crystalline ice films grown at higher temperatures (T > 135 K) do not wet the water monolayer. These results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on Pt(111).     

Relative humidity influence on the water content and on the protonic conductivity of the phosphatoantimonic acids HnSbnP2O3n+5, xH2O (n = 1, 3, 5)

The phosphatoantimonic acids H_nSb_nP₂O_3n+5, xH₂O (n = 1, 3, 5) have been prepared from the corresponding potassium compounds by ion-exchange in acidic medium. For n = 1 and 3 they are layered materials. When n = 5 the covalent framework is three dimensional with large interconnected channels. The title acids are all hydrated and their water content, lattice parameters and protonic conductivity have been studied at 20°C as a function of the relative humidity. When n = 1 a great part of the water content is physisorbed and the electrical behavior is that of a particle hydrate. For n = 3, the compound is a true lattice hydrate and the protonic conductivity is closely related to the water content. This is also the case when n = 5; however the contribution of surface water to the proton diffusion is clearly evidenced.     

YBa2Cu3O7-δ外延薄膜的不可逆线与磁通玻璃线

在0—10T磁场范围内,系统地测量了YBa₂Cu₃O_7-δ外延薄膜处于磁场平行和垂直膜面两种情况下的R-T曲线和I-V曲线, 并对该样品的不可逆线和磁通玻璃线作了直接的比较.结果表明, 由不同约化电阻率判据给出的不可逆线和磁通玻璃线遵守相同的H∝(T(0)-T(H))ⁿ 关系.对于不可逆线, n=3/2,对于磁通玻璃线,n=4/3.不可逆线的位置不仅与判据有关而且与测量电流密度也有关,磁通玻璃线位于不可逆线的下方.探讨 关键词：     

Films made of cellulose nanofibrils: surface modification by adsorption of a cationic surfactant and characterization by computer-assisted electron microscopy

Films made of nanofibrils were modified by adsorption of a cationic surfactant directly on the film surfaces. The nanofibrils were prepared by 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-mediated oxidation and mechanical fibrillation, and were relatively homogeneous in size. The average nanofibril diameter and surface porosity was quantified based on computer-assisted field-emission scanning electron microscopy (FE-SEM). The cationic surfactant used in the adsorption was n-hexadecyl trimethylammonium bromide (cetyltrimethylammonium bromide, CTAB). The adsorption of CTAB was confirmed by Fourier transform infrared (FTIR) spectroscopy and high-resolution transmission electron microscopy (HRTEM) analyses. It was shown that the adsorbed layer of CTAB increased the hydrophobicity, without affecting the tensile index significantly. This capability, combined with the antiseptic properties of CTAB, may be a major advantage for several applications.     

A comparative study of non-linearity parameter for binary liquid mixtures

The present investigation comprises of theoretical evaluation of acoustic non-linearity parameter,B/A for equimolar binary mixtures, viz. chlorobenzene or 1-chloronaphthalene with a series of normal alkanes (n-C_n,n = 6, 8, 10, 12, 14, 16), and with a series of highly branched alkanes (br-C_n,n = 6, 8, 12, 16), viz. 2,2-dimethylbutane (br-C₆), 2,2,4-trimethylpentane (br-C₈), 2,2,4,6,6-pentamethylheptane (br-C₁₂) and 2,2,4,4,6,8,8-heptamethylnonane (br-C₁₆). Tong and Dong method, thermoacoustical method, Hartmann relation and Ballou relation have been employed to evaluateB/A. A comparative study ofB/A values obtained from the aforementioned methods has been made. The results are discussed on the basis of structural orientations of normal and branched alkanes.     

氟化铒薄膜晶体结构与红外光学性能的关系

使用热蒸发技术在锗(111)衬底上制备了氟化铒(ErF₃)薄膜. XRD衍射结果表明，随着衬底温度的增加，氟化铒薄膜发生了从非晶状态到结晶状态的转变，薄膜的表面形貌和红外光学性能也发生了显著的变化，部分结晶的氟化铒薄膜的远红外透射谱和完全非晶的薄膜基本一致，但是与结晶薄膜则没有相似之处. 晶格常数计算表明薄膜中存在压应力. 使用洛伦兹谐振子模型对薄膜的透射率曲线进行拟合计算，得到ErF₃薄膜的折射率和消光系数. 在10μm处非晶薄膜的折射率和消光系数最小值分别为1.38和0.01，结晶薄膜的折射率和消光系数最小值分别为1.32和0.006. 关键词： 氟化铒 红外光学性质 光学常数 洛伦兹谐振子模型     

Structure of monolayer silica on Mo(1 1 2) investigated by rainbow scattering under axial surface channeling

The structure of a monolayer crystalline silica film grown on a Mo(1 1 2) substrate is investigated via grazing scattering of fast atoms. For scattering along low indexed directions in the surface plane (“axial surface channeling”) the corrugation of the interaction potential leads to an azimuthal out-of plane scattering with an intensity enhancement for the maximum deflection angle, the so called “rainbow”. From the comparison of the experimental angular distributions for scattered projectiles with classical trajectory simulations we obtain information on the arrangement of atoms in the topmost surface layer. Our work provides evidence for the structural model of a two-dimensional network for the silica film.     

Orientation of a nematic liquid crystal by suitable boundary surfaces

The adsorption of a monolayer of hexadecyltrimethylammonium bromide (CTAB) on glass-slides allows the preparation of a monocrystal of p-n-methoxybenzilidene-p-butylaniline (MBBA) oriented either prependicularly to surfaces (homeotropic anchorage) or in a direction parallel to surfaces (planar anchorage). These orientations are dictated by the surface density and the structure of the adsorbed CTAB monolayers and confirm a model of orientation of a liquid crystal by physico-chemical coupling.     

Electrical performance and Raman scattering characterization of GaAs planar photoconductors

Raman scattering has been used to characterize the effects of processing techniques on the photosensitive area of GaAs planar photoconductors. The longitudinal optic (LO) phonon observed in the first-order Raman spectra was studied to probe the consequences of both chemical and plasma etching on the crystalline quality of an n-doped GaAs photosentitive area. This method has also been used to examine the stress at the interface of a Si₃N₄ film and an n-doped GaAs surface. For this purpose, three specially designed GaAs planar photoconductors (SN 353, AN 344, SN 344) have been fabricated. Electrical measurements on the photoconductors support the results of Raman study. First, as expected, increasing the number of processes leads to slightly poorer electrical performance. Secondly, a significant strain-induced shift in the GaAs LO phonon frequency has been observed under the Si₃N₄ film for the AN 344 photoconductor. However, the electrical performances of this device clearly indicate no dramatic change in the value of the n-doped surface potential.     

Rings, towers, cages of ZnO

Theoretical electronic structure studies on (ZnO)_n (n= 2–18, 21) have been carried out to show that the transition from an elementary ZnO molecule to the bulk wurtzite ZnO proceeds via hollow rings, towers, and cages. Our first principles electronic structure calculations carried out within a gradient corrected density functional framework show that small Zn_nO_n (n=2–7) clusters form single, highly stable rings. Zn₃O₃ and the symmetric cage Zn₁₂O₁₂ are shown to be particularly stable clusters. Among larger clusters, the most stable are oblong cages, Zn₁₅O₁₅, Zn₁₈O₁₈, and Zn₂₁O₂₁, which are reminiscent of nanotubes.     

Anomalous dispersion in nanocomposite films at the surface plasmon resonance

Nanocomposite thin films formed by Cu nanocrystals (NCs) embedded in an amorphous aluminium oxide (Al₂O₃) host have been prepared by alternate pulsed laser deposition. Spectroscopic ellipsometry is used to determine the effective refractive index (n=n+ik). The extinction coefficient is non-negligible and shows a broad absorption band related to the surface plasmon resonance. In the neighbourhood of this wavelength, the real part of the refractive index undergoes an anomalous dispersion, leading to a significant increase of the n value of the composite compared to that of the host. When the Cu content is low enough, about 2 at. %, the use of an effective medium approach combined with a regression method allows us to determine the metal content and film thickness from the ellipsometric measurements. For larger concentrations this approach is no longer valid. Received: 31 July 2001 / Revised version: 21 September 2001 / Published online: 15 October 2001     

Two-dimensional quasicrystals of decagonal order in one-component monolayer films

By using Monte Carlo simulations we demonstrate that atomic monolayers formed on a crystalline surface may exhibit quasicrystalline decagonal order. It is shown that the stability of two-dimensional quasicrystals (QCs) is determined by the misfit between the adsorbate and the surface lattice and by the corrugation of the surface potential. QCs may be stable at the ground state or develop from the compressed commensurate c(2 x 2) structure, via the first-order phase transition at finite temperatures. The decagonally ordered phase melts into a partially ordered liquidlike phase, which then disorders via a continuous Ising-like transition.     

Ab initio hartree-fock investigation of the interaction between hydrogen monolayers and beryllium slabs

A Hartree-Fock SCF ab initio investigation is presented in which the hydrogen chemisorption at high coverage on the beryllium (0001) surface is simulated as a symmetrical interaction of two H monolayers with both sides of the Be film or as a nonsymmetrical model with one H monolayer interacting with one surface of a Be film. Relatively strong chemisorption bonds are found at two (“open” and “eclipsed” positions) of the three high-symmetry chemisorption sites. The results of this investigation are compared with the results obtained with the approaches based on cluster models