Structural and microwave properties of (Ba,Sr)TiO3 films grown by pulsed laser deposition

The relationship between the structure and the microwave dielectric properties of epitaxial Ba_0.5Sr_0.5TiO₃ (BST) films has been investigated. Single-phase BST films (40-160 nm) have been deposited onto (100) MgO substrates by pulsed laser deposition. As-deposited films show a significant tetragonal distortion. The in-plane lattice parameters (a) are always larger than the surface normal lattice parameters (c). The tetragonal distortion depends on the thickness of the films and the post-deposition annealing conditions. Films annealed at 900 °C show less tetragonal distortion than the as-deposited film and the films annealed at higher temperatures. The distortion in the film is due to stress caused by the lattice mismatch and thermal expansion coefficient differences between the film and the substrate. The dielectric constant and its change with dc bias voltage of BST films on MgO at microwave frequencies increase with increasing annealing temperature from 900 °C to 1200 °C, which corresponds to an increase in the tetragonal distortion.     

Electrostatic Nature of Size Dependences of Adsorption Properties of Ytterbium Nanofilms Grown on the Surface of Silicon: the CO–Yb–Si(111) System

The adsorption of CO molecules onto ytterbium nanofilms with their thickness varying from 1 to 16 monolayers is studied. The dependences of the number of adsorbed CO molecules (adsorption isotherms) and the work function of ytterbium films on the dose of carbon monoxide are examined. It is demonstrated that both the number of adsorbed molecules and the work function depend (under equal conditions) on the nanofilm thickness; in other words, a size effect is revealed. It is found that this size effect is induced by the electrostatic interaction between the conduction electrons of ytterbium and the electrons localized on the nanofilm surface, which establish bonding between the surface and CO molecules. This interaction depends on the film thickness and limits the number of CO molecules that may be adsorbed onto the surface of a film with a given thickness.

     

Influence of initial oxygen on the formation of thiol layers

In this study, X-ray photoelectron spectroscopy (XPS) has been used to study thin organic films. For comparison, monolayers were formed on clean and air-exposed metal substrates. Obtained results show that thiols remove contamination oxygen from gold, silver, platinum and copper surfaces. The tightly packed thiolate layers can be formed. In addition, oxygen does not take part in the final bonding of molecules to the surfaces.     

Atomic force microscopy investigation of growth process of organic TCNQ aggregates on SiO2 and mica substrates

<正>Deposition patterns of tetracyanoquinodimethane(TCNQ) molecules on different surfaces are investigated by atomic force microscopy.A homemade physical vapour deposition system allows the better control of molecule deposition. Taking advantage of this system,we investigate TCNQ thin film growth on both SiO_2 and mica surfaces.It is found that dense island patterns form at a high deposition rate,and a unique seahorse-like pattern forms at a low deposition rate.Growth patterns on different substrates suggest that the fractal pattern formation is dominated by molecule-molecule interaction.Finally,a phenomenal "two-branch" model is proposed to simulate the growth process of the seahorse pattern.     

Langmuir-Blodgett film and nematic interface

The orientation induced by a Langmuir-Blodgett film on a Nematic Liquid Crystal (NLC) is theoretically analyzed. We show that the effective surface energy is due to different contributions connected with steric and van der Waals interactions between the nematic and the solid substrate. The analysis shows that the Langmuir-Blodgett film orientation depends on the surface density of the molecules. The initial homeotropic orientation may become unstable giving rise to a tilted film. The average orientation of the nematic molecules is also analyzed. We show that, in the event in which the steric interaction Nematic-Langmuir-Blodgett film is very large with respect to the dispersion interaction Nematic-Substrate, the nematic orientation coincides with the one of the film. On the contrary, when the two interactions are comparable, the orientation of the two media may differ. In particular, we analyze how the stable orientation depends on the surface molecular density of the film.     

Influence of surface temperature and surface modifications on the initial layer growth of para-hexaphenyl on mica (0 0 1)

Ultra-thin films of para-hexaphenyl (6P) were prepared on muscovite mica (0 0 1) utilizing organic molecular beam deposition (OMBD) under well defined ultra high vacuum (UHV) conditions. The 6P growth characteristics were studied as a function of substrate temperature and substrate surface conditions. For the initial state of layer growth, thermal desorption spectroscopy (TDS) was used to verify the existence of a wetting layer. In this monomolecular continuous wetting layer, the molecules lie flat on the surface and are rather strongly bonded. For thicker films, in-situ X-ray photoelectron spectroscopy (XPS) in combination with (TDS) was applied to reveal the kinetics of the layer growth. Ex-situ atomic-force microscopy (AFM) was used to determine the film morphology. In particular, the influence of surface modifications (carbon contamination, sputtering) on 6P layer growth was investigated. XPS and low energy electron diffraction (LEED) were used to characterize the mica surface before the film deposition. TDS and AFM revealed a considerable change in film growth, from a needle-like island growth of flat laying molecules on top of the wetting layer (for the air cleaved mica) to terrace-like film growth of standing molecules, without a wetting layer (after surface modifications).     

Structural investigation of n-hexadecanoic acid multilayers on mica surface: Atomic force microscopy study

The structure of n-hexadecanoic acid (HA) multilayers formed by spreading an ethanol solution containing this molecule onto a freshly cleaved mica surface has been studied by atomic force microscopy (AFM). AFM images of multilayers obtained with different coating time showed that HA molecules first formed some sporadic domains on mica surface. With the proceeding of the coating process, these domains gradually enlarged and coalesced, until formed a continuous film finally. It was observed that HA molecules were always adsorbed on mica surface with tilted even-numbered layers structure. The height of the repeated tilted bilayer film was measured to be approximately 3.8 ± 0.2 nm, which implied a ∼60° tilt molecular conformation of the HA bilayers on mica surface. Phase image confirmed that the HA multilayers terminated with the hydrophilic carboxylic acid groups. The formation mechanism of the HA multilayers was discussed in detail. Thus, resulted hydrophilic surfaces are of special interest for further study in biological or man-made member systems.     

Controlling molecular orientation of OMBE grown 6P thin films on mica(0 0 1)

The growth of para-sexiphenyl thin films by organic molecular beam epitaxy (OMBE) on mica(0 0 1) was investigated. The morphology of the films was qualitatively and quantitatively analyzed by atomic-force microscopy. Synchrotron radiation was used in order to extract information about orientation of the individual molecules with respect to the substrate. Controlling the growth environment inside the growth chamber allowed to reproduce one-dimensional film morphologies with partially oriented crystallite chains usually obtained by hot wall epitaxy (HWE). Following a dedicated pregrowth procedure results in terraces of upright standing molecules so far not obtained. Phase imaging was used to clearly distinguish film and substrate.     

Substrate-interaction,long-range order,and epitaxy of large organic adsorbates

Large and symmetric organic molecules (>200 amu) can form highly-ordered adsorbate layers and thin films when they are deposited by vacuum sublimation on clean reactive surfaces. In such cases covalent bonding often occurs via the molecular -system leading to a parallel orientation of the adsorbate as shown for oligothiophenes and PTCDA on Ag(1 1 1). A proper choice of the substrate and/or a preadsorbate may also cause an upright orientation with bonding via a reactive group of the molecule (example: NDCA/Ni(1 11)). Most of the used molecules yield long-range ordered monolayers with large, almost defect-free domains. The stronger the bonding and the smaller the molecule the more likely is the formation of commensurate superstructures which indicate site-specific adsorption even for such large molecules as PTCDA or EC4T. Organic epitaxy is discussed and shown for a particular system, PTCDA on Ag(1 1 1), for which the structure of the monolayer is nearly identical to that of the-modification of PTCDA crystals, whereas on other substrates (e.g. Si(1 1 1), Ge(1 0 0)) a disordered interface and hence no true epitaxy is found.     

Needles and clusters of zinc porphyrin molecules on mica

Molecular monolayers of zinc porphyrins deposited from a liquid solution on mica substrates have been characterised using atomic force microscopy (AFM). At room temperature, needle-shaped single layer porphyrin islands are formed due to an anisotropic growth rate. The average island size depends on the concentration of the porphyrin solution, but the length to width aspect ratio is rather independent of the island size. Annealing the porphyrin monolayers to 450 K leads to the reduction of the size of the monolayer islands in two different ways depending on the heating rate. With slow heating rate, the island size was found to shrink by losing molecules from the edge of the island. With high heating rate, nanoscale molecular clusters were found to form.     

Strain analysis of epitaxial ultrathin films on Pt(111)

X-ray photoelectron diffraction (XPD) measurements have demonstrated that Ni ultrathin films on Pt(111) exhibit a single domain fcc stacking pseudomorphic to the substrate, with a consequent trigonal distortion of the Ni unit cell in order to accommodate the in-plane expansion of the Ni lattice parameter of about 11% with respect to its bulk value. We show that the amount of interlayer contraction and the strain energy resulting form the trigonal distortion are very well predicted by a strain analysis in the framework of simple linear elasticity theory. Strain analysis also allows to discuss in some detail the controversial case of Co growth on the same substrate. Finally, we discuss the dependence of the strain energy of the overlayer on the substrate crystal orientation and its effects on chemisorption experiments performed on heteroepitaxial monolayers (HML).     

Heteroepitaxial growth of gallium nitride on muscovite mica plates by pulsed laser deposition

We have grown GaN films on mica substrates using pulsed laser deposition for the first time and investigated their structural properties using electron beam and X-ray diffraction. We found that GaN (000-1) grows on mica (001) with an in-plane alignment of [11-20] GaN//[010] mica. Despite the large lattice mismatch between GaN and mica, 6 and 43% along the [100] mica and [010] mica directions, respectively, cubic GaN phase or 30° rotated domains are scarcely observed in the film. This phenomenon can be attributed to the enhanced surface migration of film precursors due to the large atomically flat terraces and the weak Van der Waals bonding on the mica surface.     

Influence of substrate temperature,deposition rate and surface conditions on the epitaxial growth of copper on mica

In this experimental investigation copper was deposited on mica substrates under UHV conditions. Both air-cleaved and vacuum-cleaved mica substrates were used. Evaporation rate and substrate temperature have been varied systematically over a wide range. The structure of the films was studied with X-ray methods (Laue-transmission photographs and rocking curves). It was observed that the polar orientation of the crystallites depends mainly on evaporation rate and substrate temperature. The azimuthal orientation, however, is strongly dependent upon the surface conditions of the substrate, i.e. a surface gas layer improves the azimuthal orientation significantly. On vacuum-cleaved substrates an improvement of the azimuthal orientation can only be achieved by high evaporation rates (> 2500 Å/s). These effects are explained by the dependence of the rotation jump frequency on the size of nuclei and clusters.     

生长温度对立方MgZnO薄膜生长取向和紫外光吸收特性的影响

利用脉冲激光沉积技术在非晶石英衬底上制备立方结构MgZnO薄膜,并研究MgZnO薄膜结晶特性、光学带隙随生长温度的变化情况。当生长温度从150℃升高到700℃时,MgZnO薄膜的生长取向由（200）向（111）转变。在600℃以下,MgZnO薄膜光学带隙的变化规律与晶格中Mg和Zn原子比例的变化趋势是一致的;而当温度升至700℃时,虽然MgZnO晶格中Mg和Zn原子比例降低,但由于平均晶粒尺寸变大,薄膜的光学带隙反而上升。在300℃和700℃晶格匹配的情况下,获得了单一（200）和（111）取向的立方MgZnO薄膜。     

Epitaxial growth of al thin films on mica and investigation of film structure

The temperature dependence of the structure of the aluminium thin film was studied. Reflection and transmission electron diffraction (RED and TED, respectively) patterns show that the changes of the polar orientation (texture) and the azimuthal orientation (due to epitaxy) arise in the film as the temperature of substrate surface is increased. The simultaneous presence of spot patterns at RED and arc or ring patterns at TED in a certain temperature range is explained by the preferred orientation of the film grains in the azimuthal direction and the nearly perfect orientation in the polar direction. The particular changes of the azimuthal orientation became at the temperature of the largest desorption of water molecules from mica so that we deduce that the presence of water molecules is necessary for the particular degree of the azimuthal orientation at lower temperature. At a higher temperature (500°C) of the substrate surface the well-oriented films were prepared with the same diffraction patterns as single crystal with orientation (111)_Al// //(001)_M and (110)_Al//(010)_M (double positioning is present in the films).The author expresses his gratitude to Dr. L.Eckertová and Dr. Z.Hájek for valuable discussions and to Dr. M.Rozsíval and Dr. P.Kratochvíl for their aid at the structure analysis.     

Structural phase transitions in langmuir liquid crystal films

The isotherms of water molecule absorption on hyperfine Langmuir films made based on liquid crystals are investigated. A sharp increase in the adsorption capacity of the films at temperatures of 75°C (for a film with a thickness of ten monolayers) and 102°C (for a film with a thickness of five monolayers) has been revealed. This behavior is explained by structural phase transitions that take place at these temperatures.     

Photoemission, NEXAFS and STM studies of pentacene thin films on Au(1 0 0)

We have investigated the initial stages of the growth of pentacene thin films on the Au(1 0 0) substrate using synchrotron radiation photoelectron spectroscopy (PES), near edge X-ray absorption fine structure (NEXAFS) and scanning tunnelling microscopy (STM). Results indicate a well-ordered structure with the pentacene molecules adopting a predominantly flat orientation with respect to the substrate for coverages of less than three monolayers. NEXAFS and photoemission data indicates the presence of a second molecular orientation for thicker films, with the introduction of a slight tilting away from planar bonding geometry at higher pentacene coverages. STM images of coverages less than three monolayers indicate a well-ordered pentacene structure allowing for the calculation of pentacene unit cell parameters. The pentacene molecular rows adopt a side-by-side bonding arrangement on the surface. For pentacene deposited at room temperature, step edges were observed to act as nucleation centres for film growth. Annealing of the substrate to 373 K was found to remove excess molecules and improve film quality, but did not otherwise change the bonding geometry of the pentacene with respect to the surface.     

IR and SFM study of PTCDA thin films on different substrates

FT-IR spectroscopy and SFM were used to investigate the growth of thin films of the organic semiconductor 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) deposited by vacuum sublimation onto various substrates, i.e. Ag(111) layers on mica, KBr(100), mica, oxidized Si, and TiO₂ nanoparticles on Si. Layer thicknesses of PTCDA varied from 10 to 1500 nm.The anhydride vibrations of PTCDA differ for the used substrates, which can be connected to the orientation of the molecules relative to the substrate surface and the film morphology as detected in the SFM pictures.     

Crystal growth rates of diblock copolymers in thin films: Influence of film thickness

Using optical and atomic force microscopy we determined the growth rate of polymer crystals in thin films of low molecular weight poly(styrene-ethyleneoxid) block copolymers. We focused in particular on films either thinner or slightly thicker than the thickness of a crystalline lamella (L). At a given temperature, three distinctly different growth rates were observed for i) crystals grown from adsorbed monolayers thinner than L, ii) primary compact lamellar crystals in films thicker than L and iii) secondary crystals formed from molecules remaining on top of ii). The growth rate of primary crystals did not indicate a dependence on film thickness. Crystals from monolayers grew by a factor of 30 more slowly than primary crystals. Secondary crystals grew faster than crystals from monolayers but slower than primary crystals and the degree of deviation from the growth rate of primary crystals was found to depend on the value of L, which varied with temperature. The mechanisms responsible for the differences in growth rate are discussed in terms of the amount of available polymers and the controlling influence of diffusion towards the crystal front.Received: 9 September 2003PACS: 61.41. + e Polymers, elastomers, and plastics - 68.47.Mn Polymer surfaces 68.55.-a Thin film structure and morphology - 81.10.Aj Theory and models of crystal growth; physics of crystal growth, crystal morphology and orientation     

DNA adsorption and desorption on mica surface studied by atomic force microscopy

The adsorption of DNA molecules on mica surface and the following desorption of DNA molecules at ethanol-mica interface were studied using atomic force microscopy. By changing DNA concentration, different morphologies on mica surface have been observed. A very uniform and orderly monolayer of DNA molecules was constructed on the mica surface with a DNA concentration of 30 ng/μL. When the samples were immersed into ethanol for about 15 min, various desorption degree of DNA from mica (0-99%) was achieved. It was found that with the increase of DNA concentration, the desorption degree of DNA from the mica at ethanol-mica interface decreased. And when the uniform and orderly DNA monolayers were formed on the mica surface, almost no DNA molecule desorbed from the mica surface in this process. The results indicated that the uniform and orderly DNA monolayer is one of the most stable DNA structures formed on the mica surface. In addition, we have studied the structure change of DNA molecules after desorbed from the mica surface with atomic force microscopy, and found that the desorption might be ascribed to the ethanol-induced DNA condensation.