Ordered thin films of alkyloxy-substituted benzaldehydes on a graphite surface studied by scanning tunneling microscopy

The molecular assembly of three different trialkyloxy-substituted benzaldehydes on graphite has been studied using scanning tunneling microscopy (STM). It is found that these benzaldehydes, which do not have intermolecular hydrogen bonding, could form a lamella arrangement in which the headgroups are aligned side by side. The effect due to the alkyl chain length on the lamella structure is also presented. Received: 21 January 2002 / Accepted: 22 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/6255-7908     

Atomic force microscopy study of thermal stability of silver selenide thin films grown on silicon

Silver selenide thin films were grown on silicon substrates by the solid-state reaction of sequentially deposited Se and Ag films of suitable thickness. Transmission electron microscopy and particle-induced X-ray emission studies of the as-deposited films showed the formation of single phase polycrystalline silver selenide from the reaction of Ag and Se films. Atomic force microscopy images of the as-deposited and films annealed at different temperatures in argon showed the film morphology to evolve into an agglomerated state with annealing temperature. The results indicate that when annealed above 473 K, silver selenide films on silicon become unstable and agglomerate through holes generated at grain boundaries.     

Observation of Si(1 0 0) surfaces annealed in hydrogen gas ambient by scanning tunneling microscopy

We investigated the cleaning process of Si(1 0 0) surfaces by annealing in H₂ gas ambient following chemical treatments by scanning tunneling microscopy. We observed the monohydride Si structure: Si(1 0 0):2 × 1-H on the surfaces annealed at 1000 °C in 2.5 × 10⁴ Pa H₂ gas ambient without conspicuous contaminants. On the sample annealed for 10 min or longer times, well-defined Si(1 0 0) structures with alternating S_A and S_B steps were observed, whereas the initial roughness still remained on the surfaces annealed for only 5 min.     

Effect of temperature on pulsed laser deposition of HgCdTe films

HgCdTe thin films have been deposited on Si(1 1 1) substrates at different substrate temperatures by pulsed laser deposition (PLD). An Nd:YAG pulsed laser with a wavelength of 1064 nm was used as laser source. The influences of the substrate temperature on the crystalline quality, surface morphology and composition of HgCdTe thin films were characterized by X-ray diffraction (XRD), selected area electron diffraction (SAED), atomic force microscopy (AFM) and energy dispersive X-ray spectroscopy (EDS). The results show that in our experimental conditions, the HgCdTe thin films deposited at 200 °C have the best quality. When the substrate temperature is over 250 °C, the HgCdTe film becomes thermodynamically unstable and the quality of the film is degraded.     

Surface defects in semiconductor lasers studied with cross-sectional scanning tunneling microscopy

Cross-sectional scanning tunneling microscopy is used to study defects on the surface of semiconductor laser devices. Step defects across the active region caused by the cleave process are identified. Curved blocking layers used in buried heterostructure lasers are shown to induce strain in the layers above them. Devices are also studied whilst powered to look at how the devices change during operation, with a numerical model that confirms the observed behavior. Whilst powered, low-doped blocking layers adjacent to the active region are found to change in real time, with dopant diffusion and the formation of surface states. A tunneling model which allows the inclusion of surface states and tip-induced band bending is applied to analyze the effects on the tunneling current, confirming that the doping concentration is reducing and defect surface states are being formed.     

Thickness-dependent surface morphology of La0.9Sr0.1MnO3 ultrathin films

La_0.9Sr_0.1MnO₃ (LSMO) ultrathin films with various thickness (in the range of 5-50 unit cells) are grown on (0 0 1) substrates of the single-crystal SrTi_0.99Nb_0.01O₃ by laser molecular-beam epitaxy (laser-MBE), and the surface morphology of these films were measured by scanning tunneling microscopy (STM). STM images of LSMO ultrathin film surface reveal that surface morphology becomes more flat with increasing film thickness. This study highlights the important effect of strain caused by the lattice mismatch between substrates and ultrathin films. And the results should be useful to the investigations on growing manganite perovskite materials.     

Preparation and structural properties of MgO films grown on GaAs substrate

Epitaxial MgO thin films have been grown on semiinsulating GaAs (0 0 1) substrates using electron beam (e-beam) evaporation. X-ray diffraction indicates c-axis oriented MgO with (0 0 2) reflection only and rocking curve widths ∼2.2-3°. Transmission electron microscopy (TEM) analyses confirm an epitaxial growth of the MgO films. We study the microstructure and the defects at the interface between the MgO film and the GaAs substrate. Auger electron Spectroscopy (AES) concentration depth profiles reveal no contamination of the MgO films by As and Ga at different temperatures of the deposition process.     

Morphology of sol-gel produced composite films for optical oxygen sensors

The results reported concern the characterization of thin layer SiO₂-based matrices with an oxygen sensing component Ru(II)-tris(4,7-diphenyl-1,10-phenanthroline) immobilized, when a sol-gel process along with dip- and spin-coating deposition methods are used.SEM, TEM and AFM study, assisted by X-ray energy dispersive microanalysis reveals the influence of the precursors used, sol treatment and the coating conditions on the films morphology and Ru distribution in the matrices. Uniform and smooth surface is produced from tetraethoxysilane (TEOS). The presence of ormosils (methyltriethoxysilane, MtEOS and octyltriethoxysilane, OtEOS) significantly increases the surface roughness exhibited as dots on the SEM image. Their surface concentration and size depend on the number of immersions and withdrawal speed at the dip coating. Spin deposition leads to rather different morphology of the films, based on TEOS/OtEOS. Following commonly used sol preparation procedure (with 1.25-2.5 g Ru-complex/dm³ sol) microcrystallization of the complex occurs with formation of randomly distributed crystals 100-400 nm in size. The ultrasound treatment of the sol by means of ultrasound disintegrator leads to homogeneous distribution of the complex without observable crystallization and significant improvement of the film sensing properties (increase of Stern-Volmer constant and better linearity of the Stern-Volmer plots both in gaseous and aqueous media).     

Investigation of heteroepitaxial diamond films by atomic force and scanning tunneling microscopy

We report on Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM) investigations on chemical vapour deposited heteroepitaxial diamond films. Besides the good macroscopic crystal morphology a statistical tilt up to ±5.2° of the oriented crystallites has been found relative to the silicon substrates. By optimizing the process conditions, however, the crystal tilt of the films can be reduced, resulting in an improved film perfection. On crystallite (001)-surfaces a substructure of growth facets or islands has been found and high resolution STM images have established a 2×1 surface reconstruction on these growth facets. AFM and SEM were applied to study the morphology of diamond nuclei initially grown on the silicon substrate. Strong island like (Volmer-Weber) growth has been found, with a nucleus height to diameter ratio of 1:1. While the islands are growing in size with respect to time of nucleation, its aspect ratio does not change, due to the high surface free energy of the diamond relative to silicon.     

Pulse train measurement of ferroelectric switching in thin films of vinylidene fluoride/trifluoroethylene copolymer

Ferroelectric switching processes in thin films of a vinylidene fluoride/trifluoroethylene copolymer are investigated by pulse train measurement that involves the application of a train of unipolar on–off electric field pulses and the analysis of charge response. It is found that polarization reversal is considerably delayed as pulse width becomes shorter than the conventional switching time, but is eventually completed even if pulse width is reduced to 0.2 μs. The charge response in each on–off cycle was resolved into dielectric and ferroelectric components. The former is nonlinear as well as relaxational and reflects changes in dipolar fluctuation and orientation during polarization reversal. The latter is responsible for polarization reversal that progresses forward and backward depending on the on–off period. Net polarization reversal occurs first gradually and then rapidly, corresponding to nucleation and growth, respectively. The switching current responsible for these processes can be reproduced by the sum of power and lognormal functions. The implications of these results are discussed in relation to the switching process that occurs as a result of the rotation of chain molecules. PACS 42.70.Jk; 77.80.Fm; 77.22.Ej     

Interface morphologies and magnetization characteristics of Co70Fe30 thin films deposited on conjugated polymer thin films

The surface and interface morphology and magnetization characteristics of Co₇₀Fe₃₀ thin films deposited on bare glass and p-Si/SiO₂ substrates and on conjugated polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films on such substrates have been studied by atomic force microscopy and magneto-optic Kerr effect. It was found that the average absolute magnitude of the coercive field of Co₇₀Fe₃₀ correlates with the roughness of the underlayer prior to Co₇₀Fe₃₀ deposition. P3HT deposited on p-Si/SiO₂ substrates possesses an increased surface roughness as compared to the p-Si/SiO₂ surface, but displays a decreased surface roughness as compared to the one of a bare glass substrate.     

The effect of substrate material on pulsed laser deposition of HgCdTe films

This paper describes some recent results of the HgCdTe thin film grown directly on different substrates (sapphire, GaAs and Si) by pulsed laser deposition (PLD). The influences of the substrate material on the properties of HgCdTe thin films were investigated by X-ray diffraction (XRD), selected area electron diffraction (SAED), atomic force microscopy (AFM) and energy dispersive X-ray spectroscopy (EDS). It was found that the quality of the HgCdTe film has a strong relation to the structure and properties of the substrate. The experiment results indicate that the HgCdTe epitaxial thin films grown directly on the sapphire substrates have a high quality, and the composition of the films is close to that of the target. While the quality of the HgCdTe films deposited on the Si and GaAs substrates are not very good.     

Morphology of pentacene films deposited on Cu(1 1 9) vicinal surface

We investigate the morphology of a pentacene (C₂₂H₁₄) film adsorbed on the Cu(1 1 9) vicinal surface by scanning tunnelling microscopy (STM). Thermal treatment of a thick film of molecules generates a long-range ordered structure. Series of molecular rows are alternated with areas where the molecules assume two equivalent orientations. STM data analysis suggests that the ordered structure can be described by a rippled morphology. The behaviour of the film at different annealing temperatures suggests a possible explanation of the film structure as due to an adsorbate-induced modification of the substrate.     

Preparation of superhydrophobic membranes by electrospinning of fluorinated silane functionalized poly(vinylidene fluoride)

Fluorinated silane functionalized poly(vinylidene fluoride) (PVDF) is synthesized by graft polymerization of 3-trimethoxylpropyl methylacrylate with PVDF followed by coupling of fluorinated silanes. Flat membrane prepared using this functionalized PVDF has a water contact angle of 140°. Superhydrophobic PVDF membrane with a contact angle larger than 150° is prepared by the electrospinning of the fluorinated silane functionalized PVDF. The morphologies of the membranes are characterized using scanning electron microscopy. The surface composition of the membranes is analyzed using FTIR and the contact angles and water drops on the surface of the membrane are measured using video microscopy.     

Adsorption of cobalt phthalocyanine on a Si(1 0 0) surface with Bi-line structures as evaluated by scanning tunneling microscopy

We report the reaction dynamics of cobalt phthalocyanine (CoPc) molecules with Bi-line structures (BLSs) on a Si(1 0 0) surface, investigated using scanning tunneling microscopy (STM). When CoPc molecules were deposited on a Si(1 0 0) surface with BLSs at room temperature, single-spot protrusions were observed in the STM image instead of four-spot images corresponding to CoPcs flat molecular structure. Moreover, domains with a c(4 × 4) periodicity appeared on the terraces of the Si(1 0 0) surface. This indicates that CoPc molecules may have decomposed on the surface by catalytic reaction with Bi atoms.     

Growth of Bi-Sb alloy thin films and their characterization by TEM, PIXE and RBS

Polycrystalline bulk materials of Bi₉₃Sb₇ Bi₈₈Sb₁₂, Bi₈₅Sb₁₅ and Bi₈₀Sb₂₀ were synthesized by melt-quench technique starting from the stoichiometric mixture of constituent elements. The phase purity and compositional uniformity of bulk materials were investigated using powder X-ray diffraction (XRD) and proton induced X-ray emission (PIXE) experiments. The single phase formation and the compositional analysis of thin films were confirmed by transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy (RBS). X-ray diffraction studies confirmed the phase homogeneity of the materials. Atomic concentration ratio of constituent elements (Bi and Sb) determined by PIXE and RBS revealed that near-stoichiometric composition is nearly the same in the bulk as well as in thin film forms.     

Structural and electrical properties of zinc oxides thin films prepared by thermal oxidation

We report on zinc oxide (ZnO) thin films (d = 55-120 nm) prepared by thermal oxidation, at 623 K, of metallic zinc films, using a flash-heating method. Zinc films were deposited in vacuum by quasi-closed volume technique onto unheated glass substrates in two arrangements: horizontal and vertical positions relative to incident vapour. Depending on the preparation conditions, both quasi-amorphous and (0 0 2) textured polycrystalline ZnO films were obtained. The surface morphologies were characterized by atomic force microscopy and scanning electron microscopy. By in situ electrical measurements during two heating-cooling cycles up to a temperature of 673 K, an irreversible decrease of electrical conductivity of as flash-oxidized Zn films was revealed. The influence of deposition arrangement and oxidation conditions on the structural, morphological and electrical properties of the ZnO films is discussed.     

Phase transitions and low-frequency dielectric dispersion in ferroelectric Langmuir-Blodgett films of the copolymer vinylidene fluoride with trifluoroethylene

The dielectric properties of multilayer ferroelectric Langmuir-Blodgett films based on the copolymer vinylidene fluoride with trifluoroethylene with 70/30 composition are investigated. Good agreement with theoretical models on the basis of the phenomenological Landau-Ginzburg approach is demonstrated for the first time for ultrathin films. Expressions describing the temperature variation of the permittivity in the temperature range of hysteresis and giving quantitative agreement with experimental data are obtained. It is shown that the Langmuir-Blodgett films are conducting. This conductivity does not depend on the frequency of the field. The results are explained by the fact that the motion of charge in the films is not bounded by domain walls. The jumps observed in the frequency dispersion at volume and low-temperature (surface) phase transitions are explained by a sharp increase in the relaxation times at the transition into the ferroelectric state.     

Haloform adsorption on crystalline copolymer films of vinylidene fluoride with trifluoroethylene

Reversible bromoform adsorption on crystalline polyvinylidene fluoride with 30% of trifluoroethylene, P(VDF-TrFE 70:30) was examined by photoemission and inverse photoemission spectroscopies. The adsorption of bromoform on this polymer surface is associative and reversible. Molecular bromoform adsorption appears to be an activated process at 120 K with enhanced adsorption following the initial adsorption of bromoform. Strong intermolecular interactions are also implicated in the presence of a weak shake off or screened photoemission final state, whose intensity scales with the unscreened photoemission final state.     

Microstructural and surface characterization of thin gold films on n-Ge (1 1 1)

Thin gold films were fabricated by vacuum resistive deposition on the n-Ge (1 1 1) wafers. The films were annealed between 300 and 600 °C. These resulting thin films were then characterised using scanning electron microscopy (field emission and back-scattering modes), Rutherford back scattering spectroscopy and time of flight secondary ion mass spectroscopy (TOF-SIMS). For temperatures below the eutectic temperature the distribution of both the gold and the germanium on the surface are uniform. Above the eutectic temperature, the formation of gold rich islands on the surface of the Germanium were observed. These changes in the microstructure were found to correspond to changes in the electrical characteristics of the diodes.