The ordered thin-film growth of organic semiconductor on Ag(110)

Growth of ordered perylene thin films on the Ag(110) surface has been investigated with scanning tunneling microscope. By saturating the surface with a monolayer of perylene molecules, two kinds of ordered structures are simultaneously formed with flat-lying perylene molecules on the Ag(110) surface, in which one is commensurate relative to the Ag substrate with a periodicity of while the other is commensurate with a periodicity of (-2724). There is one molecule within the former unit cell with a surface coverage of 0.1 molecule per Ag atom, while there are two molecules within the latter unit cell, which gives a slightly lower surface coverage of 0.091 molecule per Ag atom. Ab initio calculations have been carried out to identify the adsorption geometry and bonding sites.     

Temperature Controlled Lateral Pattern Formation in Confined Polymer Thin Films

The thermal induced topography change in a model system consisting of a polymer film on a Si substrate capped by a thin metal layer has been studied by using AFM. Regular lateral patterns over large areas were observed on the surface when the system was heated to a sufficiently high temperature. 2D-FFT analysis to the AFM images indicates that the patterns are isotropic and have well defined periodicities. The periodicities of the characteristic patterns are found to depend strongly on the annealing temperature. The study of the ki-netics of the formation reveals that such a topography forms almost instantaneously once the critical tempera-ture is reached. It is suggested that this wave-like surface morphology is driven by the thermal expansion co-efficient mismatch of the different layers. This method for generating regular wave-like patterns could be used as a general method for patterning various organic materials into micro/nanostructures.     

Homochiral conglomerates and racemic crystals in two dimensions: tartaric acid on Cu(110)

Two-dimensional lattice structures formed by racemic tartaric acid on a single crystalline Cu(110) surface have been studied and compared with the enantiopure lattices. At low coverage, the doubly deprotonated bitartrate species is separated into two-dimensional conglomerates showing opposite enantiomorphism. At higher coverage, however, a singly deprotonated monotartrate species forms a heterochiral, racemic crystal lattice. While the enantioseparated bitartrate system undergoes decomposition at the same temperature as the enantiopure system, the racemic monotartrate lattice has a lower thermal stability than the enantiopure lattice of identical periodicity and surface density. At monolayer saturation coverage, the pure enantiomers form a denser lattice than the racemate. This is in contrast to the three-dimensional tartaric acid crystals, where the racemate crystallizes in a lattice of higher density, which is also more thermally stable than the enantiopure tartaric acid crystals.     

Achieving structural control with thin polystyrene-b-polydimethylsiloxane block copolymer films: The complex relationship of interface chemistry,annealing methodology and process conditions

The structure of thin microphase-separated polystyrene-block-polydimethylsiloxane (PS–PDMS) films has been studied using state-of-the-art top-down and cross-sectional electron microscopy. This is the first time that the profile of PS–PDMS films has been measured in situ and these measurements allowed us to image the shape of the PDMS domains within the film as well as examine the wetting behavior of the block copolymer film on a variety of substrates. It was found that for each polymer, substrate chemistry and annealing method combination examined, there was a small range of film thicknesses whereby the films exhibited the optimal characteristics of high levels of ordering without dewetting or multilayering. Specifically, the optimum thickness for films treated by thermal annealing was greater than that for the equivalent solvent annealed film; a change that was correlated with morphology variations related to solvent swelling of the solvent annealed films. The surface chemistry also induced changes in the optimum film thickness. Selective surfaces were shown to control whether a PDMS wetting layer was formed or not, leading to either thicker or thinner wetting optimum film thicknesses; while undulating morphologies were observed for less selective surfaces. Concomitant changes in the periodicity were then hypothesized to occur as a result of confinement effects and the selectivity of the surface.     

Molecular dynamic simulation of platinum heater and associated nano-scale liquid argon film evaporation and colloidal adsorption characteristics

A novel 'fluid-wall thermal equilibrium model' for the wall-fluid heat transfer boundary condition has been developed in this paper to capture the nano-scale physics of transient phase transition of a thin liquid argon film on a heated platinum surface and the eventual colloidal adsorption phenomenon as the evaporation is diminishing using molecular dynamics. The objective of this work is to provide microscopic characterizations of the dynamic thermal energy transport mechanisms during the liquid film evaporation and also the resulting non-evaporable colloidal adsorbed liquid layer at the end of the evaporation process. A nanochannel is constructed of platinum (Pt) wall atoms with argon as the working fluid. The proposed model is validated by heating liquid argon between two Pt walls and comparing the thermal conductivity and change in internal energy to thermodynamic properties of argon. Later on, phase change process is studied by simulating evaporation of a thin liquid argon film on a Pt wall using the proposed model. Gradual evaporation of the liquid film occurs although the film does not vaporize completely. An ultra-thin layer of liquid argon is noticed to have "adsorbed" on the platinum surface. An analysis similar to the theoretical study by Hamaker (1937) is performed for the non-evaporating film and the value of the Hamaker-type constant falls in the typical range. This analysis is done to quantify the non-evaporating film with an attempt to use molecular dynamics simulation results in continuum mechanics.     

原子力显微镜研究高分子超薄膜结晶机理及功能化调控

近十年来,随着功能高分子单晶(含单层或寡层片晶)工程及应用研究的不断深入,除了纳米尺度结晶形貌的表征以外,多功能原子力显微镜还被用于研究分子结构、结晶条件和后处理条件对功能高分子晶体性能(电、热、光、磁等)的影响,进一步还可采用扫描探针加工技术(机械刻蚀、电致刻蚀和热致刻蚀等)对其性能进行调控以构筑功能化聚集态结构和微图案.另一方面,超薄膜中单层或寡层片晶可为研究高分子结晶提供合适的模型体系,与原子力显微镜相结合,不但可以原位、实空间、高分辨地研究高分子的成核与生长过程(生长形态演变和生长动力学),还可以用于研究亚稳态折叠链片晶厚度和形态随热处理温度与时间的演化,从而加深对片晶内有序差异、片晶增厚与熔融行为和自诱导成核的认识.     

Fabrication of Gold Nano‐Structures with Azopolymer Templates

Fabrication of gold nano‐patterns has been demonstrated employing surface relief structures created on films of an azobenzene‐functionalized polymer as templates. The surface relief templates were photoinscribed on the azopolymer films in one‐step with two laser beams. Thin layers of gold were over‐coated on the polymer templates by thermal evaporation. Gold lines of a few hundred nanometer width were successfully fabricated by pyrolyzing the azobenzene polymer. Sub‐micron gold dots were also created. The resulting gold structures exhibited the same periodicity as the polymer templates.     

Thermal Marangoni Convection of a Fluid Film Coating a Deformable Membrane

The thermal Marangoni instability of a fluid film coating a deformable membrane has been investigated by taking into account the deformation of the fluid free surface. Numerical calculations for different thermal boundary conditions are presented. The prestressed membrane is supposed to be very thin and therefore its behavior is similar to that of an isothermal fluid free surface with a surface tension but with a different mechanical boundary condition; that is, the fluid should stick on its surface and thus the fluid velocity is zero. An important assumption is that the membrane has no temperature dependence and therefore that only one Marangoni number exists for the free surface of the fluid. Numerical results are presented for stationary and oscillatory thermocapillary instability in both the sinuous and the varicose modes. It is shown that membrane deformation has important implications on the Marangoni instability of the fluid layer for positive and negative Marangoni numbers. Copyright 2001 Academic Press.     

Band-state interpretation of lattice thermal conductivity and microhardness of ternary chalcopyrite semiconductors

A new approach utilising the concept of band states and periodicity has been used to explain the lattice thermal conductivity and microhardness of ternary chalcopyrite crystals. The experimental values agree quite well with the calculated values using our model. A single fitting parameter used in each case explains the uniqueness of the model.     

Scanning thermal microscopy techniques for polymeric thin films using temperature contrast mode to measure thermal diffusivity and a novel approach in conductivity contrast mode to the mapping of thermally conductive particles

Advances in production are leading to increasing use of polymeric thin films in applications such as automotive bearings. Two approaches have been developed to study the thermophysical properties of these thin films: The first technique based on Flash theory uses a scanning thermal microscopy (SThM) tip in temperature contrast mode to measure thermal diffusivity over a nano-scale area. The SThM tip is in contact with the upper surface of the film to detect a heat pulse delivered by a microelectromechanical heater platform from the lower surface. The second technique is a conductivity contrast mode SThM based approach for measuring the size and distribution of thermally conducting particles in thin film polymeric coatings. Topographical and thermal conductivity data are combined to produce a “correlation analysis value” 3D particle map of the coating. Good practice and a case study are highlighted.     

Study by scanning tunneling microscopy of hydrogen adsorption and desorption on Si111 7 x 7 at room temperature and at high temperature

An overview is given on the use of scanning tunneling microscopy (STM) for investigation of the adsorption of hydrogen on Si(111)7 x 7 both at room temperature and at elevated temperature to finally obtain a hydrogen-saturated surface of Si(111). The initial stages are characterized by high reactivity of Si adatoms of the 7 x 7 structure. After adsorption of hydrogen on the more reactive sites in the beginning of the adsorption experiments a regular pattern, which is different for room and elevated temperature, is observed for the less reactive sites. In agreement with previous work, local 1 x 1 periodicity of the rest atom layer and the presence of di- and trihydride clusters is observed for hydrogen-saturated surface. STM has also been used to characterize surfaces from which the hydrogen atoms have been removed by thermal desorption. Finally, tip-induced desorption by large positive sample-bias voltages and by increasing the tunneling current will be described.     

Surface-induced morphologies in thin films of a rod-coil diblock copolymer

A block copolymer containing a rodlike block is studied for its adsorption and formation of nanostructured thin films on the substrate surface. The block copolymer is poly(styrene-b-3-triethoxysilylpropylisocyanate) (PS-b-PIC) of which the PIC chain consists of repeating amide units with triethoxysilyl side groups. As the copolymer chains are adsorbed onto silica surfaces, the PIC blocks pack laterally on the plane in a smectic manner, and the PS chains segregate along the ordered PIC chains, resulting in stripe patterns. The width of the stripes formed on the silica surface appeared to be much larger that on the carbon surface. This was accounted for by the bilayered smectic packing of the rod blocks that is induced by rod-surface attractive interaction. The periodicity of the stripe pattern on the carbon surface indicates that interdigitated packing is preferred by the copolymers on the hydrophobic surface in a manner similar to those in the bulk state of rod-coils. Excess rod-coils on the bilayered smectic layer resulted in a terraced morphology due to large difference in the periodicity between the bilayered smectic layer at the substrate surface and the interdigitated smectic layer in the bulk.     

Functionalization of cellulose dialysis membranes for chiral separation using beta-cyclodextrin immobilization

A membrane-based chiral separation system for the separation of racemic tryptophan solutions is developed by the covalently binding beta-cyclodextrin onto the surface of commercial cellulose membranes. The immobilization process is monitored by XPS. AFM demonstrates the evolutionary transition of membrane surface morphology before and after the CD immobilization. Due to their different complexation with immobilized CD, dialysis transport experiments show d-tryptophan preferential permeability through the immobilized CD membranes, and the enantioselectivity is 1.10. A model based on the existence of a thin chiral solution layer of amino acid at the interface between the feed solution and the membrane has been proposed. This chiral separation model has been verified using the chiral separation results of racemic amino acids and binding constants of amino acids with CD. The effect of membrane's pore size on enantioselectivity has also been investigated. The immobilized CD membrane, having MWCO 1000, exhibits the highest enantioselectivity to the racemic tryptophan solution.     

Robust surface nano-architecture by alkali-carboxylate ionic bonding

Ionic bonding in supramolecular surface networks is a promising strategy to self-assemble nanostructures from organic building blocks with atomic precision. However, sufficient thermal stability of such systems has not been achieved at metal surfaces, likely due to partial screening of the ionic interactions. We demonstrate excellent stability of a self-assembled ionic network on a metal surface at elevated temperatures. The structure is characterized directly by atomic resolution scanning tunneling microscopy (STM) experiments conducted at 165 °C showing intact domains. This robust nanometer-scale structure is achieved by the on-surface reaction of a simple and inexpensive compound, sodium chloride, with a model system for carboxylate interactions, terephthalic acid (TPA). Rather than distinct layers of TPA and NaCl, angle resolved X-ray photoelectron spectroscopy experiments indicate a replacement reaction on the Cu(100) surface to form Na-carboxylate ionic bonds. Chemical shifts in core level electron states confirm a direct interaction and a +1 charge state of the Na. High-temperature STM imaging shows virtually no fluctuation of Na-TPA island boundaries, revealing a level of thermal stability that has not been previously achieved in noncovalent organic-based nanostructures at surfaces. Comparable strength of intermolecular ionic bonds and intramolecular covalent bonds has been achieved in this surface system. The formation of these highly ordered structures and their excellent thermal stability is dependent on the interplay of adsorbate-substrate and ionic interactions and opens new possibilities for ionic self-assemblies at surfaces with specific chemical function. Robust ionic surface structures have potential uses in technologies requiring high thermal stability and precise ordering through self-assembly.     

Superhydrophilic surface modification of copper surfaces by Layer-by-Layer self-assembly and Liquid Phase Deposition of TiO(2) thin film

A new method has been developed for the superhydrophilic surface modification of copper using versatile solution-based fabrication techniques. The high surface area of TiO(2) nanoparticles was exploited to create a thin film with increased surface energy that transformed copper materials from relatively hydrophobic to superhydrophilic. Copper exposed to ambient conditions resulting in a thin layer of copper oxide has a water contact angle near 90°, but following TiO(2) modification, the contact angle dropped to 0°. The thin film responsible for this drastic improvement in wettability proved durable by retaining its excellent properties throughout an extended application of thermal stress. SEM and Raman Spectroscopic analysis confirmed the structural integrity of the film before and after a durability test.     

Study of solvents effect on distribution of configurational and conformational isomers of apo-tirucallol theoretically and experimentally

The adsorption of carbon monoxide on the rutile (1 1 0) and the anatase (1 0 1) surfaces has been investigated by a periodic approach using hybrid-exchange density functional theory; the quantum-mechanical study on the CO adsorption on the anatase surface has been carried out for the first time. The full optimization of the systems has been performed by considering different surface coverages and periodicities. The adsorption energetics have been investigated, on the basis of a recently proposed model, in terms of interaction, distortion and binding energies and the lateral effects have been analyzed using a simple model of nearest and next-nearest neighboring molecules. The experimental adsorbed CO stretching frequencies have been compared with the computed ones obtained by considering the highest investigated periodicity. The Lewis acidity of the two surfaces is comparable and the rutile (1 1 0) acid site seems to be slightly more electrophilic than that of the anatase (1 0 1) one.     

Lamellar diffusion during heat treatment in meltcrystallized low-density polyethylene

The annealing behavior of low-density melt-crystallized polyethylene is discussed in terms of an unprecedented lamellar diffusion mechanism. For this purpose a combined small-angle x-ray diffraction (SAXS) and differential scanning calorimetry (DSC) study has been carried out. The presence of an initial double-lamellar population is directly evidenced by electron microscopy of freeze-cut and stained sections. The intercalation of thinner lamellae within the dominant wide-lamella population gives rise to an x-ray periodicity of 220 Å (structure I). The independent stacking of thinner lamellae within the material contributes to a second periodicity at 110 Å (structure II). During annealing at successively higher temperatures T_A, the low-molecular-weight components from the thin lamellae progressively diffuse out of the double population, reinforcing the stacks of thinner lamellae. At the melting temperature of the thinner crystals a complete segregation of these lamellae takes place, and a new periodicity (structure III) corresponding to stacks of collapsed thick lamellae emerges. The periodicity of structure III increases further with T_A. The thin lamellae of structure II can be extracted by a solvent, removing the corresponding SAXS peak and DSC maxima. The partial removal of thin lamellae from structure I by means of solvent extraction concurrently yields a decrease of the SAXS period. The present results suggest that molecular segregation of a low-molecular-weight fraction, contributing to the population of thinner lamellae, occurs during annealing.     

Numerical Modeling of Thin Film Deposition in Expanding Thermal Plasma

Expanding thermal plasma (ETP) is a widely used technique for deposition of a thin layer of ceramic materials and metal oxide on a substrate for a wide range of applications including abrasion resistance, UV absorption, as well as conductive and optical coatings. The coating quality is found to be dependent on operating parameters as well as reactor designs. In this article, we have presented a CFD based model of the ETP process to simulate the deposition of silica-like coatings on a polycarbonate substrate. Along with the flow-thermal model of plasma jet expansion process, the study also reports the development of a simplified gas phase and surface reaction model to simulate the coating phenomena. The model has been used further to study the effect of various operating conditions on the coating thickness, viz. reactor pressure, reagent flow rate, distance of the substrate from the arc and substrate alignment.     

Surface structure of semicrystalline polyimide films

The surface structure of thin films based on poly[4,4′-bis(4″-N-phenoxy)diphenyl]amic acid of 1,3-bis(3′,4-dicarboxyphenoxy)benzene and the product of its thermal imidization—a semicrystalline polyimide—poly[4,4′-bis(4″-N-phenoxy)diphenyl]imide of 1,3-bis(3′,4-dicarboxyphenoxy)benzene—at various stges of thermal imidization and after melting and subsequent annealing has been studied by methods of transmission, scanning electron, and atomic force microscopies. The topological structure of the film surface has been described in terms of the discrete cluster model. Under heating to 200 and 280°C, a continuous network of the infinite cluster appears; subsequent annealing leads to disintegration of the network to discrete fragments that practically correspond to clusters in the starting poly(amic acid) film. The polyimide film heated to 280°C crystallizes in the form of needle crystals stable to the argon plasma. The surface morphology of polyimide films recrystallized from melt is of the spherulite character.     

A momentum space view of the surface chemical bond

Well-ordered and oriented monolayers of conjugated organic molecules can offer new perspectives on surface bonding. We will demonstrate the importance of the momentum distribution, or symmetry, of the adsorbate molecules' π orbitals in relation to the states available for hybridization at the metal surface. Here, the electronic band structure of the first monolayer of sexiphenyl on Cu(110) has been examined in detail with angle-resolved ultraviolet photoemission spectroscopy over a large momentum range and will be compared to measurements of a multilayer thin film and to density functional calculations. In the monolayer, the one-dimensional intramolecular band structure can still be recognized, allowing an accurate determination of orbital modification upon bonding and the relative energetic positions of the electronic levels. It is seen that the character of the molecular π orbitals is largely maintained despite strong mixing between Cu and molecular states and that the lowest unoccupied molecular orbital (LUMO) is filled by hybridization with Cu s,p states rather than through a charge transfer process. It is also shown that the momentum distribution of the substrate states involved and the periodicity of the molecular overlayer play a large role in the final E(k) distribution of the hybrid states. The distinct momentum distribution of the LUMO, interacting with the Cu substrate s,p valence bands around the gap in the surface projection of the bulk band structure, make this system a particularly illustrative example of momentum resolved hybridization. This system demonstrates that, for hybridization to occur, not only do states require overlap in energy and space, but also in momentum.