Strategies for two-dimensional growth of organic molecular films

After a brief summary of the thermodynamic background on which strategies for driving molecular film growth can be pointed out, we present some recent results on the growth of oligothiophene thin films by organic molecular beam deposition, as obtained studying the film morphology by atomic force microscopy and monitoring the film growth in situ and in real-time by reflectance anisotropy spectroscopy. Three-dimensional and two-dimensional nucleation and growth are obtained by properly tuning the process conditions, such as, in particular, the substrate type and temperature, the molecular flux, and the substrate surface quality. Finally, the tuning of these parameters is used for reaching the condition of a stable layer-by-layer epitaxial growth at room temperature, fully equivalent to that well assessed for inorganics.     

Electrochemical synthesis and nucleation and growth mechanism of Prussian blue films on p-Si(100) electrodes

In this study, we examined the synthesis of Prussian blue onto p-Si(100). The Prussian blue formation was carried out by means of the deposition of a Fe film and then its dissolution in presence of potassium hexacyanoferrate(II). In the first stage, a study by cyclic voltammetry was carried out, and then, using the potential step method, the corresponding nucleation and growth mechanism were determined. Likewise, a morphologic analysis of the deposits obtained at different potential values by means of atomic force microscopy was carried out. The results are consistent with a 3D progressive nucleation with diffusion-controlled growth. Finally, this research is oriented to construct electrochemical storage devices which can be in situ loaded by the photovoltaic action of the semiconductor base material doped silicon.     

Growth of organic n-conductors on thin polymer films for use in organic field effect transistors

Thin films of different polymers - poly(styrene) (PS), poly(methylmethacrylate) (PMMA), poly(vinylcarbazole) (PVCz), poly(vinylchloride) (PVC) and poly(vinylidene fluoride) (PVDF) - were deposited by spin-coating or by vapor deposition. On these polymers, thin films of (hexadecafluorophthalocyaninato)-oxovanadium (F₁₆PcVO) were prepared by physical vapor deposition. The growth of these films was monitored in situ by optical spectroscopy. The optical absorbance spectra were analyzed based on the coupling of transition dipoles to obtain information on the intermolecular arrangement of chromophores in the films. In all of these samples, the molecules are oriented with their molecular plane preferentially perpendicular to the substrate surface. This gives the desired overlap of the π-systems for electric conductance parallel to the substrate. Differences in the interactions were detected when deposition temperatures below or above the glass transition temperature of a given polymer were compared. The morphology of the polymer films and the deposited semiconductors were investigated by atomic force microscopy and scanning electron microscopy. The influence of the chosen substrate on the film structure is determined. The optical and electric properties of the films could thereby be influenced and the applicability of such films as active layers in organic thin film transistors is discussed.     

分级聚丙烯的取向结晶过程

聚丙烯熔体在剪切或应变应力作用下 ,分子链发生取向形成伸直链纤维晶 ,这些先取向形成的纤维晶成为其后结晶的晶核 .这种线形排列的特殊自晶核被称作排核 ( Row nuclei) [1] .排核诱导的结晶温度高于异相核和均相核 .折叠链片晶在排核上附生生长 ,形成具有柱状对称性的超分子结构 ,称为柱状晶 ( Cylindrite) [2 ,3] .聚合物的分子量 ,剪切温度和剪切速度等因素对柱状晶的生成有很大影响 [4 ,5] .本文选用不同级分的聚丙烯样品 ,利用高分子 (特别是取向结晶 )的记忆效应 [6,7] ,研究了剪切后薄膜试样在熔融重结晶过程中柱状晶的形成和发展…     

Nanoclusters in polymer matrices prepared by co-deposition from a gas phase

This paper reviews the fabrication of organic and metal nanoclusters in polymer matrices by three co-deposition techniques. In particular, the structure and properties of polytetrafluoroethylene (PTFE), polychlortrifluoroethylene (PCTFE), polyparaphenylene sulphide (PPS), polystyrene (PS) and polyparaxylylene (PPX) films, containing gold (Au) and dye clusters are discussed. For the first time, dye-filled polymers and multi-component films, consisting of both Au nanoparticles and dye molecules, dispersed in the PTFE matrix were studied. A low temperature plasma was used for film structure modification. Cluster formation process was studied using optical spectroscopy in situ. Transmission electron microscopy (TEM), atomic force microscopy (AFM) and ellipsometry were used for characterisation of the grown films. During Au-PTFE film growth plasmon band shifted from 460-480 nm to 560 nm. Au cluster diameter was in the 3-7 nm range. Plasma treatment of the vapours led to formation of smaller, but more aggregated clusters. During Au-PPS film deposition a two-step growth mechanism was discovered. At the beginning of film growth the plasmon band at 540 nm appeared, but as thickness increased, the band at 430 nm dominated. Without plasma treatment a disordered mixture was deposited, while with plasma treatment large Au aggregates confined with PPS matrix having plasmon band at 620 nm were formed. Dye cluster formation depends on the dye ability to aggregate, its concentration and the properties of the polymer matrix. But cluster formation can also be tuned by varying the deposition conditions. Laser beam evaporation promoted cluster formation, while plasma treatment and dilution in a polymer matrix prevented cluster formation. In all cases both equilibrium and non-equilibrium film structure can be formed using kinetic factor. Asymmetric molecules with bulky substituents were oriented in polymer matrices by applying an electric field in situ or by corona poling. These molecules did not aggregate even at high dye load. The films exhibited second harmonic generation, which demonstrated chromophore orientation in the polymer matrices.     

57Fe Mössbauer Spectroscopy and Electron Microscopy Study of Metal Extraction from CuFe2O4 Electrodes in Lithium Cells

Active CuFe(2)O(4) electrode materials for lithium cells are produced by thermal decomposition of a citrate precursor. The precipitation of the metal citrate is carried out by a freeze-drying procedure. A tetragonally distorted spinel structure is prepared by the decomposition of a citrate precursor. Samples free of impurities are obtained depending on the annealing temperature. The sample heated at 800 degrees C performed at 470 mAh g(-1) after 50 cycles. Electron microscopy is used as the ultimate technique to monitor the morphological changes upon the reversible conversion reaction. Detachment of metallic particles from the starting material, the formation of a polymeric organic film, and the subsequent removal on charging are discussed as determining factors in the electrochemical behaviour of this oxide as an electrode versus lithium. The growth of metallic iron aggregates is inferred from the (57)Fe M?ssbauer spectra.     

Anomalous roughness evolution of rubrene thin films observed in real time during growth

We study the growth and structure of thin films of the organic semiconductor rubrene during organic molecular beam deposition (OMBD) on silicon oxide in situ and in real time using X-ray scattering. Using in situ grazing incidence diffraction (GID) we find a small degree of local order but an otherwise largely disordered structure, consistent with out of plane scans. Monitoring the surface morphology in real time during growth, we find relatively smooth films (surface roughness sigma below approximately 15 A for thicknesses up to at least 600 A) and a significant delay before the onset of roughening. This anomalous roughening in the beginning and crossover to normal roughening later during growth may be related to conformational changes of rubrene in the early stages of growth.     

Paramagnetic pyrrole-based semiconductor molecular material

The electrochemical oxidation of hexa-N-pyrrolylbenzene in organic media leads, via intramolecular coupling of the pyrrole residues, to the deposition of a molecular semiconductor film on an electrode surface. In situ electron spin resonance–electrochemical experiments reveal that the semiconductor is endowed with both properties of conducting polymers (i.e., reversible oxidation) and polyaromatic molecular materials (i.e., highly paramagnetic). The material, which is easy to process as soft homogeneous thin film, shows a tunable 0 to 1 spin concentration per molecule at room temperature by controlling the electrochemical potential. Contribution to the Fall Meeting of the European Materials Research Society, Symposium D: 9th International Symposium on the Electrochemical–Chemical Reactivity of Metastable Materials, Warsaw, 17th–21st September, 2007. An erratum to this article can be found at     

Layer by layer buildup of polysaccharide films: physical chemistry and cellular adhesion aspects

The formation ofpolysaccharide films based on the alternate deposition of chitosan (CHI) and hyaluronan (HA) was investigated by several techniques. The multilayer buildup takes place in two stages: during the first stage, the surface is covered by isolated islets that grow and coalesce as the construction goes on. After several deposition steps, a continuous film is formed and the second stage of the buildup process takes place. The whole process is characterized by an exponential increase of the mass and thickness of the film with the number of deposition steps. This exponential growth mechanism is related to the ability of the polycation to diffuse "in" and "out" of the whole film at each deposition step. Using confocal laser microscopy and fluorescently labeled CHI, we show that such a diffusion behavior, already observed with poly(L-lysine) as a polycation, is also found with CHI, a polycation presenting a large persistence length. We also analyze the effect of the molecular weight (MW) of the diffusing polyelectrolyte (CHI) on the buildup process and observe a faster growth for low MW chitosan. The influence of the salt concentration during buildup is also investigated. Whereas the CHI/HA films grow rapidly at high salt concentration (0.15 M NaCl) with the formation of a uniform film after only a few deposition steps, it is very difficult to build the film at 10(-4) M NaCl. In this latter case, the deposited mass increases linearly with the number of deposition steps and the first deposition stage, where the surface is covered by islets, lasts at least up to 50 bilayer deposition steps. However, even at these low salt concentrations and in the islet configuration, CHI chains seem to diffuse in and out of the CHI/HA complexes. The linear mass increase of the film with the number of deposition steps despite the CHI diffusion is explained by a partial redissolution of the CHI/HA complexes forming the film during different steps of the buildup process. Finally, the uniform films built at high salt concentrations were also found to be chondrocyte resistant and, more interestingly, bacterial resistant. Therefore, the (CHI/HA) films may be used as an antimicrobial coating.     

Atomistic simulation of the formation of nanoporous silica films via molecular chemical vapor deposition on nonporous substrates

To distinguish thin deposited film characteristics clearly from the influence of substrate morphological properties, the growth mechanism and the macroscale and nanoscale properties of nanoporous SiO(2) films deposited on nonporous silica (SiO(2)) substrates from chemical precursors Si(OH)(4) and TEOS (tetraethoxysilane) via low-pressure chemical vapor deposition are the primary targets of this study. This work employs a kinetic Monte Carlo (KMC) simulation method coupled to the Metropolis Monte Carlo method to relax the strained silica structure. The influence of the deposition temperature (473, 673, and 873 K) on the properties of the SiO(x) layers is addressed via analysis of the film growth rates, density profiles of the deposited thin films, pore size distributions, carbon depth profiles (with respect to TEOS), and voidage analysis for layers of different thicknesses (8-18 nm). A comparison of simulation with experimental results is also carried out.     

Crystal growth, structural, optical, spectral and thermal studies of tris(L-phenylalanine)L-phenylalaninium nitrate: a new organic nonlinear optical material

Tris(L-phenylalanine)L-phenylalaninium nitrate, C(9)H(12)NO(2)(+)·NO(3)(-)·3C(9)H(11)NO(2) (TPLPN), a new organic nonlinear optical material was grown from aqueous solution by slow evaporation solution growth at room temperature. The grown crystals were subjected to powder X-ray diffraction and single crystal X-ray diffraction studies to confirm the crystalline nature and crystal structure. The modes of vibration of different molecular groups present in TPLPN have been identified by FTIR spectral analysis. The presence of hydrogen and carbon in the grown crystal were confirmed by using proton and carbon nuclear magnetic resonance (NMR) spectral analyses. The optical transmission spectral study establishes good transmitting ability of the crystal in the entire visible region. The thermogravimetric (TG) and differential thermal analyses (DTA) were carried out to understand the thermal stability of the sample. The nonlinear optical property of the compound observed using Kurtz powder second harmonic generation test assets the suitability of the grown material for the frequency conversion of laser radiation of Nd:YAG.     

Nanoscale Assembly of Paramagnetic Organic Radicals on Au(111) Single Crystals

The successful thin‐film deposition of a pyrene‐substituted nitronyl nitroxide radical under controlled conditions has been demonstrated. The electronic properties, chemical environment at the interface, and morphology of the thin films have been investigated by a multitechnique approach. Spectroscopic and morphological analyses indicate a Stranski–Krastanov growth mode and weak physisorption of molecules onto the metallic surface. Electron spin resonance (ESR) spectroscopy shows that evaporation processes and deposition do not affect the paramagnetic character of the molecules. Useful concepts for the engineering of new, purely organic‐based magnets, which may open the way to fruitful exploitation of organic molecular‐beam deposition for assembly on solid surfaces in view of future technological applications, are presented.     

Reusable thin film photocatalyst of Fe‐doped TiO2 deposited by ECR plasma

The paper presents an improved method of depositing nanocrystalline thin films of Fe‐doped TiO₂ to be used as a reusable cyclic photocatalyst for degrading the organic pollutants. The technique of electron cyclotron resonance plasma‐enhanced chemical vapor deposition was employed with titanium tetra‐isopropoxide (C₁₂H₂₈O₄Ti) and ferrocene (C₁₀H₁₀Fe) as precursors of Ti and Fe, respectively. Optical emission spectroscopy was used to identify the reactive species, to determine the electron temperature and the ion density during deposition. The films were characterized using optical absorption and photoluminescence spectra, whereas the morphological analysis was carried out with scanning electron microscopy. Strong adhesion of the deposited films with the substrate ruled out any possibility of TiO₂ particles being leached out. It was confirmed by observing the degradation rate of the same film repeatedly. Cyclic use of the film for the catalytic reactions thus makes the process much user friendly for the water treatment. Copyright © 2014 John Wiley & Sons, Ltd.     

Evolution of the Pt layer deposited on MgO(001) by pulsed laser deposition as a function of the deposition parameters: a scanning tunneling microscopy and energy dispersive X-ray diffractometry/reflectometry study

A combined ultrahigh vacuum scanning tunneling microscopy (STM-UHV) and energy dispersive X-ray diffractometry/reflectometry (EDXD/EDXR) study of the evolution of face-centered cubic (fcc) Pt layer growth on MgO(100) by pulsed laser deposition as a function of the process parameters such as deposition temperature and deposition duration has been carried out. The aim of this study is to define the best experimental conditions to obtain a controlled film deposition selective on the Pt growth direction (either [111] or [002]). The evolution of the Pt surface morphology as a function of the deposition temperature (T(dep)) from 300 to 700 degrees C has been studied with STM and ED techniques. Results show that the Pt surface, characterized at T(dep) = 300 degrees C by a 3D island morphology, evolves at higher temperatures to a morphology in which the original islands coexist with a distribution of orthogonal 2D stripes. The two features can be associated with the [111] and [002] Pt growth directions of the fcc phase, respectively. For T(dep) = 700 degrees C, the island morphology of the (111) face completely disappears, while the merging process of the (002) stripes reaches completion. The evolution of the morphology at T(dep) = 600 degrees C as a function of the deposition time and thickness has then been studied with STM-UHV, revealing an initial growth of mosaic-like 3D islands. These independent islands, already interconnected, expand along two orthogonal directions and, for longer deposition times, lead to the texture of orthogonal stripes. The EDXR characterization providing the morphological parameters of the films, i.e., thickness and roughness, confirms the above observation and quantifies the effect of such morphological changes on the surface roughness of the Pt film, an important parameter for applications of Pt films as underlayer in magnetic recording media.     

Effect of molecular weight on the exponential growth and morphology of hyaluronan/chitosan multilayers: a surface plasmon resonance spectroscopy and atomic force microscopy investigation

The layer-by-layer growth of multilayer assemblies of two polysaccharides, the polyanion hyaluronan (HA) and the polycation chitosan (CH), was investigated using atomic force microscopy (AFM) and surface plasmon resonance (SPR) spectroscopy, with primary emphasis on the effect of the polysaccharide molecular weights on the film thickness and surface morphology. The HA/CH multilayers exhibit an exponential increase of the optical film thickness with the number of deposited bilayers. We show that the multilayer thickness at a given stage depends on the size of both CH, the diffusing polyelectrolyte, and HA, the non-diffusing species. Assemblies (12 bilayers) of high molecular weight polysaccharides (HA, 360,000; CH, 160,000) were twice as thick (approximately 900 nm vs approximately 450 nm) as those obtained with low molecular weight polymers (HA, 30,000; CH, 31,000), as assessed by AFM scratch tests. The exponential growth rate is the same for the high and low molecular weight pairs; the larger film thicknesses observed by SPR and by AFM arising from an earlier onset of the steep exponential growth phase in the case of the high molecular weight pair. In all cases, isolated islets form during the deposition of the first CH layer onto the underlying HA. Upon further film growth, individual islets coalesce into larger vermiculate features. The transition from distinct islands to vermiculate structures depends on the molecular weights of the polysaccharides and the lower molecular weight construct presents larger worm-like surface domains than the high molecular weight pair.     

功能性超薄有序分子沉积膜的制备及其结构研究

1991年G.Decher等首次探讨了阴阳离子与聚电解质交替沉积制备有机超薄膜的方法。我们在完善成膜技术和发展成膜基质的基础上,详细研究了其成膜过程与膜的结构,并定义这种新的自组装超薄有序膜为分子沉积膜——MD膜。MD膜是利用阴阳离子的静电吸附反应特性,通过相反离子体系的交替分子沉积制备的层状有序自组装多层超薄膜。需要指出的是,分子沉积既是有机超薄膜的制备技术,本身又是一种自组装超薄有序膜。MD膜制备工艺简单,热稳定性和长期稳定性好,不受基体形状与面积限制。     

Oligomer orientation in vapor-molecular-layer-deposited alkyl-aromatic polyamide films

The surface-limited molecular-layer deposition of alkyl-aromatic polyamide films using sequential doses of 1,4-butane diamine (BDA) and terephthaloyl dichloride (TDC) is characterized using in situ quartz crystal microbalance and ex situ spectroscopy analysis. For the first time, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to offer insight into molecular orientation in films deposited via molecular-layer deposition (MLD). The results show that the oligomer units are lying nearly parallel to the surface, which differs from the linear vertical growth mode often used to illustrate film growth.     

Growth of crystalline rubrene films with enhanced stability

The growth of well-oriented crystalline films of rubrene (C42H28) on SiO2 and Au111 substrates is achieved by employing "hot wall" deposition whereas organic molecular beam deposition (OMBD) only yields rather amorphous layers or poly-crystalline dendritic networks at elevated temperature. This pronounced difference in film growth is related to the conformational change of rubrene molecules involving a loss of chirality upon crystallization and the enhanced diffusion which becomes possible at high temperature and large vapor pressure. Moreover, it is demonstrated that the crystalline rubrene films reveal an enhanced thermal and chemical stability as compared to the OMBD grown films.     

TEM investigation of formation mechanism of monocrystal-thick b-oriented pure silica zeolite MFI film

The first direct transmission electron microscopic (TEM) observation has been carried out on the continuous monocrystal-thick b-oriented pure silica zeolite MFI films produced by in situ crystallization. The self-supporting film samples for TEM study were fabricated by dissolving the steel substrate with acid. This TEM study is free of those artifacts that are typically associated with TEM sample preparations, and allows us to investigate the "true" structure and texture of a very large area of the film and at the same time to focus at will on each individual zeolite crystal in the film. Abundant TEM information including crystallographic orientation relationships among crystals in the film (both out-of-plane and in-plane), grain boundaries, and each crystal grain was obtained. This TEM investigation provides direct unambiguous new evidence to support the homogeneous nucleation mechanism, by which the films form through homogeneous nucleation and crystal growth in the bulk to form equal-sized disk-shape crystals, followed by self-assembly of these crystals onto the substrate to produce a two-dimensional close-packed structure. The last stage of the film formation involves simultaneous space-limited growth and rotation of the individual crystals to realize the in-plane crystallographic control within the film.