Behavior of Pentacene Initial Nucleation on Various Dielectrics and Its Effect on Carrier Transport in Organic Field-Effect Transistor

The influence of dielectric surface energy on the initial nucleation and the growth of pentacene films as well as the electrical properties of the pentacene-based field-effect transistors are investigated. We have examined a range of organic and inorganic dielectrics with different surface energies, such as polycarbonate/SiO2, polystyrene/SiO2, and PMMA/SiO2 bi-layered dielectrics and also the bare SiO2 dielectric. Atomic force microscopy measurements of sub-monolayer and thick pentacene films indicated that the growth of pentacene film was in Stranski-Kranstanow growth mode on all the dielectrics. However, the initial nucleation density and the size of the first-layered pentacene islands deposited on different dielectrics are drastically influenced by the dielectric surface energy. With the increasing of the surface energy, the nucleation density increased and thus the average size of pentacene islands for the first mono-layer deposition decreased. The performance of fabricated pentacene-based thin film transistors was found to be highly related to nucleation density and the island size of deposited Pentacene film, and it had no relationship to the final particle size of the thick pentacene film. The field effect mobility of the thin film transistor could be achieved as high as 1.38 cm(2)/Vs with on/off ratio over 3 x 10(7) on the PS/SiO2 where the lowest surface energy existed among all the dielectrics. For comparison, the values of mobility and on/off ratio were 0.42 cm(2)/Vs and 1 x 10(6) for thin film transistor deposited directly on bare SiO2 having the highest surface energy.

Homogeneous Epitaxial Growth of N,N '-di(n-butyl)quinacridone Thin Films on Ag(110)

The structural evolution of the ordered N-N' dibutyl-substituted quinacridone (QA4C) multilayers (3 MLs) has been monitored in situ and in real time at various substrate temperatures using low energy electron diffraction (LEED) during organic molecular beam epitaxy (MBE). Experimental results of LEED patterns clearly reveal that the structure of the multilayer strongly depends on the substrate temperature. Multilayer growth can be achieved at the substrate temperatures below 300 K, while at the higher temperatures we can only get one ordered monolayer of QA4C. Two kinds of structures, the commensurate and incommensurate one, often coexist in the QA4C multilayer. With a method of the two-step substrate temperatures, the incommensurate one can be suppressed, and the commensurate, on the other hand, more similar to the (001) plane of the QA4C bulk crystal, prevails with the layer of QA4C increasing to 3 MLs. The two structures in the multilayers are compressed slightly in comparison to the original ones in the first monolayer.

Structure and properties of InAs/AlAs quantum dots for broadband emission

The InAs quantum dots (QDs) on an AlAs layer are grown on GaAs substrates by molecular beam epitaxy technique. The properties of materials and optics of such QD structures have been investigated by cross sectional transmission electron microscopy and photoluminescence (PL) techniques. It is discovered that the inhomogeneous strain filed mainly exists below InAs QDs layers in the case of no wetting layer. The full width at half maximums (FWHMs) and intensities of PL emission peaks of InAs QDs are found to be closely related to the thickness of the thin AlAs layers. The InAs QDs on an eight monolayer AlAs layer, with wide FWHMs and large integral intensity of PL emission peaks, are favorable for producing broadband QD superluminescent diodes, external-cavity QD laser with large tuning range.

Wettability designing by ZnO periodical surface textures

A facile and effective aqueous chemical synthesis approach towards well control of periodical ZnO textures in large-scale areas is reported, by which considerable adjusting of surface wettability can be realized. With the assistance of polystyrene spheres monolayer template and morphology control agent, we succeeded in preparing a series of ordered ZnO microbowls with different sag height. It was found that the contact angle could be well adjusted by changing geometry of microbowl. Such novel, ordered arrays are expected to exploit the great potentiality in waterproof or self-cleaning micro/nanodevices, and even microfluidic devices. (C) 2010 Elsevier Inc. All rights reserved.

Optimization of evanescent wave imaging for the visualization of protein adsorption layers

We have optimized the settings of evanescent wave imaging for the visualization of a protein adsorption layer. The enhancement of the evanescent wave at the interface brought by the incident angle, the polarized state of light beam as well as a gold layer is considered. In order to improve the image contrast of a protein monolayer in experiments, we have optimized three factors-the incident angle, the polarization of light beam, and the thickness of an introduced thin gold layer with a theoretical simulation.

An ATP-dependent mechanism mediates intercellular calcium signaling in bone cell network under single cell nanoindentation

To investigate the roles of intercellular gap junctions and extracellular ATP diffusion in bone cell calcium signaling propagation in bone tissue, in vitro bone cell networks were constructed by using microcontact printing and self-assembled monolayer technologies. In the network, neighboring cells were interconnected through functional gap junctions. A single cell at the center of the network was mechanically stimulated by using an AFM nanoindenter. Intracellular calcium ([Ca2+](i)) responses of the bone cell network were recorded and analyzed. In the untreated groups, calcium propagation from the stimulated cell to neighboring cells was observed in 40% of the tests. No significant difference was observed in this percentage when the intercellular gap junctions were blocked. This number, however, decreased to 10% in the extracellular ATP-pathway-blocked group. When both the gap junction and ATP pathways were blocked, intercellular calcium waves were abolished. When the intracellular calcium store in ER was depleted, the indented cell can generate calcium transients, but no [Ca2+](i) signal can be propagated to the neighboring cells. No [Ca2+](i) response was detected in the cell network when the extracellular calcium source was removed. These findings identified the biochemical pathways involved in the calcium signaling propagation in bone cell networks. Published by Elsevier Ltd.

Intercellular calcium wave propagation in linear and circuit-like bone cell networks

In the present study, the mechanism of intercellular calcium wave propagation in bone cell networks was identified. By using micro-contact printing and self-assembled monolayer technologies, two types of in vitro bone cell networks were constructed: open-ended linear chains and looped hexagonal networks with precisely controlled intercellular distances. Intracellular calcium responses of the cells were recorded and analysed when a single cell in the network was mechanically stimulated by nano-indentation. The looped cell network was shown to be more efficient than the linear pattern in transferring calcium signals from cell to cell. This phenomenon was further examined by pathway-inhibition studies. Intercellular calcium wave propagation was significantly impeded when extracellular adenosine triphosphate (ATP) in the medium was hydrolysed. Chemical uncoupling of gap junctions, however, did not significantly decrease the transferred distance of the calcium wave in the cell networks. Thus, it is extracellular ATP diffusion, rather than molecular transport through gap junctions, that dominantly mediates the transmission of mechanically elicited intercellular calcium waves in bone cells. The inhibition studies also demonstrated that the mechanical stimulation-induced calcium responses required extracellular calcium influx, whereas the ATP-elicited calcium wave relied on calcium release from the calcium store of the endoplasmic reticulum.

运用含有偶氮苯的自组装单层膜在光化学条件下可逆地控制细胞黏附

细胞生物学研究的一个重要方向是动态地控制细胞在基底上的黏附。最近，随着表面化学的研究深入，尤其是对烷基硫醇在金基底上形成自组装单层膜（self-assembled monolayers, SAMs）这一体系的研究，使得人们能在分子水平的表面上控制细胞黏附。精氨酸-甘氨酸-天冬氨酸（arginine-glycine-aspartate, RGD）序列首先是从细胞外基质蛋白中分离出来的，能够识别并非共价结合细胞膜表面的整合素受体，从而促进细胞黏附。以前的一些工作已经证实，将含有RGD的肽链连接到SAMs表面之后，能够生物特异性地黏附动物细胞。已有的手段比如光照、电压、加热、微电极、微流控以及表面纳米形貌的梯度变化，都不能真正实现可逆地控制细胞黏附，原因是这些方法所用的化学有限；这些方法也不能得到完全抗拒细胞黏附的表面，原因是这些方法产生的表面缺陷等不完整。用两种不同波长的光（紫外光和可见光）照射偶氮苯，偶氮苯会发生可逆的光致异构变化，因此，偶氮苯的光致异构性质可以用来可逆地控制细胞在表面黏附。运用含有偶氮苯的混合SAMs，偶氮苯的末端连接GRGDS肽，混合SAMs中是以末端为六聚乙二醇的硫醇为背景，该SAMs修饰而成的表面能够黏附或者抗拒细胞黏附，其表面黏附性质取决于SAMs中偶氮苯的构象。该方法提供了一种在分子水平的表面上我们所了解到的唯一能可逆控制细胞黏附的方法，该方法需要用到的光源来自于标准荧光显微镜所配置的汞灯。 为了实现在金基底表面可逆的控制细胞黏附，我们合成了如下三个化合物： 由于化合物1的溶解性很差，几乎在所有溶剂里都不溶，所以不能直接用化合物1制备SAMs；化合物2能高效地抗拒细胞的黏附；化合物3的偶氮苯末端是活化酯，能够连接GRGDS肽，从而控制细胞黏附。 将化合物2和化合物3以一定的比例均匀混合在金基底表面形成SAMs，然后将GRGDS肽连接到偶氮苯(反式)的末端（通过GRGDS肽的甘氨酸上的伯胺基与偶氮苯末端的活化酯反应），从而得到细胞黏附的表面。用紫外光照射该细胞黏附表面5-10小时，随着偶氮苯的构象由反式变为顺式，偶氮苯末端的GRGDS肽淹没在化合物2的六聚乙二醇中，得到抗拒细胞黏附的惰性表面。再用可见光照射该惰性表面1个小时，随着偶氮苯的构象由顺式变为反式，原先埋没在六聚乙二醇中的GRGDS肽伸展至单层膜的末端，又得到了细胞黏附的表面。因此，该表面能完全可逆地控制细胞在金表面黏附。 An important area in cell biology is the dynamic control of cell adhesion on substrates. Recent advancements in surface chemistry, in particular, self-assembled monolayers (SAMs) of alkanethiols on gold substrates, have permitted unprecedented control of cell adhesion via molecularly defined surfaces. The tri-peptide sequence arginine-glycine-aspartate (RGD), initially isolated from the extracellular matrix (ECM) proteins, can recognize and non-covalently bind with integrin receptors on cell membranes to promote cell adhesion. Some previous work has demonstrated that RGD peptide grafted on SAMs can allow bio-specific adhesion of mammalian cells that mimic natural adhesion. Existing technologies such as light, voltage, heat, microelectrodes, microfluidic systems and surface gradient of nanotopography, either cannot realize fully reversible control of cell adhesion, due to the limitation in the chemistry used, or cannot yield a surface completely resistant against cell adhesion, due to the imperfection of surfaces. Azobenzenes undergo reversible photo-induced isomerization rapidly at two different wavelengths of light (UV and visible light), it therefore potentially allows the reversible control of cell adhesion on a surface. By using a mixed SAMs presenting azobenzene groups terminated in GRGDS peptides in a background of hexa(ethylene glycol) groups, the surface can either accommodate or resist cell adhesion depending on the conformation of the azobenzene embedded in SAMs. This method provides the only means we know to control cell adhesion reversibly on a molecularly well-defined surface by using light generated by a mercury lamp equipped on standard fluorescence microscopes. To realize the reversible control of cell adhesion on gold surface, we synthesized three kinds of compounds as following, We found that it was difficult to obtain SAMs directly from compound 1 because of its poor solubility in almost all kinds of solvents; compound 2 can resist cell adhesion efficiently; compound 3 presents an azobenzene terminated with NHS-activated ester, which can couple with a GRGDS peptide to control cell adhesion. After coating a gold surface with compound 2 and 3 in appropriate ratios to form a SAM followed by coupling the GRGDS peptides with NHS-activated esters at the end of azobenzene (E configuration) resulted in a cell-adhesive SAM. Irradiating this cell-adhesive SAM with UV light for 5-10 h converted the E configuration of azobenzene into the Z form, the GRGDS peptides becoming masked in the PEG, resulting in a cell-resistant surface. These SAM could again support cell adhesion as a result of the conformational switch of azobenzene from Z to E with the irradiation of visible light for 1 h. This surface, therefore, allows completely reversible control of cell adhesion on a gold surface.

Chemisorption and reaction characteristics of methyl radicals on Cu(110)

Methyl radicals are generated by pyrolysis of azomethane, and the condition for achieving neat adsorption on Cu(110) is described for studying their chemisorption and reaction characteristics. The radical-surface system is examined by X-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, temperature-programmed desorption, low-energy electron diffraction (LEED), and high-resolution electron energy loss spectroscopy under ultrahigh vacuum conditions. It is observed that a small fraction of impinging CH3 radicals decompose into methylene possibly on surface defect sites. This type of CH2 radical has no apparent effect on CH3(ads) surface chemistry initiated by dehydrogenation to form active CH2(ads) followed by chain reactions to yield high-mass alkyl products. All thermal desorption products, such as H-2, CH4, C2H4, C2H6, and C3H6, are detected with a single desorption peak near 475 K. The product yields increase with surface coverage until saturation corresponding to 0.50 monolayer of CH3(ads). The mass distribution is, however, invariant with initial CH3(ads) coverage, and all desorbed species exhibit first-order reaction kinetics. LEED measurement reveals a c(2 x 2) adsorbate structure independent of the amount of gaseous exposure. This strongly suggests that the radicals aggregate into close-packed two-dimensional islands at any exposure. The islanding behavior can be correlated with the reaction kinetics and is deemed to be essential for the chain propagation reactions. Some relevant aspects of the CH3/Cu(111) system are also presented. The new results are compared with those of prior studies employing methyl halides as radical sources. Major differences are found in the product distribution and desorption kinetics, and these are attributed to the influence of surface halogen atoms present in those earlier investigations.

Cyclopropanation on a highly active heterogeneous catalyst: CuO/TiO2-Al2O3

Some heterogeneous catalysts, cupric oxide supported on different supports, were prepared and employed to catalyze the cyclopropanation of styrene and 2,5-dimethyl-2,4-hexadiene with ethyl diazoacetate (EDA). The catalytic performance for cyclopropanation strongly depends on the nature of the support. A novel catalyst, CUO/TiO2-Al2O3, in which Al2O3 is modified with a monolayer TiO2, is found to be most active and selective for the cyclopropanation reaction. The yields of 93 and 94% cyclopropanes are obtained for styrene and 2,5-dimethyl-2,4-hexadiene at 40 degreesC as the substrates, respectively. The activity and selectivity in cyclopropanes are optimized with a monolayer dispersion of cupric oxide on the corresponding supports. (C) 2002 Elsevier Science B.V. All rights reserved.

Catalytic decomposition of hydrazine over supported molybdenum nitride catalysts in a monopropellant thruster

The catalytic decomposition of hydrazine over a series of MoNx/gamma-Al2O3 catalysts with different Mo loadings was investigated in a monopropellant thruster (10 N). When the Mo loading is equal to or higher than the monolayer coverage of MoO3 on gamma-Al2O3, the catalytic performance of the supported molybdenum nitride catalyst is close to that of the conventionally used Ir/gamma-Al2O3 catalyst. The MoNx/gamma-Al2O3 catalyst with a loading of about 23wt% Mo (1.5 monolayers) shows the highest activity for hydrazine decomposition. There is an activation process for the MoNx/gamma-Al2O3 catalysts at the early stage of hydrazine decomposition, which is probably due to the reduction of the oxide layer formed in the passivation procedure.

One-step preparation of hybrid materials of polyacrylamide networks and gold nanoparticles

Hybrid materials of polyacrylamide networks and gold nanoparticles were prepared by directly heating an aqueous solution containing HAuCl4, acrylamide, N,N'-methylenebisacrylamide, and sodium sulfite (Na2SO3). Acrylamide, N,N'-methylenebisacrylamide, and Na2SO3 were used as monomers, crosslinking agent, and initiator, respectively.

Monodisperse raspberry-like gold submicrometer spheres: Large-scale synthesis and interface assembling for colloid sphere array

We first suggested a one-pot method to synthesize monodisperse raspberry-like submicrometer gold spheres (MRSGS) with high yield. The resulting gold spheres were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersed X-ray spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and electrochemical technology. It was found that the rough structure provided by raspberry-like gold spheres led to a tremendous electrochemical active area, which was very important because these novel hierarchical gold spheres will probably find important applications in biosensors, electrocatalysis, and others.