EFFECT OF SURFACE-STRUCTURES UPON ULTRATHIN-FILM INTERFERENCE-FRINGES

Effect of surface structures upon ultrathin film interference fringes generated from extremely thin films or epitaxial layers grown on semiconductor wafers has been studied. Since dark regions of fringes correspond to the places where the thin films are destroyed or absent, the fringes are investigated to detect uneven surfaces with undesired structures. Therefore, surface microstructures can be detected and characterized effectively by the modification of the fringes.

Ultrathin-film growth of para-sexiphenyl (II): Formation of large-size domain and continuous thin film

The large-size domain and continuous para-sexiphenyl (p-6P) ultrathin film was fabricated successfully on silicon dioxide (SiO2) substrate and investigated by atomic force microscopy and selected area electron diffraction. At the optimal substrate temperature of 180 degrees C, the first-layer film exhibits the mode of layer growth, and the domain size approaches 100 mu m(2). Its saturated island density (0.018 mu m(-2)) is much smaller than that of the second-layer film (0.088 mu m(-2)), which begins to show the Volmer-Weber growth mode.

Ultrathin-film growth of para-sexiphenyl (I): Submonolayer thin-film growth as a function of the substrate temperature

The para-sexiphenyl (p-6P) monolayer film induces weak epitaxy growth (WEG) of disk-like organic semiconductors, and their charge mobilities are increased dramatically to the level of the corresponding single crystals [Wang et al., Adv. Mater. 2007, 19, 2168]. The growth behavior and morphology of p-6P monolayer film play decisive roles on WEG. Here, we investigated the growth behavior of p-6P submonolayer film as a function of the substrate temperature. Its growth exhibited two different mechanisms at high and low substrate temperature.

An organic-inorganic hybrid ultrathin film: preparation and characterization of copper phthalocyanine derivative-ferric oxide nanoparticles

A nanoparticulate ferric oxide-copper tris(2,4-di-tert-amylphenoxy)-8-quinolinolylphthalocyanine hybrid ultrathin film was constructed from alternate layers by the Langmuir-Blodgett technique. The composition, morphology and structure of the film were studied by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy, atomic force microscopy, small-angle X-ray diffraction, visible spectroscopy and polarized UV-Vis spectroscopy. All the above analyses suggest that the thin film is a kind of one-dimensional superlattice, composed of organic and inorganic components. The XPS data reveal that the nanoparticulate ferric oxide exists as an alpha-Fe2O3 phase in the films. Gas-sensing measurements show that the hybrid LB film has very fast response-recovery characteristics towards 2 ppm C2H5OH vapor.

Study of ferric oxide nanoparticles-tris-(2,4-di-t-amylphenoxy)-(8-quinolinolyl) copper phthalocyanine composite LB film

The ferric oxide nanoparticles-tris-(2,4-di-t-amylphenoxy)-(8-quinolinolyl) copper phthalocyanine (CuPcA(2)) composite ultrathin film was obtained by LB (Langmuir-Blodgett) technique. Structure of the composite LB film was characterized by X-ray photoelectron spectra, transmission electron microscopy, infrared spectra and visible spectra. Gas sensitivity measurements indicate that the composite LB film is sensitive to 100-200 ppm C2H5OH at room temperature. (C) 2000 Elsevier Science S.A. All rights reserved.

六联苯超薄膜与弱取向外延生长

有机半导体薄膜的形态结构对器件性能具有非常重要的影响，近年来高质量有机半导体薄膜的制备成为有机半导体薄膜器件研究的核心内容之一。弱取向外延生长能够获得类单晶迁移率的高质量有机半导体薄膜。而制备出高取向、大尺寸、低缺陷、连续的高质量外延生长基底是获得类单晶外延薄膜的前提条件，这样就需要深入了解它们的生长行为、生长机理及薄膜相态特性。因此，高质量外延基底生长和弱取向外延生长行为及机理的研究就具有十分重要的理论价值和实用价值，是选择和扩展弱取向外延生长材料体系的基础。由此，本论文中工作主要分为两部分，第一部分工作是对外延基底六联苯超薄膜生长的研究：一方面生长出大尺寸、高取向、连续的超薄膜为弱取向外延生长提供高质量的外延基底，另一方面丰富和发展了有机半导体薄膜生长理论。第二部分工作是对酞菁化合物在六联苯超薄膜上弱取向外延生长行为和机理的研究，为弱取向外延材料体系的选择与扩展提供实验依据和理论指导。 首先，研究了六联苯超薄膜生长行为。研究结果表明：(1) 六联苯超薄膜在高温(＞60 oC)和低温(≤60 oC)的二氧化硅(SiO2)基底上生长具有不同的生长机理：高温时薄膜生长符合扩散受限凝聚生长(DLA)机理，低温时薄膜生长是由低有序薄膜经过亚稳分解后重组向团状岛转变。(2) 六联苯超薄膜的生长行为和相结构表明单层和双层薄膜是两种不同的相态：单层薄膜是高取向的具有液晶特性的薄膜相态，双层薄膜是长程有序的近似体相β-phase结构的结晶相。(3) 通过优化基底温度和生长参数，可以制备出高取向、大尺寸、连续的六联苯超薄膜，即可以为酞菁化合物的弱取向外延生长提供高质量的外延基底。 然后，以平面型自由酞菁(H2Pc)及酞菁锌(ZnPc)和非平面型酞菁氧钒(VOPc)为例，深入研究了酞菁化合物在六联苯超薄膜上的弱取向外延生长行为及机理。研究结果表明：(1) 弱取向外延生长的酞菁分子在六联苯超薄膜上立着生长，p-p共轭的方向平行于基底，同时酞菁分子在薄膜平面内具有规则的取向织构。这种高取向的酞菁化合物薄膜有利于载流子在薄膜平面内的传输，其迁移率达到了相应的单晶水平。(2) 由于六联苯双层及单层薄膜结构和相态的差别，平面型酞菁化合物表现出不同的外延生长行为：在六联苯双层薄膜上生长的酞菁化合物薄膜在薄膜平面内只有一种取向，对应于有公度外延生长(Commensurate Epitaxy)；在六联苯单层薄膜上生长的酞菁化合物薄膜在薄膜平面内有三种取向，同时兼具有公度外延生长和无公度外延生长(Incommensurate Epitaxy)。但非平面型VOPc由于分子排列方式及三斜晶体结构的本质，在六联苯单层及双层薄膜上都只表现出无公度外延生长。(3) 六联苯(001)晶面上突起的氢原子所形成的[110]、[1-10]和[010]沟道对酞菁分子具有强烈的预取向作用，从而形成取向的单分子柱晶核。然后在晶格匹配效应和基底沟道效应下，分别形成有公度外延生长和无公度外延生长。即晶格匹配关系和基底沟道效应为寻找弱取向外延生长有机半导体材料体系提供了理论指导依据。

Enhanced amplified spontaneous emission using layer-by-layer assembled cowpea mosaic virus

Layer-by-layer assembly technique was used to construct ultrathin film of cowpea mosaic virus (CPMV) by electrostatic interactions, and the film was employed as a precursor on which an OF8T2 film was deposited by spin coating. Amplified spontaneous emission (ASE) was observed and improved for the OF8T2 film. Compared with OF8T2 film on quartz, the introduction of CPMV nanoparticles reduced the threshold and loss, and remarkably increased the net gain. The threshold, loss, and gain reached 0.05 mJ/ pulse, 6.9 cm(-1), and 82 cm(-1), respectively. CPMV nanoparticles may enormously scatter light, resulting in a positive feedback, thus the ASE is easily obtained and improved.

A method for cathodic polymerization of aniline by in situ electrogenerated intermediate at gold surface

Polyaniline (PANI) was cathodically synthesized at an evaporated gold electrode using an in situ electrogenerated intermediate as oxidant during reduction of the dissolved oxygen. The obtained PANI layer showed an electrochemical response similar to that synthesized by the conventionally anodic polymerization, and the average rate for the growth of PANI layer at polycrystalline gold electrode was 1.59 nm h(-1), while that at the Au (111) electrode was 4.93 nm h(-1). Based on these results, the thickness of the resulted layer can be easily controlled at molecular level for potential nanodevice applications. The obtained PANI layer showed morphology from an island-like nanostructure to an ultrathin film, depending on the crystal orientation of the electrode used.

Oriented polyoxometalate-polycation multilayers on a carbon substrate

In this paper, a new method of fabricating multilayers on a carbon substrate is presented. First, a uniformly charged carbon surface was prepared through molecular design. Then an ultrathin film consisting of layer-pairs of oppositely charged polymeric cationic poly(diallyldimethylammonium chloride) (PDDA) and silicotungstate, SiW12O404- (SiW12), was grown layer-by-layer onto the grafted carbon substrate using a molecular self-assembly technique and an electrochemical method. The technique allows one to prepare highly adherent, dense and smooth films of polyoxometalates with special properties. By combining cyclic voltammetry (CV) and X-ray (XR) reflectometry, it was determined that the average surface density of SiW12 was 2.10 x 10(-10) mol cm(-2), and the thickness increase per adsorption of PDDA-SiW12 was 1.7 +/- 0.2 nm, indicating that the amount of SiW12 anion per one layer adsorption corresponded to a monolayer coverage. Atomic force microscopy (AFM) was also used to examine the surface morphology and determine the grain size distribution and roughness for multilayer films. An increase in root-mean-square (RMS) surface roughness from 7 to 9 Angstrom was observed as the number of layer-pairs in the film increased from 2 to 6. FTIR results showed that the good stability of the multilayer films was due to Coulomb interactions between the SiW12 anion and the polymeric cations PDDA. Moreover, the multilayer films, in acidic aqueous solution, showed good electrocatalytic activity toward the reduction of NO2-, and the catalytic currents increased with increasing the layer numbers of SiW12 adsorption. These characteristics of the multilayer films might find potential applications in the field of sensors and microelectronics devices.

Preparation and characterization of DBSA doped polyaniline thin films

Stable monolayer of the polyaniline(PAn) doped with dodecyl benzenesulfonic acid(DBSA) can form on the pure water surface. The multilayer ultrathin film can be successfully deposited by Langmuir-Blodgett(LB) technique onto CaF2 substrate. The limiting mean molecular area and collapse pressure observed are 0.066 nm(2) and 35 mN m(-1), respectively. The multilayer LB film and casting film were all characterized by TR and UV-Vis-NIR spectroscopies.

Characterization and improvement of GaAs layers grown on Si using an ultrathin a-Si film as a buffer layer

GaAs epilayers grown on Si by metalorganic chemical vapor deposition (MOCVD) using an ultrathin a-Si buffer layer were characterized by deep-level transient spectroscopy (DLTS). Six electron traps with activation energies of 0.79, 0.67, 0.61, 0.55, 0.53 and 0.32 eV below the conduction band were determined by fitting the experimental spectra. Two of the levels, C (0.61 eV) and F (0.32 eV), were first detected in GaAs epilayers on Si and identified as the metastable defects M3 and M4, respectively. In order to improve the quality of GaAs/Si epilayers, another GaAs layer was grown on the GaAs/Si epilayers grown using MOCVD. The deep levels in this regrown GaAs epilayer were also studied using DLTS. Only the EL2 level was found in the regrown GaAs epilayers. These results show that the quality of the GaAs epilayer was greatly improved by applying this growth process.

Morphology development of ultrathin symmetric diblock copolymer film via solvent vapor treatment

We have followed the time development of the microdomain structure in symmetric diblock copolymer poly(styrene-b-methyl methacrylate), P(S-b-MMA), ultrathin films via PMMA-selective solvent vapor treatment by atomic force microscopy (AFM). After preparation on a substrate preferentially attracting the PMMA block, PS forms a continuous layer at a film's free surface. With subsequent solvent vapor treatment, the film gradually shows a well-ordered hexagonally packed nanocylinders structure. It is shown that only when the film thickness is less than the 1/2L(0) (lamellar repeat spacing), and exposed to PMMA block selective solvent for an appropriate time, can the well-ordered hexagonally packed nanocylinders form. On an extended solvent vapor treatment, a mixed morphology containing nanocylinders and stripes appears, followed by the striped morphologies. When the annealing time is long enough, the film comes back to the flat surface again, however, with PMMA instead of PS dominating the free surface.

Super-resolution with a nonlinear thin film: Beam reshaping via internal multi-interference

In laser applications, the size of the focus spot can be reduced beyond the diffraction limit with a thin film of strong nonlinear optical Kerr effect. We present a concise theoretical simulation of the device. The origin of the super-resolution is found to be mainly from the reshaping effect due to the strongly nonlinear refraction mediated multi-interference inside the thin film. In addition, both diffraction and self-focusing effects have been explored and found negligible for highly refractive and ultrathin films in comparison with the reshaping effect. Finally, the theoretic model has been verified in experiments with single Ge2Sb2Te5 film and SiN/Si/SiN/Ge2Sb2Te2 multilayer structures. (c) 2006 American Institute of Physics.

The origin of the super-resolution via a nonlinear thin film

In laser applications, resolutions beyond the diffraction limit can be obtained with a thin film of strong optical nonlinear effect. The optical index of the silicon thin film is modified with the incident laser beam as a function of the local field intensity n(r) similar to E-2(r). For ultrathin films of thickness d << lambda the transmitted light through the film forms a profile of annular rings. Therefore, the device can be related to the realization of super-resolution with annular pupils. Theoretical analysis shows that the focused light spot appears significantly reduced in comparison with the diffraction limit that is determined by the laser wavelength and the numerical aperture of the converging lens. Analysis on the additional optical transfer function due to the thin film confirms that the resolving power is improved in the high spatial frequency region. (C) 2007 Published by Elsevier B.V.

Investigation of GaN layer grown on Si(111) substrate using an ultrathin AlN wetting layer

High-quality GaN epilayers were grown on Si (1 1 1) substrate by metalorganic chemical vapor deposition. The growth process was featured by using an ultrathin AlN wetting layer (WL) in combination with a low-temperature (LT) GaN nucleation layer (NL). The full-width at half-maximum (FWHM) of the X-ray rocking curve for the GaN (0 0 0 2) diffraction was 15 arcmin. The dislocation density estimated from TEM investigation was found to be of the order of 10(9)cm(-2). The FWHM of the dominant band edge emission peak of the GaN was measured to be 47 meV by photoluminescence measurement at room temperature. The ultrathin AlN WL was produced by nitridation of the aluminium pre-covered substrate surface. The reflection high-energy electron diffraction showed that the AlN WL was wurtzite and the surface morphology was like the nitridated surface of sapphire by the atomic force microscopy measurement. X-ray photoelectron spectroscopy measurement showed that Si and SixNy at a certain concentration were intermixed in the AlN WL. This study suggests that by employing an appropriate WL combined with a LT NL, high-quality heteroepitaxy is achievable even with large mismatch. (C) 2002 Elsevier Science B.V. All rights reserved.