Selective-Area Atomic Layer Deposition

Atomic layer deposition (ALD) is a method to deposit thin films from gaseous precursors to the substrate layer-by-layer so that the film thickness can be tailored with atomic layer accuracy. Film tailoring is even further emphasized with selective-area ALD which enables the film growth to be controlled also on the substrate surface. Selective-area ALD allows the decrease of a process steps in preparing thin film devices. This can be of a great technological importance when the ALD films become into wider use in different applications. Selective-area ALD can be achieved by passivation or activation of a surface. In this work ALD growth was prevented by octadecyltrimethoxysilane, octadecyltrichlorosilane and 1-dodecanethiol SAMs, and by PMMA (polymethyl methacrylate) and PVP (poly(vinyl pyrrolidone) polymer films. SAMs were prepared from vapor phase and by microcontact printing, and polymer films were spin coated. Microcontact printing created patterned SAMs at once. The SAMs prepared from vapor phase and the polymer mask layers were patterned by UV lithography or lift-off process so that after preparation of a continuous mask layer selected areas of them were removed. On these areas the ALD film was deposited selectively. SAMs and polymer films prevented the growth in several ALD processes such as iridium, ruthenium, platinum, TiO2 and polyimide so that the ALD films did grow only on areas without SAM or polymer mask layer. PMMA and PVP films also protected the surface against Al2O3 and ZrO2 growth. Activation of the surface for ALD of ruthenium was achieved by preparing a RuOX layer by microcontact printing. At low temperatures the RuCp2-O2 process nucleated only on this oxidative activation layer but not on bare silicon.

Interferometry with a novel composite material

An interesting application of optical phase conjugation is phase conjugate interferometry. We report here a new approach to real-time interferometry which combines the high phase conjugate efficiency of photorefractive crystals and the fast response times offered by dye-doped polymer films by using a composite structure. The ability of this material to generate two independent but overlapping phase conjugate waves. which can interfere to reveal the phase changes in a test object, is discussed and demonstrated with a specific example.

The determination of stem cell fate by 3D scaffold structures through the control of cell shape

Stem cell response to a library of scaffolds with varied 3D structures was investigated. Microarray screening revealed that each type of scaffold structure induced a unique gene expression signature in primary human bone marrow stromal cells (hBMSCs). Hierarchical cluster analysis showed that treatments sorted by scaffold structure and not by polymer chemistry suggesting that scaffold structure was more influential than scaffold composition. Further, the effects of scaffold structure on hBMSC function were mediated by cell shape. Of all the scaffolds tested, only scaffolds with a nanofibrous morphology were able to drive the hBMSCs down an osteogenic lineage in the absence of osteogenic supplements. Nanofiber scaffolds forced the hBMSCs to assume an elongated, highly branched morphology. This same morphology was seen in osteogenic controls where hBMSCs were cultured on flat polymer films in the presence of osteogenic supplements (OS). In contrast, hBMSCs cultured on flat polymer films in the absence of OS assumed a more rounded and less-branched morphology. These results indicate that cells are more sensitive to scaffold structure than previously appreciated and suggest that scaffold efficacy can be optimized by tailoring the scaffold structure to force cells into morphologies that direct them to differentiate down the desired lineage. Published by Elsevier Ltd.

Optical, Electrochemical, and Structural Properties of Spray Coated Dihexyl Substituted Poly (3,4 propylene dioxythiophene) Film for Optoelectronics Devices

The dihexyl substituted poly (3,4-propylenedioxythiophene) (PProDOT-Hx(2)) thin films uniformly deposited by cost effective spray coating technique on transparent conducting oxide coated substrates. The electro-optical properties of PProDOT-Hx(2) films were studied by UV-Vis spectroscopy that shows the color contrast about 45% with coloration efficiency of approximate to 185cm(2)/C. The electrochemical properties of PProDOT-Hx(2) films were studied by cyclic voltammetry and AC impedance techniques. The cyclic voltammogram shows that redox reaction of films are diffusion controlled and ions transportation will be faster on the polymer film at higher scan rate. Impedance spectra indicate that polymer films are showing interface charge transfer process as well as capacitive behavior between the electrode and electrolyte. The XRD of the PProDOT-Hx(2) thin films revealed that the films are in amorphous nature, which accelerates the transportation of ions during redox process.

Design and synthesis of new low band gap organic semiconductors for photovoltaic applications

Donor-acceptor (D-A) conjugated polymers have attracted a good deal of attention in recent years. In D-A systems, the introduction of electron withdrawing groups reduces E-g by lowering the LUMO levels whereas, the introduction of electron donating groups reduces E-g by raising the HOMO levels. Also, conjugated polymers with desired HOMO and LUMO energy levels could be obtained by the proper selection of donor and acceptor units. Because of this reason, D-A conjugated polymers are emerging as promising materials particularly for polymer light emitting diodes (PLEDs) and polymer solar cells (PSCs). We report the design and synthesis of four new narrow band gap donor-acceptor (D-A) conjugated polymers, PTCNN, PTCNF, PTCNV and PTCNO, containing electron donating 3,4-didodecyloxythiophene and electron accepting cyanovinylene units. The effects of further addition of electron donating and electron withdrawing groups to the repeating unit of a D-A conjugated polymer (PTCNN) on its optical and electrochemical properties are discussed. The studies revealed that the nature of D and A units as well as the extent of alternate D-A structure influences the optical and the electrochemical properties of the polymers. All the polymers are thermally stable up to a temperature of 300 degrees C under nitrogen atmosphere. The electrochemical studies revealed that the polymers possess low-lying HOMO energy levels and low-lying LUMO energy levels. In the UV-Vis absorption study, the polymer films displayed broad absorption in the wavelength region of 400-700 nm. The polymers exhibited low optical band gaps in the range 1.70 - 1.77 eV.

In-situ synthesized poly(vinyl butyral)/MMT-clay nanocomposites: The role of degree of acetalization and clay content on thermal, mechanical and permeability properties of PVB matrix

Poly(vinyl butyral) - MMT clay nanocomposites were synthesized in situ with three different degrees of acetalization and with varying clay content for each vinyl butyral polymer ratio. The clay nano-platelet galleries were expanded, as determined by X-ray diffraction and TEM analysis. The glass transition temperature of the polymer nanocomposites were found to be similar to 56 degrees C and similar to 52 degrees C for the neat polymer and the 4% clay loaded samples, respectively. The 4 wt% clay loaded film showed higher strength and low strain to failure. The dynamic mechanical analysis also confirmed the improved stability of the matrix. The matrix with 0.5 butyral to alcohol ratio for 4 wt% clay exhibited good water vapor transmission compared to all other compositions. The encapsulated devices with 2.5 and 4 wt% clay loaded films increases the device life time and the efficiencies of these films were 50% higher than their encapsulated pristine polymer films. (C) 2015 Elsevier Ltd. All rights reserved.

Enzymatic degradation of polycaprolactone-gelatin blend

Blends of polycaprolactone (PCL), a synthetic polymer and gelatin, natural polymer offer a optimal combination of strength, water wettability and cytocompatibility for use as a resorbable biomaterial. The enzymatic degradation of PCL, gelatin and PCL-gelatin blended films was studied in the presence of lipase (Novozym 435, immobilized) and lysozyme. Novozym 435 degraded the PCL films whereas lysozyme degraded the gelatin. Though Novozym 435 and lysozyme individually could degrade PCL-gelatin blended films, the combination of these enzymes showed the highest degradation of these blended films. Moreover, the enzymatic degradation was much faster when fresh enzymes were added at regular intervals. The changes in physico-chemical properties of polymer films due to degradation were studied by scanning electron microscopy, Fourier transform infrared spectroscopy and differential scanning calorimetry. These results have important implications for designing resorbable biomedical implants.

Enzymatic degradation of polycaprolactone-gelatin blend

Blends of polycaprolactone (PCL), a synthetic polymer and gelatin, natural polymer offer a optimal combination of strength, water wettability and cytocompatibility for use as a resorbable biomaterial. The enzymatic degradation of PCL, gelatin and PCL-gelatin blended films was studied in the presence of lipase (Novozym 435, immobilized) and lysozyme. Novozym 435 degraded the PCL films whereas lysozyme degraded the gelatin. Though Novozym 435 and lysozyme individually could degrade PCL-gelatin blended films, the combination of these enzymes showed the highest degradation of these blended films. Moreover, the enzymatic degradation was much faster when fresh enzymes were added at regular intervals. The changes in physico-chemical properties of polymer films due to degradation were studied by scanning electron microscopy, Fourier transform infrared spectroscopy and differential scanning calorimetry. These results have important implications for designing resorbable biomedical implants.

Rapid electrohydrodynamic lithography using low-viscosity polymers.

This study explores a number of low-viscosity glass-forming polymers for their suitability as high-speed materials in electrohydrodynamic (EHD) lithography. The use of low-viscosity polymer films significantly reduces the patterning time (to below 10 s) compared to earlier approaches, without compromising the high fidelity of the replicated structures. The rapid pace of this process requires a method to monitor the completion of EHD pattern formation. To this end, the leakage current across the device is monitored and the sigmoidal shape of the current curve is correlated with the various stages of EHD pattern formation.

Magnetic alignment of high-aspect ratio microwires into vertical arrays

Fundamental studies of magnetic alignment of highly anisotropic mesostructures can enable the clean-room-free fabrication of flexible, array-based solar and electronic devices, in which preferential orientation of nano- or microwire-type objects is desired. In this study, ensembles of 100 micron long Si microwires with ferromagnetic Ni and Co coatings are oriented vertically in the presence of magnetic fields. The degree of vertical alignment and threshold field strength depend on geometric factors, such as microwire length and ferromagnetic coating thickness, as well as interfacial interactions, which are modulated by varying solvent and substrate surface chemistry. Microwire ensembles with vertical alignment over 97% within 10 degrees of normal, as measured by X-ray diffraction, are achieved over square cm scale areas and set into flexible polymer films. A force balance model has been developed as a predictive tool for magnetic alignment, incorporating magnetic torque and empirically derived surface adhesion parameters. As supported by these calculations, microwires are shown to detach from the surface and align vertically in the presence of magnetic fields on the order of 100 gauss. Microwires aligned in this manner are set into a polydimethylsiloxane film where they retain their vertical alignment after the field has been removed and can subsequently be used as a flexible solar absorber layer. Finally, these microwires arrays can be protected for use in electrochemical cells by the conformal deposition of a graphene layer.

Design and process selection for joints in flexible materials

The application of high performance textiles has grown significantly in the last 10 to 15 years. Various research groups throughout the United Kingdom, such as the Department of Trade and Industry, have identified technical textiles as a field for future development. There is little design guidance for joining of flexible materials or general property models that can be applied to theses materials. This lack is due to the large diversity of properties, structures and resulting behaviours of the materials that are classified as "Flexible Materials". This dissertation explores the issues that are involved in characterising the materials at the fibre, bulk and textile levels. Different units of measurement are used for each stage of the manufacturing process of flexible materials and this disparity creates problems when trying to make general comparisons (e.g. comparing textiles to polymer films). Thus, a possible solution to this is to create selection charts that allow designers to compare the strength of materials for a given mass per unit area. A design tool was created using the Cambridge Engineering Selector (CES) software to enable the selection of joining processes for material. The tool is effective in selecting a reduced number of viable joining processes. Through case studies it was shown that designers are required to examine the selected processes (identified by the software) in greater detail - in particular the economics and geometry of the joint - in order to identify the optimum joining process.

Magnetically actuated artificial cilia: the effect of fluid inertia.

Natural cilia are hairlike microtubule-based structures that are able to move fluid on the micrometer scale using asymmetric motion. In this article, we follow a biomimetic approach to design artificial cilia lining the inner surfaces of microfluidic channels with the goal of propelling fluid. The artificial cilia consist of polymer films filled with superparamagnetic nanoparticles, which can mimic the motion of natural cilia when subjected to a rotating magnetic field. To obtain the magnetic field and associated magnetization local to the cilia, we solve the Maxwell equations, from which the magnetic body moments and forces can be deduced. To obtain the ciliary motion, we solve the dynamic equations of motion, which are then fully coupled to the Navier-Stokes equations that describe the fluid flow around the cilia, thus taking full account of fluid inertial forces. The dimensionless parameters that govern the deformation behavior of the cilia and the associated fluid flow are arrived at using the principle of virtual work. The physical response of the cilia and the fluid flow for different combinations of elastic, fluid viscous, and inertia forces are identified.

Magnetically-actuated artificial cilia for microfluidic propulsion.

In this paper we quantitatively analyse the performance of magnetically-driven artificial cilia for lab-on-a-chip applications. The artificial cilia are fabricated using thin polymer films with embedded magnetic nano-particles and their deformation is studied under different external magnetic fields and flows. A coupled magneto-mechanical solid-fluid model that accurately captures the interaction between the magnetic field, cilia and fluid is used to simulate the cilia motion. The elastic and magnetic properties of the cilia are obtained by fitting the results of the computational model to the experimental data. The performance of the artificial cilia with a non-uniform cross-section is characterised using the numerical model for two channel configurations that are of practical importance: an open-loop and a closed-loop channel. We predict that the flow and pressure head generated by the artificial cilia can be as high as 18 microlitres per minute and 3 mm of water, respectively. We also study the effect of metachronal waves on the flow generated and show that the fluid propelled increases drastically compared to synchronously beating cilia, and is unidirectional. This increase is significant even when the phase difference between adjacent cilia is small. The obtained results provide guidelines for the optimal design of magnetically-driven artificial cilia for microfluidic propulsion.

Formation of GaAs/AlGaAs and InGaAs/GaAs nanorings by droplet molecular-beam epitaxy

GaAs/AlGaAs lattice-matched nanorings are formed on GaAs (100) substrates by droplet epitaxy. The crucial step in the formation of nanorings is annealing Ga droplets under As flux for proper time. The observed morphologic evolution of Ga droplets during annealing does not support the hypothesis that As atoms preferentially react with Ga around the periphery of the droplets, but somehow relates to a dewetting process similar to that of unstable films. Photoluminescene (PL) test results confirm the quantum-confinement effect of these GaAs nanorings. Using similar methods, we have fabricated InGaAs/GaAs lattice-mismatched rings. (c) 2005 American Institute of Physics.

染料掺杂聚合物薄膜的激光行为研究

自20世纪60年代发展到现在，激光技术发展的速度十分惊人，应用的范围不断拓展，近年来随着有机/聚合物电致发光材料在有机发光二极管上的应用以及有机晶体管和有机太阳能电池的研制成功，科学家们开始了有机/聚合物材料放大自发发射和激光发射行为的研究。到目前为止，已经开发出了这种廉价、可以大面积成膜的、具有更广泛应用范围的有机/聚合物固体激光材料及光泵浦激光器。有机/聚合物激光器的出现不仅向传统激光理论提出了新的挑战，而且具有诸多潜在的应用价值。可以断言，在21世纪知识经济的大潮中，有机/聚合物激光器的研究必将推动传统学科的发展和新兴学科形成，也必将为人类带来巨大的经济效益。新的有机激光材料不断涌现、器件结构不断推陈出新、新的激发原理不断提出并得到修正已经成为有机/聚合物固体激光研究领域的三大特点。本论文进行了利用Förster能量传递对荧光染料DCJTB放大自发发射行为的优化、基于放大自发发射的红光染料DCJTB掺杂聚合物薄膜的白光发射、多孔结构对荧光染料放大自发发射行为的优化以及基于纳米结构的荧光染料DCJTB掺杂聚合物薄膜的激光行为等方面的研究工作，具体研究内容如下: 1、利用Förster能量传递理论，系统地研究了两种或三种染料共掺杂聚合物薄膜的放大自发发射（ASE）行为。研究表明，两种染料共掺杂显著改善了掺杂聚合物薄膜的ASE阈值、增益和损耗特性，而三种染料共掺杂，由于更多的Förster能量传递，使掺杂聚合物薄膜的ASE阈值、增益和损耗性能得到了进一步的改善。将Alq3和C545T两种绿光染料同时掺杂到DCJTB：PS中，通过利用Alq3和C545T同时的能量传递效应，已经使Alq3:C545T:DCJTB：PS薄膜的阈值、增益和损耗分别达到了0.007 mJ/pulse、52 cm-1和7 cm-1。 2、将红色荧光染料DCJTB掺杂到蓝色聚合物PFO中，通过控制DCJTB在PFO中的浓度，我们获得了具有放大自发发射的白光发射，当DCJTB在PFO的掺杂浓度为0.3%时显示了最好的白光ASE特性， 白光中DCJTB和PFO发射的阈值、增益和损耗分别达到了0.072 mJ pulse-1，0.035 mJ pulse-1；36.3 cm-1，22.35 cm-1和7.39 cm-1，15.88 cm-1。我们的结果表明，DCJTB掺杂聚合物PFO体系是实现ASE白光发射的有效方法，拓展了ASE的应用范围。 3、开发出了二维多孔SBA-15和三维TiO2反蛋白石光子晶体两种实现有机ASE有效发射的两种结构，通过利用有序结构的SBA-15的量子限域效应优化了蓝光染料C151的ASE阈值、增益和损耗特性，而通过利用三维TiO2反蛋白石光子晶体的量子限域效应，也使包埋其中的绿光染料C545T的ASE特性得到了明显改善。研究表明，无论是二维SBA-15多孔结构还是三维TiO2反蛋白石光子晶体结构，通过其量子限域效应都能很好地优化包埋其中的荧光染料的ASE特性，为进一步优化有机半导体的ASE特性提供了新的思路。 4、将聚苯乙烯纳米球分散到DCJTB:PS薄膜中和把DCJTB:PS薄膜旋涂在ZnO纳米柱阵列上两种方法，我们已经成功地研制出了多模随机有机激光发射器件。详细研究表明，多模随机有机激光发射特性显著地与聚苯乙烯纳米球的尺寸和浓度以及ZnO纳米柱的疏密程度密切相关，优化后的聚苯乙烯纳米球掺杂DCJTB:PS薄膜的阈值已经达到了0.06 mJ pulse-1cm-2，而ZnO纳米柱包埋DCJTB:PS薄膜的阈值达到了0.375 mJ pulse-1cm-2。我们的结果表明，聚苯乙烯纳米球和ZnO纳米柱都是实现随机有机激光的非常好的散射介质材料。