High Vacuum Techniques Research Articles

Annealing-dependent growth and nonlinear electrical properties of fractal Ge nanojoints based on Pd matrix

The Pd/Ge bilayer films with interesting fractal Ge nanojoints were successfully prepared by high-vacuum thermal evaporation techniques. It was found that Ge nanojoints embedded in Pd matrix showed fascinating fractal morphologies with average size of fractal clusters at 3.5μm and fractal dimension at 1.77 when the films were annealed at 550°C for 15min. The fractal clusters formed Ge nanojoints with average size at 80nm in width and 450nm in length. The annealing-dependent growth model responsible for the fractal Ge nanojoints is discussed reasonably. The electrical measurements confirmed that the annealed Pd/Ge film with interesting fractal clusters showed a nonlinear voltage–current behavior.

Investigations of Structure Sensitivity in Heterogeneous Catalysis: From Single Crystals to Monodisperse Nanoparticles

Metal surface structure is often a crucial component in determining the activity and selectivity of heterogeneous catalytic reactions. Many important industrial reactions, such as ammonia synthesis, catalytic combustion, Fischer–Tropsch synthesis, and hydrocarbon reforming have been labeled as structure-sensitive. Metal single crystal studies utilizing ultra high vacuum techniques have repeatedly shown the importance of surface structure in reaction kinetics. Recent advances in the field of colloidal synthesis allow for fine control of the size and shape of metal nanoparticles, which permits catalytic studies of structure sensitivity to be performed on nanometer sized catalysts. It is clear that in order to optimize the performance of a catalyst, a complete molecular level understanding of the role of surface structure in the reaction of interest is essential. This article aims to review the importance of surface structure in heterogeneous catalysts, ranging from single crystals to size and shape controlled nanocatalysts.

Micellization behavior of model asymmetric miktoarm star copolymers of the AA′B type, where A is polyisoprene and B is polystyrene

Model asymmetric miktoarm star copolymers of the type AA′B, where A and A′ are polyisoprenes (PIs) and B is deuterated polystyrene (PS), were synthesized by anionic polymerization high-vacuum techniques. Their micellization behavior was studied in n-decane, a selective solvent for the PI blocks, and N,N-dimethylacetamide (DMA), selective for the PS blocks. Utilizing static and dynamic light-scattering techniques along with dilute solution viscometry, parameters such as the aggregation number, Nw, the hydrodynamic and viscometric radii were determined. Based on these results, structural parameters of the micelles, that is, core and corona radii, as well as core-corona interfacial area, were calculated. The thermal stability of the micelles was also examined in both selective solvents. The macromolecular architecture was found to have a considerable effect on the micellization behavior of the block copolymers. Static light-scattering techniques were utilized to study the micellization behavior of model asymmetric miktoarm star copolymers of the type AA′B, where A and A′ are polyisoprenes (PIs) and B is deuterated polystyrene (PS). In n-decane, a selective solvent for the PI blocks the comparison between linear PS–PI diblocks, symmetric PS(PI)2 and asymmetric PS(PI)(PI′) miktoarm stars, and confirms that the aggregation numbers (Nw) and, in general, the micellar properties can be manipulated at will, and that tailor-made micelles can be constructed changing the macromolecular architecture.

Formation and Third-Order Optical Nonlinearities of Fractal Ge Nanocrystals Embedded in Au Matrix

Nonequilibrium growth processes of low-dimensional materials have attracted considerable attention, which controlled micro/nanostructures, properties, and various performances of materials. Semiconductor Ge in contact with some metals, e.g., Au, Pd, and Al, etc., is a class of distinctive materials with noninteger dimensions that differ from integer dimensional materials such as nanoparticles, nanowires, nanorods, nanotubes, nanoribbons, and thin films. Here, Au/Ge bilayer films with interesting fractal Ge nanocrystals were successfully prepared by high-vacuum thermal evaporation techniques. It was found that Ge nanocrystals embedded in Au matrix showed fascinating fractal morphologies with average size of fractal clusters at 550 nm and fractal dimension at 1.756 when the films were annealed at 150 °C for 30 min. Third-order optical nonlinearities of the annealed Au/Ge bilayer films were investigated in detail by Z-scan technique using a femtosecond laser. Experimental results indicated that the nonlinear...

Effect of dilution gas on hardness of hydrogenated amorphous carbon-based films synthesized by atmospheric pressure plasma enhanced CVD

The deposition of hydrogenated amorphous carbon (a-C:H)-based films at atmospheric pressure is a cost-effective process without using a high-vacuum technique. In this study, we synthesized a-C:H-based films by atmospheric pressure-plasma enhanced CVD method from C2H2/He and C2H2/N2 mixture gas and investigated the effect of dilution gas on the hardness of the films. The film deposited from C2H2/N2 contained nitrogen elements with forming C-N bonds and the film was softer than a-C:H film deposited from C2H2/He. The hardness of the film deposited from C2H2/He was 1.1 GPa and twice higher than that from C2H2/N2.

Synthesis, characterization (molecular–morphological) and theoretical morphology predictions of linear triblock terpolymers containing poly(cyclohexadiene)

The synthesis via anionic polymerization of six linear triblock terpolymers with various sequences of blocks such as PS (polystyrene), PB [poly(butadiene)], PI [(poly(isoprene)] and PCHD [poly(1,3-cyclohexadiene)] is reported. The synthesis of the terpolymers was carried out by the use of anionic polymerization with high-vacuum techniques and sequential monomer addition. Molecular characterization of the samples was performed via size exclusion chromatography (SEC) and membrane osmometry (MO) to measure the polydispersity indices and the number-average molecular weights, respectively. Proton nuclear magnetic resonance spectroscopy (1H NMR) was employed to verify the microstructure type for the polydienes, as well as, to calculate the molar composition. Structural characterization of the terpolymers was performed via transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS), and several morphologies were observed including one which has not been reported previously. Real-space self-consistent field theory (SCFT) without a priori knowledge about the symmetry of the periodic structures was used to elucidate the thermodynamics of the synthesized triblock copolymers.

Synthesis and characterization of an exact comb polyisoprene with three branches having the middle branch twice the molecular weight of the other two identical external branches

An exact comb polyisoprene (PI) with three branches, with the middle branch having twice the molecular weight of the two other identical external branches, was synthesized by using anionic polymerization high vacuum techniques and appropriate chlorosilane chemistry. The synthetic approach involves (a) the selective replacement of the two chlorines of 4-(dichloromethylsilyl)diphenylethylene (DCMSDPE, key molecule) with identical PI chains by titration with PILi, (b) the addition of sec-BuLi to the double bond of DPE followed by the polymerization of isoprene from the newly created anionic site to form a 3-arm living star PI, (c) the selective replacement of the two chlorines of trichloromethylsilane with 3-arm star PI to form an H-shape intermediate, and (d) the replacement of the remaining chlorine of trichloromethylsilane by linear PI chains with double the molecular weight. All intermediate and final products were characterized via size exclusion chromatography, temperature gradient interaction chromatography and 1H-NMR spectroscopy. As expected, due to the inability to control the exact stoichiometry of the linking reactants, the main product (exact comb PI) is contaminated by a few by-products, despite the fact that anionic polymerization is the most efficient way to produce well-defined polymers.

Influence of (1,3‐phenylene)bis(3‐methyl‐1‐phenyl pentylidene)dilithium initiator concentration on the modality of polybutadiene

AbstractThe strong influence of (1,3‐phenylene)bis(3‐methyl‐1‐phenyl pentylidene)dilithium initiator (DLI) concentration on the modality of polybutadiene (PBd) in the presence of lithium s‐butoxide (s‐BuOLi) in benzene, at room temperature, has been studied. The quality of DLI has been evaluated by gas chromatography‐mass spectrometry (GC‐MS) and 1H NMR. Keeping s‐BuOLi/C‐Li ratio (R) close to unity, at relatively high DLI concentrations (C > 7 × 10−4 M), monomodal high 1,4‐PBds with polydispersity index less than 1.07 were obtained, whereas bimodal ones at lower concentrations (C < 6 × 10−4 M). The effect of C‐Li concentration on the modality of PBd has been evaluated using size exclusion chromatography on samples taken during and at the end of the polymerization. Viscosity observations have also been used to further support the results. The bimodality of PBd has been attributed to partially terminated difunctional species because of the inevitable presence of protic impurities in the polymerization solution, although high vacuum technique was used, which becomes more significant at low initiator concentrations. Moreover, the strong influence of s‐BuOLi on the microstructure of PBd has been demonstrated by 1H NMR. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Morphologies and Nonlinear Optical Properties of Fractal Ge Nanocrystals Embedded in Pd Matrix

An integrated device or component for the semiconductor industry is highly desirable for versatile advanced applications. Group IV semiconductors such as germanium (Ge) and silicon (Si) are unique materials of widespread technological applications, particularly in the field of microelectronic devices and optoelectronic components. Herein, Pd/Ge bilayer films with interesting Ge clusters, which have noninteger dimensions and are called fractals, were successfully prepared by high-vacuum thermal evaporation techniques. It was found that Ge nanocrystals embedded in Pd matrix showed fascinating fractal morphologies with average size of fractal clusters at 680 nm and fractal dimension at 1.708 when the films were annealed at 350 °C for 30 min. The fractal clusters consisted of Ge nanocrystals with diameters ranging from 25 to 55 nm. Third-order optical nonlinearities of the annealed Pd/Ge bilayer films were investigated in detail by Z-scan techniques using a femtosecond laser. Experimental results indicated that the nonlinear absorption coefficient and refractive index of the fractal Ge nanocrystals embedded in Pd matrix were in the ranges from 3.4 to 4.3 × 10–7 cm/W and from 4.2 to 5.0 × 10–12 cm2/W, respectively, when the input irradiance (Ip) ranged from 0.58 to 1.65 GW/cm2. This nonlinear optical material may be tailor-made for a large number of applications such as high-speed microelectronics and infrared optical micro/nanodevices.

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

The effect of a SiC cap on the growth of epitaxial graphene on SiC in ultra high vacuum

Thin and homogeneous graphenes with excellent thickness uniformity were produced on the carbon-rich surface of a SiC crystal using an ultra high vacuum technique. The sample surface was capped by another SiC substrate with a silicon-rich face to form a shallow cavity between them. During the graphene growth by high temperature annealing, silicon atoms sublimated from the capped sample were trapped inside the cavity between the two substrates. The confined vapor phase silicon maintains a relatively high partial pressure at the sample surface which significantly reduces the extremely high growth rate of epitaxial graphene to an easily controllable range. The structure and morphology of the graphene samples grown with this capping method are characterized by low energy electron diffraction and Raman spectroscopy and the results are compared with those of layers grown on an uncapped sample surface. The results show that capping yields much thinner graphene with excellent uniformity.

Nanohybrids based on polymeric ionic liquid prepared from functionalized MWCNTs by modification of anionically synthesized poly(4-vinylpyridine)

We report the synthesis of a composite material comprised of poly(4-vinylpyridine) (P4VP) grafted on multiwall carbon nanotubes (MWCNTs) and the preparation of a nanohybrid via quaternization of the nitrogen atom per monomeric unit of the polymer chains. 4-Vinylpyridine was polymerized anionically using high vacuum techniques and was reacted with MWCNTs under vacuum to be grafted on the polymer segments. The composite material was soluble in common solvents and the dispersion of the carbon nanotubes was improved after quaternization due to the formation of polymeric ionic liquid (PIL) of the MWCNTs-g-[P4VP-r-poly(4ViEtPy+Br-)] type. The successful synthesis was confirmed with Fourier-transform infrared and Raman spectroscopies, whereas differential scanning calorimetry was adopted to verify the stability of the polymer's glass transition temperature before and after grafting on the MWCNTs. Moreover, thermogravimetric analysis was used for examining the thermal stability and the PIL formation of the composite. Energy dispersive spectroscopy measurements confirm the precipitation of silver bromide when the MWCNTs-g-[P4VP-r-poly(4ViEtPy+Br-)] is reacted with silver nitrite indicating the successful quaternization and formation of the appropriate PIL. High temperature size exclusion chromatography was used for the determination of the molecular characteristics (average molecular weight by number , polydispersity I) of the homopolymer obtained from the filtration of the composite material. Finally, field-emission scanning electron microscopy was used to verify the successful grafting of the polymer to the MWCNTs. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

High-Vacuum Evaporation of n-CuIn3Se5 Photoabsorber Films for Hybrid PV Structures

Thin films of Cu-In-Se (CISe) photoabsorber with an overall composition of CuIn3Se5 were deposited onto glass/indium tin oxide (ITO) substrates from a polycrystalline bulk CuIn3Se5 source using the high-vacuum evaporation technique. Thermal conditions for the substrates during the evaporation process and the subsequent annealing in vacuum were selected to prepare polycrystalline n-CuIn3Se5 photoabsorber layers for use in hybrid photovoltaic structures based on an inorganic photoabsorber and conductive polymer functional layers. The CISe layers were deposited at a substrate temperature of 200°C and were annealed at temperatures from 300°C to 500°C in vacuum. Part of the as-deposited CISe was annealed twice, in argon and in vacuum at 500°C. These layers exhibited high photosensitivity and photoconductivity when illuminated with white light at an intensity of 100 mW/cm2. The results showed that the chalcopyrite structure of the prepared CISe photoabsorber films adhered well to the glass/ITO substrate. The average value of charge carrier concentration and the profile of charge carrier concentration in the annealed CISe photoabsorber layer were calculated using impedance spectroscopy.

Synthesis and characterization of poly[(4-vinylphenyl)dimethyl- silane]-b-polybutadiene-b-poly[(4-vinylphenyl)dimethylsilane] (PVPDMS-b-PBd-b-PVPDMS)

Abstract The novel functionalized triblock copolymers, poly[(4-vinylphenyl)dimethylsilane]-b-polybutadiene-b-poly[(4-vinylphenyl)- dimethylsilane] (PVPDMS-b-PBd-b-PVPDMS), were synthesized by anionic polymerization method using high vacuum technique. The hydrocarbon-soluble dilithium initiator synthesized from 1,3-di[1-(methylphenyl)ethenyl]benzene (MPEB) was used to synthesize polybutadiene (PBd) precursors and the triblock copolymers in the presence of sec-BuOLi. The precursors and copolymers were characterized by size exclusion chromatography (SEC), 1HNMR and DSC techniques.

Poly[(sodium sulfamate/carboxylate)isoprene‐b‐2‐vinyl pyridine] block polyampholytes: Synthesis and self‐assembly in aqueous media

AbstractWe report on the synthesis of poly[(sodium sulfamate/carboxylate) isoprene‐b‐2‐vinyl pyridine] block polyampholytes (SCPI‐P2VP), utilizing anionic polymerization and post polymerization functionalization reactions. The precursor poly(isoprene‐b‐(2‐vinylpyridine)) diblock copolymers (PI‐P2VP), containing a polyisoprene (PI) block with high 1,4 microstructure, were prepared by anionic polymerization high vacuum techniques, in two steps, involving change of the polymerization solvent. Subsequent functionalization of the PI block with chlorosulfonyl isocyanate, introduced sulfamate and carboxylate groups in the polymer chains and produced the desired block polyampholytes. The successful synthesis of the polyampolytes was corroborated by elemental analysis and IR spectroscopy measurements. The self‐assembly behaviour of the aforementioned polyampholytes was studied in aqueous solutions as a function of pH, by aid of dynamic and static light scattering, zeta potential, fluorescence spectroscopy and atomic force microscopy. Experimental results indicate that the block polyampholytes form micellar structures with P2VP cores and SCPI coronas at pH > 6, whereas more compact nanoparticles are formed at pH < 4 from the complexation of positively charged P2VP and SCPI, stabilized by excess negative charges of uncomplexed SCI segments. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Controllable Growth and Unexpected Effects of Ge Nanocrystals

Semiconductor nanostructured Ge is a unique material of widespread technological applications, particularly in the field of microelectronics. New strategies for controllable growth and an understanding of the effects of Ge nanocrystals inducted by Au nanoparticles in Au/Ge bilayer films are of fundamental importance in the development of micro/nanodevices. After an extensive search in the published literature, it is found that the previous nanostructures are one-dimensional nanomaterials such as nanowires, nanorods, nanobelts/nanoribbons, and nanotubes, two-dimensional nanoscale thin films, or zero-dimensional nanoparticles, which all have integer dimensions. Herein, the noninteger dimensional Ge nanostructures, which are called nanofractals, were successfully assembled by high-vacuum thermal evaporation techniques. A widely applicable annealing route to produce Ge nanocrystals in the form of nanoparticles, nanorings, and nanofractals is presented in detail. To our surprise, we found that a Au/Ge bilayer film with an interesting nanofractal showed a nonlinear voltage–current behavior. This result suggested that the Au/Ge bilayer film may be a promising material to facilitate future design improvement of micro/nanodevices for microelectronic applications.

Polycondensation-type Ge nanofractal assembly

The group IV semiconductors such as silicon (Si) and germanium (Ge) are unique materials with a wide range of technological applications. A versatile integrated device for the semiconductor industry is highly desirable for advanced applications. Notwithstanding the widespread application of Ge its use is not as extensive as that of Si, and nebulous domains in our understanding of its precise technical functions still remain. Previous nanostructures have either been one-dimensional nanomaterials such as nanowires, nanorods, nanobelts/nanoribbons, nanotubes, two-dimensional nanoscale thin films, or zero-dimensional nanoparticles, which all have integer dimensions. Herein, the non-integer dimensional Ge nanostructures, referred to as nanofractals, were successfully assembled by high-vacuum thermal evaporation techniques. We have found that the thermodynamically driven assemblies of Ge nanocrystals possess amazing nanostructures such as polycondensation-type Ge nanofractals with non-integer dimensions, thick branches and smooth edges, metastable gamma-Au0.6Ge0.4 nanocrystals, and a variety of interesting micro/nanometer-sized features. The results of computer simulations using a ripening mechanism of non-uniform grains agree very well with the patterns formed in experiments.

Hydrogenation of N over Fe{111}

Over the past five decades, ultra high vacuum (uhv) techniques applied to well-defined single-crystal samples (the “surface science paradigm”) have transformed our understanding of fundamental surface chemistry. To translate this success to the world of realistic heterogeneous catalysis, however, requires one seriously to address the fact that real heterogeneous catalysts usually operate under near-ambient or higher pressures. Nevertheless, the surface science paradigm can undoubtedly provide crucial insights into catalytic processes, so long as care is exercised in the design of experiments. Forging a secure link between two radically different pressure regimes is the major challenge, which we illustrate here with reference to the vitally important ammonia synthesis reaction, achieved industrially only under extremely high pressure.

A New Approach to the Living Anionic Polymerization of 4-Cyanostyrene

A new approach to the synthesis of well-defined homopolymers of 4-cyanostyrene (4CNS) via anionic polymerization using high vacuum techniques is described. This synthetic route is based on the use of a solvent mixture of tetrahydrofuran and N,N-dimethylacetamide (THF/DMA, 4/1 v/v) in order to overcome the previously reported polymer insolubility issue and long reaction times. Our system afforded well-defined linear living P4CNSs having molecular weights up to 98,500 g/mol in less than 30 min and can be applied to the synthesis of P4CNS-based homopolymers and copolymers of various molecular architectures.

Synthesis of Monodispersed Tantalum(V) oxide Nanospheres by an Ethylene Glycol Mediated Route

Tantalum(V) oxide (Ta2O5) nanospheres have been synthesized by a very simple two-step process mediated via the formation of glycolate nanoparticles and their subsequent hydrolysis at room temperature in an acetone−water mixture and calcination at 700 °C. The synthesized particles were characterized by XRD, TEM, SEM, and DLS for their phase, morphology, and particle size distribution. The particle sizes could be tailored from 50 to 500 nm or more by changing the reaction conditions. The larger particles are actually constituted by crystallites of around 15−16 nm that could be seen from the X-ray diffraction pattern as well as the TEM micrographs of the calcined samples. The particles obtained are highly dispersed under appropriate reaction conditions, and the process does not require any specific equipment or high vacuum technique for the production of the nanoparticles.