882 research outputs found
Small scale lateral superlattices in two-dimensional electron gases prepared by diblock copolymer masks
A poly(styrene-block-methylmethacrylate) diblock copolymer in the hexagonal
cylindrical phase has been used as a mask for preparing a periodic gate on top
of a Ga[Al]As-heterostructure. A superlattice period of 43 nm could be imposed
onto the two-dimensional electron gas. Transport measurements show a
characteristic positive magnetoresistance around zero magnetic field which we
interpret as a signature of electron motion guided by the superlattice
potential.Comment: 3 pages, 3 figure
Statistical mechanical description of liquid systems in electric field
We formulate the statistical mechanical description of liquid systems for
both polarizable and polar systems in an electric field in the
-ensemble, which is the pendant to the thermodynamic description in
terms of the free energy at constant potential. The contribution of the
electric field to the configurational integral in
the -ensemble is given in an exact form as a factor in the
integrand of . We calculate the contribution of the
electric field to the Ornstein-Zernike formula for the scattering function in
the -ensemble. As an application we determine the field induced
shift of the critical temperature for polarizable and polar liquids, and show
that the shift is upward for polarizable liquids and downward for polar
liquids.Comment: 6 page
Fluctuation effects in the theory of microphase separation of diblock copolymers in the presence of an electric field
We generalize the Fredrickson-Helfand theory of the microphase separation in
symmetric diblock copolymer melts by taking into account the influence of a
time-independent homogeneous electric field on the composition fluctuations
within the self-consistent Hartree approximation. We predict that electric
fields suppress composition fluctuations, and consequently weaken the
first-order transition. In the presence of an electric field the critical
temperature of the order-disorder transition is shifted towards its mean-field
value. The collective structure factor in the disordered phase becomes
anisotropic in the presence of the electric field. Fluctuational modulations of
the order parameter along the field direction are strongest suppressed. The
latter is in accordance with the parallel orientation of the lamellae in the
ordered state.Comment: 16 page
Surface induced orientation and vertically layered morphology in thin films of poly(3-hexylthiophene) crystallized from the melt
The presence of interfaces and geometrical confinement can have a strong influence on the structure and morphology of thin films of semicrystalline polymers. Using surface-sensitive grazing incidence wide angle X-ray scattering and atomic force microscopy to investigate the vertical structure of thin films of poly(3-hexylthiophene) crystallized from the melt, we show that highly oriented crystallites are induced at the air/polymer interface and not as sometimes assumed at the interface to the substrate. These crystallites are oriented with their crystallographic a-axis perpendicular to the plane of the film. While the corresponding orientation dominates in thinner films, for sufficiently thick films (>60 nm) a layer containing unoriented crystals is present below the surface layer. Due to the anisotropic charge transport properties, the observed effects are expected to be of special relevance for potential applications of semiconductor polymers in the field of organic photovoltaics for which vertical transport in thicker films plays an important role
Manipulating Semicrystalline Polymers in Confinement
Because final properties of nanoscale polymeric structures are largely
determined by the solid-state microstructure of the confined polymer, it is
imperative not only to understand how the microstructure of polymers develops
under nanoscale confinement but also to establish means to manipulate it. Here
we present a series of processing strategies, adapted from methods used in bulk
polymer processing, that allow to control the solidification of polymer
nanostructures. Firstly, we show that supramolecular nucleating agents can be
readily used to modify the crystallization kinetics of confined poly(vinylidene
fluoride) (PVDF). In addition, we demonstrate that microstructural features
that are not traditionally affected by nucleating agents, such as the
orientation of crystals, can be tuned with the crystallization temperature
applied. Interestingly, we also show that high crystallization temperatures and
long annealing periods induce the formation of the modification of PVDF, hence
enabling the simple production of ferro/piezoelectric nanostructures. We
anticipate that the approaches presented here can open up a plethora of new
possibilities for the processing of polymer-based nanostructures with tailored
properties and functionalities
The Controlled Synthesis of Carbon Tubes and Rods by Template-Assisted Twin Polymerization
The application of porous carbon is versatile. It is used for high-performance catalyst support, electrode material in batteries, and gas storage. In each of these application fields nanostructuring improves the material properties. Supercapacitors store a high energy density. Exactly adapted carbon structures increase the life of lithium batteries and protect catalysts with increasing reaction rate and selectivity. Most of porous carbon materials have a spherical shape. To the best of our knowledge, there is no procedure to synthesize nanostructured cylindrical porous carbon systematically. Here, template glass fibres and SiO2-tubes were modified with nanostructured SiO2/phenolic resin and SiO2/poly(furfuryl alcohol) layers by surface twin polymerization (TP) of 2,2′-spirobi[4H-1,3,2-benzodioxasiline] and tetrafurfuryloxysilane. Afterwards the SiO2/polymer layer on the template is thermally transformed into a defect-free nanostructured SiO2/carbon layer. After completely removing the SiO2 components microporous carbon tubes or rods are finally achieved. The diameters of the carbon rods and the inner as well as the outer diameter of the carbon tubes are adjustable according to the shape and size of the template. Thus, a huge variety of microporous carbon materials can be easily produced by template-assisted TP
Coherent X-ray Scattering from Manganite Charge and Orbital Domains
We report coherent x-ray scattering studies of charge and orbital domains in
manganite systems. The experiments were carried out on LaMnO_3 and
Pr_{0.6}Ca_{0.4}MnO_3, with the incident photon energy tuned near the Mn K
edge. At room temperature, the orbital speckle pattern of LaMnO_3 was observed
to be constant over a timescale of at least minutes, which is indicative of
static orbital domains on this timescale. For Pr_{0.6}Ca_{0.4}MnO_3, both
charge and orbital speckle patterns were observed. The observation of the
latter rules out the presence of fast orbital fluctuations, while long time
series data-- on the order of several minutes-- were suggestive of slow dynamic
behavior. In contrast, the charge order speckle patterns were static.Comment: 6 pages, 4 figure
Sacrificial-Post Templating Method for Block Copolymer Self-Assembly
A sacrificial-post templating method is presented for directing block copolymer self-assembly to form nanostructures consisting of monolayers and bilayers of microdomains. In this approach, the topographical post template is removed after self-assembly and therefore is not incorporated into the final microdomain pattern. Arrays of nanoscale holes of different shapes and symmetries, including mesh structures and perforated lamellae with a bimodal pore size distribution, are produced. The ratio of the pore sizes in the bimodal distributions can be varied via the template pitch, and agrees with predictions of self consistent field theory.Semiconductor Research CorporationFENA CenterSemiconductor Research Corporation. Nanoscale Research InitiativeSingapore-MIT AllianceNational Science Foundation (U.S.)Tokyo Electron LimitedTaiwan Semicondcutor Manufacturing Compan
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