Showing papers by "Bobby G. Sumpter published in 2013"
TL;DR: In this paper, the authors studied ordered polymorphs of Nb2O5 and detail the mechanism for the intrinsic high rates and energy density observed for this class of materials, finding that the intercalating atom (lithium) forms a solid solution adsorbing at specific sites in a network of quasi-2D NbOx faces with x = {1.3, 1.67, or 2}.
Abstract: Pseudocapacitors aim to maintain the high power density of supercapacitors while increasing the energy density towards those of energy dense storage systems such as lithium ion batteries. Recently discovered intercalation pseudocapacitors (e.g. Nb2O5) are particularly interesting because their performance is seemingly not limited by surface reactions or structures, but instead determined by the bulk crystalline structure of the material. We study ordered polymorphs of Nb2O5 and detail the mechanism for the intrinsic high rates and energy density observed for this class of materials. We find that the intercalating atom (lithium) forms a solid solution adsorbing at specific sites in a network of quasi-2D NbOx faces with x = {1.3, 1.67, or 2}, donating electrons locally to its neighboring atoms, reducing niobium. Open channels in the structure have low diffusion barriers for ions to migrate between these sites (Eb ∼ 0.28–0.44 eV) comparable to high-performance solid electrolytes. Using a combination of complementary theoretical methods we rationalize this effect in LixNb2O5 for a wide range of compositions (x) and at finite temperatures. Multiple adsorption sites per unit-cell with similar adsorption energies and local charge transfer result in high capacity and energy density, while the interconnected open channels lead to low cost diffusion pathways between these sites, resulting in high power density. The nano-porous structure exhibiting local chemistry in a crystalline framework is the origin of high-rate pseudocapacitance in this new class of intercalation pseudocapacitor materials. This new insight provides guidance for improving the performance of this family of materials.
134 citations
TL;DR: The epitaxial growth and preferred molecular orientation of copper phthalocyanine molecules on graphene has been systematically investigated and compared with growth on Si substrates, demonstrating the role of surface-mediated interactions in determining molecular orientation.
Abstract: The epitaxial growth and preferred molecular orientation of copper phthalocyanine (CuPc) molecules on graphene has been systematically investigated and compared with growth on Si substrates, demonstrating the role of surface-mediated interactions in determining molecular orientation. X-ray scattering and diffraction, scanning tunneling microscopy, scanning electron microscopy, and first-principles theoretical calculations were used to show that the nucleation, orientation, and packing of CuPc molecules on films of graphene are fundamentally different compared to those grown on Si substrates. Interfacial dipole interactions induced by charge transfer between CuPc molecules and graphene are shown to epitaxially align the CuPc molecules in a face-on orientation in a series of ordered superstructures. At high temperatures, CuPc molecules lie flat with respect to the graphene substrate to form strip-like CuPc crystals with micrometer sizes containing monocrystalline grains. Such large epitaxial crystals may po...
133 citations
TL;DR: This work demonstrates that the rigid polymer chains have higher averaged Voronoi polyhedral volumes and significantly wider distribution of the volume due to frustration in the chain packing and discusses the advantage of the rigid polymers for possible enhancement of transport properties, e.g. for enhancing ionic conductivity in solid polymer electrolytes.
Abstract: Frustration in chain packing has been proposed to play an important role in thermodynamic and dynamic properties of polymeric melts and glasses. Based on a quantitative analysis using Voronoi tessellations and large scale molecular dynamics simulations of flexible and semi-flexible polymers, we demonstrate that the rigid polymer chains have higher averaged Voronoi polyhedral volumes and significantly wider distribution of the volume due to frustration in the chain packing. Using these results, we discuss the advantage of the rigid polymers for possible enhancement of transport properties, e.g. for enhancing ionic conductivity in solid polymer electrolytes.
60 citations
TL;DR: A key finding of this study is that, in contrast to a previous report, the annealing process does not converge at the short time scales reported and it is found that the self-assembly of the blends is characterized by three rate dependent stages that require much longer simulations to approach convergence.
Abstract: Organic photovoltaics (OPVs) are a topic of extensive research because of their potential application in solar cells. Recent work has led to the development of a coarse-grained model for studying poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends using molecular simulations. Here we provide further validation of the force field and use it to study the thermal annealing process of P3HT:PCBM blends. A key finding of our study is that, in contrast to a previous report, the annealing process does not converge at the short time scales reported. Rather, we find that the self-assembly of the blends is characterized by three rate dependent stages that require much longer simulations to approach convergence. Using state-of-the-art high performance computing, we are able to study annealing at length and time scales commensurate with devices used in experiments. Our simulations show different phase segregated morphologies dependent on the P3HT chain length and PCBM volume fraction in the blend. For short chain lengths, we observed a smectic morphology containing alternate P3HT and PCBM domains. In contrast, a phase segregated morphology containing domains of P3HT and PCBM distributed randomly in space is found for longer chain lengths. Theoretical arguments justifying stabilization of these morphologies due to shape anisotropy of P3HT (rod-like) and PCBM (sphere-like) are presented. Furthermore, results on the structure factor, miscibility of P3HT and PCBM, domain spacing and kinetics of phase segregation in the blends are presented in detail. Qualitative comparison of these results with published small-angle neutron scattering experiments in the literature is presented and an excellent agreement is found.
58 citations
TL;DR: A facile synthetic strategy based on a grafting through approach to prepare well-defined molecular bottlebrushes composed of regioregular poly(3-hexylthiophene) (rr-P3HT) as the conjugated polymeric side chain is reported.
Abstract: We report a facile synthetic strategy based on a grafting through approach to prepare well-defined molecular bottlebrushes composed of regioregular poly(3-hexylthiophene) (rr-P3HT) as the conjugated polymeric side chain. To this end, the exo-norbornenyl-functionalized P3HT macromonomer was synthesized by Kumada catalyst transfer polycondensation (KCTP) followed by postpolymerization modifications, and the resulting conjugated macromonomer was successfully polymerized by ring-opening metathesis polymerization (ROMP) in a controlled manner. The P3HT molecular bottlebrushes display an unprecedented strong physical aggregation upon drying during recovery, as verified by several analyses of the solution and solid states. This remarkably strong aggregation behavior is attributed to a significant enhancement in the number of π–π interactions between grafted P3HT side chains, brought about due to the bottlebrush architecture. This behavior is qualitatively supported by coarse-grained molecular dynamics simulations.
48 citations
TL;DR: A set of boron-related parameters for commonly used empirical force fields is determined to describe the nonbonded and bonded interactions between linear boronic acid linkers and graphene sheets of GOF materials, showing significant variations in structural flexibility depending on the linker density.
Abstract: We use quantum mechanical calculations to develop a full set of force field parameters in order to perform molecular dynamics simulations to understand and optimize the molecular storage properties inside graphene oxide frameworks (GOFs). A set of boron-related parameters for commonly used empirical force fields is determined to describe the nonbonded and bonded interactions between linear boronic acid linkers and graphene sheets of GOF materials. The transferability of the parameters is discussed and their validity is quantified by comparing quantum mechanical and molecular mechanical structural and vibrational properties. The application of the model to the dynamics of water inside the GOFs reveals significant variations in structural flexibility depending on the linker density, which is shown to be usable as a tuning parameter for desired diffusion properties.
35 citations
TL;DR: The work reveals that the triblock terpolymer melts containing a high percentage of 1,2-microstructures in the PCHD block are disordered at 110 °C for all the samples, independent of sequence and volume fraction of the blocks.
Abstract: We have synthesized linear ABC triblock terpolymers containing poly(1,3-cyclohexadiene), PCHD, as an end block and characterized their morphologies in the melt. Specifically, we have studied terpolymers containing polystyrene (PS), polybutadiene (PB), and polyisoprene (PI) as the other blocks. Systematically varying the ratio of 1,2- /1,4-microstructures of poly(1,3-cyclohexadiene), we have studied the effects of conformational asymmetry among the three blocks on the morphologies using transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and self-consistent field theory (SCFT) performed with PolySwift++. Our work reveals that the triblock terpolymer melts containing a high percentage of 1,2-microstructures in the PCHD block are disordered at 110 °C for all the samples, independent of sequence and volume fraction of the blocks. In contrast, the triblock terpolymer melts containing a high percentage of 1,4-microstructure form regular morphologies known from the literature. The accuracy of the SCFT calculations depends on calculating the χ parameters that quantify the repulsive interactions between different monomers. Simulations using χ values obtained from solubility parameters and group contribution methods are unable to reproduce the morphologies as seen in the experiments. However, SCFT calculations accounting for the enhancement of the χ parameter with an increase in the conformational asymmetry lead to an excellent agreement between theory and experiments. These results highlight the importance of conformational asymmetry in tuning the χ parameter and, in turn, morphologies in block copolymers.
31 citations
TL;DR: In this article, the impact of block connectivity on the morphologies of four block copolymers of varying architecture containing polystyrene (PS) and polyisoprene (PI) has been studied.
Abstract: The impact of block connectivity on the morphologies of four block copolymers of varying architecture containing polystyrene (PS) and polyisoprene (PI) has been studied. The volume fraction of PS and molecular weight are held constant while varying the architecture from a linear PS–PI diblock copolymer to three different miktoarm star architectures: PS2PI, PSPI2, and PS2PI2. Morphologies of the PS2PI and PSPI2 miktoarm stars are different from those observed for the linear copolymer and dependent on the connectivity of the copolymer blocks. The change in morphology with connectivity indicates that combining two chains at a junction point leads to chain crowding, where subsequent excluded volume effects drive the change in morphology for each sample. The PS2PI2 miktoarm star exhibits the same morphology as the linear diblock but with a reduction in the size of the domains. The extent of the decrease in domain size indicates that chain stretching impacts the formation of this morphology. Experimentally obse...
31 citations
TL;DR: In this paper, the Penn State Center for Nanoscale Science (MRSEC; NSF grant number DMR-0820404), for a seed grant on defect engineering in layered materials, is grateful to the Japanese regional Innovation Strategy Program by the Excellence.
Abstract: M.T. thanks JST-Japan for funding the Research Center for Exotic NanoCarbons, under the Japanese regional Innovation Strategy Program by the Excellence. M.T. is grateful to the Penn State Center for Nanoscale Science (MRSEC; NSF grant number DMR-0820404), for a seed grant on “Defect Engineering in Layered Materials”. H.T. acknowledges support of CAPES, Brazil, through its Foreign Scientist Invited program. F.J.R.M., F.L.U., and E.M.S. acknowledge CONACYT (Mexico) grants CB-2008-SEP-107082, 60218-F1 and 48300 S-3907, respectively. X.J. and M.S.D. acknowledge the MURI grant ONR-N00014-09-1-1063. R.M.G. was supported by MCINN, project number FIS2009-12721-C04-01 and scholarship AGAUR “FI-DGR 2011”. This work was supported by CONACYT Ph.D. scholarships 223807 (J.O.M.) and 223824 (M.L.G.B.), as well as financial research support from PSU. J.O.M. thanks complementary support from the Graduate Complementary Scholarship program (DGRI-SEP, Mexico). B.G.S. was supported by the Center for Nanophase Materials Sciences (CNMS), sponsored at Oak Ridge National Laboratory by the Division of Scientifi c User Facilities, U.S. Department of Energy.
30 citations
TL;DR: Local interactions at the open edges are found to dictate stacking configurations that are different from Bernal (AB) stacking that are known to be otherwise absent in edge-free two-dimensional graphene.
Abstract: High-resolution transmission electron microscopy studies show the dynamics of small graphene platelets on larger graphene layers. The platelets move nearly freely to eventually lock in at well-defined positions close to the edges of the larger underlying graphene sheet. While such movement is driven by a shallow potential energy surface described by an interplane interaction, the lock-in position occurs via edge-edge interactions of the platelet and the graphene surface located underneath. Here, we quantitatively study this behavior using van der Waals density functional calculations. Local interactions at the open edges are found to dictate stacking configurations that are different from Bernal (AB) stacking. These stacking configurations are known to be otherwise absent in edge-free two-dimensional graphene. The results explain the experimentally observed platelet dynamics and provide a detailed account of the new electronic properties of these combined systems.
30 citations
TL;DR: The QTES-DFTB dynamics approach is employed to study the role of quantum nuclear effects on the interaction of hydrogen with a model graphene sheet, revealing that neglect of nuclear effects can lead to an overestimation of adsorption.
Abstract: A massively parallel, direct quantum molecular dynamics method is described. The method combines a quantum trajectory (QT) representation of the nuclear wave function discretized into an ensemble of trajectories with an electronic structure (ES) description of electrons, namely using the density functional tight binding (DFTB) theory. Quantum nuclear effects are included into the dynamics of the nuclei via quantum corrections to the classical forces. To reduce computational cost and increase numerical accuracy, the quantum corrections to dynamics resulting from localization of the nuclear wave function are computed approximately and included into selected degrees of freedom representing light particles where the quantum effects are expected to be the most pronounced. A massively parallel implementation, based on the message passing interface allows for efficient simulations of ensembles of thousands of trajectories at once. The QTES-DFTB dynamics approach is employed to study the role of quantum nuclear effects on the interaction of hydrogen with a model graphene sheet, revealing that neglect of nuclear effects can lead to an overestimation of adsorption.
TL;DR: In this paper, the authors identify pristine zigzag silicon carbide nanoribbons (zSiC-NRs) with bare edges as a metal-free monolayered material that exhibits intrinsic half-metallic behavior without chemical doping or an external electric field.
Abstract: Half-metallic nanoscale conductors, highly sought after for spintronic applications, are usually realized through metal elements, chemical doping, or external electric fields. By means of local and hybrid density functional theory calculations, we identify pristine zigzag silicon carbide nanoribbons (zSiC-NRs) with bare edges as a metal-free monolayered material that exhibits intrinsic half-metallic behavior without chemical doping or an external electric field. Ab initio molecular dynamics simulations indicate that the half-metallicity is robust at room temperature. We also demonstrate that edge termination with O and S atoms transforms the zSiC-NRs into a full metal or a semiconducting material, respectively, due to the presence of O dimerization only on the Si edge and of S trimerization on both Si and C edges, the latter being driven by an unusual Peierls-like distortion along the functionalizing S atoms. The rich electronic properties displayed by zSiC-NRs may open new perspectives for spintronic app...
TL;DR: A controllable surface-coordinated linear polymerization of long-chain poly(phenylacetylenyl)s that are self-organized into a “circuit-board” pattern on a Cu(100) surface is demonstrated, demonstrating the possibility of precisely accessing and controlling conjugated chain-growth polymerization at low temperature.
Abstract: We demonstrate a controllable surface-coordinated linear polymerization of long-chain poly(phenylacetylenyl)s that are self-organized into a “circuit-board” pattern on a Cu(100) surface. Scanning tunneling microscopy/spectroscopy (STM/S) corroborated by ab initio calculations, reveals the atomistic details of the molecular structure, and provides a clear signature of electronic and vibrational properties of the poly(phenylacetylene)s chains. Notably, the polymerization reaction is confined epitaxially to the copper lattice, despite a large strain along the polymerized chain that subsequently renders it metallic. Polymerization and depolymerization reactions can be controlled locally at the nanoscale by using a charged metal tip. This control demonstrates the possibility of precisely accessing and controlling conjugated chain-growth polymerization at low temperature. This finding may lead to the bottom-up design and realization of sophisticated architectures for molecular nano-devices.
TL;DR: In this paper, a theoretical study of the electronic, thermal, and structural properties of a series of graphene oxide frameworks (GOFs) using first-principles calculations based on density functional theory is presented.
Abstract: We report a theoretical study of the electronic, thermal, and structural properties of a series of graphene oxide frameworks (GOFs) using first-principles calculations based on density functional theory. The molecular structure of GOFs is systematically studied by varying the nature and concentration of linear boronic acid pillars, and the thermal stability is assessed using ab initio molecular dynamics. The results demonstrate that GOFs are thermally stable up to 550 K and that electronic properties, such as their band gap, can be modified controllably by an appropriate choice of pillaring unit and pillar concentration. The tunability of the electronic structure using nonchemical means, e.g., mechanical strain, is also quantified. Overall, this class of materials is predicted to offer highly tunable materials electronic properties ranging from metallic to semiconducting.
TL;DR: In this paper, the structural properties of single-layer sheets, multilayer stacks, and single-walled nanotubes (SWNTs) of cadmium chalcogenides CdX (X = S, Se, Te) have been studied using ab initio density functional theory, along with spin-orbit coupling, van der Waals (vdW) interactions, and the GW approximation.
Abstract: Geometric structures, energetics, and electronic properties of single-layer sheets, multilayer stacks, and single-walled nanotubes (SWNTs) of cadmium chalcogenides CdX (X = S, Se, Te) have been studied using ab initio density functional theory, along with spin–orbit coupling, van der Waals (vdW) interactions, and the GW approximation. Methodologies applied to the rationally designed materials have been validated through the experimental structural parameters and band gaps of 3D bulk zinc blende and wurtzite phases of CdX. The 2D single-layer sheet of CdS is found to be completely planar, while those of CdSe and CdTe are slightly corrugated, all showing a honeycomb lattice. The 2D sheets are destabilized with respect to the bulk zinc blende and wurtzite phases, but can be significantly stabilized by forming 3D multilayer stacks as a result of interlayer interactions. 1D (5,5) armchair and (9,0) zigzag SWNTs are also stabilized from their single-layer sheet counterparts. Both SWNTs consist of two concentric...
TL;DR: Current-voltage characterization shows a significant enhancement in short circuit current, suggesting the potential of these novel nanostructured buffer layers to replace the PEDOT:PSS buffer layer typically applied in traditional P3HT-PCBM solar cells.
Abstract: Buffer layers that control electrochemical reactions and physical interactions at electrode/film interfaces are key components of an organic photovoltaic cell. Here the structure and properties of layers of semi-rigid poly(3-hexylthiophene) (P3HT) chains tethered at a surface are investigated, and these functional systems are applied in an organic photovoltaic device. Areal density of P3HT chains is readily tuned through the choice of polymer molecular weight and annealing conditions, and insights from optical absorption spectroscopy and semiempirical quantum calculation methods suggest that tethering causes intrachain defects that affect co-facial π-stacking of brush chains. Because of their ability to modify oxide surfaces, P3HT brushes are utilized as an anode buffer layer in a P3HT-PCBM (phenyl-C61-butyric acid methyl ester) bulk heterojunction device. Current–voltage characterization shows a significant enhancement in short circuit current, suggesting the potential of these novel nanostructured buffer layers to replace the PEDOT:PSS buffer layer typically applied in traditional P3HT–PCBM solar cells.
05 Mar 2013
TL;DR: In this paper, the ionic transport of room-temperature ionic liquids (RTILs) through nanopores is studied using atomistic simulations, and it is shown that in nanopores wetted by RTILs, a gradual dewetting transition occurs upon increasing the applied voltage, which is accompanied by a sharp increase in ionic current.
Abstract: Electrically driven ionic transport of room-temperature ionic liquids (RTILs) through nanopores is studied using atomistic simulations. The results show that in nanopores wetted by RTILs a gradual dewetting transition occurs upon increasing the applied voltage, which is accompanied by a sharp increase in ionic current. These phenomena originate from the solvent-free nature of RTILs and are in stark contrast with the transport of conventional electrolytes through nanopores. Amplification is possible by controlling the properties of the nanopore and RTILs, and we show that it is especially pronounced in charged nanopores. The results highlight the unique physics of nonequilibrium transport of RTILs in confined geometries and point to potential experimental approaches for manipulating ionic transport in nanopores, which can benefit diverse techniques including nanofluidic circuitry and nanopore analytics.
TL;DR: In situ current-voltage measurements of cup-stacked carbon nanotubes (CSCNTs) are used to establish reversible strain induced (compressive bending) semiconducting to metallic behavior, and it is concluded that an edge-to-edge interlayer transport mechanism results in conduction channels at the compressed side of the C SCNTs due to electronic density overlap between individual cups, thus making CSC NTs more conducting.
Abstract: We have used in situ current–voltage measurements of cup-stacked carbon nanotubes (CSCNTs) to establish reversible strain induced (compressive bending) semiconducting to metallic behavior. The corresponding electrical resistance decreases by two orders of magnitude during the process, and reaches values comparable to those of highly crystalline multi-walled carbon nanotubes (MWCNTs) and graphite. Joule heating experiments on the same CSCNTs showed that the edges of individual cups merge to form “loops” induced by the heating process. The resistance of these looped CSCNTs was close to that of highly deformed CSCNTs (and crystalline MWCNTs), thus suggesting that a similar conduction mechanism took place in both cases. Using a combination of molecular dynamics and first-principles calculations based on density functional theory, we conclude that an edge-to-edge interlayer transport mechanism results in conduction channels at the compressed side of the CSCNTs due to electronic density overlap between individual cups, thus making CSCNTs more conducting. This strain-induced CSCNT semiconductor to metal transition could potentially be applied to enable functional composite materials (e.g. mechanical sensors) with enhanced and tunable conducting properties upon compression.
TL;DR: Simulations suggest that the nanoparticles self-encapsulate to form single-wall nanooysters (SWNOs) by assisting the assembly of dangling carbon bonds, accompanied by migration of the metal particle inside the carbon structure.
Abstract: The processes by which single-wall carbon nanohorns are transformed by iron nanoparticles at high temperatures to form “nanooysters”, hollow graphene capsules containing metal particles that resemble pearls in an oyster shell, are examined both experimentally and theoretically. Quantum chemical molecular dynamics (QM/MD) simulations based on the density-functional tight-binding (DFTB) method were performed to investigate their growth mechanism. The simulations suggest that the nanoparticles self-encapsulate to form single-wall nanooysters (SWNOs) by assisting the assembly of dangling carbon bonds, accompanied by migration of the metal particle inside the carbon structure. These calculations indicate that the structure of the oyster consists primarily of hexagons along with a few pentagons that are predominantly formed near the former nanohorn edges as a result of their fusion. Experimental observations of large diameter nanoparticles inside multiwall carbon shells indicate that migration and coalescence of many iron particles must occur, perhaps by the convergence of smaller SWNOs or carbon-coated Fe-nanoparticles, whereby the void space is generated by the corresponding increase in the carbon shell surface area to metal nanoparticle volume.
Journal Article•
TL;DR: In this article, a field-theory was developed to study charge regulation and local dielectric function in planar polyelectrolyte brushes, which is used to study a polyacid brush, comprised of chains end-grafted at the solid-fluid interface.
Abstract: Understanding the effect of inhomogeneity on the charge regulation and dielectric properties, and how it depends on the conformational characteristics of the macromolecules is a long-standing problem. In order to address this problem, we have developed a field-theory to study charge regulation and local dielectric function in planar polyelectrolyte brushes. The theory is used to study a polyacid brush, which is comprised of chains end-grafted at the solid-fluid interface, in equilibrium with a bulk solution containing monovalent salt ions, solvent molecules, and pH controlling acid. In particular, we focus on the effects of the concentration of added salt and pH of the bulk in determining the local charge and dielectric function. Our theoretical investigations reveal that the dipole moment of the ion-pairs formed as a result of counterion adsorption on the chain backbones play a key role in affecting the local dielectric function. For polyelectrolytes made of monomers having dipole moments lower than the solvent molecules, dielectric decrement is predicted inside the brush region. However, the formation of ion-pairs (due to adsorption of counterions coming from the dissociation of added salt) more polar than the solvent molecules is shown to increase the magnitude of the dielectric function with respect to its bulk value. Furthermore, an increase in the bulk salt concentration is shown to increase the local charge inside the brush region.
TL;DR: In this paper, a statistical mechanical basis was developed for designing random AB copolymers to optimize fullerene dispersion at intermediate copolymer compositions, and pair correlation calculations revealed a strong sensitivity of interfacial packing near the fullerenes to copolym composition and adsorption energy mismatch.
Abstract: We combine polymer integral equation theory and computational chemistry methods to study the interfacial structure, effective interactions, miscibility and spatial dispersion mechanism of fullerenes dissolved in specific random AB copolymer melts characterized by strong noncovalent electron donor–acceptor interactions with the nanofiller. A statistical mechanical basis is developed for designing random copolymers to optimize fullerene dispersion at intermediate copolymer compositions. Pair correlation calculations reveal a strong sensitivity of interfacial packing near the fullerene to copolymer composition and adsorption energy mismatch. The potential of mean force between fullerenes displays rich trends, often nonmonotonic with copolymer composition, reflecting a nonadditive competition between direct filler attractions and polymer-mediated bridging and steric stabilization. The spinodal phase diagrams are in qualitative agreement with recent solubility limit experimental observations on three systems, ...
TL;DR: In this article, ionic decomposition of the charge transfer salt β″-(BEDT-TTF)2SF5CH2CF2SO3 into two surface-supported phases on Ag(111), each of which has a cation/anion ratio different from the 2:1 stoichiometry of the bulk.
Abstract: Epitaxial growth of organic charge-transfer salts composed of molecular cations and anions will potentially allow the synthesis of thin films of molecular conductors with strong electron correlations and electron–phonon interactions, highly distinct properties compared to thin organic semiconductors and traditional self-assembled monolayers. Here, we report on ionic decomposition of the charge transfer salt β″-(BEDT-TTF)2SF5CH2CF2SO3 into two surface-supported phases on Ag(111), each of which has a cation/anion ratio different from the 2:1 stoichiometry of the bulk. The films were grown by thermal evaporation of the bulk crystal. Subsequent reassembly of the constituent bis(ethylenedithio)tetrathiafulvalene molecules (BEDT-TTF) and the SF5CH2CF2SO3 anions into long-range ordered structures on the Ag(111) surface produced well-ordered molecular islands with either 1:1 or 3:1 stoichiometric ratios of BEDT-TTF:SF5CH2CF2SO3. Tunneling spectroscopy revealed that both surface structures could be considered insu...
TL;DR: In this paper, 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.
TL;DR: In this paper, an end-tethered conjugated poly(para-phenylene) brush is created in situ by aromatizing well-defined, end-to-end poly(1,3-cyclohexadiene) (PCHD) precursor brushes.
Abstract: Well-defined conjugated polymers in confined geometries are challenging to synthesize and characterize, yet they are potentially useful in a broad range of organic optoelectronic devices such as transistors, light emitting diodes, solar cells, sensors, and nanocircuits. Herein we report a systematic study of optoelectrical properties, grafting density effects, and nanopatterning of a model, end-tethered conjugated polymer system. Specifically, poly(para-phenylene) (PPP) brushes of various grafting density are created in situ by aromatizing well-defined, end-tethered poly(1,3-cyclohexadiene) (PCHD) “precursor brushes”. This novel precursor brush approach provides a convenient way to make and systematically control the grafting density of high molecular weight conjugated polymer brushes that would otherwise be insoluble. This allows us to examine how grafting density impacts the effective conjugation length of the conjugated PPP brushes and to adapt the fabrication method to develop spatially patterned conjugated brush systems, which is important for practical applications of conjugated polymer brushes.
09 Aug 2013
01 Jan 2013
TL;DR: In this paper, the authors showcase selected illustrations of various manifestations of nanoscale and molecular electronic effects as investigated by quantum mechanical methods, including graphitic nanoribbons and carbon nanotubes.
Abstract: This chapter showcases selected illustrations of various manifestations of nanoscale and molecular electronic effects as investigated by quantum mechanical methods. The examples include results demonstrating (1) how graphitic nanoribbons can be assembled into multiterminal networks and the influence on electron transport; (2) how the position of a single embedded molecule can be modified to change the overall conduction state of a nanowire; (3) how carbon nanotubes can be assembled into complex covalent arrays and how these can be obtained experimentally; (4) how quantum interference can be understood as emerging from the presence of multiple levels of confinements in carbon nanorings; (5) how new functionality emerges at the nanoscale due to the interplay of magnetic, electronic, and structural properties of individual graphitic nanoribbons assembled into wiggle-like structures; and (6) how quantum chemical modeling can lead to the design of electrodes with enhanced interfaces for molecular coupling.
TL;DR: In this article, it was shown that when poly(styryl)lithium anions are reacted with stringently purified C60, stars with greater than six arms can be attained.
Abstract: Structure–property relationships of polymeric materials require careful characterization of macromolecular properties. In this work we use 2-dimensional chromatography, coupling temperature gradient interaction and size exclusion chromatography, as well as density functional theory calculations to clarify that when poly(styryl)lithium anions are reacted with stringently purified C60, stars with greater than six arms can be attained, in contrast to prior reports. We attribute this apparent contradiction to the limited ability of size exclusion chromatography to separate polymers having branched architectures. These results clarify the chemistry of anionically modified fullerenes and also show that more sophisticated separation techniques are necessary when characterizing architecturally complex polymers.