Dependence of electrical performance on structural organization in polymer field effect transistors

Organic semiconductors (OSCs) are strong contenders for use in printed, flexible electronics. Although organic electronic materials have been studied for many years, the physics of charge transport is still under investigation. This is in part due to variability resulting from the large variety of molecules that can be synthesized and inconsistency in electrical characterization due to device and processing conditions. Molecular ordering in OSCs is known to alter the charge transport characteristics and attention to long range and short range ordering provides clues as to the nature of transport pathways. Here, we study ordered regioregular poly(3‐hexylthiophene‐2,5‐diyl) films carefully prepared to obtain a set of three samples with incrementally increasing order on identical transistor architectures. Ordering was characterized using a variety of short and long range techniques to probe the coherence and number of crystallites formed during processing, and the correlation between these different measures of order are quantified. We observe three changes in transistor behavior that show a shift from non‐ideal to more textbook‐like characteristics with increasing order: reduction of the contact resistance, shift to field‐independent mobility, and a shift from a diode‐like (S‐shaped) to linear response at low lateral fields. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1063–1074     

Tuning the Surface Hydrophobicity of Keplerate {Mo<Subscript>72</Subscript>Fe<Subscript>30</Subscript>} Porous Molecular Capsules by Surface Ligand-Replacement Process

A spontaneous ligand-replacement process at the external surface of Keplerate polyoxometalate molecular nanocapsules {Mo₇₂Fe₃₀} occurs when dissolving {Mo₇₂Fe₃₀} in pure methanol, with the water ligands coordinated to the Fe(III) centers of {Mo₇₂Fe₃₀} gradually being replaced by CH₃OH ligands, resulting in the formation of partially hydrophobic nanocapsules. The new nanocapsules have negligible deprotonation capability and different solution behaviors. The methanol ligands can be easily exchanged back to water ligands when a small amount of water is added into the system. The exchange of water ligands dramatically changed the self-assembly behaviors of the {Mo₇₂Fe₃₀} clusters in solution.     

Dual input–output pairs for modeling hysteresis inspired by mem-models

We call attention to a dual-pair concept for modeling hysteresis involving instantaneous switching: Specifically, there are two input–output pairs for each hysteresis model under one specific input, namely a differential pair and an integral pair. Currently in engineering mechanics, only one pair is being recognized and utilized, not the other. Whereas this dual-pair concept is inherent in the differential and algebraic forms of memristors and memcapacitors, the concept has not been carried over to memristive system theory, nor to memcapacitive system theory. We show that the “zero-crossing” feature in memristors, memcapacitors, and memristive/memcapacitive models (i.e., the “mem-models”) is also a feature of the differential pairs of well-known non-mem-models, examples of which are Ramberg–Osgood, Bouc–Wen, bilinear hysteresis, and classical Preisach. The dual-pair concept thus connects mem-models and non-mem-models, thereby facilitating the modeling of hysteresis, and raising a set of scientific questions for further studies that might not otherwise come to awareness.     

Stereospecific Palladium-Catalyzed Acylation of Enantioenriched Alkylcarbastannatranes: A General Alternative to Asymmetric Enolate Reactions

We report the development of a Pd-catalyzed process for the cross coupling of unactivated primary, secondary, and tertiary alkylcarbastannatrane nucleophiles with acyl electrophiles. Reactions involving optically active alkylcarbastannatranes occur with exceptional stereofidelity and with net retention of absolute configuration. Because the stereochemistry of the resulting products is entirely reagent-controlled, this process may be viewed as a general, alternative approach to the preparation of products typically accessed via asymmetric enolate methodologies. Additionally, we report a new method for the preparation of optically active alkylcarbastannatranes, which should facilitate their future use in stereospecific reactions.     

Rhodium-Catalyzed Enantioselective Radical Addition of CX4 Reagents to Olefins

We describe the synthetically useful enantioselective addition of Br−CX₃ (X=Cl or Br) to terminal olefins to introduce a trihalomethyl group and generate optically active secondary bromides. Computational and experimental evidence supports an asymmetric atom-transfer radical addition (ATRA) mechanism in which the stereodetermining step involves outer-sphere bromine abstraction from a [(bisphosphine)Rh^IIBrCl] complex by a benzylic radical intermediate. This mechanism appears unprecedented in asymmetric catalysis.     

Molecular organization in MAPLE‐deposited conjugated polymer thin films and the implications for carrier transport characteristics

The morphological structure of poly(3‐hexylthiophene) (P3HT) thin films deposited by both Matrix Assisted Pulsed Laser Evaporation (MAPLE) and solution spin‐casting methods are investigated. The MAPLE samples possessed a higher degree of disorder, with random orientations of polymer crystallites along the side‐chain stacking, π–π stacking, and conjugated backbone directions. Moreover, the average molecular orientations and relative degrees of crystallinity of MAPLE‐deposited polymer films are insensitive to the chemistries of the substrates onto which they were deposited; this is in stark contrast to the films prepared by the conventional spin‐casting technique. Despite the seemingly unfavorable molecular orientations and the highly disordered morphologies, the in‐plane charge carrier transport characteristics of the MAPLE samples are comparable to those of spin‐cast samples, exhibiting similar transport activation energies (56 vs. 54 meV) to those reported in the literature for high mobility polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 39–48     

Effects of Heterogeneous Fracture Aperture on Multiphase Production from Shale Reservoirs

Production simulation from fractured shale reservoirs is often performed by simplifying the hydraulic fractures as rectangular planes with homogeneous aperture. This study investigates the effects of heterogeneous fracture aperture and proppant distribution in realistic, non-rectangular fractures on the multi-phase production from shales. The heterogeneous hydraulic fractures are generated with the GEOS multiphysics simulator under realistic 3D stress field. These fractures are embedded into the TOUGH+ multi-phase flow simulator for production simulation. The results emphasize the importance of flow barriers within the hydraulic fractures, due both to low-aperture regions caused by the stress-shadow effect and the settling of proppant. The production rate is particularly sensitive to aperture heterogeneity if the flow barriers are close to the wellbore such that a great portion of fracture volume is isolated from the well. A stage-to-stage comparison reveals that production from different stages could vary significantly because the local stress field leads to different fracture area and aperture. The use of proppant prevents fracture closure, but if the propped regions are far from the well, they do not enhance production because flow barriers between these regions and the well act as bottlenecks. The present study highlights the importance of incorporating aperture heterogeneity into production simulation, provides insights on the relationship between flow barriers, proppant concentration, and well production, and proposes a practical method to mitigate numerical difficulties when modeling heterogeneous fractures.

     

On the Relationship Between the Statistics of the Resolved and True Rate of Dissipation of Mixture Fraction

The relationship between the one-point probability-density-function (PDF) of the dissipation rate of mixture fraction fluctuations and the corresponding resolved quantity available in large eddy simulation (LES) is analyzed. The investigation pursues two fronts: an a priori study using direct numerical simulation (DNS), and an analytic development that, using common turbulence physics simplifications, relates the one-point statistics of the resolved and true scalar dissipations. Particularly, the analysis reveals the connection between the multi-point correlations of the mixture fraction gradient and the one-point PDF of the resolved scalar dissipation. A DNS of a temporally evolving shear layer with and without heat release is used to quantify the accuracy of the analytical result. It is verified, both by filtering the DNS and from the theory, that increasing the filter cutoff width reduces the magnitude of the resolved scalar dissipation fluctuations, as expected and observed experimentally. Comparison with DNS indicates that the analytical relationship predicts the behavior of the resolved scalar dissipation PDF well at the center planes of the shear layer, where turbulence is locally more isotropic and homogeneous. Large-scale anisotropy and inhomogeneities in the DNS degrade the accuracy of the approximate analytical result close to the edges of the shear layer. These results may be improved with future investigations to account fully for the missing statistics in LES, which have the potential to allow a more accurate quantification of finite-rate chemistry effects in reacting flows.     

Shear-wave speeds and elastic moduli for different liquids Part 3. Experiments-update

Tables of values of shear-wave speeds, shear moduli and relaxation times for 18 new liquids are presented, supplementing the tables for 51 liquids given in Part 2. A brief discussion of errors and analysis of the oscilloscope traces is presented. The relation of the effective moduli measured on the wave-speed meter to independent measurements using phase-modulated birefringence and delayed die swell is discussed. A method of measuring wave speeds and rigidities for sheared media is proposed.