Molecular Orientation Structure Research Articles

Heteroepitaxy in Organic/TMD Hybrids and Challenge to Achieve it for TMD Monolayers: The Case of Pentacene on WS2 and WSe2.

The intriguing photophysical properties of monolayer stacks of different transition-metal dichalcogenides (TMDs), revealing rich exciton physics including interfacial and moiré excitons, have recently prompted an extension of similar investigations to hybrid systems of TMDs and organic films, as the latter combine large photoabsorption cross sections with the ability to tailor energy levels by targeted synthesis. To go beyond single-molecule photoexcitations and exploit the excitonic signatures of organic solids, crystalline molecular films are required. Moreover, a defined registry on the substrate, ideally an epitaxy, is desirable to also achieve an excitonic coupling in momentum space. This poses a certain challenge as excitonic dipole moments of organic films are closely related to the molecular orientation and film structure, which critically depend on the support roughness. Using X-ray diffraction, optical polarization, and atomic force microscopy, we analyzed the structure of pentacene (PEN) multilayer films grown on WSe2(001) and WS2(001) and identified an epitaxial alignment. While (022)-oriented PEN films are formed on both substrates, their azimuthal orientations are quite different, showing an alignment of the molecular L-axis along the and directions. This intrinsic epitaxial PEN growth depends, however, sensitively on the substrates surface quality. While it occurs on exfoliated TMD single crystals and multilayer flakes, it is hardly found on exfoliated monolayers, which often exhibit bubbles and wrinkles. This enhances the surface roughness and results in (001)-oriented PEN films with upright molecular orientation but without any azimuthal alignment. However, monolayer flakes can be smoothed by AFM operated in contact mode or by transferring to ultrasmooth substrates such as hBN, which again yields epitaxial PEN films. As different PEN orientations result in different characteristic film morphologies (elongated mesa islands vs pyramidal dendrites), which can be easily distinguished by AFM or optical microscopy, this provides a simple means to judge the roughness of the used TMD surface.

Effects of HAT-CN Layer Thickness on Molecular Orientation and Energy-Level Alignment with ZnPc.

Efficient energy-level alignment is crucial for achieving high performance in organic electronic devices. Because the electronic structure of an organic semiconductor is significantly influenced by its molecular orientation, comprehensively understanding the molecular orientation and electronic structure of the organic layer is essential. In this study, we investigated the interface between a 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) hole injection layer and a zinc-phthalocyanine (ZnPc) p-type organic semiconductor. To determine the energy-level alignment and molecular orientation, we conducted in situ ultraviolet and X-ray photoelectron spectroscopies, as well as angle-resolved X-ray absorption spectroscopy. We found that the HAT-CN molecules were oriented relatively face-on (40°) in the thin (5 nm) layer, whereas they were oriented relatively edge-on (62°) in the thick (100 nm) layer. By contrast, ZnPc orientation was not significantly altered by the underlying HAT-CN orientation. The highest occupied molecular orbital (HOMO) level of ZnPc was closer to the Fermi level on the 100 nm thick HAT-CN layer than on the 5 nm thick HAT-CN layer because of the higher work function. Consequently, a considerably low energy gap between the lowest unoccupied molecular orbital level of HAT-CN and the HOMO level of ZnPc was formed in the 100 nm thick HAT-CN case. This may improve the hole injection ability of the anode system, which can be utilized in various electronic devices.

Molecular simulation of the rheological properties and shear thinning principles of supramolecular drilling fluids at different burial depths.

In order to investigate the rheological properties and shear thinning principles of supramolecular drilling fluids, the salt-responsive supramolecular ionomer polymers with different components were designed and the change in shear viscosity of supramolecular polymer drilling fluid system with shear rate was studied using the molecular dynamics simulation method. The result indicated that the ionic supramolecular polymer drilling fluid system exhibits better self-assembly performance than the nonionic acrylamide drilling fluid system. Moreover, the drilling fluid system exhibits the best rheological properties and self-assembly performance when the feeding ratios of the three monomers in the two polymers are m : n : o = 5 : 90 : 5 and m : n : o = 30 : 40 : 30, respectively. The shear viscosity recovery rate of the #3 ionic supramolecular polymer drilling fluid system at different burial depths (1-5 km) is >87%, where the shear viscosity is mainly determined at ambient pressure. The shear thinning phenomenon of the supramolecular polymer drilling fluid system occurs because of the combined effect of the polymer molecular orientation and entanglement structure. When the shear rate is above a critical value, the polymer molecules are oriented along the flow field direction, decreasing the shear viscosity. However, when the shear rate is very high, the entanglement structure of the molecules is opened and the mesh structure of the fluids is disrupted, decreasing the shear viscosity of the drilling fluid.

Customizing 3D thermally conductive skeleton by 1D aramid Nanofiber/2D graphene for high-performance phase change composites with excellent solar-to-thermal conversion ability

Three-dimensional (3D) graphene skeletons show the high potential using in encapsulation of phase change materials (PCM), yet still suffer from the high contact thermal resistance between graphene sheets in skeleton. In this work, aramid nanofiber (ANF) with high molecular chain orientation and rigid conjugate aromatic structure was firstly used to assemble graphene nanoplates (GNP) into a 3D thermally conductive skeleton. Typically, ball-milling treatment was used to improve the interfacial interaction between ANF and GNP, followed by unidirectional freezing technique was used to achieve 3D ANF/GNP skeleton with highly oriented honeycomb-like porous structure. As an encapsulation for PCM, ANF/GNP skeleton not only provides the thermal conduction path, but also endows excellent solar-thermal conversion ability, which greatly expands the application in solar energy utilization. Thus, the thermal conductivity of PCM composite can reach to 3.9 W/mK at only 4.26 vol% of GNP content, which gives the composite with rapid and stable heat charging/discharging ability during phase change process. Moreover, the honeycomb channel structure of ANF/GNP skeleton with huge capillary and surface tension forces not only reinforce the shape stability during phase change process, but also accommodate sufficient PCM for energy storage. Therefore, this work demonstrates the high-efficiency of ANF in constructing 3D thermally conductive graphene skeleton, which reveals a high potential using in encapsulating PCM.

Conjugated Polymer Films Having a Uniaxial Molecular Orientation and Network Structure Prepared by Electrochemical Polymerization in Liquid Crystals.

A new method for fabricating conjugated polymer films was developed using electrochemical polymerization in liquid crystals and magnetic orientation. A uniaxial main chain orientation and a crosslinked network structure were achieved with this method. By employing eight types of monomers, the influence of the crosslinking for the film was investigated. The crosslinking was found to improve the solvent resistance of the conjugated polymer films. This new method is expected to be useful in various applications, such as high-powered organic electronic devices with durability.

(Invited) Side-Chain Engineering of Small-Molecular Organic Semiconductors for Organic Electronic Devices

Molecular orientation and packing structure of small molecular organic semiconductors are very important for the charge carrier mobility of organic thin-film transistors (OTFT). Furthermore, the miscibility of p-and n-type materials and the grain size of each material in bulk heterojunction are critical for the performance of organic photovoltaics (OPVs). Benzoporphyrins (BPs) are one of the outstanding p-type organic semiconductors and have been studied for OTFT and OPV materials [1]. For practical use of organic electronic devices, solution process is desirable but BPs are not soluble in organic solvents.To achieve the solution process of BPs, two strategies have been studied in the last two decades. One is a precursor approach: as-spun films of soluble precursors of BPs are in-situ converted into BP thin-films by retro-Diels–Alder reaction. With this method, the film of intact BP is available but the mixed film of edge-on and face-on grains are obtained because of the too-good crystallinity of BPs. The other is a substituent engineering: proper substituents induced to the BP framework can improve the solubility and processability of the film as well as the morphology and orientation of the molecules. Recently we were successful to obtain edge-on dominant film of BP derivatives by a simple dip-coating. 5,15-triisopropylsilyl-ethynyl(TIPS)-BP could make brickwork-type packing to achieve 1.1 cm2 V-1 s-1 FET charge carrier mobility (m FET) [2].This side-chain engineering was also effective for the control of packing structure of bulk-hetero junction OPV. The alkyl chain length drastically influenced the miscibility of p- and n-materials and the short-circuit current density [3]. We also found open-circuit-voltage shift of over 0.5 V in organic photovoltaic cells induced by a minor structural difference in alkyl substituents of acene-type p-materials [4]. In this presentation, the relation between control of packing structure and the OPV performance will be discussed.

Evolution of Monoclinic CaAl2O4 Nanocrystallites via Chemical Route and Its Optical Investigations

Nanocrystalline calcium aluminate (CaAl2O4) powder is an adequate phosphor material. Calcium aluminate is a versatile member belongs to a category of alkaline earth aluminates phosphor. It has multifaceted applications in different areas such as photocatalysis, sensing, optoelectronic devices, displays, and imaging etc. In the present study, calcium aluminate has been prepared by well-known citrate sol-gel technique. X-ray diffraction (XRD) method has been used to analysis the amorphous and crystalline behavior of the prepared samples. The sample annealed at 800° indicates the monoclinic phase with enhanced intensity of the prominent diffraction peaks. By using Debye-Scherrer formula the grain size of crystalline powder is calculated ~10 nm. FTIR spectra confirmed the molecular orientation and bond structure in the prepared nanomaterial. Surface morphology and elemental composition present in prepared samples has been examined by Scanning electron microscope and energy dispersive spectroscopy. Uv-vis spectroscopy result showed red shift in the nanopowder with thermal treatment. In material science, due to their long-lasting photo-luminescence properties in visible region these aluminates are very attractive for evolution of new generation inorganic phosphor materials.

Formation of Large Crystalline Domains in a Semiconducting Polymer with Semi-fluorinated Alkyl Side Chains and Application to High-Performance Thin-Film Transistors.

The semi-fluorinated alkyl (SFA) side chain introduced thienylenevinylene (TV)-based p-type polymer, PC12TVC5F7T, was synthesized for use in organic thin-film transistors (OTFTs). Herein, we investigated the influence of SFA side chains on the morphology, molecular orientation, and crystalline structure using a combination of atomic force microscopy (AFM), scanning electron microscopy (SEM), two-dimensional (2D) grazing-incidence wide-angle X-ray scattering (GIWAXS), and density functional theory (DFT) calculations. Interestingly, the incorporation of SFA side chains led to the evolution of plate-like large-sized domains and also strongly intermolecular stacked high crystalline structures. Furthermore, due to the strong interactions between SFA side chains, several (00h) peaks could be observed for PC12TVC5F7T, in spite of their fairly large dihedral angle. As a result, due to the well-developed microstructure of PC12TVC5F7T, the OTFT devices based on it exhibited a high hole mobility of 1.91 cm2 V-1 s-1, which is an outstanding value among the poly(thiophene) derivative polymers. These observations indicate that large-sized domains and strongly intermolecular stacked high crystalline structures, which are beneficial for charge carrier transport, could be attained by the introduction of SFA side chains, further enhancing the performance of the OTFTs.

The Difference in Molecular Orientation and Interphase Structure of SiO2/Shape Memory Polyurethane in Original, Programmed and Recovered States during Shape Memory Process.

In order to further understand the shape memory mechanism of a silicon dioxide/shape memory polyurethane (SiO2/SMPU) composite, the thermodynamic properties and shape memory behaviors of prepared SiO2/SMPU were characterized. Dynamic changes in the molecular orientation and interphase structures of SiO2/SMPU during a shape memory cycle were then discussed according to the small angle X-ray scattering theory, Guinier’s law, Porod approximation, and fractal dimension theorem. In this paper, a dynamic mechanical analyzer (DMA) helped to determine the glass transition start temperature (Tg) by taking the onset point of the sigmoidal change in the storage modulus, while transition temperature (Ttrans) was defined by the peak of tan δ, then the test and the calculated results indicated that the Tg of SiO2/SMPU was 50.4 °C, and the Ttrans of SiO2/SMPU was 72.18 °C. SiO2/SMPU showed good shape memory performance. The programmed SiO2/SMPU showed quite obvious microphase separation and molecular orientation. Large-size sheets and long-period structures were formed in the programmed SiO2/SMPU, which increases the electron density difference. Furthermore, some hard segments had been rearranged, and their gyration radii decreased. In addition, several defects formed at the interfaces of SiO2/SMPU, which caused the generation of space charges, thus leading to local electron density fluctuations. The blurred interphase structure and the intermediate layer formed in the programmed SiO2/SMPU and there was evident crystal damage and chemical bond breakage in the recovered SiO2/SMPU. Finally, the original and recovered SiO2/SMPU samples belong to the surface fractal system, but the programmed sample belongs to the mass fractal and reforms two-phase structures. This study provides an insight into the shape memory mechanism of the SiO2/SMPU composite.

Thermodynamics and molecular orientation structure in mixtures of fluids with strongly different polarity

Thermodynamics and molecular orientation structure in mixtures of fluids with strongly different polarity

Photoelectron ionization time of aligned molecules clocked by attosecond angular streaking

Attoclock is a powerful technique to resolve laser-driven electron dynamics on an attosecond time scale. Previously, it was mainly limited to atomic targets. Here we apply the attoclock technique to aligned molecules to study the subcycle ionization dynamics. By controlling the alignment of the molecule relative to the major axis of a strong elliptically polarized laser field, we demonstrate that the electron ionization time in aligned molecules can be precisely resolved by the attoclock technique. We find that the ionization time corresponding to the peak of the photoelectron momentum distribution shifts several tens of attoseconds relative to the laser field peak depending on the molecular orientation and molecular orbital structure. A significant difference between the measurement and molecular Ammosov-Delone-Krainov model is found. We further show that the photoelectron ionization time of molecules is sensitive to angular-dependent ionization rate, opening up opportunities for probing attosecond electron dynamics in complex polyatomic molecules.

Impact of Graphene Work Function on the Electronic Structures at the Interface between Graphene and Organic Molecules.

The impact of graphene work function (WF) on the electronic structure at the graphene/organic interface has been investigated. WF manipulation of graphene is realized using self-assembled monolayers (SAMs) with different end groups. With this method, the upper surface of the functionalized graphene remains intact, and thus precludes changes of molecular orientation and packing structures of subsequently deposited active materials. The WF of NH2-SAM functionalized graphene is ~3.90 eV. On the other hand, the WF of graphene increases to ~5.38 eV on F-SAM. By tuning the WF of graphene, an upward band bending is found at the ZnPc/graphene interface on F-SAM. At the interface between C60 and NH2-SAM modified graphene, a downward band bending is observed.

Face-on vs. edge-on: tuning the structure of tetrathiafulvalene monolayers with solvent

Molecular orientation and supramolecular structure of tetrathiafulvalene derivative on graphite can be controlled by the change of deposition solvent, as revealed by scanning probe microscopy study.

Comprehensive Understanding of Structure-Controlling Factors of a Zinc Tetraphenylporphyrin Thin Film Using pMAIRS and GIXD Techniques.

The performance of an organic electronic device is significantly influenced by the anisotropic molecular structure in the film, which has long been difficult to predict especially for a solution process. In the present study, a zinc tetraphenylporphyrin (ZnTPP) thin film prepared by a solution process was chosen to comprehensively explore the molecular-arrangement mechanism as a function of representative film-preparation parameters: solvent, film-preparation technique, and thermal annealing. The anisotropic structure was first analyzed by using a combination of infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) and grazing incidence X-ray diffraction (GIXD), which readily revealed the molecular orientation and crystal structure, respectively. As a result, the real dominant factor was found to be the evaporation time of the solvent that determines the initial two different molecular arrangements, types-I and -II, while the thermal annealing was found to play an additional role of improving the molecular order. The correlation between the molecular orientation and the crystal structure was also revealed through the individual orientation analysis of the porphyrin and phenyl rings.

The effect of thermal annealing on the layered structure of smectic liquid crystalline organic semiconductor on polyimide gate insulator and its OFET performance

The electrical performance of organic field-effect transistors (OFETs) depends on the quality of thin-film organic semiconductors, which is significantly affected by solution-processing conditions and additional processes. We investigated the effects of post-thermal annealing on the thin-film morphologies of liquid crystalline organic semiconductors on polyimide gate insulator surface and the FET performances of the films. The active material selected for the OFETs was 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10), which shows a highly ordered smectic E (SmE) mesophase and polycrystalline thin films feature very high mobility. We aimed to produce well defined molecular orientation and crystal structure in thin-film Ph-BTBT-C10 on polyimide gate insulator not typical SiO2 gate insulator via thermal annealing process. Uniform bilayer- or monolayer-structured polycrystalline thin films were obtained on polymer gate insulator after thermal annealing at a SmE (over 148°C) and SmA (over 213°C) liquid crystalline phase temperature, respectively. The OFET using bilayer-structured thin film showed high performance with a mobility of 2.27cm2/Vs.

Origins of polarization-dependent anisotropic X-ray scattering from organic thin films.

Organic thin films that have no overall in-plane directional ordering often nonetheless produce anisotropic scattering patterns that rotate with the polarization of incident resonant X-rays. Isotropic symmetry is broken by local correlations between molecular orientation and domain structure. Such examples of molecular alignment at domain interfaces and within the bulk of domains, which are both critical to fields such as organic electronics, are simulated and compared with experimental scattering. Anisotropic scattering patterns are found to allow unambiguous identification of the mechanism of local molecular orientation correlations and, as such, promise to be both distinct and complementary to isotropic scattering intensity as a general measure of thin film microstructure.

Surface structure of aqueous ionic surfactant solutions and effects of solvent therein—a computer simulation study

The current paper reports a simulation study on the molecular structure of aqueous solutions of anionic surfactant under the environment of Material Studio 6.0. From the simulated configurations, the concentration-depth profiles of each constituent in the surface layer have been analyzed. With that, the structure of electrical double layer and molecular orientation of surface active dodecyl sulfate ion dependent on its surface density have been derived straightforward. Thereby, the surface pictures of the investigated solutions are mapped out in angstrom scale and become visible. The results demonstrate that the structure of electrical double layer is heavily dependent on the specific counter-ions. However, this dependence is opposite to its experimental counterpart and reversed to that found in the formamide solution. This contradictory finding can be explained by the salvation effect of the counter-ions. Additionally, the orientation of dodecyl sulfate ion is found only subjected to its surface density but hardly influenced by its counter-ion specificity, which keeps coincident to the past investigation on formamide solution.

Two-in-One Composite Fibers With Side-by-Side Arrangement of Silk Fibroin and Poly(l-lactide) by Electrospinning

1D fibers of Bombyx mori silk fibroin (SF) and poly(l-lactide) (SF-s-PLLA) with side-by-side parallel arrangement of the two components in a single fiber made by electrospinning are presented. The side-by-side arrangement in both randomly oriented and aligned two-in-one fibers was confirmed by scanning electron and confocal laser scanning microscope studies. The molecular orientation and secondary structure of SF and PLLA were dependent on the fiber alignment and annealing conditions. The two sides retained their individual secondary structure before and after annealing without affecting each other in a significant way. The two-in-one fibers after post treatment with methanol and heat at 80 °C showed tensile strength 16.5 ± 1.4 MPa, modulus 205 ± 20.6 MPa, and an elongation at break of 53 ± 8%.

Highly crystalline and oriented high‐strength poly(ethylene terephthalate) fibers by using low molecular weight polymer

ABSTRACTHigh‐strength poly(ethylene terephthalate) (PET) fibers were obtained using low molecular weight (LMW) polymervia horizontal isothermal bath (hIB), followed by postdrawing process. We investigated the unique formations of different precursors, which differentiated in its molecular orientation and crystalline structures from traditional high‐speed spinning PET fibers. Sharp increase in crystallinity was observed after drawing process even though the fibers showed almost no any crystallinity before the drawing. Properties of as‐spun and drawn hIB and control filaments at different process conditions were compared. As would be expected, performances of resulted treated undrawn and drawn fibers have dramatically improved with developing unique morphologies. Tenacities more than 8 g/d for as‐spun and 10 g/d for drawn treated fibers after just drawn at 1.279 draw ratio were observed. These performances are considerably higher than that of control fibers. An explanation of structural development of high‐strength fibers using LMW polymer spun with hIB is proposed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42747.

Molecular orientation and electronic structure at organic heterojunction interfaces

Due to the highly anisotropic nature of π-conjugated molecules, the molecular orientation in organic thin films can significantly affect light absorption, charge transport, energy level alignment (ELA) and hence device performance. Synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy represents a powerful technique for probing molecular orientation. The aim of this review paper is to provide a balanced assessment on the investigation of molecular orientation at the organic–organic heterojunction (OOH) interface by NEXAFS, as well as the gap-states mediated orientation dependent energy level alignment at OOH interfaces. We highlight recent progress in elucidating molecular orientation at OOH interfaces dominated by various interfacial interactions, gap-states controlled orientation dependent energy level alignments at OOH interfaces, and the manipulations of molecular orientation and ELA in OOH.