Changes In Molecular Orientation Research Articles

Structure, Energetics, and Dynamics of Screw Dislocations in Even n-Alkane Crystals.

Spiral hillocks on n-alkane crystal surfaces were observed immediately after Frank recognized the importance of screw dislocations for crystal growth, yet their structures and energies in molecular crystals remain ill-defined. To illustrate the structural chemistry of screw dislocations that are responsible for plasticity in organic crystals and upon which the organic electronics and pharmaceutical industries depend, molecular dynamics was used to examine heterochiral dislocation pairs with Burgers vectors along [001] in n-hexane, n-octane, and n-decane crystals. The cores were anisotropic and elongated in the (110) slip plane, with significant local changes in molecular position, orientation, conformation, and energy. This detailed atomic level picture produced a distribution of strain consistent with linear elastic theory, giving confidence in the simulations. Dislocations with doubled Burgers vectors split into pairs with elementary displacements. These results suggest a pathway to understanding the mechanical properties and failure associated with elastic and plastic deformation in soft crystals.

The Fabrication of Nanoimprinted P3HT Nanograting by Patterned ETFE Mold at Room Temperature and Its Application for Solar Cell.

Nanoimprinting lithography (NIL) is investigated as a promising method to define nanostructure; however, finding a practical method to achieve large area patterning of conjugated polymer remains a challenge. We demonstrate here that a simple and cost-effective technique is proposed to fabricate the nanoimprinted P3HT nanograting by solvent-assisted room temperature NIL (SART-NIL) method with patterned ETFE film as mold. The patterned ETFE template is produced by embossing ETFE film into a patterned silicon master and is used as template to transfer nanogratings during the SART-NIL process. It indicates that highly reproducible and well-controlled P3HT nanograting film is obtained successfully with feature size of nanogratings ranging from 130 to 700 nm, due to the flexibility, stiffness, and low surface energy of ETFE mold. Moreover, the SART-NIL method using ETFE mold is able to fabricate nanogratings but not to induce the change of molecular orientation within conjugated polymer. The conducting ability of P3HT nanograting in the vertical direction is also not damaged after patterning. Finally, we further apply P3HT nanograting for the fabrication of active layer of OBHJ solar cell device, to investigate the morphology role presented by ETFE mold in device performance. The device performance of OBHJ solar cell is preferential to that of PBHJ device obviously.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-016-1481-y) contains supplementary material, which is available to authorized users.

Temperature and layer thickness dependent in situ investigations on epindolidione organic thin-film transistors

We report on in situ performance evaluations as a function of layer thickness and substrate temperature for bottom-gate, bottom-gold contact epindolidione organic thin-film transistors on various gate dielectrics. Experiments were carried out under ultra-high vacuum conditions, enabling quasi-simultaneous electrical and surface analysis. Auger electron spectroscopy and thermal desorption spectroscopy (TDS) were applied to characterize the quality of the substrate surface and the thermal stability of the organic films. Ex situ atomic force microscopy (AFM) was used to gain additional information on the layer formation and surface morphology of the hydrogen-bonded organic pigment. The examined gate dielectrics included SiO2, in its untreated and sputtered forms, as well as the spin-coated organic capping layers poly(vinyl-cinnamate) (PVCi) and poly((±)endo,exo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE, from the class of polynorbornenes). TDS and AFM revealed Volmer-Weber island growth dominated film formation with no evidence of a subjacent wetting layer. This growth mode is responsible for the comparably high coverage required for transistor behavior at 90–95% of a monolayer composed of standing molecules. Surface sputtering and an increased sample temperature during epindolidione deposition augmented the surface diffusion of adsorbing molecules and therefore led to a lower number of better-ordered islands. Consequently, while the onset of charge transport was delayed, higher saturation mobility was obtained. The highest, bottom-contact configuration, mobilities of approximately 2.5×10−3cm2/Vs were found for high coverages (50nm) on sputtered samples. The coverage dependence of the mobility showed very different characteristics for the different gate dielectrics, while the change of the threshold voltage with coverage was approximately the same for all systems. An apparent decrease of the mobility with increasing coverage on the less polar PNDPE was attributed to a change in molecular orientation from upright standing in the thin-film phase to tilted in the bulk phase. From temperature-dependent mobility measurements we calculated activation barriers for the charge transport between 110meV and 160meV, depending on the dielectric configuration.

Dynamics of Molecular Orientation Observed Using Angle Resolved Photoemission Spectroscopy during Deposition of Pentacene on Graphite.

A real-time method to observe both the structural and the electronic configuration of an organic molecule during deposition is reported for the model system of pentacene on graphite. Structural phase transition of the thin films as a function of coverage is monitored by using in situ angle resolved photoemission spectroscopy (ARPES) results to observe the change of the electronic configuration at the same time. A photoemission theory that uses independent atomic center approximations is introduced to identify the molecular orientation from the ARPES technique. This study provides a practical insight into interpreting ARPES data regarding dynamic changes of molecular orientation during initial growth of molecules on a well-defined surface.

Molecular orientation behavior of chiral nematic liquid crystals based on the presence of blue phases using polarized microscopic FT-IR spectroscopy

Study on molecular orientation behavior of highly twisted chiral nematic liquid crystals (N∗LCs) expressing blue phases (BPs) is important for developing new devices. This study examines the change of molecular orientation of N∗LCs due to the presence of BPs. Polarized microscopic FT-IR spectroscopy was used to study the in- and out-of-plane molecular orientations of N∗LCs that undergo a phase transition involving BPs. The band intensity ratio of CN to CH2 stretching modes (CN/CH2) in the IR spectra was used to determine the orientation of N∗LC molecules. The measured spectra indicated that the helical axis of N∗LC molecules was perpendicular to the substrate before heating and inclined on the substrate after cooling the sample which has phase transition from BP I to chiral nematic (N∗). The N∗LC molecule in the cell of rubbed orientation film exhibited the in-plane anisotropy after a heating-cooling ramp only in samples that passed through BP I. These results indicate that the changes of molecular orientation of N∗LC by phase transition are affected by BP I.

Coverage induced structural transformations of tetracene on Ag(110).

Self-assembly of tetracene on an anisotropic surface of Ag(110) has been investigated using scanning tunneling microscopy and low-energy electron diffraction. We observe multistage structural transformations of the self-assembled tetracene on Ag(110) as a function of molecular coverages, which are accompanied by the changes in molecular orientations. They are analyzed by a balance between multiple molecule-molecule and anisotropic substrate-molecule interactions.

Influence of PAN-Fiber Stretching during Thermal Treatment on the Stabilization Reactions

The stabilization of PAN-fibers without additional co-monomers was investigated with thermo-gravimetry and evolved gas analysis (FTIR-spectroscopy and MS-spectrometry). One fiber type had been drawn after spinning, while the other was used as-spun. During the thermal treatment, fiber shrinkage was either restricted or unrestricted. Investigations of influencing chemical and physical reactions regarding this restriction were conducted. Differences in the mass loss and gas emissions were observed, depending on the strained or unstrained state of the fibers. The change of crystallinity and molecular orientation of the fiber as reason of the measured variations was discussed. The emission of ammonia and other nitrogen containing gases (supposedly nitriles/ isocyanates) could be attributed to different aspects of the stabilization process. The length restriction resulted in a change in ammonia emission, associated with the cyclization reaction of poly acrylonitrile. The onset and amount of side reactions were influenced as well.

Vibrational spectra of four polycyclic aromatic hydrocarbons under high pressure: implications for stabilities of PAHs during accretion

Infrared and Raman spectra of the polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene, phenanthrene, and pyrene have been examined up to 10–55 GPa at 300 K, to probe structural changes in these materials under high-pressures, and to relate these to shock measurements on these materials. The goal is to develop an understanding of how such hydrocarbons might be processed during planetary accretion. A range of phase transitions in PAHs are observed and, in accord with previous investigations, these typically initiate at relatively low pressures (0.3–4.0 GPa): the lower-pressure transitions are likely associated with inter-molecular changes such as changes in symmetry and/or molecular orientation, charge transfer processes, or changes in π electron density, and are often sluggish. Higher-pressure (7–10 GPa) phase transitions in PAHs are likely associated with profound structural changes like dimerization, which are not always reversible. Laser-induced luminescence is encountered at pressures well below those at which PAHs amorphize, and a strong pressure-induced Fermi resonance is identified between the highest-lying inter-molecular modes and lowest-lying intra-molecular modes in each PAH examined. It is the increased strength of inter-molecular interactions under pressure that likely generates increasing overlap of π orbitals and leads to cross-linking (dimerization) of the molecules and the destruction of their planar symmetry. The first step in the amorphization of these compounds is likely dimerization, and amorphization occurs when long-range order is lost and a greater diversity of local structural environments is introduced into these materials, such as carbons being shared between rings, embayed structures, sp, sp2, and sp3 hybridized carbon atoms, a broad range of C–H bonding environments, and fewer residual resonance-stabilized C–C units. Our results are consistent with pressure producing amorphous, hydrogenated carbon material from PAH precursors: hence, impact phenomena, coupled with post-shock hydrogen loss, could provide an alternate pathway to produce amorphous carbon assemblages of the type observed within a range of meteorites. Additionally, smaller PAHs tend to be most stable under compression; as these are the most volatile of the PAHs, the combination of shock during accretion, coupled with trends in volatility, may limit the presence of PAHs within objects formed in the early solar system.

2C05 偏光顕微赤外分光法を用いたブルー相を発現するキラルネマチック液晶の相転移による分子配向変化の観察(液晶物理・物性,口頭発表,2015年日本液晶学会討論会)

2C05 偏光顕微赤外分光法を用いたブルー相を発現するキラルネマチック液晶の相転移による分子配向変化の観察(液晶物理・物性,口頭発表,2015年日本液晶学会討論会)

Experimental study on molecular arrangement of nanoscale lubricant films—A review

In order to understand lubrication mechanism at the nanoscale, researchers have used many physical experimental approaches, such as surface force apparatus, atomic force microscopy and ball-on-disk tribometer. The results show that the variation rules of the friction force, film thicknessand viscosity of the lubricant at the nanoscale are different from elastohydrodynamic lubrication (EHL). It is speculated that these differences are attributed to the special arrangement of the molecules at the nanoscale. However, it is difficult to obtain the molecular orientation and distribution directly from the lubricant molecules in these experiments. In recent years, more and more attention has been paid to use new techniques to overcome the shortcomings of traditional experiments, including various spectral methods. The most representative achievements in the experimental research of molecular arrangement are reviewed in this paper: The change of film structure of a liquid crystal under confinement has been obtained using X-ray method. The molecular orientation change of lubricant films has been observed using absorption spectroscopy. Infrared spectroscopy has been used to measure the anisotropy of molecular orientation in the contact region when the lubricant film thickness is reduced to a few tens of nanometers. In situ Raman spectroscopy has been performed to measure the molecular orientation of the lubricant film semi-quantitatively. These results prove that confinement and shear in the contact region can change the arrangement of lubricant molecules. As a result, the lubrication characteristics are affected. The shortages of these works are also discussed based on practicable results. Further work is needed to separate the information of the solid-liquid interface from the bulk liquid film.

Molecular Orientation and Structural Transformations in Phthalic Anhydride Thin Films on MgO(100)/Ag(100).

Structural control of organic thin films on dielectric substrates is the key to tailoring the physical properties of hybrid materials, for example, for application in solar energy conversion, molecular electronics, or catalysis. In this work, we investigate the molecular orientation of phthalic anhydride (PAA) films on atomically well-defined MgO(100) on Ag(100) using temperature-programmed infrared reflection absorption spectroscopy (TP-IRAS) in combination with density-functional theory (DFT). A robust procedure is presented to determine the orientation of the PAA molecules, which relies on the intensity ratios of vibrational bands only. We show that even at deposition temperatures of 110 K, the PAA multilayer grows with a specific molecular orientation; that is, the PAA molecular plane is preferentially aligned parallel with the MgO surface. No change of molecular orientation occurs up to a temperature of 145 K. Between 145 and 160 K, the film restructures adopting a nearly flat-lying molecular orientation. Between 170 and 205 K, the film undergoes a second structural transition to a crystalline phase. This transition is associated with a pronounced molecular reorientation. The molecules adopt a tilted orientation and, simultaneously, rotate around their C2 axes. The reorientation behavior suggests that the molecular orientation in the crystalline phase is controlled by the interaction with the MgO(100) substrate. At higher temperature, no further restructuring is observed until the PAA multilayer desorbs at temperatures above 230 K.

Study of metal specific interaction, F-LUMO and VL shift to understand interface of CuPc thin films and noble metal surfaces

The performances of organic electronic devices are significantly associated with their energy level alignment at organic semiconductor/metal–electrode interfaces. The electronic character of an organic semiconducting molecular over-layer on a metal surface can vary from semiconducting to metallic, depending on the nature of the molecular orbitals with respect to the Fermi level of the electrode. The general tendency of extrapolating established models for single crystal substrates to ‘real’ device substrates is highly misleading. Hence, the importance of metal specific interaction, former lowest unoccupied molecular orbital (F-LUMO) and vacuum level (VL) shift have been investigated as a function of thickness of the deposited films by means of photoelectron spectroscopy (XPS and UPS) to understand the interface between CuPc and Cu, Ag, Pt and Au foils sequentially. The XPS data provides the signature of affectability of pyrrole sites of CuPc molecules for partial charge transfer from Cu and Ag substrates while a negligible effect on pyrrole cites resulted for Pt and Au substrates. Furthermore, the appearance of F-LUMO, a hybridized state close to the Fermi level gives confirmatory information about partial charge transfer. Contrary to the general belief that vacuum level shift caused by charge transfer can partially or totally cancel that for push back effect, our observation indicates that the partial charge transfer does not play significant role in the shift of vacuum level. The entire thickness dependent electronic energy level alignment of CuPc films on all noble metal substrates is explained in terms of a combined effect of partial charge transfer and photoemission final state relaxation energy. A systematic variation of HOMO-FWHM was observed with CuPc thickness due to continuous change in molecular orientation.

Surface 2π-walls in polar free-standing smectic films

It has been found that electric field in free-standing smectic films at high temperature leads to formation of unusual 2π-walls, with quite different structure from the structure of classical 2π-walls. The walls are formed by the change of molecular orientation not only along the film plane (as in usual walls) but also in smectic layers between two surfaces of the film. These walls are nuclei of surface field-induced synclinic–anticlinic transition between states with transverse and longitudinal electric polarization.

Vibrational Stark effect (VSE) on the infrared spectrum of a poly(methyl methacrylate) thin film

Electric-field-induced infrared spectra of a poly(methyl methacrylate) (PMMA) thin film have been collected in the temperature range between 77 and 297K by the difference FT-IR method using a cell with a BaF2/Al/PMMA (500nm)/Al layered structure. The observed temperature dependence suggests that the electric-field-induced spectra collected below 150K are due to the vibrational Stark effect (VSE), whereas those collected above 150K are caused by the VSE, electric-field-induced molecular orientation changes, etc. The dependence of the VSE spectrum on the electric field was also measured between 2.0 and 2.8MV/cm at 77K. The slightly asymmetric infrared band with a peak at 1732cm−1 and its VSE spectrum were fitted with two bands with peaks at 1741 and 1731cm−1. The 1731-cm−1 band was assigned to CO stretching vibration. The values of the changes in the electric dipole moment Δμ and polarizability Δα between the ground and the first excited vibrational states of the CO stretching mode were determined to be 0.0633Df−1 and −3.3Å3f −2, respectively, where f is a local field correction factor.

Molecular-orientation-induced rapid roughening and morphology transition in organic semiconductor thin-film growth.

We study the roughening process and morphology transition of organic semiconductor thin film induced by molecular orientation in the model of molecular semiconductor copper hexadecafluorophthalocyanine (F16CuPc) using both experiment and simulation. The growth behaviour of F16CuPc thin film with the thickness, D, on SiO2 substrate takes on two processes divided by a critical thickness: (1) D ≤ 40 nm, F16CuPc thin films are composed of uniform caterpillar-like crystals. The kinetic roughening is confirmed during this growth, which is successfully analyzed by Kardar-Parisi-Zhang (KPZ) model with scaling exponents α = 0.71 ± 0.12, β = 0.36 ± 0.03, and 1/z = 0.39 ± 0.12; (2) D > 40 nm, nanobelt crystals are formed gradually on the caterpillar-like crystal surface and the film growth shows anomalous growth behaviour. These new growth behaviours with two processes result from the gradual change of molecular orientation and the formation of grain boundaries, which conversely induce new molecular orientation, rapid roughening process, and the formation of nanobelt crystals.

Molecular dynamics simulations of pore formation in stretched phospholipid/cholesterol bilayers

Molecular dynamics (MD) simulations of pore formation in stretched dipalmitoylphosphatidylcholine (DPPC) bilayers containing different concentrations of cholesterol (0, 20, 40, and 60mol%) are presented. The stretched bilayers were simulated by constant NPZA||T MD simulations with various constant areas. The effects of the cholesterol concentration on pore formation are examined in terms of the critical areal strain where the pore is formed, the processes of pore formation, and the change in molecular orientation of the DPPC molecules by analyzing the order parameters and radial distribution functions of the DPPC molecules. With increasing cholesterol concentration, the critical areal strain initially increases, peaks at 40mol%, and then decreases, which agrees well with the available experimental data. For the bilayers containing cholesterol, DPPC molecules become disordered at low areal strains, whereas the order slightly increases when the areal strain exceeds a certain value depending on the cholesterol concentration. For 40mol% cholesterol, the two monolayers in the bilayer interpenetrate under high areal strains, inducing an increase of the order parameters and the peak positions of the radial distribution function compared with their states at low areal strains, indicating the formation of an interdigitated gel-phase-like structure. The transient increasing of the order of the molecular orientations may inhibit water penetration into the bilayer, resulting in increased critical areal strain in the phospholipid/cholesterol bilayers.

Response of Linear, Branched or Crosslinked Polyethylene Structures on the Attack of Oxygen Plasma

Linear, branched and crosslinked polyethylenes (PE) were exposed to the low-pressure oxygen plasma for 2–120 s. In the following the samples were washed with solvents to remove low-molecular weight oxidized material and to excavate the subjacent polymer structure for microscopic characterization. X-ray photoelectron spectroscopy (XPS) measurements provided information about changes in elemental composition and chemical structure of PE after plasma exposure and washing. The calculation of the concentration of tertiary C atoms using XPS data was a measure of branches and crosslinking in the polymer before and after exposure to oxygen plasma. Linear PE was most sensitive towards oxygen plasma and showed the highest concentration in tertiary C atoms after plasma exposure. On the other hand branched PE types, which possess originally more tertiary carbon atoms, have lost two-third of them after 2 s oxygen plasma exposure. Branched PE show also topological changes at their surface as detected by atomic force microscopy. Differential scanning calorimetry measurements confirmed strong changes in crystallinity and molecular orientation of linear PE already after 120 s exposure to the oxygen plasma interpreted as amorphization. These effects should be interpreted as result of crosslinking caused by the recombination of dangling bond sites.

Direct Observation of Reversible Biomolecule Switching Controlled By Electrical Stimulus.

Control and reversibility of biomolecular interactions at engineered interfaces presents opportunities to develop highly effi cient substrates and devices for a wide range of biomedical applications. [ 1‐4 ] A major challenge nowadays in the fi eld of stimuli-responsive interfaces is to acquire a molecular understanding of the changes occurring at the biointerface upon external stimulation. Herein, we used in situ Sum-FrequencyGeneration (SFG) spectroscopy to study changes in molecular orientations in electrically switchable biofunctionalized selfassembled monolayers (SAMs). The bioactivity of a mixed SAM on gold consisting of a biotin-terminated positively charged oligopeptide (biotin-KKKKC) and a tri(ethylene glycol)-terminated thiol is shown to be related to a switch between upward exposure and random orientation of the biotin group in response to positive and negative applied potentials, respectively. The fi ndings reported here support the mechanism by which charged biomolecules control biomolecular interactions, for example, protein binding affi nities, and lay the foundation for future studies aiming to explore molecular conformational changes in response to electrical stimuli. Dynamic surfaces are particularly attractive for biomedical applications and are playing an increasingly important part in the development of highly sensitive biosensors, [ 5‐7 ] novel

DNA translocation through graphene nanopores: a first-principles study

With first-principles transport simulation, a biosensor device built from a graphene nanoribbon containing a nanopore is designed for DNA sequencing. The four DNA nucleobases can be distinguished from one another by detecting the transverse-currents of this device. To investigate the transport properties and mechanisms of such a device, we examine the motion effects of nucleobases. The analysis of the transmission spectra and frontier orbital energy shows that the transverse-currents variation of the device strongly results from the long-range interaction between nucleobases and the device. This interaction makes transverse-currents ultra-sensitive to the molecule inside the pore. By rotating the nucleotides inside the pore, the transverse-currents of the device vary along with the changes of molecular orientation. Due to the long-range interaction, when nucleobases chain translocates through nanopore of the device, the influences of adjacent nucleobases on transverse-currents cannot be ignored. These novel effects of nucleobases on the transport capacity of the device provide some theoretical guidance for the design of graphene-based nanopore sensor devices.

Photocurrent Enhancement in Diketopyrrolopyrrole Solar Cells by Manipulating Dipolar Anchoring Terminals on Alkyl-Chain Spacers

We chose DPP-BDT-DPP {DPP=diketopyrrolopyrrole, BDT=4,8-di-[2-(2-ethylhexyl)-thienyl]benzo[1,2-b:4,5-b']dithiophene} as a model backbone and varied the anchoring groups [C5 H11 , COOCH3 , and SiCH3 (OSiCH3 )2 ] terminated on the N-substituted alkyl-chain spacer of the DPP units to study the effect of anchoring terminals on the morphology of blend film and on the device performances of bulk heterojunction solar cells. By replacing the nonpolar C5 H11 anchoring terminal with the polar COOCH3 anchoring terminal leads to an enhancement in the short-circuit current density (Jsc ) (4.62 vs. 9.32 mA cm(-2) ), whereas the value of Jsc sharply decreases to 0.45 mA cm(-2) if the C5 H11 anchoring terminal is replaced by a SiCH3 (OSiCH3 )2 group. The changes in Jsc are associated with changes in the π-π stacking distance (3.39→3.34 Å vs. 3.39→3.45 Å) and the phase size (50→20 nm vs. 50→>250 nm) through alteration of the anchoring group from C5 H11 to COOCH3 versus from C5 H11 to SiCH3 (OSiCH3 )2 . Interestingly, the anchoring terminals bring about drastic changes in molecular orientations, which result in different out-of-plane hole transport. This is the first time this effect has been systemically demonstrated to improve photocurrent generation by manipulating the dipolar anchoring groups terminated on the alkyl-chain spacer.