Aligned Thin Films Research Articles

Uniaxial Molecular Alignment in Thin Films Composed of Discrete NDI‐T2 Oligomers Investigated by Variable‐Angle Spectroscopic and Imaging Ellipsometry

The understanding of the optical characteristics and the influence of thermally induced crystallization is vital to evaluate newly synthesized organic semiconductors for their potential applications. Variable‐angle spectroscopic ellipsometry (VASE) is employed to determine the complex dielectric function of two recently developed discrete oligomers based on naphthalene diimide (NDI) and bithiophene (T2), applying an anisotropic optical model. It is shown that the as‐cast films are optically near‐isotropic, while the annealed films exhibit pronounced anisotropies with respect to the orientation of the π−π* transition. Differences between the two distinct molecules are observed and discussed. The degree of optical anisotropy is quantified using the average molecular orientation angle φ, associated with the orientation of the π−π* transition dipole, and the related orientation order parameter S. The molecules are shown to form crystallites of several microns in size upon annealing. Imaging ellipsometry is thus additionally employed to study their optical anisotropy on a microscopic scale, and the agreement with results from macroscopic measurements is evaluated.

Interfacial-Assembly-Induced In Situ Transformation from Aligned 1D Nanowires to Quasi-2D Nanofilms.

As the dimensionality of materials generally affects their characteristics, thin films composed of low-dimensional nanomaterials, such as nanowires (NWs) or nanoplates, are of great importance in modern engineering. Among various bottom-up film fabrication strategies, interfacial assembly of nanoscale building blocks holds great promise in constructing large-scale aligned thin films, leading to emergent or enhanced collective properties compared to individual building blocks. As for 1D nanostructures, the interfacial self-assembly causes the morphology orientation, effectively achieving anisotropic electrical, thermal, and optical conduction. However, issues such as defects between each nanoscale building block, crystal orientation, and homogeneity constrain the application of ordered films. The precise control of transdimensional synthesis and the formation mechanism from 1D to 2D are rarely reported. To meet this gap, we introduce an interfacial-assembly-induced interfacial synthesis strategy and successfully synthesize quasi-2D nanofilms via the oriented attachment of 1D NWs on the liquid interface. Theoretical sampling and simulation show that NWs on the liquid interface maintain their lowest interaction energy for the ordered crystal plane (110) orientation and then rearrange and attach to the quasi-2D nanofilm. This quasi-2D nanofilm shows enhanced electric conductivity and unique optical properties compared with its corresponding 1D geometry materials. Uncovering these growth pathways of the 1D-to-2D transition provides opportunities for future material design and synthesis at the interface.

Interplay between Side Chain Density and Polymer Alignment: Two Competing Strategies for Enhancing the Thermoelectric Performance of P3HT Analogues.

A series of polythiophenes with varying side chain density was synthesized, and their electrical and thermoelectric properties were investigated. Aligned and non-aligned thin films of the polymers were characterized in the neutral and chemically doped states. Optical and diffraction measurements revealed an overall lower order in the thin films with lower side chain density, also confirmed using polarized optical experiments on aligned thin films. However, upon doping the non-aligned films, a sixfold increase in electrical conductivity was observed for the polythiophene with the lowest side chain density compared to poly(3-hexylthiophene) (P3HT). We found that the improvement in conductivity was not due to a larger charge carrier density but an increase in charge carrier mobility after doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). On the other hand, doped aligned films did not show the same trend; lower side chain density instead led to a lower conductivity and Seebeck coefficient compared to those for P3HT. This was attributed to the poorer alignment of the polymer thin films with lower side chain density. The study demonstrates that optimizing side chain density is a synthetically simple and effective way to improve electrical conductivity in polythiophene films relevant to thermoelectric applications.

Self-assembly of supramolecular complexes of charged conjugated polymers and imidazolium-based ionic liquid crystals

The use of supramolecular interactions to construct mesogens represents a versatile means of accessing new macromolecular architectures, and material properties in the resulting mesophases. Here, we demonstrate the formation of thermotropic liquid crystalline supramolecular conjugates of sulfonated poly(alkyl thiophenes) backbones coupled to biphenyl-containing 1-alkyl-3-methylimidazolium salts with bromide counter anions. Two types of liquid crystalline imidazolium salts are studied with chemical structures differing in the terminal group of the biphenyl mesogenic unit. Both ionic liquid crystals exhibit multiple phase transitions with lamellar structures, which correspond to crystal and smectic liquid crystalline transitions. Supramolecular complexes of sulfonated polythiophene and ionic liquid crystals display thermotropic liquid crystalline phases with lamellar-like nanostructures. Detailed analysis show that the lamellar periodicity increases in the supramolecular complexes, compared to those of the ionic liquid crystals. Aligned textures were produced in thin films of supramolecular complexes by shear and magnetic field alignment of the liquid crystalline mesophases. The incorporation of ionic liquid crystal units into a conjugated polymer opens opportunities for fabrication of aligned thin films of mixed electron-ion conducting polymers. Such molecular materials could be of interest in electrochemical devices.

Hyper 100 °C Langmuir-Blodgett (Langmuir-Schaefer) Technique for Organized Ultrathin Film of Polymeric Semiconductors.

In this study, we advanced the conventional Langmuir-Blodgett (LB) method to a high-temperature range (above 100 °C) using a newly manufactured LB machine, which is adaptable to a high-boiling-point subphase, as a universally usable apparatus. A sophisticated trough design, with homogeneous heating capability up to approximately 200 °C, together with automatic film compression and Langmuir-Schaefer type film transfer, enabled the fabrication of highly aligned thin films of polymeric semiconductors with uniaxial alignment of polymer backbones, which is desirable for efficient charge transport. Herein, ultrathin films of semicrystalline thiophene-based semiconductors were prepared on ethylene glycol and heated to 80 °C. The analyses of the transferred films with pressure-area isotherms, atomic force microscopy (AFM), polarized optical microscopy (POM), and grazing-incidence wide-angle X-ray scattering (GIWAXS) indicated that the proposed high-temperature LB method allows ideal deposition of high-quality ultrathin films with molecular layer precision at the selected high-temperature conditions. Furthermore, preparing thin-film donor-acceptor-type copolymers in ionic liquids at high temperatures (up to 140 °C) was a challenging task that was successfully demonstrated in this study. Highly ordered thin films of donor-acceptor polymers with a uniaxial backbone orientation were obtained only at 140 °C. The obtained semicrystalline thin films with uniaxially aligned polymer backbones significantly contribute to the two-dimensional overlap of molecular orbitals, which is likely to promote charge transport. The use of the manufactured automatic LB machines is advantageous for better quality films prepared at higher temperatures (even above 100 °C) from various technical viewpoints, including homogeneous heating, constant compression, and automatic film transfer. The novel methodology proposed herein expands the possibilities of the Hyper 100 °C Langmuir-Blodgett technique, which has not been accessible by the conventional LB method with the aqueous subphase.

Direction‐Selectable Ultra‐Highly Oriented State of Donor–Acceptor Conjugated Polymer Induced by Slow Bar Coating Process

AbstractThe molecular orientation of π‐conjugated polymers is a significant factor for the electrical properties of polymer‐based printable devices, because π‐conjugated polymers exhibit a notable fast carrier transport property along their main chain. Although various coating processes are suggested, the challenge to realize high‐degree alignment by one‐step coating without pre‐ or post‐treatments still remains. In this paper, it is reported on the fabrication of an extremely highly aligned thin film by a slow bar coating process using a donor–acceptor π‐conjugated polymer with great aggregability. The 2D order parameter achieved is 0.9, which is the highest obtained among the alignment methods with a one‐step coating process. Furthermore, the direction of the main chain is selectively oriented parallel or perpendicular to the coating direction depending on the solution concentration with high orientation degree. This unique orientation phenomenon seems to be related to the nematic liquid crystallinity of poly[2,5‐(2‐octyldodecyl)‐3,6‐diketopyrrolopyrrole‐alt‐5,5‐(2,5‐di(thien‐2‐yl)thieno [3,2‐b]thiophene)] aggregates formed by the slow coating process. The orientation dynamics is discussed by taking the structure and behavior of the aggregates in the solution into account.

Reduced contact resistance in organic field-effect transistors fabricated using floating film transfer method

This paper presents an in-depth performance-based comparison of organic field-effect transistors (OFETs) prepared using the conventional spin coating (SC) technique and a recently developed floating film transfer method (FTM). A remarkable improvement in the performance of transistors fabricated using FTM was achieved in comparison to their SC counterparts. The estimated value of width-normalized contact resistance in FTM-based OFETs was an order lower in comparison to that of transistors prepared using SC technique. The observed results were credited to a significant enhancement in the length of π-conjugation due to the presence of edge-on oriented polymer chains of active layer deposited using FTM, leading to the lowering of carrier injection barrier at the Au/P3HT interface. These results were well supported through absorption, photoluminescence and Raman measurements as well as the anisotropy measurements using polarized absorption spectra, which also pointed towards the improvement in the polymer chain alignment of thin films prepared by FTM over that prepared by the conventional SC technique. The results indicate thin film morphology as a key towards reducing the contact resistance in OFETs.

Light-Blocking Optical Alignment Rulers for Guiding Layer-by-Layer Integration

Vertical integration of device layers is a prevailing strategy to boost the performance of microchips and optical devices while maintaining a small form factor. In this paper, we introduce light blocking optical alignment rulers (LBOARs) for layer-by-layer alignment of thin films, which can be potentially applied in manufacturing three-dimensional integrated circuits. LBOAR measures the blocking of the light transmitted through two vertically stacked subwavelength aperture arrays (gratings), which are fabricated onto the separate device layers to be aligned. Here, detailed numerical studies and proof-of-concept experiments on LBOAR are presented. The simulation results reveal two different mechanisms governing the light transmission through a pair of rulers: light blocking and induced extraordinary optical transmission (EOT). In the light-blocking regime, the intensity change is very sensitive to the horizontal shift between two rulers. In contrast, EOT-based alignment is significantly disturbed by the vertical separation of the nearly contacting rulers. Therefore, LBOAR can be more advantageous than EOT-based alignment for certain applications that require only horizontal alignment between two layers of devices. To validate the light-blocking concepts, in our experiments, two-layer stacked LBOARs were fabricated using laser lithography, physical vapor deposition, and dry etching. Optical transmission measurements show that in the nearly perfect alignment condition the transmitted intensity in the light-blocking regime drops significantly providing an alignment accuracy better than 200 nm.

(Invited) Thermoelectric properties of Fermi-level tuned and Aligned Single Wall Carbon Nanotubes

Single wall carbon nanotubes (SWCNTs) are one-dimensional materials with sharp van Hove singularities. Preparation of SWCNTs with a selected electronic structure, fabrication of their aligned thin films[1] and tuning their Fermi level allow us to reveal unique phenomena, which cannot be observed or correctly understood in conventional SWCNT random network thin films. For example, we observed anomalously large optical absorption for perpendicular polarization to the tube axis in highly doped aligned SWCNTs, which is related to intersubband transition plasmon phenomena in SWCNTs.[2] Here in this talk, we would like to discuss the thermoelectric phenomena in fermi level tuned and aligned SWCNTs. In our society, a large amount of heat at relatively low temperature, which is emitted from factories, houses, human bodies, and so on, is left unused. Development of high-performance flexible thermoelectric devices is crucial to efficiently convert such unused waste heat into electric power. Understanding of relationships between electrical conductivity, Seebeck coefficient, and thermal conductivity in thin films of flexible materials and tuning of these parameters is crucial to improve the performance. Since the seminal study by Hicks and Dresselhaus,[3] the thermoelectric properties of low dimensional materials have been intensively studied, but it still has been of great importance to experimentally clarify how the one dimensional electronic structures can influence and enhance its thermoelectric properties, although there are several important theoretical predictions for enhancement of thermoelectric properties in one-dimensional electronic systems.[3,4] SWCNTs are a model of one-dimensional flexible materials, and will play important roles for realization of high-performance flexible thermoelectric devices. Previously, we revealed how the location of Fermi-level influences the thermoelectric properties of semiconducting SWCNTs with diameter of 1.4 nm using electrolyte gating approaches.[5] However, in the previous study, the purity of chirality was not enough to clarify the intrinsic characteristics of semiconducting SWCNTs. In this talk, we will discuss the relationships between electronic structures, Fermi-level and thermoelectric properties using single chirality SWCNTs. In addition, we will discuss how the thermoelectric properties and conductivities exhibit isotropic or an-isotropic properties in aligned SWCNT thin films.[6] Acknowledgment: K.Y. acknowledges support by JST CREST through Grant Number JPMJCR17I5, Japan

π-Conjugated polymer nanowires: advances and perspectives toward effective commercial implementation

π-Conjugated polymers have continued to demonstrate their relevance and ability to be used in next-generation electronic and optoelectronic devices. Nanowires formed from semiconducting polymers can be easily produced and aligned, which considerably enhances the polymer’s properties, such as charge transport. This review discusses recent advances in the formation and alignment of semiconducting polymer nanowires. This includes whisker growth and self-assembly methods, as well as methods of nanoscopic confinement through composites and blends. Next, this review explores how semiconducting polymer nanowires have been successfully implemented in field-effect devices, sensors, and thermoelectrics. Recent developments for nanowires fabricated from self-assembled π-conjugated polymers are reviewed. Particular attention is paid to advances in techniques for scalable production of highly ordered nanowires as well as their deposition and alignment in thin films and composites. The benefits of using π-conjugated polymer nanowires, rather than thin films, in electronic devices is examined. Devices investigated include organic field-effect transistors (OFETs), chemical sensors, and thermoelectric devices.

Influence of Lewis Bases on the Mesogenic and Luminescent Properties of Homogeneous Films of Europium(III) Tris(β‐diketonate) Adducts

A new series of neutral protomesogenic EuIII tris(β‐diketonate) adducts with different Lewis bases have been synthesized. Only one of the complexes is liquid‐crystalline and displays smectic A and nematic mesomorphism over a broad temperature range. Quantum‐chemistry and small‐angle X‐ray scattering methods have permitted the evaluation of the influence of the structure of the Lewis base on the anisometry of EuIII adduct molecules and their subsequent supramolecular organization. The structural features of the presented compounds allow the creation of vitrified optically isotropic and self‐organized films with effective red luminescence. The luminescence spectra of aligned thin films of a liquid‐crystalline EuIII complex are also described.

Crowded Star Mesogens: Guest-Controlled Stability of Mesophases from Unconventional Liquid-Crystal Molecules.

The molecular design of crowded hexasubstituted star mesogens based on a benzene core and alternating substitution with oligo(phenylenevinylene) arms and aryl units generates free space between the conjugated arm scaffolds. Various arylcarboxy building blocks, decorated with alkoxy chains, have been incorporated in the void by mixing, hydrogen bonding or covalent bonds to the aryl groups. The mesogens assemble in columnar stacks ranging from soft crystals to rectangular and hexagonal columnar liquid crystals, revealed by polarized optical microscopy, differential scanning calorimetry, X-ray scattering and modelling. The stability of the mesophases is crucially influenced by the binding mode of the arylcarboxy guest building blocks. The origin of the variation in clearing temperature is unravelled by modelling, cohesive energy density considerations and solid-state NMR spectroscopy. The control over the transition temperature is important for the formation of aligned thin films and thus for potential applications.

Ferromagnetic Nanorods in Applications to Control of the In‐Plane Anisotropy of Composite Films and for In Situ Characterization of the Film Rheology

Ferromagnetic nanorods play important roles as active fillers in multifunctional composite films. Many composite films used as inductors and antennae, as recording media, or electromagnetic (EM) filters and polarizers, require the nanorod alignment in a certain direction. The strategy for the in‐plane alignment of nanorods has not been established yet. Since the composite assumes a multistep processing when the material rheological properties change in time, in situ characterization of the film is required. Magnetic rotational spectroscopy (MRS) with ferromagnetic nanorods offers flexibility and accuracy providing desired spatial and temporal resolution in characterization of submicron thick films. Herein, recent progress in understanding of the basic physical principles is presented guiding the nanorod alignment in thin films by external magnetic field and characterization of these films by MRS.

Progression of Alignment in Thin Films of Cylinder-Forming Block Copolymers upon Shearing

Application of shear stress at the surface of a block copolymer thin film has been shown to substantially orient the microdomains in the direction of the applied shear. The present work systematically examines the influence of key material, film, and process parameters on this alignment behavior using a series of cylinder-forming polystyrene–poly(n-hexyl methacrylate) copolymers. A parallel plate rheometer applies a radially dependent stress gradient to the film’s surface through a viscous nonsolvent overlayer. The degree of alignment is assessed using atomic force microscopy and examined as a function of the applied stress. To quantitatively compare the alignment process across different block copolymer films, a melting–recrystallization model is fit to the data, which allows for the determination of two key alignment parameters: the critical stress needed for alignment and an orientation rate constant. For films containing a monolayer of cylindrical domains, as polystyrene weight fraction or overall mol...

Advanced physical chemistry of carbon nanotubes.

The past decade has seen a surge of exciting research and applications of carbon nanotubes (CNTs) stimulated by deeper understanding of their fundamental properties and increasing production capability. The intrinsic properties of various CNTs were found to strongly depend on their internal microstructures. This review summarizes the fundamental structure-property relations of seamless tube-like single- and multiwalled CNTs and conically stacked carbon nanofibers, as well as the organized architectures of these CNTs (including randomly stacked thin films, parallel aligned thin films, and vertically aligned arrays). It highlights the recent development of CNTs as key components in selected applications, including nanoelectronics, filtration membranes, transparent conductive electrodes, fuel cells, electrical energy storage devices, and solar cells. Particular emphasis is placed on the link between the basic physical chemical properties of CNTs and the organized CNT architectures with their functions and performance in each application.

Influence of fluorination in π-extended backbone polydiketopyrrolopyrroles on charge carrier mobility and depth-dependent molecular alignment

The degree of fluorination in π-extended polydiketopyrrolopyrroles is correlated with semiconductor properties in transistors and an improved molecular alignment in thin films using depth-dependent grazing incidence X-ray scattering.

Cylinder Orientation and Shear Alignment in Thin Films of Polystyrene–Poly(n-hexyl methacrylate) Diblock Copolymers

Thin films of cylinder-forming polystyrene–poly(n-hexyl methacrylate) diblock copolymers, PS–PHMA, are attractive as nanolithographic templates, an application which demands good control over the thin-film structure—yet the quality of order achieved to date in PS–PHMA films has fallen short of this ideal. In the present work, a series of PS–PHMA diblocks of varying composition, all forming PS cylinders, are synthesized and their morphology studied as a function of film thickness in both nonsheared and shear-aligned films. In nonsheared films, the cylinder axis orientation relative to the surface switches from parallel to perpendicular as a function of film thickness; this oscillation is damped out as the fraction of the cylinder-forming block (PS) increases, away from the sphere–cylinder phase boundary. In aligned films, thicknesses which possess the highest coverage of parallel cylinders prior to shear show the highest quality of alignment postshear, as measured by the in-plane orientational order parameter of the cylinder axis. In well-aligned samples of optimal thickness, the quality of alignment is limited by isolated dislocations, whose density is highest at high PS contents, and by undulations in the cylinders’ trajectories, whose impact is most severe at low PS contents; consequently, polymers whose compositions lie in the middle of the cylinder-forming region exhibit the highest quality of alignment.

High-throughput analysis of molecular orientation on surfaces by NEXAFS imaging of curved sample arrays.

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy provides detailed information about the orientation and alignment of thin films. NEXAFS is a synchrotron-based technique-the availability of beam-time per user is typically limited to no more than a few weeks per year. The limited availability is currently a true barrier for using NEXAFS in combinatorial studies of molecular alignment. We have recently demonstrated how large area full field NEXAFS imaging allows users to pursue combinatorial studies of surface chemistry. Now we report an extension of this approach which allows the acquisition of orientation information from a single NEXAFS image. An array with 80 elements (samples), containing eight series of different surface modifications, was mounted on a curved substrate allowing the collection of NEXAFS spectra with a range of orientations with respect to the X-ray beam. Images collected from this array show how hyperspectral NEXAFS data collected from curved surfaces can be used for high-throughput molecular orientation analysis.

Meridional Orientation in Biaxially Aligned Thin Films of Hairy-Rod Polyfluorene

The uniaxial chain alignment and equatorial patterning of poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (Mn= 29 kg/mol, Mw = 68 kg/mol) thin films atop rubbed polyimide substrates have been studied by grazing incidence X-ray diffraction. This specific molecular weight yields among the highest observed levels of chain alignment and optical anisotropy and, after thermal annealing, the sample undergoes transformation to a highly textured crystalline hexagonal phase. The two dominant equatorial orientations (i.e., types I and II crystallites) are found to have almost identical meridional orientation distributions. The X-ray deduced orientation is in quantitative agreement with that obtained by optical absorption measurements. It is also shown that the equatorial ordering is paracrystalline in nature, and both type I and type II crystallites are similar in this respect. This equatorial scattering is superimposed on a background of a hexagonal phase polymer with a cylindrically isotropic orientation (type III).

In-plane Molecular Alignment in Thin Films of Pentacene Grown by Solution Casting and Performance of Thin Film Transistors

AbstractWe have fabricated solution-processed thin films of pentacene by casting solution on a substrate and vaporizing solvent. The films with large oriented platelet domains were obtained by directionally grown condition. Molecular alignment in the directionally grown grains has been studied by several kinds of structural analysis. Oriented domains with the width of several hundreds microns and the length in an order of cm of the films were confirmed by polarized microscopy. In-plane crystalline structure of the domain has been studied by grazing incidence X-ray diffraction (GIXD) and strong anisotropy of in-plane crystalline structure was confirmed. Crystalline growth direction of the film was determined to be b-axis from both transmission electron diffraction and GIXD. Thin films transistors (TFTs) with directionally oriented domains of the films were fabricated on electrode patterned substrate. The observed maximum carrier mobility of 2.7 cm2/Vs was comparable to that of single crystal, which indicated that the quality of the film was almost identical with the single crystal. Correlation between FET performance and growth direction was studied and preferred performance of TFTs with the film grown perpendicularly to the channel was observed.