Nonpolar Phase Research Articles

Cosmetically Approved Short-Chain Alcohol/Triethyl Citrate/Water Surfactant-Free Microemulsions and Potential Application to Transdermal Penetration of α-Arbutin.

Surfactant-free microemulsions (SFMEs) exhibited remarkable advantages and potential, attributed to their similarity to traditional surfactant-based microemulsions and the absence of surfactants. Herein, a novel SFME was developed utilizing cosmetically approved materials, such as short-chain alcohol as an amphi-solvent, triethyl citrate (TEC) as the nonpolar phase, and water as the polar phase. 1,2-Pentanediol (PtDO)/TEC/water combination can form the largest monophasic zone, accounting for ∼74% of the total phase diagram area, due to an optimal hydrophilic (water)-lipophilic (TEC) balance. Comparable to surfactant-based microemulsion, PtDO/TEC/water SFME can also be categorized into three types: water-in-oil, discontinuous, and oil-in-water. As TEC or water is increased, or PtDO is decreased, the nanoaggregates in PtDO/TEC/water SFME grow from <5 nm to tens of nanometers. The addition of α-arbutin (ABN) does not disrupt PtDO/TEC/water SFME, but rather enhances its formation, resulting in a larger monophasic area and consistent size (2.8-3.8 nm) through participating in interface assembly. Furthermore, ABN-loaded PtDO/TEC/water SFME exhibits remarkable resistance to dilution, exceptional stability, and minimal irritation. Notably, PtDO/TEC/water SFME significantly boosts ABN's solubility in water by 2 times, its percutaneous penetration rate by 3-4 times, and enables a slow-release DPPH• radical scavenging effect. This SFME serves as a safe and cosmetically suitable nanoplatform for the delivery of bioactive substances.

Brillouin light scattering and second harmonic generation in barium strontium niobate films

Temperature dependences of Brillouin light scattering spectra from Ba0.5Sr0.5 Nb2O6 (SBN 50) film were obtained. The deviations of the Brillouin shift dependence from linear near ferroelectric phase transition are due to electric fields in the local regions with symmetry breaking. A maximum of Brillouin line width is more than twice of the corresponding width in a bulk crystal of the same composition. This broadening is attributed to the confinement of the acoustic phonons. Optical second harmonic generation measurements (SHG) showed that the transition from the polar to the nonpolar phase occurred at a temperature characteristic of SBN-50.

Enhancing piezoelectric properties of PVDF-HFP composite nanofibers with cellulose nanocrystals

With the increasing interest in flexible self-powering energy devices, piezoelectric polymers such as poly(vinylidene fluoride) (PVDF), PVDF-trifluoroethylene (PVDF-TrFE and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) have garnered substantial research attention. Among these, PVDF-HFP has demonstrated immense research potential in harvesting mechanical energy and converting it to electric energy. It is also particularly attractive due to its low cost compared to some other PVDF co-polymers. In this study, PVDF-HFP nanofibers were produced by electrospinning with cellulose nanocrystal (CNC) employed as a filler to improve their piezoelectric performance. Smooth and bead-free fibers were obtained with up to 1 wt% of CNC in the solution. The inclusion of CNC increased the uniformity of fiber diameter and improved thermal stability. Higher CNC concentration (2–3 wt%) engendered agglomeration and adversely impacted the conversion of non-polar phase to electroactive phase for PVDF-HFP molecules. At the optimal CNC concentration of 1 wt%, the voltage and current sensitivities for 1 cm2 samples were obtained as 0.146±0.048 mV/mN and 0.009±0.003 nA/mN, respectively. The open-circuit voltage output of 2.69±1.11 V was achieved under an average impact force of 40 N, and this increased with increased magnitude and frequency of the applied force. The mechanical properties of composite nanofibers maximized at 0.5 wt% CNC with tensile strength of 37.85±4.35 MPa, elastic modulus of 124.96±42.17 MPa, and strain at failure of 82.32±14.84 %. The improved piezoelectric and mechanical properties of the CNC-aided electrospun PVDF-HFP fibers will open new pathways for flexible energy harvesting systems.

Reversible CsPbBr3 ↔ CsPb2Br5 Transformation via Reverse Micellar Aqueous Solution.

Lead halide perovskites suffer from water and moisture instability due to the highly ionic nature of the crystal structures, though a few groups took advantage of it for chemical transformation via water-assisted strategy. However, direct exposure of the perovskite to bulk water leads to uncontrolled chemical transformation. Here, we report a controlled chemical transformation of CsPbBr3 to CsPb2Br5 triggered by nanoconfined water by placing CsPbBr3 in the nonpolar phase within a reverse micelle. The chemical transformation reaction is probed by using steady-state and time-resolved optical spectroscopy. We observe absorption and photoluminescence in the UV region stemming clearly from the CsPb2Br5 phase upon interaction with the reverse micellar aqueous solution. Transmission electron microscopy and X-ray diffraction measurements further provided the structure and morphology. Our results direct the formation of CsPbBr3-CsPb2Br5 nanocomposite under dry conditions while the chemically transformed CsPb2Br5 phase exists only in moist conditions, which we explain via the CsBr-stripping mechanism.

Unravelling the role of poly(methyl methacrylate) (PMMA) molecular weight in poly(vinylidene fluoride) (PVDF)/PMMA/expanded graphite (ExGr) blend nanocomposites: Insights into morphology, thermal behavior, electrical conductivity, and wetting property

Poly(vinylidene fluoride) (PVDF) based conducting polymer composites with carbon nanomaterials can be used for mechanical energy harvesting through piezoelectric or triboelectric effect. This study aims to investigate the influence of PMMA molecular weight on the electrical, thermal, and wetting properties of PVDF/40 wt.% PMMA blend nanocomposites reinforced with expanded graphite (ExGr). The blend nanocomposites with 40 wt.% PMMA have been prepared by solution blending method by using two different molecular weights of PMMA whose melt flow indices are 2 g/10 min and 2.3 g/10 min. The coexistence of the electroactive gamma and non-polar alpha phases of PVDF in the blend nanocomposites has been confirmed by X-ray diffraction, Fourier transform infrared spectroscopy and differential scanning calorimetry analyses. While overall crystallinity (%) of low molecular weight PMMA employed blend nanocomposites is lower than that of high molecular weight PMMA blended nanocomposites, the electroactive gamma phase has been found to increase in the former blend nanocomposites. The dispersion of graphite nanosheets has been observed to be better in high molecular weight PMMA employed blend nanocomposites which results in higher electrical conductivity. Impedance analysis of PVDF-40 wt.% PMMA-2 wt.% ExGr blend nanocomposite with high molecular weight PMMA results in enhanced interjunction capacitance (74.5 pF) in comparison to low molecular weight PMMA mixed blend nanocomposites (68 pF). Water contact angle (WCA) increases with molecular weight of PMMA and ExGr loading level. Thermogravimetric analysis has shown that the char content (above 500°C) is slightly higher for the blend with low molecular weight PMMA than with high molecular weight PMMA.

Interface engineering for facile switching of bulk-strong polarization in Si-compatible vertical superlattices

Ferroelectric thin films incorporating different compositional layers have emerged as a promising approach for enhancing properties and performance of electronic devices. In recent years, superlattices utilizing various interactions between their constituent layers have been used to reveal unusual properties, such as improper ferroelectricity, charged domain walls, and negative capacitance in conventional ferroelectrics. Herein, we report a symmetry scheme based on the interface engineering in which the inherent cell-doubling symmetry allowed atomic distortions (phonons) in any vertically aligned superlattice activate novel interface couplings among atomic distortions of different symmetries and fundamentally improve the ferroelectric properties. In a materialized case, the ionic size difference between Hf4+ and Ce4+ in the HfO2/CeO2 (HCO) ferroelectric/paraelectric superlattice leads to these couplings. These couplings mitigate the phase boundary between polar and non-polar phases, and facilitate polarization switching with a remarkably low coercive field (Ec\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${E}_{c}$$\\end{document}) while preserving the original magnitude of the bulk HfO2 polarization and its scale-free ferroelectric characteristics. We show that the cell-doubled distortions present in any vertical superlattice have unique implications for designing low-voltage ferroelectric switching while retaining bulk-strong charge storing capacities in Si-compatible memory candidates.

Metal-organic framework/polyurea composite microreactors prepared in oil-in-oil emulsions

A new technique was developed to create composite microcapsules consisting of metal–organic framework (MOF) and polyurea. This method employs oil-in-oil (O/O) emulsion as a template for forming these microcapsules. It involves emulsifying a polar solvent containing MOF reagents within a nonpolar phase containing a suitable surfactant and an isocyanate monomer. Subsequently, the addition of the amine monomer initiates two simultaneous processes: the formation of MOF crystals in the core of polar droplets and the interfacial polymerization to create polyurea shell. Optimization of the method involved adjusting various parameters affecting the composite microcapsules' structure and morphology. These parameters include the surfactant’s type and concentration, the emulsion phases’ type and concentration, and quantity of the MOF component.This method was applied to create palladium-based MOF/polyurea microreactors by encapsulating sodium tetrachloropalladate and then reducing the palladium salt with hydrogen to generate reactive palladium species. The produced catalysts were utilized in the selective hydrogenation of terminal alkynes. It was observed that the selectivity of the catalyst is influenced by the quantity of encapsulated MOF, the amount of encapsulated palladium, and the solvent type used for the reaction.

Pressure-dependent phase transitions in hybrid improper ferroelectric Ruddlesden-Popper oxides

The temperature-dependent phase transitions in Ruddlesden-Popper oxides with perovskite bilayers have been under increased scrutiny in recent years due to the so-called hybrid improper ferroelectricity that some chemical compositions exhibit. However, little is currently understood about the hydrostatic pressure dependence of these phase transitions. Herein we present the results of a combined high-pressure powder synchrotron x-ray diffraction experiment and abinitio study on the bilayered Ruddlesden-Popper phases Ca3Mn2O7 and Ca3Ti2O7. In both compounds we observe a first-order phase transition, that in combination with our density functional theory calculations, we can confidently assign as being between polar A21am and nonpolar Acaa structures. Interestingly, we show that while the application of pressure ultimately favors a nonpolar phase, as is commonly observed for proper ferroelectrics, regions of response exist where pressure actually acts to increase the polar mode amplitudes. The reason for this can be untangled by considering the varied response of octahedral tilts and rotations to hydrostatic pressure and their trilinear coupling with the polar instability. Published by the American Physical Society 2024

Universal responses in nonmagnetic polar metals

We demonstrate that two phenomena, the kinetic magnetoelectric effect and the nonlinear Hall effect, are universal to polar metals as a consequence of their coexisting and contraindicated polarization and metallicity. We show that measurement of the effects provides a complete characterization of the nature of the polar metal, in that the nonzero response components indicate the direction of the polar axis, and the coefficients change signs on polarization reversal and become zero in the nonpolar phase. We illustrate our findings for the case of electron-doped PbTiO3 using a combination of density functional theory and model Hamiltonian-based calculations. Our model Hamiltonian analysis provides crucial insight into the microscopic origin of the effects, showing that they originate from inversion-symmetry-breaking-induced interorbital hoppings that correlate to an asymmetric charge density quantified by odd-parity charge multipoles. Our paper both heightens the relevance of the kinetic magnetoelectric and nonlinear Hall effects, and broadens the platform for investigating and detecting odd-parity charge multipoles in polar metals. Published by the American Physical Society 2024

Phase transitions in ferroelectric ZrO2 thin films

In this work, the formation of the orthorhombic phase of ZrO2 together with the minor monoclinic phase were elucidated by X-ray diffraction and transmission electron microscopy. Moreover, oxidized W can be responsible for the formation of oxygen vacancies in the ZrO2 through oxygen scavenging. The ferroelectric properties of the W/ZrO2/W film capacitors were investigated through piezoresponse force microscopy (PFM) and polarization-voltage measurements.A second-order phase transition from the polar orthorhombic phase to the non-polar tetragonal phase was observed. Density functional theory calculations confirm our experimental results and propose that oxygen vacancies are responsible for the Curie–Weiss temperature of 130 °C, significantly lower than the theoretical value for the bulk.

Headspace – Solid phase microextraction vs liquid injection GC-MS analysis of essential oils: Prediction of linear retention indices by multiple linear regression

AbstractDue to the relative independence from the operational parameters, the linear retention indices (LRIs) are useful tool in gas chromatography-mass spectrometry (GC-MS) qualitative analysis. The aim of the current study was to develop a multiple linear regression (MLR) model for the prediction of LRIs as a function of selected molecular descriptors. Liquid injection GC-MS was used for the analysis of Essential oils (Rose, Lavender and Peppermint) separating the ingredients by a semi-standard non-polar stationary phase. As a result, a sum of 103 compounds were identified and their experimental LRIs were derived relying on reference measurements of a standard mixture of n-alkanes (from C8 to C20). As a next step, a set of molecular descriptors was generated for the distinguished chemical structures. Further, a stepwise MLR was applied for the selection of the significant descriptors (variables) which can be used to predict the LRIs. From an exploit set of over 2000 molecular descriptors, it was found that only 16 can be regarded as significant and independent variables. At this point split validation was applied: the identified compounds were randomly divided into training (85%) and validation (15%) sets. The training set (87 compounds) was used to derive two MLR models by applying i) the ‘enter’ algorithm (R2 = 0.9960, RMSЕ = 17) and ii) the ‘stepwise’ one (R2 = 0.9958, RMSЕ = 17). The predictive power was assessed by the validation set (16 compounds) as follows i) q2F1 = 0.9896, RMSE = 25 and ii) q2F1 = 0.9886, RMSE = 26, respectively. The adequateness of both regression approaches was further evaluated. Newly developed headspace-solid phase microextraction (HS-SPME) procedures in combination with GC-MS were used for an alternative analysis of the studied Essential oils. Twelve additional compounds, not previously detected by the liquid sample introduction mode of analysis, were identified for which the values of the significant descriptors were within the working range of the developed MLRs. For the last compounds, the LRIs were calculated and the experimental data was used as an external set for assessment of the regression models. The predictive power for both regression approaches was assessed as follows: Enter RMSE = 41, q2F2 = 0.9503 and Stepwise RMSE = 40, q2F2 = 0.9521.

A general procedure for finding potentially erroneous entries in the database of retention indices

BackgroundThe NIST retention index database is one the most widely used sources of retention indices. In both untargeted analysis and machine learning studies filtering for potential errors is rather lacking or nonexistent. According to our estimates about 80% of the compounds from both NIST 17 and NIST 20 retention index databases have only one RI value per stationary phase, which makes searching for erroneous values with statistical methods impossible. Manual inspection is also impractical because the database contains more than 300 000 entries. ResultsWe suggest a two-step procedure to find potentially erroneous retention indices based on machine learning. The first step is to use five predictive models to obtain predicted retention index values for the whole database. The second one is to compare these predicted values against the experimental ones. We consider a retention index erroneous if its accuracy (the difference between predicted and experimental value) is in the bottom 5% for each of the five models simultaneously. Using this method, we were able to detect 2093 outlier entries for standard and semi-standard non-polar stationary phases in the NIST 17 retention index database, 566 of those were corrected or removed by the developers in the NIST 20. SignificanceThis is a novel approach to find potentially erroneous entries in a large-scale database with mostly unique entries, which can be applied not only to retention indices. The procedure can help filter and report mishandled data to improve the quality of the dataset for machine learning applications and experimental use.

From NdNiO2 to a Mott Multiferroic BiNiO2

Motivated by the recently discovered superconductivity in Sr-doped nickelate oxides NdNiO2 reduced from NdNiO3 in experiments, we predict a Mott multiferroic BiNiO2 that may be obtained from BiNiO3. There is a ferroelectric structural transition for BiNiO2 from the nonpolar phase to the polar phase with the P4mm space group, which is driven by the lone pair on Bi. The lowest energy ground-state for polar BiNiO2 is a Hubbard Mott insulator with the G-type antiferromagnetic spin configurations. The replacement of Nd by Bi serves as a connecting link between a parent compound of high-temperature superconductor and a Mott multiferroic.

Magnetron-Sputtered Lead Titanate Thin Films for Pyroelectric Applications: Part 2-Electrical Characteristics and Characterization Methods.

Pyroelectric materials are naturally electrically polarized and exhibits a built-in spontaneous polarization in their unit cell structure even in the absence of any externally applied electric field. These materials are regarded as one of the ideal detector elements for infrared applications because they have a fast response time and uniform sensitivity at room temperature across all wavelengths. Crystals of the perovskite lead titanate (PbTiO3) family show pyroelectric characteristics and undergo structural phase transitions. They have a high Curie temperature (the temperature at which the material changes from the ferroelectric (polar) to the paraelectric (nonpolar) phase), high pyroelectric coefficient, high spontaneous polarization, low dielectric constant, and constitute important component materials not only useful for infrared detection, but also with vast applications in electronic, optic, and MEMS devices. However, the preparation of large perfect and pure single crystals PbTiO3 is challenging. Additionally, difficulties arise in the application of such bulk crystals in terms of connection to processing circuits, large size, and high voltages required for their operation. In this part of the review paper, we explain the electrical behavior and characterization techniques commonly utilized to unravel the pyroelectric properties of lead titanate and its derivatives. Further, it explains how the material preparation techniques affect the electrical characteristics of resulting thin films. It also provides an in-depth discussion of the measurement of pyroelectric coefficients using different techniques.

Ultrasound-assisted production of biodiesel from oils with saturated and unsaturated fatty acids

The reaction of biodiesel or FAME (Fatty Acid Methyl Esters) production can become especially slow because of the mass transfer between the non-polar phase (oil) and polar (alcohol) is one of the critical factors. Thus, different technologies have been developed to reduce the reaction time . One of them is ultrasound. Ultrasound is an acoustic wave with a frequency greater than the upper limit of human hearing. In the present work biodiesel from oils rich in unsaturated fatty acids (rapeseed and soybean) and saturated (coconut and palm) has been synthesized using ultrasound as auxiliary energy. For this, an ultrasonic probe at a fixed frequency of 20 kHz and 450 W of power was used. Moreover, it attempted to determine if the chemical composition affects the ultrasound assisted biodiesel synthesis... FAME yields were evaluated in the different oils. The rapeseed, soybean and palm yields achieved above 90 % wt., close to meet with European standard UNE EN 14003, but coconut oil was just over 80% wt. conversion. Thus, it was observed that the differences between the oil yields are determined by chain length of the fatty acids that form part of the triglyceride. Coconut oil has mainly short chain fatty acids opposed to rapeseed, soy and palm fatty acids having 16 carbons or higher. Regarding the reaction time, it was reduced significantly compared to conventional transesterification.

Candidate ferroelectrics via ab initio high-throughput screening of polar materials

Ferroelectrics are a class of polar and switchable functional materials with diverse applications, from microelectronics to energy conversion. Computational searches for new ferroelectric materials have been constrained by accurate prediction of the polarization and switchability with electric field, properties that, in principle, require a comparison with a nonpolar phase whose atomic-scale unit cell is continuously deformable from the polar ground state. For most polar materials, such a higher-symmetry nonpolar phase does not exist or is unknown. Here, we introduce a general high-throughput workflow that screens polar materials as potential ferroelectrics. We demonstrate our workflow on 1978 polar structures in the Materials Project database, for which we automatically generate a nonpolar reference structure using pseudosymmetries, and then compute the polarization difference and energy barrier between polar and nonpolar phases, comparing the predicted values to known ferroelectrics. Focusing on a subset of 182 potential ferroelectrics, we implement a systematic ranking strategy that prioritizes candidates with large polarization and small polar-nonpolar energy differences. To assess stability and synthesizability, we combine information including the computed formation energy above the convex hull, the Inorganic Crystal Structure Database id number, a previously reported machine learning-based synthesizability score, and ab initio phonon band structures. To distinguish between previously reported ferroelectrics, materials known for alternative applications, and lesser-known materials, we combine this ranking with a survey of the existing literature on these candidates through Google Scholar and Scopus databases, revealing ~130 promising materials uninvestigated as ferroelectric. Our workflow and large-scale high-throughput screening lays the groundwork for the discovery of novel ferroelectrics, revealing numerous candidates materials for future experimental and theoretical endeavors.

Hosohedral nodal-line superconductivity in hexagonal ABC Dirac semimetals

The recently identified hexagonal non-polar phase of KZnBi, an archetypal topological semimetal, has been found to cohost superconductivity on the surface. We propose that KZnBi can realize an unconventional topological superconductor featuring the hosohedral formation of nodal lines and Bogoliubov Fermi surface emerging under a magnetic field. Our density functional theory (DFT)-based low-energy model shows that the nonsymmorphic band degeneracy of the Dirac bands generically triggers topological nodal line superconductivity fostered by inter-band Coulomb interaction. In particular, the nodal lines of the gap resemble a hexagonal hosohedron with the Schläfli symbol of {2, 6}. Remarkably, the holohedral nodal line superconductor defines the topological phase boundary of the Bogoliubov Fermi surface in the limit where time-reversal symmetry is restored. Our results demonstrate that line nodes readily inflate to the Bogoliubov Fermi surface under an external magnetic field. We provide an experimentally verifiable explanation for the observed superconductivity and suggest a feasible platform for observing topological superconductivity in the hexagonal ABC ternary systems class.

Relevance of Sulfate Functionalization and Porous Structure of TiO2 Ceramic Foams in Photocatalytic Degradation of Dye Molecules

A one-step route was developed for sulfate functionalization of titania foam, consisting of integrating the sol-gel process with an emulsion template. The resulting material was used to evaluate the relevance of the porous structure and surface sulfate groups in the photocatalytic degradation of Rhodamine B molecules in water under UV irradiation. X-ray diffraction (XRD) analysis showed that the anatase crystalline phase was metastabilized by functionalization of the surface with sulfate species, which caused the crystallite size to decrease from 21 to 10 nm, while the specific surface area increased from 4 to 42 cm2 g-1. Small-angle X-ray scattering (SAXS) measurements during heating of the non-sulfated (E-TiO2) and sulfated (E-TiO2/SO42-) sols emulsified with sodium dodecyl sulfate (SDS) as surfactant and isopropyl myristate as the nonpolar phase, revealed formation of the liquid crystalline structure. Elimination of this mesophase template allowed the preparation of ceramic foams with hierarchical structure formed by meso- and macropore families. The kinetics of photodegradation reaction of Rhodamine B followed a pseudofirst order mechanism. The most efficient photocatalytic activity (higher rate constant and shorter half-life time) was obtained for the foam templated using 20% of SDS (E-TiO2/SO42- 20SDS), which exhibited the highest values for porosity (93%) and specific surface area (80 m3 g-1).

Magnetron Sputtered Lead Titanates Thin Films for Pyroelectric Applications: Part 1: Epitaxial Growth, Material Characterization.

Pyroelectric materials, are those materials with the property that in the absence of any externally applied electric field, develop a built-in spontaneous polarization in their unit cell structure. They are regarded as ideal detector elements for infrared applications because they can provide fast response time and uniform sensitivity at room temperature over all wavelengths. Crystals of the perovskite Lead Titanate (PbTiO3) family show pyroelectric characteristics and undergo structural phase transitions. They have a high Curie temperature (the temperature at which the material changes from the ferroelectric (polar) to the paraelectric (nonpolar) phase), high pyroelectric coefficient, high spontaneous polarization, low dielectric constant, and constitute important component materials not only useful for infrared detection, but also with vast applications in electronic, optic, and Micro-electromechanical systems (MEMS) devices. However, the preparation of large perfect, and pure single crystals of PbTiO3 is challenging. Additionally, difficulties arise in the application of such bulk crystals in terms of connection to processing circuits, large size, and high voltages required for their operation. A number of thin film fabrication techniques have been proposed to overcome these inadequacies, among which, magnetron sputtering has demonstrated many potentials. By addressing these aspects, the review article aims to contribute to the understanding of the challenges in the field of pyroelectric materials, highlight potential solutions, and showcase the advancements and potentials of pyroelectric perovskite series including PbZrTiO3 (PZT), PbxCa1-x (PZN-PT), etc. for which PbTiO3 is the end member. The review is presented in two parts. Part 1 focuses on material aspects, including preparation methods using magnetron sputtering and material characterization. We take a tutorial approach to discuss the progress made in epitaxial growth of lead titanate-based ceramics prepared by magnetron sputtering and examine how processing conditions may affect the crystalline quality of the growing film by linking to the properties of the substrate/buffer layer, growth substrate temperature, and the oxygen partial pressure in the gas mixture. Careful control and optimization of these parameters are crucial for achieving high-quality thin films with desired structural and morphological characteristics.

Polar Polymorphism: A New Intermediate Structure toward the Thin-Film Phase in Asymmetric Benzothieno[3,2-b][1]-benzothiophene Derivatives.

Understanding structure and polymorphism is relevant for any organic device optimization, and it is of particular relevance in 7-decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) since high carrier mobility in Ph-BTBT-10 thin films has been linked to the structural transformation from the metastable thin-film phase to the thermodynamically stable bilayer structure via thermal annealing. We combine here a systematic nanoscale morphological analysis with local Kelvin probe force microcopy (KPFM) that demonstrates the formation of a polar polymorph in thin films as an intermediate structure for thicknesses lower than 20 nm. The polar structure develops with thickness a variable amount of structural defects in the form of individual flipped molecules (point defects) or sizable polar domains, and evolves toward the reported nonpolar thin-film phase. The direct experimental evidence is supported by electronic structure density functional theory calculations. The structure of the film has dramatic effects on the electronic properties, leading to a decrease in the film work function (by up to 1 eV) and a considerable broadening of the occupied molecular orbitals, attributed to electrostatic disorder. From an advanced characterization point of view, KPFM stands out as a valuable tool for evaluating electrostatic disorder and the conceivable emergence of polar polymorphs in organic thin films. The emergence of polar assemblies introduces a critical consideration for other asymmetric BTBT derivatives, which may be pivotal to understanding the structure-property relationships in organic field-effect transistors (OFETs). A precise determination of any polar assemblies close to the dielectric interface is critical for the judicious design and upgrading of high-performance OFETs.