Polarized Absorption Measurements Research Articles

Retrievals of Aerosol Size Distribution, Spherical Fraction, and Complex Refractive Index From Airborne In Situ Angular Light Scattering and Absorption Measurements

AbstractAerosol models, composed of size distribution, complex refractive index, and spherical fraction, are derived from a new synergistic retrieval of airborne in situ angular scattering measurements made by the Polarized Imaging Nephelometer and absorption measurements from the Particle Soot Absorption Photometer. The data utilized include phase function (F11), degree of polarization (−F12/F11), and absorption coefficient (βabs) measured at low relative humidities during the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) and Deep Convection Clouds and Chemistry (DC3) field campaigns. The Generalized Retrieval of Aerosol and Surface Properties (GRASP) is applied to these measurements to obtain summaries of particle properties that are optically consistent with the original measurements. A classification scheme is then used to categorize the corresponding retrieval results. Inversions performed on the DC3 measurements indicate the presence of a significant amount of dust‐like aerosol in the inflow of storms sampled during this campaign, with the quantity of dust present depending strongly on the underlying surface features. In the SEAC4RS data, the retrieved size distributions were found to be remarkably similar among a range of aerosol types, including urban and industrial, biogenic, and biomass burning (BB) emissions. These aerosol types were found to have average fine mode volume median radii 0.155 ≤ rvf ≤ 0.163μm and lognormal standard deviations 0.32 ≤ σf ≤ 0.36. There were, however, consistent differences between the angular scattering patterns of the BB samples and the other particle types. The GRASP retrieval predominantly attributed these differences to elevated real and imaginary refractive indices in the BB samples (m532nm≈1.55+0.007i) relative to the two other categories (m532nm≈1.51+0.004i).

Single-crystalline thin-film growth via solution-mediated polymorphic transformation of octahexyl-substituted phthalocyanine and its optical anisotropy

Abstract The fabrication of a single-crystalline thin film of non-peripheral octahexyl-phthalocyanine (C6PcH2) was realized by performing a tetrahydrofuran (THF) vapor treatment on a the spin-coated film. The solution-mediated polymorphic transformation of C6PcH2 was promoted in the THF vapor, and a single-crystalline thin film with α-phase crystal grew in the thin film when setting an appropriate temperature. The thin film exhibited optical anisotropy, such as a high dichroic ratio, and the color of the transmitted light varied from green to transparent depending on the polarization direction. The molecular packing structure on the substrate was determined by X-ray diffraction and polarized absorption measurements.

Direct evaluation of anisotropic carrier mobility in uniaxially aligned polymer semiconductor film by time-resolved microscopic optical second-harmonic generation measurement

Mobility anisotropy in uniaxially-aligned fluorene co-polymer thin film was directly observed by using time-resolved microscopic optical second-harmonic generation (TRM-SHG) imaging. Main-chain orientation of fluorene co-polymer was determined by polarized absorption measurement, and the mobilities in the direction parallel and perpendicular to the main-chain were respectively estimated as cm2 Vs−1 and cm2 Vs−1 from the visualized carrier motion starting from a round-shape electrode. These results indicate that the mobility anisotropy of this sample was 4.0. Activation energy for each direction was also evaluated by the temperature dependence measurement as 117 and 94 meV, respectively. The TRM-SHG method enables us to estimate mobility and activation energy of the oriented polymer film in all directions at once.

Enhancement of Out-of-Plane Mobilities of Three Poly(3-alkylthiophene)s and Associated Mechanism

Poly(3-alkylthiophene) (P3AT) is a popular family of π-conjugated polymers used to study polymer-based flexible electronic devices. To enhance the performance of a smart material for wearable or stretchable-skin devices, it is crucial to optimize both the carrier mobility and the film structure. Here, we present the enhancements of hole mobilities of three P3ATs films along the out-of-plane direction using photoconductivity measurements by rubbing. The three P3ATs have the same conjugated backbone but different alkyl side chain (CnH2n+1) lengths of n = 4, 6, and 12. As the alkyl chain length becomes longer, the hole mobility increased up to 5-fold after rubbing. Polarized absorption and grazing-incidence X-ray diffraction measurements indicate the P3AT (n = 12) film has a highly oriented structure in the in-plane direction and increased π–π stacking in the out-of-plane direction after rubbing due to a low tensile modulus from the long alkyl side chain.

Aligned Carbon Nanotube Thin Films from Liquid Crystal Polyelectrolyte Inks.

Single walled carbon nanotube thin films are fabricated by solution shearing from high concentration sodium nanotubide polyelectrolyte inks. The solutions are produced by simple stirring of the nanotubes with elemental sodium in dimethylacetamide, and the nanotubes are thus not subject to any sonication-induced damage. At such elevated concentrations (∼4 mg mL(-1)), the solutions exist in the liquid crystal phase and during deposition this order is transferred to the films, which are well aligned in the direction of shear with a 2D nematic order parameter of ∼0.7 determined by polarized absorption measurements. Compared to similarly formed films made from superacids, the polyelectrolyte films contain smaller bundles and a much narrower distribution of bundle diameters. After p-doping with an organic oxidizer, the films exhibit a very high DC electrical to optical conductivity ratio of σ(DC)/σ(OP) ∼ 35, corresponding to a calculated DC conductivity of over 7000 S cm(-1). When very thin (T550 ∼ 96%), smooth (RMS roughness, R(q) ∼ 2.2 nm), and highly aligned films made via this new route are used as the front electrodes of carbon nanotube-silicon solar cells, the power conversion efficiency is almost an order of magnitude greater than that obtained when using the much rougher (R(q) ∼ 20-30 nm) and less conductive (peak σ(DC)/σ(OP) ∼ 2.5) films formed by common vacuum filtration of the same starting material, and having the same transmittance.

Ordering Behavior and Linear Dichroism of Pure and Doped Liquid Crystals

In the present paper, C7 liquid crystals' refractive indices have been measured throughout the nematic and isotropic phase, in order to determine the order parameter by using Vuks method and characterization of anisotropic behaviors. The temperature dependence of refractive indices, birefringence and order parameters in the anisotropic phase, has been investigated for this liquid crystal. Furthermore, the dichroic ratios of doped C7 by linear azo dye, obtained on the basis of polarized absorption measurements. Then, the orientational ordering of the doped liquid crystal is analyzed by determining the order parameters, in different dye concentrations. The ordering behavior of a doped liquid crystal mixture is studied. It is found that even in very low compositional percentage of the dyes, the obtained order parameters are strongly affected by increasing the dye concentration.

Enhancement of Out-of-plane Mobility in P3HT Film by Rubbing: Aggregation and Planarity Enhanced with Low Regioregularity

Poly(3-hexylthiophene) (P3HT) is used as a conjugated polymer material for the fabrication of organic solar cells. To enhance the performance of the solar cell, it is crucial to improve the charge transport properties of the out-of-plane direction, which corresponds to the direction normal to the cell. The out-of-plane hole mobility in P3HT films before and after rubbing was investigated. The mobility was measured by the time-of-flight method and was enhanced by a factor of 8 with rubbing. The enhancement of mobility is significantly dependent on the regioregularity (RR) of P3HT and is the largest with low RR (91%). The mobility of low RR becomes almost equal to that of high RR (98%) after rubbing. Polarized absorption and Raman spectral measurements, and optical, atomic force, and scanning electron microscopy observations indicate the mobility enhancement can be attributed to aggregation and molecular planarity enhanced by rubbing. Three enhancement factors (EFs) for the mobility, aggregation, and planarity were quantified by changing the RR. It was thus determined that subtle aggregation produces a large mobility enhancement. According to the results of grazing-incidence X-ray diffraction measurement, the increase of face-on component of P3HT by rubbing was observed and corresponds to the increase of π–π staking of thiophene ring along the out-of-plane direction. Evolution from the disordered to the ordered P3HT structure by rubbing significantly improves hole transport in the out-of-plane direction.

Synthesis and characterization of perylene diimide based molecular multilayers using CuAAC: towards panchromatic assemblies

Three perylene diimides (PDI) were synthesized with propargyl groups at the diimide positions to enable the fabrication of molecular multilayer thin films via sequential copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) coupling reactions in a layer-by-layer (LbL) fashion. The new PDIs with phenoxy (PhO-PDI), dodecylthiol (Thiol-PDI), and pyrrolidine (Pyrr-PDI) substituents in bay positions have optical absorption spectra that span a large portion of the visible region. Multilayer growth on 11-azidoundecylsiloxane coated silica glass and indium tin oxide (ITO) surfaces was demonstrated up to 10 bilayers of each PDI with 1,3,5-tris(azidomethyl)-benzene (N3Mest) via optical and infrared spectroscopy. Polarized UV-visible absorption measurements showed a strong dependence on the orientation of the film in the light path and was consistent with a preferential orientation of the PDI molecules in the film at 40–42° with respect to the substrate surface. Grazing-angle attenuated total reflectance (GATR) infrared spectra of the films show an increase in content of unreacted azides during multilayer growth from N3Mest as a result of the smaller molecular footprint as compared to the PDI components. Electrochemical scans of the multilayer films grown on ITO exhibited reversible two-electron reduction waves for the PDIs with linear increases in charge with bilayer growth. The electrochemically derived surface coverages of PDIs averaged 1.2–1.3 × 1014 molecules per cm2 for each layer added to the multilayer films. The flexibility of the CuAAC based LbL assembly methodology is highlighted by the assembly of a mixed multilayer film containing five layers of each PDI, resulting in a panchromatic film that is a summation of individual chromophores.

Optically induced anisotropy and electrooptics in ferroic organic nanocomposites

The occurrence of linear electrooptics and optical anisotropy is observed in the ferroic crystals \([\hbox {NH}_{2}(\hbox {C}_{2}\hbox {H}_{5})_{2}]_{2} \hbox {CuCl}_{4 }\) embedded into polymethylmethacrylate polymer matrix. The studies were performed for the optically treated bulk crystals and the large-size nanocrystals (with sizes higher than 50 nm) embedded into the polymer matrix. The specimens were both in a form of bulk as well as in the form of the composite films. To understand the mechanisms giving the principal role to the effect we have performed polarized absorption measurements for the samples treated in the external dc-electric field and bicolour 532 nm/1,064 nm laser treatments. The behaviour of the birefringence and of the linear electrooptics coefficients were studied within a temperature range covering the incommensurate phase existence.

Photoinduced Symmetry‐Breaking Charge Separation: The Direction of the Charge Transfer

Even flow: Photoinduced symmetry-breaking charge separation takes place in a few picoseconds in a 1,3-bis(perylene)propane dyad in polar solvents. Polarized transient absorption measurements show that the direction of the charge flow is random and entirely governed by the fluctuations of the solvent orientation around the dyad.

Orientation of spheroid-shaped silver nanoparticles in mesostructured silica films studied by polarized absorption and low-frequency Raman spectroscopy

Orientation of spheroid-shaped silver (Ag) nanoparticles (NPs) embedded in mesostructured silica sol gel films is studied by polarized absorption measurements and low-frequency Raman scattering, two complementary techniques well-adapted to characterize nanostructures in amorphous matrix. Comparison is made with analysis of mesostructured silica sol gel films containing nanospheres. Polarization-dependent absorption spectra clearly evidence spheroid-shaped NP orientation and their mean aspect ratio is estimated from corresponding polar plots. Mean diameter of spherical NPs is deduced from low-frequency Raman scattering spectra as well as size of elongated spheroid NPs.

Development of Magnetically Aligned Single-Walled Carbon Nanotubes-Gelatin Composite Films

Magnetically aligned and highly dispersed single-walled carbon nanotubes (SWNTs)-gelatin composite films were fabricated on a quartz substrate of newly designed disassemblable sample cells. After an SWNTs-gelatin composite sol in each sample cell was exposed to vertical DC magnetic fields to 28 T during the sol-gel transition, a side plate with a polytetrafluoroethylene film attached to the inner surface was removed from the sample cell carefully, and the composite gel left on the other side plate made of quartz was dehydrated in a nonmagnetic circumstance. We have found that the alignment of SWNTs is retained in uniform composite films obtained after the nonmagnetic drying process. Both polarized absorption measurements and polarized Raman scattering measurements were performed in order to confirm the alignment of SWNTs in the composite films consistently. The optical anisotropy of the composite films increased somewhat superlinearly as a function of the magnetic field applied during the sol-gel transition and reached to 0.13 at 28 T. The nonmagnetic drying process would make it easy to apply higher magnetic fields to the fabrication of aligned SWNTs films as it consumes resources to keep operating high field magnets such as hybrid magnets during the drying process.

A Versatile Molecular Layer-by-Layer Thin Film Fabrication Technique Utilizing Copper(I)-Catalyzed Azide−Alkyne Cycloaddition

We have developed a rapid and versatile layer-by-layer (LbL) thin film fabrication method using copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) or "click" chemistry in the construction of molecular multilayer assemblies. Multilayers containing synthetic porphyrins, perylene diimides, and mixtures of the two have been constructed in order to highlight the versatility of this method. Characterization of thin films using UV-vis absorption, water contact angle, and electrochemical techniques indicate that multilayer growth is consistent over tens of layers. Preliminary X-ray reflectivity measurements yield an average bilayer thickness of 2.47 nm for multilayers of 1 and 3 grown on glass. Polarized absorption measurements suggest that the dense thin films exhibit moderate ordering in their molecular structure with partial alignment with respect to the surface normal.

Spontaneous Orientational Ordering of Liquid Crystal Layer During Evaporation onto Silica

The ordering process of nematic liquid crystal (LC) during evaporation onto silica substrate was investigated. The optical second-harmonic generation and polarized absorption measurements were employed to determine the orientational order parameters S 1 and S 2. A sudden increase of S 1 was observed during the first layer deposition while S 2 gradually increased. Interestingly, S 1 was nearly constant after the completion of the first layer deposition, while S 2 was similar to that in the first layer deposition. The results suggest that spontaneous orientational ordering process is induced during the first monolayer formation.

Orientational ordering of 4-pentyl-4′-cyanobiphenyl molecules evaporated on multi-layered polyimide film

Using optical second-harmonic (SH) generation and polarized absorption (PA) measurements, the orientational ordering process of 4-pentyl-4′-cyanobiphenyl (5CB) was studied during evaporation onto the alignment layer of polyimide (PI) Langmuir–Blodgett films. The intensity of SH signal increased in the beginning of the deposition, but it saturated after the first 5CB monolayer was formed. The PA measurement suggested that a layer-by-layer growth was induced after the formation of 5CB monolayer with accompanying the orientational ordering. The orientational order of the first 5CB monolayer was studied in terms of the orientational order parameters, S 1 = 〈cos θ〉 and S 2 = 〈(3cos 2 θ − 1)/2〉, using the SHG and PA measurements. It was found that S 1 decreased as the thickness of PI alignment layer increased from 0.4 to 4.4 nm, while S 2 was nearly constant. These results indicate that the Coulomb interaction between the permanent dipole of 5CB and its image dipole makes a significant contribution to the orientational ordering of deposited 5CBs.

Chiral Specificity of Doubly Resonant Sum-Frequency Generation in An Anisotropic Thin Film

The chiral specificity of infrared-visible sum-frequency generation (SFG) is investigated under doubly resonant (DR) conditions. An anisotropic thin film made of stacks of chiral supramolecular assemblies is investigated as a function of incoming and outgoing light polarizations. SFG experiments, crossed with in situ polarized absorption measurements, show that the double resonance process leads to a selection of the nonvanishing components of the chiral second-order susceptibility tensor. Specifically, it is demonstrated that a chiral component overcomes the achiral ones. This result is linked to the model for DR-SFG and to the selection rules of the vibration modes and electronic transition involved in the process.

Optical Anisotropic Properties of m-Plane GaN Film Grown by Metalorganic Chemical Vapor Deposition

Spontaneous and piezoelectric polarization could lower the efficiency of GaN-based light-emitting diodes. In order to eliminate or reduce this undesirable effect, m-plane GaN film was prepared by metalorganic chemical vapor deposition (MOCVD) on LiAlO 2(100) substrate with a GaN buffer layer. Since the c axis of the m-plane GaN lays in the grown plane, the breakage of in-plane symmetry gave rise to an optical anisotropy, which was revealed by a difference in refractive indexes of E-field parallel and perpendicular to the c axis of the m-plane GaN measured by polarized reflection measurement. In addition, in-plane strain anisotropy due to the lattice mismatch between the GaN film and the LiAlO 2 substrate changed the Electronic Band Structure (EBS). The change of EBS also led to an in-plane optical anisotropy. Polarized absorption and Photoluminescence (PL) measurements showed a split in energy of 32 meV for optical absorption edge and 37 meV for PL peak energy respectively.

Observation of orientational ordering processes of 4- n-pentyl-4′-cyanobiphenyl molecules during evaporation onto polyimide Langmuir–Blodgett films by optical polarized absorption measurements

Orientational ordering processes of 4- n-pentyl-4′-cyanobiphenyl (5CB) molecules were examined, in situ, using optical polarized absorption technique whilst 5CB molecules were evaporated onto synthetic silica substrates coated with n-layer Kapton-type polyimide Langmuir–Blodgett film ( n = 3, 5, 7, and 11). It was found that the second orientational order parameter S 2 of 5CBs deposited onto the 3-layer polyimide substrate is lower than S 2 of 5CB deposited onto the 5-, 7-, and 11-layer polyimide-coated substrates when the amount of the deposited LC molecules is 10 times as much as that of a single monolayer. The results were discussed in terms of anchoring phenomena.

Crystal growth and spectroscopic analysis of codoped (Ho,Tm):KGd(WO 4 ) 2

We present the crystal growth of monoclinic KGd(WO4)2 codoped with holmium and thulium by a top-seeded solution growth and slow cooling method. Macrodefect-free crystals with sizes about 8.5×5×12 (\(a^{\ast}\times{b}\times{c}\), dimensions in mm) were obtained. A complete study of the spectroscopic parameters of the holmium emission \({} 5 I_7\to{} 5 I_8\), around 2 microns, sensitized by the thulium ion, was made. Polarized absorption measurements were performed to characterize the optimum pumping wavelength in the system. The maximum value of the cross section is 8.0×1020 cm2 with E//Np direction at a wavelength value of 794 nm. The emission cross section of the 2-micron holmium transition has been calculated by the reciprocity method. The experimental lifetime of the 5I7 level has been measured in relation to holmium and thulium concentrations.

Monolayer alignment on azobenzene surfaces during UV light irradiation: Analysis of optical polarized absorption measurement results and theoretical treatment

The influence of the charge separation during the trans-cis conformational change on the surface of azobenzene 6Az10PVA monolayer on the polar liquid-crystal monolayer film, such as 4-n-pentyl-4'-cyanobiphenyl(5CB), is investigated. The effective anchoring energy (in the Rapini-Papolar form) is phenomenologically described in the framework of the molecular model, which takes into account the interaction between the surface polarization and surface electric field, for number of conformational states of the boundary surface. It is shown, using the experimental data for the voltage across the 6Az10PVA+5CB film, provided by the surface-potential technique, that the charge separation during the conformational changing, caused by the UV irradiation, may lead to changing of the surface alignment of liquid-crystalline molecules. The influence of the photoisomerization process on the orientational order parameter S2(t) using the optical polarized absorption measurement is also investigated.