Orientation In Langmuir-Blodgett Films Research Articles

Unidirectional Langmuir-Blodgett-Mediated Alignment of Polyaniline-Functionalized Multiwalled Carbon Nanotubes for NH3 Gas Sensor Applications.

Herein, we report the formation of well-aligned ultrathin films of polyaniline-functionalized multiwalled carbon nanotubes (PANI@MWCNTs) with a high orientational order over a macroscopic area by Langmuir-Blodgett (LB) technique and its enhanced ammonia gas sensing properties. During the interfacial assembly process, the PANI@MWCNTs gradually align to form small ordered blocks at the air-water interface, which further organize as a well-defined oriented monolayer. The orientation and alignment of PANI@MWCNTs in Langmuir films at the air-water interface were systematically studied as a function of interface temperature using transmission electron microscopic analysis. Surface functionalization of MWCNTs with polyaniline was found to overcome the 3D aggregation of CNTs leading to an oriented assembly of PANI@MWCNTs. The formation and stability of the compact monolayer/multilayer structures of PANI@MWCNTs-based LB films have been extensively studied using a π-A isotherm analysis and thermodynamic approach. For the first time, such highly oriented LB films of PANI-functionalized MWCNTs have been employed for ammonia gas sensing applications at room temperature. The sensor was found to exhibit outstanding sensitivity toward NH3 at room temperature compared to random networks, which is attributed to the directed electron transport through the aligned PANI@MWCNTs. The ultrathin LB film allows fast analyte diffusion due to the adequate molecular accommodation in the oriented assembly of the active sensing layer. The large-scale alignment of PANI@MWCNTs demonstrated in this investigation would enable the fabrication of high-density MEMS (micro-electromechanical system)-based nanoscale sensor arrays for high-performance NH3 gas sensor applications.

Controlling Electronic Transitions in Fullerene van der Waals Aggregates via Supramolecular Assembly.

Morphologies crucially determine the optoelectronic properties of organic semiconductors. Therefore, hierarchical and supramolecular approaches have been developed for targeted design of supramolecular ensembles of organic semiconducting molecules and performance improvement of, e.g., organic solar cells (OSCs), organic light emitting diodes (OLEDs), and organic field-effect transistors (OFETs). We demonstrate how the photonic properties of fullerenes change with the formation of van der Waals aggregates. We identified supramolecular structures with broadly tunable absorption in the visible spectral range and demonstrated how to form aggregates with targeted visible (vis) absorption. To control supramolecular structure formation, we functionalized the C60-backbone with polar (bis-polyethylene glycol malonate-MPEG) tails, thus yielding an amphiphilic fullerene derivative that self-assembles at interfaces. Aggregates of systematically tuned size were obtained from concentrating MPEGC60 in stearic acid matrices, while different supramolecular geometries were provoked via different thin film preparation methods, namely spin-casting and Langmuir-Blodgett (LB) deposition from an air-water interface. We demonstrated that differences in molecular orientation in LB films (C2v type point group aggregates) and spin-casting (stochastic aggregates) lead to huge changes in electronic absorption spectra due to symmetry and orientation reasons. These differences in the supramolecular structures, causing the different photonic properties of spin-cast and LB films, could be identified by means of quantum chemical calculations. Employing supramolecular assembly, we propounded that molecular symmetry in fullerene aggregates is extremely important in controlling vis absorption to harvest photons efficiently, when mixed with a donor molecule, thus improving active layer design and performance of OSCs.

The optical characterization of metal-mediated aggregation behaviour of amphiphilic Zn(ii) phthalocyanines.

Ag(+) mediated aggregation behaviour of two different amphiphilic zinc phthalocyanines (Zn-Pcs) (symmetric and asymmetric) has been investigated in solution, at the air-water interface and in Langmuir-Blodgett (LB) films transferred onto glass substrates. A 4 : 1 ([Ag(+)]/[Pc]) complexation was observed for both symmetric and asymmetric Zn-Pcs at high [Ag(+)] concentrations which led to the formation of H-aggregates in solution. At the air-water interface, both symmetric and asymmetric Zn-Pcs showed a tilted, edge-on orientation in columnar stacks. Addition of Ag(+) to the subphase enhanced the order in the monolayers. LB films on glass substrates showed a split Q-band indicating the presence of "herring-bone" type aggregation consisting of both H- and J-aggregates. For LB films deposited from a Ag(+) containing subphase, a shoulder appeared in the absorption spectra at longer wavelengths which indicates that the fraction of J-aggregates was enhanced by Ag(+) in LB films. The molecular orientation in LB films was investigated by polarized absorption spectroscopy and a tilt angle was calculated to be 49° with the substrate normal for symmetric and 74.6° for the asymmetric Zn-Pc. The presence of Ag(+) cations in the subphase decreased the tilt angles slightly by 4-5°. These results indicate that Ag(+) induced the aggregation of Zn-Pcs and acted as a linker between Pc molecules in the ultrathin films of both symmetric and asymmetric Zn-Pcs. These results are important in inducing J-aggregates for the fabrication of molecular devices based on phthalocyanine thin films.

The electrical and optical properties of oriented Langmuir-Blodgett films of single-walled carbon nanotubes

Thin, transparent, electrically-conductive films containing single-walled carbon nanotubes have been fabricated using the Langmuir-Blodgett method. No additional surfactant was used and films up to 99 layers in thickness (300 nm) could readily be built-up. Both optical and electrical measurements revealed anisotropy in the film plane, suggesting alignment of the tubes during the deposition process. For the majority of films, DC conductivity measurements over the range 77–300 K showed an increase in the conductivity with increasing temperature, i.e. semi-conducting characteristics. However, some samples exhibited the opposite effect. Such metallic films reverted to semi-conducting behaviour following an electrical annealing process.

Solid-state Structure and Molecular Orientation in Organized Films of Ternary Comb Copolymers Containing N-vinylcarbazole

The fine structures in the solid state and molecular orientation in Langmuir-Blodgett films of newly synthesized ternary comb copolymers with N-vinylcarbazole were investigated by wide-angle X-ray diffraction (XRD), small angle X-ray scattering, in-plane XRD, out-of plane XRD and atomic force microscopy. From the results of XRD in bulk, two kinds of short spacing peaks based on the formation of the sub-cell for fluorinated and hydrogenated side-chains were confirmed. Further, these ternary copolymers formed highly ordered layer structure. In addition, these ternary copolymers formed extremely condensed monolayers on the water surface. From the out-of plane XRD measurements of multilayers, formation of highly ordered layer structure was conformed. From the results of in-plane XRD and AFM, it was found that phase-separated structures between fluorinated and hydrogenated side-chains could not form in two-dimensional films. These structural changes may be cause by the  interaction between the carbazole rings.

Anisotropy in the optical properties of oriented Langmuir–Blodgett films of OC 1OC 6-PPV

Light emission control is reported in anisotropic Langmuir–Blodgett films from a poly( p-phenylene vinylene) derivative. The films are considered as containing long conjugated segments in the dipping direction, which are efficient emitters, with the short conjugated segments perpendicular to the dipping direction. Because the latter mainly transfer energy to adjacent chains, an interplay between light emission and energy transfer is exploited in further controlling emission, which depends on the polarization of the light.

Langmuir−Blodgett Mixed Films of Titanyl(IV) Pthalocyanine and Arachidic Acid. Molecular Orientation and Film Structure

Langmuir and Langmuir−Blodgett (LB) films of neat titanyl(IV) phthalocyanine (TiOPc) and mixed (TiOPc)−arachidic acid (AA) films were studied. The surface pressure−area isotherms (π−A) were recorded, and an unusual expansion of the surface area per molecule was detected for mixed TiOPc−AA Langmuir monolayers. The expansion is attributed to the TiOPc molecular orientation in the neat films being different from that in the mixed films with arachidic acid. The molecular orientation in LB films was monitored using reflection−absorption infrared spectroscopy (RAIRS). It was found that the TiOPc molecules are tilted with respect to the substrate in the neat film, changing to a preferential face-on orientation in the TiOPc−AA mixed films. Complementary information about molecular organization and film structure was obtained using UV−visible absorption and micro-Raman imaging.

Interface properties of organic materials investigated by using ultraviolet photoelectron spectroscopy

The properties of the interface between a thin layer of native SiO 2 and a low molecular compound were studied by means of ultraviolet photoelectron spectroscopy (UPS). The investigated material, an amphiphilic derivative of 2,5-diphenyl-1,3,4-oxadiazole (NADPO), was prepared by using the Langmuir–Blodgett (LB) technique. It is of interest since its electronic properties do not depend on the preparation conditions and it is also being candidate for use in optoelectronic devices. The UPS measurements were performed at the beam line Seya F2.2 at HASYLAB (Hamburg, Germany). This beamline is of a 1 m Seya–Namioka monochromator type, equipped with two gratings, which cover the photon energy range from 4 to 40 eV. The photoelectrons were collected with a Vacuum Generators ADES 400 angle resolving spectrometer system at room temperature. The base pressure during the experiment was 1×10 −10 mbar. We investigated the interface properties depending on the number of monolayers (2, 4, 6 and 8) of well oriented LB films, monitoring the highest occupied molecular orbital (HOMO) energy position.

Controlled orientation of Langmuir-Blodgett films of substituted phthalocyanines

Attempts have been made to induce perpendicular stacking in Langmuir-Blodgett multilayers of phthalocyanine derivatives. Normally the largely hydrophobic phthalocyanines will lie on their edges to minimise hydrophobic/hydrophilic repulsion with the aqueous subphase. This study has prepared hydrophilic phthalocyanine derivatives that lie flat on the aqueous subphase. Our aim is to maximise the electro-optical effects in the direction perpendicular to the substrate.

Study of nonlinear optical properties of multilayer Langmuir-Blodgett films containing bacteriorhodopsin.

Multilayer oriented Langmuir-Blodgett films of bacteriorhodopsin were prepared and their nonlinear optical properties, including second harmonic generation and photoresponse at a two phase-modulated beams mixing, were investigated. The nonlinear component of refractive index of the films was measured.

Quantitative Estimation of Molecular Orientation in Langmuir-Blodgett Films of Octadecyldimethylamine Oxide and Dioctadecyl-dimethylammonium Chloride

The molecular orientation in the Langmuir-Blodgett (LB) films of octadecyl-dimethylamine oxide (C18DAO) and dioctadecyldimethylammonium chloride (2C18DAC) was estimated by a newly proposed calculation method for the infrared reflection-absorption (RA) spectral data. The Y- and X-type multilayer LB films of both compounds were prepared respectively as LB (Y) and LB(X) for the RA measurements. There found a large difference in thermal stability between the LB(Y) and LB(X) films for both compounds. Above all, the 9-monolayer LB(Y) films showed a structurally heterogeneous property. The molecular orientation in the heterogeneous structure was explored by introducing a new analyzing method. The method revealed that the first monolayer directly deposited on a gold surface had a specific molecular structure arising from the interaction with the substrate. The structure is different from that in the rest layers. The LB(X) films were found to be thermally very stable; the thermal recovery was notably good comparing to the LB(Y) films.

Second harmonic generation investigation on molecular orientation in Langmuir-Blodgett films poled by an electric field

DC electric field induced molecular reorientation in hemicyanine Langmuir-Blodgett multilayers has been investigated by rotation-angle second harmonic generation (SHG) technique. The hemicyanine molecules could align their chromophore axes towards the field direction along the substrate normal, leading to reduction of the dipping-induced in-plane anisotropy and enhancement of the macroscopic optical nonlinearity upon poling at room temperature. With increasing poling field, the SHG intensity increased quadratically and reached an enhancement of an order of magnitude at a field of 1.33×10 6 V/m.

A Proposed Strategy To Design Molecules Predisposed to In-Plane Orientation in Langmuir−Blodgett Films

Assuming that in-plane orientation in Langmuir-Blodgett (LB) films can be obtained from compounds having an anisotropic shape within the plane of the Langmuir film, this particular alignment is expected for calamitic compounds with a side-on polar head or with a hydrophilic function linked to the end of a tilted rodlike group. According to such hypothesis, various calamitic molecules were synthesized and showed indeed in-plane orientation. Clearly, these molecules can be described as two-dimensional calamitic compounds forming a new family ofmolecules with a different orientation than that of the classical disklike derivatives. These experiments demonstrate that analysis of the molecular shape anisotropy enables one to predict in-plane orientation in LB films and to control the main orientation of the compounds within the plane of the multilayers.

Study of Molecular Orientation in Langmuir-Blodgett Films by FTIR

The structure information of orientation and packing of molecular chains can be obtained from infrared transmission and reflection-absorbance (RA) spectra. In the present paper, on the basis of Umemura et al., their FORTRAN program of minicomputer was developed and can be run on 486 personal computer. By comparison of infrared transmission and RA intensities, surface enhancement factors and molecular orientation angle were calculated using the above program, and the influence of complex refractive index, angle of incidence, and thickness of LB film were discussed. These results are consistent with that of Umemura et al.

Spectroscopic and electrochemical studies of N, N′-bis(decamethylcarboxylic)-3,4,9,10-perylene-bis(dicarboxyimide)

The vibrational and electronic characterization of N, N′-bis(decamethylcarboxylic)-3,4,9,10-perylene-bis(dicarboxyimide) prepared by the Langmuir-Blodgett (LB) technique was accomplished, and the average molecular orientation in LB films was calculated from the transmission and RAIRS data. It was found that the short and long chromophore axes are tilted to the perpendicular by about 70° and 28°, respectively. Using electronic spectra together with steady state fluorescence, it was concluded that the packing behavior of perylene tetracarboxylic diimide is mainly determined by II-II aromatic interactions. The electrochemical measurements show the presence of a non-negligible amount of pinhole defects in the prepared films. Monolayers of arachidic acid were used as spacer layers on silver island films, and resonance Raman scattering from the molecules was studied as a function of their spacing from the rough silver layer. A long range SERRS effect with an enhancement dropping off between 11 and 13 nm was obtained.

Initial charge separation kinetics of bacterial photosynthetic reaction centers in oriented Langmuir-Blodgett films in an applied electric field

Abstract The free energy of the initial charge separation in bacterial photosynthetic reaction centers has been modified by placing oriented Langmuir-Blodgett films of the purified protein between external electrodes in a planar capacitor and applying a field of nearly 106 V/cm. The near-infrared transient absorption changes associated with the decay of the excited state of the bacteriochlorophyll dimer and the initial charge separation was measured with 300 fs time resolution with and without applied field. The surprisingly small field induced rate changes of the oriented systems compared to unoriented systems suggest that modulation of the energy gap between excited bacteriochlorophyll dimer and the charge separated state with bacteriochlorophyll monomer reduced is the principal influence of electric field on rate. The field induced quantum yield failure observed at longer timescales appears to be associated with modulation of the bacteriopheophytin to bacteriochlorophyll dimer charge recombination.

Quantitative Analysis of Uniaxial Molecular Orientation in Langmuir-Blodgett Films by Infrared Reflection Spectroscopy

In analyzing optical properties of multilayered Langmuir-Blodgett (LB) films with the uniaxial anisotropy around the surface normal, a new method for calculating infrared reflection absorbances was developed, by extending Drude's anisotropic calculation theory for a two-phase system to Hansen's optical theory for thin isotropic multilayers. With this method, infrared external reflection spectra of a nine-monolayer LB film of cadmium stearate prepared on a gallium arsenide substrate were analyzed to obtain molecular orientations. The result was in fair agreement with that obtained by X-ray diffractometry. Reflection-absorbance spectra of the same LB film on a silver-evaporated slide glass at various temperatures were also analyzed by the same method, and the orientation angle of each molecular group was quantitatively obtained, clarifying the process of disordering with the increase of temperature. Further, the dependence of the degree of disordering on the monolayer location in LB films was discussed in light of the isotope substitution experiment

A Spectroscopic characterization and Langmuir—Blodgett films of N,N'-dialkyl-3,4:9,10-perylenebis(dicarboximide)

The absorption and emission electronic spectra, and the fundamental vibrational frequencies of N-butyl-N'-R-3,4:9,10-perylenebis(dicarboximide) [R = H 3(BPTCDM),  2H 5(BPTCDE),  3H 7(BPTCDPr), (CH 3) 3(BPTCDI) and  5H 11(BPTCDP)] are discussed. The π-A isotherms for Langmuir layers were obtained and limiting areas are given. Langmuir—Blodgett (LB) films of these dye materials were deposited on glass, AgCl and Ag surfaces. Vibrational frequencies from LB films on metal surfaces were observed using surface enhanced resonance Raman Scattering (SERRS), reflection—absorption FTIR spectroscopy (RAIRS). Transmission FTIR spectra of KBr pellets and LB films on AgCl were obtained. The average molecular orientation in LB films was calculated from the transmission and RAIRS data for LB films. The formation of aggregates in the ground and excited state (excimer formation) and the effect of NO 2 and NH 3 gases on the fluorescence signal was also examined.

Preparation of Oriented Langmuir-Blodgett Films of Polysilanes Bearing Hydroxyalkyl or Alkoxyalkyl Groups

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPreparation of Oriented Langmuir-Blodgett Films of Polysilanes Bearing Hydroxyalkyl or Alkoxyalkyl GroupsRikako Kani, Yoshihiko Nakano, Yutaka Majima, Shuzi Hayase, Chien-Hua Yuan, and Robert WestCite this: Macromolecules 1994, 27, 7, 1911–1914Publication Date (Print):March 1, 1994Publication History Published online1 May 2002Published inissue 1 March 1994https://doi.org/10.1021/ma00085a038RIGHTS & PERMISSIONSArticle Views146Altmetric-Citations35LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (473 KB) Get e-Alerts Get e-Alerts

Crown-ether amphiphiles for the preparation of ion-binding Langmuir-Blodgett films

Two crown-ether amphiphiles bearing a hydrophobic azobenzene tail were prepared to construct ion-binding Langmuir-Blodgett (LB) films. Highly oriented LB films were obtained from the amphiphiles owing to the strong stacking interactions between the adjacent azobenzene moieties, in spite of the limited hydrophilicity of the crown-ether residues. Potentiometric measurements showed that the LB films deposited on a glassy-carbon electrode can bind alkali-metal ions selectively from solution.