Isomerization Behavior of an Azopolymer in Terms of the Langmuir−Blodgett Film Thickness and the Transference Surface Pressure

Nanofabrication Techniques in Large-Area Molecular Electronic Devices

The societal impact of the electronics industry is enormous—not to mention how this industry impinges on the global economy. The foreseen limits of the current technology—technical, economic, and sustainability issues—open the door to the search for successor technologies. In this context, molecular electronics has emerged as a promising candidate that, at least in the short-term, will not likely replace our silicon-based electronics, but improve its performance through a nascent hybrid technology. Such technology will take advantage of both the small dimensions of the molecules and new functionalities resulting from the quantum effects that govern the properties at the molecular scale. An optimization of interface engineering and integration of molecules to form densely integrated individually addressable arrays of molecules are two crucial aspects in the molecular electronics field. These challenges should be met to establish the bridge between organic functional materials and hard electronics required for the incorporation of such hybrid technology in the market. In this review, the most advanced methods for fabricating large-area molecular electronic devices are presented, highlighting their advantages and limitations. Special emphasis is focused on bottom-up methodologies for the fabrication of well-ordered and tightly-packed monolayers onto the bottom electrode, followed by a description of the top-contact deposition methods so far used.

Poly(azobenzene acrylate-co-fluorinated acrylate) spin-coated films: influence of the composition on the photo-controlled wettability

The wetting properties of spin-coated films of copolymers based on azobenzene and fluorinated units have been investigated. The copolymers, denoted as poly(Azo-co-AcRf6), have been synthesized by free-radical polymerization of different proportions of acrylate monomers bearing either an azobenzene group or a semifluorinated side chain. The UV-visible spectroscopy analysis of the different spin-coating films through a cycle of UV and visible light irradiation indicates the reversible trans-cis isomerization of azobenzene groups. Simultaneously, atomic force microscopy shows that surface roughness does not exceed 1 nm. Advancing and receding contact angles of water and diiodomethane have been measured before and after UV photoirradiation of the different surfaces. In particular, a decrease in the advancing contact angles has been observed upon trans-cis isomerization of azobenzene groups. Switching variations up to 50° have been evidenced without any introduction of surface nanoroughness. Surface free-energy evaluations have been deduced from these measurements, including dispersive and polar components. The results show that, through surface composition and UV photoirradiation, a large range of surface free-energies can be obtained, from 7 to 46 mN·m(-1).

Supramolecular architecture in Langmuir and Langmuir-Blodgett films incorporating a chiral azobenzene

This article describes the synthesis and fabrication of Langmuir and Langmuir-Blodgett (LB) films incorporating a chiral azobenzene derivative, namely, ( S)-4- sec-butyloxy-4'-[5''-(methyloxycarbonyl)pentyl-1''-oxy]azobenzene, abbreviated as AZO-C4(S). Appropriate conditions for the fabrication of monolayers of AZO-C4(S) at the air-water interface have been established, and the resulting Langmuir films have been characterized by a combination of surface pressure and surface potential versus area per molecule isotherms, Brewster angle microscopy, and UV-vis reflection spectroscopy. The results indicate the formation of an ordered trilayer at the air-water interface with UV-vis reflection spectroscopy showing a new supramolecular architecture for multilayered films as well as the formation of J aggregates. Films were transferred onto solid substrates, with AFM revealing well-ordered multilayered films without 3D defects. Infrared and UV-vis absorption spectroscopy indicate that the supramolecular architecture may be favored by the formation of H bonds between acid groups in neighboring layers and pi-pi intermolecular interactions. Circular dichroism spectra reveal chiro-optical activity in multilayered LB films.

Interfacial assembly of an achiral zinc phthalocyanine at the air/water interface: a surface pressure dependent aggregation and supramolecular chirality

The aggregation and supramolecular chirality of the interfacial assemblies of an achiral phthalcyanine derivative, zinc 2,3,9,10,16,17,23,24-octakis(octyloxy)-29 H,31 H-phthaloxyanine (ZnPc), were investigated, and a surface pressure dependent behavior was observed. It was found that ZnPc could be spread as a Langmuir film on water surface and transferred onto solid substrates by the horizontal lifting method. The compound formed mixed J- and H-aggregates in the transferred Langmuir-Blodgett (LB) films. Deconvolution of the broaden Q-band revealed three main components of the spectra, which corresponded to H- and J-aggregates and medium transition aggregates, whose relative contents could be modulated by the surface pressure at which the films were transferred. On the other hand, the transferred LB films composed of these aggregates showed Cotton effects in circular dichroism spectra when the floating film was compressed over a certain surface pressure although the compound itself was achiral. The cooperative arrangement of the macrocylic ring in a helical manner through the interfacial organization was suggested to be responsible for such optical activity in the LB films. A possible explanation based on the cooperative arrangement of the ZnPc building blocks in a helical sense stacking in the aggregates was proposed.

Structural characterization and properties of an azopolymer arranged in Langmuir and Langmuir-Blodgett films

The fabrication of Langmuir and Langmuir-Blodgett (LB) films of an acid-azopolymer (PAzCOOH) is reported. Several techniques were used in their characterization: surface pressure (pi) and surface potential (DeltaV) isotherms, UV-vis reflection spectroscopy, and Brewster angle microscopy (BAM) for the Langmuir films and contact angle measurements, UV-vis, fluorescence, IR and Raman spectroscopy and scanning electronic microscopy (SEM) for the LB films. Our study reveals that lateral chains of the polymer situate preferentially onto the water surface with the acid group in contact with the water, where aggregates are scarcely formed. Therefore, the lateral chains of PAzCOOH can be treated as individual monomers to determine structural properties of the fabricated Langmuir and LB films. Monomeric treatment has been used to interpret UV-vis reflection spectroscopy, and a monomer model has been performed to represent lateral chains using density functional theory at B3LYP 6-31G(d,p) level of theory to assign the observed vibrational spectra.