Self-assembled Multilayer Thin Films Research Articles

Micro-Patterned Polystyrene Sheets as Templates for Interlinked 3D Polyelectrolyte Multilayer Microstructures

Polystyrene (PS) is currently mainly used as a sacrificial template during pattern release upon microcontact printing or for self-assembled flat free-standing films. We hereby present a method for creating 3D microstructured free-standing self-assembled polyelectrolyte multilayer (PEM) thin films. The approach presented utilizes a sacrificial microstructured PS template which can be coated with PEM thin films, whereby dissolution of the PS facilitates a free-standing 3D microstructured PEM film. The PS templating approach allows for omitting hot embossing on silicon masters, since PS templates can be prepared by drop casting on silicon masters. The prepared PEM film offers, if gold nanoparticles are incorporated, stimuli-responsive properties like light-induced chamber opening.

High transmittance, photoluminescence properties of PANI: PSS multilayer thin films to develop a hole transport layer for OLED application

The nanocolloidal solution has been spin coated on the substrate layer-by-layer, a self-assembly technique has used to form the multilayer thin films. The optical and structural properties of self-assembled multilayer thin films were characterized by UV-visible, photoluminescence, X-ray diffraction and atomic force microscopy

Poly(aniline-co-o-toluidine):poly(styrene sulfonic acid) nanocolloidal self assembled multilayer thin films as a hole transport layer in organic solar cells

A copolymer, poly(aniline-co-o-toluidine):poly(styrene sulfonic acid) has been synthesized and used as a hole transport layer in polymer solar cells based on poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester. The water dispersible nanocolloidal particles were characterized by Fourier transform infrared spectroscopy, UV–visible spectroscopy, particle size analyzer and transmission electron microscopy. The nanocolloidal solution has been spin coated on the substrate and layer by layer a self-assembly technique has used to form multilayer thin films. The optical and structural properties of self-assembled multilayer thin films were characterized by UV–visible, photoluminescence, X-ray diffraction and atomic force microscopy. The investigation of current–voltage characteristics of the polymer solar cells has revealed that the inclusion of the hole transport layer improves the photovoltaic performance. The current density of the device has been improved by the number of layers of the nanocolloidal copolymer.

Stimuli-responsive self-assembled multilayer azo thin films: Photo-switchable absorbance and morphological characteristics

Light has been employed as a source of energy for surface modulation, tuning and control in photo-responsive azo-incorporating self-assembled polyelectrolyte multilayer thin films. A custom built setup for controlled exposure enabled successful photo-switching. Spectral changes were characterized through UV–vis spectroscopy, while scanning electron microscopy technique was employed to study structural properties and response of the films. The effects of chromophore aggregation and dispersion on thin film photo-responsiveness have been studied, with significant contributions made to a recently proposed model correlating structural and spectral properties of the azo thin films. Structural and optical photo-responses have been acquired for surface tuning and modulation, while photo-reversibility is also achieved to control surface properties cyclically.

Stimuli-responsive self-assembled multilayer azo thin films: Effect of aggregates and salt on significant spectral shifts

Layer-by-layer SAMU thin films of azo dye molecules and PEL exhibited two distinct absorbance maxima in visible and UV regions. This is attributed to the assumption that a fraction of the dye adsorbed as the monomeric form yields a visible peak, while the additional fraction of the oligomeric H-type aggregated form also appeared in the UV region. The UV peak, however, disappeared in films fabricated from salt solution to demonstrate suppression of aggregates and the stimuli-responsive effect.

Novel Azobenzene-Functionalized Polyelectrolytes of Different Substituted Head Groups 3: Control of Properties of Self-Assembled Multilayer Thin Films

Novel azobenzene polyelectrolytes have been used to fabricate biocompatible self-assembled multilayer (SAMU) thin films of variable absorbance, thickness, organization, and morphology. The prepared SAMU films are useful for directed cell growth, and this application relies directly on control of contact and surface energy, and requires the ability to tune the surface characteristics which are critical to their development. The azo polyelectrolytes employed here were similar in their degree of polymerization and repeat unit composition of acrylic acid monomer and azo monomers, and only differ from each other due to the presence of different substituted head R-groups present on the p-position of the aromatic ring of the azo chromophores. Possession of characteristics of both the self-assembly due to acrylic acid groups, and photoswitchability of the azo monomer enable the azobenzene functionalized polyelectrolytes to exhibit novel photo-reversible applications. The azo polyelectrolytes with the substituted R-group pairs of shorter-ionized hydrophilic COOH and SO3H, shorter-non-ionized hydrophobic H and OC2H5, and larger-nonionized hydrophobic octyl C8H17 and C8F17 were used as polyanions and counter charge PDAC used as polycation to fabricate the layer-by-layer SAMU films onto glass and silicon substrates. The fabricated SAMU films were also characterized by various techniques. The UV absorption maxima, λmax p of the SAMU films move to lower wavelength relative to solution to exhibit a blue shift for the hydrophobic R-groups, while this behaviour was not observed for the hydrophilic R-groups. Similarly, the thickness, organization, morphology and other properties of the thin films were found to be dependent on the type of substituted R-groups of the azo polyelectrolytes due to the inter-related factors of ionization, hydrophobicity/hydrophilicity, solubility, and aggregation of azo PEL in the dipping solutions used for fabrication of the SAMU films. Understanding and controlling the adsorption characteristics of azo multilayer thin–film of switchable functionalities are vital to explore their potential for the development and application of new devices in diverse areas of biosensor, drug delivery systems, on-chip microscale chemical process and microfluidics systems.

Novel Azobenzene-Functionalized Polyelectrolytes of Different Substituted Head Groups 1: Synthesis, Characterization and Absorption Spectroscopy Studies

A new class of a series of amphiphilic polyelectrolytes functionalized with azobenzene chromophores have been synthesized and thoroughly characterized by various techniques. A facile two stage strategy is developed, and first involved the preparation of a precursor base polymer, designated as P(APEA), by the free-radical copolymerization of the monomers of acrylic acid (AA) and 2-(phenylethylamino)ethyl acrylate (PEA). In the second step, precursor PAPEA polymers are reacted and post-polymer modified with the diazonium salts to synthesize azobenzene polyelectrolyte, PAPEA-R with different substituted R-groups present on the para position of the aromatic ring of the azo chromophores. The PAPEA-R polyelectrolytes are same in their degree of polymerization and repeat unit compositions of AA and PEA monomers, and only differ from each other by the type of R-groups. The copolymers were classified on the basis of the characteristics of their R-groups into the hydrophilic-ionizeable smaller pair of ─SO3H and ─COOH, the hydrophobic-non-ionizeable smaller pair of ─ H and ─OC2H5, and the hydrophobic-non-ionizeable larger octyl pair of ─C8H17 and ─ C8F17. The prepared copolymers are also characterized by NMR spectroscopy for structure, GPC for molecular weight, and UV-Visible spectroscopy for absorption determination. In DMF solvent, approximately similar absorption maxima, (λmax) values were observed for azo chromophore-containing monomers and after incorporating these into their corresponding polymers structures. However, for the self-assembled multilayer thin films, λmax moved to lower wavelengths to exhibit a blue shift with hydrophobic R-group of ─C8H17, while this behaviour was not observed for hydrophilic R-group of ─COOH. The shift in λmax is found to be highly dependent on the type of substituted R-group, and attributed to aggregation of hydrophobic azo chromophores in DMF:H2O mixture employed for self-assembly. The presence of the ionizeable AA and light-sensitive azo-chromophore functionalized PEA monomers in the PAPEA-R polyelectrolytes impart self-assembling and photoswitchable characteristics, respectively. Through understanding and controlling the solubility and complex solution aggregation behaviour of the different substituted R-groups of azo PEL, their adsorption, thickness, morphology, wetting, molecular-control, and photoresponsiveness can be tailored to enhance the capabilities of the self-assembled multilayer film process in diversified areas of microfluidics, sensing, and controlled release.

Vapor-Phase Molecular Layer Deposition of Self-Assembled Multilayers for Organic Thin-Film Transistor

We report a vapor-phase molecular layer deposition (MLD) of self-assembled multilayer thin films for organic thin-film transistor. In the present MLD process, alkylsiloxane self-assembled multilayers (SAMs) were grown under vacuum by repeated sequential adsorptions of C=C-terminated alkylsilane and aluminum hydroxide with ozone activation. The MLD method is a self-controlled layer-by-layer growth process, and is perfectly compatible with the atomic layer deposition (ALD) method. The SAMs films prepared exhibited good mechanical flexibility and stability, excellent insulating properties, and relatively high dielectric capacitances of 374 nF/cm2 with a high dielectric strength of 4 MV/cm. They were then used as a 12 nm-thick dielectric for pentacene-based thin-film transistors (TFTs), which showed a maximum field effect mobility of 0.57 cm2/V s, operating at -4 V with an on/off current ratio of approximately 10(3).

Rapid Vapor-Phase Fabrication of Organic−Inorganic Hybrid Superlattices with Monolayer Precision

We report a new layer-by-layer growth method of self-assembled organic multilayer thin films based on gas-phase reactions. In the present molecular layer deposition (MLD) process, alkylsiloxane self-assembled multilayers (SAMs) were grown under vacuum by repeated sequential adsorptions of C=C-terminated alkylsilane and titanium hydroxide. The MLD method is a self- limiting layer-by-layer growth process, and is perfectly compatible with the atomic layer deposition (ALD) method. The SAMs films prepared exhibited good thermal and mechanical stability, and various unique electrical properties. The MLD method, combined with ALD, was applied to the preparation of organic-inorganic hybrid nanolaminate films in the ALD chamber. The organic-inorganic hybrid superlattices were then used as active mediums for two-terminal electrical bistable devices. The advantages of the MLD method with ALD include accurate control of film thickness, large-scale uniformity, highly conformal layering, sharp interfaces, and a vast library of possible materials. The MLD method with ALD is an ideal fabrication technique for various organic-inorganic hybrid superlattices.

In situ synthesis of Ag nanoparticles in polyelectrolyte multilayers

Self-assembled multilayer thin films, fabricated by the sequential adsorption of appositivelycharged polyelectrolytes, were used as a template for the in situ nucleation and growthof silver (Ag) nanoparticles. Since weak polyelectrolytes have a pH-dependentdegree of dissociation, the multilayer film formed of polyallylamine (PAH) andpolyacryllic acid (PAA) at low pH values contains free carboxylic acid groupsthat do not electrostatically bind to the positive groups of PAH chains duringthe film assembly. These free acid groups in the polyelectrolyte multilayer filmwere used to bind the silver ions, which were subsequently reduced with aNaBH4 solution to form Ag nanoparticles. Electron-microscopy examination of the films andUV–vis spectroscopy showed that the concentration and size of the Ag nanoparticles aredetermined by the processing conditions of the multilayer assembly. Furthermore, due tothe regeneration of the acid groups upon reduction, the synthesis process can be repeatedlycycled to increase the volume fraction of Ag in the polyelectrolyte multilayerfilms. After three reaction cycles, Ag nanoparticles with an average diameter of7 nm at a volume fraction of 65% were obtained in the polyelectrolyte multilayers(PEMs).

Humidity sensor based on localized surface plasmon resonance of multilayer thin films of gold nanoparticles linked with myoglobin

Layer-by-layer self-assembled multilayer thin films of gold nanoparticles (GNPs) linked with myoglobin (Mb) show substantial sensitivity to humidity at room temperature according to measurements of localized surface plasmon resonance (LSPR) absorption that relies on the interparticle interaction present in the film. The sensor response is reversible, with response and recovery times as low as 5 s. The sensing mechanism is as follows: as the ambient humidity changes, Mb molecules change their size, making the GNP-to-GNP spacing and thereby the interparticle interaction change; the change in the interparticle interaction causes a change in the LSPR absorption of the multilayer thin film. We found that the LSPR band of the multilayer thin film was almost insensitive to both the surrounding refractive index and the adlayer thickness, rendering the multilayer-film-based humidity sensor highly immune to ambient disturbances.

A Preliminary Study of Influence of the Chain Length of Alkanethiols on the Electrochemical Behavior of Redox Self-Assembled Multilayer Thin Films

A Preliminary Study of Influence of the Chain Length of Alkanethiols on the Electrochemical Behavior of Redox Self-Assembled Multilayer Thin Films

Self-assembled monolayer and multilayer thin films on aluminum 2024-T3 substrates and their corrosion resistance study

Aluminum alloy 2024-T3 substrates were modified with self-assembled monolayer or multilayer thin films from four different alkylsilane compounds, octadecyltrichlorosilane (C 18SiCl 3), octadecyltrimethoxysilane (C 18Si(OMe) 3), octyltrimethoxysilane (C 8Si(OMe) 3) and octadecyldimethylchlorosilane (C 18SiMe 2Cl). The water contact angle of the aluminum surface increased substantially after modification by the two trimethoxysilane compounds and trichrolosilane C 18SiCl 3, but decreased after the modification with monochlorosilane C 18SiMe 2Cl. Potentiodynamic polarization test, prohesion exposure, and electrochemical impedance spectroscopic studies revealed that the corrosion resistance of the C 18SiCl 3-modified aluminum alloy substrates improved significantly compared to the unmodified and the two trimethoxysilane-modified aluminum alloy samples. The C 18SiMe 2Cl-modified substrate also exhibited an improved corrosion resistance compared to the unmodified ones. While extensive corrosion appeared on the unmodified aluminum alloy samples after only 20–30 h of prohesion exposure, early stage corrosion was observed from C 18SiCl 3-modified samples after 150–200 h of prohesion exposure and C 18SiMe 2Cl-modified samples after 80 h of prohesion exposure. Based on the results obtained so far, it is believed that a mixed monolayer or multilayer thin film with opposite molecular orientations was formed through the self-assembling of alkylsilane molecules on the aluminum 2024-T3 surface.

Self-Assembly of L-Cysteine-Copper(II)/Copper(I) Multilayer Thin Films on Gold

Self-assembled multilayer thin films have been prepared on Au substrate by alternate surface derivatization with L-cysteine hydrochloride and cupric perchlorate. The layer-by-layer structure at each step of multilayer formation was investigated by X-ray photoelectron spectroscopy. The measurements indicate that there are two structure modes in the multilayers. One is that Cu2+ sandwiches between two amino acid groups. The other one is that Cu+ is bonded through disulfide and thiolate. This process is also confirmed by cyclic voltammetry of Cu ion at different self-assembled multilayers. Steps further on will lead to repeated multilayer films.

Electrochemical Charge Injection into Immobilized Nanosized Gold Particle Ensembles: Potential Modulated Transmission and Reflectance Spectroscopy

Self-assembled multilayer thin films of nanometer-sized gold particles linked with organic dithiols have been prepared on glass, indium tin oxide, and gold substrates. The gold particles within these structures retain their integrity and no sintering occurs as demonstrated by their optical absorbance, ellipsometry, and potential modulated transmission and reflectance spectroscopy. The optical response to electrochemical charge injection into the outermost layer of gold particles is completely different from that of bulk gold electrodes, and it is concluded that the particles have to be regarded as discrete, immobilized quantum-dots.

Strong enhancement of optical absorbance from ionic self-assembled multilayer thin films of nanocluster Pt and polymer dye

The observation of unusually enhanced optical absorbance spectra of ionic self assembled multilayer (ISAM) thin films composed of alternating layers of Pt nanoclusters (<1 nm dimension) and polymer dye is reported. The first bilayer absorbance is found to be considerably larger than that of several succeeding bilayers even though there is no difference in composition for each bilayer. A layer-by-layer-dependent redshift in maximum wavelength position due to the strong coupling of metals and polymer molecules is observed. The saturation absorption is obtained for the first time when the required thickness of the ISAM film is deposited. The large and unusual enhancement effects are attributed to both the charge-transfer mechanism and very large local fields and collective phenomena near the surface of the small metal clusters/electrolytes and at the interfaces between the cluster/polymer multilayers.

Self-Assembled Gold Nanoparticle Thin Films with Nonmetallic Optical and Electronic Properties

Self-assembled multilayer thin films consisting of alternating layers of ∼6-nm Au nanoparticles and α,ω-dithiols have been prepared on glass substrates. They have been studied by UV/Vis spectroscopy, ellipsometry, scanning tunneling microscopy, and temperature-dependent conductivity measurements. The electronic and optical properties of the thin film material are nonmetallic, and the Au particles maintain their individual character without fusion to larger units. Electronic conduction within the films occurs via activated electron hopping.

Self-Assembly of Photoluminescent Copper(I)−Dithiol Multilayer Thin Films and Bulk Materials

Self-assembled multilayer thin films have been prepared on Au and mercaptosilanized glass substrates by alternate surface derivatization with Cu2+ ion and alkanedithiols. The films deposited on glass show intrinsic orange-red photoluminescence and were studied by fluorescence spectroscopy. A strongly photoluminescent solid was also obtained by reaction of 1,6-hexanedithiol with cupric perchlorate in ethanolic solution. Multilayer structures on Au were studied by ellipsometry and X-ray photoelectron spectroscopy (XPS). Both in the solid material and in the self-assembled thin films, XPS measurements suggest Cu is present in the +1 oxidation state. This was also confirmed by cyclic voltammetry of Cu ion adsorbed to a self-assembled monolayer of 1,6-hexanedithiol on gold. Metal−metal interactions in the form of clustering might contribute to the unusual photophysical properties observed in both types of materials.

New electro-active self-assembled multilayer thin films based on alternately adsorbed layers of polyelectrolytes and functional dye molecules

The layer-by-layer self-assembly of a number of quite different functional dye molecules has been accomplished via the alternate spontaneous adsorption of polyelectrolytes and ionic dyes from dilute solutions. Combinations of different functional dyes such as Ponceau SS and Infrared dye 125 were successfully incorporated into self-assembled multilayer heterostructures. Uniform and well defined multilayer films were also prepared using newly synthesized ionic ruthenium(II) polypyridyl complexes and related polymers. Multilayers containing these ionic ruthenium based complexes have been utilized to fabricate light emitting devices. The fabrication and evaluation of new heterostructure thin film devices based on these light emitting materials are discussed as well as the properties of other multilayer thin films containing dye molecules.

Self-Assembled Multilayer Thin Films of Electroactive Polymers and Light Emitting Diodes

Self-Assembled Multilayer Thin Films of Electroactive Polymers and Light Emitting Diodes