Ultrathin Organic Films Research Articles

Metal-Induced Photoluminescence Quenching of Organic Molecular Crystals

We present a detailed study of the thickness dependence of the photoluminescence of ultrathin organic films on Au(111). For the evaluation of the observed intensities a new differential analysis method is introduced. The obtained quantum yield indicates a complete quenching in the thickness range below 4 monolayers (ML) for the two crystal polymorphs and saturates above 12 ML. Several models of energy transfer were applied to our data. We conclude that only consideration of both exciton diffusion and dissociation at the organic−metal interface in combination with classical energy transfer results in a complete and satisfactory description.

Assembly, structure, and performance of an ultra‐thin film organic field‐effect transistor (OFET) based on substituted oligothiophenes

AbstractWe report on the improved assembly and characterization of a small molecule organic field‐effect transistor (OFET). Novel α,ω‐dicyano substituted β,β′‐dibutylquaterthiophene molecules (DCNDBQT) were synthesized and characterized by UV–Vis spectroscopy, differential scanning calorimetry, thermal gravimetric analysis and cyclic voltammetry. The ultra‐thin organic film formation on TiO2 templates was effectively promoted through the specifically designed bifunctional self assembly molecules (SAM) 5‐cyano‐2‐(butyl‐4‐phosphonic acid)‐3‐butylthiophene (CNBTPA). Excellent structural properties were found for up to 9 DCNDBQT molecule thick films prepared through UHV vacuum sublimation as investigated with UHV non‐contact atomic force microscopy (nc‐AFM) and X‐ray diffraction. Both X‐ray and nc‐AFM data indicate that the DCNDBQT molecules form a well‐ordered terraced structure exhibiting step heights of 1.5 nm to 2.0 nm layers. Hence, the DCNDBQTmolecules are linked to the functional SAM interface layer by H‐bond interactions (see structure model) standing quasi perpendicular to the TiO2 template, and thus providing optimal orbital overlap neigh‐bouring thiophene rings.The vacuum sublimated DCNDBQT molecules form a closed packed and dense molecular layer that was used to construct and operate a nanoscopic OFET‐structure. The resulting field mobilities of 10–5 cm2 V–1 s–1 reflect a high current density in our ultrathin but highly ordered structure. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface-Enhanced Infrared Absorption: Infrared Ellipsometry of Au Evaporated Ultrathin Organic Films

Surface-Enhanced Infrared Absorption: Infrared Ellipsometry of Au Evaporated Ultrathin Organic Films

Use of ultra-thin organic silane films for the improvement of gold adhesion to the silicon dioxide wafers for (bio)sensor applications

A mono-functional silane reagent, 3-mercaptopropyltrimethoxysilane (MPS) was used to modify the surface of silicon wafers. The structure of the SAMs formed with the MPS was investigated by contact angle measurements, ellipsometry, AFM, and X-ray photoelectron spectroscopy (XPS). The deposition of a metallic gold layer via ultra-high vacuum (UHV) evaporation reveals good adhesion properties on Au/MPS/SiO 2/Si structure. The “chemisorption” between the SAM and the gold evaporated layer is confirmed by adhesion tests and optimum curing treatment is found 1 h at 100 °C). This very simple methodology, avoiding the usage of Cr and other metals as undercoating layers and could be proposed further for (bio)sensors applications.

Studies of electrochemically grafted thin organic layers on inorganic surfaces with infrared spectroscopic ellipsometry

We present IR spectroscopic ellipsometry (IRSE) measurements of electrochemically grafted organic ultrathin films down to monolayer thickness. The formation of organic layers of 4-methoxybenzene (anisole) on TiO2, Au, and Si(111) surfaces was confirmed from observation of the respective absorption bands in the ellipsometric spectra. We discuss the orientation of molecules in a thin film on an Au substrate and in a thick film grafted on a TiO2 substrate. The appearance of silicon oxide related bands in the IRSE spectra shows that oxidation is a side reaction of the electrochemical grafting on Si substrates. These results demonstrate the potential of IRSE application in studies of interfacial structures and thin film engineering.

Controlled fabrication of multilayered 4-(pyrrole-1-yl) benzoate supported poly(3,4-ethylenedioxythiophene) linked hybrid films of Prussian blue type nickel hexacyanoferrate

The ability of 4-(pyrrole-1-yl) benzoic acid (PyBA) to form monolayer-type carboxylate-derivatized ultra-thin organic films on solid electrode surfaces was explored here to attract coordinatively and immobilize Ni 2+ ions at the electrode/electrolyte interface. In the next step, the system was exposed to Fe(CN) 6 3− or Fe(CN) 6 4− solution to form a robust nickel hexacyanoferrate (NiHCF) layer. By repeated and alternate treatments in solutions of PyBA, Ni 2+ cations, and Fe(CN) 6 3− or Fe(CN) 6 4− anions, the amount of the material could be increased systematically in a controlled fashion to form three-dimensional multilayered NiHCF-based assemblies. The layer-by-layer method was also extended to the growth of hybrid conducting polymer stabilized NiHCF films in which the initial PyBA-anchored NiHCF layer (formed on glassy carbon) was subsequently exposed (a desired number of times) through alternate immersions to the monomer (3,4-ethylenedioxythiophene), Fe(CN) 6 3− and Ni 2+ solutions. During voltammetric potential cycling (electropolymerization) in the external supporting electrolyte solution, poly(3,4-ethylenedioxythiophene) or PEDOT linked NiHCF-based multilayered films were produced. They were characterized by good stability and high dynamics of charge transport.

Perspective on the Development of Microplasma Technology: The Role of Oxygen in Downstream Microwave Plasma

AbstractWe studied the role of a minor fraction of oxygen in nitrogen and argon downstream microwave plasma in view of plasma‐processing of ultrathin organic films ‐ self‐assembled monolayers (SAMs). As test systems we used SAMs of alkanethiolates on gold substrate. The plasma‐induced processes in the films, including their chemical modification, oxidation, and partial decomposition, were monitored in detail. These processes could be directly correlated with the presence of reactive, oxygen‐derived species in the plasma. The major plasma‐induced processes could be well described by the first order kinetics; the respective reaction rates were also derived. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Tunable Electrochemical Switches Based on Ultrathin Organic Films

We have performed electroreduction of bisthienyl aminobenzene and thienyl aminobenzene diazonium salts on glassy carbon and polycrystalline gold and generated ultrathin organic films. This method allows covalent bonding between the corresponding aromatic radical and the electrode. Electrochemical behaviors of these modified electrodes toward reversible outer-sphere redox species show unusual properties. No current is observed in the potential range where redox probe reactions on bare electrodes usually occur. The organic layer totally blocks the electrode in such a potential window. Above a threshold voltage, the current dramatically increases and an “irreversible” wave is observed. These results reveal that ultrathin organic layers constitute a reversible conductance switch. Moreover, the threshold voltage of this organic layer can be tuned by nature of the π-conjugated molecules grafted onto the electrode. Such junctions based on conjugated oligomers with a well-defined metal/oligomer interface retainin...

Polarized Near-Field Optical Study of Cavity Modes of Lasing and Amplified Spontaneous Emissions

To elucidate the optical coupling nature of near-field scanning optical microscope (NSOM) probes we observed by a NSOM the lasing field of commercial laser diodes (LDs) on their cleaved surface that was expected to be an ideal single TE00 mode and be polarized perpendicular to the probe axis. A clear dark region was observed at the lasing active region of the LD, indicating that the optical probe coupled only the target near-field polarized parallel to the probe's long axis. Many concentric bright rings, detected in the near-field emission profile extending outside of the optical guide layer, should be polarized perpendicular to the TE00 mode, and were ascribed to amplified spontaneous emissions from recombination between an electron and a light hole, based on the symmetry of radiation field. Another possibility was an optical coupling between the optical probe and the laser diode cavity, which might modify the original field distribution inside the laser diode cavity. In both cases the observation suggests that a near-field probe works as a novel tool to manipulate the cavity electromagnetic field, which is a common issue to many applications of cavities made from organic ultrathin films.

Immobilization of Biomaterials to Nano-Assembled Films (Self-Assembled Monolayers, Langmuir-Blodgett Films, and Layer-by-Layer Assemblies) and Their Related Functions

For utilization of highly sophisticated functions of biomaterials in nano-scale functional systems, immobilization of biomaterials on artificial devices such as electrodes via thin film technology is one of the most powerful strategies. In this review, we focus on three major organic ultrathin films, self-assembled monolayers (SAM), Langmuir-Blodgett (LB) films, and layer-by-layer (LBL) assemblies, and from the viewpoints of biomaterial immobilization, typical examples and recent progresses in these film technologies are described. The SAM method allows facile contact between biomaterials and man-made devices, and well used for bio-related sensors. In addition, recent micro-fabrication techniques such as micro-contact printing and dip-pen nanolithography were successfully applied to preparation of biomaterial patterning. A monolayer at the air-water interface, which is a unit structure of LB films, provides a unique environment for recognition of aqueous biomaterials. Recognition and immobilization of various biomaterials including nucleotides, nucleic acid bases, amino acids, sugars, and peptides were widely investigated. The LB film can be also used for immobilization of enzymes in an ultrathin film on an electrode, resulting in sensor application. The LBL assembling method is available for wide range of biomaterials and provides great freedom in designs of layered structures. These advantages are reflected in preparation of thin-film bio-reactors where multiple kinds of enzymes sequentially operate. LBL assemblies were also utilized for sensors and drug delivery systems. This kind of assembling structures can be prepared on micro-size particle and very useful for preparation of hollow capsules with biological functions.

Abnormal pinning of the Fermi and vacuum levels in monomolecular self-assembled films

Ultrathin organic monomolecular films, while electric insulators, are generally believed to be in electrical contact with a photoelectron spectrometer when adsorbed on metal substrates due to electron tunnelling that results in alignment of the Fermi levels of the film and the spectrometer. We show, however, that this generally applicable model is incorrect for these systems and that energy level alignment in photoemission experiments occurs in a complex fashion, involving both vacuum and Fermi levels of the monomolecular film, conductive substrate, and the spectrometer.

Reduced intermolecular interaction in organic ultrathin films

Submonolayer sensitivity was achieved using in situ ellipsometry to monitor the evolution of the dielectric function of tris(8-hydroxyquinoline)-aluminum(III) (Alq3) layers from submonolayer coverage to thick bulklike layers. The Alq3 layers were deposited under ultrahigh vacuum conditions onto hydrogen passivated silicon. The characteristic vacuum-ultraviolet (VUV) absorption lines of Alq3 were detected using synchrotron radiation as light source. In such ultrathin films the absorption lines corresponding to molecular transitions of the Alq3 are found to be spectrally blueshifted with respect to bulklike layers. We attribute the shift to the effect of reduced intermolecular interaction in the submonolayer regime.

Deposition of ultrathin organic films on various carbon surfaces using vacuum vapor deposition

AbstractIn this report, we describe the bonding characteristics of a model lubricant, perfluoro‐1‐decanol (P1D), having a hydroxyl end group, toward N and Si incorporated amorphous carbon surfaces. The P1D is deposited on the carbon surfaces using vacuum vapor deposition, and the P1D bonding toward the carbon surfaces is compared to that deposited by immersion, a traditional deposition method. The P1D molecules are strongly bonded to the carbon surfaces by the vacuum vapor deposition, whereas the bondings are minimal in the case of immersion. The N and Si incorporation enhances the bonding of P1D toward the carbon surfaces. This is due to the change in the surface structure of the hydrogen‐terminated carbon surfaces by N and Si incorporation into the carbon network, which increases the adsorption sites on the carbon surfaces. Copyright © 2006 John Wiley & Sons, Ltd.

In situ X-ray synchrotron study of organic semiconductor ultra-thin films growth

In this work we present an X-ray diffraction study of the early stages of growth of an organic semiconductor (sexithiophene, T6) thin film prepared by high vacuum sublimation. Specular reflectometry and grazing incidence X-ray diffraction were used to monitor the formation of T6 films on silicon oxide. Our results show that T6 grows as a crystalline layer from the beginning of the evaporation. The reflectometry analysis suggests that, in the range of rates and temperatures studied, the growth is never layer by layer but rather 3D in nature. In-plane GIXD has allowed us to observe for the first time a thin film phase of T6 formed of molecules standing normal to the substrate and arranged in a compressed unit cell with respect to the bulk, i.e. the unit cell parameters b and c are relatively smaller. We have followed the dynamics of formation of this new phase and identified the threshold of appearance of the bulk phase, which occurs above ≈5–6 monolayers. These results are relevant to the problem of organic thin film transistors, for which we have previously demonstrated experimentally that only the first two monolayers of T6 films are involved in the electrical transport. The layers above the second one do not effectively contribute to charge mobility, either because they are more “disordered” or because of a screening of the gate field.

Dendrimers at the air-water interface: surface dynamics and molecular ordering

In recent years considerable effort has been devoted to the fabrication of molecular devices by exploiting ultra thin organic films. Our attention has been focused on dendrimers, one of the most promising candidates for use in preparing highly functionalised devices due to their flexible architecture and unique properties. Thin films of dendrimers provide better control over the inter-distance and the density of the functional groups. In order to generate highly ordered ultra thin films of dendrimers, a prior understanding of their molecular behaviour is required. This review highlights recent investigations and the contribution of Korean researchers to the study of the molecular behaviour and dynamics of dendrimers at interfaces. Here we emphasise the importance of phase isotherms (air-water interface) in understanding the molecular behaviour, molecular interactions, and molecular assembly, together with factors that permit the molecular behaviour to be tuned and controlled.

Ultrathin Film Deposition by Liquid CO2 Free Meniscus CoatingUniformity and Morphology

Ultrathin organic films of sucrose octaacetate (SOA) were deposited on 12.5 cm diameter silicon wafer substrates using high-pressure free meniscus coating (hFMC) with liquid CO2 (l-CO2) as a coating solvent. The dry film thickness across the wafer and the morphology of deposited films were characterized as a function of coating conditions-withdrawal velocity, solution concentration, and evaporation driving force (deltaP). When no evaporation driving force was applied (deltaP = 0), highly uniform films were deposited with thickness in the range of 8-105 angstroms over the entire concentration range (3-11 wt%). Uniform films were also obtained at low concentrations (3-5 wt%) with a low evaporation driving force (deltaP = 0.0138 MPa). However, films deposited at medium to high concentrations (7-11 wt%) were thicker (110-570 angstroms) and less uniform, with larger nonuniformities at higher applied evaporation driving forces. Optical microscopy and atomic force microscopy (AFM) were used to characterize film morphology including drying defects and film roughness. Films deposited without evaporation had no apparent drying defects and very low root-mean-square (RMS) roughness (1.4-3.8 angstroms). Spinodal-like dewetting morphologies including holes with diameters in the range of 100-300 nm, and surface undulations were observed in films deposited at medium concentration (7 wt%) and low deltaP (0.0138-0.0276 MPa). At higher concentrations and higher evaporative driving forces, spinodal-like dewetting morphologies disappeared but concentric ring defect structures were observed with diameters in the range 20-125 microm. The film thickness and morphology of SOA films deposited from 1-CO2 hFMC were compared to those deposited from toluene and acetone under normal dip coating. Films deposited from l-CO2 hFMC were much thinner, more uniform, and exhibited much fewer drying defects and lower RMS roughness.

Grazing incidence diffraction of cadmium arachidate multilayers at the solid-liquid interface

Abstract Using complementary X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD), we report structural studies of supported thin-organic layers in contact with water and air. Using a monochromatic synchrotron beam to penetrate 10 mm of liquid, we have characterized buried films composed of 12.5 repeating bilayers of arachidic acid (C20H40O2) complexed with cadmium ions (cadmium arachidate, CdAr2). We found that the layered structure of the CdAr2 multilayers do not exhibit rearrangement after exposure to water with negligible water penetration into the inner layers when compared to their dry state. These findings are consistent with the formation of extremely robust CdAr2 multilayers that can withstand multiple rinses in strong organic solvents, acid, and mechanical wear. The second goal was to demonstrate the feasibility of similar experiments to study much thinner single bilayer bio-membranes. These studies are the first successful GIXD experiments of ultra thin-organic film composed of a few layers at the solid-liquid interface.

On the origin of multilevel conductance and memory in ultrathin organic films

Conductivity of certain organic molecules switch to a high-state via electroreduction. Different high-states or multilevel conductivity in organics has been due to different density of high-conducting molecules in a device. We have studied how the population distribution of reduced molecules changes in achieving different conductivity levels. In devices based on a few molecular layers, we have observed that the number of conductivity levels can exceed the number of Langmuir–Blodgett layers. The results showed that the distribution of high-conducting molecules did not increase layer-by-layer, but throughout the volume of the device enabling large number of conductivity levels for higher level (multibit) applications.

Formation of ultra-thin amorphous conversion films on zinc alloy coatings: Part 2: Nucleation, growth and properties of inorganic–organic ultra-thin hybrid films

Within the two parts of this contribution a detailed investigation of the formation of ultra-thin amorphous conversion coatings on hot dip galvanised steel is reported. The second part deals with the nucleation, growth and the barrier properties of these ultra-thin films on model substrates. The chemical composition as well as the film growth kinetics of the phosphoric acid and complexing organic macromolecules within the conversion layer system on a hot dip galvanised steel substrate have been determined by infra-red reflection absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS). Film nucleation was studied by atomic force microscopy (AFM) that revealed the initial formation of individual nanoscopic islands, which depends strongly on the initial surface chemistry. Cyclovoltammetry (CV) and potentiostat measurements were performed to characterise the blocking of ion transfer and electron transfer reactions on the modified surface. An excellent correlation with the barrier properties of the film with the state of film growth and the composition of the inorganic/organic nanocomposite layer was observed. The investigations are a basis for the understanding and future development of thin conversion films on zinc alloy surfaces. Finally, Scanning Kelvin Probe measurements showed the inhibiting effect of the conversion treatment under the conditions of atmospheric corrosion.

Luminescence decay of oriented phenylene nanofibers

The temporal decay of femtosecond UV light (266 nm) induced luminescence of arrays of hundreds of micrometers long organic nanofibers from phenylenes and substituted phenylenes with nanometric widths and heights is investigated. From a fit to the experimental curves monomolecular decay and exciton annihilation rate constants are determined. The excitonic lifetimes depend on the excitation power, the homogeneity of the fibers, and the environment\char22{}e.g., a dielectric coating of the nanofibers. In the limit of dense, quasicontinuous films lifetimes of the order of 800 ps are found, nearly independent of exact film thickness. For films made of well-oriented and well-separated aggregates the luminescence lifetimes are about a factor of 2 longer\char22{}i.e., approximately 2 ns. The observed luminescence spectra shift by $\ensuremath{\approx}7\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\mathrm{ns}$ to the blue with increasing decay time but they do not degrade due to the crystalline perfection of the nanofibers. This temporal decay characteristic contrasts that found from previous measurements on nonoriented thin para-hexaphenylene films of laying molecules and points to the possibility of varying the temporal response of ultrathin organic films by forming oriented nanoaggregates.