Ultrathin Organic Films Research Articles

Electrical conduction of organic ultrathin films evaluated by an independently driven double-tip scanning tunneling microscope

The electrical transport properties of organic thin films within the micrometer scale have been evaluated by a laboratory-built independently driven double-tip scanning tunneling microscope, operating under ambient conditions. The two tips were used as point contact electrodes, and current in the range from 0.1 pA to 100 nA flowing between the two tips through the material can be detected. We demonstrated two-dimensional contour mapping of the electrical resistance on a poly(3-octylthiophene) thin films as shown below. The obtained contour map clearly provided an image of two-dimensional electrical conductance between two point electrodes on the poly(3-octylthiophene) thin film. The conductivity of the thin film was estimated to be (1–8) × 10−6 S cm−1. Future prospects and the desired development of multiprobe STMs are also discussed.

Characterization of organic ultra-thin film adhesion on flexible substrate using scratch test technique

The mechanical properties of interfaces and more precisely the adhesion are of great importance for the understanding of the reliability of thin film devices. Organic thin film transistors (OTFT) on flexible substrate are a new class of electronic components. Since these devices are flexible and intended for different fields of application like sensors and displays, they will undergo a lot of mechanical and thermal stress during their useful life. Moreover, interfaces play an important role in the electrical stability of these transistors. In this context, the adhesion of two organic submicron thin films, semi conducting and dielectric respectively, deposited on polymeric substrate were investigated by scratch test method. This study demonstrates the feasibility and selectivity of the scratch test as a tool for assessing the adhesion and the damage behaviour of ultra-thin organic film on flexible plastic substrate. The semi-crystalline substrate presents a brittle cracking damage from a given strain, whereas when covered by the semi-conducting thin film, the sample exhibits a more ductile behaviour. Moreover, this technique has proven to be sensitive enough to highlight the effects of a plasma treatment prior to deposition.

Fluorescence spectroscopy of ultrathin molecular organic films on surfaces

We review our recent experiments by linear optical spectroscopy in the visible spectral range on interface controlled monolayers and thin films of π-conjugated organic molecules on well-defined surfaces. In particular, cw fluorescence and fluorescence excitation spectroscopy are considered. We discuss two interesting aspects: the possibility to prepare interface controlled films with non-bulk like structures, and consequently different and possibly superior optical properties, and the use of optical spectroscopy as an analytical tool to investigate properties of interfacial organic films.

Direct Observation and Measurement of Mobile Charge Carriers in a Monolayer Organic Semiconductor on a Dielectric Substrate

We report the electrical characterization of a single layer of an organic semiconductor grown on a dielectric surface. The dynamic response of the charge carriers in the monolayer film of pentacene was characterized through the electrostatic interactions between an electric force microscope (EFM) probe and pentacene islands of various sizes. These islands were formed in situ by segmenting a coalesced pentacene monolayer into separated regions. The size-dependent dielectric responses of the pentacene islands suggest that mobile charges exist in the organic monolayer. Local capacitance spectroscopy revealed that the charge carriers in the p-type pentacene monolayer could be depleted at high bias voltages, enabling a further determination of the charge-carrier concentration in the organic semiconductor ultrathin film.

Layer-by-layer assembly of electroactive dye/inorganic matrix film and its application as sensor for ascorbic acid

A novel inorganic–organic composite ultrathin film was fabricated by layer-by-layer assembly of naphthol green B (NGB) and layered double hydroxides (LDHs) nanoplatelets, which shows remarkable electrocatalytic behavior for oxidation of ascorbic acid. LDHs nanoplatelets were prepared using a method involving separate nucleation and aging steps (particle size: 25±5nm; aspect ratio: 2–4) and used as building blocks for alternate deposition with NGB on indium tin oxide (ITO) substrates. UV–vis absorption spectroscopy and XRD display regular and uniform growth of the NGB/LDHs ultrathin film with extremely c-orientation of LDHs nanoplatelets (ab plane of microcrystals parallel to substrates). A continuous and uniform surface morphology was observed by SEM and AFM image. The film modified electrode displays a couple of well-defined reversible redox peaks attributed to Fe2+/Fe3+ in NGB (ΔEp=68mV and Ia/Ic=1.1). Moreover, the modified electrode shows a high electrocatalytic activity towards ascorbic acid in the range 1.2–55.2μM with a detection limit of 0.51μM (S/N=3). The Michaelis–Menten constant was calculated to be KMapp=67.5 μM.

Photovoltaic properties of interfaces of organic films of substituted perylene with TiO2 and SnO2 surfaces

The photovoltaic effect has been detected and studied in structures based on ultrathin vacuum-deposited organic films of perylene-3,4,9,10-tetracarboxylic acid dianhydride on the titanium and tin dioxide surfaces. The interfacial potential barrier shape in these structures is studied by low-energy electron total current spectroscopy. Changes in the surface potential upon exposure to visible light are recorded in situ using an electron-beam probe with energies from 0 to 25 eV. The photovoltage is detected at incident photon energies of 1.5–2.5 eV, which corresponds to the organic film absorption range and simultaneously to the transmission band of titanium and tin dioxides. An analysis of the spectral distributions and transient responses shows that two components of the observed photovoltage can be distinguished. The relation of one of the components to the excitation of interband transitions in the organic film and another component to electronic transitions involving interfacial energy states are discussed.

MEIS and ToF‐SIMS analyses of organic ultrathin films for absolute quantification of functional groups

AbstractComplementary use of medium‐energy ion‐scattering spectroscopy (MEIS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) analyses was proposed as a new methodology for absolute quantification of functional groups in organic ultrathin films. First, MEIS analysis was used to determine the absolute areal densities of Ru dye molecules in organic ultrathin films, which were made by spin coating Ru 535‐bis TBA (C58H86O8N8S2Ru) solutions with varying concentrations. The thicknesses of the films were less than 2 nm each and the densities of these ultrathin layers were linearly correlated with the solution concentrations. The determined absolute areal densities of the Ru organic molecules were then used to calculate the areal densities of the stoichiometric functional groups. Secondly, the ToF‐SIMS intensities of the characteristic peaks such as Ru+, C16H35N+, CO2−, CSN−, CSNRu−, etc, associated with the functional groups of a Ru 535‐bis TBA molecule were correlated by the densities of the Ru organic molecules. Our results show that the correlation curves obtained through the complementary use of MEIS and ToF‐SIMS techniques could be useful to quantify functional groups on organic ultrathin films. Copyright © 2010 John Wiley & Sons, Ltd.

Ultrasonic force microscopy: Detection and imaging of ultra-thin molecular domains

The analysis of the formation of ultra-thin organic films is a very important issue. In fact, it is known that the properties of organic light emitting diodes and field effect transistors are strongly affected by the early growth stages. For instance, in the case of sexithiophene, the presence of domains made of molecules with the backbone parallel to the substrate surface has been indirectly evidenced by photoluminescence spectroscopy and confocal microscopy. On the contrary, conventional scanning force microscopy both in contact and intermittent contact modes have failed to detect such domains. In this paper, we show that Ultrasonic Force Microscopy (UFM), sensitive to nanomechanical properties, allows one to directly identify the structure of sub-monolayer thick films. Sexithiophene flat domains have been imaged for the first time with nanometer scale spatial resolution. A comparison with lateral force and intermittent contact modes has been carried out in order to explain the origins of the UFM contrast and its advantages. In particular, it indicates that UFM is highly suitable for investigations where high sensitivity to material properties, low specimen damage and high spatial resolution are required.

Micelle-assisted bilayer formation of cetyltrimethylammonium bromide thin films studied with combinatorial spectroscopic ellipsometry and quartz crystal microbalance techniques

We report on a combinatorial approach to study the formation of ultra-thin organic films using in-situ spectroscopic ellipsometry and quartz crystal microbalance methods. In contrast to the quartz crystal microbalance, which is sensitive to the total mass attached to the surface, including coupled and entrapped solvent, spectroscopic ellipsometry only measures the amount of adsorbent on the surface. By using these two techniques in tandem, we define and determine the solvent fraction of cetyltrimethylammonium bromide thin films adsorbed onto a gold-coated quartz crystal. Cetyltrimethylammonium bromide thin films grown from aqueous solutions above the critical micelle concentration reveal critical phases in thickness and porosity evolution. We relate these effects to the mechanisms of formation and removal and the structure of cetyltrimethylammonium bromide films, which we determine to have systemic defects due to the presence of micelles.

ChemInform Abstract: Optical and Surface-Analytical Methods for the Characterization of Ultrathin Organic Films.

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Passivation of Si(111) surfaces with electrochemically grafted thin organic films

Ultra thin organic films (about 5 nm thick) of nitrobenzene and 4-methoxydiphenylamine were deposited electrochemically on p-Si(111) surfaces from benzene diazonium compounds. Studies based on atomic force microscopy, infrared spectroscopic ellipsometry and x-ray photoelectron spectroscopy showed that upon exposure to atmospheric conditions the oxidation of the silicon interface proceed slower on organically modified surfaces than on unmodified hydrogen passivated p-Si(111) surfaces. Effects of HF treatment on the oxidized organic/Si interface and on the organic layer itself are discussed.

Combined ellipsometry and X-ray related techniques for studies of ultrathin organic nanocomposite films

Ultrathin nanocomposite films of nitrobenzene on silicon were analyzed by Infrared Spectroscopic Ellipsometry (IRSE), X-ray reflectivity (XRR) and X-ray standing waves (XSW) before and after evaporation of gold. Infrared Spectroscopic Ellipsometry measurements were performed for identification of adsorbates and for investigation of the molecular orientation. Results for film thickness were correlated with XRR measurements. Further, XSW measurements of elements incorporated in nitrobenzene (C, N, and O) were performed with soft X-rays. The combination of the different methods allowed to confirm a model for the electrochemically deposited nitrobenzene films before and after gold evaporation. The characterization by XRR and XSW scans using hard X-rays showed that gold had penetrated into the nitrobenzene film and thus changed density and optical properties of this layer significantly. A depth profile correlated to the electron density is deduced from the XRR measurements. This profile allows to localize—in vertical direction—gold islands within the composite film.

In‐situ IR synchrotron mapping ellipsometry on stimuli‐responsive PAA‐b‐PS/PEG mixed polymer brushes

AbstractA binary polymer brush consisting of weak polyelectrolytes was investigated with infrared synchrotron mapping ellipsometry in‐situ under the influence of different aqueous solutions. Thickness of the brush layer in dry state was ∼15 nm. The brush, consisting of poly(ethylene glycol) and poly(acrylic acid)‐b‐poly(styrene) in a 50/50 composition was switched between two different states by changing the pH of the solution. An IR mapping ellipsometer at the IRIS beamline located at the BESSY II synchrotron facility in Berlin, Germany, was used for high lateral resolution in‐situ measurements. The results show strong chemical changes in the brush layer due to COOH – COO– conversion of the PAA's carboxylic groups. Measurements with spot sizes of ∼1 mm on different positions on the samples proved good homogeneity of the brush layer and the qualification of this method for investigation of ultrathin organic films in aqueous solutions in‐situ with IR ellipsometry. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Residue-free room temperature UV-nanoimprinting of submicron organic thin film transistors

In this study we report on an innovative nanoimprint process for the fabrication of entirely patterned submicron OTFTs in a bottom-gate configuration. The method is based on UV-Nanoimprint Lithography (UV-NIL) combined with a novel imprint resist whose outstanding chemical and physical properties are responsible for the excellent results in structure transfer. In combination with a pretreated stamp the UV-curable resist enables residue-free imprinting thus making etching obsolete. A subsequent lift-off can be done with water. The UV-NIL process implies no extra temperature budget, is time saving due to short curing times, eco-friendly due to a water-based lift-off, simple because it is etch-free and completely r2r compatible. It works perfectly even if ultra-thin organic and hybrid films are used as gate dielectrics. On this basis entirely patterned functional submicron OTFTs with pentacene as the semiconductor are fabricated showing clear saturation, low switch-on voltage (∼3 V) and a sufficiently high on–off ratio (10 3).

DNA adsorption measured with ultra-thin film organic field effect transistors

Organic ultra-thin film field effect transistors (FET) are operated as label-free sensors of deoxyribonucleic acid (DNA) adsorption. Linearized plasmid DNA molecules (4361 base pairs) are deposited from a solution on two monolayers thick pentacene FET. The amount of adsorbed DNA is measured by AFM and correlated to the concentration of the solution. Electrical characteristics on the dried DNA/pentacene FETs were studied as a function of DNA concentration in the solution. Shift of the pinch-off voltage across a wide range of DNA concentration, from very diluted to highly concentrated, is observed. It can be ascribed to additional positive charges in the semiconductor induced by DNA at a rate of one charge for every 200 base pairs. The sensitivity 74 ng/cm 2, corresponding to 650 ng/ml, is limited by the distribution of FET parameters upon repeated cycles, and is subjected to substantial improvement upon standardization. Our work demonstrates the possibility to develop label-free transducers suitable to operate in regimes of high molecular entanglement.

Structure of Organic Ultra-thin Films Observed by Grazing Incidence X-ray Diffractmetry(<Special Issue>Crystal Growth of Organic Semiconductors toward Applications in Electronics)

Structure of Organic Ultra-thin Films Observed by Grazing Incidence X-ray Diffractmetry(<Special Issue>Crystal Growth of Organic Semiconductors toward Applications in Electronics)

Electrochemical Switches Based on Ultrathin Organic Films: From Diode-like Behavior to Charge Transfer Transparency

Ultrathin layers of thiophene derivatives were covalently attached to GC electrodes by electroreduction of diazonium salts. The films are densely packed structures and are able to mediate electron transfer above a threshold voltage tuned by the nature of the grafted molecules onto the electrode. We investigate the electron transfer properties of these layers by using electroactive probes having various redox potentials. Voltammetry and SECM measurements clearly show that for redox probes with low redox potentials diode-like behavior is observed and the current can flow in only one direction across these organic layers whereas, when the potential of the external redox probe increases, the transparency of the layers toward electron transfer in both direction increases (within the time range investigated). This behavior is not compatible with an ECcat mechanism, with electroactive immobilized centers characterized by a single redox potential E°Im, and clearly demonstrates the switching of the layer conductivity. In this sense, these layers are better modeled as covalently grafted conducting oligomers densely packed on the surface and capable of changing their charge transfer characteristics upon charge injection (i.e., doping of the grafted conjugated oligomers). The charge transfer mechanism of these ultrathin films is thus very different from that most of SAMs bearing an electroactive group, in which electron tunnelling through the monolayer is the main charge transfer mechanism.

Effects of Organic Lithium Salt Ultrathin Films on the Electron Injection Efficiency in OLED

We investigated the effects of the ultrathin film of a new cathode underlayer material, lithium tert-butyl cyclopentadienide (Li-TBCPD), on the electron injection efficiency and performance of the Alq3-based OLED. The current density-voltage-luminance (I-V-L) and the luminous efficiency characteristics of the [ITO/α-NPB (40 nm)/Alq3 (60 nm)/Li-TBCPD (1.0 nm)/Al] device were examined. The device with the Li-TBCPD/Al bilayer cathode demonstrated the current density of 360 A/m2 at 12 V and the maximum luminance of 8,100 cd/m2, while the device with the LiF/Al cathode showed 70 A/m2 and 5,800 cd/m2, respectively. This improvement seems to originate from the relatively lower bond dissociation energy of Li-TBCPD and hence the easiness of lithium layer formation beneath the aluminum cathode.

Vacuum ultraviolet ellipsometry investigation of ultrathin organic films and their heterostructures

Thin layers and multilayer structures of the DNA bases guanine and cytosine on ZnO substrates were prepared by organic molecular beam deposition under ultra-high vacuum conditions and measured in situ by means of vacuum ultraviolet spectroscopic ellipsometry at the synchrotron source BESSY. Using the dielectric function of the individual layers the optical response of the G/C and C/G heterostructures was modeled. Deviations between simulated and experimental data were mainly attributed to the ordering of cytosine over guanine in the G/C and C/G structures and to the influence of the substrate.

Imaging thin and ultrathin organic films by scanning white light interferometry

We present the results of a feasibility study of the capabilities of scanning white light interferometry (SWLI) to map large (macroscopic) areas of thin and ultrathin organic films deposited on mica and borosilicate glass substrates. We have shown that SWLI can provide useful characteristics (such as thickness and homogeneity) of polymer monolayers and surface patterns prepared by microcontact printing. We present the principle of operation of the technique and discuss the conditions under which SWLI can give reliable results, its strengths and its limitations.