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The electric transport properties of flexible and transparent conducting bilayers, realized by sputtering ultrathin gold nanometric layers on sodium–alginate free-standing films, were studied. The reported results cover a range of temperatures from 3 to 300 K. In the case of gold layer thicknesses larger than 5 nm, a typical metallic behavior was observed. Conversely, for a gold thickness of 4.5 nm, an unusual resistance temperature dependence was found. The dominant transport mechanism below 70 K was identified as a fluctuation-induced tunneling process. This indicates that the conductive region is not continuous but is formed by gold clusters embedded in the polymeric matrix. Above 70 K, instead, the data can be interpreted using a phenomenological model, which assumes an anomalous expansion of the conductive region upon decreasing the temperature, in the range from 300 to 200 K. The approach herein adopted, complemented with other characterizations, can provide useful information for the development of innovative and green optoelectronics.

Herein, we describe the use of soft X-ray reflectivity at the carbon K-edge to study the molecular organization (orientation, structure, and morphology) of pentacene active films in a top-contact transistor geometry. This technique is not affected by sample charging, and it can be applied in the case of insulating substrates. In addition, the sampling depth is not limited to the near-surface region, giving access to buried device interfaces (metal/organic and dielectric/organic). Spectral lineshape simulations, based on ab-initio calculations using a realistic 3D layer-by-layer model, allow us to unravel the details of the molecular organization in all the specific and crucial areas of the active film, overcoming the limitations of conventional approaches. The tilt angle of the long molecular axis in the whole film is found to progressively decrease with respect to the substrate normal from 25° to 0° with the increasing film thickness. A full vertical alignment, optimal for in-plane charge hopping, is reached only after the complete formation of the first five monolayers. Remarkably, starting from the first one in contact with the dielectric substrate, all the monolayers in the stack show a change in orientation with the increasing thickness. On the other hand, at the buried interface with a gold top-contact, the molecules assume a flat orientation that only propagates for two or three monolayers into the organic film. Top-contact devices with the highest performances can thus be obtained using films of at least ten monolayers. This explains the observed thickness dependence of charge mobility in pentacene transistors.

The synthesis of two new thieno(bis)imide (TBI, N) end functionalized oligothiophene semiconductors is reported. In particular, trimer (NT3N) and pentamer (NT5N) have been synthesized and characterized. Two different synthetic approaches for their preparation were tested and compared namely conventional Stille cross coupling and direct arylation reaction via C-H activation. Theoretical calculations, optical and electrochemical characterization allowed us to assess the role of the ?-conjugation extent, i.e., of the oligomer size on the optoelectronic properties of these materials. In both {TBI} ended compounds, due to the strong localization of the {LUMO} orbital on the {TBI} unit, the {LUMO} energy is almost insensitive to the oligomer size, this being crucial for the fine-tailoring of the energy and the distribution of the frontier orbitals. Surprisingly, despite its short size and contrarily to comparable TBI-free analogues, {NT3N} shows electron charge transport with mobility up to ?N = 10-4 cm2 V-1 s-1, while increasing the oligomer size to {NT5N} promotes ambipolar behavior and electroluminescence properties with mobility up to ?N = 0.14 cm2 V-1 s-1 and to ?P = 10-5 cm2 V-1 s-1.

We performed constant voltage stress on oligothiophene-based p- and n-type organic thin-film-transistors, in the accumulation regime. The stress induced charge trapping, mobility degradation and defect generation. The specific kinetics depends not only on the applied bias value and polarity, but also on the semiconductor type. Stress on PMMA capacitors (without the semiconductor film) revealed that most of the degradation is largely originated by the interaction with the semiconductor layer, which enhances charge injection. (C) 2013 Elsevier Ltd. All rights reserved.

Organic light-emitting transistors (OLETs) are multifunctional optoelectronic devices that hold great promise for a variety of applications, including flat panel displays, integrated light sources for sensing and optical communication systems. The narrow illumination area within the device channel is considered intrinsic to the device architecture and is a severe technological drawback for all those applications where a controlled, wide and homogeneous emission area is required. Here it is shown that not only the position but also the extension of the emission area is voltage-tunable, and the entire channel of the transistor can be homogeneously illuminated. The modeling of the exciton distribution within the channel at the different bias conditions coupled to the modeling of the device emission profile highlights that excitons are spread through the entire channel width and across the bulk of the central emission layer of the p-channel/emitter/n-channel trilayer active heterostructure.

The realization of organic light-emitting transistors (OLETs) based on α,ω-dihexylcarbonylquaterthiophene (DHCO4T), an intrinsically ambipolar and luminescent semiconductor, is reported. In this de...

Organic molecular semiconductors are key components for a new generation of low cost, flexible, and large area electronic devices. In particular, ambipolar semiconductors endowed with electroluminescent properties have the potential to enable a photonic field-effect technology platform, whose key building blocks are the emerging organic light-emitting transistor (OLET) devices. To this aim, the design of innovative molecular configurations combining effective electrical and optical properties in the solid state is highly desirable. Here, we investigate the effect of the insertion of a thieno(bis)imide (TBI) moiety as end group in highly performing unipolar oligothiophene semiconductors on the packing, electrical, and optoelectronic properties of the resulting materials. We show that, regardless of the HOMO-LUMO energy, orbital distribution, and molecular packing pattern, a TBI end moiety switches unipolar and nonemissive oligothiophene semiconductors to ambipolar and electroluminescent materials. Remarkably, the newly developed materials enabled the fabrication of single layer molecular ambipolar OLETs with optical power comparable to that of the equivalent polymeric single layer devices.

Optoelectronic properties of light-emitting field-effect transistors (LETs) fabricated on bottom-contact transistor structures using a tetracene film as charge-transport and light-emitting material are investigated. Electroluminescence generation and transistor current are correlated, and the bias dependence of the LETs external quantum efficiency is determined. The device's performance degrade rapidly upon gate biasing. The effect is attributed to charge trapping, which can be prevented by operating the devices in pulsed mode. A model for the electron injection mechanism in a p-type organic transistor is proposed. On the basis of this model, electrical and optical characteristics, as well as the dependence of the external quantum efficiency on drain- and gate-bias, are well reproduced.

The interplay between atomic geometry and magnetic properties of Fe atoms at the Fe–Ni(1 1 1) interface was studied. To this end dichroic optical absorption and reflectivity (both in energy and θ–2θ angular scan mode) were carried out in the L 2 , 3 edge region of Fe in (Fe 6 A /Ni 24 A (1 1 1)) 10 multilayer while the atomic geometry of the Fe site at the interface was studied for 1 atomic layer of Fe on Ni(1 1 1) by photo-electron diffraction (in angular scan mode and back-scattering regime). The Fe atom was found to be chemisorbed in an fcc hollow site and to stack pseudomorphically to Ni(1 1 1). Its magnetic moment was found to be in the range 2.9±0.9–1.93±0.6 μ B . Possibility of surface anisotropy is discussed.

An analytical model for the electric field associated with a periodic array of alternating p- and n-doped stripes lying in a half-plane, tilted with respect to the specimen edges and thus better representing the actual experimental set-up is presented. With respect to a previous treatment, relative to the case of stripes perpendicular to the edge, a more physical derivation is outlined, and the calculated phase shift is used to interpret the main features of holography and Lorentz microscopy images, allowing a quantitative assessment of the influence of the specimen edge on them.

Despite their favorable electronic and structural properties, the synthetic development and incorporation of thiazole-based building blocks into n-type semiconductors has lagged behind that of other pi-deficient building blocks. Since thiazole insertion into pi-conjugated systems is synthetically more demanding, continuous research efforts are essential to underscore their properties in electron-transporting devices. Here, we report the design, synthesis, and characterization of a new series of thiazolethiophene tetra- (1 and 2) and hexa-heteroaryl (3 and 4) co-oligomers, varied by core extension and regiochemistry, which are end-functionalized with electron-withdrawing perfluorohexyl substituents. These new semiconductors are found to exhibit excellent n-channel OFET transport with electron mobilities (mu(e)) as high as 1.30 cm(2)/(V center dot s) (I-on/I-off > 10(6)) for films of 2 deposited at room temperature. In contrary to previous studies, we show here that 2,2'-bithiazole can be a very practical building block for high-performance n-channel semiconductors. Additionally, upon 2,2'- and 5,5'-bithiazole insertion into a sexithiophene backbone of well-known DFH-6T, significant charge transport improvements (from 0.0010.021 cm(2)/(V center dot s) to 0.200.70 cm(2)/(V center dot s)) were observed for 3 and 4. Analysis of the thin-film morphological and microstructural characteristics, in combination with the physicochemical properties, explains the observed high mobilities for the present semiconductors. Finally, we demonstrate for the first time implementation of a thiazole semiconductor (2) into a trilayer light-emitting transistor (OLET) enabling green light emission. Our results show that thiazole is a promising building block for efficient electron transport in ?-conjugated semiconductor thin-films, and it should be studied more in future optoelectronic applications.

Herein is described a multidisciplinary approach to understand the performance limitations of small molecule organic light emitting transistors (OLETs) based on a layered architecture, an innovative architecture potentially competitive with the state of the art and more flexible for spectral emission control. The processes of charge injection and field-effect transport at metal/organic and organic/organic interfaces are analysed using microscopic and spectroscopic techniques in coordination. Atomic force microscopy and ultrasonic force microscopy are employed to characterize the interface morphology and the initial growth stages of organic films where charge transport actually occurs. X-ray diffraction and near edge X-ray dichroic absorption with linearly polarised light allow to determine the unit cell packing and the molecular orientation at the active organic interfaces, as well as the amount of non-ordered domains. Moreover, chemical reactivity at the interfaces is measured by X-ray photoelectron spectroscopy. It is found that a strong reaction occurs at the metal-organic interfaces, with molecular fragmentation. Additionally, the transport properties strongly depend on the nature of the materials forming the organic stack. Specifically, amorphous conjugated films as bottom layers can promote an increased molecular disorder in the upper active layer, with a concomitant deterioration of the conduction properties.

In this work, we report on the fabrication and characterization of organic light-emitting transistors (OLETs) within an indium–tin-oxide (ITO)-free platform, using graphene-based transparent conductive electrodes in place of ITO as gate electrode. A direct comparison between twin bottom-gate/top-contacts OLETs, where a standard ITO layer is replaced with a film made of a few graphene layers, shows that comparable electrical characteristics can be obtained along with a clear improvement in the electroluminescence generation characteristics. Our experimental findings pave the way to the exploitation of graphene-based transparent conductive electrodes within this class of emerging devices on flexible substrates, further promoting the novel era of flexible organic electronics.

A novel method for mapping the charge density spatial distribution in organic field-effect transistors based on the electro-modulation of the photoluminescence is demonstrated. In field-effect transistors exciton quenching is dominated by exciton-charge carrier interaction so that it can be used to map the charge distribution in different operating conditions. From a quantitative analysis of the photoluminescence quenching, the thickness of the charge carrier accumulation layer is derived. The injection of minority charge carriers in unipolar conditions is unexpectedly evidenced, which is not displayed by the electrical characteristics.

Target compound (V) is alternatively prepared via Stille coupling of bromo thiophene (I) with 5,5′-bis(tributylstannyl)-2,2′-bithiophene.

Interest in electronic properties of organic materials has been stimulated by emerging prospects of their application in electronics and photonics [1–4]. Among potential applications, one may envisage use of organic materials as active elements of molecular switches. According to a general definition, a switch is a mechanical, electrical, electronic, or optical device for opening or closing a circuit, or for diverting energy or charge from one part of a circuit to another. In this context, photochromic materials have been extensively studied.

1.1 OLED Vs. OLET Plastic electronics, i.e. electronics based on organic polymer and molecular semiconductors, is a key low-cost technology for new vast application areas. The scope of applications in plastic electronics is virtually endless. It ranges from opto-electronic devices and flexible organic light-emitting diodes (OLEDs) displays, to ultra-cheap, radio-frequency identification tags that can replace the now ubiquitous bar-codes, and to wearable computing and electronic bio-interfaces. The first demonstration of the OLED dates back to the 1960s when electrically driven light emission from non-crystalline organic materials was first observed. After that, several studies were carried out by academic groups and companies (Kodak, Pioneer, Motorola, NEC, etc...) both for fundamental physics comprehension and application purposes. Nowadays electronic products containing displays are becoming more and more portable. Therefore, they need some peculiarities like lightweightness, flexibility, brightness, etc... These, with many others, are the strong points of the OLEDs. In fact they are thinner, lighter and more flexible with respect to their inorganic counterpart. Moreover, OLEDs can be as bright as LEDs and they consume much less power. Due to the organic processability, they are easier to produce and can be made on larger area. Finally OLEDs have large fields of view, about 170 degrees, a significantly advantage over, for example, liquid crystal displays. Of course, these devices present also some disadvantages: they have typically shorter lifetime (in particular lifetime of the blue emitter is critical, about 1.000 hours), they are not very stable and can easily be contaminated by water or oxygen. In general, with respect to OLEDs, organic light-emitting transistors (OLETs) present some fascinating characteristics which overcome many physical and technical drawbacks in the realization of nano-scale integrated electroluminescent devices. The main difference between the vertical (OLED) and planar (OLET) device geometry is a direct consequence of the different device structures. In OLED, charge transport occurs perpendicular to the organic layers (bulk charge transport) while in OLET the transport occurs horizontally (field-effect charge transport). Under the typical biasing conditions, the electron and hole mobility can be about four orders of magnitude higher in OLETs than in OLEDs, thus affecting directly the material lifetime and exciton emission. In a typical OLED structure, the minority carriers travel only few tens

We report on an organic field effect transistor (OFET) with a photochromic dielectric layer, operating as an opto-electrical switch device. The structure contained a photochromic material dissolved in the polymer dielectric metastable forms; poly(methyl methacrylate) was a polymeric layer. The photochromic material was spiropyran exhibiting a large difference of the dipole moments of the stable and insulator; and an n-type perylene derivative was used as the organic semiconductor. Illumination of the structure with UV light resulted in a reversible increase of the source drain current, accompanied by a reversible decrease of the threshold voltage. The initial parameters were restored by a thermal relaxation in the dark or by illumination with visible light. The photoswitching ratio was found to be dependent on the gate voltage ranging between ca. 2 just above the threshold voltage and ca. 1.3 at the highest voltage employed (90 V). The switching has been attributed to reversible changes of dielectric properties of OFET's insulator (dielectric layer) due to a reversible light-triggered reaction of polar photochromic species, dissolved in the bulk of the dielectric layer. The contribution of dipoles aggregated on the semiconductor dielectric interface was estimated to be negligible at gate voltages exceeding ca. 10 V.

In this Article, we present a comprehensive study of organic field-effect transistors (OFETs) made of thin films of methyl, n-butyl, and n-hexyl end-substituted quaterthiophenes on a transparent substrate platform. This particular platform has been already used for organic light-emitting diodes (OLEDs) but rarely employed for OFETs. In perspective, this is a very promising route for the development of field-effect photonic applications such as organic light-emitting transistors (OLETs). A systematic characterization of the organic films has been made by means of atomic force microscopy (AFM) and X-ray diffraction (XRD) to correlate morphology, crystallinity and charge mobility to the alkyl chain length. In particular, a charge mobility value of 0.09 cm(2)/(V s) has been obtained in transparent OFETs with a large area channel for DH4T grown at room temperature. This mobility exceeds the one obtained on silicon-oxide substrates and is likely due to a more favorable interaction of the DH4T molecules with the PMMA layer employed as gate dielectric.

Organic field-effect transistors (OFETs) are alternative emerging device structures for efficient light generation, that could provide a novel architecture to address open issues like exciton-contact and exciton-charge quenching, that still limit the OLEDs efficiency and brightness. Recently, it has been introduced by our research group the model of a tri-layer organic heterostructure implemented in a field-effect configuration, that allows preventing at one time the exciton-metal as well as the exciton-charge quenching in an organic electroluminescence generating device. The device active region is formed by a central optical layer sandwiched between an electron and a hole field-effect conducting film. In order to understand the complex phenomena that happens at the interfaces, with the target to fabricate the most balanced ambipolar structure with high morphological compatibility and high mobility in a vertical heterojunction geometry, we made a preliminary study of a single layer and bi-layer OFET structure composed by ?,?-dihexylquaterthiophene (DH4T) and ?,?-diperfluoroquaterthiophene (DHF4T). By means of this study we showed a new highly balanced ambipolar OFET made of these materials, a first step toward their implementation in a more complex structure as the tri-layer is. © 2010 Elsevier Ltd. All rights reserved.

We report on the characteristics of a host-guest lasing system obtained by coevaporation of an oligo(9,9-diarylfluorene) derivative named T3 with the red-emitter 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye (DCM). We demonstrate that the ambipolar semiconductor T3 can be implemented as an active matrix in the realization of a host-guest system in which an efficient energy transfer takes place from the T3 matrix to the lasing DCM molecules. We performed a detailed spectroscopic study on the system by systematically varying the DCM concentration in the T3 matrix. Measurements of steady-state photoluminescence (PL), PL quantum yield (PLQY), time-resolved picosecond PL, and amplified spontaneous emission (ASE) threshold are used to optimize the acceptor concentration at which the ASE from DCM molecules takes place with the lowest threshold. The sample with a DCM relative deposition ratio of 2% shows an ASE threshold as low as 0.6 kW/cm(2) and a net optical gain measured by femtosecond time-resolved pump-and-probe spectroscopy as high as 77 cm(-1). The reference model system Alq(3):DCM sample measured in exactly the same experimental conditions presents an one-order-of-magnitude higher ASE threshold. The ASE threshold of T3:DCM is the lowest reported to date for a molecular host-guest energy-transfer system, which makes the investigated blend an appealing system for use as an active layer in lasing devices. In particular, the ambipolar charge transport properties of the T3 matrix and its field-effect characteristics make the host-guest system presented here an ideal candidate for the realization of electrically pumped organic lasers.

The potential of organic semiconductor-based devices for light generation is demonstrated by the commercialization of display technologies based on organic light-emitting diodes (OLEDs). Nonetheless, exciton quenching and photon loss processes still limit OLED efficiency and brightness. Organic light-emitting transistors (OLETs) are alternative light sources combining, in the same architecture, the switching mechanism of a thin-film transistor and an electroluminescent device. Thus, OLETs could open a new era in organic optoelectronics and serve as testbeds to address general fundamental optoelectronic and photonic issues. Here, we introduce the concept of using a p-channel/emitter/n-channel trilayer semiconducting heterostructure in OLETs, providing a new approach to markedly improve OLET performance and address these open questions. In this architecture, exciton-charge annihilation and electrode photon losses are prevented. Our devices are >100 times more efficient than the equivalent OLED, >2x more efficient than the optimized OLED with the same emitting layer and >10 times more efficient than any other reported OLETs.

We report on the characteristics of a host-guest lasing system obtained by co-evaporation of an oligo(9,9-diarylfluorene) derivative named T3 with the red-emitter 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye (DCM). We demonstrate that the ambipolar semiconductor T3 can be implemented as active matrix in the realization of a host-guest system in which an efficient energy transfer takes place from T3 matrix to the lasing DCM molecules. We performed a spectroscopic study on the system by systematically varying the DCM concentration in the T3 matrix. Measurements of steady-state photoluminescence (PL), PL quantum yield (PLQY) and amplified spontaneous emission (ASE) threshold are used to optimize the acceptor concentration at which the ASE from DCM molecules takes place with the lowest threshold. Organic light-emitting diodes (OLEDs) implementing the DCM:T3 host-guest system as recombination layer are fabricated for verifying the optical properties of the optimised blend in real working devices. Indeed, the very low ASE threshold of T3:DCM makes the investigated blend an appealing system for use as active layer in lasing devices. In particular, the ambipolar charge transport properties of the T3 matrix and its field-effect characteristics make the host-guest system presented here an ideal candidate for the realization of electrically-pumped organic lasers.

We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect transistors based on polyphenylenevinylene derivatives. Through a systematic investigation of the most common dielectric surface treatments, a direct correlation of their effect on the field-effect electrical parameters, such as charge carrier mobility, On/Off current ratio, threshold voltage, and current hysteresis, has been established. It is found that the presence of OH groups at the dielectric surface, already known to act as carrier traps for electrons, decreases the hole mobility whereas it does not substantially affect the other electrical characteristics. The treatment of silicon dioxide surfaces with gas phase molecules such as octadecyltrichlorosilane and hexamethyldisilazane leads to an improvement in hole mobility as well as to a decrease in current hysteresis. The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the semiconductor polymer can be related not only to the OH groups density but also to the interaction between the dielectric and the semiconductor molecules. Specifically, the elimination of the OH groups produces the same effect observed with hexamethyldisilazane. The hole mobility values obtained with hexamethyldisilazane and polymer dielectrics are the highest reported to date for PPV-based field-effect transistors.

It is shown how the same physically appealing method can be applied to find analytic solutions for two difficult and apparently unrelated problems in optics and electrostatics. They are: (i) the diffraction of a plane wave at a perfectly conducting thin half-plane and (ii) the electrostatic field associated with a parallel array of stripes held at opposite potentials and lying in a half-plane. In the latter case, the solution of the problem is essential in order to calculate the electron optical phase shift experienced by the electron beam in electron microscopy experiments, where the model mimics an array of reverse biased p–n junctions. This paper is intended, in particular, for the undergraduate student who has completed his programme of calculus and electromagnetism, but it can also be of interest to the general physicist.

We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect transistors based on Poly-phenylenevinylene derivatives. We observe a direct influence of the dielectric surface on the field-effect mobility as well as on the charge injection at the source electrode, despite the fact that we used a top contact transistor structure. We find that the presence of traps at the dielectric surface, decreases the hole mobility and increases the threshold voltages. By treating the silicon dioxide dielectric surface with gas phase molecules such as octadecyltrichlorosilane (OTS) and hexamethyldisilazane (HMDS) the hole mobility improves and the threshold voltage slightly increases. The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the semiconductor polymer can be related to the density of the oxydryl groups (-OH ), which are the most efficient traps for the charges flowing in the device. We use different polymer species such as polyvinylalchol (PVA), polymethylmetacrilate (PMMA) and a cyclotene derivative (B-staged bisbenzocyclobutene or BCB). The elimination of the -OH groups and of other traps, produces the same effect observed with HMDS coupled to a more pronounced enhancement of the threshold voltage, with the exception of PMMA. The electrical characteristics obtained with HMDS and PMMA polymer dielectrics are the highest reported to date for PPV-based field-effect transistors. We confirm that the purification of the active material is crucial to enhance the device performances and to achieve a better device to device reproducibility. We also investigated the effect of the dispersion of a phosphorescent dye into the active polymeric material. The electrical characteristics of OFETs with HMDS or PMMA dielectric with and without dye doping are compared.

Organic light-emitting field-effect transistors are a new class of electrooptical devices that could provide a novel architecture to address open questions concerning fundamental optoelectronic phenomena in organic semiconductors, and can be potentially used as key components in optical communication systems, advanced display technology, solid-state lighting and organic lasers. The realisation of Organic Light-Emitting Transistors (OLETs) with high quantum efficiency and fast switching time is crucial for the development of highly integrated organic optoelectronic systems. Organic molecular materials having intrinsically ambipolar transport and high charge mobility values are restricted in number and show poor light-emission efficiency. Here, we describe the device operation principles of OLETs and report on the approach of combining p-type and n-type molecular materials in a layered structure to achieve ambipolar transport and light emission. Imaging of the individual layers and a correlation between active layer structure and device electrical performances is achieved by means of the Laser Scanning Confocal Microscopy.

Photoluminescence spectral features of α-sexithiophene (α-T 6 ) thin films have been demonstrated to be directly linked to film morphology and temperature. Ultra-thin films at low temperature show vibronic resolved photoluminescence (PL) spectra typical of intrinsic excitonic emission. As film thickness increases, PL spectra show additional emission at low energy, which is attributed to trap states. The nature of the trap states is to date elusive as well as the non-radiative mechanism involving exciton and trap states. Here, we report a phenomenological model that allows to determine the photophysical rate constants of the excitonic and trap state transitions and the density of traps in α-T 6 thin films. Our model accounts for the thickness dependence of α-T 6 films optical emission and is well in agreement with the observed variation of photoluminescence as a function of temperature in the range of 4–300 K.

A structural study of Fe ultrathin epitaxial films, grown at room temperature on Ni(111), has been performed in the 1.5--18 ML coverage range by angle-scanned photoelectron diffraction. Both backscattering and forward-scattering energy regimes have been employed, in order to enhance the structural sensitivity at lower and higher film thicknesses, respectively. Modeling of the experimental data has been performed with multiple scattering calculations. We found indications that Fe atoms in the first layer occupy fcc hollow sites and stack with a pseudomorphic fcc structure up to 2 ML. Concerning the growth mode at these early stages, data suggest that a good substrate wetting and a sharp Fe/Ni interface take place. Between 3 and 6 ML, transition to a bcc(110) phase develops. By quantitative R-factor analysis, we found that Nishiyama-Wassermann (NW) in-plane orientation of the bcc(110) cell $(〈001{〉}_{\mathrm{bcc}}\ensuremath{\parallel}〈11\ifmmode\bar\else\textasciimacron\fi{}0{〉}_{\mathrm{fcc}})$ is favored over the Kurdjumov-Sachs $(〈11\ifmmode\bar\else\textasciimacron\fi{}1{〉}_{\mathrm{bcc}}\ensuremath{\parallel}〈11\ifmmode\bar\else\textasciimacron\fi{}0{〉}_{\mathrm{fcc}})$ orientation. The best-fit vertical interlayer distance between bcc(110) planes is ${d}_{\mathrm{NW}}=2.11\AA{}$ (+3.9% expansion) at 6 ML and relaxes to ${d}_{\mathrm{NW}}=2.05\AA{}$ (+1.0%) at 18 ML, in agreement with the angular shift observed for the forward-focusing features. In the same coverage range, the angle between bcc(110) surface basis vectors changes from 67.7\ifmmode^\circ\else\textdegree\fi{} to 69.0\ifmmode^\circ\else\textdegree\fi{}, corresponding to -1.7% and -1.0% contractions of the surface cell area, respectively.

In this paper, we present a study of the effects of the influence of the substrate platform on the properties of a three-layer vertical hetero-junction made of thin films of α, ω-diperfluorohexyl-4T (DHF4T), a blend of tris(8-hydroxyquinoline)aluminium (Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) and α, ω-dihexyl-quaterthiophene (DH4T). The hetero-junction represents the active component of an organic light-emitting transistor (OLET). The substrate platforms investigated in this study are glass/indium-tin-oxide/poly(methyl-methacrylate) (PMMA) and Si++/silicon oxide (SiO2)/PMMA. The first platform is almost completely transparent to light and therefore is very promising for use in OLET applications. The second one has been chosen for comparison as it employs standard microelectronic materials, i.e. Si++/SiO2. We show how different gate materials and structure can affect the relevant field-effect electrical characteristics, such as the charge mobility and threshold voltage. By means of an atomic force microscopy analysis, a systematic study has been made in order to correlate the morphology of the active layers with the electrical properties of the devices.