Isabelle Seguy

Ultrathin films of homeotropically aligned columnar liquid crystals on indium tin oxide electrodes

We report the achievement of very thin films (thickness of about 50 nm) of thermotropic columnar liquid crystal in homeotropic (columns normal to the interface) orientation on indium tin oxide (ITO) electrodes. The face-on alignment of the discotic compound has been obtained by thermal annealing without any intermediate coating between the mesophase and the ITO substrate. Such a columnar mesophase alignment is thus shown on a substrate of technological interest in open supported thin film reaching the thickness range suitable for organic photovoltaic devices.

Red organic light emitting device made from triphenylene hexaester and perylene tetraester

Saturated red light emission from organic light emitting diodes is less common than emission in the green or the blue. Most organic red light emitting devices are based on rare earth complexes, mainly europium, which are known to exhibit stability problems. The present article describes new diodes made of indium tin oxide-coated glass/triphenylene hexaether/perylene tetraester/aluminum. The band diagram was determined by ultraviolet photoemission spectroscopy, cyclic voltammetry, scanning tunneling microscopy, and absorbance measurements. The interfaces between electrodes and organic layers were investigated by x-ray photoelectron spectroscopy. The current–voltage and luminance–voltage characteristics are very reproducible from device to device, with an emission peak at 620 nm and a full width at half maximum of 80 nm, a current rectification ratio of about 30, I∼V2 at low voltages and I∼Lum∼V6 at higher voltages.

An all-columnar bilayer light-emitting diode

Abstract The first organic light-emitting diode with both an electron-rich columnar liquid crystal as hole transport material and an electron-deficient fluorescent columnar liquid crystal as electron transport material is presented. Red fluorescence is observed above 10 V from the configuration indium–tin oxide (ITO)/hexabutoxy-triphenylene/tetraethyl perylenetetracarboxylate/Al. The excellent charge transport properties of columnar phases are thus exploited for light-emitting diodes. Both organic materials exhibit their columnar liquid crystal phase well above room temperature and the underlying ordered solid columnar phase is used in the diode. The I ( V ) and L ( V ) characteristics are very reproducible from device to device, with an emission peak at 620 nm and an FWHM of 80 nm, a current rectification ratio of about 30, I ∼ V 2 at low voltages and I ∼ L ∼ V 7 at higher voltages. Initial lifetime measurements are encouraging.

Characterization of ITO/CuPc/AI and ITO/ZnPc/Al structures using optical and capacitance spectroscopy

Metal-substituted phthalocyanine (MPc) thin films as zinc- or copper-phthalocyanine are often used as charge injection layers for organic electroluminescent or photovoltaic devices. It is then important to characterize their electronic defect density and band structure near their gap. In this work the monolayer structures were prepared by vacuum sublimation of the organic thin film sandwiched between indium tin oxide (ITO) and aluminum electrodes. Electrically active defects were investigated with space-charge capacitance spectroscopy, as a function of temperature and frequency, in the range 80–330 K and 40 Hz to 10 MHz, respectively. Organic materials are best described on the basis of individual molecular orbital (HOMO and LUMO) energies instead of valence and conduction band. Such energies were derived from cyclic voltammetry and optical absorption spectroscopy measurements. Experimental results were correlated to the electrical J–V characteristics of these MPc based devices to gain more insight on the charge injection processes and their limitations.

Current Switching Coupled to Molecular Spin-States in Large-Area Junctions

The fabrication of large-area vertical junctions with a molecular spin-crossover complex displaying concerted changes of spin degrees of freedom and charge-transport properties is reported. Fabricated devices allow spin-state switching in the spin-crossover layer to be triggered and probed by optical means, while detecting associated changes in electrical resistance in the junctions.

Microelectromechanical systems integrating molecular spin crossover actuators

Silicon MEMS cantilevers coated with a 200 nm thin layer of the molecular spin crossover complex [Fe(H2B(pz)2)2(phen)] (H2B(pz)2 = dihydrobis(pyrazolyl)borate and phen = 1,10-phenantroline) were actuated using an external magnetic field and their resonance frequency was tracked by means of integrated piezoresistive detection. The light-induced spin-state switching of the molecules from the ground low spin to the metastable high spin state at 10 K led to a well-reproducible shift of the cantilevers resonance frequency (Δfr = −0.52 Hz). Control experiments at different temperatures using coated as well as uncoated devices along with simple calculations support the assignment of this effect to the spin transition. This latter translates into changes in mechanical behavior of the cantilever due to the strong spin-state/lattice coupling. A guideline for the optimization of device parameters is proposed so as to efficiently harness molecular scale movements for large-scale mechanical work, thus paving the road...

Photoemission study of the ITO/triphenylene/perylene/Al interfaces

Abstract The interface formation in a ITO-coated glass/triphenylene hexaether/perylene tetraester/aluminum heterostructure was studied by ultra-violet photoemission spectroscopy (UPS). The interfaces were built step by step by successive evaporation of thin (few nm) material layers. Each step was followed by UPS characterization which allowed determination of the evolutions of the valence bands (VBs) and that of the vacuum level. An electronic energy diagram has been deduced giving the metal/organic barriers and the band offset between the two organic semiconductors. Major differences were observed between the two metal/organic interfaces which can have consequences for light emitting diodes based on these materials.

Room temperature current modulation in large area electronic junctions of spin crossover thin films

We report large-area (∼3 mm2), pinhole free crossbar junctions of thin films of the molecular complex [Fe(HB(tz)3)2] displaying spin transition around 336 K. The charge transport in the thinner junctions (10 and 30 nm) occurs by a tunneling mechanism, which is not affected substantially by the spin transition. The thicker junctions (100 and 200 nm) exhibit rectifying behavior and a reproducible drop of their electrical resistance by ca. 65–80% when switching the molecules from the high-spin to the low-spin state. This current modulation is ascribed to a bulk-limited charge transport mechanism via a thermally activated hopping process. The demonstrated possibility of resistance switching in ambient conditions provides appealing prospects for the implementation of molecular spin crossover materials in electronic and spintronic devices.

Spin transport in benzofurane bithiophene based organic spin valves

In this paper we present spin transport in organic spin-valves using benzofurane bithiophene (BF3) as spacer layer between NiFe and Co ferromagnetic electrodes. The use of an AlO x buffer layer between the top electrode and the organic layer is discussed in terms of improvements of stacking topology, electrical transport and oxygen contamination of the BF3 layer. A study of magnetic hysteresis cycles evidences spin-valve behaviour. Transport properties are indicative of unshorted devices with non-linear I-V characteristics. Finally we report a magnetoresistance of 3% at 40 K and 10 mV in a sample with a 50 nm thick spacer layer, using an AlO x buffer layer.

Spin-polarized transport in NiFe/perylene-3,4,9,10-tetracarboxylate/Co organic spin valves

We have prepared organic spin valves based on the perylene derivative perylene-3,4,9,10-tetracarboxylate (PTCTE) with NiFe and Co ferromagnetic electrodes (with direct or off-axis sample configuration for the Co upper electrode deposition). Transmission electron miscroscopy is employed to study the stacking of the NiFe/PTCTE/Co vertical structures. Tunneling atomic force microscopy reveals that direct deposition of Co damages the PTCTE layer. Moreover, fluorescence and I-V studies of PTCTE evidence changes of the organic layer’s transport properties that are correlated with the deposition rate of PTCTE. Finally, we report up to 3% magnetoresistance at 10 mV and 5 K on a sample with an organic spacer 300 nm in thickness and an off-axis deposited cobalt counter electrode. We analyze this result in terms of spin diffusion and spin precession in PTCTE.