G. Guillaud

Characterization of metallophthalocyanine‐metal contacts: Electrical properties in a large frequency range

The dark electrical properties of metallophthalocyanine‐metal contacts are studied in an extended frequency range (10−3–105 Hz). The measurements show that the standard Schottky model cannot be applied to metallo‐organic semiconductors. Three different contributions to the admittance may be distinguished. Over the very first few angstroms of the semiconductor extends a surface‐charge layer associated with a high capacitive term. The surface‐charge capacitance is only slightly dependent upon superimposed dc voltages. The so‐called space‐charge region extends below the surface‐charge layer over approximately 2000–4000 A. The properties of this space‐charge region are, however, dramatically different from those found in monocrystalline inorganic semiconductors. The corresponding capacity is almost independent of any dc superimposed voltage. At the same time, the resistance of the space‐charge region is highly dependent on externally applied dc voltages. The I‐V relationship seems to indicate a Frenkel–Poole ...

Molecular material‐based junctions: Formation of a Schottky contact with metallophthalocyanine thin films doped by the cosublimation method

A new method for obtaining thin films of doped molecular materials in a complete absence of oxygen is described. It consists in simultaneously subliming under vacuum the electroactive compound—a metallophthalocyanine—and the electron acceptor or electron donor doping agent. The extent of doping is controlled by the relative rates of sublimation. The ac and dc electrical properties of thin films are determined under high vacuum as to eliminate the influence of oxygen which has been previously shown to be of the utmost importance. Both p‐ and n‐type dopings have been achieved; the increase of conductivity reaches six orders of magnitude for p‐type doping with dichlorodicyanoquinone (DDQ). For the first time a rectifying contact between doped phthalocyanine thin films and aluminum has been observed in strict absence of oxygen.

Molecular semiconductors and junction formation: Phthalocyanine derivatives☆

Abstract Three complementary aspects of the semiconducting behavior of molecular materials have been examined : the intrinsic semiconductivity, the possibility of doping and the realization of an extended space charge layer in a junction. This study is focused on the phthalocyanine derivatives. Molecular materials are all insulators when undoped. In contrast an intrinsic semiconducting behavior has been observed on two radicalar phthalocyanines: Pc2Lu (substituted or not). From their magnetic, optical and electrical properties, they may be described as intrinsic semiconductors (for Pc 2 Lu : σ RT ⋍ 10 −5 ohm −1 cm −1 ; activation energy : 0.33 eV). Controlled doping can be achieved by cosublimation of acceptors or donors (DDQ, TCNQ, TTF, DIPS). The formation of a space charge region in M1/PcM/M2 cells (M = Zn, Ni ; M1 = Au; M2 = Al, Cr) has been shown to be occur by transferring electrons from the rectifying electrode M2 to the oxygen molecules bound at the surface of the crystallites of PcM. The extent of the space charge region cannot be varied when an external voltage is superimposed. This uncontrolled rectifying behavior is linked to the migration of the ionized dopant within the molecular material. By using, at a low doping level, larger molecular weight acceptor molecules as dopant to prevent migration at room temperature, it has been possible to observe a diode behavior in strict absence of oxygen. A localized molecular model of the formation of junctions is proposed and the potentialities of molecular materials in the realization of such devices are discussed.

Transient behaviour of thin film transistors based on nickel phthalocyanine

Abstract Measurements of transient currents have been made on evaporated thin film transistors based on nickel phthalocyanine. A time delay has been observed for the establishment of the drain current. The results are consistent with the theoretical model described by Burns. A study of this delay as a function of the drain bias voltage indicates that the mobility is field dependent and that the threshold voltage is close to zero.

Calixarene membranes on semiconductor substrates for E.I.S. chemical sensors

Thin p-tert-butyl calix[4]arene and p-tert-butyl calix[6]arene films were deposited using the technique of thermal evaporation under vacuum to fabricate chemical microsensors based on an E.I.S. (Electrolyte-Insulator-Semiconductor) structure. Electrochemical capacity measurements were made to test and calibrate physico-chemical sensors with regards to their sensitivity and selectivity. The sensing properties of calix[4]arene and calix[6]arene towards alkaline and heavy metal ions were tested. Both devices have shown Nernstian response and a wide working pH range. The former responded well to sodium ions and the latter to nickel ions, thus partial selectivity for these ions is shown. The improved lifetime (compared with membranes prepared by physical adsorption techniques) and sensitivity observed for these films indicates that these structures are worthy of further developments.

Study of ion-selective evaporated calixarene film used as a sensitive layer on ISFET sensors

Abstract Thin calix[4]arene and calix[6]arene films were deposited using the technique of thermal evaporation under vacuum to fabricate chemical ion-selective field effect transistor (ISFET) microsensors. A high-performance liquid-phase chromatography (HPLC) method was used to verify the non-degradation of the calixarene layers, after the evaporation process. The sensing properties of these ionophores towards alkaline and heavy metal ions were tested. Both devices have shown Nernstian response and a wide working pH range. The former responded well to sodium ions and the latter to nickel ions, thus partial selectivity for these ions is shown. The improved lifetime membranes and sensitivity observed for these films indicates that these structures are worthy of further developments.

Electrical properties of molybdenum disulfide MoS2. Experimental study and density functional calculation results

Abstract Molybdenum disulfide is a very interesting material with numerous applications. However, to our knowledge, very few experimental works involving electrical measurements have been performed with this material up to now. This paper is devoted to the characterization of conduction mechanisms in thin layers and pressed pellets. The electronic structure related to the lubricating properties and the activation energy are determined using theoretical calculations (density functional theory) and experimental methods. A good agreement is found between the theoretical predictions and the experimental results. D.c. and a.c. conductivities are measured in the frequency range 10–107 Hz. The electronic transport mechanism is discussed in terms of Mott hopping in samples. The a.c. conductivity at high frequencies can be expressed by the formula σac(ω) = Aω11, where the slope n is close to 0.8.

Study of the membrane morphology and investigation of sensitivity to ions for sensors based on calixarenes

Abstract Many investigations on dielectric, electronic and spectroscopic characterisation have been achieved on molecular materials for their eventual application as sensors or electronic devices. Calixarenes exhibit an interesting supramolecular structure on account of their receptor properties; they represent receptor molecules of widely varying size for metal cations and organic molecules. They are comparatively new cyclic condensation products of para-substituted phenols and formaldehyde; they form a series of well-defined cyclic oligomers. Therefore, the variation of cavity size according to the requirements (i.e. ion sensitivities) of different guests is possible. The macro cycles are characterised by the phenolic unit number (4,6,8,…). p -tert-Butyl calix[ n ]arene ( n = 4 and 8), used as a sensitive membrane incorporating the receptor molecule on the top of the gate oxide, was characterised. The calixarene was thermally evaporated on the oxide to form a semiconductor-oxide-film capacitance structure. It is shown, by means of structural and spectroscopic studies, that films have good adherence to the oxide and are chemically very stable allowing the use of such a structure as a chemical sensor in liquid media. The studies of ISFET structure using calix[ n ]arene membranes are in progress, showing an interesting selectivity and stability.

Cu2+-ISFET type microsensors based on thermally evaporated p-tert-butylcalix[9 and 11]arene thin films1

Abstract Large size odd-numbered calixarenes were used for the first time as ionophoric agents for the functionalization of ISFET microsensors and EIS structures through thermal evaporation process. Both calixarenes have shown a nernstian sensitivity over three decades towards only copper (II) activities. Very low selectivity coefficients were observed for K + and Ca 2+ whereas Cd 2+ and Pb 2+ (less than 10 −3 ) can be considered as interfering ions. Lifetime of around three months for the microsensors were obtained.

Membranes Containing New Large Size Calixarenes on Semiconductor Substrates for Chemical Microsensors.

Abstract New large size calixarenes have been synthesized and tested as ionophores. The applications of these new even-membered macrocycles in the field of chemical sensors have been studied. Thin p-tert-butyl calix[10 and 12]arenes film were deposited using the technique of thermal evaporation under vacuum on ISFET (Ion Sensitive Field Effect Transistor) and EIS structures (Electrolyte Insulator Semiconductor) surfaces. It was verified that by liquid chromatography the calixarene layers, after the sublimation process did not undergo any degradation. Electrochemical measurements with EIS structures and ISFETs were made to test the sensing properties of these calixarene films towards heavy metals. These large size calixarenes have shown a Nernstian sensitivity and a good selectivity towards iron(III) ions for the p-tert-butyl calix[12]arene and silver(I) ions for the p-tert-butyl calix[10]arene. A long lifetime was obtained for these sensors when tested over a large pH range. These materials present a real and great promise for further developments.