Films Of Charge-transfer Complexes Research Articles

Novel fabrication process of charge transfer complex thin films for large area by thermal diffusion and their n-channel organic transistor operation

We have proposed a new fabrication process of donor and acceptor charge transfer complex thin films using thermal diffusion. 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4-TCNQ) was vacuum-deposited on 2,7-didecylbenzothienobenzothiophene (C10-BTBT) thin films fabricated by spin-coating, and F4-TCNQ molecules were diffused into C10-BTBT thin films by thermally annealing. The X-ray diffraction pattern and field effect transistor of thermally annealed thin films indicated the crystal structure of the charge transfer complex of C10-BTBT and F4-TCNQ and n-channel operation in air, respectively. We conclude that it is good process to use the thermal diffusion for the fabrication of the large area thin films of charge transfer complex.

1/f Noise in conducting Langmuir-Blodgett films of quasi-one-dimensional charge-transfer complexes

The noise characteristics of conducting Langmuir-Blodgett films of charge-transfer complexes (C16H33-TCNQ)0.4(C17H35−DMTTF)0.6 (surface-active complex based on a 1.5: 1.0 mixture of heptadecyldimethyltetrathiafulvalene (C17H35−DMTTF) and hexadecyltetracyanoquinodimethane (C16H33-TCNQ)) are studied. It is demonstrated that the excess (with respect to the thermal noise) electrical noise whose spectrum is close to 1/f dominates in the frequency range 1–104 Hz. This noise has a bulk nature and arises owing to fluctuations of the film conductivity. The noise level is almost three orders of magnitude higher than the level predicted by the Hooge formula. A model that explains the film conductivity fluctuations by the superposition of relaxation processes of the on/off switching of conducting channels is proposed. In the framework of this model, a relationship that makes it possible to represent the level of the 1/f noise in terms of the parameters of a sample is derived. It is shown that the application of this relationship for the calculation of the 1/f noise magnitude in specific samples of conducting Langmuir-Blodgett films and TTF-TCNQ crystals yields results that are close to the experimental data.

Green-Light Static Rewritable Optical Storage Properties of aNovel CuTCNQ Derivative Thin Film

A novel charge-transfer complex film: copper-(n-propyl ester 7,7,8,8-tetracyanoquinodimethane-2,5-ylene-(3-propionic acid))(Cu-TCNQ(C2H4COOC3H7)2) was preparedby spin-coating. Absorption spectra, green-light (514.5 nm) staticrewritable optical recording properties and rewritable mechanism ofthis film were studied. The results show that there are two strongabsorption peaks at 388 nm and 675 nm, which can be assigned toelectronic transitions in anion radicalTCNQ(C2H4COOC3H7)2−.Green-light optical storage experimental results of this filmwere as follows: write-in power was 9 mW, pulse duration was 80 ns;erasing power was 4 mW, pulse duration was 500 ns; the reflectivitycontrast C⩾15%; number of write–erase cycles N⩾100. Itis found that the realization of rewritable optical storage of theCu-TCNQ(C2H4COOC3H7)2 film isrelated to the reversible changes of the optical properties, which iscaused by the reversible charge transfer between copper and n-propylester 7,7,8,8- tetracyanoquinodimethane-2,5-ylene-(3-propionic acid) inthe complex through inducement of laser irradiation.

Novel Method for Preparing a Mixed-Stack Charge-Transfer Film of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and 3,3‘,5,5‘-Tetramethylbenzidine: The Use of a Mixed Langmuir−Blodgett Film of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and Stearic Acid as Starting Material for the Fabrication of the Charge-Transfer Film

The present paper reports a novel method for preparing a mixed-stack charge-transfer (CT) film having 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) as an electron acceptor and the possibility of the control of molecular orientation in the CT film. A seven-layer mixed Langmuir−Blodgett (LB) film of C18TCNQ and stearic acid is deposited onto a CaF2 plate or a gold-evaporated glass slide at the surface pressure of 15 mN m-1. Dipping the mixed LB film into a petroleum ether solution of 3,3‘,5,5‘-tetramethylbenzidine (TMB) results in the formation of a mixed-stack CT film of TMB·C18TCNQ. Upon the formation of the CT film, the dilution molecules, namely stearic acid, are removed completely from the film. The appearance of a broad absorption band in the near-infrared region and the frequency shift of the CN stretching band (2208 cm-1) verify that the CT complex film of TMB·C18TCNQ is formed by the TMB doping. IR spectra of the CT films do not show any bands assignable to stearic acid, suggesting that stearic acid molecules are removed from the thin films after the dipping. The band at 3496 cm-1 due to the antisymmetric stretching mode of the NH2 group does not appear in the IR transmission spectrum. Thus, it seems that the chromophore plane of TMB component is nearly perpendicular to the substrate surface with its long axis lying on the substrate in the CT films. The molecular structure and orientation in the CT films of TMB·C18TCNQ prepared by the present method and those fabricated by dipping the LB films of C18TCNQ alone into the TMB solution have been compared. The comparison shows that the orientation of the chromophore planes of both donor and acceptor components and that of the alkyl chain of C18TCNQ in the CT films are controlled mainly by the doping process.

Control of molecular orientation and conposition ratio of TTF-TCNQ films

We have fabricated TTF-TCNQ charge-transfer (CT) complex films on PTFE rubbed films by evaporation techniques, and measured the molar ratio of the films by varying the growth temperature. In contract, the molecular orientation of TTF-TCNQ on PTFE rubbed films was investigated by in-situ polarized optical measurement. TTF-TCNQ CT complex films with a molar ratio of 1:1 were obtained under the appropriate source and growth temperatures, and the molecular orientation of the TTF-TCNQ crystal long axis perpendicular to the PTFE chain was observed.

Electrical and Nonlinear Optical Properties of Langmuir-Blodgett Films of Charge Transfer Complexes

A series of donors and acceptors are synthesized and, by combing these molecules, Langmuir-Blodgett (LB) films of the charge transfer (CT) complexes are prepared. Ionic CT complexes with partial CT state exhibit high conductivity in the form of LB films. The second nonlinear optical properties of the films are examined using electro-apsorption method.

Raman Study of Mixed-Stack Charge Transfer Films of 2-Octadecyl-7,7,8,8-Tetracyanoquinodimethane and 3,3′,5,5′-Tetramethylbenzidine Prepared by the Langmuir-Blodgett Technique and Donor Doping

Raman, FT-Raman and ultraviolet-visible-near infrared (UV-vis-NIR) spectra have been measured for mixed-stack charge transfer (CT) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) doped with 3,3′,5,5′-tetramethylbenzidine (TMB) deposited on CaF2 plates and Au-evaporated glass slides. The 1604 nm-excited FT-Raman spectra can be observed clearly even for the one-layer CT films deposited on CaF2 plates because the excitation wavelength is located within a broad CT band centered near 1550nm. The degree of the charge transfer determined by a shift of the C≡N stretching band of the TCNQ chromophore suggests that the CT complex films are in a quasi-neutral state.

Charge-transfer interactions and non-linear optical properties of tetrathiafulvalene-based Langmuir–Blodgett films

Charge-transfer (CT) interactions of tetrathiafulvalene (TTF)-based Langmuir–Blodgett (LB) films with non-centrosymmetrical structure were examined from the viewpoint of nonlinear optics. Five kinds of LB films of CT complexes were prepared using TTF-based electron donors and acceptors of 7,7,8,8-tetracyanoquinodimethane and p-benzoquinone derivatives. The difference of the dipole moments between ground and excited states of these LB films was evaluated from the measurements of electroabsorption spectra.

Characterizing reactions to fabricate thin films of charge transfer complexes by synchrotron photoelectron spectroscopy: A case study of DCNQI-Cu

Ultraviolet photoelectron spectroscopy with various photon energies using synchrotron radiation was used to characterize chemical reactions associated with thin film growth of organic charge transfer complex (DMe-DCNQI) 2Cu. Other molecular systems H 2Pc, CuPc and C 60 were also studied to clarify the origin of the systematic relation between the spectra and the incident photon energy. Characteristic photon energy dependence of the photo-ionization cross section of molecular orbitals is useful to analyze the intermolecular reactions.

Thermal Behavior of Mixed-Stack Charge Transfer Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and 3,3‘,5,5‘-Tetramethylbenzidine Prepared by the Langmuir−Blodgett Technique and Donor Doping. 1. Dependence of Thermal Behavior on the Number of Layers Studied by Ultraviolet−Visible−Near Infrared and Infrared Spectroscopies

The dependence of thermal behaviors on the number of layers in Langmuir−Blodgett (LB) films of the mixed-stack charge transfer (CT) complex of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) doped by 3,3‘,5,5‘-tetramethylbenzidine (TMB) has been investigated by using ultraviolet−visible−near infrared (UV−vis−NIR) and infrared (IR) spectroscopies. The appearance of a broad CT absorption band in the NIR region reveals that the mixed-stack CT films are formed by donor doping. The degree of the charge transfer determined by a shift of the C⋮N stretching band of the TCNQ chromophore verifies that the CT complex films are in a quasi-neutral state. Their temperature-dependent UV−vis−NIR and IR spectra show that donor molecules (TMB) dedope from the CT complexes in the LB films, resulting in the restoration of acceptor molecules (octadecyl-TCNQ) to their neutral states at their respective dedope temperatures. The dedope temperature of the CT complex films increases with the number of layers up to a ...

Thermal Behavior of Mixed-Stack Charge Transfer Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and 3,3‘,5,5‘-Tetramethylbenzidine Prepared by the Langmuir−Blodgett Technique and Donor Doping. 2. Morphology and Annealing Effects of the Films Investigated by Atomic Force Microscopy and Ultraviolet−Visible−Near Infrared and Infrared Spectroscopies

Morphology and annealing effects of Langmuir−Blodgett (LB) films of a mixed-stack charge transfer (CT) complex of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) doped by 3,3‘,5,5‘-tetramethylbenzidine (TMB) have been investigated by atomic force microscopy (AFM) and ultraviolet−visible−near infrared (UV−vis−NIR) and infrared (IR) spectroscopies. AFM images of the LB films of the CT complex scanned at room temperature show that the films consist of numerous one-dimensional needle-like microcrystals and that a periodic structure exists inside these needle-like microcrystals. A cyclic thermal treatment on one-, three-, and 11-layer films of the CT complex up to 110 °C, which is below their dedope temperatures, makes the needle-like microcrystals thinner and flatter while the molecular arrangement and structure in the crystals are nearly unchanged. However, when the temperature of the CT complex films is elevated above their respective dedope temperatures, the needle-like microcrystals become m...

Langmuir-blodgett films of charge transfer complexes of bisethylenedioxytetrathiafulvalene-alkyltetracyanoquinodimethane

The LB films of the charge transfer complexes of oxygen-containing BEDT-TTF type donor bisethylenedioxytetrathiafulvalene (BO) and long-chain derivatives of tetracyanoquinodimethane (C nTCNQ) are prepared with and without matrix molecules. The measurements of π-A isotherm for the complex-icosanoic acid mixed system indicated the formation of mixed monolayers at the air-water interface. This was confirmed by the AFM measurements showing a granular structure of the film with the grain size of 100 nm in diameter and 3–4 nm in height. These grains are probably attributable to the BO complex. The conductivity of the LB films were strongly affected by the film-deposition method, the spreading solvent and the matrix. The mixed LB films of BO-C 10TCNQ and BO-C 14TCNQ with icosanoic acid under suitable conditions showed conductivities as high as 10 and 3 S/cm, respectively. The former LB film showed a metallic temperature dependence of conductivity down to 250 K. The conduction mechanism is discussed in terms of the film granuality.

Synthesis and photoelectrical properties of poly[2,6‐(p‐phenoxy)‐4‐phenylquinoline

AbstractNew heteroaromatic polymer containing quinoline unit in the main chain, poly[2,6‐(p‐phenoxy)‐4‐phenyl quinoline] (PQ), was synthesized by the acid‐catalyzed self‐condensation reaction [Friedländer quinoline synthesis] of 4‐amino‐4′‐acetyl‐3‐benzoyldiphenyl ether, and its chemical structure was characterized by IR and NMR spectra and also by elemental analysis. The PQ was soluble in common organic solvents such as chloroform or tetrahydrofuran; thus a strong coherent film of this polymer could be easily prepared from the solution by the casting or spin‐coating technique. Attributing to the wholly aromatic chemical structure, PQ appeared an extremely high thermal stability as to show no weight loss up to 550°C both in air and nitrogen atmosphere. From the UV–VIS absorption spectra, we found that PQ had a strong tendency to form the charge transfer (CT) complex with electron acceptors such as 2,4,7‐trinitrofluorenone (TNF). The CT complex film of PQ–TNF showed a good photoconductivity even under the ambient room illumination and was utilized for the fabrication of photovoltaic device whose conversion efficiency was around 10−2%.

Thin film growth characteristics of charge transfer complexes

Growth characteristics for thin films of mixed stack charge transfer (CT) complex phenothiazine-tetracyanoquinodimethane (PTZ-TCNQ) by vacuum evaporation have been investigated. It is clarified that both direct evaporation of the complex and coevaporation of its constituents cause growth of the CT complex film with a 1:1 molecular composition. The molecular planes of the two constituents in the complex film grown on HOPG were almost parallel to the substrate surface. To clarify the growth mechanism, the change of growth characteristics with substrate temperature of the complex were compared with those of the constituents. The results indicate that critical changes of the growth behavior of the complex coincide with that of TCNQ, suggesting that the complex film grows by nucleating TCNQ molecules. It is also suggested that the orientation of the complex film on HOPG originates from the parallel orientation of the admolecules of TCNQ.

Metal/organic junction characteristics of vacuum-evaporated films of charge transfer complexes

Several donor-acceptor charge transfer (CT) complexes, having different degrees of charge transfer (ϱ) and different CT absorption bands(νCT), were successfully evaporated onto SiO2/n-Si substrates, yielding the device structures of metal/CT complex /SiO2/Si. The metal/organic junction characteristics were then investigated by measuring the displacement currents(Id), which are ascribale to the changes in charge densities for the devices, with respect to applying ac biases at low frequency. Different carrier injection behaviors were observed for the complexes examined. Perylene(Per)-TCNQ exhibited a predominantly electron-injective property, while for Dimethylphenazine (M2P)-TCNQ and Phenothiazine (PTZ)-TCNQ; whose absorption energies were much smaller than ca. 0.6 eV, both electron- and hole-injective characteristics were found. The results can be explored by taking into account the participation of charge carriers injected from metal electrodes. Also the carriers stem from thermal excitation within the CT complexes.

Temperature dependence of conductivity of charge-transfer complex C16H33 - TCNG - C17H35-DMTTF LB films

We prepared conducting Langmuir-Blodgett (LB) films of charge-transfer complexes using (C16H33-TCNQ)1−x(C17H35-DMTTF)x mixture and measured conductivity in a wide temperature range and Hall constant at the room temperature. Electron microscopy and electron diffraction technique were used to make more clear the causes of some peculiarities of temperature dependencies of conductivity.

Electron spin resonance study of Langmuir-Blodgett films of the complexes of alkylammonium and metal(dmit) 2 anion

Recently a new class of conductive Langmuir-Blodgett (LB) films has been generated using M(dmit) 2 ( M = Au or Ni, H 2dmit = 4, 5-dimercapto-1, 3-dithiol-2-thione) as an electron acceptor. One member of this class, the LB film of 3C 10-Au(dmit) 2 (3C 10 = trialkymethylammonium), has after electro-oxidation been reported as the first example of a macroscopically metallic LB film system (σ ≈ 30 S cm −, because of the observed increase in its conductivity with decreasing temperature down to 200 K. Electron spin resonance (ESR) measurements have been performed on LB films of alkylammonium-M(dmit) 2 complexes as well as powder samples. The ESR spectra observed for LB and powder samples are different from each other, suggesting a change in molecular arrangements. ESR spectra are also dependent on the central metal species and the number of alkyl chains. The comparatively low spin density in the electro-oxidized film of 3C 10(dmit) 2 observed at low temperatures suggests a lower amount of disorder and/or a higher degree of dimensionality of the system than for other conductive LB films of charge transfer complexes reported so far.

Conductive Langmuir–Blodgett Film Containing Mesogenic Unit. Phase Transition Accompanied with the Change in Conductivity

Abstract Conductive Langmuir–Blodgett films of charge transfer complex containing azobenzene group in the hydrophobic part exhibited a sharp discontinuity in the temperature dependent conductivity. This discontinuity was accompanied with the change in the aggregation form and the morphology of the films.