Organic Charge-transfer Complexes Research Articles

Fabrication of organic conductive wires and molecular break junction

Break junction have been proposed as one of methods for making a molecular-scale nano-gaps between the two metallic electrodes. Here we had made organic metallic wire bridges being consist of tetrathiafulvalene-tetracyanouinodimethane (TTF-TCNQ) organic charge transfer complex, and then gradually disconnected the bridged molecular wires by a Joule heating to make a molecular-size gap or single-molecular junction. The observed conductance of the initially high conductive wires decreased by a direct current (DC) Joule heating. The conductance exhibited several different behaviors with the stage of the thermal desorption of molecules, and finally disconnected. Significant increase and decrease of the conductance were observed just before the disconnection. After the DC treatment and before the disconnection of the wires, we switched to alternation current (AC) Joule heating to make a single-molecular junction which would work as a single-molecular transistor. During the AC Joule heating, many discrete steps were found in the time evolution of the conductance. The steps were approximately 1 pS or it's multiple, and the duration of the step were nearly the same (approx. 300 - 400 s) in the final stage of the AC Joule heating. We conclude that the step height corresponds to the single-molecular conductance and the duration reflects the probability of single TCNQ molecular desorption.

Why are the Interaction Energies of Charge-Transfer Complexes Challenging for DFT?

The description of ground state charge-transfer complexes is highly challenging. Illustrative examples include large overestimations of charge-transfer by local and semilocal density functional approximations as well as inaccurate binding energies. It is demonstrated here that standard density functionals fail to accurately describe interaction energies of charge-transfer complexes not only because of the missing long-range exchange as generally assumed but also as a result of the neglect of weak interactions. Thus, accounting for the missing van der Waals interactions is of key importance. These assertions, based on the evaluation of the extent of stabilization due to dispersion using both DFT coupled with our recent density-dependent dispersion correction (dDsC) and high-level ab initio computations, reflect the imperfect error-cancellation between the overestimation of charge-transfer and the missing long-range interactions. An in-depth energy decomposition analysis of an illustrative series of four small ambidentate molecules (HCN, HNC, HF, and ClF) bound together with NF3 provides the main conclusions, which are validated on a prototypical organic charge-transfer complex (i.e., tetrathiafulvalene-tetracyanoquinodimethane, TTF-TCNQ). We establish that the interaction energies for charge-transfer complexes can only be properly described when using well-balanced functionals such as PBE0-dDsC, M06-2X, and LC-BOP-LRD.

Interface Modification of Cathode Electrode Using Dimmethyldicyanoquinonediimine as a Charge Transfer Layer in Organic Photovoltaic Cell

Various metals for the cathode electrode of organic electronic devices have been used in order to improve carrier injection and contact resistance etc. However, metal electrodes have some disadvantages such as rough surfaces, inadequate interfacial durability and unsuitable work functions. In the present work, we have fabricated an organic photovoltaic cell consisting of ITO/PEDOT:PSS/P3HT:PCBM/DMDCNQI/Al. The dimmethyldicyanoquinonediimine (DMDCNQI) compound was used as an organic n-type charge transfer complex between the cathode electrode and an organic active layer to improve contact resistance and electron transport ability. The prepared device shows a high short-circuit current density of 10.39 mA/cm2 and a maximum power conversion efficiency of 3.10%.

Device Arrays: Mass‐Production of Single‐Crystalline Device Arrays of an Organic Charge‐Transfer Complex for its Memory Nature (Small 4/2012)

The cover image shows organic single-crystalline devices and device arrays, which are challenging to fabricate in situ. Single-crystalline lamellae of a charge-transfer complex, copper 7,7,8,8-tetracyano-p-quinodimethane (CuTCNQ), are synthesized in a controlled manner, and devices and device arrays of the lamellae based on symmetrical gold electrodes are efficiently mass-produced in situ. All the devices exhibit promising semiconductor properties, which are important for understanding the switching nature of the material. Moreover, the in situ fabrication of the single-crystalline devices and device arrays are attractive for the field of organic electronics, and they open up prospects for large-scale production of high-quality organic devices and circuits. For more information, please read the Communication “Mass-Production of Single-Crystalline Device Arrays of an Organic Charge-Transfer Complex for its Memory Nature” by Z. Tang, W. Hu, and co-workers, beginning on page 557.

Controllable one-dimension nanostructures of CuTNAP for field emission properties

One-dimensional nanostructures of the organic charge-transfer (CT) complex CuTNAP (copper 11,11,12,12-tetracyano-2,6-naphthoquinodimethane) were successfully synthesized by an organic vapor-solid phase reaction. The morphologies and field-emission properties of the CuTNAP nanostructures can be easily tuned by controlling the reaction conditions. It was observed that the field emission property is dependent on the morphology. The current density of a CuTNAP film of nanowires reaches up to 13.1 mA cm(-2), which is the highest among the organic semiconductors, even higher than most inorganic materials. These results demonstrate that the CuTNAP complex nanostructures are excellent potential candidates as field emitters.

Electron-vibrational model for an organic dimer based on cation radicals TTF+ and anion radicals TCNQ−

The electron-vibrational model for an organic dimer proposed in this paper allows one to take into account not only the electron correlation (the organic charge transfer complexes are strongly correlated systems) through the dimerized Hubbard model but also the internal structure of the dimer and each of its monomers by introducing an additional term into the Hamiltonian to describe the interaction between the electrons and the vibrational subsystem. Thus, it provides ample opportunities to minutely describe the polarization and conduction properties of organic dimer systems.

AC heat capacities of κ-(BEDT-TTF)2Cu2(CN)3 measured by microchip calorimeter

Thermodynamic measurements of an organic spin liquid compound of κ-(BEDT-TTF)2Cu2(CN)3 where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene were performed by the ac calorimetry technique using a microchip device of TCG3880. This technique is effective to measure relative temperature and magnetic-field dependences of heat capacity for tiny single crystal samples less than 1μg. Broad hump structures in Cp vs T which are known as so-called 6 K anomaly were observed in κ-(BEDT-TTF)2Cu2(CN)3 and its deuterated compound. The hump temperatures are evaluated as 5.7 K in both compounds. This result demonstrates that the TCG3880 is useful for performing thermodynamic investigations of such kind of organic charge transfer complexes with much reduced sample quantity than the conventional techniques and that the existence of hump structure is intrinsic for κ-(BEDT-TTF)2Cu2(CN)3. The in-plane angular dependence of the magnetic field of 7 T applied parallel to the two dimensional layer is also studied and absence of in-plane anisotropy of the hump structure is discussed in both pristine and deuterated compounds.

Optical and structural studies of a two-dimensional organic Mott insulator dimethylphenazine-tetrafluorotetracyanoquinodimethane

Electronic structure of an organic charge-transfer complex, M${}_{2}$P-TCNQF${}_{4}$ (M${}_{2}$P: 5,10-dihydro-5,10-dimethylphenazine; TCNQF${}_{4}$: 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), was investigated by means of optical reflection spectroscopy and x-ray structural analyses. This is an ionic compound and has a unique quasi-two-dimensional (2D) crystal structure in which donor ($D$) molecules of M${}_{2}$P and acceptor ($A$) molecules of TCNQF${}_{4}$ stack respectively along the [100] direction, and $D$ and $A$ molecules also stack alternately along the [111] direction. We evaluated the transfer energy $t$($AA$) between the neighboring $A$ molecules, $t$($DD$) between the neighboring $D$ molecules along the [100] direction, and $t$($DA$) between the neighboring $D$ and $A$ molecules along the [111] direction to be 43 meV, 29 meV, and 67 meV at room temperature, respectively. This demonstrates that an anisotropic 2D electronic structure is formed. By comparing the spectra of the imaginary part of the dielectric constant ${\ensuremath{\varepsilon}}_{2}$ obtained from the polarized reflectivity spectra with the absorption spectrum of K-TCNQF${}_{4}$ and the ${\ensuremath{\varepsilon}}_{2}$ spectra of M${}_{2}$P-PF${}_{6}$, which are composed of 1D $A$ stacks and $D$ stacks, respectively, it was revealed that in M${}_{2}$P-TCNQF${}_{4}$ the optical gap corresponds to the Mott gap transition from ${A}^{\ensuremath{-}}$ to ${A}^{\ensuremath{-}}$, and the ${D}^{+}$ to ${D}^{+}$ transition and the ${A}^{\ensuremath{-}}$ to ${D}^{+}$ transition or equivalently the charge-transfer (CT) transition located at the higher energies. Below the structural and magnetic phase transition temperature ${T}_{c}$ \ensuremath{\sim} 122 K, spin-singlet states are formed via displacements of both $D$ and $A$ molecules. The pattern of molecular displacements was found to be very unique, indicating that the phase transition cannot be attributed to a spin-Peierls-like mechanism. The nature of the phase transition is discussed from the temperature dependences of molecular displacements as well as of the Mott-gap transition and the CT transition.

High Performance Organic Thin Film Transistors with Solution Processed TTF-TCNQ Charge Transfer Salt as Electrodes

Fabrication of high-performance organic thin film transistors (OTFTs) with solution processed organic charge transfer complex (TTF-TCNQ) film as bottom contact source-drain electrodes is reported. A novel capillary based method was used to deposit the source-drain electrodes from solution and to create the channel between the electrodes. Both p- and n-type OTFTs have been fabricated with solution deposited organic charge transfer film as contact electrodes. Comparison of the device performances between OTFTs with TTF-TCNQ as source-drain electrodes and those with Au electrodes (both top and bottom contact) indicate that better results have been obtained in organic complex film contacted OTFT. The high mobility, low threshold voltage, and efficient carrier injection in both types of OTFTs implies the potential use of the TTF-TCNQ based complex material as low-cost contact electrodes. The lower work function of the TTF-TCNQ electrode and better contact of the complex film with the organic thin film owing to the organic-organic interface results in efficient charge transfer into the semiconductor yielding high device performance. The present method having organic metal as contact materials promises great potential for the fabrication of all-organics and plastic electronics devices with high throughput and low-cost processing.

Microchip-calorimetry of organic charge transfer complex which shows superconductivity at low temperatures

We carried out thermodynamic measurements of organic charge transfer complex of κ-(BEDT-TTF)2Cu[N(CN)2]Br, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene by TCG3880 chip device in order to examine capability of the chip calorimeter at low temperature region and under magnetic fields. TCG3880 chip is mounted on a 3He cryostat available in combination with a superconductive magnet up to 7T. Thermal anomalies related to the glass-like freezing of ethylene groups of BEDT-TTF molecules and the superconductive transition were observed. A frequency dependence of the thermal anomaly of the glass formation and a magnetic fields dependence of the thermal anomaly of the superconductive transition are reported. The results presented in this work demonstrate that the TCG3880 is quite useful for thermodynamic investigations of the organic charge transfer complex with much reduced sample quantity as compared with those of relaxation and adiabatic calorimetry.

Field emission and electrical bistable properties of CuTCPQ nanostructures

The organic charge-transfer (CT) complex nanostructures of CuTCPQ (copper tetracyanopentacenequinodimethane) were first successfully fabricated by organic solid phase reaction. The morphologies of CuTCPQ nanosrods can be prepared by controlling the reaction temperature. The electron field-emission properties on these nanostructures were investigated. The current density and turn-on field are 3.5 mA cm(-2) and 2.70 V μm(-1) for the nanorods of CuTCPQ. The results demonstrate that the nanorods of CuTCPQ are potential candidates for field emission cathodes. Stable and reproducible current-controlled electrical switch has been observed in amorphous organic nanorods of CuTCPQ films. The current-voltage characteristics reveal an abrupt decrease in impedance form 1.2 MΩ to less than 1.1kΩ. The maximum ON/OFF ratio of CuTCPQ nanorod arrays is up to 1100.

A stable electrochemically active copper interface for room-temperature ionic liquid via surface modification to a metal–organic charge-transfer complex

A novel electrochemical interface between metal electrode and room-temperature ionic liquid (RTIL) was prepared by a procedurally facile method. A morphologically uniform and stable metal–quinone charge-transfer complex film of copper-2,3-dicyano-5,6-dichloro-1,4-benzo-quinone (Cu-DDQ) was found to possess lithium active interface against 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide ([EMIm][Tf2N]), an RTIL. The cyclic voltammetry of the film showed reversible ionic charge transfers occurring across the interface. The electrochemical impedance spectroscopy indicated that the interfacial impedance may become as low as ∼1000 Ω cm−2. The bias-induced reversible transport of lithium ion was further confirmed by the XPS method. These results suggest that metal–organic CT complex salts prepared on metal electrodes by a simple surface modification may become an excellent electrochemically active interface with RTIL.

ChemInform Abstract: Conductive Charge-Transfer Complexes of Alkoxy Substituted Tetrathiafulvalene, BEDO-TTF

Abstract BEDO-TTF which is one of the oxygen substituted BEDT-TTF analogues, gave conductive organic charge transfer complexes in contrast to BEDT-TTF and other BEDT-TTF derivatives. Especially many complexes exhibit metallic temperature dependences even with the compressed disks. These facts suggest the remarkable tendency of BEDO-TTF to aggregate themselves.

High-mobility copper-phthalocyanine field-effect transistors with tetratetracontane passivation layer and organic metal contacts

We investigate ambipolar charge transport in organic field-effect transistors (OFETs) with copper-phthalocyanine (CuPc) as active material. It is shown that charge carrier mobilities can be increased by at least one order of magnitude using the long-chain alkane tetratetracontane (TTC) as a passivation layer on top of silicon dioxide. TTC and CuPc films are characterized by atomic force microscopy and x-ray diffraction. TTC forms a highly crystalline layer that passivates electron traps on the SiO2 surface very efficiently and serves as a template for the growth of CuPc films with significantly improved crystallinity. High electron mobilities comparable to the values reported on single crystals are reached. We show that the contact resistance for hole transport as determined by the transmission line method can be reduced considerably by using organic charge-transfer complexes as top contacts in OFETs based on CuPc.

Charge-Lattice-Coupled Quantum Fluctuations in DMTTF–2,6-QBr2Cl2

79 Br NQR measurement is performed to determine the low-energy charge-transfer/dimerization fluctuations in the organic charge-transfer complex DMTTF–2,6-QBr 2 Cl 2 . With decreasing temperature, NQR line width gradually increases and the spin–lattice relaxation rate 1/ T 1 shows a large enhancement from the two-phonon relaxation law ( T 7 ). This behavior is consistent with the picture that the present material is situated in the critical region of the charge-lattice-coupled quantum transition.

Linear tuning of charge carriers in graphene by organic molecules and charge-transfer complexes

The electronic interactions of several organic donor/acceptor molecules or combination of these molecules (hereafter known as ``charge-transfer complexes'') with graphene have been investigated by first-principles calculations and photoemission spectroscopy experiments. It is found that the doping behavior of graphene can be controlled by the ratio of donor and acceptor molecules in the form of their charge-transfer complexes. The interfacial electronic structure of molecules adsorbed on graphene is dominated by the charge-transfer mechanism. The changes in work function of graphene modified by organic molecules are discussed in comparison with experiments and they are in good agreement with experimental data. Our results show that functionalization of graphene by acceptor/donor molecules and their charge-transfer complexes is an efficient way to control the doping behavior of graphene.

Ferroelectricity in a one-dimensional organic quantum magnet

In magnetically controllable ferroelectrics1,2,3, electric polarization is induced by charge redistribution or lattice distortions that occur to minimize the energy associated with both the magnetic order and interaction of spins with an applied magnetic field. Conventional approaches to designing materials that exploit such spin-mediated behaviour have focused mainly on developing the cycloidal spin order4,5, and thereby producing ferroelectric behaviour through the so-called antisymmetric Dzyaloshinskii–Moriya interaction6,7,8. However, engineering such spin structures is challenging. Here we suggest a different approach. Direct measurements of magnetic-field-dependent variations in the polarization of the one-dimensional organic quantum magnet, tetrathiafulvalene-p-bromanil, suggest a spin–Peierls instability has an important role in its response. Our results imply that one-dimensional quantum magnets, such as organic charge-transfer complexes, could be promising candidates in the development of magnetically controllable ferroelectric materials.

Double-shot inkjet printing of donor–acceptor-type organic charge-transfer complexes: Wet/nonwet definition and its use for contact engineering

We developed the double-shot inkjet printing technique for manufacturing the uniform polycrystalline thin films of organic charge-transfer complexes with a well-defined patterning on the substrate surfaces. The technique utilizes the wet/nonwet surface modification to confine the intermixed droplets of individually-printed donor and acceptor inks in a predefined area, which results in the picoliter-scale instantaneous complex formation. By the method, we obtained synthetic-metal patterned deposits of tetrathiafulvalene–tetracyanoquinodimethane with smooth thickness distribution and highly c-axis orientation, where the preference for the 1:1 complex formation is clearly demonstrated. We also discuss the use of the patterned deposits for the contact engineering of organic thin-film transistors.

Heat capacities of antiferromagnetic dimer-Mott insulators in organic charge-transfer complexes

Heat capacity measurements of quasi-two-dimensional Mott insulating compounds consisting of BEDT-TTF (bisethylendithiotetrathiafulubalene) donor molecules and counter anions were performed by the thermal relaxation calorimetry technique for single crystal samples. No distinct thermal anomalies at the predicted antiferromagnetic transition temperatures in κ-(BEDT-TTF)2Cu[N(CN)2]Cl (T N = 27 K) and β′-(BEDT-TTF)2ICl2 (T N = 22 K) were observed. These results demonstrate that the Mott insulating state of the organic salts which are dominated by the strong two-dimensional intra-layer antiferromagnetic interactions between neighboring S = 1/2 spins shows somewhat different features from the simple quasi-two-dimensional Heisenberg model with S = 1/2. The strong quantum fluctuations produced by the electron correlations suppress the long-range character of the spin correlations, which seems to be an important aspect of this kind of Mott insulating materials.

Thin-film phases of organic charge-transfer complexes formed by chemical vapor deposition

Thin films of organic charge-transfer salts, (TTF)(TCNQ) and (TTF)(DMDCNQI), are prepared by the chemical vapor deposition method, where TTF is tetrathiafulvalene, TCNQ is tetracyanoquinodimethane, and DMDCNQI is dimethyldicyanoquinonediimine. Depending on the substrate temperatures, we have obtained randomly oriented polycrystalline phases composed of relatively large crystals and microcrystalline thin-film phases, which sometimes contain well-grown nanowires. The latter shows much different conducting properties from the bulk crystals, and particularly the (TTF)(DMDCNQI) film is nearly as conductive as the (TTF)(TCNQ) film in spite of the bulk insulating property coming from the mixed-stack crystal structure.