Preparation and properties of polycrystalline films of (TTF)(TCNQ)

Thin films of the highly conductive organic complex tetrathiofulvalinium tetracyanoquinodimethane (TTF)(TCNQ) have been prepared on a variety of substrate materials and at substrate temperatures in the range 77–310 K by thermal sublimation of bulk crystals in vacuum. Analysis of the chemical and physical structure of the films indicates that this method of film deposition yields polycrystalline (TTF)(TCNQ) with the grain size and crystalline orientation sensitively dependent on the substrate temperature. In-plane electrical conductivity of the films as a function of temperature and pressure shows behavior indicative of an appropriately weighted average of the single crystal a-axis and b-axis conductivities, with the temperature dependence above the metal-insulator transition temperature influenced by a weakly activated process attributed to crystalline grain boundaries. Comparison of our results with those for sublimation of 1:1 mixtures of TTF with TCNQ suggests that (TTF)(TCNQ) vaporizes primarily as a complex rather than as the individual neutral components.     

Crystal structures and transistor properties of phenyl-substituted tetrathiafulvalene derivatives

The crystal structures, thin-film properties, and field-effect transistor (FET) characteristics of tetrathiafulvalene (TTF) derivatives with two phenyl groups are systematically investigated. The highest mobility, 0.11?cm(2)?V(-1)?s(-1), is observed in biphenyl-substituted TTF (1). The correlation between the crystal structures and the FET properties demonstrates that good transistor properties are associated with two-dimensional intermolecular interaction, which is achieved when the molecules are standing nearly perpendicular to the substrate. Since these TTF derivatives are strong electron donors, the use of a metallic charge-transfer salt (TTF)(TCNQ) as the source and drain electrodes has resulted in a considerable reduction of the off current (TCNQ: tetracyanoquinodimethane).     

Distinction of the two phases of CuTCNQ by scanning electrochemical microscopy

The two known phases of CuTCNQ and TCNQ (TCNQ = 7,7',8,8'-tetracyanoquinodimethane) have been probed by scanning electrochemical microscopy (SECM) in the feedback mode. The first use of this technique for distinguishing differences in the electronic properties of semiconductor phases exploits the large differences in conductivity that exist between CuTCNQ and the parent TCNQ material and also between the CuTCNQ phases I and II. However, the packing density of the individual CuTCNQ crystals in a film structure also is shown to influence the SECM feedback response. Finally, it is shown that films of pure phase II material or mixtures of the phases can be mapped using feedback mode SECM. The SECM method provides valuable insights for elucidating properties of semiconducting solids that are mounted on insulating substrates.     

聚甲基丙烯酸甲酯掺杂剂对7,7,8,8-四氰基苯醌二甲烷薄膜特性的影响

采用旋涂法制备了7,7,8,8-四氰基苯醌二甲烷(TCNQ)及聚甲基丙烯酸甲酯(PMMA)掺杂后的TCNQ敏感薄膜,探讨了PMMA掺杂剂及不同转速对薄膜性能的影响.结果表明,TCNQ/PMMA复合薄膜中TCNQ颗粒变得更细小和均匀,但对NH3的响应不稳定;纯TCNQ薄膜对NH3具有相当高的灵敏度,但响应和恢复时间较长.     

An analogy of an ion-selective electrode sensor based on the voltammetry of microcrystals of tetracyanoquinodimethane or tetrathiafulvalene adhered to an electrode surface

The voltammetry of solid 7,7,8,8-tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (ITF) at an electrode-microparticle-aqueous (electrolyte) interface generates characteristic current-potential profiles associated with solid-solid-phase transformations. During the reactions, electrolyte ions are included into the TCNQ (cations) and TTF (anions) lattice sites as part of the charge neutralization process. Consequently, electrolyte ion concentration is associated with the reversible potential of the TCNQ0/- and TTF0/+ reactions, making these processes candidates for the development of novel voltammetric cation and anion sensors, respectively. Electrode potential-analyte ion concentration dependence studies exhibited highly reproducible potential shifts of 45 (+/- 1) mV/decade change in ion analyte concentration for both the TCNQ cation sensor and the TTF anion sensor. When presented with mixed-analyte solutions, both ion-sensing systems exhibited a degree of ion selectivity. Ion selectivity trends may be modeled using equations based on a Nicolsky-type selectivity relationship, in accordance with the concept that these are the voltammetric analogies of potentiometric ion-selective membrane electrodes.     

K(TCNQ)薄膜的制备及其电双稳特性

在真空环境下,首次利用固态化学置换反应制备了K(TCNQ)薄膜,其分子结构与已报道过的K(TCNQ)单晶、多晶相同.但不同的是,在300K以上,K(TCNQ)薄膜具有可逆的双稳特性.因此预计在光电开关和电双稳存储器方面具有广泛的应用前景.     

真空蒸发沉积聚苯胺—TCNQ复合薄膜的STM研究

利用扫描隧道显微镜对真空蒸发沉积的聚苯胺－ＴＣＮＱ（ＰＡＮＩ－ＴＣＮＱ）复合薄膜、纯聚苯胺薄膜及纯ＴＣＮＱ薄膜试样进行了对比分析。研究发现,纯ＰＡＮＩ薄膜和纯ＴＣＮＱ薄膜都是绝缘膜,而用ＴＣＮＱ挽杂获得具有较高导电特性的ＰＡＮＩ－ＴＣＮＱ复合薄膜。而且与表面粗糙不连续的ＰＡＮＩ薄膜和ＴＣＮＱ薄膜相比,ＰＡＮＩ－ＴＣＮＱ复合薄膜易形成较大面积的表面结构完善的连续膜。傅里叶变换红外光谱（ＦＴＩＲ）分析     

Theoretical investigations on low energy surfaces and nanowires of MgH(2)

The phase stability, chemical bonding, and electronic structure of MgH(2) nanowires and possible low energy surfaces of α-MgH(2) thin films have been investigated using the ab initio projected augmented plane-wave method. Structural optimizations based on total energy calculations predicted that, for the α-MgH(2) phase, the (101) surface is more stable among the possible low energy surfaces. The electronic structure study reveals that the nanowires also have nonmetallic character similar to that of the bulk and thin film phases. Bonding analysis shows that the character of chemical bonding in nanowires has been considerably changed compared with that in bulk phases. Similarly, the bond distances in the surfaces of nanowires are found to be higher than in the bulk material, suggesting that it is possible to remove hydrogen from the nanowires considerably more easily than from bulk crystals.     

Nanoscale “Noise‐Source Switching” during the Optoelectronic Switching of Phase‐Separated Polymer Nanocomposites

A method is developed to directly map nanoscale “noise‐source switching” phenomena during the optoelectronic switching of phase‐separated polymer nanocomposites of tetrathiafulvalene (TTF) and phenyl‐C₆₁‐butyric acid methyl ester (PCBM) molecules dispersed in a polystyrene (PS) matrix. In the method, electrical current and noise maps of the nanocomposite film are recorded using a conducting nanoprobe, enabling the mapping of a conductivity and a noise‐source density. The results provide evidence for a repeated modulation in noise sources, a “noise‐source switching,” in each stage of a switching cycle. Interestingly, when the nanocomposite is “set” by a high bias, insulating PS‐rich phases shows a drastic decrease in a noise‐source density which becomes lower than that of conducting TTF‐PCBM‐rich phases. This can be attributed to a trap filling by charge carriers generated from a TTF (donor)–PCBM (acceptor) complex. In addition, when the film is exposed to UV, an optical switching occurs due to chemical reactions which lead to irreversible changes on the noise‐source density and conductivity. The method provides a new insight on noise‐source activities during the optoelectronic switching of polymer nanocomposites and thus can be a powerful tool for basic noise research and applications in organic memory devices.     

Growth of epitaxial films of the organic conductor (TTF)(TCNQ)

Singly-positioned epitaxial films of the organic conductor (TTF) (TCNQ) were grown by sublimation on cleaved (100) NaCl substrates subjected to compressive stress along a 〈110〉 direction, and on polar (010) faces of ferroelectric triglycine sulfate. The conditions for obtaining thin films of this material, as well as the limitations of the epitaxial technique for obtaining large area thick films are presented and discussed.     

All solution processable organic photovoltaic cells using DMDCNQI as an organic N-type buffer layer

Organic photovoltaic cells consisting of ITO/PEDOT-PSS/P3HT:PCBM/TiO(x)/DMDCNQI/Al have been fabricated by using dip-coated DMDCNQI layer as a cathode buffer material. We have investigated the physical effects of charge transfer complex and wettability of DMDCNQI between TiO(x)/P3HT:PCBM layer and Al cathode electrode on the performance of organic photovoltaic cell. The photovoltaic cell fabricated with a dip-coated DMDCNQI layer exhibited almost similar performance compared to the device using conventional evaporated DMDCNQI layer. Especially, the power conversion efficiency of the prepared organic photovoltaic cell using TiO(x)/DMDCNQI layer was improved to 3.1%, which is mainly due to the decrease in the low contact resistance of organic-metal interface.     

Fabrication and characterization of tetracyanoquinodimethane/phthalocyanine solar cells

Fabrication and characterization of heterojunction solar cells of tetracyanoquinodimethane (TCNQ)/copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) were carried out. The light-induced charge separation with charge transfer was investigated by light-induced current density and optical absorption. In both cases of the TCNQ/CuPc and TCNQ/ZnPc solar cells, the TCNQ thin film worked for strong electron-accepting layer as n-type semiconductor. These behaviors would be originated in charge transfer of excited electron from CuPc and ZnPc to TCNQ. The photovoltaic mechanism was discussed on the basis of the experimental results.     

Characteristics and fabrication of vertical type organic light emitting transistor using dimethyldicyanoquinonediimine (DMDCNQI) as a n-type active layer and light emitting polymer

We have investigated electrical properties of vertical type organic transistor using dimethyldicyanoquinonediimine (DMDCNQI) as a n-type active layer. And also, the contact resistance Ro for charge injection from a metal electrode to an organic semiconductor layer was studied by transfer line method. The radiance of organic light emitting transistor (OLET) consisting of glass/ITO (drain)/PEDOT:PSS/MEH-PPV/DMDCNQI/Al (gate)/DMDCNQI/Au (source) can be effectively controlled by applying gate voltage like depletion mode.     

Conventional Fabrication Method for TCNQ Radical Anion Salts Molecular Wires

NMP-TCNQ (NMP=N-methylphenazinium, TCNQ=7,7,8,8-tetracyanoquinodimethane) and (NEt₄)TCNQ gave molecular wires on a glass substrate by recrystallization from the acetone solution coexistence with the substrate. The width of the wires largely depends on the evaporation ratio of the solvent through recrystallization. When evaporating ratio of NMP-TCNQ solution was 167 μl/min, the width was minimized to 35 nm. Comparison of the IR spectra and XRD patterns with the corresponding single crystals clearly indicates that these wires have same crystallinity as the single crystals.     

Photovoltaic characteristics of TCNQ-incorporated CuPc-poly(p-phenylene) composite films

Composite films (CuPc–PPP–TCNQ) were produced by simultaneous deposition using copper phthalocyanine (CuPc) as a carrier generation material, poly(p-phenylene) (PPP) as a hole transport material, and tetracyanoquinodimethane (TCNQ) as a incorporation material. Schottky barrier photovoltaic cells, consisting of a semitransparent aluminum and the CuPc–PPP–TCNQ composite films, were fabricated. The junction properties and the photovoltaic properties on Al/CuPc–PPP–TCNQ/ITO sandwich cells were investigated. As well as a composite film of the CuPc and the PPP (CuPc–PPP), the conductivity of the CuPc–PPP–TCNQ composite film is improved as the TCNQ is simultaneously deposited in the CuPc–PPP composite film. Therefore, it is proven that the short circuit photocurrent density (J _sc) and the photovoltaic property increases significantly. The J _sc of the Al/CuPc–PPP–TCNQ/ITO cell is 2.60 A/cm², and it is found that the J _sc is about 20 times that of an Al/CuPc/ITO cell and double that of an Al/CuPc–PPP/ITO cell. Consequently, the power conversion efficiency of the Al/CuPc–PPP–TCNQ/ITO cell obtained was 3.68%.     

Extracting the electromechanical coupling constant of piezoelectric thin film by the high-tone bulk acoustic resonator technique

In this paper, a new approach is proposed to rapidly and accurately measure the electromechanical coupling constant K(t)(2) of thin film piezoelectric material, which is critically important for real-time quality control of the piezoelectric film growth in mass production. An ideal lossy bulk acoustic resonator (LBAR) model is introduced and the theory behind the method is presented. A high-tone bulk acoustic resonator (HBAR) was fabricated on a silicon wafer. The impedance response of the resonator was measured, from which the K(t)(2) of the piezoelectric material was extracted. To illustrate the potential of the proposed technique to extract material properties, two HBAR devices employing AlN as the piezoelectric material were fabricated using an RF sputter system with known good and bad deposition conditions; the extracted K(t)(2) values of the piezoelectric material are compared.     

真空蒸发沉积聚苯胺晶态薄膜的研究

利用傅里叶变换红外光谱、透射电子显微镜、高分辨扫描电子显微镜和扫描隧道显微镜等多种分析技术 ,对用真空蒸发沉积技术制备的不同聚苯胺 (PANI)薄膜试样进行了综合表征。研究发现 ,PANI薄膜是无定形态的绝缘膜 ;经HCl掺杂的PANI薄膜尽管其电导率有很大提高 ,但仍是非晶态的薄膜 ;而PANI TCNQ薄膜确是多晶薄膜 ,且薄膜的电导率较PANI薄膜提高几个数量级。研究发现PANI TCNQ薄膜中的PANI聚合链与TCNQ分子之间存在着电荷转移。与PANI薄膜和TCNQ薄膜相比 ,PANI TCNQ复合薄膜的结构更加完善 ,表面更加平整     

Electrical properties of new organic composites obtained by mechanochemical synthesis

We present new results on organic semiconductive and metallic composites obtained by direct solid-solid charge-transfer (CT) reaction. By this method, samples of arbitrarily large size are readily achievable. In the present study, we consider composites formed from the reaction between the following pairs of donors and acceptors: TTF and iodine, BEDT-TTF and TCNQ, BEDT-TTF and TCNE, BEDT-TTF and AuI as well as BEDT-TTF and AuI₃. Most of the composites show semiconducting properties only. Two of them, however, (BEDT-TTF)/(AuI) and (BEDT-TTF)₂/(AuI₃), exhibit a metallic behavior.     

Nano-phases of NaBH4 and KBH4

Phase stability and chemical bonding of beta-NaBH4 and beta-KBH4 derived nano-structures and possible low energy surfaces of them from thin film geometry have been investigated using ab initio projected augmented plane wave method. Structural optimizations based on total energy calculations predicted that, for beta-NaBH4 and beta-KBH4 phases, the (011) and (101) surfaces are more stable among the possible low energy surfaces. The predicted critical size of the nano-cluster for beta-NaBH4 and beta-KBH4 is 1.35 and 1.8 nm, respectively. The corresponding critical diameter for the nano-whisker is 2.6 and 2.8 nm respectively for beta-NaBH4 and beta-KBH4. Structural optimization based on total energy calculations show that the bond distances in the surfaces of nano-whisker are found to be higher than that in the bulk material and the calculated H site energies and bond overlap population analysis suggesting that it is considerably easier to remove hydrogen from the surface of the clusters and nano-whiskers than that from the bulk crystals.     

Semifluorinated TTF derivatives in Langmuir-Blodgett films

New semifluorinated tetrathiafulvalene (TTF) derivatives were prepared and used in Langmuir and Langmuir-Blodgett (LB) films. Transfer onto a solid substrate of such pure monolayers can be easily achieved leading to organized LB films in which the fluorinated chains are tilted with respect to the normal of the substrate as demonstrated by infrared and X-ray experiments. Doping of such films by iodine does not induce change in the layer spacing on the contrary to already published studies concerning alkyl derivatives of TTF. Such structural stability and values of the layer thickness suggest some interdigitation between TTF groups within the LB film. Depending on the exact chemical structure of the TTF moiety, the iodination is stabilized or not. When the doped film is stabilized in a mixed valency state, conductivity of approx. 10⁻² S cm⁻¹ was measured at room temperature.