New Development in the Preparation of Micro/Nano-Wires of Molecular (Magnetic) Conductors

A lot of molecular (magnetic) conductors are prepared largely using charge-transfer (CT) salts of donor molecules with acceptor molecules or nonmagnetic or magnetic anions such as metal halides and oxides; their CT salts are usually obtained as bulk crystals, which are used to elucidate the electrical conducting (magnetic) properties. In contrast, a small number of micro/nano-crystals of the molecular (magnetic) conductors, especially micro/nano-wires, are known, of which highly conducting nanowires are necessary as a key component in the development of the next generation of nano-size transistors and spin-transistors. Very recently, we succeeded in preparing highly conductive micro/nano-wires of CT salts between bent donor molecules developed by one of the author’s group and magnetic FeX4– (X = Cl, Br) ions: (1) by electrochemical oxidation of the bent donor molecules with a silicon wafer electrode coated with a phospholipid multi-lamellar structure as well as, (ii) by electrochemical oxidation of the bent donor molecules with a large arc structure, in the presence of NBu4FeX4 supporting electrolytes. This article reviews template-free and template-assisted methods developed so far for the preparation of micro/nano-wires of molecular (magnetic) conductors along with our new methods. The conducting properties of these micro/nano-wires are compared with those of the corresponding bulk crystals.

expected to be used as conducting wires bridging between two electrodes in nano-size molecular transistors and spin-transistors. This article reviews efficient preparation methods of micro/nano-wires of molecular (magnetic) conductors, some of which have very recently been developed by the authors' groups.

Preparation Methods of Micro/Nano-Wires of Molecular (Magnetic) Conductors
To prepare micro/nano-wires of molecular (magnetic) conductors, two methods are so far developed; one which uses a template and one that does not in the mixing between donor and acceptor molecules or in the electrochemical oxidation of donor molecules. The former method includes mixing of highly-dilute donor and acceptor solutions, deposition of donor and acceptor molecules by a dip-coating onto stainless steel, and electrochemical deposition of donor molecules onto conventional native silicon wafer and platinum rod or nano-size electrodes. The latter method uses nano-size channels of supramolecular network composed of counter halide anions and iodine-containing neutral molecules, and of porous alumina and phospholipid multi-lamellar membranes coated on gold, silver or silicon wafer in the electrochemical oxidation of donor molecules. helical dendrites as prepared at the drip rates of 1, 20, 40 and 500 μL s -1 , respectively. The FT-IR spectra of the TTF•TCNQ complex morphologies also provide evidence about the complex formation between TTF and TCNQ molecules. All of the TTF•TCNQ complex morphologies are single crystals, as evidenced by the indexing of the spots in their SAED patterns. In addition to the solution concentration in the reaction, the temperature is also the other key factor influencing the nucleation and growth. At low temperatures, the absolute growth rate of the TTF•TCNQ complex is low. The π-π stacking interactions between TTF molecules and between TCNQ molecules quickly lead to the formation of TTF•TCNQ nanowires. With increasing reaction temperature, the increase absolute growth rates of TTF•TCNQ complexes may result in a significant strain on the surface of the nanowires, which is likely to result in the transformation of straight nanowires into helical nanowires, eventually evolving into complex growth patterns. As expected, the four kinds of TTF•TCNQ complex morphologies as described above are obtained at different reaction temperatures. The reaction at -50 °C gives the TTF•TCNQ nanowires with the diameter of 100-500 nm and the length of a few micrometers to tens of micrometers. As the temperature increases from -50 °C to -10 °C, 0 °C, and 50 °C, the helical wires (diameter = 300-800 nm and length = 3-18 μm), helical dendrites, and the complicated helical dendrites are formed respectively, all of which have high crystallinity.  For this device, conductivities at room temperature are 3.8 × 10 -4 S cm -1 at bias voltages below 1 V, and 1.05 × 10 -2 S cm -1 at bias voltages above 4 V ( Figure 5(b)). On the other hand, for the device in Figure 5(c), the Pt electrodes are placed at the open cut at both ends of the nanowire to connect the electrodes and the nanowire together. For this device, the room-temperature conductivity is 0.26 S cm -1 at bias voltages below 1 V, and increases to 295 S cm -1 at bias voltages above 4 V ( Figure 5(d)). These results are well explained by considering that the long axis of the nanowire corresponds to the direction parallel to the TTF•TCNQ stacking, as also observed in the growth of the single crystal [8], which has large anisotropy in the resistivities (the ratio is 1:160:500 for the TTF•TCNQ intracolumnar, TTF•TTF/TCNQ•TCNQ intercolumnar and TTF•TCNQ stacking directions). The increase in the conductivities at the bias voltages above 4 V is due to the possible electron transport through the energy barrier between the nanowire chains.

Deposition on Stainless Steel Conversion Coating (SSCC) Substrate
SSCCs grow on austenitic stainless steel-sheets through a combined chemical/electrochemical process. The resulting coatings were identified as magnetite and maghemite phases [9]. They have been known to exhibit advanced adsorption properties due to their fractal-like nano-structured surface ( Figure 6(a)) and applied to fix dyes or to improve the adherence of further coatings. The TTF•TCNQ nanowires are prepared by the successive immersion of this SSCC in CH 3 CN solutions (10 -2 M) of TTF and TCNQ at room temperature [10]. Adsorption of TTF was realized at first. Then, immersion of the TTF-coating surface in the TCNQ solution resulted in the formation of nanowires and few platelets. As shown by the SEM image in Figure 6(b), the nanowires are anchored on the SSCC surface, and some of them bridge the grain boundaries of the conversion coating (boundary separation = 1-2 μm). The nanowires were easily separated from the substrate and used for TEM, AFM and I-V measurements. From the TEM observation in Figure 6(c), the nanowires are actually ribbons having an average thickness of 20 nm and a width between 20 and 200 nm. These ribbons are 20 μm long, and occasionally produce small loops (Figures 6(d) and 6(e)). The I-V characteristics are measured for a bundle of fibers deposited with a micropipette on a metal-insulator-metal nano-junction ( Figure 7). As the fibers are not parallel to the electrodes, the conductivity deduced from the curve is dominated by the perpendicular conductivity. The room-temperature conductivity is about 1 S cm -1 , and the shape of the I-V curve is similar to that observed for single crystals of TTF•TCNQ deposited on an alkali halide surface and studied by use of scanning tunneling microscopy [11], a method that also gives a characterization perpendicular to the wires.
The above dipping process on the SSCC substrate with nanopores also gave nanowires of TTF•[Ni(dmit) 2 ] 2 [12]. When the substrate is first dipped in a CH 3 Figure 8. The diameter of these nanowires is 50 to spectra of the nanowires and single crystals shows that the nanowires have the same molecular composition as the single crystals, whose structure consists of segregated stacks of TTF and [Ni(dmit) 2 ] units ( Figure 9) from comparison of Raman spectra between the nanowires and single crystals. The nanowires were only located on such a small part of the substrate surface that they could not be removed from the substrate even by use of a micropipette. Moreover, the conductivity of the nanowires located on the substrate could not be measured due to the conducting character of substrate.    length of 20-60 μm were deposited on the silicon wafer [13]. The SEM image is shown in Figure 11. This deposit showed the same X-ray diffraction pattern, X-ray photoelectron spectrum and Raman spectrum as those of the single crystals of TTF•[Ni(dmit) 2 ] 2 , which exhibits a metallic behavior down to 4 K, and a transition to a superconducting state at 1.6 K under a pressure of 7 kbar. Resistivity measurement of the bundles was performed between ambient pressure and 7.7 kbar. Whatever the pressure applied, the resistivity is nearly constant in the temperature range of 100 to 300 K, and significantly increases below 100 K, being characteristic of a semiconducting behavior. Between 100 and 300 K, inter-bundle contacts do not affect too much the overall conducting behavior. The room-temperature conductivities are 9 S cm -1 at ambient pressure and 24 S cm -1 under 7.7 kbar. A broad drop of the resistivity is observed below 0.8 K under 7.7 kbar ( Figure 12), but the transition observed is incomplete, presumably due to the contribution of grain boundaries. The occurrence of superconductivity is confirmed by applying magnetic field perpendicular to the silicon substrate plane.
The drop of the resistivity gradually becomes smaller with increasing magnetic field, and at 1.9 T the transition completely disappears. The critical magnetic field is estimated to be 0.45 T, a lower value compared with that (2.5 T) of the single crystals due to the inter-bundle resistivity contribution [14,15].  The electrolysis of a CH 2 Cl 2 solution of TMTSF and NBu 4 •[Co(dcbdt) 2 ] gave a deposition of microwires on silicon wafer [3]. The SEM image is shown in Figure 13. The microwires are <3 μm wide and >100 μm long. The molecular formula was determined as (TMTSF) 5 •[Co(dcbdt) 2 ] 4 by X-ray photoelectron spectroscopy (XPS). The Raman spectral measurement indicates that the charge residing on the TMTSF molecules is +0.8 based on a linear relationship between the C=C stretching frequency due to the TMTSF molecule and the charge and this result is in good agreement with the stoichiometry determined by XPS. The room-temperature conductivity of the microwires is comparatively high (2 S cm -1 ), but the conducting behavior is semiconducting and the activation energy is 56 meV.
The above two molecular conductor wires were of micrometer size. The actual nano-size molecular conductor wires were obtained with (perylene) 2 •[Au(mnt) 2 ] [16]. A CH 2 Cl 2 solution of perylene and NBu 4 •[Au(mnt) 2 ] was electrochemically oxidized at constant current density of 0.30 μA cm -2 using a silicon wafer electrode. A black-colored deposit made of nanowires was obtained on the silicon anode, as shown by the SEM image in Figure 14.

Electrochemical Deposition on Platinum Electrode
An example of the formation of molecular conductor nanowires by use of a conventional platinum rod electrode was found by our group [17]. A bent donor molecule with a relatively large arc, Figure 15) and its related derivatives, tend to form highly one-dimensional stacks in the radical cation salts [18].  length and thickness of the single crystals. The crystal structure is not solved because of the too small thickness. The powder X-ray diffraction patterns for the micro/nano-wires and the single crystals are almost the same, suggesting that the micro/nano-wires have the same stacking structure to that of the single crystals.
The AFM measurement of a single micro/nano-wire was performed. From the AFM image in   Of course, this method can be applied to the other donor molecules which show the strong tendency to stack along a specific direction. Figure 19. The chemical structure of ethylenedioxy-ethylenediseleno-tetrathiafulvalenoquinone-1,3-dithiolemethide (EDO-EDSe-TTFVO).

Electrochemical Deposition on Nano-Size Electrode in Solution
Chemical and electrochemical methods to get molecular conductor micro/nano-wires directly on silicon substrate were developed [19]. In the chemical method Ti (2 nm  .

DMe-DCNQI DMe-DCNQI-d 7
On the other hand, in the electrochemical method the electrode is prepared by deposition of Ti   The bulk crystal of (DMe-DCNQI) 2 •Ag is known to exhibit a rapid increase in resistivity below 100 K. A similar behavior is obtained also for the single micro/nano-wire, which however exhibits no weakly metallic behavior near room temperature as observed in the bulk crystal, probably due to the influence of contact resistance. On the other hand, the conducting behavior of the single nanowire of (DMe-DCNQI-d 7 )•Cu (DMe-DCNQI-d 7 is a derivative of DMe-DCNQI (Figure 20), whose seven hydrogen atoms are replaced by deuterium atoms) presents a striking contrast to that of the bulk crystal, as shown in Figure 23. The nanowire exhibits a continuous decrease in resistivity with decreasing temperature without metal-insulator (M-I) transition at 80 K, which occurs in the bulk crystal. The metallic character is kept down to low temperature for the nanowire. In this measurement the contact resistance is very low (ca. 1 kΩ) in spite of the small contact area (ca. 0.1 μm 2 ).
The electrochemical method using the above nano-size electrode was also applied to the preparation of micro/nano-crystals of radical cation salts of TTF derivatives, BEDT-TTF, TMTSF and

Electrochemical Deposition on Nano-size Electrode in Vacuum Evaporation
In the foregoing section a useful preparation method of molecular conductor nanowires was described, which uses electrochemical reaction of donor or acceptor molecules with nano-size electrodes in solution. The corresponding dry method, the co-evaporation of TTF and TCNQ with electric field was developed [21]. In this method, two Au electrodes on the glass substrate are settled with a separation distance of 20 or 100 μm, and the substrate is kept at 40-45 °C ( Figure 26) [22,23]. A DC electric field of 6-35 kV cm -1 was applied between the two electrodes during the co-evaporation of TTF and TCNQ. The surface morphology of TTF•TCNQ was observed using an optical microscope. Figure 27(a) shows the optical microscopy image of TTF•TCNQ obtained with zero electric field (the electrode gap is 100 μm) at 42 °C. Randomly-oriented microcrystals of TTF•TCNQ are formed on the glass substrate, particularly around the electrodes. When the average electric field of 11 or 35 kV cm -1 is applied between the electrodes under the same conditions as above, highly-oriented wire-like TTF•TCNQ crystals are grown from the electrodes and aligned along the electric field. Some of a pair of the wires make connection at their tops ( Figure 27(b)). In general, TTF•TCNQ wires grown from the high-voltage electrode tend to be longer than those from the zero-voltage electrode. Increased amount of the wires bridged between the electrodes can be achieved by optimizing the growth conditions such as electrode separation distance, electric field, and growth temperature. sharp contrast to that for the bulk TTF•TCNQ single crystal exhibiting metallic behavior above 53 K.
One conceivable cause of this behavior is that the wire is not pure TTF•TCNQ, but partly contains a nonstoichiometric component of (TTF) 1-δ •TCNQ with a high-doped semiconducting behavior. Interestingly, different growth of TTF•TCNQ wires from the cathode and anode was confirmed by AFM potentiometry. The wires grown from the cathode are good conductors with σ RT > 100 S cm -1 , while those from the anode are semiconductors with σ RT ∼ 0.3 S cm -1 . Moreover, the connection point of the two wires from the cathode and anode shows extremely high resistivity (∼2.8 × 10 6 Ω).
Obviously, metallic and semiconducting areas co-exist in the TTF•TCNQ wires obtained by this method. Accordingly, the tips of the two wires independently grown from the cathode and anode are hard to connect with keeping uniform molecular stacking at the contact point, where resistivity becomes too large. As evidenced from the high conductivity of the wires grown from the cathode, the long axis of the wire corresponds to the stacking direction of TTF and TCNQ molecules. However, there remains as a serious problem whether or not wires with completely metallic TTF•TCNQ composition can be prepared by this method.

Electrochemical Reaction in the Presence of Template Molecule Coordinated with Counteranion
Halide anions are well known to form anionic supramolecular assemblies having an infinite repeating structure by coordination with iodine-containing neutral molecules [24].

Electrochemical Deposition on Gold Wafer Electrode Coated with Porous Alumina Sheet
Porous alumina membranes prepared by anodizing Al in the presence of an acidic electrolyte have ordered honeycomb structure characterized by an excellent uniformity in diameter (20 nm to several hundred nm) and spacing of the holes [28,29]. A thin Au or Ag film was sputter-deposited on one side of the porous alumina membrane to serve as an electrode within nano-size reaction space. By electrochemical reactions using Au or Ag electrodes coated with nanoporous alumina membrane, Au and Bi 2 Te 3 nanowires with a diameter of 40 to 280 ± 30 nm were deposited into the alumina holes [30,31,32]. This method was applied to the preparation of molecule conductor nanowires and     Figure 37(b)). The slope of the straight line in the I-V curve (Figure 37(b)) gives the electrical conductivity of about 2.9 × 10 -5 S cm -1 . The temperature dependence of the normalized resistivity measured along the length direction of the nanotube arrays shows a metallic behavior in the temperature range of 10 K to room temperature (Figure 37(c)). Almost the same electrical conductivities between the nanotube array and the single nanotube are observed. This implies that the nanotube array is composed of many single nanotubes being connected to both electrodes in parallel, so their electrical conductivities are simply cumulative. However, the electrical conductivity measured by either the nanotube array or the single nanotube is 10 6 -times lower than that of the single crystal. Two main reasons are considered to explain this feature. Firstly, since the length direction of the nanotube is parallel to the c-axis, the I-V characteristics are measured along the c-axis, which is the direction of low electrical conductivity (the highly-conducting direction is on the ab-plane). Secondly, the crystalline nature of the nanotubes is not as perfect as that of a single crystal, and this may lower the conductivity across the grain boundaries. Furthermore, some effects of the measurement set-up may also be responsible for the decreasing conductivity.

Electrochemical Deposition on Silicon Wafer Electrode Coated with Phospholipid Membrane
The use of silicon wafer electrode could occasionally lead to the formation of micro/nano-wires of molecular conductors, as described in 2.  The calculated Fermi surface is two-dimensional and has a closed-ellipse.

EDT-TTFVO EDO-TTFVO
The nanowires and nanosticks show the same Raman spectra to those of the corresponding single crystals (Figures 45(a) and 45(b)), so their molecular formulas also apply to the nano-size materials.
The growth of the nano-size materials is considered to occur by the following process: the donor molecules migrate from the solution/membrane interface to the silicon electrode surface via the nano-size channels delimited by the long alkyl chains of DC 8,9 PC molecules. They are oxidized on the silicon electrode surface to produce the conducting salts by combination with    with the nano-size materials sticking out of the membrane surface, but it was not successful because of resistivity drift. To know the intrinsic conducting properties of a single nanowire or nanostick, it is necessary to fabricate the following device: the part stuck out of the membrane is removed by an appropriate method, and the fattened surface is covered with a gold film. The resistivity of nanowires or nanosticks incorporated into the membrane between the silicon and gold electrodes should be measured, which corresponds to the resistivity of a single nanowire or nanostick separated from each other by insulating phospholipid molecules.

Crystals of Molecular (Magnetic) Conductors and the Plausible Causes
As mentioned for several micro/nano-wires of molecular (magnetic) conductors, the micro/nano-wires and the corresponding single crystals exhibited different conducting properties. In almost all cases, a semiconducting character in the single crystals is also kept in the micro/nano-wires, the room-temperature conductivities and activation energies however becoming very lower and markedly increased, respectively. On the other hand, a metallic character in the single crystals is changed to be semiconducting in the micro/nano-wires. In some cases the reverse case is seen for (DMe-DCNQI-d 7 ) 2 •Cu, where a metallic character is kept down to low temperature for the micro/nano-wires in contrast to the occurrence of M-I transition for the single crystals.
In view of these results the following causes are conceivable for the different conducting properties between micro/nano-wires and the corresponding single crystals of molecular (magnetic) conductors, that is, "the size effect on the conductivity." In general, molecular (magnetic) conductors based on CT salts possess large anisotropy on the conductivity compared to that of inorganic conductors, because the component donors and acceptors are planar molecules and stack along one-or two-dimensional directions to grow up the crystals of CT salts. The high conductivity is obtained along these directions.
The long direction of micro/nano-wires usually corresponds to the stacking direction of donor or acceptor molecules, so the highest conductivity is observed along this direction. The single crystals contain more defects in a larger size than in a smaller size. Since the micro/nano-wires are single crystals as small as possible, the number of defects is supposed to be smallest, giving an ideal molecular conducting wire. However, in the micro/nano-wires a fair part of the mass is used at the surface, whose volume becomes comparable to that of the bulk phase with decreasing the size. It happens that conducting electrons or holes flow much more through the surface than inside the bulk phase. In that case the defect problem will not become so important. Thermal contraction has also a great influence on the electron or hole conduction in micro/nano-wires, which offer a large area in the contact with the electrode. In addition, the different work functions between the molecular conductor and the electrode result in partial doping of electrons or holes into the molecular conductor, and the doping influence is extremely large in micro/nano-wires with small mass. It is still not understood sufficiently which of these causes participates in the` difference in conducting properties between the micro/nano-wires and the corresponding single crystal of each molecular (magnetic) conductor.
Further investigation is necessary to clear the causes in more details.

Conclusion and Prospect
We have presented a review of new development in the preparation of micro/nano-wires of molecular (magnetic) conductors based on CT salts. During this decade a variety of methods using a template or not have emerged to efficiently give molecular micro/nano-wires, which are isolated separately or deposited on a silicon or gold wafer substrate. Nevertheless, such a facile and reliable method is still not available that any CT salt can be obtained as micro/nano-wires. In addition, there are only a few molecular micro/nano-wires exhibiting metallic or narrow bandgap-semiconducting properties. In relation to the conducting property it remains to be not thoroughly understood about size effect due to minutiazation of a bulk crystal. Deeper understanding may become possible by investigating in combination of an acquisition of a good quality of a single nanowire and the corresponding bulk crystal, and detailed conductivity measurements with a theoretical calculation analysis. When these molecular nanowires are intended to be applied to the fabrication of a nano-size transistor, it is very useful to make the molecular nanowires load parallel or perpendicular to a silicon wafer substrate. About 10 4 pieces of the molecular nanowire-based transistors can be loaded on a silicon wafer substrate with an area of 1 × 1 cm 2 in the parallel arrangement, and the number of the pieces loaded increases by around 10 3 -times in the perpendicular arrangement, leading to development of a molecule-based high-speed and large information-processing computer. To realized this next generation of computer a facile and reliable method must also be developed to make molecular nanowires load densely and regularly on a silicon wafer substrate. Without a doubt, this field remains a lot of objects to be solved and awaits many researchers to be participated.