Molecular Assembly of Nanowires Part 1 (Nanotechnology)

INTRODUCTION

Fabrication of one-dimensional (1-D) fundamental nano-scale structures from molecular systems through bottom-up approach should be one of the most important steps to realize nanoscale electronic devices.[1-3] Because 1-D systems are the structures with the lowest dimension that permit efficient electron transport, the nanowires are expected to be critical to functionalize and integrate the nanoscale electronic devices. Nanowires are important units in constructing electronic circuits, particularly in electrical conducting; thus a variety of nanowires have been the focus of extensive studies aimed toward nanoscale electronic systems.[1-4] At present, a wide range of compounds from inorganic metals, semiconductors, and carbon nanotubes have been employed as nanowires (Fig. 1).a The diameter, length, and electronic structure of these nanowires varied significantly. The electronic properties of nanowires range from insulating, semiconducting, metallic, to superconducting. Electrical conduction within nanowires is dominated by the carriers at around Fermi level of the band structure, just as in bulk metals and semiconductors (Fig. 1).[5] Although the electronic density of states along the short axis of nano-wire has a discrete character because of the restricted lattice translation, that along the wire direction is approximately represented as the band structure.[6] The doped polymers, single-walled carbon nanotubes (SWCNTs), semiconductor, and metal nanowires may have Fermi surfaces. On the other hand, DNAs are the insulator with large band gap. Semiconductor nanowires are more important than metal nanowires from the viewpoint of device application.[7]

Although only a few examples are reported for the preparation of nanowires of molecular assemblies,[8-10] we consider that these molecular nanowires should have an important role in the complete bottom-up manufacture of the molecular electronics. Such nanowires can be assembled from p-molecules through molecule-by-molecule p-stacking. The researches in the field of ”molecular conductors” will offer a guiding principle in constructing electrical conducting molecular nano-wires.[11-15] The anisotropic charge-transfer (CT) interaction in molecular conductors is advantageous to form the 1-D p — p stacking nanowire structure. In addition, ”supramolecular chemistry” will offer powerful tools to fabricate molecular nanowires through the self-assembly process.[16-18] Appropriate design of molecule to orient and integrate the molecular-assembly nanowires on the substrate surface should be effective to realize molecular-assembly electronic devices. Furthermore, the techniques of ”Langmuir-Blodgett (LB) Films” are very useful methods to fabricate nanoscale molecular-assembly structures on a variety of substrate surfaces.[19-21] To realize molecular-assembly nanoscale devices through bottom-up chemical approach, three types of scientific concepts— molecular conductor, supramolecular chemistry, and surface science—should be linked together.

Molecular Conductors

A large number of molecular conductors, ranging from semiconductors to metals and superconductors, was prepared.[11-15] In general, a stable organic molecule has a closed-shell electronic structure without conduction carriers, thereby making molecular solids highly insulating. Conduction carriers can be generated via the intermolecular CT interaction between the highest occupied molecular orbital (HOMO) of the electron donor (D) and the lowest occupied molecular orbital (LUMO) of the electron acceptor (A) molecules. Fig. 2a shows the molecular structures of typical D and A molecules utilized in the field of molecular conductors. Among them, tetrathiafulvalene (TTF) and 7,7,8,8-tetracyano-p-quino-dimethane (TCNQ) are the well-known D and A molecules, respectively, which gave the first molecular metal of (TTF+ 0 59)(TCNQ— 0 59). The degree of CT (d) in the binary (D+d)(A—d)x CT complex depends on the ionization potential of D and electron affinity of A molecules. The planar p-conjugated D and A molecules have a tendency to form the 1-D columnar structure through the p — p stacking interaction, which also formed 1-D p-band structure (Fig. 2b). Because the p-orbitals exist orthogonal to the molecular plane, the direction of p — p interaction is highly anisotropic. Therefore, the 1-D p — p interaction in the molecular conductors is suitable for obtaining molecular nanowires.

Fig. 1 Nanowires constructed from doped-polymer, SWCNT, semiconductor, and DNA. The electronic band structures of these are illustrated below.

Langmuir-Blodgett Technique

Langmuir-Blodgett (LB) method is one of the conventional fabrication techniques of nanoscale thin-film structures utilizing the air-water interface.[19-21] Fig. 3 illustrates typical procedures to obtain Langmuir mono-layer at the air-water interface and the film-forming processes of LB multilayers on substrate. Amphiphilic molecules having hydrophilic and hydrophobic moieties are dissolved into conventional organic solvents, which were spread at the air-water interface. The monolayer with a thickness of molecular scale can form a variety of molecular-assembly structures such as gas-, liquid-, and solid-like short- and long-range orders through the control of surface pressure (F, mN m— 1). Increase in the F enhances the magnitude of intermolecular interactions between the molecules. The chemical designs of hydro-philicity and hydrophobicity of the component molecules are important to obtain stable monolayer at the air-water interface.

Stable monolayers at the air-water interface can be transferred onto hydrophilic or hydrophobic substrates via dipping of the substrate. Glass, quartz, CaF2, Si, and mica are utilized as typical hydrophilic substrates. The surface wetting of these substrates largely influences the surface morphologies of the transferred LB films. Another conventional deposition technique is the horizontal-lifting method, which directly transfers the monolayer at the air-water interface onto the hydrophobic substrate.

Electrically active thin films have been fabricated by the LB techniques.[22-24] For example, metallic and semiconducting LB films have been obtained from amphiphilic molecular conductors based on TTF and TCNQ derivatives.1-22-24-1 Although the LB films possess a periodicity along the film-forming direction, random distribution of two-dimensional crystalline domains on the substrate surface dispels bulk periodicity within the substrate surface. To realize nanoscale electronic devices through bottom-up self-assembly approach, the orientation and size control of these electrical active domains within the substrate surface are two of the important problems that need to be overcome.

Supramolecular Chemistry

The term of supramolecular chemistry is a last key science to realize the bottom-up self-assembly approach.[16-18] Supramolecule is defined as an entity composed of several or large numbers of molecules, which are connected to each other through the noncovalent weak intermolecular interactions such as van der Waals (dipole-dipole, dipole-induced dipole and dispersion interactions), charge-transfer, hydrogen-bonding interactions, etc. The design of these intermolecular interactions is essential to obtain self-assembly molecular nanowires. Self-assembled supra-molecules bearing the desired structure can be achieved by using programmed molecules that are appropriately designed. From the flexibility in supramolecular designs together with the rich physics in molecular conductors, electrical active molecular nanowires are considered as very promising candidates for constructing future nano-scale devices.

A large number of complex supramolecular assemblies have already been constructed.[16-18] A simple complex between cation and crown ether is one of the typical model systems of supramolecules[25,26] (Fig. 4). Crown ethers such as 12-crown-4, 15-crown-5, and 18-crown-6 have hydrophilic cavity to bind cation through metal-oxygen interatomic interactions. According to the size of hydrophilic cavity, cations can be selectively included into the cavity. For example, 15-crown-5 and 18-crown-6 molecules show high Na+ and K+ affinity, respectively. We already introduced two design concepts of molecular conductors and LB technique to fabricate molecular-assembly nanowires. For an effective design of molecular nanowires, we further introduced the supramo-lecular approach to obtain nanowire orientation on the substrate surface.

Fig. 2 Molecular structures of typical a) electron donor (D) and acceptor (A) molecules employed in molecular conductors. b) The 1-D columnar structure through the p — p stacking interaction in crystals (left) and metallic 1-D p-band structure of (TTF)(TCNQ) (right).

RESULTS AND DISCUSSION

Design of Molecules for Fabricating Molecular-Assembly Nanowires

Amphiphilic bis(tetrathiafulvalene) [bis(TTF)] macrocy-cle 1 was designed from the concepts of molecular conductors, Langmuir-Blodgett films, and supramolecu-lar chemistry.[27,28] The molecule has two redox-active TTF units that are linked via a [24]crown-8 macrocycle and two long hydrophobic decylthio-chains (Fig. 5). Two TTF units within the molecule 1 can act as electron donor for realizing intermolecular CT interaction with electron acceptors, which forms electrically conducting 1-D p-p stacks. The second structural point is the introduction of two hydrophobic chains (-SC10H21), which were introduced into one side of the TTF unit. By introducing these hydrophobic chains, the molecule has amphiphilic character to apply the LB technique. The last designing point is carried out via the supramolecular approach—the introduction of ion-recognizing crown ether moiety into the molecule. The ion-recognizing property can be employed to fabricate oriented molecular-assembly nano-wires on the substrate surface.

Fig. 3 Langmuir-Blodgett technique to fabricate nanoscale thin-film structures. a) Langmuir monolayer at the air-water interface. Amphiphilic molecule has hydrophilic (red head) and hydrophobic tail. Surface pressure (F, mN m— 1) can be controlled by moving barriers. b) Transfer process of Langmuir monolayer onto substrate surface.

Compound 1 was synthesized by the cyanoethyl protected TTF building block.[29,30] Stepwise deprotec-tion/alkylation procedure was performed by CsOH • H2O/ 2,6-bis(2-iodoethoxy)ethane. Because a TTF molecule possesses a two-step redox process (TTF! TTF+ and TTF+! tTF2+),[11-15] donor 1 possesses a four-step redox process. The cyclic voltammetry (CV) diagram of donor 1 in 1,2-dichloroethane (C2H4Cl2) vs. SCE showed the two-step, two-electron oxidation waves at 0.56 and 0.90 V, respectively.1-27,28-1 The two TTF units in donor 1, linked via [24]crown-8 unit, independently exhibited redox reaction.

The CT complex between one molecule of donor 1 and two molecules of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-quinodimethane (F4-TCNQ), (1)(F4-TCNQ)2 was pre-pared to fabricate oriented molecular-assembly nano-wires. The electronic ground state of the CT complexes can be discussed in terms of the differences between the redox potentials, DE = E1/2(donor) —E1/2(F4-TCNQ), for electron donor 1 and F4-TCNQ.[31,32] Partial CT states, (D+d)(A—d) with 0.5 < d <1, are necessary for the formation of metallic CT complexes having a segregated-stack structure, which is achieved when DE has a value between — 0.02 and 0.34 V.[31,32] Fully ionic (D+)(A—) and neutral (D0)(A0) CT complexes are typically observed when DE < — 0.02 V and DE > 0.34 V, respectively. In the case of donor 1 and F4-TCNQ (E1/2(1) = 0.73 V), the DE values of CT complex of (1)(F4-TCNQ)2 was — 0.17 V. Because these values were far from the conditions of partial CT complexes, fully ionic ground state (12+)(F4-TCNQ—)2 was expected for the molecular nanowires.

Fig. 4 Supramolecular complex between the cation and crown ether. Cations are complexed into the crown ether cavity through cation-oxygen interactions.

Monolayer of Charge-Transfer Complexes at the Air-Water Interface

The CT complex (1)(F4TCNQ)2 was prepared in situ by mixing donor 1 and two equivalent of F4-TCNQ in CHCl3/CH3CN (9:1, v/v). Concentration of the spreading solution was fixed at 1 mM with respect to (1)(F4-TCNQ)2. Surface pressure (F)-area per molecule (A) isotherms were recorded at 291.5 K with a barrier speed of 50 cm2 min—l. The LB monolayers were transferred onto freshly cleaved mica surfaces by a single up-stroke drawing. Because the macrocyclic moiety may recognize ions introduced into the subphase upon the formation and deposition of the monolayers, the K+ ions were introduced into the subphase for realizing ion recognition at first.

The F-A isotherms of the floating monolayers of the CT complex of (1)(F4-TCNQ)2 on pure water and aqueous 0.01 M KCl subphase are shown in Fig. 6. The surface areas of CT complexes extrapolated at 0 mN m— 1 on pure water and 0.01 M KCl subphase are almost consistent with each other (A0^ 1.2 nm2). Because these values are larger than that of neutral 1 (A0=0.6 nm2), the formation of CT complex causes much expansion of the surface area. The films were deposited under a controlled surface pressure of 10 mN m— 1, at which point the surface areas of (1)(F4- TCNQ)2 on pure water and 0.01 M KCl (A10~ 1.1 nm2) were almost identical to each other.

Fig. 5 Molecular design of amphiphilic bis(tetrathiafulvalene) [bis(TTF)] macrocycle 1 from the viewpoints of molecular conductor (TTF), supramolecular chemistry (crown ether), and Langmuir-Blodgett films (hydrocarbons).

Fig. 6 F-A isotherms of CT complex of (1)(F4-TCNQ)2 on a) pure water and b) 0.01 M KCl subphase. The films were transferred at 10 mN m— 1.