Unimolecular and Supramolecular Electronics I

Modern optoelectronic devices, such as light-emitting diodes, field-effect transistors and organic solar cells require well controlled motion of charges for their efficient operation. The understanding of the processes that determine charge transport is therefore of paramount importance for designing materials with improved structure-property relationships. Before discussing different regimes of charge transport in organic semiconductors, we present a brief introduction into the conceptual framework in which we interpret the relevant photophysical processes. That is, we compare a molecular picture of electronic excitations against the Su-Schrieffer-Heeger semiconductor band model. After a brief description of experimental techniques needed to measure charge mobilities, we then elaborate on the parameters controlling charge transport in technologically relevant materials. Thus, we consider the influences of electronic coupling between molecular units, disorder, polaronic effects and space charge. A particular focus is given to the recent progress made in understanding charge transport on short time scales and short length scales. The mechanism for charge injection is briefly addressed towards the end of this chapter.

We review the development of conductive organic molecular assemblies including organic metals, superconductors, single component conductors, conductive films, conductors with a switching function, and new spin state (quantum spin liquid state). We emphasize the importance of the ionicity phase diagram for a variety of charge transfer systems to provide a strategy for the development of functional organic solids (Mott insulator, semiconductor, superconductor, metal, complex isomer, neutral-ionic system, alignment of chemical potentials, etc.). For organic (super)conductors, the electronic dimensionality of the solids is a key parameter and can be designed based on the self-aggregation ability of a molecule. We present characteristic structural and physical properties of organic superconductors.

With the constant growing complexity of electronic devices, the top-down approach used with silicon based technology is facing both technological and physical challenges. Carbon based nanomaterials are good candidates to be used in the construction of electronic circuitry using a bottom-up approach, because they have semiconductor properties and dimensions within the required physical limit to establish electrical connections. The unique electronic properties of fullerenes for example, have allowed the construction of molecular rectifiers and transistors that can operate with more than two logical states. Carbon nanotubes have shown their potential to be used in the construction of molecular wires and FET transistors that can operate in the THz frequency range. On the other hand, graphene is not only the most promising material for replacing ITO in the construction of transparent electrodes but it has also shown quantum Hall effect and conductance properties that depend on the edges or chemical doping. The purpose of this review is to present recent developments on the utilization carbon nanomaterials in molecular electronics.

This chapter describes the fundamental study of charge transport through single layers of ?-conjugated molecules organized to form the semiconducting channels of field-effect transistors (FETs). Physical and chemical methods of evaporation, Langmuir-Blodgett assembly and transfer, and self-assembly have been used by the community to realize single molecular monolayers on the gate or gate dielectric surface of FETs. Advancements in molecular design and chemical modification of FET interfaces continue to improve measured charge transport properties in FETs. These monolayer FETs have been integrated in electronic circuitry and demonstrated as chemical sensors, where they promise the ultimate in performance as the entire molecular monolayer is modulated by the applied gate field and is accessed by analytes, respectively.

Metal vapor deposition to form ohmic contacts is commonly used in the fabrication of organic electronic devices because of significant manufacturability advantages. In the case of single molecular layer devices, however, the extremely small thickness, typically ~1–2nm, presents serious challenges in achieving good contacts and device integrity. This review focuses on recent scientific aspects of metal vapor deposition on monolayer thickness molecular films, particularly self-assembled monolayers, ranging across mechanisms of metal nucleation, metal-molecular group interactions and chemical reactions, diffusion of metal atoms within and through organic films, and the correlations of these and other factors with device function. Results for both non-reactive and reactive metal deposition are reviewed. Finally, novel strategies are considered which show promise for providing highly reliable and durable metal/organic top contacts for use in metal–molecule–metal junctions for device applications.

This chapter reviews tunneling of spin-polarized electrons through molecules positioned between ferromagnetic electrodes, which gives rise to tunneling magnetoresistance. Such measurements yield important insight into the factors governing spin-polarized electron injection into organic semiconductors, thereby offering the possibility to manipulate the quantum-mechanical spin degrees of freedom for charge carriers in optical/electrical devices. In the first section of the chapter a brief description of the Jullière model of spin-dependent electron tunneling is reviewed. Next, a brief description of device fabrication and characterization is presented. The bulk of the review highlights experimental studies on spin-polarized electron tunneling and magnetoresistance in molecular junctions. In addition, some experiments describing spin-polarized scanning tunneling microscopy/spectroscopy on single molecules are mentioned. Finally, some general conclusions and prospectus on the impact of spin-polarized tunneling in molecular junctions are offered.