Abstract

This chapter focuses on molecular tunnel junctions (MTJ), the basic building block of molecular electronics (ME), which consist of either a single molecule or an ensemble of molecules in the form of a self-assembled monolayer (SAM) sandwiched between two electrodes. MTJs based on SAMs find practical applications such as diode rectifiers, switches, and molecular memory devices. The predominant charge transport mechanism in two-terminal junctions is tunneling; therefore, perturbances in the bond length scale will translate into nonlinear electrical responses, allowing MTJ to induce and control electronic activity on nanoscopic length scales with various inputs. For this reason, the subject is now progressing to devices based on finite ensembles of molecules, and many studies are underway to develop devices that can augment and complement traditional semiconductor-based electronics. SAM-based tunnel junctions are like single molecular junctions, demonstrating effects like quantized conductance, tunneling, hopping, and rectification; they also possess a unique set of properties. In addition, several new problems that need to be addressed arise from the unique characteristics of SAM-based junctions. General aspects of the two terminal molecular junctions, roles of the electrode, molecule, and molecule electrode interfaces, and how to differentiate the components of a molecular junction using impedance spectroscopy are discussed in this chapter. Different testbeds to measure the charge transport in SAM-based tunnel junctions are discussed, and a comparison of the reported charge transport data on alkanethiolate SAMs is presented. Finally, the molecular rectifiers are briefly discussed.