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.
Keywords: Charge transport, Impedance spectroscopy, Molecular tunnel junctions, Molecular rectifier, Self-assembled monolayers.
