Electrical and optical studies of gap states in self-assembled molecular aggregates

We fabricated a variety of two-terminal devices using self-assembled monolayers (SAM) of solid-state mixtures comprised of molecular "wires" [1,4-methane-benzenedithiol (Me-BDT)] and molecular insulator "spacers" [1-pentanethiol], which were prepared at various molar concentrations ratio, r of wires/spacers, and sandwiched between two gold electrodes. The devices' electrical transport was investigated at several r values using the bias voltage (V) dependencies of the conductance and differential conductance at various temperatures. In parallel, we also studied the UV-visible absorption and photoluminescence (PL) emission spectra of the SAM mixtures grown on silica transparent substrates. For r > 10(-3) we found that two-dimensional (2D) Me-BDT aggregates are formed in the SAM films leading to novel properties compared to SAM films of isolated Me-BDT molecules at concentrations 10(-8)< r < 10(-4), which we studied before [V. Burtman, A. S. Ndobe, and Z. V. Vardeny, J. Appl. Phys. 98, 034314 (2005)]. First, an Ohmic response in the current-voltage (I-V) characteristics is obtained up to V similar to 0.5 V, which results in a new band in the differential conductance spectrum around V=0. Second, a new subgap absorption band is formed at similar to 2.4 eV, which is related to a new yellow/red PL emission band. The novel optical and electrical properties of the 2D Me-BDT aggregates are explained by the formation of an electronic continuum band in the Me-BDT energy gap, which is caused by weak in-plane charge delocalization among the molecules forming the aggregates. To verify this model we also studied SAM molecular aggregate diodes using Al electrodes. The 1-eV difference in the electrode work function between Au and Al metals results in a pronounced E-F shift with respect to the aggregate-related continuum band in the gap, and consequently, dramatically changes the device I-V characteristics. (c) 2007 American Institute of Physics.