Charge Transport in Self-assembled Nanoparticle-molecule Systems

Charge transport in self-assembled gold nanoparticle (NP)-alkanedithiol (HS(CH2)nSH) systems are investigated using break-junctions. A remarkably simple and reproducible method to fabricate break-junctions using electromigration is described. Using the break-junctions, self-assembled NP systems are studied in two limits: (1) at the single-NP and (2) at the NP-array limits. Single-NP devices exhibit Coulomb-blockade (CB) conductance suppressions at low temperatures. Contrary to predictions of an Orthodox theory, temperature-dependence of conductance inside CB exhibits multiple activation energies (Ea): A small Ea at low temperatures, and a larger Ea at high temperatures. The small Ea is independent of NP size and is attributed to an energy state at the metal--molecule contact, whereas the larger Ea scales with NP size and is attributed to NPs' charging energy. Importantly, a significant (~5-100fold) discrepancy is observed between values of charging energies obtained from Ea and CB thresholds. To account for the discrepancy, a new model is proposed in which electrons can temporarily be localized at the energy states at the contacts and lose energy. The model is supported by ultraviolet photoelectron spectroscopy which shows energy states close to Fermi level likely arising from gold-thiolate bonds. A suitably modified Orthodox theory can successfully explain the experimental observations. These results underscore the critical role of metal--molecule contacts in influencing energy-profiles of molecular junctions. Resistance-temperature dependencies of alkanedithiol-linked NP films show evidence of a metal-insulator transition (MIT) as n is varied. The MIT occurs at n = 5 and is explained in the context of a Mott-Hubbard model. Furthermore, all metallic films exhibit temperature coefficients of resistance that are smaller than that of bulk gold, and all insulating films exhibit a universal behavior, R ~ exp[(T0/T)^p], with p = 0.65. These observations are discussed in terms of temperature-independent elastic scattering and competitive thermally activated processes, respectively. The ability to tune properties of NP films thru an MIT implies that materials near the transition may be viewed as semiconductors. To explore this analogy, application of these materials in fabricating field-effect transistors is briefly described. These results highlight the utility of NP films as a platform for studying charge transport.Ph