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

Influence of linker molecules on charge transport through self-assembled single-nanoparticle devices

We investigate electrical characteristics of single-electron electrode/nanoisland/electrode devices formed by alkanedithiol assisted self-assembly. Contrary to predictions of the orthodox model for double tunnel junction devices, we find a significant (∼fivefold) discrepancy in single-electron charging energies determined by Coulomb blockade (CB) voltage thresholds in current-voltage measurements versus those determined by an Arrhenius analysis of conductance in the CB region. The energies do, however, scale with particle sizes, consistent with single-electron charging phenomena. We propose that the discrepancy is caused by a multibarrier junction potential that leads to a voltage divider effect. Temperature and voltage dependent conductance measurements performed outside the blockade region are consistent with this picture. We simulated our data using a suitably modified orthodox model. © 2005 The American Physical Society

Segregation of Sublattice Domains in Nitrogen-Doped Graphene

Atomic-level details of dopant distributions can significantly influence the material properties. Using scanning tunneling microscopy, we investigate the distribution of substitutional dopants in nitrogen-doped graphene with regard to sublattice occupancy within the honeycomb structure. Samples prepared by chemical vapor deposition (CVD) using pyridine on copper exhibit well-segregated domains of nitrogen dopants in the same sublattice, extending beyond 100 nm. On the other hand, samples prepared by postsynthesis doping of pristine graphene exhibit a random distribution between sublattices. On the basis of theoretical calculations, we attribute the formation of sublattice domains to the preferential attachment of nitrogen to the edge sites of graphene during the CVD growth process. The breaking of sublattice symmetry in doped graphene can have important implications in its electronic applications, such as the opening of a tunable band gap in the material

Local Atomic and Electronic Structure of Boron Chemical Doping in Monolayer Graphene

We use scanning tunneling microscopy and X-ray spectroscopy to characterize the atomic and electronic structure of boron-doped and nitrogen-doped graphene created by chemical vapor deposition on copper substrates. Microscopic measurements show that boron, like nitrogen, incorporates into the carbon lattice primarily in the graphitic form and contributes ∼0.5 carriers into the graphene sheet per dopant. Density functional theory calculations indicate that boron dopants interact strongly with the underlying copper substrate while nitrogen dopants do not. The local bonding differences between graphitic boron and nitrogen dopants lead to large scale differences in dopant distribution. The distribution of dopants is observed to be completely random in the case of boron, while nitrogen displays strong sublattice clustering. Structurally, nitrogen-doped graphene is relatively defect-free while boron-doped graphene films show a large number of Stone-Wales defects. These defects create local electronic resonances and cause electronic scattering, but do not electronically dope the graphene film