Molecular electronics using diazonium-derived adlayers on carbon with Cu top contacts: critical analysis of metal oxides and filaments

Abstract

Evaporation of Cu metal onto thin (less than 5 nm) molecular layers bonded to conductive carbon substrates results in electronic junctions with an ensemble of molecules sandwiched between two conductors. The resulting devices have previously been characterized through analysis of current density-voltage (j-V) curves for several different molecular layers and as a function of layer thickness. The approach represents an 'ensemble' rather than 'single molecule' technique, in which the electronic response represents that of a large number of molecules (10(6)-10(12)) in parallel as well as the conducting contacts contained in the molecular junction. In this paper, we extend a more detailed investigation of two critical issues: the possibility of conduction by metal filaments, and the potential role of top contact oxidation contributing to the electronic properties of the junctions. The results show that the conductance of the junctions can be modulated by changes in the deposition environment, but that the changes are not related to Cu oxide in the top contact. Based on these results, we propose that the conditions during top contact deposition change the way in which the molecules contact the metal, leading to differences in the effective junction area. Finally, through systematic studies using variation of the temperature, we show that metal filament conduction is distinct from that observed for the molecular junctions and that if the current observed experimentally passed through nanoscopic metal filaments the Joule heating would lead to rapid melting. For a series of junctions with structurally related aromatic molecules (including biphenyl, nitrobiphenyl, fluorene, and nitroazobenzene), the electron transfer mechanism is briefly investigated using area-independent analysis methods. It is shown that field emission and/or transport through bands formed by the molecular layer is likely, based on the weak temperature dependence of junction conductance.

Molecules on Si: Electronics with Chemistry

“…[149,159,161,162] One approach postulates that if the applied bias is larger than the barrier height, the transport mechanism changes from direct tunneling to field-emission. [34,161,162] Experimentally, it is fairly simple to identify this transition bias (V T ) and indeed, in the study cited, [34,161,162] V T correlated well with the expected barrier height, the difference of the molecular level (highest occupied molecular orbital, HOMO) and the electrode Fermi level as deduced independently by UPS (see, however, Section 5.2). [52,161,162] This approach assumes a roughly linear J-V relation for tunneling transport [161] (i.e., positive ln(J/V 2 ) vs. 1/V slope), and in that case V T indeed indicates a change in transport mechanism to field-emission.…”

“…The upside is that it is more robust against substrate oxidation, can withstand heating [33] and that metal atom diffusion during top contact deposition is less likely. [34] Amongst other binding chemistries, only CÀC bonding to graphite (or burned photo-resist, see Ref. [34]) is comparable to the SiÀC or SiÀOÀC binding mode.…”

“…[34] Amongst other binding chemistries, only CÀC bonding to graphite (or burned photo-resist, see Ref. [34]) is comparable to the SiÀC or SiÀOÀC binding mode. Phosphonate binding to GaAs (GaÀOÀPÀ) is also very stable but GaAs suffers from an ill-defined (hydr)oxide layer, unless preparation and measurement are in ultra-dry ambient.…”

“…also Section 7.2, below). [34,67,69,70,92,[94][95][96] Beyond structure and morphology, several surface analyses, mainly spectroscopic ones, can provide information on the electronic structure of the system. As such information cannot be extracted directly from the transport data, the use of complementary methods is crucial for understanding transport mechanisms.…”

“…Reported temperature dependence of transport varies from none [55,172,174,175] to positive (i.e., thermal activation). [34,55,96] In MIM junctions, a pseudo-metallic temperature response is generally attributed to conductance via metallic filaments penetrating the monolayer. [34,171] This is very unlikely here, because if we change the insulator by electron irradiation [164] (or ''dope'' it, [139] ) the metal-like dependence is replaced by a thermally activated behavior, as shown in Figure 7b (dotted lines).…”