Intermolecular electrical conductance in self-assembled monolayers

Abstract

Intermolecular interaction was investigated by measuring the properties of self-assembled monolayers (SAMs) in the lateral direction. We formed SAMs of octadecyltrichlorosilane (OTS) and phenethyltrichlorosilane with Ag-or Au-patterned electrodes on mica surfaces and measured the conductance under various bias voltages. Using an atomic force microscope with a conductive cantilever, we could not detect current through the SAMs in the vicinity of the electrodes about 85 nm from the edge. The resistivity of the OTS film measured using the substrates with interdigitated (comb) electrodes was strongly dependent on the humidity. From the result in a dry N 2 atmosphere, the resistivity of a SAM of OTS in a lateral direction was estimated to be ∼ 9 × 10 10 Ω cm.Self-assembled monolayers (SAMs) are one of the promising candidates for sensors, ultra-thin passivating and insulating layers, resists for nanometer-scale lithography, and new devices, because their properties can be controlled by changing the species of constituent molecules. So far, a great number of studies have been made on the structure, formation process, physical properties, and so on. The study of the electrical properties has just started from both experimental and theoretical points of view. Most studies have been concerned with the electrical conduction along the molecular axis [1-4], but there have been few studies on the electrical conduction in the lateral direction, parallel to the surface of SAMs. Possible ways to measure the conductivity of SAMs are to make films on an insulating substrate with patterned electrodes (Fig. 1a), to form electrodes on the films (Fig. 1b), and to form films on the flat substrate with buried conductive regions (Figs. 1c,d).In this study, we formed SAMs on the metal-patterned substrates as shown in Fig. 1a and measured the electrical conductivity of SAMs in the lateral direction using atomic force microscopy (AFM) with a conductive cantilever as a counterelectrode. Generally, it is not easy to form SAMs on the patterned substrates, because their formation is quite sensitive to * Permanent address: Electrotechnical Laboratory, Tsukuba, Ibaraki 305. the nature of the substrate surfaces, e.g. its chemical properties, contaminations, and roughness. However, with a careful cleaning process, residuals in the process for forming electrodes were removed thoroughly, and we could form SAMs of the same quality as those on the pristine surface. Adding the electrode after the SAM formation (Fig. 1b) might cause some damage to the film. We also carried out macroscopic measurements of the conductivity of SAMs under controlled S A M electrode substrate (a) (b) conductive region (c) (d) conductive layer Fig. 1a-d. Schematic drawings for possible structures of SAMs on substrates with electrodes. a A SAM on the electrode-patterned substrate. b Electrode pattern on the substrate coated by a SAM c, d SAM on the flat substrate with a buried conductive region or layer. Substrates are insulative

Conductive Probe AFM Measurements of Conjugated Molecular Wires

“…CP‐AFM requires that the current is shared by many molecules. Since the molecules under the tip are experiencing different pressures; there may also be a (5) change of intermolecular interaction 48. For typical alkanethiol SAMs, as reported elsewhere,19,20 (1), (3), and (4) are likely to be factors that affect the observed current change.…”

“…For typical alkanethiol SAMs, as reported elsewhere,19,20 (1), (3), and (4) are likely to be factors that affect the observed current change. Since the intermolecular electrical conduction of the alkyl chain is negligible48 in a vacuum, (5) is not likely to exert an effect.…”

Patterning and Functionalizing Self-Assembled Monolayers

“…32,33) (comb) electrodes. 46) The measured conductivity of the OTS film was dependent on the humidity, but from the result in a dry N 2 atmosphere, the in-plane conductivity of OTS SAM was estimated to be ∼10 −11 S/cm.…”

Mechanical and electroconductive properties of spatially distributed double stranded DNA arrays on Au (111)

“…Conductive AFM has been used to investigate force interactions and conductivity through nanowire arrays, thin-layered organic films and polymers [23][24][25][26][27][28][29][30][31] and has recently been applied to single molecule studies in measurements of conductivity through single carotene molecules [28] and λ-DNA [9]. DNA monolayers formed by thiol bonding to gold surfaces [32][33][34][35][36][37][38][39][40][41][42][43] were used to investigate conductivity by electrochemical methods and were also found to provide a good substrate for probing the conductivity through DNA molecules by SPM.…”