Self‐Assembled‐Monolayer Formation of Long Alkanedithiols in Molecular Junctions

Abstract

Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. The orientation of alkanedithiol molecules in self-assembled monolayers (SAMs) is of vital importance for their transport properties in molecular junctions. It is demonstrated that a too-low concentration of long alkanedithiols in ethanol leads to the formation of looped molecules, resulting in a 50-fold increase of the current through the SAM. X-ray photoelectron spectroscopy measurements show that high-concentration dithiol solutions result in a preferential standing-up phase. To obtain an almost full standing-up phase of 1,14-tetradecanedithiol (C14) a 30 mm concentration in ethanol is required, whereas a 0.3 mm concentration leads to a highly looped monolayer. The conduction through the full standing-up phase of C14 and C16 is in accordance with the exponential dependence on molecule length as obtained from shorter alkanedithiols.

Odd−Even Effects in Charge Transport across Self-Assembled Monolayers

“…We, and others, 50,[56][57][58][59][60][61][62][63][64][65][66][67][68] have begun an effort to develop and prove reliable protocols for measuring charge transport across SAMs. Our approach features a semiconformal (mechanically compliant on the micron-scale but probably not conformal on smaller scales) liquid top-electrode, EGaIn (a liquid eutectic alloy of gallium and indium), whose surface is largely, or entirely, covered with a thin oxide film (predominantly gallium oxide).…”

“…contacts, and to protect the organic molecules from damage due to exposure of evaporated metal atoms. [62][63][64][65][66][67]106 The conductive polymer film may be protective and provide a good contact; unfortunately, it also introduces unresolved ambiguities in the system and seems to produce measurements that are rather different from other methods (β even = 0.66 ± 0.04 per carbon, where consensus value from other methods is β even = ~ 1 per carbon, n C -1 , for n-alkanethiol SAMs). 63 Although a wide range of methods have been employed to measure charge transport across SAMs, all successful techniques share the characteristic that they have a non-metallic layer between the SAM and the top electrode: a thin film of insulating metal oxide, a small gap of air or vacuum between a probe tip and the SAM, a second SAM on Hg, or a layer of conductive polymer between the SAM and an evaporated Au electrode.…”

Stability of large-area molecular junctions

“…Here we investigate the electrical transport in molecular junctions as a function of storage time and at elevated temperatures. Discrete large-area molecular junctions were fabricated according to previously published procedure [2,[13][14][15][16]. By photolithography, vertical interconnects (vias) are created on top of the gold bottom electrodes.…”

Supramolecular tunneling junctions

“…In an important study, [20] de Boer and co-workers, investigated charge transport through SAMs of alkanedithiols (2) In an extended study, [43] de Boer and co-workers, measured the J(V) characteristics of SAMs of longer alkanedithiols, HSC 14 SH (2e) and 1,16-hexadecanedithiol (HSC 16 SH) (2f). In this study, they found that using 3 mM solutions to form long alkanedithiol SAMs (the standard concentration used to form the SAMs in the previous study [20] ) caused HSC 14 SH (2e) to produce slightly asymmetric J(V) curves (as could already be seen in the previous study [20] ), and HSC 16 SH (2f) to exhibit a higher value of J than HSC 14 SH (2e) (which in theory should not occur as C 16 is a thicker insulating layer than C 14 ).…”

“…The authors attributed this to the longer carbon chains of these alkanethiols being able to loop/backbend over themselves, which they described as a 'looped phase'. [43] This causes thinner layers of SAMs to be formed, allowing the electrons to tunnel through a shorter distance. By decreasing or increasing the concentration of HSC 14 SH (2e) in solution by 100 times (0.3 mM to 30 mM), the authors found that it was possible to change the value of J obtained for the tunneling junctions.…”