Electron tunneling through alkanedithiol self-assembled monolayers in large-area molecular junctions

Abstract

The electrical transport through self-assembled monolayers of alkanedithiols was studied in large-area molecular junctions and described by the Simmons model [Simmons JG (1963) J Appl Phys 34:1793-1803 and 2581-2590] for tunneling through a practical barrier, i.e., a rectangular barrier with the image potential included. The strength of the image potential depends on the value of the dielectric constant. A value of 2.1 was determined from impedance measurements. The large and well defined areas of these molecular junctions allow for a simultaneous study of the capacitance and the tunneling current under operational conditions. Electrical transport for octanedithiol through tetradecanedithiol self-assembled monolayers up to 1 V can simultaneously be described by a single effective mass and a barrier height. There is no need for additional fit constants. The barrier heights are in the order of 4 -5 eV and vary systematically with the length of the molecules. Irrespective of the length of the molecules, an effective mass of 0.28 was determined, which is in excellent agreement with theoretical predictions. and consist of a saturated carbon backbone with one (or two) thiol end groups. Experimentally, the tunneling current through a monolayer of alkane(di)thiols was shown to be temperatureindependent and to decrease exponentially with increasing molecular length (4, 5). The transport has been interpreted in terms of the classical tunneling model through a thin insulating film as provided by Simmons (24,25). In this model the tunneling current depends on the mean value of the barrier height, allowing for a simplification of the problem of an arbitrarily shaped potential barrier to that of a rectangular barrier. This model has been applied to junctions based on SAMs (4, 26), but an extra fit parameter ␣ is needed to obtain a fit to the measured data. However, as already explained by Simmons (24, 25), for a practical tunnel junction the image potential has to be taken into account. This effect has been neglected in the literature so far.The system studied in this article is a tunnel junction with an alkanedithiol SAM as the insulating film, a bottom gold electrode, and a highly conducting polymer as a top contact. The polymeric top contact allows for the fabrication of devices with a yield of almost unity for areas up to 100 m in diameter (5). The highly conducting polymer used is PEDOT:PSS, a waterbased suspension of poly(3,4-ethylenedioxythiophene) stabilized with poly(4-styrenesulfonic acid). The polymer acts as a cushion for the thermally evaporated metal atoms to land on and prevents the metal atoms from penetration into the molecular layer. Consequently, the formation of electrical shorts is prevented (27). Simmons ModelThe tunneling current density J through a rectangular potential barrier with height 0 is given by (4, 24, 26):wherewhere ⌬s is the barrier width at the Fermi level of the electrodes, here equal to the total length s of the tunneling path between the electrodes, m e is the bare electron mass, V is th...

Model of self assembled monolayer based molecular diodes made of ferrocenyl-alkanethiols

“…This value is in accordance with experimental measurements 21 and with values obtained through numerical simulations. 13,15 The energy position of the HOMO and the LUMO of the ferrocene moieties is À3.27 eV and À4.97 eV, respectively, while their broadenings (C) is varied between 0.1 eV and 0.7 eV. These values are in accordance with numerical simulations performed using the density functional theory (DFT) for an HSC 11 Fc based molecular diode in which both electrodes are made of gold (Au).…”

“…14 The potential variation (V (x)) within the junctions has been expressed as follows: Because the relative dielectric constant of organic materials is low ($2), the image potential screening is weak within the molecular junctions. 15,16 Thus, an electron in the vicinity of an electrode "feels" an additional image potential, which results in barrier lowering. The solution of the image force problem results in an expression for the image potential V image (x), which is an infinite series 17…”

“…The relative dielectric constant of the ferrocenyl-alkanethiols has been considered to be equal to the dielectric constant of the alkyl chains (e r ¼ 2.1). 15 This expression can be accurately expressed by an hyperbolic function as follows: 17,18 V image x ð Þ ¼ À1:15:…”

Molecules on Si: Electronics with Chemistry

“…We fit the data (dots) in Figure 5a to the commonly used Simmons model, [29,[153][154][155] over the forward bias range (>0 V, solid black lines), using a single parameter set (in contrast to, e.g., Refs. [29,155]) with a tunnel barrier h ¼ 0.28 eV, a carrier effective mass m* ¼ 3.2 and a contact area of 30 mm 2 (compared to the geometric contact area of 5 Â 10 4 mm 2 ; see below). [29] Although the J-V curves appear rather symmetric, we cannot fit both bias polarities to a single parameter set.…”

“…The important conclusion is that fitting molecular transport relations using theoretical J C values (e.g., from the Simmons model [153] ) can lack clear physical relevance even though a good data fit is obtained. [29,149,155] Furthermore, while tunneling is generally accepted as the transport mechanism (Eq. (6)), a more detailed mechanism is lacking.…”