Current-voltage characteristics of single-molecule diarylethene junctions measured with adjustable gold electrodes in solution

Abstract

We report on an experimental analysis of the charge transport through sulfur-free photochromic molecular junctions. The conductance of individual molecules contacted with gold electrodes and the current-voltage characteristics of these junctions are measured in a mechanically controlled break-junction system at room temperature and in liquid environment. We compare the transport properties of a series of molecules, labeled TSC, MN, and 4Py, with the same switching core but varying side-arms and end-groups designed for providing the mechanical and electrical contact to the gold electrodes. We perform a detailed analysis of the transport properties of TSC in its open and closed states. We find rather broad distributions of conductance values in both states. The analysis, based on the assumption that the current is carried by a single dominating molecular orbital, reveals distinct differences between both states. We discuss the appearance of diode-like behavior for the particular species 4Py that features end-groups, which preferentially couple to the metal electrode by physisorption. We show that the energetic position of the molecular orbital varies as a function of the transmission. Finally, we show for the species MN that the use of two cyano end-groups on each side considerably enhances the coupling strength compared to the typical behavior of a single cyano group.

Keywords: diarylethene; mechanically controllable break-junction; molecular electronics; photoswitching; single-molecule junctions.

Figure 1

(a) Scanning electron micrograph of the MCBJ device with an illustration of a Au–4Py–Au junction. (b) Sketches of open (left) and closed (right) forms of photochromic molecules (difurylethene); R indicates the extended side-arms and end-groups. (c) Structures of three different molecules, 4Py (black), TSC (red), MN (green) investigated in this study.

Figure 2

Examples of I–V characteristics of open and closed isomers. (a) Open form of TSC, (b) closed form of TSC. (c) Closed form of 4Py, (d) closed form of MN. The colored curves are experimental data, and the black curves are the fits to Equation 1.

Figure 3

Transmission histograms of TSC. The open columns represent the transmission values from all I–V’s, the closed columns only from those I–V’s that are successfully described by the single-level model (symmetric as well as asymmetric). Panels (a) and (b) are for the closed isomer, panels (c) and (d) for the open isomer. The data shown in panels (a) and (c) were recorded during stretching of the junctions, the data in panels (b) and (d) during relaxing of the junctions.

Figure 4

Fitting parameters Γ and |E₀| for (a) open form of TSC, (b) closed form of TSC, (c) closed form of 4Py, and (d) closed form of MN as a function of the transmission for symmetric I–V’s. The various colors in panels (b) and (c) refer to the individual samples used for recording the data. We define those I–V’s as “symmetric” for which the ratio of Γ_L to Γ_R is between 0.5 and 2.

Figure 5

Fitting parameters Γ_L and Γ_R and |E₀| for (a) open form of TSC, (b) closed form of TSC, (c) closed form of 4Py, and (d) closed form of MN as a function of transmission for asymmetric I–V's.

Figure 6

Break-junction setup for use in liquid environment. A PDMS-sealed glass pipette, in which the molecular solution circulates, is pressed onto the central part of the MCBJ chip with the help of a plug screwed to the sample holder. The electrical contacts are realized in this case through spring-borne contacts outside the gasket.