High electronic couplings of single mesitylene molecular junctions

Abstract

We report on an experimental analysis of the charge transport properties of single mesitylene (1,3,5-trimethylbenzene) molecular junctions. The electronic conductance and the current-voltage characteristics of mesitylene molecules wired into Au electrodes were measured by a scanning tunnelling microscopy-based break-junction method at room temperature in a liquid environment. We found the molecular junctions exhibited two distinct conductance states with high conductance values of ca. 10(-1) G 0 and of more than 10(-3) G 0 (G 0 = 2e (2)/h) in the electronic conductance measurements. We further performed a statistical analysis of the current-voltage characteristics of the molecular junctions in the two states. Within a single channel resonant tunnelling model, we obtained electronic couplings in the molecular junctions by fitting the current-voltage characteristics to the single channel model. The origin of the high conductance was attributed to experimentally obtained large electronic couplings of the direct π-bonded molecular junctions (ca. 0.15 eV). Based on analysis of the stretch length of the molecular junctions and the large electronic couplings obtained from the I-V analysis, we proposed two structural models, in which (i) mesitylene binds to the Au electrode perpendicular to the charge transport direction and (ii) mesitylene has tilted from the perpendicular orientation.

Keywords: break junction; charge transport; mesitylene; scanning tunnelling microscopy (STM); single molecular junction.

Figure 1

Schematic illustration of the experimental setup. The inset represents structural formula of mesitylene.

Figure 2

Conductance traces (a–c) and semi-logarithmic conductance histograms (d,e) during STM-BJ rupture process with (black) and without (red) adding mesitylene solution. The bias voltage was set to 20 mV. Conductance windows are 0–3G₀ for (a) and 0–0.3G₀ for (b,c). In (b) traces display high and low conductance steps around 0.1G₀ and 0.03G₀, which are marked by dotted circles. In (c) the two types of steps are magnified. Dotted guide lines are drawn at 0.1G₀ and 0.03G₀ in (c–e). Histograms in (d) and (e) are constructed from a choice of 200 conductance traces. A bin size of Δlog(G/G₀) = 0.02 is used.

Figure 3

Stretch length histograms on a semi-log scale constructed from 2000 of all conductance traces taken at the bias voltage of 20 mV for (a) high and (b) low conductance states (For a detail see main text). The bin size is log (stretch length/nm) = 0.1. Solid curves are results of Gaussian fits. Peak center positions are indicated by dotted lines ((a) 0.044 nm and (b) 0.079 nm).

Figure 4

(a) 2D histogram of I–V curves of the mesitylene molecular junctions. The histogram is built from 1000 I–V curves. The bin size is 16 mV × 100 nA. Two current windows at 0.3 V are indicated by arrows of (i) and (ii). The current ranges of (i) and (ii) are 700–1500 nA (0.03–0.065G₀) and above 2400 nA (above 0.1G₀), respectively. The conductance windows are chosen to separate I–V curves into two states with the conductance values of 0.1G₀ and 0.03G₀ observed in Figure 2. The mean current values at 0.3 V are 1100 and 4600 nA in the current windows (i) and (ii), respectively. Deviations are 200 and 1500 nA for (i) and (ii), respectively. (b,c) Typical I–V curves of the mesitylene molecular junctions in the current windows of (ii) and (i).

Figure 5

Proposed structural models of the mesitylene molecular junctions for (a) high-conductance and (b) low-conductance states. Mesitylene is oriented perpendicular to the charge transport direction in (a). In (b) stretching of the junction leads to a tilted orientation of the mesitylene molecule. White, grey and yellow balls represent H, C and Au atoms, respectively.