The development of molecular-scale electronic devices has made considerable progress over the past decade, and single-molecule transistors, diodes and wires have all been demonstrated. Despite this remarkable progress, the agreement between theoretically predicted conductance values and those measured experimentally remains limited. One of the primary reasons for these discrepancies lies in the difficulty to experimentally determine the contact geometry and binding configuration of a single-molecule junction. In this Article, we apply a small-amplitude, high-frequency, sinusoidal mechanical signal to a series of single-molecule devices during junction formation and breakdown. By measuring the current response at this frequency, it is possible to determine the most probable binding and contact configurations for the molecular junction at room temperature in solution, and to obtain information about how an applied strain is distributed within the molecular junction. These results provide insight into the complex configuration of single-molecule devices, and are in excellent agreement with previous predictions from theoretical models.
a, Schematic of the experimental set-up showing a C6-DT molecule bound to two Au electrodes as a small, sinusoidal mechanical modulation is applied while withdrawing the tip electrode. b, Example of a current versus distance trace obtained during tip withdrawal (blue trace) showing no steps in the current. The normalized current (α) at the modulation frequency (2 kHz) is extracted from this trace (grey curve). This experiment was performed on C6-MT; no steps occur in the decay curve and the amplitude of α is approximately constant because the molecule is incapable of forming a junction. c, Histogram from thousands of C6-MT traces similar to those shown in b. The black trace shows the most probable value of α at each conductance. The green trace shows the value of β obtained by averaging thousands of current versus distance traces and taking the derivative. d, Representative conductance versus distance (blue) and α versus distance (grey) curves measured during electrode separation for a C6-DT junction. e, 2D histogram for C6-DT showing a maximum population at the conductance of ~3 × 10−4G0 and the corresponding α of ~0.8 nm−1. G0 is the conductance quantum equal to 2e2/h, where e is electron charge and h is Planck’s constant.
Figure 2: STM break-junction measurements of the alkanes.
a–c, 2D histogram for C8-DT showing α as a function of conductance (a). The total counts in a can be projected onto each axis to determine the most likely value of α (b) or conductance (c). The red dashed lines are guides to the eye. d, The natural logarithm of conductance is plotted versus molecular length for both the dithiol (blue) and diamine (red) families from C6 to C10. Error bars represent the standard deviation of the maximum conductance values obtained from six experiments for each molecule. The slopes from the linear fittings of each family are used to determine β.
