Design and control of electron transport properties of single molecules

Abstract

We demonstrate in this joint experimental and theoretical study how one can alter electron transport behavior of a single melamine molecule adsorbed on a Cu (100) surface by performing a sequence of elegantly devised and well-controlled single molecular chemical processes. It is found that with a dehydrogenation reaction, the melamine molecule becomes firmly bonded onto the Cu surface and acts as a normal conductor controlled by elastic electron tunneling. A current-induced hydrogen tautomerization process results in an asymmetric melamine tautomer, which in turn leads to a significant rectifying effect. Furthermore, by switching on inelastic multielectron scattering processes, mechanical oscillations of an N-H bond between two configurations of the asymmetric tautomer can be triggered with tuneable frequency. Collectively, this designed molecule exhibits rectifying and switching functions simultaneously over a wide range of external voltage.

Fig. 1.

STM image and adsorption structure. (A) Topographic image of isolated melamine molecules adsorbed on Cu (100) (V = 0.2 V, I = 0.5 nA). Atomically resolved image of substrate is given in the insert (V = 5 mV and I = 20 nA). The scale for apparent high of the molecule is given. (B) Adsorption structure of single melamine molecule. Hydrogen atoms within the red dashed circle are dissociated when the molecule is adsorbed on Cu (100) at RT, resulting in the dehydrogenated melamine, whose top and side views, as well as simulated STM image, are shown in C–E, respectively.

Fig. 2.

Rectifying and switching behavior of a modified single melamine molecule. (A and B) STM topographic images of the same area in Fig. 1A (V = 0.2 V, I = 0.5 nA) in which molecules 1 and 3 are activated by applying a + 2.4 V pulse on each of them. The black dots marked on the images give the STM tip position at where all conductance measurements are taken. Insert, STM images acquired with a sample bias of −0.8 V. The scale for apparent high of the molecule is given. (C) I-V Curves measured for Cu (black), melamine (red), and melamine-C (blue). Fluctuation features are shown in the inserts. (D and E) Top and side views, respectively, of the computational model for the melamine adsorbed on Cu (100). The dashed line represents the unit cell. Calculated STM images of the optimized structures of melamine-C1 (F) and melamine-C2 (H) are given in G and I, respectively (at V = −0.6 V and height of 7 Å from metal substrate). (J) The calculated I-V curves for dehydrogenated melamine, melamine-C1 and melamine-C2 on Cu (100).

Fig. 3.

Switching behavior with tuneable rates. (A and B) Current traces continuously measured at bias of −0.6 V and −0.9 V. (C) Switching rate, R, in the high conductance state as a function of tunneling current, I, under various sample biases. The solid lines are least-square fits to the data, which follow a power-law, R ∝ I^N. (D) Number of possible inelastic electrons under different bias. L and H refer to the low and high conductance states. The calculated results are presented in dashed-lines. The observed large difference between the positive and negative biases is discussed in SI Text, Figs. S4–S6, and Table S1.

Fig. 4.

Transition path for tautomerization and bond rotation processes. (A) Energy profile for tautomerization and rotation of N-H bond of dehydrogenated melamine molecule adsorbed on Cu (100) surface. (B) Schematic representation of rotation of N-H bond between the two stable configurations. (C) Calculated double well potential connected by melamine-C1 and melanmine-C2 structures and the key parameters that control the inelastic tunneling processes. P is the probability of inelastic tunneling process at external bias E_p, τ is the lifetime of the intermediate state at where the N-H bond has tendency to relax back to its origin. T is the transmission probability.