Charge Redistribution and Transport in Molecular Contacts

The forces between two single molecules brought into contact, and their connection with charge transport through the molecular junction, are studied here using non contact AFM, STM, and density functional theory simulations. A carbon monoxide molecule approaching an acetylene molecule (${\mathrm{C}}_2{\mathrm{H}}_2$) initially feels weak attractive electrostatic forces, partly arising from charge reorganization in the presence of molecular . We find that the molecular contact is chemically passive, and protects the electron tunneling barrier from collapsing, even in the limit of repulsive forces. However, we find subtle conductance and force variations at different contacting sites along the ${\mathrm{C}}_2{\mathrm{H}}_2$ molecule attributed to a weak overlap of their respective frontier orbitals.

Figure 1

(a), (b) Constant current STM images taken on CO and ${\mathrm{C}}_2{\mathrm{H}}_2$ molecules in different sample positions at $V=100\mathrm{mV}$ and $I=0.15\mathrm{nA}$. These images [(a) and (b)] were measured, respectively, with the tip terminations schematically drawn in (d): a sharp Cu tip (I) and a CO tip (II). The appearance of a ${\mathrm{C}}_2{\mathrm{H}}_2$ with a CO tip and its adsorption geometry from [28] are shown in (c). (e) Topography profiles taken along the yellow lines measured on the ${\mathrm{C}}_2{\mathrm{H}}_2$ in (a) and (b).

Figure 2

(a) Frequency shift ($\mathrm{\Delta }f$) and the corresponding vertical force [20] measured with a CO functionalized tip at $V=0\mathrm{mV}$ as a function of the tip-sample distance ($z$) over the $\mathrm{C}═\mathrm{C}$ bond of a ${\mathrm{C}}_2{\mathrm{H}}_2$ molecule (further details in [22]). $\mathrm{\Delta }f$ is corrected for the macroscopic influence of the tip as explained in [22]. The calculated force is included as dots for comparison. (b) Development of the induced electron density $\mathrm{\Delta }\rho ={\rho }_{\mathrm{tot}}-{\rho }_{\mathrm{surf}}-{\rho }_{\mathrm{tip}}$ and (c) its projection into $z$, $\mathrm{\Delta }{\rho }^{1D}=\int \int \mathrm{\Delta }\rho dxdy$ at three CO-acetylene distances. In (b), we plot $\mathrm{\Delta }\rho$ isosurfaces at $0.0005e/\AA$, where blue and red mean depletion and accumulation of electrons, respectively. Dashed black and yellow lines in (c) show induced electron density of noninteracting free standing acetylene-surface and CO tip, when an electric field $0.1\mathrm{eV}/\AA$ is applied.

Figure 3

(a), (b) Conductance and force maps recorded simultaneously along a ${\mathrm{C}}_2{\mathrm{H}}_2$ molecular axis while applying a bias of $V=80\mathrm{mV}$ to the sample. Similar force maps are obtained at zero bias [22]. The plot in (b) shows the value of the conductance at zero force. $G$ is plotted in logarithmic scale in units of the quantum of conductance $G_0=2e^2/h=(12906\mathrm{\Omega }{)}^{-1}$. Both force and conductance are obtained after deconvolution of the tip oscillation [20, 37]. (c) Conductance, force, and energy curves taken at the $\mathrm{C}═\mathrm{C}$ bond with a CO tip. A change in conductance occurs at the energy minimum.

Figure 4

(a) Development of the electrostatic (Hartree) potential felt by electrons in the gap between the tip and the acetylene molecule on the surface as the tip gradually approaches to the surface. The potentials are plotted along the CO molecular axis. Dashed black and red lines plot the electrostatic potential of the free standing surface and CO tip, respectively. The position of individual atoms of the acetylene molecule on the surface and oxygen atom on the tip at different tip-sample distances is schematically presented. All potentials are rendered with respect to the Fermi level. (b) Spatial distribution of the dominant eigenchannels at the Fermi energy for straight and bent CO configuration at the same tip-sample distance $z=0\AA$.