Intramolecular Force Contrast and Dynamic Current-Distance Measurements at Room Temperature

Scanning probe microscopy can be used to probe the internal atomic structure of flat organic molecules. This technique requires an unreactive tip and has, until now, been demonstrated only at liquid helium and liquid nitrogen temperatures. We demonstrate intramolecular and intermolecular force contrast at room temperature on PTCDA molecules adsorbed on a $\mathrm{Ag}/\mathrm{Si}(111)\text{−}(\sqrt{3}\times \sqrt{3})$ surface. The oscillating force sensor allows us to dynamically measure the vertical decay constant of the tunneling current. The precision of this method is increased by quantifying the transimpedance of the current to voltage converter and accounting for the tip oscillation. This measurement yields a clear contrast between neighboring molecules, which we attribute to the different charge states.

Figure 1

AFM constant height images of PTCDA with (a) intermolecular resolution ($V_b=-0.125\mathrm{V}$, $A=50\mathrm{pm}$) and (b) intramolecular resolution ($V_b=0.1\mathrm{V}$, $A=40\mathrm{pm}$).

Figure 2

(a) STM topography, in which neighboring molecules appear to be similar and (b) simultaneously acquired $\kappa$ image in which there is clear contrast between neighboring molecules ($V_b=-1.2\mathrm{V}$, ${⟨I⟩}_T=200\mathrm{pA}$, $A=50\mathrm{pm}$).

Figure 3

(a) Schematic model of the tip oscillating at an amplitude $A$ and bias $V_b$ over a PTCDA island. (b) Simulated plot of the tunneling current vs time dependence $I(t)$ (black solid line), and the Fourier series of $I$ up to the zeroth order (red dashed line) and first order (green dash-dotted line).