Noncontact to contact tunneling microscopy in self-assembled monolayers of alkylthiols on gold

The mechanical interaction between a scanning tunneling microscopy (STM) probe and hexadecane (C16) alkylthiol molecules in a self-assembled monolayer was investigated by sensing the force during constant current mode STM imaging. The force regime changed from attractive to repulsive over the insulating molecule islands under feedback control of the current. The repulsive force on the molecule was strongly dependent on the setpoint value of the current during STM operation. In our experiments, the threshold for contact was found at a tunneling current of 1 pA when the sample bias is 2 V. At higher current, the apparent height of molecular islands changed logarithmically with current. In addition, the current as a function of applied load revealed a stepwise increase, indicative of discrete molecular tilting events. A tunneling decay constant beta of =0.53+/-0.02 A(-1) was obtained based on the measurement of the height of molecules and the tunneling current.