High Voltage STM Imaging of Single Copper Phthalocyanine

In this chapter experiments done to investigate the scanning tunneling microscope (STM) imaging at near field emission voltages of single Copper Phthalocyanine (CuPc) molecules deposited on Au(111) are presented. An imaging bias voltage range is explored exceeding the standard tunneling imaging conditions going from the threshold of the tunneling junction barrier up to −10.0 V. At this voltage regime current transmitted through the tip-molecule–substrate junction is made not only of tunneling electrons but also of electrons overcoming the tunneling barrier and behaving like free electrons. Our interpretation of the process, enabling the visualization of the electronic cloud of single organic molecules under these conditions, is presented.