Scanning tunneling microscopy of 1, 2, and 3 layers of electroactive compounds

Bilayer and trilayer organic films grown on Au(111) were studied by scanning tunneling microscopy (STM). Studies were carried out under UHV conditions with the sample cooled to either 80 or 100K. Cobalt(II)phthalocyanine [CoPc] was deposited from vapor onto Au(111), followed by vanadyl phthalocyanine, VOPc. CoPc coverages studied were 0.5 and 1 monolayer, while VOPc coverages were about 0.5 monolayer. Constant current images were acquired at high tunneling gap resistance, of the order of 30GOmega. Two different physical structures were observed for VOPc on CoPc, and each had a characteristic I(V) curve indicating significantly different unoccupied state density. It is also demonstrated that the transmission factor for two layers of VOPc is not simply the product of the transmission factors for each layer.