PHYSICAL PARAMETERS THAT CONTROL THE IMAGING OF PURPLE MEMBRANES WITHTHE SCANNING TUNNELING MICROSCOPE

The dominant physical and geometrical parameters for imaging hydrated purple membranes with the scanning tunneling microscope are identified and characterized. These are;sample voltage, current density, and tip radius. Purple membranes are imaged with vertical and lateral resolut ion of 0.7 nm and 15 nm, respectively. We also found that the apparent height of the membranes depends on the tip-substrate separation. Thes e experiments pose the problem of electron transport through 5-10 nm t hick insulating materials. We propose a model where the contrast mecha nism is controlled by two factors, the electric field at the interface and the transmission through empty states in the membrane.