Imaging of uncoated purple membrane by scanning tunneling microscopy

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Abstract
Scanning tunneling microscopy images of uncoated purple membrane are easily obtained at tunneling currents lower than 1 pA and tunneling voltages above approximately 7 V. However, the images sometimes show positive, sometimes negative contrast. This effect appears to depend only on the shape of the tunneling tip. A simple model, assuming a constant voltage drop across the purple membrane, is proposed which can explain this observation.