Influence of tunneling voltage on the imaging of carbon nanotube rafts by scanning tunneling microscopy

The influence of bias voltage on the quality of scanning tunneling microsco py images of carbon nanotube "rafts" was investigated in the range from -1 to 1 V in combination with scanning tunneling spectroscopy (STS) measuremen ts. While for positive tip polarity only a slight voltage dependence was fo und in the image quality, for negative polarity a strong increase of the no ise was observed with increasing voltage. STS showed that, for negative tip polarity, the tunneling current may be different in different locations by several orders of magnitude. (C) 1998 American Institute of Physics. [S000 3-6951(98)05151-1].