Molecular-resolution imaging of insulating macromolecules with the scanning tunneling microscope via a nontunneling, electric-field-induced mechanism

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Abstract
The scanning tunneling microscope (STM) has been used to obtain molecular-resolution images of a film of hydroxypropylcellulose at least hundreds of angstroms thick in ultrahigh vacuum. Microfibrils with a periodicity of 34 Å were observed directly. The pulsing current used to obtain these images was drastically different from the stable direct current in conventional STM imaging of conducting substrates based on tunneling. An electric-field-induced impurity-mediated conduction in insulating thin-film materials is invoked to explain the occurrence of the pulsing current used for imaging. The experimental resolution in the images was shown to be consistent with the proposed mechanism.