Isolating, imaging, and electrically characterizing individual organic molecules on the Si(100) surface with the scanning tunneling microscope

Molecular electronics shows great potential as an approach for fabricating nanoelectronic devices and circuits. Despite this potential, many fundamental problems remain unsolved. This article outlines a three pronged approach that addresses key molecular electronic issues for molecules supported on ultrahigh vacuum scanning tunneling microscopy (UHVSTM) patterned hydrogen passivated Si(100) surfaces. First, feedback controlled lithography (FCL) has been developed as a reliable technique for making templates of individual dangling bonds on the Si(100)-2x1:H surface. FCL detects individual H desorption events while patterning, thereby compensating for variations in tip structure. When the surface is then exposed to a flux of molecules, they bind individually to the prepatterned sites. With this technique, norbornadiene and copper phthalocyanine (CuPc) molecules have been intentionally isolated into predefined patterns. STM images reveal intramolecular detail and suggest mechanical behavior such as molecular rotation. Second, using STM spectroscopy, molecules' electronic properties have been revealed. Filled state tunneling conductance maps of CuPc molecules exhibit an enhanced density of electronic states. However, in empty states, a ring of reduced local density of states surrounds each CuPc molecule. Finally, an all-UHV scheme for isolating and, ultimately, electrically contacting STM-patterned nanostructures has been developed that utilizes a predefined p-n junction on a Si(100) substrate. With STM potentiometry, the junction is easily located, allowing for efficient registration of nanostructures after intermediate processing steps. In addition, by STM patterning across the depletion region, the electrical properties of selectively deposited nanostructures can be directly evaluated when the p-n junction is reverse biased. (C) 2000 AmericanVacuum Society. [S0734-2101(00)02604-X].