MIT Department of Electrical Engineering & Computer Science

Silicon-based Molecular Nanotechnology

Dr. Mark C. Hersam
University of Illinois at Urbana-Champaign

Thursday, April 13, 2000
2:30 PM
Marlar Lounge, Room 37-252

Abstract

Molecular electronics shows great potential as an approach for fabricating nanoelectronic devices and circuits. Despite this potential, many fundamental problems remain unsolved. This paper outlines a three pronged approach that addresses key molecular electronic issues for molecules supported on ultra-high 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 pre-patterned sites. With this technique, norbornadiene (NBE), copper phthalocyanine (CuPc), and C60 molecules have been intentionally isolated into predefined patterns. STM images reveal intramolecular detail and suggest mechanical behavior such as molecular rotation.

Secondly, using STM spectroscopy, molecules' electronic properties have been revealed. Filled state tunneling conductance maps of CuPc molecules exhibit an enhanced density of electronic states. In empty states, the four-fold symmetry and central copper atom of CuPc are clearly observed. The spatial tunneling conductance maps of CuPc at positive sample bias illustrate charge transfer from the surrounding substrate when the molecule is bound to the surface via its central copper atom. C60 molecules also display intramolecular structure in topographic images and spectroscopic data. The local density of states (LDOS) of C60 clearly shows the location of the lowest unoccupied molecular orbital (LUMO), which suggests that the highest occupied molecular orbital (HOMO) is located within ~0.3 eV of the fermi level.

Finally, an all-UHV scheme for isolating and, ultimately, electrically contacting STM-patterned nanostructures has been developed that utilizes a pre-defined 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.