Silicon based molecular nanotechnology-free research paper
One potential application of molecular nanotechnology is the integration of molecular electronic function with advanced silicon technology. One step in this process is the tethering of individual molecules at speciﬁc locations on silicon surfaces. This paper reports the fabrication of arrays of individual organic molecules on H-passivated Si(100) surfaces patterned with an ultrahigh vacuum scanning tunnelling microscope (STM). Feedback controlled lithography (FCL) is used to create templates of individual silicon dangling bonds. Molecules introduced in the gas phase then spontaneously assemble onto these atomic templates. Norbornadiene (NBE), copper phthalocyanine (CuPc), and C60 molecular arrays have been made by this technique and studied by STM imaging and spectroscopy. Both NBE and CuPc molecules appear as depressions in empty states images, whereas in ﬁlled states images they are nearly indistinguishable from Si dangling bonds. Furthermore, the fourfold symmetry and central copper atom of CuPc are clearly observed at positive sample bias. Spatial tunnelling conductance maps of CuPc illustrate charge transfer from the surrounding substrate when the molecule is bound to the surface via its central copper atom. On the other hand, when the CuPc molecule interacts with the substrate via an outer benzene ring, molecular rotation is observed. C60 molecules display intramolecular structure in topographic images and spectroscopic data. The local density of states of C60 clearly shows the location of the lowest unoccupied molecular orbital, which suggests that the highest occupied molecular orbital is located within 0.3 eV of the fermi level.
Molecular electronics is currently one area being studied as a potential successor to conventional silicon-based electronics technology . One aspect of molecular electronics is the fabrication of devices whose function is governed by single molecules. Though quite promising, single molecule devices present fundamental new challenges, thereby placing estimates of their mainstream applications around the middle of the 21st century . Nevertheless, tools such as the ultra-high vacuum scanning tunnelling microscope (UHV STM) are available to begin exploring this fascinating ﬁeld. This paper describes a new approach for creating arrays of individual molecules with atomic precision on the technologically important Si(100) surface. The merger of molecular electronics with silicon-based technology represents a hybrid approach with potential nearterm applications. The approach described in this paper is that of utilizing the chemical contrast between clean and hydrogen passivated Si(100) to create templates for molecular adsorption. For this purpose a variant of the hydrogen resist STM nanolithography technique , termed feedback controlled lithography (FCL) is used to generate † To whom correspondence should be addressed. Figure 1. Timeline of anticipated eras for future electronic technologies. This ﬁgure is reproduced from the Proceedings of the research . arbitrary arrays of individual dangling bonds on the Si(100)- 2X1:H surface. The surface is then dosed with appropriately chosen organic molecules that will bind only at the prepatterned dangling bond sites . Previous STM studies of norbornadiene (NBE), copper phthalocyanine (CuPc) , and C60 suggest the compatibility of these molecules with the experiments of this paper