Investigating the Dynamics of Saturated Hydrocarbons on Si(100) with Scanning Tunneling Microscopy and Density Functional Theory

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A novel cryogenic variable temperature UHV STM has been constructed and utilized to investigate the dynamical behavior of isolated organic molecules covalently bound to silicon surfaces. The microscope can be operated from 8-300 K, and exhibits extremely low drift rates. A new design has been implemented for the rails (used in the inertial translation of a sample holder), utilizing a TiN coating to reduce wear. This microscope was used to study the desorption of saturated hydrocarbons on the Si(100) surface. This class of molecules has been reported to be stable during current flow for applied voltages between -2 and -4 V. Cyclopentene is a prototypical example of a saturated hydrocarbon, losing its p-character after reacting with the silicon dimer. Unexpectedly, desorption is observed at voltages as low as -2.5 V, albeit the desorption yields are a factor of 500 to 1000 lower than previously reported for unsaturated molecules. The desorption data is consistent with a single electron resonance mechanism, and a wide variety of computational techniques were employed to model this process. The low threshold voltage can be attributed to hybridization of the molecule with the silicon surface, which results in low-lying ionic resonances within 2-3 eV of the Fermi level. These resonances are long-lived, spatially localized on the silicon dimer, and displaced in equilibrium with respect to the neutral state. Electronic excitation of the molecule results in symmetric (positive ion) or asymmetric (negative ion) motion of the silicon dimer. Excitation of a cyclopentene molecule also occasionally results in the dissociation of C5H8 into C5H7 and hydrogen. To achieve dissociation without subsequent desorption of the products, a second feedback loop was employed to detect sudden changes in the molecule and immediately stop the flow of electrons. In the case of cyclopentene dissociation, this technique directly enabled the subsequent study of the byproducts, and the results were compared to DFT calculations. Feedback control has also been utilized to trap cyclopentene in a local minimum state following excitation. Overall, these results highlight the importance of nuclear dynamics in silicon-based molecular electronics, and hint at guidelines for the control of such dynamics.

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  • 08/30/2018
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