Paper SS2-MoA6
Feedback-Controlled Single Molecule Surface Chemistry

Monday, October 15, 2007, 3:40 pm, Room 611

In the past 25 years, the scanning tunneling microscope (STM) has enabled the detailed study of the chemistry and physics of single molecules on surfaces. Electron-driven processes (including desorption and dissociation) are especially advantageous because they offer the possibility of rapidly exciting a molecule far from equilibrium with exceptional spatial localization of the excitation.¹ Attaining precise control over the electron dose requires a method for both detecting the desired events and rapidly terminating the flow of electrons to prevent overdosing. A significant advance in this area was Feedback Controlled Lithography (FCL),² which involved the creation of isolated reactive sites on H:Si(100) through the controlled desorption of hydrogen. Since reaction products could also be susceptible to electrons, the ability to both detect a molecular conformational change and immediately terminate the flow of electrons is fundamentally relevant to the study of single-molecule processes. In this study, we apply this technique to the investigation of the byproducts of cyclopentene desorption³ from clean Si(100). Experiments were performed using a cryogenic ultra-high vacuum (UHV) STM operating at 8 K and 80 K. At low temperatures, cyclopentene molecules are controllably desorbed, and a feedback loop is utilized to detect the desorption event and halt electron flow. At the desorption conditions of –4 V and 2 nA, the desorption reaction alternately results in three distinct surface features: a clean silicon dimer (55 %), a half-dimer dark feature (30 %), and fully darkened silicon dimer (15 %). Additionally, the radial and angular distributions of the byproduct binding sites were also measured. The desorption products were often observed at significant distances from the initial desorption site, with some features as far as 3 dimer rows (~23 Å) away. The dark desorption products are attributed to hydrogen-passivated silicon atoms resulting from the dissociation of a cyclopentene C-H bond and the subsequent bonding of the ejected hydrogen with the reactive silicon surface. Finally, tunneling electrons from the STM tip were used to induce hopping and desorption of hydrogen from the partially passivated silicon dimers.

¹A. J. Mayne et al., Chemical Reviews 106, 4355 (2006).
²M. C. Hersam et al., Nanotechnology 11, 70 (2000).
³N. L. Yoder et al., Physical Review Letters 97, 187601 (2006).