Compact, single-tube scanning tunneling microscope with thermoelectric cooling.

We have designed and built a scanning tunneling microscope with a compact inertial-approach mechanism that fits inside the piezoelectric scanner tube. Rigid construction allows the microscope to be operated without the use of external vibration isolators or acoustic enclosures. Thermoelectric cooling and a water-ice bath are used to increase temperature stability when scanning under ambient conditions.

The role of defects in the reaction of chlorine atoms with alkanethiol self-assembled monolayers.

Scanning tunneling microscopy (STM) in ultra-high-vacuum is used to investigate the reaction of gas-phase atomic chlorine with octanethiolate self-assembled-monolayers on Au(111). Exposure to Cl atoms results in the formation of a variety of surface defects, and eventually leads to a complete loss of order within the alkanethiolate monolayer. X-ray photoelectron spectroscopy and thermal desorption mass spectrometry show that these morphological changes are accompanied by significant chlorination of the monolayer as well as a ~30% decrease in the amount of adsorbed sulfur. The rate of reaction is measured through the analysis of sequences of STM images, and coverage-vs.-exposure data shows that the average reactivity of any given molecule within the monolayer decreases as the reaction progresses. Working with the assumption that monolayer defects created by Cl-atom reaction will affect the reactivity of neighboring molecules, a kinetic Monte Carlo simulation shows the data are consistent with defect sites inhibiting reaction rate by a factor of 5 or more. This behavior is opposite to that found for hydrogen-atom reactions, where edge and defect sites were far more reactive. The dynamics of chlorine-atom reactivity are described primarily in terms of the formation and subsequent reaction of surface-adsorbed radicals, with surface defects providing sites where these radicals can be quenched.