B. C. Stipe

A variable-temperature scanning tunneling microscope capable of single-molecule vibrational spectroscopy

The design and performance of a variable-temperature scanning tunneling microscope (STM) is presented. The microscope operates from 8 to 350 K in ultrahigh vacuum. The thermally compensated STM is suspended by springs from the cold tip of a continuous flow cryostat and is completely surrounded by two radiation shields. The design allows for in situ dosing and irradiation of the sample as well as for the exchange of samples and STM tips. With the STM feedback loop off, the drift of the tip–sample spacing is approximately 0.001 A/min at 8 K. It is demonstrated that the STM is well-suited for the study of atomic-scale chemistry over a wide temperature range, for atomic-scale manipulation, and for single-molecule inelastic electron tunneling spectroscopy (IETS).

Atomistic studies of O2 dissociation on Pt(111) induced by photons, electrons, and by heating

The adsorption and subsequent dissociation of O2 on Pt(111) was studied by variable temperature scanning tunneling microscopy in the temperature range of 40 to 215 K. Tight clustering of bridge site molecules is observed on terraces between 40 and 70 K, indicating a highly mobile precursor to chemisorption. Coexistence of bridge and fcc hollow site molecules in fractal-shaped islands is observed after dosing between 70 and 95 K. Dissociation of these species was induced by uv radiation, inelastic tunneling electrons, and heating. In all three cases, two O atoms are found within two lattice constants of the original molecule and one to three lattice constants apart.

Atomically resolved adsorption and scanning tunneling microscope induced desorption on a semiconductor: NO on Si(111)-(7×7)

Using a variable-temperature, ultrahigh vacuum scanning tunneling microscope (STM), we have studied the adsorption and STM induced desorption of NO from Si(111)-(7×7). NO adsorbs preferentially on faulted corner sites, followed by faulted center sites, unfaulted corner sites and unfaulted center sites. The preference for the different adsorption sites is independent of temperature and correlates well with the local density of states at these sites. NO can be desorbed from Si(111) with the STM. We present data that suggest the desorption is induced by the electric field under the STM tip. The threshold positive electric field for desorption of NO is 0.114 ± 0.009 V/A. For sufficiently small tip–surface distances, NO can be desorbed locally without affecting the neighboring adsorbates.

Inducing and imaging single molecule dissociation on a semiconductor surface: H2S and D2S on Si(111)-7×7

Using a variable-temperature, ultrahigh vacuum scanning tunneling microscope (STM), we have induced and imaged the dissociation of H2S and D2S on Si(111)-7×7. H2S and D2S adsorb dissociatively at low coverage, from 50 to 300 K. Individual HS (or DS) fragments can be further dissociated with the STM at low temperatures without affecting neighboring adsorbates. The hydrogen (deuterium) atom either desorbs or re-attaches to a nearby silicon atom. Near room temperature (297 K) and above, DS dissociates thermally, with an activation barrier of 0.73±0.15 eV. The activation barrier was calculated from atomistic studies of the dissociation rates at temperatures between 297 and 312 K.

Imaging the atomically resolved dissociation of D2S on Si(100) from 80 to 300 K

Using a variable-temperature, ultrahigh vacuum scanning tunneling microscope (STM), we have induced and imaged and dissociation of D2S on Si(100). D2S dissociates into DS and D below 200 K. Individual DS fragments can be dissociated with the STM at low temperatures. The deuterium atom attaches to a neighboring silicon dimer. At 200 K or above, D2S dissociates into S and two Ds. D2S adsorption affects the surface reconstruction on Si(100), from the buckled dimer configuration to the dynamically flipping configuration and vice versa. We discuss our results in the context of other experiments on the same and similar systems.