A novel mechanical micropositioner has been developed for producing exact linear displacements in the nm range. Incorporated in a low temperature scanning tunneling microscope (STM), it is used as a coarse approach mechanism for the tunneling tip. The advantage of the design is the high accuracy of the linear motion and the absence of backlash. The design principle is discussed and its positioning accuracy and stability for STM imaging are demonstrated.

Scanning tunneling microscopy at T=4 K has been used to determine directly the binding site of a molecule chemisorbed on a metal surface, namely, ethene on Cu{110}, by simultaneous imaging of the adsorbate and the underlying lattice. The molecule is found to bond in the short bridge site on the close-packed rows with its C-C axis oriented in the [110] direction.

The clean Au{100} surface is known to be reconstructed, forming a pseudohexagonal (5×27) outermost layer. This structure is observed both in ultrahigh vacuum (UHV) and in the electrochemical environment at potentials corresponding to small negative surface electronic charges. Using a UHV scanning tunneling microscope (STM) at 77 K we have observed that the reconstruction can be lifted at large positive sample biases. The 20% less dense bulk-terminated surface is produced and the excess material appears as irregularly shaped gold clusters. Over a period of a few minutes, however, the surface relaxes back to the pseudo-hexagonal phase, a process that can also be followed with the STM.

Using a low temperature scanning tunneling microscope (STM) we have imaged isolated oxygen-related features on the unreconstructed surface between (2 × 1)-O regions on Cu{110}. On the assumption that these are individual oxygen atoms, they appear to occupy mostly hollow sites in the troughs. We suggest that such adatoms constitute an active form of oxygen known to participate readily in deprotonation reactions on copper surfaces.

We observe clearly resolved diffraction peaks in the scattering of argon from a 2H-W(100) surface, demonstrating the importance of quantum effects even for relatively heavy atoms. The argon diffraction peak intensities decrease exponentially with substrate temperature in a manner consistent with a surface Debye temperature of ~ 400 K. For large incidence angles, (e.g. θi = 60°), angular distributions are observed to consist of a single quasi-specular lobe which narrows substantially with incidence energy. In contrast, for θi = 30°, a rainbow distribution is found with the angular separation of the two lobes decreasing with increasing energy. These observations are discussed in terms of classical and quantum models of gas-surface scattering.

We have obtained images of individual Xe atoms absorbed on a Ni(110) surface using a low-temperature scanning tunneling microscope (STM). The atom-on-jellium model has been used to calculate the apparent height of a Xe atom as imaged with the STM and the result is found to be in good agreement with experiment. We conclude that the Xe 6s resonance, although lying close to the vacuum level, is the origin of the Fermi-level local state density which renders Xe "visible" in the STM.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation C. T. Rettner, C. B. Mullins, D. S. Bethune, D. J. Auerbach, E. K. Schweizer, W. H. Weinberg; Molecular beam studies of trapping dynamics. Journal of Vacuum Science & Technology A 1 May 1990; 8 (3): 2699–2704. https://doi.org/10.1116/1.576653 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAVS: Science & Technology of Materials Interfaces and ProcessingJournal of Vacuum Science & Technology A Search Advanced Search |Citation Search

SINCE its invention in the early 1980s by Binnig and Rohrer1,2, the scanning tunnelling microscope (STM) has provided images of surfaces and adsorbed atoms and molecules with unprecedented resolution. The STM has also been used to modify surfaces, for example by locally pinning molecules to a surface3 and by transfer of an atom from the STM tip to the surface4. Here we report the use of the STM at low temperatures (4 K) to position individual xenon atoms on a single-crystal nickel surface with atomic pre-cision. This capacity has allowed us to fabricate rudimentary structures of our own design, atom by atom. The processes we describe are in principle applicable to molecules also. In view of the device-like characteristics reported for single atoms on surfaces5,6, the possibilities for perhaps the ultimate in device miniaturization are evident.

Thermal energy helium and neon diffraction have been used to follow the phase transition of the W(100) surface which reconstructs to a c(2×2) structure on cooling below room temperature. In the He case we have confirmed previous observations that the reconstruction-induced half-order peaks broaden and shift towards the specular peak on heating through this transition. However, the corresponding Ne diffraction peaks show no appreciable shift or broadening under identical conditions, indicating that thermal He and Ne atoms probe surface structures with different periodicities. Since we would expect that the impinging He and Ne atoms probe quite similar potentials, this result is surprising and the first report for such behavior. These findings are consistent with an anomalous He scattering structure factor or the diffraction of He from a charge-density wave.

Helium and neon diffraction have been used to follow the phase transition of the W(100) surface, which reconstructs to a c(2 × 2) structure on cooling below room temperature. For the case of He, we find that the reconstruction-induced half order peaks broaden and shift towards the specular peak on heating through this transition, in agreement with the observations of others. However, the corresponding Ne diffraction peaks show no appreciable shift or broadening under identical conditions, indicating, that thermal He and Ne atoms probe surface structures with different periodicities.

Molecular-beam techniques have been employed to probe various aspects of the dynamics of the dissociative chemisorption of N2 on W(100). The dissociative chemisorption probability has been measured over a wide range of incidence energies Ei, angles θi, and surface temperatures Ts. This probability is found to fall rapidly with increasing Ei in the range 26–450 meV, where it is also found to fall rapidly with Ts, and to be relatively insensitive to surface coverage at low Ts. In addition, resu1ts are found to be surprisingly insensitive to the incidence angle over the range 0°–70°. This behavior has been interpreted in terms of a precursor model, noting that the trapping process must scale quite closely with the total incidence energy. Support for these conclusions comes from careful measurements of the angular and velocity distributions of scattered molecules which has also revealed that increasing Ts primarily serves to reduce the fraction of precursor molecules that go on to dissociate, by biasing the kinetics in favor of desorption. In contrast, the trapping probability into the precursor state is found to be relatively insensitive to Ts, accounting for ≲20% of the observed effects. Measurements have also been made of the trapping probability of N2 on the W(100) surface covered with N and N2 species. We find that while trapping is insensitive to coverage for Ei <0.1 eV, it varies appreciably with coverage at higher energies; being highest on the surface saturated with both atoms and molecules followed by the clean surface, with the lowest trapping probability on the surface saturated with atoms.

Molecular beam techniques have been used to probe the dynamics of the trapping and trapping–desorption of Ar at a hydrogen-saturated W(100) surface. Trapping probabilities have been measured as a function of incidence energy Ei, and angle θi for a surface temperature Ts of 85 K. We find that this probability scales approximately with Ei cos θi, rather than Ei or the so-called ‘‘normal energy’’ Ei cos2 θi. Trapping probabilities approach unity for low energies, falling to 0.5 and 0.05 for Ei cos θi ∼30 and 100 meV, respectively. The time-of-flight distributions of scattered Ar are clearly bimodal in many cases, having both direct–inelastic and trapping–desorption components. The latter component has been characterized over a wide range of conditions to provide information on the desorption of Ar from this surface. We find that desorbing species emerge with a near-cosine angular distribution for Ts ≂85 K. However, these distributions become increasingly noncosine as Ts is raised, becoming substantially broader than cosine. In addition, at the lowest temperature employed (∼85 K), the velocity distributions of the desorbing atoms are well described by Maxwell–Boltzmann distributions characteristic of the surface temperature. At higher temperatures, these distributions are still approximately Boltzmann, but the characteristic temperature falls below Ts. The ‘‘lag’’ between this effective temperature and Ts increases with Ts and is most pronounced for atoms desorbing at angles close to the normal. We show that the desorption results are very close to the predictions of a model in which angular and velocity distributions for desorption are synthesized by applying detailed balance arguments to the trapping data. Similarly the trapping results are close to trapping curves extracted from the desorption data.

Angular and velocity distributions have been obtained for the scattering of argon and N2 from a W(100) surface for incidence energies, Ei, in the range 0.03–5.5 eV, and surface temperatures, Ts, from 90 to 1700 K. For Ei < 0.1 eV, we find broad angular distributions (> 60° FWHM) which are relatively insensitive to Ts and velocity distributions which are inconsistent with parallel momentum conservation, indicating a relatively corrugated interaction potential. Increasing Ei first causes a rapid narrowing in these distributions, but as Ei exceeds ~ 2 eV, they broaden again, as the effective corrugation again becomes large.

The dissociative chemisorption of N2 on W(100) is found to fall rapidly with increasing kinetic energy Ei in the range 26 to 450 meV. For a surface temperature Ts of 300 K, the initial dissociative chemisorption probability S0 falls from ∼0.8 at Ei=26 meV to 0.15 at 450 meV. Over this range of energies the dissociation probability is also found to fall rapidly with Ts, and to be relatively insensitive to surface coverage at low Ts, strongly suggesting that dissociation occurs in this system via a precursor under these conditions. This picture is supported by angular distribution measurements of the scattered molecules which are consistent with an appreciable cosine component, which also becomes smaller as Ei increases. Results are found to be surprisingly insensitive to the incidence angle over the range 0° to 70°, indicating that the trapping process scales quite closely with the total incidence energy.