O. Custance

Atomic force microscopy as a tool for atom manipulation

During the past 20 years, the manipulation of atoms and molecules at surfaces has allowed the construction and characterization of model systems that could, potentially, act as building blocks for future nanoscale devices. The majority of these experiments were performed with scanning tunnelling microscopy at cryogenic temperatures. Recently, it has been shown that another scanning probe technique, the atomic force microscope, is capable of positioning single atoms even at room temperature. Here, we review progress in the manipulation of atoms and molecules with the atomic force microscope, and discuss the new opportunities presented by this technique.

Initial stages of Sn adsorption on Si(1 1 1)-(7×7)

Abstract By means of scanning tunneling microscopy (STM) we have performed a detailed analysis of the initial stages of Sn adsorption on Si(1xa01xa01)-(7×7) at room temperature (RT). At very low coverages ( Θ ≈ 0.01 ML) single Sn atoms as well as clusters of two Sn atoms can be resolved inside the (7×7)-half-cells. According to our STM measurements, single Sn atoms are highly mobile at RT inside the (7×7)-half-cells. This thermally activated motion, faster than the scanning speed, results in a characteristic fuzzy appearance of the half-cell. This behavior was first reported for the Pb/Si(1xa01xa01)-(7×7) system and has since then been observed in the initial stages of growth at RT of other adsorbates on Si(1xa01xa01)-(7×7) surfaces. For the Sn/Si(1xa01xa01)-(7×7) system, by combining real-time STM observations with measurements of cluster size distributions we have been able to gain some insight into the mechanisms involved in the first stages of growth. In particular, the differences observed in relation to the Pb/Si(1xa01xa01)-(7×7) are discussed.

Low temperature phases of Pb/Si(1 1 1)

Abstract In this work the low temperature phases of Pb/Si(1xa01xa01) have been investigated by means of variable temperature scanning tunneling microscopy (STM). Our STM measurements, performed in the range of 55–215 K, show that two different phases, i.e. Pb/Si(1xa01xa01)-(3×3) and Pb/Si(1xa01xa01)- 3 2 −1 1 , coexist for samples with a nominal Pb coverage below 1 ML. The general morphology as well as the atomic structure of both surface reconstructions are discussed and compared to previous results.

Field emission interferometry with the scanning tunneling microscope

A scanning tunneling microscope, operated in the near field emission regime, is used to obtain the phases of very low energy electrons reflected from a sample surface. A simple theoretical model shows that the spectrum of the electron standing waves, formed in the vacuum gap between the tip probe and the sample, is directly related to the complex amplitudes of the reflected electron waves. The surface sensitivity of the interferometric spectra is demonstrated in the analysis of different reconstructions of the Pb/Si(111) system.

STM study of dynamical effects on submonolayer phases of Pb/Si(1 1 1)

Abstract By means of STM we have been able to analyze in real time the complex dynamical processes that take place at room temperature in the interface between Pb/Si(1xa01xa01)-(1×1) and 1/3 ML α-Pb/Si(1xa01xa01)-(√3×√3) R 30° reconstructions. We have found that the border between these reconstructions is highly mobile, showing a fluctuating character between 1×1 regions and defect free areas presenting the α-(√3×√3) R 30° reconstruction. These fluctuations involve a large number of atoms in cooperative movement as well as atom recombination in the border of the two reconstructions. The intrinsic character of these effects versus possible STM influence has been analyzed and discussed, and a field-induced diffusion mechanism is tentatively suggested.

Scanning tunnelling microscopy and spectroscopy on organic PTCDA films deposited on sulfur passivated GaAs(001)

Deposition of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on sulfur passivated gallium arsenide S-GaAs(001) surfaces is investigated by scanning tunnelling microscopy and scanning tunnelling spectroscopy. The surface morphology and the film structure are studied for the multilayer growth of the organic molecules. Spectroscopic results for both clean substrate and ordered areas of PTCDA are shown in this work. We have measured I–V plots at different tip–sample distances, avoiding deformation of the organic layer. Under proper experimental conditions, a gap value of 2.2 eV has been measured on PTCDA crystals, in good agreement with the expected value for PTCDA (2.2–2.55 eV).