R. Czajka

Preparation of an Ultraclean and Atomically Controlled Hydrogen-Terminated Si(111)-(1× 1) Surface Revealed by High Resolution Electron Energy Loss Spectroscopy, Atomic Force Microscopy, and Scanning Tunneling Microscopy: Aqueous NH4F Etching Process of Si(111)

We propose an improved wet chemical process for preparing a high-quality hydrogen-terminated Si(111)-(1× 1) surface and show an atomically ordered and ultraclean surface without carbon and oxygen contamination. The vibrational properties and surface morphology are investigated by high-resolution electron energy loss spectroscopy (HREELS), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). The HREELS spectra and images of AFM and STM reveal the precise aqueous NH4F etching process of Si(111) and indicate the high controllability of steps and terraces at the atomic scale. The surface cleanliness and morphology strongly depend on the etching time. At the etching time of 10 min, we obtain an ultraclean and atomically ordered surface with wide terraces of 36±7 nm step distance. It is confirmed by AFM and STM that 1.0% ammonium sulfite is useful for removing dissolved oxygen in the 40% NH4F etching solution and for preparing a high-quality H:Si(111)-(1× 1) surface with a low density of etch pits. The onset of tunneling current and the gap of 1.39 eV are measured by scanning tunneling spectroscopy. There is no peak at -1.3 eV in comparison with the previous report [Phys. Rev. Lett. 65 (1990) 1917].

The solid state reaction of Fe with the Si(111) vicinal surface: splitting of bunched steps

The solid state reaction of deposited Fe (four monolayers, ML) with vicinal Si(111) substrate induced by subsequent thermal treatment has been studied using scanning tunnelling microscopy. At the lower range of annealing temperatures up to 400u2009°C the bunched steps of bare substrate are reproduced by the surface of the covering iron silicide layer. At 400u2009°C the onset of three-dimensional growth of iron silicide islands is observed. In comparison to the samples covered with smaller amounts of Fe it appears at a lower annealing temperature. Above 500u2009°C the bunched steps split into lower ones but more densely distributed due to proceeding reactions between Fe-rich iron silicide and Si substrate. As a consequence, at 700u2009°C the well-developed three-dimensional nanocrystallites of iron silicide are randomly distributed on the Si surface. This observation is in contrast to the formation of a regular array of iron silicide crystallites upon deposition of 2xa0ML of Fe.

Deposited microclusters and their interaction with substrate

Au, C and Sb clusters of nanometer size deposited on silicon and graphite substrates are investigated by means of a scanning tunnelling microscopy (STM). Both deposition and measurements were carried out in an ultrahigh vacuum (UHV) system at room temperature. The apparent height of the clusters, which reflects not only geometry but also their electronic density of states, changes markedly while scanning at different voltage biases. Such behaviour depends significantly on the cluster size, their chemical composition and the type of substrate. We discuss the properties of clusters in terms of their size and modifications by the substrate.

Gold clusters deposited on highly oriented pyrolytic graphite by pulse laser ablation and liquid metal ion source

Abstract We compare the shape and size distribution of supported Au clusters on highly oriented pyrolytic graphite prepared by two kinds of cluster beam techniques, liquid metal ion source and pulsed laser ablation. Scanning tunneling microscopy was applied in air to give topographic images of supported Au clusters. The liquid metal ion source technique gives rise to the quasi-two-di-mensional supported Au clusters with a narrow size distribution centered at 5.3 nm at the low coverage, which is explained by a high growth rate at the cluster periphery, resulting from the large diffusion of metallic atoms on the graphite surface. In contrast, the laser ablation technique yields the three-dimensional and smaller clusters formed by solidifying the incident hot Au clusters onto the substrate during the laser ablating process.

TUNNELING SPECTROSCOPY OF NANOMETER-SIZE CLUSTERS DEPOSITED ON GRAPHITE

The electronic structures of Sb, Ni, and Au clusters of nanometer size are discussed. Clusters were deposited on a graphite substrate and analyzed with a scanning tunneling microscope in an ultrahigh-vacuum system. Scanning at constant-current mode at different bias voltages was used as a method of spectroscopic measurements. Disappearing of the clusters images at certain range of voltage biases is attributed to occurrence of the energy gap in electronic structure of clusters, suggesting their nonmetallic behavior.

Au clusters deposited on Si(111) and graphite surfaces

Abstract The shape, size and electronic properties of Au cluster deposited onto Si(111) and graphite substrates are analyzed by scanning tunneling microscopy (STM) in ultrahigh vacuum. Deposits are sputtered from our Au STM tip by applying a sequence of voltage pulses between the tip and the substrate. Their shapes are dome type with well-controlled diameter in the nanometer scale. This technique enables us to generate arrays of Au cluster with desired periodicity and size. Such structures are stable and observable many days after deposition. Our results provide a practical example that one can create nanometer-scale structures by STM. The electronic structures of Au deposits is investigated by means of scanning tunneling spectroscopy. The I-V characteristics and CITS measurement reveal peculiar properties because of reduced cluster dimensions.

Structure and stability of microcluster lattice systems

Abstract This paper presents our experimental investigation of the adsorption and interaction of microclusters on a crystalline surface to form regular arrangements. Microclusters were produced and deposited up to a monolayer coverage on the c- plane of graphite substrates by laser ablation or thermal evaporation. Their structures and stabilities were analyzed by mass spectrometry, optical measurements and scanning tunneling microscopy (STM). Results on C, Sb and Se show that clusters, each consisting of several to several hundred atoms, tend to deposit onto the substrate in random shapes and at random sites. The coalescence of clusters begins when their coverage on the substrate approaches a monolayer. Generally speaking, our STM measurements show no definite evidence for particular lattice structures or defects on the substrate for preferential deposition sites of clusters. In some cases, however, clusters of approximately equal size and separation deposit regularly in one or two dimensions. For Se, for example, some STM images show a two-dimensional rectangular lattice consisting of Se clusters in the configuration of eight-membered rings. The structure of point defects and grain boundaries found in the rectangular lattice suggest that the regular arrangement is accomplished by mutual interaction among clusters as well as their interactions with the substrate lattice.

STM/AFM Observations of Co/Cu Magnetic Multilayers

UHV deposited magnetic Co/Cu multilayers were investigated by means of scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Surface of the sample i.e. upper covering layer in plane configuration and individual sublayers in cross-section configuration were investigated. A possibility of structure characterization of metallic multilayers by STM and AFM in the cross-section configuration is demonstrated. PACS numbers: 68.65+g , 61.16.Ch, 68.55.Jk

FORMATION AND MODIFICATION OF MESOSCOPIC STRUCTURES ON GRAPHITE (HOPG) AND SILICON SURFACES BY MEANS OF SCANNING TUNNELING MICROSCOPY

This paper presents results of our experimental investigations of the adsorption and interaction of microclusters on some crystalline surfaces to form regular arrangements. Microclusters were produced and deposited up to a monolayer coverage on the c-plane of graphite (HOPG) or Si(111) substrates by thermal evaporation, laser ablation, or deposition from STM tip. A rectangular lattice arrangement of Sen(n=5–8) ring cluster has been fabricated for the first time on the HOPG. Also, arrays of Au clusters with a well-controlled diameter, desired periodicity, and size have been obtained by applying a sequence of voltage pulses between the STM tip and the substrate. A variety of carbon clusters have been produced by laser modification of C60 fullerenes, and observed by means of scanning tunneling microscope (STM). Finally, various nanometer-scale structures have been modified by applying different bias voltages (between tip probe and sample) or induced by thermal treatment.