Field Ion-scanning Tunneling Microscope Research Articles

Field Ion Scanning Tunneling Microscopy and Its Application to Oxygen Adsorption on the Ag(110) 1x1 Surface

Field Ion Scanning Tunneling Microscopy and Its Application to Oxygen Adsorption on the Ag(110) 1x1 Surface

Atom and Electron Dynamics at Surfaces and Effect on Growth of Nanostructures

The dynamics of surface atoms becomes an important issue when the size of material structures is reduced to the nanometer scale. Additionally, quantum effects start to dominate both their properties and their growth behavior at low temperature. Using field ion microscopy (FIM) and scanning tunneling microscopy (STM), we directly observe the atoms, molecules, clusters and electron dynamics on nano surfaces. We focus mostly on studying how the atom and electron dynamics affect the dynamics as well as the growth behavior of nano-islands and clusters on silicon and metal surfaces. Here I summarize our study of the diffusion behavior of H, O2, O and Si magic clusters on the Si(111)-7×7 surface, and also how using the chemical gradient induced surface diffusion of surface atoms, one can create a chemically inert and thermally stable single atom sharp pyramidal tip. In addition, we observe that the low temperature growth of Pb nano islands on the Si(111)-7×7 surface is affected by the electronic standing wave states in the normal direction of these islands.

Toward Single Atom Chemical Analysis with STM

An important function of microscopy is the chemical analysis of the sample. Chemical analysis with an atomic resolution microscope can mean two different things. First, for a sample of known composition or adsorbed species, it means distinguishing the chemical species from the atomic image. Second, for a sample of unknown composition, it means the identification of the chemical components from the atomic resolution image. Here I compare available methods of chemical identification in field ion microscopy (FIM) and scanning tunneling microscopy (STM), and report our progress in achieving true atomic resolution for a non-destructive chemical analysis of a sample surface using STM.[DOI: 10.1380/ejssnt.2003.102]

Atomic, molecular and cluster dynamics on flat and stepped surfaces

Surface diffusion is responsible for transport of atoms at the surface. It is therefore essential for understanding many surface phenomena where transport of atoms and molecules are involved. Using the field ion microscope and scanning tunneling microscope, we have directly observed many elementary surface atomic and molecular processes at terraces and steps and have measured the activation barrier heights of these processes, and have also studied their atomic mechanisms in detail. For Si(111) surface, transport of atoms above 450°C appears to be achieved by magic clusters of about 10–15 atoms in size, and their diffusion behavior is very different from those of individual atoms and molecules. We discuss how atom dynamics affects the growth behavior, island shape transitions, and growth modes in the growth of crystals and epitaxial films.

Si- and C-rich structure of the 6H-SiC(0001) surface

The reconstructions of the 6H-SiC(0001) surface under both Si-rich and C-rich conditions were studied using field ion-scanning tunneling microscopy (FI–STM). The sample was cleaned by in situ Si beam etching at 900–1000 °C. The as-cleaned surface showed a (√3×√3) structure. The Si-rich phases were produced by annealing the sample in a Si flux. With increasing Si concentration, (2×2), (2√3×6√3), (3×3), and (7×7) reconstructions were observed. Reaction of the Si-rich phases with C2H2 molecules at 1050 °C resulted in the formation of a C-rich surface, which exhibited a (2×2)/(6×6) reconstruction. A structure model for (√3×√3) reconstruction is proposed, and possible applications of using the surface reconstruction to selectively grow SiC polytype is discussed.

Field-ion scanning tunneling microscopy study of the atomic structure of 6H–SiC(0001) surfaces cleaned by in situ Si molecular beam etching

Field ion-scanning tunneling microscopy has been used to study 6H–SiC(0001) surfaces with Si adlayers on the Si-terminated surface, formed by in situ Si molecular beam etching at 950 °C. The as-cleaned surface showed a (∛×∛) reconstruction. The (2×3), (2∛×6√6), and (3×3) phases were formed by evaporating Si on this clean 6H–SiC(0001)-(∛×∛) surface. A Si(111) film 6 monolayers thick was also epitaxially grown on the 6H–SiC(0001)-(∛×∛) surface at 800 °C, and the surface exhibited the Si(111)–(7×7) reconstruction. A surface vacancy model for the (∛×∛) reconstruction is proposed, and a possible application of utilizing the various surface reconstructions to control the growth of different polytypes of SiC on the 6H–SiC substrate is discussed.

FULLERENES ADSORPTION ON Cu(111) AND Ag(111) SURFACES

Initial stage adsorption and film growth of C 60, C 70, C 60(x) C 70(1−x), and Sc 2@ C 84 fullerene molecules on Cu (111) and Ag (111) surfaces have been investigated by field-ion scanning tunneling microscopy. Fullerene molecules are mobile on the terrace and initially segregate to the step edges. A well-ordered monolayer films form with a close-packed arrangement upon annealing the fullerence-covered surfaces. The (4×4) phases form in the case of the C 60, C 70, and C 60(x) C 70(1−x) adsorption on the Cu (111)-(1×1) surface, indicating a strong interaction between the Cu substrate and fullerenes. Intramolecular structures of the C 60 and C 70 molecules are observed and interpreted as the mappings of the local electron density of states. On the Ag (111) surface, C 60 and Sc 2@ C 84 monolayer films show several phases that have almost identical nearest neighbor distance but are rotated from each other, indicating weaker interaction between the Ag substrate and fullerenes compared with the case of Cu .

Structures of 6H-SiC Surfaces

We have systematically studied reconstructions of the 6H SiC(0001) and (0001) surface under both Si rich and C rich condition using field ion-scanning tunneling microscopy (FI-STM). The sample was cleaned by in situ Si beam etching at 900-1000 °C. The Si rich and C rich phases were produced by annealing the sample in a Si flux and C 2 H 2 , respectively. On the (0001) surface, the as-cleaned surface showed (√3x√3) reconstruction. With increasing Si concentration, (2x2), (2√3x6√3), (3x3), and (7x7) reconstructions were observed. Reaction of the Si rich phases with C 2 H 2 molecule at 1050 °C resulted in the formation of C rich surface, which exhibited (2x2)/(6x6) reconstruction. On the (0001) surface, (2√3x2√3) reconstruction was observed after cleaning. Under Si rich condition, (2x2), (3x3), and (7x7) reconstructions were observed. Annealing the surface in C 2 H 2 beam at 1050 °C led to C rich phase with (1 x 1) structure. Structure model for (√3x√3) and (2√3x2√3) reconstruction are proposed, and possible applications of using the surface reconstruction to selectively grow the polytype SiC is discussed.

Scanning tunneling microscopy observation of copper-phthalocyanine molecules on Si(100) and Si(111) surfaces

Molecules of Cu-phthalocyanine (CuPc) deposited on Si(100) and Si(111) surfaces have been observed by an ultra high vacuum field ion scanning tunneling microscope (FI-STM). On a Si(100) surface, STM images with four-fold symmetry are observed, which reflect the shape of the CuPc molecule. The STM pictures show that CuPc molecules are deposited with the molecular plane parallel to the substrate surface and have three kinds of adsorption configurations on the dimer-row of Si(100). The images of the CuPc are modified by the electronic state of the Si(100) surface. This behavior suggests strong interaction between the molecule and the substrate. The molecular images on the Si(111) surface have a unique bias-voltage dependence. At a sample bias of 1.6 V, the molecule looks transparent by STM, and becomes dark like a vacancy at 1.2 V. From the bias dependence, the electronic interaction between the CuPc molecule and the Si surface is discussed.

Surface structure of GaAs(001)-(2 × 4) α, β and γ phases

The As-rich GaAs(001)-(2 × 4) α, β and γ phases grown by molecular-beam epitaxy (MBE) and migration enhanced epitaxy (MEE) have been investigated by field ion-scanning tunneling microscopy and in-situ reflection high-energy electron diffraction (RHEED). The high-resolution STM images show that the α, β and γ phases all have the same unit structure in the outermost surface layer which consists of two As dimers and two As dimer vacancies. The structure models are examined based on the STM observations and dynamical RHEED calculation. We propose a new structure model: The α phase is the two-As-dimer model proposed by Farrell and Palmstrom with relaxation incorporated by Northrup and Froyen. The β phase is the two-As-dimer model proposed by Chadi. The γ phase consists of the local structure same as the β phase and the open areas with a disordered As double-layer structure similar to that of the c(4 × 4) phase.

Surface structure of GaAs(001)-(2×4) α, β and γ phases

The As-rich GaAs(001)-(2 × 4) α, β and γ phases grown by molecular-beam epitaxy (MBE) and migration enhanced epitaxy (MEE) have been investigated by field ion-scanning tunneling microscopy and in-situ reflection high-energy electron diffraction (RHEED). The high-resolution STM images show that the α, β and γ phases all have the same unit structure in the outermost surface layer which consists of two As dimers and two As dimer vacancies. The structure models are examined based on the STM observations and dynamical RHEED calculation. We propose a new structure model: The α phase is the two-As-dimer model proposed by Farrell and Palmstrom with relaxation incorporated by Northrup and Froyen. The β phase is the two-As-dimer model proposed by Chadi. The γ phase consists of the local structure same as the β phase and the open areas with a disordered As double-layer structure similar to that of the c(4 × 4) phase.

Adsorption of fullerenes on Cu(111) and Ag(111) surfaces

We have studied the initial stage adsorption and film growth of C 60, C 70, and C 60( x) C 70(1− x) on Cu(111) and Ag(111) surfaces using field-ion scanning tunneling microscopy. Fullerene molecules are mobile on the terrace of the metal surfaces and initially segregate to the step edges. A well-ordered two-dimensional overlayer forms with a close-packed arrangement upon annealing the fullerene covered surfaces.

Determination of the surface structures of the GaAs(001)-(2×4) As-rich phase

Field-ion scanning tunneling microscopy (STM) and in situ reflection high-energy electron diffraction (RHEED) have been used to study the atomic structures of the As-rich GaAs(001) surfaces grown by molecular-beam epitaxy and migration-enhanced epitaxy. The (2\ifmmode\times\else\texttimes\fi{}4) \ensuremath{\alpha}, \ensuremath{\beta}, and \ensuremath{\gamma} phases and the c(4\ifmmode\times\else\texttimes\fi{}4) phase have been investigated in a systematic manner, controlling the As surface coverage. The high-resolution STM images show that the (2\ifmmode\times\else\texttimes\fi{}4) \ensuremath{\alpha}, \ensuremath{\beta}, and \ensuremath{\gamma} phases all have the same unit structure in the outermost surface layer, which consists of two As dimers and two As dimer vacancies. Various structure models proposed for the (2\ifmmode\times\else\texttimes\fi{}4) phases are examined based on the STM observations and dynamical calculation of the RHEED spot intensities. We now propose the following model: The \ensuremath{\alpha} phase is the two-As-dimer model proposed by Farrell and Palmstrom with relaxation incorporated by Northrup and Froyen. The surface coverage of the \ensuremath{\alpha} phase is 0.5 ML of As. The \ensuremath{\beta} phase is the two-As-dimer model proposed by Chadi. The surface coverage of the \ensuremath{\beta} phase is 0.75 ML of As. The so-called \ensuremath{\gamma} phase is characterized by its small domains separated by open areas. The domains consist of the same local structure as the \ensuremath{\beta} phase and the open areas between them have a disordered As double-layer structure similar to that of the c(4\ifmmode\times\else\texttimes\fi{}4) phase. The \ensuremath{\gamma} phase is, thus, no more than the mixture of the \ensuremath{\beta} phase and the c(4\ifmmode\times\else\texttimes\fi{}4) phase with the surface As coverage varying between 1.75 and 0.75 ML depending on the growth conditions. Our structure model for the As-rich GaAs(001) surface is consistent with most of the observations reported.

Intramolecular structures of C60 adsorbed on the Cu(111)1×1 surface studied by the field ion-scanning tunneling microscopy

The adsorption of C60 molecules on the Cu(111)1×1 surface has been investigated by field ion- scanning tunneling microscopy (FI-STM). At the initial stage adsorption, C60 molecules are mobile on the terrace at room temperature and segregate to the steps to form linear chains. With increasing coverage, two-dimensional islands form with a close-packed configuration. Upon monolayer adsorption, a highly ordered Cu(111)-(4×4)-C60 overlayer forms. Bias voltage-dependent scanning tunneling microscope images of the individual C60 molecules show unique intramolecular structures. The adsorption geometry of C60 is determined by analyzing the scanning tunneling microscope images and the intramolecular structures are interpreted as the local density of the states of C60 interacting with the substrate.

Scanning tunneling microscopy of the GaAs(001) surface morphology prepared by migration enhanced epitaxy

Employing field ion-scanning tunneling microscopy (FI-STM), we have studied the GaAs(001) surface prepared by the migration enhanced epitaxy (MEE) technique in a wide range of growth temperatures. The STM images clearly show the advantageous effect of enhanced Ga migration of MEE on the surface morphology of the GaAs(001) surface.

Vibrational and structural studies of oxygen adsorption on the Ag(110) surface

Adsorption of oxygen on the Ag(110)1×1 surface is studied by field ion-scanning tunneling microscopy (FI-STM) and high resolution electron energy loss spectroscopy (HREELS). Linear chain (added-row) formation in the [001] direction upon oxygen adsorption is observed. The step movement and step bunching are observed at the exposure lower than 60 L. Another type of chain formation are observed between the added rows with the local 3×1 configuration. The coverage and time-dependent HREELS data shows rather complicated dynamics in the case of room temperature adsorption. The HREELS data with the resolution of 4 meV (FWHM) shows an extra broad, weak peak at 15 meV, which has a maximum peak value at the exposure near 1800 L corresponding to the completion of the 3×1 phase.

FI-STM study of the structure of Sc-encapsulated fullerenes

We report our recent study of the electronic and geometric structure of Sc-encapsulated fullerenes (metallofullerenes) ScC 74, Sc 2C 74, and Sc 2C 84 adsorption on the Si(100)2×1 surface with field ion scanning tunneling microscopy. A comparison with the pristine C 60 and C 84 confirmed the assumption that the Sc atoms are encapsulated inside the carbon cage. The charge transfer from the Sc atoms to the carbon cage is evidenced. The presented results also show that these metallofullerenes do not prefer to form dimers or trimers.

Thermal stability of fullerene (C 70) on the Si(100) 2×1 surface studied with FI-STM

We have studied the thermal stability of fullerene C 70 adsorbed on the Si(100)2×1 surface with the field ion-scanning tunneling microscope. The results show that the first layer of C 70 adsorbed considerably strongly due to the existence of Si dangling bonds on the surface. The first layer passivated the surface efficiently so that the multilayer crystalline film could be grown as close-packing array. The C 70 above the first layer can be easily desorbed at temperatures above approximately 250°C, which is similar to that of bulk C 70. In contrast to that, the first layer remains stable without any evidence of cage breaking or reordering until at temperatures above 1000°C, resulting in well-ordered islands: β-SiC(100)3×2 surface, according to our STM observations. The present work reveals a novel fullerene property on the Si(100)2×1 surface.

Interaction of atomic hydrogen with the Si(100)2�1 surface

The interaction of atomic hydrogen with the Si(100)2×1 surface has been investigated in detail by a field ion-scanning tunneling microscope (FI-STM). At low exposure, hydrogen atoms reside singly on top of the dimerised Si atoms, and are imaged brightly. The hydrogen chemisorption induces the buckling of dimers, indicating the strong bonding between Si and hydrogen atoms. The adsorption geometry changed from the (2×1) monohydride phase to the (1×1) dihydride phase with increasing exposure of hydrogen. The former is imaged dark compared with the unreacted Si dimers due to the reduction of the density of electronic states near the Fermi level. Surface etching was also observed during the formation of the dihydride phase. The behavior of hydrogen desorption from the H-saturated Si(100) surface was investigated as a function of annealing temperatures. Our STM results suggest that the desorbing H2 molecules are formed by two hydrogen atoms on the same dihydride species.

Fi-Stm Study of Fullerenes

ABSTRACTAdsorption of C60, C70, C60(x)C70(1-x) and Sc2C84 on the Cu(111)1×1 and Ag(l11)1×1 surfaces has been investigated by field ion-scanning tunneling microscopy (FI-STM). On the Cu(111) and Ag(111) surfaces, the fullerene molecules are mobile on the terrace at room temperature and segregate to the steps to form linear chains then two-dimensional islands with a close-packed configuration. Upon monolayer adsorption, highly ordered 4×4 commensurate phases form in the case of the C60, C70 and C60(x)C70(1−x) adsorption on the Cu(111) surface, while close-packed hexagonal multiple phases, with nearest-neighbour distance equal to that of the bulk phase, form in the case of the C60 and Sc2C84 adsorption on the Ag(111) surface. These observations imply the competition between the adsorbate-adsorbate and adsorbate-substrate interactions. The intramolecular structures observed in the STM images of the C60 and C70 molecules in the Cu(111)-4×4 phases are analyzed to determine the adsorption geometry and are interpreted as the local density of the states of the C60 and C70 molecules.