Electron-induced H atom desorption patterns created with a scanning tunneling microscope: Implications for controlled atomic-scale patterning on H-Si(1 0 0)

We have used scanning tunneling microscopy (STM) to explore the details of single and multiple H atom desorption from the H-Si(1 0 0)-2 × 1 surface induced by the inelastic scattering of electrons from an STM tip. The desorption of pairs of H atoms from individual Si dimers is rarely observed. Two-H atom desorption most often involves pairs of dimers, in the same or adjacent rows. This suggests that recombinative H₂ desorption via an interdimer reaction pathway, like that observed recently under nanosecond laser heating, may also be operative for electron-induced excitation using STM. Repeatable fabrication of desired size-selected dangling bond (DB) clusters is also achieved. The single atomic precision of the fabrication is a result of the intrinsically unfavorable paired H atom desorption from a single dimer, but does not result from the spatial localization of excitation energy of the Si-H bond under the STM tip as suggested in previous studies.     

A scanning tunneling microscopy study of water on Si(111)-(7 × 7): adsorption and oxide desorption

Scanning tunneling microscopy and temperature-programmed desorption have been used to investigate the chemistry of water on Si(111)-(7 × 7) substrates which were misoriented 2° toward the [ 10] direction. Upon room temperature exposure to water, the adatoms of the (7 × 7) unit cell are still evident even after high exposures, implying that major modifications of the substrate do not occur. At high coverages, the distribution of reacted adatoms shifts from one controlled by dissociative adsorption across the adatom-rest atom pair to a statistical distribution based on the availability of dangling bonds. Desorption of the oxide layer which remains after water adsorption and the desorption of hydrogen have also been characterized. The oxide desorption occurs along well-defined wavefronts which originate at step edges and advance in directions consistent with the underlying substrate symmetry, primarily the [ 2] direction (i.e. the wave vector points in the [ 2] direction). In regions of the surface where the oxide has desorbed, the (7 × 7) unit cell can be seen clearly. Vacancies resulting from the loss of surface silicon atoms (via the etching) coalesce into islands in the clean regions of the terraces, but unlike desorption of oxide layers from Si(100), the desorption does not occur from the boundaries of these vacancy islands.     

Electron stimulated desorption of cesium atoms from germanium-covered tungsten

The electron stimulated desorption (ESD) yield and energy distributions for Cs atoms from cesium layers adsorbed on germanium-covered tungsten have been measured for different Ge film thicknesses, 0.25-4.75 ML (monolayer), as a function of electron energy and cesium coverage Θ. The measurements have been carried out using a time-of-flight method and surface ionization detector. In the majority of measurements Cs is adsorbed at 300 K. The appearance threshold for Cs atoms is about 30 eV, which correlates well with the Ge 3d ionization energy. As the electron energy increases the Cs atom ESD yield passes through a wide maximum at an electron energy of about 120 eV. In the Ge film thickness range from 0.5 to 2 ML, resonant Cs atom yield peaks are observed at electron energies of 50 and 80 eV that can be associated with W 5p and W 5s level excitations. As the cesium coverage increases the Cs atom yield passes through a smooth maximum at 1 ML coverage. The Cs atom ESD energy distributions are bell-shaped; they shift toward higher energies with increasing cesium coverage for thin germanium films and shift toward lower energies with increasing cesium coverage for thick germanium films. The energy distributions for ESD of Cs from a 1 ML Ge film exhibit a strong temperature dependence; at T = 160 K they consist of two bell-shaped curves: a narrow peak with a maximum at a kinetic energy of 0.35 eV and a wider peak with a maximum at a kinetic energy of 0.5 eV. The former is associated with W level excitations and the latter with a Ge 3d level excitation. These results can be interpreted in terms of the Auger stimulated desorption model.     

Adsorption of pyrimidine molecules on Pd(110) observed by scanning tunneling microscopy

Adsorbed states of pyrimidine molecules on Pd(110) have been studied by a scanning tunneling microscope (STM). The pyrimidine molecules are preferentially adsorbed on terraces, not at steps. The isolated pyrimidine molecule shows a 0.6 nm × 0.6 nm rectangular shape with two parts of elongated protrusions. Two adsorption sites are observed: on-top site of the Pd[1 0] row and the midway between two [1 0] rows. Pyrimidine molecules show a strong tendency to form dimers even at a low coverage (0.01 ML), indicating that there is an attractive interaction between two adsorbed molecules.     

Scanning tunneling microscopy of N2H4 on silicon surfaces

The chemistry of N₂H₄ on Si(100)2 × 1 and Si(111)7 × 7 has been studied using scanning tunneling microscopy. At low coverages on Si(100)2 × 1 at room temperature the adsorption sites are distributed randomly on the surface and are imaged as dark spots in the dimer row by the STM. Upon annealing the substrate at 600 K, both isolated reaction products, as well as clusters of reaction products are formed on the surface. The STM images show that the majority of the isolated reaction products are adsorbed symmetrically across the dimers. Based on previous HREELS data, these are most likely NH_x groups. However, the clusters are not well resolved. Because of this we speculate that they are not simply symmetrically adsorbed NH_x groups, but likely have a more complicated internal structure. At higher coverages, the STM images show that the predominant pathway for adsorption is with the N---N bond parallel to the surface, in agreement with HREELS studies of this system. On Si(111)7 × 7, the molecule behaves in a manner which is similar to NH₃. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms.     

Electroluminescence of diamond films induced by a scanning tunneling microscope

We report the scanning tunneling microscope induced band-A emission from boron-doped polycrystalline diamond films fabricated by chemical-vapor deposition (CVD). The broad blue emission occurs at a bias above ±3.4 V with double peaks at 410 and 450 nm and is attributed to the dislocation-related defect centers. Greatly enhanced green emissions around 530 nm are observed at high positive bias. This, together with strongly bias- and polarity-dependent emission intensities and spectra, leads us to propose that the boost in the green emission at high bias is probably related to the minority electron injection into the boron-related acceptor states in the subsurface.     

The adsorption of C6H5Cl on Si(111)7 × 7 studied by STM

The adsorption of chlorobenzene on Si(111)7 × 7 at room temperature was studied by scanning tunneling microscopy (STM). Selective chemisorption was observed at different adatom sites. It was found that the center adatoms were more reactive than the corner adatoms, and the faulted half of the unit cell was more reactive than the unfaulted. The mechanism is discussed in terms of the electronic and atomic structures in Si(111)7 × 7. Both preferences indicate that chlorobenzene was present initially in a mobile precursor state.     

Probing dynamics of a phase transition of two-dimensional nano-domains with STM imaging and manipulation

A reversible, temperature-driven structural surface phase transition of Pb/Si(1 1 1) nano-domains is studied with a variable-temperature scanning tunneling microscope (STM). Finite-size effects of the transition are clearly demonstrated. Most importantly, structural fluctuations in the low-temperature phase can be induced by the direct interaction between the tip atoms and the surface atoms. The structural changes reveal dynamics in the low-temperature phase. Amazingly, the largest size of the domains that can be manipulated decreases with decreasing sample temperature.     

Metrological orientation-confirmation of Si(h h k) using scanning tunneling microscopy

For the periodicity-modulation of the Si(h h k) template between (0 0 1) and (1 1 1), it is necessary to prepare the surface with any orientation within this range, most especially for fabricating useful one-dimensional nanostructures. Especially, when there are no strong X-ray signals using the standard Cu K-α source in the vicinity of any arbitrarily chosen (H H K), it turns out that the line-profile analysis on the topographic image of scanning tunneling microscopy can be a unique way for confirming the orientation of the prepared surface. Though there are a number of small-width facets on the reconstructed surface, if any of well-defined facets, such as (1 1 1), (3 3 7), (1 1 2), and (3 3 5), are included in these facets it is possible to determine the orientation using the weighted-average method.     

Vibration-induced structures in scanning tunneling microscope light emission spectra of Ni(1 1 0)-streaky (1 × 2)-H

We have measured the scanning tunneling microscope (STM) light emission spectra of Ni(1 1 0)-streaky (1 × 2) surfaces. When the tip was fixed over atomic hydrogen adsorbed on the surfaces, two types of vibration-induced structure were observed in the STM light emission spectra. One is the periodic fine structures that were already reported in our previous paper [Y. Uehara, S. Ushioda, Phys. Rev. Lett. 92 (2004) 066102] and the other newly found in the present experiments is a stepwise structure that is located at the vibrational energy of hydrogen below the cutoff energy of the STM light emission. They are ascribed to different excitation mechanisms of the vibration in the STM light emission process; the periodic fine structures appear when the vibrating motion is directly excited by the electrons injected from the tip. Conversely, the stepwise structure is observed when it is excited by the electromagnetic fields confined in the tip-sample gap, i.e., by localized surface plasmons.     

Self-regulated Ni cluster formation on the TiO2(1 1 0) terrace studied using scanning tunneling microscopy

We have studied the growth mode and morphology of Ni clusters on a TiO₂(1 1 0) surface with a wide terrace using scanning tunneling microscopy (STM) at a low coverage (less than 3 atoms nm⁻²). The Ni clusters are formed on the terrace at the low coverage of 0.2 atoms nm⁻². Their average dimensions are constant in three directions up to 1 atoms nm⁻². The Ni clusters have an oval shape with average sizes of 1.8 nm (along [0 0 1]) × 1.4 nm (along (in the [1 1 0] directions). Above the coverage of 1.0 atoms nm⁻², an increase in the cluster height occurs, retaining an almost constant lateral size. It is proposed that the interaction of the Ni cluster and the support surface regulates the Ni cluster size.     

Structures of sulfur on TiO2(1 1 0) determined by scanning tunneling microscopy, X-ray photoelectron spectroscopy and low-energy electron diffraction

The temperature dependent adsorption of sulfur on TiO₂(1 1 0) has been studied with X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). Sulfur adsorbs dissociatively at room temperature and binds to fivefold coordinated Ti atoms. Upon heating to 120°C, 80% of the sulfur desorbs and the S 2p peak position changes from 164.3±0.1 to 162.5±0.1 eV. This peak shift corresponds to a change of the adsorption site to the position of the bridging oxygen atoms of TiO₂(1 1 0). Further heating causes little change in S coverage and XPS binding energies, up to a temperature of 430°C where most of the S desorbs and the S 2p peak shifts back to higher binding energy. Sulfur adsorption at 150°C, 200°C, and 300°C leads to a rich variety of structures and adsorption sites as observed with LEED and STM. At low coverages, sulfur occupies the position of the bridging oxygen atoms. At 200°C these S atoms arrange in a (3×1) superstructure. For adsorption between 300°C and 400°C a (3×3) and (4×1) LEED pattern is observed for intermediate and saturation coverage, respectively. Adsorption at elevated temperature reduces the substrate as indicated by a strong Ti³⁺ shoulder in the XPS Ti 2p_3/2 peak, with up to 15.6% of the total peak area for the (4×1) structure. STM of different coverages adsorbed at 400°C indicates structural features consisting of two single S atoms placed next to each other along the [0 0 1] direction at the position of the in-plane oxygen atoms. The (3×3) and the (4×1) structure are formed by different arrangements of these S pairs.     

Highly resolved scanning tunneling microscopy study of Si(0 0 1) surfaces flattened in aqueous environment

A surface preparation method with fine SiO₂ particles in water is developed to flatten Si(0 0 1) surfaces on the nanometer scale. The flattening performance of Si(0 0 1) surfaces after the surface preparation method is investigated by scanning tunneling microscopy. The observed surface is so flat that 95% of the view area (100 × 100 nm²) is composed of only three atomic layers, namely, one dominant layer occupying 50% of the entire area and two adjacent layers. Furthermore, a magnified image shows the outermost Si atoms regularly distributed along the 〈1 1 0〉 direction on terraces.     

Formation mechanism of the Si(1 1 1)7×7 reconstruction studied by scanning tunneling microscopy: Zipper-like restructuring in the sequential size changes of isolated single faulted-halves

The formation mechanism of the 7×7 reconstruction on annealed Si(1 1 1) surfaces has been demonstrated at the atomic level. In situ observations of unreconstructed regions (‘1×1’) on terraces after rapid quenching to 380 °C were done using scanning tunneling microscopy (STM) with a scanning speed of 1.7 s per frame. In the narrow ‘1×1’ regions, we imaged isolated single-faulted (F) halves of the dimer-adatom-stacking-fault (DAS) structure from “birth” to “death”. During “life”, the isolated single F-halves frequently changed their size. The size changes between odd-sized F-halves always took place through even-sized F-halves of intermediate sizes: 5×5-F^′↔6×6-F↔7×7-F^′↔8×8-F↔9×9-F^′↔10×10-F↔11×11-F^′↔12×12-F↔13×13-F^′, where the 5×5-F^′, 7×7-F^′ and so forth are irregular-type structures of the odd-sized F-halves. Even-sized F-halves and the irregular-type structures are necessary in the size changes, whereas the regular-type structures have never been involved. Lifetimes of the 10×10-F, 8×8-F, and 6×6-F at 380 °C are about 10.5, 6, and 2-3 s, respectively, which are much shorter than those of the isolated irregular-type structures of the odd-sized F-halves. With the aid of room temperature STM images of a rapidly quenched surface, we determined the atomic structures of the even-sized F-halves. We have proposed a sequential size change (SSC) model, including undiscovered parts of the size changes ‘1×1’ ↔2×2-F↔3×3-F^′↔4×4-F↔5×5-F^′, as the formation and decay mechanism of isolated single F-halves in the ‘1×1’ region. The SSC model has the following sequence: ‘1×1’ ↔ 2×2-F↔3×3-F^′↔4×4-F↔5×5-F^′↔6×6-F↔7×7-F^′↔8×8-F↔9×9-F^′↔10×10-F↔11×11-F^′↔12×12-F↔13×13-F^′. It was found by collecting statistics of size-change directions that one of two equivalent sides of the irregular-type structures, which have a mirror symmetry, is involved in the size changes thus indicating that other parts of the F-halves remain unchanged. Based on such findings, we have proposed the atomic processes for bond-rearrangements in the SSC model. The bond-rearrangements proceed along one side of a triangular F-half by breaking the existing dimers and forming new dimers like a “zipper”. Proposed atomic processes of the zipper-like restructuring are illustrated by a ball-and-stick model. The reason for the appearance of the even-sized F-halves and the irregular-type structures of the odd-sized ones is discussed in terms of the energy barrier heights along a reaction path in the size change of single F-halves.     

Scanning tunneling microscopy study on initial stage of atomic hydrogen adsorption on the Si(1 1 1) 7 × 7 surface

Initial hydrogen adsorption on the Si(1 1 1) 7 × 7 surface was studied by scanning tunneling microscopy (STM) in an ultrahigh vacuum. Room temperature adsorbed hydrogen on the adatom in the 7 × 7 reconstruction led to depression of adatoms in the STM images. The hydrogen uptake curve at the adatom site as a function of hydrogen exposure time was well represented by Langmuir adsorption. No preferential adsorption was seen among four inequivalent adatoms in the 7 × 7 reconstruction. Adsorption of the adjacent center and corner adatoms respectively showed ∼10% higher adsorption. Even though the number of reacted adatoms in the half unit of the 7 × 7 reconstruction was statistically random, the number of reacted adatoms in the nearest neighbor half unit was enhanced as the number of reacted sites increased in the half unit.     

Electron stimulated desorption of CO molecules from chemisorbed CO layers on Ru(001): a comparison of the distributions of translational and internal energies

The translational and internal energies of CO desorbed by electron impact from well-ordered × layers on Ru(001) are investigated with time-of-flight techniques and resonance enhanced multiphoton ionization (REMPI) spectroscopy. We discriminate four desorption channels with different translational and internal energy distributions. Translational and vibrational energies are positively, crrelated; rotational and vibrational–translational energies are negatively correlated. The microscopic origins of these correlations are discussed.     

Desorption of chlorine atoms on Si (1 1 1)-(7 × 7) surfaces induced by hole injection from scanning tunneling microscope tips

We investigated desorption of chlorine atoms on Si (1 1 1)-(7 × 7) surfaces induced by hole injection from scanning tunneling microscope tips. The hole-induced desorption of chlorine atoms had a threshold bias voltage corresponding to the energy position of the S3 surface band originated in Si backbonds. The chlorine atom desorption rate was almost proportional to the square of the tunneling current. We have discussed possible mechanisms that two holes injected into Si surface states get localized at the backbonds of chlorinated Si adatoms, which induces the rupture of Cl-Si bonds to result in chlorine atom desorption.     

Polymerization and depolymerization of fullerenes induced by hole injection from scanning tunneling microscope tips

Reversible polymerization and depolymerization reactions of fullerene (C₆₀) molecules are induced by tunnel injection of holes from tips of scanning tunneling microscopes similarly to the reactions induced by tunneling electrons. The bi-directional and bi-polar features of the hole-induced and electron-induced reactions can be interpreted by a unified model considering ionic reactions over symmetry-imposed barriers in the [2+2] cyclo-addition and the reverse reactions.     

Field-induced transfer of lithium in a scanning tunneling microscope

Field-induced transfer of Li atoms from a [110]-oriented W tip to a Pt sample is realized in the scanning tunneling microscope (STM) under UHV conditions, using a working principle of the new solid state Li surface diffusion metal ion source (Li-SDMIS), in which the supply of Li to the apex of the tip occurs via surface diffusion. By applying 5–10 μs voltage pulses to the Li-covered W tip placed within tunneling range, single Li hillocks (400 × 400 Ų) were formed on the chosen area of the Pt(110) sample in a well-rep way. The pulse magnitude necessary for Li emission displays a distinct threshold character. A comparative analysis of the energetics of Li field desorption in the STM and Li-FDM (lithium field desorption microscope) modes indicates chemically-assisted field desorption of Li as the field-induced transfer mechanism.     

STM study of low pressure adsorption of silane on Si(111)7 × 7

We report a study of silane adsorption on the Si(111)7 × 7 surface. We have been interested in the first stages of chemisorption at room temperature. Reactive sites of the unit cell have been clearly identified on Scanning Tunneling Microscopy (STM) images: the reaction involves the rest atom and the adjacent adatom of the DAS structure with preferential adsorption on the center adatom. We propose an original chemisorption mechanism which leads to the formation of two SiH₂ species by chemisorption and involves the breaking of Si---Si backbonds of the adatom.