Silicon Dimer Research Articles

Chemical vapor deposition of amorphous silicon carbide thin films on metal surfaces using monomethylsilane gas at low temperatures

The low temperature chemical vapor deposition process of depositing silicon carbide on a metal surface, such as aluminum and stainless steel, was developed using monomethylsilane gas. In order to prepare the reactive substrate surface, two alternative methods were separately used. The first was the formation of a silicon interlayer containing silicon dimers. The other method was the argon plasma etching for producing dangling bonds. After the reactive surface preparation, the silicon carbide thin film was formed at room temperature using monomethylsilane gas. Because the silicon–carbon bond remained after the deposition, the silicon carbide coating technology is expected to be applicable for various metals.

Atomic-scale control of hydrogen bonding on a bare Si(100)-2×1 surface

The control of the dissociative adsorption of individual hydrogen molecules is performed on the silicon surface at the atomic scale. It is achieved using the tip of a low-temperature (9 K) scanning tunneling microscope (STM) exposed to 10${}^{\ensuremath{-}6}$ torr of H${}_{2}$ and by probing the bare Si(100)-2 \ifmmode\times\else\texttimes\fi{} 1 surface at a positive bias. This effect is very localized and is induced by the tunnel electrons. The statistical study of this process reveals an activation energy threshold matching the creation of ${{\mathrm{H}}_{2}}^{\ensuremath{-}}$ at the surface of the STM tip. Our results are supported by ab inito density functional calculations of a hydrogenated silicon dimer.

Relativistic potential energy surfaces of initial oxidations of Si(100) by atomic oxygen: The importance of surface dimer triplet state

The non-relativistic and relativistic potential energy surfaces (PESs) of the symmetric and asymmetric reaction paths of Si(100)-2×1 oxidations by atomic oxygen were theoretically explored. Although only the singlet PES turned out to exist as a major channel leading to "on-dimer" product, both the singlet and triplet PESs leading to "on-top" products are attractive. The singlet PESs leading to the two surface products were found to be the singlet combinations (open-shell singlet) of the low-lying triplet state of surface silicon dimer and the ground (3)P state of atomic oxygen. The triplet state of the "on-top" product can also be formed by the ground singlet state of the surface silicon dimer and the same (3)P oxygen. The attractive singlet PESs leading to the "on-dimer" and "on-top" products made neither the intersystem crossings from triplet to singlet PES nor high energy (1)D of atomic oxygen necessary. Rather, the low-lying triplet state of surface silicon dimer plays an important role in the initial oxidations of silicon surface.

Ene-like Reaction of Cyclopentene on Si(001)-2 × 1: An XPS and NEXAFS Study

The control and the understanding of single-molecule covalent coatings on silicon surfaces is increasingly important in designing nanoscale electrical elements, such as organic/inorganic semiconductor hybrid structures. In this respect, ordered arrays of cyclopentene deposited on Si(001)-2 × 1 appear as promising buffer layers for further molecular crystal growth on the substrate. In this work, we examine the adsorption of cyclopentene on Si(001)-2 × 1 at 130 and 280 °C by means of C 1s XPS and NEXAFS. Until now cryogenic and room-temperature adsorption studies tended to prove that cyclopentene adsorption results from a formal cycloaddition ([2 + 2]-like) reaction of the C═C double bond with a silicon dimer. Our XPS/NEXAFS study reveals that an ene-like reaction competes with the [2 + 2]-like reaction channel, leading to the formation of products bearing a C═C bond, already at deposition temperature as low as ∼130 °C. This work helps to determine the optimum conditions leading to optimal chemical order in...

Effect of the tip state during qPlus noncontact atomic force microscopy of Si(100) at 5 K: Probing the probe.

Background: Noncontact atomic force microscopy (NC-AFM) now regularly produces atomic-resolution images on a wide range of surfaces, and has demonstrated the capability for atomic manipulation solely using chemical forces. Nonetheless, the role of the tip apex in both imaging and manipulation remains poorly understood and is an active area of research both experimentally and theoretically. Recent work employing specially functionalised tips has provided additional impetus to elucidating the role of the tip apex in the observed contrast.Results: We present an analysis of the influence of the tip apex during imaging of the Si(100) substrate in ultra-high vacuum (UHV) at 5 K using a qPlus sensor for noncontact atomic force microscopy (NC-AFM). Data demonstrating stable imaging with a range of tip apexes, each with a characteristic imaging signature, have been acquired. By imaging at close to zero applied bias we eliminate the influence of tunnel current on the force between tip and surface, and also the tunnel-current-induced excitation of silicon dimers, which is a key issue in scanning probe studies of Si(100).Conclusion: A wide range of novel imaging mechanisms are demonstrated on the Si(100) surface, which can only be explained by variations in the precise structural configuration at the apex of the tip. Such images provide a valuable resource for theoreticians working on the development of realistic tip structures for NC-AFM simulations. Force spectroscopy measurements show that the tip termination critically affects both the short-range force and dissipated energy.

Structural characterization of the lithium silicides Li15Si4, Li13Si4, and Li7Si3 using solid state NMR

Local environments and lithium ion dynamics in the binary lithium silicides Li(15)Si(4), Li(13)Si(4), and Li(7)Si(3) have been characterized by detailed variable temperature static and magic-angle spinning (MAS) NMR spectroscopic experiments. In the (6)Li MAS-NMR spectra, individual lithium sites are generally well-resolved at temperatures below 200 K, whereas at higher temperatures partial or complete site averaging is observed on the ms timescale. The NMR spectra also serve to monitor the phase transitions occurring in Li(7)Si(3) and Li(13)Si(4) at 235 K and 146 K, respectively. The observed lithium isotropic shift ranges of up to approximately 50 ppm indicate a significant amount of electronic charge stored on the lithium species, consistent with the expectation of the extended Zintl-Klemm-Busmann concept for the electronic structure of these materials. The (29)Si MAS-NMR spectra obtained on isotopically enriched samples, aided by double-quantum spectroscopy, are well suited for differentiating between the individual types of silicon sites within the silicon frameworks, and in Li(13)Si(4) their identification aids in the assignment of individual lithium sites via(29)Si{(7)Li} cross-polarization/heteronuclear correlation NMR. Variable temperature static (7)Li NMR spectra reveal motional narrowing effects, illustrating high lithium ionic mobilities in all of these compounds. Differences in the mobilities of individual lithium sites can be resolved by temperature dependent (6)Li MAS-NMR as well as (6)Li{(7)Li} rotational echo double resonance (REDOR) spectroscopy. For the compound Li(15)Si(4) the lithium mobility appears to be strongly geometrically restricted, which may result in a significant impediment for the use of Li-Si anodes for high-performance batteries. A comparison of all the (6)Li and (7)Li NMR spectroscopic data obtained for the three different lithium silicides and of Li(12)Si(7) previously studied suggests that lithium ions in the vicinity of silicon clusters or dimers have generally higher mobilities than those interacting with monomeric silicon atoms.

Dimer-T3reconstruction of the Sm/Si(100)(2×3) surface studied by high-resolution photoelectron spectroscopy and density functional theory calculations

Silicon dimer--containing reconstructions on Si(100) can be induced by submonolayer amounts of rare earth (RE) metals. The tilt of dimer bonds in such reconstructions can be controlled by the coverage and electronic properties of RE adsorbates. In this study, we have utilized improved high-resolution photoelectron spectroscopy with the synchrotron radiation and density functional theory (DFT) calculations to exploit the structural and electronic properties of the Sm/Si(100)(2 $\ifmmode\times\else\texttimes\fi{}$ 3) system. A careful analysis of photoelectron spectra, in combination with DFT calculations of surface core-level shifts for silicon atoms in energetically plausible structural models, has allowed us to establish the favorable atomic configuration of Sm/Si(100)(2 $\ifmmode\times\else\texttimes\fi{}$ 3) with a buckled Si dimer and to explain characteristic features of Si 2$p$ line shape in detail. It is shown that the dimer buckling leads to a significant core-level binding-energy splitting of the first-layer Si atoms, affecting the lower-binding-energy region of Si 2$p$ spectra drastically. An interpretation of the Si 2$p$ line shape for RE/Si(100)(2 $\ifmmode\times\else\texttimes\fi{}$ 3) that is based on combined initial state and complete screening data is suggested. The mechanism underlying the buckling and symmetrization of silicon dimers in RE/Si(100) reconstructions is discussed.

Atomic-scale study of the adsorption of calcium fluoride on Si(100) at low-coverage regime

We investigate, experimentally and theoretically, the initial stage of the formation of Ca/Si and Si/F structures that occurs during the adsorption of CaF${}_{2}$ molecules onto a bare Si(100) surface heated to 1000 K in a low-coverage regime (0.3 monolayer). A low-temperature (5 K) scanning tunneling microscope (STM) is used to observe the topographies and the electronic properties of the exposed silicon surfaces. Our atomic-scale study reveals that several chemical reactions arise during CaF${}_{2}$ deposition, such as dissociation of the CaF${}_{2}$ molecules and etching of the surface silicon dimers. The experimental and calculated STM topographies are compared using the density functional theory, and this comparison enables us to identify two types of reacted structures on the Si(100) surface. The first type of observed complex surface structure consists of large islands formed with a semiperiodic sequence of 3 \ifmmode\times\else\texttimes\fi{} 2 unit cells. The second one is made of isolated Ca adatoms adsorbed at specific sites on the Si(100)-2 \ifmmode\times\else\texttimes\fi{} 1 surface.

Cooperative Chiral Adsorption of Styrene Molecules on the Si(001)-c(2 × 4) Surface: First-Principles Investigation of Reaction Mechanisms

Detailed reaction mechanisms for the chiral adsorption of styrene molecules on the reconstructed Si(001)-c(2 × 4) surface are investigated using first principles calculations based on density functional theory. The most stable configuration corresponds to the R chirality on the so-called “on-top” site, where the molecule is adsorbed on a single silicon dimer. The on-top styrene adsorption does not encounter reaction barriers, but there is a small energy barrier following an intermediate metastable state associated with the styrene adsorption on bridge sites between two Si dimers (the “single-end-bridge” site, or SEB). Our calculations show that when the first styrene molecule is adsorbed in the SEB configuration, there is no energy barrier associated with the adsorption of the second molecule. This cooperative effect leads to the experimentally observed “paired end-bridge” configuration.

The local adsorption of pyridine on Si(100) a combined PES and XPD study

The chemical and geometrical properties of the system pyridine on Si(100) are investigated in a combined photoelectron spectroscopy (XPS) and photoelectron diffraction (XPD) study. Synchrotron radiation was applied to achieve high spectral resolution and a high surface sensitivity. Our studies were performed at saturation coverage of pyridine on silicon. The XPS and XPD results, including diffraction patterns for all spectral resolved components, clearly show that pyridine is reacting with silicon dimer atoms of the (2 × 1)-reconstructed surface. We propose a tetra-σ-bonded structure model and provide detailed structure parameters.

Hydrogen-anion formation near a (2×1)-reconstructed Si(100) surface: Substrate-electronic-structure and trajectory dependence

We calculated the yield of outgoing hydrogen negative ions after the reflection of 1-keV neutral hydrogen atoms from a $(2\ifmmode\times\else\texttimes\fi{}1)$-reconstructed Si(100) surface. We find that the charge-transfer dynamics at the reconstructed surface is dependent on both the surface-electronic structure and orientation of the projectile trajectory relative to the crystal azimuthal directions. Our results are in good quantitative agreement with the measured $\mathrm{H}{}^{\ensuremath{-}}$ fractions of Maazouz and Esaulov [Surf. Sci. 398, 49 (1998)] for scattering trajectories that are aligned perpendicularly to rows of silicon dimers.

Mechanism of Silicon Carbide Film Deposition at Room Temperature Using Monomethylsilane Gas

The mechanism of silicon carbide chemical vapor deposition on a silicon surface using a monomethylsilane gas at room temperature was studied. Because the silicon carbide film surface tended to have large hillocks at the high monomethylsilane gas concentrations, the surface chemical reaction for the silicon carbide film formation was considered to include a three-dimensional pass, even at room temperature. The composition of the obtained film was consistent with that expected from the film formation process. Because the lowest temperature of hydrogen annealing for enabling the silicon carbide film formation coincided with the lowest temperature for practically removing the native oxide film by the hydrogen annealing, the silicon dimer at the silicon surface formed by the hydrogen annealing and the cooling in ambient hydrogen was concluded to play a key role.

Electron Transport Suppression from Tip−π State Interaction on Si(100)-2 × 1 Surfaces

We investigate the electron transport between a scanning tunneling microscope tip and Si(100)-2 × 1 surfaces with four distinct configurations by performing calculations using density functional theory and the nonequilibrium Green's function method. Interestingly, we find that the conducting mechanism is altered when the tip-surface distance varies from large to small. At a distance larger than the critical value of 4.06 Å, the conductance is increased with a reduction in distance owing to the π state arising from the silicon dimers immediately under the tip; this in turn plays a key role in facilitating a large transmission probability. In contrast, when the tip is closer to the substrate, the conductance is substantially decreased because the π state is suppressed by the interaction with the tip, and its contribution in the tunneling channels is considerably reduced.

Surface-mediated chain reaction through dissociative attachment

Chain reactions on a surface offer an important route to linear nanopatterning. We recently reported cooperative reactions on a silicon surface in which the reaction of one halogen atom with a silicon atom of a silicon dimer induced the halogenation of its neighbouring silicon atom through surface-mediated charge transfer. The reaction was unable to propagate further but here we describe how, by chemically bridging the gaps between the rows of these silicon dimers, this mechanism is able to form extended chains. The agents for chain growth are CH(3)Cl molecules that dissociatively attach CH(3) groups and chlorine atoms to silicon atoms from different dimers. By means of charge transfer through the surface, this gives rise to dangling bonds adjacent to the CH(3) groups and chlorine atoms (in effect, 'free radicals') that dissociate further incoming CH(3)Cl molecules, thereby providing the growing points for chains of indefinite length. This versatile mechanism of chain growth is examined in experiments and using ab initio theory.

A Synchrotron Radiation X-ray Photoemission Spectroscopy Study of n-Propyltriethoxysilane Adsorption on Si(001)-2 × 1 at Room Temperature

The reaction of n-propyltriethoxysilane (PTES) with clean Si(001)-2 × 1 at room temperature is studied by synchrotron radiation X-ray photoemission spectroscopy (XPS). It is shown that PTES dissociatively adsorbs on the surface via the scission of at least two Si−O bonds. Adsorbate geometries are proposed accounting for the XPS data, and possible reaction paths are discussed, considering the zwitterionic nature of the surface silicon dimers. The understanding of the reactivity of the triethoxysilane termination on clean silicon paves the way to its possible use as an anchoring unit enabling the grafting of complex multifunctional molecules.

Atomic structure ofYb/Si(100)(2×6): Interrelation between the silicon dimer arrangement andSi 2pphotoemission line shape

Combining photoelectron spectroscopy and density-functional theory calculations, we have studied the atomic geometry of Yb/Si(100)(2 X 6) reconstruction and the mechanisms responsible for its stabilization as well as the influence of this reconstruction on Si 2p core-level photoemission. The analysis of measured and calculated surface core-level shifts supports the recently proposed model of the Yb/Si(100)(2 X 6). It involves, in agreement with valence-band measurements, unbuckled (symmetrical) silicon dimers, leading to unusually narrowed Si 2p line shape as compared to those of related systems. The origin of the symmetrical dimers in the (2 X 6) structure is discussed in the context of previous results in literature.

Structure and stability of Si 6Li 6: Aromaticity vs polarizability

Detailed exploration of the Si 6Li 6 potential energy surface shows that the most stable form of such cluster is not planar, it is a tridimensional structure formed by a silicon tetrahedron interacting with a silicon dimer through four lithium atoms. In order to understand stability, both polarizability and aromaticity were used. Given that the most stable structure is non-planar, aromaticity evaluation is not easy. Gratifyingly, the minimum polarizability principle is satisfied.

Diffusion of Atomic Oxygen on the Si(100) Surface

The processes of etching and diffusion of atomic oxygen on the reconstructed Si(100)-2 × 1 surface are investigated using an embedded cluster QM/MM (Quantum Mechanics/Molecular Mechanics) method, called SIMOMM (Surface Integrated Molecular Orbital Molecular Mechanics). Hopping of an oxygen atom along the silicon dimer rows on a Si15H16 cluster embedded in an Si136H92 MM cluster model is studied using the SIMOMM/UB3LYP (unrestricted density functional theory (UDFT) with the Becke three-parameter Lee−Yang−Parr (B3LYP) hybrid functional) approach, the Hay−Wadt effective core potential, and its associated double-ζ plus polarization basis set. The relative energies at stationary points on the diffusion potential energy surface were also obtained with three coupled-cluster (CC) methods, including the canonical CC approach with singles, doubles, and noniterative quasi-perturbative triples (CCSD(T)), the canonical left-eigenstate completely renormalized (CR) analogue of CCSD(T), termed CR-CC(2,3), and the linear scaling variant of CR-CC(2,3) employing the cluster-in-molecule (CIM) local correlation ansatz, abbreviated as CIM-CR-CC(2,3). The pathway and energetics for the diffusion of oxygen from one dimer to another are presented, with the activation energy estimated to be 71.9 and 74.4 kcal/mol at the canonical CR-CC(2,3)/6-31G(d) and extrapolated, CIM-based, canonical CR-CC(2,3)/6-311G(d) levels of theory, respectively. The canonical and CIM CR-CC(2,3)/6-31G(d) barrier heights (excluding zero point vibrational energy contributions) for the etching process are both 87.3 kcal/mol.

Photoelectron spectroscopy (PES) and photoelectron diffraction (XPD) studies on the local adsorption of cyclopentene on Si(100)

We investigated the chemical and geometrical characteristics of the system cyclopentene on Si(100) in a combined photoelectron spectroscopy (PES) and photoelectron diffraction (XPD) investigation. In this study synchrotron radiation was applied to achieve a high resolution with high surface sensitivity. Our PES and XPD results show that the cyclopentene reacts with the silicon dimers of the (2 × 1)-reconstructed surface, changing the planar molecule to a diverse tilted molecule after the adsorption.

Phenylacetylene One-Dimensional Nanostructures on the Si(100)-2 × 1:H Surface

Using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM), many olefins have been shown to self-assemble on the hydrogen-passivated Si(100)-2 x 1 surface into one-dimensional nanostructures. This paper demonstrates that similar one-dimensional nanostructures can also be realized using alkynes. In particular, UHV STM, sum frequency generation (SFG), and density functional theory (DFT) are employed to study the growth mechanism and binding configuration of phenylacetylene (PA) one-dimensional nanostructures on the Si(100)-2 x 1:H surface. Molecular-resolution UHV STM images reveal the binding position and spacing of PA with respect to the underlying silicon dimer rows. Furthermore, UHV STM characterization of heteromolecular one-dimensional nanostructures of styrene and PA shows distinct electronic contrast between the two molecules, which is confirmed using simulated STM images derived from DFT and provides insight into the nature of PA binding to silicon. Additional evidence from SFG measurements corroborates the conclusion that the terminal carbon atoms of PA retain pi-conjugation following reaction to the Si(100)-2 x 1:H surface.