Buckled Dimers Research Articles

External stress-induced chemical reactivity ofO2on Si(001)

In the dissociation reaction of ${\text{O}}_{2}$ on the $\text{Si}(001)\text{\ensuremath{-}}c(4\ifmmode\times\else\texttimes\fi{}2)$ surface, a trapping-mediated reaction is accelerated by external tensile stress. This alteration of the reactivity is more than an order of magnitude greater than estimations based on the alteration of the electronic structure caused by strain of the surface lattice. It was found that on the $\text{Si}(001)\text{\ensuremath{-}}c(4\ifmmode\times\else\texttimes\fi{}2)$ surface, the stress destabilized the antiferromagnetic ordering of topmost Si atoms forming buckled dimer and induced the collective phason-flip motion of the dimer. The alteration of the reactivity caused by such a collective instability of the arrangement of surface atoms is discussed.

Effect of Surface Stress around the SA Step of Si(001) on the Dimer Structure Determined by Noncontact Atomic Force Microscopy at 5 K

Using noncontact atomic force microscopy (NC-AFM) at 5 K, we investigate the effect of surface stress around the S A step of Si(001) on the buckling of a surface dimer. Both topographic and dissipation images show different buckled dimer phases between the terraces, namely, the asymmetric c(4×2) phase in the upper terrace and the symmetric p(2×1) one in the lower terrace. In addition, we find, for the first time, that dimers in the vicinity of the step in the upper terrace were compressed by 3.6% in the dimer bond direction, as determined by the averaged profile analysis of the image. A variation in the surface dimer structure reflecting surface stress is demonstrated by dissipation image analysis, which points to the potential of NC-AFM for studying surface stress.

Stability and structure of rare-earth metal and Ba-induced reconstructions on a Si(100) surface

We have studied, by means of ab initio calculations, the energetics and the atomic and electronic structures of various reconstructions induced by rare-earth metals ($\text{RE}=\text{Eu}$, Nd, Sm, and Yb) and Ba on Si(100) in the coverage range up to 0.5 monolayer. It is shown that Si dimer buckling is an important structural element for such systems, leading frequently to oblique surface lattice symmetries. The strong metal atom---silicon binding favors the increased amount of metal atoms per unit surface area, i.e., the $(2\ifmmode\times\else\texttimes\fi{}3)$ reconstruction with two metal atoms per unit cell is found to be energetically unstable with respect to the $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstruction with three metal atoms per the same surface area [Eu/Si(100) and Yb/Si(100)]. The influence of the atomic size and the valence of the adsorbates is also investigated. In particular, it is found that an increase in atomic size stimulates the metal-metal repulsion, stabilizing the $(2\ifmmode\times\else\texttimes\fi{}3)$ configuration [Ba/Si(100)]. In the case of trivalent metals, the stabilization of the $(2\ifmmode\times\else\texttimes\fi{}3)$ is mediated by the loss of semiconducting state in the competing phases [Sm/Si(100) and Nd/Si(100)]. Our results demonstrate the importance of many factors, which account for the abundance of RE/Si(100) reconstructions. Finally, prominent atomic models are proposed for $(2\ifmmode\times\else\texttimes\fi{}3)$ and $(2\ifmmode\times\else\texttimes\fi{}6)$ reconstructions, and the character of the wavy ``$(1\ifmmode\times\else\texttimes\fi{}2)$'' reconstruction is discussed. The simulated scanning tunneling microscopy images for the proposed $(2\ifmmode\times\else\texttimes\fi{}6)$ reconstruction are in a particularly good agreement with the complex experimental images.

QPlus atomic force microscopy of the Si(100) surface: Buckled, split-off, and added dimers

Dimer configurations at the Si(100) surface have been studied with noncontact atomic force microscopy in the qPlus mode at 77 K, using both large (10 nm peak to peak) and small (0.5 nm peak to peak) oscillation amplitudes. In addition to the p(2×1), p(2×2), and c(4×2) reconstructions of the pristine surface, a variety of defect types including ad-dimers, vacancies, and split-off dimers have been imaged. Our data appear at odds with the currently accepted structural model for split-off dimers. At low oscillation amplitudes the degree of apparent dimer buckling can be “tuned” by varying the frequency shift set point.

Adsorbate-induced buckling switch of Si dimers in dissociated phenylamine on Si(001)

Using first-principles density-functional calculations, we investigate the adsorption structures of phenylamine, ${\text{NH}}_{2}({\text{C}}_{6}{\text{H}}_{5})$, on the Si(001) surface. We find that the N atom initially bonds to the down Si atom of the buckled dimer and subsequently, adsorbed ${\text{NH}}_{2}({\text{C}}_{6}{\text{H}}_{5})$ dissociates into $\text{NH}({\text{C}}_{6}{\text{H}}_{5})$ and H species. Unlike the case of ammonia where the buckling of Si dimers remains unchanged through ammonia dissociation, we find that the N-H dissociation in adsorbed phenylamine gives rise to the buckling switch of the neighboring Si dimers, thereby inducing a charge polarization in neighboring dimers such that the electrophilic (down-buckled) and nucleophilic (up-buckled) sides are oriented adjacent to the $\text{Si-NH}({\text{C}}_{6}{\text{H}}_{5})$ and Si-H moieties, respectively. This adsorbate-induced buckling switch in dissociated phenylamine is caused by a greater electronegativity of the phenyl group compared to the H atom in dissociated ammonia.

Electronic control of kink propagation at Ge(0 0 1) surface with STM: A perspective on the article: Flip motion of heterogeneous buckled dimers on Ge(0 0 1) by electron injection from STM tip by K. Tomatsu et al.

Electronic control of kink propagation at Ge(0 0 1) surface with STM: A perspective on the article: Flip motion of heterogeneous buckled dimers on Ge(0 0 1) by electron injection from STM tip by K. Tomatsu et al.

Flip motion of heterogeneous buckled dimers on Ge(0 0 1) by electron injection from STM tip

Surface motion of a topological defect between p ( 2 × 2 ) and c ( 4 × 2 ) structures, a “kink”, across buckled Sn–Ge and Si–Ge dimers on Ge(0 0 1) surfaces was investigated using scanning tunneling microscopy. Energy thresholds of π ∗ electrons for flipping these dimers in the kink are obtained by analyzing the kink surface motion. Electronic states of these systems and energy barriers for flipping the dimers are examined by first-principles calculations for considering elementary processes of the electronically-excited flip motion of the dimers. We propose that the flip motion is caused by a resonant scattering of the π ∗ electrons with localized electronic states at the kink.

Remarkably long-ranged repulsive interaction between adsorbed CO molecules on Pt modified Ge(001)

We have studied the adsorption of CO molecules on a Pt modified Ge(001) surface at 77 K. The CO molecules preferentially adsorb on the self-organized Pt chains rather than on the terraces. We have identified two different adsorption sites at 77 K: one either on the short-bridge site or on top of one of the atoms of a buckled Pt dimer, and the other one on the long-bridge site in between two Pt dimers. The CO molecules are mobile at room temperature but immobile at 77 K. A statistical analysis of the nearest-neighbor spacing of the CO molecules reveals that adsorbed CO molecules interact repulsively along the Pt-dimer chains. This repulsive interaction, composed of electrostatic terms and a strain-mediated term, gradually fades away over, for simple molecules, a remarkably long distance of 3--4 nm.

First-principles study of thermal and electron-activated dissociation of acetone on Si(001)

Using first-principles density-functional calculations, we investigate the reaction of acetone on the Si(001) surface, which exhibits the conversion from a kinetically controlled reaction to thermodynamically controlled one by means of thermal anneal or the highly confined electron beam of the scanning tunneling microscopy (STM) tip. We identified the four different reaction pathways forming not only two kinds of di-sigma structures on top of a single Si dimer (termed as the [2+2] cycloaddition structure) and across the ends of two adjacent Si dimers but also two bridge-bonded dissociative structures (termed the "end-bridge" and "dimer-bridge" structures) involving two adjacent Si dimers. Our calculated energy profiles for the reaction pathways show not only that formation of the [2+2] cycloaddition structure is kinetically favored because of its low-energy barrier, but also that, as temperature increases, the kinetically favored [2+2] cycloaddition structure is converted to the more thermodynamically stable end-bridge and dimer-bridge structures via an intermediate state where the O atom forms a dative bond to the down Si atom of the buckled dimer. In addition, we find that the Si-C bonding (antibonding) states of the [2+2] cycloaddition structure appear at about 1-2 (2-3) eV below (above) the Fermi level, in which injected holes (electrons) through the STM tip can be created (trapped) to give rise to a Si-C bond breakage. These results manifest that the kinetically controlled reaction of acetone on Si(001) is associated with the [2+2] cycloaddition structure, rather than the alpha-H cleavage structure proposed by a recent STM experiment.

First-principles molecular dynamics of the initial oxidation of Si{001} by ozone

Ozone has recently received considerable attention as an alternative oxidant in the low-temperature oxidation of silicon. Here, we use density-functional theory to elucidate the initial oxidation stages of ${\text{O}}_{3}$ on Si{001}. Ozone favors a barrierless dissociation with two surface reaction centers involving either the up $({\text{Si}}^{u})$ or the down $({\text{Si}}^{d})$ dimer atom of the buckled Si dimer. The ${\text{Si}}^{u}$ site exhibits a strong steering effect which leads typically to partial dissociation, whereas ${\text{Si}}^{d}$ initially interacts only weakly with the molecule, resulting sometimes in complete dissociation. We discuss the underlying electronic structure of the reaction, with particular emphasis on the spin evolution.

Model systems for exploring electron correlation effects in the buckling of SiSi dimers on the Si(100) surface

Si2H4 and Si7H8 cluster models are constructed for studying the role of high-order electron correlation effects on the buckling of SiSi dimers on the Si(100) surface. CASPT3, multi-reference coupled-pair functional, and the diffusion Monte Carlo methods are used to examine whether correlation effects not recovered in CASPT2 calculations are important for the buckling of the dimers. The calculations show that such high-order correlation effects are indeed important for determining the relative stability of the buckled and unbuckled structures.

Density-Functional Calculations of the Adsorption and Reaction of Acetic Acid on Ge(001)

The adsorption and reaction of acetic acid on the Ge(001) surface are investigated by first-principles density-functional calculations. We find that the carbonyl O atom initially bonds to the down Ge atom of the buckled dimer, and subsequently, O-H dissociation takes place through the intradimer or interdimer transfer of the H atom. The resulting monodentate (MD) structure after the intradimer H-atom transfer remains stable, while that after the interdimer H-atom transfer proceeds to formation of the end-bridged (EB) bidentate structure across the ends of two adjacent dimers by forming an additional Ge-O bond on the opposite side of the H-atom transfer. It is also possible that the latter MD structure proceeds to formation of the on-top (OT) bidentate structure on a single Ge dimer, but it is kinetically difficult because of the existence of a repulsive interaction between an O lone pair and a single Ge dangling bond. However, our calculated energy profiles for the reaction pathways predict that, as temperature increases, the relatively less stabilized MD and EB structures are converted into the most stable OT structure, as observed by a recent scanning tunneling microscopy experiment.

Dehydrogenation of 1,4-cyclohexadiene on Si(001): A first-principles study

The adsorption and reaction of 1,4-cyclohexadiene on the Si(001) surface are investigated by first-principles density-functional calculations. We find that there are two kinds of single di-$\ensuremath{\sigma}$ bonding configurations: One is the on-top (OT) structure on top of a single Si dimer and the other is the end-bridge (EB) structure across the ends of two adjacent Si dimers in the same dimer row. Formation of the OT and EB structures is kinetically facilitated through the $\ensuremath{\pi}$-complex precursor state in which the electron-deficient down Si atom of the buckled dimer attracts the $\ensuremath{\pi}$ bond of 1,4-cyclohexadiene. However, there exists a drastic difference in dehydrogenation kinetics of the two single di-$\ensuremath{\sigma}$ bonding configurations. Dehydrogenation of the OT structure hardly takes place because of the existence of a high activation barrier of $1.34\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, whereas that of the EB structure is kinetically feasible at room temperature with a relatively lower activation barrier of $0.68\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Thus, we can say that the recently observed dehydrogenation process in the single di-$\ensuremath{\sigma}$ bonding configuration would be associated with the EB structure rather than the OT structure. We also discuss the recent experimental data about the desorption of produced benzene and the formation of double di-$\ensuremath{\sigma}$ bonding configuration.

CAICISS and STM study of [formula omitted] and [formula omitted] tin phases on Si(1 0 0)

Tin (Sn) adsorption on Si(100) surface has been studied by scanning tunneling microscopy (STM) and co-axial impact collision ion scattering spectroscopy (CAICISS). The c(8×4) and (5×1) phases were analyzed to reveal their atomic structure. It has been confirmed that the c(8×4) structure consists of buckled Sn dimers. The (5×1) structure has been found to be partially irregular along (5×1) rows and to consist of double Sn layer. The Sn atoms in the first layer are undimerised while those in second layer form asymmetric dimers.

Kinematic simulation of convergent beam low-energy electron diffraction patterns

Kinematic simulations are used to investigate the geometrical properties of convergent beam low-energy electron diffraction patterns from an Si(0 0 1) surface. Compression of a low-energy electron microscope immersion lens is included explicitly and the sensitivity of patterns to surface reconstruction and dimer buckling is investigated. Key pattern features and whole pattern symmetries are identified from the simulations and interpreted geometrically. Advantages over conventional low-energy electron diffraction techniques are identified.

Development of Noncontact Atomic Force Microscopy Operating at Low Temperatures

Noncontact atomic force microscopy (NC-AFM) using frequency modulation detection method has been widely used to investigate the various surfaces with atomic resolution. In this paper, we introduce the measurement technique of NC-AFM operating at low temperatures (LTs). First, we theoretically discuss the enhancement of the force sensitivity in NC-AFM operating at LTs. Then, we present the design and performance of LT-NC-AFM using fiber optic interferometer with quick sample and cantilever exchange mechanism. We also show the present status of the LT-NC-AFM imaging. In detail, we show the experimental results to investigate the influence of the surface stress around an SA step of Si(001) surface onto the buckled dimer at 5 K. We demonstrate that the LT-NC-AFM has a capability to detect the surface stress with atomic resolution.

First-principles study of the adsorption and reaction of 2,3-butanediol on Si(001)

The adsorption and reaction of 2,3-butanediol on the Si(001) surface are investigated by first-principles density-functional calculations within the generalized-gradient approximation. We consider the two different reaction pathways forming adsorptions on top of a single dimer (termed the ``on-top'' structure) and across the ends of two adjacent dimers in the same dimer row (termed the ``end-bridge'' structure). For both reaction pathways, the O atom of the hydroxyl group initially bonds to the down Si atom of the buckled dimer and, subsequently, O-H dissociation takes place. However, the reaction of another hydroxyl group shows a drastic difference between the two reaction pathways. Along the former reaction pathway, it is difficult to form the second $\mathrm{Si}\mathrm{O}$ bond because of a repulsive interaction between an O lone pair and a single Si dangling bond, thereby prohibiting formation of the on-top structure. On the other hand, along the latter reaction pathway, an O lone pair of the hydroxyl group is attracted to the down Si atom of a neighboring dimer, proceeding to the formation of the end-bridge structure. These results provide an explanation for a recent scanning tunneling microscope observation that 2,3-butanediol occupies predominantly the end-bridge structure.

Energetics and Rate Constants of Si2H6 and Ge2H6 Dissociative Adsorption on Dimers of SiGe(100)-2 × 1

Dissociative adsorption of Si2H6 and Ge2H6 on various buckled dimers of SiGe(100)-2 × 1 surface has been studied using density functional theory at the B3LYP level. Two paths, individually leading to (I) two chemisorbed SiH3 or GeH3 and (II) surface hydrides plus a gas-phase SiH4 or GeH4, are considered in the energetics calculation. Contrary to the intuitive mechanism involving a four-center transition state (TS) that yields two SiH3(a) or GeH3(a) in one step, the intermolecular bond scission on a dimer is accomplished in two consecutive steps of three-center TS, in which the first step is rate-limiting. The calculated energy barrier of the first step in Si−Si bond scission is higher than that of the H−Si bond scission on Si*−Si dimer by 9 kcal/mol. Therefore, in contrast to the conventional wisdom of favoring Si−Si bond cleavage over H−Si as the first step in chemisorption on Si(100)-2 × 1, the calculation result supports Niwano and co-workers' experiments that concluded that Si2H6 adsorption without breaking the Si−Si bond was preferred. On the other hand, the Ge alloying on the surface reduces the barrier height difference between these two paths. Hence, the energy blockade between dehydrogenation and intermolecular bond scission decreases when Ge2H6 is added in Si2H6 chemical vapor deposition. Rate constants are also calculated and the results are qualitatively in line with the Ge2H6 reactivity on Si(100) measured by Lam et al.

Surface reconstruction and core distortion of silicon and germanium nanowires

We report the results of molecular dynamics simulations for structures of pristine siliconnanowires and germanium nanowires with bulk cores oriented along the [110]direction and bounded by the (100) and (110) surfaces in the lateral direction. Wefound that the (100) surfaces for both silicon and germanium nanowires undergo2 × 1 dimerization while their (110) surfaces do not show reconstruction. The direction of the dimerrows is either parallel or perpendicular to the wire axis depending on the orientation of thesurface dangling bonds. The dimer length for Si is in good agreement with the result obtainedby first-principles calculations. However, the geometry of Si dimers belongs to the symmetrical2 × 1 reconstruction rather than the asymmetrical buckled dimers. We also show that surfacereconstruction of a small nanowire induces significant change in the lattice spacing for theatoms not on the (100) surface, resulting in severe structural distortion of the core of thenanowire.

Molecular interactions and decomposition pathways ofNH3on Si(001)

Ammonia adsorbs dissociatively on the Si(001) surface above $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The majority dissociation products are -H and -$\mathrm{N}{\mathrm{H}}_{2}$ groups, adsorbed on the dangling bonds of the Si dimers. At low coverages, the ammonia fragments are observed to form clumps, which extend both along and across dimer rows, suggesting interactions between molecules: either direct interactions, or those mediated by the buckled dimers on the Si(001) substrate. The detailed reactions in the adsorption and dissociation of $\mathrm{N}{\mathrm{H}}_{3}$ on Si(001) have been investigated by density functional theory calculations, and compared to data from scanning tunneling microscope images. The buckling of the Si dimers provides a mechanism for preferential adsorption next to existing adsorbed species, along a single dimer row. We find that direct H-bonding interactions between chemisorbed $\mathrm{N}{\mathrm{H}}_{2}$ groups and incoming $\mathrm{N}{\mathrm{H}}_{3}\phantom{\rule{0.3em}{0ex}}$ molecules are also significant, including an interaction across dimer rows, which explains the observed clumping behavior.