1,721,082 research outputs found
Theory of the site-selective reaction of NH3 with Si(111)-(7x7)
No full textAdsorption of NH3 molecules on the Si(111)-(7x7) surface has been studied by density-functional theory calculations. We find that the molecular adsorption is strongly site selective: the active site is Si adatoms. Si restatoms appear repulsive towards NH3 molecules, contrary to experimental suggestions and a recent theoretical prediction. The adsorbed molecule on an adatom undergoes a low-barrier dissociation process, which is readily activated by an adatom backbond breaking and ends up with capping an adjacent restatom by the dissociated H species. We also find that the 2-to-1 adatom-restatom population ratio of Si(111)-(7x7) makes the precursor-mediated dissociation process self-limiting and becomes the origin of the intriguing experimental reaction behavior, an early saturation of restatom features and a partial saturation of adatom features.open1118sciescopu
Hexagonal indium double layer on Si(111)-root 7 x root 3
No full textDensity-functional calculations are used to verify the atomic structure of the hexagonal In/Si(111)-(root 7 x root 3) surface, which has been considered to represent an ultimate two-dimensional (2D) limit of metallic In overlayers. Contrary to the prevailing assumption, this surface consists of not a single layer but a double layer of In atoms, which corresponds to a hexagonal deformation of the well-established rectangular In double layer formed on Si(111)-(root 7 x root 3) [Park and Kang, Phys. Rev. Lett. 109, 166102 (2012)]. The same double-layer thickness accounts well for the typical coexistence of the hexagonal and rectangular phases and their similar 2D electronic structures. It is thus conclusive that, regardless of rectangular or hexagonal, the In/Si(111)-(root 7 x root 3) surface does not represent a one-atom-thick In overlayer.1133sciescopu
Single-Layer Limit of Metallic Indium Overlayers on Si(111)
No full textDensity-functional calculations are used to identify one-atom-thick metallic In phases grown on the Si(111) surface, which have long been sought in quest of the ultimate two-dimensional (2D) limit of metallic properties. We predict two metastable single-layer In phases, one root 7 x root 3 phase with a coverage of 1.4 monolayer (ML; here 1 ML refers to one In atom per top Si atom) and the other root 7 x root 7 phase with 1.43 ML, which indeed agree with experimental evidences. Both phases reveal quasi-1D arrangements of protruded In atoms, leading to 2D-metallic but anisotropic band structures and Fermi surfaces. This directional feature contrasts with the free-electron-like In-overlayer properties that are known to persist up to the double-layer thickness, implying that the ultimate 2D limit of In overlayers may have been achieved in previous studies of double-layer In phases.1153sciescopu
Atomic origins of Si 2p and Au 4f surface core-level shifts on Au/Si(111)-(5 x 2)
No full textWe have performed density functional theory calculations to investigate the core-level shifts on the Au/Si(111)-(5 x 2) surface. The employed structural model, which was recently proposed as containing seven Au atoms per (5 x 2) unit cell [Phys. Rev. Lett. 113, 086101 (2014)], is found to well reproduce the Si 2p and Au 4f surface core-level shifts reported by previous x-ray photoelectron spectroscopy studies, thereby clarifying their atomic origins.1131sciescopu
First-principles study of the dissociative adsorption of NH3 on the Si(100) surface
No full textWe have studied the energetics and bonding geometry of the ammonia adsorbed Si(100) surface by performing density-functional total-energy calculations within generalized gradient approximation. Ammonia molecules are found to adsorb on the down atoms of buckled Si dimers with an adsorption energy of 1.2 eV, but then exists a low-energy (of about 0.6 eV) activation pathway to the more stable dissociative chemisorption where NH2 (H) bonds to the down (up) Si atom. This energy diagram implies that the molecular physisorption may survive at sufficiently low temperature, but thermal activation at higher temperatures possibly carries the system into the dissociated state over the energy barrier. Especially, the calculated adsorption energy (of 2.0 eV) and bonding geometry for the dissociated state is in good agreement with the measurements in recent desorption and photoelectron diffraction experiments.open1165sciescopu
Electronic and vibrational properties of initial-stage oxidation products on Si(111)-(7x7)
No full textChemisorption of O-2 molecules on the adatom site of Si(111)-(7X7) has been studied by density-functional theory calculations for all possible dissociation configurations. Structures possessing an oxygen atom on top of the Si adatom are all found to be metastable and account well for the metastable electronic and vibrational spectra observed in previous experiments, while structures with only oxygen atoms inserted into the adatom back bonds appear quite stable. The present structural models therefore are all compatible with either the metastable or the stable O-2 reactions products found in this system. The calculated decay pathways of the metastable structures provide additional informations useful for identifying the experimental metastable structures.open1126sciescopu
Adsorption structure of pyrazine on Si(100): Density-functional calculations
No full textWe have studied the adsorption structure of pyrazine (C(4)H(4)N(2)) on the Si(100) surface by using density-functional theory calculations within a slab representation. We found that the adsorption energetics strongly depends on the C(4)H(4)N(2) coverage. At 0.25 monolayer (one monolayer is defined as one pyrazine molecule per Si dimer), two different molecular configurations are equally favored in energy: the end-on model where C(4)H(4)N(2) adsorbs on the down atom of a Si dimer and the cross-row model where C(4)H(4)N(2) connects two dimer rows. At 0.5 monolayer, the interactions between adsorbed molecules results in a far stabilized cross-row configuration in which molecules arrange in series along the row-perpendicular direction. This cross-row chain configuration is in good accordance with the images of a recent scanning tunneling microscopy experiment. When including the van der Waals interactions in our calculations, the cross-row structures were found to have larger energy gains, thus being more favored in energy than the end-on structures. The adsorption picture of the present slab-model calculations differs from previous cluster-model calculations, which questions the accuracy of the cluster representation of this adsorbed system.open111111sciescopu
Identification of the Au Coverage and Structure of the Au/Si(111)-(5 x 2) Surface
No full textWe identify the atomic structure of the Au/Si(111)-(5 x 2) surface by using density functional theory calculations. With seven Au atoms per unit cell, our model forms a bona fide (5 x 2) atomic structure, which is energetically favored over the leading model of Erwin et al. [Phys. Rev. B 80, 155409 (2009)], and well reproduces the Y-shaped and V-shaped (5 x 2) STM images. This surface is metallic with a prominent half filled band of surface states, mostly localized around the Au-chain area. The correct identification of the atomic and band structure of the clean surface further clarifies the adsorption structure of Si adatoms and the physical origin of the intriguing metal-to-insulator transition driven by Si adatoms.open111918sciescopu
Site-selective chemisorption of CH3OH on si(111)-(7x7): Density-functional theory calculations
No full textChemisorption of CH3OH on the Si(111)-(7x7) surface has been investigated by using density-functional theory calculations. We find that the Si adatom site is reactive towards CH3OH while the Si restatom site appears to be repulsive. At the adatom site, the adsorbed CH3OH molecule dissociates into CH3O + H without any appreciable energy barrier, leading to the final products CH3O-Si(adatom) and H-Si(restatom). This reaction picture explains well the dissociation geometry reported by a scanning-tunneling-microscopy study, but differs from the prediction of previous studies that the restatom is more reactive than the adatom. The origin of the discrepancy between the present and previous studies will be discussed.X114sciescopuskc
Atomic structure of alkali metal (Li, Na, K) adsorbed Ge(111)-(3x1) surfaces
No full textWe have studied the atomic structure of alkali metal (AM) adsorbed Ge(111)-(3x1) surfaces using the pseudopotential and density-functional theory. Our total-energy calculations for Li, Na, and K demonstrate that the AM/Ge(111)-(3x1) surfaces have the same ground-state structure as the AM/Si(111)-(3x1) surfaces, which is referred to as the honeycomb-chain-channel model, and show systematic variations in surface bonding geometry over the AM adsorbates. Details of the structural changes are reported and discussed in connection to the questions raised by recent scanning tunneling microscopy and x-ray photoelectron spectroscopy studies.open1119sciescopu
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