Half Unit Cell Research Articles

Investigation of 1,1-dichloroethene interacting with the Si(1 1 1)–7 × 7 surface studied by scanning tunneling microscopy

Adsorption of 1,1-dichloroethene (1,1-DCE) at the Si(111)–7×7 surface has been investigated using scanning tunneling microscopy. 1,1-DCE dissociates upon adsorption by breaking one or both CCl bonds. The appearance of reacted adatoms in the 7×7 reconstruction is found to vary for both positive and negative sample bias voltages in the range of 0.8V to 2.5V. Dissociated Cl atoms bond to adatom sites and appear bright for bias voltages higher than ±1.4V. The other dissociated species appear dark for bias voltages below ±1.85V with a preference of 2:1 for bonding to center relative to corner adatom sites. The faulted half unit cell is preferred. It is demonstrated that rest atoms are active in the dissociation of two-thirds of the 1,1-DCE molecules.

Spontaneous formation of Mn nanocluster arrays on aSi(111)−7×7surface observed with STM

An extreme low-deposition rate has been used to fabricate highly ordered Mn nanocluster arrays on A Si(111)-7x7 surface. Scanning tunneling microscopy (STM) studies reveal that the Mn nanoclusters prefer to occupy the faulted half-unit cells of the Si(1110)-7x7 with preference fraction similar to 95%. High-resolution STM images demonstrate that two types of three-dimensional Mn nanoclusters coexist in the arrays; one is an identical triangle cluster and the other is an off-center pear cluster. A planar identical Mn cluster has been obtained at the initial deposition stages, indicating the coverage-dependent Mn clustering. Careful analysis of the initial adsorption stage of Mn reveals that individual Mn atoms prefer to occupy the bridging B(2) sites around Si rest atoms and the substitution site of Si center adatoms. The formation mechanism of Mn nanocluster arrays is attributed to the delicate balance between thermodynamic tendency and kinetic limitation.

Self-Assembly of Chiral Molecular Honeycomb Networks on Au(111)

Self-assembly of the specifically designed 3-fold symmetric 1,3,5-trikis(4'-carboxylphenyl)-2,4,6-trikis(4'-tert-butylphenyl)-benzene molecule on a Au(111) surface is investigated by means of in situ ultrahigh vacuum scanning tunneling microscopy. A variety of chiral honeycomb networks with increasing interpore distance is observed. Experiments and force field calculations reveal that the formation of the two-dimensional hexagonal porous networks is driven by a balance of molecular close-packing within half-unit cells and cyclic hydrogen bonding dimerization between half-unit cells. The honeycomb networks are shown to be chiral owing to asymmetric molecular close-packing within the half-unit cells.

Defects and Pd growth on the reduced SnO 2(1 0 0) surface

Scanning tunneling microscopy experiments on a clean, reduced SnO 2(1 0 0)-(1 × 1) surface reveal surface defects with zero-, one-, and two-dimensions. Point defects consist of missing SnO/SnO 2 units. Line defects are probably crystallographic shear planes that extend to the surface and manifest themselves as rows of atoms, shifted half a unit cell along the [0 1 0] direction. Their ends act as preferential nucleation sites for the formation of Pd clusters upon vapor deposition. Areas of a more reduced surface phase, still with a (1 × 1) structure and a half-unit cell deep, form at [0 0 1]-oriented step edges.

Adsorption structure of a single C 60 molecule on Si(1 1 1)-(7 × 7): Density-functional calculations

We have studied the structure and bonding nature of a single C 60 molecule adsorbed on the Si(1 1 1)-(7 × 7) surface using density-functional theory calculations. Based on total-energy calculations and scanning-tunneling-microscopy (STM) simulations, we propose a structural model for the most representative adsorption configuration reported by STM experiments. In this model, C 60 adsorbs at a position about 1 Å apart from the middle of the half unit cell with a C–C bond shared by pentagon and hexagon rings facing down toward the surface. We find in the calculated surface density of states that the highest occupied molecular orbital (HOMO) peak of C 60 is resolved by the interaction with the surface into two levels, in good agreement with photoelectron spectroscopy data. It is shown in the charge characters of the affected electronic states that the nature of the molecule-surface interaction is basically the C–Si covalent bondings.

Magic clusters and [formula omitted] local structure formed in a half-unit cell of the Si(1 1 1)- [formula omitted] surface by Tl adsorption

Formation of a ( 2 × 1 ) local structure is found at submonolayer growth of Tl on the Si(1 1 1)- ( 7 × 7 ) surface by means of scanning tunneling microscopy (STM). At low coverages, the ( 7 × 7 ) grid remains and the ( 2 × 1 ) structure is formed inside of the half-unit cells of the ( 7 × 7 ) . The ( 2 × 1 ) coexists with other structures, such as of “magic” clusters observed in the case of all other group III metals. Based on our STM observations we propose an atomic arrangement of the ( 2 × 1 ) structure.

Metal-cluster growth with asymmetric diffusion and reversible aggregation on Si(111)7×7

The nucleation and growth behavior of metal nanoclusters on a Si(111)7×7 reconstructed surface is studied by using a kinetic Monte Carlo simulation. The main atomistic processes included in the model are diffusion of adatoms between half-unit cells of the 7×7 surface, and aggregation and decay of clusters occurred inside the half-unit cells. Considering the structural difference between faulted and unfaulted half-cells, the model introduces an asymmetric adatom aggregation mechanism by setting different effective binding energies of clusters in the two different types of half-cells, which gives rise to preferential occupation of clusters for the faulted half-cells. The simulation of the model produces growth mode transitions from hit and stick regime at low temperatures to self-organized growth regime at high temperatures. A comparison with relevant growth experiments demonstrates good qualitative agreement.

STM study of charge transfer and the role of rest-atoms in the binding of benzene to Si(1 1 1)7 × 7

Chemisorbed benzene was studied by room temperature STM at low coverage on Si(1 1 1)7 × 7. Images taken of both filled and unfilled states show effects on adatoms near the benzene molecules. Filled states imaging shows a significant amount of what we believe to be charge transfer throughout the involved half unit cell. For a benzene molecule bound to a middle adatom there is also significant charge transferred from the adjacent middle adatom of the neighbouring half unit cell. Symmetry considerations allow for the conclusion that benzene binds in a di-σ bridge configuration between an atom and rest-atom. Evidence has been obtained that charge transfers away from the involved rest-atom to adjacent adatoms. Empty states imaging of the π ∗ state also provides evidence for the involvement of a rest-atom in the binding.

Influence of substrate temperature on submonolayer Au adsorption on anSi(111)-(7 × 7) surface studied by scanning tunneling microscopy

The influence of substrate temperature on the adsorption of submonolayer Au on anSi(111)-(7 × 7) surface was investigated using a scanning tunneling microscope. With a small amount of Au depositedat room temperature, Au atoms form small clusters located inside the half unit cells (HUCs) of theSi(111)-(7 × 7) surface while most parts of the surface are undisturbed. When the substrate temperatureduring deposition of the same Au amounts increases from room temperature up to565 °C,the small Au clusters grow, coalesce into larger clusters, incorporate Si atoms, and finally form a(5 × 2) reconstruction.The Si(111)-(5 × 2)–Au reconstruction takes place in the form of island–hole pairs on a terrace, whereas onlyholes are formed in the upper terrace at a step. Both the islands and the holes display the(5 × 2) reconstruction. The occurrence of the island–hole pairs of the(5 × 2) reconstruction results from the fact that the Si atom density in the(5 × 2) reconstruction islower than in the (7 × 7) reconstruction.

Chemical and structural variations at augite (100) deformation twin boundaries

It is commonly found that deformation twinning response to plastic deformation results in structural variations at a twin boundary (TB). Chemical heterogeneity at a TB, compared to the host, however, has rarely been reported. An augite sample with a high density of deformation twinning lamellae from Lofoten, Norway was examined using high-resolution transmission electron microscopy (HRTEM) and energy-filtered TEM (EFTEM) techniques to characterize both structural and chemical variations at a TB. HRTEM experimental results combined with high-resolution image simulation unambiguously resolve the augite deformation TB structure as a half unit cell of orthopyroxene. EFTEM images at TB demonstrate that Ca is depleted, whereas Fe is enriched at this sub-nanometer scale (0.9 nm) interface. In addition, distance-least-squares (DLS) program was applied to verify that the TB structure is geometrically stabilized by substitution of Fe for Ca.

Intermediate surface structure of Al nanoclusters restricted to Si(111) half-unit cells observed via scanning tunneling microscopy

We have observed an intermediate surface structure between two different surface phases of $\mathrm{Al}∕\mathrm{Si}$ (111) using the scanning tunneling microscope. The intermediate structure between the Al nanocluster and the $\ensuremath{\gamma}$ phase shows a large cluster with the size of a half-unit cell. The structural model of the $\ensuremath{\gamma}$-phase $\mathrm{Al}∕\mathrm{Si}$ (111) has been reasonably proposed through the intermediate Al cluster.

Diffusion of Water in a Synthetic Clay with Tetrahedral Charges by Combined Neutron Time-of-Flight Measurements and Molecular Dynamics Simulations

The dynamics of water molecules confined in the interlayer space of a synthetic Na saponite clay with a tetrahedral layer charge of 0.7 per half unit cell were studied by combining time-of-flight quasi-elastic neutron experiments (QENS) and molecular dynamics simulation along the water adsorption isotherms. In the monolayer regime, three different adsorbed amounts corresponding to increasing fillings of the interlayer space were investigated whereas two situations (adsorption and desorption) were investigated in the bilayer region. In a first step, molecular dynamics were used to reexamine some of the approximations classically used in the analysis of water motion by QENS (i.e., separation between rotational and translational diffusion, preferred orientation of molecules, and contribution to the elastic incoherent structure factor). A careful analysis of experimental data obtained at two wavelengths and for two orientations of the clay platelets and validated by simulations yielded rotational and translat...

A Unified Model: Self-Assembly of Trimesic Acid on Gold

Self-assembly of trimesic acid (TMA) displayed remarkable abundance over its full coverage range on gold under ultrahigh vacuum conditions. Experiments showed that previously well-reported “chicken wire” and “flower” structures were actually two special cases within its full coverage. All observed assembling structures formed hexagonal porous networks that could be well-described by a unified model in which the TMA molecules inside the half unit cells (equilateral triangles) were bound via trimeric hydrogen bonds and all half unit cells were connected to each other via dimeric hydrogen bonds. These porous networks possessed pores of 1.1 ± 0.1 nm in diameter, and the interpore distance was tunable from 1.6 nm on at a step size of ∼0.93 nm. Energetics analysis unveiled that the assembling structures less than one molecular layer was optimally driven by maximization of the dimeric hydrogen bonds.

Formation and stabilization of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template

We demonstrate the growth of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template by in situ scanning tunneling microscopy (STM). These clusters form near a dimer row at one side of the half-unit cell (HUC); and with three different equivalent orientations. A cluster model comprising three top layer Si atoms bonded to six Fe atoms at the next layer in the 7 × 7 faulted-half template is proposed. The optimized cluster structure determined by first-principles total-energy calculation shows an inward-shifting of the three center Fe atoms. The clusters and the nearby center-adatoms of the next HUCs appear with a significantly reduced height below bias voltages 0.4 V in high resolution empty-state STM images, suggesting an energy gap opening near the Fermi level at these localized cluster and adatom sites. We explain the stabilization of the clusters on the 7 × 7 template using the gain in electronic energy as the driving force for cluster formation.

Dynamics of copper atoms on Si(1 1 1)-7 × 7 surfaces

This study investigated the dynamics of copper atoms adsorbed on Si(1 1 1)-7 × 7 surfaces between 300 K and 623 K using a variable-temperature scanning tunneling microscope (STM). The diffusion behavior of copper clusters containing up to ∼6 atoms into a particular half unit cell of the 7 × 7 reconstructed Si(1 1 1) surface was considered. The movements and the formation of copper clusters were tracked in detail. The activation energies and pre-exponential factors for various diffusion paths were estimated. Finally, the Cu-etching-Si process and the quasi-5 × 5 incommensurated phase of Cu/Si islands were discussed.

Si nucleation on Si(1 1 1)-7 × 7: From cluster pairs to 2D islands

Nucleation of 2D islands in Si/Si(1 1 1)-7 × 7 molecular beam epitaxy is studied using scanning tunneling microscopy (STM). A detailed analysis of the population of small amorphous clusters coexisting on the surface with epitaxial 2D islands has been performed. It is shown that small clusters tend to form pairs. The pairs serve as precursors for 2D islands as confirmed by direct STM observations of the smallest 2D islands covering two adjacent half-unit cells of the 7 × 7 reconstruction. It is proved with scaling arguments that the critical nucleus for 2D island formation consists not only of the pair itself, but also includes additional adatoms not belonging to the stable clusters.

Formation and evolution of a self-organized hierarchy of Ge nanostructures onSi(111)−(7×7): STM observations and first-principles calculations

We report scanning-tunneling-microscopy observations and first-principles calculations for the formation and evolution of self-organized Ge nanostructures on $\mathrm{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ surfaces for Ge coverages up to 0.5 ML. We show that individual Ge atoms initially form a triangular lattice. At higher coverages, Ge nanoparticles $1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter gradually form in both the faulted and unfaulted half unit cells with an initial preference in the faulted halves, ultimately driving ordered hexagonal arrays. The underlying $7\ifmmode\times\else\texttimes\fi{}7$ surface periodicity, the triangular single-Ge lattice, and the nanoparticle hexagonal superstructures coexist. Charge transfer from Si adatoms to Ge nanoparticles is shown to play a key role in the self-organization.

The atomistic growth of silver clusters on a Si(111)7 × 7 surface

We repot here the growth of silver nanostructures on a Si(111)7 × 7 surface using scanning tunneling microscopy(STM). It is found that the each Ag atom hops each half unit cells of 7 × 7 structure and forms dimers, trimers, tetramers and pentamers inside the half unit cells. We estimate intercell hopping rates of (1.39±0.16) × 10−4 1/s for trimers and (1.33±0.14) × 10−31/s for tetramers. We also observed the intracel motion of trimers.

Patterned Growth of Nanoscale In Clusters on the Si(111)-7×7 and Si(111)-Ge(5×5) Reconstructions

Results of a study designed to investigate the possibility of using the Si(111)- Ge(5×5) surface reconstruction as a template for In cluster growth are described. As with Si(111)-7×7, the In adatoms preferentially adsorb in the faulted half-unit cell, but on Si(111)- Ge(5×5) a richer variety of cluster geometries are found. In addition to the clusters that occupy the faulted half-unit cell, clusters that span two and four half-unit cells are found. The latter have a triangular shape spanning one unfaulted and three, nearest neighbor, faulted half-unit cells, Triangular clusters in the opposite orientation were not found. Many of the faulted halfunit cells have a streaked appearance consistent with adatom mobility.

Hydrothermal synthesis and characterization of dioctahedral smectites: A montmorillonites series

The aim of this study is to synthesize and finely characterize montmorillonite samples, dioctahedral smectites without tetrahedral charges (structural formulae Nax(Al(2−x)Mgx)Si4O10(OH)2), to allow their use as reference samples in clay science. The montmorillonites synthesis under hydrothermal conditions at different pressures and with various layer charge deficit has been attempted. The temperature was fixed at 320 °C, the pressure parameter values were 20 MPa, 80 MPa, 120 MPa and 200 MPa. The Mg content varied from 0.25 to 0.60 per half unit cell. The reaction products have been characterized with multi-technique analyses (ICP-AES, EMP, CEC, XRD, FTIR, NMR and TGA).Montmorillonite phase was only produced at 120 and 200 MPa.At 20 and 80 MPa, the results suggest that a 0.33 and 0.16-tetrahedral charge deficit exist in the formed samples. Moreover, the octahedral occupancies are higher than two (2.15 and 2.07 at 20 and 80 MPa respectively). In these experimental conditions, the synthetic smectites are mixtures between montmorillonite, beidellite and saponite.At 120 MPa and for a Mg content of 0.25 or higher than 0.33, the synthetic products were also mixtures of smectites. Tetrahedral charge deficits of 0.11, 0.11 and 0.15 were found for Mg contents of 0.25, 0.50 and 0.60 respectively. The octahedral occupancy was also higher than 2.00.A montmorillonite phase with only octahedral charges and an octahedral occupancy near 2.00 was synthesized for a Mg content of 0.33 and at pressures equal to or higher than 120 MPa. This low charge reference smectite shows a very low amount of accessory minerals and an octahedral charge deficit only created by the presence of magnesium in the structure. This montmorillonite can be compared structurally to the most studied natural one: the montmorillonite SWy-2 from Wyoming.