Si Rest Atoms Research Articles

Clarification of Au dimer adsorption sites on Si (111)-7 × 7 surface by AFM/KPFM

Atomic-scale surface adsorption has been a significant research topic in recent years, with a particular emphasis on the adsorption properties of Au/Si(111)-7 × 7, which are vitally important for pioneering future novel semiconductor devices. Here, we investigated the adsorption of Au dimers on the Si(111)-7 × 7 surface with atomic resolution using non-contact atomic force microscopy and Kelvin probe force microscopy at room temperature. Our results show that the Au dimer adsorbs in the vicinity of the Si rest atoms, exhibiting a distinct localized electron distribution. In density functional theory calculations, three candidate Au dimer adsorption sites have been identified, and the most stable site of Au dimer adsorption aligns with experimental findings. Furthermore, the local electron transfer of Au dimer adsorption has been analyzed, confirming the distribution of electrons around the Au dimer adsorption site. This research reveals that the structure and charge transfer of adsorbed Au dimers on Si(111)-7 × 7 provide insight into the mechanism of the metal-semiconductor system.

Si(775)-Au atomic chains: Geometry, optical properties, and spin order

The geometry and electronic structure of self-assembled atomic scale Au wires on Si(775) are investigated within density functional theory. The calculated surface diagram indicates the existence of two stable configurations with Au coverages of 0.32 and 0.96 monolayers, respectively. The low-coverage structure is predicted to host an antiferromagnetic spin chain localized at the Si rest atom dangling bonds, while the high-coverage structure is characterized by a Au-induced $\ensuremath{\beta}\text{\ensuremath{-}}\sqrt{3}$-like structure on the terrace. These structural models are supported by the comparison of measured and calculated surface optical anisotropies.

Initial stages of Lutetium growth on Si (111)-7 × 7 probed by STM and core-level photoelectron spectroscopy

The interaction of small amounts of Lutetium with the Si (111)-7 × 7 reconstructed surface was investigated in detail using a combination of Scanning Tunneling Microscopy (STM) and Photoelectron Spectroscopy (XPS and UPS). Various immobile and also fastly moving atoms and nanocluster were found in the initial growth of the Lu/Si interface. Density functional theory calculations and photoelectron spectroscopy results suggest that the most attractive adsorption sites for the Lu atoms are basins around Si rest-atoms and there is no strong interaction between Lu and Si at the initial steps of film growth. However Lu nanocluster could also be found on other adsorption sites which results in a different voltage dependence in STM. Coverage-dependent STM images reveal the growth of a closed Lu metal overlayer by joining of the clusters. The existence of a stoichiometric Lu silicide compound was not detected on the surface in the initial growth for deposition at room temperature.

Comparative STM analysis of C60 and C70 fullerene adsorption sites on pristine and Al-modified Si(111)7 × 7 surfaces

Using scanning tunneling microscopy observations, preferable adsorption sites of C60 and C70 fullerenes on pristine Si(111)7 × 7 and Al-modified Si(111)α–7 × 7-Al surfaces at 300 and 450 K were determined. The C60 and C70 molecules display similar, albeit not identical, adsorption behavior while the most essential variance is related with the hosting surfaces, pristine 7×7 or α–7 × 7-Al. Both C60 and C70 prefer to occupy positions above the Si rest atoms upon adsorption onto Si(111)7 × 7 at 300 K and change their preferable sites for those located at the Si edge adatom at 450 K, that is plausibly accompanied by releasing the Si adatom. Upon C60 and C70 adsorption onto Si(111)α–7 × 7-Al surface at 300 K, the fullerenes prefer to occupy the asymmetric sites at the dimer rows. At 450 K, the C70 fullerenes preserve this location, while about half of the C60 fullerenes change it for that at the corner Si adatom position. The present experimental data set serves as a useful basis for theoretical analysis.

Inhibiting molecular array formation on Si(111)-7×7using site-selective Ge/Si exchange

The Si(111)-$7\ifmmode\times\else\texttimes\fi{}7$ surface reconstruction can be used to template an array of mesitylene (1,3,5-trimethyl-benzene) molecules in which each molecule bridges a corner Si adatom and a neighboring Si rest atom. The molecules adsorb in corner sites to minimize a weak steric interaction associated with nearest-neighbor bridging sites. We demonstrate that the formation of the molecular array can be blocked by adding submonolayer amounts of Ge to the surface. The Ge atoms block the formation of the array because they substitute preferentially for Si adatoms located in corner sites, and the Ge--C bond strength is $\ensuremath{\approx}$0.8 eV weaker than the Si--C bond strength.

Growth of nanosize Ag dots with uniform height on a Si(111)-7×7-C2H5OH surface, and their electronic properties

C2H5OH adsorbs by dissociating on Si-adatom/Si-rest atom pair sites on Si(111)-7×7 surfaces. A half of six Si adatoms and three Si rest atoms are changed to Si-OC2H5 and Si-H in every half unit cell at the saturation. When an Ag atom was deposited on this surface, it was stabilized on an intact Si adatom remained in the half unit cell and it did not migrate by hopping. With the increasing number of deposited atoms, uniform height with ca. 5-nm size Ag dots were grown in wide area. A similar growth mode was observed by depositing Ga and Zn on this surface. We deduced that the uniform height growth of 5-nm dots may be given by a layer-by-layer growth of dots in the natural templates composed of six half unit cells. Scanning tunneling spectroscopy indicated that one-monolayer Ag dots had nonmetallic energy gap of ca. 2.2 V at the Fermi level, but the energy gap became narrower with the increasing number of layers and became metallic at eight or nine layers.

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.

Vibrational properties of Na nanoclusters self-assembled on Si(1 1 1) surface

We have investigated structural and vibrational properties of Na nanoclusters self-assembled on the Si(1 1 1)- 7 × 7 surface at room temperature mainly using high-resolution electron-energy-loss spectroscopy. We observe three characteristic loss peaks L 1, L 2, and L 3 ascribed to an interband transition, a local atomic vibration, and another interband transition from an Na nanocluster-induced state, respectively. The spectral change of L 1 with Na coverage θ suggests that the S 1 dangling bond band is gradually filled up to open a band gap with increasing θ up to 1.1 eV when all three loss peaks completely disappeared. The relatively high loss energy E l = 243 meV of L 2 with a narrow linewidth of 32 meV indicates the only Na–Si atomic vibrational mode with Na atoms occupying the tilted on-top sites above Si rest atoms. Furthermore the extremely weak loss peak L 3 visible at a coverage range unique only to the Na nanoclusters proves the presence of a Na nanoclusters-induced electronic state N 2. These observations provide explanation to most unresolved spectral behavior of earlier photoemission study and evidence for the atomic structure of the Na nanocluster.

Photoemission study of CCl 4 adsorption on Si(1 1 1)-7 × 7

Adsorption of carbon tetrachloride (CCl 4) on Si(1 1 1)-7 × 7 at room temperature (RT) and low temperature (LT) was investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). It was demonstrated that at RT CCl 4 dissociates on the Si(1 1 1)-7 × 7 surface leaving the surface extensively adsorbed with atomic Cl species. The dissociated Cl shows site preference to Si restatoms resulting in quick extinction of dangling bonds at the Si restatoms. At LT (around 120 K), both molecular and dissociative adsorption of CCl 4 occurs, which produces Cl, CCl x ( x ⩽ 3), and CCl 4 on the surface. The dangling bonds at the restatoms and adatoms were simultaneously quenched upon the LT CCl 4 adsorption. The site selectivity of restatoms to adatoms for molecule adsorption on the Si(1 1 1)-7 × 7 surface is discussed.

Controlled growth of Zn nano-dots on a Si(111)-7×7 surface saturated with C2H5OH

Metal atoms bonded with Si adatoms on the Si(111)-(7x7) surface undergo migration by hopping adjacent Si-rest atoms with dangling bond. By saturated adsorption of Si(111)-(7x7) surface with C(2)H(5)OH, the whole Si-rest atoms and a half of Si adatoms are occupied with Si-H and Si-OC(2)H(5), so that the Zn atoms adsorbed on this surface cannot migrate by hopping. When Zn atoms were deposited on this surface, ca. 5 nm Zn dots were grown in the hexagonal spacing of ca. 5.4 nm width around the corner holes, which work as a mold. This is quite different from the growth of honeycomb layers composed of Zn(3) clusters on the clean Si(111)-(7x7) surface. The dots grow up to nine (1.97 nm) to 13 layers (2.64 nm) by keeping their size, which implies a layer-by-layer growth of dots in the mold, where the growth is controlled by the kinetics instead of energetic feasibility.

Theoretical study of adsorption of gallium and gallium nitrides on Si(1 1 1)

Adsorption of gallium (Ga, Ga +) and gallium nitrides (GaN, GaN +, GaN 2, GaN 2 + ) on a model Si(1 1 1) surface was studied by density functional theory calculations. Significant binding is found for all the species considered, with binding energies up to 6.13 eV. For GaN, GaN 2, and GaN 2 + the lowest energy structures are those with the adsorbate forming a bridge between the Si adatom and rest atom, while for GaN + it is a vertical structure connecting to Si via N. Furthermore, it is found that adsorbed GaN and GaN + are essentially the same species, resembling free GaN −.

THE ADSORPTION OF Au ON Si(111)-7 × 7 SURFACE: A FIRST-PRINCIPLES STUDY

The initial stage of Au adsorption on the Si (111)-7 × 7 surface has been investigated by the first-principles calculations. Different configurations have been considered and the high coordination sites were determined as the most possible places for the Au adsorption. The diffusion barriers for Au atoms on the Si surface have also been evaluated. The results indicated that the Au atoms could diffuse from the top of the Si rest atoms to the high coordination sites only by overcoming a small barrier of 0.11 eV.

Growth of nano-crystalline metal dots on the Si(1 1 1)-7 × 7 surface saturated with C 2H 5OH

Metal atom on the Si(1 1 1)-7 × 7 surface undergoes migration by hopping among Si-adatom and Si-rest atom. If the hopping migration is prohibited, how change the deposited metals? In this paper, we studied the deposition of metals on the Si(1 1 1)-7 × 7 surface saturated with C 2H 5OH, on which the whole Si-rest atoms are changed to Si–H so that the hoping migration of metals will be prohibited. We found the growth of ca. 5 nm of crystalline dots by the deposition of Sn, Zn and Ag. Interestingly, Ag dots undergo layer-by-layer growth so that the surface is covered with 5 nm size dots with uniform height. When the hopping migration is prohibited, growth of dots is controlled by the kinetics of precursor state atoms instead of the lattice energy relating to lattice matching or strain.

DFT study of NH 3 dissociation on Si(1 1 1)-7 × 7. The role of intermolecular interactions

The adsorption of NH 3 molecule on the Si(1 1 1)-7 × 7 surface modelled with a cluster has been studied using density functional theory (DFT). The results indicate the existence of a precursor state for the non-dissociative chemisorption. The active site for the molecular chemisorption is the adatom; while the NH 3 molecule adsorbs on the Si restatom via this preadsorbed state, the adsorption on the Si adatom is produced practically without an energy barrier. The ammonia adsorption on the adatom induces an electron transfer from the dangling bond of this atom to the dangling bond of the adjacent Si restatom, hindering this site for the adsorption of a second NH 3 incoming molecule. However, this second molecule links strongly by means of two H-bonds. The dissociative chemisorption process was studied considering one and two ammonia molecules. For the dissociation of a lonely NH 3 molecule an energy barrier of ∼0.3 eV was calculated, yielding NH 2 on the adatom and H on the restatom. When two molecules are adsorbed, the NH 3–NH 3 interaction yields the weakening of a N–H bond of the ammonia molecule adsorbed closer the Si surface. As a consequence, the dissociation barrier practically disappears. Thus, the presence of a second NH 3 molecule at the adatom–restatom pair of the Si(1 1 1)-7 × 7 surface makes the dissociative reaction self-assisted, the total adsorption process elapsing with a negligible activation barrier (less than 0.01 eV).

Formation of copper clusters on a thiophene mediated Si(111)-(7×7) surface via molecular anchors

Thiophene selectively binds to the pair of adjacent Si adatom and Si rest atom on the Si(111)-(7×7) surface, leading to the covalent attachment of C–S–C linkages onto the surface. Cu atoms are found to preferentially adsorb onto the S atoms of the functional C–S–C groups in the formation of copper nanoclusters on the thiophene-mediated Si(111)-(7×7) surface.

Theoretical study of halogen‐substituted benzene at a Si(111)7×7 surface

AbstractThe adsorption of halobenzene (for halogens F, Cl, Br, and I) on Si(111)7×7 was investigated using AM1 quantum mechanical calculations. First the 1,4‐cyclohexadiene type of chemisorbed structures with two C—Si bonds at C atoms 1 and 4 have been calculated. Generally, the calculated binding energy increases with the size of the halogen atom, in the series F, Cl, Br, and I. The sp2 carbon positions are the most favorable for the halogen atom, and for bromobenzene and iodobenzene there is significant additional stabilization for the geometry that allows interaction of the halogen atom with a nearby Si adatom. This stabilization hinders the transfer of the heavier halogen atoms, Br and I, to the surface. Other chemisorbed structures, involving formally divalent halogen atoms in a C‐X‐Si type of binding, have also been found to correspond to energy minima in the AM1 calculations. Furthermore, it was possible to calculate physisorbed structures for chlorobenzene and bromobenzene once the Si rest atoms were capped with H atoms. Two types of binding structures are suggested for chemisorption in excess of three molecules per half‐unit cell: one is a radical structure binding by a single C‐Si adatom bond, and the second one is binding by a single C‐X‐Si adatom interaction. Both of these singly‐bonded structures have been calculated to correspond to energy minima with binding energies smaller than the 1,4‐cyclohexadiene type of chemisorbed structure.

Si(111)2×2-Fe surface reacted with nitric oxide

We have studied a Si(111)2×2-Fe surface exposed to nitric oxide at room temperature with Auger electron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy (STM). The surface local structure of Si(111)2×2-Fe has been modeled to be similar to that of Si(111)7×7; both structures have Si adatom and Si first-layer restatom, but the restatom in the former surface bonds to Fe atoms. It is known that NO molecules dissociatively adsorb on Si(111)7×7 surface, especially atop adatoms. Before the exposure to the Si(111)2×2-Fe surface, STM images showed 2×2 islands on the 2×2 terrace. After the exposure, the 2×2 islands were eroded at the island edge while the 2×2 terrace was stable against nitric oxide. We ascribed the inert property of the 2×2 terrace, and the reactive property of the 2×2 island edges and the 7×7 terrace, primarily to the Si restatom with different neighbor atoms.

Adsorbate lone-pair-electron stimulated charge transfer between surface dangling bonds: methanol chemisorption on Si(1 1 1)-7 × 7

Unrestricted spin density functional theory in the flavor of B3LYP is used to explore the reaction pathways for CH 3O–H dissociation over an adatom–rest atom pair site on Si(1 1 1)-7 × 7. We find that an adatom–rest atom pair site should be best regarded as a di-radical pair and that adsorbate lone-pair electrons stimulate efficiently the charge transfer from Si adatoms to Si rest atoms and vice versa. Our calculations show that rest atoms are more active than adatoms towards the dissociation of a lone-pair electron donor adsorbate.

Theory of the site-selective reaction ofNH3withSi(111)−(7×7)

Adsorption of ${\mathrm{NH}}_{3}$ molecules on the $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)$ 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 ${\mathrm{NH}}_{3}$ 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 $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)$ 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.

Reactivity of halogens on a Si(111) surface studied by surface differential reflectivity

The adsorption of Br on a $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ surface at room temperature and the isothermal desorption of silicon bromides from a Br-terminated Si(111) rest atom surface at about 900 K have been investigated by means of in situ real-time surface differential reflectivity (SDR) spectroscopy in order to elucidate the fundamental processes of Br-etching. The present results are compared with those of previous SDR studies on chlorine adsorption and chloride desorption. In the adsorption process, both the sticking probability on adatom dangling bonds and the breaking probability of adatom back bonds are larger than those for Cl adsorption. In the desorption process, the activation energy for bromide desorption (1.8 eV) is smaller than that for chloride desorption. The experimental results can be well interpreted in terms of larger repulsive interaction between Br adsorbates. The activation energy for the formation of the dimer-adatom-stacking fault structure (3.5 eV) is larger than that for the surface after chloride desorption.