Adatom Reconstruction Research Articles

Mo-edge reconstructions in MoSe2 and MoS2 : Reexamination of the mechanism

For zigzag nanoribbons of transition metal dichalcogenides (TMDCs), experiments show a band gap with different gap size at the Mo edges, while theoretical models often predict metallic edges. Such an inconsistency could be attributed to the inadequate understanding of the possible mechanisms of edge reconstructions. Here, we revisit the mechanisms of different edge reconstructions along the Mo edge of $\mathrm{Mo}{X}_{2}$ ($X=\mathrm{Se}$, S) zigzag nanoribbons by first-principles calculations. We demonstrate that the valency of $\frac{1}{3}$ edge Mo increases to 5+ with $X$ adatom reconstruction, or all edge Mo decreases to 3+ with a Mo trimer reconstruction. Based on these Mo valence reconstructions, thermodynamically stable reconstructions are proposed for the Mo edge in $6\ifmmode\times\else\texttimes\fi{}$ and $4\ifmmode\times\else\texttimes\fi{}$ periodicities, which are comparable with the recent experimental observation of a small energy gap (of 0.36 eV) at the Mo edge of zigzag $\mathrm{Mo}{\mathrm{Se}}_{2}$ nanoribbons. The edge Mo valency reconstruction leads to quasi-one-dimensional Peierls distortion and charge- and spin-density waves at the edge. These findings should be applicable to other TMDCs.

Superconductivity in a Hole-Doped Mott-Insulating Triangular Adatom Layer on a Silicon Surface.

Adsorption of one-third monolayer of Sn on an atomically clean Si(111) substrate produces a two-dimensional triangular adatom lattice with one unpaired electron per site. This dilute adatom reconstruction is an antiferromagnetic Mott insulator; however, the system can be modulation doped and metallized using heavily doped p-type Si(111) substrates. Here, we show that the hole-doped dilute adatom layer on a degenerately doped p-type Si(111) wafer is superconducting with a critical temperature of 4.7±0.3 K. While a phonon-mediated coupling scenario would be consistent with the observed T_{c}, Mott correlations in the Sn-derived dangling-bond surface state could suppress the s-wave pairing channel. The latter suggests that the superconductivity in this triangular adatom lattice may be unconventional.

Depletion Layer Built‐In Field at (1−100), (0001), and (000−1) GaN/Water Junction and Its Role in Semiconductor Nanowire Water Splitting

AbstractThe nature of specific GaN plane/water interfaces under external bias and illumination can influence photoelectrolysis efficiency using GaN nanowires. Studies of Ga‐polar, N‐polar, and m‐plane GaN interfaces with deionized water allow determining differences between surfaces corresponding to different nanowire facets. They are investigated under external bias conditions to reveal the profile of Fermi level localization through analysis of Franz–Keldysh oscillations using electrolyte electroreflectance (EER) technique in a specially designed measurement chamber. Calculation of the potential barrier height is also possible. EER study shows differences between surface densities of states (SDOS) at distinct GaN planes. One broad SDOS is identified near the conduction band in case of ±c‐plane and related to Ga adatom reconstruction and β‐Ga2O3 presence at the GaN electrode. Two narrow SDOS singularities are found at the m‐plane one of which is localized near the middle of the bandgap and allows to generate approximately two times higher surface potential barrier than in case of polar surfaces at zero‐bias conditions. This suggests that n‐type GaN nanowires can enhance carrier separation at sidewalls and refine the oxygen evolution rate. Additionally, a voltage‐controlled hysteresis loop of Fermi level localization is detected at the Ga‐face GaN/water interface.

On the rotational alignment of graphene domains grown on Ge(110) andGe(111)

AbstractWe have used low-energy electron diffraction and microscopy to compare the growth of graphene on hydrogen-free Ge(111) and Ge(110) from an atomic carbon flux. Growth on Ge(110) leads to significantly better rotational alignment of graphene domains with the substrate. To explain the poor rotational alignment on Ge(111), we have investigated experimentally and theoretically how the adatom reconstructions of Ge interact with graphene. We find that the ordering transition of the Ge(111) adatom reconstruction is not significantly perturbed by graphene. Density functional theory calculations show that graphene on reconstructed Ge(110) has large-amplitude corrugations, whereas it is remarkably flat on reconstructed Ge(111). We argue that the absence of corrugations prevents graphene islands from locking into a preferred orientation.

Phase diagrams of polar surface reconstructions of zinc oxide

Due to the formation of surface dipoles the truncated-bulk like {0001} surfaces of zinc oxide (ZnO) are electrostatically unstable. These dipoles can neither be compensated by the formation of surface dimers nor by a relaxation of near surface layers. Hence, the driving force for the reconstruction of ZnO must be a deviation of the surface stoichiometry. To study this stabilization effect, we have performed density functional theory calculations on (2×2) and (3×3) periodic non-stoichiometric reconstructions of the ZnO polar surfaces. Among the 24 possible (2×2) periodic structures that preserve bulk symmetry, a class of ad-atom reconstructions shows the lowest energy of formation. To facilitate comparability with experimental conditions we calculate the Gibbs free energy of each reconstruction as a function of temperature and chemical conditions. However, the established calculation method is ambiguous in case of non-stoichiometric surfaces and must be extended by taking into account the temperature dependence of the ZnO enthalpy of formation. From the resulting data phase diagrams of the stable reconstructions of ZnO polar surfaces are constructed which are discussed in terms of epitaxial growth.

Thermodynamic driving forces governing assembly of disilicide nanowires

Driving forces are investigated for assembling low dimensional, metallic, erbium and dysprosium disilicide nanowires on Si(001), using both scanning probe microscopy and density functional theory. Side-by-side comparison between emulated and measured scanning tunneling microscopy images allows establishment of reliable atomic models for complex adatom surface reconstructions of Er/Si(001) and Dy/Si(001) that are precursors to high aspect ratio disilicide nanowires. Peculiar surface reconstructions and relaxation of Si bonds are identified as the key factors for nucleation of these disilicide nanowires in parallel arrays on vicinal Si(001). Stable nanowire widths and heights are calculated with predicted atomic models that are consistent with experimental observations. A clear understanding of the nanowire–substrate interface is determined by correlating adatom reconstruction patterns with nanowire formation that is imperative to the development of unique procedures for massive fabrication of monodisperse nanosystems.

The structure of the Au(111)/methylthiolate interface: New insights from near-edge x-ray absorption spectroscopy and x-ray standing waves

The local structure of the Au(111)(square root(3) x square root(3))R30 degrees-methylthiolate surface phase has been investigated by S K-edge near-edge s-ray absorption fine structure (NEXAFS) both experimentally and theoretically and by experimental normal-incidence x-ray standing waves (NIXSW) at both the C and S atomic sites. NEXAFS shows not only excitation into the intramolecular sigma(*) S-C resonance but also into a sigma(*) S-Au orbital perpendicular to the surface, clearly identifying the local S headgroup site as atop a Au atom. Simulations show that it is not possible, however, to distinguish between the two possible adatom reconstruction models; a single thiolate species atop a hollow-site Au adatom or a dithiolate moiety comprising two thiolate species bonded to a bridge-bonded Au adatom. Within this dithiolate moiety a second sigma(*) S-Au orbital that lies near parallel to the surface has a higher energy that overlaps that of the sigma(*) S-C resonance. The new NIXSW data show the S-C bond to be tilted by 61 degrees relative to the surface normal, with a preferred azimuthal orientation in <211>, corresponding to the intermolecular nearest-neighbor directions. This azimuthal orientation is consistent with the thiolate being atop a hollow-site Au adatom, but not consistent with the originally proposed Au-adatom-dithiolate moiety. However, internal conformational changes within this species could, perhaps, render this model also consistent with the experimental data.

Effect of Ag Adatoms on High-Coverage Alkanethiolate Adsorption on Au(111)

Alkanethiol adsorption on Ag-adatom-modified Au(111) surfaces is studied by means of electrochemical techniques combined with Auger electron spectroscopy and periodic density functional (DF) calculations. The bimetallic surfaces are prepared by Ag underpotential deposition, and only the alkanethiolate high-coverage regime is considered. Alkanethiolate electrodesorption from Ag-modified Au(111) surfaces requires potentials shifted 0.3 eV in the negative direction with respect to the value corresponding to desorption from Au(111) surfaces. In agreement with this experimental result, DF calculations show that alkanethiolates prefer to be laterally adsorbed to Ag adatoms, the interaction energy being 0.3 eV larger than that corresponding to the same thiolate on Au(111). Therefore, it is concluded that alkanethiolates adsorbed on Au are likely to be observed only when the Ag adatom surface density is small enough or all Ag sites are occupied. The preference for alkanethiolate adsorption on adatoms is also found for a model surface containing Au adatoms on Au(111). This indicates that no significant differences in electrodesorption potentials should be expected between Ag-adatom- and Au-adatom-rich surfaces. These findings and the analysis of experimental results strongly suggest that, under experimental conditions, the Au(111) surface does not exhibit a significant amount of Au adatoms, thus casting reasonable doubts on the adatom reconstruction models proposed for thiolates on Au(111).

Structural elements on reconstructed Si andGe(110)surfaces

We present ab initio studies of possible reconstruction elements on Si and $\text{Ge}(110)$ surfaces. Using $2\ifmmode\times\else\texttimes\fi{}2$, $3\ifmmode\times\else\texttimes\fi{}2$, and $6\ifmmode\times\else\texttimes\fi{}2$ unit cells we optimize models with buckled atomic chains, dimers, different adatom distributions, and interstitial atoms which may exist on the larger $\text{Si}(110)16\ifmmode\times\else\texttimes\fi{}2$ or $\text{Ge}(110)16\ifmmode\times\else\texttimes\fi{}2∕c(8\ifmmode\times\else\texttimes\fi{}10)$ surface reconstructions. We show that adatom reconstructions gain energy. Only the adatom model which seemingly leaves no dangling bonds cannot occur on Si and $\text{Ge}(110)$ surfaces. An adatom-rest atom electron transfer mechanism is more favorable. An adatom-tetramer-interstitial $3\ifmmode\times\else\texttimes\fi{}2$ model also stabilizes the Si and Ge surfaces and leads to a semiconducting behavior (at least for Si). Simulated scanning tunneling microscopy (STM) images of empty states of this reconstruction look like the pentagon structures observed on $\text{Si}(110)16\ifmmode\times\else\texttimes\fi{}2$. A $6\ifmmode\times\else\texttimes\fi{}2$ reconstruction with five-membered adclusters is energetically completely unfavorable, though it also reproduces the empty-state pentagonlike STM images.

Sb/Si(110) 2×3 surface studied with scanning tunneling microscopy: Evidence for adatom reconstruction

Scanning tunneling microscopy (STM) has been used to study the $\mathrm{S}\mathrm{b}/\mathrm{S}\mathrm{i}(110) 2\ifmmode\times\else\texttimes\fi{}3$ surface at room temperature. The STM filled-state image shows the presence of four protrusions per unit cell while the empty-state one presents two broader protrusions. These results are consistent with single Sb adatoms saturating three Si dangling bonds.

Theory of interfaces and surfaces in wide-gap nitrides

A selection of the results of a theoretical investigation of the properties of interfaces and surfaces of the wide-gap III–V nitride semiconductors is reviewed. The electronic properties of wurtzite heteroepitaxial interfaces of AlN and GaN, incorporating the effects of strain, are discussed. In particular, we find that this interface is of type I and have calculated the valence-band offset to be −0.57 eV. The surface energies and atomic geometries of the 2×2 reconstructions of the (0001) face of GaN are also presented. In conditions which are rich in a given species, an adatom reconstruction of that species is found to be the most energetically favorable: for gallium-rich conditions, the reconstruction with a gallium adatom on a T3 site is the most stable, while for nitrogen-rich conditions the reconstruction with a nitrogen adatom on the H3 site is energetically the most favorable.

Adsorption of Sn on Si(111)7 × 7: reconstructions in the monolayer regime

Different monolayer phases of Sn on Si(111)7 × 7 have been studied by means of scanning tunneling microscopy (STM), core-level photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS). The STM results show that 3 × 3 reconstructions are obtained for room-temperature deposition of 1 3 ML of Sn followed by sample annealing in a broad temperature range. A T 4 Sn adatom 3 ×3 phase is formed for temperatures between 500 and 800°C, with a concentration of defects that is strongly dependent on the temperature and which is as high as 25% for the lowest temperatures. Above 825°C a second 3×3 adatom reconstruction is formed, a mosaic-like phase with a 1:1 mixture of Si and Sn atoms in T 4 positions. The results from investigations of the higher coverage 2 3 × 2 3 reconstruction by XPS and RBS support the theory that this phase is a two-layer epitaxial Sn structure with all Si(111) dangling bonds saturated. The Sn coverage for this phase was determined to be between 1 and 1.2 ML.

Formation of an iodine zig-zag chain c(2 × 4) reconstruction on the Ge(111) surface

Room temperature adsorption of I 2 on the Ge(111) surface is found to break the initial c(2×8) adatom reconstruction as iodine preferentially occupies the on top substrate site, breaks the backbonds between the adatom and the substrate and thus removes the Ge adatoms. As this process continues a c(2×4), and modifications thereof, reconstruction is formed consisting of chains of I 2 molecules bound in almost T 1 sites with a I-I bond length of 3.6 A

Dynamic observation of Si (111) surface using a fast scanning tunneling microscope

Atomic structures of a Si (111) surface are dynamically observed at a 2 s/frame velocity over a scanning field about 150 Å×150 Å using a fast scanning tunneling microscope (FSTM). A FSTM has been developed and it features a compensation method for probe tip servo position error in the constant current mode. The compensation value is derived from the ratio of tunnel current fluctuation and tunnel current (Δ I/I) in differential-type tunnel current equation. The FSTM provides first dynamic observation of a residual gas molecule adsorption on atomic defects in 7×7 Si adatom reconstructions using a video tape recorder. In addition, a thermal drift of about 10 Å /s, about 30 min after direct electric flash heating of the sample for Si surface cleaning, can be easily observed.