Bulk-like Termination Research Articles

On the work function of the surface Mo(0 0 1) and its temperature dependence: an ab initio molecular dynamics study

Ab initio molecular dynamics simulations in NVT ensemble have been performed to investigate the finite temperature structure of the Mo(0 0 1) surface and its effect on work function (ϕ). In accord with previous experimental and theoretical work, our simulations predict that a termination with a stable reconstruction pattern is formed at T = 123 K. This pattern vanishes when temperature is increased to 423 K or 623 K and a disordered surface phase is formed whose time average corresponds to a bulk-like termination. Our results demonstrate that the surface relaxation is an important factor contributing to thermal variation of ϕ. At the lowest temperature, at which a stable reconstruction pattern is formed, the work function is found to increase by ∼0.23 eV compared to relaxed unreconstructed surface. The disappearance of stable reconstruction pattern at elevated temperatures leads to a decrease of ϕ by ∼0.07 eV. In contrast, the values computed for a non-reconstructing surface Mo(1 1 0) at T = 123 K, 423 K and 623 K are found to be nearly identical to the zero temperature value, which is a consequence of restricted atomic motion due to high packing density in this surface.

Magnetite Fe3O4 (111) Surfaces: Impact of Defects on Structure, Stability, and Electronic Properties

We present a comprehensive investigation, via first-principles density functional theory (DFT) calculations, of various surface terminations of magnetite, Fe3O4 (111), a major iron oxide that also has a number of applications in electronics and spintronics. We compare the thermodynamic stability and electronic structure among the different surfaces terminations. Interestingly, we find that surfaces modified with point defects and adatoms are close in surface energy and that they can be more stable than bulk-like terminations in the oxygen-rich and -poor regimes. These surfaces show different surface chemistry and electronic structures as well as distinctive spin polarization features near the Fermi level with regard to those previously considered in the literature. Our studies provide an atomic level insight for magnetite surfaces, which is a necessary step to understanding their interfaces with organic layers in OLEDs and spintronic devices.

Determination of the surface structure of CeO2(111) by low-energy electron diffraction

We determine the atomic structure of the (111) surface of an epitaxial ceria film using low-energy electron diffraction (LEED). The 3-fold-symmetric LEED patterns are consistent with a bulk-like termination of the (111) surface. By comparing the experimental dependence of diffraction intensity on electron energy (LEED-I(V) data) with simulations of dynamic scattering from different surface structures, we find that the CeO2(111) surface is terminated by a plane of oxygen atoms. We also find that the bond lengths in the top few surface layers of CeO2(111) are mostly undistorted from their bulk values, in general agreement with theoretical predictions. However, the topmost oxygen layer is further from the underlying cerium layer than the true bulk termination, an expansion that differs from theoretical predictions.

Epitaxial oxide growth on polar (111) surfaces

Obtaining atomically smooth surfaces and interfaces of perovskite oxide materials on polar (111) surfaces presents a particular challenge as these surfaces and interfaces will reconstruct. Here, the effect of the use of screening buffer layers on the epitaxial growth on such polar surfaces is investigated. Both transmission electron microscopy and in situ reflective high energy electron diffraction data imply that the buffer layers, SrRuO3 or LaAlO3, restore a near bulk-like termination at growth temperature, allowing for coherent growth of BiFeO3 and CaTiO3 for all deposited unit cell layers of the film material.

Atomic-Scale Sharpening of Silicon Tips in Noncontact Atomic Force Microscopy

The atomic-scale stability of clean silicon tips used in noncontact atomic force microscopy (NC-AFM) is simulated by ab initio calculations based on density functional theory. The tip structures are modeled by silicon clusters with and termination. For the often assumed Si(111)-type tip we observe the sharpening of the initially blunt tip via short-range chemical forces during the first approach and retraction cycle. The structural changes corresponding to this intrinsic process are irreversible and lead to stable NC-AFM imaging conditions. In opposition to the picture used in literature, the Si(001)-type tip does not exhibit the so-called "two-dangling bond" feature as a bulklike termination suggests.

Medium-energy ion scattering studies of two-dimensional rare-earth silicides

Medium-energy ion scattering (MEIS) has been used to determine the atomic structure of two-dimensional (2D) rare-earth silicides on Si(111). In the case of the Si(111)1\ifmmode\times\else\texttimes\fi{}1-Er 2D silicide surface, the MEIS results refine previously published results, but in the case of the Si(111)1\ifmmode\times\else\texttimes\fi{}1-Ho surface, this work represents to our knowledge the first full quantitative structural analysis that reveals a structure similar to that of the Si(111)1\ifmmode\times\else\texttimes\fi{}1-Er surface and directly supports a model in which a rare-earth monolayer resides below a Si bilayer close to bulklike termination.

Comparative studies on the surface structures of NiSi 2 and epitaxially formed on Si(111)

On the basis of a set of structural parameters obtained from low energy electron diffraction analyses together with a set of Debye temperatures, characteristics of the bonding on the NiSi 2 (NS) and Co- and Si-rich surfaces of CoSi 2(111) (CSC and CSS) have been explored. Despite the similar structure with a bulk-like termination, the CSC is more flexible than the NS for the displacement of the topmost Si atom. On the NS, both the first and second interlayer spacing has been found contracted by 11 and 8%, respectively. On the CSC, surface relaxation is completed with only the first spacing being contracted noticeably by 17%, while the underlying layers remain almost intact. The CSS, which is capped with an additional Si bilayer on top of the CSC, is highly strained because of the backbond of the topmost Si atom, involving large expansion of the first interlayer spacing by 17% and contraction of the third one by 14%.

Ab initiocalculation of the stoichiometry and structure of the (0001) surfaces of GaN and AlN

We have investigated the stoichiometry and the atomic and electronic structure of the anion- and cation-terminated (0001) surfaces of wurtzite-phase GaN and AlN, using ab initio local-orbital calculations based on the local-density approximation and the pseudopotential method. All stable surface configurations studied differ in atomic composition and periodicity from the ideal bulklike termination. We compare the total energy computed for various $p(2\ifmmode\times\else\texttimes\fi{}2)$ geometries of GaN and AlN(0001). Vacancy structures are found to be the most stable configurations for the anion- and cation-terminated surfaces. For metal-rich growth conditions, our calculations favor the adsorption of metal atoms on the cation-terminated surface. Anion- and cation-derived dangling-bond states appear in the bulk band gap as a result of the formation of vacancies or the adsorption of group-III atoms. Flat surfaces of both types are found to be stabilized by a $\frac{3}{4}$ ML adsorption of hydrogen.

Oxygen adsorption and oxide formation on Ni3Al (111)

The interaction of oxygen with the ordered Ni3Al (111) surface has been investigated in the temperature range from 300 to 1000 K using high-resolution electron-energy-loss spectroscopy (HREELS) and low-energy electron diffraction (LEED). The “2×2” LEED pattern of the clean Ni3Al (111) surface indicates a bulklike termination. After oxygen adsorption at 300 K the LEED pattern is diffuse suggesting the formation of an amorphous overlayer. The HREELS spectra show evidence for oxygen interaction with both aluminum and nickel atoms. At 600 K adsorption temperature the fcc surface order is restored, however, the observed (1×1) LEED pattern indicates the loss of chemical order. Again HREELS spectra suggest interaction of oxygen with both aluminum and nickel. For an adsorption temperature of 800 K LEED shows an unrotated oxygen induced superstructure with a lattice spacing of 2.93 Å in addition to the (1×1) substrate spots. The HREELS spectra exhibit an intense loss at 81.9 meV, which is also known from oxygen in threefold hollow sites on Al (111). Since such sites are not available on the Ni3Al (111) surface, we conclude the buildup of an oxygen covered aluminum overlayer. Finally, during oxygen exposure at 1000 K we observe the growth of a γ′-Al2O3 structure on the reordered Ni3Al (111) substrate surface. This structure has been identified by means of the characteristic vibrational losses in HREELS at 54.6, 80.3, and 112.8 meV together with the emergence of overlayer spots in LEED exhibiting the lattice spacing of γ′-Al2O3 (3.02 Å). For oxygen exposures at 800 and 1000 K an island growth of the overlayer has been found.

The role of Si atoms in In/Si(111) surface phase formation

The deposition of In onto the ▪ surface appears to result, depending on the temperature, in the formation of three different reconstructions, Si(111)-(2 × 2)-In (20–100°C), Si(111)-(4 × 1)-In (∼200°C) and ▪ (∼450°C). The formation of each surface phase has been determined to be controlled by the mobility of the top Si atoms. At the ▪ to (2 × 2) transition, the bulk-like termination of the Si(111) substrate is preserved. At the formation of the Si(111)-(4 × 1)-In phase, Si atoms reorder via a bond-switching process. The formation of the ▪ phase involves surface Si mass transport. The applicability of the results obtained to other submonolayer adsorbate/Si systems is discussed, and the criterion for revealing the surface phases with a reconstructed substrate is formulated.

Structural Analysis of the Fivefold Symmetric Surface of the Al70Pd21Mn9Quasicrystal by Low Energy Electron Diffraction

The atomic structure of the fivefold symmetric quasicrystal surface of A${\mathrm{l}}_{70}$P${\mathrm{d}}_{21}$M${\mathrm{n}}_{9}$ was investigated by means of a dynamical low energy electron-diffraction analysis. Approximations are developed to address the high structural complexity of quasicrystals, yielding average layer-by-layer structural and compositional information. A mix of several relaxed bulklike terminations is found, consistently favoring dense Al-rich outermost atomic layers. These results can be understood in terms of principles known to govern surface structures of simpler, crystalline metals.

Medium-energy backscattered electron diffraction studies of TiO 2(110): relation to surface structure

Medium-energy backscattered electron diffraction (MEED) studies are reported for the rutile TiO 2(110) surface. Two-dimensional diffraction images are obtained using a retarding field analyzer with a microchannel plate-intensified detector. Background removal is accomplished using digital image processing techniques. We find that the backscattered pattern for 750 eV incident electrons is predominantly due to diffraction of the outgoing electrons (subsequent to backscattering from subsurface atoms) by overlying lattice atoms. We also find effects related to the orientation of the incident beam relative to the surface. These effects are ascribed to diffraction of the incoming electrons (prior to backscattering) and possibly the focusing of electron intensity on second-layer atoms. Single-scattering cluster (SSC) calculations are used to model the outgoing diffraction process. The results are qualitatively consistent with a bulk-like termination of the TiO 2(110) surface.

Medium-energy backscattered electron diffraction studies of TiO 2(110): relation to surface structure

Abstract Medium-energy backscattered electron diffraction (MEED) studies are reported for the rutile TiO 2 (110) surface. Two-dimensional diffraction images are obtained using a retarding field analyzer with a microchannel plate-intensified detector. Background removal is accomplished using digital image processing techniques. We find that the backscattered pattern for 750 eV incident electrons is predominantly due to diffraction of the outgoing electrons (subsequent to backscattering from subsurface atoms) by overlying lattice atoms. We also find effects related to the orientation of the incident beam relative to the surface. These effects are ascribed to diffraction of the incoming electrons (prior to backscattering) and possibly the focusing of electron intensity on second-layer atoms. Single-scattering cluster (SSC) calculations are used to model the outgoing diffraction process. The results are qualitatively consistent with a bulk-like termination of the TiO 2 (110) surface.

Epitaxial growth and surface structure of NiSi 2{0 0 1} on Si{0 0 1}

LEED (low-energy electron diffraction) intensity measurements were done to monitor the formation of epitaxial NiSi 2 on the {0 0 1} surface of Si. The resulting silicide surface, although possibly reconstructed, was subjected to LEED intensity analysis as NiSi 2{0 0 1}1×1. Several structural models involving bulk-like terminations, and variants thereof, failed the LEED test. However, one model, with Si adatoms in the four-fold symmetrical sites of the Si bulk-like termination, provides reasonable agreement with experiment and is therefore tentatively proposed as containing the main features of the real structure. The model may explain the failure of double epitaxy, i.e., the failure to grow epitaxial Si films on the silicide surface.

Magnetic properties, conductivity and ionic ordering in Fe 1- xCu xCr 2S 4

Magnetic properties, conductivity and ionic ordering in Fe 1- xCu xCr 2S 4