Surface Rumpling Research Articles

High temperature stress and its influence on surface rumpling in NiCoCrAlY bond coat

The objective of this work is to develop a methodology to measure the high temperature stress in the bond coat, and investigate its role on the surface rumpling. We first presented an analytical model to evaluate the high temperature stress using X-ray sin2Ψ technique coupling with the curvature measurement at room temperature. A typical NiCoCrAlY bond coat with a Hastelloy-X substrate was employed as a model sample. During exposure at 1150 °C, the bond coat was under tension at high temperature, increasing parabolically from 1.05 MPa after 12 h to 3.81 MPa after 120 h. To understand its effect on the surface rumpling, the bond coat surface roughness was recorded as a function of time, and compared with a bulk NiCoCrAlY alloy. A strong correlation between the bond coat stress and the surface roughness was identified. In addition, the origination of the bond coat stress and the rumpling mechanism were discussed. It was revealed that the high temperature stress in the bond coat was caused by the volume shrinkage from β-γ transformation, mainly due to the inter-diffusion. The grain sliding accompanied with diffusional creep in response to the bond coat stress controls the roughening.

Surface Relaxations, Surface Energies and Electronic Structures of BaSnO3 (001) Surfaces: Ab Initio Calculations

The surface relaxations, surface energies and electronic structures of BaO- and SnO2-terminated BaSnO3 (001) surfaces have been studied by employing the first-principles density functional theory. For both terminations, we find that the upper-layer Ba and Sn atoms move inward, whereas upper-layer O atoms move outward from the surface. Moreover, the largest relaxations are occurred on the first-layer atoms of both terminations. The surface rumpling of BaO-terminated BaSnO3 (001) is slightly less than that of the SnO2-terminated BaSnO3 (001) surface. The surface energies show that both terminated surfaces are energetically stable and favorable. Finally, the surface band gap is slightly decreased for the BaO termination, while it is dramatically decreased for the SnO2 termination.

A mechanistic understanding on rumpling of a NiCoCrAlY bond coat for thermal barrier coating applications

We present a series of experimental observations on surface rumpling of an initially flat NiCoCrAlY coating deposited on a Ni-based superalloy during cyclic oxidation at 1150 °C. The extent of rumpling of the coating depends on the thermal history, coating thickness and exposed atmosphere. While the coating surface progressively roughens with cyclic oxidation, the bulk NiCoCrAlY alloys with the same nominal composition are much less susceptible to rumpling under the same oxidation conditions. The coatings, especially the thin ones, experience substantial degradation (e.g. β to γ phase transformation) induced by oxidation and coating/subsatrate interdiffusion. The observations together suggest that rumpling of the NiCoCrAlY coating is driven by a combination of the lateral growth of the thermally grown oxide and coating/substrate thermal mismatch. The results in this work are further discussed and compared with the rumpling behaviour of a β-(Ni,Pt)Al bond coat reported in the literature to illustrate the importance of possible factors in governing the development of rumpling in the NiCoCrAlY coating.

First-principles calculations on the structural and electronic properties of cubic KCaF3 and NaCaF3 (001) surfaces

First-principles density functional theory (DFT) calculations have been used to investigate the structural and electronic properties of the cubic KCaF3 and NaCaF3 (001) surfaces with MF (M = K or Na) and CaF2 terminations. For both KCaF3 and NaCaF3 (001) surfaces, the MF termination has stronger surface rumpling than the CaF2 termination. All the computed band gaps for the KCaF3 and NaCaF3 (001) surfaces are smaller than those of the bulks. Furthermore, separated bands that originate from surface layer Fp states are introduced at the top of the valance band of MF-terminated surfaces, indicating the emergence of the surface states. The calculated surface energies show that the MF-terminated surface is energetically more favorable than the CaF2-terminated surface.

First-principles study of the cubic CaHfO3(001) surface

The geometric and electronic structures of the cubic CaHfO3(001) surfaces have been studied using first-principles density functional theory (DFT) calculations. Two different terminations, CaO- and HfO2-terminated surfaces, were investigated. It has been found that Ca atom has the largest relaxation for both kinds of terminations, and the surface rumpling of the CaO termination is far more prominent than that of the HfO2-terminated surface. Both the bandgaps of the CaO- and HfO2-terminated surfaces were calculated to be smaller than that of the CaHfO3bulk. It was also shown that the CaO-terminated surface has a lower energy than the HfO2-terminated surface.

Interface properties of Ge on cubic SrHfO3 (001)

High quality Ge-on-high-k oxide interface is essential to facilitate the high performance metal-oxide semiconductor field-effect transistors and monolithically integrated optoelectronics device performance. The atomic structure and electronic properties of Ge on perfect and defective (001) SrHfO3 are investigated by first-principle calculations. The amplitude of the surface rumpling for the SrO-terminated surface is much larger than that for HfO2-terminated surface, although both SrO- and HfO2-terminated surfaces are stable for a comparable range of the HfO2 chemical potential. The distance between the first and second planes compresses while that of the second and third planes expands due to the relaxation of the slab. We investigated systematically the specific adsorption sites and the atomic structure at the initial growth stage of Ge on the SrHfO3 (001) substrate. The top sites of the oxygen atoms are favorable for 1/2 (1/3) monolayer Ge adsorbate at SrO (HfO2)-terminated surface. We calculated the surface grand potential and presented the complete surface phase diagram. We also pointed out the energetically favorable interfaces among the atomic arrangements of the Ge/SrHfO3 (001) interfaces. The atomic structure and electronic properties of the intrinsic point defects were calculated and analyzed for the Ge/SrHfO3 (001) interfaces.

Theoretical investigation of surface states and energetics of PtSi surfaces

Platinum silicide (PtSi) is highly promising material for applications in microelectronic devices. In this article, the surface electronic structure, surface energetics and work functions of stoichiometric and non-stoichiometric PtSi(010) surfaces are explored within the framework of first-principle density functional theory. The surface rumpling is found to be significant only for the top surface layer. The computed values of the rumpling parameter for the top three layers are ~11.0%, ~0.9% and ~1.9%. Further, the interlayer relaxation is found to be largest for the top layer and decreases rapidly for inner layers. Localized surface states are obtained in the valence band at ~9.0eV below the Fermi level. Under rich Pt and Si growth conditions, nonstoichiometric (010) terminations are found to have the lowest surface energies, whereas stoichiometric termination has the lowest surface energy (~1.74J/m2) under mixed conditions. The work function of stoichiometric (010) termination is computed to be 5.15eV and differ as much as by ±0.5eV for nonstoichiometric terminations.

The comparative theoretical study of the LaBO3 (001) (B = Mn, Fe, Co, and Ni) surface properties and oxygen adsorption mechanisms

In this paper, the surface properties of the (001) surface LaBO3 (B = Mn, Fe, Co, and Ni) perovskites and oxygen adsorption mechanism are investigated using first-principles calculations based on density functional theory (DFT) and pseudopotential method. Surface rumpling and Mulliken effective atomic charges have been obtained. The surface rumpling displays a big wave and increases following the sequence Fe < Co < Mn < Ni at the BO2-terminated surface. The calculated effective charges show that the excess electronic charges at the LaO-terminated surface transfer to the inner atom of the surface. We also report a comparative study of oxygen molecular adsorption properties at BO2- and LaO-terminated surfaces of LaBO3 (001). The optimized geometries of the adsorbed oxygen species, adsorption energies, bond lengths, bond populations, and effective charges have been analyzed. The results show the oxygen molecule adsorption at the hollow site of the LaO-terminated surface has much higher adsorption energy and dissociation probability than that at other sites. Oxygen species has a strong tendency for getting adsorbed at the site and thereby filling it. The comparative study of the LaBO3 (001) surface properties and oxygen adsorption mechanism may guide us to search for high-efficiency cathode materials for solid oxide fuel cells (SOFC).

Water adsorption induced in-plane domain switching on BaTiO3 surface

In this study, the influences of the adsorption of water molecules on the changes in the atomic and electric structures of BaTiO3 surface were investigated using ab initio calculation. Water molecules are molecularly and dissociatively adsorbed on the BaTiO3 surface, which makes electrons transfer from water molecules to the BaTiO3 surface. The redistribution of electrons in the BaTiO3 surface layers weakens the Ba-O interactions and strengthens the Ti-O interactions, so that the Ti atom shifts in TiO2 plane, i.e., an in-plane domain switching. The adsorption of water molecules on BaTiO3 surfaces also results in a reduction in the surface rumpling.

Comparative First-Principles Calculations of SrTiO3, BaTiO3, PbTiO3 and CaTiO3 (001), (011) and (111) Surfaces

I present the results of my ab initio calculations for SrTiO3, BaTiO3, PbTiO3 and CaTiO3 neutral (001), as well as for polar (011) and (111) surfaces using the hybrid description of exchange and correlation. AO and BO2-terminations for the nonpolar (001) surface and A, BO, and O terminations of the polar (011) surface, as well as B and AO3-terminations of the polar (111) surface were calculated. In the case of the neutral AO-terminated (001) surface for SrTiO3, BaTiO3, PbTiO3 and CaTiO3 perovskites, all upper-layer A atoms relax inward towards the bulk, while all second layer atoms relax outwards. For the BO2-terminated (001) surface, in most cases, the largest relaxations are on the second-layer metal atoms. For practically all ABO3 perovskites, the surface rumpling is considerably larger for the AO-terminated than for the BO2-terminated (001) surface, but their surface energies always are almost equal. Just opposite, different terminations of the (011) ABO3 surface lead to very different surface energies for the O-terminated, A-terminated, and BO-terminated (011) surface, respectively. The surface energies for all calculated (111) surfaces are considerably larger than even for (011) surfaces. A considerable increase in the Ti-O chemical bond covalency near the (011) surface as compared to the bulk and to the (111) and (001) surfaces in considered ABO3 perovskites were predicted.

Lattice-mismatched heteroepitaxy of IV-VI thin films on PbTe(001): Anab initiostudy

The chalcogenides of tin and lead (SnS, SnSe, SnTe, PbS, PbSe, and PbTe) have applications ranging from solar cells to thermoelectrics. Taking rocksalt structured PbTe(001) as the substrate, we explore the coherent (001)-oriented heteroepitaxy of the other five IV-VI semiconductors through first-principles electronic structure calculations. We investigate the effects of lattice strain and its relationship to the interface energy, as well as the electron redistribution, as a function of the epilayer thickness [from 1 to 5 monolayers (ML)] below the dislocation critical point. Analysis of the chemical bonding explains trends including work function shifts and surface rumpling. Among the five combinations studied SnTe/PbTe(001) has the smallest lattice mismatch, resulting in the most stable coherent interface and unique charge transfer behavior. Here, we perform orbital-resolved band structure calculations (with spin-orbit coupling effects) for the SnTe/PbTe(001) system, highlighting its potential use in topological spintronic thin-film devices.

Surface polarization, rumpling, and domain ordering of strained ultrathinBaTiO3(001) films with in-plane and out-of-plane polarization

${\mathrm{BaTiO}}_{3}$ ultrathin films (thickness $\ensuremath{\approx}1.6$ nm) with in- and out-of-plane polarization are studied by first-principles calculations. Out-of-plane polarization is simulated using the method proposed by Shimada et al. [Phys. Rev. B 81, 144116 (2010)], which consists in building a supercell containing small domains with alternating up and down polarization. This allows one to investigate the properties of defect free ${\mathrm{BaTiO}}_{3}$ ultrathin films with polarization perpendicular to the surface, as a function of in-plane lattice constant, i.e., epitaxial strain. The configurations with polarization perpendicular to the surface $(c$ phase) are found stable under compressive strain, while under tensile strain, the polarization tends to lie in-plane ($aa$ phase), along [110]. In the $c$ phase, the most stable domain width is predicted to be 1 to 2 lattice constants, and the magnitude of the surface rumpling varies according to the direction of the polarization (upwards versus downwards), though its sign is unchanged, the oxygen anions pointing in all cases outwards. Finally, all the surfaces studied are found to be insulating. Analysis of the atom-projected electronic density of states gives insight into the surface contributions to the electronic structure. An important reduction of the Kohn-Sham band gap is predicted at ${\mathrm{TiO}}_{2}$ terminations in the $c$ phase ($\ensuremath{\approx}1$ eV with respect to the $aa$ phase). The Madelung potential at the surface plays the dominant role in modifications of the surface electronic structure.

Oxidation Behavior of Hf-Modified Aluminide Coatings on Inconel-718 at 1050°C

Simple β-NiAl, Hf-modified β-NiAl, Pt-diffused, Pt-modified β-(Ni,Pt)Al + ξ-PtAl2, and Hf-Pt-modified β-(Ni,Pt)Al were cyclic oxidation tested at 1050°C in air on Inconel-718 substrates for up to 4370h. The Pt-diffused specimen failed most quickly, &lt; 100 h, while the simple β-NiAl aluminide maintained a positive weight change for ~1300 h. The Pt-modified aluminides clearly improved the cyclic oxidation behavior of both simple and Hf-modified aluminides, sustaining a zero weight change only after 3600 and 4000 h, respectively. The Hf additions did not immediately appear to produce as strong an improvement as expected, however, they were more highly ranked when normalized by coating thickness. They also decreased surface rumpling, important for TBC durability. Hf-rich NiAl grain boundaries, formed during coating processing, resulted in HfO2 particles in the scales and oxide pegs at the metal interface, all suggesting some level of over-doping. The high sulfur content of the substrate influenced spalling to bare metal and re-healing to less protective Ni(Al,Cr)2O4 spinel-type and (Ti,Cr,Nb)O2 rutile scales. The evolution of these surface features have been documented over 100 to 4370 h of exposure. The coating aluminum content near failure was ~2-3 wt. %.

Ab initio calculations of SrTiO3, BaTiO3, PbTiO3, CaTiO3, SrZrO3, PbZrO3 and BaZrO3 (001), (011) and (111) surfaces as well as F centers, polarons, KTN solid solutions and Nb impurities therein

In this paper, the review of recent results of calculations of surface relaxations, energetics, and bonding properties for ABO 3 perovskite (001), (011) and (111) surfaces using mostly a hybrid description of exchange and correlation is presented. Both AO and BO 2-terminations of the nonpolar (001) surface and A , BO , and O terminations of the polar (011) surface, as well as B and AO 3-terminations of the polar (111) surface were considered. On the AO -terminated (001) surface, all upper-layer A atoms relax inwards, while all second layer atoms relax outwards. For the BO 2-terminated (001) surface, in most cases, the largest relaxations are on the second-layer metal atoms. For almost all ABO 3 perovskites, the surface rumpling is much larger for the AO -terminated than for the BO 2-terminated (001) surface, but their surface energies are always quite similar. In contrast, different terminations of the (011) ABO 3 surface lead to very different surface energies for the O -terminated, A -terminated, and BO -terminated (011) surface, respectively. A considerable increase in the Ti – O or Zr – O , respectively, chemical bond covalency near the (011) surface as compared both to the bulk and to the (001) surface in ABO 3 perovskites were predicted. According to the results of ab initio calculations for Nb doped SrTiO 3, Nb is a shallow donor; six nearest O ions are slightly displaced outwards from the Nb ion. The F center in ABO 3 perovskites resembles electron defects in the partially-covalent SiO 2 crystal rather than usual F centers in ionic crystals like MgO and alkali halides. The results of calculations for several perovskite KNb x Ta 1-x O 3 (KTN) solid solutions, as well as hole and electron polarons in ABO 3 perovskites are analyzed.

Effects of atomic scale roughness at metal/insulator interfaces on metal work function

We evaluate the performance of different van der Waals (vdW) corrected density functional theory (DFT) methods in predicting the structure of perfect interfaces between the LiF(001), MgO(001), NiO(001) films on the Ag(001) surface and the resulting work function shift of Ag(001). The results demonstrate that including the van der Waals interaction is important for obtaining accurate interface structures and the metal work function shift. The work function shift results from a subtle interplay of several effects strongly affected by even small changes in the interface geometry. This makes the accuracy of theoretical methods insufficient for predicting the shift values better than within 0.2 eV. Most of the existing van der Waals corrected functionals are not particularly suited for studying metal/insulator interfaces. The lack of accurate experimental data on the interface geometries and surface rumpling of insulators hampers the calibration of existing and novel density functionals.

Structural stability and electronic properties of LaO- and NiO2-terminated LaNiO3 (0 0 1) surface

LaO- and NiO2-terminated LaNiO3 (001) surfaces were investigated by first principle calculations. For NiO2-terminated surface, Ni atoms exhibit an abnormal relaxation. Larger surface rumpling emerges in the topmost layer of LaO termination, which is almost twice as large as that of NiO2 termination. LaO-terminated surface has the lowest surface energy and stable structure. Non-polar surfaces exhibit remarkably different electronic structures and more Ni atoms exposed to the surface will contribute to higher density of states at Fermi level, which provides a guide to grow novel ultra-thin heterostructures with controlled electronic state of metal ions.

First-principles calculations of the structural and electronic properties of the cubic CaZrO3 (001) surfaces

The (001) surfaces of cubic perovskite CaZrO3 with two different terminations (CaO and ZrO2) were studied using the first principles density functional theory. The structural, electronic and energetic properties of each surface have been calculated; the differences between the properties of the bulk and slab materials were identified. In particular, changes of the band gap and density of states distributions from atoms located in different layers were revealed. Calculations of the surface rumpling, effective Mulliken charges, and surface energy were compared with data existing for other cubic perovskites. It was also shown that the surface with the CaO-termination has a lower energy than with the ZrO2 termination.

Charge transfer at organic–inorganic interface of surface-activated PbS by DFT method

Electronic structure of a surface-activated PbS by a bio-active molecule 4-aminothiophenol (4-ATP) was investigated using density functional theory (DFT). The obtained results demonstrated the importance of charge transfer which accounted for the flipping of surface rumpling and the nature of the binding between the activated surface and the capping agent. The influence of 4-ATP–PbS topology on bonding nature between surface atoms was discussed. The capping-induced bonding nature shift was interpreted as surface core level shifts (SCLSs) of PbS.

First-Principles Investigation of Hexagonal WB2 Surfaces

Using the first-principles method with the generalized gradient approximation, we have investigated the surface stability, structural relaxation, and electronic structure of WB 2 surfaces. The calculated surface free energy indicates that the (001)-BB surface is most energetically favorable over the range of the chemical potential for boron. Large surface rumpling appears on the (100) type surfaces and (110) surface due to the unsaturated chemical bonds and the unbalanced atomic interaction at the surface area. An analysis of calculated DOS reveals that surface state is mainly formed by the first-layer atomic orbitals for each surface.

First-principles study of the (001) and (110) surfaces of superhard ReB2

Structural relaxations, electronic properties, and surface energies of ReB2 (001) and (110) surfaces with various terminations are investigated with a first-principles method. It is found that the surface interatomic spacings of ReB2 (001) and (110) surfaces are different from those of the bulk structure. The vertical spacings between the first and second layers of the studied surfaces are contracted. The (001)-Re surface is likely to be stable without introducing a large relaxation. Among these surfaces, only the (110) surface has surface rumpling, and the Re atoms on its first layer are apt to move inward. After atomic relaxation, some covalent bonds formed by the outmost atoms of the relaxed surfaces are shorter than those of the bulk system, which indicates that the covalent B–B and Re–B bonds of the surface layer have been strengthened. An analysis of surface energies shows that after relaxation, the (001)-Re surface is more stable than other types of surfaces.