Growth and atomic structure of chromium overlayers on W(110) and W(100)
Abstract
The growth, structure and thermal stability of chromium overlayers vapor-deposited onto W(110) and W(100) substrates have been studied using Auger electron spectroscopy, temperature programmed desorption, work function measurements and low energy electron diffraction. Layer-by-layer growth is observed on both substrates in the 120–400 K temperature range, despite a 9% Cr : W bulk lattice mismatch. On W(110), a pseudomorphic (1 × 1) Cr monolayer (ML) forms at 100 K and remains stable up to desorption at 1290 K. A metastable surface structure of (2 × 2) symmetry is observed for in the 500 < T < 800 K range, for which a quad-cluster model is proposed. Cr coverages above ~2 ML are thermally unstable above 400 K and form three-dimensional clusters which co-exist with the pseudomorphic (1 × 1) monolayer and (2 × 2) bilayer. An (11 × 11) coincidence lattice is observed for multilayer films confirming the epitaxial Cr(110):W(110) relationship reported in earlier field emission microscopic studies. On W(100), the clean surface (2 × 2) structure reverts to (1 × 1) symmetry with the formation of a pseudomorphic Cr monolayer, which remains stable up to desorption at 1400 K. A 2 ML overlayer exhibits no long range order but remains stable up to 1100 K, while thicker films are thermally unstable above 500 K, forming three-dimensional clusters on top of the pseudomorphic monolayer.
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Cited by (44)
Effective Work Functions of the Elements: Database, Most probable value, Previously recommended value, Polycrystalline thermionic contrast, Change at critical temperature, Anisotropic dependence sequence, Particle size dependence
2022, Progress in Surface ScienceAs a much-enriched supplement to the previous review paper entitled the “Effective work functions for ionic and electronic emissions from mono- and polycrystalline surfaces” [Prog. Surf. Sci. 83 (2008) 1–165], the present monograph summarizes a comprehensive and up-to-date database in Table 1, which includes more than ten thousands of experimental and theoretical data accumulated mainly during the last half century on the work functions (, and ) effective for positive-ionic, electronic and negative-ionic emissions from mono- and polycrystalline surfaces of 88 kinds of chemical elements (1H–99Es), and also which includes the main experimental condition and method employed for each sample specimen (bulk or film) together with 490 footnotes. From the above database originating from 4461 references published to date in the fields of both physics and chemistry, the most probable values of , and for substantially clean surfaces are statistically estimated for about 600 surface species of mono- and polycrystals. The values recommended for together with and in Table 2 are much more abundant in both surface species and data amount, and also they may be more reliable and convenient than those in popular handbooks and reviews consulted widely still today by great many workers, because the latter is based on less-plentiful data on published generally before 1980 and also because it covers no value recommended for and . Consequently, Table 1 may be more advantageous as the latest and most abundant database on work functions (especially ) for quickly referring to a variety of data obtained under specified conditions. Comparison of the most probable values of recommended for each surface species between this article and other literatures listed in Tables 2 and 3 indicates that consideration of the recent work function data accumulated particularly during the last 40 years is very important for correct analysis of these surface phenomena or processes concerned with either work function or its changes. On the basis of our simple model about the work function of polycrystal consisting of a number of patchy faces (1–i) having each a fractional area (F) and a local work function (), its values of both and are theoretically calculated and also critically compared with a plenty of experimental data. In addition, the “polycrystalline thermionic work function contrast” () well-known as the thermionic peculiarity inherent in every polycrystal is carefully analyzed as a function of the degree of monocrystallization () corresponding to the largest (F) among F’s (Tables 4–6 and Fig. 1), thereby yielding the conclusions as follows: (1) const (>0) holds for the generally called “polycrystalline” surfaces (usually < 50%), (2) ranges from 0.3 eV (Pt) to 0.7 eV (Nb) depending upon the polycrystalline surface species, (3) in the case of the “submonocrystal” (50 < < 100%) tentatively named here, decreases parabolically down to zero as increases from 50% up to 100% (monocrystal), (4) applies to a clean and smooth monocrystalline surface ( 100%) alone, (5) regarding negative ion emission, on the other hand, our theoretical prediction of is experimentally verified to hold for any surface species under any surface conditions (Table 7), (6) every polycrystal (usually, < 50%) may be concluded in general to have a unique value of characteristic of its species with little dependence upon , (7) this conclusion affords us first a sound basis for supporting theoretically the experimental fact (Table 2) that every species of polycrystal has a nearly constant value of as well as (usually within the uncertainty of 0.1 eV) depending little upon the difference in the surface components (F and ) among specimens so long as < 50%, (8) on the contrary to polycrystal ( < 50%), any submonocrystal (50 < < 100%) has such an anomaly that it does not possess the unique value of work function characteristic of the surface species itself, because its as well as changes considerably depending upon , (9) consequently, submonocrystal must be taken as another type (category) different from both poly- and monocrystals, (10) in this way, acts as the key factor mainly governing the work functions in the different mode between poly- and submonocrystals with lower and higher than the “critical point” of 50%, respectively, (11) on the contrary to , belonging to has a differential effect on both and , but their values remain nearly constant so long as < 50% and, thus interestingly, (12) the complicate governance of and by both and and also the anomaly of submonocrystal (cf. (8) above) observed first by our theoretical analysis may be considered as a new contribution to the work function studies developed to date. Together with brief comments and experimental conditions, typical data on and/or are summarized from the various aspects of (1) examination of the work function dependence upon the surface atom density of low-Miller-index monocrystals of typical metals such as Al, Ni, W and Re (Table 8), (2) demonstration of the above dependence usually called the “anisotropic work function dependence sequences” of both (110) > (100) > (111) and (110) > (100) > (111) for various bcc-metals (e.g., Nb, Mo, Ta and W) exactly obeying the Smoluchowski rule (Table 9), (3) substantiation of both (111) > (100) > (110) for a variety of fcc-metals (except Al and Pb) and (111) > (100) > (110) for Ni strictly following the above rule (Table 10), (4) verification of the quantitative relations between work function and surface energy and also melting point of the three low index planes of several metals (typically, Ni), (5) examination of the work function change () due to allotropic transformation from to or to phase (Table 11) together with a concise outline of the Burgers orientation relationship, (6) evaluation of due to liquefying (Table 12), (7) estimation of due to transformation from ferro- to paramagnetic state (Table 13) in addition to a brief description of the Curie point dependence upon metastable metal film thickness above one monolayer, (8) estimation of due to transition from normal to superconducting state (Table 14), (9) study of the work function dependence on the Wigner–Seitz radius and also comparison between its theoretical values (by Kohn) and experimental data (Fig. 2), (10) inspection of the annealing effect on work function for layers or films, (11) verification of the coincidence of work function values among different experimental methods, and (12) inquisition of the work function dependence upon the size of fine particles (20–100 Å in radius) studied by theory and experiment.
Surface magnetism in vanadium overlayers on W(100)
2020, Journal of Magnetism and Magnetic MaterialsLocalized surface magnetism in materials with non-magnetic bulk is a subject of considerable interest. In particular, in the case of strained transition metal overlayers on transition metals substrates, the interaction overlayer-substrate can lead to a fascinating magnetic behavior where the competition between strain and hybridization controls spin-polarized states. In this work, we have studied the structural, electronic, and magnetic properties of V overlayers on W(100). We report full potential linearized augmented plane wave (FLAPW) calculations, within the local density approximation (LDA), the generalized gradient approximation (GGA) and the hybrid functional B3LYP for the exchange-correlation part of the total energy, including spin-orbit coupling. A magnetic to non-magnetic transition was found in the surface layer as a function of the number of atomic vanadium layers. For the monolayer and bilayer cases, we found that the strong hybridization between the overlayer -orbitals and the substrate -orbitals define the characteristics of the surface density of states, resulting in a magnetic moment of and at the surface layer of and in the LDA, GGA and B3LYP, respectively. In contrast, for 3V/W(100), the surface density of states presents a strong localization due to the strain generated by the substrate; in this case, the magnetism disappears. The effects of the strain and the hybridization of the - orbitals of the overlayer-substrate on the local electronic band structure of the surface layers are discussed.
Growth and oxidation of Cr films on the W(1 0 0) surface
2006, Surface ScienceCitation Excerpt :Additionally, we have observed the metastability of 3 ML Cr films far above the clustering temperature and identified stress relief mechanisms [(4 × 4) inclusions and morphological instabilities during the growth of the third layer] for the first time. Surprisingly, the stability of 2 ML ps Cr films, which was seen before [2] and is verified here, contradicts the recent prediction based on first principles calculations that only one ps Cr layer is thermodynamically stable on W(1 0 0) [23]. These calculations also predict a ferromagnetic ground state for a 1 ML ps Cr film on W(1 0 0).
The growth and oxidation of Cr films on the W(1 0 0) surface have been studied with low energy electron microscopy (LEEM) and diffraction (LEED). Cr grows in a Stranski–Krastanov (SK) mode above about 550 K and in a kinetically limited layer-by-layer mode at lower temperature. Stress relief in the highly strained pseudomorphic (ps) Cr film appears to be achieved by the formation of (4 × 4) periodic inclusions during the growth of the third layer between 575 and 630 K and by growth morphological instabilities of the third layer at higher temperature. Kinetic or stress-induced roughening is observed at lower temperature. In the SK regime, three-dimensional (3D) Cr islands nucleate after the growth of three Cr layers. 3D island nucleation triggers dewetting of one layer from the surrounding Cr film. Thus, two ps Cr layers are thermodynamically stable. However, one and two layer ps Cr films are unstable during oxidation. 3D clusters, that produce complex diffraction features and are believed to be Cr2O3, are formed during oxidation of one Cr layer at elevated temperature, T ⩾ 790 K. The single layer Cr film remains intact during oxidation at T ⩽ 630 K. 3D bulk Cr clusters are formed predominantly during oxidation of two ps Cr layers.
Anisotropic growth of Cr Ad-layer: A one-dimensional nanostructure
2003, Thin Solid FilmsThe growth behavior of Cr ad-layer on W(110) surface was studied with scanning tunneling microscopy. Two-dimensional islands are formed at Cr coverage of ∼0.3 monolayer, suggesting typical homogeneous nucleation. The growth mode undergoes a transition to the formation of organized nano-scale lines at the coverage of >0.5 monolayer. The grown lines are aligned along [1̄11] and [11̄1] directions at room temperature. At an elevated temperature, trenches are formed in a two-dimensional wetting layer with high aspect ratios. Possible microscopic origins for the observed structures are discussed.
Adsorption studies of Cr on W (1 1 0) plane by probe hole field emission microscopy
2002, Surface ScienceA probe hole field emission microscope has been used to study adsorption of chromium (Cr) on microscopic (1 1 0) plane of tungsten (W) tip. Adsorption is studied by measuring variation in workfunction (Φ) with Cr deposition (θ) at two temperatures, 722 and 300 K. For the 722 K tip, Φ decreases linearly by about 0.5 eV till completion of one monolayer and a very small increase is observed thereafter, indicating that the formation of Cr monolayer is by two-dimensional islanding. The average dipole moment associated with Cr–W pair is calculated to be 6.0×10−31 C m. On a tip held at 300 K, Cr atom population on the (1 1 0) plane is low. Therefore, only a small (0.3 eV) decrease in Φ is observed. Also the ΔΦ–θ behaviour is found to be erratic, indicating crystallite formation on the (1 1 0) plane. These observations are compared with those of Berlowitz and Shinn [Surf. Sci. 209 (1989) 345], who used macroscopic W surfaces to study Cr/W system. The results are discussed in view of the existing literature.
An ultra-thin PdMn intermetallic compound on W(1 1 0)
2001, Surface ScienceA study of Pd and Mn co-deposited on W(1 1 0) is presented. Using high resolution core level photoelectron spectroscopy and low energy electron diffraction it is shown that heating of a sandwich consisting of 1 ML each of Pd and Mn yields a double-layer PdMn compound with a c(2×2) structure. The layers are suggested to be [1 1 0] planes of a PdMn compound with CsCl structure. This relies on the fact that such a phase exists in the bulk, with a lattice parameter nearly identical to that of W. The results demonstrate the feasibility for growth of ultra-thin ordered Pd–Mn films where the stoichiometry and the geometric structure can be controlled.
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Present address: EXXON Research and Engineering Co., Linden, NJ, USA.