Cr Islands Research Articles

Atomic structures and electronic properties of Cr-doped ZnO() surfaces

An integrated approach combining scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) is used to investigate the atomic structures and electronic properties of Cr-doped ZnO(▪) surfaces. When deposited at 300 K, Cr at low surface coverage (< 0.1 ML) appeared either as isolated atoms on the surface terrace of ZnO(▪) or substituting Zn atoms in the ZnO lattice. Their structural models could be identified from atomic-resolution STM images and their oxidation states were found as Cr3+ based on XPS measurements. Rectangular islands nucleated at step edges along the [0001] direction could also be observed during the initial growth of Cr at 300 K and were assigned as Cr islands. The density of Cr islands as well as their average size increased with the increasing of Cr surface loading. Thermal treatments at above 600 K could facilitate the decomposition of Cr islands and the re-dispersion of Cr atoms into the ZnO lattice, indicating a strong interaction between Cr and ZnO. The adsorption of CO at 78 K showed no preferential adsorption at Cr3+ sites embedded in the surface lattice of ZnO. However, the re-dispersion of Cr atoms into the ZnO bulk at above 600 K could induce a significant upward band bending, causing a negative shift of core level XPS peaks of Zn 2p and O 1s by ~0.5–0.7 eV. Our study has thus constructed a model catalyst for Cr-doped ZnO and provided atomic insight for understanding ZnO-based catalysts.

Parameter dependent fabrication of Chromium nano-structures on Au(111) surface

Circular and rectangular Cr islands of nanometre size and monoatomic height exhibit an impromptu long range order on locally stepped Au(111) surface with narrow terraces. The circular islands are detached from the {100} microfaceted Au(111) step edges and nucleate between a pair of discommensuration lines of the herringbone reconstruction on ≈ 9.5 nm wide terraces. In contrast, the rectangular islands are attached to the {111} microfaceted step edges and are formed on even narrower terraces of ≈ 6 nm width. Both types of islands prefer the hcp region of the substrate as the nucleation centre. Cr deposition at elevated temperatures such as 470 K results in formation of triangular islands and spiral dislocations on wide substrate terraces. Interestingly however, at higher coverages of 3–4 ML, huge elongated ridges that are 90–260 nm in length, 10–20 nm in width and up to almost 3 nm height are observed. These ridges occur primarily along <11¯0> direction, with the rest forming at an angle of 60 or 120° with respect to this direction.

Study of optical and luminescence properties of silicon — semiconducting silicide — silicon multilayer nanostructures

By method of in situ differential spectroscopy it was established that at the formation of monolayer Fe, Cr, Ca, Mg silicide and Mg stannide islands on the atomically clean silicon surface an appearance of loss peaks characteristic for these materials in the energy range of 1.1-2.6 eV is observed. An optimization of growth processes permit to grow monolithic double nanoheterostructures (DNHS) with embedded Fe, Cr and Ca nanocrystals, and also polycrystalline DNHS with NC of Mg silicide and Mg stannide and Ca disilicide. By methods of optical spectroscopy and Raman spectroscopy it was shown that embedded NC form intensive peaks in the reflectance spectra at energies up to 2.5 eV and Raman peaks. In DNS with β-FeSi2 NC a photoluminescence and electroluminescence at room temperature were firstly observed.

Scanning tunneling microscopy study of Fe, Co and Cr growth on Re(0001)

Atomically flat terraces of the Re(0001) surface with a contaminant density below 0.5% have been obtained by oxygen annealing followed by a flash to higher temperature. This Re(0001) single crystal has been used as a substrate for the deposition of Fe, Co and Cr atoms. Scanning Tunneling Microscopy experiments characterize the growth mode for the submonolayer coverage regime. Co, Cr and Fe atoms self-assemble to form monolayer high islands. Despite a large lattice mismatch between film and substrate, Co and Fe grow pseudomorphically up to half a monolayer. Cr islands are pseudomorphic only for a size below 10nm. Higher coverage leads to reconstructed islands with an element-dependent reconstruction pattern. Scanning Tunneling Spectroscopy measurements at 8K reveal the electronic properties of Fe and Re. Differential conductance measurements on the Re(0001) show the presence of standing waves, possibly due to a rhenium surface state. Atomic resolution images of Fe attached to a Re step edge lead to the conclusion that the Fe atoms occupy hcp hollow sites. A Néel magnetic ordered state of the Fe hcp monolayer is revealed with Spin-Polarized Scanning Tunneling Microscopy and Magnetic Atom Manipulation Imaging.

Observation of a spin-density wave node on antiferromagnetic Cr(110) islands

We have performed a detailed study of the evolution of the charge-density wave (CDW) and spin-density wave (SDW) on the surface of nanoscale Cr islands grown on W(110). Using low-temperature scanning tunneling microscopy (STM), we find a striking nonmonotonic thickness dependence of the CDW wavelength at 50 K. As the local island thickness $\ensuremath{\Theta}$ decreases from about 50 to 5.2 nm, the CDW wavelength gradually increases from the bulk value by approximately 30$%$. We find a gap without any CDW at coverages ${\ensuremath{\Theta}}_{\mathrm{gap}}$ between 5.2 and 3.7 nm. Spin-resolved STM data reveal that within this CDW gap the SDW modulation also disappears on the Cr island surface. At $\ensuremath{\Theta}&lt;3.7$ nm the CDW reappears. This unusual behavior of CDW and SDW in Cr nanoislands can be understood by a reorientation of the SDW wave vector Q which is potentially driven by the pinning of the SDW node at the island surface at ${\ensuremath{\Theta}}_{\mathrm{gap}}$.

Numerical Analysis of Electron Emission Site Distribution of Carbon Nanofibers for Field Emission Properties

To obtain optimal field emission (FE) properties, it is important to evaluate FE parameters including the electron emission site α and the field enhancement factor β. However, it is difficult to evaluate α quantitatively because the emitting electrons cannot be observed directly. The authors have aimed to analyze this site using an original architecture with a computation system tool based on the surface charge method, and a three-dimensional model has been employed to calculate FE properties with high accuracy. In this study, to analyze α for determining FE properties, each carbon nanofiber (CNF) model separated by Cr islands which include the minimum area for calculating electric fields by the surface charge method was constructed on the surface of a Ni catalyst. The FE current was simulated with a Fowler-Nordheim formula using the calculated electric fields, followed by a simulation performed using all CNFs on a field emitter cathode. The electron emission site α was determined by comparing the simulation and experimental results of the FE current. It was found that α depends on the morphology of the CNF bundles, and a close quantitative correspondence between the experimental and the computation results of FE properties was obtained. In summary, a method of analyzing FE properties was established using an original architecture, making it possible to predict FE properties with a computational tool based on the surface charge method.

Chromium nanostructures formed by dewetting of heteroepitaxial films on W(100)

In this paper, we report the surprising formation of square-based facetted islands with linear dimension of the order of 500 nm upon dewetting of a Cr multilayer on W(100). We show that these square islands are composed of inclined facets surrounding a depressed center such that the facet slopes inward with the outer edges of the islands thicker than the centers. The islands' shapes do not represent traditional equilibrium crystal shapes as expected for a Wulf construction. In situ UV and x-ray photoelectron emission microscopy allied to spatially resolved spectroscopy throws considerable light on the nature of the dewetting and shows that the metal surface between the islands remains covered by a thin pseudomorphic wetting layer of \ensuremath{\sim}1 ML. Low-energy electron diffraction and scanning tunneling and atomic force microscopies allow quantification of facet slopes, and we identify a predominance of tilted Cr(100) facets \ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{} off of the substrate normal bound by (210) planes at \ensuremath{\sim}26\ifmmode^\circ\else\textdegree\fi{}. The epitaxial Cr islands adopt the bulk Cr lattice constant but are tilted with respect to the surface normal. We suggest that the Cr crystallite tilting creates a vicinal-like interface structure that determines the island morphology.

Spin-polarized scanning tunneling microscopy and spectroscopy study of chromium on aCr(001) surface

Several tens of chromium layers were deposited at250 °C on a Cr(001) surface and investigated by spin-polarized scanning tunneling microscopy(SP-STM), Auger electron spectroscopy (AES) and scanning tunneling spectroscopy (STS).Chromium is found to grow with a mound-like morphology resulting from the stacking ofseveral monolayers which do not uniformly cover the whole surface of the substrate. Theterminal plane consists of an irregular array of Cr islands with lateral sizes smaller than20 × 20 nm2. Combined AES and STS measurements reveal the presence of a significant amount ofsegregants prior to and after deposition. A detailed investigation of the surface shows thatit consists of two types of patches. Thanks to STS measurements, the two types of areahave been identified as being either chromium pure or segregant rich. SP-STM experimentshave evidenced that the antiferromagnetic layer coupling remains in the chromium moundsafter deposition and is not significantly affected by the presence of the segregants.

Two-dimensional vacancy islands induced by the growth of Cr on Cu(111)

Surface structures induced by the growth of Cr on Cu(111) at room temperature are studied with scanning tunneling microscopy. At sub-monolayer Cr coverage, islands and vacancy islands with lateral sizes of about 10 nm in length are observed. The islands show one- or two-monolayer heights with irregular shapes, while vacancy islands have one-monolayer depth and hexagonal shapes with openings. Atomic structures inside and near the vacancy islands are the same as those of bare Cu(111), implying that Cu atoms detach from the vacancy islands and attach to other step edges and islands. Possible origins and atomic processes for the observed structures are discussed.

Low-Temperature Scanning Tunneling Microscopy and Spectroscopy Investigation of the Electronic Surface State of Self-Organized Cr Islands on Au(111)

We have investigated the morphologic and electronic properties of self-organized nanosized Cr islands grown on atomically flat Au(111) surfaces by means of low-temperature scanning tunneling microscopy and spectroscopy under ultra-high vacuum conditions. We observe the existence of an electronic surface state on the Cr islands, which manifests itself through the formation of standing wave patterns within the interior of the atomically flat islands that gain in complexity for increasing bias voltages. The patterns are only weakly observable when compared to similar standing waves formed within self-organized Co islands on Au(111). This difference is attributed to the more irregular shape of the Cr islands. Furthermore, we have found that the presence of the Cr surface state is reflected by the appearance of a distinct electronic state in the -200 mV to -150 mV range, resembling to previous findings for self-organized Co islands on Au(111).

How metal films de-wet substrates—identifying the kinetic pathways and energetic driving forces

We study how single-crystal chromium films of uniform thickness on W(110) substrates are converted to arrays of three-dimensional (3D) Cr islands during annealing. We use low-energy electron microscopy (LEEM) to directly observe a kinetic pathway that produces trenches that expose the wetting layer. Adjacent film steps move simultaneously uphill and downhill relative to the staircase of atomic steps on the substrate. This step motion thickens the film regions where steps advance. Where film steps retract, the film thins, eventually exposing the stable wetting layer. Since our analysis shows that thick Cr films have a lattice constant close to bulk Cr, we propose that surface and interface stress provide a possible driving force for the observed morphological instability. Atomistic simulations and analytic elastic models show that surface and interface stress can cause a dependence of film energy on thickness that leads to an instability to simultaneous thinning and thickening. We observe that de-wetting is also initiated at bunches of substrate steps in two other systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are converted into patterns of unidirectional stripes as the trenches that expose the wetting layer lengthen along the W[001] direction. Finally, we observe how 3D Cr islands form directly during film growth at elevated temperature. The Cr mesas (wedges) form as Cr film steps advance down the staircase of substrate steps, another example of the critical role that substrate steps play in 3D island formation.

Growth and oxidation of Cr films on the W(1 0 0) surface

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 Cr 2O 3, 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.

An STM study of growth and alloying of Cr on Ru(0 0 0 1) and CO adsorption on the alloy

We have investigated the growth of Cr on Ru(0 0 0 1) using scanning tunneling microscopy (STM). In the submonolayer regime ( θ Cr = 0.25 ML) monatomically high islands are formed at 300 K, accompanied by step decoration. The island density decreases upon heating to 700 K. Small amounts of Cr are incorporated in the Ru substrate at this temperature. For higher Cr coverages (2 ML), three-dimensional growth is observed at 300 K. Annealing this Cr layer to 500 and 700 K leads to the formation of Cr islands (4–5 layers high) with an elongated shape and bcc(1 1 0) structure with a pseudomorphic Cr monolayer covering the remaining substrate. As shown by atomically resolved images, the Cr bcc(1 1 0) islands have Kurdjumov–Sachs orientation with respect to the Ru(0 0 0 1) substrate. After annealing at 1000 K, the formation of a hexagonal, chemically disordered CrRu alloy is observed, in agreement with a previous study [Engelhardt et al., Surf. Sci. 512 (2002) 107]. The room-temperature STM investigation of CO adsorption on the CrRu alloy with approx. 34% Cr in the first layer shows that adsorbed molecular CO resides only on Ru atoms, not on Cr, mostly in an on-top geometry. The CO occupation of Ru sites increases slightly with the number of neighbouring Cr atoms, indicating a weak ligand effect. We also find indications that the few threefold hollow sites surrounded by three Cr atoms become occupied by atomic C or O, due to dissociation of a small fraction of the adsorbed CO molecules; thus, we cannot unambiguously determine whether molecular CO can bind to these threefold sites at room temperature.

New aspects of the induction period of ethene polymerization using Phillips CrO x/SiO 2 catalyst probed by XPS, TPD and EPMA

In this work, an industrial Phillips CrO x /SiO 2 catalyst engaged in the induction period through interaction with ethene at room temperature (RT) was studied by the combination of surface analytical methods: X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and electron probe microanalysis (EPMA). XPS method disclosed the reduction of surface chromate species (expressed as Cr(VI)O x,surf ) into surface-stabilized Cr 3+ and Cr 2+ species (expressed as Cr(III)O x,surf and Cr(II)O x,surf , respectively) by monomer during the induction period and the simultaneous formation of two kinds of surface carbon species, namely, formaldehyde and unsaturated hydrocarbons. The hydrocarbon species were supposed to be composed of not only coordinated ethene, but also adsorbed longer olefin chains. TPD measurement confirmed the formation of formaldehyde. EPMA characterization revealed that the surface Cr aggregation occurred in a low extent during the induction period resulting in a few Cr islands in sizes of several microns on catalyst surface. It was demonstrated that reduction and alkylation of Cr species as well as initiation of ethene insertion had already occurred even in the induction period. A mechanism concerning the reactions occurred during the induction period on the Phillips catalyst was proposed. The Cr(II)O x,surf species coordinated with one formaldehyde and one ethene is postulated to be the active sites precursor for the alkylation as well as initiation of the ethene insertion. The coordinatively-adsorbed formaldehyde molecules on the catalyst are supposed to be the very reason for the hindered ethene oligomerization rather than normal polymerization, as well as for the induction of the surface Cr aggregation.

Magnetic dichroism and spin-resolved photoemission from rough interfaces

The magnetic structure of ultrathin Cr films on Fe is analyzed by taking into account the roughness of the surface and of the interface. An algorithm for the epitaxial growth of the film simulates the near-surface structure, and the magnetic moment distribution is calculated self-consistently in a periodic Anderson model. For rough interfaces we find that the layered antiferromagnetic structure of the Cr adlayer is quenched. Within this model we determine the spin polarization and the value of the magnetic linear dichroism to be expected in angle-resolved photoemission. Comparing with experimental data it is concluded that Cr grows on the Fe(001) in a Stranski-Kastranoff mode. We propose an explanation based on the confinement of the itinerant electrons within the Cr islands on the Fe substrate.

Onset of C(2×2) ferrimagnetic order in Cr islands deposited on Fe(001) as a function of island size

We present a study of the magnetic properties of compact Cr clusters deposited on Fe(001). A Cr monolayer deposited on Fe(001) is found to be c(2×2) ferrimagnetic (AP) and the moments of a single adatom and very small clusters are antiferromagnetically aligned with the Fe substrate moments (P). Therefore a transition from P to AP order is expected when the cluster size increases. An Ising model is used to model the magnetic energy of two geometric configurations, a square-shaped and a diamond-shaped cluster. Self-consistent tight-binding calculations allow us to determine the P to AP transition which is obtained for a 400 adatoms square-shaped cluster. The pair magnetic energies have been estimated to −105.7 meV for the first neighbors Fe–Cr interaction and to −111.3 meV for the second neighbors Cr–Cr interaction in the Ising model.

Initial growth mode of Cr on Cu(100) and Ag(100): a comparison

The initial growth mode of Cr on Cu(100) and Ag(100) surfaces is studied by photoemission methods. In particular photoemission is used as a titration method allowing to determine the clean surface area left after Cr deposit. For Cu(100) it is found that at room temperature, or above, body centered cubic (bcc) Cr grows initially in the form of bilayer or multilayer islands with (110) orientation and with 90% substrate coverage by ∼ 4 monolayers. This contrasts with the growth mode observed on Ag(100) where, at moderate substrate temperatures (430–450 K), only monolayer bcc Cr islands with (100) orientation are formed initially. This behaviour is confirmed by Cr 2p core level photoemission measurements which indicate that in the monolayer range Cr is already in bulk-like environment on Cu(100) in contrast to Ag(100) where a flat monolayer with strongly enhanced magnetism is obtained.

Evolution of a surface state into an interface state: A probe of the buried epitaxial Cr/Au(100) interface.

Angle-resolved synchrotron-radiation photoemission results are presented that examine the evolution of the Au(100)-(1\ifmmode\times\else\texttimes\fi{}1) surface state at the epitaxial Cr/Au(100) interface as a function of Cr thickness. At submonolayer coverages, two-dimensional Cr islands form and stabilize the (1\ifmmode\times\else\texttimes\fi{}1) termination of the substrate, as judged by the simultaneous detection of both the Au-(1\ifmmode\times\else\texttimes\fi{}1) and -(5\ifmmode\times\else\texttimes\fi{}20) surface states. The Au(100)-(1\ifmmode\times\else\texttimes\fi{}1) surface state persists in the outer Au layer for 1 monolayer (ML) of Cr or Au(100). As the Cr overlayer thickness is increased from 1 to 2 ML, the interface state is modified and its binding energy changes from 1.9 to 2.6 eV. For Cr coverages in excess of 3 ML, the Au state couples with bulk states and vanishes. The Au surface-interface state is reestablished by the growth of an ultrathin Au layer over the 2--3-ML-thick epitaxial Cr layers, and its binding energy shifts back toward ${E}_{F}$. The results are discussed in terms of changes in the surface-interface potentials during the evolution of the overlayer.