| 1. |
- Annese, Emilia, et al.
(författare)
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Nonvortical Rashba Spin Structure on a Surface with C1h Symmetry
- 2016
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Ingår i: Physical Review Letters. - 0031-9007. ; 117:1
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Tidskriftsartikel (refereegranskat)abstract
- A totally anisotropic peculiar Rashba-Bychkov (RB) splitting of electronic bands was found on the Tl/Si(110)-(1×1) surface with C1h symmetry by angle- and spin-resolved photoelectron spectroscopy and first-principles theoretical calculation. The constant energy contour of the upper branch of the RB split band has a warped elliptical shape centered at a k point located between Γ and the edge of the surface Brillouin zone, i.e., at a point without time-reversal symmetry. The spin-polarization vector of this state is in-plane and points almost the same direction along the whole elliptic contour. This novel nonvortical RB spin structure is confirmed as a general phenomenon originating from the C1h symmetry of the surface.
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| 2. |
- Bliem, Roland, et al.
(författare)
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Adsorption and incorporation of transition metals at the magnetite Fe3O4(001) surface
- 2015
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 92:7
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of Ni, Co, Mn, Ti, and Zr at the (root 2 x root 2)R45 degrees-reconstructed Fe3O4(001) surface was studied by scanning tunneling microscopy, x-ray and ultraviolet photoelectron spectroscopy, low-energy electron diffraction (LEED), and density functional theory (DFT). Following deposition at room temperature, metals are either adsorbed as isolated adatoms or fill the subsurface cation vacancy sites responsible for the (root 2 x root 2)R45 degrees reconstruction. Both configurations coexist, but the ratio of adatoms to incorporated atoms depends on the metal; Ni prefers the adatom configuration, Co and Mn form adatoms and incorporated atoms in similar numbers, and Ti and Zr are almost fully incorporated. With mild annealing, all adatoms transition to the incorporated cation configuration. At high coverage, the (root 2 x root 2)R45 degrees reconstruction is lifted because all subsurface cation vacancies become occupied with metal atoms, and a (1 x 1) LEED pattern is observed. DFT+U calculations for the extreme cases, Ni and Ti, confirm the energetic preference for incorporation, with calculated oxidation states in good agreement with photoemission experiments. Because the site preference is analogous to bulk ferrite (XFe2O4) compounds, similar behavior is likely to be typical for elements forming a solid solution with Fe3O4.
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| 3. |
- Burkitt-Gray, Mary, et al.
(författare)
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Structural investigations into colour-tuneable fluorescent InZnP-based quantum dots from zinc carboxylate and aminophosphine precursors
- 2022
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 15:4, s. 1763-1774
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Tidskriftsartikel (refereegranskat)abstract
- Fluorescent InP-based quantum dots have emerged as valuable nanomaterials for display technologies, biological imaging, and optoelectronic applications. The inclusion of zinc can enhance both their emissive and structural properties and reduce interfacial defects with ZnS or CdS shells. However, the sub-particle distribution of zinc and the role this element plays often remains unclear, and it has previously proved challenging to synthesise Zn-alloyed InP-based nanoparticles using aminophosphine precursors. In this report, we describe the synthesis of alloyed InZnP using zinc carboxylates, achieving colour-tuneable fluorescence from the unshelled core materials, followed by a one-pot ZnS or CdS deposition using diethyldithiocarbamate precursors. Structural analysis revealed that the “core/shell” particles synthesised here were more accurately described as homogeneous extended alloys with the constituent shell elements diffusing through the entire core, including full-depth inclusion of zinc.
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| 4. |
- Eriksson, Johan, et al.
(författare)
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Atomic and electronic structures of the ordered 2√3 × 2√3 andthe molten 1×1 phase on the Si(111):Sn surface
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:23, s. 235410-
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Tidskriftsartikel (refereegranskat)abstract
- The Si(111) surface with an average coverage of slightly more than one monolayer of Sn, exhibits a 2√3 × 2√3 reconstruction below 463 K. In the literature, atomic structure models with 13 or 14 Sn atoms in the unit cell have been proposed based on scanning tunneling microscopy (STM) results, even though only four Sn atoms could be resolved in the unit cell. This paper deals with two issues regarding this surface. First, high resolution angle resolved photoelectron spectroscopy (ARPES) and STM are used to test theoretically derived results from an atomic structure model comprised of 14 Sn atoms, ten in an under layer and four in a top layer [Törnevik, et al., PRB 44, 13144 (1991)]. Low temperature ARPES reveals six occupied surface states. The calculated surface band structure only reproduces some of these surface states. However, simulated STM images show that certain properties of the four atoms that are visible in STM are reproduced by the model. The electronic structure of the Sn atoms in the under layer of the model does not correspond to any features seen in the ARPES results. New STM images are presented which indicate the presence of a different under layer consisting of eight Sn atoms, that is not compatible with the model. These results indicate that a revised model is called for. The second issue is the reversible transition from a 2√3 × 2√3 phase below 463 K to a 1×1 phase corresponding to a molten Sn layer, above that temperature. It is found that the surface band structure just below the transition temperature is quite similar to that at 100 K. The surface band structure undergoes a dramatic change at the transition. A strong surface state, showing a 1×1 periodicity, can be detected above the transition temperature. This state resembles parts of two surface states which, already before the transition temperature is reached, has begun a transformation and lost much of their 2√3×2√3 periodicities. Calculated surface band structures obtained from 1×1 models with 1 ML of Sn are compared with ARPES and STM results. It is found that the strong surface state present above the transition temperature shows a dispersion similar to that of a calculated surface band originating from the Sn-Si interface with the Sn atoms in T1 sites.
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| 5. |
- Gerhold, Stefan, et al.
(författare)
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Nickel-Oxide-Modified SrTiO3(110)-(4 x 1) Surfaces and Their Interaction with Water
- 2015
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:35, s. 20481-20487
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Tidskriftsartikel (refereegranskat)abstract
- Nickel oxide (NiO), deposited onto the strontium titanate (SrTiO3) (110)-(4 X 1) surface, was studied using photoemission spectroscopy (PES), X-ray absorption near edge structure (XANES), and low-energy He+ ion scattering (LEIS), as well as scanning tunneling microscopy (STM). The main motivation for studying this system comes from the prominent role it plays in photocatalysis. The (4 X 1) reconstructed SrTiO3(110) surface was previously found to be remarkably inert toward water adsorption under ultrahigh-vacuum conditions. Nickel oxide grows on this surface as patches without any apparent ordered structure. PES and LEIS reveal an upward band bending, a reduction of the band gap, and reactivity toward water adsorption upon deposition of NiO. Spectroscopic results are discussed with respect to the enhanced reactivity toward water of the NiO-loaded surface.
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| 6. |
- Inagaki, Shunsuke, et al.
(författare)
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Effects of adsorbed molecular ordering to the superconductivity of a two-dimensional atomic layer crystal
- 2023
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Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 7:2
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) adsorption on the physical properties of the two-dimensional (2D) atomic layer superconductor (ALSC) In/Si(111)-(7×3) has been studied by angle-resolved photoelectron spectroscopy, transport measurements, and scanning tunneling microscopy. Hole doping from the adsorbed molecules has been reported to increase the superconducting transition temperature Tc of this ALSC, and the molecular spin tends to decrease it. Owing to its large electron affinity and its nonexistent spin state, the adsorption of PTCDA was expected to increase Tc. However, the PTCDA adsorption dopes only a small number of holes in the In layers and causes a suppression of Tc with a sharp increase in the normal-state sheet resistance followed by an insulating transition. Taking the disordering of the arrangement of PTCDA into account, we conclude that the increase in resistance is due to the localization effect originating from the random potential that is induced by the disordered PTCDA molecules. The present result also indicates the importance of the crystallinity of a 2D molecular film adsorbed on ALSCs.
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| 7. |
- Makela, Jaakko, et al.
(författare)
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Comparison of chemical, electronic, and optical properties of Mg-doped AlGaN
- 2016
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:50, s. 28591-28597
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Tidskriftsartikel (refereegranskat)abstract
- Hydrogen, carbon, and oxygen are common unintentional impurities of Al(x)Ga(1−x)N crystals. This impurity structure and its interplay with Mg impurities in Al(x)Ga(1−x)N semiconductors are relevant to develop the p-type nitride crystals for various devices (e.g, LEDs, transistors, gas sensors) but are still unclear. Here we have investigated Mg-doped Al0.5Ga0.5N before and after postgrowth annealing with valence-band and core-level photoelectron spectroscopy, photoluminescence, and resistivity measurements. First, it is found that a surface part of the Al0.5Ga0.5N crystal is surprisingly inert with air and stable against air exposure-induced changes. Thus, the relatively surface-sensitive photoelectron spectroscopy measurements reflect in this case also the bulk crystal characteristics. The measurements reveal the presence of deep states up to 1 eV above valence-band maximum before and after the annealing and that oxygen and carbon occupy N lattice sites (i.e., ON and CN). The model where CN-induced acceptor states in the band gap participate in the blue emission (photoluminescence) is supported. Furthermore, the presented Mg 2p core-level spectra demonstrate that part of Mg atoms forms direct bond(s) with oxygen in the bulklike structure of Al0.5Ga0.5N and that the chemical environment of Mg atoms is much richer than was expected previously.
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| 8. |
- Makela, J., et al.
(författare)
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Line shape and composition of the In 3d(5/2) core-level photoemission for the interface analysis of In-containing III-V semiconductors
- 2015
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Ingår i: Applied Surface Science. - : Elsevier BV. - 1873-5584 .- 0169-4332. ; 329, s. 371-375
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Tidskriftsartikel (refereegranskat)abstract
- The In 3d(5/2) photoelectron spectroscopy peak has been widely used to determine the interface structures of In-containing III-V device materials (e.g., oxidation states). However, an unclear parameter affecting the determination of the energy shifts and number of the core-level components, and therefore, the interpreted interface structure and composition, is still the intrinsic In 3d(5/2) peak line shape. It is undecided whether the line shape is naturally symmetric or asymmetric for pure In-containing III-V compounds. By using high-resolution photoelectron spectroscopy, we show that the In 3d(5/2) asymmetry arising from the emission at high binding-energy tail is not an intrinsic property of InAs, InP, InSb and InGaAs. Furthermore, it is shown that asymmetry of In 3d(5/2) peaks of pure III-V's originates from the natural surface reconstructions which cause the coexistence of slightly shifted In 3d(5/2) components with the symmetric peak shape and dominant Lorentzian broadening. (C) 2015 Elsevier B.V. All rights reserved.
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| 9. |
- Osiecki, Jacek, et al.
(författare)
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Alloying of Sn in the surface layer of Ag(111)
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87:7
-
Tidskriftsartikel (refereegranskat)abstract
- It is found that 1/3 monolayer (ML) of Sn forms a surface alloy with 2/3 ML of Ag on Ag(111). This highly ordered alloy layer shows a √3×√3 structure. By employing experimental and theoretical tools (scanning tunneling microscopy [STM], angle resolved photoelectron spectroscopy, low-energy electron diffraction, and density functional theory), an atomic model has been obtained that reproduces the experimental electronic structure in both real and reciprocal space. Detailed surface band dispersions, constant energy contours, and STM images, obtained experimentally and theoretically, are compared in order to verify the model. Similar, 1-layer-thick alloys on Ag(111) with Pb, Bi, or Sb exhibit measurable spin–orbit interactions. However, no such spin split could be detected in the case of Sn in this study.
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| 10. |
- Osiecki, Jacek, et al.
(författare)
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Experimental and Theoretical Evidence of a Highly Ordered Two-Dimensional Sn/Ag Alloy on Si(111)
- 2012
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 109:5, s. 057601-
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Tidskriftsartikel (refereegranskat)abstract
- The existence of a highly ordered, two-dimensional, Sn/Ag alloy on Si(111) is reported in this study. We present detailed atomic and electronic structures of the one atomic layer thick alloy, exhibiting a 2 x 2 periodicity. The electronic structure is metallic due to a free-electron-like surface band dispersing across the Fermi level. By electron doping, the electronic structure can be converted into a semiconducting state. A rotated Sn trimer constitutes the key structural element that could be identified by a detailed analysis of constant energy contours derived from the free-electron-like band.
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| 11. |
- Osiecki, Jacek, et al.
(författare)
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Formation of a root 3 x root 3 surface on Si/Ge(111) studied by STM and LEED
- 2009
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Ingår i: SURFACE SCIENCE. - : Elsevier BV. - 0039-6028. ; 603:16, s. 2532-2536
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Tidskriftsartikel (refereegranskat)abstract
- We have performed a detailed study of the formation and the atomic structure of a root 3 x root 3 surface on Si/Ge(111) using both scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Both experimental methods confirm the presence of a root 3 x root 3 periodicity but unlike the Sn/Ge(111) and the Sn/Si(111) surfaces, the Si/Ge(111) surface is not well ordered. There is no long range order on the surface and the root 3 x root 3 reconstruction is made up of double rows of silicon atoms separated by disordered areas composed of germanium atoms.
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| 12. |
- Osiecki, Jacek R., et al.
(författare)
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Periodic corner holes on the Si(111)-7×7 surface can trap silver atoms
- 2022
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- Advancement in nanotechnology to a large extent depends on the ability to manipulate materials at the atomistic level, including positioning single atoms on the active sites of the surfaces of interest, promoting strong chemical bonding. Here, we report a long-time confinement of a single Ag atom inside a corner hole (CH) of the technologically relevant Si(111)-7×7 surface, which has comparable size as a fullerene C60 molecule with a single dangling bond at the bottom center. Experiments reveal that a set of 17 Ag atoms stays entrapped in the CH for the entire duration of experiment, 4 days and 7 h. Warming up the surface to about 150 °C degrees forces the Ag atoms out of the CH within a few minutes. The processes of entrapment and diffusion are temperature dependent. Theoretical calculations based on density functional theory support the experimental results confirming the highest adsorption energy at the CH for the Ag atom, and suggest that other elements such as Li, Na, Cu, Au, F and I may display similar behavior. The capability of atomic manipulation at room temperature makes this effect particularly attractive for building single atom devices and possibly developing new engineering and nano-manufacturing methods.
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| 13. |
- Razado-Colambo, Ivy, et al.
(författare)
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STM studies of the Sn/Ge(111)√3x√3 and 3x3 surfaces
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Scanning tunneling microscopy (STM) was used to study the room temperature (RT) √3x√3 and the low temperature (LT) 3x3 surfaces of Sn/Ge(111). The Sn/Ge(111)√3x√3 surface was prepared under different conditions. Nine sample preparations were performed with various Sn coverage and annealing treatments. The conditions that produced the best √3x√3 surface (low defect density and minimal area covered by islands and disorder) were: i) Sn coverage of 0.38 ML, ii) sample temperature slightly above that corresponding to the c(2x8) to 1x1 transition. This optimum preparation was used for the STM study of the LT 3x3 phase. The apparent height distribution of the Sn atoms in the 3x3 phase was analysed in detail and discussed in relation to the Sn 4d core-level line shape. Two peaks were observed in the apparent height distribution of the Sn atoms corresponding to the up and down atoms constituting the 3x3 reconstruction. Simulated Sn 4d core-level spectra were generated from the distribution by assuming a linear relation between the apparent height and the core-level binding energy. The simulated spectra are compared to experimental spectra appearing in the literature.
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| 14. |
- Sakamoto, Kazuyuki, et al.
(författare)
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Valley spin polarization by using the extraordinary Rashba effect on silicon
- 2013
-
Ingår i: Nature Communications. - : Nature Publishing Group: Nature Communications. - 2041-1723. ; 4:2073
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Tidskriftsartikel (refereegranskat)abstract
- The addition of the valley degree of freedom to a two-dimensional spin-polarized electronic system provides the opportunity to multiply the functionality of next-generation devices. So far, however, such devices have not been realized due to the difficulty to polarize the valleys, which is an indispensable step to activate this degree of freedom. Here we show the formation of 100% spin-polarized valleys by a simple and easy way using the Rashba effect on a system with C-3 symmetry. This polarization, which is much higher than those in ordinary Rashba systems, results in the valleys acting as filters that can suppress the backscattering of spin-charge. The present system is formed on a silicon substrate, and therefore opens a new avenue towards the realization of silicon spintronic devices with high efficiency.
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| 15. |
- Sohail, Hafiz Muhammad, et al.
(författare)
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Electronic and atomic structures of a 3x3 surface formed by a binary Sn/Ag overlayer on the Ge(111)c(2x8) surface: ARPES, LEED, and STM studies
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 85:20, s. 205409-
-
Tidskriftsartikel (refereegranskat)abstract
- The electronic and atomic structures of a well-ordered 3x3 periodicity of a binary Sn/Ag overlayer on Ge(111) have been studied. The ordered binary overlayer was formed by depositing 0.75 monolayer of Sn on an Ag/Ge(111) root 3x root 3 surface. Annealing at 330 degrees C resulted in a low-energy electron diffraction pattern that exhibited sharp spots. A detailed electronic structure investigation was performed by angle-resolved photoelectron spectroscopy. The Sn/Ag/Ge(111) 3x3 surface shows a rich band structure. There are seven bands which are positively identified as 3x3 surface bands, all within 1.5 eV below the Fermi level (E-F). The upper two bands disperse across E-F exhibiting steep almost linear dispersions down to a minimum energy of approximate to 0.40 eV below E-F at the (Gamma) over bar point (approximate to 0.30 eV at the (K) over bar point). Constant energy contours have been mapped in the 3x3 surface Brillouin zone (SBZ) in order to study an intriguing split observed in the band structure related to the two upper bands. It turned out that the two upper bands are degenerate along the (Gamma) over bar - (K) over bar and (M) over bar - (K) over bar symmetry lines of the 3x3 SBZ but separated along (Gamma) over bar - (M) over bar. Scanning tunneling microscopy images obtained at approximate to 40 K show essentially a hexagonal structure except for a honeycomb structure in a limited bias range imaging empty states. Core-level spectroscopy shows a narrow Sn 4d spectrum consistent with the high degree of structural order.
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| 16. |
- Sohail, Hafiz Muhammad, et al.
(författare)
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First principles study of electronic and atomic structures of a √3x√3 superstructures induced by Ag on Si(111) and Ge(111)
- 2014
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We have employed first principles density functional theory (DFT) based calculations (WIEN2k) to study the electronic and atomic structures of the reconstruction induced by Ag on Si(111) and Ge(111). The Ag/Si(111) surface, in particular, has acted as a model system when it comes to the interaction between adsorbed metals and semiconductor surfaces. Two models have been studied, i.e., the honeycomb-chained-triangle (HCT) and the in-equivalenttriangle (IET) model. The band structures of these models were calculated using density functional theory within the generalized gradient approximation (GGA) and the local density approximation (LDA). The band structures calculated from the fully relaxed versions of the two models were found to be quite similar except for the occupancy of the free electron like band at the - point. The IET model gives a slightly lower energy minimum compared to the HCT model for both Si and Ge. Further, we find that the energy minima are deeper for Ge when comparing the results with Si for the HCT and IET models, respectively. The theoretical surface band structure is qualitatively in good general agreement with the experimental dispersions of the main surface states, while the theoretical band widths are approximately half of the experimental ones. The calculated band structures show a gap between the two uppermost, fully occupied, bands at the - point only when the IET model is used to account for the electronic structure of Ag/Si(111) . Neither the IET nor the HCT model resulted in a gap when applied to Ag/Ge(111) .
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| 17. |
- Tuominen, M., et al.
(författare)
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Oxidized crystalline (3 x 1)-O surface phases of InAs and InSb studied by high-resolution photoelectron spectroscopy
- 2015
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 106:1
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Tidskriftsartikel (refereegranskat)abstract
- The pre-oxidized crystalline (3 x 1)-O structure of InAs(100) has been recently found to significantly improve insulator/InAs junctions for devices, but the atomic structure and formation of this useful oxide layer are not well understood. We report high-resolution photoelectron spectroscopy analysis of (3 x 1)-O on InAs(100) and InSb(100). The findings reveal that the atomic structure of (3 x 1)-O consists of In atoms with unexpected negative (between -0.64 and -0.47 eV) and only moderate positive (In2O type) core-level shifts; highly oxidized group-V sites; and four different oxygen sites. These fingerprint shifts are compared to those of previously studied oxides of III-V to elucidate oxidation processes. (C) 2015 AIP Publishing LLC.
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| 18. |
- Virojanadara, Chariya, et al.
(författare)
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Substrate orientation : A way towards higher quality monolayer graphene growth on 6H-SiC(0001)
- 2009
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 603:15, s. L87-L90
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Tidskriftsartikel (refereegranskat)abstract
- The influence of substrate orientation on the morphology of graphene growth on 6H-SiC(0 0 0 1) was investigated using low-energy electron and scanning tunneling microscopy (LEEM and STM). Large area monolayer graphene was successfully furnace-grown on these substrates. Larger terrace widths and smaller step heights were obtained on substrates with a smaller mis-orientation from on-axis (0.03 degrees) than on those with a larger (0.25 degrees). Two different types of a carbon atom networks, honeycomb and three-for-six arrangement, were atomically resolved in the graphene monolayer. These findings are of relevance for various potential applications based on graphene-SiC structures.
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| 19. |
- Wagner, Margareta, et al.
(författare)
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Resolving the Structure of a Well-Ordered Hydroxyl Overlayer on In2O3(111) : Nanomanipulation and Theory
- 2017
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 11:11, s. 11531-11541
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Tidskriftsartikel (refereegranskat)abstract
- Changes in chemical and physical properties resulting from water adsorption play an important role in the characterization and performance of device-relevant materials. Studies of model oxides with well-characterized surfaces can provide detailed information that is vital for a general understanding of water-oxide interactions. In this work, we study single crystals of indium oxide, the prototypical transparent contact material that is heavily used in a wide range of applications and most prominently in optoelectronic technologies. Water adsorbs dissociatively already at temperatures as low as 100 K, as confirmed by scanning tunneling microscopy (STM), photoelectron spectroscopy, and density functional theory. This dissociation takes place on lattice sites of the defect-free surface. While the In2O3(111)-(1 × 1) surface offers four types of surface oxygen atoms (12 atoms per unit cell in total), water dissociation happens exclusively at one of them together with a neighboring pair of 5-fold coordinated In atoms. These O-In groups are symmetrically arranged around the 6-fold coordinated In atoms at the surface. At room temperature, the In2O3(111) surface thus saturates at three dissociated water molecules per unit cell, leading to a well-ordered hydroxylated surface with (1 × 1) symmetry, where the three water OWH groups plus the surface OSH groups are imaged together as one bright triangle in STM. Manipulations with the STM tip by means of voltage pulses preferentially remove the H atom of one surface OSH group per triangle. The change in contrast due to strong local band bending provides insights into the internal structure of these bright triangles. The experimental results are further confirmed by quantitative simulations of the STM image corrugation.
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| 20. |
- Wagner, Margareta, et al.
(författare)
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Well-ordered in adatoms at the In2 O3 (111) surface created by Fe deposition
- 2016
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Ingår i: Physical Review Letters. - 0031-9007. ; 117:20
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Tidskriftsartikel (refereegranskat)abstract
- Metal deposition on oxide surfaces usually results in adatoms, clusters, or islands of the deposited material, where defects in the surface often act as nucleation centers. Here an alternate configuration is reported. After the vapor deposition of Fe on the In2O3(111) surface at room temperature, ordered adatoms are observed with scanning tunneling microscopy. These are identical to the In adatoms that form when the sample is reduced by heating in ultrahigh vacuum. Density functional theory calculations confirm that Fe interchanges with In in the topmost layer, pushing the excess In atoms to the surface where they arrange as a well-ordered adatom array.
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| 21. |
- Walton, Alex S., et al.
(författare)
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Interface Controlled Oxidation States in Layered Cobalt Oxide Nanoislands on Gold
- 2015
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 9:3, s. 2445-2453
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Tidskriftsartikel (refereegranskat)abstract
- Layered cobalt oxides have been shown to be highly active catalysts for the oxygen evolution reaction (OER; half of the catalytic "water splitting" reaction), particularly when promoted with gold. However, the surface chemistry of cobalt oxides and in particular the nature of the synergistic effect of gold contact are only understood on a rudimentary level, which at present prevents further exploration. We have synthesized a model system of flat, layered cobalt oxide nanoislands supported on a single crystal gold (111) substrate. By using a combination of atom-resolved scanning tunneling microscopy, X-ray photoelectron and absorption spectroscopies and density functional theory calculations, we provide a detailed analysis of the relationship between the atomic-scale structure of the nanoislands, Co oxidation states and substrate induced charge transfer effects in response to the synthesis oxygen pressure. We reveal that conversion from CO2+ to Co3+ can occur by a facile incorporation of oxygen at the interface between the nanoisland and gold, changing the islands from a Co-O bilayer to an O-Co-O trilayer. The O-Co-O trilayer islands have the structure of a single layer of beta-CoOOH, proposed to be the active phase for the OER, making this system a valuable model in understanding of the active sites for OER. The Co oxides adopt related island morphologies without significant structural reorganization, and our results directly demonstrate that nanosized Co oxide islands have a much higher structural flexibility than could be predicted from bulk properties. Furthermore, it is clear that the gold/nanoparticle interface has a profound effect on the structure of the nanoislands, suggesting a possible promotion mechanism.
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| 22. |
- Wang, Weimin, et al.
(författare)
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Broken symmetry induced band splitting in the Ag2Ge surface alloy on Ag(111)
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 89:12, s. 125410-1-125410-6
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Tidskriftsartikel (refereegranskat)abstract
- We report a study of the atomic and electronic structures of the ordered Ag2Ge surface alloy containing ⅓ monolayer of Ge. Low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and angle-resolved photoelectron spectroscopy (ARPES) data reveal a symmetry breaking of the expected √3 × √3 periodicity, which is established for other Ag2M alloys (M = Bi, Sb, Pb, and Sn). The deviation from a simple √3 × √3 structure manifests itself as a splitting of diffraction spots in LEED, as a striped structure with a 6× periodicity including a distortion of the local hexagonal structure in STM, and as a complex surface band structure in ARPES that is quite different from those of the other Ag2M alloys. These results are interesting in view of the differences in the atomic and electronic structures exhibited by different group IV elements interacting with Ag(111). Pb and Sn form √3 × √3 surface alloys on Ag(111), of which Ag2Pb shows a surface band structure with a clear spin-orbit split. Si and C form silicene and graphene structures, respectively, with linear band dispersions and the formation of Dirac cones as reported for graphene. The finding that Ag2Ge deviates from the ideal (√3 × √3) Ag2Sn and Ag2Pb surface alloys makes Ge an interesting “link” between the heavy group IV elements (Sn, Pb) and the light group IV elements (Si, C).
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| 23. |
- Watcharinyanon, Somsakul, et al.
(författare)
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Hydrogen intercalation of graphene grown on 6H-SiC(0001)
- 2011
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 605:17-18, s. 1662-1668
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Tidskriftsartikel (refereegranskat)abstract
- Atomic hydrogen exposures on a monolayer graphene grown on the SiC(0001) surface are shown to result in hydrogen intercalation. The hydrogen intercalation induces a transformation of the monolayer graphene and the carbon buffer layer to bi-layer graphene without a buffer layer. The STM, LEED, and core-level photoelectron spectroscopy measurements reveal that hydrogen atoms can go underneath the graphene and the carbon buffer layer and bond to Si atoms at the substrate interface. This transforms the buffer layer into a second graphene layer. Hydrogen exposure results initially in the formation of bi-layer graphene islands on the surface. With larger atomic hydrogen exposures, the islands grow in size and merge until the surface is fully covered with bi-layer graphene. A (root 3 x root 3)R30 degrees periodicity is observed on the bi-layer areas. ARPES and energy filtered XPEEM investigations of the electron band structure confirm that after hydrogenation the single pi-band characteristic of monolayer graphene is replaced by two pi-bands that represent bi-layer graphene. Annealing an intercalated sample, representing bi-layer graphene, to a temperature of 850 degrees C. or higher, re-establishes the monolayer graphene with a buffer layer on SiC(0001). (C) 2011 Elsevier B.V. All rights reserved.
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