| 1. |
- Amft, Martin, et al.
(författare)
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A Molecular Mechanism for the Water-Hydroxyl Balance during Wetting of TiO2
- 2013
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:33, s. 17078-17083
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Tidskriftsartikel (refereegranskat)abstract
- We show that the formation of the wetting layer and the experimentally observed continuous shift of the H2O-OH balance toward molecular water at increasing coverage on a TiO2(110) surface can be rationalized on a molecular level. The mechanism is based on the initial formation of stable hydroxyl pairs, a repulsive interaction between these pairs, and an attractive interaction with respect to water molecules. The experimental data are obtained by synchrotron radiation photoelectron spectroscopy and interpreted with the aid of density functional theory calculations and Monte Carlo simulations.
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| 2. |
- Amft, Martin, et al.
(författare)
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Adsorption of Cu, Ag, and Au atoms on graphene including van der Waals interactions
- 2011
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 23:39
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Tidskriftsartikel (refereegranskat)abstract
- We performed a systematic density functional (DF) study of the adsorption of copper, silver, and gold adatoms on pristine graphene, especially accounting for van der Waals (vdW) interactions by the vdW-DF and PBE + D2 methods. In particular, we analyze the preferred adsorption site (among top, bridge, and hollow positions) together with the corresponding distortion of the graphene sheet and identify diffusion paths. Both vdW schemes show that the coinage metal atoms do bind to the graphene sheet and that in some cases the buckling of the graphene layer can be significant. Only the results for silver are qualitatively at variance with those obtained with the generalized gradient approximation, which gives no binding in this case. However in all three cases, we observe some quantitative differences between the vdW-DF and PBE + D2 methods. For instance the adsorption energies calculated with the PBE + D2 method are systematically higher than the ones obtained with vdW-DF. Moreover, the equilibrium distances computed with PBE + D2 are shorter than those calculated with the vdW-DF method.
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| 3. |
- Amft, Martin, et al.
(författare)
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Calculated magneto-optical Kerr spectra of the half-Heusler compounds AuMnX (X = In, Sn, Sb)
- 2007
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 19:31, s. 315216-
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Tidskriftsartikel (refereegranskat)abstract
- The ferromagnetic ground states of the half-Heusler compounds AuMnX (X ≤ In, Sn, Sb) have been calculated in the framework of the local spin-density approximation (LSDA) to density functional theory (DFT). AuMnSn is computed to be a half-metallic ferromagnet, whereas AuMnIn and AuMnSb are not half-metallic, due to their different band filling. The computed relativistic electronic structures served as inputs to calculate the magneto-optical Kerr rotations and ellipticities for all three materials. In the case of AuMnSn the largest, zero-temperature, polar Kerr rotation has been found to be -0.45° at about 1eV photon energy. The computed MOKE spectra of AuMnSn are in qualitative agreement with recent experiments. The largest Kerr rotations of AuMnIn and AuMnSb have been calculated to be +0.64° at 4.3eV and -0.85° at 0.9eV, respectively.
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| 4. |
- Amft, Martin, et al.
(författare)
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Catalytic activity of small MgO-supported Au clusters towards CO oxidation : A density functional study
- 2010
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 81:19, s. 195443-
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Tidskriftsartikel (refereegranskat)abstract
- In order to explain the experimentally found catalytic characteristics of Au1-4 /MgO (100) we have performed a comprehensive density functional study of these systems and their ability to (co)adsorb CO and O2 molecules. Starting from the carefully determined ground-state structures we have analyzed binding mechanisms, the influence of spin-orbit coupling, and charge redistributions in Au1-4 /MgO+CO (O2). Experimentally Au1,2 /MgO were found to be inactive under a mixed atmosphere. We show that O2 strongly binds to Au1 /MgO that prevents coadsorption. Although a catalytic reaction cycle towards CO oxidation, analogous to the gas phase reaction involving Au 2-, is energetically possible for Au2 /MgO, the cluster will get blocked by a strongly bound CO. On the other hand, the catalytic activity of Au3,4 /MgO could be explained by their ability to coadsorb CO and O2, hence indicating the occurrence of a Langmuir- Hinshelwood-type reaction mechanism for these clusters.
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| 5. |
- Amft, Martin
(författare)
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Density Functional Theory Studies of Small Supported Gold Clusters and Related Questions : What a Difference an Atom Makes
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During the last decades the specific manipulation of matter on the (sub-) nanometer scale, also known as nanoscience, became possible by technologies such as the scanning tunneling microscope. Nanocatalysts, i.e. catalytic active structures of up to a few nanometers in size, belong to this rather new class of materials. Unlike ordinary ’macroscopic’ catalytic materials, the performance of nanocatalysts does not simply scale, for instance, with the surface to volume ratio of the active material. In this Thesis model nanocatalysts are investigated by means of ab-initio density functional theory calculations. In paper I, we explain the experimentally observed catalytic characteristics of small gold clusters, Au1-4, on a regular magnesium oxide terrace towards the oxidation of carbon monoxide by thoroughly studying the adsorption of CO and O2 on these clusters. In the subsequent paper II, we study the feasibility of a catalytic water-mediated CO oxidation reaction on Au1-4/MgO and find that this reaction mechanism is not assessable for Au2,4/MgO and unlikely for Au1,3/MgO. Papers III and IV concentrate on the reactivity of clusters in the gas phase. Particularly, we focus on the relative stability of Au13 isomers and its potential for O2 dissociation (paper III). We find the lowest energy isomers, which contain a triangular prism at their center surrounded by a ring of the remaining seven atoms, to be generally stable upon O2 adsorption. The dissociation of O2 at certain sites of Au13 is found to be exothermic. In paper IV we performed scans of the Born-Oppenheimer potential energy surfaces of neutral and charged Cu3, Ag3, and Au3 to explore the thermally excited vibrations of these trimers. While the Born-Oppenheimer surface of Cu3 exhibits one fairly deep energy minimum, it is comparatively flat with two shallow minima in the case of Ag3. Hence for Ag3 there exist many thermally accessible geometries in a wide range of angles and bond lengths. For Au3, two distinct energy minima appear, being well-separated by a barrier of 180 meV. Already at room temperature, we find bond lengths changes of up to 5% for the studied trimers. Choosing Au3 as a case study for the changed reactivity of thermally excited modes, we find CO to bind up to 150 meV stronger to the excited cluster. Gold deposited on graphene and graphite was observed to form larger aggregates. In paper V, we study the electronic structures, high mobility, and substrate-mediated clustering processes of Au1-4 on graphene. Already in the 1970s is was speculated that dispersion forces, i.e. van der Waals forces, significantly contribute to the adsorption energies of gold atoms on graphite. We accounted for van der Waals interactions in our density functional theory calculations (paper VI) and investigated the influence of these dispersion forces on the binding of copper, silver, and gold adatoms on graphene. While copper and gold show a mixed adsorption mechanism, i.e. chemical binding plus attraction due to the van der Waals forces, silver is purely physisorbed on graphene.
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| 6. |
- Amft, Martin, et al.
(författare)
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Does H2O improve the catalytic activity of Au1−4/MgO towards CO oxidation?
- 2010
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The present density functional theory study addresses the question whether the presence of H2O influences the catalytic activity of small gold clusters, Au1-4/MgO(100), towards the oxidation of carbon monoxide. To this end, we studied the (co-)adsorption of H2O and CO/O2 on these gold clusters. The ground state structures in the presence of all three molecular species, that we found, are Au1O2/MgO and Au2-4CO/MgO with H2O adsorbed on the surface in the proximity of the clusters-molecule complex. In this configuration the catalytic activity of Au1-4/MgO is indifferent to the presence of H2O. We also found that a stable, highly activated hydroperoxyl-hydroxyl complex, O2H ·· OH, can be formed on Au1,3/MgO. For the catalytic active system Au8/MgO, it has been predicted that this complex opens an alternative catalytic reaction pathway towards CO oxidation. Our results suggest that this water mediated catalytic cycle is unlikely to occur on Au1,3/MgO. In the case of Au1/MgO the cycle is interrupted by the dissociation of the remaining (OH)2 complex after forming the first CO2 molecule. On Au3 /MgO the O2H ·· OH complex is likely to decompose upon the impact of a CO molecule, since three of its bond dissociation energies are comparable to the reaction barrier of the CO to CO2 oxidation.
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| 7. |
- Amft, Martin, et al.
(författare)
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First-principles calculations of optical and magneto-optical properties of Ga1-xMnxAs and MnAs
- 2009
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Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 404:20, s. 3782-3788
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Tidskriftsartikel (refereegranskat)abstract
- We present a first-principles density-functional investigation of the optical and magneto-optical properties of MnGa1-xAs (with x = 0,0.0625, and 1) systems. Our calculated dielectric function, magneto-optical Kerr effect, and magnetic circular dichroism spectra agree reasonably well with existing experimental results. A comparison of the optical and magneto-optical spectra of MnAs in the naturally occurring hexagonal phase and in the cubic zinc-blende structure is made. The differences in the spectral properties of these two phases could aid detection of MnAs in the zinc-blende structure. (C) 2009 Elsevier B.V. All rights reserved.
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| 8. |
- Amft, Martin, et al.
(författare)
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Influence of the cluster dimensionality on the binding behavior of CO and O(2) on Au(13)
- 2012
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 136:2, s. 024312-
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Tidskriftsartikel (refereegranskat)abstract
- We present an ab initio density functional theory study of the binding behavior of CO and O(2) molecules to two-and three-dimensional isomers of Au(13) in order to investigate the potential catalytic activity of this cluster towards low-temperature CO oxidation. First, we scanned the potential energy surface of Au(13) and studied the effect of spin-orbit coupling on the relative stabilities of the 21 isomers we identified. While spin-orbit coupling increases the stability of the three-dimensional more than the two-dimensional isomers, the ground state structure at 0 K remains planar. Second, we systematically studied the binding of CO and O(2) molecules onto the planar and three-dimensional structures lowest in energy. We find that the isomer dimensionality has little effect on the binding of CO to Au(13). O(2), on the other hand, binds significantly to the three-dimensional isomer only. The simultaneous binding of multiple CO molecules decreases the binding energy per molecule. Still, the CO binding remains stronger than the O(2) binding. We did not find a synergetic effect due to the co-adsorption of both molecular species. On the three-dimensional isomer, we find O(2) dissociation to be exothermic with an dissociation barrier of 1.44 eV.
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| 9. |
- Amft, Martin, et al.
(författare)
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Small gold clusters on graphene, their mobility and clustering : a DFT study
- 2011
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Ingår i: Physical Review B Condensed Matter. - : IOP Publishing. - 0163-1829 .- 1095-3795. ; 23:20, s. 205301-
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Tidskriftsartikel (refereegranskat)abstract
- Motivated by the experimentally observed high mobility of gold atoms on graphene and their tendency to form nanometer-sized clusters, we present a density functional theory study of the ground state structures of small gold clusters on graphene, their mobility and clustering. Our detailed analysis of the electronic structures identifies the opportunity to form strong gold-gold bonds and the graphene-mediated interaction of the pre-adsorbed fragments as the driving forces behind gold's tendency to aggregate on graphene. While clusters containing up to three gold atoms have one unambiguous ground state structure, both gas phase isomers of a cluster with four gold atoms can be found on graphene. In the gas phase the diamond-shaped Au-4(D) cluster is the ground state structure, whereas the Y-shaped Au-4(Y) becomes the actual ground state when adsorbed on graphene. As we show, both clusters can be produced on graphene by two distinct clustering processes. We also studied in detail the stepwise formation of a gold dimer out of two pre-adsorbed adatoms, as well as the formation of Au-3. All reactions are exothermic and no further activation barriers, apart from the diffusion barriers, were found. The diffusion barriers of all studied clusters range from 4 to 36 meV only, and are substantially exceeded by the adsorption energies of -0.1 to -0.59 eV. This explains the high mobility of Au1-4 on graphene along the C-C bonds.
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| 10. |
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| 11. |
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| 12. |
- Cheng, Jingyuan, et al.
(författare)
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Textile Building Blocks:Toward Simple, Modularized, and Standardized Smart Textile
- 2017
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Ingår i: Smart Textiles. - Cham : Springer, Cham. - 9783319501239 - 9783319501246 ; , s. 303-331
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Bokkapitel (refereegranskat)abstract
- Textiles are pervasive in our life, covering human body and objects, as well as serving in industrial applications. In its everyday use of individuals, smart textile becomes a promising medium for monitoring, information retrieval, and interaction. While there are many applications in sport, health care, and industry, the state-of-the-art smart textile is still found only in niche markets. To gain mass-market capabilities, we see the necessity of generalizing and modularizing smart textile production and application development, which on the one end lowers the production cost and on the other end enables easy deployment. In this chapter, we demonstrate our initial effort in modularization. By devising types of universal sensing fabrics for conductive and non-conductive patches, smart textile construction from basic, reusable components can be made. Using the fabric blocks, we present four types of sensing modalities, including resistive pressure, capacitive, bioimpedance, and biopotential. In addition, we present a multi-channel textile–electronics interface and various applications built on the top of the basic building blocks by ‘cut and sew’ principle.
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| 13. |
- Elgazzar, S, et al.
(författare)
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Hidden order in URu2Si2 originates from Fermi surface gapping induced by dynamic symmetry breaking
- 2009
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Ingår i: Nature Materials. - 1476-1122 .- 1476-4660. ; 8:4, s. 337-341
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Tidskriftsartikel (refereegranskat)abstract
- Spontaneous, collective ordering of electronic degrees of freedom leads to second-order phase transitions that are characterized by an order parameter driving the transition. The notion of a 'hidden order' has recently been used for a variety of materials where a clear phase transition occurs without a known order parameter. The prototype example is the heavy-fermion compound URu2Si2, where a mysterious hidden-order transition occurs at 17.5 K. For more than twenty years this system has been studied theoretically and experimentally without a firm grasp of the underlying physics. Here, we provide a microscopic explanation of the hidden order using density-functional theory calculations. We identify the Fermi surface 'hot spots' where degeneracy induces a Fermi surface instability and quantify how symmetry breaking lifts the degeneracy, causing a surprisingly large Fermi surface gapping. As the mechanism for the hidden order, we deduce spontaneous symmetry breaking through a dynamic mode of antiferromagnetic moment excitations.
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| 14. |
- Hild, K., et al.
(författare)
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Magnetic Circular Dichroism in Two-Photon Photoemission
- 2009
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:5, s. 057207-
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Tidskriftsartikel (refereegranskat)abstract
- We report the observation of magnetic circular dichroism (MCD) in two-photon photoemission (2PPE). The Heusler alloys Ni2MnGa and Co2FeSi were investigated by excitation with femtosecond laser light, showing MCD asymmetries of A = (3.5 +/- 0.5) x 10(-3) for Ni2MnGa and of A = (2.1 +/- 1.0) x 10(-3) for Co2FeSi, respectively. A theoretical explanation is provided based on local spin-density calculations for the magnetic dichroic response; the computed 2PPE MCD agrees well with the experiment. The observed 2PPE magnetic contrast represents an interesting alternative for future time-resolved photoemission studies on surface magnetism practicable in the laboratory.
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| 15. |
- Pálsson, Gunnar K., et al.
(författare)
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Hydrogen site occupancy and strength of forces in nanosized metal hydrides
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:19, s. 195407-
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Tidskriftsartikel (refereegranskat)abstract
- The dipole force components in nanosized metal hydrides are quantitatively determined with curvature and x-ray diffraction measurements. Ab initio density functional theory is used to calculate the dipole components and the symmetry of the strain field. The hydrogen occupancy in a 100-nm-thick V film is shown to be tetrahedral with a slight asymmetry at low concentration, and a transition to octahedral occupancy is shown to take place at around 0.07 [H/V] at 360 K. When the thickness of the V layer is reduced to 3 nm and biaxially strained, in a Fe0.5V0.5/V superlattice, the hydrogen unequivocally occupies octahedral z-like sites, even at and below concentrations of 0.02 [H/V].
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| 16. |
- Pálsson, Gunnar K., et al.
(författare)
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Proximity effects on the H absorption in ultra-thin V layers
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:4, s. 045420-
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Tidskriftsartikel (refereegranskat)abstract
- We discuss proximity effects on the hydrogen uptake in 14 atomic layers of vanadium. The enthalpy and entropy of solution were measured and compared to ab initio density functional calculations. We show that there exists a large difference in the hydrogen uptake of V in Cr/V and Fe/V superlattices, in which the V is under close to identical strain states. The calculations show that neither local strain effects nor charge redistributions can be the cause for the observed effect. This leaves magnetic and long ranged elastic strain fields-neither captured by the current calculations-as possible mechanisms for the observed effects.
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| 17. |
- Zhang, Rui, et al.
(författare)
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A generic sensor fabric for multi-modal swallowing sensing in regular upper-body shirts
- 2016
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Ingår i: Proceedings of the 2016 ACM International Symposium on Wearable Computers. - New York, NY, USA : ACM Digital Library. - 9781450344609 ; , s. 46-47
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Konferensbidrag (refereegranskat)abstract
- We investigate a generic fabric material as basis for resistive pressure and bio-impedance sensors and apply the fabric in a shirt collar for swallowing spotting. A pilot study confirmed the signal performance of both sensor types.
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