| 1. |
- Elgammal, Karim, et al.
(författare)
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Density functional calculations of graphene-based humidity and carbon dioxide sensors : effect of silica and sapphire substrates
- 2017
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Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 663, s. 23-30
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Tidskriftsartikel (refereegranskat)abstract
- We present dispersion-corrected density functional calculations of water and carbon dioxide molecules adsorption on graphene residing on silica and sapphire substrates. The equilibrium positions and bonding distances for the molecules are determined. Water is found to prefer the hollow site in the center of the graphene hexagon, whereas carbon dioxide prefers sites bridging carbon-carbon bonds as well as sites directly on top of carbon atoms. The energy differences between different sites are however minute - typically just a few tenths of a millielectronvolt. Overall, the molecule-graphene bonding distances are found to be in the range 3.1-3.3 (A) over circle. The carbon dioxide binding energy to graphene is found to be almost twice that of the water binding energy (around 0.17 eV compared to around 0.09 eV). The present results compare well with previous calculations, where available. Using charge density differences, we also qualitatively illustrate the effect of the different substrates and molecules on the electronic structure of the graphene sheet.
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| 2. |
- Hugosson, Håkan Wilhelm, et al.
(författare)
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Surface energies and work functions of the transition metal carbides
- 2004
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 557:03-jan, s. 243-254
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Tidskriftsartikel (refereegranskat)abstract
- We have performed an ab initio study of the surface energies, surface electronic structures and work functions for the (10 0) surface of the, existent and hypothetical, cubic 3d (Sc-Cu), 4d (Zr-Ag) and 5d (La-Au) transition metal carbides. The calculated surface energies have been compared to predictions using a so-called bond-cutting model and a model based on the so-called bonding energies. The absolute values and rough trends of the surface energies are fairly well predicted within the simple bond-cutting model, as compared to fully self-consistent calculations, while both trends and absolute values are well reproduced within the bonding energy model. The electronic structure (densities of states) of the transition metal carbides at the surface and in the bulk have been calculated. The trends are discussed in relation to the behavior of the surface energy and the work function across the series.
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| 3. |
- An, Siwen, et al.
(författare)
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Signal-to-Noise Ratio Optimization in X-ray Fluorescence Spectrometry for Chromium Contamination Analysis
- 2021
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Ingår i: Talanta. - : Elsevier BV. - 0039-9140 .- 1873-3573. ; 230
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Tidskriftsartikel (refereegranskat)abstract
- In most cases, direct X-ray fluorescence (XRF) analysis of solutions entails technical difficulties due to a high X-ray scattering background resulting in a spectrum with a poor signal-to-noise ratio (SNR). Key factors that determine the sensitivity of the method are the energy resolution of the detector and the amount of scattered radiation in the energy range of interest. Limiting the width of the primary spectrum by the use of secondary targets, or filters, can greatly improve the sensitivity for specific portions of the spectrum. This paper demonstrates a potential method for SNR optimization in direct XRF analysis of chromium (Cr) contamination. The suggested method requires minimal sample preparation and achieves higher sensitivity compared to existing direct XRF analysis. Two states of samples, fly ash and leachate from municipal solid waste incineration, were investigated. The effects of filter material, its absorption edge and filter thickness were analyzed using the combination of Monte Carlo N-Particle (MCNP) code and energy-dispersive XRF spectrometry. The applied filter removes primary photons with energies interfering with fluorescence photons from the element of interest, thus results in lower background scattering in the spectrum. The SNR of Cr peak increases with filter thickness and reaches a saturation value when further increased thickness only increases the measurement time. Measurements and simulations show that a Cu filter with a thickness between 100 μm and 140 μm is optimal for detecting Cr by taking into account both the SNR and the exposure time. With direct XRF analysis for solutions, the limit of quantitation (LOQ) of the achieved system was 0.32 mg/L for Cr, which is well below the allowed standard limitation for landfills in Sweden. This work shows that XRF can gain enough sensitivity for direct monitoring to certify that the Cr content in leachate is below environmental limits.
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| 4. |
- Hugosson, Håkan Wilhelm, et al.
(författare)
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Sulfur- and Oxygen-Induced Alterations of the Iron (001) Surface Magnetism and Work Function : A Theoretical Study
- 2013
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:12, s. 6161-6171
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and magnetic properties of atomic sulfur and oxygen adsorbed on the iron (001) surface are examined using density functional theory (DFT). The sulfur/oxygen coverage is considered from a quarter of one monolayer (ML) to a full monolayer. The work function change, magnetic properties, and density of states are determined and compared. We find that the work function increases with sulfur coverage in agreement with experiment. We also find that sulfur interacts strongly with the surface layer and that the magnetic moment of the Fe surface decreases as the sulfur coverage increases. In the case of oxygen adsorption, we find that the magnetic moment of the surface Fe atoms instead increases. We show that the difference in surface magnetic moment between sulfur adsorption and oxygen adsorption can be simply explained combining the Slater-Pauling rigid band model linking d-occupation and magnetic moment with an electronegativity argument. Moreover, the work function of the Fe surface as a function of oxygen coverage is found to be very sensitive to overlayer arrangement, here seen in the cases of 0.5 ML c(2 x 2) and 0.5 ML p(2 x 1). This is shown to result from large differences in the surface dipole moment change induced by the oxygen adsorption in the two different overlayer arrangements.
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| 5. |
- Smith, Anderson D., et al.
(författare)
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Resistive graphene humidity sensors with rapid and direct electrical readout
- 2015
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 7:45, s. 19099-19109
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate humidity sensing using a change of the electrical resistance of single-layer chemical vapor deposited (CVD) graphene that is placed on top of a SiO2 layer on a Si wafer. To investigate the selectivity of the sensor towards the most common constituents in air, its signal response was characterized individually for water vapor (H2O), nitrogen (N-2), oxygen (O-2), and argon (Ar). In order to assess the humidity sensing effect for a range from 1% relative humidity (RH) to 96% RH, the devices were characterized both in a vacuum chamber and in a humidity chamber at atmospheric pressure. The measured response and recovery times of the graphene humidity sensors are on the order of several hundred milliseconds. Density functional theory simulations are employed to further investigate the sensitivity of the graphene devices towards water vapor. The interaction between the electrostatic dipole moment of the water and the impurity bands in the SiO(2)d substrate leads to electrostatic doping of the graphene layer. The proposed graphene sensor provides rapid response direct electrical readout and is compatible with back end of the line (BEOL) integration on top of CMOS-based integrated circuits.
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| 6. |
- Mirsakiyeva, Amina, et al.
(författare)
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Experimental and density functional theory studies of some novel piperidine-containing acetylene glycols
- 2016
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Ingår i: ARKIVOC. - : Arkat. - 1551-7004 .- 1551-7012. ; , s. 86-99
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Tidskriftsartikel (refereegranskat)abstract
- Synthesis routes of novel piperidine-containing acetylenes are presented. The new molecules are expected to exhibit plant growth stimulation properties. In particular, the yield in a situation of drought is expected to increase. Our synthesis makes use of the Favorskii reaction between cyclohexanone/piperidone and triple-bond containing alcohols. The structures of the obtained molecules were determined using nuclear magnetic resonance (NMR). The electronic structure and geometries of the molecules were studied theoretically using first-principles calculations based on density functional theory. The calculated geometries agree very well with the experimentally determined ones, and also allow us to determine bond lengths, angles and charge distributions inside the molecules.
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| 7. |
- Bustad, Johnny
(författare)
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Shake-up studies of model systems for adsorbates : Department of Quantum Chemistry, Uppsala University
- 1993
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Shake-up studies of model systems for adsorbates are presented. The molecule CO adsorbed on transition metal surfaces belong to one of the most studied systems in surface science. The interest is motivated by fundamental questions about the adsorbate-substrate interaction as well as technical applications, such as catalysis. A good starting point in order to get more detailed understanding of the CO-metal interaction is to study the transition metal carbonyls, where CO is coordinated to metal atoms. It has been found that many aspects of the adsorbate systems can be modelled by these molecules. These are the main reasons for studying the molecules Mo(CO)6 and Cr(CO)6, for which results are reported in this thesis.
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| 8. |
- Hugosson, Håkan Wilhelm, et al.
(författare)
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Surface segregation of transition metal impurities on the TiC(100) surface
- 2005
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 585:02-jan, s. 101-107
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Tidskriftsartikel (refereegranskat)abstract
- The segregation energies of 3d (Sc-Cu), 4d (Y-Ag) and 5d (La-Au) transition metal impurities on the (10 0) surface of TiC have been obtained using first-principles electronic structure calculations. The results are in agreement with available experimental data and show that the difference in atomic size between the impurity and host species, as well as the difference in surface energies determines if the impurity will segregate towards the surface or not. The results indicate that the difference in size is the dominant factor for the trends in segregation of transition metal impurities towards the (100) surface of TiC.
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| 9. |
- Hugosson, Håkan Wilhelm, 1972-, et al.
(författare)
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Theoretical studies of substitutional impurities in molybdenum carbide
- 1999
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 60:22, s. 15123-15130
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Tidskriftsartikel (refereegranskat)abstract
- First-principles full-potential linear muffin-tin orbital calculations have been used to study the effect on the cohesion and electronic structure of cubic δ-MoC when 25% of the carbon is substituted for boron, nitrogen, or oxygen and when 25% of the molybdenum is substituted for niobium, tungsten, or ruthenium. A thorough study of the changes in the electronic structure and the effect of these on the properties of the compounds is made. Special attention is paid to the character (ionic, covalent, or metallic) of the states becoming occupied (or unoccupied) due to the substitution. A study is also made on the properties of the quaternary alloy Mo0.75W0.25C0.75N0.25. This substitution is shown to harden δ-MoC.
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| 10. |
- Lefez, B., et al.
(författare)
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Application of reflectance spectrophotometry to the study of copper (I) oxides (Cu2O and Cu3O2) on metallic substrate
- 1994
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Ingår i: Surface and Interface Analysis. - : Wiley. - 0142-2421 .- 1096-9918. ; 22:1-12, s. 451-455
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study is to characterize the compounds grown on copper during the oxidation at low temperature (T < 523 K) by optical methods: photoluminescence and UV‐Visible‐NIR diffuse reflectance spectroscopy. Two cuprous oxides Cu2O and Cu3O2 have been studied. The absorption of Cu2O films in the range 450–630 nm is mainly due to non‐stoichiometry bands associated with copper and oxygen vacancies. Cu3O2 is characterized by an optical band gap greater than that of Cu2O (respectively 2.25 and 1.95 eV) and by an intense luminescence emission at 760–780 nm. Cu3O2 may be considered as a gross defect structure of Cu2O (a Cu 2O = 0.427 nm, a Cu 3O 2 = 0.431 nm). The experimental approach of the oxidation mechanism reveals that at 423 K Cu2O is the primary product which later on is transformed into Cu3O2. Experimental and calculated optical absorption curves disclose the nucleation of CuO inside the cuprous oxides layer for oxidation in the range 473–523 K.
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