| 1. |
- Dou, Yangpeng, et al.
(författare)
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Effects of reducibility of graphene oxide nanosheets on preparation of AgNPs/GO nanocomposites and their electrocatalytic performance
- 2015
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Ingår i: Journal of nanoparticle research. - : Springer Science and Business Media LLC. - 1388-0764 .- 1572-896X. ; 17:12, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- Silver nanoparticles/graphene oxide (AgNPs/GO) nanocomposites were prepared in a solution of AgNO3 and GO. The GO serves not only as a reductant but also as a substrate to support the as-reduced silver nanoparticles. The reducibility of GO was investigated by analyzing the influence factors such as pH, duration, the reaction temperature, and the weight ratio of AgNO3 and GO in the AgNP/GO nanocomposite mixture, which were evaluated by the UV–vis absorption spectroscopy. The results demonstrated that Ag nanoparticles with an average diameter of 5–10 nm were uniformly dispersed on the surface of GO nanosheets under the optimum synthesis conditions of pH between 8 and 11, weight ratio of AgNO3 and GO between 55 % and 60 %, and at 80 °C for 6 h. Moreover, the obtained AgNPs/GO nanocomposites exhibit good electrocatalytic activity for the reduction of p-nitrophenol to 4-(hydroxyamino) phenol.
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| 2. |
- Emanuelsson, Christian, et al.
(författare)
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Delicate Interactions of PTCDI molecules on Ag/Si(111)-√3×√3
- 2018
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Ingår i: Journal of Chemical Physics. - New York : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 149:16
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Tidskriftsartikel (refereegranskat)abstract
- PTCDI molecules were evaporated onto a Ag/Si(111)√3×√3 surface and studied using scanning tun-resolution STM images are used to identify the delicate molecule/molecule and molecule/substrate interactions and the shapes of the molecular orbitals. The results show that the substrate/molecule interaction strongly modifies the electronic configuration of the molecules as their orbital shapes are quite different at 1 and 2 monolayer (ML) coverage. Simple models of molecular HOMO/LUMO levels and intermolecular hydrogen-bondings have been made for 1 and 2 ML PTCDI coverages to explain the STM images. Changes due to the interaction with the substrate are also found in ARUPS as extra states above the regular HOMO level at 1 ML PTCDI coverage. The ARUPS data also show that the electronic structure of the substrate remains unchanged after the deposition of molecules as the dispersion of the substrate related bands is unchanged. The changes in electronic structure ofthe molecules are discussed based on aHOMO/LUMO split.
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| 3. |
- Emanuelsson, Christian
(författare)
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Electronic Structure and Film Morphology Studies of PTCDI on Metal/Semiconductor Surfaces
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Organic semiconductors have received increasing attention over the last decades as potential alternatives for inorganic semiconductors. The properties of these films are highly dependent on their structural order. Of special interest is the interface between the film and its substrate, since the structure of the interface and the first few layers decide the growth of the rest of the film. The interface structure is determined by the substrate/molecule interactions, the intermolecular interactions and the growth conditions.In this thesis, thin films of the organic semiconductor PTCDI have been studied using complementary microscopy and spectroscopy techniques on two metal-induced surface reconstructions, Ag/Si(111)-√3×√3 and Sn/Si(111)-2√3×2√3. These surfaces were chosen because they have different reactivities and surface periodicities. On the weakly interacting Ag-terminated surface, the film growth is mainly governed by the intermolecular interactions. This leads to well-ordered films that grow layer-by-layer. The interaction with the substrate is through electron charge transfer to the molecules from the substrate. This results in two different types of molecules with different electronic structure, which are identified using both STM images and PES spectra. On the more strongly interacting Sn-terminated surface the molecules adsorb in specific adsorption geometries and form 1D rows. At around 0.5 ML coverage the rows also interact with each other and form a 4√3×2√3 reconstruction and beyond one ML coverage the growth is characterized as island growth. The interaction with the substrate is mainly due to heavy electron charge transfer from the Sn atoms in the substrate to the C atoms in the imide group, but also the N atoms and the perylene core in PTCDI are involved. In these systems, the interactions with the surfaces result in new states inside the HOMO-LUMO gap, and the intermolecular interactions are dominated by O···H and O···H-N hydrogen bondings.
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| 4. |
- Emanuelsson, Christian, et al.
(författare)
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Photoelectron spectroscopy studies of PTCDI on Ag/Si(111)-√3 x √3
- 2018
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 149:4
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Tidskriftsartikel (refereegranskat)abstract
- 3,4,9,10-perylene tetracarboxylic diimide molecules were evaporated onto a Ag/Si(111)-√3 x √3 surface and studied using photoelectron spectroscopy and near edge X-ray absorption fine structure (NEXAFS). All core levels related to the imide group of the molecules showed a partial shift to lower binding energies at low coverages. In NEXAFS spectra, the first transitions to the unoccupied states were weaker at low coverages compared to thicker films. Also, extra states in the valence band between the regular highest occupied molecular orbital and the Fermi level were found at low coverages. These changes were explained by two types of molecules. Due to charge transfer from the surface, these two types have different interactions between the imide group and the substrate. As a result, one type has a partially filled lowest unoccupied molecular orbital while the other type does not.
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| 5. |
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| 6. |
- Emanuelsson, Christian, et al.
(författare)
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Scanning tunneling microscopy study of PTCDI on Sn/Si(111)-2√3×2√3
- 2019
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Ingår i: Journal of Chemical Physics. - New York : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 150:4
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Tidskriftsartikel (refereegranskat)abstract
- Perylene tetracarboxylic diimide molecules were evaporated onto a Sn/Si(111)-2 root 3 x 2 root 3 surface and studied using scanning tunneling microscopy (STM) and low energy electron diffraction. At low coverages, single molecules are locked into specific adsorption geometries, which are investigated in detail using high resolution STM. The electronic structure of these individual molecules was studied using bias dependent STM images. The molecules form 1D rows that become more common with increasing coverages. Possible intermolecular O center dot center dot center dot H interactions within the rows have been identified. At around half of a monolayer (ML), the rows of molecules interact with each other and form a commensurate 4 root 3 x 2 root 3 reconstruction. In a complete monolayer, several structures emerge as molecules fill in the space between the 4 root 3 x 2 root 3 stripes. Possible intermolecular interactions within the 1 ML structures have been discussed. At coverages above 1 ML, the growth is characterized by island growth, where the molecules are arranged according to the canted structure within the layers.
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| 7. |
- Emanuelsson, Christian, et al.
(författare)
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Scanning tunneling microscopy study of thin PTCDI films on Ag/Si(111)-root 3 x root 3
- 2017
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Ingår i: Journal of Chemical Physics. - : AMER INST PHYSICS. - 0021-9606 .- 1089-7690. ; 146:11
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Tidskriftsartikel (refereegranskat)abstract
- 3,4,9,10-perylene tetracarboxylic diimide molecules were evaporated onto a Ag/Si(111)-root 3 x root 3 surface and studied by scanning tunneling microscopy/spectroscopy and low energy electron diffraction (LEED). The growth mode was characterized as layer-by-layer growth with a single molecular unit cell in a short range order. The growth of the first two monolayers involves a molecule/substrate superstructure and a molecule/molecule superstructure. At higher coverages, the molecules in each layer were found to align so that unit cells are on top of each other. The experimentally obtained LEED pattern is described as a combination of patterns from the molecular unit cell and the molecule/substrate superstructure. The electronic structure was found to be strongly dependent on the film thickness for the first few layers: Several extra states are found at low coverages compared to higher coverages resulting in a very small pseudo gap of 0.9 eV for the first layer, which widens up to 4.0 eV for thicker films.
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| 8. |
- Ericsson, Leif K E, 1964-, et al.
(författare)
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AFM and STM Study of ZnO Nanoplates
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The surface morphology and electronic structure of hexagonal ZnO nanoplates have been studied by Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM). It was found that these nanoplates are terminated by their polar (0001) surfaces. The AFM investigation was performed in the ambient conditions with the nanocrystals “as grown”. Surprisingly, the AFM images of the top surfaces revealed an interesting triangular reconstruction, which was earlier observed only after cycles of sputtering and annealing of the ZnO(0001) surface in Ultra High Vacuum (UHV) systems. The surface atomic and electronic structures of these nanoplates have been further studied by STM and Scanning Tunneling Spectroscopy (STS) in UHV. The STM images also showed a triangular structure with single atomic steps. In addition, a 2x2 surface reconstruction has been observed with high resolution STM. This reconstruction agrees well with the recently proposed model that involves the removal of 1/4 of the topmost Zn atoms on the ZnO(0001) surface.
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| 9. |
- Ericsson, Leif, 1964-, et al.
(författare)
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Photoemission study of ZnO nanocrystals : Thermal annealing in UHV and induced band bending
- 2013
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Ingår i: Surface Science. - Amsterdam : Elsevier. - 0039-6028 .- 1879-2758. ; 612, s. 10-15
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Tidskriftsartikel (refereegranskat)abstract
- ZnO nanocrystals distributed by spin-coating on SiO2/Si surfaces were annealed in UHV and studied in situ by synchrotron radiation based X-ray Photoelectron Spectroscopy. Changes in chemical composition and electronic structure of ZnO nanocrystal surfaces were found with increasing annealing temperatures. Annealing at 650 °C reduces the surface contaminant levels without any observed de-composition of ZnO. After annealing at 700 °C an initial de-composition of ZnO together with further reduction of contaminants was observed. As a result, 650 °C is found to be the optimal annealing temperature for thermal cleaning of ZnO nanocrystals. Chemical changes and induced point defect formation cause changes in the band structure of the ZnO/SiO2/Si system. An upward band bending of 0.7 eV on the surfaces of the ZnO nanocrystals was found after annealing at 300 °C. The bands on the surfaces of ZnO nanocrystals gradually bend downwards with increasing annealing temperatures. A downward band bending of 1.4 eV is the result after annealing at 750 °C for 1 h. This large downward band bending is explained as due to the change in balance of oxygen vacancies and zinc vacancies on the surfaces of ZnO nanocrystals.
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| 10. |
- Gurnett, Michael, et al.
(författare)
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Photoemission study of the Li/Ge(111)-3×1 reconstruction
- 2009
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Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 603:4, s. 727-735
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Tidskriftsartikel (refereegranskat)abstract
- In this article we report our findings on the electronic structure of the Li induced Ge(1 1 1)–3 × 1reconstruction as determined by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and core-level spectroscopy using synchrotron radiation. The results are compared to the theoretical honeycomb-chain-channel (HCC) model for the 3 × 1reconstruction as calculated using density functional theory (DFT). ARUPS measurements were performed in both the and directions of the 1 × 1 surface Brillouin zone at photon energies of 17 and 21.2 eV. Three surface related states were observed in the direction. In the direction, at least two surface states were seen. The calculated band structure using the single-domain HCC model for Li/Ge(1 1 1)–3 × 1 was in good agreement with experiment, allowing for the determination of the origin of the experimentally observed surface states. In the Ge core-level spectra, two surface related components were identified, both at lower binding energy with respect to the Ge bulk peak. Our DFT calculations of the surface core-level shifts were found to be in fair agreement with the experimental results. Finally, in contrast to the Li/Si(1 1 1)–3 × 1 case, no double bond between Ge atoms in the top layer was found
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