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Träfflista för sökning "WFRF:(Zhang Hanmin 1966 ) "

Sökning: WFRF:(Zhang Hanmin 1966 )

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1.
  • Emanuelsson, Christian (författare)
  • Electronic Structure and Film Morphology Studies of PTCDI on Metal/Semiconductor Surfaces
  • 2018
  • 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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2.
  • Emanuelsson, Christian, et al. (författare)
  • Scanning tunneling microscopy study of thin PTCDI films on Ag/Si(111)-root 3 x root 3
  • 2017
  • Ingår i: Journal of Chemical Physics. - : AMER INST PHYSICS. - 0021-9606 .- 1089-7690. ; 146:11
  • 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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4.
  • Dou, Yangpeng, et al. (författare)
  • Effects of reducibility of graphene oxide nanosheets on preparation of AgNPs/GO nanocomposites and their electrocatalytic performance
  • 2015
  • Ingår i: Journal of nanoparticle research. - : Springer Science and Business Media LLC. - 1388-0764 .- 1572-896X. ; 17:12, s. 1-10
  • 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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5.
  • Emanuelsson, Christian, et al. (författare)
  • Delicate Interactions of PTCDI molecules on Ag/Si(111)-√3×√3
  • 2018
  • Ingår i: Journal of Chemical Physics. - New York : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 149:16
  • 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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6.
  • Emanuelsson, Christian, et al. (författare)
  • Photoelectron spectroscopy studies of PTCDI on Ag/Si(111)-√3 x √3
  • 2018
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 149:4
  • 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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7.
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8.
  • Emanuelsson, Christian, et al. (författare)
  • Scanning tunneling microscopy study of PTCDI on Sn/Si(111)-2√3×2√3
  • 2019
  • Ingår i: Journal of Chemical Physics. - New York : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 150:4
  • 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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9.
  • Ericsson, Leif K E, 1964-, et al. (författare)
  • AFM and STM Study of ZnO Nanoplates
  • 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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10.
  • Ericsson, Leif, 1964-, et al. (författare)
  • Photoemission study of ZnO nanocrystals : Thermal annealing in UHV and induced band bending
  • 2013
  • Ingår i: Surface Science. - Amsterdam : Elsevier. - 0039-6028 .- 1879-2758. ; 612, s. 10-15
  • 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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11.
  • Gurnett, Michael, et al. (författare)
  • Photoemission study of the Li/Ge(111)-3×1 reconstruction
  • 2009
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 603:4, s. 727-735
  • 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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12.
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13.
  • Hirvonen Grytzelius, Joakim, 1975-, et al. (författare)
  • Coverage dependence and surface atomic structure of Mn/Si(111)-√3×√3 studied by scanning tunneling microscopy and spectroscopy
  • 2009
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - Washington : American Institute of Physics (AIP). - 1098-0121 .- 1550-235X. ; 80, s. 235324-1-235324-6
  • Tidskriftsartikel (refereegranskat)abstract
    • Thin manganese silicide films of different thicknesses on Si(111) have been studied in detail by low-energy electron diffraction (LEED), scanning tunneling microscopy, and scanning tunneling spectroscopy (STM/STS). Up to a Mn coverage of 3–4 monolayers (ML), island formation is favored. For higher Mn coverages up to 12 ML uniform film growth is found. The silicide film morphology at low coverages supports a layered Mn-Si film structure. The silicide surfaces displayed a √3×√3 LEED pattern. STM images recorded from the √3×√3 surfaces mostly show a hexagonal pattern but a honeycomb pattern has also been observed. A surface atomic structure based on chained Mn triangles is proposed. Our STM results are in good agreement with a recent theoretical model. The high-quality STS spectra recorded from the different surfaces show a clear metallic character at 1.5 ML and higher coverages. The filled-state features in the STS spectra at surfaces with 3–4 ML Mn coverages are similar to earlier published angle-resolved photoelectron spectroscopy data.
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15.
  • Hirvonen Grytzelius, Joakim, 1975-, et al. (författare)
  • Mn5Ge3 film formation on Ge(111)c(2×8)
  • 2012
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - New York : American Physical Society. - 1098-0121 .- 1550-235X. ; 86:12, s. 125313-1-125313-7
  • Tidskriftsartikel (refereegranskat)abstract
    • Thin manganese germanide films with different thicknesses on Ge(111) have been studied in detail by low-energy electron diffraction (LEED), scanning tunneling microscopy, and core-level spectroscopy (CLS). Annealing of the deposited Mn on Ge(111)c(2×8) between 330-450 C resulted in well-ordered Mn5Ge3 surfaces as seen by intense 3×3 LEED spots. Up to a coverage of 24 monolayers (ML), island formation is favored. At a coverage of 32 ML a well ordered Mn5Ge3 film was found to fully cover the surface. High-resolution Ge 3d CLS spectra were recorded with photon energies between 50 and 110 eV at normal and 60 emission angles. In contrast to earlier results, three components have been used in the line-shape analysis to achieve a consistent fit over the energy and angular range. In addition, three components have been identified for the Mn 2p CLS spectra. The two major components fit well with a layered Mn germanide structure suggested in the literature.
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16.
  • Hirvonen Grytzelius, Joakim, 1975-, et al. (författare)
  • Surface atomic and electronic structure of Mn5Ge3 on Ge(111)
  • 2011
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - New York : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:19, s. 195306-1-195306-6
  • Tidskriftsartikel (refereegranskat)abstract
    • The atomic and electronic structure of the Mn5Ge3(001) surface grown on Ge(111) c(2×8) has been studied in detail by angle-resolved photoelectron spectroscopy (ARPES), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy. ARPES spectra recorded from the Γ̅ -K̅ -M̅ and Γ̅ -M̅ -Γ̅ directions of the surface Brillouin zone show six surface-related features. The STM images recorded at biases higher/lower than ±0.4 V always show a honeycomb pattern with two bright protrusions in each unit cell. At lower biases, a hexagonal, intermediate transition, and a honeycomb pattern are observed. These can be explained as arising from Mn and Ge atoms in the sublayer arranged in triangular structures and Mn atoms in the top layer arranged in a honeycomb structure, respectively. The photoemission and STM data from the germanide surface are discussed and compared to earlier published theoretical, photoelectron spectroscopy, and scanning tunneling microscopy studies.
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17.
  • Hirvonen Grytzelius, Joakim, 1975-, et al. (författare)
  • Surface electronic structure of Mn/Si(111)-√3×√3
  • 2008
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - New York : American Institute of Physics. - 1098-0121 .- 1550-235X. ; 78:15, s. 155406-1-155406-6
  • Tidskriftsartikel (refereegranskat)abstract
    • The Mn/Si(111)-√3×√3 surface has been studied in detail by low energy electron diffraction (LEED), angle-resolved photoelectron spectroscopy (ARPES), and core-level photoelectron spectroscopy (CLS). Annealing of the deposited manganese resulted in a well-ordered surface as seen by intense √3×√3 LEED spots. ARPES spectra recorded in the Γ̅ -K̅ -M̅ direction of the √3×√3 surface Brillouin zone show five surface related features in the band gap while in the Γ̅ -M̅ -Γ̅ direction four surface features are observed. The high-resolution Si 2p CLS data were recorded at photon energies between 108–140 eV both at normal and 60° emission angle. The bulk component was identified from the bulk sensitive spectrum recorded at a photon energy of 108 eV. To achieve a consistent core-level fitting over the whole energy and angular range, five components were introduced in the line-shape analysis. The photoemission data from the √3×√3 surface have been discussed and compared with a recent theoretical model. The findings here support a layered Mn silicide film structure.
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18.
  • Sakamoto, Kazuyuki, et al. (författare)
  • Band structure of the Ca/Si(111)-(2×1) surface
  • 2003
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 68:24, s. 245316-
  • Tidskriftsartikel (refereegranskat)abstract
    • We have investigated the electronic structure of the Ca/Si(111)-(2×1) surface using angle-resolved photoelectron spectroscopy. Two semiconducting surface states were clearly observed in the bulk band gap, and one was found in a pocket of the bulk band projection. Qualitatively, the dispersions of the two surface states observed in the band gap agree well with theoretical dispersions for a clean Si(111)-(2×1) surface with the Seiwatz structure. Taking this result into account, we conclude that the two surface states in the band gap originate from orbitals of Si atoms that form a Seiwatz structure, and that two electrons are donated from Ca to Si per (2×1) unit cell. This conclusion supports the structure of the Ca/Si(111)-(2×1) surface proposed in the literature.
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19.
  • Sakamoto, Kazuyuki, et al. (författare)
  • Electronic structure of the Ca/Si(111)-(3×2) surface
  • 2004
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 69:12, s. 1253211-1253217
  • Tidskriftsartikel (refereegranskat)abstract
    • The electronic structure of the Ca/Si(111)-(3×2) surface has been investigated by angle-resolved photoelectron spectroscopy. Five surface states, none of which crosses the Fermi level, were observed in the bulk band gap, and one surface state was observed in a bulk band pocket. The dispersion features of three of the surface states in the band gap agree well with results from monovalent atom adsorbed Si(111)-(3×1) surfaces along the chain direction. The close resemblance indicates that the origins of the surface states are the same as or quite similar to those of the (3×1) surface. The two other states observed in the band gap have not been reported in the literature, and they are interpreted as surface states that occur on Ca/Si(111)-(3×2) due to the lower coverage (1/6 monolayer of Ca). Further, based on the finite surface state dispersion in the direction perpendicular to the Ca chains, we conclude that the electronic character of this surface is not completely one dimensional.
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20.
  • Sakamoto, Kazuyuki, et al. (författare)
  • Photoemission study of metastable oxygen adsorbed on a Si(111)-(7×7) surface
  • 2004
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 70:3, s. 035301-
  • Tidskriftsartikel (refereegranskat)abstract
    • We have investigated the metastable oxygen species adsorbed on a Si(111)-(7×7) surface at room temperature using real time valence-band and core-level photoemission measurements. The dosage- and time-dependent changes in the intensity of the metastable 3.8 eV peak observed in the valence-band spectra were different from those of the metastable O 1s components. Further, although no metastable O 1s components were observed in the core-level measurement after annealing the sample at 600 K, the 3.8 eV peak was still observed. These results indicate that both the stable and the metastable oxygen species produce the 3.8 eV valence-band peak, and we therefore conclude that the contradicting lifetimes reported in the literature result from a misinterpretation of the 3.8 eV peak.
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21.
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23.
  • Starfelt, Samuel, et al. (författare)
  • Initial quantum well states in Ag thin films on the In/Si(111)- √3 × √3 surface
  • 2020
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 692, s. 1-7
  • Tidskriftsartikel (refereegranskat)abstract
    • Silver thin films have been formed by room temperature deposition of Ag on In/Si(111)-√3 × √3. The Ag films have been investigated using both angle-resolved photoelectron spectroscopy (ARPES) and scanning tunnelingmicroscopy and spectroscopy (STM/STS). This creates a powerful link between the electronic structures and the film morphology. The valence band spectra show a clear evidence of quantum well state (QWS) formation already for a 2 monolayer (ML) film. This QWS moves towards the Fermi level for the 3 ML film, which also reveals a second QWS. The QWSs’ dispersions have been plotted along the ΓM and ΓK symmetry lines of the 1×1 surface Brillouin zone (SBZ), where the ΓM direction shows the umklapp-mediated QWSs. The valence band spectra for the 3 ML Ag film also show a strong Ag sp band close to the edge of the Ag(111) 1×1 SBZ. In the STS spectrum from 2 ML, two peaks are visible below the Shockley surface state. These peaks are compared with the ARPES data and attributed to different features of the QWS
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24.
  • Starfelt, Samuel, et al. (författare)
  • Layer-by-layer control of Ag film growth on Sn/Si(111)-(√3 ×√3)-R30°
  • 2020
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 701, s. 1-9
  • Tidskriftsartikel (refereegranskat)abstract
    • Silver thin films have been formed on the Sn/Si(111)-(√3 ×√3)-R30 degrees surface at room temperature. The film morphologies, growth and electronic structures have been studied by scanning tunneling microscopy/spectroscopy (STM/STS), low-energy electron diffraction (LEED) and angle-resolved photoelectron spectroscopy (ARPES). The first layer of Ag forms an interface which consists of atomic rows with three-fold symmetry oriented along the Si(111)-1 x 1 directions. On top of the interface, Ag grows as an uniform thin film, following a layer-by-layer mode. The electronic structures of the films have been studied by STS for coverages between 1-5 monolayers (MLs). The STS spectra show peaks in the occupied electronic states which move towards the Fermi level with increased film thicknesses. These peaks have been attributed to quantum well states. ARPES measurements have been performed for 1 and 2 ML Ag coverage on Sn/Si(111)-(√3 ×√3)-R30 degrees, where the resulting thicknesses were confirmed by STM. The spectra reveal that quantum well states appear first for the 2 ML film.
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25.
  • Starfelt, Samuel, et al. (författare)
  • Quantum-well states in thin Ag films grown on the Ga/Si(111)-√3 x √3 surface
  • 2018
  • Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:19, s. 1-9
  • Tidskriftsartikel (refereegranskat)abstract
    • Silver thin films have been created by room temperature deposition on a Ga/Si(111)-√3 x √3 surface and their valence band structures and core levels have been measured by angle-resolved photoelectron spectroscopy (ARPES). Discrete quantum-well states (QWSs) quantized from the Ag sp valence band are observed already at 3 monolayers (ML). The characteristics of the QWSs have been examined in the phase accumulation model for thicknesses between 3 and 12 ML. The phase shift and QWSs binding energies dependence with Ag film thicknesses have all been consistently derived. In-plane energy dispersion follows a parabolic curve, and the effective mass of the QWSs shows an increasing trend with binding energies as well as with reduced film thicknesses. Furthermore, the ARPES measurements reveal umldapp mediated QWSs around the (M)over-bar points of the Si(111) 1 x 1 surface Brillouin zone. The study confirms that the Ga/Si(111)-√3 x √3 surface is a good substrate for growing uniform ultrathin Ag films in room temperature conditions.
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26.
  • Starfelt, Samuel (författare)
  • Scanning tunneling microscopy and photoemission studies of Ag films on metal/semiconductor surfaces
  • 2020
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The research presented in this thesis has been focused on the study of thin Ag films, grown on metal-reconstructed Si(111) and Ge(111) surfaces.The films have been grown at room temperature, and the morphologiesand electronic structures of the films have been investigated using scanning tunneling microscopy and spectroscopy (STM/STS), low-energy electron diffraction (LEED) and angle-resolved photoelectron spectroscopy(ARPES).On the Ga-, In-, and Sn/Si(111)-√3x√3 surfaces, the first layer of Ag forms a special interface which consists of short atomic rows, with slightly different appearances depending on the base surface. Starting from two monolayers (MLs), Ag grows as a thin film with bulk-like lattice parameters.The electronic structures of the films reveal the behavior of the intrinsic quantum well states (QWSs). STS data show peaks in the filled states which move towards the Fermi level with increased thicknesses. These peaks have been compared with ARPES spectra and linked to the QWSs. The evolution of the QWSs with film thicknesses has been examined within, and extending upon, the established theoretical framework. The results point towards metal-reconstructed Si(111) surfaces using group III and IV elements as strong candidates for uniform film growth, and open up new avenues for studying electronic coupling effects between film/substrate.On Sn/Si(111)-√3x√3, the Ag films grow in domains with two different lattice orientations, rotated 30° from each other. This is due to the interface consisting of two different structures, as revealed by the STM. One of the interfacial phases is a 3×3 honeycomb structure, and the other a line structure of short atomic rows with a three-fold symmetric 2√3×√3-R30° unit cell. Atomic models for the two interface phases have been proposed, based on two different spin configurations of the Sn/Si(111)-√3x√3 surface. The presence of two interfaces makes this system highly attractive for the study of interface related phenomena, and the difference in Ag filmson Sn/Ge(111) compared with Sn/Si(111) highlights the importance of electronic effects for the film growth.
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27.
  • Starfelt, Samuel, et al. (författare)
  • Spin coupling in Ag film growth on Sn/Ge(111)-√3×√3
  • 2022
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 719, s. 122043-122043
  • Tidskriftsartikel (refereegranskat)abstract
    • Silver has been deposited on the Sn/Ge(111)-(√3×√3)-R30° surface at room temperature. The Ag growth and resulting surface morphology have been investigated using scanning tunneling microscopy. The first layer of silver forms an interface with domains of two different phases. One structure consists of short atomic rows with three-fold symmetry, oriented in the directions of the √3×√3 surface. These rows are separated by a distance equal to  and are found to fit a 2√3×√3 unit cell. The other phase is a 3×3 honeycomb structure, oriented in the Ge(111) 1 × 1 directions. Atomic structural models for the two interface phases are proposed, based on two different spin arrangements of the Sn/Ge(111)-√3×√3 surface. The results highlight the topological coupling of the two interface faces. Both interface structures are preserved with additional silver deposition. The second layer of Ag grows with a bulk-like lattice thickness on top of both interfaces. Low-energy electron diffraction on a mostly two layer Ag film reveals that it consists of domains where Ag grows in different orientations. These domains are rotated 30° with respect to each other, and thus mirror the symmetry directions of the two interfacial phases.
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28.
  • Starfelt, Samuel, et al. (författare)
  • STM/STS and photoemission study of Ag thin films on Ga/Si(111)(√3 × √3)R30°
  • 2019
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 682, s. 25-32
  • Tidskriftsartikel (refereegranskat)abstract
    • Silver thin films have been formed by room temperature deposition of Ag on a Ga/Si(111) (√3 × √3)R30° surface. Scanning tunneling microscopy and spectroscopy (STM/STS) have been used to study both the clean Ga/Si(111) (√3 × √3)R30° surface and Ag films with different coverages. For the film formation, Ag first grows into 2D islands on Ga/Si(111) (√3 × √3)R30°. The first layer of the islands forms atomic lines with three-fold symmetry in the 1 × 1 direction of the underlying Si(111) substrate. From a 2 ML coverage, the growth becomes layer-by-layer mode. STS measurements have been performed on Ag films with different coverages inorder to investigate their electronic structures. Between 2–4 ML, the STS spectra reveal 2 and 3 peaks below the Fermi level. These peaks move towards the Fermi level with increased film thicknesses and thus share the same behavior as those of quantum well states (QWSs). The energy positions of the peaks have been compared with valence band photoelectron spectra in order to assign them to various parts of the QWSs. In addition, the photoemission results also confirm the existence of QWSs for a 2 ML Ag film on Ga/Si(111) (√3 × √3)R30°.
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32.
  • Zhang, Hanmin, 1966-, et al. (författare)
  • Band splitting of quantum wells of thin Ag films on Sn/Si(111)√3×√3
  • 2017
  • Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:4, s. 041402-1-041402-6
  • Tidskriftsartikel (refereegranskat)abstract
    • High-resolution valence band spectra of ultrathin Ag films on Sn/Si(111)√3×√3 show intrinsic splitting of the quantum-well states (QWSs). Especially at low coverages, the QWSs of such a system display delicate coupling characters with the bulk bands from the substrate. The observed QWS splitting agrees well with the result of the theoretical calculation. We found that the splitting originates from an interface with a finite thickness. In addition, the interface also causes a large sp band splitting due to the Umklapp scattering in the Γ−M direction of the Ag(111) surface Brillouin zone.
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33.
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35.
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36.
  • Zhang, Hanmin, 1966-, et al. (författare)
  • Electronic structure of PTCDA on Sn/Si(111)-2√3×2√3
  • 2014
  • Ingår i: Chemical Physics. - : Elsevier. - 0301-0104 .- 1873-4421. ; 439, s. 71-78
  • Tidskriftsartikel (refereegranskat)abstract
    • The electronic structures of PTCDA on the Sn/Si(111)-2√3×2√3 surface have been thoroughly studied by high-resolution photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS). Upon deposition of PTCDA, there is an unusual charge transfer from the Sn/Si(111)-2√3×2√3 surface to the molecules. This is clearly shown by a new component in the Sn 4d core-level spectra that shifts towards higher binding energy. In contrast to the literature, the charge provided by Sn is donated to the carbonyl C instead of the O atoms. This is revealed by a new component in the C 1s core-level spectra that shifts towards lower binding energy. The charge transfer causes a splitting of the HOMO level in the valence band spectra. As indicated in the NEXAFS spectra, it also induces a splitting of the LUMO level of the molecules. For thick films the NEXAFS results suggest a layer by layer growth mode.
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37.
  • Zhang, Hanmin, 1966-, et al. (författare)
  • Electronic structure of PTCDA on Sn/Si(111)-√3×√3
  • 2011
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - USA : American Chemical Society (ACS). - 1098-0121 .- 1550-235X. ; 84:20, s. 205420-1-205420-8
  • Tidskriftsartikel (refereegranskat)abstract
    • The electronic structures of PTCDA films on Sn/Si(111)-√3×√3 have been studied by high-resolution photoelectron spectroscopy (PES) and near-edge x-ray absorption fine structure (NEXAFS). There is a clear chemical interaction between PTCDA molecules and the Sn/Si(111)-√3×√3 surface, as indicated by new components in Sn 4d core levels that are shifted to higher binding energies. The chemical interaction is also evidenced by new components in the O-1s and C-1s core-level spectra. The new components are shifted to lower binding energies and have their origins from two reactions between Sn dangling bonds and the PTCDA molecules. One reaction is located at the anhydride O atom and the other is at the carbonyl C atom. These reactions also induce modifications in the HOMO and LUMO levels seen in the valence band and NEXAFS spectra. For thin films, the NEXAFS results suggest a tilting molecular configuration with respect to the substrate, while for the thicker films, there is an improved ordering of the molecular orientation to the substrate.
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38.
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39.
  • Zhang, Hanmin M., 1966- (författare)
  • Electronic and atomic structure of metal induced reconstructionson Si(lll) and Ge(lll) surfaces studied by PES and STM
  • 2003
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Deposition of metal atoms on semiconductors leads in many cases to reconstructed surfaces. The lack of symmetry in the direction normal to the surface allows for a large variety in the atomic arrangements of the surface reconstructions. In the case of silicon, more than 300 kinds of adsorbate-induced surface reconstructions have been found. The atomic and electronic structures of these surfaces have attracted a lot of attention in the field of surface science. Detailed investigations of some surfaces of current interest are presented in this thesis.The interest in the Sn/Ge(111) system increased very much a few years ago when it was realized that the room temperature ✓3x✓3 phase changes to a 3x3 phase at low temperature. This transition was described as a charge density wave transition in an early report. However, photoemission data gave a different picture since the room temperature phase shows clear similarities with the low temperature phase regarding both valence band and core-level spectra. By employing low energy electron diffraction (LEED), core-level, and valence bandspectroscopy the correct Sn 4d line shape for the Sn/Ge(111) surface was established. At 70 K three 4d components were identified on the 3x3 phase. The two major components are attributed to the two types of Sn atoms in the 3x3 unit cell. The results presented in this study put an end to the discussion about the core-level line shape.An apparent 3x3 LEED pattern was observed for the Sn1-xSix/Si(111)✓3x✓3 surface. The origin of this additional diffraction has been investigated in detail by scanning tunneling microscopy (STM). The 3x3 diffraction, which appears after annealing, is associated with the arrangement of Sn and Si substitutional atoms in the surface layer, forming many local structures such as honeycombs, hexagons, and atomic lines. As revealed by Fourier-transformsof the STM-images, these local structures are the origins of the 3x3 diffraction and a weak 2✓3x✓3 streaky background superposed on the ✓3x✓3 LEED pattern.Metal to semiconductor transitions have recently attracted much attention by the surface physics community. Except for the well-known temperature-dependent transitions, there exist other types such as coverage-dependent transitions. The surface electronic structures of the ✓3x✓3, ✓39x✓39, and 6x6 phases of Ag/Ge(111) have been studied by angle-resolved photoelectron spectroscopy and LEED. An interesting transition from the metallic ✓39x✓39 phase to a semiconducting 6x6 phase was observed. On the 6x6 surface, the upper metallic band is missing in the photoemission spectra. This change results in a gap of around 0.2 eV with respect to the Fermi level.Despite the extensive research on Ag/Si(111) surfaces (like ✓3x✓3 and ✓21x✓21-Au), the surface electronic structures of the Ag-induced ✓21x✓21 and 6x6 phases were reported for the first time in this study. The basic character of these reconstructions formed by Ag on theSi(111) or Ge(111) surfaces is very similar although a different periodicity is observed for the intermediate phases. In contrast to the two metallic bands discussed in earlier reports on✓21x✓21-Au, only one surface metallic band was found on the ✓21x✓21-Ag surface. The 6x6surface has four surface state bands without any band crossing the Fermi level. The result is a semiconducting character of the 6x6 phase. Thus, by depositing small amounts of Ag atoms onthe ✓3x✓3 surface, the surface electronic structure shows an interesting change from a semiconducting to a metallic and back to a semiconducting state.A crystal to glass-like transition was investigated in the Au/Si(111) system by means of LEED, core-level and valence photoelectron spectroscopy. The α-✓3x✓3 phase shows Si 2p spectra that are different from the quenched β-✓3x✓3 and 6x6 phases. A similarity between the 6x6 and quenched β-✓3x✓3 phases is evident from the decomposition of the Si 2p spectra. Eight surface state bands are found on the 6x6 surface in contrast to two surface state bands reported in the literature. The reconstructions of the α-✓3x✓3 phase, the 6x6 and the quenched β-✓3x✓3 phases are discussed in terms of extra Au adatoms on the ✓3x✓3 surface described by the honeycomb-chain-trimer (HCT) model. Seven surface bands are broadened into three bands, when the 6x6 phase is transformed into the quenched β-✓3x✓3 just by annealing followed by quick cooling. This behavior indicates a crystal to glass-like transition between the 6x6 surface and the quenched β-✓3x✓3 surface.The ✓3x✓3, ✓39x✓39 and 6x6 phases of Ag/Ge(111) have been studied by STM. Four types of configurations that contain 6 additional Ag adatoms per unit cell are found on the 6x6 surface, which show mirrored and complementary relations. On the ✓39x✓39 surface, the protrusions from extra Ag atoms are extended into neighboring Ag trimers. The surface structure models of the ✓39x✓39 and 6x6 phases have been proposed based on the HCT model. An interesting transition between the different types of 6x6 configurations was observed. By applying a combination of negative and positive tip biases, one can transform the surface reconstruction between the ✓39x✓39 and the 6x6 phases. Two types of domains were found on the ✓39x✓39 surface, and transformations between these two phases are also possible. Thus byusing different tip biases, it is possible to manipulate the different types of surface reconstructions.
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43.
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44.
  • Zhang, Hanmin, 1966-, et al. (författare)
  • PTCDA induced reconstruction on Sn/Si(111)-2√3×2√3
  • 2012
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - New York : American Physical Society. - 1098-0121 .- 1550-235X. ; 85:24, s. 245317-1-245317-5
  • Tidskriftsartikel (refereegranskat)abstract
    • The electronic structures of the Sn/Si(111)-2√3×2√3 surface and deposited 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) have been studied by use of high-resolution photoelectron spectroscopy and scanning tunneling microscopy. On deposition, PTCDA molecules form a 4√3×2√3 periodicity superposed on the substrate. The new reconstruction is caused by a charge transfer between the Sn/Si(111)-2√3×2√3 surface and the molecules, as indicated by a new component of the Sn 4d core level that is shifted toward higher binding energy. In contrast to earlier reports, the charge provided by Sn is given to carbonyl C atoms instead of O atoms. This is evidenced by a new component in the C 1s core-level spectra, which is shifted toward lower binding energy. The charge transfer also induces a splitting in the highest occupied molecular orbital level of PTCDA seen in the valence band structure.
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45.
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47.
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48.
  • Zhang, Hanmin, 1966-, et al. (författare)
  • STM study of PTCDA on Sn/Si(111)-2 root 3 x 2 root 3
  • 2016
  • Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 144:12
  • Tidskriftsartikel (refereegranskat)abstract
    • The electronic structures of perylene tetracarboxylic dianhydride on Sn/Si(111)-2 root 3 x 2 root 3 have been studied by scanning tunneling microscopy and spectroscopy. Individual molecules have been investigated at 0.15 ML, while at 0.3 ML molecules formed short rods. At 0.6 ML, the molecular rods interacted with each other, coupling with the substrate and forming a new 4 root 3 x 2 root 3 super structure. At 0.9 ML, the surface was further reconstructed and consisted of strips with two and three rods of molecules. We found that these surface structures are strongly modified by the molecule/substrate and the intermolecular interactions. As a result, the HOMO-LUMO gaps of these molecules change with respect to the phases and the thickness. For a single molecular layer of the 4 root 3 x 2 root 3 phase, the HOMO-LUMO levels were split with a gap of approximately 2.1 eV, which is caused by charge transfer from the substrate to the molecules. (C) 2016 AIP Publishing LLC.
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49.
  • Zhang, Hanmin, 1966-, et al. (författare)
  • STM study of the electronic structure of PTCDA on Ag/Si(111)-√3×√3
  • 2010
  • Ingår i: Chemical Physics Letters. - Amsterdam : Elsevier. - 0009-2614 .- 1873-4448. ; 485:1-3, s. 69-76
  • Tidskriftsartikel (refereegranskat)abstract
    • PTCDA molecules were evaporated onto the Ag/Si(111)-√3×√3 surface. Simple HOMO and LUMO models have been made to explain the STM images of the square and herringbone phases. The charge transfer from the substrate and the H-bonding between the molecules play crucial roles for the formation of these phases. We found that their electronic structures are strongly modified by the moleculesubstrate and intermolecular interactions. As a result, the HOMO-LUMO gaps of the PTCDA films are different for different phases and thicknesses. The increase in the HOMO-LUMO gaps with film thicknesses is discussed based on the surface and the interface polarization effect
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50.
  • Zhang, Hanmin, 1966-, et al. (författare)
  • Surface atomic structure of Ag/Si(111)√3 × √3
  • 2006
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 74:20
  • Tidskriftsartikel (refereegranskat)
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