Browsing by Author "Tsuei, KD"

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Experimental electronic structure of Be2C
(1998-09-15)
Tzeng, CT
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Tsuei, KD
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Lo, WS
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光電工程研究所
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Institute of EO Enginerring
The insulating Be2C thin films have been successfully prepared on a Be surface. A low-energy electron diffraction pattern shows that the films have (100) orientation along the surface normal. We have used angle-resolved photoemission to map out the occupied bulk band dispersion along the T-X direction. The band-gap edges at the X point are 6.7 and 11.5 eV below the valence-band maximum, which is located at the Gamma point. These values are in good agreement with theoretical calculations. The unoccupied bulk electronic structure is measured using C Is near-edge x-ray-absorption spectroscopy. The spectrum is similar in shape to the energy-loss spectrum and the calculated p-partial density of states, while the peak positions are different.
Photoemission and photoabsorption study of C-60 adsorption on Cu(111) surfaces
(1997-12-15)
Tsuei, KD
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Yuh, JY
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Tzeng, CT
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Chu, RY
;
Chung, SC
We have carried out an extensive study of C-60 adsorption on Cu(111) surfaces using low-energy electron diffraction, photoemission, and x-ray-absorption spectroscopy. It is found that in valence-band photoemission a state forms right below the Fermi energy for an annealed, well-ordered monolayer, similar to the case of K-doped C-60. This peak disperses across the Fermi energy at off normal emission geometry. The spectra of carbon core-level photoemission show that the line shape is highly asymmetric with a metalliclike tail. The carbon near-edge absorption spectra show that the lowest unoccupied molecular orbital (LUMO) is attenuated, and a clear Fermi edge jump appears at the absorption onset. This evidence indicates that charge transfers from the substrate to the C-60 molecular orbitals and the overlayer becomes metallic. The amount of charge transfer can be determined to be 1.5-2 electrons per molecule from both the area of the occupied LUMO in photoemission and the peak shift in near-edge absorption spectra. It has been reported that many metal surfaces with originally different work functions covered by a monolayer of C-60 have a similar work function of about 5 eV. We suggest that the measured work functions are due to the metallic C-60 overlayers and are similar regardless of the metal substrates. This is in line with the reported alignment of monolayer energy levels to substrate Fermi energy. Since the work functions are similar, the energy levels with respect to the vacuum level are also similar. Finally we compare near-edge x-ray-absorption with inverse photoemission spectroscopy to address the screening effects.
Photoemission study of surface state intensity resonance and dispersion of Be2C(100)
(1999-03-10)
Tzeng, CT
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Yuh, JY
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Lo, WS
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Chu, RY
;
Tsuei, KD
We have carried out an extensive study on the surface state properties of an ionic, insulating Be2C(100) surface using angle-resolved photoemission spectroscopy. Two surface states were observed and characterized. One (S1) is derived at the Gamma-point above the valence band maximum in the fundamental gap. The other (S2) is located in the gap at the X-point. Resonance in the photoemission intensity of surface state S2 shows multiple peaks due to a surface umklapp process, similar to bulk transitions, because of large corrugation of the ionic surface. The penetration lengths deduced are about 4 Angstrom for both surface states. The dispersions of these states are very small. These two results indicate strong localization of the. surface state wave function to individual surface C atoms owing to the ionic character of this compound. That a surface state (S2) exists in a non-inverted Shockley gap is attributed to the compound structure. (C) 1999 Elsevier Science B.V. All rights reserved.
Photoemission, near-edge x-ray-absorption spectroscopy, and low-energy electron-diffraction study of C-60 on Au(111) surfaces
(2000-01-15)
Tzeng, CT
;
Lo, WS
;
Yuh, JY
;
Chu, RY
;
Tsuei, KD
The geometric and electronic structures of C-60 adsorption on Au(111) surfaces have been studied py low-energy electron diffraction (LEED), angle-resolved photoemission, and near-edge x-ray-absorption spectroscopy. An irreversible structural transition of the C-60 overlayer on Au(111) was observed by LEED upon successive annealing. These structures are 38 x 38 "in phase," R14 degrees and (2 root 3 x 2 root 3)R30 degrees, with the latter phase predominating after annealing to 350 degrees C. Valence-band photoemission spectra reveals a state right below the Fermi level for an annealed, ordered monolayer. This peak disperses across the Fermi energy that indicates the C-60 overlayer becomes metallic. Its intensity shows a resonance that primarily follows the behavior of highest occupied molecular orbitals, identified unambiguously as lowest unoccupied molecular orbitals (LUMO's) filled by charge transfer from the substrate. An asymmetric distribution of LUMO charge is observed. The thermal-desorption energy of the monolayer is estimated from annealing experiments to be 1.9 eV, which is 0.5 eV larger than the desorption energy from multilayers. Comparison with available spectroscopic data indicates that interaction of C-60 with Au(111) is slightly weaker than with Au(110), and much weaker than with Cu(111). The amount of charge transfer estimated from photoemission is 0.8 electrons per C-60 molecule on Au(111), compared to 1.6 electrons on Cu(111). We argue that charge transfer is determined by the bulk sp density of states at the Fermi energy scaled by the size of the C-60 molecule, and also modified by a clean surface electronic structure, and that charge transfer is the dominant interaction in these systems.