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ABSTRACT: The adsorption of maleic anhydride on the Si(001) surface has been investigated using the first-principles pseudopotential formalism. Our total-energy calculations suggest that maleic anhydride (C2H2-C2O3) adsorbs preferentially through a [2+2] cycloaddition of the C=C bond ([2+2]) with an adsorption energy of around 42 kcal/mol. Besides the [2+2] configuration we have also considered other possible coverages and adsorption models, including the adsorption on inter-row and intrarow dimer sites. Based on the analysis of the relative stability of different adsorption models, we propose the formation of mixed domains, containing the [2+2] unit and an interdimer unit. The comparison of our calculated electronic band structure, vibrational modes, and scanning tunneling microscopy images for the [2+2] and the favored interdimer adsorbed structures corroborate our proposed mixed domain model.
The Journal of Chemical Physics 09/2005; 123(7):074708. · 3.33 Impact Factor
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ABSTRACT: Using a first-principles pseudopotential technique, we have investigated the adsorption of CH3OH on the Si(001) surface. We have found that, in agreement with the overall experimental picture, the most probable chemisorption path for methanol adsorption on silicon (001) is as follows: the gas phase CH3OH adsorbs molecularly to the electrophilic surface Si atom via the oxygen atom and then dissociates into Si–OCH3 and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. Other possible adsorption models and dissociation paths are also discussed. Our calculations also suggest that the most probable methanol coverage is 0.5ML, i.e., one molecule per Si–Si dimer, in agreement with experimental evidences. The surface atomic and electronic structures are discussed and compared to available theoretical and experimental data. In addition, we propose that a comparison of our theoretical STM images and calculated vibrational modes for the adsorbed systems with detailed experimental investigations could possibly confirm the presented adsorption picture.
Surface Science - SURFACE SCI. 01/2005; 575(3):287-299.
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ABSTRACT: Using a first-principles pseudopotential method we have compared the adsorption and dissociation of the common n-type dopant molecule PH3 on the Si(001)-(21) and Ge(001){(21) surfaces. We find that the dissociated state is energetically more favourable than the molecular state by 1.70(0.81) eV, whereas the latter is 0.58(0.25) eV more stable than the system composed of the free silicon(germanium) surface and PH3(g). The chemisorbed system is characterised by elongated Si{Si(Ge{Ge) dimers that are symmetric in the dissociative case and asymmetric in the molecular case and by the fact that the Si(Ge){PH2 as well as the PH3(ads) groups retain the pyramidal geometry of the phosphine molecule. Our dissociative adsorption model is further supported by our calculated vibrational modes, which are in good agreement with available experimental works.
Brazilian Journal of Physics. 01/2002;
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ABSTRACT: Using a first-principles pseudopotential method we have studied the adsorption and dissociation of NH3, PH3, and AsH3 on the Si(001)–(2×1) surface. Apart from the existence of a barrier for the adsorption of the precursor state for arsine, we observe that the global behavior for the chemisorption of the XH3 molecules considered in this work is as follows: the gas phase XH3 adsorbs molecularly to the electrophilic surface Si atom and then dissociates into XH2 and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. The energy barrier, corresponding to a thermal activation, is much smaller than the usual growth temperature, indicating that all three molecules will be observed in their dissociated states at room temperature. All adsorbed systems are characterized by elongated Si–Si dimers that are (almost) symmetric in the dissociative case but asymmetric in the molecular case. According to our first-principles calculations, all XH3 and XH2 systems retain the pyramidal geometry observed for the gas molecules. Our calculated vibrational spectra further support the dissociative model for the XH3 molecules considered here. © 2001 American Institute of Physics.
The Journal of Chemical Physics 05/2001; 114(21):9549-9556. · 3.33 Impact Factor
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ABSTRACT: Using a first-principles pseudopotential method we have studied the adsorption and dissociation of the common n-type dopant molecule PH3 on the Si(001)-(2×1) surface. We have found that phosphine adsorbs molecularly on one side of the Si–Si dimer and, at temperatures around 110 K, fully dissociates into PH2+H, with each component attached to one side of the dimer. After dissociation, the surface is characterized by an elongated dimer, symmetric for the dissociated case and asymmetric for the molecular case. The H–P–H angles and H–P bond lengths for the dissociative case have exactly the same values as obtained for the PH3 molecule. However, for the molecular case, while the H–P bond lengths are the same as observed for the PH3 molecule, the H–P–H angles are ∼8% bigger. Our dissociative adsorption model is further supported by our calculated vibrational modes, which are in good agreement with available experimental works.
Surface Science - SURFACE SCI. 01/2001; 482:160-165.
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ABSTRACT: The authors report self-consistent pseudopotential calculations for the atomic geometry of the ZnSe (110) surface. Using the methods of total energy and Hellmann-Feynman forces, within the local density approximation, they find that the equilibrium surface geometry corresponds to a top layer rotation of 25 degrees rather than 4 degrees . Bond lengths change up to 4% and the top layer atomic displacements parallel to the surface are 0.14 and 0.39 AA for Se and Zn, respectively. The surface-state energies are in good agreement with angle-resolved photoemission data.
Journal of Physics C Solid State Physics 11/2000; 19(30):5987.
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ABSTRACT: The atomic structures of four possible models for the adsorption of Zn on GaAs(001)–(2×4) are investigated by means of a first-principles pseudopotential technique. Our calculations suggest that Zn atoms adsorb preferentially in trench (third layer) sites. All structures are characterized by the breaking of the arsenic dimer and the formation of two mixed dimers in the vertical plane containing the original As–As dimer. The Zn atom lies 0.16 Å higher than the As atoms, and the mixed dimer has a bond length of 2.31 Å. All other structural features for the four models studied (e.g., minimum interplanar distance, and remaining As dimer bond lengths) retain the characteristics of the free surface. © 2000 American Institute of Physics.
Applied Physics Letters 06/2000; 76(25):3735-3737. · 3.84 Impact Factor
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ABSTRACT: Theoretical investigations of atomic relaxation and electronic states in ultrathin (Ge2)n(ZnSe)n superlattices, for n=1, 2, 3, in (001) and (110) growth directions, have been made using the pseudopotential method and a local density scheme. The bond lengths at relaxed interfaces are in agreement with the concept of Pauling's covalent radii. The nonpolar interface is found to be more stable than the polar interface. The formation enthalpies and band structures of the polar and nonpolar superlattices are discussed in terms of different spatial arrangements of nonocted 'wrong bonds' at the interfaces.
Semiconductor Science and Technology 12/1998; 8(1):67. · 1.72 Impact Factor
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ABSTRACT: The authors report on the electronic band structure of (GaAs)n(AlAs)n superlattices. Using the self-consistent pseudopotential method they conclude that the band diagram of ultrathin as well as multilayer superlattices is similar to that presented in recent experiments. However, some differences are pointed out with regard to the variation of the valence band discontinuity at the GaAs/AlxGa1-xAs heterojunction as a function of x.
Semiconductor Science and Technology 12/1998; 1(3):169. · 1.72 Impact Factor
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ABSTRACT: We have investigated the effects of density gradient and non-linear core corrections, within the first-principles pseudopotential method, on structural and electronic properties of GaN bulk and (001) surfaces. We find that the combined use of the generalized gradient approximation and non-linear core correction for exchange and correlation (NGGA) produces important changes in structural properties. The calculated bulk valence band electronic structure shows much better agreement with experiment when the NGGA scheme is used than when the Ga 3d electrons are considered explicitly as a part of the valence shell. We have discussed the atomic structure and chemical bonding on the gallium terminated (1×1),(2×2),c(2×2) and (1×4) cubic-GaN(001) surface reconstructions, and find that the most stable of these, viz. the (1×4) structure, is characterised by a linear Ga tetramer with an energy gain of 0.29 eV per (1×1) cell over the unreconstructed (1×1) structure in agreement with previous results by Neugebauer and co-workers.
Physica B: Condensed Matter.
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ABSTRACT: Using a first-principles pseudopotential method we have studied the adsorption and dissociation of the common n-type dopant molecule PH3 on the Si(0 0 1)-(2×1) surface. We have found that phosphine adsorbs molecularly on one side of the Si–Si dimer and, at temperatures around 110 K, fully dissociates into PH2+H, with each component attached to one side of the dimer. After dissociation, the surface is characterized by an elongated dimer, symmetric for the dissociated case and asymmetric for the molecular case. The H–P–H angles and H–P bond lengths for the dissociative case have exactly the same values as obtained for the PH3 molecule. However, for the molecular case, while the H–P bond lengths are the same as observed for the PH3 molecule, the H–P–H angles are ∼8% bigger. Our dissociative adsorption model is further supported by our calculated vibrational modes, which are in good agreement with available experimental works.
Surface Science.
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ABSTRACT: In this work we have performed an ab initio theoretical study of the In-covered GaAs(001) surface. Our total-energy calculations indicate that for an In “rich” environment, with a coverage of 0.75 monolayer, the ζ(4×2) model represents the energetically most stable structure. The β2(4×2) model is dismissed based upon energetic analysis and theoretical scanning tunneling microscopy (STM) images. It is argued that the α model is not expected to be stabilized, in contrast to the interpretation of recent STM images.
Phys. Rev. B. 67(4).
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ABSTRACT: We have studied the phonon dispersion and density-of-states on the (110) surface of II-VI compounds CdTe, ZnTe, and ZnS in their zinc-blende phase by applying the adiabatic bond charge model. The relaxed surface atomic geometry and the corresponding bond charge information is obtained from the application of an ab initio pseudopotential calculation. The origins of various surface phonon modes are discussed and their variation for different compounds are analyzed in terms of the reduced mass and total mass differences. Furthermore, we have related some differences between surface modes on II-VI (110) and III-V (110) in terms of the ionicity factor.
Phys. Rev. B. 62(23).
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ABSTRACT: Using a first-principles pseudopotential technique, we have investigated the adsorption of CH3OH on the Si(0 0 1) surface. We have found that, in agreement with the overall experimental picture, the most probable chemisorption path for methanol adsorption on silicon (0 0 1) is as follows: the gas phase CH3OH adsorbs molecularly to the electrophilic surface Si atom via the oxygen atom and then dissociates into Si–OCH3 and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. Other possible adsorption models and dissociation paths are also discussed. Our calculations also suggest that the most probable methanol coverage is 0.5 ML, i.e., one molecule per Si–Si dimer, in agreement with experimental evidences. The surface atomic and electronic structures are discussed and compared to available theoretical and experimental data. In addition, we propose that a comparison of our theoretical STM images and calculated vibrational modes for the adsorbed systems with detailed experimental investigations could possibly confirm the presented adsorption picture.
Surface Science.
