M. Onchi

The adsorbed states of ethylene on Si(100)c(4×2), Si(100)(2×1), and vicinal Si(100) 9°: Electron energy loss spectroscopy and low‐energy electron diffraction studies

The adsorbed states of ethylene on the Si(100)c(4×2), Si(100)(2×1), and the Si(100) 9° vicinal surfaces have been studied using high resolution electron energy loss spectroscopy (EELS) and low‐energy electron diffraction (LEED). Ethylene is nondissociatively chemisorbed on the Si(100) surface in the wide temperature range between 77 and ∼600 K, and is rehybridized to have a near sp3 hybridization state. The adsorbed structure is proposed in which ethylene is di‐σ bonded to two adjacent Si atoms of the dimer at the Si(100) surface. The thermal decomposition of chemisorbed ethylene and the influence of steps on the adsorbed states of ethylene are discussed.

The adsorption and thermal decomposition of acetylene on Si(100) and vicinal Si(100)9

The adsorption and decomposition of acetylene on Si(100) and vicinal Si(100)9° have been studied using high resolution electron energy loss spectroscopy and low-energy electron diffraction. Acetylene is predominantly chemisorbed molecularly in the temperature range between 80 and ~ 300 K; a small amount of acetylene is partially dissociated. The molecularly-chemisorbed acetylene is rehybridized, and has the hybridization state near sp 3 . It is proposed that molecular acetylene is di-σ bonded to adjacent Si atoms of a dimer on Si(100).The thermal decomposition of chemisorbed acetylene, and the (inactive) role played by steps are discussed.

Reactions of atomic hydrogen with the Si(111) (7×7) surface by high resolution electron energy loss spectroscopy

High resolution electron energy loss spectroscopy (vibrational and electronic excitations) and monochromatic low‐energy electron diffraction (LEED) have been applied to the study of atomic‐hydrogen adsorption on the Si(111) (7×7) surface at ∼300 K. Adsorbed states of hydrogen are differentiated by taking the vibrational loss spectra of the Si(111) surface exposed to various amounts of hydrogen and of the same surface subsequently heated to high temperatures. The existence of four adsorbed states is proposed. Hydrogen adsorption is considered to proceed as follows. In the very early stage (fractional hydrogen coverage θ≲0.3), the SiH species are produced by the covalent bond formation of hydrogen atoms with the dangling bonds of the Si(111) surface atoms. Then, the Si–Si bond breaking and the desorption of SiH4 and, possibly, SiH3 occur, and the corrosion‐induced SiH (with the bond axes parallel to and away from the surface‐normal direction), SiH2, and SiH3 species are also produced. The hydrogen coverage ...

Reactions of no with the Si(111) (7 × 7) surface: EELS, LEED and AES studies

High-resolution vibrational electron energy loss spectroscopy, low-energy electron diffraction and Auger electron spectroscopy are used to study the reactions of NO with the Si(ll1) (7 x7) surface. At 300 K, NO is adsorbed dissociatively on the (7 X 7) surface in the disordered structure. The N and 0 atoms are chemisorbed in the on-top sites forming the Si-N and Si-0 single bonds which are characterized by the vibrational losses at 118-125 and loo-105 meV, respectively. The maximum fractional coverages of N and 0 atoms are both 6’ - 0.5. By heating the NO-saturated surface at 1150-1300 K, the adsorbed oxygen is removed from the surface and the N-covered surface is reconstructed to form the “(8 X 8)“-N structure which is characterized by the losses at 91 and 120 meV. This structure is proposed to arise from the overlayer in which each N atom is bonded to three Si atoms with the sp3-like bonds forming a hexagonal lattice with unit mesh vectors of S/11 times those of the unreconstructed Si(ll1) surface. As this surface is further heated at 1300-1350 K, the “quadruplet” structure which is characterized by the losses at 61 and 120 meV is formed. It is proposed that this structure has planar geometry with each N atom bonded to three Si atoms forming the sp2-like bonds. The Si-N bond length is estimated to be - 1.7 A. The local structure of the quadruplet surface is very similar to that of the bulk silicon nitride (Si,N,).

Oxidation of the Si(111) (7×7) surface: Electron energy loss spectroscopy, low‐energy electron diffraction, and Auger electron spectroscopy studies

High‐resolution vibrational/electronic‐transition electron energy loss spectroscopy, low‐energy electron diffraction, and Auger electron spectroscopy have been used to study the oxidation of the Si(111) (7×7) surface at 300 K. From the initial stage (O2 exposure ∼1 L, fractional oxygen coverage θ∼0.2), an Si–Si bond breaking occurs, and atomic oxygen is adsorbed in the bridge site (between the first and second layers of Si substrate) as well as in the on‐top site. Some molecular species (superoxide‐like species) are also existent. With the increase in O2 exposure up to 100 L where θ∼1.5 is reached, the number of the Si–O–Si species in the selvedge region of Si substrate is greatly increased. By heating the surface pre‐exposed to 1–100 L O2 at 850 K, the superoxide‐like species are removed and the number of oxygen atoms in the on‐top sites is decreased. With the increase in O2 exposure (0→100 L) and by heating (300→850 K), the bond angle of the Si–O–Si species is increased towards that of the vitreous SiO2...

Chemisorption and thermal decomposition of ethylene on Pd(110): electron energy loss spectroscopy, low-energy electron diffraction, and thermal desorption spectroscopy studies

The adsorbed state of ethylene on Pd(110) at 90 K and its thermal decomposition in the temperature region between 90 and 600 K have been studied by the use of high resolution electron energy loss spectroscopy (EELS), low‐energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). At 90 K, ethylene is π bonded to the Pd(110) surface and is adsorbed almost disorderedly. The c(2×2)‐C2H4 patches are formed near the saturation coverage (which corresponds to 0.58 C2H4 molecule per surface Pd atom). By heating the C2H4‐saturated Pd(110) surface to 260 K, some C2H4 admolecules are desorbed intact and the remaining admolecules rearrange their adsorbed sites to form the c(2×2)‐C2H4 structure. At above 300 K, almost all the C2H4 admolecules are dehydrogenated, and the ethynyl (CCH) species, H adatoms and unstable dehydrogenated species [possibly, vinyl (CHCH2) species] are formed; the C2H4 desorption occurs by the recombination of H adatoms and dehydrogenated species. The remaining H adatoms are d...

High-resolution electron energy-loss spectroscopy of CO on an Ni(110) surface☆

Abstract High-resolution vibrational electron energy-loss spectra of CO on an Ni(110) surface were studied at 300 K with the in-situ combination of LEED, Auger electron spectroscopy and work-function change measurement. The observed peaks are at 436 cm−1, 1855 cm−1 (shifting to 1944 cm−1 with increasing coverage) and at 1960 cm−1 (shifting to 2016 cm−1 with increasing coverage). The experimental results indicate that CO is adsorbed non-dissociatively at all coverages. Three adsorbed states of CO have been found. At fractional CO coverages less than θ ~ 0.9 where the disordered adsorbed structure dominates, CO is adsorbed in two inequivalent sites (short- and long-bridge sites) at random with its axis oriented perpendicular to the surface. At high coverages (θ > 0.9) where the (2 × 1) structure develops, our results indicate that the adsorbed CO molecules may occupy the distorted long-bridge sites forming zig-zag chains which lie essentially in the troughs of the (110) surface.

Vibrational electron energy loss spectroscopy of the Si(111)(7×7)–H2O(D2O) system

High‐resolution electron energy loss spectroscopy (EELS) has been applied to the study of the Si(111)(7×7)–H2O(D2O) system. At 300 K, H2O(D2O) is partially dissociated on the Si(111) surface to form the SiOH(SiOD) and SiH(SiD) species. Angle and primary‐electron‐energy dependences of the vibrational loss intensities were measured. Relative contributions to the vibrational excitations of the dipole, impact, and resonance mechanisms were estimated. The O–H(O–D) stretching and Si–O–H (Si–O–D) bending vibrations are partly excited by the resonance mechanism in the primary energy region of Ep ≂2–7 eV. EELS spectra of the Si(111) surface exposed to H2O(D2O) at 300 K and of the same surface heated to ∼700–900 K are presented, and surface reaction mechanisms are discussed.

Electron energy-loss spectroscopy study of oxygen chemisorption and initial oxidation of Fe(110)

Abstract The initial stages of the interaction of oxygen with an Fe(110) surface have been studied at 300 K by electron energy-loss spectroscopy with in-situ combined low energy electron diffraction, Auger electron spectroscopy and work-function change measurement. From all the results, four different stages of the oxygen interaction are distinguished: (I) a first dissociative chemisorption up to 3 L, characterized by the c(2×2)-O structure, (II) a second dissociative chemisorption between 3 and 7 L, characterized by the c(3×1)-O structure, (III) incorporation of O adatoms into the selvage between 7 and 30 L, and (IV) oxidation above 30 L leading to the formation of FeO(111), characterized by the diffuse hexagonal diffraction pattern. The sticking probability was found to be initially near unity and fall off rapidly to a minimum value of ≈0.05 at ≈1 L. Particular emphasis was placed upon the investigation of the change in surface electronic properties from those characteristic of them metal to those of the oxide. In stage (I) an energy-loss peak, being attributed to the transition from the 2p orbital of the chemisorbed oxygen, was observed at 6.0 eV, while in stage (II) two additional peaks of the same origin appeared at 7.5 and 9.3 eV due to the formation of the O 2p band. The energy-loss spectrum in the oxide phase was characterized by the peaks at 4.8 and 7.5 eV due to the O2− 2p → Fe2+3d charge-transfer transitions and by a peak at 2.4 eV due to the ligand-field d → d transitions of an Fe2+ ion in FeO. It is shown that the Fe 3dyz,zx and 4sp electrons play a major role in the chemisorption bond (O adatoms located in the long-bridge site), and that for the incorporation process the Fe 3dy2 electrons are also involved in bonding by the symmetry breaking. The change in the Fe 3p-excitation spectrum during oxidation was also investigated. The differences between the experimental results on Fe(100) and (110) surfaces are summarized.

Interaction of ethylene with the Si(111)(7×7) surface- A vibrational study

Abstract The adsorption of ethylene on the Si(111)(7×7) surface has been studied at 300 K using high-resolution electron energy loss spectroscopy. Assignments of the observed loss peaks are attempted. Ethylene is predominantly adsorbed non-dissociatively and is rehybridized to have a near sp3 hybridization state. The adsorbed structure is proposed in which ethylene is di-σ bonded to two adjacent surface Si atoms saturating the dangling bonds.