High Resolution Electron Energy Loss Spectroscopy Study Research Articles

A BPTDS and HREELS study of the interaction of cyclohexane with the platinum (111) surface

The surface chemistry of cyclohexane (c-C{sub 6}H{sub 12}) adsorbed on the Pt(111) surface has been investigated using bismuth postdosing thermal desorption mass spectroscopy (BPTDS) and high-resolution electron energy loss spectroscopy (HREELS). Cyclohexane was found to adsorb molecularly at 95 K in agreement with others. Cyclohexane decomposition begins at {approximately} 195 K and is complete by {approximately}230 K as determined by BPTDS. Between 230 and 290 K, the dominant species on the surface are c-C{sub 6}H{sub 9,a} and H{sub a}, with small amounts of adsorbed benzene also present. The c-C{sub 6}H{sub 9,a} converts to adsorbed benzene in the temperature range 290-340 K, almost simultaneous with the desorption of H{sub a} as H{sub 2}. 25 refs., 4 figs.

Silicon oxinitride and aluminum films interface: Rutherford backscattering and high resolution electron-energy-loss spectroscopic studies

In this study, we report on the growth and characterization of SiOxNy/Al films used as coating for static radiative cooling devices. Aluminum films were deposited by e-beam evaporation in high vacuum conditions. Silicon oxinitride overcoating aluminum was grown by RF reactive plasma sputtering in a mixed oxygen-nitrogen atmosphere. For the Rutherford backscattering spectroscopic (RBS) study, a carbon disc was the substrate. As for the highresolution electron-energy loss spectroscopy (HREELS) experiments, silicon substrates were used. RBS and HREELS studies showed that the silicon oxinitride/aluminum interface consists of two clearly distinct regions: one is a thin silicon nitride layer and the other one is aluminum oxide. This means that a dynamic phenomenon takes place at the interface. This phenomenon induces oxygen migration towards aluminum. Then the first silicon oxinitride monolayers are depleted in oxygen, leaving silicon nitride and forming a thin film of aluminum oxide.

Model surface studies of metal oxides: Adsorption of water and methanol on ultrathin MgO films on Mo(100)

Model surface studies of magnesium oxide have been carried out using surface sensitive techniques. Ultrathin MgO films have been synthesized under ultrahigh vacuum (UHV) conditions by thermally evaporating Mg onto Mo(100) in the presence of oxygen. Low-energy electron diffraction (LEED) studies indicate that the MgO films grow epitaxially with the (100) face of MgO oriented parallel to Mo(100). The MgO films, prepared under optimum synthesis conditions, have essentially one-to-one stoichiometry, are nearly free from pointlike surface defects, and have properties essentially identical to those of bulk, single-crystal MgO. Adsorption of water and methanol onto the MgO films has been studied using high-resolution electron energy-loss spectroscopy (HREELS) and temperature programmed desorption (TPD). In order to circumvent the difficulty associated with intense multiple surface optical phonon (Fuchs–Kliewer modes) losses, a new approach to acquisition of HREELS data has been demonstrated. This new approach enables the direct observation of weak loss features due to excitation of the adsorbates without serious interference from multiple phonon losses. Our HREELS studies show that water and methanol undergo heterolytic dissociation, leading to the formation of hydroxyl and methoxy species, respectively.

Interaction of slow electrons with organic films: theoretical and experimental HREELS studies on selectively deuterated molecules

The scattering processes of slow electrons in high-resolution electron-energy-loss spectroscopy (HREELS) at the surface of organic solids have been studied, both theoretically and experimentally. We used selectively deuterated molecules which were prepared in highly ordered films by the Langmuir-Blodgett (LB) technique. In the framework of a microscopic approach the intensity ratios for CD- and CH-stretching vibrations in the dipolar and impact limit were estimated. The broad angular distribution of elastically reflected electrons scattered at organic surfaces as well as the anisotropy of the films were taken into account. In a first set of experiments the influence of the substitution of hydrogen by deuterium on the intensity of the corresponding methyl and methylene stretching vibrations was measured. Comparison of the results with the theoretical considerations supports a dipolar-dominated scattering at around 5 eV with an increase of the impact contribution at lower energies. Second, the information depth and its dependence on the impact energy of the electrons was studied. The strong increase in the information depth towards lower electron energies can be correlated with a decrease of the dipolar scattering intensity and an enhancement of short-range interaction.

Induced ordering of ethylidyne on the Pd(111) surface by the preadsorption of oxygen: a LEED and HREELS study

Overlayers of oxygen coadsorbed with acetylene and ethylene on a Pd(111) surface were studied by low-energy electron diffraction (LEED) and high-resolution electron-energy-loss spectroscopy (HREELS) in the temperature range 150–320 K. Low temperature adsorption of both C 2H 2 and C 2H 4 resulted in non-dissociatively chemisorbed molecules, and the preadsorption of oxygen did not change the LEED or HREELS data. At room temperature both C 2H 2 and C 2H 4 formed ethylidyne (C 2H 3), which poorly ordered in a (√3 × √3)R30° structure. We found that preadsorption of oxygen induced better ordering for C 2H 3 formed from acetylene exposure, but not for C 2H 3 derived from ethylene adsorption. We propose that preadsorbed oxygen helped the ordering of the C 2H 3 overlayer by: (a) efficiently removing surface hydrogen and (b) attractively interacting with C 2H 3 to give coadsorbate induced ordering.

A HREELS study of the UV photon-induced chemistry of C 6H 5Cl adsorbed on Ag{111}

The UV photochemistry of both monolayer and multilayer C 6H 5Cl adsorbed on Ag{111} surfaces has been studied using high resolution electron energy loss spectroscopy (HREELS). Photon-induced dissociation via the cleavage of the CCl bonds was observed as a common feature. For the monolayer, chemisorbed biphenyl appears to be formed on the surface after photolysis at 110 K and subsequent annealing to 300 K. The two phenyl rings are found to lie parallel to the metal surface. The photon-induced dissociation of multilayer C 6H 5Cl leads, however, to photopolymerization as shown by the high thermal stability of the surface species formed and the detection of simple additive products in thermal desorption.

A HREELS study of the UV photon-induced chemistry of C 6H 5Cl adsorbed on Ag{111}

The UV photochemistry of both monolayer and multilayer C 6H 5Cl adsorbed on Ag{111} surfaces has been studied using high resolution electron energy loss spectroscopy (HREELS). Photon-induced dissociation via the cleavage of the CCl bonds was observed as a common feature. For the monolayer, chemisorbed biphenyl appears to be formed on the surface after photolysis at 110 K and subsequent annealing to 300 K. The two phenyl rings are found to lie parallel to the metal surface. The photon-induced dissociation of multilayer C 6H 5Cl leads, however, to photopolymerization as shown by the high thermal stability of the surface species formed and the detection of simple additive products in thermal desorption.

HREELS studies of formamide on Pt(111): hydrogen bonding interactions between adsorbates

The adsorption properties of formamide, HCONH 2, on Pt(111) have been studied using high-resolution electron energy loss spectroscopy (HREELS). At low dosages, dissociative adsorption of formamide at 170 K results in the formation of coadsorbed NH 3 and CO. The onset of molecular adsorption occurs just prior to formation of a monolayer. At saturation monolayer coverages, hydrogen-bonding interactions between the chemisorbed molecules result in red-shifting of the ν(NH 2) vibrational frequency from the gas phase value of 3570 to 3307 cm −1. The linewidths of the ν s(NH 2) and ν a(NH 2) modes are broadened into a singular peak due to enhanced coupling with NH 2 modes of lower frequency. In the case of HCOND 2, the ν s(ND 2) and ν a(ND 2) peaks are distinctly resolved. Upon heating multilayer coverages of formamide, the hydrogen-bonding interactions lead to autocatalytic desorption of formamide at 223 K.

The effects of surface damage on surface plasmon excitations in doped InSb(100)

High-resolution electron energy loss spectroscopy (HREELS) has been used to study the (100) surface of n-type InSb (Te doped, n ≈ 1 x 10 18 cm -3). Clean, ordered surfaces were prepared by cycles of low-energy argon ion bombardment and annealing. Throughout the annealing process, a series of surface reconstructions were observed as a function of annealing temperature, by low-energy electron diffraction (LEED). Specular HREELS studies, carried out at 300 K, showed that the plasmon loss energy changed dramatically with post-bombardment annealing temperature. These changes in plasmon energy are discussed in relation to the particular surface reconstruction and the degree of surface damage induced by the ion sputtering process.

AES and HREELS studies of the Cu/MgO(100) interface

AbstractWe have studied the formation of the Cu/MgO interface with Auger electron spectroscopy and high‐resolution electron energy‐loss spectroscopy (HREELS). Copper overlayers ranging in thickness from 1 to 20 Å were formed on an MgO single‐ crystal substrate by evaporation from a Knudsen effusion cell. Direct Auger spectra for the O KLL, Cu LMM and Mg KLL lines were collected. The evolution of the Auger peak intensities shows that the copper initially forms a complete monolayer, followed by island formation (i.e. Stransky–Krastanov growth). Low energy electron diffraction patterns recorded after Cu deposition confirmed the ordering of the first monolayer.A significant decrease in energy of the surface MgO phonon peak was observed by HREELS following deposition of 20 Å of copper. Simultaneously with this shift of the phonon energy, we observed a decrease of the intensity ratio of the phonon‐loss and elastic peaks. These results have been interpreted using the dielectric theory. Theoretical spectra of MgO covered by a layer of Cu2O obtained by using dielectric constants taken from the literature were able to reproduce qualitatively the decreases in energy and intensity of the phonon‐loss peak with increasing copper thickness.

UPS and HREELS studies of early stages of adsorption of CuCl on TiO 2(110)

The valence-band electronic structures and vibrational modes of molecular-beam CuCl deposited upon 1 × 1 TiO 2(110) surfaces at 293 K have been studied by means of ultraviolet photoelectron spectroscopy (UPS) and high-resolution electron energy-loss spectroscopy (HREELS). Initially, CuCl molecules are adsorbed on the surfaces with the Cu end of the molecule oriented toward the surface. Rehybridization between the adsorbate Cu 3d levels and the substrate O 2p levels occurs. A CuCl vibrational mode is found at 186 cm −1.

HREELS study of the Pt-Rh(100) alloy single crystal surface during NO + H 2 reaction

The catalytic reduction of NO by H2 on a Pt0.25 Rh0.75(100) alloy single crystal was investigated by high-resolution electron energy loss spectroscopy (HREELS) while the reaction was proceeding. Adsorption of NO at 500 K formed a surface with a p(3 × 1) structure. This p(3 × 1) overlayer consists of adsorbed oxygen, though it seems to be generated only with SiO2 impurities on the surface. The NO + H2 reaction formed a c(2 × 2) overlayer of N(a). Hydrogenated N in form of NHx(a) was observed in the presence of gas-phase H2. The reaction between N(a) + H2 and NHx(a) was reversible, and faster than the complete hydrogenation to NH3(g).

HREELS study of epitaxial Si(111)/WSi 2: Correlation with optical properties in the infrared energy range

Electronic properties of epitaxial WSi 2 layers on top of the Si(111)(7 × 7) surface have been studied by means of high resolution electron energy loss spectroscopy (HREELS) in the loss energy range below 1 eV. A characteristic electronic-type excitation is observed and its origin explained, in the framework of the dielectric theory, through consideration of the IR-optical properties of bulk single crystal WSi 2. HREELS spectra measured for different WSi 2 overlayer thicknesses allow the identification of contributions from electron gas excitations at the surface and at the interface. AES and LEED analyses have also been carried out for characterization.

LEED and HREELS studies of the coadsorbed CO + Ethylidyne and NO + Ethylidyne systems on the Rh(111) crystal surface

The CO + C 2H 3 (ethylidyne) and NO + C 2H 3 ordered coadsorption systems on a Rh(111) crystal surface were studied by low energy electron diffraction (LEED) surface crystallography and high resolution electron energy loss spectroscopy (HREELS). In both surface structures the ethylidyne occupies one type of three-fold hollow site and CO or NO the other type. For the case of CO + C 2H 3 CO is adsorbed in a hcp hollow site forcing C 2H 3 into the fcc site, while for NO + C 2H 3 NO is adsorbed at the fcc hollow site pushing C 2H 3 into the hcp hollow site. Interactions between CO or NO and C 2H 3 are likely to be responsible for both the site choices of the coadsorbates and for their c(4 × 2) ordering since the molecular adsorhates alone on Rh(111) prefer different adsorption sites and ordering. CO and NO bond to the metal through the carbon and nitrogen atoms, respectively. The molecular axis of the NO is perpendicular to the metal surface, while that of the CO is tilted by about 5° from the surface normal. The C-C bond in ethylidyne is perpendicular to the metal surface. Coadsorbate-induced ordering should be of great importance in investigating the interactions between coadsorhed molecules.

HREELS study of the interaction of oxygen with a Mo(111) surpace

High resolution electron energy loss spectroscopy (HREELS) is used to characterize the adsorption sites occupied during the chemisorption of oxygen on Mo(111) at room temperature. At low exposures (≤ 1×10 −4 Pa s), the vibrational spectrum shows two losses at 407 and 616 cm −, which are interpreted as due to a symmetric and an asymmetric mode of oxygen adsorbed at a threefold site. When the exposure increases above 1×10 −4 Pa s, a new loss appears in the spectrum which is assigned to on-top adsorbed oxygen. At expossures above 4×10 −4 Pa s, a decrease in frequency of the symmetric mode and an increase in frequency of the asymmetric mode of oxygen at a threefold site occur, which is assumed to be due to oxygen penetration in the surface layer of the metal. After oxygen exposures at elevated temperatures, a structure with a distinct peak at 725 cm −1 is observed in the spectrum and is attributed to the formation of a surface oxide, probably MoO 2.

Investigation of electronic properties of semiconductor interfaces and layer systems by electron energy-loss spectroscopy

Knowledge about space-charge layers and transport parameters in thin films is necessary for an understanding of the electrical properties of semiconductor heterostructures and metal-semiconductor junctions. The application of high-resolution electron energy-loss spectroscopy (HREELS) for the investigation of such problems is briefly discussed. As an example for the study of a space-charge layer the coupled surface plasmon/phonon polaritons measured in HREELS yield information about carrier density and band bending in the depletion layer generated by oxygen adsorption on InSb(110). HREELS studies of Sn films deposited on InSb(110) illustrate how information about transport parameters of the overlayers can be obtained. Some other results about thin metal films on semiconductor surfaces are also presented.

Multiple vibrational excitations of H 2O and D 2O on Si(100)(2 × 1): A HREELS study

The HREELS (high-resolution electron energy-loss spectroscopy) spectra from H 2O and D 2O chemisorbed on the clean Si(100)(2 × 1) surface have been investigated in the energy region between about 500 and 1400 meV. For the first time, second overtones have been observed for a chemisorbed species. This observation is believed to be due to the enhancement of the O-H stretching fundamental and overtone modes because of the temporary formation of a negative ion that occurs when the incident electron is briefly trapped in a shape resonance. Additional multiple and combination modes are observed involving the resonance enhanced overtones as well as the fundamental modes. Furthermore, the dissociation energy of the hydroxyl group has been determined and found to be much larger than the actual energy needed to dissociate hydroxyl groups on the surface, indicating that the driving force for the observed atomic rearrangements is probably Si-O bond formation rather than O-H dissociation.

A high-resolution electron energy loss spectroscopy study of the surface structure of benzene adsorbed on the rhodium(111) crystal face

Benzene adsorption on the Rh(111) crystal surface has been studied by HREELS (high-resolution electron energy loss spectroscopy), LEED (electron diffraction), and TPD (temperature programmed desorption). The vibrational spectra indicate that benzene adsorbs molecularly at 300 K and is ..pi..-bonded to the surface with the ring plane parallel to the surface plane. Recent dynamic LEED calculations together with the angle-dependent HREELS studies reported here establish a C/sub 3v/(sigma/sub d/) bonding symmetry for the c(2..sqrt..3 x 4)rect-C/sub 6/H/sub 6/ structure. Several other ordered benzene overlayers can be formed between 300 and 400 K depending on the benzene coverage. No large changes occur in the chemisorption bonding mode or geometry coincident with the two-dimensional ordering phase transitions in this temperature range. The vibrational spectra show that two molecular adsorption sites can be populated. Benzene adsorption is only partially reversible; less than 20% of the adsorbed benzene desorbs molecularly upon heating. The remaining benzene irreversibly decomposes, evolving hydrogen and leaving a carbon-covered surface. The TPD and HREELS data on Rh(111) and other single-crystal surfaces show correlations between the metal-benzene bond strength, the work function of the clean surface, and the frequency shifts of some of the molecular benzene vibrational modes. 44 references, 10 figures,more » 4 tables.« less

A high resolution EELS study of free-carrier variations in H+2/H+ bombarded (100)GaAs

High resolution electron energy loss spectroscopy (EELS) has been used to examine whether thermal recovery of the near-surface free-carrier concentration in Te-doped (100) GaAs is accomplished following low energy (250–1500 eV) hydrogen ion bombardment. For hydrogen ion impact energies below 500 eV, the nominal bulk free-carrier density is recovered by annealing at 725 K for 2 h. For comparable ion doses, the net free-carrier concentration decreases monotonically at higher impact energies under similar annealing conditions. The threshold for damage retention occurs close to the value of transmitted energy which is necessary to create either a Ga or an As interstitial point defect.

Tilted CO chemisorbed on Pt {110}

A variety of techniques, including LEED, AES, thermal desorption (for surface characterization and comparison with previous work), high resolution electron energy loss spectroscopy (HREELS), and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) (for local structure determination) have been used to study CO on Pt {110}. Both the Pt {110}; (1 × 1)-CO and the Pt {110} (2 ×1) p1g1-CO structures were examined. The HREELS study revealed absorption bands for both structures at 476 cm−1 (PtCO stretch) and at 2097 cm−1 (CO stretch), assigned to chemisorbed CO bonded end-on a single Pt atom. ARUPS results show an unusually clear distinction between 4σ, 1π and 5σ-d orbitals in the chemisorbed complex, and polar plots obtained with the analyser motion in the orthogonal plane to the incidence plane reveal that the CO axis is strongly tilted away from the surface normal.