Vibrational Electron Energy Loss Spectroscopy Research Articles

Investigating the Spatial Resolution of Vibrational Electron Energy-Loss Spectroscopy

Journal Article Investigating the Spatial Resolution of Vibrational Electron Energy-Loss Spectroscopy Get access Kartik Venkatraman, Kartik Venkatraman School for the Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287-6106, USA Search for other works by this author on: Oxford Academic Google Scholar Qianlang Liu, Qianlang Liu School for the Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287-6106, USA Search for other works by this author on: Oxford Academic Google Scholar Toshihiro Aoki, Toshihiro Aoki LeRoy Eyring Center for Solid State Science, Arizona State University, Tempe, Arizona 85287-1704, USA Search for other works by this author on: Oxford Academic Google Scholar Peter Rez, Peter Rez Department of Physics, Arizona State University, Tempe, Arizona 85287-1704, USA Search for other works by this author on: Oxford Academic Google Scholar Peter A Crozier Peter A Crozier School for the Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287-6106, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 22, Issue S3, 1 July 2016, Pages 992–993, https://doi.org/10.1017/S1431927616005808 Published: 25 July 2016

Detection of water and its derivatives on individual nanoparticles using vibrational electron energy-loss spectroscopy

Understanding the role of water, hydrate and hydroxyl species on nanoparticle surfaces and interfaces is very important in both physical and life sciences. Detecting the presence of oxygen-hydrogen species with nanometer resolution is extremely challenging at present. Here we show that the recently developed vibrational electron energy-loss spectroscopy using subnanometer focused electron beams can be employed to spectroscopically identify the local presence and variation of OH species on nanoscale surfaces. The hydrogen-oxygen fingerprint can be correlated with highly localized structural and morphological information obtained from electron imaging. Moreover, the current approach exploits the aloof beam mode of spectral acquisition which does not require direct electron irradiation of the sample thus greatly reducing beam damage to the OH bond. These findings open the door for using electron microscopy to probe local hydroxyl and hydrate species on nanoscale organic and inorganic structures.

Importance of Time Scale and Local Environment in Electron-Driven Proton Transfer. The Anion of Acetoacetic Acid

Anion photoelectron spectroscopy (PES) and electron energy-loss spectroscopy (EELS) probe different regions of the anionic potential energy surface. These complementary techniques provided information about anionic states of acetoacetic acid (AA). Electronic structure calculations facilitated the identification of the most stable tautomers and conformers for both neutral and anionic AA and determined their relative stabilities and excess electron binding energies. The most stable conformers of the neutral keto and enol tautomers differ by less than 1 kcal/mol in terms of electronic energies corrected for zero-point vibrations. Thermal effects favor these conformers of the keto tautomer, which do not support an intramolecular hydrogen bond between the keto and the carboxylic groups. The valence anion displays a distinct minimum which results from proton transfer from the carboxylic to the keto group; thus, we name it an ol structure. The minimum is characterized by a short intramolecular hydrogen bond, a significant electron vertical detachment energy of 2.38 eV, but a modest adiabatic electron affinity of 0.33 eV. The valence anion was identified in the anion PES experiments, and the measured electron vertical detachment energy of 2.30 eV is in good agreement with our computational prediction. We conclude that binding an excess electron in a π* valence orbital changes the localization of a proton in the fully relaxed structure of the AA(-) anion. The results of EELS experiments do not provide evidence for an ultrarapid proton transfer in the lowest π* resonance of AA(-), which would be capable of competing with electron autodetachment. This observation is consistent with our computational results, indicating that major gas-phase conformers and tautomers of neutral AA do not support the intramolecular hydrogen bond that would facilitate ultrarapid proton transfer and formation of the ol valence anion. This is confirmed by our vibrational EELS spectrum. Anions formed by vertical electron attachment to dominant neutrals undergo electron autodetachment with or without vibrational excitations but are unable to relax to the ol structure on a time scale fast enough to compete with autodetachment.

Sequential adjacent Si dimer dechlorination mechanism of perchloroethylene adsorption on Si(1 0 0) with temperature evolution

To elucidate the peak shifting of C-1s in X-ray photoelectron spectrum (XPS) of perchloroethylene (PCE) adsorption on Si(100), the sequential adjacent Si dimer dechlorination mechanism was proposed based on first principles calculations. The highly-symmetric Cl atoms of PCE induce three possible initial di-dechlorination processes occurring on intra, inter-dimer and iso intra-dimer, respectively, to yield three tetra-σ states. These tetra-σ states are identified to coexist at room temperature (RT) due to the relatively low reaction barriers (<0.59eV). However, their further di-dechlorination to form intra and inter-dimer hexa-σ states requires much higher activation barriers (>1.08eV), which leads to hexa-σ states only exist at elevated temperatures although they are found to be the most stable in terms of energetics. The calculated ionization energies (IEs) of C-1s core electron and vibrational frequencies of various potential adspecies are well consistent with the experimental data observed by XPS and vibrational electron energy loss spectroscopy (EELS), which further corroborates the sequential dechlorination processes of PCE on Si(100).

Adsorption and thermal decomposition of acetic acid on Si(111)7×7 studied by vibrational electron energy loss spectroscopy

Vibrational electron energy loss spectroscopy (EELS) has been used to characterize the adsorption of acetic acid on Si(111)7 x 7 at room temperature and as a function of annealing temperature. At room temperature, acetic acid is found to undergo O-H dissociative adsorption to form predominantly unidentate adstructure. The equilibrium geometry and the corresponding characteristic vibrational wavenumbers of the adstructures were obtained by density functional theory calculations and are found to be in good accord with the vibrational EELS data. Annealing the sample near 473 K marked the onset of C-O dissociation of the acetate adstructure with the emergence of Si-O-Si vibrational modes at 720 and 1020 cm(-1). Further annealing to 673 K caused a marked intensity reduction in the C-C stretch at 930 cm(-1) and in the CH(3) vibrational features at 1360 and 2990 cm(-1), suggesting further dissociation of the adstructures. The complete removal of the blueshifted Si-H stretching mode at 2275 cm(-1) upon further annealing to 773 K is consistent with the recombinative desorption of H(2) from Si monohydrides expected in this temperature range. The emergence of the Si-C stretching mode at 830 cm(-1) at 873 K is in good accord with the formation of SiC. Similar thermal evolution of the vibrational features have also been observed for acetic acid adsorption on a sputtered Si(111) surface.

Two-Dimensional Self-Assembly and Catalytic Function: Conversion of Chiral Alcohols into Self-Assembled Enols on Pt(111)

The chemical transformation and subsequent self-assembly of chiral alcohols on platinum was studied using three different pairs of prochiral ketones and their alcohol products. The ketones were chosen because they represent three different types of substrates in the asymmetric hydrogenation on chirally modified platinum catalysts. Scanning tunneling microscopy and high-resolution electron energy loss vibrational spectroscopy data were combined to show that methyl lactate transforms into the enol tautomer of methyl pyruvate on Pt(111) at room temperature. Specifically, the chiral alcohol undergoes dehydrogenation leading to the same adsorbed enol assemblies that are formed directly through the adsorption of the prochiral α-ketoester. Similarly, 1-phenylethanol transforms into assemblies of the enol tautomer of acetophenone. The interrelationship between surface reactivity and self-assembly was further explored by studying the oxidation of 1-phenyl-2,2,2-trifluoroethanol to form CH···O bonded 2,2,2-trifluor...

Dissociative adsorption and thermal evolution of carbon tetrachloride on Si(1 1 1)7 × 7

The adsorption of CCl 4 on Si(1 1 1)7 × 7 at room temperature has been characterized by electronic and vibrational electron energy loss spectroscopy (EELS). CCl 4 was found to adsorb dissociatively on Si(1 1 1)7 × 7 with the formation of Si Cl bond and adsorbed CCl x species. At high exposures, CCl 4 appears to more readily undergo a greater degree of dechlorination. Using a Si 16H 18 cluster to model only the adatom–restatom site, our density functional theory calculations suggest two plausible adstructures involving the dissociated Cl and CCl 3 fragments. The wavenumbers of the observed EELS features are found to be in general accord with the corresponding calculated wavenumbers. Thermal evolution of the adsorbed fragments has been followed by both vibrational and electronic EELS as a function of the sample annealing temperature. A new EELS feature at 920–960 cm −1 attributed to SiC film or alloy formation is found to emerge at 573 K, while the Si Cl stretch at 550 cm −1, corresponding to dissociated surface Cl, is removed upon annealing to ∼883 K. Furthermore, the electronic EELS spectra reveal an energy loss at 9.3 eV that can be assigned to a single-electron transition from the Cl(p z ) bonding state to the Cl(p z ) antibonding state produced by the Si Cl bond.

Dissociative adsorption and thermal evolution of dichloroethylenes on Ni(1 0 0)

The room temperature (RT) adsorption and thermal evolution of cis- and trans-dichloroethylene (DCE) and their structural isomer, iso-DCE, on Ni(1 0 0) have been studied by vibrational electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES) and thermal desorption spectrometry (TDS). For RT adsorption, both cis- and trans-DCE exhibit very similar EELS features that are different from those found for iso-DCE. These differences indicate the formation of different fragments upon RT adsorption. In particular, the primary adspecies for cis- and trans-DCE are ethane-1,1,2,2-tetrayl ( H C ¨ – C ¨ H ) and acetylide-like ( ▪) adspecies along with a small amount of chlorovinyl adspecies, while ethylylidyne ( ▪) is the more plausible adspecies for iso-DCE. The differences in the adstructures upon dissociative adsorption at RT underline the important isomeric effects. Furthermore, both AES and TDS results for all three DCE isomers show that most of the Cl atoms produced by dechlorination remain on the surface and its surface concentration remains unchanged upon annealing the samples above 500 K. Upon further annealing to 550 K, the EELS spectra of all three isomers exhibit a broad feature near 1600 cm −1, which suggests the formation of carbon clusters on the surface. The presence of surface Cl atoms therefore appears to prevent the C C bond cleavage during thermal evolution of the adspecies on Ni(1 0 0).

Reactivity induced at 25 K by low-energy electron irradiation of condensed NH3–CH3COOD (1 : 1) mixture

Chemical reactivity is observed following electron irradiation of a binary mixture of ammonia (NH(3)) and acetic acid (CH(3)COOD) at 25 K, without any subsequent thermal activation, as evidenced by vibrational high resolution electron energy loss spectroscopy (HREELS). Analysis of the HREEL spectra and comparison with infrared and Raman data of different molecules are compatible with glycine formation in its zwitterionic form. The onset for electron induced reaction is found to be at about approximately 13 eV. The mechanisms may involve NH radicals interaction with CH(3)COOD molecules. Then glycine formation does not imply any displacement of reactants, so that it involves only NH(3) and CH(3)COOD neighboring molecules.

Isomeric effects on room-temperature chemisorption and thermal evolution of iso-, cis- and trans-dichloroethylene on Si(1 1 1)7 × 7

The room-temperature adsorption and thermal evolution of iso-, cis- and trans-dichloroethylene (DCE) on Si(1 1 1)7 × 7 have been studied by vibrational electron energy loss spectroscopy and thermal desorption spectrometry (TDS). The presence of the Si–Cl stretch at 510 cm −1 suggests that, upon adsorption, all three isomers dissociate via C–Cl bond breakage on the 7 × 7 surface to form mono-σ bonded chlorovinyl ( H C ˙ CHCl or Cl C ˙ CH 2 ) , which could, in the case of iso-DCE, further dechlorinate to vinylidene (:C CH 2) upon insertion into the back-bond. The higher saturation exposure for the Si–Cl stretch at 510 cm −1 observed for cis- and trans-DCE than iso-DCE suggests that Cl dissociation via the CHCl group in the cis and trans isomers is less readily than the CCl 2 group in iso-DCE. Our TDS data show remarkable similarities in both molecular desorption near 360 K and thermal evolution of the respective adstructures for all three isomers on Si(1 1 1)7 × 7. In particular, upon annealing to 450 K, the mono-σ bonded chlorovinyl adspecies is found to further dechlorinate to either vinylene ( H C ˙ C ˙ H ) di-σ bonded to the Si surface or acetylene to be released from the surface. Above 580 K, vinylene could also become gaseous acetylene or undergo H abstraction to produce hydrocarbon or SiC fragments. All three DCE isomers also exhibit TDS features attributable to an etching product SiCl 2 at 800–950 K and recombinative desorption products HCl at 700–900 K and H 2 at 650–820 K. The stronger Cl-derived TDS signals and Si–Cl stretch at 510 cm −1 over 450–820 K for trans-DCE than those for cis-DCE indicate stronger dechlorination for trans-DCE than cis-DCE, which could be due to less steric hindrance resulting from the formation of the chlorovinyl adspecies for trans-DCE during the initial adsorption/dechlorination process. Finally, our density functional calculations qualitatively support the thermodynamic feasibility and relative stabilities of the proposed adstructures involving chlorovinyl, vinylidene, and vinylene adspecies.

Thermal and Ion-Induced Surface Reactions of 1,1-Difluoroethylene on Si(111)7×7 and Vitreous SiO2

Thermal and ion-induced reactions of 1,1-difluoroethylene (1,1-C2H2F2 or iso-DFE) on Si(111)7 x 7 and vitreous SiO2 surfaces have been investigated by vibrational electron energy loss spectroscopy and thermal desorption spectrometry. Like ethylene, iso-DFE predominantly chemisorbs via a [2 + 2] cycloaddition mechanism onto the 7 x 7 surface as a di-sigma-bonded difluoroethane-1,2-diyl adstructure, which undergoes H abstraction and defluorination, producing hydrocarbon fragments and SiF(x) (x = 1-3) upon annealing to >700 K. Ion irradiation of Si(111)7 x 7 in iso-DFE at 50 eV impact energy appears to substantially enhance the production of hydrocarbon fragments and SiF(x)(), leading to stronger SiF4 desorption products over an extended temperature range (400-900 K). The observed SiC and SiF(x) produced on the 7 x 7 surface by ion irradiation in iso-DFE are found to be similar to those obtained by ion irradiation in the fluoromethane homologues, CF4 and CH2F2. The production of higher relative concentrations for the larger SiF(x) and C2-containing fragments is evidently favored on the 7 x 7 surface. On a vitreous SiO2 surface, ion irradiation in iso-DFE, unlike that in CF4 and CH2F2, appears to produce less SiF(x) than that on the 7 x 7 surface, which indicates that surface O does not interact strongly with the C2-containing fragments. The presence or absence of a C=C bond and the relative F-to-C ratio of the sputtering gas could therefore produce important effects on the resulting surface products obtained by low-energy ion irradiation.

Room-Temperature Chemisorption and Thermal Evolution of 1,1-Dichloroethylene and Monochloroethylene on Si(111)7×7: Formation of Vinylidene and Vinylene Adspecies

The room-temperature (RT) adsorption and thermal evolution of 1,1-dichloroethylene (1,1-C2H2Cl2 or iso-DCE) and monochloroethylene (C2H3Cl or MCE) on Si(111)7 x 7 have been studied by vibrational electron energy loss spectroscopy and thermal desorption spectrometry (TDS). The presence of the Si-Cl stretch at 510 cm(-1) suggests that upon adsorption iso-DCE dissociates via C-Cl bond breakage on the 7x7 surface to form mono-sigma-bonded 1-chlorovinyl (ClC=CH2) and/or di-sigma-bonded vinylidene (: C=CH(2)) adspecies. Upon annealing to 450 K, the 1-chlorovinyl adspecies undergoes further dechlorination to vinylidene adspecies, which may be converted to di-sigma-bonded vinylene (HC=CH) before dehydrogenating to hydrocarbon fragments above 580 K. TDS studies reveal both molecular desorption of iso-DCE near 350 K and C2H2 fragments near 700 K, and the presence of the latter confirms the existence of the di-sigma-bonded vinylene adspecies. Like the other chlorinated ethylene homologues, iso-DCE also exhibits TDS features of an etching product SiCl2 at 800-950 K and a dehydrochlorination product HCl at 700-900 K. Unlike iso-DCE, MCE is found to adsorb on the 7 x 7 surface predominantly through a [2 + 2] cycloaddition mechanism at RT, with similar di-sigma bonding structure as ethylene. The thermal evolution of MCE however follows that of iso-DCE, with the formation of vinylene above 580 K. Despite the lack of TDS feature attributable to HCl, weaker SiCl2 TDS feature could be observed at 800-950 K. For both iso-DCE and MCE, strong recombinative desorption of H2 is observed near 780 K. The differences in the Cl content among iso-DCE, MCE, and ethylene therefore play a key role in the RT chemisorption and thermally driven chemical processes on Si(111)7 x 7.

Vibrational EELS and DFT study of propionic acid and pyruvic acid on Ni(100): Effects of keto group substitution on room-temperature adsorption and thermal chemistry

Abstract The room-temperature adsorption and thermally induced processes of propionic acid and pyruvic acid on Ni(1 0 0) have been investigated by electron energy loss spectroscopy (EELS). Computational vibrational analysis of the optimized bidentate structures for acid–Ni model complexes (involving the organic acid and a Ni atom) has been performed by using the two-layer ONIOM method with the Density Functional Theory and used to interpret the vibrational EELS data. Dehydrogenation of the hydroxyl group is found to result in bonding of the carboxylate group in the propionate and pyruvate adspecies to either a single Ni surface atom in a bidentate configuration or two neighbouring Ni atoms in a bridge configuration. Given the similarities in the total energies and related vibrational frequencies obtained by the calculations in the case of pyruvate adspecies, it is difficult to differentiate the alternate adsorption structure, in which the keto O and hydroxyl O atoms are bonded to a Ni atom in a five-member chelate ring configuration. Furthermore, temperature-dependent EELS studies show that both the propionate and pyruvate adspecies could decompose upon annealing to above 400 K and further dissociate to CO adspecies above 550 K and to C and/or O above 600 K.

Room-temperature chemisorption and thermal evolution of perchloroethylene and trichloroethylene on Si(1 1 1)7 × 7: Formation of chlorinated vinylene and vinylidene and acetylide adspecies, and thermal etching reactions

The prominent surface species and their corresponding bonding structures of perchloroethylene (PCE, C 2Cl 4) and trichloroethylene (TCE, C 2HCl 3) upon room-temperature adsorption and thermal excitation on Si(1 1 1)7 × 7 have been examined by vibrational electron energy loss spectroscopy (EELS) and thermal desorption spectrometry (TDS). The presence of the Si–Cl stretch at 510 cm −1 shows that both PCE and TCE undergo dechlorination on Si(1 1 1)7 × 7, while the presence of the C C stretch near 1500 cm −1 indicates that the C C bond remains intact upon adsorption. The mono-σ bonded trichlorovinyl ( Cl C ˙ CCl 2 ) and 1,2-dichlorovinyl ( Cl C ˙ CHCl ) adspecies have been proposed as plausible adsorption intermediates for PCE and TCE, respectively. Furthermore, the trichlorovinyl adspecies is found to further dechlorinate at 350 K, and becomes carbon clusters and/or SiC after complete dechlorination above 450 K. On the other hand, further dechlorination of the dichlorovinyl adspecies above 350 K is found to produce a di-σ bonded chlorovinylene adspecies that is stable up to 450 K. As supported by the presence of the C C stretch at 3300 cm −1, an additional adspecies in the form of acetylide ( C ˙ CH ) is also observed. Above 580 K, hydrocarbon fragments and/or SiC are evident, likely due to dehydrogenation of the chlorovinylene and acetylide adspecies. In addition to these novel adspecies, our TDS data reveal desorption of SiCl 2 over 800–950 K and HCl over 700–900 K for both PCE and TCE, which supports thermal etching and recombinative dehydrochlorination reactions at a higher temperature. The TDS data also provide evidence for weak molecular adsorption, likely related to dative bonding as a result of inductive effects.

Adsorption of silane and methylsilane on gold surfaces

The adsorption of silane and methylsilane on the (1 1 0) and polycrystalline surfaces of gold is examined using vibrational electron energy loss spectroscopy (VEELS), angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and X-ray photoelectron spectroscopy (XPS). Adsorption of silane onto the Au(1 1 0) surface at low temperatures is dissociative and yields an SiH 2 and possibly also SiH 3 surface species. Further dissociation occurs at room temperature to yield adsorbed SiH, which is tilted on the surface, with complete dissociation to Si occurring by 110 °C. The similarity in the UP spectra for silane adsorbed on the polycrystalline sample suggests that the same surface species are present over that temperature range. Above 200 °C, spectral changes suggest rearrangement of the Si atoms, which, by 350 °C, have diffused into the bulk. Adsorption of methylsilane onto the (1 1 0) surface at low temperatures initially produces adsorbed CH 3SiH or CH 3SiH 2, with undissociated methylsilane physisorbing at higher exposures. By room temperature, desorption and decomposition leaves (or direct adsorption yields) only adsorbed CH 3Si. After further heating, the hydrogen–carbon bonds of the CH 3 group break to leave an adsorbed SiC species. On the polycrystalline surface, methylsilane adsorption is the same at low temperatures as on (1 1 0). In contrast to the latter, though, the UP spectra indicate that direct exposures at room temperature yield adsorbed Si or SiC initially, with CH 3Si again adsorbing at higher exposures. Upon further heating to 330 °C, little if any methyl-groups remain on the surface and the Si has started to diffuse into the bulk.

Room-temperature chemisorption of chloroethylenes on Si(1 1 1)7 × 7: formation of surface vinyl, vinylidene and their chlorinated derivatives

Chemisorption of a family of six chloroethylenes (C 2H 3Cl, 1,1-C 2H 2Cl 2, cis-1,2-C 2H 2Cl 2, trans-1,2-C 2H 2Cl 2, C 2HCl 3, and C 2Cl 4) on Si(1 1 1)7 × 7 at room temperature (RT) has been investigated by vibrational electron energy loss spectroscopy (EELS). The characteristic vibrational EELS features have been used to identify the prominent surface species upon RT adsorption. Like ethylene, C 2H 3Cl has been found to predominantly adsorb in a di-σ bonding geometry to the Si surface, while 1,1-C 2H 2Cl 2, cis- and trans-1,2-C 2H 2Cl 2, C 2HCl 3 and, to a lesser extent, C 2Cl 4 appear to undergo dechlorination upon adsorption to form chlorinated vinyl adspecies involving single-σ bonding structures. Evidence of vinylidene (>CCH 2) has been obtained for the first time on a semiconductor surface for the adsorption of 1,1-C 2H 2Cl 2. The present work illustrates that the molecular structure and the Cl content of chloroethylenes play a crucial role in controlling not only the adsorption geometry but also the extent of dechlorination and the resulting adspecies upon RT adsorption on Si(1 1 1).

Adsorption of methylsilane on copper surfaces

The adsorption of methylsilane on the (1 1 1), (1 1 0) and polycrystalline surfaces of copper is examined using vibrational electron energy loss spectroscopy, angle-resolved ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. On all surfaces, low exposures of methylsilane below −140 °C yield chemisorbed CH 3Si species, while undissociated methylsilane physisorbs at higher exposures. This physisorbed layer then dissociates at higher temperatures to leave CH 3Si on the surface by room temperature. Above room temperature, the VEELS and ARUP spectra suggest that Si–C bond cleavage occurs on the (1 1 1) surface, whereas the C–H bonds break to leave an SiC molecular species on the (1 1 0) surface. A surface–molecule bonding model is constructed to describe the adsorbate–substrate interactions and suggests that CH 3Si adsorbs in a hollow site on the (1 1 1) surface and in a long-bridge site on the (1 1 0) surface.

Adsorption of SiH 4 on copper ( [formula omitted]) and ( [formula omitted]) surfaces

The adsorption of silane on Cu(1 1 1) and Cu(1 1 0) is examined using vibrational electron energy loss spectroscopy, angle-resolved ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. At room temperature, SiH adsorbs on Cu(1 1 1), whereas complete dissociation occurs on Cu(1 1 0) to leave adsorbed silicon. Below room temperature, an SiH x ( x=2 or 3) species exists on both surfaces. A surface-molecule bonding model is constructed to describe the adsorbate–substrate interactions and suggests that both Si and SiH adsorb in a substitutional or hollow site on the (1 1 1) surface and Si adsorbs in one of the same two sites on the (1 1 0) surface.

UHV Study of Hydrogen Atom Induced Etching of Amorphous Hydrogenated Silicon Thin Films

Amorphous hydrogenated silicon (a-Si:H) films were deposited at 300 K by the ion-beam-deposition (IBD) method on a Pt(111) single crystal and characterized by means of Auger electron spectroscopy (AES), electronic and vibrational electron energy loss spectroscopy (EELS, HREELS), and thermal desorption spectroscopy (TDS). The a-Si:H films appear to grow in a two-dimensional mode, and exhibit a polymer-like structure with a hydrogen content of approximately 45 at. %, bonded in monohydride (SiH) and dihydride (SiH2) groups. The films are stable up to 500 K; above this temperature the evolution of hydrogen and the formation of platinum silicides were observed. Exposure to atomic hydrogen leads to the formation of silane, disilane, and higher silicon hydrides. Between 100 and 300 K a constant etching rate of Si/H = ∼0.01 was observed. At higher temperatures the etching rate decreased due to the beginning instability of higher hydrides and the competing process of silicide formation.

Electronic and vibrational electron energy loss studies of the formation of SiO 2 and carbonaceous layers on Si(111)7×7 by thermal and ion-assisted deposition techniques

Electronic and vibrational electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES) were used to characterize the formation of SiO 2 and SiC on Si(111)7×7. The differences between thermal deposition and ion-assisted deposition of these films were investigated. Thermally formed SiO 2 layers obtained by annealing the silicon sample in O 2 at 700 K can be characterized by the electronic transitions near 5 and 7 eV and by the blue shifts in its vibrational features from that of atomic oxygen adsorption. The formation of SiO 2 could also be achieved by ion irradiation at room temperature (RT). Carbonaceous layers formed at different ethylene dosages and sample temperatures were characterized by EELS and AES. Like the oxide formation, a higher efficiency in producing the carbonaceous layer was obtained by using ion-assisted deposition than thermal deposition. In particular, a SiC layer could be obtained by cycles of ethylene ion irradiation at RT and annealing to 800 K, whereas a very large dosage of ethylene ions at RT was found to produce a graphite layer.