High-resolution Electron Energy Loss Spectroscopy Spectra Research Articles

Assessing Oxygen Vacancies in Bismuth Oxide through EELS Measurements and DFT Simulations

Pioneering electron energy loss spectroscopy (EELS) measurements of α-Bi2O3 are performed on three samples obtained through different synthesis methods. Experimental low-loss and core-loss EELS spectra are acquired. By combining them with detailed structural characterization and Density Functional Theory (DFT) simulations, we are able to detect and evaluate the presence of oxygen vacancies in the samples. This type of information has not been accessed previously from EELS data in bismuth oxide, because high-resolution EELS spectra or how vacancies reflect in Bi2O3 spectra were unreported. This novel measurement is further validated through comparison with photoluminescence data. Therefore, the technique has the ability to probe oxygen vacancies in Bi2O3 at an unprecedented resolution, which might allow solving material science and technological issues related to this material.

C-H activation and metalation at electrode surfaces: 2,3-dimethyl-1,4-dihydroxybenzene on Pd(pc) and Pd(111) studied by TLE, HREELS and DFT.

Previous studies, based on thin-layer electrochemistry (TLE), in situ scanning tunneling microscopy (EC-STM), high-resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) computations, on the chemical adsorption of hydroquinone from aqueous solutions onto atomically smooth Pd (and Pt) electrode surfaces indicated two modes of attachment that depended upon the solution concentration. At low activities, the diphenol was oxidatively chemisorbed as benzoquinone in a flat orientation, suggestive of a Pd(2,3,5,6-η-C6H4O2) surface complex; at higher concentrations, vertical chemisorption was effected via two C-H bond activations (or metalations) at the 2 and 3 ring positions, evocative of an o-phenylene organopalladium compound. We have extended the work to 2,3-dimethyl-1,4-dihydroxybenzene on Pd(pc) and Pd(111) electrodes to probe the effect of two methyl substituents on only one side of the diphenol ring. Surface coverage and adsorbed-molecule cross section data from TLE and HREELS measurements revealed non-random concentration-dependent adsorbate orientations similar to the oxidative chemisorption of hydroquinone: flat at low concentrations and edgewise at elevated concentrations. The DFT results suggested that, for the flat structure, surface coordination is via the two double bonds of the quinone ring as in [Pd(2,3,5,6-η)-2,3-dimethyl-p-quinone]. For the edge-vertical orientation, a structure analogous to an o-phenylene compound is generated in which C-H bonds at the 5 and 6 ring positions are activated and then metalated. DFT-simulated HREELS spectra helped identify the observed peaks that distinguish the surface-coordinated quinone from the surface-metalated diphenol.

Adsorbed states of iron(II) phthalocyanine on Ag(111) studied by high‐resolution electron energy loss spectroscopy

We investigated the adsorption states of iron(II) phthalocyanine (FePc) molecules on Ag(111) by using high‐resolution electron energy loss spectroscopy (HREELS). The HREELS spectra were taken as a function of the coverage of FePc. At 1 monolayer, vibrational modes assigned as A2u and A1g representations were observed mainly by the dipole scattering mechanism, which indicates that the first‐layer molecules adsorb with the molecular planes parallel to the surface. The vibration associated with the motion of the central Fe2+ ion perpendicular to the molecular plane shows a considerable shift of the vibration energy. This reflects significant bonding interactions between the FePc molecule and the substrate. In the high coverage regime, multilayers were grown, and the vibration modes attributed to not only A2u but also Eu representation were observed. The FePc molecules in the upper layers tilt the molecular planes to the substrate. The vibrational energy is almost identical to those in the gas phase, indicating weaker interactions between the molecules. Copyright © 2014 John Wiley & Sons, Ltd.

Grafting of Cobaltic Protoporphyrin IX on Semiconductors toward Sensing Devices: Vibrational and Electronic High-Resolution Electron Energy Loss Spectroscopy and X-ray Photoelectron Spectroscopy Study

Cobaltic protoporphyrin IX (CoPP) was grafted on GaAs(100), Si(111), and highly oriented pyrolytic graphite, HOPG(0001), surfaces using different preparation conditions. Modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS) recorded in vibrational and electronic domains. XPS confirms the adsorption of the porphyrin on all of the substrates through its main fingerprint, the photoelectron Co 2p region. HREELS spectra also attest to the presence of CoPP on the substrates. The presence of oxides is also clearly detected. Analysis of the corresponding cross sections confirms that vibration excitations are mostly produced via an electron impact mechanism. Analysis of the electronic domain allowed observation of the Q, Soret, N, and C bands present in UV/vis spectra. Quantitative results for relative differential cross sections of vibrational and electronic excitations suggest that negative-ion resonances may play a role in the excitation.

Structural and compositional characterization of ultrathin titanium oxide films grown on Pt3Ti(111)

We have investigated the growth of ultrathin titanium oxide (TiOx) films on a Pt3Ti(111) single crystal surface as a function of oxidation temperature (300–1000 K) and oxygen exposure (up to 4500 l) by means of Auger electron spectroscopy, low-energy electron diffraction, ultraviolet photoelectron spectroscopy and high-resolution electron energy loss spectroscopy (HREELS). Both the surface composition and the surface structure of the resulting TiOx films exhibit a strong dependence on the preparation conditions. Loss of the chemical order and Ti segregation are observed at the Pt3Ti(111) surface upon oxygen exposures of more than 135 l at 1000 K. Increasing oxygen exposure enhances Ti segregation and oxide growth. At a threshold of ≈220 l (at 1000 K) a transition in the oxide structure occurs, namely from a (6 × 3√3) rectangular structure (a = 16.6 Å, b = 14.4 Å) below 220 l to a (7 × 7)R21.8° hexagonal structure (a = b = 19.3 Å) above 220 l. Two additional incommensurate rectangular metastable structures are observed for the highest oxygen exposures (above 900 l) at intermediate oxidation temperatures (800–900 K). In all cases the changes in the valence band spectra and the work function with respect to the clean Pt3Ti(111) surface are independent of the chosen oxidation parameters. Based on their HREELS spectra we identify the (6 × 3√3) and (7 × 7)R21.8° structures grown at 1000 K with a stoichiometric TiO phase, while the other and less stable oxide phases grown at 800–900 K exhibit more complex phonon structures that could not simply be associated with any of the stoichiometric phases TiO, Ti2O3 or TiO2. Our results are rather similar to those found by Granozzi et al for the deposition of Ti onto a Pt(111) surface in an oxygen atmosphere, except a few interesting deviations as a consequence of the different preparation conditions.

High-resolution electron energy loss spectroscopy of anions chemisorbed on electrode surfaces: The effect of counter cations

In the course of experiments that included Auger electron spectroscopy (AES) and high-resolution electron energy loss spectroscopy (HREELS) on cation exchange at benzoquinone sulfonate chemisorbed on a Pd(111) electrode, it was found that, whereas the AES spectra remained invariant as the counter cation was varied from H+ to K+ to Cs+, profound changes occurred in the HREELS spectra. Specifically, the intensity of the spectral features decreased noticeably when H+ was replaced with K+. And, when the K+ ions were exchanged with Cs+, nothing but a flat-line (dead) spectrum was observed; even the elastic peak was completely attenuated. When the Cs+ ions were displaced by protons, the initial undiminished spectrum was fully restored. This outcome, while unrelated to cation-exchange selectivity, is of exceptional significance in surface electron spectroscopy. It appears that the positive ions on the surface attracted the low-energy incident electrons such that backscattering towards the energy analyzer was hindered; partially by K+ but totally by the larger Cs+ ion. The use of HREELS to examine the molecular integrity of chemisorbed anionic species must thus take cognizance of the possibility that the counter cation chosen to preserve interfacial-layer electroneutrality can have a profound effect. To circumvent such complication, low-valent and small-radii cations will have to be employed. In addition, although subject to instrument limitations, higher incident-electron energies could be adopted. AES, with incident-electron energies in the kV range, is impervious to the presence of counter cations.

Influences of Ion-Induced Defects on Growth of Copper-Phthalocyanine Film on Graphene Substrates

Influences of argon ion-induced defects on growth of copper-phthalocyanine (CuPc) films on graphene were studied with atomic force microscope, X-ray diffraction, and high-resolution electron energy-loss spectroscopy (HREELS). Points defects and edges on graphene were found to be favorable sites for initial nucleation of CuPc. On as-grown graphene substrate with low defect density, CuPc nucleation begins at edges of the graphene sheets and then further grows into large islands with regular edges and smooth surfaces. On graphene subjected to argon ion treatment, which is known to induce surface defects, CuPc begins to nucleate at the defect positions, giving rise to crystals of much smaller sizes than those grown on a pristine graphene substrate. Such ion-induced modifications are correlated to changes in HREELS spectra of the corresponding CuPc films.

Selective Attachment of 4-Bromostyrene on the Si(111)-(7 × 7) Surface

The covalent attachment of 4-bromostyrene on the Si(111)-(7 × 7) surface was investigated using X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory calculations. The HREELS spectra suggest that 4-bromostyrene covalently binds to the silicon surface through a [2 + 2]-like cycloaddition pathway between the external vinyl group and the adjacent adatom–rest atom pair of the Si(111)-(7 × 7) surface, forming 4-bromoethylbenzene-like binding configuration. The XPS results further confirm that only the vinyl double bond participates in the surface binding reaction, whereas the bromine atom remains unchanged during the adsorption process. The resulting 4-bromoethylbenzene-like structure on the Si(111)-(7 × 7) surface can be employed as a precursor for further chemical modification and functionalization.

Spectroscopic study of propargyl chloride attachment on Si(1 0 0) − 2 × 1

The vibrational and electronic features of propargyl chloride (HC C CH 2Cl) attached on the Si(1 0 0) − 2 × 1 surface have been investigated using high resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The disappearance of the C Cl stretching mode and the existence of C sp H and C C stretching modes in the HREELS spectra of chemisorbed molecules strongly demonstrate the cleavage of C–Cl bonds and the retention of C CH linkage upon the chemisorption, further supported by the changes of the electronic features in chemisorbed XPS spectra.

Electronic and vibrational properties at the ZnPc/Ag(110) interface

The geometrical, electronic, and vibrational properties of one monolayer of Zinc-phthalocyanine (ZnPc) adsorbed on Ag(110) are studied by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and high-resolution electron energy-loss spectroscopy (HREELS). STM and LEED revealed that the molecules lie flat on the surface, ordered in a compact arrangement with a supercell defining a coincidence mesh with the substrate lattice. By comparing the HREELS spectra of one monolayer to those of a multilayer film, in which the molecules are weakly interacting, it was found that the electronic and vibrational properties of the molecular film are sensibly perturbed at the interface. The $Q$ and $B$ bands corresponding to optical interband excitations measured for the multilayer are not detected for the monolayer film and an intense low-energy Drude-like plasmon loss in the infrared region is observed. The vibrational features are also modified: several Raman modes of the isolated molecule were found to become infrared active for the monolayer because of the lowering of the molecular symmetry induced by the interaction with the substrate. Moreover a sizeable vibrational softening was measured for the selected modes indicating a charge transfer from the substrate to the molecules. Finally the strong asymmetric line shape observed for one of the Raman modes is discussed in terms of interfacial dynamical charge transfer and electron-phonon coupling.

Selective Dissociation of 4-Chloroaniline on the Si(111)-7×7 Surface through N−H Bond Breakage

The covalent attachment of 4-chloroaniline on the Si(111)-7×7 surface was investigated by using a combination of X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations. The HREELS spectra suggest that one of the N−H bonds dissociates to form Si−N and Si−H bonds with the phenyl ring and the C−Cl bond unperturbed upon chemisorption. The XPS results confirm that only the NH2 group participates in the surface binding. This binding mode and surface reaction pathway are further supported by the DFT calculation. The resulting chlorobenene-like structure on Si(111)-7×7 can be employed for further photochemical modification and functionalization.

High-resolution electron energy loss spectroscopy as a probe of SAMs

High-resolution electron energy loss spectroscopy (HREELS) as a probe of self-assembled monolayers (SAMs) obtained from thiols is here reviewed. It is shown how this surface selective spectroscopy can deliver information about these molecular hybrid systems. The layer ordering and organization into domains is probed through the analysis of the broadening of the elastic peak. Interfacial bonding between the thiol group and the substrate as well as the orientation of different parts of the grafted molecule is studied through the analysis of the vibrations induced by incident electrons interacting by dipolar mechanisms. The nature of outermost molecular groups exposed to the vacuum is inferred through the interactions by electron impact. HREELS spectra can provide additional information on interband transitions, excitation thresholds, the values of the gap and, besides, about intermolecular interactions. The interest of HREELS for studying strands of DNA ordered in SAMs, as models of DNA, is evinced.

Vibrational properties of perfluoropentacene thin film

We have studied the vibrational properties of perfluoropentacene (PFP) thin films on highly oriented pyrolytic graphite (HOPG) substrates by high-resolution electron energy loss spectroscopy (HREELS) and Raman spectroscopy. The HREELS spectra showed slight but clear increase in the vibrational energies with increasing film thickness as well as with decreasing temperature of the multilayer films. In the polarization-dependent Raman spectra of a multilayer film, the depolarization ratios of all detected vibrations with the Ag irreducible representation were found to be larger than 1, indicating that the molecular structure is distorted in the multilayer. Both results suggest that there is a fairly strong intermolecular interaction in PFP multilayer films.

Preparation and Characterization of Ultraclean H:Si(111)-(1*1) Surfaces Studied by HREELS, AFM and STM-STS

We review a wet chemical process to prepare the high quality hydrogen-terminated Si(111)-(1×1) surface and show that the two key issues for the high reproducibility are the etching time and the oxygen in the etching solution. We add ammonium sulfite to remove the oxygen according to the previous report [Appl. Surf. Sci. 130, 146 (1998)]. To elucidate the optimal etching time, the vibrational properties and the surface morphology are investigated by high-resolution electron-energy-loss spectroscopy (HREELS), atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The HREELS spectra and images of AFM and STM reveal the precise aqueous NH4F etching process of Si(111) and indicate the high controllability of steps and terraces with atomic scale. The surface cleanliness and morphology strongly depend on the etching time. At the etching time of 10 min, we obtain an ultra-clean and atomically ordered surface with wide terraces of 36 ± 7 nm step distance. It is confirmed by AFM and STM that the 1.0% ammonium sulfite is useful to remove dissolved oxygen in the 40% NH4F etching solution and to prepare a high quality H:Si(111)-(1×1) surface with low density of etch pits. The measurement of scanning tunneling spectroscopy(STS) shows the Schottky diode character of the surface. This surface is well applied to the initial stages of the nano-particle and film growth of Ag atoms and Pentacene molecules. [DOI: 10.1380/ejssnt.2009.557]

The dissociative adsorption of unsaturated alcohols on Si(1 1 1)-7×7

High-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations were used to investigate the attachment of allyl and propargyl alcohols on Si(1 1 1)-7×7 under ultra-high vacuum conditions. The HREELS spectra of chemisorbed allyl alcohol (AA) show the concurrent appearance of characteristic stretching vibrations of Si−H (2104 cm −1) and Si−O (795 cm −1) coupled with the retention of vibrational features of C C stretching (1657 cm −1) and (sp 2)C−H stretching (3012 and 3102 cm −1). These results clearly demonstrate the dissociative reaction nature via the hydroxyl group for the chemisorption of AA on Si(1 1 1)-7×7, which was further supported by XPS and DFT studies. A similar reaction pathway was found for propargyl alcohol (PA) adsorbed on the same Si(1 1 1)-7×7 surface. Our studies suggest that OH dissociation is highly favorable compared to [2+2]-like cycloadditions via C C/C C for organic reactions on silicon surfaces, which may be explained by the large spatial separation between the adjacent adatom-rest atom pair on Si(1 1 1)-7×7.

Self-trapping nature of Tl nanoclusters on the Si(111)-7×7 surface

We have studied properties of thallium (Tl) nanoclusters formed on the Si(111)-7×7 surface at room temperature (RT) by utilizing photoemission spectroscopy (PES) and high-resolution electron-energy-loss spectroscopy (HREELS) combined with first principles calculations. Our PES data reveal that the surface states stemming from the Si substrate remain quite inert with Tl adsorption producing no Tl-induced state until saturation at Tl coverage θ=0.21 monolayers. Such a behavior, in sharp contrast with the extremely reactive surface states upon the formation of Na or Li nanoclusters, together with the presence of a unique Tl-induced loss peak in HREELS spectra suggests no strong Si–Tl bonding, and is well understood in terms of gradual filling of Si dangling bonds with increasing θ. Our calculation further indicates the presence of several metastable atomic structures of Tl nanoclusters at RT rapidly transforming from one to another faster than 1010 flippings per second. We thus conclude that the highly mobile Tl atoms form self-trapped nanoclusters within the attractive basins of the Si substrate at RT with several metastable phases. The mobile and multi-phased nature of Tl nanoclusters not only accounts for all the existing experimental observations available at present, but also provides an example of self-trapping of atoms in a nanometre-scale region.

New Insights in Adsorption and Dehydrogenation of Cyclohexene on Pt(111) and Ordered Pt−Sn Surface Alloys: Experiment and Theory

The adsorption of cyclohexene (C6H10) on Pt(111) and two ordered PtnSn/Pt(111) surface alloys has been investigated experimentally using high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and temperature programmed desorption (TPD) as well as theoretically by ab initio density functional theory (DFT) calculations. On Pt(111) and Pt3Sn/Pt(111) a di-σ-bonding of cyclohexene has been found both experimentally and in the DFT calculations. In contrast, on the Pt2Sn/Pt(111) a mixture of weakly bound species (di-σ, π, and physisorbed) has been found. Whereas on Pt(111) a considerable fraction of the cyclohexene dehydrogenates during thermal treatment, the adsorption is completely reversible on the two surface alloys. A detailed theoretical study of the structure and vibrational spectra of the dehydrogenated species has been performed on Pt(111). This allowed the interpretation of the variations in the HREELS spectra when the flash temperature increases by the formation of some dehydrogenated products.

Preparation of an Ultraclean and Atomically Controlled Hydrogen-Terminated Si(111)-(1× 1) Surface Revealed by High Resolution Electron Energy Loss Spectroscopy, Atomic Force Microscopy, and Scanning Tunneling Microscopy: Aqueous NH4F Etching Process of Si(111)

We propose an improved wet chemical process for preparing a high-quality hydrogen-terminated Si(111)-(1× 1) surface and show an atomically ordered and ultraclean surface without carbon and oxygen contamination. The vibrational properties and surface morphology are investigated by high-resolution electron energy loss spectroscopy (HREELS), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). The HREELS spectra and images of AFM and STM reveal the precise aqueous NH4F etching process of Si(111) and indicate the high controllability of steps and terraces at the atomic scale. The surface cleanliness and morphology strongly depend on the etching time. At the etching time of 10 min, we obtain an ultraclean and atomically ordered surface with wide terraces of 36±7 nm step distance. It is confirmed by AFM and STM that 1.0% ammonium sulfite is useful for removing dissolved oxygen in the 40% NH4F etching solution and for preparing a high-quality H:Si(111)-(1× 1) surface with a low density of etch pits. The onset of tunneling current and the gap of 1.39 eV are measured by scanning tunneling spectroscopy. There is no peak at -1.3 eV in comparison with the previous report [Phys. Rev. Lett. 65 (1990) 1917].

A selective [4 + 2]-like cycloaddition of α, β-unsaturated ketone on Si(1 1 1)-7 × 7

The interaction of ethyl vinyl ketone (EVK) with Si(1 1 1)-7 × 7 has been investigated using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. The disappearance of both stretching vibrations of CH 2 (3099 cm −1) and C O (1684 cm −1) coupled with the appearance of new C C stretching mode (1660 cm −1) in the HREELS spectra of chemisorbed EVK clearly demonstrates the direct involvement of conjugated C C and C O bonds to form a SiC 1H 2C 2H C 3(C 4H 2C 5H 3)OSi surface species via [4 + 2]-like cycloaddition in a highly selective manner. In addition, XPS studies show that the C1s binding energies of C 1/C 2 and C 3 upon chemisorption display chemical downshifts of 0.8 eV and 2.2 eV, respectively, further confirming the proposed [4 + 2]-like cycloaddition reaction for the EVK/Si(1 1 1)-7 × 7 system. DFT theoretical calculations suggest that the proposed [4 + 2]-like cycloadduct is thermodynamically most favorable.

Temperature-Dependent Electronic and Vibrational Structure of the 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide Room-Temperature Ionic Liquid Surface: A Study with XPS, UPS, MIES, and HREELS

The near-surface structure of the room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide has been investigated as a function of temperature between 100 and 620 K. We used a combination of photoelectron spectroscopies (XPS and UPS), metastable induced electron spectroscopy (MIES), and high-resolution electron energy loss spectroscopy (HREELS). The valence band and HREELS spectra are interpreted on the basis of density functional theory (DFT) calculations. At room temperature, the most pronounced structures in the HREELS, UPS, and MIES spectra are related to the CF3 group in the anion. Spectral changes observed at 100 K are interpreted as a change of the molecular orientation at the outermost surface, when the temperature is lowered. At elevated temperatures, early volatilization, starting at 350 K, is observed under reduced pressure.