High Resolution Electron Energy Loss Research Articles

Reprint of “The low temperature oxidation of lithium thin films on HOPG by O2 and H2O”

Lithiated graphite and lithium thin films have been used in fusion devices. In this environment, lithiated graphite will undergo oxidation by background gases. In order to gain insight into this oxidation process, thin (<15monolayer (ML)) lithium films on highly ordered pyrolytic graphite (HOPG) were exposed to O2(g) and H2O(g) in an ultra-high vacuum chamber. High resolution electron energy loss spectroscopy (HREELS) was used to identify the surface species formed during O2(g) and H2O(g) exposure. Auger electron spectroscopy (AES) was used to obtain the relative oxidation rates during O2(g) and H2O(g) exposure. AES showed that as the lithium film thickness decreased from 15 to 5 to 1ML, the oxidation rate decreased for both O2(g) and H2O(g). HREELS showed that a 15ML lithium film was fully oxidized after 9.7L (L) of O2(g) exposure and Li2O was formed. HREELS also showed that during initial exposure (<0.5L) H2O(g), lithium hydride and lithium hydroxide were formed on the surface of a 15ML lithium film. After 0.5L of H2O(g) exposure, the H2O(g) began to physisorb, and after 15L of H2O(g) exposure, the 15ML lithium film was not fully oxidized.

The use of bimetallics to control the selectivity for the upgrading of lignin-derived oxygenates: Reaction of anisole on Pt and PtZn catalysts

The adsorption and reaction of anisole on Pt and PtZn catalysts were investigated using both model single crystal and high surface area supported metal catalysts. Temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) studies of the interaction of anisole with Pt(111) demonstrated that there is a strong interaction between the phenyl ring of anisole and the surface, resulting in CO and CH bond scission at relatively low temperatures. In contrast, anisole was observed to bond to a Zn-modified Pt(111) surface primarily via the oxygen at Zn sites or possibly adjacent Pt sites, with the phenyl ring tilted away from the surface. Such bonding configuration facilitated selective CO bond cleavage producing phenyl groups and methoxide groups with the latter being bonded to the Zn sites. These results suggested that PtZn may be an effective catalyst for hydrodeoxygenation (HDO) of lignin-derived aromatic oxygenates with low activity for ring hydrogenation. This hypothesis was then tested and verified by investigating the reaction of anisole and H2 over high surface area carbon-supported Pt and PtZn catalysts.

Surface Analysis of Li-Ion Electrode-Electrolyte Interfaces: Insights By a Surface Science Approach

Electrode-electrolyte interfaces play a key role for the performance and degradation of electrochemical devices such as lithium-ion cells. Side reactions at cathode-electrolyte interfaces result in the formation of a solid-electrolyte interface (SEI) layer, resulting in loss or performance due to lithium loss and increase in surface layer resistance. Over the past years, the cathodic SEI layer has been subject to extensive investigation, focused on the correlation between composition and battery cycling parameters. Yet, open questions remain with regard to formation mechanisms and properties. Most investigations on SEI layers have been performed by post mortem analysis of composite electrode surfaces by photoelectron spectroscopy (XPS). XPS gives information on the chemical structure by way of core level electron binding energies. Despite of the standard use of XPS for SEI analysis, unambiguous identification of the different phases is often not straightforward as binding energy shifts are encountered. Moreover, the post mortem analysis of composite materials is not ideally suited to investigate the SEI on the active material, as emissions from the other materials in the electrode (carbon, binder) add complexity. Nevertheless, such data on the SEI composition of composite electrodes are often used to propose reaction mechanisms on SEI-formation and side reactions (electrolyte oxidation) occurring at the surface of the active materials. Improved insights into SEI-composition and side reactions require experiments that address binding energies and interaction of single material surfaces with components of the electrolyte. We prepare model interfaces of LCO (LiCoO2) thin film electrodes with components of the SEI, artificial SEI-layers, as well as with solvents, and analyze them with surface analysis methods [1]. In such experiments, the SEI or electrolyte-related components are deposited as top phase either by thin film deposition or low temperature adsorption, and analysis is performed by photoelectron spectroscopy (XPS, UPS, SXPS) and high resolution electron energy loss spectroscopy (HREELS). Usually, the preparation is performed stepwise with intermediate analysis in order to investigate the interface formation in detail with respect to chemical structure, electronic structure and space charge effects. In this contribution, we present our results with respect to the deposition of several solid compounds such as lithium oxide and LiPON, as well as the exposure to several solvents (water, diethyl carbonate, etc.). In most cases, the formation of interface and/or gradient layers is observed. For the substrate-related binding energies we usually note a shift to higher values which is attributed to space charge layer formation in the LiCoO2 (band bending downwards). Also, we find indications that binding energies from nominally identical overlayer compounds and those related to the substrate are differently positioned against each other depending on the preparation method. Possible causes for this are charging effects, differences in the overlayer compound´s Fermi-level, or presence of interfacial dipoles. Mechanistic insights regarding the formation of cathode-electrolyte interfaces and of ionic interfaces in general are discussed [2][3]. [1] R. Hausbrand, D. Becker, and W. Jaegermann, Progress in Solid State Chemistry 42 (2014) [2] A. Schwöbel, W. Jaegermann, and R. Hausbrand, Solid State Ionics, in press. [3] D. Becker, et al., Journal of Physical Chemistry C, 118 (2014)

Chemical and Structural Evolution of Layered Lithium-Transition Metal Oxide Cathode Material upon Cycling

With the rapid development of electronic devices and hybrid-electric/electric vehicles (H/EVs), lithium-ion batteries have become the most promising electrical storage system due to its high volumetric and gravimetric energy density. However, the high energy density of lithium-ion batteries is difficult to maintain after extended cycles due to capacity fading, which can be attributed to multiple possible reasons including SEI formation,[1] electrolyte decomposition,[2] and structural changes in electrode materials.[3] These issues generate important concerns to the stability and lifetime of the battery. Layered lithium-transition metal oxides represent a major type of cathode materials that are widely used in the commercial market. However, these materials are suggested to suffer structural transformation during electrochemical cycling. Previous studies suggest the possible surface structural transitions from the layered structure to spinel[4], and/or rock-salt structures,[5] or possible further decomposition of the transformed phase.[6]A clear explanation of these surface phenomenon is still under debate. An in-depth investigation of the structural reconstruction is necessary to elucidate the degradation mechanisms of the layered cathode materials. In the present study, we investigated the chemical evolution and structural transformation of a prominent layered cathode material for lithium-ion batteries, LiNi1/3Mn1/3Co1/3O2 (NMC). The redox reaction of NMC cathode during charge-discharge process was analyzed in detail using high-resolution electron-energy loss spectroscopy (EELS) and 7Li magic-angle spinning (MAS) NMR. Our results suggest that the charge compensation of the NMC cathode during the delithiation-lithiation process is mainly achieved by the oxidation and reduction of Ni2+↔Ni4+, whereas Mn4+ and Co3+ remain mostly unchanged. This is consistent with the NMR findings, from which a trend to lower chemical shift upon lithium extraction is well-correlated with the oxidation change from paramagnetic Ni2+ to diamagnetic Ni4+. Furthermore, the electronic structure of the NMC cathode during initial charging process is found to be inhomogeneous from the particle surface to the bulk using spatially resolved STEM-EELS technique (Figure 1a-1d). It is revealed that the particle surface is at lower oxidation state compared with the bulk region, indicating that the surface evolution of NMC cathode material occurs during the initial cycle. This surface evolution is further analyzed at different numbers of cycles using atomic-resolution high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) imaging combined with EELS and nano-beam electron diffraction (NBED). Figure 1e shows a HAADF-STEM image of the NMC particle after 50 cycles. It can be clearly seen from the image that the bulk of the cycled NMC particle shows identical layered structure symmetry, the NBED (Figure 1h) obtained from the bulk can be indexed to the R-3m space group. In contrast, the Li layers are occupied by transition metal ions at the particle surface, and this surface reconstruction layer thickens with extended cycles. When probing to the outer-most surface, the corresponding NBED can be indexed to an Fm-3m rock-salt structure, as shown in Figure 1f. The valence change of the transition metal cations is analyzed using STEM-EELS. The results indicate that the transition metal ions are reduced to divalent states at the surface. In addition, the TM:O ratio increases almost twice at the surface than the bulk. The results indicating that the surface layer is a MO-type rock-salt phase with small amount of residual Li ions and/or vacancies. In addition, a transition zone is observed (Figure 1e) between the bulk layered region and the surface rock-salt layer, where the Li sites are partially occupied by the transition metal ions (some are marked by the red arrows), and the diffraction spots corresponding to the alternating arrangement of Li-containing layers and transition metal layers (some are marked with arrows) become weaker (Figure 1g). 7Li NMR studies of the local Li-environments, following ten cycles and ending at the top of charge, indicate that the order of the TM layers remains unchanged in the bulk of the material. This highlights the importance of complementary studies that are sensitive to different length scales. Further data, including EELS and NMR of the NMC cathode at different cycling state and electrolyte effect will be discussed in the presentation. [1] K. Edström, T. Gustafsson, J. Thomas, Electrochim. Acta 2004, 50, 397. [2] L. Terborg, S. Weber, F. Blaske, et al. J. Power Sources 2013, 242, 832. [3] B. Xu, C.R. Fell, M. Chi, et al. Energy Environ. Sci. 2011, 4, 2223. [4] A. Boulineau, L. Simonin, J.F. Colin, et al. Chem. Mater. 2012, 24, 3558. [5] F. Lin, I. M. Markus, D. Nordlund, et al. Nat. Commun. 2014, 5, 3529. [6] J. Zheng, M. Gu, J. Xiao, et al. Nano Lett. 2013, 13, 3824. Figure 1

CO Adsorption on Au Nanoparticles Grown on Hexagonal Boron Nitride/Rh(111)

CO adsorption on size-controlled Au nanoparticles grown on an h-BN/Rh(111) nanomesh surface has been examined to probe their potential catalytic properties. A combination of high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations demonstrate that the CO adsorption strength depends heavily on the Au deposition coverage and particle morphology. Particles resulting from low Au coverages deposited at the liquid nitrogen temperature exhibit significantly enhanced CO binding relative to bulk crystalline Au. The resulting CO TPD spectra, and the significantly red-shifted C–O stretching frequency and negative charging of the Au nanoparticles as evidenced by HREELS and DFT, all correspond to those reported for catalytically active Au nanoparticles grown on reactive metal oxides, even though the h-BN/Rh(111) surface is free of carbon, oxygen, or defects. DFT modeling further suggests that the enhanced CO adsorption occurs ...

The low temperature oxidation of lithium thin films on HOPG by O2 and H2O

Lithiated graphite and lithium thin films have been used in fusion devices. In this environment, lithiated graphite will undergo oxidation by background gases. In order to gain insight into this oxidation process, thin (<15monolayer (ML)) lithium films on highly ordered pyrolytic graphite (HOPG) were exposed to O2(g) and H2O(g) in an ultra-high vacuum chamber. High resolution electron energy loss spectroscopy (HREELS) was used to identify the surface species formed during O2(g) and H2O(g) exposure. Auger electron spectroscopy (AES) was used to obtain the relative oxidation rates during O2(g) and H2O(g) exposure. AES showed that as the lithium film thickness decreased from 15 to 5 to 1ML, the oxidation rate decreased for both O2(g) and H2O(g). HREELS showed that a 15ML lithium film was fully oxidized after 9.7L (L) of O2(g) exposure and Li2O was formed. HREELS also showed that during initial exposure (<0.5L) H2O(g), lithium hydride and lithium hydroxide were formed on the surface of a 15ML lithium film. After 0.5L of H2O(g) exposure, the H2O(g) began to physisorb, and after 15L of H2O(g) exposure, the 15ML lithium film was not fully oxidized.

Transformation-deformation bands in C60 after the treatment in a shear diamond anvil cell

The C60 fullerene has been investigated by high-resolution transmission electron microscopy and electron energy loss spectroscopy in a shear diamond anvil cell after applying pressure and shear deformation treatment of fcc phase. Shear transformation-deformation bands are revealed consisting of shear-strain-induced nanocrystals of linearly polymerized fullerene and polytypes, the triclinic, monoclinic, and hcp C60, fragments of amorphous structures, and voids. Consequently, after pressure release, the plastic strain retains five high pressure phases, which is potentially important for their engineering applications. Localized shear deformation initially seems contradictory because high pressure phases of C60 are stronger than the initial low pressure phase. However, this was explained by transformation-induced plasticity during localized phase transformations. It occurs due to a combination of applied stresses and internal stresses from a volume reduction during phase transformations. Localized phase transformations and plastic shear deformation promote each other, which produce positive mechanochemical feedback and cascading transformation-deformation processes. Since the plastic shear in a band is much larger than is expected based on the torsion angle, five phase transformations occur in the same region with no transformation outside the band. The results demonstrate that transformation kinetics cannot be analyzed in terms of prescribed shear, and methods to measure local shear should be developed.

Layer specific optical band gap measurement at nanoscale in MoS2 and ReS2 van der Waals compounds by high resolution electron energy loss spectroscopy

Layer specific direct measurement of optical band gaps of two important van der Waals compounds, MoS2 and ReS2, is performed at nanoscale by high resolution electron energy loss spectroscopy. For monolayer MoS2, the twin excitons (1.8 and 1.95 eV) originating at the K point of the Brillouin zone are observed. An indirect band gap of 1.27 eV is obtained from the multilayer regions. Indirect to direct band gap crossover is observed which is consistent with the previously reported strong photoluminescence from the monolayer MoS2. For ReS2, the band gap is direct, and a value of 1.52 and 1.42 eV is obtained for the monolayer and multilayer, respectively. The energy loss function is dominated by features due to high density of states at both the valence and conduction band edges, and the difference in analyzing band gap with respect to ZnO is highlighted. Crystalline 1T ReS2 forms two dimensional chains like superstructure due to the clustering between four Re atoms. The results demonstrate the power of HREELS technique as a nanoscale optical absorption spectroscopy tool.

Hydrogenated graphene on Ir(111): A high-resolution electron energy loss spectroscopy study of the vibrational spectrum

Hydrogen atom adsorption on high-quality graphene on Ir(111) [gr/Ir(111)] is investigated using high-resolution electron energy loss spectroscopy. The evolution of the vibrational spectrum, up to 400 meV, of gr/Ir(111) upon increasing hydrogen atom exposures is measured. The two dominant binding configurations of atomic hydrogen are identified as (1) graphanelike hydrogen clusters on the parts of the graphene more strongly interacting with the Ir(111) surface and (2) dimers bound more weakly to the freestanding parts of the graphene. The graphanelike surface structures lead to increased corrugation of the graphene sheet, yielding graphane-related phonon components. Additionally, a recent theoretical prediction of the existence of a bending character for a LO/TO graphane chair phonon mode is experimentally verified. No clear evidence was found for hydrogen bound on both sides of a high-quality graphene sheet and phonon features strongly suggest interactions between graphanelike hydrogen clusters and Ir atoms in the substrate.

Reaction pathways of furfural, furfuryl alcohol and 2-methylfuran on Cu(111) and NiCu bimetallic surfaces

Hydrodeoxygenation (HDO) is an important reaction for converting biomass-derived furfural to value-added 2-methylfuran, which is a promising fuel additive. In this work, the HDO of furfural to produce 2-methylfuran occurred on the NiCu bimetallic surfaces prepared on either Ni(111) or Cu(111). The reaction pathways of furfural were investigated on Cu(111) and Ni/Cu(111) surfaces using density functional theory (DFT) calculations, temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) experiments. These studies provided mechanistic insights into the effects of bimetallic formation on enhancing the HDO activity. Specifically, furfural weakly adsorbed on Cu(111), while it strongly adsorbed on Ni/Cu(111) through an η2(C,O) configuration, which led to the HDO of furfural on Ni/Cu(111). The ability to dissociate H2 on Ni/Cu(111) is also an important factor for enhancing the HDO activity over Cu(111).

Absolute vibrational cross sections for 1-19 eV electron scattering from condensed tetrahydrofuran (THF).

Absolute cross sections (CSs) for vibrational excitation by 1-19 eV electrons impacting on condensed tetrahydrofuran (THF) were measured with a high-resolution electron energy loss spectrometer. Experiments were performed under ultra-high vacuum (3 × 10(-11) Torr) at a temperature of about 20 K. The magnitudes of the vibrational CSs lie within the 10(-17) cm(2) range. Features observed near 4.5, 9.5, and 12.5 eV in the incident energy dependence of the CSs were compared to the results of theoretical calculations and other experiments on gas and solid-phase THF. These three resonances are attributed to the formation of shape or core-excited shape resonances. Another maximum observed around 2.5 eV is not found in the calculations but has been observed in gas-phase studies; it is attributed to the formation of a shape resonance.

Influence of electronic structure, plasmon-phonon and plasmon-polariton excitations on anomalously low heat conductivity in TiAlN/Ag nanoscale multilayer coatings

TiAlN/Ag multilayer coatings with different number of bilayers and thicknesses of individual layer were fabricated by DC magnetron sputtering. Thermal conductivity and electronic structure features were measured on a set of samples with different number of bilayers and individual layer thickness from a 5–150 nm for total coating thickness of ∼0.5 or 1 μm. It was found that thermal conductivity of the multilayer insulator/metal structure becomes lower than for monolithic TiAlN coatings with the same thickness when Ag layers are thinner than 25 nm. The physical nature of thermal barrier properties of the nanolaminate TiAlN/Ag films is studied using X-ray photoelectrons (XPS) and high-resolution electron-energy loss (HREELS) spectroscopies. Based on these results, a physical model for the anomalous decrease in heat conductivity for nanoscale multilayers was developed on base of analysis of plasmon and lattice vibrations inside the layers and at the interfaces. Anomalously low heat transfer is attributed to attenuation of longitudinal acoustic phonons vibrations, decreasing of plasmon-polariton penetration on interfaces, and plasmon states splitting in Ag nanolayers due to confinement effects at multilayer coatings nanostructuring.

Transmission electron microscopy investigation of oxidation of (110)NiAl single crystal with wedge-shaped profile

Low energy electron diffraction (LEED), high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray (EDX), and electron energy loss spectroscopy (EELS) investigations of oxidation processes in (110)NiAl single crystal of wedge like shape, i.e., on the sample’s areas of different thickness, were carried out. It was found that in the result of several cycles of ion etching, annealing and oxidation the upper layer of (110)NiAl is enriched with Ni. With the increase of Ni concentration from 50 to 100 at. %, the stoichiometry of the near surface area changes and the new phases of Ni3Al and Ni with Al doping are formed one after another. Up to Ni content of 75 at. % the defects concentration in the near-surface area increases and above 75 at. % it drops again. This leads to the change in orientation and azimuth direction of aluminum oxide (alumina). By varying the conditions of γ-Al2O3 epitaxial growth on (110)NiAl with (100), (110), and (111) orientations, we found that this oxide can be grown with different azimuthal directions, for example [440](111)γ-Al2O3 ∥ [002](110)NiAl and [440](111)γ-Al2O3 ∥ [022](110)NiAl.

Metallosupramolecular Assembly of Cr and p-Terphenyldinitrile by Dissociation of Metal Carbonyls on Au(111)

Metal carbonyl complexes have been widely investigated for use in heterogeneous catalysis as a method for the formation of metal clusters. However, their use as a metal source for metallosupramolecular assemblies has not been widely considered. In this study, the combination of Cr(CO)6 with p-terphenyldinitrile on Au(111) creates ordered 1D chains, which are thermally stable up to 423 K. The bond between the nitrile linkers and metal nodes undergoes a unique π-interaction, which thus far has been observed only in a small set of organometallic complexes. The structure and composition of the metal–organic coordination network have been elucidated and characterized using scanning tunnelling microscopy, high-resolution electron energy loss spectroscopy, and density functional theory calculations.

The growth and thermal properties of Au deposited on Rh(111): formation of an ordered surface alloy.

Scanning tunnelling microscopy (STM), low energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS) were applied for studying Au deposited on the Rh(111) surface. Both the deposition of Au at different substrate temperatures (400-800 K) and the effect of annealing Au deposited at 500 K were investigated. Gold deposition at 500 K, investigated by STM and LEIS methods, revealed that up to half monolayer Au the system exhibits clearly layer-by layer growth; however, above this coverage a slight deviation was identified, mainly due to kinetic and morphological effects. A continuous cover layer of Au was formed only above ∼2.5 monolayers (ML). Below this coverage, the pseudomorphic character of the Au overlayer was clearly proven by STM, but this feature disappears at 4 ML coverage. A moderate (5-10%) surface mixing of the two metals was observed only above 600 K, for both annealing the Au layer formed at lower temperatures and performing the deposition at elevated temperatures. Above 600 K a clear step-flow growth mechanism was verified. Depending on the Au coverage, a more extended mixing of the top layer and the sublayer was observed at even higher temperatures. In this case, nano-range ordering of the alloyed layer was detected by STM, where the lateral extension of the uniform commensurate (2 × 1) domains was around 4 × 4 nm2. In this case, the local intralayer mixing of Rh and Au can locally reach a value of 50%. The proposed structural model for the (2 × 1) alloy phase was also corroborated by HREELS investigations on CO adsorption.

Graphite-to-diamond (&lt;SUP align="right"&gt;13&lt;/SUP&gt;C) direct transition in a diamond anvil high-pressure cell

13C-graphite treated in a diamond anvil high-pressure cell was studied by high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and electron energy loss spectroscopy. It was found that 13C-diamond and lonsdaleite originate after the treatment under high pressure and shear deformation.

Observation of nanotwinning and room temperature ferromagnetism in sub-5 nm BiFeO3 nanoparticles: a combined experimental and theoretical study.

Particle size significantly affects the properties and therefore the potential applications of multiferroics. However, is there special particle size effect in BiFeO3, which has a spiral modulated spin structure? This is still under investigation for sub-5 nm BiFeO3. In this report, the structural, electronic and magnetic properties are investigated for chemically synthesized BiFeO3 nanoparticles with an average size of 3 nm. We observed nanotwinning features in the specific size regime of the nanoparticles (2-4 nm). A weak Bi-O-Fe coordination and weak covalent nature has been observed in the nanoparticles through high-resolution electron energy loss spectroscopy and theoretical analysis, confirming that BiFeO3 nanoparticles a retain rudimentary R3c phase even at sub-5 nm dimensions. The R3c phase of sub-5 nm BiFeO3 nanoparticles has also been confirmed using Raman spectroscopy and Raman mapping of the vibrational modes. The nanoparticles display cluster spin glass, room temperature ferromagnetism, and a metamictization-davidite phase. The observation of weak magnetic entropy features confirmed the presence of a weak correlation between the magnetic and ferroelectric components. To support our experimental observations, we have simulated a sub-5 nm BiFeO3 nanocluster. Using density functional theory, the ferromagnetic ground state and the presence of a weak covalent nature in the nanocluster is established considering the first Brillouin zone, thus confirming our experimental results. Finding of new physicochemical features in sub-5 nm BiFeO3 would be beneficial for the understanding of the fundamental physical and chemical science as well as potential device development.

Analyzing Dirac Cone and Phonon Dispersion in Highly Oriented Nanocrystalline Graphene.

Chemical vapor deposition (CVD) is one of the most promising growth techniques to scale up the production of monolayer graphene. At present, there are intense efforts to control the orientation of graphene grains during CVD, motivated by the fact that there is a higher probability for oriented grains to achieve seamless merging, forming a large single crystal. However, it is still challenging to produce single-crystal graphene with no grain boundaries over macroscopic length scales, especially when the nucleation density of graphene nuclei is high. Nonetheless, nanocrystalline graphene with highly oriented grains may exhibit single-crystal-like properties. Herein, we investigate the spectroscopic signatures of graphene film containing highly oriented, nanosized grains (20-150 nm) using angle-resolved photoemission spectroscopy (ARPES) and high-resolution electron energy loss spectroscopy (HREELS). The robustness of the Dirac cone, as well as dispersion of its phonons, as a function of graphene's grain size and before and after film coalescence, was investigated. In view of the sensitivity of atomically thin graphene to atmospheric adsorbates and intercalants, ARPES and HREELS were also used to monitor the changes in spectroscopic signatures of the graphene film following exposure to the ambient atmosphere.

Electron Processing at 50 eV of Terphenylthiol Self-Assembled Monolayers: Contributions of Primary and Secondary Electrons.

Aromatic self-assembled monolayers (SAMs) can serve as platforms for development of supramolecular assemblies driven by surface templates. For many applications, electron processing is used to locally reinforce the layer. To achieve better control of the irradiation step, chemical transformations induced by electron impact at 50 eV of terphenylthiol SAMs are studied, with these SAMs serving as model aromatic SAMs. High-resolution electron energy loss spectroscopy (HREELS) and electron-stimulated desorption (ESD) of neutral fragment measurements are combined to investigate electron-induced chemical transformation of the layer. The decrease of the CH stretching HREELS signature is mainly attributed to dehydrogenation, without a noticeable hybridization change of the hydrogenated carbon centers. Its evolution as a function of the irradiation dose gives an estimate of the effective hydrogen content loss cross-section, σ = 2.7-4.7 × 10(-17) cm(2). Electron impact ionization is the major primary mechanism involved, with the impact electronic excitation contributing only marginally. Therefore, special attention is given to the contribution of the low-energy secondary electrons to the induced chemistry. The effective cross-section related to dissociative secondary electron attachment at 6 eV is estimated to be 1 order of magnitude smaller. The 1 eV electrons do not induce significant chemical modification for a 2.5 mC cm(-2) dose, excluding their contribution.

Fabrication of graphitic layers in diamond using FIB implantation and high pressure high temperature annealing

We have used high pressure high temperature annealing (HPHT) for graphitisation of implanted layers in diamond created by 30keV Ga+ focused ion beam. Electron microscopy has been used to investigate the implanted layers. It has been revealed that, unlike annealing at vacuum pressure, the graphitization during HPHT annealing occurred through epitaxial growth of graphite (002) planes parallel to (111) diamond planes. High quality of graphite was confirmed by high resolution electron microscopy and electron energy loss spectroscopy.