Combination Of Auger Electron Spectroscopy Research Articles

AES and ToF-SIMS combination for single cell chemical imaging of gold nanoparticle-labeled Escherichia coli.

A chemical fingerprint of the Escherichia coli cell surface labeled by gelatin coated gold nanoparticles was obtained by combining Auger Electron Spectroscopy (AES) for single cell level chemical images, and Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) Tandem MS for unambiguous molecular identification of co-localized species.

Femtosecond Charge Transfer Dynamics in Monomolecular Films in the Context of Molecular Electronics.

A key issue of molecular electronics (ME) is the correlation between the molecular structure and the charge transport properties of the molecular framework. Accordingly, a variety of model and potentially useful molecular systems are designed, to prove a particular function or correlation or to build a prototype device. These studies usually involve the measurements of the static electric conductance properties of individual molecules and their assembles on solid supports. At the same time, information about the dynamics of the charge transport (CT) and transfer in such systems, complementary in the context of ME and of a scientific value on its own, is quite scarce. Among other means, this drawback can be resolved by resonant Auger electron spectroscopy (RAES) in combination with core hole clock (CHC) approach, as described in this Account. The RAES-CHC scheme was applied to a variety of aliphatic and aromatic self-assembled monolayers (SAMs), adsorbed on Au(111) over the thiolate and selenolate docking groups. Electron transfer (ET) from a suitable terminal tail group to the substrate, across the molecular framework, was monitored, triggered by resonant excitation of this group (nitrile in most cases) by narrow-band X-ray radiation. This resulted in the quantitative data for the characteristic ET time, τET, in the femtosecond domain, with the time window ranging from ∼1 fs to ∼120 fs. The derived τET exhibit an exponential dependence on the molecular length, mimicking the behavior of the static conductance and suggesting a common physical basis behind the static CT and ET dynamics. The dynamic decay factors, βET, for the alkyl, oligophenyl, and acene molecular "wires" correlate well with the analogous parameters for the static CT. Both τET and βET values exhibit a distinct dependence on the character of the involved molecular orbital (MO), demonstrating that the efficiency and rate of the CT in molecular assemblies can be controlled by resonant injection of the charge carriers into specific MOs. This dependence as well as a lack of correlation between the molecular tilt and τET represent strong arguments in favor of the generally accepted model of CT across the molecular framework ("through-bond") in contrast to "through-space" tunneling. Comparison of the SAMs with thiolate and selenolate docking groups suggests that the use of selenolate instead of thiolate does not give any gain in terms of ET dynamics or molecular conductance. Whereas a certain difference in the efficiency of the electronic coupling of thiolate and selenolate to the substrate cannot be completely excluded, this difference is certainly too small to affect the performance of the entire molecule to a noticeable extent. The efficient electronic coupling of the thiolate docking group to the substrate was verified and the decoupling of the electronic subsystems of the substrate and π-conjugated segment by introduction of methylene group into the backbone was demonstrated. No correlation between the molecular dipole or fluorine substitution pattern (at the side positions) and the ET efficiency was recorded. Several representative examples for the resonantly addressable tail groups are given, and perspectives for future research in the context of ET dynamics in molecular assemblies are discussed.

Experimental measurement of diffusion coefficients and assessment of diffusion mobilities in HCP Mg–Li–Al alloys

An atomic mobility database was established for the ternary HCP Mg–Li–Al system as a part of an ongoing effort to enable rapid development of novel lightweight Mg alloys. Three sets of three diffusion couples were assembled and annealed at temperatures ranging from 400 to 500 °C. Li concentration profiles were measured using a combination of Auger electron spectroscopy (AES) and inductively coupled plasma optical emission spectrometry (ICP-OES), while Al composition profiles were acquired using electron probe microanalysis (EPMA). The forward-simulation analysis (FSA) was employed to extract both interdiffusion and impurity diffusion coefficients from the collected experimental composition profiles. These measured diffusivity data were used to assess and iteratively optimize mobility parameters for the Mg–Li–Al system using the diffusion module within the Thermo-Calc Software package (DICTRA). The reliability of the assessed mobility parameters was further confirmed by two validation diffusion couples that were annealed at 425 and 475 °C for 96 and 48 h, respectively. It was observed that additions of Li increased the diffusivity of Al in HCP Mg, whereas additions of Al decreased the diffusivity of Li in HCP Mg.

Surface Characterization of MoS2 Atomic Layers Mechanically Exfoliated on a Si Substrate.

Mo disulfide overlayers with the thickness exceeding 1.77 nm were obtained on Si substrates through mechanical exfoliation. The resulting Mo disulfide flakes were then analyzed ex situ using combination of Auger electron spectroscopy (AES), elastic-peak electron spectroscopy (EPES) and scanning electron microscopy (SEM) in order to characterize their surface chemical composition, electron transport phenomena and surface morphology. Prior to EPES measurements, the Mo disulfide surface was sputter-cleaned and amorphized by 3 kV argon ions, and the resulting S/Mo atomic ratio varied in the range 1.80–1.88, as found from AES measurements. The SEM images revealed single crystalline small-area (up to 15 μm in lateral size) Mo disulfide flakes having polygonal or near-triangular shapes. Such irregular-edged flakes exhibited high crystal quality and thickness uniformity. The inelastic mean free path (IMFP), characterizing electron transport, was evaluated from the relative EPES using Au reference material for electron energies E = 0.5–2 keV. Experimental IMFPs, λ, determined for the AES-measured surface compositions were approximated by the simple function λ = kEp, where k = 0.0289 and p = 0.946 were fitted parameters. Additionally, these IMFPs were compared with IMFPs resulting from the two methods: (i) present calculations based on the formalism of the Oswald et al. model; (ii) the predictive equation of Tanuma et al. (TPP-2M) for the measured Mo0.293S0.551C0.156 surface composition (S/Mo = 1.88), and also for stoichiometric MoS2 composition. The fitted function was found to be reasonably consistent with the measured, calculated and predicted IMFPs. We concluded that the measured IMFP value at 0.5 keV was only slightly affected by residual carbon contamination at the Mo disulfide surface.

Oxygen dissolution and surface oxide reconstructions on Nb(100)

The ever-present native oxide layer on niobium plays a fundamental role in the performance of Nb superconducting radio frequency (SRF) cavities for particle accelerators and light sources. Using Nb(111) and Nb(100) as model systems, oxygen dissolution and surface structural evolution as a function of thermal treatments in ultrahigh vacuum (UHV) were studied using combined Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM), providing novel, real space information regarding the complex evolution of the Nb surface oxide. The surface crystallographic orientation was shown to influence oxide surface structure; Nb(111) displayed disordered surface oxide domains while Nb(100) was comprised of well-ordered, (n × 1)-O domains. The temperature-dependent dissolution of the native oxide layer on Nb(100) was characterized to ascertain the energetics for oxygen dissolution, and evolution of the surface oxide superlattice structure following thermal annealing was observed with STM. By understanding the kinetic behavior of oxygen dissolution during thermal annealing and subsequent structural evolution of the Nb(n × 1)-O superlattice on Nb(100), a more thorough understanding of the complex interactions driving chemical composition and cavity surface structure under Nb SRF cavity processing and operating conditions may be understood.

Pyridine as a Resonantly Addressable Group to Study Electron-Transfer Dynamics in Self-Assembled Monolayers

Resonant Auger electron spectroscopy (RAES) in combination with the core-hole clock (CHC) approach represents a unique tool to study femtosecond electron transfer (ET) dynamics in molecular systems and self-assembled monolayers (SAMs) in particular. A most promising experimental strategy to apply this approach to SAMs involves the decoration of the target films with a group, which can be resonantly excited by narrow-band X-rays. Using a series of well-defined SAMs with a terminal pyridine moiety, we demonstrate here that pyridine can be used as such a group, as an alternative to the nitrile group, which has been utilized for this purpose in the past. For these SAMs, we evaluate the characteristic time for ET from the terminal nitrogen atom of the pyridine group through the molecular framework to the substrate and discuss the results in terms of conjugation and coupling of the relevant electronic states in the molecules. The obtained ET times correlate well with the literature values obtained for the analo...

Surface characterization of low-temperature grown yttrium oxide

The step-by-step growth of yttrium oxide layer was controlled in situ using X-ray photoelectron spectroscopy (XPS). The O/Y atomic concentration (AC) ratio in the surface layer of finally oxidized Y substrate was found to be equal to 1.48. The as-grown yttrium oxide layers were then analyzed ex situ using combination of Auger electron spectroscopy (AES), elastic-peak electron spectroscopy (EPES) and scanning electron microscopy (SEM) in order to characterize their surface chemical composition, electron transport phenomena and surface morphology. Prior to EPES measurements, the Y oxide surface was pre-sputtered by 3 kV argon ions, and the resulting AES-derived composition was found to be Y0.383O0.465C0.152 (O/Y AC ratio of 1.21). The SEM images revealed different surface morphology of sample before and after Ar sputtering. The oxide precipitates were observed on the top of un-sputtered Y oxide layer, whereas the oxide growth at the Ar ion-sputtered surface proceeded along defects lines normal to the layer plane. The inelastic mean free path (IMFP) characterizing electron transport was evaluated as a function of energy in the range of 0.5–2 keV from the EPES method. Two reference materials (Ni and Au) were used in these measurements. Experimental IMFPs determined for the Y0.383O0.465C0.152 and Y2O3 surface compositions, λ, were uncorrected for surface excitations and approximated by the simple function λ = kEp at electron energies E between 500 eV and 2000 eV, where k and p were fitted parameters. These values were also compared with IMFPs resulting from the TPP–2 M predictive equation for both oxide compositions. The fitted functions were found to be reasonably consistent with the measured and predicted IMFPs. In both cases, the average value of the mean percentage deviation from the fits varied between 5% and 37%. The IMFPs measured for Y0.383O0.465C0.152 surface composition were found to be similar to the IMFPs for Y2O3.

Quantitative evaluation of sputtering induced surface roughness and its influence on AES depth profiles of polycrystalline Ni/Cu multilayer thin films

The polycrystalline Ni/Cu multilayer thin films consisting of 8 alternating layers of Ni and Cu were deposited on a SiO2 substrate by means of electron beam evaporation in a high vacuum. Concentration-depth profiles of the as-deposited multilayered Ni/Cu thin films were determined with Auger electron spectroscopy (AES) in combination with Ar+ ion sputtering, under various bombardment conditions with the samples been stationary as well as rotating in some cases. The Mixing-Roughness-Information depth (MRI) model used for the fittings of the concentration-depth profiles accounts for the interface broadening of the experimental depth profiling. The interface broadening incorporates the effects of atomic mixing, surface roughness and information depth of the Auger electrons. The roughness values extracted from the MRI model fitting of the depth profiling data agrees well with those measured by atomic force microscopy (AFM). The ion sputtering induced surface roughness during the depth profiling was accordingly quantitatively evaluated from the fitted MRI parameters with sample rotation and stationary conditions. The depth resolutions of the AES depth profiles were derived directly from the values determined by the fitting parameters in the MRI model.

High-resolution characterization of hexagonal boron nitride coatings exposed to aqueous and air oxidative environments

Hexagonal boron nitride (h-BN) is believed to offer better passivation to metallic surfaces than graphene owing to its insulating nature, which facilitates blocking the flow of electrons, thereby preventing the occurrence of galvanic reactions. Nevertheless, this may not be the case when an h-BN-protected material is exposed to aqueous environments. In this work, we analyzed the stability of mono and multilayer h-BN stacks exposed to H2O2 and atmospheric conditions. Our experiments revealed that monolayer h-BN is as inefficient as graphene as a protective coating when exposed to H2O2. Multilayer h-BN offered a good degree of protection. Monolayer h-BN was found to be ineffective in an air atmosphere as well. Even a 10–15 layers-thick h-BN stack could not completely protect the surface of the metal under consideration. By combining Auger electron spectroscopy and secondary ion mass spectrometry techniques, we observed that oxygen could diffuse through the grain boundaries of the h-BN stack to reach the metallic substrate. Fortunately, because of the diffusive nature of the process, the oxidized area did not increase with time once a saturated state was reached. This makes multilayer (not monolayer) h-BN a suitable long-term oxidation barrier. Oxygen infiltration could not be observed by X-ray photoelectron spectroscopy. This technique cannot assess the chemical composition of the deeper layers of a material. Hence, the previous reports, which relied on XPS to analyze the passivating properties of h-BN and graphene, may have ignored some important subsurface phenomena. The results obtained in this study provide new insights into the passivating properties of mono and multilayer h-BN in aqueous media and the degradation kinetics of h-BN-coated metals exposed to an air environment.

Circular Dichroism in Cu Resonant Auger Electron Diffraction

Abstract Upon a core level excitation by circularly polarized light (CPL), the angular momentum of light, i.e. helicity, is transferred to the emitted photoelectron. This phenomenon can be confirmed by the parallax shift measurement of the forward focusing peak (FFP) direction in a stereograph of the atomic arrangement. The angular momentum of the emitted photoelectron is the sum of CPL helicity and the magnetic quantum number (MQN) of the initial state that define the quantum number of the core hole final state. The core hole may decay via Auger electron emission, where in this two electron process the angular momentum has to be conserved as well. Starting from a given core hole, different Auger decay channels with different final state energies and angular momenta of the emitted Auger electrons may be populated. Here we report the observation and formulation of the angular momentum transfer of light to Auger electrons, instead of photoelectrons. We measured photoelectron and Auger electron intensity angular distributions from Cu(111) and Cu(001) surfaces as a function of photon energy and photoelectron kinetic energy. By combining Auger electron spectroscopy with the FFP shift measurements at absorption threshold, element- and MQN-specific hole states can be generated in the valence band.

Large conductance switching nonvolatile memories based on p-ZnS nanoribbon/n-Si heterojunction

A new structure nonvolatile memory, with large conductance switching (on/off ratio > 106), has been constructed from a p-ZnS nanoribbon (NR)/n-Si heterojunction. The p-type ZnS NRs were obtained using cuprous sulfide (Cu2S) as the Cu dopant. Excellent Ohmic contact to p-type ZnS NRs was achieved by using a Cu/Au bilayer electrode, which contributed to the formation of the thin Cu2S interfacial layer between the electrode and the NR, as confirmed from the combined X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analysis. These devices exhibit a stable and reproducible hysteresis and excellent memory characteristics with a long retention time of 1 × 105 s and good endurance >6 months at room temperature. The electrical switching behavior could be attributed to the charge trapping and detrapping in interface states at the junction. The approach could potentially provide a viable way to create new advanced nonvolatile memory devices with simple structure and to fabricate large conductance switching and large-capacity data storage.

Segregation and morphology on the surface of ferritic stainless steel (0 0 1)

The temperature dependence of the segregation and morphology of ferritic steel (001) surfaces has been examined by a combination of Auger electron spectroscopy, low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and low-energy electron loss spectroscopy. Upon annealing ferritic steel at 500°C, the topmost layer was observed to be mainly composed of Fe–Cr alloy. Oxygen segregation was also detected locally in the STM images. LEED showed a (1×1) pattern and a weak (√5×√5)R27° reconstruction corresponding to Fe and Cr4O5, respectively. Upon annealing at 600°C, carbon and chromium co-segregated to the surface, forming two different regions composed of CrC and Cr-based steel, while the Cr4O5 domains disappeared. Upon annealing at 700°C, nitrogen segregated to the surface, and the topmost layer was observed to be mainly composed of CrN domains with local CrC domains.

Spin-polarized scanning tunneling microscopy and spectroscopy study of chromium on aCr(001) surface

Several tens of chromium layers were deposited at250 °C on a Cr(001) surface and investigated by spin-polarized scanning tunneling microscopy(SP-STM), Auger electron spectroscopy (AES) and scanning tunneling spectroscopy (STS).Chromium is found to grow with a mound-like morphology resulting from the stacking ofseveral monolayers which do not uniformly cover the whole surface of the substrate. Theterminal plane consists of an irregular array of Cr islands with lateral sizes smaller than20 × 20 nm2. Combined AES and STS measurements reveal the presence of a significant amount ofsegregants prior to and after deposition. A detailed investigation of the surface shows thatit consists of two types of patches. Thanks to STS measurements, the two types of areahave been identified as being either chromium pure or segregant rich. SP-STM experimentshave evidenced that the antiferromagnetic layer coupling remains in the chromium moundsafter deposition and is not significantly affected by the presence of the segregants.

On the modeling of electrochemical systems with simultaneous gas evolution. Case study: The zinc deposition mechanism

Electrodeposition of zinc with simultaneous hydrogen evolution in an aqueous acidic sulphate solution is investigated in an Inverted Rotating Disk Electrode (IRDE) reactor. A simulation tool, based on the MITReM (Multi-Ion Transport and Reaction Model) model is used to describe the electrochemical behavior. This model can account for mass transport by convection, diffusion and migration and for the presence of homogeneous reactions. The electron transfer at the electrode is described by a Butler–Volmer relation. A reaction mechanism is proposed from surface analysis of the deposited zinc layer. Field Emission Auger Electron Spectroscopy (FE-AES) in combination with Factor Analysis (FA) is used to determine the compositional structure of the formed zinc layer. The model parameters such as diffusion coefficients, rate constants and transfer coefficients are determined by comparison of experimental and simulated polarization curves and current efficiency profiles for different rotation speeds of the IRDE and concentrations of supporting electrolyte. The importance of the homogeneous reactions is shown. The influence of gas bubble evolution (induced micro-stirring effects, reduced electrolyte conductivity and surface blockage) on the electrodeposition process is elucidated.

Ferroelectric polarization dependent interactions at Pd–LiNbO3(0001) interfaces

A combination of Auger electron spectroscopy and temperature-programed desorption was used to characterize the growth and interaction of Pd films with positively and negatively terminated ferroelectric LiNbO3(0001) surfaces. The growth mode of vapor-deposited Pd layers at 300K was found to be dependent on the direction of the ferroelectric polarization with layer-by-layer growth occurring on the negative (c−) surface and particle formation occurring on the positive (c+) surface. The Pd metal layers were also found to be more thermally stable on the c− surface relative to the c+ surface. These results provide another example of how the polarization orientation in ferroelectric materials affects adsorption and reaction on their exposed surfaces.

Interdiffusion and stress development in Ni-Cu thin film diffusion couples

Thin film Ni-Cu diffusion couples (individual layer thicknesses: 50 mn) have been prepared by direct-current magnetron sputtering on silicon substrates. The microstructural development and the stress evolution during diffusion annealing have been investigated employing ex-situ and in-situ X-ray diffraction, transmission electron microscopy and Auger-electron spectroscopy (in combination with sputter-depth profiling). Annealing at relatively low temperatures (175 degrees C to 350 degrees C) for durations up to about 100 hours results in considerable diffusional intermixing. In addition to thermal stresses due to mismatch of the coefficients of thermal expansion of layers and substrate, tensile stress contributions in the sublayers arise during diffusion anneals. The obtained stress data are discussed in terms of possible mechanisms of stress generation.

Activity Enhancement of Dense Strontium-Doped Lanthanum Manganite Thin Films under Cathodic Polarization: A Combined AES and XPS Study

cathodes used in solid oxide fuel cells have exhibited enhanced catalytic activity after polarization; however, the origin of this enhancement is not understood. In this study, dense thin-film microelectrodes fabricated by sputtering were cathodically or anodically polarized, and electrochemical impedance spectroscopy was used to examine the changes in impedance for oxygen reduction reaction (ORR) before and after polarization. Scanning electron and atomic force microscopy revealed surface microstructural changes after polarization. Surface compositions of thin films and microelectrodes were analyzed by auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), respectively. Annealed, quenched were found to have surfaces significantly enriched in La. Cathodic polarization considerably decreased electrode impedance, which was associated with reduced surface La and increased surface Sr and Mn fractions on the cathode adjacent to the three-phase boundaries. Surface particles of appeared on the surface regions, where changes in the surface chemical composition of microelectrodes were detected. Acid-etching similarly lowered surface La contents. However, only cracks were found and no change in the surface composition was detected after anodic polarization. The mechanism of ORR activity enhancement induced by cathodic and anodic polarization was discussed.

Interdiffusion, phase formation, and stress development in Cu–Pd thin-film diffusion couples: Interface thermodynamics and mechanisms

Cu–Pd thin-film diffusion couples (individual layer thicknesses of 50nm) have been prepared by dc-magnetron sputtering on silicon substrates coated with a thin amorphous Si3N4 layer. Stress evolution, microstructural development, and phase formation during interdiffusion have been investigated employing Auger-electron spectroscopy (in combination with sputter-depth profiling), x-ray diffraction, wafer curvature measurements and transmission electron microscopy. Upon annealing at relatively low temperatures (175–250°C) for durations up to 10h, considerable diffusional intermixing occurs. Interdiffusion is accompanied by sequential formation of new phases. First, Cu3Pd forms; subsequently, CuPd forms and grows at the expense of Cu3Pd, which has been interpreted as a consequence of interface thermodynamics. Annealing leads to a slight sharpening of the pre-existing {111}-fiber textures and a little increase in the average grain size. A combination of ex situ (x-ray diffraction) and in situ (wafer curvature) stress measurements revealed that tensile stresses are generated during annealing. The obtained stress results have been discussed in the light of possible mechanisms of stress generation, and the dominant sources of stress buildup and relaxation have been identified.

Interdiffusion and stress development in Cu–Pd thin film diffusion couples

Thin film Cu–Pd diffusion couples (individual sublayer thicknesses: 50 nm) were prepared by direct current magnetron sputtering on silicon substrates coated with amorphous diffusion/reaction barriers (Si 3N 4 on top of SiO 2). The microstructural variations, phase formation and stress evolution during diffusion annealing were investigated employing Auger-electron spectroscopy in combination with sputter-depth profiling, transmission electron microscopy and in-situ X-ray diffraction. Upon annealing at relatively low temperatures (185 °C) for durations up to 17 h, considerable diffusional intermixing occurs. Interdiffusion is accompanied by the formation of a compound phase (ordered Cu 3Pd (α′)). The stress changes in the bilayer system during heating and isothermal annealing were investigated employing in-situ X-ray diffraction stress measurements and were compared to corresponding results obtained for single layers of the components in the Cu–Pd bilayer system, produced under conditions identical to those employed for the sublayers of the diffusion couple. The specific residual stresses that emerge due to diffusion (and reactions) between the (sub)layers in the bilayer could then be identified by comparing the stress evolutions upon annealing of the single layers with those recorded for the sublayers of the bilayer.

Dependence of the properties of compositionally graded Pb(Zr ,Ti)O 3 ferroelectric films of the bottom electrode

Compositionally graded Pb(Zr ,Ti)O 3 thin films were prepared on the Pt(1 1 1)/Ti/SiO 2/Si, LNO/Si(1 0 0) and LNO/Pt(1 1 1)/Ti/SiO 2/Si substrates by a modified sol–gel method and rapid heat-treatment. The composition depth profile of a typical up-graded film was determined using a combination of auger electron spectroscopy and Ar-ion etching. The crystallographic orientation and the microstructure of the resulting graded PZT thin films on the different substrates were characterized by XRD. The dielectric and ferroelectric properties of the graded PZT films were discussed. The graded PZT films on LNO/Pt/Ti/SiO 2/Si and LNO/Si(1 0 0) substrates have larger dielectric constant and remnant polarizations compared to that grown on Pt/Ti/SiO 2/Si substrates.