Ultrahigh Vacuum Atmosphere Research Articles

Atomic-scale observation of strain-dependent reversible topotactic transition in La0.7Sr0.3MnOx films under an ultra-high vacuum environment

Reversible topotactic phase transition between perovskite (PV) and oxygen-vacancy-ordered brownmillerite (BM) structures provides an effective platform for realizing the control of physical properties in complex transition metal oxides. However, such reversibility always requires extreme external conditions, that is, a high temperature and vacuum environment during the PV-BM transition, while an oxidizing atmosphere and relatively low temperature vice versa. Here, we experimentally observe the reversible process in strained La0.7Sr0.3MnOx films at atomic scale by using in-situ heating aberration-corrected scanning transmission electron microscopy (STEM). Apart from the conventional reduction reaction of creating oxygen-vacancy-ordered frameworks after heating in the TEM, the inverse process of BM-to-PV transition is unexpectedly discovered under such an ultra-high vacuum atmosphere (∼10−9 Torr) at room temperature. Moreover, this abnormal behavior is strain-dependent. The large compressive strain is found to be detrimental to the inverse phase transition. The density functional theory (DFT) calculations show that the high oxygen affinity of La0.7Sr0.3MnO2.5 is responsible for the reversible transitions. Our findings provide a new insight into the redox reactions of manganite and might be further utilized for potential applications in solid fuel cells, oxygen sensors or resistive switching memories.

In-situ electron loss spectroscopy reveals surface dehydrogenation of hydrated ceria nanoparticles at elevated temperatures

Ceria (CeO2) exhibits high reversible oxygen storage capacity at intermediate temperatures (500–800 °C) related to an extraordinary and not fully understood reduction of its surfaces. We have investigated pristine and alcohol-dispersed commercially available ceria nanoparticles by in-situ scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) to examine the dynamic changes during the initial redox reaction process of ceria nanoparticles in an ultra-high vacuum atmosphere using an in-situ heating holder. High spatially resolved EELS was used to estimate the amounts of Ce3+ and Ce4+ in the nanoparticles as a function of temperature, based on the white-line ratios M5/M4 of the EELS spectra. The results show a nm-range thick surface layer rich in Ce3+ on pristine particles prior to heating. During heating, this oxidises to Ce4+. Heating in high vacuum should normally not lead to oxidation, but the observed results can be understood if the surface layer has an oxyhydroxide composition such as CeOOH, which by heating in the vacuum dehydrogenates and hence oxidises to CeO2, a process that requires diffusion of hydrogen only. This process occurred for all samples, but was more pronounced for the particles that were previously dispersed in ethanol. Thermogravimetric analysis (TGA) by heating the pristine powder in dry atmosphere yielded a considerable weight loss confirming the content of hydroxide and probably water in and on the CeO2 particles. The results suggest that CeO2 surfaces are reduced to a layer of oxyhydroxide by hydrogen-containing molecules like water vapour or alcohols.

(Invited) On-Surface Synthesis of Higher Acenes Using Precursors

Acenes are polycyclic aromatic hydrocarbons made up of linearly fused benzene rings. They are the smallest units of graphene and graphene nanoribbons, and have been extensively studied as organic semiconductor materials. Higher acenes are expected to have smaller band gaps required for practical use as organic electronic materials, but those with more than six benzene rings are poorly soluble in common organic solvents and unstable in aerobic condition to be synthesized and purified with traditional organic synthesis method. To overcome the obstacles, a precursor approach is a useful technique, where the soluble and stable precursors are purified enough and then converted to less-soluble and unstable target acenes in-situ.[1,2]Recently surface-assisted synthesis of higher acenes using precursors have attracted much attentions. Under ultra-high vacuum atmosphere, unstable acenes can be prepared from precursors by annealing, photoirradiation, or scanning tunneling microscopy (STM)-tip treatment on metal surface. We were successful to synthesize bis-diketone precursors of heptacene and nonacene. [3-6] The precursors were vacuum-deposited on Au(111) surface and converted to heptacene and nonacene by photoirradiation. Through combined STM, non-contact atomic force microscopy(nc-AFM) and scanning tunneling spectroscopy (STS) measurements, together with state-of-the-art first principles calculations, insight into the chemical and electronic structure of these elusive compounds was successful. It was revealed that nonacene has open-shell biradical structure in the ground state on Au(111). We were also successful to prepare tetraazaundecacene on Au(111) surface from bicyclo[2.2.2]octadiene (BCOD) precursor. [7] The tip-induced release of the protecting group can be employed to produce the targeted tetraazaundecacene species. The experimental frontier orbital gap, derived from the difference between PIR and NIR, is 1.35 eV. In this talk, the attraction and possibility of the “precursor approach” for the synthesis and application of acene compounds will be overviewed.References M. Suzuki, T. Aotake, Y. Yamaguchi, N. Noguchi, H. Nakano, K. Nakayama, H. Yamada, J. Photochem. Photobiol. C: Photochem. Rev. 2014, 18, 50-70.H. Yamada, D. Kuzuhara, M. Suzuki, H. Hayashi, N. Aratani, Bull. Chem. Soc. Jpn. 2020, 93, 1234-1267.J. I. Urgel, S. Mishra, H. Hayashi, J. Wilhelm, C. A. Pignedoli, M. Di Giovannantonio, R. Widmer, M. Yamashita, N. Hieda, P. Ruffieux, H. Yamada, R. Fasel, Nat. Commun. 2019, 10, 861–864.J. I. Urgel, H. Hayashi, M. Di Giovannantonio, C. A. Pignedoli, S. Mishra, O. Deniz, M. Yamashita, T. Dienel, P. Ruffieux, H. Yamada, R. Fasel, J. Am. Chem. Soc. 2017, 139, 11658–11661.J. I. Urgel, M. Di Giovannantonio, G. Gandus, Q. Chen, X. Liu, H. Hayashi, P. Ruffieux, S. Decurtins, A. Narita, D. Passerone, H. Yamada, S.-X. Liu, K. Müllen, C. A. Pignedoli, R. Fasel, ChemPhysChem 2019, 20, 2360-2366.C. G. Ayani, M. Pisarra, J. I. Urgel, J. J. Navarro, C. Díaz, H. Hayashi, H. Yamada, F. Calleja, R. Miranda, R. Fasel, F. Martín, A. L. Vázquez de Parga, Nanoscale Horiz. 2021, 6, 744-750.K. Eimre, J. I. Urgel, H. Hayashi, M. D. Giovannantonio, P. Ruffieux, S. Sato, S. Ohtomo, Y. S. Chan, N. Aratani, D. Passerone, O. Gröning, H. Yamada, R. Fasel, C. A. Pignedoli, Nat. Commun. 2021. in press.

Annealing Condition Effects on the Structural Properties of FePt Nanoparticles Embedded in MgO via Pulsed Laser Deposition.

FePt nanoparticles (NPs) were embedded into a single-crystal MgO host by pulsed laser deposition (PLD). It was found that its phase, microstructures and physical properties were strongly dependent on annealing conditions. Annealing induced a remarkable morphology variation in order to decrease its total free energy. H2/Ar (95% Ar + 5% H2) significantly improved the L10 ordering of FePt NPs, making magnetic coercivity reach 37 KOe at room temperature. However, the samples annealing at H2/Ar, O2, and vacuum all showed the presence of iron oxide even with the coverage of MgO. MgO matrix could restrain the particles’ coalescence effectively but can hardly avoid the oxidation of Fe since it is extremely sensitive to oxygen under the high-temperature annealing process. This study demonstrated that it is essential to anneal FePt in a high-purity reducing or ultra-high vacuum atmosphere in order to eliminate the influence of oxygen.

Wire-based additive manufacturing using an electron beam as heat source

Many standard welding processes, such as gas metal arc-, laser-, or electron-beam welding, can be used for additive manufacturing (AM) with only slight adaptions. Wire-based additive manufacturing provides an interesting alternative to powder-based processes due to their simplicity and comparatively high deposition rates. The use of an electron beam as heat source for AM offers unique possibilities for construction of components due to its inherent flexibility. It is possible to efficiently build bigger parts with comparably fine features and high complexity. Furthermore, additional working steps such as preheating, surface modification, welding, or heat treatments can be implemented into the additive manufacturing process and thereby alleviate the bottleneck of the evacuation of the vacuum chamber. Aside from this, the ultra high vacuum atmosphere can be beneficial, when working with reactive materials such as Ti or Mo. The intrinsic complexity of electron-beam additive manufacturing (EBAM) can make a stable and reproducible process control quite challenging. In this study, the influence of the main process parameters, such as heat input, energy distribution, wire feed, and their complex interactions are investigated. Based on single beads on a mild steel substrate using an unalloyed metal core wire (G4Si1), the correlation between the process parameters such as beam current, acceleration voltage, speed, wire feed rate and position, and the resulting bead geometry, height, width and penetration was studied. These findings were used to successfully establish a multi pass layout consisting of one to six beads next to each other and up to ten layers in height. For basic characterization, metallographic analysis as well as hardness measurements were performed.

Structural investigation of nanoporous alumina film with grazing incidence small angle X-ray scattering

Nanoporous alumina films (NPAF) have been elaborated by anodization of an aluminum film on silicon wafer. Ex situ structural characterization of the films has been achieved with grazing incidence small angle X-ray scattering under ultra high vacuum atmosphere and using a synchrotron source. The comparison of the experimental patterns with suitable modeling confirms the cylindrical geometry of the pores well as the good local hexagonal order. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Visualization of hydrogen gas evolution during deformation and fracture in SCM 440 steel with different tempering conditions

Abstract In the present study, the hydrogen gas evolution behavior was investigated in SCM 440 steel by using a hydrogen microprint technique (HMT) and a testing machine equipped with a quadrupole mass spectrometer (QMS) in a ultrahigh vacuum (UHV) atmosphere. SCM 440 steels prepared by varying the tempering temperature over the range 200–700 °C were evaluated in order to elucidate the relationship between the hydrogen gas evolution and the tempered microstructures in the deformation. Cathodic hydrogen charging was carried out with a current density of 100 A/m2 for 1 h at room temperature. For comparison, a tensile specimen was prepared without hydrogen charging. The HMT showed that silver particles, which are indicative of the hydrogen emission sites, were present mainly in the matrix as well as on the slip lines after the deformation. It is believed that the silver particles on the slip lines represent the effect of hydrogen transportation due to mobile dislocations. In addition, accumulation of silver particles around non-metallic inclusions such as Al2O3 was also identified. This tendency was observed for different tempering conditions. From the relationship between the stress–strain curves and the hydrogen evolution, determined by using QMS under a UHV atmosphere, it was found that the hydrogen gas evolution behavior varied with the deformation stage.

Water Desorption from Magnesium Oxide Films Fabricated by Chemical Vapor Deposition

Magnesium oxide (MgO) films are utilized for the anti-plasma sputtering coating with excellent ability of secondary electron emission in plasma display panels (PDP). These properties are degraded by the impurities adsorbed on the film surface. Therefore, we should obtain impurity-free surface during the PDP manufacturing process. We have synthesized whisker and continuous film types of metal oxide using a chemical vapor deposition (CVD) method operated under atmosphere. In this study, a temperature programmed desorption method has been applied to detect residual species adsorbed on the surface of the present films in the ultra-high vacuum atmosphere. The amount of water adsorption was determined by this method.

In situ characterisation of Sr2FeMoO6 films prepared by pulsed laser deposition

Thin epitaxial films of the ferromagnetic double perovskite Sr 2 FeMoO 6 (SFMO) have been prepared by pulsed laser deposition on SrTiO 3 (100) substrates under different oxygen pressures. The surface electronic structure of the as-deposited films has been analysed by XPS without exposure to the atmosphere. The films consist mainly of the c-axis oriented SFMO phase over a wide range of oxygen pressure from 10 -8 to 10 -4 mbar. However, the analysis of the films by XPS and XRD revealed the presence of different secondary phases depending on the preparation conditions. The films prepared under ultra-high vacuum conditions show the presence of metallic Fe. The films prepared under pure oxygen gas flow at pressures above 10 -6 mbar did not show any trace of metallic Fe, but Fe 3 O 4 phase. In these films XPS analysis revealed the formation of the SrFeO 3 phase (SFO), or another non reported Sr-containing phase, which is not observed by XRD. At higher oxygen pressures above 10 -4 mbar there is a clear formation of SrMoO 4 phase (SMO). In conclusion, a ultra-high vacuum atmosphere favours the formation of metallic Fe, whereas the increase in oxygen pressure during deposition induces the decomposition of the SFMO phase to SFO and SMO phases. The conditions for the preparation of pure SFMO phase have not been attained so far, which would lie within a very narrow oxygen pressure window. Our best films (with no any other ferromagnetic phase than SFMO) show a saturation magnetisation of about 1 μ B /f.u. at 10 K, and T c close to 200 K. The resistivity of the films has a metallic-like behaviour and remains almost constant in the temperature range from 10 to 350 K to about 5 to 8 10 -4 Ω cm. It was measured a magnetoresistance of about -10% at 10 T and 10 K, which coincides with the values reported in the literature for SFMO films.

Investigation of the bonding strength and interface current of p-Si/n-GaAs wafers bonded by surface activated bonding at room temperature

Equivalent bulk strength of the interface between p-Si/n-GaAs bonded through the surface activated bonding (SAB) method is found. The interface current was extensively investigated. Nonideal behavior of the pn junction current is found to be due to the tunneling current between the conduction band and valence band across the transition region associated with band gap states. Interface current decreases with increasing sputtering time and energy and vice versa. Irradiation time and energy dependent behavior indicates that the accumulation of radiation induced defects associated with the doping controls the interface current of p-Si/n-GaAs. Moreover, strong impact of the exposure to an ultrahigh vacuum atmosphere of the activated surfaces on the interface current of p-Si/p-Si is found. Finally it can be suggested that a laser diode can be fabricated by the bonding between p-Si and n-GaAs through the SAB method, because of the achievement of equivalent bulk strength of the interface.

High density molecule-like species formed in surface layers of C 8K

The surface of C 8K is very active at room temperature and it is easily deteriorated even under ultrahigh vacuum atmosphere. Freshly cleaved surfaces of C 8K were exposed to H 2, H 2O or O 2 gas systematically, and investigated with low-energy electron energy loss spectroscopy and Auger electron spectroscopy. It is proved that at room temperature the C 8K surface reacts strongly to oxygen, and a high density of molecular species, presumably K 2O or its related compound molecules, are formed on the surface. This results in the very easy deterioration of the C 8K surface. The surface of C 8K at 140 K, on the other hand, is much less active when it is exposed to gasses.

Chemical and Physical Roles of Individual Reactive Ions in Si Dry Etching

Chemical and physical sputtering yields of silicon by fluorocarbon ions such as F+, CF+, CF2+, CF3+ and C+ were investigated as a function of incident ion energy. Yield measurements over the 100–3000 eV incident ion energy range were performed through the use of mass-selected single species ion beam etching and in situ quartz crystal oscillator in an ultrahigh vacuum atmosphere. It was found that Si surface was fully covered with carboneous polymer and carbon in the cases of CF+ and C+ irradiation in the energy ranges of 100–700 eV and 100–2500 eV, respectively. Small quantity of carbon accumulation was also observed in the CF2+ ion beam irradiation of silicon in the 100–200 eV region, on the other hand, no carbon accumulation took place for F+ and CF3+ ion beam cases. In conventional Si dry etching by a CF4 gas plasma, since low energy (less than 1000 eV) fluorocarbon ions impinge the etched surface, it can be concluded that suppression of silicon etch rates mainly arises from the incidence of C+ and CF+ ion species.

Electron Energy-Loss Spectra of CdS, CdSe, and CdTe

A study has been made of the electron energy-loss spectra of the three $\mathrm{II}B\ensuremath{-}\mathrm{VI}A$ compounds, CdS, CdSe, and CdTe. These spectra have been obtained by both electron reflection and electron transmission. The electron-reflection measurements were made over an energy-loss range of 0 to 50 eV on single crystals cleaved in an ultrahigh vacuum atmosphere. Changes in these spectra were then observed as the surface was slowly contaminated. The electrontransmission measurements were made using as-grown platelets or thin films. The spectra of these compounds consist of a series of low-lying peaks between 0 and 9 eV associated with interband transitions from the valence band and a peak between 12 and 15 eV associated with an interband transition from the $d$ band. There also exists a peak between 9 and 12 eV and another between 15 and 19 eV. The former has been tentatively identified as a surface-plasmon peak, the latter as a bulk plasmon. These results are compared with optical reflectivity measurements and induced-electron-emission measurements.