Long-time Air Exposure Research Articles

Unusual re-entrant spin-glass-behavior and enhanced diamagnetism in sp3-rich sulfur/oxygen doped highly oriented pyrolytic graphite

The recent observations of unusual ferromagnetic and superconductive phenomena in sulfur-doped amorphous carbon and highly oriented pyrolytic graphite (HOPG) have attracted significant attention. In this work we present a novel in-depth investigation on the relationship between magnetic-ordering and sulfur-doping-induced structural modifications in pyrolytic graphite. We demonstrate the nucleation of an unusual amorphous carbon film with sp3-rich characteristics as a result of chemical interactions between sulfur and HOPG. Investigations were performed at the controlled temperatures (T) of ~300, 350, 500 °C, with the residence time parameter varying from 1 min to 15 h. Magnetic characterization revealed a re-entrant spin-glass behavior, possibly originating from vacancy-defects created by sulfur migration. Noticeably, long-time air-exposure of the samples (3 weeks) was found to significantly enhance the diamagnetic component. Density functional theory (DFT) calculations applied to a sp3-rich amorphous‑carbon-system in presence of sulfur-bonding revealed a significant variation of the system-metallicity. We report the presence of insulating states up to sulfur concentrations of ~6.25% (atom %), which then vanish at ~7.5% (atom %) of sulfur-doping. Interestingly, inclusion of the sulfur-migration effect within the DFT calculations revealed significant contributions arising from magnetic-moments generated by electron localization at the vacancy sites. Total spin isosurface analyses (spin up - spin down) highlighted the presence of ferromagnetic contributions arising from carbon-atoms, due to dangling bonds generated by point defects (with the largest magnetic moment values being 0.42 μΒ and 0.379 μΒ respectively).

Effect of introducing manganese as additive on microstructure, hydrogen storage properties and rate limiting step of Ti–Cr alloy

TiCr2 with adding different amount of Mn (0, 2, 4 and 8 wt.%) alloys have been investigated. All alloys have C14-type main phase (gray color in SEM) and Ti minor phase (dark gray color in SEM). Rietveld fitting results proved that the lattice parameter a and cell volume of C14-type phase decreased with increasing Mn content. The first hydrogenation measurement manifest that all alloys have best activation properties and could be activated without any prior heat treatment and hydrogen exposure. However, introducing Mn led to the decrease of the first hydrogen absorption rate of TiCr2 alloy, which may be due to the decrease of cell volume of C14-type main phase. The first hydrogenation properties at low temperature and effect of air exposure of the alloy were discussed. The results showed that the maximum hydrogen absorption capacity at 0 °C was obviously higher than that at room temperature. In addition, TiCr2 alloy doped with minor amounts of Mn after long-time air exposure showed better hydrogenation performance. This may be due to the Mn additive acting as a deoxidizer. Finally, the first hydrogenation kinetic mechanisms of all alloys at different temperature were also studied by using the rate limiting step.

Control of the hydrophobicity of rare earth oxide coatings deposited by solution precursor plasma spray by hydrocarbon adsorption

The basis of the hydrophobicity of lanthanide rare earth oxides (REOs) has been the subject of considerable debate. To explore this question, the wetting behaviors and surface compositions of hierarchically-structured Yb2O3 (one of the REOs) coatings and non-REO Al2O3 coatings deposited via solution precursor plasma spray process were investigated in this work. The Yb2O3 coatings were subjected to a number of post-deposition treatments including vacuum (1−15 Pa) treatment, Ar-plasma treatment, heat treatment (400 °C), long-time air exposure and ultra-high vacuum (1 × 10−7 Pa) treatment. Subsequent characterization showed that different post-deposition treatments resulted in different wetting behavior for the Yb2O3 coatings which correlated with the content of hydrocarbon on the surface. Yb2O3 coatings exhibited reversible transitions between superhydrophobicity after vacuum treatment and superhydrophilicity after Ar-plasma or heat treatment, linked to hydrocarbon adsorption onto and desorption from the surface. Yb2O3 coatings after long-time air exposure and ultra-high vacuum treatment both remained hydrophilic and showed a smaller hydrocarbon content than coatings after vacuum treatment. Al2O3 coatings with hierarchical surface structures similar to the Yb2O3 coatings showed an increase in WCA to only ∼17° after the same vacuum treatment, indicating the REO has a much higher affinity for hydrocarbon adsorption than Al2O3, and that the content of hydrocarbon adsorbed on the surface of the REO determined the wetting behavior.

X-Ray Photoelectron Spectroscopy Estimation of Cobalt Seed Layer Reactivity Toward Air Exposure: A Challenge?

For the last several generations of CMOS technology, copper has been used widely as an interconnect material in the back end of the line (BEOL). [1] [2] [3] As the current nodes technology are decreasing continuously, several copper scaling challenges appeared such as the Cu barrier thinning, the copper electromigration... [4] [5] Hence, cobalt and ruthenium are proposed as replacement materials. Recently, Co has made inroads in microelectronics, specifically on-chip metallization, serving as a capping layer for Cu to diminish electromigration but also provide a better conductivity for line width inferior to 10 nm [6] [7] [8].In this very challenging context, the control of Co surface oxidation process is strategic. Conformal Co seed characteristics (i.e electronic conductivity), preceding the following filling chemistry, are partially related to its surface oxide. Its understanding, then its control, will be determinant to access to a perfect and reliable filling of the nodes. To collect quantitative information, we carefully investigate the behavior of a pure Co metallic surface which is exposed to air oxidation. For this purpose, we have used a 400nm thick Co layer electrochemically plated using Kari© aveni acidic chemistry. The Co layer is electrochemically deposited on Co seed (3nm) /Ta/TaN barrier/Tetraethyl orthosilicate (TEOS). The latter, with its native oxide, is introduced in ultra-high vacuum (UHV) of a XPS Nexsa ThermoFisher spectrometer. Then, a totally deoxidized Co surface is obtained with a argon beam sputtering. The initial free oxide bulk metallic Co is then checked by XPS. Starting from this surface, we explore again by XPS, consequences of different air exposure times using the entrance prepation chamber of the spectrometer. According to the time air-exposure, the kinetic of the oxide growth mechanism can be evaluated. Our work shows clearly that the metallic Co surface evolves very quickly to an ultra-thin (<nm) oxide capping layer whose thickness will increase slowly, progressively. However, the metallic signal of the buried metallic Co is always present even for very long-time air-exposure. These results indicate a phenomenological trend over the oxide thickness limit. In our paper, these kinetic studies which are very reproducible in time and spectral modifications will be analyzed as all the different Co oxide phases thanks to the Co2p and O1s spectra from XPS (Figure.1). Moreover, a simulation of behaviors of this oxide surface coverage immersed in acidic solution will be proposed to understand how an ultra-thin Co seed layer interacts in Co filling solution. For both cases (air-exposure and cobalt filling solution) the results comparison and a discussion will be proposed .

Effect of air exposure on hydrogen storage properties of catalyzed magnesium hydride

Air exposure of the magnesium hydride materials can lead to severe degradation on the hydrogen storage properties, and therefore is one of the key remaining challenges for its application. In this work, the effects of direct air exposure and solvent-protected air exposure for catalyzed MgH2 on the hydrogen storage properties are systematically investigated. Results show that the direct air exposure of catalyzed MgH2 leads to reduction of the hydrogen storage capacity, and moderate deterioration of the hydrogen absorption/desorption kinetics, but has limited impact on reversibility and cycle stability. A short-time air exposure (15 min) causes the decrease of hydrogen storage capacity of MgH2–5%VTiCr by 0.2 wt%, and long-time air exposure (2400 min) significantly deteriorates the hydrogen storage capacity by 1.95 wt%. During the direct exposure in ambient air, MgH2 reacts with oxygen and moisture, and the surface forms a layer consisting of Mg(OH)2 and MgO. Various solvents, hexane, acetone, and ethanol, are used to protect MgH2–5%TiMn2 material. The hydrogen storage properties of the solvent-protected materials after air exposure for 1500 min are evaluated, both the hydrogen storage capacity and kinetics are degraded after exposure. The acetone-protected MgH2–5%TiMn2 shows better kinetics and capacities compare to those protected by hexane and ethanol.

Large area growth of few-layer In2Te3 films by chemical vapor deposition and its magnetoresistance properties

In this work we report a facile route to grow large area, uniform, continuous and few-layer α-In2Te3 film via chemical vapor deposition (CVD) methods. The characterizations show the large area of CVD-grown few-layer α-In2Te3. This method guarantees the precise control of thickness down to few layers and large area preparation. The magnetoresistance (MR) properties of few-layer In2Te3 was investigated from 2 to 300 K and its MR stability under long exposure to ambient air was studied for the first time. Few-layer of α-In2Te3 shows a positive MR and the largest transverse MR was observed to about 11% at 2 K and a high stability of MR to long time exposure in air up to 21 weeks.

Solution phase surface functionalization of PbS nanoparticles with organic ligands for single-step deposition of p-type layer of quantum dot solar cells

Semiconducting p-type layers of heterojunction quantum dot solar cells (QDSCs) are often prepared by the layer-by-layer (LBL) method. Due to the intermittent nature of quantum dot (QD) deposition, solid-state ligand exchange and washing steps, the LBL process typically involves a high waste of materials and a relatively low surface quality of the applied layers. To introduce a simplified method for deposition of QD layer, this paper investigates the preparation of pre-exchanged QD inks for the single-step deposition of QD layers of QDSCs. Various pre-exchanged QD inks were prepared through the solution phase exchange of oleate ligands with organic ligands of different functional groups, chain lengths and chain structures. Oxidation and colloidal stability of the prepared inks during long-time air exposure storage were investigated by the means of UV–Vis-NIR spectroscopy, photoluminescence (PL) and zeta potential measurements. Transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), nuclear magnetic resonance (NMR) and combustion CHNS analyses were used to study the surface chemistry of the exchanged QDs and the mechanism of ligand exchange. Various inks of pre-exchanged QDs were deposited on different substrates in a single-step method. The best surface topography and the lowest oxidation rate were achieved for layers prepared by the single-step deposition of 3-mercaptopropionic acid (MPA)-capped QD ink. The QDSCs fabricated by such layers had higher efficiency than those fabricated by the common LBL method. Furthermore, annealing the MPA-capped QD layer increases the power conversion efficiency of the ink based solar cells up to 3.22% under simulated AM1.5 solar illuminations.

Identification and characterization of apoptosis regulator Bax involved in air-exposure stress of the mud crab, Scylla Paramamosain Estampador, 1949

The mud crab, Scylla paramamosain Estampador, 1949, is a widely farmed commercial species in the South-East coastal areas of China. The crabs are captured and placed in a water-free container for further transportation. The long-time air exposure makes the crabs suffer from oxidative stress, triggering cell apoptosis and leading to the crab’s death. In this study, the homologue of the apoptosis regulator BAX was firstly identified in S. paramamosain and named as SpBAX. The coding region of SpBAX yielded a polypeptide of 278 amino acids, consisting of the defining motif of the BAXs family including Bcl-2 homologous BH1, BH2, BH3 regions and a transmembrane (TM) domain. Subcellular prediction suggested that SpBAX was located in the cytoplasm. The three-dimensional structure showed that SpBAX contained 8 helical regions. Two central α-helices (α5 and α6) flanked on one side by α3 and α4, and on the other side by α1, α2 and α7, which showed the same structure as BAXs in mammals. The highest expression level of SpBAX was detected in hepatopancreas tissue. The expression level of SpBAX was up-regulated in 12 h in hepatopancreas and in 24 h in haemocytes after air exposure. Meanwhile, a flow cytometry assay revealed that the proportion of apoptotic haemocytes exceeded 65% after air exposure for 36 h, showing a significant difference with the control group. These results indicated that the cloned SpBAX might be involved in the response to air exposure stress by causing cell apoptosis. This study may thus be helpful to clarify the mechanisms of the air-exposure stress response in the mud crab.

Inertness and degradation of (0001) surface of Bi2Se3 topological insulator

Inertness of the cleaved (0001) surface of the Bi2Se3 single crystal, grown by modified Bridgman method, to oxidation has been demonstrated by X-ray photoelectron spectroscopy, scanning tunneling microscopy, and by ab initio DFT calculations. No intrinsic bismuth and selenium oxides are formed on the low-defect, atomically flat Bi2Se3(0001)-(1×1) surface after a long-time air exposure. The inertness of Bi2Se3(0001) to O2 and NO2, as well as bismuth-oxygen bonding formation under molecular adsorption in the Se vacancy was supported by DFT calculations.

Stress characteristics of suspended porous silicon microstructures on silicon

The mechanical properties of porous silicon (PS) microstructures on crystalline silicon (c-Si) were investigated. Micro-Raman spectroscopy was applied to probe stresses in PS micro-hotplates in the form of cantilevers. Important information was obtained for the stress distribution across cantilevers supported at one edge. The structural stability of thermally treated PS after long time exposure in air was also investigated. The results can be used for optimization of microsensors based on the PS microstructures.

Modeling thermal exposure effects on fatigue of gamma titanium aluminides

Aircraft engine applications of gamma titanium aluminide alloys require these materials to undergo extended periods of high temperature exposure in air. In order to determine whether gamma alloys will function satisfactorily in such applications, the effects of long-time air exposure need to be understood, and quantified. To date, investigations in this area have been conducted on cast Ti–48Al–2Nb–2Cr and wrought Alloy 395. Both alloys showed reductions in fatigue capability that tended to peak in the intermediate temperature (∼540 °C) range, with smaller or no reductions at lower or higher temperatures. This paper develops a model for fatigue of thermally exposed gamma alloys based on propagation of surface cracks initiating from an embrittled surface layer. The model successfully predicted the observed maximum effect at intermediate temperature and provides quantitative agreement with the observed post-exposure fatigue capability.

Laser coloration and bleaching of amorphous WO3 thin film

Laser-induced chromism of amorphous WO3 thin films prepared by pulsed laser deposition (PLD) has been investigated. The original films could be colored from light brown to purple by a single pulse of KrF excimer laser irradiation at 248 nm and subsequently bleached to brown by a single pulse of Nd–yttrium–aluminum–garnet laser at 1.06 μm in air. Spectroscopic investigations were applied to the films at three different states: original, colored, and bleached. The measurements by ellipsometry spectroscopy showed an increase in the refractive index (n) and decrease in extinction coefficient (k) in the luminous range of films from the colored state to the bleached state. Scanning tunneling microscopy and Raman spectroscopy showed slight crystallization in the films after coloration, with both the grain dimension and the surface roughness around tens of nanometers. The films were very stable to maintain the same color after long-time exposure in air, or in oxygen. X-ray photoelectron spectroscopy was used to study the chromism mechanisms. While some W5+ states were introduced in the original films by PLD, W4+ states were produced when the films were colored by the KrF excimer laser, along with the decrease of W5+ states and the increase of W6+ states. In contrast, laser bleaching was accompanied with increasing W4+ states in the films. Therefore, it is believed that the purple color is due to the polaron transition between the W4+ and W5+ states. Photochemical activation and photothermal oxidation are ascribed to the coloring and the bleaching processes, respectively.

Contactless capacitance–voltage and photoluminescence characterization of ultrathin oxide–silicon interfaces formed on hydrogen terminated (111) surfaces

Electronic properties of the interfaces between Si and ultrathin (≲10 Å) oxides formed by various low-temperature processes were characterized in contactless fashion, using contactless capacitance–voltage and photoluminescence surface state spectroscopy techniques together with x-ray photoelectron spectroscopy measurement. Hydrogen (H) terminated Si(111) surfaces were used as the initial surface. Ultrathin oxides were formed at low temperatures by chemical oxidation processes (hot HNO3, H2SO4+H2O2), long-time air exposure, and low-temperature oxidation processes below 350 °C. The initial H-terminated surfaces showed presence of Fermi-level pinning at E0=EV+0.65 eV due to high density of amphoteric discrete state probably originating from Si dangling bonds. On the other hand, all the ultrathin oxide–Si interfaces exhibited very limited capacitance variations with voltage at low capacitance levels similar to GaAs metal–insulator–semiconductor systems, and indicated that the Fermi level is pinned near the hybrid orbital charge neutrality level EHO due to presence of interface states with narrow U-shaped continuous distributions. Low-temperature oxidation at 350 °C slightly weakens such pinning. The present work indicates difficulty of realizing unpinned ultrathin oxide–silicon interface by low-temperature processes.