RESUMEN

Carbon contamination from graphite molds during spark plasma sintering (SPS) considerably affects the properties of the sintered materials, especially transparent ceramics. Herein, transparent Y3Al5O12 (YAG) ceramics were prepared via SPS using Mo and Ta foils, separately and in tandem, as protective barriers against carbon contamination. The effects of Ta and Mo foils on the transparency and microstructure of the ceramics, and their protection mechanisms were studied. Experimental results show that a reaction layer formed at the Ta-YAG interface with a YTaO4-Al2O3 eutectic composition suppresses carbon penetration into the ceramic, increasing its transparency. By contrast, Mo foils, when used as protective barriers, allow carbon diffusion into the ceramic, resulting in the formation of nonuniform microstructural features. However, it does not form a reactive layer and, hence, is removed easily from the YAG surface. Multilayered Ta-Mo barrier exhibits improved outcomes if the Ta thickness is more than ∼100 µm. This behavior is attributed to the interior diffusion-blocking mechanism of Ta. Similar optical performance was demonstrated by both approaches. The results prove that carbon contamination in SPS-derived samples can be effectively prevented. Additionally, this study reports on a novel strategy of bonding oxide ceramics to metals by adding a Ta layer at the joint interface.

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It is well known that carbon present in scanning electron microscopes (SEM), Focused ion beam (FIB) systems and FIB-SEMs, causes imaging artefacts and influences the quality of TEM lamellae or structures fabricated in FIB-SEMs. The severity of such effects depends not only on the quantity of carbon present but also on its bonding state. Despite this, the presence of carbon and its bonding state is not regularly monitored in FIB-SEMs. Here we demonstrated that Secondary Electron Hyperspectral Imaging (SEHI) can be implemented in different FIB-SEMs (ThermoFisher Helios G4-CXe PFIB and Helios Nanolab G3 UC) and used to observe carbon built up/removal and bonding changes resulting from electron/ion beam exposure. As well as the ability to monitor, this study also showed the capability of Plasma FIB Xe exposure to remove carbon contamination from the surface of a Ti6246 alloy without the requirement of chemical surface treatments.

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In this work, we investigate ethanol (EtOH)-sensing mechanisms of a ZnO nanorod (NRs)-based chemiresistor using a near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS). First, the ZnO NRs-based sensor was constructed, showing good performance on interaction with 100 ppm of EtOH in the ambient air at 327 °C. Then, the same ZnO NRs film was investigated by NAP-XPS in the presence of 1 mbar oxygen, simulating the ambient air atmosphere and O2/EtOH mixture at the same temperature. The partial pressure of EtOH was 0.1 mbar, which corresponded to the partial pressure of 100 ppm of analytes in the ambient air. To better understand the EtOH-sensing mechanism, the NAP-XPS spectra were also studied on exposure to O2/EtOH/H2O and O2/MeCHO (MeCHO = acetaldehyde) mixtures. Our results revealed that the reaction of EtOH with chemisorbed oxygen on the surface of ZnO NRs follows the acetaldehyde pathway. It was also demonstrated that, during the sensing process, the surface becomes contaminated by different products of MeCHO decomposition, which decreases dc-sensor performance. However, the ac performance does not seem to be affected by this phenomenon.

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The ambient-pressure endstation and branchline of the Versatile Soft X-ray (VerSoX) beamline B07 at Diamond Light Source serves a very diverse user community studying heterogeneous catalysts, pharmaceuticals and biomaterials under realistic conditions, liquids and ices, and novel electronic, photonic and battery materials. The instrument facilitates studies of the near-surface chemical composition, electronic and geometric structure of a variety of samples using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy in the photon energy range from 170 eV to 2800 eV. The beamline provides a resolving power hν/Δ(hν) > 5000 at a photon flux > 1010 photons s-1 over most of its energy range. By operating the optical elements in a low-pressure oxygen atmosphere, carbon contamination can be almost completely eliminated, which makes the beamline particularly suitable for carbon K-edge NEXAFS. The endstation can be operated at pressures up to 100 mbar, whereby XPS can be routinely performed up to 30 mbar. A selection of typical data demonstrates the capability of the instrument to analyse details of the surface composition of solid samples under ambient-pressure conditions using XPS and NEXAFS. In addition, it offers a convenient way of analysing the gas phase through X-ray absorption spectroscopy. Short XPS spectra can be measured at a time scale of tens of seconds. The shortest data acquisition times for NEXAFS are around 0.5 s per data point.

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Two thickness measurement methods using an electron energy loss spectroscopy (EELS) and 10a convergent beam electron diffraction (CBED) were compared in an Fe-18Mn-0.7C alloy. The thin foil specimen was firstly tilted to satisfy 10a two-beam condition. Low loss spectra of EELS and CBED patterns were acquired in scanning transmission electron microscopy (STEM) and TEM-CBED modes under the two-beam condition. The log-ratio method was used for measuring the thin foil thickness. Kossel-Möllenstedt (K-M) fringe of the [Formula: see text] diffracted disk of austenite was analyzed to evaluate the thickness. The results prove the good coherency between both methods in the thickness range of 72 ~ 113 nm with a difference of less than 5%.

RESUMEN

Deposition of synchrotron-radiation-induced carbon contamination on beamline optics causes their performance to deteriorate, especially near the carbon K edge. The photon flux losses due to carbon contamination have spurred researchers to search for a suitable decontamination technique to restore the optical surface and retain its performance. Several in situ and ex situ refurbishing strategies for beamline optics are still under development to solve this serious issue. In this work, the carbon contamination is removed from a large (340 mm × 60 mm) Au-coated toroidal mirror surface using a capacitively coupled low-pressure RF plasma. Before and after RF plasma cleaning, the mirror was characterized by Raman spectroscopy, soft X-ray reflectivity (SXR) and atomic force microscopy (AFM) techniques. The Raman spectra of the contaminated mirror clearly show the G (1575-1590 cm-1) and D (1362-1380 cm-1) bands of graphitic carbon. The SXR curve of the contaminated mirror shows a clear dip near the critical momentum transfer of carbon, indicating the presence of carbon contamination on the mirror surface. This dip disappears after removal of the contamination layer by RF plasma exposure. A decrease in the intensities of the CO bands is also observed by optical emission spectrometry during plasma exposure. The AFM and SXR results suggest that the root-mean-square (r.m.s.) roughness of the mirror surface does not increase after plasma exposure.

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RESUMEN

The choice of a reflective optical coating or filter material has to be adapted to the intended field of application. This is mainly determined by the required photon energy range or by the required reflection angle. Among various materials, nickel and rhodium are common materials used as reflective coatings for (soft) X-ray mirrors. Similarly, aluminium is one of the most commonly used materials for extreme ultraviolet and soft X-ray transmission filters. However, both of these types of optics are subject to carbon contamination, which can be increasingly problematic for the operation of the high-performance free-electron laser and synchrotron beamlines. As an attempt to remove this type of contamination, an inductively coupled plasma source has been used in conjunction with N2/O2/H2 and N2/H2 feedstock gas plasmas. Results from the chemical surface analysis of the above materials before and after plasma treatment using X-ray photoelectron spectroscopy are reported. It is concluded that a favorable combination of an N2/H2 plasma feedstock gas mixture leads to the best chemical surface preservation of Ni, Rh and Al while removing the carbon contamination. However, this feedstock gas mixture does not remove C contamination as rapidly as, for example, an N2/O2/H2 plasma which induces the surface formation of NiO and NiOOH in Ni and RhOOH in Rh foils. As an applied case, the successful carbon removal from ultrathin Al filters previously used at the FERMI FEL1 using an N2/H2 plasma is demonstrated.

RESUMEN

In deep X-ray lithography (DXRL), synchrotron radiation is applied to pattern polymer microstructures. At the Synchrotron Laboratory for Micro and Nano Devices (SyLMAND), Canadian Light Source, a chromium-coated grazing-incidence X-ray double-mirror system is applied as a tunable low-pass filter. In a systematic study, the surface conditions of the two mirrors are analyzed to determine the mirror reflectivity for DXRL process optimization, without the need for spectral analysis or surface probing: PMMA resist foils were homogeneously exposed and developed to determine development rates for mirror angles between 6 mrad and 12 mrad as well as for white light in the absence of the mirrors. Development rates cover almost five orders of magnitude for nominal exposure dose (deposited energy per volume) values of 1 kJ cm-3 to 6 kJ cm-3. The rates vary from case to case, indicating that the actual mirror reflectivity deviates from that of clean chromium assumed for the experiments. Fitting the mirror-based development rates to the white-light case as a reference, reflectivity correction factors are identified, and verified by experimental and numerical results of beam calorimetry. The correction factors are related to possible combinations of a varied chromium density, chromium oxidation and a carbon contamination layer. The best fit for all angles is obtained assuming 7.19 g cm-3 nominal chromium density, 0.5 nm roughness for all involved layers, and an oxide layer thickness of 25 nm with a carbon top coat of 50 nm to 100 nm. A simulation tool for DXRL exposure parameters was developed to verify that the development rates for all cases do coincide within a small error margin (achieving a reduction of the observed errors by more than two orders of magnitude) if the identified mirror surface conditions are considered when calculating the exposure dose.

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Atomistic control over the growth of semiconductor thin films, such as aluminum nitride, is a long-sought goal in materials physics. One promising approach is plasma-assisted atomic layer epitaxy, in which separate reactant precursors are employed to grow the cation and anion layers in alternating deposition steps. The use of a plasma during the growth-most often a hydrogen plasma-is now routine and generally considered critical, but the precise role of the plasma is not well-understood. We propose a theoretical atomistic model and elucidate its consequences using analytical rate equations, density functional theory, and kinetic Monte Carlo statistical simulations. We show that using a plasma has two important consequences, one beneficial and one detrimental. The plasma produces atomic hydrogen in the gas phase, which is important for removing methyl radicals left over from the aluminum precursor molecules. However, atomic hydrogen also leads to atomic carbon on the surface and, moreover, opens a channel for trapping these carbon atoms as impurities in the subsurface region, where they remain as unwanted contaminants. Understanding this dual role leads us to propose a solution for the carbon contamination problem which leaves the main benefit of the plasma largely unaffected.

RESUMEN

Focused electron beam induced deposition (FEBID) is a versatile tool for the direct-write fabrication of nanostructures on surfaces. However, FEBID nanostructures are usually highly contaminated by carbon originating from the precursor used in the process. Recently, it was shown that platinum nanostructures produced by FEBID can be efficiently purified by electron irradiation in the presence of water. If such processes can be transferred to FEBID deposits produced from other carbon-containing precursors, a new general approach to the generation of pure metallic nanostructures could be implemented. Therefore this study aims to understand the chemical reactions that are fundamental to the water-assisted purification of platinum FEBID deposits generated from trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPtMe3). The experiments performed under ultrahigh vacuum conditions apply a combination of different desorption experiments coupled with mass spectrometry to analyse reaction products. Electron-stimulated desorption monitors species that leave the surface during electron exposure while post-irradiation thermal desorption spectrometry reveals products that evolve during subsequent thermal treatment. In addition, desorption of volatile products was also observed when a deposit produced by electron exposure was subsequently brought into contact with water. The results distinguish between contributions of thermal chemistry, direct chemistry between water and the deposit, and electron-induced reactions that all contribute to the purification process. We discuss reaction kinetics for the main volatile products CO and CH4 to obtain mechanistic information. The results provide novel insights into the chemistry that occurs during purification of FEBID nanostructures with implications also for the stability of the carbonaceous matrix of nanogranular FEBID materials under humid conditions.

RESUMEN

Monolayer doping (MLD) involves the functionalization of semiconductor surfaces followed by an annealing step to diffuse the dopant into the substrate. We report an alternative doping method, oxide-MLD, where ultrathin SiO2 overlayers are functionalized with phosphonic acids for doping Si. Similar peak carrier concentrations were achieved when compared with hydrosilylated surfaces (∼2 × 1020 atoms/cm3). Oxide-MLD offers several advantages over conventional MLD, such as ease of sample processing, superior ambient stability, and minimal carbon contamination. The incorporation of an oxide layer minimizes carbon contamination by facilitating attachment of carbon-free precursors or by impeding carbon diffusion. The oxide-MLD strategy allows selection of many inexpensive precursors and therefore allows application to both p- and n-doping. The phosphonic acid-functionalized SiO2 surfaces were investigated using X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy, whereas doping was assessed using electrochemical capacitance voltage and Hall measurements.

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A prototype Peltier thermoelectric cooling unit has been constructed to cool a cold finger on an electron microprobe. The Peltier unit was tested at 15 and 96 W, achieving cold finger temperatures of -10 and -27°C, respectively. The Peltier unit did not adversely affect the analytical stability of the instrument. Heat conduction between the Peltier unit mounted outside the vacuum and the cold finger was found to be very efficient. Under Peltier cooling, the vacuum improvement associated with water vapor deposition was not achieved; this has the advantage of avoiding severe degradation of the vacuum observed when warming up a cold finger from liquid nitrogen (LN2) temperatures. Carbon contamination rates were reduced as cooling commenced; by -27°C contamination rates were found to be comparable with LN2-cooled devices. Peltier cooling, therefore, provides a viable alternative to LN2-cooled cold fingers, with few of their associated disadvantages.

RESUMEN

Carbon-free chromium-coated optics are ideal in the carbon K-edge region (280-330 eV) because the reflectivity of first-order light is larger than that of gold-coated optics while the second-order harmonics (560-660 eV) are significantly suppressed by chromium L-edge and oxygen K-edge absorption. Here, chromium-, gold- and nickel-coated mirrors have been adopted in the vacuum ultraviolet and soft X-ray branch beamline BL-13B at the Photon Factory in Tsukuba, Japan. Carbon contamination on the chromium-coated mirror was almost completely removed by exposure to oxygen at a pressure of 8 × 10(-2) Pa for 1 h under irradiation of non-monochromated synchrotron radiation. The pressure in the chamber recovered to the order of 10(-7) Pa within a few hours. The reflectivity of the chromium-coated mirror of the second-order harmonics in the carbon K-edge region (560-660 eV) was found to be a factor of 0.1-0.48 smaller than that of the gold-coated mirror.

RESUMEN

The effect of carbon contamination on the analysis of carbon-coated silicate minerals at 5 kV for X-ray energies 0.7-4 keV is examined. For individual spot analyses, carbon is found to deposit adjacent to the beam spot forming ring-shaped deposits with no impact on the analysis. Carbon contamination becomes important for closely spaced analyses such as multipoint transects, where each subsequent analysis overlaps the carbon ring of the previous analysis. X-ray intensity loss due to contamination is most severe for low-overvoltage elements such as Ca K consistent with carbon deposition effectively reducing beam energy. Rates of contamination are calculated and the use of a liquid nitrogen cold trap is shown to greatly reduce the amount of carbon deposited. A complimentary empirical correction is developed to correct for X-ray intensity loss from measured carbon, assuming the carbon is a film, and is compared with corrections derived from thin film calculations. PENELOPE electron probe microanalysis (PENEPMA) calculations confirm that asymmetry of the carbon deposition can be ignored for X-ray energies where intensity loss is predominantly through energy loss of beam electrons. Using a cold trap and/or an empirical correction high spatial resolution analysis (ca. 400 nm between points) is achievable with analytical errors of ca. 1-3%.

RESUMEN

Although the graphitic carbon contamination of synchrotron beamline optics has been an obvious problem for several decades, the basic mechanisms underlying the contamination process as well as the cleaning/remediation strategies are not understood and the corresponding cleaning procedures are still under development. In this study an analysis of remediation strategies all based on in situ low-pressure RF plasma cleaning approaches is reported, including a quantitative determination of the optimum process parameters and their influence on the chemistry as well as the morphology of optical test surfaces. It appears that optimum results are obtained for a specific pressure range as well as for specific combinations of the plasma feedstock gases, the latter depending on the chemical aspects of the optical surfaces to be cleaned.

RESUMEN

Investigations of X-ray photoelectron spectra from solid samples need corrections for the surface charging effect. For powder samples such as heterogeneous catalysts and their supports, the C-(C,H) component of the C 1s peak is often used as an internal standard for the calibration of the binding energy scale. Although this method is widely recognized as suitable for the study of heterogeneous catalysts, we show that a significant calibration bias can be encountered upon comparing samples with different bulk composition. In this paper, a series of SiO2-Al2O3 supports and Pd/SiO2-Al2O3 catalysts with various Si/Al ratios were studied. The spectra issued from these samples were processed with the classical calibration method on the basis of the carbon peak. Important discrepancies in the relative position of the photoelectron peaks were noticed. After systematically discarding instrument-related issues, a true chemical influence of the bulk matrix on the analyzed surface species was evidenced. The extent of this chemical effect was dependent on the composition of the sample and more precisely on its ionicity. Two possible mechanisms for this chemical effect were proposed and discussed. Finally, an alternative calibration method was offered.