We report results of a combined experimental and computational model study on the interaction of the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI) with a Mg thin film model electrode grown on a Ru(0001) substrate, which aims at a fundamental understanding of the solid electrolyte interphase formation at the electrode–electrolyte interface in postlithium batteries. Scanning tunneling microscopy, x-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy were employed for the characterization of the Mg thin film model electrode, revealing oxygen-free and atomically flat Mg films. Room temperature XPS measurements after vapor deposition of a (sub)monolayer of BMP-TFSI on the Mg film revealed the formation of a “contact layer” on Mg(0001), created by the reactive decomposition of the IL. In agreement with computationally determined core level binding energies of stable reaction products (dispersion corrected density functional theory calculations), we identified mainly inorganic MgF2-, MgO-, and MgS-like surface compounds, but also other more complex (Mg2+-free) F-, O-, and/or S-containing “TFSI-like” and carbon-containing adsorbed species. The deposition of higher IL amounts (up to 6 monolayers) results in the overgrowth of the direct “contact layer” by molecularly adsorbed BMP-TFSI. Heating of the adsorbate covered surface to around 470 K leads to desorption of multilayer BMP-TFSI and the partial desorption and transformation of adsorbed (Mg2+-free) “TFSI-like” decomposition products on the Mg substrate into MgF2-, MgO-, and MgS species or the respective adsorbed Fad, Oad, and Sad species.
We report uniform liquid crystal (LC) alignment on a polyacrylamide (PAM) film via ion beam (IB) treatment. The IB incidence angle is adjusted from 15° to 75°. Physicochemical modifications caused by the IB process are investigated by atomic force microscopy and x-ray photoelectron spectroscopy. Low IB incidence results in biased and bumpy surfaces, whereas high IB incidence forms isotropic and smooth surfaces; both of these are unsuitable for uniform LC alignment. The 45° IB incidence induces anisotropic surface chemical reformations, and these modifications induce van der Waals forces between the LCs and modified PAM, thereby leading to uniform LC alignment. The LC alignment state is verified by polarized optical microscopy and pretilt angle. The electro-optical characteristic of the modified PAM showed excellent switching performance in twisted-nematic LC system. Thus, the IB-treated PAM film is a good candidate for LC alignment layers and suitable for LC device applications.
In doped manganites, a substantial tuning of the magnetic and electrical transport properties can be realized by engineering the concentration of oxygen vacancies. To date, most oxygen-deficient La1−xSrxMnO3−δ (0 ≤ x ≤ 1) films are synthesized by after-growth treatments. However, the direct growth of La1−xSrxMnO3−δ films remains challenging due to the metastability of this material. Here, we report the epitaxial growth of high quality single crystalline La0.67Sr0.33MnO3−δ films with an extremely large out-of-plane lattice parameter of 4.26 Å by reactive oxide molecular beam epitaxy. To stabilize this metastable phase, Sr3Al2O6 buffer layers are used to block the oxygen diffusion from the SrTiO3 substrate to the film during the growth process. This work provides an efficient way to obtain metastable La0.67Sr0.33MnO3−δ films.
Thin films of silver were deposited on nonalkali glass substrates at substrate temperatures ranging from room temperature (28 °C) to 150, 200, 300, 400, and 500 °C at discharge pressures of 0.40, 1.20, and 2.00 Pa using direct current magnetron sputtering. On the basis of the measured cross-sectional and surface morphologies, crystallographic structures, and film properties, I discuss the dependence of the film structure and properties on the substrate temperature. The x-ray diffraction measurements showed that the <111> orientation was preferred for all deposition conditions. Scanning electron microscope observations revealed a microstructure of convex-shaped fine grains for a substrate at room temperature, while laterally growing, mound-shaped grains with flat-topped surfaces appeared at substrate temperatures of 400 and 500 °C. Atomic force microscopy also showed an increase in the lateral size and height of the mound-shaped structures with increasing substrate temperature. The lateral grain size evaluated from the areal particle density obtained from atomic force microscopy increased significantly with increasing substrate temperature, reaching 600–800 nm at a substrate temperature of 500 °C. The film stress also changed from compressive to tensile with increasing substrate temperature. The relative density, defined as the ratio of the deposited amount-of-substance to the physical-thickness, decreased significantly with increasing substrate temperature and, at a substrate temperature of 500 °C, was approximately 0.6 times as large as that obtained for thin films deposited at room temperature. The high surface diffusivity of the Ag adatoms induces the growth of laterally growing, mound-shaped grains. Besides, the energy accumulated in the thin films during sputter deposition induces the void formation to increase the efficiency of energy release in the form of heat.
Equipment such as the Mars rover whose energy absorption conversion function of the power supply system can reduce or even get completely lost in a dusty environment is always affected by the adhesion of dust particles. The antidust self-cleaning film is an effective method to reduce the deposition of dust and ensure the operational performance of the equipment. In this study, perfluorocarbon (PFC) film can be used as a potential self-cleaning film because of its excellent properties such as stability and low surface energy and was prepared using plasma enhanced chemical vapor deposition. The dust removal efficiency of the PFC film-coated sample and the bare glass substrate sample was measured and calculated by three different masses of the dust vertical free-fall approach. The results demonstrated that, by comparison, the self-cleaning film exhibited excellent dust removal performance. Specifically, for the PFC film, the dust removal efficiency was 88.86%, 92.80%, and 98.11%, and optical transmittance was about 92%; these values are better than that of the glass substrate. Moreover, the dust adhesion and resistance mechanism was explored and analyzed, which indicated that decreasing the surface roughness and surface energy can effectively reduce the adhesion of the dust on the sample surface. The results will be of major theoretical significance and practical engineering value for long-term service in power systems that are placed in a dusty environment.
Area-selective atomic layer deposition (ALD) of dielectrics on chemically similar growth and nongrowth surfaces is very challenging. In this study, we use aminosilane inhibitors to achieve selective blocking of ALD of Al2O3 on plasma-deposited SiO2 versus plasma-deposited SiNx. The SiO2 and SiNx surfaces were exposed to bis(dimethylamino)dimethylsilane followed by (N,N-dimethylamino)trimethylsilane through the vapor phase at 150 °C. At the same substrate temperature, Al2O3 films were grown by ALD using dimethylaluminum isoproxide and H2O. In situ surface infrared spectroscopy shows that aminosilane inhibitors react with almost all the surface −SiOH groups on SiO2, but reaction with surface −NHx groups on the SiNx surface is incomplete, thereby leaving potential growth sites for ALD of Al2O3. In situ ellipsometry results shows that a ∼2.7 nm Al2O3 film can be selectively deposited on SiNx versus SiO2. Upon exposure of the plasma-deposited SiNx to the atmosphere, a higher attachment of aminosilanes and longer nucleation delay during the ALD of Al2O3 were observed, indicating the need to remove the native surface oxynitride prior to functionalization. This study shows that while fully passivating the nongrowth surface is necessary for achieving growth inhibition, ALD can initiate on a partially passivated growth surface.
The spontaneous etching of boron oxide (B2O3) by hydrogen fluoride (HF) gas is important during thermal atomic layer etching after BCl3 converts the surface of various metal oxides to a B2O3 layer. In this study, the chemical vapor etching (CVE) of B2O3 by HF was experimentally monitored using Fourier transform infrared (FTIR) spectroscopy and quadrupole mass spectrometry (QMS). The spontaneous etching of B2O3 by HF gas was also analyzed using density functional theory (DFT). B2O3 films were grown using B2O3 atomic layer deposition with BCl3 and H2O as the reactants at 40 & DEG;C. FTIR spectroscopy then observed the CVE of B2O3 by HF at 150 & DEG;C. B2O3 etching was monitored by the loss of absorbance for B-O stretching vibration in B2O3 films. FTIR spectroscopy studies also observed B-F stretching vibrations from BFx species on the B2O3 surface after HF exposures. In addition, the QMS analysis was able to identify the etch products during the spontaneous etching of B2O3 by HF gas at 150 & DEG;C. The QMS studies observed the main volatile etch products as BF3, BF2(OH), and H2O. Additional volatile etch products were also detected including B3O3F3 and other boroxine ring compounds. The DFT predictions were consistent with the spontaneous etching of B2O3 by HF gas. DFT confirmed that CVE was likely because the energetics of the spontaneous etching reaction B2O3(s) + 6HF(g) & RARR; 2BF(3)(g) + 3H(2)O(g) were more favorable than the self-limiting reaction B2O3(s) + 6HF(g) & RARR; 2BF(3)(s) + 3H(2)O(g). The spontaneous etching of B2O3 was predicted at temperatures above -163 & DEG;C for an HF reactant pressure of 0.2 Torr and BF3 and H2O product pressure of 0.01 Torr.
Barium strontium oxide-coated carbon nanotubes (CNTs) were implemented as a work function lowering and field enhancing functional coating on a coiled tungsten filament to create a new thermionic cathode. This cathode resembles conventional oxide cathodes in structure. It has the same coiled tungsten filament as a conventional oxide cathode but uses barium strontium oxide-coated CNTs instead of the traditional barium strontium calcium oxide powder mixture as an emissive coating. The cathode produces a strong thermionic emission. At 1395 K and 2.5 V/μm, the thermionic emission current of 0.87 A or current density of 2.9 A/cm2 was obtained from this oxide-coated CNT cathode. This level of emission is about three times as large as a conventional oxide cathode operating at similar temperature and field strength. Strong thermionic emissions from the cathode also lead to a large thermionic cooling effect. Temperature reduction as large as 90° was observed from the cathode surface when it was emitting electrons. Strong thermionic emission and a large cooling effect obtained are the result of the combination of the low work function of barium strontium oxide (1.6 eV) and the large field effect induced by the CNTs. Plasma enhanced chemical vapor deposition was used to grow CNTs, and magnetron sputtering deposition was used to deposit the barium strontium oxide functional coating; details of the cathode fabrication are presented to illustrate both the versatility of the processing techniques and the adaptability of barium strontium oxide-coated CNTs as a functional coating. Measurements on thermionic emission and thermionic cooling of the cathode are also presented.
Metal-oxide thin films and interfaces exhibit numerous fascinating electronic transport properties that are not found in conventional semiconductor materials. There has been much interest in engineering them to improve their functionalities, and an improved fundamental understanding of the phenomena that occur in oxide films and heterostructures is critical. In this review, an innovative approach to strontium titanate and zinc oxide-based heterostructures using state-of-the-art scanning tunneling microscopy and photoemission spectroscopy systems, as well as electrical measurements are presented. The results show that atomic-scale bottom-up processes with greater care provide excellent opportunities for improving material properties and classifying complicated conductivity.
Copper oxides CuO and Cu2O are promising p-type semiconductors for low cost and environmentally friendly solar energy conversion applications. To design optimized devices, a precise control of their optoelectonic properties is necessary. Direct current reactive magnetron sputtering was used to synthesize CuO and Cu2O thin films, and their optoelectronic properties were assessed and compared. It is shown how a heated or biased substrate can modify the electrical properties of sputtered copper oxides. Applying a negative bias mostly increases the hole density and conductivity of the material by one order of magnitude, with a moderate increase of mobility, while heating the substrate (523 K) leads to a clear increase of the hole mobility (up to 0.3 and 4.5 cm2/V s for CuO and Cu2O, respectively) and a decrease of the hole density. A hole density of 1015 cm−3 was reached for Cu2O, while for CuO, it remained as high as 1019 cm−3.
Surfaces of correlated electron oxides are of significant interest from both fundamental and applied perspectives. Many such oxides feature a near-surface region (NSR) that differs from the bulk’s properties. The NSR can significantly affect the interpretation of the material’s electronic structure, especially for those in thin film form, and have detrimental effects for applications such as field effect devices and catalysts. In this work, we study the changes in the composition and the electronic structure of the NSR of SrVO3 (SVO) thin films. We employ x-ray photoelectron spectroscopy (XPS) and compare TiOx-capped SVO films to identical uncapped films that were exposed to ambient conditions. The significant overoxidation of the SVO surface in the bare film, illustrated by a primary V5+ component, is prevented by the TiOx layer in the capped film. The capped film further exhibits a decrease in Sr surface phases. These results demonstrate the importance and potential of such capping layers in preserving the bulk properties of correlated oxides in their NSR, enabling more accurate probes for their underlying physics and offering a route for their integration into devices.
We have combined extensive density functional theory calculations with an evolutionary algorithm to investigate possible structural models for two-dimensional (2D) Pb films supported on the Al13Co4(100) quasicrystal approximant surface. The minimization of the total energy while maximizing the atomic density in the layer leads to 2D atomic arrangement with pentagonal motifs, reflecting the symmetry of the substrate. Our findings show that the 2D structure can be interpreted as a stable structure with 16 Pb atoms per surface cell in the film, in line with the measured coverage. This conclusion is also supported by the reasonable agreement between the experimental scanning tunneling microscopy images and those simulated using this structural model. Alternatively, a metastable 2D film made of 15 Pb atoms fits with the experimental observations. This study opens a route toward the prediction of supported complex 2D films.
Er3+-doped Bi2O3 films were sputter deposited on Si(100) substrates at room temperature with H2O vapor as an oxygen source gas. Crystal phases appearing after postannealing in an O2 atmosphere included single-phases of α-Bi2O3, γ-Bi2O3, and δ-Bi2O3, as well as a mixed phase of α-Bi2O3 and γ-Bi2O3. Selection of the crystal phase was possible in terms of H2O pressure and postannealing temperature. Photoluminescence spectra from Er3+ ions excited at a laser wavelength of 532 nm revealed distinct spectral features specific to the crystal phases. A clear crystal-field splitting feature consisting of eight emission lines was observed in PL spectra from sufficiently oxidized α-Bi2O3:Er films, indicating that Er3+ ions occupied low-symmetry C2v sites of Bi3+. The optimum annealing temperature for optical activation of Er3+ ions was between 400 and 450 °C. The emission intensity of α-Bi2O3:Er deposited with H2O was seven times higher than that of α-Bi2O3:Er deposited with O2 probably because larger numbers of Er3+ could substitute Bi3+ sites under reduced condition. Increasing deposition temperature lowered the emission intensity due to the loss of OH and H species from the as-deposited films. The emission spectra of γ-Bi2O3:Er were featureless and its emission intensity was one order of magnitude lower than that of α-Bi2O3:Er. The low-temperature phase of δ-Bi2O3:Er turned out to be entirely emission inactive. Reactions at the interface between the Bi2O3 film and the Si substrate were promoted upon postannealing at 500 °C. The resulting Bi2SiO5:Er exhibited a weak emission spectrum with four emission lines, which reflected occupation at tetragonal Bi3+ sites in the (Bi2O2)2− layers.
An in-depth understanding of charge transfer processes at the electrochemical interfaces is a critical knowledge gap impeding the design of energy storage materials. X-ray photoelectron spectroscopy plays an important role in analyzing electronic structures of heterogeneous interfaces, such as electrode-electrolyte interphases. Correspondingly, ex situ studies based on postmortem analysis of electrode materials using x-ray techniques are widely reported in the literature but often fail to capture intermediate and transient species, which are critical for a predictive understanding of the charge transfer process. The lack of extensive in situ/operando x-ray analysis of buried interfaces in energy storage systems can be mainly attributed to technical limitations, such as the requirement of high vacuum conditions. However, in the past decade, considerable efforts have been devoted to overcoming these technical barriers and enable investigation of the solid/solid and solid/liquid interfaces. This review catalogs some of the recent progresses and new experimental designs in the application of in situ and operando x-ray photoelectron spectroscopy toward characterizing interfacial processes and emergent properties, which can help build the design strategy for advanced batteries. The remaining challenges and future research directions are also discussed, as potential paths forward in this field.
We investigated the reduction of a CeO2(111) surface after the deposition of size-selected Pt8 clusters on it via x-ray photoelectron spectroscopy (XPS). Electron transfer from Pt to CeO2(111) results in the reduction of Ce4+ to Ce3+ and the subsequent formation of Ptδ+ atoms at the interface. We quantitatively estimated the reduction from the Ce3+/(Ce3+ + Ce4+) ratio determined from the Ce3d spectra. Approximately 30% of Ce4+ under or in the perimeter of the Pt8 cluster was reduced to Ce3+. The XPS spectra showed an increase in the ratio of Ce3+/(Ce3+ + Ce4+) with an increase in the amount of Pt8 clusters deposited on the CeO2(111) surface. The reduction of 3.7 Ce4+ to Ce3+ per Pt8 clusters, followed by the formation of 3.7 Ptδ+ atoms per Pt8 clusters was observed at 0.006 ML (mono layer) Pt coverage. The actual Ce3+ formation influenced by Pt deposition was approximately 2.5%. We estimate that approximately 30% of Ce4+ under or in the perimeter of a Pt8 cluster was reduced to Ce3+. At a higher coverage of 0.015 ML, the reduction of 2.2 Ce4+ to Ce3+ per Pt8 cluster, followed by the formation of 3.2 Ptδ+ atoms per Pt8 cluster was observed. This implies the aggregation of Pt clusters occurred at a higher Pt coverage. Atomically precise quantification of the electron transfer across the Pt–CeO2(111) interface can enable an improved understanding of the metal-support interaction.
We develop a resonant scattering technique to measure cation stoichiometry of lanthanum-doped BaSnO3 (BSO) thin films on a DyScO3 substrate. Samples are grown by a hybrid molecular beam epitaxy method and display high room-temperature carrier mobilities. The measured thin films are grown with widely differing cation arrival rates, with Ba being evaporated from an elemental source and Sn from a SnO2 source. Differences in mobilities in these films may arise from differences in Ba/Sn cation stoichiometry. Owing to the similar scattering strength of the Ba and Sn cations, odd-order Bragg peaks of BSO are particularly sensitive to the material's cation stoichiometry, i.e., the Ba/Sn ratio. Sensitivity to cation stoichiometry is further enhanced using the technique of resonant x-ray scattering, which changes the scattering strength of a single element across the Ba L absorption edges. We determine that the Ba/Sn cation stoichiometry varies from unity by less than 1% for films of mobility ranging from 84.8 to 144 cm(2)/(V s) and conclude that the mobility of these films scales with film thickness and growth rate rather than stoichiometry.
A novel precursor, 1,1,1-tris(dimethylamino)disilane {TADS, [(H3C)2N]3Si2H3}, is used to deposit silicon dioxide (SiO2) films in a temperature range of 115–480 °C by thermal atomic layer deposition (tALD) and plasma-enhanced atomic layer deposition (PEALD) techniques. Compared to tris(dimethylamino)silane (TDMAS), the additional Si-Si bond in TADS is expected to enhance the reactivity of the molecule due to the polarization of the bond. In the tALD process, TADS gives a growth rate of 0.06 nm/cycle, which is approximately 20% higher than that of TDMAS, and an excellent conformality (>95% step coverage) in high aspect ratio nanotrenches (6:1). In the case of the PEALD process, TADS leads to not only a higher or at least comparable growth rates (0.11 nm/cycle), but also a higher bulk film density (∼2.38 g/cm3). As a result, the PEALD SiO2 films of TADS show a wet-etch rate down to 1.6 nm/min in 200:1 HF, which is comparable to that of the thermal oxide. Analyzed with Fourier-Transform Infrared (FTIR), the SiO2 films contain predominant Si−O bonds and a low level of Si−H and O−H bonds, consistent with the observed high wet-etch resistance. Furthermore, the PEALD SiO2 films deposited at 310 °C have at least 75% step coverage in high aspect ratio nanotrenches, suggesting that TADS is applicable for forming high-quality SiO2 films on both planar and patterned surfaces.
Hybrid MoS2-based molybdenum thiolate thin films with selected organic motifs are grown using molecular layer deposition (MLD), allowing for tunable optoelectronic film properties. The thin films are deposited at 170 °C using the metal precursor molybdenum hexacarbonyl and one of the three organic precursors: 1,2-ethanedithiol, 1,4-butanedithiol, and 1,4-benzenedithiol. The resulting Mo-ethanethiolate, Mo-butanethiolate, and Mo-benzenethiolate films show saturating growth with a growth per cycle of 1.2, 1.0, and 1.5 Å/cycle, respectively. Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, Raman spectroscopy, x-ray absorption spectroscopy, and x-ray diffraction are used to characterize the as-deposited films. Results show that by changing the organic precursor, the film composition as well as the optical and electronic properties can be tuned. The Mo-thiolate films grown with benzenedithiol exhibit the lowest resistivity, which at 12 mΩ cm is ∼400 times more conductive than Mo-thiolates grown with aliphatic organic linkers. All three backbone chemistries of the Mo-thiolates show an optical bandgap between 2.3 and 2.4 eV and mild photoconductivity response. The MLD of these Mo-thiolate films demonstrates the synthesis of transition metal-organosulfur thin films with tunable properties.
Ceria-supported Ni has shown unique catalytic activity due to unique properties of small Ni particles and strong metal-support interaction. Identification of adsorption sites and understanding the chemical interaction over Ni-ceria at the fundamental level provide crucial insights into the reaction pathways of complex catalytic processes. In this study, to probe the surface sites, the adsorption of CO was carried out with model Ni/ceria systems consisting of Ni nanoparticles vapor-deposited on well-ordered CeOx(111) (1.5 < x < 2) thin films using infrared reflection absorption spectroscopy (IRRAS) and temperature-programmed desorption (TPD) under ultrahigh vacuum (UHV) conditions. Fully oxidized CeO2(111) (Ce4+) and partially reduced CeO1.75(111) (Ce4+/Ce3+) thin films were grown on Ru(0001) to examine the role of the ceria support. Ni with low coverages (e.g., 0.2 ML) grows forming small two-dimensional particles on ceria at 300 K, which develop into three-dimensional clusters after heating to 700 K. In the absence of Ni, CO adsorption at 1 mTorr at 100 K shows distinct IR bands at 2158 cm−1 on CeO2 and 2165 cm−1 on CeO1.75. Bridging and atop IR bands associated with CO adsorption over metallic Ni were observed on the Ni-CeO1.75 surface at 300 K under UHV conditions. CO adsorption over Ni0 was also observed over as-deposited Ni on CeO2. However, a new IR band at 2146 cm−1 due to CO adsorption over Ni2+ species was detected at 100 K over the annealed Ni particles on CeO2. CO IRRAS data suggest the oxidation of Ni to Ni2+ on CeO2 and the formation of predominant Ni2+ species with heating, which is further confirmed with CO TPD data and previous x-ray photoelectron spectroscopy results.
