AVS 70 Session CA-ThP: Chemical Analysis and Imaging of Interfaces Poster Session

Complex oxide interfaces have captivated the research community in the last 15 years due to the potential emergent nanoscale phenomena, owing to the strong interaction among charge, spin, orbital, and structural degrees of freedom.Studying these interfaces across a few unit cells is challenging with conventional methods, as optical probes tend to be around 1 μm in diameter. This makes it’s impossible to study an individual interface of a vertically stacked transition metal oxide from a cross-sectional sample as the optical probe collects an averaged signal from each layer. In this talk, electron probes are used to overcome the diffraction-limit set by optical probes. This is done by converging electrons to a focused beam to collect spectroscopic signal from an oxide-interface with electron energy loss spectroscopy (EELS). A monochromated aberration corrected scanning transmission electron microscopy (MAC-STEM) is used to couple unprecedented energy resolution and spatial resolution to do vibrational spectroscopy at atomic resolution. When the EELS aperture is in conventional settings, where the bright field disc is perfectly aligned with the entrance aperture, the EELS signal is delocalized and dominated by optical selection rules. Accordingly, the bright field disc is electrostatically shifted by the projector lens so that the dark field disc is highlighted, which describes a technique known as off-axis EELS. We use this to create a localized signal dominated by impact scattering so that the long-range contributions are suppressed, and band edges can be resolved without plasmon interference. Ultimately, we aim to introduce a novel technique to determine the structure-property relationship of complex oxides to bring oxide-electronics to new heights.

The biological secretions from bacterial biofilms contain fatty acids, metabolites, and other components of the extracellular polymeric substances (EPS). These secretions can result in corrosion of hard surfaces. Fatty acids produced by Paenibacillus sp. 300A, Acidovorax JHL-9, and Shewanella ondenesis MR-1 are thought to degrade glasses, polymers, and other materials. Observation of these fatty acids and other metabolites like quinolones, flavonoids, and quorum sensing molecules help reveal the metabolic process that can contribute to material degradation. The depth profiling capabilities of ToF-SIMS are particularly useful for distinguishing processes at the air-biofilm interface from those at the biofilm-material interface. However, ToF-SIMS depth profiling by gaseous cluster ion beams is only reliable for elucidating biofilm composition and structures down to a depth of a few micrometers and biofilms can grow to thicknesses surpassing a millimeter [1]. Ablation with 800 nm, ~75 fs laser pulses (fs-LA) has used for layer removal and depth profiling at much greater depths and without damage to the underlying biofilm [2, 3]. Femtosecond-LA combined with X-ray photoelectron spectroscopy has also been demonstrated for depth profiling in polymers [1]. Stand-alone fs-LA is used here to expose buried regions of biofilms of Paenibacillus sp. 300A biofilms cultured on Si wafers and other substrates for analysis using ToF-SIMS. Comparison of the mass spectra of ablated vs. non-ablated regions shows an absence of laser-induced changes in their respective chemical compositions, demonstrating the feasibility of fs-LA for probing buried regions of biofilms that are not readily accessible to gaseous cluster ion beams.

[1]M.A. Baker… R.G. White, Appl. Surf. Sci. 654 (2024) 159405.

[2]S. Milasinovic,… L. Hanley, J. Vac. Sci. Technol. A 28 (2010) 647.

[3]Y. Cui,… L. Hanley, ACS Appl. Mater. Interf. 5 (2013) 9269.

Superconducting devices are important for the detection of microwave to x-ray radiation. Identifying the factors affecting oxide thickness, uniformity, and chemical composition will lead to the development of more reliable and lower noise detectors. We compare x-ray absorption spectroscopy (XAS) measurements and depth-dependent x-ray photoemission spectroscopy (XPS). We have studied two types of superconducting thin films: Al/Nb and Al /Nb-Ti-N. Depth-dependent niobium oxide (Nb₂O₅, NbO₂ and NbO) formation has been evaluated. We explore the impact of different etching and oxide cleaning treatments to the presence of these oxides. This research is supported by NASA EPSCoR Award# 80NSSC22M0173. We acknowledge the use of the research facilities at the Lawrence Berkeley National Laboratory and West Virginia University Shared Research Facilities.

Introduction

Buried interfaces in electronic devices such as batteries, solar cells, transistors etc. are highly chemically sensitive and reliable instrumentation is necessary for material development. While X-ray photoelectron spectroscopy (XPS) is a powerful method to investigate the chemical nature of surfaces, buried interfaces in device electronics are more difficult to investigate due to scattering. Hard X-rays giving increased information depth, have therefore been increasingly used in the photoelectron spectroscopy field.

HAXPES and XPS applications

Having access to XPS and HAXPES X-ray sources enables measurements of core levels with different resulting kinetic energy of the photoelectrons. With a soft X-ray XPS source, the kinetic energy is low and thus the obtained information is very surface sensitive. With Hard X-rays it is possible to be both surface and bulk sensitive, as electrons stemming from deep core levels will have lower kinetic energy and contain more surface sensitive information. Similarly, electrons stemming from shallow “XPS” core levels will have higher kinetic energy and contain more bulk sensitive information. This is especially valuable when detecting artefacts formed by sample exposure to different environments (e.g. air, moisture, heat, cold etc.) or by preparation steps known to induce chemical changes on the surface (e.g. sputtering). Scienta Omicron’s HAXPES Lab uses both an XPS Al Kα source and a monochromatic Ga Kα MetalJet HAXPES source with excitation energy of 9.25 keV, therefore enabling artefact-free investigations with superior information depth, which clearly extends beyond limits of conventional XPS surface analysis. This unlocks a comprehensive and effective characterisation of layered materials. Combined with a hemispherical electron analyzer with a ±30 degree acceptance angle, investigation of buried interfaces, operando devices and real-world samples becomes achievable. (Fig.1). Over the years it has proven invaluable in research of semiconductor materials in thin film electronic devices including the operando characterization of bias induced changes in chemical composition of material interfaces. Other applications include polymer materials, metal surfaces and coatings. This presentation will give an overview of HAXPES applications with focus on buried interfaces in electronic devices such as passivation of luminescent quantum dots studied through a 35 nm overlayer, and semiconductor device band alignment by operando measurement with the Si substrate signal as reference detected through 20 nm TiN + 4 nm ZrO stack without sputtering and risk of artifacts.

Recent progress towards innovation in power electronics has largely been through implementation of new wide-bandgap (WBG) semiconductor materials, like SiC or GaN. Alternatively, diamond is another WBG material with a high theoretical hole mobility (1300 cm²*V^-1*s^-1), thermal conductivity, and breakdown voltage that rivals SiC and GaN, while also offering improved chemical and thermal stability over its WBG counterparts. Recently, diamond electronic devices have been fabricated displaying impressive electrical performance and thermal management, however little has been investigated for these model devices at high temperatures in combination with heavily oxidizing or reducing (“harsh”) environments. In this work, we fabricated model diamond devices that possess metal/Al₂O₃/metal/diamond, and metal/Al₂O₃/diamond interfaces by depositing the standard metal contact stack (Ti/Pt/Au) or pure Pt on un-doped and non–hydrogenated diamond followed by growth of a 40-50 nm Al₂O₃ film via ALD as a gate dielectric. We then investigated the dielectric properties of this un-gated device by conducting I-V and capacitance measurements prior to any exposure. Next, the sample surface chemistry and morphology are investigated with x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), respectively. The device is then annealed to 600 °C, followed by exposures to H₂ and O₂ partial pressures of up to 10 kPa also at elevated temperature, where XPS, SEM, and 2-point I-V are all collected at each intermediate step in situ without breaking vacuum. Overall, we defined the stability range of a model diamond interfaces at high temperature and reactive environments.

Under application of a voltage bias, asymmetric ion-movement of a mixed ionic liquid (IL), having the same anion [bis(trifluoromethylsulfonyl)imide / TFSI^-] but two different cations [1-butyl-1-methylpyrrolidinium / BMP⁺ and Rb⁺/ through a multi-layered graphene (MLG) electrode, has been detected by x-ray photoelectron spectroscopy (XPS), via recording the intensities of the C1s, N1s, F1s and Rb3d core level peaks. Accordingly, upon increasing the bias gradually from 1 to 3.5 V, we have observed up to 3-fold increase in the Rb3d peaks signal, accompanied with parallel F1s and N1s peak intensity changes, albeit to a lesser degree. The C1s peak of the graphene layer was observed to decrease, clearly indicating the increase in the surface composition of the IL. Moreover, a discernable differential change in the N1s peaks of the anion (N^-) and the cation (N⁺) was also observed.Additionally, bias dependent binding energy shifts, extracted through the changes in the positions of F1s, N1s and C1s and Rb3d peaks of the IL mixture and the C1s of the graphene electrode indicate that multi-faceted and distinct solid-liquid interfaces develop throughout the entire intercalation process with an additional and pertinent evidence for finite potential drops. Details of the experimental set-up, and dynamics of bias-induced ion movement will be presented and discussed.

MXene is a class of two-dimensional metal carbides with an empirical formula of M_n+1X_nT_x, where M is an early transition metal, X is a carbon or nitrogen, and T is a surface functional group (such as F^- or OH^-). In our recent studies^[1,2], incipient wetness impregnation was used to load platinum (Pt) onto Mo₂TiC₂ MXene to synthesize a 0.5% (wt.) Pt/Mo₂TiC₂ MXene catalyst. The catalyst containing Pt nanolayers exhibited excellent catalytic activity in alkane dehydrogenation with turnover frequencies (TOFs, converted molecules per surface Pt atom) of 0.4~1.2 s^-1 for converting methane^[1] and ethane^[2], respectively. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements, including surface spectra, mass spectral imaging, secondary electron imaging, and depth profiling, were used to investigate the surface and subsequent monolayer structures of both unsupported Mo₂TiC₂ MXene and 0.5% Pt/Mo₂TiC₂ MXene catalysts. These analyses were used to derive the structure-activity relationship between Pt/Mo₂TiC₂ catalysts and their catalytic dehydrogenation performance in this work.

[1]Li Z.,et al., Nano Research, 17 (2024) 1251–1258.

[2]Li Z,et al., Nature Catalysis,10 (2021) 882–891.

Keywords: ToF-SIMS, MXene, Catalyst, Alkane Dehydrogenation, Methane Activation

Acknowledgements:

This work was in part supported by the Louisiana Experimental Program to Stimulate Competitive Research (EPSCoR), funded by the U.S. National Science Foundation (NSF) under award number OIA-1946231 and the Board of Regents Support Fund. T.K.M. and Y.X. additionally appreciate the financial support from NSF's CBET division for the Engineering Research Initiation (ERI) award under grant number CBET-2347475 and the start-up grant from the College of Engineering and Science of Louisiana Tech University.

Support for G.B. and X.Y.Y. was provided by the strategic Laboratory Directed Research and Development (LDRD) of the Physical Sciences Directorate of the Oak Ridge National Laboratory (ORNL). ORNL is managed by UT-Battelle, LLC, for the U. S. Department of Energy (DOE) under contract number DE-AC05-00OR22725. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non -exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Complex interfaces are prevalent in the natural and industrial environments, including water pipelines, batteries, geochemical transformations, and environmental processes. Surface chemistry occurs through adsorption, reduction-oxidation, and catalysis at the air/liquid or liquid/solid interface. Uncovering these reactions are essential for addressing environmental challenges, such as rising CO₂ levels. Mitigating CO₂ levels are addressed by geological sequestration, through capture and conversion to carbonates. Iron is an earth abundant catalyst that is found in soils and dust as iron oxides. In the environment, iron interfaces interact with ions, particularly chloride, thus initiating corrosion, altering the physical structure. Here we show how surface corrosion, spontaneous reduction-oxidation, and deposition lead to film formation on iron interfaces for CO₂ capture mechanisms.

This poster will summarize our group's research on our advanced method to measure surface vibrational signatures of CO₂ adsorption and conversion to carbonate films. Polarized modulated – infrared reflection absorption spectroscopy (PM-IRRAS) was used to probe through an ultra-thin liquid layer to observe in situ reactions at the air/solution/iron interface. We observe iron surface oxidation and mineral formation, using the vibrational modes to track air adsorption and mineral film growth. The results are corroborated with XPS and ATR-FTIR spectroscopy to show the films produce signatures of different natural carbonates. In situ liquid AFM was used to verify the corrosion rate and resulting morphologies after air oxidation.

The electrolyte (from alkali and alkaline chlorides) is found to influence the type and rate of film grown at the air/electrolyte/iron interface. The divalent cations (from MgCl₂, CaCl₂, FeCl₂) are compared with the monovalent cations (from KCl, NaCl) as well as electrolytes of various pH’s (HCl, NaOH). The Cl ions are found to initiate the corrosion, pitting through the native oxide layer, thus producing nucleation sites for film growth. The adsorption of atmospheric CO₂, and O₂, results in carbonate film growth, resulting in a specific-cation effect. The Fe²⁺ that dissolves during corrosion competes for adsorption with the other cations suggesting other surface-solution mediated mechanisms exist. These findings are relevant for understanding surface chemistry impacting mechanisms of CO₂ capture, geochemical formations, and other processes at complex air/liquid/solid interfaces.