ALD Zn(O,S) Thin Films’ Interfacial Chemical and Structural Configuration Probed by XAS

The ability to precisely control interfaces of atomic layer deposited (ALD) zinc oxysulfide (Zn(O,S)) buffer layers to other layers allows precise tuning of solar cell performance. The O K- and S K-edge X-ray absorption near edge structure (XANES) of ∼2–4 nm thin Zn(O,S) films reveals the chemical and structural influences of their interface with ZnO, a common electrode material and diffusion barrier in solar cells. We observe that sulfate formation at oxide/sulfide interfaces is independent of film composition, a result of sulfur diffusion toward interfaces. Leveraging sulfur’s diffusivity, we propose an alternative ALD process in which the zinc precursor pulse is bypassed during H2S exposure. Such a process yields similar results to the nanolaminate deposition method and highlights mechanistic differences between ALD sulfides and oxides. By identifying chemical species and structural evolution at sulfide/oxide interfaces, this work provides insights into increasing thin film solar cell efficiencie

The interface of SiO2/ZnS films studied by high resolution X-ray photoluminescence

Sharp interfaces in optoelectronic devices are key for proper band alignment. Despite its benefits as buffer layer, ZnS deposited via atomic layer deposition (ALD) renders intermixed interfaces to its substrate, which can be detrimental for device performance. Here, we are attempting to elucidate the chemical species deriving from this metal-oxide to metal-sulfide transition studying ultrathin film ZnS on SiO2 using high resolution X-ray photoluminescence spectroscopy (XPS). Regarding the S 2p spectra after a deposition of only three cycles of ZnS, we discover the many different chemical species in which S is present. These include intermediate oxides such as SO42-. These species become more obvious as we tilt the sample in the XPS chamber to shallower angles. Comparing the Si 2p and S 2p high resolution peaks in the depth profile, one can clearly uncover the confinement of SO42- to the interface of the underlying substrate. This may indicate that SiO2/ZnS interfaces contain interfacial sulphates that likely alter the electronic configuration of this interface

Comparing physiologically relevant corrosion performances of Mg AZ31 alloy protected by ALD and sputter coated TiO2

The utilization of Mg alloys for biomedical applications is so far underexplored due to the accelerated corrosion hampering patient recovery post implantation. Here, we explore the effectiveness of corrosion reduction of an AZ31 alloy in Simulated Body Fluid when coated with a 40 nm sputtered TiO2 layer and compare it to a similar coating made by Atomic Layer Deposition (ALD). Potentiodynamic polarization and hydrogen evolution experiments were performed on coated samples having different surface roughness and 3D topologies. Interestingly, ALD layers reduce corrosion current density by 94% on Ra = 118.6 ± 5.1 nm and 93% on Ra = 4794.3 ± 49.4 nm, whereas sputtered only by 84% on Ra = 118.6 ± 5.1 nm and 60% on Ra = 4794.3 ± 49.4 nm. Particularly on 3D aspects, the ALD coatings are superior, where a scaffold of 85% porosity with 1 mm pore sizes released 68% lower hydrogen compared to the sputtered counterparts. We relate these observations to the higher surface integrity, adhesion strength and lower line-of-sight restrictions of ALD compared to sputter deposition. The results can be interesting for researchers and practitioners aiming to make Mg alloys more commonplace as temporary metallic implant materials

Improving stress corrosion cracking behavior of AZ31 alloy with conformal thin titania and zirconia coatings for biomedical applications

Magnesium and its alloys have been widely studied as materials for temporary implant devices. However, corrosion-assisted cracking phenomena such as stress corrosion cracking (SCC) continue to prevent their mainstream use. For the first time, we explore the SCC susceptibility of Atomic Layer Deposition (ALD) coated AZ31 alloys in Simulated Body Fluid (SBF). Conformal 100 nm coatings of titania and zirconia were deposited on standard dogbone specimens and subjected to slow strain rate tests at 3.5 10-6 s-1 and a temperature of 37 °C. Remarkably, the SCC susceptibility index IUTS was reduced by 6% and 40% and the Iε was reduced by more than 70% and 76% with a titania and zirconia coating, respectively. Potentiodynamic polarization, hydrogen evolution and fracture behavior of the samples revealed the drastic corrosion reduction to be the main reason for the susceptibility reduction. We discuss the observed SCC behavior of our samples in light of the coatings’ electrochemical activities, wettabilities, surface integrities and mechanical properties. This straightforward conformal surface treatment can be useful as a workaround for one of the major bottlenecks of biomedical Mg based implants and hence provides a possible pathway for making them more commonplace in the field

Relating Electronic and Geometric Structure of Atomic Layer Deposited BaTiO3 to its Electrical Properties

Atomic layer deposition allows the fabrication of BaTiO3 (BTO) ultrathin films with tunable dielectric properties, which is a promising material for electronic and optical technology. Industrial applicability necessitates a better understanding of their atomic structure and corresponding properties. Through the use of element-specific X-ray absorption near edge structure (XANES) analysis, O K-edge of BTO as a function of cation composition and underlying substrate (RuO2 and SiO2) is revealed. By employing density functional theory and multiple scattering simulations, we analyze the distortions in BTO’s bonding environment captured by the XANES spectra. The spectral weight shifts to lower energy with increasing Ti content and provides an atomic scale (microscopic) explanation for the increase in leakage current density. Differences in film morphologies in the first few layers near substrate–film interfaces reveal BTO’s homogeneous growth on RuO2 and its distorted growth on SiO2. This work links structural changes to BTO thin-film properties and provides insight necessary for optimizing future BTO and other ternary metal oxide-based thin-film devices

Energy States of Ligand Capped Ag Nanoparticles: Relating Surface Plasmon Resonance to Work Function

The work function (WF) and surface plasmon resonance (SPR) of organic ligand capped Ag nanoparticles (NPs) have been studied experimentally and computationally. Experimental observations reveal a significant increase in WF as the size of ligand-capped Ag NPs increases, a trend contrary to that previously observed for bare Ag NPs. Computational results confirm the effect on the WF from simplified ligand molecules and relate it to charge transfer between the Ag core and surrounding ligands. We also observe a possible coupling between increases in WF and decreases in SPR transition energy, supported by computational results and attributed to the interplay between the 4d and 5s electron states of the system. These results, along with our observations of WF dependence on ligand choice, indicate the ability to strongly engineer the electronic structure of metal NPs through size and ligand control

On the evaluation of ALD TiO2, ZrO2 and HfO2 coatings on corrosion and cytotoxicity performances

Magnesium alloys have been widely studied as materials for temporary implants, but their use has been limited by their corrosion rate. Recently, coatings have been proven to provide an effective barrier. Though only little explored in the field, Atomic Layer Deposition (ALD) stands out as a coating technology due to the outstanding film conformality and density achievable. Here, we provide first insights into the corrosion behavior and the induced biological response of 100 nm thick ALD TiO2, HfO2 and ZrO2 coatings on AZ31 alloy by means of potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), hydrogen evolution and MTS colorimetric assay with L929 cells. All three coatings improve the corrosion behavior and cytotoxicity of the alloy. Particularly, HfO2 coatings were characterized by the highest corrosion resistance and cell viability, slightly higher than those of ZrO2 coatings. TiO2 was characterized by the lowest corrosion improvements and, though generally considered a biocompatible coating, was found to not meet the demands for cellular applications (it was characterized by grade 3 cytotoxicity after 5 days of culture). These results reveal a strong link between biocompatibility and corrosion resistance and entail the need of taking the latter into consideration in the choice of a biocompatible coating to protect degradable Mg-based alloys

Ultra-Cheap Renewable Energy as an Enabling Technology for Deep Industrial Decarbonization via Capture and Utilization of Process CO2 Emissions

Rapidly declining costs of renewable energy technologies have made solar and wind the cheapest sources of energy in many parts of the world. This has been seen primarily as enabling the rapid decarbonization of the electricity sector, but low-cost, low-carbon energy can have a great secondary impact by reducing the costs of energy-intensive decarbonization efforts in other areas. In this study, we consider, by way of an exemplary carbon capture and utilization cycle based on mature technologies, the energy requirements of the “industrial carbon cycle”, an emerging paradigm in which industrial CO2 emissions are captured and reprocessed into chemicals and fuels, and we assess the impact of declining renewable energy costs on overall economics of these processes. In our exemplary process, CO2 is captured from a cement production facility via an amine scrubbing process and combined with hydrogen produced by a solar-powered polymer electrolyte membrane, using electrolysis to produce methanol. We show that solar heat and electricity generation costs currently realized in the Middle East lead to a large reduction in the cost of this process relative to baseline assumptions found in published literature, and extrapolation of current energy price trends into the near future would bring costs down to the level of current fossil-fuel-based processes