Diffusion Of Metal Atoms Research Articles

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Impacts of terminal molecules on metal-atom diffusion into alkane self-assembled-monolayer films: first-principles study

The effects of terminal molecules on metal-atom diffusion into alkane self-assembled-monolayer (SAM) films were studied using first-principles calculations. It was shown that the diffusion of Al and Au atoms easily occurs when the alkane molecules have a CH3 termination, while the diffusion is suppressed by COOK terminal molecules for Au atom and by COOH and COOK molecules for Al atom. This is in good agreement with experiments. We showed that such suppression is caused by the orbital-hybridization-induced interaction between a metal atom and terminal molecules: the ionic bonding between negatively charged Au and surrounding positively charged K atoms, and the covalent-like bonding between Al and neighboring O atoms. We also showed that, once the metal atoms form films on the SAM surface, the metal atoms bond to each other and stabilize, thus also suppressing the diffusion into the SAM substrate.

Thermally stable Ni@SiO2 core-shell nanoparticles for high-temperature solar selective absorber

Improving the thermal stability of solar absorbers persists to be the challenge for solar-thermal technologies operated at high temperatures, such as concentrated solar power trough system. Herein, we report a rationally designed solar selective absorber based on Ni@SiO2 core–shell nanoparticles with significantly improved thermal stability at temperatures up to 700 °C. Ni nanoparticles were encapsulated into dense ceramic SiO2 nanoshells, which effectively prevented the formation of Ni-alloys and oxides by inhibiting diffusion of both inclusion Ni and metallic atoms from the substrate at high temperatures. Besides, the spectral absorption properties of the Ni@SiO2 solar absorber can be optimized by tuning the sizes of Ni cores and absorbing structures. As a result, high solar absorptance (∼0.913) but low thermal emittance (∼0.085) have been achieved for a double-layer Ni@SiO2 absorber with Ni cores of 40 nm and 80 nm. The excellent thermal stability and high absorption selectivity of the Ni@SiO2 solar absorber indicate its great potential for high-temperature solar-thermal conversion applications.

Study of Pt growth on Si, Al2O3, Au, and Ni surfaces by plasma enhanced atomic layer deposition

Atomic layer deposition is a powerful technique for achieving atomic-level control in the deposition of thin films and nanoparticles. The ultrathin noble metal films can be applied in many functional devices, but it is a challenge to obtain such films since the island growth mode generally happens instead of the layer growth mode. In this work, the nucleation and growth of platinum on Si, Al2O3, Au, and Ni substrates were studied using (MeCp)PtMe3 and O2 plasma as a precursor and a co-reactant, respectively. The evolution of Pt coverage on different surfaces was observed and discussed based on the experimental results by x-ray photoelectron spectra. The chemical adsorption of the precursor and the following processes like metal atom diffusion on substrate surfaces and up-stepping onto the existing metal islands were considered to dominate the growth before continuous films formed. The chemisorption determined the metal coverage on bare substrate surfaces, and the total coverage was influenced by metal atom diffusion and up-stepping behaviors that are determined by surface energy relationships between the deposited metal and substrate surfaces. Pt films were easier to form on Al2O3 and Ni surfaces compared with on Si and Au surfaces, respectively. A model was proposed to help to understand the mechanisms in the nucleation and growth processes, involving the chemisorption, diffusion, and up-stepping, which contributed to prepare ultrathin continuous Pt films on different substrates.

Vacancy-cluster and off-lattice metal-atom diffusion mechanisms in transition metal carbides

Using ab initio simulations, we report potential metal atom diffusion mechanisms in the group IVB and VB transition metal carbides. By computing the metal vacancy formation energies of vacancy clusters, we find that a metal vacancy surrounded by six carbon vacancies is the lowest metal vacancy formation energy structure for the group IVB carbides while the lowest energy for the group VB carbides is a metal vacancy surrounded by two carbon vacancies. The vacancy cluster mechanisms reveal activation energies that are consistent with experiments in both TiC (group IVB) and TaC (group VB). We also report that an off-lattice diffusion mechanism, that is only energetically favorable in the group IVB transition metal carbides, has a lower formation energy than the regular vacancy cluster mechanism. This new mechanism shows lower formation energies for a given carbon vacancy concentration which indicates this off-lattice mechanism might be the most dominant metal-atom diffusion mechanism among the existing mechanisms proposed for the group IVB metal carbides.

Sulfur stabilizing metal nanoclusters on carbon at high temperatures

Supported metal nanoclusters consisting of several dozen atoms are highly attractive for heterogeneous catalysis with unique catalytic properties. However, the metal nanocluster catalysts face the challenges of thermal sintering and consequent deactivation owing to the loss of metal surface areas particularly in the applications of high-temperature reactions. Here, we report that sulfur—a documented poison reagent for metal catalysts—when doped in a carbon matrix can stabilize ~1 nanometer metal nanoclusters (Pt, Ru, Rh, Os, and Ir) at high temperatures up to 700 °C. We find that the enhanced adhesion strength between metal nanoclusters and the sulfur-doped carbon support, which arises from the interfacial metal-sulfur bonding, greatly retards both metal atom diffusion and nanocluster migration. In catalyzing propane dehydrogenation at 550 °C, the sulfur-doped carbon supported Pt nanocluster catalyst with interfacial electronic effects exhibits higher selectivity to propene as well as more stable durability than sulfur-free carbon supported catalysts.

Transient increase of Tc and Jc in superconducting/metallic heterostructures

The presence of a metallic layer can influence the properties of high-temperature superconductors underneath. We investigate the influence of metallic structures deposited in form of nanoparticles or continuous layers by electron beam evaporation or ion beam sputtering on the properties of Y1Ba2Cu3O7-x (YBCO) thin films. To generally avoid diffusion of metal atoms an additional barrier layer is introduced. Detailed measurements of the magnetic moment of the superconductor as a function of temperature and magnetic field have been performed using SQUID magnetometry. It is found that the modification of the superconducting properties of coated YBCO strongly depends on the deposition method of the metal on top rather than the type of metal (Ni or Ag), its magnetic properties (ferromagnetic or paramagnetic) or its morphology (nanoparticles or thin film). The main result is a transient increase of the critical temperature Tc and critical current density Jc that was observed for samples prepared by electron beam evaporation.

Enhancing sintering resistance of atomically dispersed catalysts in reducing environments with organic monolayers

Atomically dispersed precious metal catalysts maximize atom efficiency and exhibit unique reactivity. However, they are susceptible to sintering. Catalytic reactions occurring in reducing environments tend to result in atomically dispersed metals sintering at lower temperatures than in oxidative or inert atmospheres due to the formation of mobile metal-H or metal-CO complexes. Here, we develop a new approach to mitigate sintering of oxide supported atomically dispersed metals in a reducing atmosphere using organophosphonate self-assembled monolayers (SAMs). We demonstrate this for the case of atomically dispersed Rh on Al2O3 and TiO2 using a combination of CO probe molecule FTIR, temperature programmed desorption, and alkene hydrogenation rate measurements. Evidence suggests that SAM functionalization of the oxide provides physical diffusion barriers for the metal and weakens the interactions between the reducing gas and metal, thereby discouraging the adsorbate-promoted diffusion of metal atoms on oxide supports. Our results show that support functionalization by organic species can provide improved resistance to sintering of atomically dispersed metals with maintained catalytic reactivity.

Contact Melting of Metals Explained via the Theory of Quasi-Liquid Layer

It has been shown that the contact melting rate for metals is determined by the fact that at least one of them has a quasi-liquid layer on the surface. As a result, the diffusion of metal atoms occurs in the liquid phase, and not in the solid phase, which determines the characteristic contact melting time (seconds and minutes).

Enhanced thermal stability of the metal/dielectric multilayer solar selective absorber by an atomic-layer-deposited Al2O3 barrier layer

To improve the thermal stability of multilayered solar selective absorber, an atomic-layer-deposited (ALD) Al2O3 layer was adopted to suppress the diffusion of metal atoms from the bottom reflection layer. The designed film structure was as that: Cu (>100.0 nm)/Al2O3 (59.8 nm)/Cr (17.8 nm)/SiO2 (66.3 nm)/Cr (4.4 nm)/SiO2 (83.0 nm). Multi-target magnetron sputtering method was employed to fabricate Cu, SiO2, and Cr layers, while for the Al2O3 layer, it was prepared by ALD. The sample has a solar absorptance of about 95.4% and a thermal emittance of 0.196 at the temperature of 773 K. The sample with the Al2O3 barrier layer work stably at 500 °C for 72 h, demonstrating that the proposed multilayer solar selective absorber is suitable for applications at middle-temperature solar thermal conversion systems.

Size-controlled in situ synthesis of metal–polymer nanocomposite films using a CO2 laser

In situ synthesis of metal–polymer nanocomposite films by irradiating a CO2 laser for several seconds is a new alternative to fabricate metal–polymer nanocomposite films. The main features of this method are that the number density of the synthesized metal nanoparticles is very high so that the optical density easily exceeds 0.5 ~ 1.5 for the film thickness of ~ 200 nm, and owing to the short fabrication time and the use of non-focused laser beam, large-scale processing is possible. For this technique to be applicable for a variety of purposes, an important question is how and how much we can control the film properties. In this work, we demonstrate that the size and size distribution of metallic nanoparticles in the synthesized nanocomposite films can be well controlled by the choice of the laser power and irradiation time as well as the concentrations of nanoparticle precursor. Properties of the synthesized films can be roughly understood by considering the diffusion of metallic ions, atoms, and nanoparticles in the polymer film under the elevated temperature induced by the CO2 laser.

A sintering-avoiding support

Catalysis Heterogeneous catalysts containing dispersed metal nanoparticles (NPs) on supports such as metal oxides or carbon can undergo sintering at high temperatures, a process in which the diffusion of metal atoms results in larger NPs with smaller surface areas. Chen et al . reduced a melt of NaBH4, NaNH2, and noble metal salts (Au, Pd, and Pt) to form metal NPs encapsulated by hexagonal boron nitride nanosheets. The supported NPs were released by washing away excess salt and could be loaded onto conventional supports such as alumina. The strong interaction of Pd NPs with these nanosheet supports allowed them to resist sintering up to 950°C and enabled their use for CO oxidation in the presence of water and hydrocarbons. ACS Cent. Sci. 6 , 1617 (2020).

Stability of heterogeneous single-atom catalysts: a scaling law mapping thermodynamics to kinetics

Heterogeneous single-atom catalysts (SACs) hold the promise of combining high catalytic performance with maximum utilization of often precious metals. We extend the current thermodynamic view of SAC stability in terms of the binding energy (Ebind) of single-metal atoms on a support to a kinetic (transport) one by considering the activation barrier for metal atom diffusion. A rapid computational screening approach allows predicting diffusion barriers for metal–support pairs based on Ebind of a metal atom to the support and the cohesive energy of the bulk metal (Ec). Metal–support combinations relevant to contemporary catalysis are explored by density functional theory. Assisted by machine-learning methods, we find that the diffusion activation barrier correlates with (Ebind)2/Ec in the physical descriptor space. This diffusion scaling-law provides a simple model for screening thermodynamics to kinetics of metal adatom on a support.

Phase formation in magnetron sputtered CrMnFeCoNi high entropy alloy

Thin films of the CrMnFeCoNi high entropy alloy were deposited by magnetron sputtering from a sintered equimolar target. The substrate temperature and bias were varied during deposition, and the structure, morphology and elemental distribution were studied in detail. All films formed phase mixtures of multiple crystal structures. This contrasts with studies on the bulk alloy, where it typically forms a single phase with a simple cubic closed packed (ccp) structure, with other phases precipitating only after long annealing times. For higher substrate temperatures, we observed a mixture of phases with ccp and bcc (body centered cubic) structures, and the intermetallic phases σ-phase and L10, the first three being the predicted equilibrium phases at the deposition temperature. For room temperature depositions, we found evidence of very limited diffusion of metal atoms during the deposition. These films formed a mixture of a ccp and the intermetallic χ-phase. Two mechanisms can be distinguished that govern the phase formation at lower and higher temperatures. From the present results and comparisons with the literature, we also discuss why the small grain size, the low process temperature, and the fast surface diffusion during synthesis causes magnetron sputtering to yield different results compared to bulk synthesis from the melt. These principles explain why it is easier to form the equilibrium phases by sputtering, and why a single ccp phase should not be expected as a rule for this deposition method. Following the thermodynamic principles of high entropy alloys, this may also be the case in other high entropy alloy systems.

Investigating the Formation of MoSe2 and TiSe2 Films from Artificially Layered Precursors.

The reaction of ultrathin layers of Mo and Ti with Se was investigated, and significantly different reaction pathways were found. However, in both systems postdeposition annealing results in smooth dichalcogenide films with specific thicknesses determined by the precursor. X-ray diffraction (XRD) patterns of as-deposited Mo|Se films around a 1:2 ratio of Mo to Se contain weak, broad reflections from small and isolated MoSe2 crystallites that nucleated during deposition and a sharper intensity maximum resulting from the composition modulation created from the alternating deposition of Mo and Se layers. In contrast, as-deposited Ti|Se films around a 1:2 ratio of Ti to Se contain narrow and intense 00l reflections from TiSe2 crystallites and do not contain a Bragg reflection from the sequence of deposited Ti|Se layers. The as-deposited TiSe2 crystallites have a larger c-axis lattice parameter than was previously reported for TiSe2, however, which suggests a poor vertical interlayer registry and/or high defect densities including interstitial atoms. In-plane XRD patterns show the nucleation of both TiSe2 and Ti2Se during deposition, with the Ti2Se at the substrate. For both systems, annealing the precursors decreases the peak width and increases the intensity of reflections from crystalline TiSe2 and MoSe2. Optimized films consist of a single phase after the annealing and show clear Laue oscillations in the specular XRD patterns, which can only occur if a majority of the diffracting crystallites in the film consist of the same number of unit cells. The highest quality films was obtained when an excess of ∼10% Se was deposited in the precursor, which presumably acts as a flux to facilitate diffusion of metal atoms to crystallite growth fronts and compensates for Se loss to the open system during annealing.

Suppressing Ion Migration Enables Stable Perovskite Light‐Emitting Diodes with All‐Inorganic Strategy

AbstractStability issue is one of the major concerns that limit emergent perovskite light‐emitting diodes (PeLEDs) techniques. Generally, ion migration is considered as the most important origin of PeLEDs degradation. In this work, an all‐inorganic device architecture, LiF/perovskite/LiF/ZnS/ZnSe, is proposed to address this imperative problem. The inorganic (Cs1−xRbx)1−yKyPbBr3 perovskite is optimized with achieving a photoluminescence quantum yield of 67%. Depth profile analysis of X‐ray photoelectron spectroscopy indicates that the LiF/perovskite/LiF structure and the ZnS/ZnSe cascade electron transport layers significantly suppress the electric‐field‐induced ion migrations of the perovskite layers, and impede the diffusion of metallic atoms from cathode into perovskites. The as‐prepared PeLEDs display excellent shelf stability (maintaining 90% of the initial external quantum efficiency [EQE] after 264 h) and operational stability (half‐lifetime of about 255 h at an initial luminance of 120 cd m−2). The devices also exhibit a maximum brightness of 15 6155 cd m−2 and an EQE of 11.05%.

Probing pristine and defective NiB6 monolayer as promising anode materials for Li/Na/K ion batteries

Two-dimensional material NiB6 has attracted attention and predicted to be a potential candidate for next-generation nanoscale devices duo to its excellent mechanical and electronic properties. In this section, by using first-principles calculations, the pristine and defect-containing NiB6 substrates as a potential anode for alkali metal ion batteries are systematically studied. Our calculated results show that the pristine NiB6 monolayer might has promising application for alkali-ion batteries due to its large adsorption energy, high specific capacity (1301.61 mA·h·g−1), and fast migration capability (energy barrier is 0.43/0.23/0.14 eV for Li/Na/K). Meanwhile, pristine NiB6 monolayer can also provide an appropriate average open circuit voltage (0.96/0.71/0.69 V for Li/Na/K). Alkali metal atoms adsorption and diffusion mechanism on the selective defective NiB6 substrates show that defect sites tend to act as metal trapping sites, which improve the NiB6 monolayer adsorption ability of alkali metal atoms but hinder the metal atoms diffusion around the defects. The present study lays a foundation for ion battery applications of NiB6 monolayer and offers a better understanding of the defect effects on alkali metal atoms adsorption and diffusion properties on low-dimensional materials.

Anti-Corrosion Layer Embedded Top Cathode for Semi-Transparent Perovskite Solar Cells

Power conversion efficiency (PCE) of perovskite solar cells (PSCs) has exceeded 25%, which is comparable to that of silicon solar cells and makes PSCs promising candidates for the next generation photovoltaics. However, their commercialization is still seriously hindered by the low device stability. Besides the well-known thermal and chemical factors, another major cause of the instability is the interaction of top metal electrodes with the Iodine-containing compounds decomposed from perovskite absorber layers. Both these volatile compounds and metal atoms can diffuse towards each other through hole transport layers and consequently lead to the degradation of device performance. In order to resolve this problem, an anti-corrosion layer against undesired mass transport is required to be inserted between the metal electrode and the perovskite absorber layer. Molybdenum is intrinsically resistant to the corrosive reaction of Mo to MoI3 because of its higher Metal-Metal bonding energy (435.5 kJ/mol) than Metal-Iodine bonding energy (266.9 kJ/mol). Besides, it has also been proven that Mo is stable against metal atom diffusion into perovskite by theory analysis. In this work, Mo was chosen to act as the anti-corrosion layer embedded in Oxide/Metal/Oxide (OMO) transparent cathodes for inverted semi-transparent PSCs. Bottom indium zinc tin oxide (IZTO) layers with a thickness of 30nm were fabricated via facing target sputtering using a low power before the deposition of magnetron sputtered Mo to protect organic layers from sputter damage. The electrical and optical characteristics of IZTO/Mo/IZTO (IMI) transparent electrodes were analyzed with dependence on Mo thickness and top IZTO thickness. Inverted semi-transparent PSCs integrated with the optimized IMI top cathode were assembled, in which the diffusion of iodine ions and metal atoms were more efficiently blocked compared to the reference device without a Mo anti-corrosion layer.

Catalytic Etching of Platinoid Gauzes during the Oxidation of Ammonia by Air: Etching of the Frontal Surface of the Platinoid Gauze in the Course of NH3 Oxidation at 1133 K

The morphology, microstructure, and chemical composition of the surface and near-surface layers of polycrystalline wire of an industrial platinoid gauze composed of Pt (81 wt %), Pd (15 wt %), Rh (3.5 wt %), and Ru (0.5 wt %) are investigated by scanning electron microscopy and energy dispersive X-ray spectroscopy. After oxidizing NH3 with air at T = 1133 K under a pressure of 3.6 bar for 50 h, a continuous rough layer of the cauliflower-type agglomerates formed during catalytic etching is detected on the frontal surface of the gauze. On the surface of wire fragments from 100 to 150 μm in size with a smooth micrograined structure, nanometer-size etch pits are detected at a concentration of 1.0 × 108–6.0 × 108 cm–2, which may be etching sites of the hotspot type. The growth of etch pits and the formation of crystalline terraces on the grain surface are caused by the surface diffusion of metal atoms. The continuous etching layer contains porous crystalline agglomerates (cauliflowers) with a linear size of 3 to 18 μm (mean size about 10 μm) at a concentration of 4.9 × 105 cm–2. Pores with a diameter of 0.1 to 1.7 μm are detected on the surface of agglomerates at a concentration of 1.3 × 107 cm–2. The specific surface area of the platinoid gauze, which is calculated from microscopic images taking into account the surface area of agglomerates and pores, is about 260 cm2/g. In the process of highly exothermic oxidation of NH3 with oxygen, on the surface of agglomerates with a low concentration of defects and in pore voids 5–15 μm in width and up to 10 μm in depth with an increased specific surface area and a high concentration of defects, vapor of volatile oxides and metals that are formed at hot regions of the bottom of pore voids can be condensed on the overlaying cold regions of the surface of agglomerates and single crystals. These processes give rise to the formation of a continuous etching layer of porous crystalline agglomerates, massive single crystals, and deep pore voids. The formed etching layer substantially increases the specific surface area of the catalyst, which leads to an increase in the volumetric rate of NH3 oxidation that accelerates the etching process.

Overview of Bias Temperature Instability in Scaled DRAM Logic for Memory Transistors

Continued scaling of DRAM technologies has required a limitation of the power dissipation from the logic components on-chip, while downscaling both transistor oxide thickness and gate length. One route to enable further scaling, while circumventing excessive leakage currents, is the integration of high-permittivity ( ${\kappa }$ ) metal-gate (HKMG) components into the logic and high-voltage (e.g., I/O) devices. The requirement of a gate-first flow for devices in the peripheral region introduces significant reliability challenges. Even though Negative Bias Temperature Instability (NBTI) performance of CMOS and memory thermal budget compatible transistors are aligned with conventional HKMG integration with thin oxide devices, it is not the case for thick oxide devices. In particular, it will be shown that strong NBTI lifetime degradation is observed as soon as high- ${\kappa }$ layers are deposited on top of the thick interfacial layer. In this work, a review of the impact of these high- ${\kappa }$ layers on the NBTI high voltage logic for memory devices is presented. The stress induced degradation is correlated to a diffusion of metal atoms from the HKMG gate stack towards the silicon surface. Directions for reliability improvements are then defined. The presence of Nitrogen throughout the HKMG stack can originate either from high- ${\kappa }$ processing or metal-nitride gate electrode. It is shown that preventing nitrogen diffusion towards the Si/SiO2 interface region, together with AlOx and/or F incorporation at the HKMG interface, can tune device threshold voltage and modulate access to donor trap-defect bands. The result of these effects is a vast improvement in NBTI performance. A detailed study of NBTI-degradation, supported by physical analysis, assessing the impact of various tuning components within the stack (interface layer, high- $\kappa $ fluorination and/or cap, metal gate) will be presented. Potential solutions for this reliability challenge will be reported.