Impact Of Stoichiometry Research Articles

Quantifying relaxation time constants in MoS2xSe2(1-x) alloys: Impact of Stoichiometry and Si/SiO2 interference

Despite a decade of extensive research on two-dimensional transition metal dichalcogenides, there are still some gaps in our understanding of their remarkable properties, limiting their potential applications. One such gap relates to the properties of these alloys, which offer tunable band gaps and can serve as the basis for various optoelectronic and nonlinear optical devices. A comprehensive understanding of the ultrafast charge carrier excitation and relaxation dynamics as well as their characteristic relaxation time constants, is crucial for the further use of these materials in specific applications. In this work, we experimentally investigate the dynamics of ultrafast relaxation processes in monolayer MoS2xSe2(1-x) alloys on a Si/SiO2 substrate using time-resolved spectroscopy. We determine the characteristic relaxation time constants for structures with different stoichiometric compositions and refine these constants by accounting for interference effects arising from the substrate.

The impact of stoichiometry on the initial steps of crystal formation: Stability and lifetime of charged triple-ion complexes.

Minerals form in natural systems from solutions with varying ratios of their lattice ions, yet non-stoichiometric conditions have generally been overlooked in investigations of new formation (nucleation) of ionic crystals. Here, we investigated the influence of cation:anion ratio in the solution on the initial steps of nucleation by studying positively and negatively charged triple ion complexes and subsequent particle size evolution. Our model systems are carbonates and sulfates of calcium and barium, as it was recently shown that solution stoichiometry affects the timing and rate of their nucleation. Molecular dynamics (MD) simulations and dynamic light scattering (DLS) flow experiments show that nucleation correlates with the stability and lifetime of the initial complexes, which were significantly impacted by the cation:anion stoichiometry and ion type. Specifically, was found to have higher association constants and its lifetime was twofold longer than . Similar trends were observed for and . Contrastingly, for , was found to have lower association constants and its lifetime was shorter than . These trends in stability and lifetime follow the same asymmetrical behaviour as observed experimentally for particle formation using techniques like DLS. This suggests a causal relationship between the stability and lifetime of the initial charged complexes and the nucleation under non-stoichiometric conditions.

Impact of Stoichiometry on the Network Structure, Properties, and Processing Relationships of an Epoxy Tannic Acid Resin System

Impact of Stoichiometry on the Network Structure, Properties, and Processing Relationships of an Epoxy Tannic Acid Resin System

The Impact of Stoichiometry and Method of Synthesis of Powder Cu–Fe–Al Precursor on Stability and Activity of Ceramometals CuFeAlO/CuFeAl in High Temperature WGSR

Ceramometal catalysts CuFeAlO/CuFeAl, obtained from various powdered Cu–Fe–Al precursors differing in stoichiometry and preparation method, were characterized by physicochemical methods and studied in the water gas shift reaction (CO + H2O ⇌ CO2 + H2). The catalysts are a monolith porous composites, in which metal particles are covered with an oxide shell. The sample Cu : Fe : Al = 45 : 22 : 33, synthesized from powder precursor in two stages was shown to be the most stable. At the first stage the mechanochemical melting of Cu and Fe powders was done. The mechanochemical treatment of obtained product and Al powder was performed at the second stage. This procedure provides the most homogeneous distribution of the components in the precursor. Initial and tested at 400°C ceramometal catalysts were characterized with XRD, SEM and XPS methods. It was revealed that the most active at 350°C catalysts after reductive treatment have Cu1+ and Fe3+ sites. The least active catalysts are completely reduced to Cu0 and partially – to Fe0. It was found that the activity at 330–400°C is determined not only by iron, but also by copper active centers on the surface of the catalysts, or their combined action. Two-stage mechanical activation, apparently, leads to a deeper chemical interaction of the components – Fe and Cu, which provides a higher activity of chromium-free CuFeAl ceramic-metal catalysts.

Impact of stoichiometry and strain on Ge1−x Sn x alloys from first principles calculations

We calculate the electronic structure of germanium-tin (Ge1−x Sn x ) binary alloys for using density functional theory (DFT). Relaxed alloys with semiconducting or semimetallic behaviour as a function of Sn composition x are identified, and the impact of epitaxial strain is investigated by constraining supercell lattice constants perpendicular to the [001] growth direction to the lattice constants of Ge, zinc telluride, or cadmium telluride substrates. It is found that application of 1% tensile strain reduces the Sn composition required to bring the (positive) direct band gap to zero by approximately 5% compared to a relaxed Ge1−x Sn x alloy having the same gap at Γ. On the other hand, compressive strain has comparatively less impact on the alloy band gap at Γ. Using DFT calculated alloy lattice and elastic constants, the critical thickness for Ge1−x Sn x thin films as a function of x and substrate lattice constant is estimated, and validated against supercell DFT calculations and experiment. The analysis correctly predicts the Sn composition range at which it becomes energetically favourable for Ge1−x Sn x /Ge to become amorphous. The influence of stoichiometry and strain is examined in relation to reducing the magnitude of the inverted (‘negative’) – band gap, which is characteristic of semimetallic alloy electronic structure. Based on our findings, strategies for engineering the semimetal-to-semiconductor transition via strain and quantum confinement in Ge1−x Sn x nanostructures are proposed.

The Impact of Stoichiometry on the Photophysical Properties of Ruddlesden–Popper Perovskites

Abstract2D Ruddlesden–Popper perovskites are interesting for a variety of applications owing to their tunable optical properties and their excellent ambient stability. As these materials are processable from solution, they hold the promise of procuring flexible and cost‐effective films through large‐scale fabrication techniques. However, such solution‐based deposition techniques often induce large degrees of heterogeneity due to poorly controlled crystallization. The microscopic properties of films of (PEA)2PbI4 cast from precursor solutions of different stoichiometry are therefore investigated. The stoichiometry of the precursor solution is found to have a large impact on the crystallinity, morphology, and optical properties of the resulting thin films. Even for films cast from stoichiometric precursors, differences in photoluminescence intensities occur on a subgranular level. The heterogeneity in these films is found to be thermally activated with an activation energy of 0.4 eV for the emergence of local variations in nonradiative recombination rates. The spatial variation in the distribution of trap states is attributed to local fluctuations in the stoichiometry. In line with this, the surface can successfully be passivated by providing an excess of phenylethylammonium iodide (PEAI) to an as‐cast film, enhancing the photoluminescence by as much as 85% without significantly altering the film's morphology.

(Invited) Impact of Stoichiometry and Interface Configuration on the Time Stability and the Speed-Limiting Step in Memristive SrTiO3 Cells

Resistive switching oxides are promising candidates for future non-volatile memory and for functional units of neuromorphic computing. Epitaxial SrTiO3 thin films can be regarded as model system for resistive switching thin films due to their well-known defect chemistry and the absence of grain boundaries. We have recently shown that so-called eightwise switching in SrTiO3 can be attributed to the release and reincorporation oxygen at the interface to a Pt electrode [1,2]. In the light of this knowledge, we studied in detail the impact of the Sr/Ti stoichiometry on the filament formation process, the filament stability [3] and the switching kinetics. We observed that Sr rich thin films exhibit an improved retention of the low resistive state (LRS) at low current compliance values with respect to the stoichiometric thin films. In order to identify the underlying processes, we investigated the filament formation process of SrTiO3 thin films with different stoichiometry by photoelectron emission microscopy. Stoichiometric thin films form a stable LRS state as soon as the current compliance is sufficient to trigger SrO segregation at the surface [4]. We attribute this to the impeded reoxidation of the oxygen deficient filament by the presence of the SrO at the surface. We could mimic this effect by intentionally depositing additional SrO, or other oxygen blocking layers such as Al2O3 on the surface of stoichiometric thin films and were thereby able to improve the memory window as well as the LRS retention. We furthermore investigated the switching kinetics of the SET process for different Sr/Ti stoichiometries and interface layers. We compared these results to analytical models for the switching kinetics based on ion movement, modeled by the Mott-Gurney-Law, and an oxygen exchange reaction at the interface, modeled by the Butler-Volmer-Equation, as the rate-determining steps in combination with simulated temperature and electrical potential data from finite element simulations, as shown in Figures 1 ((a) stoichiometric, (b) Sr rich). For stoichiometric SrTiO3 cells, oxygen transfer at the oxide-metal interface is determined as the rate-determining step. In contrast, for Sr-rich STO cells we propose oxygen diffusion within the SrO layer at the metal-STO interface as rate-limiting. We will discuss possible reasons for the different rate-determining steps, such as different vacancy concentrations and exchange probabilities.

Impact of Stoichiometry and Fluorine Atoms on the Charge Transport of Perylene-F4TCNQ.

The charge-transport properties of charge-transfer complexes (CTCs) play a key role in the potential applications toward novel optoelectronic devices. We have systematically studied the charge-transport properties of perylene-F4TCNQ CTCs with different stoichiometric ratios by first-principles calculations. Our calculated results showed that 1P1F4 (perylene-F4TCNQ 1:1) exhibits a higher charge-carrier mobility than 3P2F4 (perylene-F4TCNQ 3:2) due to the strong interlayer interactions in 3P2F4. Compared with the perylene-TCNQ CTC, the higher charge-carrier mobility in perylene-F4TCNQ CTC indicates that introducing fluorine atoms can enhance the charge-carrier mobility due to stronger intermolecular interactions. More importantly, the experimental measurements carried out with 1P1F4- and 3P2F4-based field-effect transistors are consistent with the theoretical predictions. Our study reveals that tuning the charge-transport properties in CTCs by controlling the stoichiometry between the donor and acceptor is a promising strategy in accelerating the development of high-performance organic electronic materials.

Multi-scale study of Ti3SiC2 thin film growth mechanisms obtained by magnetron sputtering

In the present work we focus on the mechanisms involved in Ti3SiC2 MAX phase thin-film formation and on the impact of stoichiometry and thickness/element quantity correlation. TiAlx thin-films, with x=0--2, 100–300 nm thick, were deposited by magnetron sputtering on a SiC-4H (0001) substrate. Samples were annealed at 1000 ∘C for 30 min and analyzed by XRD, AFM, SEM and TEM. The MAX phase Ti3SiC2 was formed at 1000 ∘C in accordance with thermodynamic considerations. Moreover, an epitaxial relation between the film and the substrate occurred as follows (0001)MAX//(0001)SiC and [121¯0]MAX//[121¯0]SiC due to the formation of an earlier TiC phase by species interdiffusion. The catalytic role of Al has been demonstrated and explained by the formation of Ti2AlC as precursor for Ti3AlC2 and for Ti3SiC2. Finally, second phases and continuity of films have been controlled by x and thickness factors. Indeed, the MAX phase film continuity is obtained for Al richer films, whereas the purest MAX phase have been synthesized for the thinnest layer of TiAlx deposited.

Impact of Stoichiometry on the Mechanism and Kinetics of Oxidative Dissolution of UO2 Induced by H2O2 and γ-Irradiation

Radiation-induced oxidative dissolution of uranium dioxide (UO2) is one of the most important chemical processes of U driven by redox reactions. We have examined the effect of UO2 stoichiometry on ...

Impact of Stoichiometry and Size on the Magnetic Properties of Cobalt Ferrite Nanoparticles

Cobalt ferrite nanoparticles (NPs) are promising candidates for application as nanomagnets. The challenge is to lower the size of the cobalt ferrite NPs such that the remanent magnetization remains stable at room temperature and above. Wet chemical synthesis allows for the production of monodisperse cobalt ferrite (CoxFe3–xO4) NPs in which the stoichiometry of the metal cations can be varied in a controlled way. Here we experimentally show that NPs with the cobalt stoichiometry in the range of 0.5 < x < 0.7 exhibit the largest magnetocrystalline anisotropy constant and room-temperature remanent magnetization. Moreover, we experimentally demonstrate that the commonly observed bimagnetic behavior of the magnetization in cobalt ferrite NPs is due to interparticle dipolar interactions. At the optimum cobalt stoichiometry, we tune the size of the cobalt ferrite NPs through a systematic change in the reaction parameters. We experimentally demonstrate that 10 nm is the smallest particle diameter at which cobalt ferrite NPs show remanent magnetization that is stable at room temperature. The smallest NPs size translates to a maximum packing density in excess of 1012 nanomagnets/cm2, which can be advantageously used for magnetic nonvolatile memories and mass information storage applications.

Impact of stoichiometry on the linear and nonlinear optical response of SnOx thin films

SnO is a promising p-type oxide semiconductor materials for applications such as transparent electronics and solar cells. However, further improvement of its performance is hindered by its diverse stoichiometry. We investigated the nonlinear and saturable absorption characteristics of pristine SnO and O-rich SnOx films by femtosecond degenerate pump-probe measurements at 515 nm. UV-Vis absorption data indicate bandgap blueshift with increasing oxygen concentration. Pristine SnO film exhibit saturable absorption while nonlinear absorption is observed in O-rich SnOx films. Our results shed light on the utilization of SnO in future device applications.

Impact of Stoichiometry on Structural and Optical Properties of Sputter Deposited Multicomponent Tellurite Glass Films

Multicomponent TeO2–Bi2O3–ZnO (TBZ) glass thin films were prepared using RF magnetron sputtering under different oxygen flow rates. The influences of oxygen flow rate on the structural and optical properties of the resulting thin films were investigated. We observed that thin films sputtered in an oxygen‐rich environment are optically transparent while those sputtered in an oxygen‐deficient environment exhibit broadband absorption. The structural origin of the optical property variation was studied using X‐ray diffraction, X‐ray photoelectron spectroscopy, Raman Spectroscopy, and transmission electron microscopy which revealed that the presence of under‐coordinated Te leads to the observed optical absorption in oxygen‐deficient films.

The impact of O/Si ratio and hydrogen content on ArF excimer laser ablation of fused silica

The impact of stoichiometry and hydrogen content on the ArF excimer laser ablation characteristics of fused silica is investigated. Near-surface substoichiometic SiOx with X &lt; 2 diminishes the ablation threshold as a result of increased absorption. The ablation rate is raised by an elevated hydrogen content. As confirmed by mass spectrometric analysis, the laser-induced formation of substoichiometric near-surface layers within the ablation spot sustains absorption and ablation for consecutive laser pulses.

Impact of Stoichiometry on the Electronic Structure of PbS Quantum Dots

Although the stoichiometry of bulk lead sulfide (PbS) is exactly 1:1, that of quantum dots (QDs) can be considerably different from this crystalline limit. Employing first-principles calculations, we show that the impact of PbS QD stoichiometry on the electronic structure can be enormous, suggesting that control over the overall stoichiometry in the QD will play a critical role for improving the efficiency of optoelectronic devices made with PbS QDs. In particular, for bare PbS QDs, we find that: (i) stoichiometric PbS QDs are free from midgap states even without ligand passivation and independent of shape, (ii) off stoichiometry in PbS QDs introduces new states in the gap that are highly localized on certain surface atoms, and (iii) further deviations in stoichiometry lead to QDs with "metallic" behavior, with a dense number of energy states near the Fermi level. We further demonstrate that this framework holds for the case of passivated QDs by considering the attachment of ligand molecules as stoichiometry variations. Our calculations show that an optimal number of ligands makes the QD stoichiometric and heals unfavorable electronic structure, whereas too few or too many ligands cause effective off stoichiometry, resulting in QDs with defect states in the gap.

Influence of the structure of MgO·nAl2O3 spinel lattices on transparent ceramics processing and properties

Abstract It is demonstrated that a complete elimination of pores on sintering is governed not only by the size of the ceramic powder particles and by the homogeneity of their mutual coordination but similarly strongly by the state of the crystal lattice: with different cation disorder at fixed stoichiometry ( n = 1) the sintering temperatures may differ by as much as 200 °C at constant powder particle size and equal homogeneity of the green bodies. Additionally, the impact of stoichiometry was investigated over the range between n = 1 and n = 3 with retarded reactive sintering at moderately increased Al 2 O 3 concentrations but promoted densification of alumina-rich compositions. Taking advantage of the observed effects, sintered spinel ceramics were derived by reactive sintering of undoped MgO/Al 2 O 3 mixtures resulting in an in-line transmittance which equals spinel single crystals of similar composition from 200 nm wave length up to the IR range.

Oxidation of Acetone and Its Interaction with Nitric Oxide

An experimental and kinetic modeling study of the oxidation of acetone, both in the absence and the presence of NO, has been performed under flow reactor conditions in the temperature range of 700−1500 K and for different stoichiometries, ranging from fuel-rich to fuel-lean environments. The impact of stoichiometry on flow reactor data and ignition delay times has been studied, whereas the influence of NO has been analyzed under flow reactor conditions. A literature kinetic detailed model, slightly modified in the present work, has been used to simulate and explain the experimental findings.

Impact of stoichiometry on L1 0 ordering in FePt and FePtCu thin films

The L1 0 ordering kinetics strongly depend on the films initial Fe/Pt atomic ratio. Fe x Pt 100− x films with x<50 at.% benefit from Cu addition since the Cu is expected to alloy with the FePt as an Fe atom replacement. The addition of Cu is less remarkable in films that start with an equiatomic Fe and Pt composition. In that case, Cu decreases the ultimate film coercivity and does not significantly improve the L1 0 ordering process. Large fields (9 T) are required to fully saturate the (1 1 1) textured L1 0 ordered FePt(Cu) films. It has been theoretically suggested that the moment of ordered FePt decreases due to ferrimagnetic alignment of the Fe and Pt moments; however, in these experiments the M s value did not change significantly with annealing treatment and subsequent L1 0 ordering.

Effect Of Mg, Zn, Si, And O On The Lattice Constant of Gallium Nitride Thin Films

AbstractThis study analyzes the impact of most common impurities and dopants on the c lattice parameter for thin films of Gallium Nitride (GaN) deposited on basal plane sapphire. Both Mg (∼1017 cm-3) and Zn (∼3 × 1020 cm-3) doping were found to expand the c lattice parameter as much as +0.38% and +0.62%, respectively. On the contrary, Oxygen up to concentrations 9 1021 cm-3 is shown to replace N in GaN thin films reducing the c parameter only by a small amount. Incorporation of Si leads to a large decrease of the c parameter which can not be attributed to the different size of Ga and Si atom. It is suggested that doping alters the film stoichiometry by a predicted Fermi level dependence of defect formation energies. The impact of stoichiometry on c lattice parameter and the effect of hydrostatic strain on resistivity in undoped and doped GaN is discussed.