Atomic Stoichiometry Research Articles

An approach for full space inverse materials design by combining universal machine learning potential, universal property model, and optimization algorithm

We present a full space inverse materials design (FSIMD) approach that fully automates the materials design for target physical properties without the need to provide the atomic composition, chemical stoichiometry, and crystal structure in advance. Here, we used density functional theory reference data to train a universal machine learning potential (UPot) and transfer learning to train a universal bulk modulus model (UBmod). Both UPot and UBmod were able to cover materials systems composed of any element among 42 elements. Interfaced with optimization algorithm and enhanced sampling, the FSIMD approach is applied to find the materials with the largest cohesive energy and the largest bulk modulus, respectively. NaCl-type ZrC was found to be the material with the largest cohesive energy. For bulk modulus, diamond was identified to have the largest value. The FSIMD approach is also applied to design materials with other multi-objective properties with accuracy limited principally by the amount, reliability, and diversity of the training data. The FSIMD approach provides a new way for inverse materials design with other functional properties for practical applications.

Atomic structure and electronic properties of Ce + O films co-adsorbed on Mo(112)

The atomic structure, stoichiometry and electron work function of Ce + O films co-adsorbed on Mo(112) were studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and contact potential difference (CPD) methods. The films with a significant predominance of one major species show the structure characteristic of the main component, which remains stable up to 15−50% of the minor species content. We report the c(4 × 4) and c(2 × 6) superstructures for Ce + O films with single layer Ce coverage as well as c(4 × 6) and (2 × 2) for double layer Ce coverage. They are exclusively specific to the (Ce + O)/Mo(112) system and have not been previously observed neither for cerium oxides nor pure cerium/pure oxygen adfilms. No noticeable electron work function change was detected due to oxygen addition to the Ce/Mo(112) films. Both oxygen-enriched and oxygen-depleted (Ce + O) films show similar typical electropositive properties.

Optical properties of chromium-selenide films designed for terahertz applications

Herein the effects of indium substrates on the properties of chromium selenide thin films are reported. Chromium selenide thin films and indium substrates are prepared by the thermal evaporation technique under a vacuum pressure of 10−5 mbar. It is observed that indium sublayers alters the atomic stoichiometry of chromium selenide. It induces the formation of Cr2Se3 phase instead of CrSe2 phase which grow onto glass substrates. Both of the glass/CrSe2 and In/Cr2Se3 films displayed direct and indirect energy band gaps. The respective gaps are 2.60 eV and 3.19 eV and 2.25 eV and 2.83 eV. The Urbach tails states exhibited a width of 2.24 eV in glass/CrSe2 and showed value of 0.85 eV in In/Cr2Se3. In addition a light absorbability enhancement of more than 45% is reached at the In/Cr2Se3 interfaces in the visible range of light. Moreover, as terahertz resonators, In/Cr2Se3 films showed larger optical conductivity. It also displayed a spectral terahertz cutoff frequency in the range of 3.0–35 THz. Furthermore, when utilized as terahertz resonators, In/Cr2Se3 films demonstrate heightened optical conductivity and a spectral terahertz cutoff frequency ranging from 3.0 to 35 THz. Analyzing these terahertz oscillators using Drude-Lorentz models reveals that the density of free charge carriers increases with higher oscillator energy, while the drift mobility of these carriers within the terahertz resonators varies between 1.10 and 3.09 cm²/Vs. The array of features showcased by In/Cr2Se3 thin films, including their performance as terahertz resonators, positions them as strong contenders for applications in optical and terahertz technologies.

Effect of Substitutional Oxygen on Properties of Ti3 C2 Tx MXene Produced Using Recycled TiO2 Source.

MXenes are an emerging class of 2D materials with unique properties including metallic conductivity, mechanical flexibility, and surface tunability, which ensure their utility for diverse applications. However, the synthesis of MXenes with high crystallinity and atomic stoichiometry in a low-cost process is still challenging because of the difficulty in controlling the oxygen substitute in the precursors and final products of MXenes, which limits their academic understanding and practical applications. Here, a novel cost-effective method is reported to synthesize a highly crystalline and stoichiometric Ti3 C2 Tx MXene with minimum substitutional oxygen impurities by controlling the amount of excess carbon and time of high-energy milling in carbothermal reduction of recycled TiO2 source. The highest used content (2wt%) of excess-carbon yields TiC with the highest carbon content and minimal oxygen substitutes, which leads to the Ti3 AlC2 MAX phase with improved crystallinity and atomic stoichiometry, and finally Ti3 C2 Tx MXene with the highest electrical conductivity (11738 S cm-1 ) and superior electromagnetic shielding effectiveness. Additionally, the effects of carbon content and substitutional oxygen on the physical properties of TiC and Ti3 AlC2 are elucidated by density-functional-theory calculations. This inexpensive TiO2 -based method of synthesizing high-quality Ti3 C2 Tx MXene can facilitate large-scale production and thus accelerate global research on MXenes.

XPS Core-Level Chemical Shift by Ab Initio Many-Body Theory.

X-ray photoemission spectroscopy (XPS) provides direct information on atomic composition and stoichiometry by measuring core-electron binding energies. Moreover, from the shift of the binding energy, the so-called chemical shift, the precise chemical type of bonds can be inferred, which brings additional information on the local structure. In this work, we present a theoretical study of the chemical shift first by comparing different theories, from Hartree-Fock and density functional theory to many-body perturbation theory approaches like the GW approximation and its static version (COHSEX). The accuracy of each theory is assessed with respect to a carbon 1s chemical shift experimental benchmark measured on a set of gas-phase molecules. More importantly, by decomposing the chemical shift into different contributions according to terms in the total Hamiltonian, classical electrostatics is identified as the major contributor to the chemical shift, one order of magnitude larger than the correlation.

Off‐Stoichiometry Quaternary Heusler‐Like Semiconductors with Magnetism and Disorder

AbstractThe theoretical finding of a large group of intrinsic off‐stoichiometry quaternary Heusler‐like semiconductors with mixing and tunable occupation of 4c‐4d atomic sites, such as FexCoyTiSb with non‐integer x + y > 1 are reported. Those semiconductors are never reported before, and they radically break the well‐recognized 18‐ (or 24‐) valence electron counting (VEC) rule and the exact 4c‐ (or both 4c‐ and 4d‐) atomic sites stoichiometry for the traditional quaternary half‐Heusler (or full‐Heusler) semiconductors. Physically, the novel semiconductors can be designed by following a d‐orbital‐determined compensation rule. The extra atoms fill in the tetrahedral vacancies of the half‐Heusler structure, introduce symmetry‐constrained d orbitals near the Fermi level, large crystal field splitting, and then form a bandgap at appropriate fractional compositions, which leads to an exact compensated electronic structure. The newly established compensation rule reveals an abundant phase space of the quaternary Heusler‐like (2x + 3y = 3 for VECY+Z = 9, 2x + 3y = 4 for VECY+Z = 8, and 2x + 3y = 5 for VECY+Z = 7) compounds. High‐throughput screenings reveal many new FexCoyYZ semiconductors, all of which possess strong disorder, weak magnetism, spin compensation, and wide composition‐range tunability. The intertwined orders may either find functionalities surpassing the existing materials or give rise to new potential applications.

Optical and dielectric dispersion in the Ge/In2Se3/Ga2S3 interfaces

"In this article, the optical and dielectric performance of the Ge, Ge/In2Se3 and Ge/In2Se3/Ga2S3 interfaces are reported and discussed. The growth nature of the physically vacuum deposited thin film layers is investigated by means of X-ray diffraction and energy dispersive X-ray spectroscopy. Each 200 nm thick layer exhibited an amorphous type of crystallization with appropriate atomic stoichiometry. Optically, the Ge/In2Se3/Ga2S3 system is found to exhibit a conduction and a valence band offsets of values of 0.53 and 0.47 eV at the Ge/In2Se3 and of values of 0.30 and 0.70 eV at the In2Se3/Ga2S3 interfaces, respectively. The values are high enough to actualize quantum confinements in the heterojunction device. The formed double and three layers displayed higher light absorbability than single layers. On the other hand, the dielectric dispersion analysis has shown a wide tunability in the dielectric property in visible light and near IR regions. The dielectric responses at the Ge/In2Se3 and at the Ge/In2Se3/Ga2S3 interfaces are linear below 2.10 eV and 1.53 eV, respectively. The modeling of the dielectric function revealed the optical conductivity parameters presented by the drift mobility, scattering time, plasmon frequency and free electron density. It was observed that the quantum condiment at the Ge/In2Se3 interfaces improved both of the drift mobility and made the scattering time longer at femtosecond levels. The establishing of the second quantum confinement at the second interface In2Se3/Ga2S3 raised the drift mobility more and extended the scattering time further. With the estimated plasmon frequencies, the formation of the Ge/In2Se3/Ga2S3 interface appears to be promising for use in optoelectronic device production especially in photodetection issues. "

Substitution of ‘small’ divalent cations (e.g. Mg) for Si and Al in the nepheline tetrahedral framework: 2. The occurrence of Mg-rich nepheline and kalsilite

AbstractNatural nepheline usually contains very small amounts of MgO (<0.1 wt.%), although these examples are mainly from Mg-poor alkaline igneous rocks such as nepheline syenites. However, this work shows that nepheline and kalsilite with much higher MgO concentrations can occur in the groundmass of strongly SiO2-undersaturated, feldspar-free, mafic volcanic rocks (i.e. olivine-rich foidites). Furthermore, a strong positive correlation is evident between their Mg and Fe contents. The occurrence of Mg-rich nepheline and kalsilite seems to be related to their derivation from Mg-rich magmas when compared to most of the host rocks investigated to date. Additionally, the physicochemical conditions of crystallisation seem to have an important role in the incorporation of ‘small’ divalent cations by these minerals. The prevalence of Mg-rich nepheline and kalsilite as late magmatic phases and the divergent Mg and Fe relationships for phenocrysts and ‘quenched’ groundmass crystals support this hypothesis. The positive correlation between Mg and Fe contents reflects their strong geochemical affinity and the entrance of Fe3+, Fe2+ and Mg2+ cations into the same crystallographic site of nepheline and kalsilite structures. The calculation of atomic formulae and stoichiometry parameters for nepheline-group minerals where data for the T2+ cations (e.g. Mg2+) are incorporated gives more reliable compositional parameters (see Paper 1). Calculated excess silica values (Si′) are affected significantly when the coupled substitution 2Al3+ = Mg2+ + Si4+ is considered. Thus, specific analyses of ‘small’ divalent cations are essential to obtain more realistic values of excess Si′, in particular, for nepheline and kalsilite that crystallised from Mg-rich, Si-poor, mafic–ultramafic alkaline lavas.

A General Concurrent Template Strategy for Ordered Mesoporous Intermetallic Nanoparticles with Controllable Catalytic Performance

AbstractWe report a general concurrent template strategy for precise synthesis of mesoporous Pt‐/Pd‐based intermetallic nanoparticles with desired morphology and ordered mesostructure. The concurrent template not only supplies a mesoporous metal seed for re‐crystallization growth of atomically ordered intermetallic phases with unique atomic stoichiometry but also provides a nanoconfinement environment for nanocasting synthesis of mesoporous nanoparticles with ordered mesostructure and rhombic dodecahedral morphology under elevated temperature. Using the selective hydrogenation of 3‐nitrophenylacetylene as a proof‐of‐concept catalytic reaction, mesoporous intermetallic PtSn nanoparticles exhibited remarkably controllable intermetallic phase‐dependent catalytic selectivity and excellent catalytic stability. This work provides a very powerful strategy for precise preparation of ordered mesoporous intermetallic nanocrystals for application in selective catalysis and fuel cell electrocatalysis.

Injection Effect of Anti-CD3 Monoclonal Antibody on Primo Vessel in Lymph Vessel of Rabbit with Lipopolysaccharide-Induced Inflammation.

The primo vascular system can be viewed as a circulatory system that plays a therapeutic function in regenerating the body tissue. The anti-CD3 monoclonal antibody was used as an immunotherapeutic agent to treat the novel coronavirus infection (COVID-19). Objectives: In this study, we observed the effect of injecting lymph nodes with Foralumab, an anti- human CD3 epsilon therapeutic monoclonal antibody, on primo vessels. The structure and atomic stoichiometry of the antibody were determined by transmission electron microscopy and energy dispersive spectroscopy. Alcian blue dying solution was injected into the lymph nodes of the abdominal vena cava of rabbits, and the solution further flowed into the lymph vessels. A primo vessel with primo nodes stained with Alcian blue was clearly visible in the lymph vessel. By injecting Foralumab into lymph nodes of rabbits with lipopolysaccharide-induced inflammation, the floating primo vessel in the lymph vessel appeared thicker and was distinctly visible. The observation of the primo vessel post-treated with Foralumab in the inflamed lymphatic system suggests the possibility of a functional role of the primo vascular circulatory system in pathophysiological conditions.

Methods for neutron diffraction studies on hydride superconductors and other metal hydrides

High-pressure neutron diffraction is an extremely useful technique in the quest for making and understanding novel hydride superconductors. Neutron diffraction can be used to directly determine elemental stoichiometries and atomic positions of many light elements such as hydrogen or deuterium, even in the presence of heavy elements such as rare-earth metals. Here, we report on the current status and ongoing developments on high-pressure neutron diffraction for hydride superconductors and other metal hydrides with a special focus on current advancements at the Spallation Neutrons and Pressure (SNAP) beamline of the Spallation Neutron Source at Oak Ridge National Laboratory. For broader context, an overview of high-pressure neutron diffractometers and pressure cells is included together with insight into critical sample considerations. There, attention is given to the requirements for powdered hydride samples and the need for deuterium rather than hydrogen. Additionally, the advantages of angular access and data representation as possible at SNAP are described. We demonstrate the current capability for high-pressure neutron diffraction on two different samples created via hydrogen gas loading, specifically pure deuterium and nickel-deuteride. The deuterium example highlights the usefulness of adding sample materials that facilitate the formation of a good powder while the nickel-deuteride example demonstrates that atomic deuterium positions and stoichiometry can be directly determined. Both examples highlight the importance of large scattering apertures. These enable investigation of the data resolved by scattering angle that is needed to identify parasitic peaks and background features. Finally, future directions beyond current high-pressure neutron powder diffraction are also discussed.

Increasing fish meal replacement level with feather meal in golden pompano Trachinotus ovatus diet through gamma‐ray irradiation

This study evaluated the potential to replace dietary fish meal with gamma-ray (γ)-irradiated feather meal and non γ-irradiated feather meal for golden pompano Trachinotus ovatus. The basal diet (R) contained 250 g/kg fish meal, and 20% and 40% of the fish meal were replaced by either γ-irradiated feather meal (IF20 and IF40) or non γ-irradiated feather meal (F20 and F40). Fish were fed the test diets in inshore net pens for eight weeks. The weight gain of fish fed diet R did not significantly differ from those fed diet IF20, but was higher than those fed diets F20, F40 and IF40. No significant differences were found in feed intake, feed conversion ratio, retention efficiencies of carbon and nitrogen, condition factor, hepatosomatic index and wastes of carbon and nitrogen between fish fed diet R and diets F20, F40, IF20 and IF40. This study reveals that γ-irradiated feather meal could replace more fish meal in golden pompano diet than that replaced by non γ-irradiated feather meal, and dietary fish meal content could be reduced to 200 g/kg by inclusion of 43 g/kg γ-irradiated feather meal as a fish meal substitute. Replacing dietary fish meal with either γ-irradiated feather meal or non γ-irradiated feather meal could shift carbon–nitrogen–phosphorus atomic stoichiometry in the coastal waters for golden pompano aquaculture.

Growth performance, feed cost and environmental impact of largemouth bass Micropterus salmoides fed low fish meal diets

This study evaluated the effect of replacing dietary fish meal with poultry by-product meal (PBM), cottonseed-protein concentrate (CPC) and their blend on growth, feed utilization, body composition, waste outputs and carbon-nitrogen-phosphorus atomic stoichiometry in wastes of largemouth bass, Micropterus salmoides. A diet containing 40 % fish meal served as reference (RF), and 60 % and 80 % of the fish meal was replaced by PBM (P60 and P80) and CPC (C60, C80) alone or in combination (CP60 and CP80). Moreover, a cost-effective diet (CPS80) was formulated by replacing 80 % of the fish meal with a blend of PBM, CPC and soybean meal. Largemouth bass weighing 24.0 ± 0.3 g were fed the test diets for eight weeks. Weight gain, retention efficiencies of nitrogen and phosphorus were lower, while wastes of carbon, nitrogen and phosphorus were higher, in fish fed diet RF than in fish fed diets P60, P80, C60, C80, CP60, CP80 and CPS80, and no significant differences were found in condition factor and hepatosomatic index between fish fed diet RF and the low fish meal diets. At the same fish meal replacement level, weight gain was slightly high in fish fed diets P60, P80, CP60 and CP80, relative to fish fed diets C60 and C80. Fish fed diets CP80 and CPS80 exhibited high weight gain and minimum feed cost, fish meal reliance and waste outputs. Carbon-nitrogen-phosphorus atomic stoichiometry in wastes was (50–86):(10–17):1, and C : P and N : P in wastes of fish fed diet CPS80 were high relative to that of fish fed diet RF and diets C60, C80, P60, P80, CP60 and CP80. This study indicates that dietary fish meal level for largemouth bass could be reduced to 80 g/kg by using PBM and CPC in combination as a fish meal substitute, and feeding diet CP80 and CPS80 can significantly reduce feed cost and fish meal reliance without increasing negative effect on growth and environment.

Characterization of InP Surfaces Perturbations after Ar+ Bombardment By Combining Electrochemistry and X-Ray Photoemission Spectroscopy

Over the past decades, the perturbations induced by ionic bombardment on semi-conductors have been explored, raising questions such as the deepness affected inside the material. Here, we focus on one well-known semi-conductor: InP, under Ar bombardment induced by an ionic sputtering performed directly inside the X-Ray photoemission spectroscopy (XPS) chamber using an ion gun. We already proved that, after a bombardment by Ar ions, InP surfaces suffer from several perturbations, degrading the initial properties of this semi-conductor: chemical ones (oxidation degree, atomic network organization and stoichiometry) [1], but also morphological and optical ones [2]... To get a better insight on the damages induced on the surfaces of InP semi-conductors, we present a study where electrochemistry is coupled to XPS analysis. Electrochemistry is indeed a very efficient probe to detect modification of the electrical and electrochemical properties of the materials but also a quantitative tool to provide information bulk and surface properties of any electrode material.In our approach, n- & p- type samples were investigated toward their modified electrochemical responses. We observed that, on p-type InP, after bombardment, the surface modification is evidenced by a lower dark open circuit potential compared to the one measured on pristine surface. Moreover, cyclic voltammetry performed at pH 10 shows initial cathodic features on the surface, related to an oxidized metallic In-enriched layer onto InP surface. This feature agrees with XPS analysis. By continuing the cyclic voltammetry, a more pronounced In rich overlayer formation is observed, due to specific oxidation /reduction phenomenon.[3] This behavior is very similar to the one obtained by cathodic decomposition onto InP surface. We will also explore the response of a n-type InP surface under similar conditions.In conclusion, we will discuss how the XPS and electrochemical characterizations combination is able to provide complete and complementary diagnostic of the stoichiometry loss and its electrochemical reactivity.[1] Aureau et al., SIA, 2018, 50 (11) 1163-1167 ; D. Aureau et al., ECS Trans., 2019, 89, 9[2] Béchu et al., JVST B, 2019, 27, 062902[3] Béchu et al., Electrochem. Comm., 2020, 117, 106766

Characterizing the Tensile Strength of Metastable Grain Boundaries in Silicon Carbide Using Machine Learning

The local atomic structure, local chemistry, and stoichiometry of grain boundaries control in part the strength and fracture toughness of silicon carbide components. The predictions of the structure and properties of these grain boundaries are generally limited to their ground-state configurations. We investigated the tensile strength behavior of metastable grain boundaries in silicon carbide using high-throughput atomistic simulations combined with machine learning techniques. We analyzed and compared the ∑5 ⟨100⟩{120} and ∑9 ⟨110⟩{122} tilt grain boundary metastable configurations to identify structural and chemical attributes that dominate their tensile strength. We characterized these metastable grain boundaries using a set of microscopic descriptors representing the local grain boundary atomic structure and the local grain boundary stoichiometry and chemical-bound types. We used a boosted regression tree surrogate model for the successful prediction of metastable grain boundary strength as a function of these descriptors. Our results show that the tensile strength of generic (i.e., any random grain boundary from the entire grain boundary population), metastable grain boundaries is primarily dominated by the grain boundary excess free volume, closely followed by the type of structure composing the boundary and the amount of C–C bonds. The 5% strongest metastable grain boundaries have particular characteristics with a low amount of free volume and the highest density of C–C bonds. Our results reveal that the 5% strongest and weakest metastable grain boundaries are most sensitive to the local stoichiometry, regardless of the local atomic structure composing the grain boundary as compared to any other generic metastable grain boundaries. We show that the strongest and weakest metastable grain boundary configurations can be identified as specific regions in a low-dimensional-representation space of their microscopic descriptors. Taken together, these findings showcase the effectiveness and validity of using a low-dimensional representation of the grain boundary structure and machine-learned surrogate models to rapidly assess metastable grain boundary strength without the need to perform actual tensile simulations.

Spatially Controlled Fabrication and Mechanisms of Atomically Thin Nanowell Patterns in Bilayer WS2 Using in Situ High Temperature Electron Microscopy.

We show controlled production of atomically thin nanowells in bilayer WS2 using an in situ heating holder combined with a focused electron beam in a scanning transmission electron microscope (STEM). We systematically study the formation and evolvement mechanism involved in removing a single layer of WS2 within a bilayer region with 2 nm accuracy in location and without punching through to the other layer to create a hole. Best results are found when using a high temperature of 800 °C, because it enables thermally activated atomic migration and eliminates the interference from surface carbon contamination. We demonstrate precise control over spatial distributions with 5 nm accuracy of patterning and the width of nanowells adjustable by dose-dependent parameters. The mechanism of removing a monolayer of WS2 within a bilayer region is different than removing equivalent sections in a monolayer film due to the van der Waals interaction of the underlying remaining layer in the bilayer system that stabilizes the excess W atom stoichiometry within the edges of the nanowell structure and facilitates expansion. This study offers insights for the nanoengineering of nanowells in two-dimensional (2D) transitional metal dichalcogenides (TMDs), which could hold potential as selective traps to localize 2D reactions in molecules and ions, underpinning the broader utilization of 2D material membranes.

One-step electrodeposition of ligand-free PdPt alloy nanoparticles from water droplets: Controlling size, coverage, and elemental stoichiometry

We present a robust and facile method to produce metal nanoparticle (NP) alloys in a one-step synthesis using direct electrodeposition onto highly oriented pyrolytic graphite (HOPG). Precursor salts, H2PtCl6 and Pd(NO3)2, were dissolved in a 1 mM sodium dodecylsulfate (SDS) water droplet with 1× phosphate buffered saline solution and suspended in a dichloroethane (DCE) continuous phase. Tetrabutylammonium perchlorate was added to the DCE continuous phase to maintain charge balance during electrodeposition. NP fabrication via electrodeposition was driven by droplet collisions onto HOPG, which was biased at a potential where the metal precursor salts would reduce to their respective zero-valent atoms. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) were used to study the size, coverage, and morphology of the NPs as well as the atomic stoichiometry. EDX mapping indicated homogeneous distribution of Pd and Pt at the single NP level. Homogeneously alloyed PdPt NPs were realized from this study with demonstrated control over metal composition, surface coverage, and NP size.

Optoelectronic properties of the InSe/Ga2S3 interfaces

Abstract In this work, the nature of formation, the structural, optical and optoelectronic properties of the InSe/Ga2S3 interfaces are explored by the Scanning electron microscopy, energy dispersive X-ray, X-ray diffraction, optical spectrophotometry and temperature dependent dark and photoconductivity. The InSe/Ga2S3 interfaces that are prepared by the vapor deposition technique are observed to exhibit a physical nature of stacking with correct atomic stoichiometry of the layers. The deposition of the Ga2S3 onto InSe substrate is induced to exhibit a polycrystalline nature. In addition, the optical characterizations have shown that the presence of Ga2S3 onto the top surface of InSe enhances the absorbability significantly and causes valence and conduction band offsets of 0.35 and 1.25 eV, respectively. On the other hand, the temperature dependent photoconductivity analysis in the range of 330–160 K, have shown that the conduction is dominated by the thermionic and variable range hopping transport mechanisms. The photoexcitation process tunes the current transport mechanisms. When the InSe/Ga2S3 interface devices were exposed to laser light irradiations of wavelengths of 406 and 850 nm, promising photocurrent characteristics were observed. Namely, the linear growth of the photocurrent, responsivity, external and internal quantum efficiencies with increasing biasing voltage under these two laser lights nominates the InSe/Ga2S3 interfaces for optoelectronic applications.

Low-temperature, plasma assisted, cyclic synthesis of MoS2

Thin film reaction based synthesis techniques are promising for large area, uniform two-dimensional transition metal dichalcogenide (TMD) layers such as MoS2. In this work, the impact of the initial molybdenum film composition (metallic versus oxidized) is explored. Alternating steps of Mo sputtering and H2S soaks are used in conjunction with plasma assisted synthesis techniques to synthesize films at low temperatures. Raman, photoluminescence, x-ray photoelectron spectroscopy, and atomic force microscopy are used to physically characterize the films' atomic structure, stoichiometry, and topography, while devices were fabricated to characterize their electronic properties. MoS2 synthesized from metallic Mo films were found to exhibit better atomic and electronic structure than MoS2 synthesized from MoOx films. Additionally, slowing the rate of synthesis by segmenting growth into repeating cycles resulted in much higher film quality. To understand the impact of atomic structure and stoichiometry on device performance, films synthesized at low temperature were exposed to various high temperature annealing conditions to induce changes in film structure and composition. Physical and electrical characterization reveal that stoichiometry has a significantly weaker influence on electronic performance than grain size and atomic structure. These results provide valuable information on the optimization of low temperature thin film reactions for TMD syntheses.

Ligand-induced symmetry breaking, size and morphology in colloidal lead sulfide QDs: from classic to thiourea precursors

Colloidal lead chalcogenide quantum dots (CQDs) exhibit promising optoelectronic properties for applications in solar cell devices and as thermoelectrics. Herein, we report and discuss a ferroelectric structural distortion, at the picometer scale resolution, in PbS CQDs prepared using both classic and new synthetic pathways. The investigation was performed using synchrotron X-ray total scattering data and advanced methods of analysis that rely on a homo-core-shell model and evaluate the atomic arrangement, stoichiometry, size and morphology of nanocrystals. The CQDs show comparable size-dependent relative elongation, up to 0.7 % of one body diagonal of the cubic rock-salt structure, which corresponds to a rhombohedral lattice deformation. The findings suggest a joint role for the oleate ligands (which induce surface tensile strain) and the Pb(II) lone pair as the driving forces of the deformation. Pb displacements along the [111] direction, which provoke a ferrolectric distortion related to the lattice change, fall in the 0.0 – 0.1 Å. Overall, the findings suggest the local nature of the metal off-centering, leading to different average displacements which depend on the synthetic conditions.