In-plane and vertical heterostructures from 1T’/2H transition-metal dichalcogenides

Objectives An avalanche of research has been carried out on two-dimensional (2D) transition metal dichalcogenides (TMDs) due to their potential applications in advanced electronics and flexible devices. To take full use of the emerging 2D TMDs materials, their in-plane/vertical heterostructures have been explored, enabling effective tuning of their physical and chemical properties. However, structural differences between the various phases impede the formation of functional heterostructures. Therefore, robust synthesis strategies for heterostructures with different phases have been explored in this study. Methods A chemical vapor deposition process has been proposed in which the key parameters like reaction sources, deposition sites, etc. have been carefully adjusted, trying to achieve simultaneous synthesis of 1T’/2H in-plane and vertical heterostructures. Results Consequently, 2D in-plane RexMo1-xS2/MoS2 and vertical ReS2/MoS2 heterostructures have been produced in different regions at the same time. Atomic-resolution Z-contrast images reveal the detailed atomic structure of the 1T’/2H interfaces. The lateral interface is found to contain Mo atoms with only 5-fold coordination with S due to the phase mismatch. Conclusion This work demonstrates a route to exploit heterostructures of different phases and opens the possibility to build more complicated 2D heterostructures using CVD.

Thin film growth of MAX phases as functional materials

Layered nanolaminate ternary carbides, nitrides and carbonitrides with general formula Mn+1 AXn or MAX (n = 1, 2, or 3, M is an early transition metal, A is mostly group 13 or 14 element, and X is C and/or N) has revolutionized the world of nanomaterials, due to the coexistence of both ceramic and metallic nature, giving rise to exceptional mechanical, thermal, electrical, chemical properties and wide range of applications. Although several solid-state bulk synthesis methods have been developed to produce a variety of MAX phases, however, for certain applications, the growth of MAX phases, especially in its high-quality epitaxial thin films form is of increasing interest. Here, we summarize the progress made thus far in epitaxial growth and property evaluation of MAX phase thin films grown by various deposition techniques. We also address the important future research directions to be made in terms of thin-film growth. Overall, in the future, high-quality single-phase epitaxial thin film growth and engineering of chemically diverse MAX phases may open up interesting new avenues for next-generation technology.

Ultrathin Metal-Organic Framework Nanosheets and Devices

A few recent findings on ultrathin two-dimensional (2D) metal-organic frameworks (MOFs) were discussed in this spotlight review. MOFs are a class of materials with intriguing properties for possible applications in several fields ranging from catalysis to sensors and functional devices. To date, several synthesis strategies have been explored to derive crystalline 2D MOF structures. However, most synthetic strategies to obtain such materials remain underexplored. This highlighted review evaluated select synthesis strategies focused on deriving micron-sized 2D MOF crystals, emphasizing their rich chemistries. More importantly, the possibility of integrating the synthesized ultrathin 2D crystalline MOFs into the functional device and their electrical conductivity measurements are reviewed. Overall, this review provides the most recent outcomes in the ultrathin 2D MOF community and its influence on electronic devices.

Alkali Metal Cation Incorporated Ag3BiI6 Absorbers for Efficient and Stable Rudorffite Solar Cells

Objectives Toxic lead and poor stability are the main obstacles of perovskite solar cells. Lead-free silver bismuth iodide (SBI) was first attempted as solar cells photovoltaic materials in 2016. However, the short-circuit current of the SBI rudorffite materials is commonly below 10 mA/cm2, limiting the overall photovoltaic performance. Here, we present a chemical composition engineering to enhance the photovoltaic performance. Methods In this study, we incorporated a series of alkali metal cations (Li+, Na+, K+, Rb+, and Cs+) into Ag3BiI6 absorbers to investigate the effects on the photovoltaic performance of rudorffite solar cells. Results Cs+ doping improved VOC and Na+ doping showed an obvious enhancement in JSC. Therefore, we co-doped Na+ and Cs+ into SBI (Na/Cs-SBI) as the absorber and investigated the crystal structure, surface morphology, and optical properties. The photo-assisted Kelvin probe force microscopy (photo-KPFM) was used to measure surface potential and verified that Na/Cs doping could reduce the electron trapping at the grain boundary and facilitate electron transportation. Conclusion Na/Cs-SBI reduced the electron-holes pairs recombination and promoted the carrier transport of rudorffite solar cells. Finally, the Na/Cs-SBI rudorffite solar cell exhibited a PCE of 2.50%, a 46.0% increase to the SBI device (PCE = 1.71%), and was stable in ambient conditions for over 6 months.

Density Functional Theory of Material Design: Fundamentals and Applications - I

This article is part-I of a review of density-functional theory (DFT) that is the most widely used method for calculating electronic structure of materials. The accuracy and ease of numerical implementation of DFT methods has resulted in its extensive use for materials design and discovery and has thus ushered in the new field of computational material science. In this article we start with an introduction to Schrödinger equation and methods of its solutions. After presenting exact results for some well-known systems, difficulties encountered in solving the equation for interacting electrons are described. How these difficulties are handled using the variational principle for the energy to obtain approximate solutions of the Schrödinger equation is discussed. The resulting Hartree and Hartree-Fock theories are presented along with results they give for atomic and solid-state systems. We then describe Thomas-Fermi theory and its extensions which were the initial attempts to formulate many-electron problem in terms of electronic density of a system. Having described these theories, we introduce modern density functional theory by discussing Hohenberg-Kohn theorems that form its foundations. We then go on to discuss Kohn-Sham formulation of density-functional theory in its exact form. Next, local density approximation is introduced and solutions of Kohn-Sham equation for some representative systems, obtained using the local density approximation, are presented. We end part-I of the review describing the contents of part-II.

A comprehensive review of the pyrolysis process: from carbon nanomaterial synthesis to waste treatment

Thermally induced chemical decomposition of organic materials in the absence of oxygen is defined as pyrolysis. This process has four major application areas: (i) production of carbon materials, (ii) fabrication of pre-patterned micro and nano carbon-based structures, (iii) fragmentation of complex organic molecules for analytical purposes and (iv) waste treatment. While the underlying process principles remain the same in all cases, the target products differ owing to the phase and composition of the organic precursor, heat-treatment temperature, influence of catalysts and the presence of post-pyrolysis steps during heat-treatment. Due to its fundamental nature, pyrolysis is often studied in the context of one particular application rather than as an independent operation. In this review article an effort is made to understand each aspect of pyrolysis in a comprehensive fashion, ensuring that all state-of-the-art applications are approached from the core process parameters that influence the ensuing product. Representative publications from recent years for each application are reviewed and analyzed. Some classical scientific findings that laid the foundation of the modern-day carbon material production methods are also revisited. In addition, classification of pyrolysis, its history and nomenclature and the plausible integration of different application areas are discussed.

Laser additive manufacturing of metallic glasses: issues in vitrification and mechanical properties

Bulk metallic glasses (BMGs), on account of their attractive properties, have now begun to witness a few commercial applications, e.g. in coatings and micro-gears. Additive manufacturing (AM) or 3D printing, although established for crystalline alloys, has only recently been used for synthesising BMG components. The issues arising in 3D printing of BMGs are of current relevance, and this review focuses on the key scientific aspects, namely vitrification (or crystallisation) during printing, mechanical properties of printed glassy alloys and the use of AM in identifying newer BMGs. Available data on crystallisation during printing of a variety of BMGs are analysed in terms of schematic TTT diagrams and the complex interplay between thermal cycles, the presence of quenched-in nuclei in the glass and oxygen contamination in a way that is hoped to be broadly applicable to most alloy systems. Also reviewed are three key factors influencing mechanical properties of printed BMGs, i.e. porosity, crystallinity and oxygen contamination and thereby potential strategies for improvement are suggested. The review concludes with a discussion on the use of AM for combinatorial alloy development aimed at identifying better glass-forming compositions, which may in turn facilitate greater use of AM in manufacturing glassy components with desired properties.

Dendrite-Free Lithium Electrodeposition Enabled by 3D Porous Lithiophilic Host toward Stable Lithium Metal Anodes

Lithium metal is a promising anode utilized in cutting-edge high-energy batteries owing to the low density, low electrochemical potential, and super high theoretical capacity. Unfortunately, continuous uncontrollable lithium dendrite growth and ‘dead’ lithium result in capacity decay, low coulombic efficiency, and short circuit, severely hindering the practical utilization of lithium anode. Herein, we propose a three-dimensional porous lithiophilic current collector for lithium storage. The conductive 3D structure constructed by carbon fiber (CF) can well accommodate the deposited lithium, eliminating volume change between the lithium depositing/stripping process. Moreover, the polydopamine (PDA) coating on the CF surface possesses a large number of polar groups, which can homogenize Li+ ions distribution and apply as the sites for lithium deposition, decreasing nucleation overpotential. As a result, under the 1 mA cm−2 current density, the PDA coated CF (PDA@CF) electrode exhibits high CE (∼98%) for 1000 cycles. Galvanostatic measurements demonstrate that the Li anode using PDA@CF achieves 1000 h cycling life under 1 mA cm−2 with a low overpotential (<15 mV). The LiFePO4 full cell shows enhanced rate performance and stable long-term cycling.

Engineering of Layered Metal Dichalcogenides: Introducing Imperfections to Make it Perfect

In the past decade, the surge in research of layered metal dichalcogenides (LMD) has already demonstrated the tremendous potentiality of this particular category of materials towards technology. But in parallel, it is also established that to make them technology-perfect meticulous engineering to impose ‘imperfections’ within the materials is inevitable. So exploring different LMD with inexorable and appropriate engineering techniques for the enhancement of their functionality is the burning issue for materials scientists. This review comprehensively focuses on different pathways of introducing ‘imperfections’ within various LMDs, mainly by engineering the thickness, morphology, defect, doping and phase. Based on recent progress thickness and shape engineering of LMDs have been discussed with their success and modulation by defect has been examined in detail. Doping and phase engineering of LMDs have also been illustrated with the light of development till now. Finally, challenges and opportunities associated with this research direction are highlighted.

Sensitivity and Spatial Resolution for Thermal Conductivity Measurements using Non-contact Scanning Thermal Microscopy with Thermoresistive Probes under Ambient Conditions

Thermoresistive probes are increasingly popular in thermal conductivity characterization using Scanning Thermal Microscopy (SThM). A systematic analysis of the thermal conductivity measurement performance (sensitivity and spatial resolution) of thermoresistive SThM probe configurations that are available commercially is of interest to practitioners. In this work, the authors developed and validated 3-Dimensional Finite Element Models (3DFEM) of non-contact SThM with self-heated thermoresistive probes under ambient conditions with the probe-sample heat transfer in transition heat conduction regime for the four types of SThM probe configurations resembling commercially available products: Wollaston wire (WW) type probe, Kelvin Nanotechnology (KNT) type probe, Doped Silicon (DS) type probe, and Nanowire (NW) type probe. These models were then used to investigate the sensitivity and spatial resolution of the WW, KNT, DS and NW type probes for thermal conductivity measurements in non-contact mode in ambient conditions. The comparison of the SThM probes performance for measuring sample thermal conductivity and for the specific operating conditions investigated here show that the NW type probe has the best spatial resolution while the DS type probe has the best thermal conductivity measurement sensitivity in the range between 2-10 W·m−1·K−1. The spatial resolution is negatively affected by large probe diameters or by the presence of the cantilever in close proximity to the sample surface which strongly affects the probe-sample heat transfer in ambient conditions. An example of probe geometry configuration optimization was illustrated for the WW probe by investigating the effect of probe wire diameter on the thermal conductivity measurement sensitivity, showing ∼20% improvement in spatial resolution at the diameter with maximum thermal conductivity measurement sensitivity.