Optoelectronic crystal of artificial atoms in strain-textured molybdenum disulphide

The isolation of the two-dimensional semiconductor molybdenum disulphide introduced a new optically active material possessing a band gap that can be facilely tuned via elastic strain. As an atomically thin membrane with exceptional strength, monolayer molybdenum disulphide subjected to biaxial strain can embed wide band gap variations overlapping the visible light spectrum, with calculations showing the modified electronic potential emanating from point-induced tensile strain perturbations mimics the Coulomb potential in a mesoscopic atom. Here we realize and confirm this ‘artificial atom’ concept via capillary-pressure-induced nanoindentation of monolayer molybdenum disulphide from a tailored nanopattern, and demonstrate that a synthetic superlattice of these building blocks forms an optoelectronic crystal capable of broadband light absorption and efficient funnelling of photogenerated excitons to points of maximum strain at the artificial-atom nuclei. Such two-dimensional semiconductors with spatially textured band gaps represent a new class of materials, which may find applications in next-generation optoelectronics or photovoltaics

Light interstitials in iron under extreme mechanical conditions

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2019Cataloged from PDF version of thesis.Includes bibliographical references (pages 99-108).Addition of small amounts of light interstitial elements to iron can alter its physio-chemical characteristics to a great degree. The most crucial of these elements being carbon, overcomes many of iron deficiencies such as lack of hardenability, tensile strength and so on. It is owing to this element that iron in form of steel has become the most commonly used material in modern industry. However, not all interstitial elements have a positive impact on iron's performance, nor their presence is desirable. Due to its high diffusivity, hydrogen can travel inside iron with relative ease and interact with already formed, or forming defects such as dislocations and vacancies. It is believed that this interaction impacts the formation and evolution process of defects significantly. From macroscopic perspective, this is manifested in form of embrittlement of iron, usually referred to as hydrogen embrittlement (HE). Super-ferrite is a newly discovered phase of iron supersaturated in carbon.It is usually formed under extreme mechanical conditions like severe plastic deformation, from iron and a commonly found form of carbide in steel, namely cementite. The first part of this document delves into many aspects of super-ferrite using atomistic simulations and density functional theory. Of the crucial findings of said chapter, one is the process of super-ferrite formation, which involves a secondary intermediate phase. Another is careful analysis of its structure and its comparison with the more common supersaturated phase, martensite. The second part is devoted to careful examination of a newly proposed HE mechanism in iron. Using the concrete framework of thermodynamics and statistical mechanics, complemented by numerical methods such as molecular dynamics, grand canonical Monte Carlo, and density functional theory, many aspects of this theory are scrutinized.It is concluded although viable for iron under extremely high hydrogen pressure, this mechanism is not applicable to HE that is commonly observed in industry. As a by product of this part, the iron hydrogen phase diagram is extended to temperatures as low as 100 Kelvin.by Sina Moeini Ardakani.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Civil and Environmental Engineerin

Highly efficient parallel grand canonical simulations of interstitial-driven diffusion-deformation processes

Diffusion of interstitial alloying elements like H, O, C, and N in metals and their continuous relocation and interactions with their microstructures have crucial influences on metals properties. However, besides limitations in experimental tools in capturing these mechanisms, the inefficiency of numerical tools also inhibits modeling efforts. Here, we present an efficient framework to perform hybrid grand canonical Monte Carlo and molecular dynamics simulations that allow for parallel insertion/deletion of Monte Carlo moves. A new methodology for calculation of the energy difference at trial moves that can be applied to many-body potentials as well as pair ones is a primary feature of our implementation. We study H diffusion in Fe (ferrite phase) and Ni polycrystalline samples to demonstrate the efficiency and scalability of the algorithm and its application. The computational cost of using our framework for half a million atoms is a factor of 250 less than the cost of using existing libraries

Origin of micrometer-scale dislocation motion during hydrogen desorption

Hydrogen, while being a potential energy solution, creates arguably the most important embrittlement problem in high-strength metals. However, the underlying hydrogen-defect interactions leading to embrittlement are challenging to unravel. Here, we investigate an intriguing hydrogen effect to shed more light on these interactions. By designing an in situ electron channeling contrast imaging experiment of samples under no external stresses, we show that dislocations (atomic-scale line defects) can move distances reaching 1.5 μm during hydrogen desorption. Combining molecular dynamics and grand canonical Monte Carlo simulations, we reveal that grain boundary hydrogen segregation can cause the required long-range resolved shear stresses, as well as short-range atomic stress fluctuations. Thus, such segregation effects should be considered widely in hydrogen research. ©2020 The Authors.JSPS KAKENHI (JP16H06365)JSPS KAKENHI (JP20H02457)Swiss National Science Foundation grant (P300P2_171423)NSF CMMI-1922206Department of the Navy, ONR (N00014-18-1-2284