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1. Modeling for Structural Engineering and Synthesis of Two Dimensional WSe2 Using a Newly Developed ReaxFF Reactive Force Field

   Nayir, Nadire; Wang, Yuanxi; Shabnam, Sharmin; Hickey, Danielle Reifsnyder; Miao, Leixin; Zhang, Xiaotian; Bachu, Saiphaneendra; Alem, Nasim; Redwing, Joan; Crespi, Vincent H.; et al (December 2020, Journal of physical chemistry)

   null (Ed.)

   Atomistic simulation techniques have become an indispensable tool to acquire a fundamental understanding of growth and structural characteristics of two-dimensional (2D) materials of interest, thereby accelerating experimental research in the same field. A new ReaxFF reactive force field presented here is the first comprehensive empirical potential that is explicitly designed to capture the most prominent features of 2D WSe2 solid-phase chemistry, such as defect formation as a function of local geometry and chalcogen chemical potential, vacancy migration and phase transition, thus enabling cost-effective and reliable characterization of 2D WSe2 at large length scales and time scales much longer than what is accessible by first-principles theory. This potential, validated using extensive first-principles energetics data on both periodic and nonperiodic systems and experimental measurements, can accurately describe the mechanochemical coupling between monolayer deformations and vacancy energetics, providing valuable atomistic insights into the morphological evolution of a monolayer in different environments in terms of loading conditions and various concentrations and distributions of defects. Since understanding how growth is affected by the local chemical environment is vital to fabricating efficient and functional atomically thin 2D WSe2, the new ReaxFF description enables investigations of edge-controlled growth of single crystals of 2D WSe2 using reactive environments closely matching experimental conditions at a low computational cost. 

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2. Room temperature electrically pumped topological insulator lasers

   https://doi.org/10.1038/s41467-021-23718-4  

   Choi, Jae-Hyuck; Hayenga, William E.; Liu, Yuzhou G.; Parto, Midya; Bahari, Babak; Christodoulides, Demetrios N.; Khajavikhan, Mercedeh (December 2021, Nature Communications)

   null (Ed.)

   Abstract Topological insulator lasers (TILs) are a recently introduced family of lasing arrays in which phase locking is achieved through synthetic gauge fields. These single frequency light source arrays operate in the spatially extended edge modes of topologically non-trivial optical lattices. Because of the inherent robustness of topological modes against perturbations and defects, such topological insulator lasers tend to demonstrate higher slope efficiencies as compared to their topologically trivial counterparts. So far, magnetic and non-magnetic optically pumped topological laser arrays as well as electrically pumped TILs that are operating at cryogenic temperatures have been demonstrated. Here we present the first room temperature and electrically pumped topological insulator laser. This laser array, using a structure that mimics the quantum spin Hall effect for photons, generates light at telecom wavelengths and exhibits single frequency emission. Our work is expected to lead to further developments in laser science and technology, while opening up new possibilities in topological photonics. 

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3. Bound states at partial dislocation defects in multipole higher-order topological insulators

   https://doi.org/10.1038/s41467-022-29785-5  

   Yamada, Sasha S.; Li, Tianhe; Lin, Mao; Peterson, Christopher W.; Hughes, Taylor L.; Bahl, Gaurav (April 2022, Nature Communications)

   Abstract The bulk-boundary correspondence, which links a bulk topological property of a material to the existence of robust boundary states, is a hallmark of topological insulators. However, in crystalline topological materials the presence of boundary states in the insulating gap is not always necessary since they can be hidden in the bulk energy bands, obscured by boundary artifacts of non-topological origin, or, in the case of higher-order topology, they can be gapped altogether. Recently, exotic defects of translation symmetry called partial dislocations have been proposed to trap gapless topological modes in some materials. Here we present experimental observations of partial-dislocation-induced topological modes in 2D and 3D insulators. We particularly focus on multipole higher-order topological insulators built from circuit-based resonator arrays, since crucially they are not sensitive to full dislocation defects, and they have a sublattice structure allowing for stacking faults and partial dislocations. 

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4. Scalable nanomanufacturing of holey graphene via chemical etching: an investigation into process mechanisms

   https://doi.org/10.1039/d1nr08437b  

   Bi, Kun; Wang, Dini; Dai, Rui; Liu, Lei; Wang, Yan; Lu, Yongfeng; Liao, Yiliang; Ding, Ling; Zhuang, Houlong; Nian, Qiong (March 2022, Nanoscale)

   Graphene with in-plane nanoholes, named holey graphene, shows great potential in electrochemical applications due to its fast mass transport and improved electrochemical activity. Scalable nanomanufacturing of holey graphene is generally based on chemical etching using hydrogen peroxide to form through-the-thickness nanoholes on the basal plane of graphene. In this study, we probe into the fundamental mechanisms of nanohole formation under peroxide etching via an integrated experimental and computational effort. The research results show that the growth of nanoholes during the etching of graphene oxide is achieved by a three-stage reduction–oxidation–reduction procedure. First, it is demonstrated that vacancy defects are formed via a partial reduction-based pretreatment. Second, hydrogen peroxide reacts preferentially with the edge-sites of defect areas on graphene oxide sheets, leading to the formation of various oxygen-containing functional groups. Third, the carbon atoms around the defects are removed along with the neighboring carbon atoms via reduction. By advancing the understanding of process mechanisms, we further demonstrate an improved nanomanufacturing strategy, in which graphene oxide with a high density of defects is introduced for peroxide etching, leading to enhanced nanohole formation. 

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5. Edge-Mediated Annihilation of Vacancy Clusters in Monolayer Molybdenum Diselenide (MoSe2) under Electron Beam Irradiation

   https://doi.org/DOI: 10.1002/smll.202105194  

   Xu Zhang, Xiang Zhang (January 2021, Its a small small small small world)

   Annihilation of vacancy clusters in monolayer molybdenum diselenide (MoSe2) under electron beam irradiation is reported. In situ high-resolution transmission electron microscopy observation reveals that the annihilation is achieved by diffusion of vacancies to the free edge near the vacancy clusters. Monte Carlo simulations confirm that it is energetically favorable for the vacancies to locate at the free edge. By computing the minimum energy path for the annihilation of one vacancy cluster as a case study, it is further shown that electron beam irradiation and pre-stress in the suspended MoSe2 monolayer are necessary for the vacancies to overcome the energy barriers for diffusion. The findings suggest a new mechanism of vacancy healing in 2D materials and broaden the capability of electron beam for defect engineering of 2D materials, a promising way of tuning their properties for engineering applications. 

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