Grain boundaries are observed and characterized in chemical vapor deposition-grown sheets of hexagonal boron nitride (h-BN) via ultra-high-resolution transmission electron microscopy at elevated temperature. Five- and seven-fold defects are readily observed along the grain boundary. Dynamics of strained regions and grain boundary defects are resolved. The defect structures and the resulting out-of-plane warping are consistent with recent theoretical model predictions for grain boundaries in h-BN.
Atomically precise tailoring of graphene can enable unusual transport pathways and new nanometer-scale functional devices. Here we describe a recipe for the controlled production of highly regular "5-5-8" line defects in graphene by means of simultaneous electron irradiation and Joule heating by applied electric current. High-resolution transmission electron microscopy reveals individual steps of the growth process. Extending earlier theoretical work suggesting valley-discriminating capabilities of a graphene 5-5-8 line defect, we perform firstprinciples calculations of transport and find a strong energy dependence of valley polarization of the charge carriers across the defect. These findings inspire us to propose a compact electrostatically gated "valley valve" device, a critical component for valleytronics.2 Atomically-precise modification of low-dimensional materials such as graphene is exceedingly challenging since existing experimental techniques rarely achieve atomic precision. Nevertheless, if successful, atomic manipulations could have dramatic impact on graphene's electrical, magnetic, optical, mechanical, chemical, and thermal properties [1][2][3][4] , leading to novel functionalities that could be exploited in nanoscale devices.A recently emerging field is "valleytronics", a zero-magnetic-field analog to spintronics which exploits the quantum mechanical "valley" degree of freedom of charge carriers in graphene 2, 5-10 .At low energies the band structure of single-layer graphene is composed of two energetically degenerate valleys ("Dirac cones"), separated by ~30 nm -1 . 11 The intervalley coupling is quite weak in high quality graphene even at room temperature 12,13 , and hence this additional degree of freedom is a good quantum number. Valley polarization could be used for information processing much as the electron spin degree of freedom is used in spintronics, with the added benefit of temperature insensitivity.Generally, two approaches have been suggested for lifting the degeneracy and thus achieving graphene valley polarization: 1) application of external magnetic field, and 2) using local modifications of the crystalline lattice. The growth process of the 5-5-8 linear defect is intriguing, and we examine it in some detail here. The metastable 5-6 pair generated at the growth-leading end is the key to the growth mechanism. Figures 3a-3c illustrate critical formation steps as determined by TEM. Each experimental image is constructed from an average of 12 single shot TEM images taken in rapid succession to reduce the background noise and to include all possible configurations of the defect. As the line defect grows by one octagon, one carbon atom (marked by a blue dot in the illustrations) is ejected, and a new bond is formed between its nearest neighbors (marked by yellow dots). This process also creates a new 5-6 termination pair, which serves as a seed for continued growth. Since an 6 isolated pentagon cannot be sustained in otherwise ideal graphene 23 , an extended structural irre...
The size distribution of particles, which is essential for many properties of nanomaterials, is equally important for the mechanical behaviour of the class of alloys whose strength derives from a dispersion of nanoscale precipitates. However, particle size distributions formed by solid-state precipitation are generally not well controlled. Here we demonstrate, through the example of core-shell precipitates in Al-Sc-Li alloys, an approach to forming highly monodisperse particle size distributions by simple solid-state reactions. The approach involves the use of a two-step heat treatment, whereby the core formed at high temperature provides a template for growth of the shell at lower temperature. If the core is allowed to grow to a sufficient size, the shell develops in a 'size focusing' regime, where smaller particles grow faster than larger ones. These results suggest strategies for manipulating precipitate size distributions in similar systems through simple variations in thermal treatments.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.