First-Principles Study of the Electron Transport Properties of Graphene-Like 2D Materials

First-principles density functional theory and non-equilibrium Green function calculations have been conducted to explore the electronic properties of the graphene-like 2D materials. It is found that zigzag boron phosphide nanoribbons (zBPNRs) exhibit non-magnetic direct bandgap semiconducting property and bandgap is about 1 eV. The heterostructure zSiC-BP-SiC nanoribbons are found to display an obvious negative differential resistance (NDR), which are tunable by changing the length of BPNRs. It is also found that for the armchair MoS2/WS2NRs heterostructures, with the number of WS2NR unit cell increasing, the NDR effect can be modulated. Especially for the case of M(edge) with W atoms doping on the edges, it not only exhibits a significant NDR effect but also owns a fast current transport. Therefore, these graphene-like 2D materials may possess potential for the application in logic transistor

The formation and stability of junctions in single-wall carbon nanotubes

The structure and stability of molecular junctions, which connect two single-wall carbon nanotubes (SWCNTs) of different diameters and chiral angles, (n(1), m(1))-(n(2), m(2)), are systematically investigated by density functional tight binding calculations. More than 100 junctions, which connect well-aligned SWCNTs, were constructed and calculated. For a highly stable junction between two chiral (n(1), m(1)) and (n(2), m(2)) SWCNTs with opposite handedness, the number of pentagon-heptagon (5/7) pairs required to build the junction can be denoted as vertical bar vertical bar n(2) - n(1)vertical bar - vertical bar m(2) - m(1)vertical bar vertical bar + min{vertical bar n(2) - n(1)vertical bar, vertical bar m(2) - m(1)vertical bar} with (n(2), m(2)) rotating pi/3 angle or not. While for a junction connected by two zigzag, armchair or two chiral SWCNTs with the same handedness, the number of 5/7 pairs is equal to vertical bar n(1) - n(2)vertical bar + vertical bar m(1) - m(2)vertical bar. Similar to the formation energies of grain boundaries in graphene, the curve of the formation energies vs. chiral angle difference present an 'M' shape indicating the preference of similar to 30 degree junctions. Moreover, the formation energies of the zigzag-type and armchair-type junctions with zero misorientation angles are largely sensitive to the diameter difference of two sub-SWCNTs

Segment Anything Model for Medical Image Analysis: an Experimental Study

Training segmentation models for medical images continues to be challenging due to the limited availability and acquisition expense of data annotations. Segment Anything Model (SAM) is a foundation model trained on over 1 billion annotations, predominantly for natural images, that is intended to be able to segment the user-defined object of interest in an interactive manner. Despite its impressive performance on natural images, it is unclear how the model is affected when shifting to medical image domains. Here, we perform an extensive evaluation of SAM's ability to segment medical images on a collection of 11 medical imaging datasets from various modalities and anatomies. In our experiments, we generated point prompts using a standard method that simulates interactive segmentation. Experimental results show that SAM's performance based on single prompts highly varies depending on the task and the dataset, i.e., from 0.1135 for a spine MRI dataset to 0.8650 for a hip x-ray dataset, evaluated by IoU. Performance appears to be high for tasks including well-circumscribed objects with unambiguous prompts and poorer in many other scenarios such as segmentation of tumors. When multiple prompts are provided, performance improves only slightly overall, but more so for datasets where the object is not contiguous. An additional comparison to RITM showed a much better performance of SAM for one prompt but a similar performance of the two methods for a larger number of prompts. We conclude that SAM shows impressive performance for some datasets given the zero-shot learning setup but poor to moderate performance for multiple other datasets. While SAM as a model and as a learning paradigm might be impactful in the medical imaging domain, extensive research is needed to identify the proper ways of adapting it in this domain.Comment: Link to our code: https://github.com/mazurowski-lab/segment-anything-medica

Project Shanghai: An Experimental Short Film Commenting on Income Inequality in Shanghai

Project Shanghai is an experimental short film that addresses the problem of wealth inequality in Shanghai and draws attention to the lives of the lower class. Since the economic reform launched in the last century that brought China unprecedented growth, Shanghai has developed into the financial hub of East Asia and the gateway to mainland China. However, it has also become a city where one can see the extremes of income inequality. Influenced by the Six-Generation Chinese directors and their persistent attention to the marginalized city dwellers, Project Shanghai attempts to advocate the basic human rights for the victims of income inequality. With visual storytelling that illustrates the intimate details, rich textures, and vivid human stories in the city corners below the iconic skyscrapers, the film also addresses other socio-economic problems including geographic disparity and gender inequality

Hydrogenation of bilayer graphene: A small twist makes a big difference

The effect of twist angle on the hydrogenation of bilayer graphene (BLG) is systematically explored by density functional theory (DFT) calculations. We found that a twist between the upper and lower layers of the graphene BLGs, either big or small, interferes with the formation of inter-layer C-C covalent bonds and this leads to strong resistance to hydrogenation. In addition, the electronic properties of stable, hydrogenated twisted BLG with different twist angles and degrees of H coverage were investigated. This study paves the way to the selective functionalization of BLG for various applications

Strategies, Status, and Challenges in Wafer Scale Single Crystalline Two-Dimensional Materials Synthesis

The successful exfoliation of graphene has given a tremendous boost to research on various two-dimensional (2D) materials in the last 15 years. Different from traditional thin films, a 2D material is composed of one to a few atomic layers. While atoms within a layer are chemically bonded, interactions between layers are generally weak van der Waals (vdW) interactions. Due to their particular dimensionality, 2D materials exhibit special electronic, magnetic, mechanical, and thermal properties, not found in their 3D counterparts, and therefore they have great potential in various applications, such as 2D materials-based devices. To fully realize their large-scale practical applications, especially in devices, wafer scale single crystalline (WSSC) 2D materials are indispensable. In this review, we present a detailed overview on strategies toward the synthesis of WSSC 2D materials while highlighting the recent progress on WSSC graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenide (TMDC) synthesis. The challenges that need to be addressed in future studies have also been described. In general, there have been two distinct routes to synthesize WSSC 2D materials: (i) allowing only one nucleus on a wafer scale substrate to be formed and developed into a large single crystal and (ii) seamlessly stitching a large number of unidirectionally aligned 2D islands on a wafer scale substrate, which is generally single crystalline. Currently, the synthesis of WSSC graphene has been realized by both routes, and WSSC hBN and MoS2 have been synthesized by route (ii). On the other hand, the growth of other WSSC 2D materials and WSSC multilayer 2D materials still remains a big challenge. In the last section, we wrap up this review by summarizing the future challenges and opportunities in the synthesis of various WSSC 2D materials

Mechanisms of the epitaxial growth of two-dimensional polycrystals

In the epitaxial growth of two-dimensional (2D) materials on substrates, 2D polycrystals with various shapes have been broadly observed, but their formation mechanisms are still highly elusive. Here we present a complete study on the formation mechanisms of various 2D polycrystals. The structures of the 2D polycrystals are dependent on the symmetries of both the 2D material and the substrate. We build four complete libraries of 2D polycrystals for (i) threefold symmetric 2D materials on two- or six-fold symmetric substrates (i.e., family-III/II or -III/VI), (ii) threefold symmetric 2D materials on fourfold symmetric substrates (i.e. family-III/IV), (iii) fourfold symmetric 2D materials on three- or six-fold symmetric substrates (i.e., family-IV/III or -IV/VI), and (iv) sixfold symmetric 2D materials on fourfold symmetric substrates (i.e., family-VI/IV), respectively. The four libraries of 2D polycrystals are consistent with many existing experimental observations and can be used to guide the experimental synthesis of various 2D polycrystals

Etching of two-dimensional materials

Etching is one of the key steps in materials processing in the semiconducting industry and recent progress in this field has shown that etching can be used to create various structures of two-dimensional (2D) materials, and can be viewed as a complementary technique to growth. Besides, etching has many other applications in 2D materials synthesis, processing and characterization, such as helping to understand the growth mechanism in-depth, identifying point and line defects in a 2D material, improving the quality of 2D materials by combining the growth process, and for fabricating 2D heterojunctions. Here, we review the current progress on the etching of graphene and other 2D materials; the content of this review includes: (i) etching of single-crystalline 2D materials; (ii) etching of multilayer 2D materials; (iii) etching of polycrystalline 2D materials; (iv) experimental factors, such as partial pressure of etchants, etchant species, and substrate, that affect 2D materials etching; (v) applications of etching in graphene and other 2D materials synthesis, characterization and processing and (vi) the challenges and opportunities in 2D materials etching

Formation of Twinned Graphene Polycrystals

Liquid metals have been widely used as substrates to grow graphene and other 2D materials. On a homogeneous and isotropic liquid surface, a polycrystalline 2D material is formed by coalescence of many randomly nucleated single-crystal islands, and as a result, the domains in a polycrystal are expected to be randomly aligned. Here, we report the unexpected finding that only 30 degrees-twinned graphene polycrystals are grown on a liquid Cu surface. Atomic simulations confirm that the unique domain alignment in graphene polycrystals is due to the free rotation of graphene islands on the liquid Cu surface and the highly stable 30 degrees-grain boundaries in graphene. In-depth analysis predicts 30 types of possible 30 degrees-twinned graphene polycrystals and 27 of them are observed. The revealed formation mechanism of graphene polycrystals on a liquid Cu surface deepens our fundamental understanding on polycrystal growth and could serve as a guideline for the controlled synthesis of 2D materials

Spin magnetism of graphene nanoribbon modulated by triangular boron nitride flake

© 2019 Elsevier B.V.The electronic properties and spin magnetism of graphene nanoribbon doped with triangular BN flake are systematically investigated. For the spin polarized state, the size of the triangular BN flake could tailor the spin magnetism of devices. More importantly, it will intrigue property of bipolar magnetic semiconducting with the spin-filtering efficiency (SFE) nearly reaching 100% and keep good transport property when the triangular BN flake is large enough. Although the triangular vacancies also present a bipolar magnetic semiconducting property with a higher SFE, the transmission probability drops dramatically and the stability is no more than those of triangle BN flake11sciescopu