1,721,017 research outputs found
Spin magnetism of graphene nanoribbon modulated by triangular boron nitride flake
No full text© 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
Mechanisms of the epitaxial growth of two-dimensional polycrystals
No full textIn 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
Multi-stage anisotropic etching of two-dimensional heterostructures (Apr, 10.1007/s12274-022-4193-x, 2022)
No full textRegarding the reverse process of materials growth, etching has been widely concerned to indirectly probe the growth kinetics, offering an avenue in governing the growth of two-dimensional (2D) materials. In this work, interface-driven anisotropic etching mode is demonstrated for the first time to be generally applied to 2D heterostructures. It is shown that the typical in-plane graphene and hexagonal boron nitride (h-BN) heterostructures follow a multi-stage etching behavior initiated first along the interfacial region between the two materials and then along edges of neighboring h-BN flakes and finally along central edges of h-BN. By accurately tuning etching conditions in the chemical vapor deposition process, series of etched 2D heterostructure patterns are controllably produced. Furthermore, scaled formation of graphene and h-BN heterostructures arrays has been realized with full assist of as-proposed etching mechanism, offering a direct top-down method to make 2D orientated heterostructures with order and complexity. Detection of interface-driven multi-staged anisotropic etching mode will shed light on understanding growth mechanism and further expanding wide applications of 2D heterostructures.11Nsciescopu
The formation and stability of junctions in single-wall carbon nanotubes
Full text linkThe 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
Kinetics of Graphene and 2D Materials Growth
No full textDuring the last 10 years, remarkable achievements on the chemical vapor deposition (CVD) growth of 2D materials have been made, but the understanding of the underlying mechanisms is still relatively limited. Here, the current progress on the understanding of the growth kinetics of 2D materials, especially for their CVD synthesis, is reviewed. In order to present a complete picture of 2D materials' growth kinetics, the following factors are discussed: i) two types of growth modes, namely attachment-limited growth and diffusion-limited growth; ii) the etching of 2D materials, which offers an additional degree of freedom for growth control; iii) a number of experimental factors in graphene CVD synthesis, such as structure of the substrate, pressure of hydrogen or oxygen, temperature, etc., which are found to have profound effects on the growth kinetics; iv) double-layer and few-layer 2D materials' growth, which has distinct features different from the growth of single-layer 2D materials; and v) the growth of polycrystalline 2D materials by the coalescence of a few single crystalline domains. Finally, the current challenges and opportunities in future 2D materials' synthesis are summarized
Edge Reconstruction-Dependent Growth Kinetics of MoS2
No full textUnderstanding the growth mechanisms of multielement two-dimensional (2D) crystals is challenging because of the unbalanced stoichiometry and possible reconstruction of their edges. Here, we present a systematic theoretical study on the chemical vapor deposition (CVD) growth mechanism of MoS2. We found that the growth kinetics of MoS2 highly depends on its edge reconstruction determined by concentrations of Mo and S in the growth environment. Based on the calculated energies of nucleation and propagation of various MoS2 edges, we predicted the transition of a MoS2 island growth from a regime of a triangle enclosed by Mo-terminated zigzag edges that are passivated by 50% S (Mo-II edges), to a regime of continuous evolution within a triangle, hexagon, and inverted triangle with 75%-S-terminated edges (S-III edges) and Mo-II edges, and finally to a regime of triangles with Mo-terminated zigzag edges that are passivated by 100% S (Mo-III edges) by tuning the growth condition from Mo-rich to S-rich, which provides a reasonable explanation to many experimental observations. This study provides a general guideline on theoretical studies of 2D crystals' growth mechanisms, deepens our understanding on the growth mechanism of multielement 2D crystals, and is beneficial for the controllable synthesis of various 2D crystals
How graphene crosses a grain boundary on the catalyst surface during chemical vapour deposition growth
No full textThe chemical vapour deposition (CVD) growth of graphene is normally an epitaxial process, where the atomic structure of the adlayer should copy the texture of the substrate. However, it has been widely observed that single crystalline graphene grown on metal foil may cross a grain boundary (GB) of the substrate without forming any line defect, a necessary condition to change its crystalline orientation and maintain the structure registry with the substrate on the other side of the GB. Here, we present a comprehensive theoretical study on graphene growth behavior on polycrystalline metal substrates. Our density functional theory (DFT) calculations reveal that for graphene growth on most metal surfaces, the binding energy difference between the epitaxial and non-epitaxial graphene on the substrate is not large enough to compensate for the formation energy of a GB in graphene and therefore, during the CVD process, the growing graphene can pass through a GB on the metal surface without changing its crystalline orientation. Hence, graphene CVD growth cannot be strictly regarded as an epitaxial process; this conclusion is further verified by atomic simulations. The present study shows that the growth of graphene on a metal catalyst surface should be regarded rather as a quasi-epitaxial process, where a graphene domain is aligned only on the single crystalline metal facet on which it nucleates, but this structural registry with the metal substrate may be lost when the graphene crosses a GB on the metal surface
Highly stable phosphorene isomers based on a buckled honeycomb lattice
No full textDue to their high stabilities and tuneable electronic structures, two dimensional isomers of black phosphorene (BP) have drawn great attention recently. By carefully considering the bonding characteristics of phosphorus atoms, we propose a buckled honeycomb lattice strategy to search for possible highly stable phosphorene isomers. As an example, phosphorene isomers with a unit cell size no larger than 8 atoms are fully explored and 14 isomers, including the well-known ??-, ??-, ??-, ??-, and ??-phosphorene, are discovered and named from P-I to P-XIV in the order of their relative stabilities. Among these isomers, P-II and P-III are two newly found isomers with superior stability comparable to BP, whose formation energies are just 0.01 and 0.03 eV per atom higher than that of BP (??-phosphorene), respectively, and both are more stable than the well-known blue phosphorene (??-phosphorene). These new phosphorene isomers exhibit a wide range of medium energy band gaps from 0.30 to 2.66 eV, various HOMO/LUMO energy levels and, therefore, can be used for various optoelectronic and device applications
The geometry of hexagonal boron nitride clusters in the initial stages of chemical vapor deposition growth on a Cu(111) surface
No full textTo understand the nucleation process in the growth of hexagonal boron nitride (h-BN) on transition metal substrates by chemical vapor deposition (CVD), the energy of formation and stability of h-BN clusters of different geometries on a pristine Cu(111) surface were systematically investigated using density functional theory calculations. We find that unlike carbon clusters, h-BN clusters on Cu supports can undergo two possible transformations of the minimum-energy structure at a critical size of 13. Different from freestanding h-BN clusters, on a Cu(111) surface, h-BN chains are more stable than h-BN rings and thus dominate the minimum-energy structure for cluster sizes lower than the critical size. Thus, depending on the experimental conditions of CVD, one-dimensional Bn-1Nn (N-rich environment) or BnNn-1 (B-rich) chains are first created, and they transform to two-dimensional sp(2) networks or h-BN islands, but for a BnNn chain, the transformation to a two-dimensional sp(2) network h-BN island does not occur. In contrast to carbon islands where pentagons are readily formed, odd-membered rings are extremely rare in h-BN islands, where the transformation to the most stable structure occurs through a combination of trapeziums and hexagons at the edges, so as to avoid B-B and N-N bonds. Moreover, on a Cu(111) surface, trapeziums are destabilized when the four edges are connected to other hexagons because of additional curvature energy, thus favoring the nucleation of planar nuclei. A deep insight into h-BN cluster formation on a Cu support is vital to understanding the growth mechanism of h-BN on a transition metal surface in CVD experiments to further improve experimental designs in the CVD growth of h-BN
Formation of Twinned Graphene Polycrystals
No full textLiquid 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
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