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  • Küçük Resim
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    Atomic-scale probing of defect-assisted Ga intercalation through graphene using ReaxFF molecular dynamics simulations
    (Elsevier Ltd., 2022) Nayir, Nadire; Şengül, Mert Y.; Costine, Anna L.; Reinke, Petra; Rajabpour, Siavash; Bansal, Anushka; Kozhakhmetov, Azimkhan
    We report a joint theory and experimental investigation on the defect-mediated surface interactions of gallium (Ga) metals and trimethyl-gallium (TMGa) molecules with graphene. A combination of Raman spectra, X-ray photoelectron spectroscopy, scanning tunneling microscopy and spectroscopy (STM/STS) reveal defects in graphene, which can act as pathways for Ga intercalation. These experimental results are connected to ReaxFF simulations, which further confirm that the Ga and TMGa adsorption on graphene is strongly impacted by the presence and size of defects. These defects catalyze the surface reactions by lowering the temperature for Ga-deposition on the surface. Moreover, multivacancy defects promote Ga intercalation through graphene by reducing the kinetic barrier while the migration through single vacancy or 5-8-5 defect is kinetically hindered. The ReaxFF results indicate that TMGa exposure leads to defect healing by the passivation of carbon-dangling bonds by hydrocarbon and organometallic adducts, which is supported by the decreased Raman D:G ratio in Ga-intercalated graphene and by STM images. Since probing and controlling graphene defects constitutes a key step in the intercalation mechanism, this work provides an in-depth atomic scale understanding into the complex interplay between defects and precursors, thus providing an effective way to design defects for 2D metal fabrication.
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    Step engineering for nucleation and domain orientation control in WSe2 epitaxy on c-plane sapphire
    (Nature Portfolio, 2023) Zhu, Haoyue; Nayir, Nadire; Choudhury, Tanushree H. H.; Bansal, Anushka; Huet, Benjamin; Zhang, Kunyan
    Surface chemistry controls the location of WSe2 nucleation on a stepped sapphire substrate. Preferential nucleation at either the top or bottom step edge can be used to minimize mirror twin domains and produce unidirectional WSe2 monolayers. Epitaxial growth of two-dimensional transition metal dichalcogenides on sapphire has emerged as a promising route to wafer-scale single-crystal films. Steps on the sapphire act as sites for transition metal dichalcogenide nucleation and can impart a preferred domain orientation, resulting in a substantial reduction in mirror twins. Here we demonstrate control of both the nucleation site and unidirectional growth direction of WSe2 on c-plane sapphire by metal-organic chemical vapour deposition. The unidirectional orientation is found to be intimately tied to growth conditions via changes in the sapphire surface chemistry that control the step edge location of WSe2 nucleation, imparting either a 0 & DEG; or 60 & DEG; orientation relative to the underlying sapphire lattice. The results provide insight into the role of surface chemistry on transition metal dichalcogenide nucleation and domain alignment and demonstrate the ability to engineer domain orientation over wafer-scale substrates.
  • Küçük Resim
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    Toward a mechanistic understanding of the formation of 2D-GaNxin epitaxial graphene
    (American Chemical Society, 2023) Bansal, Anushka; Nayır, Nadire; Wang, Ke; Rondomanski, Patrick; Subramanian, Shruti; Kumari, Shalini
    Ultrathin 2D-GaNx can be formed by Ga intercalation into epitaxial graphene (EG) on SiC followed by nitridation in ammonia. Defects in the graphene provide routes for intercalation, but the nature and role of the defects have remained elusive. Here we examine the influence of graphene layer thickness and chemical functionalization on Ga intercalation and 2D-GaNx formation using a combination of experimental and theoretical studies. Thin buffer layer regions of graphene near steps on SiC readily undergo oxygen functionalization when exposed to air or a He/O2 plasma in contrast to thicker regions which are not chemically modified. Oxygen functionalization is found to inhibit Ga intercalation leading to accumulation of Ga droplets on the surface. In contrast, Ga readily intercalates between EG and SiC in the thicker graphene regions that do not contain oxygen. When NH3 annealing is carried out immediately after Ga exposure, 2D-GaNx formation is observed only in the oxygen-functionalized regions, and Ga intercalated under thicker nonfunctionalized graphene does not convert to GaNx. Density functional theory calculations demonstrate that oxygen functionalization of graphene alters the binding energy of Ga and NH3 species to the graphene surface. The presence of hydroxyl groups on graphene inhibits binding of Ga to the surface; however, it enhances the chemical reactivity of the graphene surface to NH3 which, in turn, enhances Ga binding and facilitates the formation of 2D-GaNx. By modifying the EG process to produce oxygen-functionalized buffer layer graphene, uniformly intercalated 2D-GaNx is obtained across the entire substrate surface.