Graphene on nickel (100) micrograins: Modulating the interface interaction by extended moiré superstructures

Abstract

Interaction with the substrate strongly affects the electronic/chemical properties of supported graphene. So far, graphene deposited by chemical vapor deposition (CVD) on catalytic single crystal transition metal surfaces -mostly 3-fold close-packed -has mainly been studied. Herein, we investigated CVD graphene on a polycrystalline nickel (Ni) substrate, focusing in particular on (100) micrograins and comparing the observed behavior with that on single crystal Ni(100) substrate. The symmetry-mismatch leads to moiré superstructures with stripe-like or rhombic-network morphology, which were characterized by atomically-resolved scanning tunneling microscopy (STM). Density functional theory (DFT) simulations shed light on spatial corrugation and interfacial interactions: depending on the misorientation angle, graphene is either alternately physi-and chemisorbed or uniformly chemisorbed, the interaction being modulated by the (sub)nanometer-sized moiré superstructures. Ni(100) micrograins appear to be a promising substrate to finely tailor the electronic properties of graphene at the nanoscale, with relevant perspective applications in electronics and catalysis. of the graphene lattice, which leads to alternate strongly-and weakly-interacting regions across the moiré supercells [13][14][15][16][17][18]. In contrast, the weak coupling between graphene and other transition metals (such as copper (Cu), iridium (Ir) and platinum) results in large interfacial spacing out of the range of chemisorption, smaller spatial corrugation of moirés with respect to strongly-coupled systems, and limited rotational alignment between graphene and the substrate [19][20][21][22][23]. From an electronic point of view, the band structures for chemisorbed graphene (such as that on Ni(111) or Ru(0001)) are fragmented or disrupted due to the hybridization of the graphene π state and the metal d orbital, while physisorbed graphene typically shows Dirac cones similar to its pristine form [24][25][26]. Therefore, the magnitude of energy gap opening, interface charge polycrystalline Ni foils are also explored, thereby bridging the material gap from single crystal to realistic, non-ideal surfaces for STM measurements. Indeed, the (100) facet is one of the most common orientations present in polycrystalline Ni foils or thin films, as reported in literature [29,37] and further corroborated in this work. In addition, nickel is among the class of most-utilized metallic catalysts for CVD growth of graphene [5][6]38]. This work is therefore of potential interest for the scalable production and applications of graphene. Our results indicate that graphene structures observed on both single-and poly-crystalline substrates are highly nanometer scale. Generally, graphene moiré originates from lattice mismatch and/or angular misorientation in two isosymmetric overlapping periodic lattices; herein the situation is further complicated by the symmetry mismatch of the two interface lattices. In figures 2(a-c), from left to right, we show three STM images with incr...

Operando atomic-scale study of graphene CVD growth at steps of polycrystalline nickel

“…The (100) facet of the Ni-poly substrate is found to be featured with flat terraces as wide as tens of nanometers, separated by step bunches which can span up to tens of atomic layers. [15] It should be noted that with the growth conditions adopted in this work (see Methods), graphene is always monolayer, as recently found also on Ni(100) thin films via ambient-pressure CVD preparation; [29] moreover, like the case of Ni(111) single crystals, graphene never grows rightly above nickel carbide, at variance with the behavior observed on early transition metals. [31][32] Similarly to what already reported for the Ni(111) single crystal, [11] two graphene growth mechanisms are active on flat (100) terraces of Ni-poly, corresponding to in-plane transformation and on-terrace adlayer growth, respectively, which will be 8 reported elsewhere.…”

“…The same series of simple pre-treatments already described in our previous work [15] was used to prepare Ni-poly foils suitable for STM measurements. Here, we focus on grains exposing the (100) facet, one of the ubiquitous orientations in Ni-poly substrates.…”

“…Here, we focus on grains exposing the (100) facet, one of the ubiquitous orientations in Ni-poly substrates. [15,[26][27] The extended moiré pattern which originates on these facets from the interface symmetry mismatch can serve as a hallmark of graphene formation and a magnification of the defects in graphene lattice even at a relatively large scan scale. [15,[28][29][30] It should be noted that this experimental approach could be easily extended to differently oriented facets in Ni and even other polycrystalline transition metal foils, whenever graphene grows at a temperature and a hydrocarbon pressure compatible with the specifics of the microscope.…”

“…First of all we tried to clarify the origin of the previously described moiré pattern adopted by an infinite graphene layer on Ni(100). [15] To this purpose, we simulate a growing graphene flake with ribbons of different width in the growing [ ] direction and infinitely long along the moiré stripes parallel to the [011] direction ( Fig. 4c, see Fig.…”

“…[12][13][14] Recently, the feasibility of STM measurements has been proved also for graphene on polycrystalline nickel (Ni-poly), after suitable sample pretreatment. [15] Operando STM investigations of graphene growth on polycrystalline substrates appears therefore to be a viable route to clarify the atomistic details of the process.…”