Structure of a Two-Dimensional Silicate Layer Formed by Reaction with an Alloy Substrate

Abstract

The ability to synthesize two-dimensional (2D) oxide layers with nonbulk structures on transition-metal surfaces has attracted attention as a method to expand the range of 2D structures and properties beyond those accessible by thinning known layered materials. In this paper, a combination of surface spectroscopies, scanning tunneling microscopy (STM), and density functional theory (DFT) show that the reaction of silicon and oxygen with a Ni–Pd alloy surface extracts Ni from the substrate to produce a non-bulk-structured 2D Ni silicate. The surface spectroscopies reveal that high-temperature annealing in an oxygen-rich environment causes substrate Ni to segregate toward the surface where it can oxidize to form a 2D crystalline layer that includes Ni–O–Si bonds. STM images of the 2D crystalline layer reveal a local honeycomb structure consistent with six-membered rings of corner-sharing SiO4 tetrahedra overlaid on a larger-scale moiré pattern that arises from the lattice mismatch between the 2D layer and the substrate. The DFT calculations identify a thermodynamically favorable 2D Ni silicate structure composed of a layer of six-membered rings of corner-sharing SiO4 tetrahedra bonded to an octahedrally coordinated Ni–O layer that can explain the observed STM contrast. The structure resembles a single layer of a dioctahedral clay with bonds to the substrate replacing O–H bonds in clays. The dioctahedral Ni clay stands in contrast to known bulk materials in which Ni is only observed in trioctahedral clays. Theory indicates that the formation of the 2D Ni silicate is not strongly dependent on the alloy composition while experiments suggest that its formation is self-limited; that is, once it forms, continued exposure to oxidizing conditions does not alter the structure. The results highlight the potential for using alloy substrates with a reactive component to form unique 2D surface layers with a broad processing window and without tight constraints on composition control.

Growth and Atomic‐Scale Characterization of Ultrathin Silica and Germania Films: The Crucial Role of the Metal Support

“…The silica bilayer today is in the tool box of 2D materials, [76] thanks to extensive characterization that includes studies of: work function, [77] band‐gap, [78] crystalline‐vitreous interface, [3] atomic [9, 11] and molecular [79, 11, 80] sieve, confined chemistry, [59, 12, 60, 61] bending rigidity, [81] 2D zeolites, [82–84] transferring from one substrate to another [6] and imaging in water [85] . With all this knowledge available, the silica bilayer becomes an interesting dielectric material for nanoelectronic devices [76]…”

“…[4] The silica bilayer also allows one to computer-model amorphouss ystems [70][71][72][73][74] and comparet hem with experimental results in real space, since the decrease in the dimensionality from the three-dimensional (3D) bulk material [1] to 2D networks renders the calculations feasible. [75] The silica bilayer today is in the tool box of 2D materials, [76] thankst oe xtensive characterization that includes studies of: work function, [77] band-gap, [78] crystalline-vitreous interface, [3] atomic [9,11] and molecular [79,11,80] sieve, confined chemistry, [59,12,60,61] bending rigidity, [81] 2D zeolites, [82][83][84] transferring from one substrate to another [6] and imaging in water. [85] With all this knowledgea vailable,t he silica bilayer becomes an interesting dielectric material for nanoelectronic devices.…”

Two-Dimensional Transition Metal Silicate Formed on Ru (0001) by Hydrogenation

“…Recently, 2D transition metal silicates such as Fe, Ni and Ti-silicates have been synthesized on metal substrates [1][2][3]. These materials have the chemical formula of M 2 Si 2 O 8 , where a honeycomb Si 2 O 5 layer with corner sharing tetrahedra is stacked on top of a transition metal oxide layer which has both edge and corner sharing 5fold coordinated, square pyramidal polyhedra as shown in Fig.…”

“…1(a). The synthesis procedure in all these three examples starts with depositing Si or SiO and the transition metal at modest temperatures on the substrate or by depositing the Si source on an alloy containing the target transition metal and then following an annealing near 950 K [2]. These transition metal silicates on metal substrates resemble crystal structures of naturally existing sheet silicates (phyllosilicates), particularly that of dehydroxylated nontronite, M 2 Si 2 O 8 [1,4].…”

“…These transition metal silicates on metal substrates resemble crystal structures of naturally existing sheet silicates (phyllosilicates), particularly that of dehydroxylated nontronite, M 2 Si 2 O 8 [1,4]. To our knowledge, there has been no concerted effort to yield a VDW layer from these materials after their syntheses: The 2D transition metal silicate forms strong chemical bonds with the substrate, which prevents the isolation of the 2D layer [2]. An efficient strategy can be to hydrogenate these materials with a hydrogen source such as hydrogen plasma or water.…”