Atomically thin half-van der Waals metals enabled by confinement heteroepitaxy

Abstract

Atomically thin two-dimensional (2D) metals may be key ingredients in next-generation quantum and optoelectronic devices. However, 2D metals must be stabilized against environmental degradation and integrated into heterostructure devices at the wafer scale. The high-energy interface between silicon carbide and epitaxial graphene provides an intriguing framework for stabilizing a diverse range of 2D metals. Here we demonstrate large-area, environmentally stable, epitaxial graphene/single-crystal 2D gallium, indium, and tin heterostructures. The 2D metals are covalently bonded to SiC below but present a non-bonded interface to graphene overlayer, i.e. they are "half van der Waals" metals with strong internal gradients in bonding character. These non-centrosymmetric 2D metals open compelling opportunities for superconducting devices, topological phenomena, and advanced optoelectronic properties. For example, the reported 2D-Ga is a superconductor that combines six strongly coupled Ga-derived electron pockets with a large nearly-freeelectron Fermi surface that closely approaches the Dirac points of the graphene overlayer.Major advances in fundamental science have followed from the exfoliation, stacking, and encapsulation of atomically thin 2D layers 1 . The next step towards technological impact of 2D layers and heterostructures is to transition sophisticated "pick and place" devices to a wafer-scale platform. However, the sensitivity of 2D systems to interfacial reactions and environmental influences -especially for two-dimensional metals or small-gap semiconductors -poses challenges for large-scale integration. Very few metals resist degradation of their top few atomic layers upon environmental exposure, and for a 2D metal, these layers constitute the entire system. A general platform for producing environmentally stable and wafer-scale 2D metals that are not prone to interfacial interactions would represent a significant advance. Inspired by the success of wide-bandgap 2D gallium nitride 2 , we turn focus onto the metal alone and demonstrate a platform dubbed confinement heteroepitaxy (CHet), where the interface between epitaxial graphene (EG) and silicon carbide (SiC) stabilizes crystalline 2D forms of Group-III (Ga, In) and group-IV (Sn) elements. Defect engineering of the graphene overlayer enables uniform, large-area intercalation at the high-energy SiC/EG interface; this interface then templates intercalant crystallization at a thermodynamically defined number of atomic layers. The unreactive nature of as-grown EG on SiC (graphene plus buffer layer) performs multiple services: (1) it only partially passivates the SiC surface underneath, thereby sustaining the high-energy interface that drives intercalation; (2) it lowers the energy of the (otherwise exposed) upper surface of the metal, thus facilitating 2D morphologies; (3) it protects the newly formed 2D metal from environmental degradation after intercalation through in situ healing of the graphene defects. Stability of these 2D metals in air over months gr...

Light–Matter Interaction in Quantum Confined 2D Polar Metals

“…We model bilayer (2L) and trilayer (3L) Ga or In bonded to Si‐terminated 6H SiC substrates, using slabs with six units of SiC per unit cell ( Figure ) (see Methods Section). Following previous studies on the energetically favored stacking configurations, [ 42 ] the first (bottom) and the second Ga/In atomic layer are aligned, respectively, with the silicon atoms and carbon atoms in SiC1 (Figure 3), while for the trilayer, the third (top) atomic layer is aligned with the hollow sites of SiC1. The resulting interlayer distances are shown in Table S3, Supporting Information.…”

“…The DFT band structure obtained for bilayer Ga/SiC (Figure 3a) agrees well with angle‐resolved photoemission spectra (ARPES), except for the position of the Fermi level which is shifted by a few tenths of eV, consistent with the findings that graphene does not have a significant effect on the band structure. [ 42 ] To investigate the effect of an error in the Fermi level on the predicted , we shift the Fermi level in the calculations by ±0.4 eV and ±0.6 eV and present the results in Figure S4, Supporting Information. The results show that the peak positions above 1.0 eV are essentially unchanged by these Fermi level shifts.…”

“…Recently, 2D polar metals were realized via confinement heteroepitaxy (CHet)—a process of intercalating metals between epitaxially grown graphene and its silicon carbide (SiC) substrate ( Figure ). [ 42 ] These 2D gallium (Ga) and 2D indium (In) structures are protected by the graphene overlayers, demonstrate a gradient in their bonding covalency along the z ‐direction, and exhibit fascinating properties like superconductivity, [ 42 ] strong plasmonic response in the visible range, [ 43 ] and strong nonlinear optical properties emerging by giant second harmonic generation in the NIR range approaching 10 nm 2 V −1 .…”

2D Polarized Materials: Ferromagnetic, Ferrovalley, Ferroelectric Materials, and Related Heterostructures

“…Although hard to be exfoliated, the vdW epitaxy technique promotes the development of 2D nonlayered materials and have been demonstrated in 2D GaN, Ga, and CdTe. [ 215–218 ] With simplified element component, 2D Cr 2 S 3 , CrSe, and MnSe have been developed. [ 127,133,219 ] By trading‐off the thickness and satisfying performance, it is challenging on balance the dangling‐bonds and polarization behavior.…”

Thermodynamic Preference for Atom Adsorption on versus Intercalation into Multilayer Graphene

“…Other examples include intercalated metals which can be used to engineer band gaps to achieve large-gap quantum-spin-Hall states 8 , potential spintronics devices 9 , 2D superconductors with the Giant Rashba effect 10 , etc. Another goal for intercalation into graphene-based materials is to stabilize 2D metals against environmental degradation and to be integrated into heterostructure devices 11 .…”