Achieving environmental stability in an atomically thin quantum spin Hall insulator via graphene intercalation

Abstract

Atomic monolayers on semiconductor surfaces represent an emerging class of functional quantum materials in the two-dimensional limit - ranging from superconductors and Mott insulators to ferroelectrics and quantum spin Hall insulators. Indenene, a triangular monolayer of indium with a gap of ~ 120 meV is a quantum spin Hall insulator whose micron-scale epitaxial growth on SiC(0001) makes it technologically relevant. However, its suitability for room-temperature spintronics is challenged by the instability of its topological character in air. It is imperative to develop a strategy to protect the topological nature of indenene during ex situ processing and device fabrication. Here we show that intercalation of indenene into epitaxial graphene provides effective protection from the oxidising environment, while preserving an intact topological character. Our approach opens a rich realm of ex situ experimental opportunities, priming monolayer quantum spin Hall insulators for realistic device fabrication and access to topologically protected edge channels.

Fig. 1. Graphene-intercalated indenene is topologically non-trivial and resilient to atmosphere.

a, b ARPES spectra at the K-point of a pristine (20 K) and b intercalated indenene (RT) measured with 21.2 eV photons and overlaid DFT (HSE06) calculations (dotted lines) of pristine indenene. Spectra in a,b were extracted along the red path in the sketched indenene BZ (blue) and share the k_∣∣-axis. The band splitting in a and b originate from the combined role of SOC and ISB as discussed in the text. c DFT (HSE06) band gap calculations as well as a guide to the eye (red line) of pristine indenene as a function of the In-Si bond length d_In−Si, the latter controlling the ISB strength λ_ISB as indicated by a black arrow. Experimentally (by X-ray standing wave (XSW) photoemission) determined d_In−Si of intercalated (yellow data point) and pristine (blue data point) indenene are placed in this diagram. d, e In 3d XPS core-level peaks of d pristine and e intercalated indenene, for the as-grown films (black; E_3/2 = −451.9 eV; E_5/2 = −444.3 eV), after exposure to 21 kL (red) of oxygen, after 10 min exposure to ambient air (green, only e), and after immersion in liquid water and subsequent mild in vacuo degas (blue) as specified in the Methods section. Panel d, e share the same energy-axis, E_F being the Fermi energy. Black arrows and dotted lines in d indicate the chemically shift of oxidized In (InO_x).

Fig. 2. Structure of intercalated indenene.

a LEED image taken at 100 eV showing diffraction spots of ($6\sqrt{3}\times 6\sqrt{3}$)R30° periodicity (orange), SiC(0001) (1 × 1) (blue) and graphene (black). Red marks indicate a selection of possible scattering vectors between SiC(0001) (1 × 1) and graphene. b ARPES spectra taken at hν=46 eV around the graphene K-point K_Gr indicate the graphene Dirac point (DP_Gr) position at ~−0.22 eV. c RT STEM image of 1 ML indium intercalated graphene revealing positions of Si (of SiC, red spheres), indium (blue spheres) and graphene (black spheres), the latter being most evident in the horizontally integrated intensity profile (red). d STM constant current topography of intercalated indenene (1 μm × 1 μm) measured at 4 V and 10 pA after immersion in water and a mild degas. Labels t₁₋₄ indicate different SiC terraces. e, f Lock-in dI/dV maps taken in constant height mode showing e the graphene lattice (black unit cell) at 50 mV and f the indenene lattice (red unit cell) at 800 mV. A white rhombus marks the ($6\sqrt{3}\times 6\sqrt{3}$)R30° moiré unit cell of both lattices in each panel. g dI/dV point spectroscopy of pristine (black) and intercalated indenene (red), both showing a sharp increase (dashed red/black lines) attributed to the onset of In 5p_z-like states. Minima in the dI/dV spectroscopy of intercalated indenene are identified as the graphene DP_Gr and an inelastic tunneling gap (PG_Gr).

Fig. 3. The band structure of pristine and graphene capped indenene.

ARPES of a pristine monolayer indium and b intercalated indenene on SiC(0001). The data were taken at RT with hν=21.2 eV. Blue arrows indicate distinct band maxima due to out-of-plane mirror symmetry breaking and orbital hybridization. The different indenene band population is illustrated by black stepped line anchored at those maxima. The top row illustrations depict the Brillouin zones of indenene (blue) and graphene (black) and the high symmetry k-path (red) along which the ARPES data are shown. Graphene and indenene band replicas in b that are consistent with electron diffraction off the In/SiC (orange) or graphene lattice (red) and replicas consistent with multiple scattering (white, green) are shown in the sketch top right.

Fig. 4. QSHI character of intercalated indenene.

a, b Potassium doping of the indenene K-point in ARPES taken with hν = 90 eV photons. Increasing doping populates the conduction band, separated by an energy gap of E_gap  ≈  100 meV from the rigidly shifted valence bands. c, d DFT (PBE) band structure (black) around the K-point of pristine c and intercalated indenene d. The blue marker radius denotes the indium character of the bands. Band splitting among VBs (CBs) is driven by the ISB strength λ_ISB (arrow), while the larger λ_SOC splitting (arrow) opens a topologically non-trivial gap. Note that the combined ($6\sqrt{3}\times 6\sqrt{3}$)R30° super cell (Fig. 2e, f) leads to band backfolding which projects graphene bands (black) into the indenene gap. Importantly, in order to disentangle the role of d_In−Si from the graphene-induced change in λ_ISB, we calculate both band structures at the In-Si bonding distance determined from XSW of intercalated indenene and for the latter with the corresponding indenene graphene distance. e, f Fourier filtered dI/dV maps of the same position, but taken at e −100 mV (VB) and f −250 mV (VB-1) reproducing the topologically non-trivial switch of the charge maximum from site B to A (arrow) (details in Supplementary Note 8).