MnBr$_2$ on the graphene on Ir(110) substrate: growth, structure, and super-moirè

TL;DR

The paper demonstrates the first experimental realization and thorough characterization of single-layer MnBr$_2$ grown on Gr/Ir(110), revealing a hexagonal MnBr$_2$ lattice with $a=3.90\,\AA$ rotated by $30^{\circ}$ to Gr and exhibiting azimuthal disorder typical of van der Waals epitaxy. It uncovers a rich growth morphology that evolves with temperature and a sizable band gap ($\sim$4.4–5.1 eV) whose apparent height in STM is governed by the MnBr$_2$ electronic structure. A complex three-lattice super-moiré arises from MnBr$_2$, Gr, and Ir(110), with a dominant virtual MnBr$_2$/Ir(110) moiré producing 2.7 nm stripes and a beating pattern around 13.5 nm due to interference with Gr/Ir(110) moiré, as shown by detailed FFT analyses and bias-dependent STM measurements. Comparative MnBr$_2$/Gr/Ir(111) studies confirm the uniqueness of the MnBr$_2$/Gr/Ir(110) virtual moiré, which originates from inhomogeneous Gr binding to Ir(110); these findings lay a foundation for exploring magnetic order and polaron physics in 2D MnBr$_2$–based systems and guide the study of super-moiré phenomena in multi-lattice heterostructures.

Abstract

Single-layer MnBr$_2$ is grown on graphene (Gr) supported by Ir(110) and investigated using low-energy electron diffraction, scanning tunneling microscopy, and spectroscopy. The structure and epitaxial relationship with the substrate are systematically characterized. The structure and epitaxial relationship with the substrate are systematically characterized. The growth morphology strongly depends on the growth temperature, evolving from fractal to dendritic and eventually to compact dendritic skeletal islands, reflecting changes in the underlying surface diffusion processes. The pronounced variation in the apparent height with tunneling conditions for the magnetic insulator is explained based on the measured electronic density of states. MnBr$_2$ on Gr/Ir(110) constitutes a three-lattice system, giving rise to a super-moiré pattern -- a moiré of moirés. The super-moiré of MnBr$_2$/Gr/Ir(110) is unique, as it involves a virtual moiré of MnBr$_2$ with the Ir(110) surface lattice -- two lattices not in contact with each other. Using a careful Fourier analysis, the known properties of Gr/Ir(110), and the results of ab initio calculations, the origin of the virtual moiré is uncovered and related to the inhomogeneous binding of Gr to Ir(110). Comparative experiments with MnBr$_2$ on Gr/Ir(111) show similar growth and structure, but highlight the unique properties of the MnBr$_2$/Gr/Ir(110) super-moiré.

Figures (14)

• Figure 1: Structural characterization of MnBr$_{2}$. (a) Contrast-inverted 90 eV LEED pattern of MnBr$_{2}$ after the growth of 1.1 ML at 400 K on Gr/Ir(110). First-order Ir, Gr, and MnBr$_{2}$ reflections are encircled in magenta, green, and orange, respectively. Reciprocal Ir, Gr, and MnBr$_2$ primitive translations are indicated. (b) STM topography corresponding to the sample in (a). Contrast enhanced insets display striped moirés highlighted by white lines. (c) Atomically resolved STM image of MnBr$_{2}$. White line highlights moiré stripe, as in (b). (d) Top and (e) side view ball models of the structure and bilayer stackings of MnBr$_{2}$. All subfigures are oriented as indicated in (a), with the [001]-direction of Ir being horizontal. MnBr$_2$ unit cells are indicated as black rhombuses in (c) and (d). STM images are obtained at 300 K with (b) $V_\mathrm{b}$ = -2 V, 50 pA and (c) $V_\mathrm{b}$ = -2 V, 500 pA. Image sizes: (b) 180 nm $\times$ 200 nm, and (c) 7.5 nm $\times$ 7.5 nm.
• Figure 1: STM topograph of MnBr$_2$ on Gr/Ir(110). Lower panel shows the height profile along the black line in the STM topograph. STM image is obtained at 300 K with (b) $V_\mathrm{b}$ = -2 V, 50 pA. Image size: 200 nm $\times$ 160 nm
• Figure 2: (a)-(c) STM topographs of MnBr$_{2}$ on Gr/Ir(110) after deposition of $\approx 0.6$ ML at 80 K, 300 K, and 400 K, respectively. The sample in (a) was briefly annealed to $\approx 150$ K during transfer to the STM and then imaged at 77 K. Inset in (a) uses double gray scale to make the moiré on Gr/Ir(110) and the moiré on the MnBr$_2$ islands simultaneously visible. Images in (b) and (c) were acquired at 300 K. The white arrows in (b) indicate the dendritic growth directions of the primary, secondary, and ternary dendrite branches. STM imaging parameters: (a) $V_\mathrm{b} = -1$ V and $I_\mathrm{t} = 50$ pA, (b,c) $V_\mathrm{b} = -2$ V and $I_\mathrm{t} = 50$ pA Image sizes: 160 nm $\times$ 160 nm.
• Figure 2: STM topograph of MnBr$_2$ grown at 400 K on (a) Gr/Ir(111) and (b) Gr(Ir(110). (b) is identical to Figure 1(b) of the main manuscript and is reproduced here for direct comparison. Tunneling parameters are for (a) $V_\mathrm{b}$ = 3.5 V, 20 pA. and for (b) $V_\mathrm{b}$ = -2 V, 50 pA. Image size for (a) and (b) is 180 nm $\times$ 200 nm.
• Figure 3: (a)-(d) The MnBr$_2$ sample of Figure 2(b) grown at 300 K is annealed in isochronal steps of 120 s successively to 370 K, 470K̇, 570 K, and 670 K, respectively. White arrows in (c) highlight two pinning centers. Imaging is conducted 300 K with $V_\mathrm{b}$ = -2 V, $I_\mathrm{t} = 50$ pA, Images sizes: 160 nm $\times$ 160 nm.