Substrate Role in Polaron Formation on Single-layer Transition Metal Dihalides

TL;DR

This work addresses how substrate choice governs polaron formation in single-layer MnBr$_2$ and demonstrates that substrate-induced potentials critically shape polaron species, densities, and mobility. Using STM/STS, the authors identify four polaron types on MnBr$_2$/Gr/Ir(110) and two on MnBr$_2$/Gr/Ir(111) and MnBr$_2$/Au(111), with mobility that is tunable by tunneling bias and strongly influenced by a substrate-derived super-moiré; polarons persist up to 300 K, indicating robust stabilization. Polaron energetics are discussed via a formation-energy framework $E_{ ext{pol}}+E_{ ext{cbe}}<0$ with a conduction-band-edge offset $E_{ ext{cbe}}\\approx 1.9$ eV, suggesting unusually large $E_{ ext{pol}}$ on metal substrates. The findings emphasize that accurate modeling of polarons in 2D insulators on conducting substrates must include the substrate, and they hint at the possibility of patterning polaron behavior through substrate engineering for potential information-processing applications.

Abstract

Single-layer transition metal dihalides grown on conducting substrates were shown to host stable polarons. Here, we investigate polarons in insulating single-layer MnBr$_2$ grown by molecular beam epitaxy on three different substrates, namely graphene on Ir(110), graphene on Ir(111), and Au(111). The number densities and species of polarons observed vary strongly as a function of the substrate. For MnBr$_2$ grown on Ir(110) the largest number of polaron species is observed, namely four, of which three show clear similarities with the species observed for CoCl$_2$ on graphite. Polarons in single-layer MnBr$_2$ are observed up to 300K. They can be created, converted, and moved by the STM tip when a tunneling current flows at a proper bias voltage. For graphene on Ir(110) as a substrate, mobile polarons in MnBr$_2$ are guided through the periodic potential imposed from the super-moiré resulting from the interaction of MnBr$_2$ with graphene and Ir(110). Our findings indicate that modeling of polarons in such single-layer insulators in contact with a conducting substrate requires to take the substrate explicitly into account.

Figures (15)

• Figure 1: Polaron mobility. (a) Single-layer MnBr$_2$ island imaged by STM with $V_\mathrm{b} = 2.5$ V, $I_\mathrm{t} = 20$ pA. At this $V_\mathrm{b}$, polarons are immobile. (b) Same island as in (a), but $V_\mathrm{b} = 3$ V, $I_\mathrm{t} = 20$ pA. Polarons fluctuate in position under the influence of the STM tip. Inset bounded by white line displays the same island as in (a), but imaged at $V_\mathrm{b} = -1$ V. At this voltage, the MnBr$_2$/Gr/Ir(110) super-moiré is visible. STM topographs are taken at 4.2 K. Images sizes: 100 nm $\times$ 85 nm.
• Figure 1: (a-i) Sequence of STM images of the same area acquired at varying $V_\mathrm{b}$ ranging from $+3$ V and $-3.5$ V, as indicated, and at $I_\mathrm{t} = 50$ pA. Selected polarons are encircled to track their position: dark blue (type-I mobile), light blue (type-I immobile), yellow (type-II 2x2), and red (type-II 1x1). From comparison of (d) and (h) it is obvious that type-I mobile polarons are also moved at $V_\mathrm{b}= -3.5$ V. Image sizes: 45 nm $\times$ 45 nm.
• Figure 2: Polarons in single layer MnBr$_{2}$. (a)-(g) Successive STM topographs of the same sample location acquired at 4.2 K, with indicated $V_\mathrm{b}$ and $I_\mathrm{t} = 50$ pA. Polaron types are labeled as follows: light blue circle (type-I immobile), dark blue circle (type-I mobile), yellow circle (type-II 2×2), and red circle (type-II 1×1). Atomically resolved images of (h) type-I immobile, (i) type-I mobile, (j) type-II 2×2, and (k) type-II 1×1 polarons. (l) Height profiles extracted along the white line in (a) and (b). Atomic resolution imaging parameters:(h,k) $V_\mathrm{b} = 2$ V and $I_\mathrm{t} = 1$ nA; (i) $V_\mathrm{b} = 1$ V and $I_\mathrm{t} = 1$ nA; (j) $V_\mathrm{b} = 2$ V and $I_\mathrm{t} = 100$ pA. Image sizes: (a)-(f) 27 nm $\times$ 27 nm; (g)-(j) 2.7 nm $\times$ 2.7 nm.
• Figure 2: STM image acquired with $V_\mathrm{b} = 2$ V, $I_\mathrm{t} = 3$ nA. Image size: 15 nm $\times$ 15 nm.
• Figure 3: Band bending induced by polarons. (a) Constant height $\mathrm{d}I/\mathrm{d}V$ point spectra taken on the polarons indicated compared to a reference spectrum without a polaron. (b) Lower panel: constant current $\mathrm{d}I/\mathrm{d}V$ linescans over the type-I mobile, type-I immobile, type-II $2\times2$, and type-II $1\times1$ polaron. Spectra are acquired with $V_{st}$ = +2.3 V, $I_{st}$ = 50 pA, $f_{mod}$ = 667 Hz, and $V_{mod}$ = 20 mV. Path of $\mathrm{d}I/\mathrm{d}V$ linescans through polarons is indicated in STM topographs in the upper panels of the respective linescan maps. STM imaging parameters: $V_\mathrm{b} = 2$ V and $I_\mathrm{t} = 100$ pA and STM images size: 6 nm $\times$ 4 nm.