Substrate Choice Advances Mobile Polaron Control in Transition Metal Dihalides at 300K

The behaviour of electrons in single-layer materials holds immense promise for future electronic devices, and understanding how these electrons interact with their supporting surfaces is crucial for realising this potential. Affan Safeer, Oktay Güleryüz, and Guangyao Miao, along with colleagues at the II. Physikalisches Institut, Universität zu Köln, now demonstrate that the underlying substrate dramatically influences the formation of polarons, quasi-particles formed when electrons couple with lattice vibrations, in single-layer manganese bromide. Their research reveals that growing this material on different substrates, specifically iridium and gold, results in varying numbers and types of polarons, with the iridium (110) surface hosting the most diverse range. Significantly, the team observed these polarons persisting up to 300 Kelvin and even manipulated them using a scanning tunnelling microscope, showing that substrate interactions must be considered when modelling these two-dimensional insulators and paving the way for controlled design of novel electronic components.

Polaron Characteristics in MnBr2 on Graphene

This supplementary information provides extensive data supporting the discovery of polarons in single-layer manganese bromide grown on graphene substrates, detailing the characteristics of different polaron types and their mobility. The data confirms the existence of distinct polaron types with varying degrees of mobility, and demonstrates that a scanning tunneling microscope (STM) tip can actively manipulate these polarons. Researchers tracked polaron positions at a range of bias voltages, confirming that mobile polarons remain active even at negative voltages, indicating mobility is not solely dependent on positive bias. Further investigation revealed that even at lower bias, mobile polarons exhibit movement when a larger tunneling current is applied, suggesting the current drives polaron motion.

Overview images of MnBr2 grown on different substrates, iridium(110), iridium(111), and gold(111), showed that second-layer MnBr2 islands can form, and the gold substrate exhibits a herringbone reconstruction partially lifted by the MnBr2 overlayer, providing context for substrate influence on polaron behavior. Detailed analysis on the iridium(111) substrate enabled differentiation between polaron types, with spectroscopic mapping characterizing the electronic properties of immobile polarons. Crucially, the team directly observed the STM tip converting static Type-II polarons into mobile Type-I polarons, demonstrating the tip’s ability to manipulate polaron state. They also demonstrated the creation of mobile Type-I polarons by applying a bias pulse, supporting the idea that the polaron state is sensitive to electric fields. Measurements of tip height above graphene and MnBr2 surfaces quantified differences in surface corrugation and electronic structure, providing insight into tip-sample interactions and the potential landscape experienced by the polarons. These findings collectively strengthen the understanding of polaron behavior and manipulation in two-dimensional materials.

MnBr2 Polaron Studies on Multiple Substrates

Scientists investigated polarons within single-layer manganese bromide grown on iridium(110), iridium(111), and gold(111) substrates, extending understanding of these phenomena to a new transition metal dihalide. The team meticulously grew single-layer MnBr2, confirming a lattice constant closely matching the bulk material, crucial for observing and characterizing polaron behavior. Employing scanning tunneling microscopy (STM) and spectroscopy, researchers visualized charge distribution and identified polaron species, observing these quasiparticles up to 300K. They actively created, converted, and moved polarons by applying a controlled tunneling current with specific bias voltages using the STM tip, demonstrating dynamic control over these localized charges.

The observation of up to four distinct polaron species on Ir(110), three similar to those previously identified in cobalt chloride on graphite, suggests common mechanisms governing polaron formation. A key innovation involved exploiting a super-moiré pattern formed between MnBr2 and Ir(110), effectively guiding mobile polarons. These experiments demonstrate that understanding polarons in single-layer insulators requires explicit consideration of the underlying substrate, highlighting its influence on polaron phenomenology and stability.

Polaron Diversity and Mobility on MnBr Substrates

Scientists have demonstrated the existence of stable polarons within single-layer manganese bromide grown on various conducting substrates, revealing a strong dependence of polaron behavior on the underlying material. Experiments conducted up to 300K show that the number and type of polarons observed vary significantly depending on whether the MnBr2 is grown on Ir(110), Ir(111), or Au(111). The largest diversity of polaron species, specifically four distinct types, was observed on the Ir(110) substrate, with three exhibiting similarities to those previously found in cobalt chloride on graphite. The team measured polaron mobility using scanning tunneling microscopy (STM), observing that the application of a bias voltage can create, convert, and move these polarons.

Specifically, STM topographs reveal that at 2.5V, polarons appear immobile, while increasing the voltage to 3.0V induces fluctuations in their position, driven by interaction with the STM tip. When MnBr2 is grown on Ir(110), mobile polarons are guided by a periodic potential created by a super-moiré pattern resulting from the interaction between the MnBr2 and the substrate, exhibiting a wavelength of approximately 2.7nm.

This super-moiré pattern is superimposed on a larger moiré wavelength of approximately 3.3nm originating from the substrate itself. Detailed analysis revealed four distinct polaron types, characterized by varying shapes and depths observed through STM imaging. Type-I mobile polarons exhibit a maximum apparent depth of approximately 1 Ångström, while type-I immobile polarons are shallower, at 0.7 Ångström.

Type-II polarons appear as either 2×2 arrangements with a side length of 7.8 Ångström or 1×1 arrangements with a side length of 3.9 Ångström, both featuring triangular protrusions. These findings demonstrate that accurate modeling of polarons in single-layer insulators requires explicit consideration of the substrate’s influence, opening new avenues for manipulating and utilizing these quasi-particles in future electronic devices.

Substrate Controls Polaron Formation and Stability

This research demonstrates the existence of stable polarons within single-layer manganese bromide when grown on various metallic substrates, revealing that the number and type of polarons formed are strongly influenced by the underlying substrate, with the most diverse range appearing on an iridium substrate. These polarons persist up to 300 Kelvin and can be manipulated using a scanning tunneling microscope, allowing for their creation, conversion, and movement through the application of a controlled electrical current. The findings reveal a critical need to consider the substrate’s properties when modeling polarons in these single-layer materials, as traditional calculations focusing solely on the monolayer itself are insufficient. The team observed that the substrate’s periodic potential can even guide the movement of these polarons, further highlighting the strong interplay between the material and its support. While the research did not find a correlation between polarons and material defects, the authors acknowledge that their understanding of polaron formation energy may be incomplete, and future work should explore this aspect further. They suggest that detailed theoretical calculations are necessary to fully describe the observed phenomena and explain the substrate dependence of polaron density and behavior.