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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - EPIC2D (Engineering Electron-Phonon Interactions of Two-Dimensional Materials from First-Principles)

Teaser

Summary

Owing to their extraordinary properties, two-dimensional (2D) materials such as graphene and transition metal dichalcogenide monolayers hold significant promise for applications in the next-generation flexible, transparent, and energy-efficient electronic and optoelectronic devices. To realize the full potential of 2D materials, it is crucial to understand and optimize their charge transport properties. In solids, charge transport properties are significantly influenced by the interactions between the electrons and lattice vibrations, namely the electron-phonon interactions (EPIs). This project aims to employ state-of-the-art computational methodologies to investigate the EPIs in 2D materials and their effects on the charge transport properties. The project further aims to investigate means to engineer the EPIs in 2D materials for optimized charge transport properties.

Specifically, a primary focus of this project is first-principles investigations of the carrier mobilities in 2D semiconductors. Carrier mobility quantifies how fast electrons can travel inside a material and directly affects the switching frequency and power efficiency of electronic devices. Therefore, carrier mobility is a critical design parameter for electronics and optoelectronics. In a clean semiconductor crystal, the carrier mobility near room temperature is limited by the EPIs in the solid. Recent methodology developments in the hostâ€™s group enable the prediction of the carrier mobility of semiconductors using a Boltzmann transport equation approach that treats the EPIs from first principles. In this project, we employ these newly developed methods to study the EPIs and carrier mobilities of 2D semiconductors.

Work performed

Our study focuses on InSe, a prototypical 2D semiconductor that receives a significant amount of attention due to its large measured carrier mobility. We find that for InSe, EPIs affect the carrier mobility mainly through the coupling between the electrons and the long-wavelength longitudinal optical phonons, namely the FrÓ§hlich interaction. Surprisingly, we find that for the electron carriers in InSe, the carrier mobility exhibits an order of magnitude increase from monolayer to bulk. We discover that such a strong dimensionality effect in carrier mobility mainly arises from the layer-dependent scattering rates of the charge carriers in InSe. The strong layer dependence of carrier scattering rates originates from an interlayer wavefunction overlap effect that gives rise to layer-dependent density of states available for carrier scattering. Importantly, we find that the dimensionality-controlled carrier mobility is universal in 2D semiconductors, and that the layer-dependent carrier mobility is sensitive to the strength of interlayer electronic coupling. Based on these findings, we propose that van der Waals epitaxy, in which a 2D material is grown on a substrate or another 2D material that maximizes the interlayer electronic coupling, can be used for the optimization of the carrier mobility in 2D semiconductors. The comprehensive study of the dimensionality effect on the carrier mobility of 2D materials has been published in the journal Nano Letters.

We have further investigated the effect of EPIs on the electron effective mass of InSe. For semiconductors, the carrier effective mass is a key parameter that affects a wide range of properties, including the carrier mobility. Many-body interactions, including both the electron-electron interactions (EEIs) and EPIs, can renormalize the carrier effective mass and lead to mass enhancement. The nature and strength of many-body interactions in InSe were not clear before. We employ many-body perturbation calculations to investigate the EEIs and EPIs in bulk InSe, as well as their influence on the renormalization of the electron effective mass. Surprisingly, we find that EEIs lead to a significant directional anisotropy in the mass enhancement, which we explain in terms of the symmetry of band-edge wavefunctions. Furthermore, we find that the main contribution to the EPI-induced mass enhancement originates from the FrÃ¶hlich interaction. The weak FrÃ¶hlich interaction in InSe leads to a weak polaronic mass enhancement, which is desirable for device applications. Our results provide important insight into the nature and strength of many-body interactions in InSe. The comprehensive study of the many-body renormalization of electron effective mass will be useful for designing electronics and optoelectronics based on InSe and other similar layered semiconductors such as GaS and GaSe. The work leading up to these results has been submitted for publication.

Final results

The research carried out within this Marie SkÅ‚odowska-Curie Fellowship has led to improved understanding of the effects of dimensionality and EPIs on the electronic structure and charge transport properties of 2D semiconductors. The knowledge and insights gained from this project will provide important guidance for the design of electronic and optoelectronic devices based on InSe and other 2D semiconductors. As 2D materials emerge as top candidates for post-Mooreâ€™s law electronics, the results from this project could have a long-lasting impact on the development of new technologies based on 2D materials.