Electronic Transport in Two-Dimensional Materials (1802.01045v1)

Abstract: Two-dimensional (2D) materials have captured the attention of the scientific community due to the wide range of unique properties at nanometer-scale thicknesses. While significant exploratory research in 2D materials has been achieved, the understanding of 2D electronic transport and carrier dynamics remains in a nascent stage. Furthermore, since prior review articles have provided general overviews of 2D materials or specifically focused on charge transport in graphene, here we instead highlight charge transport mechanisms in post-graphene 2D materials with particular emphasis on transition metal dichalcogenides and black phosphorus. For these systems, we delineate the intricacies of electronic transport including bandstructure control with thickness and external fields, valley polarization, scattering mechanisms, electrical contacts, and doping. In addition, electronic interactions between 2D materials are considered in the form of van der Waals heterojunctions and composite films. This review concludes with a perspective on the most promising future directions in this fast-evolving field.

• The paper clarifies how bandstructure modulation via material thickness and applied fields governs electronic transport in TMDCs and BP.
• It employs detailed analysis of carrier dynamics to explain transitions from indirect to direct bandgaps and quantum confinement effects.
• The study highlights device integration challenges, emphasizing the need for improved contact engineering and scalable growth methods for 2D materials.

Insights into Electronic Transport in Two-Dimensional Materials

The paper by Sangwan and Hersam provides an in-depth examination of the electronic transport mechanisms in two-dimensional (2D) materials, focusing particularly on transition metal dichalcogenides (TMDCs) and black phosphorus (BP)—the materials that have garnered substantial interest beyond graphene. Despite significant progress in the exploration of 2D materials' properties, the specifics of charge transport and carrier dynamics remain inadequately understood. This paper serves both as a tutorial and a comprehensive roadmap for future work in this domain.

Key Aspects of 2D Materials Electronic Transport

Two-dimensional materials exhibit extraordinary properties due to their reduced dimensionality and unique band structures. The electronic transport properties are intricately linked to their bandstructure, influenced by thickness, electric and magnetic fields, valley polarization, scattering mechanisms, doping, and the influence of electric contacts. In particular, TMDCs, such as MoS$_2$, WSe$_2$, and InSe, along with BP, have shown promising traits for electronic applications due to their considerable charge carrier mobilities and appropriate bandgaps. The intrinsic carrier mobility in these materials shows potential, contingent on ideal atomically flat substrate interfaces which minimize scattering mechanisms.

TMDCs exhibit interesting quantum confinement effects with thickness transition influencing the bandgap from indirect in the bulk to direct in the monolayer form. Similarly, BP shows a direct and tunable bandgap across all thicknesses. These properties suggest that the modulation of bandstructure via thickness and electric fields introduces new opportunities for device applications, though it also poses the challenge of maintaining atomic-level precision in thick film fabrication for technological reliability.

Challenges and Opportunities in Device Applications

The practical application of 2D materials in devices faces several challenges predominantly surrounding the optimization of electrical contacts, doping, and defect management. Contact engineering remains crucial due to the nontrivial interfacing between metals and 2D semiconductors. The formation of Schottky barriers, Fermi level pinning, and the existence of van der Waals tunneling barriers often complicate charge injection and collection, crucial for device performance. Novel approaches such as heterostructures with tunable interfaces and chemical doping strategies present pathways to mitigate these challenges.

Important is the exploration of van der Waals heterostructures, which leverage the weak interlayer bonding of 2D materials to create multi-layer devices with unique electronic characteristics. These vdW heterostructures provide the potential for applications in photodetectors, field-effect transistors (FETs), and photovoltaic cells due to customizable band alignment. Continued research could yield heterostructures that may outperform traditional semiconductor technologies, particularly under aggressively scaled device dimensions.

Future Research Directions

For 2D materials to transition from laboratory to commercial applications, advances are required in scalable and uniform large-area growth techniques, substitutional doping to control carrier concentration, and comprehensive defect mitigation strategies. Robust passivation techniques are needed to stabilize materials like BP in ambient conditions without degrading electronic properties.

The development of composite films consisting of 2D flakes shows promise in addressing surface roughness, although achieving the necessary film uniformity remains elusive. High-k dielectrics pose both an opportunity and a challenge, where substrate choice can significantly impact intrinsic material properties.

Conclusion

The roadmap provided by Sangwan and Hersam highlights the multifaceted exploration required to harness the full potential of 2D materials in next-generation electronic applications. This includes overcoming the current limitations in material growth and processing, understanding fundamental electron scattering and mobility constraints, and developing novel device architectures. Through comprehensive research into electronic transport mechanisms and controlled synthesis and integration techniques, the prospects of 2D materials in scalable electronics remain promising, opening avenues for advancements in both theoretical understanding and practical applications.