AVS 67 Session 2D-Invited On Demand: 2D Materials Invited On Demand Session

Due to the quantum confinement and changes in the symmetry, electrical and topological properties of atomically-thin two-dimensional (2D) materials are often largely different from those of their bulk counterparts. In this talk, I will focus on the changes in topological properties in 2D layers, which is explored by combining bottom-up growth using molecular beam epitaxy (MBE), in situ angle-resolved photoemission (ARPES), scanning tunneling microscopy/spectroscopy (STM/STS), and first principle calculations. We have found that the topological properties of 2D materials can be tuned by thickness, lateral interface, strain, and electric field. The material systems under discussion include 1T’-WTe₂ [1], 1T’-WSe₂ [2], alpha-Sn [3] and Na₃Bi [4].

[1] S. Tang et al., Nat. Phys. 13, 683 (2017). [2] M. M. Ugeda et al., Nat. Commun. 9, 3401 (2018). [3] C.-Z. Xu et al., Phys. Rev. Lett. 118, 146402 (2017); Phys. Rev. B 97, 035122 (2018). [4] J. L. Collins et al., Nature 564, 390 (2018).

As a result of their unique electronic, optical, and physical properties, two-dimensional (2D) materials are actively being explored for applications in next-generation computing [1], quantum information science [2], and energy technologies [3]. With exceptionally high surface-to-volume ratios, 2D materials are highly sensitive to their environment, resulting in a strong dependence of their properties on substrate effects, extrinsic adsorbates, and interfacial defects. Furthermore, the integration of 2D materials into heterostructure devices introduces further demands for controlling interfaces with atomic precision. With this motivation, this talk will explore emerging efforts to understand and utilize interfacial chemical functionalization to influence the properties of 2D heterostructures. For example, organic adlayers can tailor chemical reactivity to enable conformal atomic layer deposition of pinhole-free encapsulation layers that mitigate the deleterious effects of ambient exposure, particularly for ambient-unstable 2D materials such as black phosphorus and monochalcogenides [4]. The integration of organic self-assembled monolayers with 2D semiconductors also allows for tailoring of electronic and optical properties such as photoinduced charge separation in fullerene/InSe heterojunctions [5] and mixed-dimensional excitonic states in phthalocyanine/MoS₂ heterojunctions [6]. By exploiting spatially inhomogeneous surface chemistry, seamless lateral 2D heterostructures can also be realized including perylene/borophene [7] and graphene/borophene [8] heterostructures, each of which show atomically sharp electronic interfaces as confirmed by ultrahigh vacuum scanning tunneling microscopy and spectroscopy. Overall, by providing substantial tailoring of interfaces, chemical functionalization presents opportunities for improved functionality in 2D heterostructure devices.

[1] V. K. Sangwan, et al., Nature Nanotechnology, DOI: 10.1038/s41565-020-0647-z (2020).

[2] X. Liu, et al., Nature Reviews Materials, 4, 669 (2019).

[3] S. Padgaonkar, et al., Accounts of Chemical Research, 53, 763 (2020).

[4] S. A. Wells, et al., Nano Letters, 18, 7876 (2018).

[5] S. Li, et al., ACS Nano, 14, 3509 (2020).

[6] S. H. Amsterdam, et al., ACS Nano, 13, 4183 (2019).

[7] X. Liu, et al., Science Advances, 3, e1602356 (2017).

We study the potential of material simulations based on first-principles methods to predict gas sensing properties of 2D materials. This emerging class of materials is of particular interest to gas sensing applications due to high surface-to-volume ratios and chemical stability. We discuss in detail results of electron transport calculations within the Landauer-Büttiker formalism and compare the conclusions to analyses in terms of adsorption energies, charge transfers, and work functions. Specific examples include the effects of the interlayer interaction in bilayer MoS₂ and WS₂ on the gas sensing performance and the consequence of the presence of reactive Si in Si₂BN. We also address the properties of C₃N and para/meta-C₃Si. Potential of very sensitive gas sensing is demonstrated for para-C₃Si and is explained by the susceptibility of Dirac states to symmetry breaking distortions rather than by a mechanism based on charge transfer. Finally, the enhanced gas sensing performance of monovacant C₆BN is studied and it is shown that the work function changes of both pristine and monovacant C₆BN during gas adsorption do not correlate with the changes observed in the I-V characteristics.

Vertical stacking of monolayers via van der Waals (vdW) interaction opens promising routes toward engineering physical properties of two-dimensional (2D) materials and designing atomically thin devices. Increasingly, the bottleneck in this field is the controlled synthesis of these materials through methods such as chemical etching and chemical vapor deposition (CVD). In this talk, I will present insights into synthesis and growth of 2D materials developed from analytical, phase-field, and machine learning models. First, we adapt the state-of-the-art positive and unlabeled (PU) machine learning framework to predict which theoretically proposed 2D materials in the MXene family have the highest likelihood of being successfully synthesized. By considering both the MXenes and their precursors, we identify 18 MXene compounds that are highly promising candidates for synthesis. Next, we develop a general multiscale model for scalable CVD growth of layered materials and predict the necessary growth conditions for vertical (initial + subsequent layers) versus in-plane lateral (monolayer) growth. An analytic thermodynamic criterion is established for multilayer growth that depends on the sizes of both layers, the vdW interaction energies, and the edge energy of 2D layers. We connect the model to experimental controls and find that temperature and adatom flux from vapor are the primary criteria affecting the self-assembled growth. This model agrees with experimental observations of various monolayer and bilayer transition metal dichalcogenides grown by CVD. Finally, we consider CVD synthesizable transition metal dichalcogenide heterostructures as a robust platform for engineering quantum confinement of Dirac fermions using a multiscale model for electronic properties.