AVS2016 Session 2D-TuA: Novel Quantum Phenomena in 2D Materials

The spatial current distribution in H-terminated zigzag graphene nanoribbons (ZGNRs) under electrical bias is investigated using time-dependent density-functional theory solved on a real-space grid. A projected complex absorbing potential is used to minimize the effect of reflection at simulation cell boundary. The calculations show that the current flows mainly along the edge atoms in thehydrogen terminated pristine ZGNRs. When a vacancy is introduced to the ZGNRs, loop currents emerge at the ribbon edge due to electrons hopping between carbon atoms of the same sublattice. The loop currents hinder the flow of the edge current, explaining the poor electric conductance observed in recent

In order create nanoscale electronic devices, there is a need for making high quality electrical connections between functional regions or specific defects[1]. However, connecting dissimilar materials such as graphene and metals[2] may pose complications from differing densities of states and work functions, and predicting how the system is effected computationally can be challenging given the system size. To address these challenges, studies of the electrons flow through heterogeneous material junctions, using complex potentials on a real-space grid and Time-Dependent Density Functional Theory have been performed. By confining an electron into the conduction band at a single point and propagating the system in time, the wavefunction for the system in a specific conducting state can be solved. Considerations for junctions and use of injecting and absorbing potentials in regions of diminished electron density will be presented.

[1] J. Lahiri, Y. Lin, P. Bozkurt, I.I. Oleynik, M. Batzill, An extended defect in graphene as a metallic wire, Nat Nano, 5 (2010) 326-329.

[2] F. Xia, V. Perebeinos, Y.-m. Lin, Y. Wu, P. Avouris, The origins and limits of metal-graphene junction resistance, Nat Nano, 6 (2011) 179-184.

The Talbot effect for a graphene sheet as a grating, using electron matter waves, is simulated using density functional theory and solving the Helmholtz equation. The obtained Talbot images show focusing effects suggesting possible applications for reshaping electron wave-packets and interferometry [1].

[1] Salas, J. A., Varga, K., Yan, J.-A. & Bevan, K. H. Electron Talbot effect on graphene. Phys. Rev. B 93, 104305 (2016).

Two-dimensional (2D) atomic crystals, best exemplified by graphene, have emerged as a new class of material that may impact future science and technology. The reduced dimensionality in these 2D crystals often leads to novel material properties that are vastly different from that in the bulk. We refer to such a recurring scheme as “less is different”. In this talk I will illustrate this scheme with two 2D materials that we found particularly interesting – black phosphorus and 1T-TaS₂. These two layered materials have vastly different properties. Black phosphorus is a 2D semiconductor, and its superior material quality has recently enabled us to observe the quantum Hall effect. 1T-TaS₂, on the other hand, is a metal with a rich set of charge density wave phases. We explore their electronic properties while the doping and dimensionality of the 2D systems are modulated.

We use soft x-ray resonant photoemission and core-hole-clock spectroscopy to investigate the ultrafast carrier dynamics in SnS₂. We show that carriers delocalize on a time-scale of a few hundred attoseconds near the conduction band minimum, but remain localized over an order of magnitude longer in higher lying bands. On the basis of density functional theory calculations we are able to show that this is consistent with the 2D nature of SnS₂. Moreover, we show that these experiments map the wavefunction in the unoccupied bands. Our measurements represent the first of their kind on the carrier dynamics in such materials.

Transition metal dichalcogenides (TMDs) are 2D materials which belong to a class known as van der Waals materials where the adjacent layers are held together by weak van der Waal’s interactions and, in principle, have no surface dangling bonds, which permits a relaxed growth requirement in terms of lattice matching. This relaxed lattice-matching criteria allows us to couple these materials based primarily on their band alignment and electronic properties. WTe₂ is a TMD with an equilibrium structure in the distorted octahedral (1T’) phase. This 1T’ phase of WTe₂ is a semi-metal and hence may be implemented as a 2D metal in an all-2D heterostructure for new devices. Monolayer 1T’ WTe₂ has been separately predicted to be a Weyl semi-metal and to behave as a relatively wide bandgap (>0.1 eV) topological insulator, possessing helical edge states which have a number of interesting properties including time reversal symmetry, spin-momentum locking, and ballistic transport^1,2. The trigonal prismatic (2H) phase of WTe₂ is viewed as an integral part of the tunnel field effect transistor (TFET) device due to its bandgap and effective mass and has been theoretically predicted to provide low power operation and sub 60 mV subthreshold swing. WTe₂ is truly a remarkable material with intriguing electronic properties owing to its strong spin-orbit coupling and layered crystal structure.

In this work, we demonstrate the first report of WTe₂ growth by molecular beam epitaxy (MBE) on a variety of substrate materials (Bi₂Te₃, MoS₂, and graphite). We will discuss the optimal MBE growth conditions (substrate temperature, flux rates etc.) along with in-depth structural and chemical characterization of the resultant single crystal thin films. Characterization was conducted via reflection high energy electron diffraction, transmission electron microscopy, scanning tunneling microscopy/spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Challenges associated with Te incorporation and simple device and transport measurements will be presented.

This work is supported in part by the Center for Low Energy Systems Technology (LEAST), one of six centers supported by the STARnet phase of the Focus Center Research Program (FCRP), a Semiconductor Research Corporation program sponsored by MARCO and DARPA. It is also supported by the SWAN Center, a SRC center sponsored by the Nanoelectronics Research Initiative and NIST. This work was also supported in part by the Texas Higher Education Coordinating Board’s Norman Hackerman Advanced Research Program.

1. A. A. Soluyanov et. al, Nature527, 495 (2015).

2. X. Qian et. al, Science346, 1344 (2014).