Session PP04: V: 12.01.03 Virtual Talks

Probing Single Dopant Atoms in 2D Semiconductors via Scanning Probe Microscopy

Two-dimensional (2D) semiconductors provide an exciting platform to engineer atomic quantum states in a robust, yet tunable solid-state system. In this talk, I will present our efforts to unravel the interesting physics behind single dopant atoms in transition metal dichalcogenide (TMD) monolayers by means of high-resolution scanning probe microscopy [1-8].

Our recent research on transition metal doped TMDs such as (n-type) Re-doped MoS₂ and (p-type) V-doped WSe₂ reveals the significance of the charge state in the spectroscopic signature of these defects. By substrate chemical gating, we can stabilize three charge states of Re_Mo, where two of the charge states exhibit symmetry broken electronic orbitals and a distorted atomic configuration that we assign to a pseudo Jahn-Teller effect [9]. Negatively charged V dopants and dopant pairs in WSe₂ exhibit a series of occupied p-type defect states above the valence band edge, accompanied by an intriguing electronic fine-structure that we attribute to many-body electron interactions [10].

I will also introduce our efforts to probe excitation dynamics on the ultrafast time scales using a newly developed THz-pulse-driven STM [11].

 

References

Direct observation of controlled growth of oriented metal-chalcogenide nanowires

One-dimensional (1D) transition-metal chalcogenide nanowires (TMC-NWs) are flexible metallic structures with a general formula of X₆Y₆ (X = Mo, W; Y = S, Se, and Te)^1-3 and show tuneable metallic characteristics.^{1, 4-6} Benefiting from their unique structures and electrical properties, TMC-NWs show the potential to serve as conductive interconnections between nanodevices⁵.

Although significant progress has been made in the growth of TMC-NWs^6-10, their orientation control is still challenging due to its 1D nature, resulting in an isotropic distribution of NW bundles.

In this work, we successfully applied in-situ electrical bias to 2D molybdenum ditelluride (MoTe₂) and simultaneously observed a real-time structural transformation to oriented nanowires in a scanning transmission electron microscope (STEM) with atomic resolution. Under the applied bias voltage, 2D MoTe₂ gradually converts into highly oriented 1D Mo₆Te₆ NWs along zigzag edges with sharp interfaces. The novelty of the experiment can be summarized as follows:

1)    In-situ observation of structural evolution of MoTe₂ under electrical bias. This is realized by transferring the graphite-sandwiched 2D MoTe₂ heterostructure to the microelectromechanical system (MEMS) chip. With graphite layers connected to electrodes, we can apply vertical electrical bias to the confined 2D MoTe₂ flake.

2)    Atomic resolution of the electrical in-situ experiment. This is realized by the excellent stability of the MEMS-based in-situ system for applying bias and also the rational design of the 3-layer heterostructure. The graphite electrodes are quite thin compared to the MoTe₂ which makes them electron transparent.

3)    First-principles calculations suggest anisotropic migration of Te atoms. Density functional theory (DFT) calculations confirm that the growth of highly oriented NWs originates from both the confinement effect of the graphite electrodes and the local Joule heating generated by the applied bias.

Our work demonstrates alternative strategies to prepare metal-semiconductor heterojunctions in 2D materials. Moreover, we also show the feasibility of studying phase transformations of 2D materials under electrical excitation at the atomic scale by introducing graphite electrodes, which greatly expands in-situ characterization techniques.   

Topological transistors with Weyl semiconductor for low-power computing

In the post-Moore era, electronics based on traditional materials face challenges such as heat dissipation, lack of stability against defects and environmental disturbances, and high power consumption. Topological materials are known for their robustness against defects and environmental disturbances, making them promising candidates for reliable informative electronic applications. Here, we have investigated the potential of using the Weyl semiconductor for developing functional field-effect transistors. Firstly, we present low-loss topological phase change transistors (TPCTs) based on tellurium (Te). The TPCTs utilize electrostatic gate modulation to control the energy separation between the Fermi level and the Weyl point, enabling a topological phase change between Weyl and conventional semiconductors. The TPCTs exhibit low-loss transport characteristics in the ON state, providing resistance against external disturbances, while demonstrating trivial charge transport in the OFF state by placing the Fermi level within the bandgap. At low operation voltage, these TPCTs exhibit the p-type transfer curve with 10⁸ ON/OFF ratio and ultrahigh ON-state conductance (39 mS/μm), a step forward for the development of p-type transistors. Additionally, we demonstrate room-temperature valley transistors in the non-local transport structure by manipulating the non-trivial band topology of Te. Te valley transistors generate long-lived valley polarization, as indicated by observed valley-polarized diffusion lengths, which overcomes the temperature limitation of short-lifetime valley-polarized excitons in previous works. By introducing the ion insertion/extraction, we have realized non-volatile synaptic states for neuromorphic computing with low readout power (~fW) and high recognition accuracy. Based on topological materials, we have realized functional topological field-effect transistors with analogue/digital computing, promising for post-Moore chips.   

Chiral Magnonic Anomaly in Magnetic Weyl Semimetal FM-EuCd2As2

The interplay between topological band structure and magnetic ordering in magnetic topological phases can exhibit extraordinary physical properties enabling Axion insulator, magnetic Weyl semimetals, and quantum anomalous Hall insulator. The chemical tunability of EuCd₂As₂ materials has two magnetic ordering states: antiferromagnetic (AFM) and ferromagnetic (FM). While AFM-EuCd₂As₂ has an axion insulating state due to its symmetry breaking with spin configuration, FM-EuCd₂As₂ is an ideal candidate for Weyl physics studies because of the minimum number of Weyl points with opposite chirality. We use helictical-resolved Raman spectroscopy to observe phonon modes and signature of chiral magnons with applied magnetic field of a FM-EuCd₂As₂ crystal. We show the behavior of E_g and A_g phonon modes, and their dependence on magnetic ordering below the Curie temperature, at approximately 26 K. We also show that the two magnons peaks at 10.7 and 24 meV at cryogenic temperature move to blue- or red-shifts depending on the circularly polarized excitation with applied magnetic field. The energy shifting increases (1 meV/T at 4.2 K) as temperature decreases under the same field. This energy shifting behavior with each helical direction persists above the Curie temperature. Our results contribute to understanding the topological chirality of FM-EuCd₂As₂ coupled with a helictical light creating chirality imbalance by the magnetic field and allows deeper investigations of Weyl systems for novel applications.    

Search for an Antiferromagnetic Weyl Semimetal in (MnTe)m(Sb2Te3)n and (MnTe)m(Bi2Te3)n Superlattices

The interaction between topology and magnetism can lead to novel topological materials including Chern insulators, axion insulators, and Weyl semimetals. Following a systematic first principles analysis of the superlattice structures based on MnTe and Sb2Te3 building blocks [i.e., (MnTe)m(Sb2Te3)n], we predict that an antiferromagnetic Weyl semimetal (WSM) exists within a van derWaals layered material. This centrosymmetric material, Mn10Sb8Te22, shows ferromagnetic intralayer and antiferromagnetic interlayer interactions. The obtained electronic bandstructure also indicates that the spin-split bands consistent with a WSM exist with a single pair of Weyl points. The presence of the Weyl nodes is subsequently verified with Berry curvature, Wilson loop, and Weyl chirality calculations. More specifically, Mn10Sb8Te22 is a van der Waals magnetic material consisting of three layers of MnSb2Te4 (i.e., m=1, n=1) and one layer of Mn7Sb2Te10 (i.e., m=7, n=1). In turn, Mn7Sb2Te10 is constructed by adding six layers of MnTe to MnSb2Te4. Other combinations of the MnSbTe-family materials are found to be antiferromagnetic topological or normal insulators on either side of the Mn:Sb ratio, respectively, illustrating the topological phase transition as anticipated. A similar investigation in the homologous (MnTe)m(Bi2Te3)n system produces mostly non-trivial antiferromagnetic insulators due to the strong spin-orbit couplig. When realized, the antiferromagnetic WSMs in the simplest form (i.e., a single pair of Weyl nodes) are expected to provide a promising candidate for low-power spintronic applications.