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Register a new userMg(OH)2 -WS2 Heterobilayer: Electric Field Tunable Bandgap Crossover
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Mehmet Yagmurcukardes
Publication date
2016
fields
Physics
and research's language is
English
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Magnesium hydroxide (Mg(OH)2) has a layered brucite-like structure in its bulk form and was recently isolated as a new member of 2D monolayer materials. We investigated the electronic and optical properties of monolayer crystals of Mg(OH)2 and WS2 and their possible heterobilayer structure by means of first principles calculations. It was found that both monolayers of Mg(OH)2 and WS 2 are direct-gap semiconductors and these two monolayers form a typical van der Waals heterostructure with a weak interlayer interaction and a type-II band alignment with a staggered gap that spatially seperates electrons and holes. We also showed that an out-of-plane electric field induces a transition from a staggered to a straddling type heterojunction. Moreover, by solving the Bethe-Salpeter equation on top of single shot G0 W0 calculations, we show that the oscillator strength of the intralayer excitons of the heterostructure is an order of magnitude larger than the oscillator strength of the interlayer excitons. Because of the staggered interfacial gap and the field- tunable energy band structure, the Mg(OH)2 -WS2 heterobilayer can become an important candidate for various optoelectronic device applications in nanoscale.
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Van der Waals (vdW) layered transition metal dichalcogenides (TMDCs) materials are emerging as one class of quantum materials holding novel optical and electronic properties. In particular, the bandgap tunability attractive for nanoelectronics technology have been observed up to 1.1 eV when applying dielectric screening or grain boundary engineering. Here we present the experimental observation of bandgap closing at the center of the screw dislocation-driven WS2 spiral pyramid by means of scanning tunneling spectroscopy, which is validated by first-principle calculations. The observed giant bandgap modulation is attributed to the presence of dangling bonds induced by the W-S broken and the enhanced localized stress in the core of the dislocation. Achieving this metallic state and the consequent vertical conducting channel presents a pathway to 3D-interconnected vdW heterostructure devices based on emergent semiconducting TMDCs.
Motivated by recent studies that reported the successful synthesis of monolayer Mg(OH)$_{2}$ [Suslu textit{et al.}, Sci. Rep. textbf{6}, 20525 (2016)] and hexagonal (textit{h}-)AlN [Tsipas textit{et al}., Appl. Phys. Lett. textbf{103}, 251605 (2013)], we investigate structural, electronic, and optical properties of vertically stacked $h$-AlN and Mg(OH)$_{2}$, through textit{ab initio} density-functional theory (DFT), many-body quasi-particle calculations within the GW approximation, and the Bethe-Salpeter equation (BSE). It is obtained that the bilayer heterostructure prefers the $AB^{prime}$ stacking having direct band gap at the $Gamma$ with Type-II band alignment in which the valance band maximum and conduction band minimum originate from different layer. Regarding the optical properties, the imaginary part of the dielectric function of the individual layers and hetero-bilayer are investigated. The hetero-bilayer possesses excitonic peaks which appear only after the construction of the hetero-bilayer. The lowest three exciton peaks are detailedly analyzed by means of band decomposed charge density and the oscillator strength. Furthermore, the wave function calculation shows that the first peak of the hetero-bilayer originates from spatially indirect exciton where the electron and hole localized at $h$-AlN and Mg(OH)$_{2}$, respectively, which is important for the light harvesting applications.
Graphene nanoribbons (GNRs) possess distinct symmetry-protected topological phases. We show, through first-principles calculations, that by applying an experimentally accessible transverse electric field (TEF), certain boron and nitrogen periodically co-doped GNRs have tunable topological phases. The tunability arises from a field-induced band inversion due to an opposite response of the conduction- and valance-band states to the electric field. With a spatially-varying applied field, segments of GNRs of distinct topological phases are created, resulting in a field-programmable array of topological junction states, each may be occupied with charge or spin. Our findings not only show that electric field may be used as an easy tuning knob for topological phases in quasi-one-dimensional systems, but also provide new design principles for future GNR-based quantum electronic devices through their topological characters.
We study the effect of external electric fields on superconductor-semiconductor coupling by measuring the electron transport in InSb semiconductor nanowires coupled to an epitaxially grown Al superconductor. We find that the gate voltage induced electric fields can greatly modify the coupling strength, which has consequences for the proximity induced superconducting gap, effective g-factor, and spin-orbit coupling, which all play a key role in understanding Majorana physics. We further show that level repulsion due to spin-orbit coupling in a finite size system can lead to seemingly stable zero bias conductance peaks, which mimic the behavior of Majorana zero modes. Our results improve the understanding of realistic Majorana nanowire systems.
We report a rare atom-like interaction between excitons in monolayer WS2, measured using ultrafast absorption spectroscopy. At increasing excitation density, the exciton resonance energy exhibits a pronounced redshift followed by an anomalous blueshift. Using both material-realistic computation and phenomenological modeling, we attribute this observation to plasma effects and an attraction-repulsion crossover of the exciton-exciton interaction that mimics the Lennard-Jones potential between atoms. Our experiment demonstrates a strong analogy between excitons and atoms with respect to inter-particle interaction, which holds promise to pursue the predicted liquid and crystalline phases of excitons in two-dimensional materials.
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