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Register a new userGate Switchable Transport and Optical Anisotropy in 90{deg} Twisted Bilayer Black Phosphorus
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Anisotropy describes the directional dependence of a materials properties such as transport and optical response. In conventional bulk materials, anisotropy is intrinsically related to the crystal structure, and thus not tunable by the gating techniques used in modern electronics. Here we show that, in bilayer black phosphorus with an interlayer twist angle of 90{deg}, the anisotropy of its electronic structure and optical transitions is tunable by gating. Using first-principles calculations, we predict that a laboratory-accessible gate voltage can induce a hole effective mass that is 30 times larger along one Cartesian axis than along the other axis, and the two axes can be exchanged by flipping the sign of the gate voltage. This gate-controllable band structure also leads to a switchable optical linear dichroism, where the polarization of the lowest-energy optical transitions (absorption or luminescence) is tunable by gating. Thus, anisotropy is a tunable degree of freedom in twisted bilayer black phosphorus.
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We explore the possibility of using van dar Waals bonded heterostructures of stacked together 2D bilayer black phosphorus (BP) for nanoscale device applications. The electronic property of BP in AA stacking and 90{deg} twisted is studied with density functional theory. Further, we study the homogeneous nanojunction architecture of BP to use its anisotropic properties. Using the first principle simulations along with NEGF approach, we calculate quantum transport properties of the nanojunction setup. The interlayer directionally dependent current characteristics are explained in different setups. Our result revealed that 90{deg} twisted nanojucntion device would be a potential rectifier despite having no p-n junction characteristic only due to the intrinsic anisotropy of the material, making tunneling between armchair- and zigzag-directional BP sheets asymmetric.
We report the infrared transmission measurement on electrically gated twisted bilayer graphene. The optical absorption spectrum clearly manifests the dramatic changes such as the splitting of inter-linear-band absorption step, the shift of inter-van Hove singularity transition peak, and the emergence of very strong intra-valence (intra-conduction) band transition. These anomalous optical behaviors demonstrate consistently the non-rigid band structure modification created by the ion-gel gating through the layer-dependent Coulomb screening. We propose that this screening-driven band modification is an universal phenomenon that persists to other bilayer crystals in general, establishing the electrical gating as a versatile technique to engineer the band structures and to create new types of optical absorptions that can be exploited in electro-optical device application.
Optical and electronic properties of black phosphorus strongly depend on the number of layers and type of stacking. Using first-principles calculations within the framework of density functional theory, we investigate the electronic properties of bilayer black phosphorus with an interlayer twist angle of 90$^circ$. These calculations are complemented with a simple $vec{k}cdotvec{p}$ model which is able to capture most of the low energy features and is valid for arbitrary twist angles. The electronic spectrum of 90$^circ$ twisted bilayer black phosphorus is found to be x-y isotropic in contrast to the monolayer. However x-y anisotropy, and a partial return to monolayer-like behavior, particularly in the valence band, can be induced by an external out-of-plane electric field. Moreover, the preferred hole effective mass can be rotated by 90$^circ$ simply by changing the direction of the applied electric field. In particular, a +0.4 (-0.4) V/{AA} out-of-plane electric field results in a $sim$60% increase in the hole effective mass along the y (x) axis and enhances the $m^*_{y}/m^*_{x}$ ($m^*_{x}/m^*_{y}$) ratio as much as by a factor of 40. Our DFT and $vec{k}cdotvec{p}$ simulations clearly indicate that the twist angle in combination with an appropriate gate voltage is a novel way to tune the electronic and optical properties of bilayer phosphorus and it gives us a new degree of freedom to engineer the properties of black phosphorus based devices.
In this paper, the electronic properties of 30{deg} twisted double bilayer graphene, which loses the translational symmetry due to the incommensurate twist angle, are studied by means of the tight-binding approximation. We demonstrate the interlayer decoupling in the low-energy region from various electronic properties, such as the density of states, effective band structure, optical conductivity and Landau level spectrum. However, at Q points, the interlayer coupling results in the appearance of new Van Hove singularities in the density of states, new peaks in the optical conductivity and importantly the 12-fold-symmetry-like electronic states. The k-space tight-binding method is adopted to explain this phenomenon. The electronic states at Q points show the charge distribution patterns more complex than the 30{deg} twisted bilayer graphene due to the symmetry decrease. These phenomena appear also in the 30{deg} twisted interface between graphene monolayer and AB stacked bilayer.
Recently, intriguing physical properties have been unraveled in anisotropic layered semiconductors with the in-plane anisotropy often originates directly from the low crystallographic symmetry. However, little has been known about the systems where the size effect dominates the anisotropy of electronic band structures. Here, applying both experiment and theory, we show that the anisotropic energy bands of monoclinic gallium telluride (GaTe) are determined by a strong bulk-surface interaction rather than geometric factors. Bulk electronic states are found to be the major contribution to the highest valence band, whose anisotropy is yet immune to surface doping by potassium atoms. Further analysis indicates the weakened bulk-surface interaction gives rise to an inverse anisotropy of hole effective masses and the strong interlayer coupling induces a direct-indirect-direct band gap transition at transfer from mono- to few-layer GaTe. Our results thus pave the way to future applications of anisotropic layered semiconductors in nanoelectronics and optoelectronics.
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