Tiny Band Gap Research Articles

First principles calculations of the geometric structures and electronic properties of van der Waals heterostructure based on graphene, hexagonal boron nitride and molybdenum diselenide

In this work, by means of density functional theory, we systematically investigate the geometric structure and electronic properties of the vertical heterostructure by stacking graphene on top of single-layered hexagonal boron nitride and molybdenum diselenide (Gr/h-BN/MoSe2). Our results show that the interlayer coupling in the heterostructure is mainly governed by the weak van der Waals interactions. We find that in the heterostructure, a tiny band gap of 82 meV is opened around the Dirac K point of graphene due to sublattice symmetry breaking, and a minigap of 39 meV is opened between the π band of graphene and the vertical orbitals of MoSe2 due to their hybridization. This band gap opening in graphene makes it suitable for application in novel high-performance nanoeclectronic devices. Furthermore, the Gr/h-BN/MoSe2 heterostructure with inserting insulated h-BN layer forms an n-type Schottky contact with a small Schottky barrier height of 0.1 eV. These findings could provide helpful information for designing novel nanoelectronic devices by stacking graphene on top of single-layered h-BN and MoSe2.

First principles study on the electronic properties and Schottky barrier of Graphene/InSe heterostructure

Graphene-based van der Waals heterostructures by stacking graphene on other two-dimensional materials have recently attracted much attention due to their extraordinary properties and greatly extend the applications of the parent materials. By means of the density functional theory from first-principles calculations, in this work, the electronic properties and Schottky contact of the Graphene/InSe heterostructure, together with the effect of strain, are investigated systematically. Our results show that in the graphene/InSe heterostructure, graphene is very weakly bound to the InSe monolayer. Furthermore, we find that due to the sublattice symmetry breaking, a tiny band gap of 5 meV is opened in the graphene/InSe heterostructure, making it suitable for applications in electronic and optoelectronic devices. Moreover, we also find that the n-type Schottky contact is formed in the graphene/InSe heterostructure with a very small Schottky barrier height of 0.05 eV. The Schottky barrier height as well as Schottky contact types in the graphene/InSe heterostructure could be controlled by vertical strain applied perpendicularly to the heterostructure. When the interlayer distance between graphene and the topmost InSe monolayer is smaller than 2.40 Å, one can observe a transformation of the Schottky contact of the graphene/InSe heterostructure. Our results may provide helpful information for designing novel high-performance graphene-based van der Waals heterostructures and explore their potential applications in future nanoelectronic and optoelectronic devices.

Van der Waals graphene/g-GaSe heterostructure: Tuning the electronic properties and Schottky barrier by interlayer coupling, biaxial strain, and electric gating

Graphene-based van der Waals heterostructures are expected recently to design and fabricate many novel electronic and optoelectronic devices. The combination of the electronic structures of graphene and graphene-like GaSe monolayer (g-GaSe) in an ultrathin heterostructure has been realized experimentally, such as graphene/g-GaSe field effect transistor and dual Schottky diode device. In the present work, we investigate the electronic properties of the graphene/g-GaSe heterostructures under the applied electric field, in-plane strains, and interlayer coupling. Our results show that the electronic properties of the graphene/g-GaSe heterostructures are well preserved owing to a weak vdW interaction. Especially, a tiny band gap of 13 meV has opened in the presence of the g-GaSe monolayer. We found that the n-type Schottky contact is formed in the graphene/g-GaSe heterostructure with a Schottky barrier height of 0.86 eV, which can be efficiently modulated by applying the electric field, in-plane strains, and interlayer coupling. Furthermore, a transformation from the n-type to p-type Schottky contact is observed when the applied electric field is larger than 0.1 V/Å or the interlayer distance is smaller than 3.2 Å. Our results may provide helpful information to design and fabricate the future graphene-based vdW heterostructures, such as graphene/g-GaSe heterostructure and understand the physics mechanism in the graphene-based 2D vdW heterostructures.

Interlayer coupling and electric field tunable electronic properties and Schottky barrier in a graphene/bilayer-GaSe van der Waals heterostructure.

In this work, using density functional theory we investigated systematically the electronic properties and Schottky barrier modulation in a multilayer graphene/bilayer-GaSe heterostructure by varying the interlayer spacing and by applying an external electric field. At the equilibrium state, the graphene is bound to bilayer-GaSe by a weak van der Waals interaction with the interlayer distance d of 3.40 Å with the binding energy per carbon atom of -37.71 meV. The projected band structure of the graphene/bilayer-GaSe heterostructure appears as a combination of each band structure of graphene and bilayer-GaSe. Moreover, a tiny band gap of about 10 meV is opened at the Dirac point in the graphene/bilayer-GaSe heterostructure due to the sublattice symmetry breaking. The band gap opening in graphene makes it suitable for potential applications in nanoelectronic and optoelectronic devices. The graphene/bilayer-GaSe heterostructure forms an n-type Schottky contact with the Schottky barrier height of 0.72 eV at the equilibrium interlayer spacing. Furthermore, a transformation from the n-type to p-type Schottky contact could be performed by decreasing the interlayer distance or by applying an electric field. This transformation is observed when the interlayer distance is smaller than 3.30 Å, or when the applied positive external electric field is larger than 0.0125 V Å-1. These results are very important for designing new electronic Schottky devices based on graphene and other 2D semiconductors such as a graphene/bilayer-GaSe heterostructure.

Geometric and electronic structures of a two-dimensional covalent network of sp2 and sp3 carbon atoms

Based on first-principles total-energy calculations, we investigated the geometric, electronic, and spin structures of two-dimensional honeycomb networks of polymerized C40. Our calculations showed that the polymerized C40 possesses two polymeric structures that are statically and dynamically stable, retaining their covalent topologies up to 3000K. The C40 polymers are semiconductors with a tiny band gap and small dispersion bands near the gap, which causes the magnetic ordering of both ferromagnetic and antiferromagnetic states as their stable states. In addition to spin polarization, the C40 polymer possesses the electronic polarity normal to the two-dimensional covalent network because of the inhomogeneous charge density arising from the arrangement of pentagonal and hexagonal rings in the networks.

Excellent valleytronic properties and nontrivial topological phase in germanene heterostructure

In comparison with transition metal dichalcogenides (TMDs), germanene is not an ideal valleytronic material due to its very weak spin–orbit coupling (SOC) and tiny band gap. By performing density functional calculations, we studied the heterostructures of germanene/Sb and germanene/SbF in order to augment the valleytronic properties of germanene to a level similar to that of TMDs. Although germanene/Sb has large spin splittings, the system is found to be gapless. By proximity with the fluorine-adsorbed Sb layer, germanene can acquire large SOC. Germanene/SbF has a sizable direct band gap at Dirac points and the Dirac valleys of SbF and germanene are preserved in the heterostructure. The Ge–SbF interaction breaks the inversion symmetry, leading to large spin splittings and Berry curvatures, which are valley contrasting due to time reversal symmetry. We studied the interband optical transitions and found that valley polarization is achievable via valley-selective circular dichroism. The calculated Z2 topological invariant further confirms that Ge/SbF is a topological insulator. Our studies illustrate that weak SOC materials can be converted into good valleytronic material and that nontrivial topological phase and valleytronic properties can be simultaneously realized.

DFT design of polyguanidine – a unique two-dimensional material with high-energy density

ABSTRACTWe report herein a theoretical prediction and characterisation of a new two-dimensional (2D) material based on energetic polyguanidine. The structure represents a hexagonal type lattice of the P6/m space group. The material is dynamically and mechanically stable. Highly accurate band structure calculation with hybrid functional HSE06 reveals a tiny direct band gap being equal to 0.181 eV. We provide an additional spectral characterisation of the 2D polyguanidine substance including UV-vis, nuclear magnetic resonance and nuclear quadrupolar resonance parameters. The electron transport properties of a 26.6 Å wide polyguanidine ribbon are calculated in terms of tight-binding density functional theory approach. The predicted 2D material is also analysed by means of Quantum Theory of Atoms in Molecules and the aromatic character of the formed rings is estimated using nucleus-independent chemical shifts quantities.

Magnetic interactions between vacancy-induced intrinsic magnetic impurities in single-wall carbon nanotubes

Spin-half paramagnetism induced by point detects was found in graphene recently, micromechanism of this magnetic response can be explained well by the intrinsic magnetic impurity theory. In this paper, we apply this theory to two types of single-walled carbon nanotubes (SWCNs) and calculate the properties of various magnetic interactions for comparison. Interestingly, magnetic interactions have different behaviors in these systems. Following our calculation, within a short length, the interactions can be suppressed by ether size effect or a tiny band gap, and then exhibit exponentially decaying. However, in the absence of a band gap, the RKKY interaction could leave a tiny tail at long range, which determines long range magnetic order. Further more, when a tiny band gap exist in the systems, the Heisenberg coupling is the dominate one due to the expanded wavefunction. According to these result, vacancy states in different types of SWCNs could form different magnetic order, bringing abundant candidates for application.

Inversion response to optimize lumped‐element circuits for metamaterial

SUMMARYEnhancing electromagnetic research on prior light bend mechanisms is a significant issue for composite material phenomena, and one that is not available in nature. The unit cell of metamaterial can be revised, such that it is significantly smaller than the free space of the wavelength. It is essential to extend the appropriate retrieval process to measure electrical and magnetic responses to the backward wave propagation and dispersion diagram characteristics. In this paper, we propose a retrieval process inversion response between known and unknown parameters of the lumped unit‐cell circuits that optimizes the best fit parameters. By utilizing the optimization of particle swarm techniques, we provide appropriate values of periodic unit cells of CRLH transmission line circuits that matched with the microstrip implementation model. The group velocity of the wave is indicated in the parallel direction, whereas the phase velocity is in the anti‐parallel direction. The dispersion diagram displayed a specifically left‐handed and right‐handed medium, and a tiny band gap appeared in the attenuation medium. The constitutive parameters of dielectric permittivity, permeability, and refractive index were obtained from the periodic composite transmission line circuits.

Topological properties determined by atomic buckling in self-assembled ultrathin Bi(110).

Topological insulators (TIs) are a new type of electronic materials in which the nontrivial insulating bulk band topology governs conducting boundary states with embedded spin-momentum locking. Such edge states are more robust in a two-dimensional (2D) TI against scattering by nonmagnetic impurities than in its three-dimensional (3D) variant, because in 2D the two helical edge states are protected from the only possible backscattering. This makes the 2D TI family a better candidate for coherent spin transport and related applications. While several 3D TIs are already synthesized experimentally, physical realization of 2D TI is so far limited to hybrid quantum wells with a tiny bandgap that does not survive temperatures above 10 K. Here, combining first-principles calculations and scanning tunneling microscopy/spectroscopy (STM/STS) experimental studies, we report nontrivial 2D TI phases in 2-monolayer (2-ML) and 4-ML Bi(110) films with large and tunable bandgaps determined by atomic buckling of Bi(110) films. The gapless edge states are experimentally detected within the insulating bulk gap at 77 K. The band topology of ultrathin Bi(110) films is sensitive to atomic buckling. Such buckling is sensitive to charge doping and could be controlled by choosing different substrates on which Bi(110) films are grown.

Theoretical study on electronic properties of MoS2 antidot lattices

Motivated by the state of the art method for etching hexagonal array holes in molybdenum disulfide (MoS2), the electronic properties of MoS2 antidot lattices (MoS2ALs) with zigzag edge were studied with first-principles calculations. Monolayer MoS2ALs are semiconducting and the band gaps converge to constant values as the supercell area increases, which can be attributed to the edge effect. Multilayer MoS2ALs and chemical adsorbed MoS2ALs by F atoms show metallic behavior, while the structure adsorbed with H atoms remains to be semiconducting with a tiny bandgap. Our results show that forming periodically repeating structures in MoS2 can develop a promising technique for engineering nano materials and offer new opportunities for designing MoS2-based nanoscale electronic devices and chemical sensors.

Electromechanical Properties of Carbon Nanotubes

Electromechanical properties of carbon nanotubes were studied using Born–Oppenheimer molecular dynamics simulations within the QM/MM approach. The indentation of nanotubes was simulated using an AFM tip. The electronic structure and transport response to the mechanical deformations were investigated for different deflection points starting from perfect unperturbed systems up to the point where the first bonds break. We found the dependence of the force constant on the diameter size: the smaller the diameter, the larger the k. For the metallic-armchair tubes, with diameters from 8 to 13 A, the conductance decreases only slightly under radial deformation, and a tiny band gap opening of up to 50 meV was observed.

First‐principle study on the structures and electronic properties of gallium nitride nanowires

We have carried out first-principle density-functional calculations for gallium nitride (GaN) nanowires and related materials such as aluminum nitride (AlN) nanowires. The simplest model of a GaN nanowire has been devised as a fragment of the wurtzite GaN structure which has a one-dimensional periodic boundary, where the unit cell contains a Ga3N3 six-membered chair ring. We have found the most stable structure of the simplest model of a GaN nanowire, and the electronic interaction in the nanowire model has been discussed in terms of the quantum energy densities based on the regional density functional theory. Our calculations give a tiny indirect band gap of 0.07 eV for the simplest model of GaN nanowire, and it is shown the potential for band-gap control by the thickness of bundled nanowires and the mixing effect as the AlGaN nanowire due to relative higher band gap of the simplest model of AlN nanowire.