Showing papers by "Debnarayan Jana published in 2020"

A review on role of tetra-rings in graphene systems and their possible applications.

Abstract: Inspired by the success of graphene, various two-dimensional (2D) non-hexagonal graphene allotropes having sp2-bonded tetragonal rings in free-standing (hypothetical) form and on different substrates have been proposed recently. These systems have also been fabricated after modifying the topology of graphene by chemical processes. In this review, we would like to indicate the role of tetra-rings and the local symmetry breaking on the structural, electronic and optical properties of the graphene system. First-principles computations have demonstrated that the tetragonal graphene (TG) allotrope exhibits appreciable thermodynamic stability. The band structure of the TG nanoribbons (TGNRs) strongly depends on the size and edge geometry. This fact has been supported by the transport properties of TGNRs. The optical properties and Raman modes of this graphene allotrope have been well explored for characterisation purposes. Recently, a tight-binding model was used to unravel the metal-to-semiconductor transition under the influence of external magnetic fluxes. Even the introduction of transition metal atoms into this non-hexagonal network can control the magnetic response of the TG sheet. Furthermore, the collective effect of B-N doping and confinement effect on the structural and electronic properties of TG systems has been investigated. We also suggest future directions to be explored to make the synthesis of T graphene and its various derivatives/allotropes viable for the verification of theoretical predictions. It is observed that these doped systems act as a potential candidate for carbon monoxide gas sensing and current rectification devices. Therefore, all these experimental, numerical and analytical studies related to non-hexagonal TG systems are extremely important from a basic science point of view as well as for applications in sensing, optoelectronic and photonic devices.

The topology and robustness of two Dirac cones in S-graphene: A tight binding approach.

Abstract: Present work reports an elegant method to address the emergence of two Dirac cones in a non-hexagonal graphene allotrope S-graphene (SG). We have availed nearest neighbour tight binding (NNTB) model to validate the existence of two Dirac cones reported from density functional theory (DFT) computations. Besides, the real space renormalization group (RSRG) scheme clearly reveals the key reason behind the emergence of two Dirac cones associated with the given topology. Furthermore, the robustness of these Dirac cones has been explored in terms of hopping parameters. As an important note, the Fermi velocity of the SG system (vF $$\simeq $$ c/80) is almost 3.75 times that of the graphene. It has been observed that the Dirac cones can be easily shifted along the symmetry lines without breaking the degeneracy. We have attained two different conditions based on the sole relations of hopping parameters and on-site energies to break the degeneracy. Further, in order to perceive the topological aspect of the system we have obtained the phase diagram and Chern number of Haldane model. This exact analytical method along with the supported DFT computation will be very effective in studying the intrinsic behaviour of the Dirac materials other than graphene.

Twin T-graphene: a new semiconducting 2D carbon allotrope.

Abstract: Two dimensional carbon allotropes with multiple atomic layers have attracted significant interest recently. In this work a new three atomic layer thick 2D carbon allotrope, twin T-graphene, is proposed. The sp2 hybridized dynamically stable phase is a nonmagnetic semiconducting material with an indirect band gap of 1.79 eV. Thermal stability investigations indicate that the material undergoes no change in the bonding pattern at 2000 K. The study of its mechanical properties indicates that it is an elastically isotropic soft material with low values of elastic constants. The electron mobility of the semiconductor is found to be nearly 375 cm2 V-1 s-1. The material when doped with single nitrogen in the tetragonal site undergoes a drastic change in its electronic properties and manifests itself in the form of a bipolar magnetic semiconductor which is a potential spintronic material. The study of the optical properties clearly indicates an optical gap of 1.89 eV. The material shows optical response in the visible range. Two sharp characteristic EELS peaks indicate the presence of this material. All these characteristics indicate that this material can have potential photocatalytic activity and be an attractive material for the applications in field effect transistors.

Electronic and optical properties of non-hexagonal Dirac material S-graphene sheet and nanoribbons

Abstract: In this paper, we have critically explored the electronic, magnetic and optical properties of stable non-hexagonal carbon allotrope S-graphene (SG). The band structure of SG sheet exhibits two Dirac cones at distinct k → points. The ab-initio molecular dynamics study reveals the dynamical stability of the system up to 600 K. Besides, the sheet possesses appreciable mechanical stability. Furthermore, the electronic properties of the ribbons strongly depend on the edge geometry and width. The A-type S-graphene nanoribbons (SGNRs) with even N exhibit width-dependent direct bandgap semiconducting nature. Whereas, A-SGNRs with odd N are semi-metallic. In contrary, B-SGNRs are uniformly semi-metallic except for extremely narrow N = 1 and 2 case. Besides, the unexpected occurrence of Dirac point for the N = 4.5 C-SGNR has been explored. Moreover, the optical response of the sheet and nanoribbons are highly anisotropic. We have critically obtained the plasma frequency for the collective electron oscillation of the sheet at 5.37 eV under parallel incidence of electromagnetic waves. Besides, the effective number of electrons participating in direct interband transitions in sheet show a quasi-saturation between 3-10 eV. We thus expect these theoretical predictions will provide a basic understanding of the opto-electronic responses towards possible application of this non-hexagonal graphene network with multiple Dirac cones.

Electronic structural critique of interesting thermal and optical properties of C17Ge germagraphene.

Abstract: In this communication, we report a theoretical attempt to understand the involvement of the electronic structure in determining the optical and thermal properties of C17Ge germagraphene, a buckled two-dimensional material. The structure is found to be a direct bandgap semiconductor with low carrier effective mass. Our study has revealed the effect of spin–orbit coupling on the band structure, and the appearance of spin Hall current on the material. The selectively high blue to ultraviolet light absorption, and a refractive index comparable to flint glass, open up the possible applicability of this material for optical devices. From an electronic structural point of view, we investigate the reason behind its moderately high Seebeck coefficient and power factor which are comparable to traditional thermoelectric materials. Besides its narrow bandgap and relatively smaller work function of 4.361 eV, compared to graphene (4.390 eV) and germanene (4.682 eV), ensures easier removal of electrons from the surface. This material turns out to be an excellent alternative for future semiconductor applications, from optical to thermal devices.

Electronic structural critique of interesting thermal and optical properties of C$_{17}$Ge germagraphene

Abstract: In this communication, we report a theoretical attempt to understand the involvement of electronic structure in determination of optical and thermal properties of C$_{17}$Ge germagraphene, a buckled two dimensional material. The structure is found to be a direct bandgap semiconductor with low carrier effective mass. Our study has revealed that the effect of spin-orbit coupling on the band structure and in appearance of spin Hall current in the material. A selectively high blue to ultraviolet light absorption and a refractive index comparable to flint glass open up the possible applicability of this material for optoelectronic devices. From electronic structural point of view, we investigate the reason behind its moderately high Seebeck coefficient and power factor comparable to traditional thermoelectric materials. Besides its narrow bandgap, relatively smaller work function of C$_{17}$Ge ($4.361 ~eV$) than graphene ($4.390 ~eV$) and germanene ($4.682 ~eV$) assures more easily removal of electron from the surface. This material is turned out to be an excellent alternative for futuristic semiconductor application from optical to thermal device regime.

Semiconductor Physics: A Density Functional Journey

Abstract: The journey of theoretical study on semiconductors is reviewed in a non-conventional way. We have started with the basic introduction of Hartree-Fock method and introduce the fundamentals of Density Functional Theory (DFT). From the oldest Local Density Approximations (LDA) to the most recent developments of semi-local corrections [Generalised Gradient Approximation (GGA), Meta-GGAs], hybrid functionals and orbital dependent methodologies are discussed in detail. To showcase the performance of DFT, results obtained via different approximations are compared. We indicate the success of semi-local approximations in structural properties prediction. We also show how less computationally costly but withstand architecture of some semi-local DFT methods can solve the long riddle of bandgap underestimation. In semiconductor physics, the importance of not only the band structure prediction, but also, the proper calculation of Fermi energy, and, exact finding of band alignment is argued. The comparison of Fermi energy dependent properties can channelize the theoretical studies on modern age environment-friendly researches on semiconductors, like artificial photocatalysis, energy efficient opto-electronic devices, etc. This prescription on proper choice of DFT method is potentially competent to complement the experimental findings as well as can open up a pathway of advanced semiconducting materials discoveries.

Dirac Materials in a MatrixWay

Abstract: Recent years have been the platform of discovery of a wide range of materials, like d-wave superconductors, graphene, and topological insulators. These materials do indeed share a fundamental similarity in their low-energy spectra namely the fermionic excitations. There carriers behave as massless Dirac particles rather than conventional fermions that obey the usual Schrodinger Hamiltonian. A surprising aspect of most Dirac materials is that many of their physical properties measured in experiments can be understood at the non-interacting level. In spite of the large effective coupling constant in case of graphene, it has been observed that the interactions do not seem to play a major key role. Controlling the electrons at Dirac nodes in the first Brillouin zone needs the interplay of sublattice symmetry, inversion symmetry and the time-reversal symmetry. In this article, we have used explicit fundamental symmetry to understand the basic features of Dirac materials occurring in three diverse systems in a compact 2 × 2 matrix way. Furthermore, the robustness of the Dirac cones has also been explored from the scientific notion of topological physics. In addition, an elementary introduction on the three dimensional (3D) topological insulators and d wave superconductors will shed light in their respective fields. Furthermore, we have also discussed the way to evaluate the effective mass tensor of the carriers in the two dimensional (2D) Dirac materials. This methodology has also been critically extended to three dimensional (3D) topological insulators and d wave superconductors.

Correction: Electric field induced band tuning, optical and thermoelectric responses in tetragonal germanene: a theoretical approach.

Abstract: In this article, we have systematically explored the electronic, optical and thermoelectric properties of tetragonal germanene (T-Ge) using first principles calculations. The ground state geometry of pristine T-Ge is buckled and exhibits nodal line semi-metallic behaviour. In addition, we have proposed a tight binding (TB) model Hamiltonian that efficiently explains the emergence of double Dirac points at the Fermi level of T-Ge. Furthermore, a hopping relation has been explored at which both Dirac points merge and then annihilate resulting in a direct band gap at the Γ point. To exploit the buckling of the system, we have employed a transverse electric field, which invariably breaks the sublattice symmetry and removes the degeneracies at the Fermi surface. Furthermore, the band gap at the Dirac points varies linearly with the external electric field strength. Our TB Hamiltonian adequately satisfies the first principles results even in the presence of an external electric field. Moreover, we have found that T-Ge offers efficient tuning of band gaps at the Dirac points compared to other buckled systems viz. hexagonal silicene and germanene. In addition, the optical behaviour of T-Ge has been explained in accordance with the electronic states of the system. The strong optical responses in a low energy region make the material efficient for optical nanodevice applications. Moreover, T-Ge shows relatively better thermoelectric behaviour than graphene. Therefore, the external electric field induced tunable band gap and intriguing low energy optical signals pave the way to choose T-Ge as a smart choice for optoelectronic device applications. Finally we have suggested probable routes for experimental realization of the T-Ge structure.