Suman Chowdhury

A theoretical review on electronic, magnetic and optical properties of silicene

Inspired by the success of graphene, various two dimensional (2D) structures in free standing (FS) (hypothetical) form and on different substrates have been proposed recently. Silicene, a silicon counterpart of graphene, is predicted to possess massless Dirac fermions and to exhibit an experimentally accessible quantum spin Hall effect. Since the effective spin-orbit interaction is quite significant compared to graphene, buckling in silicene opens a gap of 1.55 meV at the Dirac point. This band gap can be further tailored by applying in plane stress, an external electric field, chemical functionalization and defects. In this topical theoretical review, we would like to explore the electronic, magnetic and optical properties, including Raman spectroscopy of various important derivatives of monolayer and bilayer silicene (BLS) with different adatoms (doping). The magnetic properties can be tailored by chemical functionalization, such as hydrogenation and introducing vacancy into the pristine planar silicene. Apart from some universal features of optical absorption present in all these 2D materials, the study on reflectivity modulation with doping (Al and P) concentration in silicene has indicated the emergence of some strong peaks having the robust characteristic of a doped reflective surface for both polarizations of the electromagnetic (EM) field. Besides this, attempts will be made to understand the electronic properties of silicene from some simple tight-binding Hamiltonian. We also point out the importance of shape dependence and optical anisotropy properties in silicene nanodisks and establish that a zigzag trigonal possesses the maximum magnetic moment. We also suggest future directions to be explored to make the synthesis of silicene and its various derivatives viable for verification of theoretical predictions. Although this is a fairly new route, the results obtained so far from experimental and theoretical studies in understanding silicene have shown enough significant promising features to open a new direction in the silicon industry, silicon based nano-structures in spintronics and in opto-electronic devices.

Defect induced magnetism in planar silicene: a first principles study

We study here the magnetic properties of two dimensional silicene using spin polarized density functional theory. The magnetic properties were studied by introducing monovacancy and di-vacancy, as well as by doping phosphorous and aluminium into the pristine silicene. It is observed that there is no magnetism in the monovacancy system, while there is large significant magnetic moment present for the di-vacancy system. Besides, the numerical computation reveals that the magnitude of the magnetic moment is more when the system is doped with aluminium than phosphorous. All these theoretical predictions in this two dimensional system may shed light to open a new route to design silicon based nano-structures in spintronics.

Optical properties of P and Al doped silicene: a first principles study

Here we study the optical properties of two dimensional pure, as well as doped, buckled silicene nanosheets using density functional theory in the long wavelength limit. Optical properties were studied by varying the concentration of substituted aluminium (Al), phosphorus (P) and aluminium–phosphorus (Al–P) atoms in silicene nanosheets. It has been observed that unlike graphene, no new electron energy loss spectra (EELS) peak occurs irrespective of doping type for parallel polarization. However, for perpendicular polarization two new, small yet significant, EELS peaks emerge for P doping. The origin of these new EELS peaks may be explained through the buckling effect of stable silicene. In addition, the calculations have revealed that the maximum values of the absorption coefficient of the doped system are higher than the pristine one. The study on reflectivity modulation with doping concentration has indicated the emergence of some strong peaks having the robust characteristic of a doped reflective surface for both polarizations of the electromagnetic field. Moreover, for all doped systems, the reflectivity modulation is restricted to low energy ( 8 eV) for parallel and perpendicular polarization respectively. Although no significant changes are noticed in the maximum values of optical conductivity with doping concentration in perpendicular polarization, a sudden jump appears for the Al–P codoped system at an 18.75% doping concentration. All these theoretical observations are expected to shed light on fabricating opto-electronic devices using silicene as the block material.

A real-space study of random extended defects in solids : Application to disordered Stone–Wales defects in graphene

Abstract We propose here a first-principles, parameter free, real space method for the study of disordered extended defects in solids. We shall illustrate the power of the technique with an application to graphene sheets with randomly placed Stone–Wales defects and shall examine the signature of such random defects on the density of states as a function of their concentration. The technique is general enough to be applied to a whole class of systems with lattice translational symmetry broken not only locally but by extended defects and defect clusters. The real space approach will allow us to distinguish signatures of specific defects and defect clusters.

Configuration and self-averaging in disordered systems

The main aim of this work is to present two different methodologies for configuration averaging in disordered systems. The Recursion method is suitable for the calculation of spatial or self-averaging, while the augmented space formalism averages over different possible configurations of the system. We have applied these techniques to a simple example and compared their results. Based on these, we have reexamined the concept of spatial ergodicity in disordered systems. The specific aspect, we have focused on, is the question “Why does an experimentalist often obtain the averaged result on a single sample?” We have found that in our example of disordered graphene, the two lead to the same result within the error limits of the two methods.

First principles Raman study of boron and nitrogen doped planar T-graphene clusters

Tetragonal graphene (TG) is one of the theoretically proposed dynamically stable graphene allotropes. In this study, the Raman spectra, IR spectra and some electronic properties of pristine and doped (single boron (B) and nitrogen (N)) TG have been investigated by first-principles based density functional theory (DFT) at the B3LYP/6-31G(d) level. Formation energy computation indicates that for the pristine structures, stability increases with increasing cluster size. In addition, for a particular cluster size, single B doping introduces some distortion in the system while single N doping increases the stability of it. The Raman spectrum of the N doped system is dominated by a single peak but for the B doped system several intense lines are found. For all the structures low intensity similar breathing-like modes have been observed. Besides, relatively low (high) intensity Raman lines are found for single B (N) doping compared to those of the pristine one. The vibrational study also reveals the existence of a prominent phonon Raman line for pristine clusters which hardly changes its position and nature of vibration with varying cluster size. So this mode can be used for identification of pristine TG structures. Unlike pristine TG, the doped structures possess non-zero finite dipole moments due to asymmetry in charge distribution. Large values of the HOMO–LUMO gap as well as the absence of DOS at the Fermi level lead to the semiconducting nature of all the structures. All these theoretical predictions from DFT calculations may shed light on experimental observations involving TG systems.

A first principles approach to magnetic and optical properties in single-layer graphene sandwiched between boron nitride monolayers

The dependence of the stability of single-layer graphene (SLG) sandwiched between hexagonal boron nitride bilayers (h-BN) has been described and investigated for different types of stacking in order to provide the fingerprint of the stacking order which affects the optical properties of such trilayer systems. Considering the four stacking models AAA-, AAB-, ABA-, and ABC-type stacking, the static dielectric functions (in case of parallel polarizations) for AAB-type stacking possesses maximum values, and minimum values are noticed for AAA. However, AAA-type stacking structures contribute the maximum magnetic moment while vanishing magnetic moments are observed for ABA and ABC stacking. The observed optical anisotropy and magnetic properties of these trilayer heterostructures (h-BN/SLG/h-BN) can be understood from the crystallographic stacking order and inherent crystal lattice symmetry. These optical and magnetic results suggest that the h-BN/SLG/h-BN could provide a viable route to graphene-based opto-electronic and spintronic devices.

Erratum to: Optical and Magnetic Properties of Free-standing Silicene, Germanene and T-graphene System

Abstract The physics of two-dimensional (2D) materials is always intriguing in their own right. For all of these elemental 2D materials, a generic characteristic feature is that all the atoms of the materials are exposed on the surface, and thus tuning the structure and physical properties by surface treatments becomes very easy and straightforward. The discovery of graphene have fostered intensive research interest in the field of graphene like 2D materials such as silicene and germanene (hexagonal network of silicon and germanium, respectively). In contrast to the planar graphene lattice, the silicene and germanene honeycomb lattice is slightly buckled and composed of two vertically displaced sublattices.The magnetic properties were studied by introducing mono- and di-vacancy (DV), as well as by doping phosphorus and aluminium into the pristine silicene. It is observed that there is no magnetism in the mono-vacancy system, while there is large significant magnetic moment present for the DV system. The optical anisotropy of four differently shaped silicene nanodisks has revealed that diamond-shaped (DS) silicene nanodisk possesses highest static dielectric constant having no zero-energy states. The study of optical properties in silicene nanosheet network doped by aluminium (Al), phosphorus (P) and aluminium-phosphorus (Al-P) atoms has revealed that unlike graphene, no new electron energy loss spectra (EELS) peak occurs irrespective of doping type for parallel polarization. Tetragonal graphene (T-graphene) having non-equivalent (two kinds) bonds and non-honeycomb structure shows Dirac-like fermions and high Fermi velocity. The higher stability, large dipole moment along with high-intensity Raman active modes are observed in N-doped T-graphene. All these theoretical results may shed light on device fabrication in nano-optoelectronic technology and material characterization techniques in T-graphene, doped silicene, and germanene.

Optical properties of monolayer BeC under an external electric field: A DFT approach

Abstract BeC, a two-dimensional hypercoordinated nanostructure carbon compound, has been the focus of the nanoworld because of its high value of dynamical stability, in-plane stiffness, carrier mobility and the existence of band gap. In this work, we have explored the electronic and the optical properties of this material under the influence of static external perpendicular electric field within the framework of density functional theory. Under the influence of a uniform electric field, the band gap changes within the meV range. The electron energy loss function study reveals that this material has optical band gaps which remain constant irrespective of the applied electric field strength. The optical property also exhibits interesting features when the applied field strength is within 0.4–0.5 V/Å. We have also tried to explain the optical data from the respective band structures and thus paving the way to understand qualitatively the signature of the optical anisotropy from the birefringence study.