Deciphering the Role of Alkali Metals (Li, Na, K) Doping for Triggering Nonlinear Optical (NLO) Properties of T-Graphene Quantum Dots: Toward the Development of Giant NLO Response Materials

Nanoscale nonlinear optical (NLO) materials have received huge attention of the scientists in current decades because of their enormous applications in optics, electronics, and telecommunication. Different studies have been conducted to tune the nonlinear optical response of the nanomaterials. However, the role of alkali metal (Li, Na, K) doping on triggering the nonlinear optical response of nanomaterials by converting their centrosymmetric configuration into noncentrosymmetric configuration is rarely studied. Therefore, to find a novel of way of making NLO materials, we have employed density functional theory (DFT) calculations, which helped us to explore the effect of alkali metal (Li, Na, K) doping on the nonlinear optical response of tetragonal graphene quantum dots (TGQDs). Ten new complexes of alkali metal doped TGQDs are designed theoretically. The binding energy calculations revealed the stability of alkali metal doped TGQDs. The NLO responses of newly designed complexes are evaluated by their polarizability, first hyperpolarizability (β_o), and frequency dependent hyperpolarizabilities. The Li@r8a exhibited the highest first hyperpolarizability (β_o) value of 5.19 × 10⁵ au. All these complexes exhibited complete transparency in the UV region. The exceptionally high values of β_o of M@TGQDs are accredited to the generation of diffuse excess electrons, as indicated by NBO analysis and PDOS. NCI analysis is accomplished to examine the nature of bonding interactions among alkali metal atoms and TGQDs. Our results suggest alkali metal doped TGQD complexes as potential candidates for nanoscale NLO materials with sufficient stability and enhanced NLO response. This study will open new doors for making giant NLO response materials for modern hi-tech applications.

Improvement of the hydrogen storage performance of t-graphene-like two-dimensional boron nitride upon selected lithium decoration

In recent years, search for applicable bidimensional (2D) hydrogen storage materials with high capacity and excellent H₂ physisorption properties has attracted considerable attention from scientists and researchers. According to the rational design, and using first-principles calculations, we propose a t-graphene-like boron nitride monolayer (t-B₄N₄) for hydrogen storage application by replacing C atoms in t-graphene with B and N atoms. The thermal stability and polarization mechanisms of lithium atoms adsorbed at the center of octagons on the t-B₄N₄ system were evaluated at 300 K using ab initio molecular dynamics (AIMD) calculations. Moreover, Li-decorated double-sided t-B₄N₄ can store up to 32H₂ molecules with an average hydrogen adsorption energy of 0.217 eV per H₂ and a maximum hydrogen storage capacity of 12.47 wt%. The reversibility of adsorbed hydrogen was checked and the calculated desorption temperature was 161 K, much higher than the critical point for hydrogen. Based on diffusion barriers, the H₂ molecule diffusion kinetics is faster on the t-B₄N₄ surface than that on t-graphene and graphene.

Emerging properties of carbon based 2D material beyond graphene

Graphene turns out to be the pioneering material for setting up boulevard to a new zoo of recently proposed carbon based novel two dimensional (2D) analogues. It is evident that their electronic, optical and other related properties are utterly different from that of graphene because of the distinct intriguing morphology. For instance, the revolutionary emergence of Dirac cones in graphene is particularly hard to find in most of the other 2D materials. As a consequence the crystal symmetries indeed act as a major role for predicting electronic band structure. Since tight binding calculations have become an indispensable tool in electronic band structure calculation, we indicate the implication of such method in graphene's allotropes beyond hexagonal symmetry. It is to be noted that some of these graphene allotropes successfully overcome the inherent drawback of the zero band gap nature of graphene. As a result, these 2D nanomaterials exhibit great potential in a broad spectrum of applications, viz nanoelectronics, nanooptics, gas sensors, gas storages, catalysis, and other specific applications. The miniaturization of high performance graphene allotrope based gas sensors to microscopic or even nanosized range has also been critically discussed. In addition, various optical properties like the dielectric functions, optical conductivity, electron energy loss spectra reveal that these systems can be used in opto-electronic devices. Nonetheless, the honeycomb lattice of graphene is not superconducting. However, it is proposed that the tetragonal form of graphene can be intruded to form new hybrid 2D materials to achieve novel superconducting device at attainable conditions. These dynamic experimental prospects demand further functionalization of these systems to enhance the efficiency and the field of multifunctionality. This topical review aims to highlight the latest advances in carbon based 2D materials beyond graphene from the basic theoretical as well as future application perspectives.

Electronic and thermal transport in novel carbon-based bilayer with tetragonal rings: a combined study using first-principles and machine learning approach

In this article, the structural, electronic and thermal transport characteristics of bilayer tetragonal graphene (TG) are systematically explored with a combination of first-principles calculations and machine-learning interatomic potential approaches. Optimized ground state geometry of the bilayer TG structure is predicted and examined by employing various stability criteria. Electronic bandstructure analysis confirmed that bilayer TG exhibits a metallic band structure similar to the monolayer T-graphene structure. Thermal transport characteristics of the bilayer TG structure are explored by analysing thermal conductivity, the Seebeck coefficient, and electrical conductivity. The electronic part of the thermal conductivity shows linearly increasing behaviour with temperature, however the lattice part exhibits the opposite character. The lattice thermal conductivity part is investigated in terms of the three phonon scattering rates and weighted phase space. On the other hand, the Seebeck coefficient goes through a transition from negative to positive values with increasing temperature. The Wiedemann-Franz law regarding electrical transport of the bilayer TG is verified and confirms the universal Lorentz number. Specific heat of the bilayer TG structure follows the Debye model at low temperature and constant behaviour at high temperature. Moreover, the Debye temperature of the bilayer TG structure is verified by ab initio calculations as well as fitting the specific heat data using the Debye model.

First-principles study of the optical and thermoelectric properties of tetragonal-silicene

We report the optical and thermoelectric properties of the two-dimensional Dirac material T-silicene (TS) sheet and nanoribbons (NRs) by first-principles calculations. Both the optical and thermoelectric properties of TS can be modified by tailoring the sheet into nanoribbons of different widths and edge geometries. The optical response of the structures is highly anisotropic. A π interband transition occurs in the visible range of incident light with parallel polarization. The optical response for asymmetric arm-chair TS nanoribbons (ATSNRs) is larger than for symmetric ATSNRs. The absorptions of asymmetric ATSNR are redshifted due to a decrease in the bandgap with the width of the NRs. Plasma frequencies of the sheet and the NRs are identified from the imaginary part of the dielectric function and electron energy loss spectra curves. Thermoelectric properties like electrical conductivity, Seebeck coefficient, power factor, and electronic figure of merit are also studied. Compared with graphene, the TS sheet possesses a higher electrical conductivity and a better figure of merit. Among the NRs, asymmetric ATSNRs exhibit a better thermoelectric performance. All these intriguing features of TS may shed light on fabricating smart opto-electronic and thermoelectric devices.

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

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

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 [Formula: see text] 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.

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

Transverse electric field breaks the sublattice symmetry and generates a band gap in the semi-metallic T-Ge structure.

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

Two dimensional carbon allotropes with multiple atomic layers have attracted significant interest recently.

First-principles calculation of the electronic and optical properties of a new two-dimensional carbon allotrope: tetra-penta-octagonal graphene

A novel sp² hybridized planar 2D carbon allotrope consisting of tetra, penta and octagonal (TPO) rings is proposed in this work. Its thermodynamic stability is confirmed by molecular dynamics in the canonical ensemble at 600 K and the analysis shows that it can also remain stable at 1000 K. The mechanical stability of this material has been estimated by the Born-Huang criterion. Its in-plane stiffness constants are found to be 85% of that of graphene ensuring its high strength quality. The investigation of the electronic properties reveals that the material is metallic in nature with a Dirac cone at 3.7 eV above its Fermi level at an asymmetric position in the conduction band. The study of its optical property for parallel and perpendicular polarization yields the absence of any plasma frequency. Besides, its absorption is mostly spread within 10-20 eV. Further electrical transport study shows negative differential resistance (NDR) above 3.5 V for one nano device. Nano ribbons made out of a TPO-graphene sheet exhibit metallic character. When the porous sheet of TPO-graphene is exposed to Li and S atoms, it is found that the Li atoms pass through the pores unlike the S atoms owing to the less barrier energy compared to S atoms. Substitutional doping with boron and nitrogen at different sites of TPO-graphene showed splitting of the Dirac feature. Also suitable B and N doping brings about semiconducting properties with tunability in band gap with a maximum band gap of 1.09 eV for an isoelectronic structure. All these theoretical predictions might trigger further new avenues involving this novel TPO graphene.

Acetylenic linkage dependent electronic and optical behaviour of morphologically distinct ‘-ynes’

We have critically examined the key role of acetylenic linkages (–CC–) in determining the opto-electronic responses of the dynamically stable tetragonal (T) ‘-ynes’ with the help of density functional theory.

Effect and Characterization of Stone–Wales Defects on Graphene Quantum Dot: A First-Principles Study

A first principles based density functional theory (DFT) has been employed to identify the signature of Stone–Wales (SW) defects in semiconducting graphene quantum dot (GQD). Results show that the G mode in the Raman spectra of GQD has been red shifted to 1544.21 cm − 1 in the presence of 2.08% SW defect concentration. In addition, the intensity ratio between a robust low intense contraction–elongation mode and G mode is found to be reduced for the defected structure. We have also observed a Raman mode at 1674.04 cm − 1 due to the solo contribution of the defected bond. The increase in defect concentration, however, reduces the stability of the structures. As a consequence, the systems undergo structural buckling due to the presence of SW defect generated additional stresses. We have further explored that the 1615.45 cm − 1 Raman mode and 1619.29 cm − 1 infra-red mode are due to the collective stretching of two distinct SW defects separated at a distance 7.98 Å. Therefore, this is the smallest separation between the SW defects for their distinct existence. The pristine structure possesses maximum electrical conductivity and the same reduces to 0.37 times for 2.08% SW defect. On the other hand, the work function is reduced in the presence of defects except for the structure with SW defects separated at 7.98 Å. All these results will serve as an important reference to facilitate the potential applications of GQD based nano-devices with inherent topological SW defects.