Absence Of Band Gap Research Articles

A review of the synthesis, properties, and applications of 2D transition metal dichalcogenides and their heterostructures

One of the most nascent and universally accepted categories among the 2D materials is the transition metal dichalcogenides (TMDs). The absence of bandgap in graphene fomented the researchers to quest for an alternative semiconducting 2D materials. The aberrant optical and electronic properties helped 2D TMDs to excel in fundamental research and novel device applications. This article overviews the recent advances in the preparation methods, properties, characterization, and novel multifunctional applications of TMDs and their corresponding heterostructures with special emphasis on their biosensing applications. The major portion of the review underlines the TMDs based heterostructures, which find potential applications in optoelectronics, sensors, and photovoltaics, that are devised to boost the functionality and flexibility and there by surmount the existing limitations of monolayered materials. At the end, the advancement and elevation of TMDs as biosensors over other 2D materials are also explored.

Novel atomic-scale graphene metamaterials with broadband electromagnetic wave absorption and ultra-high elastic modulus

The weak visible-infrared broadband electromagnetic absorption, absence of bandgap, and low out-of-plane Young's modulus in graphene are three long-standing challenges plaguing its applications in optoelectronic and photonic devices. Herein, we propose a novel atomic-scale graphene metamaterial via vertically crossing graphene nanosheets, showing remarkable energetical, dynamical, and mechanical stability from state-of-the-art theoretical calculations. Compared with the zero bandgaps in pristine graphene, our graphene metamaterial exhibits a gap of 107 meV. Using pressure engineering, a tunable bandgap in the range of 0–260 meV was obtained, enabling our graphene metamaterial huge potential in semiconductor-based modern electronic devices. Anisotropic Young's moduli of over 1 TPa along the [010] direction and 413 GPa along the [001] direction were demonstrated. The in-plane Young's modulus (1085 GPa) is higher than that of the state-of-the-art technical ceramics such as SiC (about 425 GPa), Al2O3 (about 400 GPa), and Si3N4 (about 300 GPa), and the out-of-plane Young's modulus (413 GPa) is significantly increased comparing with monolayer graphene (about 2 GPa). Significant enhancement of broadband electromagnetic absorption for the visible (400–800 nm) and infrared light (1–6 μm) was achieved with a value of 50–1000 times higher than that of monolayer graphene, which promises the present graphene metamaterial a potential building block for photonic and optoelectronic devices.

Effects of Grain Size on the Thermoelectric Properties of Cu2SnS3: An Experimental and First-Principles Study

Cu–Sn-based sulfides are earth-abundant and nontoxic compounds of special interest for low-cost energy harvesting applications. In the present work, we have investigated the effect of grain size on the thermoelectric properties of Cu2SnS3 (CTS). Three dense CTS samples with nanometric grains were produced by mechanical alloying combined with spark plasma sintering, preserving the small size of crystalline domains to 12, 25, and 37 nm, respectively. The experimental results show that the Seebeck coefficient (S) and electrical resistivity (ρ) decrease with decreasing domain sizes, while the thermal conductivity (κ) increases. A smaller domain size correlates with a lower resistivity and a degenerate semiconductor-like behavior due to higher carrier concentration. At the same time, our synthesis method leads to materials with very low lattice thermal conductivity, thanks to the nanometric size of grains and structural disorder. As a result, the sample with the smallest grain size exhibits the highest zT of ∼0.4 at 650 K. First-principles density functional theory (DFT) simulations on various CTS crystallite surfaces revealed localized states near the Fermi level and the absence of band gap, indicating the metallic nature of the surfaces. Various CTS systems were tested by DFT, showing the following order of increasing formation energy: stoichiometric CTS, Cu vacancy, Cu-rich, Sn vacancy, and Sn-rich.

Dynamical stability, vibrational, and optical properties of anti-perovskite A3BX (Ti3TlN, Ni3SnN, and Co3AlC) phases: A first principles study

We have investigated various physical properties including phonon dispersion, thermodynamic parameters, optical constants, Fermi surface, Mulliken bond population, theoretical Vickers hardness, and damage tolerance of anti-perovskite A3BX phases for the first time by employing density functional theory methodology based on the first principles method. Initially, we assessed nine A3BX phases in total and found that only three phases (Ti3TlN, Ni3SnN, and Co3AlC) are mechanically and dynamically stable based on the analysis of computed elastic constants and phonon dispersion along with phonon density of states. We revisited the structural, elastic, and electronic properties of the compounds to judge the reliability of our calculations. The absence of bandgap at the Fermi level characterizes the phases under consideration as metallic in nature. The values of Pugh ratio, Poisson’s ratio, and Cauchy factor have predicted the ductile nature associated with strong metallic bonding in these compounds. A high temperature feasibility study of the phases has also been performed using the thermodynamic properties, such as the free energy, enthalpy, entropy, heat capacity, and Debye temperature. The Vickers hardness of the compounds is estimated to be ∼4 GPa, which is comparable to many well-known MAX phases, indicating their reasonable hardness and easily machinable nature. The static refractive index n(0) has been found around ∼8.0 for the phases under study that appeals as a potential candidate to design optoelectronics appliances. The reflectivity is found above 44% for the Ti3TlN compound in the energy range of 0 eV–14.8 eV, demonstrating that this material holds significant promise as a coating agent to avoid solar heating.

Modulation of Negative Differential Resistance in Graphene Field-Effect Transistors by Tuning the Contact Resistances

Graphene and molybdenum disulfide are two-dimensional novel materials considered promising for nanoscale electronic devices. Due to high carrier mobility and in spite of lacking a bandgap, nanoscale graphene transistors have been demonstrated to reach a cut-off frequency above 400 GHz. The absence of bandgap in graphene leads to a remarkable band-to-band tunneling property in electron devices with negative differential resistance. Ultra-thin field-effect transistors fabricated with graphene as gate conducting channels have been shown experimentally to exhibit negative differential resistance (NDR) with widespread appeal for both digital and analog electronics. NDR devices like the Esaki p–n junction have been known to have applications for high frequency oscillators, fast logic switches, memories and low-power amplifiers. In this work, a semi-analytical model equation for transfer characteristics of graphene transistors is developed to successfully model the NDR. Data from three known experimental devices exhibiting NDR with gate length from 500 nm to 3 μm are shown to match well with theoretical modeled results. Numerical calculations using the model equation show that at a fixed gate bias, NDR can be modulated by tuning the value of contact resistance. The result also shows that separate onset of NDR in purely electron current or hole current can be modeled with this equation and matches experimental data.

Electric Field Tunable Band Gap in Bi-Axially Strained Graphene/Hexagonal Boron Nitride Super-Lattice

In this work, we have reported the tunable band gap of single-layer graphene (SLG) on hexagonal boron nitride (h-BN) monolayer arranged in B3 super-lattice form, as a function of an external electric field perpendicular to the super-lattice plane and an applied external bidirectional stress from the first principle Density Functional theory (DFT). Graphene with its intrinsic high electron mobility poses itself very promising for electronic device application, but absence of band gap and substrate induced mobility degradation limits its application in logic transistor. To reduce the effect of substrate related performance degradation graphene placed on h-BN has been reported in literature. Three super-lattice structures of this bilayer system have been proposed in literature where B3 is reported to have the greatest intrinsic band gap opening among the three but this is not sufficient for logic transistor. To tailor the band gap perpendicular electric field and biaxial stress was applied in this work. It was observed that band gap increases (65.45 meV – 84.5 meV) from compressive to tensile biaxial strain (-5% to +5%) when no electric field was applied. With the application of 5×109 V/m external electric field a band gap of 144.5 meV was found which is promising for logic FET. However, tensile stress and external electric field causes degradation in Fermi velocity and produces higher electron effective mass. Figure 1

Chiral tunneling, tunneling times, and Hartman effect in bilayer graphene

We investigate the chiral tunneling in a bilayer graphene $n\text{\ensuremath{-}}p\text{\ensuremath{-}}n$ junction in time aspect. The phase and dwell times are evaluated for various situations, including the effects of trigonal warping and band gap due to an external gate field. In the absence of band gap and for normal incidence, the chirality induces Klein effects; when the trigonal warping is excluded, the tunneling times are the same as the ordinary barrier tunneling, but including it leads to a perfect reflection and a new type of phase time is found. In the presence of band gap and/or for an oblique incidence, the Klein effects disappear and the tunneling times have peaks corresponding to resonant transmission maxima due to the Fabry-P\`erot-type interference of the oscillating waves allowed inside the barrier. The trigonal warping also leads to valley-polarized transmission for an oblique incidence in the absence or presence of band gap. As a result, the tunneling times and scattering angles of particles from one valley are different from the other valley. We observe that the Hartman effect exists only when the chirality selects pure evanescent waves as transmission channels.

Crystal structure, electronic and elastic properties for novel Hf 3AlN and Zr 3AlN ceramics explored by first principles studies

Investigations into crystal structure, electronic and elastic properties of M 3AlN (M=Hf, Zr) had been conducted by plane-wave pseudopotential calculations. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of these two compounds. The charge density distributions and density of states indicate that there exist relatively soft Al–M and strong N–M covalent bonds, which might be contributed to layered chemical bonding character of M 3AlN. By analyzing Cauchy pressure and the bulk modulus to C 44 ratio, Hf 3AlN was predicted to be more ductile than Zr 3AlN.

First-principles study of structural, electronic and elastic properties of single crystal CuZr

Investigations into the structural, electronic and elastic properties of intermetallic CuZr compound had been conducted by the plane-wave pseudopotential method. The calculated lattice constant was consistent well with the experimental value. The absence of band gap and finite value of the density of states (DOS) at the Fermi level reveal the metallic behavior of CuZr crystal, and Zr 4d states give rise to the electrical conductivity. The calculated elastic constants for single crystal CuZr at zero pressure obey the cubic mechanical stability condition, which indicates that the cubic CuZr crystal is mechanical stable at zero pressure. By analyzing the ratio between the bulk and shear moduli, we conclude that CuZr crystal is ductile in nature. The present theoretical investigations might give prediction to polycrystalline CuZr system.

Effect of high hydrostatic pressure on structural stability of Ti 3GeC 2: A first-principles investigation

An investigation into the structural stability, electronic and elastic properties of Ti 3GeC 2 under high hydrostatic pressure was conducted using first-principles calculations based on density functional theory (DFT). From the energy and enthalpy calculations, and the variations of elastic constants with pressure, we conclude that α ‐ Ti 3 GeC 2 is most stable upon compression to 100 GPa, which is not consistent with the nonhydrostatic in situ synchrotron X-ray diffraction studies. The higher structural stability was analyzed in terms of electronic level. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of all polymorphs of Ti 3GeC 2.

First-principles study on electronic structure and elastic properties of hexagonal Zr 2Sc

An investigation into the equation of state (EOS), electronic and elastic properties of Zr 2SC has been conducted by first-principles pseudopotential calculations. The calculated EOS is well consistent with the recent experimental reports. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of Zr 2SC. From the variations of elastic constants with pressure, we find that hexagonal Zr 2SC is most stable in the pressure range from 0 to 100 GPa, which is consistent with the experimental observations. The strong hybridization of Zr 4d states, S 3p states and C 2p states and the presence of pseudogap stabilize the structure of Zr 2SC. By analyzing the ratio between the bulk and shear moduli, we conclude that Zr 2SC is brittle in nature. The mechanism of brittleness of Zr 2SC originated from the large value of Zr atom occupying the internal parameter z.

Electronic Structure and Elastic Properties of Ti3AlC from First-Principles Calculations

We perform a first-principles study on the electronic structure and elastic properties of Ti3AlC with an antiperovskite structure. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of this compound. The elastic constants of Ti3AlC are derived yielding c11 = 356 GPa, c11 = 55 GPa, c44 = 157 GPa. The bulk modulus B, shear modulus G and Young's modulus E are determined to be 156, 151 and 342 GPa, respectively. These properties are compared with those of Ti3AlC2 and Ti2AlC with a layered structure in the Ti-Al-C system and Fe3AlC with the same antiperovskite structure.

First-principles study on electronic structure and elastic properties of Ti 2SC

We have performed first-principles study on electronic structure and elastic properties of Ti 2SC. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of this compound. The five independent elastic constants were derived and the bulk modulus, Young's modulus, shear modulus, and Poisson's ratio were determined. The high bulk modulus and hardness was found to be originated from the strong Ti 3 d–S 2 p hybridization. Such strong M A bonding is unusual in the MAX phases studied so far. Ti 2SC is elastically stable and exhibits highly elastic isotropy.

De Haas-van Alphen effect and the Fermi surface in κ-(BEDT-TTF)2I3

The de-Haas-van Alphen effect and the Fermi surface have been studied in the κ-typ organic superconductor (BEDT-TTF)2I3. The originally proposed Fermi surface using the tight-binding band calculation is different from the ones reported afterwards for the other κ-type salts. The Fermi surface has simple 2-dimensional cylindrical shape and no band gap exists at the zone boundary due to the crystal symmetry. The absence of band gap at the boundary is known as the band sticking effect. We confirmed the band sticking effect experimentally using the magnetization measurement. The observed quantum oscillation frequency corresponds to the 100% area of the first Brillouin zone. The Fourier analysis has revealed another frequency corresponding to the 20% area of the first zone other than the fundamental one. This frequency corresponds to the one reported for the other κ-type salts (BEDT-TTF)2Ag(CN)2H2O and (BEDT-TTF)2Cu(NCS)2. The effective mass of the fundamental frequency and the Dingle temperature are 2.7m0 and 0.9 K respectively. The many body renormalization effect has benn discussed considering mass enhancement and strong coupling model for the superconductor.