Bulk Band Gap Research Articles

Unraveling the obscure electronic transition and tuning of Fermi level in Cu substituted Bi2Te3 compound

Observation of a broad peak in the temperature (T) dependent resistivity (ρ) in Cu substituted Bi2Te3 has been reported earlier, though its origin remains obscured to date. Our investigation reveals that an electronic phase transition accompanied by Fermi level tuning primarily contributes toward the observed ρ(T). The analysis reveals that tuning of the Fermi level affects bulk transport phenomena to produce the unique ρ(T). We have proposed a model to show how the bulk bandgap can affect ρ(T). We have shown that bulk carrier contribution has a greater role in producing such ρ(T), and a detailed discussion supported by temperature dependent ARPES study, Raman study, and XPS study as well as structural study conducted by x-ray diffraction has been presented.

Disordered transmission-line networks with and without parity symmetry

Topological states are useful because they are robust against disorder and imperfection. In this study, we consider the effect of disorder and the breaking of parity symmetry on a topological network system in which the edge states are protected by Chern numbers. In the absence of periodicity, the local Chern number is adopted to characterize the topological features of the network. Our numerical results show that the local Chern number and the edge states are very robust against onsite disorder as long as the gap of the bulk state continuum remains open and survives even when the bulk band gap is closed. Breaking the parity symmetry can destroy the quantization of local Chern numbers, compromising the existence of edge modes. We observed non-integer local Chern number peaks that are non-zero inside the bulk bands but these non-zero non-integral local Chern numbers are not associated with the existence of robust edge states.

Molecular precursor-mediated facile synthesis of photo-responsive stibnite Sb2S3 nanorods and tetrahedrite Cu12Sb4S13 nanocrystals.

Stibnite Sb2S3 and tetrahedrite Cu12Sb4S13 nanostructures being economical, environmentally benign and having a high absorption coefficient are highly promising materials for energy conversion applications. However, producing these materials especially tetrahedrite in the phase pure form is a challenging task. In this report we present a structurally characterized single source molecular precursor [Sb(4,6-Me2pymS)3] for the facile synthesis of binary Sb2S3 as well as ternary Cu12Sb4S13 in oleylamine (OAm) at a relatively lower temperature. The as-prepared Sb2S3 and Cu12Sb4S13 nanostructures were thoroughly checked for their phase purity, elemental composition and morphology by powder X-ray diffraction (pXRD), electron dispersive spectroscopy (EDS) and electron microscopy techniques. pXRD and EDS studies confirm the formation of phase pure, crystalline orthorhombic Sb2S3 and cubic Cu12Sb4S13. The SEM, TEM and HRTEM images depict the formation of well-defined nanorods and nearly spherical nanocrystals for Sb2S3 and Cu12Sb4S13, respectively. The Sb2S3 nanorods and Cu12Sb4S13 nanocrystals exhibit an optical bandgap of ∼1.88 and 2.07 eV, respectively, which are slightly blue-shifted relative to their bulk bandgap, indicating the quantum confinement effect. Finally, efficient photoresponsivity and good photo-stability were achieved in the as-prepared Sb2S3 and Cu12Sb4S13 nanostructure-based prototype photo-electrochemical cell, which make them promising candidates for alternative low-cost photon absorber materials.

Second-order topological insulator state in hexagonal lattices and its abundant material candidates

We propose two mechanisms to realize the second order topological insulator (SOTI) state in spinless hexagonal lattices, viz., chemical modification and anti-Kekul\'e/Kekul\'e distortion of hexagonal lattice. Correspondingly, we construct two models and demonstrate the nontrivial band topology of the SOTI state characterized by the second Stiefel-Whitney class $w_2$ in the presence of inversion symmetry ($\textit{P}$) and time-reversal symmetry ($\textit{T}$). Based on the two mechanisms and using first-principles calculations and symmetry analysis, we predict three categories of real light element material candidates, i.e., hydrogenated and halogenated 2D hexagonal group IV materials XY (X=C, Si, Ge, Sn, Y=H, F, Cl), 2D hexagonal group V materials (blue phosphorene, blue arsenene, and black phosphorene, black arsenene), and the recent experimentally synthesized anti-Kekul\'e/Kekul\'e order graphenes and the counterparts of silicene/germanene/stanene. We explicitly demonstrate the nontrivial topological invariants and existence of the protected corner states with fractional charge for these candidates with giant bulk band gap (up to 3.5 eV), which could facilitate the experimental verification by STM. Our approaches and proposed abundant real material candidates will greatly enrich 2D SOTIs and promote their intriguing physics research.

Bulk and edge properties of twisted double bilayer graphene

The emergence of controlled, two-dimensional moiré materials1–6 has uncovered a new platform for investigating topological physics7–9. Twisted double bilayer graphene has been predicted to host a topologically non-trivial gapped phase with Chern number equal to two at charge neutrality, when half the flat bands are filled8,9. However, it can be difficult to diagnose topological states using a single measurement because it is ideal to probe the bulk and edge properties at the same time. Here we report a combination of chemical potential measurements, transport measurements and theoretical calculations that show that twisted double bilayer graphene can host metallic edge transport in addition to simultaneously being insulating in the bulk. A Landauer–Büttiker analysis of the measurements on multi-terminal samples allows us to quantitatively assess the edge-state scattering. We interpret these results as signatures of the predicted topological phase at charge neutrality, but further characterization of the edge transport is required to be certain. Twisted double bilayer graphene is predicted to be a topological insulator under certain conditions. Simultaneous bulk and edge measurements now show metallic transport with a bulk bandgap, suggestive of this prediction.

Computational study of the energy landscape of water on the ThO2 {111} surface

The bulk and surface properties of ThO2 are studied computationally using density functional theory within the LSDA+U approach. The computational method is benchmarked against bulk lattice parameter and band gap, the best combined description of which is with the PBE functional and the Liechtenstein method with U = 7 and J = 1, and without spin-orbit coupling. The optimised computational settings are used to study the reaction of water with the {111} surface, which we find to be hydrophilic. Molecular adsorption of a single water molecule is energetically favourable, and dissociation of this water to form surface hydroxyl groups is barrierless. This wetted surface can aid further water adsorption, with an even lower free energy difference than the first adsorption, although dissociation of the second water is much harder than the first. Reaction of H+, H. radical or OH− groups with the wetted {111} surface has a substantial energy barrier, and reaction with H+ is very sensitive to temperature.

Size quantization effect in water-dispersible LEEH capped ZnSe nanocrystals

Herein we report the synthesis of LEEH capped water-dispersible ZnSe (Zinc Selenide) nanocrystals using a low temp (80 °C) solution processable chemical route. The reaction parameters such as reaction time, temperature, concentrations of reactants and capping agent were tuned to achieve water dispersible ZnSe nanocrystals of average size, 2 nm with improved photophysical properties. The structure, size and optical properties of the nanocrystals are evaluated from XRD, HRTEM, optical absorption and photoluminescence studies. Both HRTEM and XRD analysis indicate polycrystalline cubic phase of ZnSe with size in the range of 2.0 nm. A strong excitonic absorption peak appearing at 385 nm in the absorption spectrum suggests the size quantization effect in ZnSe nanocrystals marked by a large blue shift from its bulk band gap. Photoluminescence of ZnSe nanocrystals at 300 K exhibit multiband spectral feature appearing with a low intensity peak at 414 nm leading to a large nonresonant stokes shift by 29 nm.

Numerical simulation of novel lead-free Cs3Sb2Br9 absorber-based highly efficient perovskite solar cell

In this paper, a novel Cs 3 Sb 2 Br 9 perovskite is presented for the absorber layer of perovskite solar cells (PSC). A planar structure as ITO/TiO 2 /Absorber/Spiro-OMeTAD/Au with Cs 3 Sb 2 Br 9 as a light harvesting material is numerically simulated using SCAPS-1D simulation software. Initially, the optimization is performed on the various material parameters of absorber layer such as thickness, bulk defect density, relative permittivity and bandgap. The obtained optimal values for these parameters are 1000 nm, 1 × 10 12 cm −3 , 10, and 2.0 eV, respectively. Finally, the proposed PSC structure with optimized absorber layer is achieved the maximum power conversion efficiency (PCE) of 15.69% whereas V OC as 1.31 V, J SC as 13.67 mA/cm 2 , FF as 87.61%, and QE of ∼100% at 390 nm wavelength near the visible range of solar spectrum. The obtained results of numerical simulations with Cs 3 Sb 2 Br 9 as absorber layer endorse the novel proposed PSC material as the main body of PSC and opens a better route towards the development of a cost effective, highly efficient (Pb) lead-free and environment friendly PSC. • A novel Cs 3 Sb 2 Br 9 absorber layer material is proposed for design of high efficiency lead-free perovskite solar cell. • Numerical analysis and optimization of novel absorber in proposed PSC structure is performed using SCAPS-1D to investigate the effect of bulk defect density, absorber thickness, energy bandgap and permittivity variations on PSC parameters. • A planar structure consisting of charge transport materials TiO 2 and Spiro-OMeTAD used as ETL and HTL, is simulated for the analysis. • The proposed PSC is promising and may offer the PCE of 15.69% with V OC of 1.31 V , J SC of 13.67 mA/cm 2 and FF of 86.78%.

Effect of Dilute Magnetism in a Topological Insulator

Three-dimensional (3D) topological insulator (TI) has emerged as a unique state of quantum matter and generated enormous interests in condensed matter physics. The surfaces of a 3D TI consist of a massless Dirac cone, which is characterized by the Z2topological invariant. Introduction of magnetism on the surface of a TI is essential to realize the quantum anomalous Hall effect and other novel magneto-electric phenomena. Here, by using a combination of first-principles calculations, magneto-transport and angle-resolved photoemission spectroscopy (ARPES), we study the electronic properties of gadolinium (Gd)-doped Sb2Te3. Our study shows that Gd doped Sb2Te3is a spin-orbit-induced bulk band-gap material, whose surface is characterized by a single topological surface state. Our results provide a new platform to investigate the interactions between dilute magnetism and topology in magnetic doped topological materials.

A -type antiferromagnetic order and magnetic phase diagram of the trigonal Eu spin- 72 triangular-lattice compound EuSn2As2

The trigonal compound EuSn2As2 was recently discovered to host Dirac surface states within the bulk band gap and orders antiferromagnetically below the Neel temperature TN = 24 K. Here the magnetic ground state of single-crystal EuSn2As2 and the evolution of its properties versus temperature T and applied magnetic field H are reported. Included are zero-field single-crystal neutron-diffraction measurements versus T, magnetization M(H,T), magnetic susceptibility chi(H,T) = M(T)/H, heat capacity Cp(H,T), and electrical resistivity rho(H,T) measurements. The neutron-diffraction and chi(T) measurements both indicate a collinear A-type antiferromagnetic (AFM) structure below TN =23.5(2) K, where the Eu{2+} spins S = 7/2 in a triangular ab-plane layer (hexagonal unit cell) are aligned ferromagnetically in the ab plane whereas the spins in adjacent Eu planes along the c axis are aligned antiferromagnetically. The chi(H{ab},T) and chi(H{c},T) data together indicate a smooth crossover between the collinear AFM alignment and an unknown magnetic structure at H ~ 0.15 T. Dynamic spin fluctuations up to 60 K are evident in the chi(T), Cp(T) and rho(H,T) measurements, a temperature that is more than twice TN. The rho(H,T) of the compound does not reflect a contribution of the topological state, but rather is consistent with a low-carrier-density metal with strong magnetic scattering. The magnetic phase diagrams for both H||c and H||ab in the H-T plane are constructed from the TN(H), chi(H,T), Cp(H,T), and rho(H,T) data.

Ideal type-II Weyl points in twisted one-dimensional dielectric photonic crystals.

We proposed an one-dimensional layer-stacked photonic crystal using anisotropic materials to realize ideal type-II Weyl points. The topological transition from Dirac to Weyl points can be clearly observed by tuning the twist angle between layers. Also, on the interface between the photonic type-II Weyl material and air, gapless surface states have been demonstrated in an incomplete bulk bandgap. By breaking parameter symmetry, these ideal type-II Weyl points would transform into the non-ideal ones, exhibiting topological surface states with single group velocity. Our work may provide a new idea for the realization of photonic semimetal phases by utilizing naturally anisotropic materials.

Aqueous synthesis of mercaptopropionic acid capped ZnSe QDs and investigation of photoluminescence properties with metal doping

An aqueous-based green route has been demonstrated for the preparation of ZnSe quantum dots (QDs) and doping of transition metals in the presence of thiol mercaptopropionic acid (MPA) as growth moderator by refluxing at 100 ​°C. The structure and morphology of synthesized ZnSe quantum dots have been investigated by using X-ray diffraction studies (XRD), Ultraviolet–Visible spectroscopy (UV–vis), Fourier Transform Infrared Spectroscopy (FTIR) and Photoluminescence (PL) Spectroscopy. XRD studies indicate the structure of the quantum dots is cubic with diameters in the range of 4–5 ​nm. Fourier Transform Infrared (FTIR) studies proves that MPA ligands were bound strongly on the ZnSe nanocrystal surface as thiolate. The band gap energy (Eg) was calculated to be 3.8 ​eV which is blue shifted from the standard value of bulk band gap (2.60–2.70eV. Photoluminescence spectra shows broad emission value ranging between 400 and 700 ​nm due to surface defects which has been reduced by doping with transition metals (Fe, Co, Ni, Cd) in aqueous medium. The effective passivation of trap luminescence is done by doping leading to increase in photoluminescence quantum yield specifically with nickel and cadmium doped ZnSe QDs.

Four-band non-Abelian topological insulator and its experimental realization

Very recently, increasing attention has been focused on non-Abelian topological charges, e.g., the quaternion group Q8. Different from Abelian topological band insulators, these systems involve multiple entangled bulk bandgaps and support nontrivial edge states that manifest the non-Abelian topological features. Furthermore, a system with an even or odd number of bands will exhibit a significant difference in non-Abelian topological classification. To date, there has been scant research investigating even-band non-Abelian topological insulators. Here, we both theoretically explore and experimentally realize a four-band PT (inversion and time-reversal) symmetric system, where two new classes of topological charges as well as edge states are comprehensively studied. We illustrate their difference in the four-dimensional (4D) rotation sense on the stereographically projected Clifford tori. We show the evolution of the bulk topology by extending the 1D Hamiltonian onto a 2D plane and provide the accompanying edge state distributions following an analytical method. Our work presents an exhaustive study of four-band non-Abelian topological insulators and paves the way towards other even-band systems.

Edge state mimicking topological behavior in a one-dimensional electrical circuit

For one-dimensional (1D) topological insulators, the edge states always reside in the bulk bandgaps as isolated modes. The emergence and vanishing of these topological edge states are always associated with the closing/reopening of the bulk bandgap and changes in topological invariants. In this work, we discover a special kind of edge state in a 1D electrical circuit, which can appear not only inside the bandgap but also outside the bulk bands with the changing of bulk circuit parameters, resembling Tamm states or Shockley states. We prove analytically that the emergence/vanishing of this edge state and its position relative to the bulk bands depends on the intersections of certain critical frequencies. Specifically, the edge mode in the proposed circuit can be mathematically described by polynomials with roots equal to some critical frequencies in the bulk circuit. From this point of view, the transition of the edge state is uniquely determined by the order of the critical frequencies in the bulk circuit. Such topological behaviors shown by the edge state in the proposed electrical circuit may indicate, in a broader sense, the presence of certain type of topology.

First principles study of the 2D Mo(S1-XTeX)2 TMD alloy adsorbed on an Al-terminated sapphire (0001)-substrate

A first principles study, was performed for a 2D, three atom thick monolayer of the Transition Metal Dichalcogenide (TMD) alloy Mo(S1-XTeX)2 adsorbed on an Al-terminated (0001)-sapphire surface. Bulk composition dependent binding energies and band-gaps, and a partial phase diagram, were calculated, using the cluster expansion method. Although the 3D Mo(S1-XTeX)2 alloy system has a phase diagram that is dominated by S-rich/Te-rich phase separation, the 2D system adsorbed on sapphire is dominated by S:Te-ordering. Five ground-state phases are predicted; all have P1 symmetry, and all disorder via contiuous (2’nd order) transitions. These results indicate that synthesis on the sapphire substrate is favorable for band-gap engineering, in which a continuous single phase solid solution allows continuous band-gap tuning, as a function of bulk composition. Whereas, bulk TMD-synthesis followed by exfoliation favors the formation of two-phase mixtures.

Topological phase transition in chiral graphene nanoribbons: from edge bands to end states

Precise control over the size and shape of graphene nanostructures allows engineering spin-polarized edge and topological states, representing a novel source of non-conventional π-magnetism with promising applications in quantum spintronics. A prerequisite for their emergence is the existence of robust gapped phases, which are difficult to find in extended graphene systems. Here we show that semi-metallic chiral GNRs (chGNRs) narrowed down to nanometer widths undergo a topological phase transition. We fabricated atomically precise chGNRs of different chirality and size by on surface synthesis using predesigned molecular precursors. Combining scanning tunneling microscopy (STM) measurements and theory simulations, we follow the evolution of topological properties and bulk band gap depending on the width, length, and chirality of chGNRs. Our findings represent a new platform for producing topologically protected spin states and demonstrate the potential of connecting chiral edge and defect structure with band engineering.

Direct band gap halide-double-perovskite absorbers for solar cells and light emitting diodes: Ab initio study of bulk and layers

We present results of a state-of-the-art computational study of the atomic and electronic structure of ${\mathrm{Cs}}_{2n+2}{M}_{n}{M}_{n}^{{}^{\ensuremath{'}}}{X}_{6n+2}$ ($M=\mathrm{Cu}$, Ag; ${M}^{\ensuremath{'}}=\mathrm{In}$, Bi; $X=\mathrm{Cl}$, Br, I) layers with up to three-unit-cell thickness ($n=3$) as well as their bulk counterparts in the search for economical and stable halide double perovskites (HDPs) with a direct band gap and strong light absorption. Among the 24 layers we have studied, seven are found to be suitable for light emitting diodes, whereas one is proposed for solar cell applications. Our results on bulk ${\mathrm{Cs}}_{2}\mathrm{Ag}M{}^{\ensuremath{'}}{X}_{6}$ (${M}^{\ensuremath{'}}=\mathrm{In}$, Bi; $X=\mathrm{Cl}$, Br) HDPs agree with previous studies. Interestingly, we find that HDP layers as well as bulk are more stable than their respective lead-halide perovskites. The Cu-based HDPs are cost effective compared with those based on Ag and are found to be equally suitable as absorber materials with a lower band gap. However, bulk ${\mathrm{Cs}}_{2}\mathrm{CuIn}{X}_{6}$ ($X=\mathrm{Cl}$, Br) HDPs are found to be dynamically unstable. Our calculations show a combined effect of confinement and spin-orbit coupling in deciding the nature of the band gap for one (two)-unit-cell thick layers of ${\mathrm{Cs}}_{4}\mathrm{AgBi}{X}_{8}$ (${\mathrm{Cs}}_{8}{\mathrm{Cu}}_{2}{\mathrm{Bi}}_{2}{X}_{14}$). Furthermore, strong confinement effects are found to be crucial in making the thinnest layers of ${\mathrm{Cs}}_{4}\mathrm{CuBi}{X}_{8}$ ($X=\mathrm{Cl}$, Br) direct band gap semiconductors. The reduction in the dimension from three to two dimensions introduces distortions in the layers of the HDPs, and it is more in the case of the Cu-based layers. The nature of the band gap becomes direct for very thin layers of ${\mathrm{Cs}}_{2n+2}{\mathrm{Ag}}_{n}{\mathrm{Bi}}_{n}{X}_{6n+2}$ ($n=1$) and ${\mathrm{Cs}}_{2n+2}{\mathrm{Cu}}_{n}{\mathrm{Bi}}_{n}{X}_{6n+2}$ ($n=1$, 2) compared with indirect band gap in bulk. The HDP layers with a direct band gap and enhanced stability also have a strong absorption coefficient of the order of ${10}^{5}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}1}$, making them interesting for solar cell applications. The effects of surfaces on the electronic structure of direct band gap layers of ${\mathrm{Cs}}_{2n+2}{\mathrm{Cu}}_{n}{\mathrm{Bi}}_{n}{\mathrm{Br}}_{6n+2}$ ($n=1$, 2) and ${\mathrm{Cs}}_{2n+2}{\mathrm{Ag}}_{n}{\mathrm{In}}_{n}{\mathrm{Cl}}_{6n+2}$ ($n=3$) are also shown with surface states. We hope that our results on HDPs will stimulate further research on these materials.

Accurate band gap determination of chemically synthesized cobalt ferrite nanoparticles using diffuse reflectance spectroscopy

Optical band gap of cuboidal Cobalt ferrite (CoF) Nanoparticles (NP), synthesized using wet chemical method, was accurately determined, by combining Diffuse Reflectance Spectroscopy (DRS) of the nanopowder, Schuster-Kubelka-Munk (SKM) formalism and the application of a detailed band gap estimation algorithm, using which absorption onsets and corresponding base line segments from the Tauc plots were precisely identified. Structural and compositional characterizations such as XRD, FE-SEM, TEM and EDX, were used to conclude that the NP exhibited cubic crystal structure with an average crystallite size of 14 nm. The values of indirect and direct band gaps were calculated to be 0.79 ± 0.01 eV and 1.5 ± 0.01 eV, respectively. The band gap shift for the nanocrystallites, from the bulk band gap, estimated using the effective mass model, came up to a value of 0.084 eV, which corresponded well to the experimentally obtained direct band gap of CoF NP. The direct and indirect band gap values were found to be well consistent with the partially inverse spinel crystal structure of the NP, emphasized using magnetization measurement.

Mechanics and strain engineering of bulk and monolayer Bi2O2Se

High-mobility semiconductive ultrathin films of bismuth oxyselenide (Bi2O2Se) have attracted great interest due to their potential applications in advanced electronic and photoelectronic devices. Enthusiasm aside, future success of such applications hinges upon the fundamental understanding of two dimensional (2D) Bi2O2Se, which is far from mature. In particular, unlike many other 2D materials with a van der Waals gap, 2D Bi2O2Se features a weak electrostatic stacking interaction, which gives rise to a significantly different mechanical response. The unique mechanical response of 2D Bi2O2Se has a strong influence on its band gap, a critical property toward its device applications. Here, using ab initio calculations, we investigate the mechanical and electronic responses of both bulk and monolayer Bi2O2Se under uniaxial and biaxial tension as well as those of monolayer Bi2O2Se under in-plane shearing. The distinct mechanical responses of bulk and monolayer Bi2O2Se under different mechanical deformation are explained by the associated study of the evolution of electron localization function and charge density analysis. We reveal that the band gap of both bulk and monolayer Bi2O2Se decreases as the applied tensile strain increases. The critical uniaxial and biaxial tensile strains, above which bulk and monolayer Bi2O2Se transform from being semiconductive to being metallic, are determined. These findings suggest fertile yet largely unexplored opportunities of strain engineering of 2D Bi2O2Se toward new device applications.

Mn-Rich MnSb2 Te4 : A Topological Insulator with Magnetic Gap Closing at High Curie Temperatures of 45-50 K.

Ferromagnetic topological insulators exhibit the quantum anomalous Hall effect, which is potentially useful for high-precision metrology, edge channel spintronics, and topological qubits. The stable 2+ state of Mn enables intrinsic magnetic topological insulators. MnBi2 Te4 is, however, antiferromagnetic with 25 K Néel temperature and is strongly n-doped. In this work, p-type MnSb2 Te4 , previously considered topologically trivial, is shown to be a ferromagnetic topological insulator for a few percent Mn excess. i) Ferromagnetic hysteresis with record Curie temperature of 45-50 K, ii) out-of-plane magnetic anisotropy, iii) a 2D Dirac cone with the Dirac point close to the Fermi level, iv) out-of-plane spin polarization as revealed by photoelectron spectroscopy, and v) a magnetically induced bandgap closing at the Curie temperature, demonstrated by scanning tunneling spectroscopy (STS), are shown. Moreover, a critical exponent of the magnetization β≈ 1 is found, indicating the vicinity of a quantum critical point. Ab initio calculations reveal that Mn-Sb site exchange provides the ferromagnetic interlayer coupling and the slight excess of Mn nearly doubles the Curie temperature. Remaining deviations from the ferromagnetic order open the inverted bulk bandgap and render MnSb2 Te4 a robust topological insulator and new benchmark for magnetic topological insulators.