Band Gap Opening Research Articles

Topological design of soft substrate acoustic metamaterial for mechanical tuning of sound propagation

The design of tunable acoustic waves is crucial in phononic crystals (PnCs), as acoustic waves exhibit continuous variation across diverse frequency ranges in practical applications. While there have been research efforts on tunable PnCs, existing design strategies predominantly rely on predetermined topology and scatterer shapes based on empirical experience. This reliance poses challenges in ensuring customized and reversible tuning of the bandgap. In this work, we present a customized tunable PnC design model and solution strategy based on soft substrates, which enables reversible tuning of a specific frequency/order bandgap. The designed structure consists of a soft substrate and crystalline columns dispersed in the soft substrate and air. The relative positions of the scatterers in the air are changed by stretching the substrate, thereby realizing the modulation of sound propagation. Since soft materials have both material nonlinearities and complex geometrical variations that are difficult to obtain design sensitivity information, the material-field series expansion topology optimization approach is employed to achieve custom tunable design of the bandgap. The paper presents simulation-based analyses and experimental verification of the customized bandgap opening and closing. The results demonstrate a close alignment between theoretical predictions of propagation curves and experimental findings. This concurrence serves as evidence that the soft-substrate PnC, derived through topology optimization, effectively facilitates the adjustment of bandgaps of arbitrary orders and enables switching between various acoustic functions.

Large dynamic scissoring mode displacements coupled to band gap opening in the cubic phase of the methylammonium lead halide perovskites

Hybrid perovskites are a rapidly growing research area, having reached photovoltaic power conversion efficiencies of over 25%. There is a increasing consensus that the structures of these materials, and hence their electronic structures, cannot be understood purely from the time and space averaged crystal structures observable by conventional methods. We apply a symmetry-motivated analysis method to analyse x-ray pair distribution function data of the cubic phases of the hybrid perovskites MAPbX 3 (X = I, Br, Cl). We demonstrate that, even in the cubic phase, the local structure of the inorganic components of MAPbX 3 (X = I, Br, Cl), are dominated by scissoring type deformations of the PbX 6 octahedra. We find these modes to have a larger amplitude than equivalent distortions in the A-site deficient perovskite ScF3 and demonstrate that they show a significant departure from the harmonic approximation. Calculations performed on an inorganic perovskite analogue, FrPbBr3, show that the large amplitudes of the scissoring modes are coupled to a dynamic opening of the electronic band gap. Finally, we use density functional theory calculations to show that the organic MA cations reorientate to accommodate the large amplitude scissoring modes.

A low-frequency pure metal metamaterial absorber with continuously tunable stiffness

To address the incompatibility between high environmental adaptability and deep subwavelength characteristics in conventional local resonance metamaterials, and overcome the deficiencies in the stability of existing active control techniques for band gaps, this paper proposes a design method of pure metal vibration damping metamaterial with continuously tunable stiffness for wideband elastic wave absorption. We design a dual-helix narrow-slit pure metal metamaterial unit, which possesses the triple advantage of high spatial compactness, low stiffness characteristics, and high structural stability, enabling the opening of elastic flexural band gaps in the low-frequency range. Similar to the principle of a sliding rheostat, the introduction of continuously sliding plug-ins into the helical slits enables the continuous variation of the stiffness of the metamaterial unit, achieving a continuously tunable band gap effect. This successfully extends the effective band gap by more than ten times. The experimental results indicate that this metamaterial unit can be used as an additional vibration absorber to absorb the low-frequency vibration energy effectively. Furthermore, it advances the metamaterial absorbers from a purely passive narrowband design to a wideband tunable one. The pure metal double-helix metamaterials retain the subwavelength properties of metamaterials and are suitable for deployment in harsh environments. Simultaneously, by adjusting its stiffness, it substantially broadens the effective band gap range, presenting promising potential applications in various mechanical equipment operating under adverse conditions.

Unlocking and optimizing the thermodynamic potential of tetragonal Si and Ge: Strategies and insights from a tight-binding simulation framework for material engineering

This study theoretically explores the effects of external fields and doping on the electronic and thermoelectric properties of tetragonal silicon (T-Si) and tetragonal germanium (T-Ge) structures using a tight-binding model, Green’s function formalism, and the Kubo formula. External parameters modify the positions and intensities of the density of states (DOS) peaks, influencing charge carrier excitation and conductivity trends. Applying bias voltage introduces zero-intensity regions in thermal conductivity due to band gap opening, reducing thermal properties. In contrast, chemical potential and magnetic field significantly enhance thermal properties by increasing intensity and shifting the peak position. T-Ge exhibits higher thermal properties than T-Si across the selected parameter ranges. The findings highlight the potential for optimizing and enhancing the thermoelectric properties of tetragonal structures through external controllable parameters.

Optimizing optoelectronic performance: impact of polar-terminated zinc oxide on MoS2 Van der Waals heterostructure

We report a study on the stability, optical absorption and modulated electronic properties of the ZnO(000 1― ) and MoS2 Van der Waals heterostructure using density functional theory. We employed a supercell of ZnO/MoS2 hybrid and specifically explored the effects of creating an interface with the O-terminated face of ZnO while considering the interlayer interaction. We observed an increase in the band gap opening of MoS2 within the hybrid structure (1.37 eV) is primarily attributed to in-plane strain, with minimal contribution from the identified charge transfer occurring from MoS2 to ZnO. Notably, the hybrid structure exhibits enhanced photo absorption in the visible and near-infrared regions, highlighting their significance for optoelectronic applications.

Comprehensive analysis of band gap modulation of hexagonal fan blade and optimized ligament structure in the low-frequency range

This paper proposed a meta material model with low-frequency wide band gap and band gap tunability, optimized its structure as a multibranch chain structure, and analyzed the band gap relationship of above two models based on Brag's theorem and finite element method. Among them, the fan base structure has a band gap of 65.92 % within 20,000 Hz, and the band gap is optimized to low-frequency after the structure is optimized, and the band gap reaches 75.94 % within 10,000 Hz. The effects of the ligament width and the thickness of the center parcel layer on the band gap distribution of the structure are also discussed, and the wave transmission characteristics in the structure are explored by group and phase velocities to verify the acoustic performance of the structure. The results show that the smaller the ligament thickness is, the lower the first band gap opening frequency of the structure is; when the thickness of the central wrapping layer is increased, the structure develops the band gap at the center frequency of 3000 Hz to a lower frequency and generates an ultra-wide band gap at the center frequency of 6000 Hz. These findings can provide a new idea for low-frequency vibration isolation and noise reduction.

Vibration attenuation and wave propagation analysis of 3D star-shaped resonant plate structures and their derivatives with ultra-wide band gap

Based on the local resonance effect of elastic waves, three single-phase acoustic metamaterials are proposed in this paper. Based on the finite element method and Bloch's theorem, the energy band structure diagrams and vibration modes are plotted, and the band gap properties and band gap opening mechanism of these structures are explored. New structures possessing lower frequency band gaps are obtained by topological optimization. The transmission curves verify the accuracy of the band gap and the vibration attenuation ability of the structure. Finally, the structural parameters were adjusted and the effect of each parameter change on the band gap characteristics was analyzed. The results show that the proposed structure has a maximum band gap coverage of 72.4 % and a strongest attenuation peak of less than −400 (dB) due to the occurrence of a local resonance. This paper provides a methodology for analyzing the vibration and noise reduction performance of single-phase material phononic crystals, as well as a three-dimensional phononic crystal with potential for practical applications.

Novel three-dimensional acoustic metamaterials with low-frequency bandgap and strong vibration and noise attenuation

The suppression of low-frequency vibration and noise has been the research focus of acoustic metamaterials. Based on the local resonance effect of the subwavelength structure, two new topology optimization methods are proposed in this paper, and two new three-dimensional single-phase material structures are obtained. Based on Bloch theorem and lattice theory, finite element simulations of the single cells with different structures are carried out to analyze and explore the bandgap properties and opening mechanism of the structures. The frequency response spectra are obtained by vibrational transport simulation of the finite-period structures. The results show that both novel metamaterial structures have good bandgap properties, the widest first bandgap width within 2000 Hz reaches 809 Hz, and the total bandgap width percentage reaches 52.7 %, and the main vibration forms of opening the bandgap are rotational and offset resonance of the internal structure and torsional resonance of part of the structure.

Effect of N doping on Electronic Band Structure and Magnetic Moments in Graphene Sheets: Density Functional Theory Study

Abstract Graphene has become a material that is often discussed recently because of its unique properties. One of the obstacles in using graphene as a nanoelectronic device is its zero band gap energy. One effort that can be made to open this energy gap is by substituting N atoms. In this research, spin-polarization density functional theory is studied. We simulated 4 x 4 single-layer graphene supercells with varying numbers of N atom substitutions (N = 0, 1, 2 atoms). The GGA – PBE function is used in this modeling to complement the exchange – correlation potential. The result obtained is an open energy band gap when the Nitrogen atom is substituted. Nitrogen substituted in the form of graphite – N also causes the emergence of a magnetic moment in graphene, with a Fermi energy value for pristine graphene of -0.4607 eV, for G1N of -1.6140 eV, and for G2N of -1.3346 eV. These results pave the way for research and development of graphene in its application as electronic nanodevices.

Magnonic bandgap openings and in-gap propagating states in domain-wall waveguides induced by periodic modulations

Magnonic bandgap openings and in-gap propagating states in domain-wall waveguides induced by periodic modulations

Ab-initio investigation of the structural, electronic and optical properties of CsPbBr3 perovskite under pressure using the DFT-1/2 method

Pressure-induced phase transitions in metal halide perovskites lead to significantly different material properties and offer great potential for diverse applications. The electronic, structural, and optical properties of Cesium lead bromide (CsPbBr3) crystals are investigated using density functional theory (DFT) to draw an integrated picture of orbital morphology, absorption spectra, and band gap values. Spin-orbit coupling and DFT-1/2 correction are used to gain a better insight into electronic properties and band splitting in the cubic phase. Calculations of pressure-induced phase transitions, electronic band gap engineering, and manipulation of optical absorption edges of CsPbBr3 are conducted. In the cubic and tetragonal crystal lattices, a band gap closure and opening occurs demonstrating a Dirac cone upon application and increase of pressure. Indeed, in the tetragonal lattice of CsPbBr3, a topological band occurs. Orthorhombic perovskite crystal lattice undergoes successive direct-indirect and indirect-direct band gap transitions, followed by a space group change at extreme pressures. However, under pressure, orthorhombic non-perovskite lattice experiences a sudden band gap breakdown. Phase and band gap transitions are explained by orbital hybridizations and structural symmetry evolution (e.g., bond length contraction/dilation and octahedral cage rotation/distortion). These results provide comprehensive guidelines for further experimental studies on pressure engineering, advanced photovoltaic and optoelectronic applications, discovering and understanding new topological band structures of perovskite materials.

Theoretical Characterization of Germanene Doped with Main Group Elements.

Herein, using density functional calculations, we studied the substitutional doping in germanene with B, C, N, O, Al, Si, P, S, Ga, As, and Se. Nitrogen is the element that can be more easily incorporated into the germanene lattice, followed by silicon, carbon, and boron. Almost all dopants were efficient in opening a band-gap. Yet, caution should be taken because this opening strongly depends on the dopant concentration. Carbon and sulfur were the most effective elements for band-gap opening. C-doping generates the lowest effective masses (me*/m0=mh*/m0=0.09). The equal me and mh values indicate an intrinsic semiconductor behavior, a characteristic shared by the chalcogenides-doped systems. Additionally, we performed a detailed analysis of the preferred disposition of dopants in the germanene lattice. In contrast with the results obtained for graphene, when multiple atoms are introduced in the germanene framework, they do not prefer to be agglomerated, adopting a random disposition, except in the case of sulfur and nitrogen, which favored specific dopant arrangement. Two sulfur dopants showed a notorious preference for replacing a Ge-Ge bond but without forming an S-S linkage, thus adopting a thiophene-like structure that may impart germanene exciting properties, as observed for S and N codoped graphene.

Modulation instability and wavenumber bandgap breathers in a time layered phononic lattice

We provide the first experimental realization of wavenumber bandgap (q−gap) breathers. Experiments are obtained in the setting of a time-periodic phononic lattice where the model and experiment exhibit good qualitative agreement. q−gap breathers are localized in time and periodic in space, and are the counterparts to the classical breathers found in space-periodic systems. We derive an exact condition for modulation instability that leads to the opening of wavenumber bandgaps in which the q−gap breathers can arise. The q−gap breathers become more narrow and larger in amplitude as the wavenumber goes further into the bandgap. In the presence of damping, these structures acquire a nonzero, oscillating tail. The controllable temporal localization that q−gap breathers make possible has potential applications in the creation of phononic frequency combs, energy harvesting or acoustic signal processing. Published by the American Physical Society 2024

Band gap opening of metallic single-walled carbon nanotubes via noncovalent symmetry breaking

Covalent bonding interactions determine the energy-momentum (E-k) dispersion (band structure) of solid-state materials. Here, we show that noncovalent interactions can modulate the E-k dispersion near the Fermi level of a low-dimensional nanoscale conductor. We demonstrate that low energy band gaps may be opened in metallic carbon nanotubes through polymer wrapping of the nanotube surface at fixed helical periodicity. Electronic spectral, chiro-optic, potentiometric, electronic device, and work function data corroborate that the magnitude of band gap opening depends on the nature of the polymer electronic structure. Polymer dewrapping reverses the conducting-to-semiconducting phase transition, restoring the native metallic carbon nanotube electronic structure. These results address a long-standing challenge to develop carbon nanotube electronic structures that are not realized through disruption of π conjugation, and establish a roadmap for designing and tuning specialized semiconductors that feature band gaps on the order of a few hundred meV.

Analysis of vibration and noise reduction performance of 3D flower-shaped resonance structure with low-frequency bandgap

In this paper, two single-phase acoustic metamaterials that can produce localized resonance effects are proposed. Dispersion curves and vibrational modes of each structure are plotted using finite element software, and the bandgap properties and bandgap opening mechanism of these structures are explored by comparing the energy band structures and vibrational forms. Then the topological optimization of the flower-shaped structure was performed to obtain two new structures with bandgaps having lower onset frequencies. Finally, the accuracy of the frequency range corresponding to the bandgap is verified by transmission curves and stress cloud diagrams, and the vibration attenuation ability of several structures is analyzed. The results show that each new structure has excellent bandgap properties and vibration attenuation performance due to the local resonance effect, and the topology optimization method of adding spheres to the circular plate has a very good improvement of bandgap properties at low frequencies. This paper provides a means for structure optimization of single-phase material phononic crystals, a method for analyzing the vibration and noise reduction properties of the structures, and several three-dimensional phononic crystals with potential for practical applications.

Blue-Emitting Cs(Pb,Cd)Br3 Nanocrystals Resistant to Electric Field-Induced Ion Segregation.

High-performance solution-processed perovskite light-emitting diodes (PeLEDs) have emerged as a good alternative to the well-established technology of epitaxially grown AIIIBV semiconductor alloys. Colloidal cesium lead halide perovskite nanocrystals (CsPbX3 NCs) exhibit room-temperature excitonic emission that can be spectrally tuned across the entire visible range by varying the content of different halogens at the X-site. Therefore, they present a promising platform for full color display manufacturing. Engineering of highly efficient PeLEDs based on bromide and iodide perovskite NCs emitting green and red light, respectively, does not face major challenges except low operational stability of the devices. Meanwhile, mixed-halide counterparts demonstrating blue luminescence suffer from the electric field-induced phase separation (ion segregation) phenomenon described by the rearrangement (demixing) of mobile halide ions in the crystal lattice. This phenomenon results in an undesirable temporal redshift of the electroluminescence spectrum. However, to realize spectral tuning and, at the same time, address the issue of ion segregation less mobile Cd2+ ion could be introduced in the lattice at Pb2+-site that leads to the band gap opening. Herein, we report an original synthesis of CsPb0.88Cd0.12Br3 perovskite NCs and study their structural and optical properties, in particular electroluminescence. Multilayer PeLEDs based on the obtained NCs exhibit single-peak emission centered at 485 nm along with no noticeable change in the spectral line shape for 30 min which is a significant improvement of the device performance.

Structural, electronic and thermoelectric properties of monolayer TiSe2.

In this work the first-principles calculations of the structural, electronic and thermoelectric properties of monolayer TiSe2 are presented. The optimized lattice parameter of monolayer TiSe2 shows excellent agreement with the experimental value. The computed band structure and density of states calculations predict metallic nature of monolayer TiSe2 with overlapping of 0.44eV between the lowest conduction band and top valance band at high symmetry point M. The position of pseudogap formed by Ti-3d orbitals near the Fermi level confirms the mechanical stability of monolayer TiSe2. Due to the influence of positive strain (tensile strain), the Ti-Se bond length increases and the layer height decreases. The applied tensile strain changes the metallic nature of TiSe2 into a semiconductor with opening of bandgap. It has also been observed that the positions of conduction band minima and valance band maxima change with strain. The charge analysis shows that charge transfer from Ti to Se atom increases when tensile strain is applied, while an opposite trend is observed with compression. The computed thermoelectric coefficientsi.e. Seeback coefficient, power factor and figure of merit are in good agreement with the experimental data.The temperature dependence of these coefficientsis also reported. The density functional theory based calculations are reported employing the PBE-GGA ansatz using the plane wave-pseudopotential method embodied in the Quantum ESPRESSO package. The self-consistent field calculations are performed over a dense Monkhorst-Pack net of 12 × 12 × 1k-points. The energy convergence criteria for the self-consistent field calculation were set to 10-6 Ry/atom with a cutoff energy of 90 Ry. The thermoelectric properties are computed by combining the band structure calculations with the Boltzmann transport equation using Boltztrap2 peckage.

First principle studies on half-metallicity and pressure-induced half-metallic band gap in 3-d transition metal-based quaternary Heusler ScVCoZ (Z = Si, Ge, Sn, P, As, and Sb) alloys

In the present work, we predict a series of 3-d transition metals-based quaternary Heusler alloys (QHA) ScVCoZ (Z = Si, Ge, Sn, P, As, and Sb) (SVCZ), by employing density functional theory (DFT) calculations. We carried out extensive band structure (BS) and density of states (DOS) analysis to explain the half-metallic and magnetic behavior of the alloys. We found that SVCZ alloys stabilize in the F 4̅ 3m space group and exhibit a ferromagnetic (FM) ground state (GS). Among the QHAs, SVCZ (Z = Si, Ge, and Sb) exhibit intriguing half-metallic properties featuring 100% spin polarization at the Fermi level (EF). Additionally, the effect of pressure indicates a band gap opening in one of the spin channels for SCVZ (Z = P, and As), and the half-metallic characteristics are retained up to a compressive pressure, respectively of 65, and 79 GPa for SCVP and SCVAs. Elastic properties calculations suggest SVCZ alloys satisfy Born stability criteria and are mechanically stable. Among the predicted QHAs, SVCZ (Z = Si, Ge, and Sn) shows brittle nature, whereas SVCZ (Z = P, As, and Sb) are ductile.

Strain-tuned optical properties of bilayer silicon at midinfrared wavelengths

Optical properties of two-dimensional bilayer silicon have been explored at midinfrared wavelengths using density functional theory. In this work, progressive atomic structural deformation and the resultant variations in the optical properties of the bilayer silicon films were investigated under external in-plane strain. A phase transformation of the atomic structure has been observed at an applied in-plane tensile strain of 5.17%, at which the atomic lattice is changed from a low buckled to a buckle-free honeycomb structure. Evaluations of the optical properties were carried out by taking into account the inter- and intraband transitions. An abrupt change in the optical refraction index was observed at the phase transition. In addition, the buckle-free honeycomb structure presents a strain-resistive absorption edge pinned at 1.14 μm wavelength. Exceeding a strain threshold of 12.26% results in the development of both direct- and indirect-energy bandgap openings. The direct bandgap induced interband optical transitions, resulting in absorption peaks at midinfrared wavelengths and a drastic increase in the refraction index. Moreover, by adjusting the strain, the optical absorptions can be tuned in a wide range of wavelength at midinfrared from 1.5 to 11.5 μm.

Spin-Selective Memtransistors with Magnetized Graphene.

Spin-polarized bands in pristine and proximity-induced magnetic materials are promising building blocks for future devices. Conceptually new memory, logic, and neuromorphic devices are conceived based on atomically thin magnetic materials and the manipulation of their spin-polarized bands via electrical and optical methods. A critical remaining issue is the direct probe and the optimized use of the magnetic coupling effect in van der Waals heterostructures, which requires further delicate design of atomically thin magnetic materials and devices. Here, a spin-selective memtransistor with magnetized single-layered graphene on a reactive antiferromagnetic material, CrI3, is reported. The spin-dependent hybridization between graphene and CrI3 atomic layers enables the spin-selective bandgap opening in the single-layered graphene and the electric field control of magnetization in a specific CrI3 layer. The microscopic working principle is clarified by the first-principles calculations and theoretical analysis of the transport data. Reliable memtransistor operations (i.e., memory and logic device-combined operations), as well as a spin-selective probe of Landau levels in the magnetized graphene, are achieved by using the subtle manipulation of the magnetic proximity effect via electrical means.