Bandgap Design Research Articles

Study on vibration bandgap characteristics of a cantilever beam type local resonance unit

This article proposes a novel phononic crystal configuration consisting of a through-hole cantilever beam and a mass block, and conducts numerical analysis and experimental verification on the bandgap characteristics of a two-dimensional periodic array plate containing this configuration. The results indicate that there are multiple bending wave band gaps in the proposed structure, and the formation of the bandgap is due to the coupling between elastic waves in the matrix and the resonance characteristics of the local resonant structure. The width of the bandgap is related to the coupling strength. Further research has also found that the proportion of effective mass of the mode is a criterion for determining whether the mode generates a bandgap. At the same time, the regulation of band gaps by the cell constants and geometric parameters of local resonance units was studied. Based on the above research, by improving the original local resonance structure, more abundant bandgap features were obtained, providing a feasible approach for the design of broadband bandgaps. Finally, the vibration transmission rate of the finite period structural plate was obtained through simulation calculations and experiments, and its attenuation frequency band was basically consistent with the bandgap range, indicating that the structure has good low-frequency vibration reduction performance, which has broad engineering application prospects in the field of vibration and noise reduction.

Outstanding comprehensive energy storage performances in multiscale synergistic regulation-engineered (Bi0.5Na0.5)TiO3-based multilayer ceramics

Environmentally friendly dielectric ceramics with superior energy storage performances (ESP) are strongly demanded in pulsed power capacitor applications. Unfortunately, it is challenging to achieve simultaneously large recoverable energy density (Wrec), high Wrec/E (E as electric field) and broad operating temperature range in those ceramics. Herein we propose a multiscale synergistic regulation strategy to enhance comprehensive ESP of (Bi0.5Na0.5)TiO3 (BNT)-based ceramics. We introduce Ca(Zr0.8Ti0.2)O3 (CZT) into the BNT to increase atomic chaos and lattice distortion, which enhances relaxor behavior and lowers domain-switching barriers, thus effectively reducing remanent polarization. Besides, both suppressed oxygen vacancy and refined submicron grains improve extrinsic resistivity, which works synergistically with the enlarged intrinsic bandgap and multilayer structure design, producing substantially enhanced breakdown strength. Encouragingly, a very large Wrec of 13.4 J/cm3, an ultrahigh Wrec/E of 160 J/(kV∙m2), and a high efficiency η of 90.2 % are simultaneously achieved in the novel (Bi0.5Na0.5)TiO3-Ca(Zr0.8Ti0.2)O3 (BNT-CZT) multilayer ceramics, together with outstanding energy storage stability over a broad working temperature range (20–180 °C). The comprehensive ESP of our multilayer ceramics are superior to most of previously reported lead-free ones. This work provides a feasible pathway for substantially improving comprehensive ESP of lead-free ceramics, and also highlights advanced energy storage potential of the BNT-CZT for high-performance pulsed power applications.

Gradient-induced long-range optical pulling force based on photonic band gap

Optical pulling provides a new degree of freedom in optical manipulation. It is generally believed that long-range optical pulling forces cannot be generated by the gradient of the incident field. Here, we theoretically propose and numerically demonstrate the realization of a long-range optical pulling force stemming from a self-induced gradient field in the manipulated object. In analogy to potential barriers in quantum tunnelling, we use a photonic band gap design in order to obtain the intensity gradients inside a manipulated object placed in a photonic crystal waveguide, thereby achieving a pulling force. Unlike the usual scattering-type optical pulling forces, the proposed gradient-field approach does not require precise elimination of the reflection from the manipulated objects. In particular, the Einstein-Laub formalism is applied to design this unconventional gradient force. The magnitude of the force can be enhanced by a factor of up to 50 at the optical resonance of the manipulated object in the waveguide, making it insensitive to absorption. The developed approach helps to break the limitation of scattering forces to obtain long-range optical pulling for manipulation and sorting of nanoparticles and other nano-objects. The developed principle of using the band gap to obtain a pulling force may also be applied to other types of waves, such as acoustic or water waves, which are important for numerous applications.

Low-frequency band gap design of acoustic metamaterial based on cochlear structure

In this paper, a new chiral spiral structure based on the cochlear structure is proposed. The chiral spiral structure consists of four orthogonally oriented cochlear structures with the same geometric parameters connected at the inner endpoints of the four cochlear structures. Based on the Bloch’s theory and finite element method, the band gap characteristics of the proposed chiral spiral structure are studied. The effects of ligament bending angle (θ), the ratio of arc radius of cochlear contour (α), the ligament thickness (tc ), and the level of the chiral spiral structure (n) on the chiral spiral structure are discussed. The results show that the two-level chiral spiral structure (n= 2) has the best band gap characteristics when θ = 180° and α = 0.45. With the decrease of tc and the increase of n, the opening frequency of the first band gap gradually decreases. When n = 22, the chiral spiral structure has the lowest opening frequency, 1.91 Hz. The existence of the band gap is verified through the low amplitude elastic wave transmission tests. The distribution of the iso-frequency lines indicates that with the increase n, the propagation of elastic waves of the chiral spiral structure shows more distinct directivity, which provides a basis for the propagation control of elastic waves. These findings can provide new design ideas and directions for low-frequency vibration and noise control.

Learning conditional policies for crystal design using offline reinforcement learning

Conservative Q-learning for band-gap conditioned crystal design with DFT evaluations – the model is trained on trajectories constructed from crystals in the Materials Project. Results indicate promising performance for lower band gap targets.

Double-order coupling bandgap design of metamaterial beams and broadband vibration reduction properties

<sec>Local-resonance bandgap and Bragg bandgap can coexist in a metamaterial beam, and their coupling effect can be used to realize ultra-wide bandgap, which has great potential applications in the field of wide-band vibration reduction. Previous studies usually considered the single-order coupling between the local-resonance bandgap and Bragg bandgap in metamaterial beams with a single array of local resonators, which can only achieve the single-order ultra-wide coupling bandgap and cannot meet the wide-band vibration reduction requirements of double/multiple target frequency bands. In this paper, metamaterial beams with double arrays of local resonators are considered, and the regulation design and analysis of double-order coupling of local-resonance and Bragg bandgaps are carried out based on an analytical model of bending wave dispersion relation. Moreover, the vibration reduction characteristics of the double-frequency-resonator metamaterial beams with double-order coupling bandgaps are studied by using spectral element method and the finite element method. The main conclusions are as follows.</sec><sec>1) A design method is proposed for realizing double-order coupling wide bandgap in a metamaterial beam with double arrays of local resonators. By using this method, the resonance frequencies of the local resonators can be quickly designed on conditions that host beam parameters, lattice constant and added mass ratio of the local resonators are given.</sec><sec>2) The double-order coupling bandgaps in a metamaterial beam carrying double arrays of local resonators are compared with the single-order coupling bandgaps in metamaterial beams with a single array of local resonators. It is found that through proper design, the total normalized width of the double-order coupling bandgap can be much broader than that of the single-order coupling bandgap, so the double-order coupling bandgap is more beneficial to wide-band vibration reduction.</sec><sec>3) It is found that for a given total added mass ratio of the double arrays of local resonators, it is necessary to optimize the mass distribution ratio of the double resonators to maximize the total normalized width of double-order coupling bandgap. An approximate formula for designing the optimal mass distribution ratio of the double resonators is further established.</sec><sec>4) The spectral element method is used to study the vibration reduction characteristics of the metamaterial beams carrying double arrays of local resonators designed based on double-order bandgap coupling. The accuracy of the spectral element method is verified by comparing with the finite element method. The results show that significant vibration reduction can be achieved in two wide frequency bands corresponding to the double-order coupling bandgaps. The influences of number of unit cells and resonator damping on the vibration reduction characteristics of the metamaterial beam are further analyzed. It is shown that the increase of number of unit cells can enhance the vibration reduction performance in the bandgap, and the increase of resonator damping can effectively broaden the vibration reduction frequency band.</sec>

Bandgap design of 3D single-phase phononic crystals by geometric-constrained topology optimization

<abstract> <p>Phononic crystals (PnCs) possessing desired bandgaps find many potential applications for elastic wave manipulation. Considering the propagating essence of three-dimensional (3D) elastic waves and the interface influence of multiphase material, the bandgap design of 3D single-phase PnCs is crucial and appealing. Currently, the main approaches for designing 3D single-phase PnCs rely on less efficient trial-and-error approaches, which are heavily dependent on researchers' empirical knowledge. In comparison, topology optimization offers a dominant advantage by transcending the restriction of predefined microstructures and obtaining topologies with desired performance. This work targeted the exploration of various novel microstructures with exceptional performance by geometric-constrained topology optimization. To deal with high-dimensional design variables in topology optimization, the unit cell structure of a PnC was confined by pyramid symmetry to maximumly deduct the variable number of the unit cell. More importantly, to alleviate mesh dependence inherent in conventional topology optimization, node-to-node and edge-to-edge connection strategies were adopted, supplemented by the insertion of cylinders to ensure the stability of these connections. Finally, unstable PnC structures were filtered out using extra geometric constraints. Leveraging the proposed framework for the optimization of 3D single-phase PnCs, various novel structures were obtained. Particularly, our results demonstrate that PnC structures with only one type of mass lump exhibit significant potential to possess outstanding performance, and geometric configurations of the ultimately optimized structures are intricately linked to the particular sequence of the bandgaps.</p> </abstract>

Bandgap design of fabricated BN/ZnO/Al2O3/TiO2 doped graphene using XPS approach

This study investigates the use of BN/ZnO/Al2O3/TiO2 nanoparticles as dopants for Graphene nanoparticles. The sintering process was employed to manufacture semiconductor materials. The bandgap serves as the central feature of an atomic heterojunction, playing a crucial role in determining the characteristics or overall quality of the semiconductor materials involved. X-ray photoelectron spectroscopy (XPS) was employed to ascertain the bandgap values of Boron (B), Nitrogen (N), and Carbon (C) at the interfaces of BN/Graphene, BN/ZnO/Graphene, BN/Al2O3/Graphene, and BN/TiO2/Graphene. The survey spectra were examined to provide evidence of atoms' presence and their respective atomic proportions. The analysis of the narrow spectra of XPS was employed to determine the binding energy of atoms within various materials. The conduction band offset (CBO) and valence band offset (VBO) of the aforementioned heterojunctions were computed. The estimation of the ratio between the conduction band offset (CBO) and the valence band offset (VBO), is denoted as ΔEc/ΔEv,. The significant change in the energy gap in the valance band concerning the change in conduction band energy demonstrates that this material possesses the characteristics of an exemplary semiconductor. The present investigation reveals that the BN/TiO2/Graphene heterojunction exhibits the highest values of ΔEv/ΔEc, namely 13.07 for nitrogen (N) and 15.07 for boron (B). The results suggest that the combination of BN/TiO2/Graphene semiconductors holds promising potential for applications in nanoelectronics.

Design and Observation of Topological Band Gaps and Edge Modes of SOI Photonic Crystal Slabs in the Mid-Infrared Range

Design and Observation of Topological Band Gaps and Edge Modes of SOI Photonic Crystal Slabs in the Mid-Infrared Range

Comparison of optimized GeSn/Si heterojunction and GaInAsSb/GaSb thermophotovoltaic cells with similar bandgaps

The bandgap of Ge1−xSnx material can be designed within 0.5 ∼ 0.6 eV with different Sn content, and the characteristic of indirect bandgap of pure Ge will change to direct, which make Ge1−xSnx become a proper and low cost thermophotovoltaic cell material. Here we investigate direct bandgap Ge1−xSnx cells with bandgaps of 0.508 ∼ 0.548 eV. Triple antireflection layers, the surface recombination rate, p–n junction depth, impurity doping concentration, etc are optimized for cell design. The optimal cell structures are adopted for cell performance evaluation under given blackbody radiation within 1000 ∼ 2000 K. Simultaneously, the output power densities of GeSn cells are compared with those of traditional GaInAsSb cells with similar bandgap designs. GeSn cells show comparable performances with GaInAsSb cells over the temperature range of 1000 ∼ 1500 K blackbody radiation, and the efficiencies are 1.01 ∼ 2.49 times over 1500 ∼ 2000 K.

Electrical Performance Analysis of High-Speed Interconnection and Power Delivery Network (PDN) in Low-Loss Glass Substrate-Based Interposers.

In this article, electrical performance analysis of high-speed interconnection and power delivery network (PDN) in low-loss glass substrate-based interposers is conducted considering signal integrity (SI) and power integrity (PI). The low-loss glass substrate is a superior alternative to silicon substrate in terms of high-speed signaling and fabrication yield. However, the low-loss of the substrate is vulnerable to power/ground noise in the PDN since the low-loss property of the substrate cannot suppress the noise naturally. In this article, an in-depth electrical performance analysis is conducted based on various measurements and simulations to fully benefit the advantages of the low-loss glass substrate. First, the fabrication process and test vehicles for the analysis are explained. Using the test vehicles, the electrical performance of the glass interposer's high-speed interconnection is compared with those of silicon and organic interposers. The insertion loss, eye-diagrams, and signal bandwidths of three interposer channels are compared and analyzed based on electromagnetic (EM) and circuit simulations. Also, the electrical performance of the through glass via (TGV) channel is measured and compared with through silicon via (TSV) channel. The high-speed interconnection of the glass interposer showed better performance for most of the parameters which is more suitable for maintaining the SI. Even though the low-loss of the glass substrate ensured the SI, power/ground noise issues in the PDN must be analyzed and solved. In this article, various cases inducing the power/ground noise in the PDN are considered, simulated, and measured. To solve the issues, ground TGV design and electromagnetic bandgap (EBG) design are proposed for an efficient broadband suppression of the noise generated in the glass interposer PDN.

A Design of Tunable Band Gaps in Anti-tetrachiral Structures Based on Shape Memory Alloy

Benefitted from the properties of band gaps, elastic metamaterials (EMs) have attracted extensive attention in vibration and noise reduction. However, the width and position of band gaps are fixed once the traditional structures are manufactured. It is difficult to adapt to complex and changeable service conditions. Therefore, research on intelligent tunable band gaps is of great importance and has become a hot issue in EMs. To achieve smart control of band gaps, a design of tunable band gaps in anti-tetrachiral structures based on shape memory alloy (SMA) is proposed in this paper. By governing the phase transition process of SMA, the geometric configuration and material properties of structures can be changed, resulting in tunable band gaps. Therein, the energy band structures and generation mechanism of tunable band gaps in different states are studied, realizing intelligent manipulation of elastic waves. In addition, the influence of different geometric parameters on band gaps is investigated, and the desired bandgap position can be customized, making bandgap control more flexible. In summary, the proposed SMA-based anti-tetrachiral metamaterial provides valuable reference for the application of SMA materials and the development of EMs.

Study on bandgap characteristics and vibration attenuation mechanism of double-oscillator power-exponent prism phononic crystal plate

This study introduces a local resonance mechanism to a periodic acoustic black hole (ABH) structure to achieve vibration control of plate structures and proposes a double-oscillator power-exponent prism phononic crystal. Results show that the periodic power-exponent prism can generate a high-frequency bandgap, the interior oscillator can generate a low-frequency bandgap, and the top oscillator can separate the frequency dispersion curve at around 700 Hz to form a bandgap with a width of 189 Hz. The double-oscillator power-exponent prism phononic crystal, composed of two types of oscillators and a power-exponent prism, can simultaneously have high-, middle-, and low-frequency bandgaps. Simulations and experiments show that it has a good attenuation effect on flexural vibration in the bandgap frequency band. The present results can provide a useful reference for bandgap design based on the combination of multiple mechanisms.

(Invited) Towards Understanding the Competition of Electron and Energy Transfer in Nanographene

Graphene has captured the imagination of researchers around the world due to its groundbreaking chemical and physical properties. Opening a band gap in graphene must be achieved without, however, compromising its exceptional properties as they are of paramount importance for its use in electronic devices. Notable is the fact that the band gap design in graphene is typically carried out by either chemical or physical methodologies. Chemical modification of graphene is mostly centered around “top-down” or “bottom-up” approaches. The earlier alters, nevertheless, the graphene lattice and, as a consequence, poorly defined structures emerge. The latter by means of, for example, organic synthesis offers a wide palette of tools to control sizes as well as geometries of the resulting “molecular” nanographenes with atomic precision. It allows the fabrication of uniform and well-defined molecular structures. Such “molecular” nanographenes are compelling choices for “on demand” molecular electronics, photovoltaic applications, hydrogen storage, and sensing. In recent years, two main strategies have been developed to fabricate “molecular” nanographenes of defined chemical structures. It is, on one hand, oxidative cyclodehydrogenation of custom-made polycyclic aromatic hydrocarbons (PAHs) and, on the other hand, on-surface cyclodehydrogenation, which enabled the preparation of atomically precise “molecular” nanographenes. To this end, the 13 fused-benzene rings of hexa-peri-hexabenzocoronene (HBC), which are arranged in a 2D disk-shaped fashion, render HBCs the smallest “molecular” nanographenes.

Exploring the correlation between the bandgap engineering and defect density toward high CTS solar cell efficiency

Ternary chalcogenide Cu2SnS3 (CTS) has emerged as a relevant compound for solar energy harvesting owing to its favorable optoelectronic properties. The aim of this study was to conduct a numerical investigation using SCAPS-1D software to determine the optimal conditions for an efficient CTS solar cell. The research focused on how the bandgap (Eg) design affects the optical properties and photovoltaic performance (PV) of a CTS solar cell. The correlation between the Eg width and bulk defect density (Nt), as well as the CTS/CdS interface defect density (Nit) of CTS thin films, was also investigated. The results revealed that the increase in Eg significantly improves the external quantum efficiency (EQE), power conversion efficiency (PCE), and open-circuit voltage (Voc) of the solar cells. In addition, a remarkable decrease in recombination rate was observed. For Eg values of 1.1 eV and 1.18 eV, the solar cell demonstrated a minimal recombination rate of 5 × 1020 cm−3 and a high PCE of 5%. The contour plot of Nt as a function of Eg confirmed that Eg and Nt values of 1.18 eV and 5 × 1016 cm−3, respectively, provide higher efficiency. Furthermore, the results highlighted the importance of limiting Nit to below 5 × 1014 cm−3 at the CTS/CdS junction. Under optimum conditions, the output parameters of the CTS solar cell were calculated to be 6.30% for the PCE, 46.64% for the fill factor (FF), 33.89 mA/cm2 for the short-circuit current (Jsc), and 398.2 mV for Voc.

The Vibration Isolation Design of a Re-Entrant Negative Poisson’s Ratio Metamaterial

An improved re-entrant negative Poisson’s ratio metamaterial based on a combination of 3D printing and machining is proposed. The improved metamaterial exhibits a superior load-carrying and vibration isolation capacity compared to its traditional counterpart. The bandgap of the proposed metamaterial can be easily tailored through various assemblies. Additionally, particle damping is introduced to enhance the diversity of bandgap design, improve structural damping performance, and achieve better vibration isolation at low and medium frequencies. An experiment and simulation were conducted to assess the static and vibration performances of the metamaterial, and consistent results were obtained. The results indicate a 300% increase in the bearing capacity of the novel structure compared to traditional structural metamaterials. Furthermore, by increasing the density of metal assemblies, a vibration-suppressing bandgap with a lower frequency and wider bandwidth can be achieved. The introduction of particle damping significantly enhanced the vibration suppression capability of the metamaterial in the middle- and low-frequency range, effectively suppressing resonance peaks. This paper establishes a vibration design method for re-entrant metamaterials, which is experimentally validated and provides a foundation for the vibration suppression design of metamaterials.

A Novel MoS2/TiO2/Graphene Nanohybrid for Enhanced Photocatalytic Hydrogen Evolution under Visible Light Irradiation

Graphene is regarded as a potential co-photocatalyst for photocatalytic hydrogen (H2) evolution, but its great photocatalytic ability requires tuning the band gap structure or design morphology of composites. In this study, MoS2/TiO2/graphene (MTG) nanohybrids were fabricated at varied ratios of graphene and served as co-photocatalysts for H2 evolution. The results exhibited that the H2 evolution of MTG-10 obtained is much better than others. The amount of hydrogen evolution was high, which was found to be 4122 μmol g−1 of H2 in 5 h with photocatalytic systems, which is almost 7.5~13.4 times greater than that of previous pristine MoS2 (548 μmol g−1) and TiO2 (307 μmol g−1) samples, respectively. This is significantly attributed to the graphene as a bridge of MoS2/TiO2 and the incorporation of graphene, suggesting the synergistic effect of the rapid electron-transferring of photoinduced electrons and holes and the powerful electron-collecting of graphene, suppressing the charge recombination rate.

Band gap characteristics of multi-functional lattice metamaterials with adjustable thermal expansion and Poisson's ratio

The current development of science and technology has put increased demands on the versatility of metamaterials. In this paper, the band gap characteristics of a lattice metamaterial that can simultaneously modulate thermal expansion and Poisson’s ratio are studied, the dispersion characteristics of elastic waves propagating in periodic lattice metamaterials are analyzed, and the effects of different structures and parameters on the band gap are discussed. The results indicate that the configuration has excellent band gap properties while satisfying the tunable coefficient thermal expansion (CTE)and Poisson’s ratio functions. Through reasonable material selection and shape design, it is expected to achieve a multi-objective win-win for specific thermal expansion properties, Poisson’s ratio, and band gap design, resulting in better tunability and versatility of the metamaterial.

Low frequency band gap and vibration suppression mechanism of innovative multiphase metamaterials

In order to achieve low frequency vibration and noise reduction, researchers have found and explored many types of metamaterial structures. Based on this, an innovative multiphase metamaterial structure composed of hard materials and soft materials is proposed in this paper. According to Bloch’s theorem, the band gap characteristics of the proposed structure are calculated using the finite element method, and the formation mechanism of the band gap is analyzed. In addition, the effects of the stiffness and density of the two materials on the band gap are studied in detail. Then, the phase velocity, group velocity and wave propagation direction are calculated according to a certain frequency of the specific dispersion curve, and the important information of wave propagation in the structure is obtained. Finally, according to the transmission characteristics of vibration in a finite periodic structure, it is verified that the existence of band gap can inhibit the vibration transmission, and it is further clarified that the proposed structure has a good performance in the design of low-frequency band gaps with large amplitude.

Oxidation State Dependent Conjugation Controls Electrocatalytic Activity in a Two-Dimensional Di-Copper Metal–Organic Framework

Molecularly defined two-dimensional conjugated metal–organic frameworks combine properties of molecular and material based electrocatalyst, enabling tunable active site and bandgap design. The rational optimization of these systems requires an understanding of the complex interplay between the various metal sites and their influence on catalysis. For this purpose, copper-phthalocyanine-based two-dimensional conjugated metal–organic framework (CuPc-CuO4 2D c-MOF) films with an edge-on layer orientation were transferred to nickel-nitrilotiacetic acid (Ni-NTA)-functionalized graphite electrodes and analyzed via electrochemical resonance Raman spectroscopy. With the help of density functional theory (DFT) calculations for the first time a detailed assignment of the vibrational bands for different Cu oxidation states could be achieved and correlated to their electrocatalytic activity in respect to oxygen reduction (ORR). Potential dependent Raman spectroscopy made it furthermore possible to determine the redox potentials of the Cu in the CuPc moieties and the Cu-catecholate nodes individually with ECuPc = −0.04 V and ECuO4 = +0.33 V versus Ag|AgCl. Electrocatalytic ORR, however, was only observed below −0.2 V where both Cu units were present in their respective CuI state. DFT calculations of bandgaps and density of states (DOS) showed a significant decrease in bandgap and increase in π-conjugation upon transition from the inactive mixed CuII/CuI state to the active CuI/CuI state, suggesting that slow electron transfer in the mixed state limits ORR catalysis. Our results indicate that the coupling between metal oxidation changes and π-conjugation of the 2D c-MOF is a key parameter toward achieving electrocatalytic activity.