Band Gap Response Research Articles

Microenvironment Modulation of Imine‐Based Covalent Organic Frameworks for CO2 Photoreduction

Comprehensive SummaryPhotocatalytic CO2 reduction to fuels and chemicals has been considered as the promising avenue to realize carbon resource recycling. Covalent organic frameworks (COFs) with pre‐designed and tailorable structures are promising platforms for studying the influence of the microenvironment of catalysts on photocatalytic CO2 reduction. Herein, we report three isomorphic COFs (TPHH‐COF, TPPD‐COF and TPBD‐COF) as heterogeneous photocatalysts for CO2 reduction and investigate the different levels of conjugation and planarity of COFs effect on the catalytic activity. Photoelectrochemical measurements show that TPPD‐COF has a narrower band gap and faster photocurrent response compared to TPHH‐COF and TPBD‐COF, probably due to the moderate conjugation and planarity. As the photocatalyst, TPPD‐COF showed the most outstanding photocatalytic activity with the production rates of 951 and 157 μmol·g–1·h–1 for CO and H2, respectively. This study illustrates the close relation between microenvironment and photocatalytic activity and provides new insights for designing high‐performance photocatalysts.

Coiled Phononic Crystal with Periodic Rotational Locking: Subwavelength Bragg Band Gaps

Phononic crystals (PnC) are spatially periodic materials with band gaps that form by Bragg scattering of elastic waves. Within the frequency range of a band gap, wave propagation is not admitted. A long-standing limitation of this class of materials is that the wavelength for band-gap formation must be on the order of the unit-cell size. This restricts the presence of band gaps to relatively high frequencies for a given lattice spacing. Locally resonant metamaterials, on the other hand, enable the opening of low-frequency, subwavelength band gaps through resonance hybridization. However, their band gaps are characteristically narrow and require large or massive local resonators to form. Here, we break both limitations using beam-based PnCs by (1) locking the rotation degree of freedom at the edges of the primitive unit cell, and (2) coiling the PnC by applying full beam-axis rotations at the locked locations. These respective kinematic and geometric transformations convert a conventional beam PnC from its extended form with a nominal lattice constant to an extremely compact coiled configuration with a greatly reduced lattice constant. With the periodic rotational locking, the band gaps remain intact and are still large, and in fact increase noticeably in size. With the subsequent coiling, the band gaps remain based on Bragg scattering and are quantitatively conserved except now appearing at lower frequencies as dictated by the ratio of the extended-to-coiled lattice constants. This ratio defines a coiling factor, which is a measure of the reduction in the PnC unit-cell length in the direction of wave transmission while maintaining the band structure of its original extended form except for the favorable changes induced by the periodic rotational locking. A coiling factor of $\ensuremath{\beta}$ lowers, by construction, the location of the normalized central frequency of any given band gap by a factor of $\ensuremath{\beta}$. The only limitation is the need for lateral space to accommodate the coiling of the beam segments. The vibration behavior of a finite version of the coiled structure is experimentally tested demonstrating a matching band-gap response, despite the reduction in length, to that obtained by finite-element analysis of the extended rotationally locked version. This concept creates effectively subwavelength Bragg band gaps. It clears the path for PnCs to serve in applications that are orders-of-magnitude smaller in scale than are currently possible, while featuring band gaps that are significantly larger than those generated by locally resonant metamaterials.

The Effect of an External Electric Field on the Electronic Properties of Defective CBN Nanotubes: A Density Functional Theory Approach

We investigated the effects of applying an external electric field on the electronic properties of Stone-Wales (SW) defective carbon-boron-nitride nanotubes (CBN) using first principles calculations. The defective CBN nanotubes were modeled by introducing Stone–Wales defects in the boron-nitride segment (BN-SW), the carbon segment (C-SW), and the carbon-boron-nitride interface segment (CBN-SW). Initially, we studied the formation energies and the structural stability for all models. As a result of adding the SW defects, the calculated bandgap values of the C-SW and CBN-SW models showed significant changes compared to the pristine CBN nanotube. Meanwhile, the BN-SW model showed a slight bandgap change because of the strong covalent bonding between the boron and nitrogen atoms. Applying a transverse electric field induced a fast bandgap closing response in all models, indicating a rapid semiconductor-to-metal phase transition. The defective C-SW and CBN-SW models demonstrated unique bandgap closing patterns in response to applied transverse and longitudinal electric fields, while pristine and BN-SW models had similar bandgap responses.

Na3Ti3O3(SeO3)4F: A Phase-Matchable Nonlinear-Optical Crystal with Enlarged Second-Harmonic-Generation Intensity and Band Gap.

A new phase-matchable nonlinear-optical material, Na3Ti3O3(SeO3)4F, has been achieved by a facile hydrothermal reaction. The anionic skeleton displays a honeycombed 3D structure with Ti6Se6 12-member polyhedral ring tunnels along the c axis. Na3Ti3O3(SeO3)4F presents a strong second-harmonic-generation (SHG) response of about 6 × KDP, which is twice that of its isostructural Ag compound. Interestingly, the band gap of Na3Ti3O3(SeO3)4F is also broader than that of the isomorph. Also, the powder laser-induced damage threshold of Na3Ti3O3(SeO3)4F is even 5.5 times larger than that of Ag3Ti3O3(SeO3)4F. Theoretical calculations revealed that the fully filled Ag 4d and empty Ag 5s states have made the difference in the band gap and SHG response of the two isostructural compounds. Our work has provided a practical way of improving the SHG efficiency and band gap simultaneously, which is usually considered to be a pair of negatively correlated parameters.

HgSO4: An excellent mid-infrared sulfate nonlinear optical crystal with wide band gap and strong second harmonic generation response

A mercury-based sulfate nonlinear optical (NLO) material, HgSO4, was successfully grown using a hydrothermal method. It crystallizes in the polar space group Pmn21 and forms a three-dimensional spatial network structure consisting of [HgO8] polyhedras and [SO4] polyhedras. The powder second harmonic generation (SHG) measurement showed that HgSO4 is a phase-matching material and has a very strong SHG efficiency of 11 times that of KDP. Such a remarkable SHG response is due to the well-ordered arrangement of the distorted [HgO8] and [SO4] polyhedras. In parallel, large bandgap (4.13 eV) and laser damage threshold (26.5 × AgGaS2) are exhibited. Further characterizations showed that this compound possesses a large birefringence and a wide transmission window from the near-UV to mid-IR, suggesting that HgSO4 may be an excellent mid-IR NLO crystal.

Static isotropic pressure induced ultra-wide band gap response of NaCaF3 fluoro-perovskite and its repercussions on optical properties: ab initio calculation

ABSTRACT A comprehensive theoretical study to investigate the outcomes of externally applied static isotropic pressure (0–50 GPa) on electronic, optical and structural properties of NaCaF3, using density functional theory based CASTEP code with ultra-soft pseudo-potential USP plane wave and Perdew Burke Ernzerhof exchange–correlation functional of Generalized Gradient Approximation, is reported. The electronic band gap shows the increasing trend 4.773–6.203 eV with increasing external pressure. The increase in band gap is significant up to 20 GPa as compared to higher external pressures. The mystery of increasing band gap is nicely decoded by the total density of states and elemental partial density of states. Optical properties have been calculated to analyse the impact of increment in band gap on them. We observed that the highest peak of energy loss function shows the blue shift which confirms the increment of band gap. At zero photon energy, for 0 GPa, the static refractive index has a value of 1.4456. After applying external pressure, there is a slight increase in which favours the ultrawide band gap behaviour of the ternary compound. The energy points at which the absorption peak is maxima, the refractive index has lowest value.

Modulation of band gap and optical response of layered MoX2 (X = S, Se, Te) for electronic and optoelectronic applications

Motivated by the fascinating behavior of monolayer MoX2 (X = S, Se, Te), we have reported the structural, phonon dispersions relations, electronic and optical properties for monolayer to quadlayer of MoX2 (X = S, Se, Te) by employing ab-initio calculations. The optimized lattice parameters, bond angle and bond length of layered MoX2 (X = S, Se, Te) increases in the order of MoS2 < MoSe2 < MoTe2 which are in good agreement with earlier reported results. The dynamic stability is confirmed by cohesive energy and phonon modes with positive frequency in phonon dispersion calculations. The observed frequency gap between high and low frequency optical mode depicts the utility of layered MoX2 (X = S, Se, Te) in selective transmission of higher frequency range. The electronic band structure of layered MoX2 (X = S, Se, Te) shows semiconducting character with degeneracy as per number of layers. However, the noticeable change in indirect band gap energy compared to direct band gap energy is observed from quadlayer to monolayer which shows interactions of d orbital in MoX2 (X = S, Se, Te). The polarizability of layered MoX2 (X = S, Se, Te) increases with the increasing number of layers. The refractive index also increases with the number of layers and reaches up to 4.34, 4.11 at energy 2.45 eV, 2.05 eV, respectively in visible region and 4.12 at energy 1.45 eV in IR region of electromagnetic spectrum for MoS2, MoSe2 and MoTe2 quadlayer, respectively. The maximum reflectivity responses of 49%, 58% and 68% are obtained for MoS2, MoSe2 and MoTe2 quadlayer, respectively in the C band of UV region.

Preparation of Bi3.64Mo0.36O6.55 by reflux method and its application in photodegradation of organic pollution

Bismuth molybdate (γ-Bi2MoO6) photocatalyst has garnered huge attention in the field of photocatalysis because of its band gap (2.5–2.8 eV) and good visible-light response (420 ≤ λ ≤ 500 nm). However, as a kind of bismuth molybdates, there are only few studies on Bi3.64Mo0.36O6.55 (BMO), thus further exploration is needed. Herein, a simple reflux method was developed to synthesize the cubic phase of BMO. This method is simple and easy to operate under atmospheric pressure, showing great potential for large-scale production. In contrast with the nanosheet structure of Bi2MoO6, the morphology of BMO is a mixture of nanorod and nanoparticle-like structure. The band structures showed that the band gap, conduction band position and the valence band position of BMO was 2.77 eV, − 0.33 eV and 2.44 eV, respectively. A new mixed phase of 3Bi2O3·2MoO3 appeared in BMO crystal, showing that the phase transition of BMO began at 400 °C. When BMO was calcined at 300 °C, photocatalytic degradation rate was up to maximum. The photocatalytic activity of visible-light range was tested and compared with γ-Bi2MoO6. BMO had better photodegradation activity than that of the Aurivillius phase γ-Bi2MoO6 due to its larger band gap and strong oxidation ability.

Morphology and Environmental Applications of Bismuth Compound Nano-Photocatalytic Materials: A Review

Many great breakthroughs have been made in the past five years in understanding the mechanism of bismuth compounds. However, the actual efficiency achieved with this material to date is far away from the theoretical conversion efficiency. The structure and morphology have been proven to be vital factors to enhance the electronic migration to influence photocatalytic performance. Bismuth compounds with different structures and morphologies have certain application prospects in environmental governance due to their small band gap and strong visible light response. This review is aimed at summarizing the recent experimental and bandgap computational breakthroughs in photocatalytic properties of bismuth oxides and halides, in the meanwhile, compared with the band gap adjustment, the recombination of photoexcited electron–hole (e−–h+) pairs is one of the most important factors for the photocatalytic performance. Although bismuth compounds photocatalyst has been used for environmental applications, our understanding on the degradation mechanism of organic matters is limited. The target location and the degree of mineralization for different pollutants are not yet clear. Moreover, photocatalytic material needs to match the potential of the photogenerated e−–h+ pairs to the potential required to degrade the pollutant to be oxidized or reduced. We aim to provide guidelines for the rational design and fabrication of highly efficient bismuth compounds materials for water treatment. Additionally, the potential applications of bismuth compounds in environment are presented for future research directions.

A novel CeO2–TiO2/PANI/NiFe2O4 magnetic photocatalyst: Preparation, characterization and photodegradation of tetracycline hydrochloride under visible light

A novel CeO2–TiO2/PANI/NiFe2O4 magnetic photocatalyst was successfully prepared by hydrothermal, sol-gel and ultrasonic impregnation method. The composition, morphological and optical property of the samples were characterized by XRD, FT-IR, UV–vis DRS, SEM, XPS and other characterization methods. The results show that CeO2–TiO2/PANI/NiFe2O4 has narrower band gap and better visible light response ability, and possesses better photo-generated electron hole pairs separation capability than that of pure TiO2, CeO2–TiO2 and CeO2–TiO2/NiFe2O4 photocatalysts, and the degradation rate of tetracycline hydrochloride can reach 92.6% under the optimal conditions. The photocatalytic reaction rate constant of CeO2–TiO2/PANI/NiFe2O4 is the largest (0.037 ​min−1), which is 9.27 times, 1.95 times and 1.28 times of that of TiO2, CeO2–TiO2 and CeO2–TiO2/NiFe2O4, respectively. The CeO2–TiO2/PANI/NiFe2O4 composite photocatalyst exhibits a superior stability after three consecutive reuses. The improvement of the photocatalytic performance of CeO2–TiO2/PANI/NiFe2O4 composite photocatalyst may be due to the fact that some Ce ions enter into the lattice of TiO2, which narrows the band gap and broadens the spectral response range. CeO2, NiFe2O4 and TiO2 form semiconductor heterojunction, which can effectively separate photo-generated electron hole pairs. In addition, a possible dual Z-scheme electron transfer mechanism of CeO2–TiO2/PANI/NiFe2O4 composite catalyst was proposed by free radical capture experiments in this study.

Cement sensors with acoustic bandgaps using carbon nanotubes

Cement is widely used in wellbores to stabilize the steel casing used in wellbore operations for oil and gas production, enhanced geothermal systems and carbon sequestration, and to limit fluid movement between sub-surface strata. Flaws such as microcracks in wellbore cement can lead to leakage along the wellbore compromising wellbore integrity. There is an increasing need for methods to monitor cement crack propagation in wellbore environments. In this study, we develop and report the first cementitious sensors capable of exhibiting high frequency acoustic bandgaps (ABGs) using carbon nanotubes (CNTs). Computational simulations of a sensor unit cell are used to design cement-multi walled carbon nanotubes (MWCNTs) sensors that show a wide bandgap. When the cement-MWCNTs sensors is embedded in cement specimens, bandgaps were measured experimentally under 300 kHz and under 600 kHz, consistent with the computationally predicted bandgaps in the range of 290–360 kHz, 410–460 kHz and 515–585 kHz. These bandgap features were absent in homogeneous cement specimens. X-ray tomographic reconstructions showed microscopic debonding at cement-MWCNTs sensor interface. Frequency response analysis of a three-dimensional computational model indicated a shift of frequency of minimum transmission due to the interface debonding, but no perturbation of bandgap response was observed. The cement-MWCNTs sensors developed in this study show the potential of a packed CNT inclusion material in cementitious matrix to create ABGs in a cement matrix.

Robust charge spatial separation and linearly tunable band gap of low-energy tube-edge phosphorene nanoribbon.

Versatile applications have been proposed for phosphorene nanoribbons (PNRs), whose properties depend strongly on the edge structures. Recently, a unique tube-reconstruction at the zigzag edge (ZZ[Tube]) of PNRs was discovered to be the lowest configuration. Therefore, studies on PNRs should be reconsidered. In this paper, we systemically explore the width and strain effects on zigzag PNRs with different edge structures, including ZZ[Tube], ZZ and ZZ[ad] edges. ZZ PNRs always have small band gaps which are nearly independent of both width and strain. A remarkable band gap exists in ZZ[ad] PNRs which increases with a decrease in the ribbon width but is not sensitive to strain. In contrast, the band gaps of ZZ[Tube] PNRs change from 1.08 to 0.70 eV as the width increases from 12 to 65 Å. In addition, the band gaps of ZZ[Tube] PNRs show a linear response under a certain range of strain. In addition, the carrier effective masses (0.50 m0 for electrons and 0.94 m0 for holes) of ZZ[Tube] PNRs are much lower than for ZZ[ad], and the VBM and CBM charges are robustly spatially separated even under strains ranging from −5% to 5%. Their ease of formation, lowest energy, light effective mass, linear band gap response to strain and robust charge spatial separation provide ZZ[Tube] PNRs with potentially excellent performance in microelectronic and opto-electric applications.

Unraveling interactions of resonances for tunable low frequency bandgap in multiphase metamaterials under applied deformation

Metamaterial with various degrees of bandgap tunability is an emerging area in manipulating elastic wave transmission characteristics for next generation phononic devices. Although several attempts are made employing multi-fields such as magnetic, electric and thermal etc., mechanical deformation based tunable bandgap is a major interest. However, achieving tunability in terms of broadening or terminating the bandgap in locally resonant acoustic metamaterial (LRAM) remains as challenging tasks. In this work, we propose LRAM by integrating a two-dimensionally periodically arranged square unit cells consisting of soft coated hard inclusion embedded in a polymer scaffold. We develop computational framework to simulate the low frequency bandgap of unit cell under external deformation. At first quasi-static analysis of unit cell has been performed to find static displacement field under prescribed deformation. Further, utilizing Bloch-Floquet wave analysis for the periodic unit cell and finite element based numerical scheme, an eigenvalue problem is formulated to predict dispersion response within irreducible Brillouin zone (IBZ). Analyzing the symmetry group of the unit cell under deformed state for a series of loading, we extract the bandgap responses. Moreover, we illustrate tunable bandgap at low frequency regimes for different inclusion shapes and sizes. To correlate extension and suppression of bandgap under the applied deformation, evolution of local resonances are described with vibration mode shapes.

Rational design of the ratiometric fluorescent probes for sulfur dioxide derivatives and the study on the sensing performance of cyano-containing groups

Sulfur dioxide (SO2) and its derivativeshave a close relationship with some physiological processes and many diseases. Herein, a series of fluorescent probes (named as NC1, NC2, and NC3) was rationally designed for SO2 derivatives by incorporating various cyano-containing groups to the thienocoumarin moiety. Among them, the probe NC2 with ethyl cyanoacetate moiety could ratiometrically determine SO2 derivatives with ultra-broad emission band gap (211 nm), high selectivity, excellent sensitivity, and fast optical response. More importantly, the cell imaging experiments demonstrated that the probe NC2 could be utilized as a powerful chemical tool for ratiometric visualizing SO2 derivates in biological systems.

Stepwise assembly of multidimensional Al–Pb based coordination polymers

The coordination chemistry of the main group elements has always been attractive. Present herein is a stepwise assembly approach toward multidimensional Al–Pb based crystalline compounds. [Al2Pb3(L)2(HL)2(μ3−OH)2]·2Hdma·2H2O·2Cl (AlOC-48) and [AlPb2(L)2(μ3−OH)]·3H2O (AlOC-49) (H3L ​= ​2,3-dihydroxybenzoic acid, Cat ​= ​catechol and dma ​= ​dimethylamine) represent the first examples of multidimensional Al–Pb based compounds. They are assembled through [Al4(L)4(Cat)2]]4- (Al4, AlOC-13) polyanions and Pb ions. During the process of coordination assembly, metal site substitution occurs on the Al4 precursors and transform into Al2Pb2 unit. AlOC-48 and AlOC-49 are two-dimensional and three-dimensional frameworks based on Al2Pb2 clusters. Moreover, the optical band gap and photocurrent response of these two compounds are studied.

Multiband and Broadband Interference-Free Metallic Package Designs for Microwave Modules

This letter presents a technique for designing metallic packages and cavities hosting RF and/or digital electronic systems for minimizing the unintentional coupling due to cavity resonances. The method involves an appropriate design of the conductive lid by adding periodic structures, unit cells (UCs), characterized by metal pins of appropriate length. The technique proposed herein, based on a quick analytical design procedure, demonstrates that the use of two different UCs arranged by following specific layouts is able to offer either multiple bandgaps or an ultrawide bandgap. Three UCs geometry are analyzed for bandgaps at $X$ -, Ku -, and $K$ -bands; they are combined to achieve the two above-mentioned bandgap responses aimed at $X$ – Ku and $X$ – $K$ band applications. Both full-wave simulations and experimental results confirm the validity of the proposed technique.

An extensive investigation of structural, electronic, thermoelectric and optical properties of bi-based half-Huesler alloys by first principles calculations

Half Huesler (HH) alloys have been a hot topic of research due to their fascinating properties and applications in several fields. There are several HH alloys been studied and this work is based on three Bismuth based HH alloys, ZrRhBi, ZrIrBi and HfRhBi, of which we have presented an extensive study on the structural, electronic, thermoelectric (TE) and optical properties using ab-initio density functional theory (DFT) calculations. The previous work on these alloys reported their stability and electronic properties stating that they possess good TE response which was done using DFT with PBE-GGA functional (J. Mater. Chem. A,5(13), 2017). In our study, we have used different exchange functionals to investigate these properties and observed that using nmBJ (new modified Becke–Johnson) potential, we can obtain enhanced band gap and TE response (high figure of merit, ZT and power factor, PF) of these alloys as compared to previous report and fascinating optical properties as well. Our results show that ZrIrBi and HfRhBi are narrow-gap and ZrRhBi is a moderate gap semiconductor. All of these alloys have excellent ZT values of around 0.7 at room temperature. The optical properties show that these compounds have low absorbance, moderate reflectivity and low optical conductivity in visible region. The contrasting behaviour of their optical properties with respect to the regions of electromagnetic spectrum and their electronic properties suggests that they can be effectively used in optoelectronics and various optical devices. Also, their high ZT values both in low and high temperatures open up a possibility to use them for various TE applications.

Electronic and optical properties of zinc based hybrid organic-inorganic compounds

There is significant interest in hybrid organic-inorganic (HOI) compounds since these materials offer multiple functionalities and properties that can be tailored at the mesoscopic and nanoscale levels. HOIs investigated for photovoltaic applications typically contain lead or mercury. There is considerably less work done on Zn-based HOIs. These could potentially be considered in biomedical applications due to presence of organic components and the biocompatibility of Zn cations. Using a systematic materials selection approach, we have carried out a detailed search of Zn-HOI compounds in two comprehensive experimental crystallographic repositories: Inorganic Crystal Structure Database and American Mineralogist Crystal Structure Database. Thirteen Zn-HOI compounds are discovered: CuZnO2(CO3), Zn(C2O4), ((CH3)2NH2)Zn3(PO4)(HPO4)2, (CH3NH3)Zn4(PO4)3, Zn(N(CH2PO3H)3)(H2O)3, (CH3NH3)Zn(HCO2)3, Zn4(CO3)2, Zn8(HPO4)16(C2H8N)8, Zn5(CO3)2, (Mg2Zn)8(CO3)2(OH), Zn7(CO3)2(OH)10, Ca3Zn2(PO4)CO3(OH).2H2O, and Zn(CO3). We have then performed first principles calculations via density functional theory with hybrid functional treatment to determine the electronic band gap and optical response of these materials. Our computations show that eleven of the thirteen compounds have insulating properties with band gaps ranging from 2.8 eV to 6.9 eV. Ten of these are found to have a high absorbance in the far ultra-violet (FUV) region of 200–112 nm wavelength. For example, the absorption coefficient of (CH3NH3)Zn(HCO2)3 is ∼0.75 × 105 cm−1 for F2 excimer laser energy (wavelength ∼157 nm) which is more than three orders higher than the average tissue absorbance (∼101.5 cm−1) and the refractive index of 1.85 is larger than typical biological matter which is in the range 1.36–1.49. These results suggest that Zn-HOIs could potentially find applications in photothermolysis and UV protection.

Strongly nonresonant four-wave mixing in semiconductors

When semiconductors are optically excited above or slightly below the band gap, the linear and nonlinear responses originate predominantly from the interband polarization. Here, we demonstrate that intraband excitations, i.e., rapidly oscillating currents that originate from the electric-field-induced acceleration of electrons and holes, contribute strongly to transient four-wave mixing when it is performed with center frequencies near half the band gap frequency. Within a two-band model we show that the presence of several pathways arising from different combinations of inter- and intraband excitations and their interference give rise to characteristic signatures in time- and spectrally resolved signals. Our approach is based on the semiconductor Bloch equations and includes the dynamics of off-resonant electron-hole excitations on a microscopic level. The predicted significant broadening and structure appearing in the four-wave-mixing spectra are in good qualitative agreement with experimental results.

Template effects in Cu(i)–Bi(iii) iodide double perovskites: a study of crystal structure, film orientation, band gap and photocurrent response

Structural dimensions, energy band gaps and thin film orientations are revealed to be amine-dependent in seven newly prepared cuprous–bismuth iodide double perovskites.