Band Structure Dispersion Research Articles

Structure, Characterizations, Photocurrent Response, and Theoretical Study of One Chained Hybrid Iodoplumbate

AbstractExploring new haloplumbate hybrids and understanding the structure‐activity relationships are of great significance for further promoting their applications in the photovoltaic fields. Herein, with the in situ‐formed [Hmd]+ (md = 2‐methyl‐1,3‐diazinane) templates, a new organic–inorganic hybrid iodoplumbate, namely [Hmd]PbI3 (1), is successfully constructed and then structurally characterized using multiple technical approaches. X‐ray crystallography studies show that compound 1 features the typical 1D chain‐like motifs of [Pb2I6]n2n−, generating the 3D supermolecular network by the extensive hydrogen bond interactions. Interestingly, compound 1 exhibits the semiconductive behavior, with an optical band gap of 2.72 eV. More attractively, the title compound has good photoelectric switching performances under the alternating light irradiation, whose photocurrent densities compete well with or surpass those of many metal halide counterparts. Further theoretical analyses reveal that the title compound has a more dispersive band structure (especially the value band) that facilitates the transport of charge carriers, which may be the main origin of its excellent optoelectronic performance. Presented in this paper also bring the studies of Hirshfeld surface, X‐ray photoelectron spectroscopy (XPS) as well as thermogravimetric analysis.

PtTe2 photodetectors with negative photoconductivity under different wavelength laser irradiation

Platinum telluride (PtTe2), as a newly emerged type II Dirac semimetal, has attracted much attention. Due to its excellent linear dispersive band structure and ultra-high carrier mobility, PtTe2 show great potential in the new quantum phenomena, topological phase transitions, unconventional superconductivity and other photodetecting fields. In this paper, we report the negative photoelectric effect of photodetectors based on the type II Dirac semimetal PtTe2 film under the laser irradiation of 405 nm, 532 nm and 808 nm. Under the condition of applying 1 V voltage, the R of the PtTe2 device under 405 nm laser irradiation was 20 A/W and the D* was 6.9×109 Jones; the R of the PtTe2 device under 532 nm laser irradiation was 1.3 A/W and the D* was 4.1×108 Jones; the R of the PtTe2 device under 808 nm laser irradiation was 0.6 A/W and the D* was 2.1×108 Jones. Under normal atmospheric pressure and room temperature conditions, we observed a negative photoconductivity phenomenon for the PtTe2 device, where the change in photoconductivity is attributed to the light-induced desorption of O or H2O molecules from the surface of the material. In addition, a single-site scanning imaging system based on a PtTe2 photodetector was designed to test the detection capability of the device. The analysis of the photocurrent effect produced by the PtTe2 device further deepens the understanding of the photoconductivity mechanism in the field of photodetection.

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.

Wave propagation in periodic nano structures through second strain gradient elasticity

The study of wave propagation in the periodic nano-waveguides is of importance to advance the development of energy and wave controlling systems. In this paper, firstly, the element discretization of nano structures is introduced based on the second strain gradient theory. The displacement field is determined through the utilization of six quintic Hermite polynomial interpolating functions. By applying Hamilton’s principle, the weak form, which incorporates the element matrices, is determined and the global dynamic stiffness matrix of a unit cell is assembled. Then, the wave finite element method, based on the inverse form, direct form and contour integral solution, is used to analyze the two-dimensional free wave propagation properties of complex nano structures by solving the eigenvalue problems. The interpretation of the frequency spectrum is confined to the irreducible first Brillouin zone, encompassing both the dispersion relation and band structure. Furthermore, the slowness surfaces and energy flow vector fields are discussed via the second strain gradient theory. The novel work, which utilizes the second strain gradient theory equipped with the wave finite element method to investigate two-dimensional complex nano structures, has revealed significant potential in elucidating the two-dimensional wave propagation characteristics of the complex nano-waveguides.

Destabilization of Sn2+ 5s2 Lone-Pair States of SnO through Dimensional Crossover.

Materials exhibiting unique electronic properties arising from a characteristic crystal structure have physical properties that are sensitive to structural dimensionality. This study involves the destabilization of Sn 5s2 lone-pair states of SnO films by decreasing their structural dimensionality in the out-of-plane direction. The inherent dispersive band structure of the SnO films remained unchanged between 80 and 11 nm. Below 11 nm, their dispersive band structure disappeared, the O/Sn ratio increased, and the carrier type changed from the p type to the n type, whereas the Sn valency remained constant at +2. These unconventional changes arose from the electronic separation corresponding to the Debye length, which is proportional to permittivity, and were attributed to weakened interactions between Sn 5s2 lone-pair electrons. Therefore, designing low-permittivity materials is beneficial for reducing the crystallite size required for stabilizing lone-pair states. These results are essential for designing emergent p-type oxides and improving their semiconducting properties and performance in transparent or high-power electronics.

Understanding the electronic, structural, optical, photovoltaic and thermoelectric properties of Cs2GeSnBr6 by first-principles and SCAPS-1D simulation

The electronic, optical, structural, thermoelectric and photovoltaic properties of Cs2GeSnBr6 are investigated by using density functional theory and SCAPS-1D simulation. There is a lack of imaginary frequency in the phonon dispersion band structure implies that Cs2GeSnBr6 is stable against lattice distortion. Also, Goldsmith tolerance factor (τ), octahedral factor (µ), and convex-hull calculations demonstrated that Cs2GeSnBr6 can be synthesized. In addition, electronic band structure calculation using GGA-PBE+mBJ+SOC shows that Cs2GeSnBr6 has a direct band gap value of 1.01 eV. Further, optical calculations revealed the higher value of absorption coefficient (α) and optical conductivity (σ) of Cs2GeSnBr6. Motivated by this, we have modelled the ITO/ZnO/Cs2GeSnBr6/Au photovoltaic cell which shows a maximum power conversion efficiency of 18.74%. Moreover, the obtained large ZT value Cs2GeSnBr6 which is originating from its good electrical conductivity with poor thermal conductivity.

Excitonic quenching of the oxygen-chain phonon in the photoinduced metal-to-insulator transition of photoexcited Sr0.95NbO3.37 studied by ultraviolet-resonance Raman scattering

The quasi-one-dimensional metal ${\mathrm{Sr}}_{0.95}{\mathrm{NbO}}_{3.37}$ shows a metal-to-insulator transition upon laser-induced pumping of the d-d exciton at 1.1 eV. The oxygen-chain-related phonon modes are highly susceptible to this metal-to-insulator transition due to their sensitivity to structural and electronic changes. We employ angle-dependent UV-resonance Raman spectroscopy to probe the nonequilibrium oxygen-chain-related phonon dynamics and the concomitant changes in the angle-dependent midinfrared reflectance. The latter shows the expected reduction of charge carrier density upon pumping, whereas the central chain mode at 680 $\mathrm{cm}{}^{\ensuremath{-}1}$ can be selectively quenched upon pumping the d-d excitons. We find that the underlying quenching mechanism for this phonon is electronically driven and related to the photo-induced bleaching of the charge-transfer transition. First-principles calculations show the connection between phonon dispersion and electronic band structure leading to the selective quenching of the oxygen-chain mode. Our results contribute to a deeper understanding of the interplay between lattice and charge degrees of freedom for exciton-based optoelectronics.

The impact of magnetic field on the surface of carbon-insulator-GaAs Semiconductors which is tunable with a frequency range in the presence of surface magneto Plasmon

In this paper, group velocity and frequency wave can be tuned with an applied external magnetic field when we increase the magnetic field from 0-4 tesla the frequency range can be reduced for given semiconductor materials. The excitation of the two layers of semiconducting material propagating band structures can be explained by the oscillations of electrons in semiconductors on applying the magnetic field, we study the effects of an external magnetic field in the band structure of C-insulator-GaAs materials in presence of surface magneto plasmons concerning plasma frequency below and above the surface band structures. The surface magneto plasmon bands get excited and show the dispersion relation with frequency range. The higher dispersion band moves in faster than the lower dispersion band structure of semiconducting material. The most energy is stored in a lower surface of magneto plasmon. When we increase the magnetic field, the surface of the semiconductor moves opposite to the lower surface of the semiconductor material. Here, we use semiconducting materials instead of metals because metal cannot support a wide frequency range on the magneto-plasmonic surface providing a good tunning property and more flexibility in this mechanism, which is widely useful in telecommunications, magneto-plasmonic devices, and data processing unit. This study is widely more promising due to its wavelength confinements of electromagnetic fields on semiconducting and insulating layers. Due to nonreciprocal effects, the dispersion of frequency waves varies with different band structures and group velocity also varies with two propagating directions among semiconductor-insulator-semiconductor layers.

Optical delay lines in topological microring resonator array

We use a periodic microring resonator array (MRRA) to realize photonic topological insulator. By tuning the coupling of resonators, two topological edge states can be observed in different bands in the dispersive band, normalized transmission spectrum and light field distribution. From the dispersion band structure, we calculate the group velocity, group velocity dispersion, normalized delay bandwidth product and other delay properties. It is demonstrated that optical delay lines (ODLs) in topological MRRA not only can obtain a comparable delay times, but also show a flatter transmission spectrum and more stable delay time, compared with the traditional coupled resonator optical waveguide. Our work may provide a theoretical reference for the topological ODLs with robust transmission.

Enhanced photocatalytic reduction of CO2 on BiOBr under synergistic effect of Zn doping and induced oxygen vacancy generation

In this work, different BiOBr powders (without and with Zn doping) were prepared. Their specific properties and photocatalytic performance were studied. Zn doped BiOBr showed higher carrier transportation ability, beneficial to high performance photocatalysis. Further analysis and theoretical calculations unveiled that Zn doping resulted in more dispersive energy band structure with improved oxygen vacancy (OV) generation due to lattice distortion. OV acted as trap centers, playing dominant role in carrier transportation enhancement, which also synergized with more dispersive energy band due to Zn doping, improving carrier separation and transfer. Besides, Zn doping would further strengthen trapping effect under OV existence, stimulating synergistic enhancement to spatial charge separation and transfer with OV. With synergy of Zn doping and OV, Zn doped samples produced 1.75 times higher CH4 generation during gas–solid photocatalytic reduction of CO2 under visible light, testifying successful conducting of Zn doping improved photocatalytic capacity on BiOBr.

High sensitivity and enhanced measurement range biosensing based on defective photonic crystal microring resonators

In this paper, we propose a defective photonic crystal microring resonator (DPhCMRR) by introducing a point defect into a conventional photonic crystal microring resonator (PhCMRR). The defective resonant wavelength within the photonic mode gap and the location of the defect mode distribution can be readily controlled. Unlike conventional PhCMRR, where the free spectral range (FSR) is limited by the dispersive band structure, our DPhCMRR can take advantage of the tunability of the defect mode within the photonic mode gap, leading to a significant increase of the measurement range. Moreover, the bulk refractive index sensitivity can reach 200 nm/RIU (refractive index unit) and the local refractive index sensitivity is about 5 to 10 times larger than that of the conventional PhCMRR. For sensing applications, our DPhCMRR can possess high sensitivity and wide measurement range simultaneously. As proof of principle, it is demonstrated that our proposed DPhCMRR can perform as a sensitive virus biosensor, which can detect a single virus and a concentration of viruses quantitatively. Therefore, our DPhCMRRs can provide a new platform for achieving high sensitivity and wide measurement range biosensing.

Optical absorption engineering in dispersive band structure of MWCNTs array: design and optimization of total absorber for NIR to MIR regime

Optical absorption engineering in dispersive band structure of MWCNTs array: design and optimization of total absorber for NIR to MIR regime

Optical phonon mode assisted thermal conductivity in p-type ZrIrSb half-Heusler alloy: a combined experimental and computational study

Half-Heusler (HH) alloys with an 18 valence electron count have attracted significant interest in the area of research related to thermoelectrics (TEs). Understanding the novel transport properties exhibited by these systems with semiconducting ground state is an important focus area in this field. The large thermal conductivity shown by most of the HH alloys poses a major hurdle to improving the figure of merit (ZT). In addition, understanding the mechanism of thermal conduction in heavy-constituent HH alloys is an interesting aspect. Here, we have investigated the high-temperature TE properties of ZrIrSb through experimental studies, phonon dispersion and electronic band structure calculations. ZrIrSb is found to exhibit a substantially lower magnitude of resistivity and Seebeck coefficient near room temperature due to the existence of anti-site disorder between Ir/Sb and vacant sites. Interestingly, in ZrIrSb, lattice thermal conductivity is governed by a coupling between the acoustic and low-frequency optical phonon modes, which originates due to heavier Ir/Sb atoms. This coupling leads to an enhancement in the Umklapp processes due to the optical phonon excitations near the zone boundary, resulting in a lower magnitude of κ L. Our studies point to the fact that the simultaneous existence of two heavy mass elements within a simple unit cell can substantially decrease the lattice degrees of freedom.

An enhanced dual-resonator metamaterial beam for low-frequency vibration suppression

Structural vibrations seriously affect the working performance of and even cause fatal hazards in host structures. The use of a metamaterial is a novel way to suppress structural vibrations across a wide frequency band. This paper presents an enhanced dual-resonator metamaterial beam (DRMB), where the free end of a traditional series dual-resonator is periodically connected to the rigid body with a spring. The mass and frequency response of the enhanced dual-resonator is derived. The theoretical dispersion relation and band structure are obtained using a transfer matrix method. The effects of the number of cells, mass ratio, and spring stiffness ratio on the bandgap and transmissibility performance of the proposed DRMB are analyzed and discussed. An experiment is carried out to verify the theoretical model. The results show that with increasing number of cells, mass ratio, and spring stiffness ratio of the DRMB, the bandgap changes significantly, thereby enhancing the vibration suppression performance in a low-frequency range. The dual-resonator with three springs suppresses beam vibration.

Strain effects on band structure and Dirac nodal-line morphology of ZrSiSe

The Dirac nodal-line semimetals are new promising materials for technological applications due to their exotic properties, which originate from band structure dispersion and nodal-line behavior. We report strain effects on the band structure of ZrSiSe Dirac nodal-line semimetal through the density functional theory calculations. We found that the kz=0 Dirac nodal-line of ZrSiSe is robust to all strains under reasonable magnitude although there are significant changes in the band oscillation amplitude, bandgap, and band occupancy due to orbital interactions and the Fermi energy shift upon strains. We also found that the effective strains to tune the nodal-line and band structure are equi-biaxial tensile, uniaxial (100) tensile, and xz-plane shear strains.

Atomistic electronic structure in the single-chain limit of organic conductors

Using density functional dynamical mean-field theory we show how electronic interactions can tune a dispersive band structure such that it develops atomistic Landau-Fermi liquidness. Thereby, we show how a relaxed single chain with electron densities slightly below half filling can improve orbital selectivity with coexisting metallic and semiconducting states. Our setup provides a well-controlled environment at the atomistic scale to understand the principles behind orbital-selective electronic reconstruction and the interplay between electron interactions and local electron-hole doping in the single-chain limit of organic conductors.

Effect of dynamic ions on band structure of plasmon excitations

In this paper, we develop a new method to study the plasmon energy band structure in multispecies plasmas. Using this method, we investigate a plasmon dispersion band structure of various quasineutral plasma systems with arbitrary degree of electron degeneracy. The linearized Schrödinger–Poisson model is used to derive an appropriate coupled pseudoforce system from which the energy dispersion structure is calculated. It is shown that the introduction of ion dynamics, as opposed to static ion assumption in the jellium model with a wide plasmon bandgap, can significantly modify the plasmon dispersion character leading to a new low-level energy band caused by the electron–ion interactions. The investigation on the effect of ion charge-state and chemical potential of electrons on the plasmonic band structure indicates some interesting features and reveals the fundamental role played by ions in the phonon assisted plasmon excitations in different kinds of plasma systems. Moreover, our study confirms that ion charge screening has a significant impact on plasmon excitations in multispecies plasmas. The plasmon band structure in pair-ion or electron–positron plasmas indicates the unique role of positive charges on collective excitations. Current research helps us to better understand the underlying mechanisms of collective interactions in charged environment and the important role played by heavy charged particles on elementary plasmon excitations, which have important applications in plasmonic devices. The method developed in this research may also be extended to study magnetized quantum plasmas as well as to investigate surface plasmon–polariton interactions in nanometallic structures.

Ultrabroadband, Fast, and Flexible Photodetector Based on HfTe5 Crystal

AbstractUltrabroad spectrum detection has a wide range of photonic and optoelectronic applications, such as spectroscopy, optical communication, imaging, and sensing. 3D topological insulator candidates are promising materials for fast high‐performance photodetectors owing to their linear dispersion band structure and high carrier mobility. In this study, an ultrabroadband photothermoelectric (PTE) self‐powered detector based on the topological insulator candidate HfTe5 is reported for the first time. The photosensitive properties are characterized in an ultrabroadband range from the ultraviolet (375 nm) to terahertz (118.8 µm) wavelengths, and the responsivities at all examined wavelengths are found to be greater than 1 V W−1 at room temperature. Owing to the Dirac band dispersion of HfTe5, the response time (τ) of the proposed detector is as short as ≈1 ms, which is 1–3 orders of magnitude faster than that of recently reported PTE detectors based on millimeter‐scale graphene, 3D graphene, EuBiSe3 single crystal, and SrTiO3 crystal. Furthermore, the sensitivity of the HfTe5 detector to the light intensity and direction of linearly‐polarized light is demonstrated. Thus, the proposed device demonstrates outstanding flexibility, air stability, and long‐term photostability as well, displaying high potential for practical applications in wearable optoelectronics.

Resonant Multi-phonon Raman scattering of black phosphorus

Black phosphorus (BP) has been attracting intense interest due to its unique anisotropic properties. The investigations on phonon dispersion and electronic band structure could expand the understanding of the properties of BP and promote its application on next generation nano-electronic devices. As the fingerprint of materials, Raman spectroscopy can provide the information of their phonon dispersion and electronic band structure. According to the Raman selection rule, Raman process involving multiple (two or more) phonons can be used to probe the phonon density of states within the whole Brillouin zone. However, the intensity of high-order Raman modes is much lower than that of the first-order Raman mode. To break through the limit of low intensity, here, we measured the resonant Raman spectroscopy of BP excited by several wavelength lasers and observed rich information about high-order Raman modes in the spectral range of 680–930 cm<sup>–1</sup>. To further investigate high-order Raman modes and avoid the birefringence effects from optical anisotropy on Raman intensity, we employ a special polarization configuration to obtain resonant Raman spectra and Raman intensity as a function of excitation wavelength. All the observed high-order Raman modes are certainly assigned, according to the phonon dispersion and symmetry analysis of related phonons. This indicates the great contribution of phonons within the Brillouin zone to the second- and third-order Raman scattering. This work proposes a general and systematical method to investigate high-order Raman modes, and paves ways for the researches of phonon dispersion and resonance Raman spectroscopy in other anisotropic materials.

DFT modeling of thermoelectric and optical features of novel MgxSn1-xSe (x = 6%, 12% & 18%)

We explore influence of Mg alloying effect on electronic band structure dispersion and thermoelectric properties of tin chalcogenide materials. Based on density functional theory (DFT) within a framework of full potential linearized augmented plane wave method (FP-LAPW), we evaluate the energy band structure and optical properties of MgxSn1-xSe (x = 6%, 12% and 18%) materials. Moreover, we extend our calculations to simulate the electrical transport properties using Boltzmann transport theory. Within the approximations employed in our calculations the theoretically predicted band energy gap values and the temperature dependence of electrical transport properties of MgxSn1-xSe compounds revealed that the Mg-alloying have enhanced thermoelectric features. To verify the quality of calculations the comparison with the experimental absorption spectra are presented. The better thermoelectric performance in MgxSn1-xSe is expected to occur for all doping concentrations, however 18% Mg-doped material exhibits higher value of Seebeck coefficient and lower thermal conductivity which is suggestive that at higher Mg concentration the holes become dominant over electrons and hence make these materials to be more promising candidates for their use in thermoelectric power generation and in cooling devices.