Band Gap Formation Research Articles

Wave bandgap formation and its evolution in two-dimensional phononic crystals composed of rubber matrix with periodic steel quarter-cylinders

The band structure of a two-dimensional phononic crystal, which is composed of four homogenous steel quarter-cylinders immersed in rubber matrix, is investigated and compared with the traditional steel/rubber crystal by the finite element method (FEM). It is revealed that the frequency can then be tuned by changing the distance between adjacent quarter-cylinders. When the distance is relatively small, the integrality of scatterers makes the inner region inside them almost motionless, so that they can be viewed as a whole at high-frequencies. In the case of relatively larger distance, the interaction between each quarter-cylinder and rubber will introduce some new bandgaps at relatively low-frequencies. Lastly, the point defect states induced by the four quarter-cylinders are revealed. These results will be helpful in fabricating devices, such as vibration insulators and acoustic/elastic filters, whose band frequencies can be manipulated artificially.

Tamm-plasmon polaritons in one-dimensional photonic quasi-crystals.

We present an investigation to ascertain the existence of Tamm-plasmon-polariton-like modes in one-dimensional (1D) quasi-periodic photonic systems. Photonic bandgap formation in quasi-crystals is essentially a consequence of long-range periodicity exhibited by multilayers and, thus, it can be explained using the dispersion relation in the Brillouin zone. Defining a "Zak"-like topological phase in 1D quasi-crystals, we propose a recipe to ascertain the existence of Tamm-like photonic surface modes in a metal-terminated quasi-crystal lattice. Additionally, we also explore the conditions of efficient excitation of such surface modes along with their dispersion characteristics.

Elucidating DFT study on structural, electronic, thermal and elastic properties of SrTcO3 by using GGA and mBJ approach

Generalized gradient approximation and Modified Becke and Johnson (mBJ) potential scheme, within density functional theory, has been implemented to evaluate the structural, electronic and thermo-elastic attributes of SrTcO3. The structural stability of the very compound has been probed from tolerance factor, elastic stability criterion and ground state optimizations. In the study of electronic properties, formation of band-gap has been resolved by using density of states and from electron spin density contour plots. It is for the first time that mechanical and thermo-dynamical properties have been studied in terms of elastic constants, melting temperature, enthalpy of formation and Debye temperature. Our results have shown that SrTcO3 exhibit a stable ductile nature that makes it a convincing candidate for high temperature electronic applications.

Enhanced Hole Injection Into Single Layer WSe2

Electronic devices with light-emitting regions composed of single layer transition metal dichalcogenides, such as tungsten diselenide (WSe2), are of great interest due to the formation of a direct bandgap at the single layer limit. Furthermore, increasing injected hole current into the single layer WSe2 is of great importance due to the resulting increase in electroluminescence. In this paper, we demonstrate a $1000\times $ increase in injected hole current into the channel region of an FET composed of a single layer WSe2 channel by incorporation of a thicker (20–30 nm) multilayer WSe2 film under the metal source contact only. By fabrication and analysis of FETs composed of single layer, bi-layer, and tri-layer WSe2 channel regions, we correlate the increase in injected hole current to a decrease in the hole Schottky barrier height at the source contact due to incorporation of the multilayer WSe2 film.

Optical evaluation of the wave filtering properties of graded undulated lattices

We investigate and experimentally demonstrate the elastic wave filtering properties of graded undulated lattices. Square reticulates composed of curved beams are characterized by graded mechanical properties which result from the spatial modulation of the curvature parameter. Among such properties, the progressive formation of frequency bandgaps leads to strong wave attenuation over a broad frequency range. The experimental investigation of wave transmission and the detection of full wavefields effectively illustrate this behavior. Transmission measurements are conducted using a scanning laser Doppler vibrometer, while a dedicated digital image correlation procedure is implemented to capture in-plane wave motion at selected frequencies. The presented results illustrate the broadband attenuation characteristics resulting from spatial grading of the lattice curvature, whose in-depth investigation is enabled by the presented experimental procedures.

Strong Correlation Effect in Ferrimagnetic Half-Metallic V2CoAl and V2CoGa Heusler Compounds

We present first-principles calculations based on the full-potential linearized augmented plane wave (FP-LAPW) method to investigate the structural, elastic, electronic, and magnetic properties of the Heusler compounds V2CoAl and V2CoGa. We explore different magnetic configurations in both regular and inverse Heusler structures: the Cu2MnAl-type and Hg2CuTi-type structures respectively. Our compounds are found to be ferrimagnets in the inverse Heusler structure. To take into account the effects of the strong correlation among the localized d-states, we perform a calculation using the GGA+U method (the general gradient approximation with the Hubbard on-site Coulomb correction U). Then we show that both compounds exhibit a half-metallic character and have a magnetic moment of 2.00 μB per formula unit. This latter statement is coherent with the generalized Slater-Pauling rule (SP). In terms of the band structure calculations, we explain the formation of the indirect energy band gaps in the spin-majority channels. We also use the mean-field approximation (MFA) to discuss the magnetic exchange couplings and predict the Curie temperature for both compounds.

Lattice-Matched Epitaxial Graphene Grown on Boron Nitride.

Lattice-matched graphene on hexagonal boron nitride is expected to lead to the formation of a band gap but requires the formation of highly strained material and has not hitherto been realized. We demonstrate that aligned, lattice-matched graphene can be grown by molecular beam epitaxy using substrate temperatures in the range 1600-1710 °C and coexists with a topologically modified moiré pattern with regions of strained graphene which have giant moiré periods up to ∼80 nm. Raman spectra reveal narrow red-shifted peaks due to isotropic strain, while the giant moiré patterns result in complex splitting of Raman peaks due to strain variations across the moiré unit cell. The lattice-matched graphene has a lower conductance than both the Frenkel-Kontorova-type domain walls and also the topological defects where they terminate. We relate these results to theoretical models of band gap formation in graphene/boron nitride heterostructures.

Crystal Growth of the Nowotny Chimney Ladder Phase Fe2Ge3: Exploring New Fe-Based Narrow-Gap Semiconductor with Promising Thermoelectric Performance

A new synthetic approach based on chemical transport reactions has been introduced to obtain the Nowotny chimney ladder phase Fe2Ge3 in the form of single crystals and polycrystalline powders. The single crystals possess the stoichiometric composition and the commensurate chimney ladder structure of the Ru2Sn3 type in contrast to the polycrystalline samples that are characterized by a complex microstructure. In compliance with the 18-n electron counting rule formulated for T–E intermetallics, electronic structure calculations reveal a narrow-gap semiconducting behavior of Fe2Ge3 favorable for high thermoelectric performance. Measurements of transport and thermoelectric properties performed on the polycrystalline samples confirm the formation of a narrow band gap of ∼30 meV and reveal high absolute values of the Seebeck coefficient at elevated temperatures. Low glass-like thermal conductivity is observed in a wide temperature range that might be caused by the underlying complex microstructure.

Modification of the transmission spectrum of the ”semiconductor-dielectric” photonic crystal in an external magnetic field

In this paper, some features of electromagnetic transmission of the waves of ”semiconductor dielectric” periodic Bragg structure with a finite number of periods have been investigated. In the absence of absorption, for a structure with p-type semiconductor layers, we have analyzed the dependences on the external magnetic field of photon spectrum and transmission coefficient spectrum. It has been shown that with an increasing magnetic field, there is a significant narrowing of bandwidth and broadening of band gaps, as well as formation of new band gaps in the resonance region. The boundaries of all forbidden and allowed bands are shifted to higher frequencies with the increase of the angle of radiation incidence.

First-Principles Study in New Hf2CoZ (Z = B, C, Al, Si, Ga, Ge, In, Sn, Tl, and Pb) Heusler Alloys

The full-potential linearized augmented plane wave method within density functional theory was used to investigate the electronic structures, magnetic properties, and half-metallicity in Hf2CoZ (Z = B,C, Al, Si, Ga, Ge, In, Sn, Tl, and Pb) alloys with AlCu2Mn- and CuHg2Ti-type structures. The AlCu2Mn-type structure is energetically favorable than the CuHg2Ti-type structure for all compounds. Hf2CoGa, Hf2CoAl, Hf2CoIn, Hf2CoTl, and Hf2CoSn compounds in the CuHg2Ti-type structure were half-metallic ferromagnets. The d-d and covalent hybridizations between transition metals Hf and Co were effective in the formation of minority band gap. The total magnetic moments of half-metallic Hf2CoZ (Z = Al, Ga, In, Sn, and Tl) compounds in the CuHg2Ti-type structure obeyed the Slater–Pauling rule (M tot = Z tot − 18). Hf2CoIn and Hf2CoTl compounds were half-metals in relatively wide ranges of lattice constants which indicates that they can be promising and interesting candidates for spintronic devices.

Understanding the synergistic effects, optical and electronic properties of ternary Fe/C/S‐doped TiO2 anatase within the DFT + U approach

AbstractAlthough TiO2 is an efficient photocatalyst, its large band gap limits its photocatalytic activity only to the ultraviolet region. An experimentally synthesized ternary Fe/C/S‐doped TiO2 anatase showed improved visible light photocatalytic activity. However, a theoretical study of the underlying mechanism of the enhanced photocatalytic activity and the interaction of ternary Fe/C/S‐doped TiO2 has not yet been investigated. In this study, the defect formation energy, electronic structure and optical property of TiO2 doped with Fe, C, and S are investigated in detail using the density functional theory + U method. The calculated band gap (3.21 eV) of TiO2 anatase agree well with the experimental band gap (3.20 eV). The defect formation energy shows that the co‐ and ternary‐doped systems are thermodynamically favorable under oxygen‐rich condition. Compared to the undoped TiO2, the absorption edge of the mono‐, co‐, and ternary‐doped TiO2 is significantly enhanced in the visible light region. We have shown that ternary doping with C, S, and Fe induces a clean band structure without any impurity states. Moreover, the ternary Fe/C/S‐doped TiO2 exhibit an enhanced photocatalytic activity, a smaller band gap and negative formation energy compared to the mono‐ and co‐doped systems. Moreover, the band edges of Fe/C/S‐doped TiO2 align well with the redox potentials of water, which shows that the ternary Fe/C/S‐doped TiO2 is promising photocatalysts to split water into hydrogen and oxygen. These findings rationalize the available experimental results and can assist the design of TiO2‐based photocatalyst materials.

Spectral properties of excitons in the bilayer graphene

In this paper, we consider the spectral properties of the bilayer graphene with the local excitonic pairing interaction between the electrons and holes. We consider the generalized Hubbard model, which includes both intralayer and interlayer Coulomb interaction parameters. The solution of the excitonic gap parameter is used to calculate the electronic band structure, single-particle spectral functions, the hybridization gap, and the excitonic coherence length in the bilayer graphene. We show that the local interlayer Coulomb interaction is responsible for the semimetal-semiconductor transition in the double layer system, and we calculate the hybridization gap in the band structure above the critical interaction value. The formation of the excitonic band gap is reported as the threshold process and the momentum distribution functions have been calculated numerically. We show that in the weak coupling limit the system is governed by the Bardeen-Cooper-Schrieffer (BCS)-like pairing state. Contrary, in the strong coupling limit the excitonic condensate states appear in the semiconducting phase, by forming the Dirac's pockets in the reciprocal space.

Formation of local resonance band gaps in finite acoustic metamaterials: A closed-form transfer function model

The objective of this paper is to use transfer functions to comprehend the formation of band gaps in locally resonant acoustic metamaterials. Identifying a recursive approach for any number of serially arranged locally resonant mass in mass cells, a closed form expression for the transfer function is derived. Analysis of the end-to-end transfer function helps identify the fundamental mechanism for the band gap formation in a finite metamaterial. This mechanism includes (a) repeated complex conjugate zeros located at the natural frequency of the individual local resonators, (b) the presence of two poles which flank the band gap, and (c) the absence of poles in the band-gap. Analysis of the finite cell dynamics are compared to the Bloch-wave analysis of infinitely long metamaterials to confirm the theoretical limits of the band gap estimated by the transfer function modeling. The analysis also explains how the band gap evolves as the number of cells in the metamaterial chain increases and highlights how the response varies depending on the chosen sensing location along the length of the metamaterial. The proposed transfer function approach to compute and evaluate band gaps in locally resonant structures provides a framework for the exploitation of control techniques to modify and tune band gaps in finite metamaterial realizations.

Pole distribution in finite phononic crystals: Understanding Bragg-effects through closed-form system dynamics.

Bragg band gaps associated with infinite phononic crystals are predicted using wave dispersion models. This paper departs from the Bloch-wave solution and presents a comprehensive dynamic systems analysis of finite phononic systems. Closed form transfer functions are derived for two systems where phononic effects are achieved by periodic variation of material property and boundary conditions. Using band structures, differences in dispersion characteristics are highlighted and followed by an analytical derivation of the eigenvalues. The latter is used to derive the end-to-end transfer function of a finite phononic crystal as a function of any given parameters. The analysis reveals intriguing features that explain the evolution of Bragg band gaps in the frequency response. It quantifies how the split of eigenvalues into sub- and super-band-gap natural frequencies contribute to band gap formation. The unique distribution of poles allows the closely packed sub-band gap natural frequencies to achieve maximum attenuation in the Bode response. At that point, the impact of the super-band-gap frequencies on the opposing side becomes significant causing the attenuation to fade and the band gap to come to an end. Finally, the effect of splitting the poles further apart is presented in both phononic systems, with material and boundary condition periodicities.

Band gap formation and Anderson localization in disordered photonic materials with structural correlations

Disordered dielectric materials with structural correlations show unconventional optical behavior: They can be transparent to long-wavelength radiation, while at the same time have isotropic band gaps in another frequency range. This phenomenon raises fundamental questions concerning photon transport through disordered media. While optical transparency in these materials is robust against recurrent multiple scattering, little is known about other transport regimes like diffusive multiple scattering or Anderson localization. Here, we investigate band gaps, and we report Anderson localization in 2D disordered dielectric structures using numerical simulations of the density of states and optical transport statistics. The disordered structures are designed with different levels of positional correlation encoded by the degree of stealthiness [Formula: see text] To establish a unified view, we propose a correlation-frequency ([Formula: see text]-[Formula: see text]) transport phase diagram. Our results show that, depending only on [Formula: see text], a dielectric material can transition from localization behavior to a band gap crossing an intermediate regime dominated by tunneling between weakly coupled states.

Engineering Graphene Quantum Dots for Enhanced Ultraviolet and Visible Light p-Si Nanowire-Based Photodetector

In this work, a significant improvement of the classical silicon nanowire (SiNW)-based photodetector was achieved through the realization of core-shell structures using newly designed GQDPEIs via simple solution processing. The poly(ethyleneimine) (PEI)-assisted synthesis successfully tuned both optical and electrical properties of graphene quantum dots (GQDs) to fulfill the requirements for strong yellow photoluminescence emission along with large band gap formation and the introduction of electronic states inside the band gap. The fabrication of a GQDPEI-based device was followed by systematic structural and photoelectronic investigation. Thus, the GQDPEI/SiNW photodetector exhibited a large photocurrent to dark current ratio (Iph/Idark up to ∼0.9 × 102 under 4 V bias) and a remarkable improvement of the external quantum efficiency values that far exceed 100%. In this frame, GQDPEIs demonstrate the ability to arbitrate both charge-carrier photogeneration and transport inside a heterojunction, leading to simultaneous attendance of various mechanisms: (i) efficient suppression of the dark current governed by the type I alignment in energy levels, (ii) charge photomultiplication determined by the presence of the PEI-induced electron trap levels, and (iii) broadband ultraviolet-to-visible downconversion effects.

The attenuation performance of locally resonant acoustic metamaterials based on generalised viscoelastic modelling

Acoustic metamaterials are known as a promising class of materials interacting with acoustic and/or elastic waves. Band gap formation is one of the most spectacular phenomena that they exhibit. Different ways to broaden the attenuated frequency ranges are still being actively explored. It turns out that material damping through intrinsic viscoelastic material behaviour, if accurately tailored, may contribute to the enhancement of the performance of a properly designed acoustic metamaterial. In this study, a locally resonant acoustic metamaterial with periodic multicoated inclusions with viscoelastic layers is investigated. Multiple attenuation regimes obtained with the considered geometry are joined for a certain level of viscosity of the coating layer. The analysis is performed using a generalised Maxwell model, which allows for an accurate description of nonlinear frequency dependent elastic properties, and thus is widely used to model the behaviour of many polymeric materials in a realistic way. The study reveals that variation of the material parameters of the rubber coating with respect to frequency influences not only the position of the band gaps but also the effectiveness of the wave attenuation in the frequency ranges around the band gaps.

Band Gap Formation and Tunability in Stretchable Serpentine Interconnects

Serpentine interconnects are highly stretchable and frequently used in flexible electronic systems. In this work, we show that the undulating geometry of the serpentine interconnects will generate phononic band gaps to manipulate elastic wave propagation. The interesting effect of “bands-sticking-together” is observed. We further illustrate that the band structures of the serpentine interconnects can be tuned by applying prestretch deformation. The discovery offers a way to design stretchable and tunable phononic crystals by using metallic interconnects instead of the conventional design with soft rubbers and unfavorable damping.

Experimental study of the effect of local atomic ordering on the energy band gap of melt grown InGaAsN alloys

We present a study of melt grown dilute nitride InGaAsN layers by x-ray photoelectron spectroscopy (XPS), Raman and photoluminescence (PL) spectroscopy. The purpose of the study is to determine the degree of atomic ordering in the quaternary alloy during the epitaxial growth at near thermodynamic equilibrium conditions and its influence on band gap formation. Despite the low In concentration (∼3%) the XPS data show a strong preference toward In–N bonding configuration in the InGaAsN samples. Raman spectra reveal that most of the N atoms are bonded to In instead of Ga atoms and the formation of N-centred In3Ga1 clusters. PL measurements reveal smaller optical band gap bowing as compared to the theoretical predictions for random alloy and localised tail states near the conduction band minimum.

Electronic structure of Fe, α-Fe2O3 and Fe(NO3)3 × 9 H2O determined using RXES

Resonant X-ray emission spectroscopy (RXES) technique was applied to probe electronic states of three Fe compounds: Fe, α-Fe2O3 (hematite) and Fe(NO3)3×9 H2O (ferric nitrate) around Fe K-absorption edge with simultaneous detection of Kβ and valence-to-core transitions. We show that deep insights on the valence and conduction band position, such as main orbital contribution to band-gap formation and ligand orbital contributions, can be retrieved from RXES data. Moreover applicability of Kβ and valence-to-core RXES measurements to extract band gap energies is demonstrated. Obtained results were supplemented with ab initio calculations allowing precise determination of orbital contributions to the measured spectral features. Good agreement with experimental results has shown that proposed approach is promising tool in further applications.