Variation Of Band Structure Research Articles

Lattice Softening and Band Convergence in GeTe-Based Alloys for High Thermoelectric Performance.

GeTe-based alloys have been studied as promising TE materials in the midtemperature range as a lead-free alternate to PbTe due to their nontoxicity. Our previous study on GeTe1-xIx revealed that I-doping increases lattice anharmonicity and decreases the structural phase transition temperature, consequently enhancing the thermoelectric performance. Our current work elucidates the synergistic interplay between band convergence and lattice softening, resulting in an enhanced thermoelectric performance for Ge1-ySbyTe0.9I0.1 (y = 0.10, 0.12, 0.14, and 0.16). Sb doping in GeTe0.9I0.1 serves a double role: first, it leads to lattice softening, thereby reducing lattice thermal conductivity; second, it promotes a band convergence, thus a higher valley degeneracy. The presence of lattice softening is corroborated by an increase in the internal strain ratio observed in X-ray diffraction patterns. Doping also introduces phonon scattering centers, further diminishing lattice thermal conductivity. Additionally, variations in the electronic band structure are indicated by an increase in density of state effective mass and a decrease in carrier mobility with Sb concentration. Besides, Sb doping optimizes the carrier concentration efficiently. Through a two-band modeling and electronic band structure calculations, the valence band convergence due to Sb doping can be confirmed. Specifically, the energy difference between valence bands progressively narrows upon Sb doping in Ge1-ySbyTe0.9I0.1 (y = 0, 0.02, 0.05, 0.10, 0.12, 0.14, and 0.16). As a culmination of these effects, we have achieved a significant enhancement in zT for Ge1-ySbyTe0.9I0.1 (y = 0.10, 0.12, 0.14, and 0.16) across the entire range of measured temperatures. Notably, the sample with y = 0.12 exhibits the highest zT value of 1.70 at 723 K.

Unveiling Variations in Electronic and Atomic Structures Due to Nanoscale Wurtzite and Zinc Blende Phase Separation in GaAs Nanowires.

Phase separation is an intriguing phenomenon often found in III-V nanostructures, but its effect on the atomic and electronic structures of III-V nanomaterials is still not fully understood. Here we study the variations in atomic arrangement and band structure due to the coexistence of wurtzite (WZ) and zinc blende (ZB) phases in single GaAs nanowires by using scanning transmission electron microscopy and monochromated electron energy loss spectroscopy. The WZ lattice distances are found to be larger (by ∼1%), along both the nanowire length direction and the perpendicular direction, than the ZB lattice. The band gap of the WZ phase is ∼20 meV smaller than that of the ZB phase. A shift of ∼70 meV in the conduction band edge between the two phases is also found. The direct and local measurements in single GaAs nanowires reveal important effects of phase separation on the properties of individual III-V nanostructures.

Fluorinated interface engineering targeting high-performance multifunctional composites of BN/aramid nanofibers

Aramid nanofibers (ANFs)-based composites have attracted considerable interest in electronic and electrical engineering due to their exceptional thermal stability, high strength, and superior insulating properties. However, the performance of ANFs-based composites is severely confined by interfacial issues. Accordingly, a novel strategy of fluorinated interface engineering is proposed herein, by which the fillers are grafted by the fluorine-containing functional groups. The introduction of interfacial fluorination yields a dense and compact composite structure which stems from the improved compatibility at the matrix-filler interfaces. The tensile strength and thermal conductivity are increased by 30% and 50%, respectively. Additionally, the band structure variation is observed in the fluorinated BNNs, where the electron trap states are formed at the interface. With the deepened deep trap depth and increased density, the breakdown strength is enhanced by 50%. The breakdown phase field simulations also indicate the high discharge energy consumption by the fluorination effect.

The study of N-polar GaN/InAlN MOS-HEMT and T-gate HEMT biosensors

The sensing performance of N-polar GaN/InAlN MOS-HEMT biosensors for neutral biomolecules was investigated and compared with the Ga-polar MOS-HEMT and N-polar T-gate HEMT by numerical simulation. The results indicate that the N-polar GaN/InAlN MOS-HEMT biosensor has higher sensing sensitivity than the Ga-polar MOS-HEMT and N-polar T-gate HEMT biosensors. Furtherly, to improve the sensing performance of N-polar MOS-HEMT, the influence of cavity dimensions, GaN channel layer thickness, and InAlN back barrier layer thickness on device performance was investigated. It is demonstrated that the sensitivity of the biosensor increases as the cavity height decreases and the cavity length increases. Therefore, the sensing performance of the N-polar MOS-HEMT device will be enhanced by thinning the GaN channel layer thickness or increasing the InAlN back barrier thickness, which can be mainly attributed to the variation of the energy band structure and two-dimensional electron gas concentration in the HEMT heterostructure. Finally, the highest sensitivity can be obtained for the N-polar MOS-HEMT with 6 nm-thick GaN channel layer, 30 nm-thick InAlN back barrier layer, and two 0.9 μm-long and 5 nm-high cavities. This work provides structural optimal design guidance for the N-polar HEMT biosensor.

Analytic modelling of quantum capacitance and carrier concentration for [formula omitted]-borophene FET based gas sensor

In this work, we investigate the physical and electronic properties of β12 - borophene FET-based gas sensor using a theoretical quantum capacitance model based on tight-binding approach. We study the impact of adsorbed NH3, NO, NO2 and CO gas molecule on its density of states, carrier concentration, quantum capacitance and I–V characteristics. We found a remarkable variation in the energy band structure and the density of states (DOS) of the β12 - borophene in the presence of the adsorbed gas molecule. The appearance of non-identical Van-Hove singularities in the DOS in the presence of adsorbed gas molecules strongly indicates the high sensitivity of β12 - borophene. We found a significant increase in the carrier concentration for NH3 gas while it decreases for all other gases. Moreover, a drastic change in quantum capacitance and current–voltage relation is also observed in adsorbed gases. The different properties of the given gas molecules are compared with the pristine borophene and found to exhibit distinct wrinkles in each case, thereby indicating the strong selectivity of our proposed gas sensor. Though β12 - borophene is found to be highly sensitive for all studied gases, the NO gas is found to be most sensitive compared to the others.

Mechanisms of Shear Band Formation in Heterogeneous Materials Under Compression: The Role of Pre‐Existing Mechanical Flaws

AbstractShear bands critically govern the shear failure processes and associated geophysical phenomena, for example, faulting, in Earth's crust. Earlier studies on homogeneous materials recognized temperature and strain rate as principal factors controlling the band growth. However, how inherent mechanical heterogeneities can influence their growth mechanisms and complex internal structures needs further investigations. The present article revisits this problem by combining field observations and laboratory experiments, supported by numerical simulations considering geological strain‐rates. Field studies in the Chotanagpur Granite Gneissic Complex of eastern India reveal varied types of macro‐scale shear zone localization. We conducted plane‐strain compression experiments on rock‐analogue models to investigate their origin. The experiments show that mechanical heterogeneities develop wide bands, localized preferentially in their neighborhood, unlike uniformly distributed, conjugate sets of closely spaced narrow bands in a homogeneous system. The wide bands eventually attain a composite structure, containing a core of densely packed band‐parallel sharp secondary bands, flanked by linear zones of closely spaced, narrow bands. We also demonstrate the effects of global strain‐rate on the band evolution in heterogeneous systems. Decreasing strain‐rates replace the composite bands by well‐defined homogeneous shear bands, containing a core of uniform shear, bordered by weakly sheared narrow zones, grading into almost undeformed walls. The experimental results are complemented with numerical models based on visco‐elasto‐plastic rheology, which establish appropriate scaling of the rock analogue to mid‐crustal deformations. This integrated approach finally leads us to conclude that inherent heterogeneities can result in large variations of shear band structures in geological terrains.

Temperature Dependent Band Gap Correction Model Using Tight-Binding Approach for UTB Device Simulations

In order to accurately determine the electrostatics of Ultra-Thin-Body (UTB) devices, the semi-empirical tight-binding (TB) approach is widely used for calculating the channel thickness dependent band structure of any material at those temperatures where TB parameters are available (generally defined at 0 K and 300 K). In this work, we analyze the variation of band structure for Si, Ge, and GaAs over different channel thicknesses at 0 K and 300 K, and show that the band curvature at the band minima remains unchanged with temperature, while the band gap changes significantly and affects the channel electrostatics. Based on this finding, we propose an approach to simulate the electrostatics of UTB devices, at any temperature between 0 K and 300 K, using the band structure obtained at 0 K, along with a suitable channel thickness and temperature-dependent band gap correction. From the results obtained for the channel charge density, we show good agreement with band structure based simulation results, at 300 K, over a wide range of channel thicknesses, for Si, Ge, and GaAs, while also showing good agreement with TCAD simulation results, at a typical intermediate temperature of 150 K, thus highlighting the accuracy, simplicity and wide applicability of the proposed approach.

Structural and electronic properties of two-dimensional titanium carbo-oxides

This work was inspired by new experimental findings where we discovered a two-dimensional (2D) material comprised of titanium-oxide-based one-dimensional (1D) sub-nanometer filaments. Preliminary results suggest that the 2D material contains considerable amounts of carbon, C, in addition to titanium, Ti, and oxygen, O. The aim of this study is to investigate the low-energy, stable atomic forms of 2D titanium carbo-oxides as a function of C content. Via a combination of first-principles calculations and an effective structure sampling scheme, the stable configurations of C-substitutions are comprehensively searched by templating different 2D TiO2 polymorphs and considering a two O to one C replacement scheme. Among the searched stable configurations, a structure where the (101) planes of anatase bound the top and bottom surfaces with a chemical formula of was of particularly low energy. Furthermore, the variations in the electronic band structure and chemical bonding environments caused by the high-content C substitution are investigated via additional calculations using a hybrid exchange-correlation functional.

Sub-nanometer mapping of strain-induced band structure variations in planar nanowire core-shell heterostructures

Strain relaxation mechanisms during epitaxial growth of core-shell nanostructures play a key role in determining their morphologies, crystal structure and properties. To unveil those mechanisms, we perform atomic-scale aberration-corrected scanning transmission electron microscopy studies on planar core-shell ZnSe@ZnTe nanowires on α-Al2O3 substrates. The core morphology affects the shell structure involving plane bending and the formation of low-angle polar boundaries. The origin of this phenomenon and its consequences on the electronic band structure are discussed. We further use monochromated valence electron energy-loss spectroscopy to obtain spatially resolved band-gap maps of the heterostructure with sub-nanometer spatial resolution. A decrease in band-gap energy at highly strained core-shell interfacial regions is found, along with a switch from direct to indirect band-gap. These findings represent an advance in the sub-nanometer-scale understanding of the interplay between structure and electronic properties associated with highly mismatched semiconductor heterostructures, especially with those related to the planar growth of heterostructured nanowire networks.

Electronic phase transitions in quasi-one-dimensional atomic chains: Au wires on Si(553)

The Si(553) surface, covered with half a monolayer of gold, forms a double strand of well-ordered atomic Au chains in each miniterrace. It represents one of the smallest possible realizations of quasi-one-dimensional (1D) systems. In this prototype system, by combining DC conductance and low-energy electron diffraction measurements with density functional calculations and ab initio Monte Carlo simulations, we demonstrate that nonlocal electronic correlations, together with thermal excitations, lead to peculiar phase transitions without long-range order. They are characterized by marginal (average) geometric relaxations, but with huge variations of the electronic band structure and concomitant strong temperature-dependent modifications of the density of states close to the Fermi level. Similar phenomena are expected in the large class of quasi-1D conductors and open a wide range of possibilities for their controlled manipulation. It is the increasing hybridization between spin-polarized Au and Si edge states that makes the Si dangling bond states at the step edge conducting, first as a transient between two insulating phases and finally opening a permanent new 1D conduction channel at high temperatures.

Two band tunneling for a pnp junction in tetralayer graphene

We investigate the band structure and the tunneling of ABCA-tetralayer graphene (ABCA-TTLG) subjected to an external potential U 0 applied between top and bottom layers. Using the tight-binding model, including the nearest t and next-nearest-neighbor t ′ hopping, low-energy model and two-band approximation model we study the band structure variation along the lines Γ − M − K − Γ in the first Brillouin zone , electronic band gap near Dirac point K , transmission properties and conductance, respectively. Our results reveal that ABCA-TTLG exhibits markedly different properties as functions of t ′ and U 0 . We show that the hopping parameter t ′ changes the energy dispersion, the position of K and breaks sublattice symmetries. A sizable band gap is created at K , which could be opened and controlled by the applied potential U 0 . This gives rise to 1D-like van Hove singularities (VHS) in the density of states (DOS). Resonant electronic transmission through graphene-based a p n p junction is studied as a function of the incident wave vector, the width and height of the barrier with and without U 0 . The resonant features in the transmission result from resonant hole states in the barrier and strongly influence the conductance. Our results are numerically discussed and compared with the literature. • Quantum tunneling of ABCA-tetralayer graphene in an potential applied between top and bottom layers is investigated. • The hopping parameter acts by changing the energy dispersion and the position of Dirac point K, also breaks the sublattice symmetries. • A sizable band gap is created at K, which could be opened and controlled by the potential giving rise to 1D-like van Hove singularities (VHS) in the density of states. • The resonant features in the transmission result from resonant hole states in the barrier and strongly influence the conductance.

The Sensing Mechanism of InAlN/GaN HEMT

The sensing mechanism of InAlN/GaN high electron mobility transistors (HEMTs) is investigated systematically by numerical simulation and theoretical analysis. In detail, the influence of additional surface charge on device performance and the dependence of surface sensing properties on InAlN barrier thickness are studied. The results indicate that the saturation output drain current Idsat and two-dimensional electron gas (2DEG) concentration increase with the increase of positive surface charge density, which decrease with the increase of negative surface charge. The influence of negative surface charge on device performance is more remarkable than that of positive surface charge. Additionally, the modulation ability of surface charge on device performance increases with the decrease ofInAlN barrier thickness. The modulation of surface charge on device performance and the influence of barrier thickness on surface sensing sensitivity are mainly attributed to the variation of the energy band structure, surface potential and 2DEG concentration in the HEMT heterostructure. This work provides important support for structural optimization design of GaN-based HEMT sensors.

Tunable Electronic Properties of Few-Layer Tellurene under In-Plane and Out-of-Plane Uniaxial Strain.

Strain engineering is a promising and fascinating approach to tailoring the electrical and optical properties of 2D materials, which is of great importance for fabricating excellent nano-devices. Although previous theoretical works have proved that the monolayer tellurene has desirable mechanical properties with the capability of withstanding large deformation and the tunable band gap and mobility conductance induced by in-plane strain, the effects of in-plane and out-of-plane strains on the properties of few-layer tellurene in different phases should be explored deeply. In this paper, calculations based on first-principles density functional theory were performed to predict the variation in crystal structures and electronic properties of few-layer tellurene, including the α and β phases. The analyses of mechanical properties show that few-layer α-Te can be more easily deformed in the armchair direction than β-Te owing to its lower Young’s modulus and Poisson’s ratio. The α-Te can be converted to β-Te by in-plane compressive strain. The variations in band structures indicate that the uniaxial strain can tune the band structures and even induce the semiconductor-to-metal transition in both few-layer α-Te and β-Te. Moreover, the compressive strain in the zigzag direction is the most feasible scheme due to the lower transition strain. In addition, few-layer β-Te is more easily converted to metal especially for the thicker flakes considering its smaller band gap. Hence, the strain-induced tunable electronic properties and semiconductor-to-metal transition of tellurene provide a theoretical foundation for fabricating metal–semiconductor junctions and corresponding nano-devices.

Expression of concern: Surface-dependent band structure variations and bond deviations of GaN

Expression of concern for 'Surface-dependent band structure variations and bond deviations of GaN' by Chih-Shan Tan et al., Phys. Chem. Chem. Phys., 2022, 24, 9135-9140, https://doi.org/10.1039/D2CP00100D.

Expression of concern: Surface-dependent band structure variations and bond-level deviations in Cu2O

Expression of concern for ‘Surface-dependent band structure variations and bond-level deviations in Cu2O’ by Chih-Shan Tan and Michael H. Huang, Inorg. Chem. Front., 2021, 8, 4200–4208, https://doi.org/10.1039/D1QI00733E.

Surface-dependent band structure variations and bond deviations of GaN.

Density functional theory (DFT) calculations on a tunable number of GaN (0001) planes give an invariant band structure, density of states (DOS) diagram, and band gap of the GaN unit cell. Dissimilar band structures and DOS diagrams are obtained for 1, 3, 5, 7, and 9 layers of GaN (101̄0) planes, but the same band structure as that of the (0001) plane returns for 2, 4, 6, and 8 (101̄0) planes. Furthermore, 1 to 4 layers of GaN (101̄1) planes exhibit dissimilar band structures, but the GaN unit cell band structure is obtained for 5 (101̄1) planes. While there are no changes to the Ga-N bond length and bond geometry for the (0001) planes, the (101̄0) planes present bond length variation and bond distortion with odd numbers of layers. Bond length and bond direction deviations are also obtained for 1 to 4 (101̄1) planes. These results suggest that slight structural deviations may be present near the GaN surface to produce facet-dependent properties, and such atomic position deviations in the surface layer can be observed in various semiconductors.

Distinct variation of electronic states due to annealing in T′ -type La1.8Eu0.2CuO4 and Nd2CuO4

We performed Cu {\it K}-edge X-ray absorption fine structure measurements on T'-type La$_{1.8}$Eu$_{0.2}$CuO$_4$ (LECO) and Nd$_2$CuO$_4$ (NCO) to investigate the variation in the electronic state associated with the emergence of superconductivity due to annealing. The X-ray absorption near-edge structure spectra of as-sintered (AS) LECO are quite similar to those of AS NCO, indicating that the ground state of AS T'-type LECO is a Mott insulator. We found a significant variation of the electronic state at the Cu sites in LECO due to annealing. The electron density after annealing ($n_{\rm AN}$) was evaluated for both superconducting LECO and non-superconducting NCO and found to be 0.40 and 0.05 electrons per Cu, respectively. In LECO but not in NCO, extended X-ray absorption fine structure analysis revealed a softening in the strength of the Cu-O bond in the CuO$_2$ plane due to annealing, which is consistent with the screening effect on phonons in the metallic state. Since the amounts of oxygen loss due to annealing ($\delta$) for LECO and NCO are comparable with each other, these results suggest distinct electron-doping processes in the two compounds. That electron-doping in NCO approximately follows the relation $n_{\rm AN}=2\delta$ can be understood if electrons are doped through oxygen deficiency, but the anneal-induced metallic nature and large $n_{\rm AN}$ of LECO suggest a variation of the electronic band structure causes self-doping of carriers. The origin of the difference in doping processes due to annealing is discussed in connection with the size of the charge transfer gap.

Floquet Theory of Photoinduced Topological Phase Transitions in the Organic Salt α-(BEDT-TTF)2I3 Irradiated with Elliptically Polarized Light

We theoretically investigate possible photoinduced topological phase transitions in the organic salt $\alpha$-(BEDT-TTF)$_2$I$_3$, which possesses a pair of inclined massless Dirac-cone bands between the conduction and valence bands under uniaxial pressure. The Floquet analyses of a driven tight-binding model for this material reveal rich photoinduced variations of band structures, Chern numbers, and Hall conductivities under irradiation with elliptically polarized light. The obtained phase diagrams contain a variety of nonequilibrium steady phases, e.g., the Floquet Chern insulator, Floquet semimetal, and Floquet normal insulator phases. This work widens a scope of target materials for research on photoinduced topological phase transitions and contributes to development of research on the optical manipulations of electronic states in matters.

Wet Chemistry Vitrification and Metal‐to‐Semiconductor Transition of 2D Gray Arsenene Nanoflakes

AbstractTo manipulate the electrical and optical properties of 2D materials via engineering their phases and crystallinity is of great significance for the construction of nanodevices with versatile functions. Herein, the controllable transformation of semimetallic gray arsenene nanoflakes into semiconducting vitreous arsenene nanoflakes via a wet chemistry vitrification method is reported. Experimental studies and theoretical simulations reveal that the vitrification of gray arsenene nanoflakes is attributed to the consumption of arsenic atoms by aqueous HF via the trigger of dissolved oxygen, resulting in a significant variation of band structure rendered by the formation of atomic structure disorderliness and arsenic atom defects/vacancies. Unlike the semimetallic features of pristine gray arsenene nanoflakes, the as‐prepared vitreous arsenene nanoflakes exhibit a strong photoluminescence peak centered at 635 nm corresponding to an optical band gap of 1.95 eV, and the field‐effect transistors based on vitreous arsenene nanoflakes also exhibit definitely p‐type semiconducting characteristics with a carrier mobility of ≈159.1 cm2 V−1 s−1. The wet chemistry induced vitrification of gray arsenene nanoflakes presents an efficient strategy to regulate the electrical and optical properties of arsenene nanoflakes, providing new insights for the interface and band structure engineering of 2D nanomaterials.

Enhanced photocurrent of self-powered ultraviolet photodetectors based on Ba1−xSrxTiO3 ceramics via ferroelectric polarization

Research of photovoltaic energy conversion devices based on ferroelectric materials has received growing attention in recent years. However, the low photocurrent of the traditional ferroelectric ceramics is the key challenge for the development of these devices. Here, the photocurrent of the self-powered ultraviolet (UV) photodetectors based on Ba1−xSrxTiO3 (BSTO) ceramics is increased by strontium doping and ferroelectric poling. At zero bias, the BSTO/electrolyte heterojunction-based photodetectors demonstrate over 460% enhancement of photocurrent at x = 0.15 compared to that at x = 0.05. Moreover, by applying poling bias from −15 V to +15 V to the BSTO (x = 0.15) sample, the photocurrent can be tuned from 6% to 260% of that of the unpoled BSTO. The variation of energy band structure in depletion region induced by surface ferroelectric polarization is proposed to be responsible for the switchable photoelectrochemical response. This work suggests that substitutional doping of metal ions and ferroelectric polarization has great promise in increasing the photoelectrochemical properties of the ferroelectric ceramics.