Lowest Direct Band Gap Research Articles

Structural electronic and thermodynamic properties of CdX(X: S, Se, and Te) cadmium chalcogenides compound

The structural and electronic properties of (CdS, CdSe, and CdTe) compounds in rock-salt, zinc-blend, and wurtzite crystal structures were calculated using ab initio calculation. In addition to these properties, the thermodynamic properties were added advantage to clarify their comportment as temperature variation. Under the context of density functional theory DFT, the calculations were carried out using the full potential linearized augmented plane wave FP-LAPW approach. The generalized gradient approximations GGA-PBE established by Perdew-Burke-Ernzerhof and the local density approximation LDA and modified Bucke Jhonson have both been employed for the exchange-correlation energy and related potential MBJ. The results show that the zinc-blend phases were the stable crystal structure for all compounds. The lowest direct band gap is found in the B3 phase for CdX, close to the experimental value. The values of band energies of CdS, CdSe, and CdTe were estimated to be 2,463 eV, 1,76 eV, and 1,532 eV, respectively. In general, this work fits well with other experimental and theoretical results. The quasiharmonic Debye theory is used to determine the impact of temperature and pressure on thermodynamic properties. This includes the calculation of pressure and temperature dependence, as well as the analysis of how heat capacity, thermal expansion, and the Debye temperature are affected by these variables.

Titanium dioxide/reduced graphene oxide nanocomposites as effective photocatalytic for hazardous 4-nitrophenol

Titanium dioxide/reduced graphene oxide nanocomposite (TiO2/rGO NCs) was prepared using various hydrothermal strategies as a highly efficient photocatalyst. The crystal structure, morphology, elemental distribution, chemical structure, optical properties, and electrical conductivity of TiO2/rGO NCs were studied to investigate the effect of the incorporation of rGO with TiO2NPs on these properties. The TiO2/rGO NCs surface exhibits thin sheets of rGO with TiO2NPs distributed on the sheets. The successful incorporation between rGO and TiO2NPs was confirmed by EDX maps, IR spectrum, and XRD patterns. Compared with TiO2NPs, a tiny reduction in the crystallite size and an increase in the microstrain was found; in addition, the TiO2/rGO NCs exhibit a low direct bandgap energy, a high electrical conductivity, and excellent photoconductivity. On the other hand, different amounts of 4-nitrophenol (4-NP) were used to investigate the photocatalyst degradation efficiency of TiO2NPs and TiO2/rGO NCs under UV irradiation. The higher degradation efficiency after 120 min was for TiO2/rGO NCs with 5 mg/L and 10 mg/L of 4-NP, with 94% and 91% values, respectively. The first-order kinetic constant (k1), the kinetic rate constant of the surface reaction (ks) and the adsorption equilibrium constant (KL) were evaluated. The KL of TiO2NPs and TiO2/rGO NCs is 0.003 and 0.182 (mgL−1)−1, and the ks is 2.53 and 0.21 mgL−1.min−1, respectively.

Structural stability, opto-electronic, magnetic and thermoelectric properties of half-metallic ferromagnets quaternary Heusler alloys CoFeXAs (X = Mn, Cr and V)

The Quaternary Heusler alloys CoFeXAs (X = Cr, Mn and V) fully spin-polarized with half-metallic stability, show low direct band gap, high absorption in the ultraviolet light, high spin polarization and adequate Seebeck coefficient. CoFeCrAs, CoFeMnAs and CoFeVAs have an integer magnetic moment of 4 μB, 5 μB and 3 μB according to the Slater-Pauling rule. The obtained minimal energy favors the type I CoFeVAs alloy over the type III CoFeCrAs and CoFeMnAs alloys. For CoFeCrAs and CoFeMnAs alloys, there is no band gap close to the Fermi level in the minority-spin, suggesting that they are direct Г–Г band gap semiconductors. Three-dimensional Co, Fe, and Mn atoms hybridized strongly, with little input from V and As atoms. Co, Fe, Cr, and Mn all have parallel magnetic moments, which results in ferromagnetic interactions between these atoms and gives these elements their ferromagnetic character.

Studies on photovoltaic properties of BFO/WO3 bilayer thin films for solar energy harvesting applications

BFO possess high remnant polarization (∼100 μC/cm2) and low direct band gap (∼2.7 eV) having potential for photovoltaic (PV) applications. In the present work, WO3 insulating layer is utilized with BFO and BFO/WO3 bilayer thin film structure is fabricated for reducing the leakage current along with possessing the low band gap and high ferroelectric polarization. In BFO/WO3 bilayer structure, the WO3 nanostructured thin film layer was fabricated utilizing rf-magnetron sputtering technique under the optimized parameters while BFO thin film layer was fabricated by altering the incident energy of excimer laser (150 mJ − 250 mJ) in Pulsed Laser deposition technique. BFO/WO3 bilayer structure prepared at 200 mJ laser energy demonstrated the superior ferroelectric properties (Ps = 63.24 µC/cm2 and Pr = 25.94 µC/cm2) with high values of short circuit photocurrent density (Jsc = ∼18 mA/cm2) and open-circuit voltage (Voc = ∼320 mV) under the UV illumination of Intensity = 160 mW/cm2 and λ = 355 nm. Obtained results suggest the utilization of prepared BFO/WO3 bilayer thin film structures on future novel energy harvesting purposes.

Optical properties of ScN layers grown on Al2O3(0001) by plasma-assisted molecular beam epitaxy

Accurate knowledge of optical constants (refractive index n and extinction coefficient k) of ScN is crucial for understanding optical properties of this binary nitride semiconductor as well as for its use in optoelectronic applications. Using spectroscopic ellipsometry in a spectral range from far infrared to far ultraviolet (0.045–8.5 eV), we determine n and k of ScN layers grown on Al2O3(0001) substrates by plasma-assisted molecular beam epitaxy. Fits of ellipsometry data return the energies of four oscillators representing critical points in the band structure of ScN, namely, 2.03, 3.89, 5.33, and 6.95 eV. As the infrared range is dominated by free carriers, the vibrational properties of the layers are examined by Raman spectroscopy. Despite the rock salt structure of ScN, several first-order phonon modes are observed, suggesting a high density of point defects consistent with the high electron density deduced from Hall measurements. Finally, photoluminescence measurements reveal an emission band slightly above the lowest direct bandgap. We attribute the redshift of the peak emission energy from 2.3 to 2.2 eV with increasing layer thickness to a reduction in the O concentration in the layers.

Topotactical Route to Multiwalled Cerium Oxide Nanotubes from MWCNTs

AbstractThe development of new strategies for synthesizing 1D cerium oxide (CeO2) hollow nanostructures has attracted much attention in recent years due to the importance of their superior properties and highly anisotropic geometry. This study reports an unpublished route of fabricating novel multiwalled CeO2‐δ nanotubes (CeO2‐δ NTs) in which the entire volume of functionalized multiwalled carbon nanotubes (f‐MWCNTs) is converted into the CeO2‐δ pseudomorph through oxidation and dehydration topotactic reactions. The stable CeO2‐δ (111) planes are topotactically grown on the curved C (002) planes, preferentially exposed along the nanotube axis. In their initial condition, the novel nanotubes consist of Ce oxyhydroxide (CeO2‐x(OH)x) with residual carbon. When heating the air up to 500 °C, CeO2‐x(OH)x transforms gradually by dehydration into CeO2‐δ, while the residual carbon is oxidized. Despite compositional changes, nanotubes maintain their multiwalled structural integrity up to ≈550 °C. The CeO2‐δ NTs exhibit an unusually high presence of Ce3+ ions and surface O vacancies, contributing to a low direct band gap ranging from 2.67 to 2.32 eV compared to their NPs counterparts (3.2 eV).

Study of structural and microstructural contributions to the optical and dielectric properties of NBBT binary relaxor composites substituted with trivalent and non-magnetic ions

Pb-free ceramic composites with tremendous energy storage performance have been optimized with alkaline-based additives. Herein, dual trivalent and nonmagnetic ions substituted binary [Na0.5Bi0.5-xLax]0.94Ba0.06Ti1-yAlyO3] relaxor ferroelectric ceramics have been designed and synthesized by conventional mixed oxide route reaction. The accumulated trivalent ions show the Morphotropic Phase Boundary (MPB) in all compositions. Moreover, the introduction of La3+ in different ratios reduces strain and increases grain size. The oxidation state of the prepared compound is tested by X-ray photoelectron spectroscopy. The lowest direct band gap was observed at an x = 0.02 ratio, which clearly indicates that this system is a photoactive material. Dielectric analysis denotes diffuse type relaxor behavior with the existence of grain and grain boundary resistance for all La3+ concentrations. The activation energy calculated from the Arrhenius law shows a low value with an increase in the ratio of rare earth (RE) ions, indicating the oxygen and Bi vacancies in the ceramic system.

B[sbnd]N counterpart of biphenylene network: A theoretical investigation

In this work, a new BN counterpart of biphenylene network (bi-BN) with 4-, 6-, and 8-membered rings is predicted with the aid of the Density Functional Theory (DFT) calculations. Its structural stability is carefully verified from different aspects. This two-dimensional (2D) material has a very low cohesive energy indicating the high synthetic possibility, compared to the other theoretical proposed 2D boron nitride (BN) layer systems. Its mechanic and electronic properties are also investigated. The monolayer bi-BN possesses a relatively low direct band gap among the 2D BN systems. The stacking form of the layered 3D bi-BN system is also discussed. The stacking form can significantly affect its electronic configuration. Its physical properties can be manipulated by applying external stress, and therefore, its application potentials can be further developed.

Structural and mechanical stability, lattice dynamics and electronic structure of the novel CrVZ (Z = S, Se, & Te) half-Heusler alloys

To meet energy demand, half-Heuslers (HH) are proved to be a cost-effective and energy-efficient choice for advanced spintronic and energy storage applications. The structural, electronic, magnetic, vibrational, elastic, thermodynamic and thermoelectric properties of the innovative HH CrVZ (where Z = S, Se, & Te) alloys are studied by using density functional theory (DFT). All materials are found to be magnetically active with 100% spin polarization where partially filled 3d-orbitals of Cr confirm the major contribution to the magnetic moment. The ferromagnetic (FM) state is observed to be the most energetically stable state among the non-magnetic (NM) and antiferromagnetic (AFM) states for all the HH CrVZ alloys with their optimized lattice parameters ranging from 5.47 Å to 6.00 Å. The HH CrVSe alloy is found to have the lowest direct bandgap (EBG) of 1.07 eV with a comparatively highest half-metallic gap (EHM) of 0.44 eV, due to the d-d hybridization. Our calculated cohesive (EC) and formation (Ef) energies indicate the chemical and thermodynamic stability of the studied HH CrVZ alloys. The phonon dispersion curves for the HH CrVSe and CrVTe alloys show that they are vibrationally stable while HH CrVS alloy shows soft modes with imaginary frequency due to the small ionic size of the S-ion. The calculated Cauchy pressure, Pugh and Poisson ratio prove that all studied compounds are hard, elastically brittle, anisotropic, and mechanically stable. The temperature fluctuations for specific heat (CV), Entropy (S) and Free Energy (F) follows the Quasi-harmonic Debye model with expected Dulong-petit limit 75 J/mol/K. The thermoelectric investigations although show the lower values of the Seebeck coefficient and power factor. Our investigated half-metallic HH CrVZ alloys can be the potential contenders for the spintronic applications.

Refractive index and polarizability of polystyrene under shock compression

The refractive index, polarizability and thermodynamic response of polystyrene under shock compression were investigated through experiments and theoretical analysis, and a relationship between the refractive index and the density, pressure and temperature of the polystyrene was obtained. Above a pressure of 20 GPa, an obvious inflexion was observed in how the refractive index of the polystyrene varied with the thermodynamic variables; in particular, it was found to depend strongly on the temperature (as obtained using a semiempirical complete equation of state). Relating the measured refractive index to the polarizability indicates that the polarizability decreased from 1.28 × 10−23 cm3 at ambient conditions, to 0.98 × 10−23 cm3 at pressure of 33 GPa, indicating that the lowest direct band gap $$ E_{t} $$ of polystyrene becomes < 2 eV, similar to that of many semiconductor materials.

Plasmonic Resonance Enhanced Polarization-Sensitive Photodetection by Black Phosphorus in Near Infrared.

Black phosphorus, a recently intensely investigated two-dimensional material, is promising for electronic and optoelectronic applications due to its higher mobility and thickness-dependent direct band gap. With its low direct band gap and anisotropic properties in nature, black phosphorus is also suitable for near-infrared polarization-sensitive photodetection. To enhance photoresponsivity of a black phosphorus based photodetector, we demonstrate two designs of plasmonic structures. In the first design, plasmonic bowtie antennas are used to increase the photocurrent, particularly in the armchair direction, where the optical absorption is higher than that in the zigzag direction. The simulated electric field distribution with bowtie structures shows enhanced optical absorption by localized surface plasmons. In the second design, bowtie apertures are used to enhance the inherent polarization selectivity of black phosphorus. A high photocurrent ratio (armchair to zigzag) of 8.7 is obtained. We choose a near-infrared wavelength of 1550 nm to demonstrate the photosensitivity enhancement and polarization selectivity, as it is useful for applications including telecommunication, remote sensing, biological imaging, and infrared polarimetry imaging.

Ultraefficient Ultraviolet and Visible Light Sensing and Ohmic Contacts in High-Mobility InSe Nanoflake Photodetectors Fabricated by the Focused Ion Beam Technique.

A photodetector using a two-dimensional (2D) low-direct band gap indium selenide (InSe) nanostructure fabricated by the focused ion beam (FIB) technique has been investigated. The FIB-fabricated InSe photodetectors with a low contact resistance exhibit record high responsivity and detectivity to the ultraviolet and visible lights. The optimal responsivity and detectivity up to 1.8 × 107 A W-1 and 1.1 × 1015 Jones, respectively, are much higher than those of the other 2D material-based photoconductors and phototransistors. Moreover, the inherent photoconductivity (PC) quantified by the value of normalized gain has also been discussed and compared. By excluding the contribution of artificial parameters, the InSe nanoflakes exhibit an ultrahigh normalized gain of 3.2 cm2 V-1, which is several orders of magnitude higher than those of MoS2, GaS, and other layer material nanostructures. A high electron mobility at room temperature reaching 450 cm2 V-1 s-1 has been confirmed to be one of the major causes of the inherent superior PC in the InSe nanoflakes. The oxygen-sensitized PC mechanism that enhances carrier lifetime and carrier collection efficiency has also been proposed. This work demonstrates the devices fabricated by the FIB technique using InSe nanostructures for highly efficient broad-band optical sensing and light harvesting, which is critical for development of the 2D material-based ultrathin flexible optoelectronics.

Synthesis and Characterization of ZnCr2O4 Nanospinel Prepared via Homogeneous Precipitation Using Urea Hydrolysis

In situ and homogeneous hydrolysis of urea was utilized to design a homogeneous precipitation method to synthesize the ZnCr2O4 functional material. The as‐prepared nanospinel was systematically studied in terms of crystal structure, morphology, texture, and light absorption. A well‐crystalline single phase of ZnCr2O4 nanospinel with a spongy network structure was obtained. Transmission electron microscopy and X‐ray diffraction observations confirmed the formation of monocrystalline nanoparticles of ZnCr2O4 with the nanosize of 13 nm. The synthesized nanospinel showed a low direct band gap value of 1.8 eV with appropriate absorption in the visible light region, which was a remarkable result for this route.

Band engineering and growth of tensile strained Ge/(Si)GeSn heterostructures for tunnel field effect transistors

In this letter, we propose a heterostructure design for tunnel field effect transistors with two low direct bandgap group IV compounds, GeSn and highly tensely strained Ge in combination with ternary SiGeSn alloy. Electronic band calculations show that strained Ge, used as channel, grown on Ge1−xSnx (x &amp;gt; 9%) buffer, as source, becomes a direct bandgap which significantly increases the tunneling probability. The SiGeSn ternaries are well suitable as drain since they offer a large indirect bandgap. The growth of such heterostructures with the desired band alignment is presented. The crystalline quality of the (Si)Ge(Sn) layers is similar to state-of-the-art SiGe layers.

Electroreflectance Study of (AlxGa1-x)0.5In0.5P Alloys

Electroreflectance spectroscopy has been applied to the study of (Alx Ga1-x )0.5In0.5P quaternary systems lattice-matched to GaAs. The measurements are made at room temperature in the photon-energy range of 1.7–5.8 eV. The composition dependence of the E0, E0+Δ0, E1, E2 and E2+δ gaps has been determined. The lowest direct band gap is found to be given by E0(x)=1.90+0.57x+0.11×2 eV. The E0+Δ0, and E1 gaps are written as similar quadratic equations. On the other hand, the variation of the E2 and E2+δ gaps is found to be approximately linear. The lowest direct-to-indirect gap crossover is also estimated to occur at x∼0.63.

Photoconductivity in solid films of C 60/70

We report photoeffects in the visible and near-infrared spectrum in submitted films of C 60/70. The peak photoefficiency (photocarrier per absorbed photon) is ≈ 10 −4. Since for wavelengths > 7000 Å, absorption in C 60/70 films is very weak, carrier photoinjection from the electrodes into the conduction states of C 60/70 must be considered. Evidence, based on the energy level structure of C 60 films, is discussed which suggests this may not be the source of the photocurrents. Alternatively, the observed threshold may be photoconductive edge associated with the reported weak absorption due to forbidden transitions in the lowest direct bandgap, which recent calculations place at 1.5 eV.

Erratum: Resonant Raman studies of confined LO modes and interface modes in a small-period GaAs/AlAs superlattice

Resonant Raman scattering has been used to study the confined LO modes and interface modes near the lowest direct band gap of a GaAs/AlAs superlattice. We have studied the polarization dependence of the resonance profiles in detail. The peak positions of the polarized and depolarized resonance profiles are not the same. When the laser is polarized along (110) or (11-bar0), we see an interference effect for some of these modes similar to that seen by Menendez and Cardona in bulk GaAs. In bulk GaAs, this interference is an intrinsic effect. In superlattices, this effect is a violation of the selection rules for confined LO modes; we attribute this ''forbidden'' scattering by confined LO modes to an impurity-induced Froehlich mechanism.

Optical Properties and Electronic Structure of ZnSiAs2

We report electroreflectance spectra for the chalcopyrite crystal ZnSi${\mathrm{As}}_{2}$. Structure in the electroreflectance spectra is observed at 2.12, 2.22, and 2.47 eV due to direct energy gaps in ZnSi${\mathrm{As}}_{2}$ corresponding to the ${E}_{0}$ and ${E}_{0}+{\ensuremath{\Delta}}_{0}$ direct energy gaps in zinc blende. The ordering and splittings of these transitions as well as their strong polarization dependences are explained quantitatively by a simple model which regards the chalcopyrite lattice as a compressed version of its binary analog. We also observe structure at 2.74 and 2.90 eV corresponding to the ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ structures in zinc blende. The splittings and polarization dependences agree with observations in stressed zinc-blende crystals. Much additional structure observed in ZnSi${\mathrm{As}}_{2}$ is attributed to "pseudodirect" band gaps which result from the doubling of the unit cell in the $Z$ direction in chalcopyrite relative to zinc blende. This change in the unit cell causes the Brillouin zone of zinc blende to be imbedded into the smaller Brillouin zone of chalcopyrite, and as a result new direct transitions can occur corresponding to indirect transitions in zinc blende. This is the first study of the electroreflectance spectra of a crystal which should have a pseudodirect transition as its lowest band gap. We do not observe this particular transition, and estimate that it is at least an order of magnitude weaker than the direct transitions derived from ${\ensuremath{\Gamma}}_{15}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1}$ in zinc blende. We have also observed the dichroism of ZnSi${\mathrm{As}}_{2}$ at a wavelength slightly below the lowest direct band gap. We find that the transmission is considerably higher for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\perp}Z$ than for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\parallel}Z$, as predicted by the quasicubic model.