Presence Of Localized States Research Articles

Computational study on the structural, electronic, lattice vibration, and magnetism in Zn(1−x)FexSeyTe(1−y) quaternary materials

In this article, we studied the structural, electrical, lattice vibrational, and magnetic properties of the quaternary compound Zn(1−x)FexSeyTe(1−y) using density functional theory. All the calculations have been performed based on first-principles calculations using Perdew–Zunger [local-density approximation (LDA)] and Hubbard parameter correction (LDA+U) functionals as employed in the Quantum Espresso package. The computed equilibrium lattice parameter for ZnTe is 6.01 Å, and the energy bandgap, Eg, is 1.362 eV, which is consistent with the experimental values as well as the previous reports, respectively. The influence of the co-doping of iron and selenium on electrical and magnetic properties in a ZnTe system is discussed in detail. The co-doping of iron and selenium affects metallic behavior in these systems by forming localized states between the conduction and valance bands. The presence of localized states is related to the metallic properties of the iron atom, specifically iron 3d orbitals. The spin-polarized density of state and band structure computations also confirmed that the iron and selenium co-doped ZnTe system exhibits significant half-metal ferromagnetic and dilute magnetic semiconductor features at room temperature. Furthermore, the phonon calculation of these systems indicated that the systems are dynamically stable and that localized frequency states are created at higher frequencies due to the presence of iron atoms. As a result, the iron and selenium co-doped ZnTe systems can be considered for magnetic and spintronic device applications at room temperature, pending further experimental research.

Comparative study on gas-sensing properties of amorphous MoS2 nanoparticles and crystalline MoS2 nanoflowers

High-performance gas sensors for the field of gas sensing have drawn a lot of attention to molybdenum sulfide materials, but there is little research on the influence of crystallization state of MoS2 on gas sensitivity performance. In this study, amorphous MoS2 and crystalline MoS2 were prepared by the hydrothermal method. Then, the gas-sensing characteristics of the as-synthesized amorphous MoS2 and crystalline MoS2 toward NO2 gas were thoroughly investigated and compared. According to the results, with an excellent response value of 1.99 toward 5 ppm NO2 at room temperature, the crystalline MoS2 gas sensor outperformed the amorphous one, while that of the amorphous MoS2 under the same condition was 1.25. However, Four times faster recovery periods were observed in the amorphous MoS2 sample compared to the crystalline MoS2 sample. At the same time, the recovery level of amorphous MoS2 is high and can be fully restored. We discovered that the primary cause of the strong recovery performance of amorphous MoS2 is the presence of localized states that facilitate the easy transition of electrons from the conduction band to the valence band, allowing for effective recombination with the holes. Additionally, the surface of the material absorbs a significant amount of oxygen, which will occupy more of the NO2 adsorption site and hasten the desorption of NO2. This work may aid in the development of gas sensors based on amorphous molybdenum sulfide.

Early formation of surface states in MOS structures under ionizing irradiation

A quantitative model of the early formation of surface states (SS) in MOS structures under ionizing irradiation (II) with a limiting stage --- dispersion transport of holes has been developed. According to the model, the main contribution to the early formation of SS occurs during the microsecond pulse II for a thin gate dielectric and after the end of the pulse for a thick field oxide. The increase in the density of early SS after the end of II is associated with the presence of localized states and the dispersion transport of holes. The late formation of SS is limited by the dispersion transport of hydrogen ions, which delays the formation of late SS from 0.1 to 104 seconds or more. Keywords: MOSFET structure, ionizing irradiation, surface conditions, dispersion transport.

High photoactivity of ZrxOy-Bi2O2(CO3) composite materials prepared by one-step synthesis for efficient photodegradation of 4-chlorophenol in water

The ZrBi composite materials were prepared in a single step by the chemical co-precipitation method. Bi2O2(CO3) joined ZrxOy modifying the mole percentage from 5 to 11 mol %. ZrBi composites dried at 80 °C were characterized by: XRD, FTIR, DRS, SEM, N2 adsorption, HR-TEM, PL, XPS and EDS-SEM. Later they were evaluated in the photodegradation of 4-Chlorophenol under UV irradiation. The degradation and mineralization percentages were determined after 150 min of reaction by UV–vis spectroscopy and Total Organic Carbon (TOC), respectively. The composite containing 9 mol % of Bi2O2(CO3) (ZrBi-9) with a specific volume of 0.03 m3/kg in solution with specific catalyst density of 1*10−3 kg/L, showed the highest photoactivity with 96 % photodegradation, a higher value compared to that obtained with the reference material TiO2-P25 (72 %). Finally, a possible reaction mechanism was proposed based on recent studies, which allow to follow the formation of active species OH, O2− and h+. The formation of OH radicals was measured by fluorescence spectroscopy while the inhibition of O2− radicals as well as the scavenge of h+ were determined by UV–vis spectroscopy. The results showed that ZrBi composite materials do not promote OH radical formation. The scavenge of h+ showed a total loss of photoactivity and the inhibition of O2− in the reaction medium results in a partial decrease of photoactivity. The formation of heterojunctions, the presence of localized states and oxygen vacancies in the synthesized materials promise an excellent alternative for the rapid photodegradation of 4-Chlorophenol under UV radiation.

Unveiling the electron-nuclear spin dynamics in an n -doped InGaAs epilayer by spin noise spectroscopy

We discuss the implications of a small indium content (3%) in a GaAs epilayer on the electron- and nuclear-spin relaxation due to enhanced quadrupolar effects induced by the strain. Using the weakly perturbative spin-noise spectroscopy, we study the electron-spin relaxation dynamics without explicit excitation. The observed temperature dependence indicates the presence of localized states, which have an increased interaction with the surrounding nuclear spins. Time-resolved spin-noise spectroscopy is then applied to study the relaxation dynamics of the optically pumped nuclear-spin system. It shows a multi-exponential decay with time components, ranging from several seconds to hundreds of seconds. Further, we provide a measurement of the local magnetic field acting between the nuclear spins and discover a strong contribution of quadrupole effects. Finally, we apply the nuclear spin diffusion model, that allows us to estimate the concentration of the localized carrier states and to determine the nuclear spin diffusion constant characteristic for this system.

Study of High field conduction in Se based Chalcogenide glasses

The present work reports the study of high field conduction in thin films of Se90Ge10-x Inx (x=2,6) because high field conduction in chalcogenide glasses is affected by the presence of localised states at the band edges as well as the defect states present in the mobility gap.To measure the density of states in these thin films , space charge limited conduction technique is used. I-V characteristics have been measured at various fixed temperatures. An ohmic behaviour is observed at low electric fields upto 102V/cm. Superohmic behaviour is observed at high electric fields (103 -104 V/cm).High field conduction theory of space charge limited conduction for uniform distribution of localised states in the mobility gap fits well with the experimental data.Using this theory, the density of defect states near Fermi level is calculated for all glassy alloys.

Wavelength-dependent generation of reactive species in the photodegradation process over pure and C-doped Nb2O5

Reactive species, such as hydroxyl radicals (·OH), holes (h+) and superoxide radicals (·O2−), are considered as the main active substances involved in the photocatalytic degradation of organic pollutants. However, to the best of our knowledge, the effect of the photon energy on the generation of these species has never been examined in detail. In this study, the photocatalytic production of ·OH, h+ and ·O2− over niobium pentoxide (Nb2O5) was investigated as a function of the excitation wavelength. The ESR results clearly show that all three active species can be generated at 355 nm for both pure and C-doped Nb2O5. At 532 nm, which is well beyond the absorption edge of Nb2O5 (399 nm), only h+ and •O2− are produced over pure Nb2O5, while at the same time all three species can be produced over C-doped Nb2O5. The results of the trapping experiments indicate that •O2− and h+ are the main reactive species that are responsible for the photocatalytic degradation. The photogenerated holes play a crucial role in the wavelength range of 365–420 nm but become less active with increasing wavelength. •O2− shows little dependence on wavelength and is consistently active at all wavelengths between 365 and 630 nm, in particular, for C-doped Nb2O5. It is concluded that the presence of localized states within the forbidden gap has a profound impact on the photocatalytic activity in the visible-wavelength region.

First principle investigation of polaronic resistive switching behavior in titania based memristors with different charge states

Resistive random-access memory (RRAM) also referred as memristors are considered a critical component of nano-electronics. In this first principle study we presented a polaron related model. We revealed polaron; a conducting species in mediating resistive switching (RS) mechanism in anatase polymorph of TiO2 bulk based memristors. We examined polaron formation after substitution of silver (Ag), copper (Cu) and nickel (Ni) in place of titanium (Ti) atom. Influence of charge addition (q- and q--), charge removal (q+ and q++) and neutral (q0) states of doped system on polaron based conducting filaments are also explored. Various values of formation energy indicated the influence of charged state variation on conductivity of the three doped systems. Findings of iso-surface charge density plots indicated polaron formed around the substitutional dopants characterized by localized states in band gap region of density of states (DOS). Presence of localized state in band gap region that confirm the achievement of RS mechanism in these doped systems. Our results predicted Cu as a best substitutional dopant for polaron as a conducting species for RS mechanism. Integrated charge density plots also confirmed these results. Interstitially doped Ag, Cu and Ni doped TiO2 systems with different charged states showed the conducting filaments formed for RS mechanism with no polaron formation. This study suggests that charged states tuned polaron in substitutional doped TiO2 will allow to construct conducting pathway for polaronic RS mechanism in nanoelectronics beyond RRAM.

Addition of SnO2 over an oxygen deficient zirconium oxide (ZrxOy) and its catalytic evaluation for the photodegradation of phenol in water

The ZrSn composites materials were prepared in one pot by chemical co-precipitation method. SnO2 was incorporated to ZrxOy modifying the molar percentage from 1 to 5 mol%. The ZrSn composites were characterized by different techniques: XRD, FTIR, DRS, SEM, N2 physisorption and HR-TEM. The ZrSn composites were dried at 80 °C and thereafter were evaluated in the photodegradation of phenol under UV irradiation. The percentages of degradation and mineralization were determined after a reaction time of 150 min by UV–Vis spectroscopy and Total Organic Carbon analysis (TOC), respectively. The composite containing SnO2 in a 3 mol% showed the highest photoactivity with a 72% of photodegradation, a higher value compared with the obtained with TiO2-P25 (62%). Finally, a possible reaction mechanism was proposed based on certain studies, which allows to follow the formation of the active species •OH, •O2− and h+. The formation of the •OH specie was measured by fluorescence spectroscopy whereas the inhibition of the species •O2− and h+ was determined by UV–Vis spectroscopy. The results showed that the ZrSn composites not promotes the hydroxyl radical formation. In addition, the holes capture showed a full-loss of the photoactivity while the minimization of (•O2−) radicals in the reaction media results in a decrement of the photoactivity. The formation of heterojunctions and the presence of localized states in the synthesized composites offer an excellent alternative for a fast photodegradation of phenol.

Co doped ZrO2 nanoparticles: An efficient visible light triggered photocatalyst with enhanced structural, optical and dielectric characteristics

In this work ZrO2 and Co doped ZrO2 (tetragonal) nanoparticles (NPs) were prepared via facile co-precipitation method. Multivalent Co-ZrO2 NPs were investigated for structural, optical, magnetic, dielectric and photocatalytic applications. A detailed structural analysis confirmed successful substitution of dopant ions in zirconia lattice with an average crystallite size of 18.0 ± 1.7 nm. SEM analysis depicted narrow, spherical like morphology for both pure ZrO2 and Co-ZrO2 NPs. DRS spectra revealed substantial tuning of band gap from 3.7 to 2.12 eV with Co doping owing to crystal defects. Moreover, incorporation of Co contents in ZrO2 matrix had significantly enhanced the dielectric constant up to 2.8 × 104 with a very low loss, which is attributed to the size of the nanoparticles along with presence of localized states between valence band and conduction band. The room temperature ferromagnetism (RTF), driven by creation of oxygen vacancies coupled with dopant concentration in the host lattice, enhanced the degradation process. Interestingly, Co-ZrO2 NPs have shown effective visible light triggered photocatalytic activity against methylene blue (MB). The significant removal of MB (96.5%) was achieved using 10% Co doped ZrO2 NPs, which was consigned to the efficient control on the recombination rate. These findings illustrate that Co-ZrO2 NPs can be a profound alternate for futuristic high frequency devices, and waste water treatment processes.

Effect of coexistence of defect and dopant on the quantum capacitance of graphene-based supercapacitors electrodes

The influence of doping (B, N, Al, Si, P, S), vacancy, and Stone-Wales defect on stability, electronic structures, quantum capacitance, and surface charge storage of graphene is explored by density functional theory calculations. The results indicate that the quantum capacitance and surface charge storage can be significantly improved by the introducing of defect and doping. Furthermore, the enhancement of doped vacancy-defected graphene is more obvious than that of doped Stone-Wales defected graphene. By analyzing the density of states, it can be obtained that the improvement of quantum capacitance is resulted from the presence of localized states around Fermi level by doping or defect. Moreover, the effect of concentration of doping and defect on the graphene is also explored, and the results suggest that the quantum capacitance increases with the increasing of concentration for most modified graphene-based substrates. The findings can provide effective approach for improving the performance of graphene-based surpercapacitors.

Electronic structure of GaP/Si(001) heterojunctions and the role of hydrogen passivation

AbstractEpitaxially grown single crystal GaP on Si is of considerable interest due to being nearly lattice matched to Si, making it attractive for III‐V/Si solar cells. GaP has been used as a buffer layer for III‐V/Si solar cells and also in selective contact Si solar cells. The performance and functionality of such devices are strongly influenced by the presence of localized states at the GaP/Si interface. Here, we examine the electronic structure of GaP/Si(001) heterojunctions and the effect of hydrogen (H) passivation at the interface, in contrast to interface mixing, through density functional theory calculations. Our calculations show that due to the heterovalent mismatch nature of the GaP/Si interface, there is a high density of localized states at the abrupt GaP/Si interface due to the excess charge associated with heterovalent bonding, as reported elsewhere. We find that the addition of H leads to additional bonding at the interface, which mitigates the charge imbalance, and greatly reduces the density of localized states, leading to a nearly ideal heterojunction. A similar result is found with a completely intermixed interface (alternating cation and anion bonding) in terms of low interface state density. However, when the intermixing occurs through single‐layer or double‐layer terraces, the benefits of intermixing are lost and the interface state density reverts more to the abrupt interface case.

Photonic systems with two-dimensional landscapes of complex refractive index via time-dependent supersymmetry

We present a framework for the construction of solvable models of optical settings with genuinely two-dimensional landscapes of refractive index. The solutions of the associated non-separable Maxwell equations in paraxial approximation are found with the use of the time-dependent supersymmetry. We discuss peculiar theoretical aspects of the construction. Sufficient conditions for existence of localized states are discussed. Localized solutions vanishing for large $|\vec{x}|$, that we call light dots, as well as the guided modes that vanish exponentially outside the wave guides are constructed. We consider different definitions of the parity operator and analyze general properties of the $\mathcal{PT}$-symmetric systems, e.g. presence of localized states or existence of symmetry operators. Despite the models with parity-time symmetry are of the main concern, the proposed framework can serve for construction of non-$\mathcal{PT}$-symmetric systems as well. We explicitly illustrate the general results on a number of physically interesting examples, e.g. wave guides with periodic fluctuation of refractive index or with a localized defect, curved wave guides, two coupled wave guides or a uniform refractive index system with a localized defect.

Photoluminescence activity of BaTiO3 nanocubes via facile hydrothermal synthesis

Free from scaling effect is highly desirable but yet challenging in lead-free BaTiO3 (denoted as BTO). In this work, we revisit the synthesis BTO nanocubes via hydrothermal route and provide an insight on the photoluminescence behavior of BTO nanocubes, whilst others focus on the reproducibility of BTO nanocubes. The crystallinity of the BTOs was enhanced when the as-synthesized powders underwent calcination at elevated temperatures (> 500 °C). A combination of X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) images affirms that the BTO nanostructures evolved from cubic to tetragonal and changing from pseudo-ellipsoid to nanocube with {100} sharp facets. The tunable optical band gaps (3.18–2.74 eV), Urbach tails in UV–Vis absorption spectra and the enhanced intensity of photoluminescence at violet wavelength (433.7 nm) indicate the presence of localized state. In order to sort out the origin of the localized state, either stems from structural disorder or surface state, time-resolved photoluminescence was carried out. The long decay of the time-resolved photoluminescence (> 100 ns) proves the dominant involvement of self-trapped states. This in turn narrows the band gap, thus facilitates photoluminescence excitation.

Unraveling the Dirac fermion dynamics of the bulk-insulating topological system Bi2Te2Se

Using femtosecond time- and angle-resolved photoemission spectroscopy, we explore the out-of-equilibrium dynamics of surface fermions in the topological system ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$. We show that the presence of localized states from defects at the surface is one of the key material parameters undergirding the long relaxation time of photoexcited Dirac electrons lying in the projected band gap of the bulk-insulating pristine compound. Doping this ternary compound with Sn substituting on Bi sites, while desirably increasing the resistivity at low temperatures decreases decay times of the excited homologous Dirac electrons. On the basis of these observations, we argue that long relaxation times can be ultimately controlled by a charge transfer to the surface.

Coupling between quantum Hall edge channels on opposite sides of a Hall bar

We investigate the coupling between quantum Hall (QH) edge channels (ECs) located at opposite sides of a 50-μm-wide Hall bar by exciting a charged wavepacket in one EC and detecting time-dependent current in the other EC. In a QH state, the current shows a peak followed by a dip, demonstrating the existence of capacitive coupling across the incompressible two-dimensional electron system (2DES). The observed magnetic field dependence of the amplitude and time delay of the current suggests that the capacitance is affected by the presence of localized states. We also show that the dominant manner of the coupling changes gradually as the system changes between the QH and non-QH states.

Discrete and selective absorption in crystalline molecular nanofilms

Recent research in nano-optical engineering and in nanomedicine as well, seeks for methods of construction of various types of nano-markers, nano-carriers, and ways to deliver drugs to the exactly determined regions of body. In this process it is important to find methods of recognition of certain types of molecules. It is obvious that optical recognition would be the easiest and the most effective way to do it. Our research presents a model of a molecular ultrathin crystalline film and generated exciton system inside it and corresponding methodology of analysis of their optical characteristics. Properties of these spatially very restricted structures are very sensitive to their surrounding surfaces. Using the two-time Green’s functions adapted for crystalline structures with symmetry breaking, and graphical-numerical software, we have calculated the energy spectra and possible exciton states. We have shown that the appearance and the presence of localized states on the surfaces and in the boundary layers of the film depend on the thickness of the film and the film surroundings, presented through the perturbation of parameters on surfaces. Optical properties in these structures demonstrate discrete and very selective resonant absorption spectra, depending on the perturbation on their surfaces.

Effect of rapid thermal annealing on the optical properties of GaAsSb alloys

GaAsSb ternary alloys are fundamental components of advanced electronic and optoelectronic devices in the future. The presence of localized states could greatly affect the optical properties in GaAsSb alloy, which depend on the fluctuation of alloy composition. In order to optimize the optical properties, GaAsSb alloys were treated by rapid thermal annealing (RTA) at different temperatures, and the optical behaviors of the annealed samples were investigated in detail. During RTA, a significant reduction of the localized states was observed by photoluminescence (PL) spectral analysis. Furthermore, the RTA process also altered the distribution of the components of the GaAsSb alloy, which caused a slight red-shift of the maximum PL peak at 150 K. The relationship between the localized states and the temperature of the RTA process was also investigated. The process involving the conversion of localized carriers to free carriers was proposed. Under the suitable RTA conditions, the Sb component was homogenized and the depth of carrier localization was decreased.

First-principles studies on the effects of halogen adsorption on monolayer antimony.

Using first-principles calculations, we carry out systematic studies on the electronic, magnetic and structural properties of halogenated β-phase antimonene. We consider two different levels of halogen adatom coverage i.e. Θ = 1/8 and Θ = 1/18. It is found that F, Cl and Br adatoms act as acceptors whereas the I adatom acts as a donor. For a high coverage of Θ = 1/8, halogenated β-phase antimonene exhibits metallic characteristics. With a lower coverage of Θ = 1/18, through the adsorption of F, Cl and Br the semiconducting unstrained antimonene becomes metallic. In contrast, I-adsorbed antimonene remains semiconducting but exhibits magnetic behavior. We further investigate the effects of bi-axial strain on the halogenated β-phase antimonene. It is found that bi-axial strain can only induce ferromagnetism on the halogenated antimonene at Θ = 1/18. However, the ferromagnetism is suppressed when the applied strain is high. We uncover that the emergence of strain-dependent magnetism is attributed to the presence of localized states in the bandgap resulting from collective effects of bi-axial strain and the adsorption of halogen atoms.

Improved optical properties of InAs quantum dots for intermediate band solar cells by suppression of misfit strain relaxation

The properties of InAs quantum dots (QDs) have been studied for application in intermediate band solar cells. It is found that suppression of plastic relaxation in the QDs has a significant effect on the optoelectronic properties. Partial capping plus annealing is shown to be effective in controlling the height of the QDs and in suppressing plastic relaxation. A force balancing model is used to explain the relationship between plastic relaxation and QD height. A strong luminescence has been observed from strained QDs, indicating the presence of localized states in the desired energy range. No luminescence has been observed from plastically relaxed QDs.