Band Structure Density Research Articles

Acetaminophen and mepirizole molecular adsorption studies on novel ζ – phosphorene nanotube based on first-principles investigation

ABSTRACT Based on the first-principles investigation, we explored the adsorption properties of two drug molecules on ζ – phosphorene nanotube (ζ-PNT) present in the aqueous environment. Firstly, the structural stableness of ζ-PNT is ensured with the support of formation energy (−5.005 eV/atom). Additionally, the electronic properties of ζ-PNT are studied by band structure and projected density of states (PDOS) spectrum. The energy band gap of ζ-PNT is found to be 1.298 eV, which confirms the semiconductor nature of the base substrate. The ζ-PNT is used as the base substrate to adsorb acetaminophen and mepirizole drug molecules in the contaminated water. The adsorption energy of ζ-PNT owing to molecular adsorption of drug molecules figures out to be in the chemisorption regime (−4.244 eV to −9.058 eV). Besides, the variation in the energy band, charge transfer, band structure, and electron density difference studies due to acetaminophen and mepirizole adsorption on ζ-PNT is reported. The outcome reveals that ζ-PNT is a possible material for acetaminophen and mepirizole removal from the contaminated water.

Mn5+-activated Ca6Ba(PO4)4O near-infrared phosphor and its application in luminescence thermometry

The near-infrared luminescence of Ca6Ba(PO4)4O:Mn5+ is demonstrated and explained. When excited into the broad and strong absorption band that spans the 500–1000 nm spectral range, this phosphor provides an ultranarrow (FWHM = 5 nm) emission centered at 1140 nm that originates from a spin-forbidden 1E → 3A2 transition with a 37.5% internal quantum efficiency and an excited-state lifetime of about 350 μs. We derived the crystal field and Racah parameters and calculated the appropriate Tanabe–Sugano diagram for this phosphor. We found that 1E emission quenches due to the thermally-assisted cross-over with the 3T2 state and that the relatively high Debye temperature of 783 K of Ca6Ba(PO4)4O facilitates efficient emission. Since Ca6Ba(PO4)4O also provides efficient yellow emission of the Eu2+ dopant, we calculated and explained its electronic band structure, the partial and total density of states, effective Mulliken charges of all ions, elastic constants, Debye temperature, and vibrational spectra. Finally, we demonstrated the application of phosphor in a luminescence intensity ratio thermometry and obtained a relative sensitivity of 1.92%K−1 and a temperature resolution of 0.2 K in the range of physiological temperatures.

In silico study of adsorption of oxide gases by MN4 (M = Be, Mg) monolayers

In response to the continuously increasing pollution and hazards due to gas leakage, density functional theory based simulation of MN4 (M = Be, Mg) monolayer (ML) was performed. The computed cohesive energy of −2.28 eV/atom for BeN4 ML as well as −2.03 eV/atom for MgN4 ML, and absence of imaginary frequencies in the phonon dispersion curves suggest dynamical stability of both the MLs. The stable structures of MN4 MLs attain monoclinic configuration. The electronic band structure and electron density of states show, the semi-metallic behaviour of both the MLs. With the adsorption energy of −1.64 eV and −2.68 eV, for BeN4 and MgN4 ML, respectively, NO2 is superiorly sensed by MN4 MLs. The charge transfer of −0.452e (BeN4 ML) and −0.646e (MgN4 ML), from gas molecule to the ML, provide assurance for strong adsorption. Also, both the MLs are good at sensing SO2, as the adsorption energies are −0.87 eV (BeN4 ML) and −1.64 eV (MgN4 ML). Both NO2 and SO2 gas molecules contribute at the Fermi level in the projected density of states (PDOS), referring to the transition of MN4 ML, from semi-metallic to metallic, after the adsorption. Besides, in the current–voltage (I-V) characteristics, the extremely sensitive behaviour of MN4 ML towards NO2 gas molecule can be observed, as the current for NO2 adsorbed MN4 ML shoots really high at voltage as low as 0.6 V, which makes MN4 MLs excellent pick as sensing arrays in NO2 gas sensors.

Species transformation and removal mechanism of various iodine species at the Bi2O3@MnO2 interface

Long-term exposure to excessive iodine via drinking water significantly increases the risk of thyroid diseases. Further, the mechanisms and feasible technologies for iodine removal are far from being well elucidated. In this study, we constructed a heterogeneous Bi2O3@MnO2 interface with oxidation and adsorption efficiency toward iodide (I−), and investigated the performance and mechanisms involved in iodine removal. Bi2O3@MnO2 at the optimized Bi/Mn ratio of 0.05:1 had a maximum adsorption capacity of 1.19, 1.21, and 1.06 mg/g toward I−, iodine elemental (I2), and iodate (IO3−), respectively. According to the density functional theory (DFT) calculation, Bi2O3@MnO2 had an adsorption energy of -2.34, -2.11, and -3.89 eV for I−, I2, and IO3−, and exhibited a better band structure and state density character for iodine removal. Based on the results of XPS, HPLC, and LC-ICP-MS characterization, Bi2O3 plays an important role in adsorbing and capturing I− whereas MnO2 dominates the moderate oxidation of I− and the adsorption of I− and I2. The adsorbed I− and I2 concentrations on the Bi2O3@MnO2 surfaces were 146.3 μg/L and 18.3 μg/L. Notably, IO3− was not detected owing to its moderate oxidation effect. The coexisting ions of chloride (Cl−) and bromide (Br−) tended to occupy the Bi2O3 lattice and form insoluble BiOCl and BiOBr. Further, reductive species, such as sulphite (SO32−), may reduce MnO2 to Mn(III) and Mn(II). The synergistic effect between moderate oxidation and adsorption led to Bi2O3@MnO2 with high iodine removal capability. Overall, this study proposes a strategy for designing suitable interfaces and adsorbents for iodine removal; however, further studies are necessary to advance its application in practice.

Ni-Decorated ZnO Monolayer for Sensing CO and HCHO in Dry-Type Transformers: A First-Principles Theory

This work implements first-principles simulations in order to investigate the Ni-decorating property on the ZnO monolayer and the sensing property of the Ni-decorated ZnO (Ni–ZnO) monolayer upon CO and HCHO molecules formed in the dry-type transformers. The results reveal that the Ni dopant is stably anchored on the TO site of the ZnO surface forming the Ni–Zn and Ni–O bonds with the binding energy (Eb) of −1.75 eV. Based on the adsorption energy (Ead) of −1.49 and −2.22 eV for CO and HCHO on the Ni–ZnO monolayer, we determined the chemisorption for two such systems. The band structure (BS) and atomic density of state (DOS) of the gas adsorbed systems are analyzed to comprehend the electronic property of the Ni–ZnO monolayer in the gas adsorptions. Besides, the change of bandgap and work function uncover the sensing potential of Ni–ZnO monolayer upon CO and HCHO detections, with admirable electrical response (15,394.9% and −84.6%). The findings in this work manifest the potential of Ni–ZnO monolayer for CO and HCHO sensing to evaluate the operation condition of the dry-type transformers.

Energy band reconstruction mechanism of Cl-doped Cu2O and photocatalytic degradation pathway for levofloxacin

Cl-doped Cu 2 O microparticles were prepared by hydrothermal synthesis . XRD, BET, SEM, TEM, UV–Vis and RSM were used to reveal the microstructure, optical properties and degradation regularity of the Cl-doped Cu 2 O microparticles. The band gap structures and state densities of Cl-doped Cu 2 O microparticles were computed based on density functional theory . The major intermediate degradation products and potential photocatalytic degradation pathways of levofloxacin were identified by HPLC–MS/MS. The photocatalytic active sites of levofloxacin and its intermediate products were calculated based on the frontier electron densities. The results showed that the prepared Cl-doped Cu 2 O microparticles increased the specific surface area and absorption efficiency of visible light compared with undoped pure Cu 2 O. The doping of Cl introduced hybrid orbitals and oxygen vacancy defects in the Cu 2 O crystal cell and decreased the forbidden bandwidth of Cu 2 O crystal microparticles, leading to a redshift of the absorption edge . Cl doping could form a shallow donor impurity energy level composed of Cl 3 p orbital in the energy gap of Cu 2 O and make the energy band denser and move to a lower energy, which could promote the electron leap to the conduction band through the impurity level . The doping of Cl can make the density of states curve of the Cu 2 O crystal shift to low-energy and the Cl-doped Cu 2 O rendered to half-metallic behavior and characteristic behavior of an n type semiconductor. The photocatalytic degradation of levofloxacin was attributed to attack of photogenerated holes and hydroxyl radicals , and the ring opening of the piperazine ring by oxidation and decarboxylation reactions from the quinolone ring might be two photocatalytic degradation pathways. This study revealed the mechanism of oxygen vacancy formation and the change rules of the band structure by doping of the nonmetal chlorine into Cu 2 O crystallites . • Migration laws of band structure and state density of Cl-doped Cu 2 O were revealed. • Doping of Cl introduced hybrid orbitals and oxygen vacancies in the Cu 2 O crystal. • Doping of Cl decreased band gap of Cu 2 O and led to red shift of absorption edge. • Holes and hydroxyl radicals are the main photocatalytic active species. • Degradation paths of levofloxacin were piperazine ring opening and decarboxylation.

Structural stability and electronic properties of Er nanowire on Si(001)

The silicon surfaces covered with rare earth metals have attracted great interest in recent years due to the promising device applications. Here we report a systematic study on the structural stability and electronic properties of the Er nanowire on Si(001) surface by ab initio calculations. It is shown that Er atomic chains with a double-core odd-membered-ring (5-7-5) structure on Si(001) surface is one of the most stable structures at lower coverage. Total energy calculations show that Er atoms prefer ferromagnetic coupling with a spin moment of 2.04–2.06 μB on Er sites derived from the 4f electrons. Electronic band structure and band-decomposed charge density distribution calculations show that the odd-membered-ring (5-7-5) structure has a semiconducting feature with a small indirect band gap of 0.13 eV. This study provides a new insight for further exploration of self-assembled nanowires of rare-earth metal on silicon surfaces.

First-principles calculations to investigate structural, elastic, electronic, and thermoelectric properties of monolayer and bulk beryllium chalcogenides

A detailed first-principles investigation on the structural, vibrational, elastic, electronic, and thermoelectric properties of the family of beryllium chalcogenides (BeX; X = O, S, Se, Te) in hexagonal monolayer (h-BeX) and bulk (zinc blende; zb-BeX) phases are studied in this work. The stabilities of the compounds are assessed by phonon dispersion analysis and elastic properties. The band structure and partial density of states predicts the nature and understanding of the electronic properties of the considered BeX compounds. The effective mass, mobility and relaxation time for carriers are also predicted for both the BeX phases. The thermoelectric properties are evaluated for a wide temperature range of 50–800 K in terms of thermal conductivity (κ), electrical conductivity (σ), Seebeck coefficient (S) and figure of merit (ZT). The considered BeX materials shows outstanding thermoelectrical promises. The present findings will undoubtedly aid experimentalists in prospective synthesis and future thermoelectric power applications of the recognized BeX compounds.

The elasticity, anisotropy and thermodynamic properties of binary and ternary B–Sc compounds: First-principles calculations

In the past few years, binary and ternary transition metal borides have attracted extensive attentions. Herein, we employed the first-principles calculations method to predict the structural properties, electronic, hardness, elastic and thermodynamic properties of binary and ternary Sc–B and Sc-X-B compounds. The calculated lattice parameters result of Sc–B and Sc-X-B compounds accord well with the available experimental results. The phonon frequencies and formation enthalpies display that all B–Sc type compounds are thermodynamically stable. Based on the criterion of mechanical stability, it can be concluded that Sc–B and Sc-X-B compounds have the mechanical stability. The analysis of band structure and state density displayed the metallic properties. ScB2 and ScB12 have the larger Young's modulus and ScB2 and ScB12 have the higher hardness. According to Poisson's ratio and the ratio for shear modulus and bulk modulus (G/B), it is proved that all Sc–B and Sc-X-B compounds are the ductile materials. Finally, the Debye temperature of B–Sc type compounds at high temperatures obeys the sequence of ScB12> ScB2> Sc3SnB > Sc3PbB > Sc3TlB > Sc3InB.

First-principles investigation of electronic and optical properties of Fe doped in CsBrO3 for enhanced photocatalytic hydrogen production

Self-consistent ab initio calculations carried out using full potential augmented plane wave (FP-LAPW) method were performed to study the electronic, optical and photocatalytic properties of CsBrO3 and Fe doped CsBrO3. Ground state and formation energy for CsBrO3-perovskite are calculated and analyzed. The magnetic moment of Cs, Br, Fe and O are calculated in CsBrO3 and CsBr0.34Fe0.66O3. The band structure, total and partial density of states (DOS) diagrams are discussed.The CsBrO3 have semiconductor character with a wide direct gap. The value of gap energy is 4.24 eV of CsBrO3. Fe doped in CsBrO3 lead to band gap narrowing 1.02 eV for spin up and 1.434 eV for spin dn , which enhances the visible light catalytic activity. The conduction band minimum (CBM) and valence band maximum (VBM) potentials vs. normal hydrogen electrode (NHE) are calculated and analyzed. The general profiles of the optical spectra and the optical properties, including the real and imaginary part of dielectric function, reflectivity, absorption and optical conductivity are discussed. Our results predict that Fe doped CsBrO3 is a promising visible light photo-catalyst for hydrogen production by water splitting.

Intelligent Bi2Se3@Cu2−xSe heterostructures with enhanced photoabsorption and photoconversion efficiency for tri-modal imaging guided combinatorial cancer therapy by near-infrared Ⅱ light

A novel nanoplatform that supports multimodal imaging has been designed for deep tumor therapy. In this study, Bi2Se3@Cu2−xSe heterojunction nanocomposites with tunable spectral absorption, effective electron–hole separation and high photothermal conversion efficiency were prepared for the combination therapy of phototherapy (PT), chemodynamic therapy (CDT) and radiotherapy (RT). By adjusting the doping ratio, the heterojunction nanoparticles show obvious tunable ability of local surface plasmon resonance and the ability to promote electron-hole separation with significantly enhanced reactive oxygen species production capacity. The band structure and charge density difference calculated by density functional theory further reveal that the change of band gap and the decrease of free carriers can regulate the spectral absorption of nanomaterials and promote electron-hole separation. In addition, the photothermal conversion properties of low carrier density semiconductors are related to their inherent deep level defects. The formation of heterojunctions making the Se atoms deviate from the Bi2Se3 lattice, resulting in more deep level defects and stronger photothermal conversion properties. Meanwhile, this nanoplatform presented features similar to catalase activities and glutathione (GSH) consumption characteristics, which was capable of effectively alleviate the tumor-specific hypoxia environment to enhance the efficacy of O2-dependent photodynamic therapy (PDT) and radiotherapy (RT) and depletion GSH to prevent the reduction of therapeutic efficacy due to the clearance of reactive oxygen species. In addition to therapeutic enhancement, heterojunction nanomaterials have excellent nuclear magnetic resonance imaging (MRI), infrared thermal imaging (IR) and computed tomography (CT) properties due to their significant paramagnetism and excellent photothermal conversion and X-ray attenuation capacities. In conclusion, our findings provide a new strategy for designing multi-function and efficient nanoplatform to treat tumor.

First principle calculations of the structural, elastic, electronic and transport properties of XRuAs (X = Ta and V)

The structural, electronic, elastic, and transport properties of Ruthenium Arsenide based Tantalum and Vanadium (XRuAs (X = Ta and V)) half heusler alloys were investigated as a promising thermoelectric material. In determining the properties of these compounds, density functional theory (DFT), as implemented in Quantum Espresso and Boltztrap codes were used. The alpha atomic position gave the lowest minimum energy and was used for all other calculations. The lattice constants, electronic band structure, total and partial density of state and the elastic properties of the compounds were determined. The electronic band structure calculation showed that TaRuAs and VRuAs were semiconductors with an indirect bandgap of 0.17 eV and 0.42 eV, respectively. The d-orbital of the Ruthenium atom in both compound were responsible for the huge peak observed in the total DOS. The elastic properties showed that the alloys are mechanically stable, ductile and elastically anisotropic. The transport properties calculated are the Seebeck coefficient, power factor, and electronic thermal conductivity. The maximum Seebeck coefficient at 1000 K is 286.30 μV/K and 212.06 μV/K, while the maximum electrical conductivity obtained at 1000 K is 4.99 S/ms and 4.11 S/ms. The power factor obtained at 1000 K for VRuAs and TaRuAs were 185.37 W/msK2 and 166.25 W/msK2. At 1000 K, an electronic thermal conductivity (TC) of 8.59 W/mK was obtained for TaRuAs, and 8.01 W/mK was obtained for VRuAs. Figures of merit of 0.58 and 0.77 are obtained for VRuAs and TaRuAs, respectively, at 1000 K which is a limit and the final value may be lower with the computation of the lattice thermal conductivity. The Electronic Fitness Functions (EFF) was determined to further screen the compounds. Values of 1.4 W5/3 ms1/3 K2 and 0.77 W5/3 ms1/3 K2 are obtained, which are comparable with the values obtained for high-performance thermoelectric materials. The results showed that while both compounds are good thermoelectric materials, TaRuAs is the better of the two compounds with a higher figure of merit. Moreover, the results obtained from the EFF calculation suggests that the TaRuAs is a good thermoelectric material. As far as we know, the results obtained for TaRuAs have not been presented in any literature.

Electronic, optical and magnetic characteristics of V doped BeS

Electronic, magnetic and optical features of V doped BeS compound are obtained with FP-LAW technique. A comparative investigation is performed among electronic band structure (BS), total and partial density of states (PDOS) is presented with 6.25, 12.5, 18.75 and 25% concentration of V. The results of electronic BS and density of states (DOS) show half metallic ferromagnetic (HMF) behavior with direct band gap (Eg). The calculated Eg are 2.8, 2.9, 3.3 and 3.7 for Be0.9375V0.0625S, Be00875V0.125S, Be0.8125V0.1875S and Be0.75V0.25S, respectively. For the optical feedback of TM doped BeS material, the imaginary and real part of dielectric function has been studied with the absorption coefficient, extinction coefficient, reflectivity, optical conductivity, and refractive index up to 15 eV. It is noted that with the increment of Eg, the static dielectric constant increases. Furthermore, the magnetic characteristics of resultant compound are examined. The HMF nature of TM doped BeS in electronic and magnetic properties may makes resultant compounds a potential candidate for optical and spintronic appliances.

First Principles Study on the Thermodynamic Properties, Elastic Properties and Electronic Structure of Li-Doped Mg2Si

The topic of this study is the thermodynamic properties, elastic properties and electronic structure of Mg2Si and Li-doped Mg2Si (Mg7Si4Li, Mg8Si3Li and Mg8Si4Li) by first principles based on density functional theory method. The analysis of lattice constants manifests that When Li is doped in Mg2Si, structure cell will generate lattice distortion, which can change lattice constants and total volumes. All the studied compounds are stable. However, the structure will be not as stable as Mg2Si, when Li is doped in. Intrinsic Mg2Si, exhibit the best stiffness and the strongest brittleness. Li-doped Mg2Si, can refine the brittleness but reduce the stiffness. Density of State, band structure, Mulliken electrons and electron density difference show that electrical conductivity is enhanced after Li doping in Mg2Si. Specially, Mg8Si3Li exhibits a worst stability and stiffness, a best plasticity and electroconductibility.

Electronic band structure and chemical bonding in trigonal Se and Te

Herein, the electronic band structure and charge density distribution are theoretically studied in trigonal Se and Te to clarify the uncertainty stemming from the different views on the types of chemical bonding in their crystals and to reconsider the role of valence s- and p-electrons in bonding. The lack of overlapping of the lower and upper bands of valence p-electrons in trigonal Se and the large band separation of valence s- and p-electrons present an opportunity to estimate the contributions of valence s- and p-electrons to the charge densities of two types of bond critical points (BCPs) in trigonal Se. Valence s-electrons and lower p-electrons significantly contribute to the charge density of BCPs of the first type, covalently connecting the nearest neighboring atoms within helical chains. In contrast, the lower and upper valence p-electrons are mainly responsible for the BCPs of the second type linking the neighboring chains in the Se and Te trigonal crystal structures. The nonlocal long-range van der Waals (vdW) correlation functional vdW-DF2, which is important for determining lattice constants, has a minimal effect on BCP parameters, which define the chemical bonding types. The exchange potential of Becke and Johnson modified by Tran and Blaha and the short-range electron–electron correlations considered in the local density approximation correctly reproduce not only the energy bandgap values but also various peculiarities in the electronic band structure of trigonal Se and Te, such as band crossings (Weyl nodes) of the valence p-electrons recently found in trigonal Te via angle-resolved photoemission spectroscopy experiments.

DFT Investigations of Aun Nano-Clusters Supported on TiO2 Nanotubes: Structures and Electronic Properties.

TiO2 nanotubes (TiO2NTs) are beneficial for photogenerated electron separation in photocatalysis. In order to improve the utilization rate of TiO2NTs in the visible light region, an effective method is to use Aun cluster deposition-modified TiO2NTs. It is of great significance to investigate the mechanism of Aun clusters supported on TiO2NTs to strengthen its visible-light response. In this work, the structures, electronic properties, Mulliken atomic charge, density of states, band structure, and deformation density of Aun (n = 1, 8, 13) clusters supported on TiO2NTs were investigated by DMOL3. Based on published research results, the most stable adsorption configurations of Aun (n = 1, 8, 13) clusters supported with TiO2NTs were obtained. The adsorption energy increased as the number of Au adatoms increased linearly. The Aun clusters supported on TiO2NTs carry a negative charge. The band gaps of the three most stable structures of each adsorption system decreased compared to TiO2NTs; the valence top and the conduction bottom of the Fermi level come mainly from the contribution of 5d and 6s-Au. The electronic properties of the 5d and 6s impurity orbitals cause valence widening and band gap narrowing.

DFT insights into the new Hf-based chalcogenide MAX phase Hf2SeC

The physical characteristics of the novel chalcogenide MAX phase Hf2SeC have been investigated using the DFT method. The obtained lattice constants and elastic constants (Cij) are compared with previous results to check the consistency of our setting parameters during calculations. Moreover, the elastic properties such as elastic constants, moduli, anisotropy, and hardness are also compared with preexisting MAX phases of its kind. The checking of the dynamical and mechanical stability have been done based on the phonon dispersions and elastic constants. The reason for the higher hardness of Hf2SeC compared to Hf2SC is explained using the density of states (DOS). The brittleness of Hf2SeC has been revealed using the Pugh ratio, Poisson's ratio, and Cauchy pressure. The electronic properties (band structure and charge density mapping) of Hf2SeC disclosed its metallic nature and bonding behavior. The anisotropy in both electronic conductivity and mechanical properties are investigated. The temperature and pressure dependence of volume, Grüneisen parameter (γ), Debye temperature (ΘD), thermal expansion coefficient (TEC), and specific heat at constant volume (Cv) are explored. In addition, minimum thermal conductivity (Kmin) and melting point (Tm) are studied to investigate its suitability for high-temperature applications.

Electronic and spectroscopic studies of rare earth doped yttrium strontium silicate fluorapatite compound

This paper reports the luminescence studies and first principle calculations based on density functional theory (DFT) using B3LYP/def2-SVP hybrid GGA function of rare earth doped yttrium strontium silicate fluorapatite (Y6-xSr4(SiO4)6F2:xEr3+) compound synthesized by solution combustion method and characterized by XRD, SEM and Raman spectroscopy. Raman spectra of synthesized compound confirms the formation of symmetric and asymmetric bending and stretching Raman active modes within SiO4 tetrahedron. Photoluminescence spectroscopy of Y6-xSr4(SiO4)6F2:xEr3+ compounds show emission spectra at 522 nm with high lumen intensity attributing to 2H11/2 – 4I15/2 electric dipole transition in the green region under 260 nm UV excitation radiation. In the thermoluminescence analysis prepared compound shows the presence of six trap levels where difference between the shallowest and deepest trap was found to be 1.396 eV, as calculated by computerised glow curve deconvolution (CGCD). The computational crystal structure shows a good agreement with experimental structural (XRD) analysis which confirms the formation of hexagonal closed packed structure along with space group P63/m. Frontier molecular orbital calculations were done and the value of ΔE = 5.05243 eV and 1.60969 eV were calculated for Y6Sr4(SiO4)6F2 and Y5ErSr4(SiO4)6F2 compounds. The Mulliken population of Er – O is more than Y – O which shows that covalency of Y5ErSr4(SiO4)6F2 is more than covalency of Y6Sr4(SiO4)6F2 compound. The electronic band structure and total density of states of Er3+ ion substituted Y6-xSr4(SiO4)6F2 shows semiconducting behaviour of compounds.

Preparation, electronic structure and optical properties of Na2GeSe3 crystals

From the first principles, in the framework of the density functional theory in LDA and LDA+U approximations, the band structure, total and partial densities of electronic states, spatial distribution of the electron charge density, also the optical functions: dielectric constant, refractive and absorption indices, reflection and absorption coefficients of Na2GeSе3 crystal have been calculated. According to the calculation results, Na2GeSе3 is a direct-gap crystal with the top of valence band and the bottom of conduction band at the point Г of Brillouin zone. The calculated band gap is Egd = 1.7 eV LDA and Egd = 2.6 eV in the LDA+U approximations. Based on the data of total and partial densities of electronic states, contributions of atomic orbitals to the crystalline ones have been determined. Also, the data of chemical bond formation in the crystals under discussion have been obtained.

A comparative study of ultrafast carrier dynamics near A, B, and C-excitons in a monolayer MoS2 at high excitation densities

With the growing demand for monolayer MoS2 in diverse optoelectronic applications, it is more important than ever to understand carrier behavior under varied excitation conditions and at different excitonic levels. In this article, we show that band structure, excitation wavelength, and excitation density all have a significant impact on the carrier dynamics in a monolayer MoS2. At the A and B excitation levels, an initial bandgap renormalization is detected, while band bleaching is found to play a stronger role at the C-excitonic state. The exciton dissociation at the band edge near A and B-exciton causes the bandgap renormalization. On the other hand, band bleaching, which occurs at the C-exciton, is caused by the exciton formation due to the high availability of states. At this excitation energy, carrier relaxation and thermalization occur through lattice interaction by releasing a high number of hot phonons, which creates a bottleneck effect. Due to the hot phonon bottlenecking effect, carrier relaxation time is prolonged.