Group VI Elements Research Articles

2D and bulk h-MgSe materials from MgmSen (m, n = 1–3) clusters: Density functional investigation

An investigation in detail on the structure, electronic and thermodynamic properties for a series of group II and VI Combined hybrid alloy clusters, viz, MgmSen (m,n = 1–3) is performed, under density functional theory (DFT). The geometries of all clusters has been optimized by employing a very popular and reliable exchange-correlation functional, viz, Becke’s 3 parameter exchange with Lee-Yang-Parr correlation functional (B3LYP). The influence of adding of group II and/or group VI elements on the electronic properties is also addressed in the present work. A sincere effort has been tendered to identify any potential cluster motif from the considered series for utilizing as the building block for prospective metal-insulator-semiconductor (MIS) materials at the bulk scale. Accordingly, with an achievement of exceptionally stable rhombus shaped and D2h symmetric Mg2Se2, we have investigated structure and electronic properties of 2D hexagonal magnesium selenide (2D h-MgSe) as well as hexagonal magnesium selenide at bulk scale (bulk h-MgSe). It is evident from the present study that although 2D planar h-MgSe reveals as an insulator with the direct band gap 4.20 eV whereas bulk h-MgSe come out to be a semiconductor material with direct band gap as 1.13 eV. The bulk h-MgSe phase is reported to be an exceptionally stable semiconductor in the magnesium selenide family.

Electronic Structure and Magnetic Properties of FeTe, BiFeO3, SrFe12O19 and SrCoTiFe10O19 Compounds

The electronic energy structures and magnetic properties of iron-based compounds with group VI elements (FeTe, BiFeO$_3$, SrFe$_{12}$O$_{19}$ and SrCoTiFe$_{10}$O$_{19}$) are studied using the density functional theory (DFT) methods. Manifestations of different types of chemical bonds in magnetism of these compounds are studied theoretically. Calculations of electronic structures of these systems were performed using the generalized gradient approximation (GGA) for description of the exchange and correlation effects within DFT. For SrFe$_{12}$O$_{19}$ and SrCoTiFe$_{10}$O$_{19}$ hexaferrites the GGA+$U$ method was also employed to deal with strongly correlated 3$d$-electrons. The calculations have revealed distinctive features of electronic structure of the investigated iron-based compounds with strongly correlated 3$d$-electrons, which can be responsible for their peculiar structural and magnetic properties.

ChemInform Abstract: Cesium Salts of Niobo‐Tungstate Isopolyanions with Intermediate Group V—Group VI Character.

AbstractCs4Na2 [Nb4W2O19]·12H2O (I) and Cs3Na [Nb2W4O19]·10H2O (II) are prepared by acidification of an aq.

Unconventional Superconductivity in the Layered Iron Germanide YFe(2)Ge(2).

The iron-based intermetallic YFe_{2}Ge_{2} stands out among transition metal compounds for its high Sommerfeld coefficient of the order of 100 mJ/(mol K^{2}), which signals strong electronic correlations. A new generation of high quality samples of YFe_{2}Ge_{2} show superconducting transition anomalies below 1.8K in thermodynamic, magnetic, and transport measurements, establishing that superconductivity is intrinsic in this layered iron compound outside the known superconducting iron pnictide or chalcogenide families. The Fermi surface geometry of YFe_{2}Ge_{2} resembles that of KFe_{2}As_{2} in the high pressure collapsed tetragonal phase, in which superconductivity at temperatures as high as 10K has recently been reported, suggesting an underlying connection between the two systems.

Direct band gaps in group IV-VI monolayer materials: Binary counterparts of phosphorene

We perform systematic investigation on the geometric, energetic and electronic properties of group IV-VI binary monolayers (XY), which are the counterparts of phosphorene, by employing density functional theory based electronic structure calculations. For this purpose, we choose the binary systems XY consisting of equal numbers of group IV (X = C, Si, Ge, Sn) and group VI elements (Y = O, S, Se, Te) in three geometrical configurations, the puckered, buckled and planar structures. The results of binding energy calculations show that all the binary systems studied are energetically stable. It is observed that, the puckered structure, similar to that of phosphorene, is the energetically most stable geometric configuration. Our results of electronic band structure predict that puckered SiO and CSe are direct band semiconductors with gaps of 1.449 and 0.905 eV, respectively. Band structure of CSe closely resembles that of phosphorene. Remaining group IV-VI binary monolayers in the puckered configuration and all the buckled monolayers are also semiconductors, but with indirect band gaps. Importantly, we find that the difference between indirect and direct band gaps is very small for many puckered monolayers. Thus, there is a possibility of making these systems undergo transition from indirect to direct band gap semiconducting state by a suitable external influence. Indeed, we show in the present work that seven binary monolayers namely SnS, SiSe, GeSe, SnSe, SiTe, GeTe and SnTe become direct band gap semiconductors when they are subjected to a small mechanical strain (<= 3 %). This makes nine out of sixteen binary monolayers studied in the present work direct band gap semiconductors. Thus, there is a possibility of utilizing these binary counterparts of phosphorene in future light-emitting diodes and solar cells.

Relativity Could Alter Superheavy Bonding

For decades, chemists have hypothesized that relativistic effects could alter the bonding properties of superheavy elements. These effects come into play when the charge on an atom’s nucleus becomes so large that its electrons orbit close to the speed of light. A new theoretical study from a team led by Jun Li at Tsinghua University adds more intrigue to the idea, suggesting that diatomic molecules made of superheavy group VI elements have a lower bond order than their lighter group VI counterparts (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.5b11793). For example, although group VI diatomic molecules such as Cr2, Mo2, and W2 sport a hextuple bond, the Tsinghua group’s model predicts that the superheavy molecule Sg2 will have only a quadruple bond. The study also suggests a similar bond order reduction for group VII superheavy diatomic molecules. The conclusions can’t be tested with today’s technology—superheavy elements have lifetimes on the

Promising Piezoelectric Performance of Single Layer Transition-Metal Dichalcogenides and Dioxides

Piezoelectricity is a unique material property that allows one to convert mechanical energy into electrical one or vice versa. Transition metal dichalcogenides (TMDC) and transition metal dioxides (TMDO) are expected to have great potential for piezoelectric device applications due to their noncentrosymmetric and two-dimensional crystal structure. A detailed theoretical investigation of the piezoelectric stress (e11) and piezoelectric strain (d11) coefficients of single layer TMDCs and TMDOs with chemical formula MX2 (where M= Cr, Mo, W, Ti, Zr, Hf, Sn and X = O, S, Se, Te) is presented by using first-principles calculations based on density functional theory. We predict that not only the Mo- and W-based members of this family but also the other materials with M= Cr, Ti, Zr and Sn exhibit highly promising piezoelectric properties. CrTe2 has the largest e11 and d11 coefficients among the group VI elements (i.e., Cr, Mo, and W). In addition, the relaxed-ion e11 and d11 coefficients of SnS2 are almost the sa...

Atomically thin layers of B-N-C-O with tunable composition.

In recent times, atomically thin alloys of boron, nitrogen, and carbon have generated significant excitement as a composition-tunable two-dimensional (2D) material that demonstrates rich physics as well as application potentials. The possibility of tunably incorporating oxygen, a group VI element, into the honeycomb sp(2)-type 2D-BNC lattice is an intriguing idea from both fundamental and applied perspectives. We present the first report on an atomically thin quaternary alloy of boron, nitrogen, carbon, and oxygen (2D-BNCO). Our experiments suggest, and density functional theory (DFT) calculations corroborate, stable configurations of a honeycomb 2D-BNCO lattice. We observe micrometer-scale 2D-BNCO domains within a graphene-rich 2D-BNC matrix, and are able to control the area coverage and relative composition of these domains by varying the oxygen content in the growth setup. Macroscopic samples comprising 2D-BNCO domains in a graphene-rich 2D-BNC matrix show graphene-like gate-modulated electronic transport with mobility exceeding 500 cm(2) V(-1) s(-1), and Arrhenius-like activated temperature dependence. Spin-polarized DFT calculations for nanoscale 2D-BNCO patches predict magnetic ground states originating from the B atoms closest to the O atoms and sizable (0.6 eV < E g < 0.8 eV) band gaps in their density of states. These results suggest that 2D-BNCO with novel electronic and magnetic properties have great potential for nanoelectronics and spintronic applications in an atomically thin platform.

Effect of the Replacement of Some Indium Atoms with Aluminum Atoms on the Behaviour of p-CuInSe2 Thin Films and Schottky Diodes

As environmental and energy resource concerns have increased, greater stress has been placed on the development of renewable energy resources such as photovoltaic electric generators. Chalcopyrite semiconductors have gained prominence as solar cell fabrication materials due to its steady progress in conversion efficiencies. These have emerged as leading absorber semiconducting material for solar power generation purposes. CulnSe2 (CIS), a prominent member of the chalcopyrite family, has emerged as a promising material for high efficiency thin film solar cells. Due to its favorable electrical and optical properties, stability and inexpensive means of production that several recent advances have been made to improve its bandgap [Eg]. The bandgap of CIS has been tailored to match the desired portion of the Solar Spectrum by alloying the group III (Ga and Al) or group VI (S and Se) elements. An extensive research work has been carried out on Cu(In,Ga)Se2 [CIGS] and CuIn(S,Se)2 [CISS] alloy systems but the efficiency of these devices with Eg >1.3 eV is reduced due to degradation of the electronic properties of the absorber layer leading to losses in the fill-factor and the open-circuit voltage. The performance of thin film CIS based solar cells has alternatively been improved by the addition of Al in place of Ga to form CuInAlSe2(CIAS) absorber layers with changed bandgap to match the selective regions of the solar spectrum. Further, Ga is scarce and expensive material and can preferably be replaced by inexpensive and abundant Al. In the present investigation, behaviour of thin film CIAS to that of CIS as an absorber layer portion of the solar cells has been studied. Both the materials possessed tetragonal structure and found oriented towards < 112 > set of planes. The addition of ‘Al’ to CIS films caused reduction in the lattice imperfections’ which resulted in the improved cystallinity of the films. The grain size increased while microstrain and dislocation density decreased in CIAS. The films demonstrated decrease in the cohesive force between film and the substrate material. The optical analysis of the films revealed that the bandgaps and absorption coefficients increased on Al substitution and, thus, CIAS films have been found to be more suitable as an absorber layer in solar cells. The electrical measurements of the films showed that both CIS and CIAS films possessed p-type conductivity. Further, a significant improvement has been observed in the conductivity as well as mobility values in the CIAS films. The phenomenon has been attributed due to the reduction in the grain boundary scattering in the CIAS in comparison to CIS films. The Schottky Diodes of both the types of films were formed using Aluminum metals. The reverse biased leakage currents measured from the I-V/C-V characteristics of CIS were found to lower than that of CIAS films. The forward biased parameters such as ideality factors and series resistance decreased while barrier heights increased in the Al/p-CuIn0.81Al0.19Se2 Schottky Diodes. Further, the temperature dependence of the I-V/C-V characteristics of the undertaken CIAS diodes has been explained on the basis of barrier inhomogenities existing over the M-S interface. However, some deviations have been observed between the data generated on the basis of Gaussian Distribution Function for barrier inhomogenities to that obtained from the experimental studies of undertaken Schottky Diodes which indicated the existence of current component due to tunneling alongwith the thermionic emission of the charge carriers existing over the M-S interface.

Synthesis of CuInS2 quantum dots using polyetheramine as solvent.

This paper presents a facile solvothermal method of synthesizing copper indium sulfide (CuInS2) quantum dots (QDs) via a non-coordinated system using polyetheramine as a solvent. The structural and optical properties of the resulting CuInS2 QDs were investigated using composition analysis, absorption spectroscopy, and emission spectroscopy. We employed molar ratios of I, III, and VI group elements to control the structure of CuInS2 QDs. An excess of group VI elements facilitated precipitation, whereas an excess of group I elements resulted in CuInS2 QDs with high photoluminescence quantum yield. The emission wavelength and photoluminescence quantum yield could also be modulated by controlling the composition ratio of Cu and In in the injection stock solution. An increase in the portion of S shifted the emission wavelength of the QDs to a shorter wavelength and increased the photoluminescence quantum yield. Our results demonstrate that the band gap of the CuInS2 QDs is tunable with size as well as the composition of the reactant. The photoluminescence quantum yield of the CuInS2 QDs ranged between 0.7% and 8.8% at 250°C. We also determined some important physical parameters such as the band gaps and energy levels of this system, which are crucial for the application of CuInS2 nanocrystals.

First-principles prediction of kink-pair activation enthalpy on screw dislocations in bcc transition metals: V, Nb, Ta, Mo, W, and Fe

The atomistic study of kink pairs on screw dislocations in body-centered cubic (bcc) metals is challenging because interatomic potentials in bcc metals still lack accuracy and kink pairs require too many atoms to be modeled by first principles. Here, we circumvent this difficulty using a one-dimensional line tension model whose parameters, namely the line tension and Peierls barrier, are reachable to density functional theory calculations. The model parameterized in V, Nb, Ta, Mo, W, and Fe, is used to study the kink-pair activation enthalpy and spatial extension. Interestingly, we find that the atomistic line tension is more than twice the usual elastic estimates. The calculations also show interesting group tendencies with the line tension and kink-pair width larger in group V than in group VI elements. Finally, the present kink-pair activation energies are shown to compare qualitatively with experimental data and potential origins of quantitative discrepancies are discussed.

Role of chemical termination in edge contact to graphene

Edge contacts to graphene can offer excellent contact properties. Role of different chemical terminations is examined by using ab initio density functional theory and quantum transport simulations. It is found that edge termination by group VI elements O and S offers considerably lower contact resistance compared to H and group VII element F. The results can be understood by significantly larger binding energy and shorter binding distance between the metal contact and these group VI elements, which results in considerably lower interface potential barrier and larger transmission. The qualitative conclusion applies to a variety of contact metal materials.

Non-orthogonal tight-binding model for tellurium and selenium

The twofold coordinated heavier group-VI elements tellurium and selenium with the trigonal crystal structure have unshared electron pairs (lone pairs) which control the interplay of the intra- and inter-chain interactions and their sensitivity on pressure and temperature. We have developed tight-binding (TB) parameters for the helical structures of tellurium and selenium using the Naval Research Laboratory Tight-Binding (NRL-TB) method. The TB parameters were derived by fitting to the band structures and total energies of density functional theory (DFT) calculations. We have applied the TB parameters to study the phase stabilities of different structures under hydrostatic pressure. We have predicted (without fitting) the volume dependence of the rhombohedral and diamond structures, the bcc to rhombohedral and rhombohedral to simple cubic phase transitions and the elastic constants of the trigonal structures, all in agreement with ab initio and experimental results. While the results for the unrelaxed vacancy formation energies and surface energies are in good agreement with the DFT values, we find large discrepancies for the relaxed values indicating that the present set of TB parameters cannot accurately capture the inter-chain interactions.

Introducing a Phenomenon of Single Junction Multiple Band-gap Solar Cell Compared to Single Junction or Multi Junction Solar Cell to Achieve High Efficiency

Besides cost, the most fundamental issue in assessing photovoltaic solar cells is efficiency. The benefits of single-junction solar cells are easier to manufacture, simpler and less expensive to make. However, they are less efficient than multi-junction solar cells. But when different semiconductors are stacked in layers to create multiple junctions, the atoms in the lower layers must not be blocked by the atoms in the upper layers or the sunlight passes by them instead of striking them. That means higher efficiency multi-junction solar cells are difficult to make. From the periodic table, group III-V and group II-VI produce the alloys of direct band-gap semiconductor. If group V anions are partially replaced with nitrogen atom or group VI element is partially replaced with oxygen atom then new semiconductor material is produced that respond to a wider spectrum of frequencies within sunlight. It will make single-junction multiple band-gap solar cell which is more efficient and less expensive to manufacture.

Configuration and binding energy of multiple hydrogen atoms trapped in monovacancy in bcc transition metals

We present a first-principles study of stable configurations of single and multiple H atoms in a monovacancy in bcc transition metals and binding energies of the H atoms to the monovacancy. Typical bcc transition metals are group-V elements (V, Nb, and Ta), group-VI elements (Cr, Mo, and W), and Fe. The most stable site for an interstitial H atom in the intrinsic bcc transition metals is a tetrahedral interstitial site (T site). On the other hand, a single or a few H atoms trapped in a monovacancy in bcc metals occupy close to octahedral interstitial sites (O sites) next to the monovacancy. However, stable configurations of four and more-than-four H atoms in the monovacancy are various and different depending on the host metals. Stable sites for H atoms are usually shifted toward the T site or diagonal interstitial site (D site) as the number of H atoms increases in the monovacancy. As a result, a maximum of six H atoms can be accommodated in a monovacancy in V, Nb, Ta, Cr, and Fe, which is in good agreement with previous computational studies, while 10 and 12 H atoms can be accommodated in a Mo and W monovacancy, respectively.

Unprecedented Bidentate Coordination of the Uranyl Cation by the Chromate Anion in the Structure of [(CH3)2CHNH3]2[UO2(CrO4)2

Abstract[(CH3)2CHNH3]2[UO2(CrO4)2] is the first uranyl chromate with a bidentate coordination mode between the uranyl and chromate ions. Although this type of coordination is typical for sulfates, it has not been previously observed for other group VI elements. Statistical analysis of the bidentate coordination geometry parameters in uranyl compounds is given.

Concentration and Annealing Effects on Luminescence Properties of Ion-Implanted Silica Layers

The development of optoelectronic or even photonic devices based on silicon technology is still a great challenge. Silicon and its oxide do not possess direct optical transitions and, therefore, are not luminescent. The remaining weak light emission is based on intrinsic and extrinsic defect luminescence. Thus the investigations are extended to ion implantation into silica layers, mainly on over-stoichiometric injection or isoelectronic substitution of both the constituents silicon or oxygen, that is, by ions of the group IV (C, Si, Ge, Sn, Pb) or the group VI (O, S, Se). The samples have been used were 500 nm thick thermally grown amorphous SiO2 layers, wet oxidized at 1100°C on a crystalline Si substrate. The ion implantations were performed with different energies but all with a uniform dose of 5 × 1016 ions/cm2. Such implantations produce new luminescence bands, partially with electronic-vibronic transitions and related multimodal spectra. Special interest should be directed to lowdimension nanocluster formation in silica layers. Implantations of group IV elements show a general increase of the luminescence in the violet-blue region and implantations of group VI elements lead to an increase in the yellow-red spectral region. Comparing cathodoluminescence, photoluminescence, and electroluminescence still too small luminescence quantum yields are obtained.

Trends in the band-gap pressure coefficients and bulk moduli in different structures of ZnGa2S4, ZnGa2Se4 and ZnGa2Te4

We have performed a first-principles investigation for the family of compounds ZnGa2X4 (X = S, Se, Te). The properties of two possible structures, defect chalcopyrite and defect famatinite are both calculated. We reveal that ZnGa2S4 and ZnGa2Se4 have direct band gaps, while ZnGa2Te4 has an indirect band gap. The local density approximation band gaps are found to be very different in two structures, while the lattice parameters and bulk moduli are similar. We extend Cohen's empirical formula for zinc-blende compounds to this family of compounds. The pressure coefficients are calculated and metallization pressures are discussed. We find that aig remains fairly constant when the group-VI element X is varied in ZnGa2X4(II–III2—VI4).

Growth and characterization of heavily selenium doped GaAs using MOVPE

High n-type doping in GaAs using silicon suffers from its amphoteric character and free carrier concentrations are practically limited to 3×10 18 cm −3. Se as group VI element should not be amphoteric and promises higher carrier concentrations. The highest Hall free carrier concentration in GaAs:Se layers grown by MOVPE is indeed approximately two times higher than when using silicon doping. However, the doping is not stable to subsequent growth at high temperatures. The effects of high Se doping of GaAs on the electrical and optical properties are discussed in conjunction with the observed change of the lattice constant.

Extrinsic point defects in aluminum antimonide

We investigate thermodynamic and electronic properties of group IV (C, Si, Ge, Sn) and group VI (O, S, Se, Te) impurities as well as P and H in aluminum antimonide (AlSb) using first-principles calculations. To this end, we compute the formation energies of a broad range of possible defect configurations including defect complexes with the most important intrinsic defects. We also obtain relative scattering cross strengths for these defects to determine their impact on charge carrier mobility. Furthermore, we employ a self-consistent charge equilibration scheme to determine the net charge carrier concentrations for different temperatures and impurity concentrations. Thereby, we are able to study the effect of impurities incorporated during growth and identify optimal processing conditions for achieving compensated material. The key findings are summarized as follows. Among the group IV elements, C, Si, and Ge substitute for Sb and act as shallow acceptors, while Sn can substitute for either Sb or Al and displays amphoteric character. Among the group VI elements, S, Se, and Te substitute for Sb and act as deep donors. In contrast, O is most likely to be incorporated as an interstitial and predominantly acts as an acceptor. As a group V element, P substitutes for Sb and is electrically inactive. C and O are the most detrimental impurities to carrier transport, while Sn, Se, and Te have a modest to low impact. Therefore, Te can be used to compensate C and O impurities, which are unintentionally incorporated during the growth process, with minimal effect on the carrier mobilities.