GeI2 monolayer: a model thermoelectric material from 300 to 600 K

ABSTRACT

In this work, the electronic structure, optical properties and thermoelectric properties of the GeI₂ monolayer are calculated by the first principles with the Boltzmann transport equation. The monolayer is calculated as an indirect band gap semiconductor with an indirect band gap of a value 2.19?eV. This GeI₂ monolayer is good for absorbing low-energy photons, and it is insensitive to high-energy photons. The material is stable at temperatures up to 600?K, so we calculated the thermal conductivity (K_L), Seebeck coefficient (S), power factor (PF) and thermoelectric figure of merit (ZT) of the GeI₂ monolayer at various carrier concentrations from 300 to 600?K. Due to the lower group velocity, the GeI₂ monolayer has a lower thermal conductivity of 0.48?W/m?K at 300K. In P-type doping, the power factor can up to 0.11?mW/m?K², and its ZT value is 4.04 at 600?K of the GeI₂ monolayer, indicating that the GeI₂ monolayer is a potential thermoelectric material.     

An experimental investigation of the photoconductivity spectrum of a monocrystal of AgInS2 grown by Bridgman and chemical transport techniques

The photoconductivity spectrum of a AgInS₂ single crystal grown both by chemical transport and by the Bridgman method has been studied at 300 K. Splitting of the valance band was clearly observed in both the chaleopyrite and pseudo-wurtzite structures. The observed splitting is in reasonable agreement with the values reported by the electroreflection technique.     

Effect of Pr-filling in binary skutterudites CoX3 (P,As and Sb) on structural,electronic, elastic and transport properties

ABSTRACT

We have studied the effect on the structural, electronic, elastic and transport properties of binary skutterudite CoX₃ (X?=?P, As, Sb) after filling void spaces by Pr atoms. All the calculations are carried out using the full-potential linearised augmented plane wave (FP-LAPW) method within the generalised gradient approximation (GGA) viz, PBE, and PBEsol. In case of binary skutterudites, equilibrium lattice parameters obtained using PBEsol functional are in good agreement with the theoretical and experimental results. The Hubbard parameter (U) has been used with PBEsol-GGA functional to see its effect on the band structures of binary and ternary compounds. Filling of Pr atoms at void positions in binary skutterudites creates the gap between the valence and conduction band. The obtained values of the elastic constants show that CoX₃, PrCo₄X₁₂ (X?=?P, As, Sb) are mechanically stable and brittle. Mechanically, PrCo₄P₁₂ and PrCo₄As₁₂ are anisotropic, but PrCo₄Sb₁₂ is isotropic. Obtained saturated Seeback coefficients are approximate ?60?μV/K (PrCo₄P₁₂), ?80?μV/K (PrCo₄As₁₂) and ?68?μV/K (PrCo₄Sb₁₂) in the spin-up channel while in the spin-down channel corresponding values are ?10, ?50 and ?120?μV/K at 800?K, respectively. These values are higher in magnitude than that in the corresponding binary compounds.     

On the Electrical and Optical Properties of Some Poly(Azomethine Sulfone)s in Thin Films

Poly(azomethine sulfone)s were synthesized by reacting 4,4′-sulfonyl bis(4-chlorophenyl) with 2,2-bis(4-hydroxyphenyl)propane and azomethine bisphenol in different molar ratios. Thin films were deposited from solution onto glass substrates. Study of the temperature dependences of the electrical conductivity, σ, and Seebeck coefficient, S, were performed in the temperature range 300 K–500 K. Thermal activation energies of electrical conduction, E_a , calculated from these dependences, ranged between 1.50 eV and 1.85 eV. The values of E_a were smaller for polymers with extended conjugation systems. The possibility to use the polymers in thermistor technology is discussed. The aspect of the temperature dependences of σ and S shows that a model based on the energy band-gap representation can be successfully used for explaining the electronic transport mechanism in the higher temperature range. In the lower temperature range, the mechanism of the electrical conduction is discussed in terms of the Mott variable range hopping conduction. The values of some optical parameters (absorption coefficient, optical band gap, etc.) were determined from transmission spectra.     

Electrical and optical properties of CuIn5S8 single crystals

Summary Several transport and optical properties have been studied onn-type CuIn₅S₈ single crystals. The energy gap at 0 K was determined from the electrical measurements to be 1.4 eV. An anisotropy of the magnetoresistance effect was found and it was suggested that the minima of the conduction band were located at points along the [100] directions ink-space. An optical-absorption band was found in an infrared region of (1÷1.6) μm and was attributed to the transitions from the lowest conduction band situated along the [100] directions to an upper conduction band. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Transport properties of CdIn2S4 single crystals

Several transport properties have been studied on CdIn₂S₄ singlê crystals with different degrees of deviation from stoichiometry. The energy gap at 0 K was determined from the electrical measurements to be 2.2 eV. The anisotropy of the magnetoresistance effect was found and it was suggested that the minima of the conduction band were located at points along the [100] directions in k space. From an analysis of the mobility data it was found that the high resistivity of the samples is due to compensation of donors by acceptors introduced by excess sulphur. Several band parameters, the carrier scattering mechanisms and the impurity levels were determined. The thermal conductivity was measured from 4 K to 300 K and analysed by Callaway's formalism.     

Detection of photoquenching in CuGaTe2

The photoconductivity spectrum of a single crystal of p type of CuGaTe₂, has been examined at 300K and 77K. Structural details, which appeared on the high energy side of The band gap transition, are attributed to the valance band structure. Pronounced photoquenching is also observed on the high energy side and it is due to the recombination of holes with the ejected electrons from the inner 3d level of copper atom. Such a photoquenching effect has not been reported so for.     

First-principles study of electronic structure and magnetism of cubic Al1−xErxN using the LSDA+U approach

First-principles calculations, by means of the full-potential augmented plane wave method using the LSDA+U approach (local spin density approximation with Hubbard-U corrections), have been carried out for the electronic structure of the Al_0.75Er_0.25N. The LSDA+U method is applied to the rare-earth 4? states. We have investigated the electronic and magnetic properties.The Al_0.75Er_0.25N is shown to be a semiconductor, where the filled ? states are located in the valence bands and the empty ones above the conduction band edge. The magnetic interaction of the rare-earth ion with the host states at the valence and conduction band edges has been investigated and discussed.     

First principles study on electronic structure of β-Ga2O3

We have studied the electronic structure of β-Ga₂O₃ using the first principles full-potential linearized augmented plane wave method. It is found that β-Ga₂O₃ has an indirect band gap with a conduction band minimum (CBM) at Γ point and a valence band maximum on the E line. The anisotropic optical properties are explained by the selection rule of the band-to-band transitions. On the other hand, the shape of the CBM is almost isotropic and, therefore, the observed electronic anisotropy in the n-type semiconducting state should not be attributed to the properties of a perfect lattice. The Burstein-Moss shift is discussed using the effect of several allowed transitions between the levels of the valence band and the CBM.     

Evidence of magnetic ordering in tellurium substituted FeSb2

We report on a⁵⁷Fe Mössbauer study of tellurium substituted FeSb₂, FeSb_2?x Tc _x (x=0.2, 0.4, 0.6), at temperatures between 4.2 K and 300 K. For all three alloys, the Mössbauer spectra at 4.2 K are characteristic of a magnetically ordered state. The hyperfine field at Fe site increases with increasing tellurium concentration. The magnetic character may be attributed to the existence of a very narrow band gap leading to fairly strong Coulomb and exchange interactions between holes in the valence band and electrons in the conduction band.     

Carrier transport mechanisms in the ZnO based heterojunctions grown by MBE

This paper presents a review of models of the current transport in different kind of heterojunctions (HJs) and their characteristics. In order to effectively deduce the dominant electron transport for the HJs based on ZnO or Zn_1?xMg_xO layers grown on Si substrate by MBE a comparison is performed – which type of the HJ exhibits better electrical properties. The current–voltage characteristics for the studied HJs were measured within 280–300 K. The transport properties of the HJs are explained in terms of Anderson model with reference to aforementioned current transport models. It is found, that the mechanisms of current transport for all of the studied HJs are similar. At a low forward voltage bias the tunneling current dominates while at medium voltage bias (0.5–1 V) multitunneling capture-emission prevails with the electron trap located at 0.1–0.25 eV below the bottom of a ZnO (Zn_1?xMg_xO) conduction band. Beyond this voltage bias space charge limited current governs the current transport.     

AlB2 and MgB2: a comparative study of their electronic,phonon and superconductivity properties via first principles

ABSTRACT

Recently, the AlB₂-type compounds (such as AlB₂ and MgB₂) which exhibit Dirac Nodal Line (DNLs) semimetal on their electronic band structure and Phononic Weyl Nodal Straight Lines (PTWNLs) on their phonon spectrum, have received wide attentions on their novel properties. Up to date, no comparative studies have been investigated on their electronic structures, phonon spectrum, and electron phonon coupling (EPC) under the conditions of carrier doping and strain engineering. Here, we systemically investigate their above properties under carrier doping and strain engineering by first-principles calculations. The results show that the superconducting transition temperature T _c can be enhanced by electron doping and tensile strain. For AlB₂, the tensile strain of 6% can enhance T _c to 10.25?K and with the doping concentrate of 0.1 e^- per cell can enhance T _c reach to 9.89?K. Moreover, the physical quantities related to superconductivity of AlB₂ are more affected by carrier doping than MgB₂. Our results provide a theoretical reference to explore the correlation between electronic and phonon topological properties in AlB₂-type materials.     

Strain effect on the electronic properties of 1T-HfS2 monolayer

We perform first-principles based on the density function theory to investigate electronic and magnetic properties of 1T-HfS₂ monolayer with biaxial tensile strain and compressive strain. The results show that HfS₂ monolayer under strains doesn’t display magnetic properties. When the strain is 0%, the HfS₂ monolayer presents an indirect band gap semiconductor with the band gap is about 1.252 eV. The band gap of HfS₂ monolayer decreases quickly with increasing compressive strain and comes to zero when the compressive strain is above −7%, the HfS₂ monolayer system turns from semiconductor to metal. While the band gap increases slowly with increasing tensile strain and comes to 1.814 eV when the tensile strain is 10%. By comparison, we find that the compressive strain is more effective in band engineering of pristine 1T-HfS₂ monolayer than the tensile strain. And we notice that the extent of band gap variation is different under tensile strain. The change of band gap with strain from 1% to 5% is faster than that of the strain 6–10%. To speak of, the conduction band minimum (CBM) is all located at M point with different strains. While the valence band maximum (VBM) turns from Γ point to K point when the strain is equal to and more than 6%.     

Modulation of band gap by normal strain and an applied electric field in SiC-based heterostructures

The structure and electronic properties of the WS₂/SiC van der Waals (vdW) heterostructures under the influence of normal strain and an external electric field have been investigated by the ab initio method. Our results reveal that the compressive strain has much influence on the band gap of the vdW heterostructures and the band gap monotonically increases from 1.330 to 1.629 eV. The results also imply that electrons are likely to transfer from WS₂ to SiC monolayer due to the deeper potential of SiC monolayer. Interestingly, by applying a vertical external electric field, the results present a parabola-like relationship between the band gap and the strength. As the E-field changes from to ?0.50 +0.20 V/Å, the band gap first increases from zero to a maximum of about 1.90 eV and then decreases to zero. The significant variations of band gap are owing to different states of W, S, Si, and C atoms in conduction band and valence band. The predicted electric field tunable band gap of the WS₂/SiC vdW heterostructures is very promising for its potential use in nanodevices.     

Carrier transport in In-doped CuO thin films

Temperature-dependent conductivity and thermopower measurements are carried out for undoped and In-doped CuO thin films. We investigate the effects of In-doping on carrier transport properties of CuO thin films in the temperature range of 300?<?T?<?400?K. Carrier transport is dominated by simple thermally activated conduction in the undoped film. On the other hand, small polarons play an essential role in the carrier transport properties in the In-doped films. By increasing the In content, the conduction behaviour transits from adiabatic limit to the non-adiabatic limit, and the conductivity decreases.     

Photoconductivity on europium oxyde single crystals and thin films

In this paper, results of photoconductivity measurements on four EuO samples are given. Low frequency photoconductivity versus temperature (10°K < T < 300°K) and magnetic field H is investigated for two wavelengths: 6600 Å and 9000 Å. The photoconductivity kinetic is also described, and is characterized by a distribution in decay times. Temperature, magnetic field and carrier concentration have small effects on this kinetic. Quenching effect is obtained by adding a continuous illumination (λ₂) to the weak modulated light (λ₁). The kinetic is strongly affected by quenching and becomes more simple. Quenching effect is maximum for the wavelength associated to the 4?⁷–4?⁶ 5d, transition. In contrast to the Penney-Kasuya[1] model we propose another one in which the conduction of equilibrium carriers as photo-excited carriers takes place in a broad band. The variation of low frequency photoconductivity versus temperature is attributed to the mobility variation. This variation agrees well with the model of mobility controlled by spin-disorder. The photoconductivity kinetic is interpreted by a three levels recombination model: the conduction band, the 4f levels and a distribution of trap levels. The lack of variation of photoconductivity decay in the range of metal-semiconductor transition is discussed.     

Effect of strain on electronic and magnetic properties of n-type Cr-doped WSe2 monolayer

Using first-principles calculations based on the density functional theory, we study the effect of strain on the electronic and magnetic properties of Cr-doped WSe₂ monolayer. The results show that no magnetic moment is induced in the Cr-doped WSe₂ monolayer without strain. For the Cr substitutions, the impurity states are close to the conduction bands, which indicate n-type doping occurs in this case. Then we applied strain (from −10% to 10%) to the doped system, and find that a little magnetic moment is induced with tensile strain from 6% to 9% and negligible. We find that the influence of strain on the magnetic properties is inappreciable in Cr-doped WSe₂. Moreover, the tensile strain appears to be more effective in reducing the band gap of Cr-doped WSe₂ monolayer than the compressive strain.     

Anomalous Hall effect in highly Mn-Doped silicon films

The transport and magnetic properties of Mn _x Si_{1 ? x} films with a high (x ≈ 0.35) content of Mn produced by laser deposition at growth temperatures of 300–350°C have been studied in a temperature range of 5–300 K in magnetic fields of up to 2.5 T. The films exhibit a hole-type metallic conductivity and a relatively weak change of magnetization in a temperature range of 50–200 K. An anomalous Hall effect with an essentially hysteretic behavior from 50 K up to ≈230 K has been discovered. The properties of the films are explained by the two-phase model, in which ferromagnetic clusters containing interstitial Mn ions with a localized magnetic moment are embedded in the matrix of a weak band MnSi_{2 ? x} (x ≈ 0.3) type ferromagnet with delocalized spin density.     

Magneto-optics and strain effects in GaInAsAlInAs and GaInAsInP quantum wells

Transitions between the Landau levels of the lowest electron and hole sub-bands have been observed in the inter-band optical absorption of a modulation doped Ga_.47In_.53AsAl_0.48In_.52As superlattice and a Ga_0.3In_.7AsInP strained layer superlattice, in magnetic fields up to 16T. In the unstrained sample, the energies of transitions up to 250 meV above the GaInAs band gap were found to be well described by a parabolic model for the heavy hole band and the model of Bowers and Yafet for the conduction band. In contrast to previous work on undoped quantum wells, no Coulomb binding effects were observed in this lightly doped sample, and this is attributed to screening of the exciton. In the strained sample, the heavy hole sub-band was found to show highly non-parabolic structure, and this is believed to be due to the effects of biaxial strain on the valence band.