Indirect Gap Semiconductor Research Articles

Tuning of electronic and optical properties of a predicted silicon allotrope: Hexagonal silicon h10 -Si

The indirect band gap of the diamondlike silicon does not allow direct travel of electrons between valence band and conduction band edges, which has limited its application and performance in optoelectronic devices. Searching new silicon allotropes with highly tunable or direct band gap becomes more and more urgent with the increasing demand on clean energy. Here, we predict a silicon allotrope, $h10$-Si, which is an indirect-gap semiconductor with a band gap of about 0.96 eV. Furthermore, the electronic and optical properties exhibit a strong dependence on strain. In particular, the indirect band gap of the predicted silicon allotrope can be switched to a direct band gap at a uniaxial tensile strain of about 8% and the light absorption can be tuned continuously in a wide range of photon energy. Besides, the single-layer counterpart of this allotrope is confirmed to be dynamically stable and more stable in energy than silicene, endowing it with potential applications in nanoscale devices.

Coexistence of phononic sixfold, fourfold, and threefold excitations in the ternary antimonide Zr3Ni3Sb4

Three-, four-, and sixfold excitations have significantly extended the subjects of condensed-matter physics. There is an urgent need for a realistic material that can have coexisting three-, four-, and sixfold excitations. However, these materials are uncommon because these excitations in electronic systems are usually broken by spin-orbit coupling (SOC) and are normally far from the Fermi level. Unlike the case in electronic systems, phonon systems with a negligible SOC effect, not constrained by the Pauli exclusion principle, provide a feasible platform to realize these excitations in a wide frequency range. Hence, in this work, we demonstrate by first-principles calculations and symmetry analysis that perfect three-, four-, sixfold excitations appear in the phonon dispersion rather than the band structures of ${\mathrm{Zr}}_{3}{\mathrm{Ni}}_{3}{\mathrm{Sb}}_{4}$, which is a well-known indirect-gap semiconductor with an ${\mathrm{Y}}_{3}{\mathrm{Au}}_{3}{\mathrm{Sb}}_{4}$-type structure. This material features a threefold-degenerate quadratic contact triple-point phonon, fourfold-degenerate Dirac point phonon, and sixfold degenerate point phonon. Moreover, these nodal point phonons are very robust to uniform strain. Two obvious phonon surface arcs of the (001) plane are extended in the whole Brillouin zone, which will facilitate their detection in future experimental studies. The current work provides an ideal model to investigate rich excitations in a single material.

The Case Studies of PtX2 (X = As, P) as High Thermo-power Materials

For thermoelectric applications, semiconductors are generally better than metals and insulators. PtAs2 and PtP2 are indirect energy gap semiconductors that have been predicted with high thermo-powers (PtP2 having higher thermopower than PtAs2). The crystal structure and the electronic structure of PtAs2 and PtP2 are similar except for the energy band gap of PtP2 that is wider than that of PtAs2. The generalized gradient approximation of the Density Functional Theory (DFT), the Density Functional Perturbation Theory (DFPT) were used to explore the full elastic tensors, phonon dispersion and the thermodynamics of PtP2 and PtAs2. This was done to understand the link, if any, between high thermo-power materials and the results. The two compounds are dynamically and elastically stable with higher mechanical properties recorded for PtP2 over PtAs2. The calculated entropy, vibration free energy and the heat capacity at constant volume for PtAs2 (PtP2) were 354.51 (264.18) J/K; -9.21 (27.84) kJ and 276.04 (250.36) J/K at 300 K respectively. The low frequency acoustic modes are between 100 - 170 cm-1 for PtAs2 while it is between100 - 190 cm-1 for PtP2. The calculated high frequency transverse optical (TO) mode for PtP2 is 410 cm-1 while it is 250 cm-1 for PtAs2. Further analysis of the phonons spectrum showed that additional bond-bending modes can be created in PtP2 than in PtAs2. All the results points toward PtP2 as better material over PtAs2 for thermoelectric application and these, with or without the knowledge of the energy bandgap can serve to guide material selection/modelling.

Indirect-to-direct band gap crossover of single walled MoS2 nanotubes

Using density functional theory, the electronic structures of single walled molybdenum disulfide nanotubes (MoS2 NTs) were investigated. The armchair MoS2 NTs are indirect gap semiconductors for diameters up to approximately 5.2 nm, while those with larger diameters are direct gap semiconductors with band edges located in the vicinity of k = 2π/3. This finding implies that MoS2 NTs with large diameters should exhibit similar photoluminescence to 2D monolayer MoS2 sheets. This indirect-to-direct band gap crossover accounts for the relative upward shift of the valence band peak at the Γ point in small diameter NTs, owing to the tensile strain arising from curvature.

Structural, elastic, electronic, and anisotropic properties of Pbca-SiC and Pbcn-SiC

The crystal structures, mechanical stability, anisotropy, electronic band structures, and effective mass of Pbca-SiC and Pbcn-SiC under pressure are calculated utilizing first-principles calculations. Pbca-SiC and Pbcn-SiC with pressures in the range of 0–50 GPa have thermodynamic and mechanical stability. Visible anisotropies are discovered by analyzing the two- and three-dimensional representations of Young’s modulus, which also change with increasing pressure. The band structure results forecast two wide bandgap semiconductors. Pbca-SiC is an indirect gap semiconductor with a value of 3.724 eV. It is worth noting that Pbcn-SiC is a direct gap semiconductor with a value of 3.639 eV, and the bandgaps of Pbcn-SiC decrease with increasing pressure, which makes the emission wavelength of Pbcn-SiC change from the near ultraviolet light zone to visible light zone. Based on the controllable direct bandgap, Pbcn-SiC has better application potential in light-emitting devices. Moreover, the carrier effective mass under ambient conditions is also calculated, and the minimum value of the electron effective mass is obtained in Pbcn-SiC with a value of 0.262m0, while the minimum value of the hole effective mass is found in Pbca-SiC with a value of −0.285m0.

Buckled hexagonal carbon selenium nanosheet for thermoelectric performance

Buckled hexagonal CSe nanosheet made of earth-abundant elements is investigated for thermoelectric performance by employing the first-principle calculations and the Boltzmann transport theory. The buckled hexagonal CSe sheet is shown to be an indirect-gap semiconductor with a lattice constant of 3.07 A and band gap of 1.51 eV. The maximum power factor of n-type (p-type) CSe sheet is 618 to 483 (9.87 to 7.42) mW m $$^{-1}$$ K $$^{-2}$$ at the temperatures from 300 to 700 K at the carrier concentration of about $$1.17\times 10^{13}$$ ( $$1.24\times 10^{13}$$ ) cm $$^{-2}$$ ). The lattice thermal conductivity of the buckled CSe sheet is in the range of 11.62 to 5.00 W m $$^{-1}$$ K $$^{-1}$$ , and the dimensionless figure of merit of n-type (p-type) CSe sheet is as high as 0.69 to 0.85 (0.18 to 0.40) at the temperatures from 300 to 700 K by choosing appropriate doping level. The high thermoelectric performance indicates that the buckled hexagonal CSe sheet is a promising n-type two-dimensional thermoelectric material.

Theoretical prediction of the PtOX (X = S and Se) monolayers as promising optoelectronic and thermoelectric 2D materials

Abstract In this paper, two new monolayers, namely PtOS and PtOSe, are theoretically predicted using first-principles calculations. Structural, electronic, optical and thermoelectric properties are explored using full-potential linearized augmented plane-wave (FP-LAPW) method and the semiclassical Boltzmann transport theory. Predicted two-dimensional (2D) materials show good dynamical, thermodynamic and structural stability. Calculated electronic structures indicate the indirect gap semiconductor nature of the PtOS and PtOSe single layers with energy gap of 1.346(2.436) and 0.978(1.978) eV as calculated with the WC(HSE06) functional, respectively. Density of states spectra and valence charge distribution maps suggest a mix of covalent and ionic characters of the chemical bonds. 2D materials at hand exhibit good absorption property in the visible regime with coefficient value reaching the order of 105/cm, even much larger in the ultraviolet, suggesting the promising optoelectronic applicability. Finally, the thermoelectric parameters including electrical conductivity, thermal conductivity, Seebeck coefficient, power factor and figure of merit are determined and analyzed. Results indicate prospective thermoelectric performance of both considered single layers as demonstrated by large figure of merit close to unity. Our work introduces two new 2D multifunctional materials that may possess potential applications in the optoelectronic and thermoelectric nano-devices.

Electronic and optical properties of armchair silicon carbide nanotubes from first principles

We calculate the electronic band gap and optical spectra of armchair (n, n) silicon carbide nanotubes (n = 9–12) using density functional theory (DFT) combined with Non equilibrium Green's function (NEGF) method as implemented in the SIESTA code. Our simulation suggests that the armchair SiCNTs are indirect-gap semiconductors and SiCNTs (11, 11) have the largest electronic band gap. The z component of the refractive index of SiCNTs (9, 9) reaches its zenith at energies below 2.5 eV. Moreover, the optical absorption takes place in the energy range ∼2−5 eV for the z polarization. The simulated armchair SiCNTs with larger diameter have the smaller first optical band gap as well.

Influence of lattice deformations on the electronic structure of the molybdenum disulfide monolayer

The possibilities and conditions for modifying the band gap and the behavior of interband transitions under compressive and tensile strains in the crystal lattice of a molybdenum disulfide monolayer have been determined by theoretical modeling. It is shown that depending on the value and direction of the strains the compound may be a direct-gap or indirect-gap semiconductor, and the conditions for such transformations are determined. The results demonstrate a potential use of the molybdenum disulfide monolayer in nanoelectronic devices of new generation in which controlled transport of charge carriers is possible

Structural and magnetic properties of two-dimensional layered BiFeO3 from first principles

Using first-principles calculations, we predict a unique $\mathrm{BiFe}{\mathrm{O}}_{3}$ (BFO) phase, which is coined $I$-BFO, with layered characteristics based on the ilmenite structure. The interactions between interlayer bismuth atoms make it easy to exfoliate monolayer BFO with a configuration in which Bi ions are above and below two-dimensional (2D) iron-oxygen octahedra sharing common edges. The calculated phonon spectra reveal the dynamic stability of both bulk and monolayer forms of $I$-BFO. Combining total energy computation and crystal structure search, we corroborate that the $I$-BFO monolayer maintains the lowest-energy ground state, as compared to other 2D BFO exfoliations. We also discover that monolayer $I$-BFO is a robust antiferromagnetic material with a N\'eel temperature (${T}_{N}$) of 4 K in the fully relaxed case. Such ${T}_{N}$ can be prominently improved by strain, for example, ${T}_{N}=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at 6% tension. Furthermore, our electronic structure calculations with PBE and hybrid HSE functionals show the $I$-BFO monolayer to be an indirect-gap semiconductor. Finally, we indicate that other ternary $AB{\mathrm{O}}_{3}$ compounds also have the potential to produce layered materials from the ilmenite structure. This work thus provides guidance to explore unconventional layered materials, and further broadens the application of BFO and other promising ilmenite oxides in two dimensions.

Tuning MoSO monolayer properties for optoelectronic and spintronic applications: effect of external strain, vacancies and doping.

Since the successful synthesis of the MoSSe monolayer, two-dimensional (2D) Janus materials have attracted huge attention from researchers. In this work, the MoSO monolayer with tunable electronic and magnetic properties is comprehensively investigated using first-principles calculations based on density functional theory (DFT). The pristine MoSO single layer is an indirect gap semiconductor with energy gap of 1.02(1.64) eV as predicted by the PBE(HSE06) functional. This gap feature can be efficiently modified by applying external strain presenting a decrease in its value upon switching the strain from compressive to tensile. In addition, the effects of vacancies and doping at Mo, S, and O sites on the electronic structure and magnetic properties are examined. Results reveal that Mo vacancies, and Al and Ga doping yield magnetic semiconductor 2D materials, where both spin states are semiconductors with significant spin-polarization at the vicinity of the Fermi level. In contrast, single S and O vacancies induce a considerable gap reduction of 52.89% and 58.78%, respectively. Doping the MoSO single layer with F and Cl at both S and O sites will form half-metallic 2D materials, whose band structures are generated by a metallic spin-up state and direct gap semiconductor spin-down state. Consequently, MoV, MoAl, MoGa, SF, SCl, OF, and OCl are magnetic systems, and the magnetism is produced mainly by the Mo transition metal that exhibits either ferromagnetic or antiferromagnetic coupling. Our work may suggest the MoSO Janus monolayer as a prospective candidate for optoelectronic applications, as well as proposing an efficient approach to functionalize it to be employed in optoelectronic and spintronic devices.

Thermoelectric Effect in Janus Monolayer InSSe

Thermoelectric effect in Janus monolayer InSSe is investigated by non-equilibrium Green's function and density functional theory. Janus monolayer InSSe is an indirect gap semiconductor with a bandgap of 1.24 eV. The value of the thermoelectric figure of merit is up to 4.39 at the temperature of 500 K and 1.38 at room temperature. We found a remarkably low and anisotropic phononic thermal conductance about 0.05987 WK–1m–1, which is originated from the soft lattice vibration modes. This low thermal conductance is responsible for the high value of the figure of merit. All the results indicate that Janus monolayer InSSe is a more suitable low dimensional thermoelectric material.

Влияние сжимающих и растягивающих напряжений на электронную структуру фосфорена

A new promising semiconductor material - phosphorene - was investigated and the possibilities of modifying of its interband transitions under compressive and tensile strains in the crystal lattice of the material were determined by means of theoretical modeling. It is shown that depending on the value and direction of the strains this material may be direct-gap or indirect-gap semiconductor. The potential for the use of phosphorene in nanoelectronic devices of new generation with controlled transport of charge carriers is shown.

Stable anisotropic single-layer of ReTe2: a first principles prediction

In order to investigate the structural, vibrational, electronic, and mechanical features of single-layer ReTe2first-principles calculations are performed. Dynamical stability analyses reveal that single-layer ReTe2crystallize in adistorted phase while its 1H and 1T phases are dynamically unstable. Raman spectrum calculations show that single-layer distorted phase of ReTe2exhibits 18 Raman peaks similar to those of ReS2and ReSe2. Electronically, single-layerReTe2is shown to be an indirect gap semiconductor with a suitable band gap for optoelectronic applications. In addition,it is found that the formation of Re-units in the crystal induces anisotropic mechanical parameters. The in-plane stiffnessand Poisson ratio are shown to be significantly dependent on the lattice orientation. Our findings indicate that single-layer form of ReTe2can only crystallize in a dynamically stable distorted phase formed by the Re-units. Single-layer ofdistorted ReTe2can be a potential in-plane anisotropic material for various nanotechnology applications.

Theoretical analysis of the HfS[formula omitted] monolayer electronic structure and optical properties under vertical strain effects

In this paper, the HfS2 monolayer electronic structure and optical properties under vertical strains are theoretically explored using density functional theory (DFT) calculations. The HfS2 single layer dynamical stability is examined by calculating the phonon dispersion curves. Based on our simulation, the studied two-dimensional (2D) material is an indirect gap semiconductor with band gap value of 1.545 eV. The band gap engineering can be effectively realized by applying the vertical strains. In this regard, the indirect-direct gap transition in the monolayer at hand may be induced by compressive strains with strength from −9%. Consequently, significant changes of the optical properties may be obtained, in particular when visible to middle ultraviolet regime radiation is incident on the sample. The HfS2 monolayer displays promising optoelectronic applicability with a high absorption coefficient reaching to 49.600 (104/cm) and 88.122 (104/cm) in the visible and ultraviolet regime, respectively. Results may suggest an effective approach to modify the optoelectronic properties of the HfS2 single layer at the time of designing its practical applications.

A first-principles Quantum Monte Carlo study of two-dimensional (2D) GaSe

Two-dimensional (2D) post-transition metal chalcogenides (PTMCs) have attracted attention due to their suitable bandgaps and lower exciton binding energies, making them more appropriate for electronic, optical, and water-splitting devices than graphene and monolayer transition metal dichalcogenides. Of the predicted 2D PTMCs, GaSe has been reliably synthesized and experimentally characterized. Despite this fact, quantities such as lattice parameters and band character vary significantly depending on which density functional theory (DFT) functional is used. Although many-body perturbation theory (GW approximation) has been used to correct the electronic structure and obtain the excited state properties of 2D GaSe, and solving the Bethe-Salpeter equation (BSE) has been used to find the optical gap, we find that the results depend strongly on the starting wavefunction. In an attempt to correct these discrepancies, we employed the many-body Diffusion Monte Carlo (DMC) method to calculate the ground and excited state properties of GaSe because DMC has a weaker dependence on the trial wavefunction. We benchmark these results with available experimental data, DFT [local-density approximation, Perdew-Burke-Ernzerhof (PBE), strongly constrained and appropriately normed (SCAN) meta-GGA, and hybrid (HSE06) functionals] and GW-BSE (using PBE and SCAN wavefunctions) results. Our findings confirm that monolayer GaSe is an indirect gap semiconductor (Γ-M) with a quasiparticle electronic gap in close agreement with experiment and low exciton binding energy. We also benchmark the optimal lattice parameter, cohesive energy, and ground state charge density with DMC and various DFT methods. We aim to present a terminal theoretical benchmark for pristine monolayer GaSe, which will aid in the further study of 2D PTMCs using DMC methods.

Pressure Effect Study on the Electronic and Optical Properties of BxIn1 – xAs Alloys Using DFT Calculation

In order to extract structural and electronic properties of BxIn1 – xAs ternary alloys and enrich the database of materials based on boron and indium, we have used full-potential augmented plane wave (FP-LAPW) method through the density function theory (DFT) and within generalized gradient approximation (GGA), local density approximation (LDA), and Tran–Blaha modified Becke–Johnson approximation (TB–mBJ). We have optimized the cohesive energy of our binary compound and ternary alloys versus volume of the unit cell firstly, and we have found that the optimum volume, lattice parameter, and the bulk modulus vary for different boron concentrations. Using DFT–mBJ calculations, we found that InAs possess direct band-gap energy and an indirect gap semiconductor for BAs and B0.75In0.25As. However, B0.25In0.75As and B0.5In0.5As ternary alloys have a metallic and semi metallic characters, respectively. We also studied the optical properties of our BAs and InAs binary and B0.75In0.25As ternary semiconductors and their behaviors are also investigated under the application of hydrostatic pressure in a range of 0 to 25 GPa. In summary, we conclude that the incorporation of boron atom in InAs increase its hardness and affects the band-gap energy considerably, and therefore provides a novel research perspective. We note that InAs binary compound loses its semiconductor character and becomes semi-metal at 5 GPa.

First-principles study on the electronic structure and optical properties of BiOBr

By adopting the first principle method based on density functional theory framework, the geometric structure, electronic structure and optical properties of BiOBr were studied by generalized gradient approximate. Theoretical calculation results have shown that BiOBr is an indirect wide-band gap semiconductor material with a band gap width of 2.19 ev. The analyses of state density and atomic charge arrangement number have shown that the valence band top of BiOBr is mainly contributed by 4p orbitals of O 2p and Br 4p, while the lowest conduction band is mainly formed by Bi 6p orbital. In addition, the charge density has revealed that BiOBr is a hybrid bond semiconductor with strong covalent and weak ionic properties, and has a strong p orbital distribution. The calculation results of optical properties have shown that two obvious dielectric peaks appear in the energy range of 0-15 eV, and the absorption band edge is 448 nm, which is consistent with the experimental results. The above outcomes have the theoretical guiding significance for exploring the potential applications of BiOBr materials, and provide theoretical basis for us to accurately monitor and control the growth of BiOBr materials.

Two-dimensional MgSiP2 with anisotropic electronic properties and good performances for Na-ion batteries

Using the global particle-swarm optimization method and density functional theory, we predict a new stable two-dimensional layered material: MgSiP2 with a low-buckled honeycomb lattice. Our HSE06 calculation shows that MgSiP2 is an indirect-gap semiconductor with a band-gap of 1.20 eV, closed to that of bulk silicon. More remarkably, MgSiP2 exhibits worthwhile anisotropy along with electron and hole carrier mobility. A ultrahigh electron mobility is even up to 1.29 × 104 cm2 V−1 s−1, while the hole mobility is nearly zero along the a direction. The large difference of the mobility between electron and hole together with the suitable band-gap suggest that MgSiP2 may be a good candidate for solar cell or photochemical catalysis material. Furthermore, we explore MgSiP2 as an anode for sodium-ion batteries. Upon Na adsorption, the semiconducting MgSiP2 transforms to a metallic state, ensuring good electrical conductivity. A maximum theoretical capacity of 1406 mAh/g, a small volume change (within 9.5%), a small diffusion barrier (∼0.16 eV) and low average open-circuit voltages (∼0.15 V) were found for MgSiP2 as an anode for sodium-ion batteries. These results are helpful to deepen the understanding of MgSiP2 as a nanoelectronic device and a potential anode for Na-ion batteries.

DFT study of gas adsorption and sensing based on noble metal (Ag, Au and Pt) functionalized boron selenide nanosheets

Adsorption of transition metal atoms on the surface of 2D layered nanomaterials plays a significant role in modulating the electronic properties of the metal functionalized systems. In this work, we examined the adsorption of Ag, Au and Pt adatoms on the surface of BSe monolayer based on the DFT method. Our band structure calculations reveal that perfect BSe monolayer displays indirect gap semiconductor property. The adsorption performance of the Au-adsorbed BSe monolayer upon NO, NH3 and NO2 gas molecules was examined to evaluate its potential use for highly efficient gas sensor devices. The Au adatom is adsorbed on the BSe monolayer with considerable adsorption energy, which indicates the strength and stability of the substrate for adsorption processes. Thus, the Au-adsorbed BSe monolayer behaves as an excellent adsorbent for sensing NO, NH3 and NO2 gas molecules. Interestingly, all the gas molecules are strongly chemisorbed on the Au site of Au–BSe system. NO2 molecule presents the most favorable adsorption on the surface with oxygen atoms coordinated to the Au atom. Our results also suggest that gas molecules adsorption on the Au–BSe monolayer can lead to remarkable variations in the electronic properties of the systems. The greatly enhanced electron overlaps between the projected density of states of the interacting atoms confirm the chemical interaction between the gas molecules and Au–BSe monolayer.