Indirect Band Gap Transition Research Articles

Effect of modifier oxides on spectroscopic and optical properties of Pr3+ doped PbO-Ro2O3–WO3–B2O3 glasses (with Ro2O = Sb2O3, Al2O3, and Bi2O3)

Praseodymium (Pr2O3) doped PbO-Ro2O3–WO3–B2O3 glasses were prepared by melt quenching method and their optical absorption and emission spectra were characterized, for three different heavy metal oxides (Ro2O3 = Sb2O3, Al2O3, and Bi2O3). The characteristics of the glass matrix are confirmed by X-ray diffraction spectra. Many structural properties can be identified by Fourier Transform infrared spectra of the glasses, such as PbO, B2O3, and WO3. From the optical absorption spectra, the absorption edges, and the values of direct and indirect band gap transitions and Urbach energies were calculated. The oscillator strengths of the absorption bands were determined by the Judd-Ofelt model, and the Judd-Ofelt intensity parameters Ω2, Ω4, and Ω6 were calculated for each of the studied glasses. Moreover, the nephelauxetic ratio β, bonding parameter δ, and optical basicity Λth were also calculated to characterize the bonding nature of the Pr3+ rare-earth ions. From the emission spectra, spontaneous radiative transition probability Aτ, total radiative transition probability AT, luminance branching ratio βr, and radiative life time τrad were calculated. From the spectra, the CIE color coordinates were also calculated for all the studied glasses. This research confirms that the studied materials are suitable for red laser applications.

Computational prediction of electronic and optical properties of Janus Ga2SeTe monolayer

Janus group III monochalcogenide structures, which are predicted to have many promising applications in optoelectronics and photocatalytic water splitting, have been of particular interest recently. In this study, electronic properties and optical characteristics of the Janus Ga2SeTe monolayer under a biaxial strain εb and electric field E were considered using the density functional theory. Our calculations demonstrated that the Janus Ga2SeTe monolayer is dynamically and thermally stable and exhibits a semiconducting characteristic with a moderate direct band gap at equilibrium. The band gap of Janus Ga2SeTe monolayer at equilibrium, which is calculated by PBE method and corrected by HSE06 hybrid functional, is smaller than that of both GaSe and GaTe monolayers. Mulliken population analysis shows that there has been a redistribution of charge during the formation of the Janus structure, especially there is a large difference in charge between the two Ga layers in Janus Ga2SeTe monolayer. The biaxial strain has greatly altered the electronic structure of the Janus Ga2SeTe monolayer and direct–indirect band gap transitions were found at appropriate strain εb. While the effect of the E on electronic properties and especially optical properties is weak, the optical absorbance of the Janus Ga2SeTe monolayer can be enhanced by strain engineering, up to 14.42 × 104 cm−1 at εb=−7% in the near-ultraviolet region. The optical absorbance of the Janus Ga2SeTe monolayer is activated in the visible light region that is following its calculated band gap value. This work not only systematically presents the electronic and optical properties of the Janus Ga2SeTe monolayer in the presence of strain engineering and electric field but can also motivate experimental studies for applications in nanoelectromechanical and optoelectronic devices.

Janus Ga2STe monolayer under strain and electric field: Theoretical prediction of electronic and optical properties

Abstract In this work, detailed investigations of the electronic and optical properties of a Janus Ga2STe monolayer under a biaxial strain and electric field have been performed using density functional theory. Via the phonon spectrum and ab-initio molecular dynamics simulations, the dynamical and thermal stabilities of the Janus Ga2STe monolayer are verified. Our obtained results showed that the Janus Ga2STe exhibits a direct semiconducting characteristic and its band gap depends greatly on the biaxial strain. While both the electronic and optical properties are very weakly dependent on the electric field, strain engineering can cause a direct–indirect band gap transitions in the Janus Ga2STe. At equilibrium, the optical absorbance of the Janus Ga2STe monolayer is activated in the infrared light region of about 0.9 eV, which is close to its band gap value. The main peak of the optical absorbance spectrum is located in the ultraviolet light region with an absorbance intensity of 11.914 × 104 cm−1 may be increased by compression strain. In particular, the absorbance intensity of the Janus Ga2STe monolayer increases rapidly in the visible light region, reaching 4.810 × 104 cm−1 and can be altered by strain. Our results not only show that the Janus Ga2STe monolayer has many promising applications in opto-electronic devices but also motivates experimental works on Janus structures in near future.

First-principles study of the band structure and optical spectra of germanium carbide under mechanical strain

By using first-principles calculations, the electronic and optical properties of GeC sheet under strain have been investigated. The results demonstrate that both biaxial compressive and tensile strain change the band gap of GeC sheet. The capability of direct to indirect band gap transition is observed by exerting the in-plane biaxial strain on GeC sheet. Also, by applying the tensile (compressive) strain, the optical spectra are globally shifted toward lower (higher) photon energies. The results demonstrate that the electronic structure and optical properties of GeC sheet can be tuned by strain engineering and the GeC sheet can be a promising material for the future nano-based electrical and optical devices.

Transfer of Direct to Indirect Bound Excitons by Electron Intervalley Scattering in Cs2AgBiBr6 Double Perovskite Nanocrystals

Lead-free halide double perovskites have emerged as a non-toxic alternative to the heavily researched lead-based halide perovskites. However, their optical properties and the initial charge carrier relaxation processes are under debate. In this study, we apply time-resolved photoluminescence and differential transmission spectroscopy to investigate the photo-excited charge carrier dynamics within the indirect band structure of Cs2AgBiBr6 nanocrystals. Interestingly, we observe a high energetic emission stemming from the direct band gap, besides the previously reported emission from the indirect band gap transition. We attribute this emission to the radiative recombination of direct bound excitons. This emission maximum redshifts nearly 1 eV within 10 ps due to electron intervalley scattering, which leads to a transfer of direct to indirect bound excitons. We conclude that these direct bound excitons possess a giant oscillator strength causing not only a pronounced absorption peak at the optical band gap energy but also luminescence to occur at the direct band gap transition in spite of the prevailing intervalley scattering process. These results expand the understanding of the optical properties and the charge carrier relaxation in double perovskites, thus, facilitating the further development of optoelectronic devices harnessing lead-free perovskites.

Electronic properties of Janus MXY/graphene (M = Mo, W; X ≠ Y = S, Se) van der Waals structures: a first-principles study.

Based on the first-principles calculations, we studied the intrinsic dipole moment and electronic properties of Janus MXY (M = Mo, W; X ≠ Y = S, Se) monolayers, bilayers and heterostructures with graphene, and the possibility of MXY encapsulating graphene. The results show that Janus MXY monolayer has an intrinsic dipole moment and a direct band gap. However, for MXY bilayers strong interlayer coupling will cause direct to indirect band gap transition, and the existence of the dipole moment leads to a significantly large interlayer band offset, being the driving force for the formation of interlayer excitons. In MXY/graphene heterostructures, changes in the direction of intrinsic dipole moment will cause a change in Schottky barrier height and even the transition between p- and n-type Schottky contacts. Independent of the interface atomic layer of Janus MXY, on one hand, the Dirac cone still exists in graphene, proving that MXY is an ideal coating material. On the other hand, the type-II band alignment will disappear as the intrinsic dipole moment disappears, confirming that the intrinsic dipole moment plays a vital role in the formation of a large band offset. Our results provide guidance for the study of interlayer excitonic states, the experimental construction of atomically thin p-n junctions and the encapsulation of graphene.

Enhanced sensitivity of band gap engineered phosphorene towards NH3 and NO2 toxic gases

Band gap tuning is an efficient approach to modify the sensing ability of phosphorene. Herein, density functional theory (DFT) calculations have been performed to investigate the effect of various group elements (III-VI) doping in phosphorene sheet for NH3 and NO2 gas sensing ability. The presence of NH3 and NO2 gas molecules on doped phosphorene nanosheet is found to alter the structural network, electronic properties hence leading to a better sensing and selectivity of the doped nanosheet. B doped phosphorene showed maximum adsorption of −2.64 eV and −3.70 eV towards the NH3 and NO2 gas molecules respectively. NH3 adsorption on C, Si and S-doped phosphorene causes direct to indirect band gap transition while NO2 adsorption shows indirect band gap for pristine and S-doped phosphorene. Simulated scanning tunneling microscopy (STM) images showed different structural phases after gas adsorption due to electronic hybridization with orbitals from the upmost phosphorous atoms and the gas molecule. After gas adsorption, some flat bands are observed and localized states appear near the Fermi level, which are readily accessible at low bias voltages, thereby indicating the improved sensory functionality of doped phosphorene.

Phase-pure iron pyrite (FeS2) micro- and nano-sized crystals synthesized by simple one-step microwave-assisted hydrothermal method

Iron pyrite micro- and nano-sized crystals are desirable as active materials in lithium ion batteries and photovoltaics, and are particularly suitable for nanocrystal inks for roll-to-roll deposited or ink-jet printed solar cells. In this paper we report the synthesis of iron pyrite micro- and nano-sized crystals via simple and convenient green one-step microwave-assisted hydrothermal (M − H) process at relatively low growth temperatures, using commonly used precursors such as FeCl3, Na2S and S8 and. The structural, morphological and optical properties of the resulting nanostructured materials have been thoroughly investigated for two typical M − H growth temperatures. X-ray diffraction (XRD) pattern and Raman data revealed good crystalline quality for the as synthesized pyrite NCs. Typical XRD patterns show the dominant peaks which can be indexed as a pure cubic phase of FeS2, with lattice constant values close to the lattice parameters reported for FeS2, and in agreement with a stoichiometric pyrite phase. XRD and Raman analysis also confirm that no other impurities phases such as hexagonal FeS, marcasite, pyrrhotite, greigite, S or Fe–O compounds were detected, confirming the high purity of the synthesized iron pyrite nanocrystals. Various shapes of pyrite like quasi-cubic, cubic and flower-like FeS2 nanocrystals have been observed, which can be modulated by using different synthetic conditions. The sizes of the pyrite micro- and nanostructures were in the range of 150 nm to 1 μm as obtained. The present study indicates that the M − H method is a facile one-step way to obtain phase pure iron pyrite micro- and nano-sized crystals. Optical characterization confirms direct bandgap transitions (values of 2.61 eV and 2.55 eV for iron pyrite NCs samples hydrothermal grown at 130 °C and 160 °C, respectively), and indirect bandgap transitions (values of 1.19 eV and 1.52 eV for samples hydrothermal grown at 130 °C and 160 °C, respectively). Optical studies show high absorbance in the entire UV–Vis wavelength range making the as-synthesized pyrite nanocrystals potential candidate as absorber in nanoscale photovoltaic solar cells.

Direct–indirect bandgap transition in monolayer MoS2 induced by an individual Si nanoparticle

MoS2 is promising for the next generation of electronic and optoelectronic devices by virtue of its unique optical, electrical and mechanical properties. Bandgap engineering of it is an interesting topic. However, the reported factors including temperature, defect, strain and external electric field are difficult to handle precisely. Here, we demonstrated direct–indirect bandgap transition in monolayer MoS2 induced by an individual Si nanoparticle. We observed photoluminescence (PL) emission with obvious spectral redshift and broadening in the MoS2/Si heterostructures after depositing Si nanoparticles onto the surface of monolayer MoS2. Raman spectra of heterostructures show measurable shifts in contrast with the bare MoS2. Energy transfer between MoS2 and Si nanoparticles did not happen, which is demonstrated by scattering spectra of MoS2/Si heterostructures. In addition, the natural oxide layer presented on the surface of Si nanoparticles can effectively prevent the carrier transferring from Si nanoparticles to MoS2. Thus, we attribute the direct–indirect bandgap transition of monolayer MoS2 to the strain induced by Si nanoparticles controlled by their sizes. The PL intensity of MoS2/Si heterostructure depends on the size of Si nanoparticles, resulting from the enhanced optical absorption of monolayer MoS2 based on Mie resonances of Si nanoparticles. The MoS2/Si heterostructure is promising for photodetector and circuit integration.

Electronic and optical properties of SrS nanosheet in 001 and 101 directions

Optical and electronic properties of SrS nanosheet were investigated via density functional theory (DFT). The calculation was performed using the generalized gradient approximation (GGA) based on the WIEN2k package. Random phase approximation (RPA) was used to derive the dielectric functions. Energy band structure, density of states, dielectric function, energy loss function, optical reflectivity, absorption coefficient index of the samples were calculated for E||x and E||z electric field polarizations. The Density of state (DOS) indicates a strong hybridization between p-orbitals (S atom) and d-orbitals (Sr atom) in the valence band. The results show that the SrS nanosheets are semiconductors with indirect band gap transition. In addition, the samples have not magnetic property. The Eg of SrS nanosheet are 2.46 eV, 3.32 eV with indirect bandgap type M→Γ within the PBE-GGA and mBJ-GGA, in 001 direction and 2.83 eV, 3.82 eV with direct bandgap type Γ→Γ and indirect bandgap Γ→Δ within the PBE-GGA and mBJ-GGA, in 101 direction. The optical analysis illustrate that the first absorption peaks for these materials occur in the visible range of the electromagnetic spectrum which can be potentially attractive for applications in optoelectronics and nanoelectronics.

Moderate strain induced indirect bandgap and conduction electrons in MoS2 single layers

MoS2 single layers are valued for their sizeable direct bandgap at the heart of the envisaged electronic and optoelectronic applications. Here we experimentally demonstrate that moderate strain values (~2%) can already trigger an indirect bandgap transition and induce a finite charge carrier density in 2D MoS2 layers. A conclusive proof of the direct-to-indirect bandgap transition is provided by directly comparing the electronic and optical bandgaps of strained MoS2 single layers obtained from tunneling spectroscopy and photoluminescence measurements of MoS2 nanobubbles. Upon 2% biaxial tensile strain, the electronic gap becomes significantly smaller (1.45 ± 0.15 eV) than the optical direct gap (1.73 ± 0.1 eV), clearly evidencing a strain-induced direct to indirect bandgap transition. Moreover, the Fermi level can shift inside the conduction band already in moderately strained (~2%) MoS2 single layers conferring them a metallic character.

Ab-initio simulations of MgTiO3 oxide at different pressure

We employ Wien2k code, an all-electron scheme based on density functional theory (DFT) to explore the structural, thermodynamic, mechanical and opto-electronic behavior of MgTiO3 (MTO) oxide in the pressure range 0–200 GPa. The structural, mechanical and thermodynamic stabilities of MTO are discussed in terms of Goldschmidt's tolerance factor, enthalpy of formation and Born stability criteria, respectively. Mechanical nature is further discussed by calculating the Debye temperature, wave velocity, Pugh's and Poisson's ratios. The electron density (n) and specific heat capacity (Cv) of electrons are explained in details. The pressure up to 200 GPa, with the step of 50 GPa, is implemented in order to tune the electronic properties where a direct to indirect band gap transition is observed. We further explored the refraction of light (ultraviolet region), dielectric constant and the optical behavior of MTO.

New Approaches in Flexible Organic Field-Effect Transistors (FETs) Using InClPc.

Organic semiconductor materials have been the center of attention because they are scalable, low-cost for device fabrication, and they have good optical properties and mechanical flexibility, which encourages their research. Organic field-effect transistors (OFETs) have potential applications, specifically in flexible and low-cost electronics such as portable and wearable technologies. In this work we report the fabrication of an InClPc base flexible bottom-gate/top-contact OFET sandwich, configured by the high-evaporation vacuum technique. The gate substrate consisted of a bilayer poly(ethylene terephthalate) (PET) and indium–tin oxide (ITO) with nylon 11/Al2O3. The device was characterized by different techniques to determine chemical stability, absorbance, transmittance, bandgap, optical properties, and electrical characteristics in order to determine its structure and operational properties. IR spectroscopy verified that the thin films that integrated the device did not suffer degradation during the deposition process, and there were no impurities that affected the charge mobility in the OFET. Also, the InClPc semiconductor IR fingerprint was present on the deposited device. Surface analysis showed evidence of a nonhomogeneous film and also a cluster deposition process of the InClPc. Using the Tauc model, the device calculated indirect bandgap transitions of approximately 1.67 eV. The device’s field effect mobility had a value of 36.2 cm2 V−1 s−1, which was superior to mobility values obtained for commonly manufactured OFETs and increased its potential to be used in flexible organic electronics. Also, a subthreshold swing of 80.64 mV/dec was achieved and was adequate for this kind of organic-based semiconductor device. Therefore, semiconductor functionality is maintained at different gate voltages and is transferred accurately to the film, which makes these flexible OFETs a good candidate for electronic applications.

Direct to indirect band gap transition in two-dimensional germanium carbide through Si substitution

In this work, the structural, electronic and optical properties of Si substituted single-layer GeC are studied by performing density functional theory calculations. Single-layer GeC possesses a direct band gap of 2.11 eV at the K point. The direct band gap feature in single-layer GeC does not change by incorporation of one or two Si atom in a (2 × 2) GeC supercell, while a direct band gap (K–K) into indirect band gap (K–Γ) transition is observed by incorporation of three Si atom in the supercell. After substitution of three Si atom in (2 × 2) GeC supercell, for parallel polarized light, the first absorption peak at 3.09 eV slightly shifted to higher energy of 3.22 eV along with an increase in peak intensity. For (E‖z), the first two peaks show small shifts to higher energies by about 0.3 eV. These results are useful for application of two-dimensional GeC in nanoelectronic and optoelectronic devices.

Strain Modulated Optoelectronic Properties of CdO Monolayer

We studied the effect of biaxial tensile and compressive strain on the electronic and optical properties using the density function theory. Electronic properties are investigated in terms of the electronic band structure and density of states. CdO monolayer exhibits a direct bandgap ∼ 1.35 eV using a modified Becke–Johnson approximation. The value of bandgap decreases under both biaxial tensile and compressive strains. Moreover, direct to indirect electronic bandgap transition is observed above 8% tensile strain, whereas direct bandgap nature is conserved under compressive strain. The effective mass of an electron is nearly insensitive to strain, whereas hole effective mass shows linear increment with strain and a sudden jump at the electronic transition point of above 8% tensile strain. The noticed low reflectivity in unstrained and compressive strained conditions suggests that the CdO monolayer may be a promising material for transparent conducting oxide applications.

SINGLE LAYER ReS2H2 : STABILITY, RAMAN ACTIVITY AND ELECTRONIC PROPERTIES

In this study, the structural, vibrational and electronic properties of the hydrogenated single layer of ReS2 are investigated by performing the first principle calculations based on density functional theory. We found that the characteristic properties of the monolayer ReS2 can be manipulated upon the hydrogen functionalization. As the monolayer ReS2, the ReS2H2 has distorted 1Tphase; however, the bonding in Re slab significantly varies with the hydrogenation. Our results demonstrate that the full-surface hydrogenation leads to an expansion in lattice and the Re4 tetramer-chains in the monolayer ReS2 are separated into two dimers in the hydrogenated monolayer. It is calculated that the dynamically stable monolayer of ReS2H2 has 26 Raman-active vibrational modes. Constant volume specific heat calculations are also performed and the results indicate that at high temperature, the monolayer ReS2 approaches to limit of 3R before the monolayer ReS2H2. By performing the electronic band structure calculations, it is shown that when the ReS2 surface is fully hydrogenated, there occurs a direct to indirect band gap transition and the semiconducting hydrogen-induced monolayer has a band gap of 0.74 eV.

Strain-mediated stability and electronic properties of WS2, Janus WSSe and WSe2 monolayers

Monolayers of transition metal dichalcogenides (TMDs) have been proposed as the next generation electronic materials for nanoscale devices. We evaluated the thermodynamic stability of WS2, Janus WSSe and WSe2 monolayers under biaxial tensile and compressive strain using density functional approach. The phonon band structures of unstrained WS2, WSSe and WSe2 monolayers confirm their thermodynamic stability. The stability of these monolayers is investigated under strain and phonon softening is observed for acoustic and optical mode upto 8% tensile strain. The bond length reduction under compressive strain resulted in out of plane deformation, leading the instability of monolayers under compressive strain. The bond length W-S(Se) and bond angle W-S(Se)-W exhibit significant contribution in coupling strength between transition metal d-orbitals and chalcogen p-orbitals that mediated the electronic properties. The spin orbit coupling showed strong effect on splitting of the valence band for unstrained monolayers. The spin splitting in valence band increases with increasing the tensile strain and decreases with increasing the compressive strain. The effective masses and mobility values are 0.57me, 0.54me, 0.47me; and 0.059, 0.062, 0.072 m2V−1s−1 for unstrained WS2, WSSe and WSe2 monolayers, respectively. The tungsten dx2-y2 orbitals are mainly contributing to the conduction and valence band electronic states in compressive strained monolayers. In contrast under tensile strain tungsten dz2 orbitals contribute to conduction and valence band electronic states, causing the direct to indirect band gap transition in these monolayers. We observed that the impact of tensile strain is more sensitive as compared to that of compressive strain.

Above 600 mV Open-Circuit Voltage BiI3 Solar Cells

Phase-pure BiI3 films obtained by versatile gas-phase iodination of Bi2S3 are investigated as an absorber in photovoltaic devices. This preparation method leads to highly crystalline BiI3 films featuring a rhombohedral phase and a high-degree of stacking order. The films are composed of micrometer-sized flat grains distributed homogeneously across the F-doped SnO2 (FTO) substrate, exhibiting an indirect band gap transition at 1.72 eV. High-level calculations based on G0W0 approximation are used to rationalize the electronic structure of BiI3, confirming the band gap value estimated experimentally. The films show p-type conductivity with an acceptor density on the order of 1015 cm–3. Solar cells with the architecture glass/FTO/TiO2/BiI3/F8/Au, where F8 is poly(9,9-di-n-octylfluorenyl-2,7-diyl), display a record open-circuit voltage above 600 mV and overall power conversion efficiency of 1.2% under AM 1.5G illumination. The large open-circuit potential is rationalized in terms of carrier lifetimes longer th...

New two-dimensional allotrope of single layer IV-V semiconductor XBi (X = Si, Ge, Sn)

Using first-principles calculations based on density functional theory, we propose a new two dimensional (2D) tetragonal allotrope of single layer IV-V semiconductor XBi (X = Si, Ge, Sn), which consists of repeated square and octagon rings. The calculated cohesive energy, phonon dispersion and ab-initio molecular dynamics simulations indicate that 2D tetragonal XBi (X = Si, Ge, Sn) are stable at room temperature and have potential to be synthesized in the future. Furthermore, due to the strong spin–orbit coupling (SOC) of Bi atom, the inclusion of SOC alters character of the fundamental band gaps of 2D tetragonal XBi (X = Si, Ge, Sn) and decreases their band gaps. When SOC is include, the electronic band structures show that 2D tetragonal SiBi is a direct band gap semiconductor, while 2D tetragonal GeBi and SnBi are indirect band gap semiconductors. Interestingly, the strain can induce 2D tetragonal XBi (X = Si, Ge, Sn) to undergo a direct to indirect band gap transition or an indirect band gap semiconductor to metal transformation.

Theoretical prediction of sandwiched two-dimensional phosphide binary compound sheets with tunable bandgaps and anisotropic physical properties

Atomic layers of GaP and InP binary compounds with unique anisotropic structural, electronic and mechanical properties have been predicted from first-principle molecular dynamics simulations. These new members of the phosphide binary compound family stabilize to a sandwiched two-dimensional (2D) crystalline structure with orthorhombic lattice symmetry and high buckling of 2.14 Å–2.46 Å. Their vibration modes are similar to those of phosphorene with six Raman active modes ranging from ∼80 cm−1 to 400 cm−1. The speeds of sound in their phonon dispersions reflect anisotropy in their elastic constants, which was further confirmed by their strong directional dependence of Young’s moduli and effective nonlinear elastic moduli. They show wide bandgap semiconductor behavior with fundamental bandgaps of 2.89 eV for GaP and 2.59 eV for InP, respectively, even wider than their bulk counterparts. Such bandgaps were found to be tunable under strain. In particular, a direct–indirect bandgap transition was found under certain strains along zigzag or biaxial orientations, reflecting their promising applications in strain-induced bandgap engineering in nanoelectronics and photovoltaics. Feasible pathways to realize these novel 2D phosphide compounds are also proposed.