Indirect-direct Band Gap Transition Research Articles

Electric field and strain effects on the electronic and optical properties of g-C3N4/WSe2 van der Waals heterostructure

Efficient carrier separation and regulable visible light absorption are important characteristics for eligible nanoscale photoelectric devices. In this work, tunable electronic structure and optical properties of novel g-C3N4/WSe2 van der Waals heterostructure under various electric fields and biaxial strains are systematically investigated by first principle calculations. The results show that the g-C3N4/WSe2 heterostructure is a direct band gap semiconductor (1.395 eV) with intrinsic type-I band alignment and exhibits good UV and visible light absorption compared to the isolated g-C3N4 and WSe2 monolayers. Applied external electric-field can effectively adjust the interlayer coupling and charge transfer, which can further alter the band structure and achieve the transition of type I to type II band alignment for the g-C3N4/WSe2 heterostructure. The biaxial strain causes charge redistribution in the interfacial regions and triggers an indirect-direct band gap transition in the g-C3N4/WSe2 heterostructure, which is strongly associated with the modification of band structure contributed by W dz2 and dxy orbitals near the Fermi level. Besides, the systematical red shift and blue shift of absorption peaks is also observed under the tensile and compressive strains, respectively. The tunable electronic structure and optical properties make g-C3N4/WSe2 vdW heterostructure potential candidates for application in the photoelectronic device.

Direct–Indirect Transition of Pressurized Two-Dimensional Halide Perovskite: Role of Benzene Ring Stack Ordering

Two-dimensional (2D) hybrid organic-inorganic metal halide perovskites (HOIPs) with considerably hydrophobic phenyl ethylammonium (PEA) organic cations have been used in highly efficient solar cells and LEDs, which are stable and enjoy a long lifetime, even when exposed to moisture. Different from other 2D HOIPs with alkyl amine cations, a benzene ring is present in the PEA cation. Until recently, an understanding of the effects of PEA on the structural, electronic, and optical properties of 2D HOIPs under pressure has remained limited. We find that there is a direct-indirect band gap transition at around 5.8 GPa and that the direct band gap recovers when the pressure is released. The stacking order of the benzene rings in the PEA cation plays a critical role in the mechanical and electronic properties. Our present work demonstrates that 2D HOIPs with organic cations containing benzene rings prove highly attractive for use in flexible optoelectronics.

Strain-engineered indirect–direct band-gap transitions of PbPdO2 slab with preferred (0 0 2) orientation

Layered transition metal oxide PbPdO2 has great potential application in electronic devices because of its unique electronic structure and large thermoelectric power at room temperature. In this work, strain effect on the electronic structure of PbPdO2 slab with preferred (0 0 2) orientation was systematically investigated using first-principles calculation. The calculated results indicate that PbPdO2 ultrathin slab possesses a small indirect gap while an indirect–direct band gap transition occurs when a moderate 2% compression or tensile strain is applied on the slab. Moreover, this strain induced indirect–direct band gap transition was analyzed in detail using the charge density difference at different point of valence band. The charge transfer and energy barrier with charge polarization resulting from the changes of bond length and angle for Pd–O bonding under the strain, have been accounted for this transition. Remarkablely, for the (0 0 2) preferred orientation PbPdO2 slab, the predicted carrier mobilities of electrons and holes are 11 645.31 and 694.60 cm2 V−1 s−1 along the x-axis direction, 935.05 and 16.05 cm2 V−1 s−1 along the y -axis direction, respectively. These calculated mobilities of electrons along the x-axis direction are larger than those for 2D MoS2 (~400 cm2 V−1 s−1), and being comparable to those for InSe (103 cm2 V−1 s−1) and black phosphorene (103–104 cm2 V−1 s−1). It is strong suggested that the (0 0 2) orientated PbPdO2 slab with high mobility should be an ideal candidate material for the application of electronics devices.

Copper(I)-Based Highly Emissive All-Inorganic Rare-Earth Halide Clusters

Summary The development of new environmentally friendly luminescent materials is crucial for future solid-state lighting, sensor, and display applications. Here, a Cu(I)-based all-inorganic rare-earth halide material, Rb 8 CuSc 3 Cl 18 , has been synthesized by a solid-state reaction method. In this compound, two Cu(I) ions are connected to three rare-earth halide octahedra to form a paddle-wheel-like cluster. The Cu(I) coordinated rare-earth halide clusters contribute to a strong blue photoluminescence emission. This Cu(I)-regulated emission can be extended to other isostructural compounds, such as Rb 8 CuY 3 Cl 18 . Moreover, the crucial role of Cu(I) has been illustrated by the isostructural non-emissive Rb 8 AgSc 3 Cl 18 . On the basis of comprehensive spectroscopy studies and density functional theory calculations, we found that Cu(I) photo-oxidation and correct orbital-energy-level alignment are crucial for the observed bright-blue emission through a proposed metal (Cu)-to-octahedra ([ScCl 6 ] 3− ) charge-transfer mechanism. The discovery of Cu(I)-based all-inorganic rare-earth halide clusters establishes a new strategy for constructing promising emissive halide materials.

RETRACTED: Engineering the optical and electronic properties of Janus monolayer Ga2SSe by biaxial strain

Abstract Here we present a comprehensive study on the electronic and optical properties of Ga2SSe monolayer under a biaxial strain using the density functional theory (DFT). We identified that Ga2SSe monolayer is an indirect semiconductor with an energy band gap of 2.04 eV at the equilibrium state. The energy band gap tends to decrease slowly when the large biaxial strains applied. It is of interest that the compression strain leads to an indirect–direct band gap transition at the strain of 2%. In addition, we demonstrate that the absorption begins in the visible region. While the absorption edge remains visible region under the effect of compression strain, the tensile strain leads to shifting the absorption edge to the near infrared region. Overall, the maximum peaks are positioned in the ultraviolet region. Hence, it can be said that Ga2SSe can be used in the ultraviolet detectors, or in the photoelectric devices. We have confidence in that the control of the optical and electronic properties of Ga2SSe monolayer by biaxial strain engineering can be beneficial in its application in optoelectronic and nanoelectronic devices.

Optical and electronic properties of dichalcogenides WX2 (X=S, Se, and Te) monolayers under biaxial strain

The effects of biaxial strain on the electronic structures and optical properties of the two-dimensional WX2 (X = S, Se, and Te) monolayers are investigated by first-principle calculations. The WX2 monolayers are direct band gap semiconductors. Spin-orbit coupling has a significant effect on the electronic structures for WX2 monolayers. The band gap, direct-indirect band gap transition, semiconductor to metal transition, dielectric constant, as well as absorption edge can be appropriately adjusted by the biaxial strain means. The superior ability of the WX2 monolayers to modulate the electronic and optical properties provides an opportunity for the application of optoelectronic devices.

Exciton Radiative Recombination Dynamics and Nonradiative Energy Transfer in Two-Dimensional Transition-Metal Dichalcogenides

We employ fluorescence lifetime imaging technology to explore exciton radiative recombination dynamics in layered transition-metal dichalcogenides (TMDCs) and nonradiative energy transfer from CdSe/ZnS quantum dot (QD) to monolayer TMDCs (MoS2, WS2, and WSe2). Owing to an indirect–direct band gap transition, exciton radiative lifetimes decrease with the TMDCs’ layer number reducing. The fastest exciton recombination rate is observed in monolayer TMDCs, which is attributed to their reduced dielectric screening. Furthermore, the effect of reduced dielectric screening on nonradiative energy transfer from QDs to monolayer TMDCs is investigated. The fastest energy transfer rate is observed in QD/WS2 heterostructure owing to weak dielectric screening of monolayer WS2, and the slowest rate in QDs/MoS2 is caused by strong dielectric screening of monolayer MoS2. Our experiments provide fundamental insights into exciton recombination dynamics in TMDCs and potentially enable new avenues for controlling motion of exc...

Tunable electronic and optical properties of arsenene/MoTe2 van der Waals heterostructures

The effects of biaxial strain and interlayer separation on the electronic structures and optical properties of arsenene/MoTe2 van der Waals heterostructures are investigated based on first principles density functional theory calculations. The six possible high-symmetry stacking methods are considered for arsenene/MoTe2 heterostructures. The type I band alignment with a 1.504 eV direct band gap is obtained in the most stable arsenene/MoTe2 heterostructure. Both biaxial strain and interlayer separation induce direct-indirect band gap transition and semiconductor–metal transition. The arsenene/MoTe2 heterostructures have appreciable adsorption of visible light and ultraviolet light. We also consider the effects of different biaxial strains and interlayer distances on the optical properties of arsenene/MoTe2 heterostructure, and find the optical properties can be effectively modulated by biaxial strains.

Tunable electronic properties of an Sb/InSe van der Waals heterostructure by electric field effects.

The electronic properties of an Sb/InSe heterostructure are investigated by using the density functional theory method. A type-II staggered-gap band alignment is achieved from the Sb/InSe vdW heterostructure with the Sb layer dominates the lowest energy holes as well as the lowest energy electrons are contributed by the InSe layer, which facilitates the spatial effective separation of photogenerated electron-hole pairs. Additionally, an indirect-direct band gap transition can be triggered via varying the interlayer distance. More fascinatingly, the characteristic of type-II band alignment is robust, while the band gap values are tunable with respect to a moderate external electric field, even leading to an intriguing semiconductor-metal transition at a strong electric field. These results are expected to provide meaningful guidelines for the design of novel nanoelectronic and optoelectronic devices based on the Sb/InSe heterostructure.

Tuning electronic properties of InSe/arsenene heterostructure by external electric field and uniaxial strain

First-principles based on density functional theory (DFT) are used to calculate the structural and electronic properties of InSe/arsenene heterostructure. The results show that the InSe/arsenene bilayer forms a type-II van der Waals heterostructure with a direct band gap of 0.876 eV, which is favorable for the separation of photogenerated electron-hole pairs. The properties of InSe/arsenene heterojunction can be effectively regulated by applying an external electric field and uniaxial strain. When an external electric field is applied, the heterostructure experiences a transition from semiconductor to metal. In addition, due to the presence of built-in electric field, the positive and negative electric fields have different effects on the electronic properties of heterostructure. When uniaxial strain is applied, the heterostructure can withstand larger tensile strain than compression strain without damaging the structure and the band gap is more easily decreased by X-direction strain. In addition, the heterostructure undergoes a direct-indirect bandgap transition under strain. When compressive strain is applied in the Y direction, especially ε < −2%, the heterostructure possesses a higher carrier mobility while maintaining direct bandgap characteristics. Our results show that the new InSe/arsenene heterostructure has great potential for applications in electronic and optoelectronic devices.

Strain impacts on commensurate bilayer graphene superlattices: Distorted trigonal warping, emergence of bandgap and direct-indirect bandgap transition

Due to low dimensionality, the controlled stacking of graphene films and their electronic properties are susceptible to environmental changes including strain. The strain-induced modification of the electronic properties such as the emergence and modulation of bandgaps crucially depends on the stacking of the graphene films. However, to date, only the impact of strain on electronic properties of Bernal and AA-stacked bilayer graphene has been extensively investigated in theoretical studies. Exploiting density functional theory and tight-binding calculation, we investigate the impacts of in-plane strain on two different classes of commensurate twisted bilayer graphene (TBG) which are even/odd under sublattice exchange (SE) parity. We find that the SE odd TBG remains gapless whereas the bandgap increases for the SE even TBG when applying equibiaxial tensile strain. Moreover, we observe that for extremely large mixed strains both investigated TBG superstructures demonstrate direct-indirect bandgap transition.

Electronic properties of WS2 and WSe2 monolayers with biaxial strain: A first-principles study

In the present work, we consider electronic properties of WX2 (X = S, Se) monolayers under a biaxial strain εb using the first principles study. Our calculations indicate that, at equilibrium, the WS2 and WSe2 monolayers are semiconductors with a direct band gap of respectively 1.800 eV and 1.566 eV while their bulk structures are indirect semiconductors. The electronic properties of the WX2 monolayers are very sensitive with the biaxial strain, especially compression strain. The biaxial strain εb is the cause of the band gap of the WX2 monolayers and especially the semiconductor-metal phase transition has occurred in the WS2 monolayer at εb=-10%. In addition, the direct-indirect band gap transition was observed in both WS2 and WSe2 monolayers at a certain elongation of biaxial strain εb. The phase transitions in these monolayers can be very useful for their applications in nanoelectromechanical devices.

A study of strain-induced indirect-direct bandgap transition for silicon nanowire applications

Recently, ultralarge (&amp;gt;10%) strain with fully reversible elastic deformation has been experimentally achieved in silicon nanowires [H. Zhang et al., Sci. Adv. 2, e1501382 (2016)]. With this breakthrough, here in this work, based on the first principles calculation, the structural and electric properties of silicon under ultralarge strain are comparatively investigated. Unlike previous theoretical/simulation investigations on silicon nanowires with only a few nanometers, bulk silicon models are employed here to provide more realistic and comparable results to our experimentally tested samples (∼100 nm diameter). Strong anisotropic effects are induced by loading strain along all different orientations. Simultaneously, the band structures evolution demonstrates electronic anisotropy with the loading strain on three orientations. Silicon keeps an indirect bandgap under increased strain along the ⟨100⟩ orientation while transforming to a direct bandgap with strain along ⟨110⟩ and ⟨111⟩ orientations. Furthermore, ultralarge strain on these two orientations would diminish the bandgap and result into metallization. These results provide insights into understanding “elastic strain engineering” of silicon nanowire applications and demonstrate the possibility of tuning the electronic and optical properties through pure mechanical straining of functional materials.

Electronic, Thermal Expanding, and Optical Absorption Properties of Transition Metal Dichalcogenides: A First-principles Study

A comprehensive investigation was made on the electronic structure, thermal expansion coefficient and light absorption spectrum of total six transition metal dichalcogenides (TMDs) compounds with formula of MX2 (M=Mo, W, Cr, X=S, Se). First, an indirect-direct band gap transition from bulk to singlelayer was declared for all the six compounds. Moreover, the detailed lattice constants and thermal expansion coefficients provided in the paper were the key information for designing MX2-based field effect transistors. Finally, the calculated optical absorption spectra demonstrate that these compounds can effectively utilize solar energy and are good photo catalyst candidates. All these present findings will benefit the design of new generation of novel two-dimensional materials.

Zero-point motion and direct-indirect band-gap crossover in layered transition-metal dichalcogenides

Two-dimensional transition-metal dichalcogendes $MX_2$ (es. MoS$_2$, WS$_2$, MoSe$_2$, \ldots) are among the most promising materials for bandgap engineering. Widely studied in these compounds, by means of ab-initio techniques, is the possibility of tuning the direct-indirect gap character by means of in-plane strain. In such kind of calculations however the lattice degrees of freedom are assumed to be classical and frozen. In this paper we investigate in details the dependence of the bandgap character (direct vs. indirect) on the out-of-plane distance $h$ between the two chalcogen planes in each $MX_2$ unit. Using DFT calculations, we show that the bandgap character is indeed highly sensitive on the parameter $h$, in monolayer as well as in bilayer and bulk compounds, permitting for instance the switching from indirect to direct gap and from indirect to direct gap in monolayer systems. This scenario is furthermore analyzed in the presence of quantum lattice fluctuation induced by the zero-point motion. On the basis of a quantum analysis, we argue that the direct-indirect bandgap transitions induced by the out-of-plane strain as well by the in-plane strain can be regarded more as continuous crossovers rather than as real sharp transitions. The consequences on the physical observables are discussed.

2D Crystals in Three Dimensions: Electronic Decoupling of Single-Layered Platelets in Colloidal Nanoparticles.

2D crystals, single sheets of layered materials, often show distinct properties desired for optoelectronic applications, such as larger and direct band gaps, valley- and spin-orbit effects. Being atomically thin, the low amount of material is a bottleneck in photophysical and photochemical applications. Here, the formation of stacks of 2D crystals intercalated with small surfactant molecules is proposed. It is shown, using first principles calculations, that the very short surfactant methyl amine electronically decouples the layers. The indirect-direct band gap transition characteristic for Group 6 transition metal dichalcogenides is demonstrated experimentally by observing the emergence of a strong photoluminescence signal for ethoxide-intercalated WSe2 and MoSe2 multilayered nanoparticles with lateral size of about 10 nm and beyond. The proposed hybrid materials offer the highest possible density of the 2D crystals with electronic properties typical of monolayers. Variation of the surfactant's chemical potential allows fine-tuning of electronic properties and potentially elimination of trap states caused by defects.

Size and strain effects on mechanical and electronic properties of green phosphorene nanoribbons

Recently, a phosphorus isomer named green phosphorus was theoretically predicted with a similar interlayer interaction compared to that of black phosphorus, thus indicating that individual layers can be mechanically exfoliated to form two-dimensional (2D) layers known as green phosphorene. In this work, we investigated the properties of green phosphorene nanoribbons along both armchair and zigzag directions with ribbon widths up to 57 Å using density functional theory. Effects of ribbon width and strain on the mechanical and electronic properties of the ribbons were studied. The Young’s modulus, effect of quantum confinement on the band gap, and effect of strain on the band structures of the ribbons were investigated. The green phosphorene ribbons were found to exhibit prominent anisotropic properties, with the Young’s modulus in the range of 10-35 GPa for the armchair green phosphorene nanoribbons (AGPNR) and 160-170 GPa for the zigzag green phosphorene nanoribbons (ZGPNR), which are the same order of magnitude as those of the 2D sheets. The work function was found to be between 5 eV ∼ 5.7 eV for the range of widths studied. Both size and strain trigger direct-indirect band gap transitions in the ribbons and their transition mechanisms were discussed.

Intriguing electronic properties of germanene/ indium selenide and antimonene/ indium selenide heterostructures

The structural and electronic properties of the germanene/indium selenide (Ge/InSe), antimonene/indium selenide (Sb/InSe) heterostructures and the effects of in-plane strains on their electronic structures are investigated by performing comprehensive DFT calculations. The results show that the hybridization of the interbedded Se- and Ge- (Sb-) p orbitals can increase the binding strength between the InSe and Ge (Sb) monolayers. It is noteworthy that the band gap of germanene is opened by realizing the Ge/InSe heterostructure. In the case of the Sb/InSe heterostructure, all the stacks have direct bandgaps varying from 126.2 meV to 563.7 meV. In addition, strain has significant influence on the band structure. In the C-stacking Ge/InSe, a suitable −2% compression and 4% stretch can trigger the semiconductor-metal transition. Remarkably, an indirect-direct bandgap and semiconductor-metal transition have been observed under sufficient strain in the elastic range in the C-stacking Ge/InSe. These theoretical results could provide useful guidelines for applications of the Ge/InSe and Sb/InSe heterostructures in many fields of nanoelectronics, such as field-effect devices, optoelectronic applications, and strain sensors.

Modulation of electronic properties of monolayer InSe through strain and external electric field

In this work, we consider systematically the influence of uniaxial strain and external electric field E on electronic properties of a monolayer InSe using ab initio approach based on density functional theory. Our calculations indicate that the monolayer InSe has a medium indirect energy bandgap of 1.38 eV at equilibrium. The calculated results also demonstrate that we can adjust the bandgap of the monolayer InSe by strain engineering or electric field. The bandgap of the monolayer InSe changes dramatically when the uniaxial strain is applied. Especially, under the compressed uniaxial strain, an indirect–direct bandgap transition has been observed at certain elongations. Within the electric field magnitude E range from 0 to 5 V/nm, the calculated results show that the negative electric field changes the bandgap of the monolayer InSe up to 23% while the positive electric field effect on its bandgap is negligible.

Tunable electronic properties and carrier mobility in binary pnictogen nanosheets, PX (X = As, Sb, and Bi)

Electronic structures and electronic transport properties of binary pnictogen nanosheets, PX, (X = As, Sb, and Bi) are studied by the density functional method and are compared with those of the mono-elemental counterpart, blue phosphorene (PP). We find that both PAs and PSb, similar to PP, are indirect band gap semiconductors. However, an interesting indirect-direct band gap transition is predicted in PX when X = Bi, which originates from the upshifted energy level of valence p electrons and the enlarged vertical distance between atomic layers of P and X as the increasing atomic number of X atoms. Moreover, the transport properties in PBi nanosheets are also significantly improved, e.g., the hole mobility along the armchair direction in PBi is almost doubled as compared with that in PP. The direct band gap and enhanced carrier mobility in PBi suggest it holds greater promise for nanoelectronic and optoelectronic devices than the mono-elemental counterpart, PP.