Conduction Band Minimum States Research Articles

Surface-Originated Weak Confinement in Tetrahedral Indium Arsenide Quantum Dots.

While the shape-dependent quantum confinement (QC) effect in anisotropic semiconductor nanocrystals has been extensively studied, the QC in facet-specified polyhedral quantum dots (QDs) remains underexplored. Recently, tetrahedral nanocrystals have gained prominence in III-V nanocrystal synthesis. In our study, we successfully synthesized well-faceted tetrahedral InAs QDs with a first excitonic absorption extending up to 1700 nm. We observed an unconventional sizing curve, indicating weaker confinement than for equivalently volumed spherical QDs. The (111) surface states of InAs QDs persist at the conduction band minimum state even after ligand passivation with a significantly reduced band gap, which places tetrahedral QDs at lower energies in the sizing curve. Consequently, films composed of tetrahedral QDs demonstrate an extended photoresponse into the short-wave infrared region, compared to isovolume spherical QD films.

Origin of the Nonmonotonic Pressure Dependence of the Band Gap in the Orthorhombic Perovskite CsPbBr3.

The perovskite CsPbBr3 exhibits an unusual nonmonotonic dependence of the band gap on increasing pressure to about 2.0 GPa as compared to conventional semiconductors. Using the first-principles calculation method, we show that under pressure, isotropic volume deformation induces considerable compression of the Pb-Br bond length and thus an enhanced interaction between atomic orbitals of the antibonding valence band maximum states and the mostly nonbonding conduction band minimum states, resulting in a monotonic decrease in the band gap. On the other hand, structural relaxation tends to reduce the strain energy by decompressing the Pb-Br bond length and simultaneously compressing the Pb-Br-Pb bond angle, which increases the band gap energy. We find that the competition between the volume deformation effect and structural relaxation effect is the origin of the nonmonotonic behavior of the dependence of the band gap on pressure.

Giant band gap bowing in two dimensional Pb[formula omitted]Sn[formula omitted]O alloys and Janus PbSnO monolayers

Two-dimensional monolayer alloys can be experimentally synthesized and the monolayer form of α-PbO and ultrathin SnO were recently synthesized. The band gap values of the materials can be tuned by varying the host atom composition by foreign atoms. Thus, desired band gap value can be obtained for various applications. In this study, we investigated electronic, mechanical and vibrational properties of energetically the most stable Pb1−xSnxO monolayer alloys and dynamically stable Janus PbSnO (J-PbSnO) monolayer using first-principles density functional theory calculations. We found that seven Pb1−xSnxO monolayer alloys can be synthesized by the exothermic reactions. However, to synthesize J-PbSnO monolayer, experimentally suitable environment conditions requires such as which is done to synthesize J-MoSSe monolayer. We found both with PBE and HSE06 functionals that PbO and Pb0.75Sn0.25 compounds have direct band gap values while the remaining compounds have indirect band gap values. In addition, the band gap values of Pb1−xSnxO monolayer alloys do not scale linearly with the changing of the composition and exhibit giant band gap bowing. This non-linearity is attributed to electronegativity and atomic radius differences between Pb and Sn atoms. In addition, we observed that in Pb1−xSnxO monolayer alloys localization of valence band maximum states are mostly distributed among Sn pz orbitals whereas the conduction band minimum states localized around Pb px and py orbitals by the increasing of Sn concentration. We believe that our theoretical predictions will guide the experimental realization of these Pb1−xSnxO monolayer alloys for various applications.

Enhanced Electron-Hole Separation in Phosphorus-Coordinated Co Atom on g-C3N4 toward Photocatalytic Overall Water Splitting.

Revealing the decoration mode of g-C3N4 and understanding the physical mechanism of overall water splitting is important for the further improvement of the photocatalytic activity of g-C3N4-based materials. With core level shift and molecular dynamics simulations based on first-principles calculations, Co1(PHx)3 anchored on the triazine of g-C3N4 is determined as a stable single-atom catalyst with high efficiency for photocatalytic overall water splitting. The separated spin-polarized charge density distribution of valence-band maximum and conduction-band minimum states is beneficial for the long lifetime of photoexcited electrons and holes. An anchored Co single atom site is the active site for oxygen evolution reaction, and nitrogen atoms act as active sites for hydrogen evolution reaction. This new decoration mode of g-C3N4 opens a possible way to functionalize g-C3N4 on both triazine and void sites to realize the separation of OER and hydrogenation reaction by water splitting.

Polar methylammonium organic cations detune state coupling and extend hot-carrier lifetime in lead halide perovskites

<h2>Summary</h2> The slow hot-carrier relaxation properties of lead halide perovskites show great promise for hot-carrier solar cells. Here, we discover that the hot-carrier relaxation could be prolonged by two orders of magnitude in lead halide perovskites. By ultrafast spectroscopy, we unravel a slow hot-carrier relaxation process mediated by an intermediate state above the conduction band minimum (CBM). This intermediate state is formed by incorporating polar methylammonium (MA) cations. The first-principle calculation further confirms that the incorporation of MA cations introduces a prolonged relaxation process from CBM + 1 to CBM state, which are the split-off states of the CBM due to the strong spin-orbit coupling. The prolonging effect is interpreted that MA cations largely increase the lattice distortions and detune their inter-state coupling. This work reveals a neglected way to prolong the hot-carrier relaxation processes and provides guidance for the hot-carrier photovoltaics to overcome the current Shockley-Queisser limit.

A comparison study of the structural, electronic and electronic transport properties of nanoribbons based on Penta-graphene, Penta-P2C and Penta-SiC2

A comparison study of the structural, electronic and electronic transport properties of the three proposed sawtooth edge nanoribbon materials based on Penta-graphene, p-P2C and p-SiC2, i.e. PG-SS, p-P2C-SS and p-SiC2-SS is done. While these systems are the same in sawtooth-edges with dangling bond atoms neutralized by H ones, in atomic lines number and their precursor models pose P-421m symmetry, their geometry structures differ mainly at boundaries and their buckles are increased compared with the corresponding precursor structures because of the quantum confinement effect. Both the electronic and electronic transport properties of these systems depend strongly on the sp2-hybridized atoms. In particular, we find out their special electronic characteristic is that conduction band minimum states are mainly contributed by sp2-hybridized atoms not located at edges. Moreover, in the case of electronic transport properties, we strongly believe that the more valence electrons are concentrated at atoms, the lower the transmission of these systems decreases, especially for sp2-hybridized atoms.

Interface controlled band alignment type in Janus SnS2/SSnSe and SnS2/SeSnS van der Waals heterojunctions

Two-dimensional (2D) Janus structures have been employed significantly considering their asymmetrical layout and multiple applications in different areas. A novel van der Waals (vdW) heterojunction is constructed based on the SnS2 monolayer and Janus SnSSe monolayer in the current study. Due to different atoms in two sides of the SnSSe monolayer, two kinds of heterojunctions, including SnS2/SSnSe and SnS2/SeSnS, can be constructed. Both heterojunctions are indirect band gap semiconductors. The kind of interface atom can affect the electronic properties of heterojunctions, resulting in interface-controlled band alignment (BA) type. Since the conduction band minimum states of the SnS2/SSnSe system contain the contribution of both SSnSe and SnS2 layers, this system is not suitable for efficient electron-hole separation. The SnS2/SeSnS heterojunction constructs a standard type-II BA, in which the holes and electrons are placed within the layers of SeSnS and SnS2, respectively. The external electric field, in-plane strain, and the adsorption of nonmetal atoms are also considered to modulate the electronic properties and BA type of the heterojunctions. According to these findings, the interface atom is an important factor influencing the electronic properties of those Janus vdW heterojunctions.

Electronic states and modulation doping of hexagonal boron nitride trilayers

We study the stabilities and geometric and electronic properties of hexagonal boron nitride $(h\text{\ensuremath{-}}\mathrm{BN})$ trilayers by using first-principles electronic-structure calculations within the framework of the density functional theory. From the results of total-energy calculations, we reveal the relative stabilities for various stacking sequences of $h$-BN trilayers. We also show that energy-band structures as well as spatial distributions of wave functions at the valence-band maximum (VBM) and the conduction-band minimum (CBM) strongly depend on the stacking sequences of the $h$-BN trilayers. We further investigate the effects of substitutional doping of a carbon atom on the electronic properties of the $h$-BN trilayers. In several stacking sequences of the C-doped $h$-BN trilayers, we find that the C-atom dopant can be spatially separated from the carrier transport layers associated with the VBM or the CBM, suggesting the possibility of realizing conduction channels only weakly disturbed by the C-atom impurity in $h$-BN trilayers. Interestingly, these donor states spatially separated from the CBM state are found to become rather shallow. This theoretical finding of ``atomically thin modulation doping'' using the $h$-BN layers may open an important way to design future layered electronic device materials.

A Linear Relationship between the Charge Transfer Amount and Level Alignment in Molecule/Two-Dimensional Adsorption Systems.

We systematically study the adsorption of tetrathiafulvalene (TTF), tetracyanoquinodimethane (TCNQ), and tetracyanoethylene (TCNE) on a variety of two-dimensional (2D) monolayers with weak van der Waals (vdW) interactions based on density functional theory. We confirm that TTF can act as an effective donor when its highest occupied molecular orbital (HOMO) level is higher than the conduction band minimum (CBM) state of 2D materials, while TCNQ and TCNE can act as effective acceptors when their lowest unoccupied molecular orbital (LUMO) levels are lower than the valence band maximum (VBM) state of 2D materials. Moreover, our calculations reveal a linear relationship between the charge transfer amount and level alignment between the molecule and 2D monolayer. In other words, the charge transfer is linearly dependent on the energy difference between the HOMO level and 2D CBM state for the donor molecule or the energy difference between the LUMO level and 2D VBM state for the acceptor molecule. The linear relationship indicates that the charge transfer is insensitive to the local binding environments due to the weak vdW interaction. However, the linear relationship cannot be applied to atoms or molecules that are chemisorbed on 2D materials.

High-Performance Photovoltaic Materials Based on the Superlattice Structures of Organic-Inorganic Halide Perovskite and Superhalogen Hybrid Perovskite.

The use of superlattice structures is an attractive strategy for expanding the family of perovskites and obtaining excellent optoelectronic materials. Mixing of cations and partial replacement of halogens by superhalogens are advantageous for improving the stability and optoelectronic properties of hybrid perovskites. Herein, the superlattice structures of the (CsPbI3)n/MAPbI2BF4, (FAPbI3)n/MAPbI2BF4, (MAPbI3)n/CsPbI2BF4, and (FAPbI3)n/CsPbI2BF4 hybrid perovskites were investigated using first-principles density functional theory calculations. The results show that these superlattice structures have tunable direct band gaps and small effective electron and hole masses. Additionally, the charge densities for the valence band maximum and conduction band minimum states are located in different regions of the superlattices. Suggesting that these structures are type-II superlattices that show greatly reduced electron-hole recombination rates. Excellent optical absorption properties for all of perovskite superlattices and the calculated power conversion efficiency of 22.77% for the single-junction solar cells based on the (FAPbI3)3/MAPbI2BF4 and (FAPbI3)3/CsPbI2BF4 perovskites were obtained.

L-Type Ligand-Assisted Acid-Free Synthesis of CsPbBr3 Nanocrystals with Near-Unity Photoluminescence Quantum Yield and High Stability.

X-type ligands, for example, the pair of oleylamine (OAm) and oleic acid (OA), have been widely used to prepare CsPbX3 nanocrystals (NCs). However, the proton exchange between coordinated OAm and OA may induce the detachment of ligands, resulting in poor performance after cleaning or long-time storage. Herein, density functional theory calculations predict that primary amines (L-type ligands) can stabilize a PbBr x-rich surface and yield a trap-free material with fully delocalized valence band maximum and conduction band minimum states, which can significantly improve the photophysical properties and stability of CsPbBr3 NCs. Along this prediction, a room-temperature reprecipitation method using L-type ligands (OAm, n-octylamine, or undecylamine) as the sole capping ligand has been developed to synthesize high-quality CsPbBr3 NCs with near-unity photoluminescence quantum yield and dramatically improved stability against purification and water treatment. The enhancement can be attributed to the strong binding of unprotonated amines to lead atoms and the effective surface passivation provided by the resulted PbBr x-rich surface, which are highly consistent with the theoretical predictions. This work not only offers an approach to synthesize high-quality perovskite NCs but also provides an in-depth understanding of the surface modification of CsPbX3 NCs for practical applications.

The localized effect of the Bi level on the valence band in the dilute bismuth GaBixAs1-x alloy

The research on the temperature dependence of the band gap energy of the dilute bismuth GaBixAs1-x alloy has been done. It is found that its temperature insensitiveness is due to the enhanced localized character of the valence band state and the small decrease of the temperature coefficient for the conduction band minimum (CBM). The enhanced localized character of the valence band state is the main factor. In order to describe the localized effect of the Bi levels on the valence band, the localized energy is introduced into the Varshni's equation. It is found that the effect of the localized Bi level on the valence band becomes strong with increasing Bi content. In addition, it is found that the pressure dependence of the band gap energy of GaBixAs1-x does not seem to be influenced by the localized Bi levels. It is due to two factors. One is that the pressure dependence of the band gap energy is mainly determined by the Г CBM of GaBixAs1-x. The Г CBM of GaBixAs1-x is not influenced by the localized Bi levels. The other is that the small variation of the pressure coefficient for the Г valence band maximum (VBM) state of GaBixAs1-x can be cancelled by the variation of the pressure coefficient for the Г CBM state of GaBixAs1-x.

A Novel Intrinsic Interface State Controlled by Atomic Stacking Sequence at Interfaces of SiC/SiO2.

On the basis of ab initio total-energy electronic-structure calculations, we find that electron states localized at the SiC/SiO2 interface emerge in the energy region between 0.3 eV below and 1.2 eV above the bulk conduction-band minimum (CBM) of SiC, being sensitive to the sequence of atomic bilayers in SiC near the interface. These new interface states unrecognized in the past are due to the peculiar characteristics of the CBM states that are distributed along the crystallographic channels. We also find that the electron doping modifies the energetics among the different stacking structures. Implication for performance of electron devices fabricated on different SiC surfaces is discussed.

Dynamic Disorder and Potential Fluctuation in Two-Dimensional Perovskite.

The structural and electronic properties of 2D perovskite (C4H9NH3)2PbBr4 are investigated from first-principles calculations. It is found that despite the existence of carbon chain, the organic molecule C4H9NH3 in 2D perovskite is able to rotate at room temperature, showing a highly dynamic behavior. The dynamic disorder of C4H9NH3 introduces dynamic potential fluctuation, which is sufficient to localize the wave function and to separate the valence band maximum and conduction band minimum states. We further showed that, rather than a pure dipole rotation model, the disorder effect of the molecules can be better described by the motion of the net charge centers of the molecules, which contributes to the potential fluctuation. Hence, polar molecule is not the necessary condition to create potential fluctuation in perovskite, which is further demonstrated by the calculations of 2D inorganic perovskite Cs2PbBr4.

Calculated hybrid and semilocal functionals andGWelectronic structure of the metal trifluoridesMF3(M=Sc, Y, Al)

A thorough investigation of the effect of exchange and correlation on the electronic structure of wide-band-gap insulators ${\mathrm{ScF}}_{3},\phantom{\rule{0.16em}{0ex}}{\mathrm{YF}}_{3}$, and ${\mathrm{AlF}}_{3}$ is carried out using local, semilocal, and hybrid functionals in the density functional theory framework and the $GW$ approximation with four current plasmon-pole models. It is shown that the hybrid functionals, which attribute more weight to electron exchange, lead to a decent agreement with the state of the art $GW$ results, whereas the Tran-Blaha semilocal functional does not improve significantly the local density approximation results of the insulating transition-metal trifluorides, because of the high localization of the conduction band minimum states which are mainly of $d$ character, and underestimate considerably the valence-band width of the studied materials.

Achieving Type I, II, and III Heterojunctions Using Functionalized MXene

In the present work, type I, II, and III heterostructures are constructed with the same base material using three representative functionalized monolayer scandium carbides (Sc2CF2, Sc2C(OH)2, and Sc2CO2) by first-principles calculations based on density functional theory. In contrast to general bilayer heterosystems composed of two-dimensional semiconductors, type I and III heterojunctions are obtained in one Sc2CF2/Sc2CO2 system and the remains of the functionalized Sc2C heterostructures, respectively. It is noteworthy that the same monolayer Sc2CF2 and Sc2CO2 constituents lead to dissimilar heterostructure types in the two Sc2CF2/Sc2CO2 systems by modifying the stacking interface. In addition, in the two Sc2CF2/Sc2CO2 systems, remarkable changes in the heterojunction type are induced by a strain, and two distinct type-II heterostructures are generated where one layer with the conduction band minimum state and the other layer including the valence band maximum level are different. The present work suggests an attractive direction to obtain all heterostructure types with the same base material for novel nanodevices in various fields such as photonics, electronics, and optoelectronics using only the two monolayer components Sc2CF2 and Sc2CO2.

Spatially separated charge densities of electrons and holes in organic-inorganic halide perovskites

Solution-processable methylammonium lead trihalide perovskites exhibit remarkable high-absorption and low-loss properties for solar energy conversion. Calculation from density functional theory indicates the presence of non-equivalent halogen atoms in the unit cell because of the specific orientation of the organic cation. Considering the 〈100〉 orientation as an example, I1, one of the halogen atoms, differs from the other iodine atoms (I2 and I3) in terms of its interaction with the organic cation. The valance-band-maximum (VBM) and conduction-band-minimum (CBM) states are derived mainly from 5p orbital of I1 atom and 6p orbital of Pb atom, respectively. The spatially separated charge densities of the electrons and holes justify the low recombination rate of the pure iodide perovskite. Chlorine substitution further strengthens the unique position of the I1 atom, leading to more localized charge density around the I1 atom and less charge density around the other atoms at the VBM state. The less overlap of charge densities between the VBM and CBM states explains the relatively lower carrier recombination rate of the iodine-chlorine mixed perovskite. Chlorine substitution significantly reduces the effective mass at a direction perpendicular to the Pb-Cl bond and organic axis, enhancing the carrier transport property of the mixed perovskite in this direction.

Two-dimensional semiconductor alloys: Monolayer Mo1−xWxSe2

Monolayer Mo1−xWxSe2 (x = 0, 0.14, 0.75, and 1) alloys were experimentally realized from synthesized crystals. Mo1−xWxSe2 monolayers are direct bandgap semiconductors displaying high luminescence and are stable in ambient. The bandgap values can be tuned by varying the W composition. Interestingly, the bandgap values do not scale linearly with composition. Such non-linearity is attributed to localization of conduction band minimum states around Mo d orbitals, whereas the valence band maximum states are uniformly distributed among W and Mo d orbitals. Results introduce monolayer Mo1−xWxSe2 alloys with different gap values, and open a venue for broadening the materials library and applications of two-dimensional semiconductors.

Electronic structures and optical properties of GaN nanotubes with MgGa–ON co-doping

Both the electronic structures and the optical properties of single-walled zigzag GaN nanotubes (NTs) with MgGa–ON co-doping are investigated using first-principles calculations. We find that the MgGa–ON defect complex can exist stably in GaN NTs. The direct band gap width of the GaN NTs can be reduced by means of the MgGa–ON co-doping. The electrons of the valence band maximum (VBM) state are localized around the N atoms bonded with the Mg atom. The imaginary part ε2 of the complex dielectric function of GaN NTs with MgGa–ON co-doping has a sharp peak closely related to the optical transitions between the VBM and conduction band minimum states.

Quasiparticle band structures ofβ-HgS, HgSe, and HgTe

The electronic structures of mercury chalcogenides in the zinc-blende structure have been calculated within the LDA, $GW$ ($G$${}_{0}$$W$${}_{0}$, ``one-shot'') and quasi-particle self-consistent $GW$ ($\text{QS}GW$) approximations, including spin-orbit (SO) coupling. The slight tendency to overestimation of band gaps by $\text{QS}GW$ is avoided by using a hybridscheme (20$%$ LDA and 80$%$ $\text{QS}GW$). The details of the $GW$ bands near the top of the valence bands differ significantly from the predictions obtained by calculations within the LDA. The results obtained by ${G}_{0}{W}_{0}$ depend strongly on the starting wave functions and are thus quite different from those obtained from QS$GW$. Within $\text{QS}GW$, HgS is found to be a semiconductor, with a ${\ensuremath{\Gamma}}_{6}$ $s$-like conduction-band minimum state above the valence top ${\ensuremath{\Gamma}}_{7}$ and ${\ensuremath{\Gamma}}_{8}$ (``negative'' SO splitting). HgSe and HgTe have negative gaps (inverted band structures), but for HgTe the ${\ensuremath{\Gamma}}_{7}$ state is below ${\ensuremath{\Gamma}}_{6}$ due to the large Te SO splitting, in contrast to HgSe where ${\ensuremath{\Gamma}}_{6}$ is below ${\ensuremath{\Gamma}}_{7}$. There appears to be significant differences, in particular for HgSe and HgS, between the ordering of the band-edge states as obtained from experiments and theory.