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Showing papers on "Effective mass (solid-state physics) published in 2021"


Journal ArticleDOI
16 Sep 2021-Nature
TL;DR: In this paper, the interaction strength of the MoTe2/WSe2 superlattices was tuned to drive a continuous metal-to-insulator transition at a fixed electron density, which is consistent with the universal critical theory of a continuous Mott transition in two dimensions.
Abstract: The evolution of a Landau Fermi liquid into a non-magnetic Mott insulator with increasing electronic interactions is one of the most puzzling quantum phase transitions in physics1–6. The vicinity of the transition is believed to host exotic states of matter such as quantum spin liquids4–7, exciton condensates8 and unconventional superconductivity1. Semiconductor moire materials realize a highly controllable Hubbard model simulator on a triangular lattice9–22, providing a unique opportunity to drive a metal–insulator transition (MIT) via continuous tuning of the electronic interactions. Here, by electrically tuning the effective interaction strength in MoTe2/WSe2 moire superlattices, we observe a continuous MIT at a fixed filling of one electron per unit cell. The existence of quantum criticality is supported by the scaling collapse of the resistance, a continuously vanishing charge gap as the critical point is approached from the insulating side, and a diverging quasiparticle effective mass from the metallic side. We also observe a smooth evolution of the magnetic susceptibility across the MIT and no evidence of long-range magnetic order down to ~5% of the Curie–Weiss temperature. This signals an abundance of low-energy spinful excitations on the insulating side that is further corroborated by the Pomeranchuk effect observed on the metallic side. Our results are consistent with the universal critical theory of a continuous Mott transition in two dimensions4,23. The interaction strength in moire superlattices is tuned to drive a continuous metal-to-insulator transition at a fixed electron density.

110 citations


Journal ArticleDOI
01 Jul 2021-Nature
Abstract: When the Coulomb repulsion between electrons dominates over their kinetic energy, electrons in two-dimensional systems are predicted to spontaneously break continuous-translation symmetry and form a quantum crystal1. Efforts to observe2–12 this elusive state of matter, termed a Wigner crystal, in two-dimensional extended systems have primarily focused on conductivity measurements on electrons confined to a single Landau level at high magnetic fields. Here we use optical spectroscopy to demonstrate that electrons in a monolayer semiconductor with density lower than 3 × 1011 per centimetre squared form a Wigner crystal. The combination of a high electron effective mass and reduced dielectric screening enables us to observe electronic charge order even in the absence of a moire potential or an external magnetic field. The interactions between a resonantly injected exciton and electrons arranged in a periodic lattice modify the exciton bandstructure so that an umklapp resonance arises in the optical reflection spectrum, heralding the presence of charge order13. Our findings demonstrate that charge-tunable transition metal dichalcogenide monolayers14 enable the investigation of previously uncharted territory for many-body physics where interaction energy dominates over kinetic energy. The signature of a Wigner crystal—the analogue of a solid phase for electrons—is observed via the optical reflection spectrum in a monolayer transition metal dichalcogenide.

107 citations


Journal ArticleDOI
05 Apr 2021
TL;DR: In this paper, the growth of bilayer beta tellurium dioxide (β-TeO2) through surface oxidation of a eutectic mixture of a mixture ofTellurium and selenium was reported.
Abstract: Wide-bandgap oxide semiconductors are essential for the development of high-speed and energy-efficient transparent electronics. However, while many high-mobility n-type oxide semiconductors are known, wide-bandgap p-type oxides have carrier mobilities that are one to two orders of magnitude lower due to strong carrier localization near their valence band edge. Here, we report the growth of bilayer beta tellurium dioxide (β-TeO2), which has recently been proposed theoretically as a high-mobility p-type semiconductor, through the surface oxidation of a eutectic mixture of tellurium and selenium. The isolated β-TeO2 nanosheets are transparent and have a direct bandgap of 3.7 eV. Field-effect transistors based on the nanosheets exhibit p-type switching with an on/off ratio exceeding 106 and a field-effect hole mobility of up to 232 cm2 V−1 s−1 at room temperature. A low effective mass of 0.51 was observed for holes, and the carrier mobility reached 6,000 cm2 V−1 s−1 on cooling to −50 °C. Bilayer beta tellurium dioxide nanosheets with p-type characteristics can be formed through the surface oxidation of a mixture of tellurium and selenium, and used to create transistors with performance that matches their n-type oxide counterparts.

55 citations


Journal ArticleDOI
TL;DR: In this paper, the discovery of superconductivity and detailed normal-state physical properties of RbV3Sb5 single crystals with V kagome lattice were reported.
Abstract: We report the discovery of superconductivity and detailed normal-state physical properties of RbV3Sb5 single crystals with V kagome lattice RbV3Sb5 single crystals show a superconducting transition at Tc ~ 092 K Meanwhile, resistivity, magnetization and heat capacity measurements indicate that it exhibits anomalies of properties at T* ~ 102 - 103 K, possibly related to the formation of charge ordering state When T is lower than T*, the Hall coefficient RH undergoes a drastic change and sign reversal from negative to positive, which can be partially explained by the enhanced mobility of hole-type carriers In addition, the results of quantum oscillations show that there are some very small Fermi surfaces with low effective mass, consistent with the existence of multiple highly dispersive Dirac band near the Fermi energy level

53 citations


Journal ArticleDOI
TL;DR: In this paper, the intrinsic charge carrier mobility of penta-graphene has been investigated within the formalism of density functional theory and using the concept of effective mass approximation.
Abstract: Within the formalism of density functional theory and using the concept of effective mass approximation, the electron-acoustic phonon scattering principle along with the deformation potential theory of Bardeen and Shockley, we have investigated the intrinsic charge carrier mobility of penta-graphene. The electron mobility of penta-graphene is significantly large compared to hole mobility and it is reasonably higher than other novel two-dimensional materials e.g. penta-X2C (X = P, As, Sb), Si-based pentagonal monolayer (SiX, X = B, C, N), penta-stanene monolayer and CaP3 which possess the highest carrier (electron) mobility among phosphorene derivatives. This high carrier mobility originates from the low effective mass, low deformation potential and high stiffness constant of this material. As our main focus is to investigate the carrier mobility which depends on band edges, a detailed study about orbital contribution to the electronic states in the vicinity of band edges has been carried out. The intrinsic wide band gap and ultrahigh carrier mobility show the possible signature of using it in the fabrication of microelectronic devices, mainly in FETs and logic circuits and in optoelectronic devices.

46 citations


Journal ArticleDOI
TL;DR: In this paper, density functional theory calculations are implemented to predict that V is an effective dopant for GeTe to enlarge the bandgap and converge the energy offset, which suppresses the bipolar conduction and increases the effective mass.
Abstract: Owing to the moderate energy offset between light and heavy band edges of the rock‐salt structured GeTe, its figure‐of‐merit (ZT) can be enhanced by the rational manipulation of electronic band structures. In this study, density functional theory calculations are implemented to predict that V is an effective dopant for GeTe to enlarge the bandgap and converge the energy offset, which suppresses the bipolar conduction and increases the effective mass. Experimentally, V‐doped Ge1−xVxTe samples are demonstrated to have an enhanced Seebeck coefficient from ≈163 to ≈191 µV K−1. Extra alloying with Bi in Ge1−x−yVxBiyTe can optimize the carrier concentration to further enhance the Seebeck coefficient up to ≈252 µV K−1, plus an outstanding power factor of ≈43 µW cm−1 K−2. Comprehensive structural characterization results also verify the refinement of grain size by V‐doping, associated with highly dense grain boundaries, stacking faults, nanoprecipitates, and point defects, reinforcing the wide‐frequency phonon scattering and in turn, securing an ultralow thermal conductivity of ≈0.59 W m−1 K−1. As a result, the Ge0.9V0.02Bi0.08Te sample shows a peak ZT of >2.1 at 773 K, with an average plateaued average ZT of >2.0 from 623 and 773 K, which extends better thermoelectric behavior for GeTe over a wider temperature range. This study clarifies the multiple benefits of V‐doping in GeTe‐based derivatives and provides a framework for a new‐type of high‐performance middle‐temperature thermoelectric material.

35 citations


Journal ArticleDOI
01 Nov 2021-Nature
TL;DR: In this paper, a two-dimensional van der Waals heterostructure for artificial heavy-fermion physics was proposed, where heavy fermions emerge from the Kondo coupling between a lattice of localized magnetic moments and itinerant electrons in a 1T/1H-TaS2 heterostructures.
Abstract: Heavy-fermion systems represent one of the paradigmatic strongly correlated states of matter1–5. They have been used as a platform for investigating exotic behaviour ranging from quantum criticality and non-Fermi liquid behaviour to unconventional topological superconductivity4–12. The heavy-fermion phenomenon arises from the exchange interaction between localized magnetic moments and conduction electrons leading to Kondo lattice physics, and represents one of the long-standing open problems in quantum materials3. In a Kondo lattice, the exchange interaction gives rise to a band with heavy effective mass. This intriguing phenomenology has so far been realized only in compounds containing rare-earth elements with 4f or 5f electrons1,4,13,14. Here we realize a designer van der Waals heterostructure where artificial heavy fermions emerge from the Kondo coupling between a lattice of localized magnetic moments and itinerant electrons in a 1T/1H-TaS2 heterostructure. We study the heterostructure using scanning tunnelling microscopy and spectroscopy and show that depending on the stacking order of the monolayers, we can reveal either the localized magnetic moments and the associated Kondo effect, or the conduction electrons with a heavy-fermion hybridization gap. Our experiments realize an ultimately tunable platform for future experiments probing enhanced many-body correlations, dimensional tuning of quantum criticality and unconventional superconductivity in two-dimensional artificial heavy-fermion systems15–17. A study demonstrates the synthesis and characterization of a two-dimensional van der Waals heterostructure hosting artificial heavy fermions, providing a tunable platform for investigations of heavy-fermion physics.

33 citations


Journal ArticleDOI
TL;DR: In this article, the effect of an applied external mechanical load on a periodic structure exhibiting band gaps induced by inertial amplification mechanism is investigated, and the results presented in this paper provide insights in the behaviour of band gap induced by an external prestress and suggest new opportunities for real-time tunable wave manipulation.

31 citations


Journal ArticleDOI
TL;DR: In this article, a review of thermoelectric argyrodites is presented, focusing on the synthesis of both single-crystalline and poly-crystaline argyrogrodites as well as the chemical compositions, phases and crystallographic features.

30 citations


Journal ArticleDOI
TL;DR: In this paper, a comparative study for the structural, electronic, and optical properties and photocatalysis of triclinic BiTaOO4, orthorhombic biTaO4 (BiTaO2O11, Bi7TaO3O18, Bi6s-O 2p) and Bi 7TaO 3O18 was performed.

30 citations


Journal ArticleDOI
22 Jul 2021
TL;DR: In this article, the authors established a direct carrier-concentration-dependent restructured single parabolic band (SPB) model, which eliminates Fermi-Dirac integrals and fermi level calculation and emerges stronger visibility and usability in experiments.
Abstract: The single parabolic band (SPB) model has been widely used to preliminarily elucidate inherent transport behaviors of thermoelectric (TE) materials, such as their band structure and electronic thermal conductivity, etc. However, in the SPB calculation, it is necessary to determine some intermediate variables, such as Fermi level or the complex Fermi-Dirac integrals. In this work, we establish a direct carrier-concentration-dependent restructured SPB model, which eliminates Fermi-Dirac integrals and Fermi level calculation and emerges stronger visibility and usability in experiments. We have verified the reliability of such restructured model with 490 groups of experimental data from state-of-the-art TE materials and the relative error is less than 2%. Moreover, carrier effective mass, intrinsic carrier mobility and optimal carrier concentration of these materials are systematically investigated. We believe that our work can provide more convenience and accuracy for thermoelectric data analysis as well as instructive understanding on future optimization design.

Journal ArticleDOI
TL;DR: In this paper, the position-dependent effective mass and non-resonant intense laser field effects on the first and third-order corrections of the absorption and relative changes of the refraction index coefficients for intersubband transition in Razavy-like quantum wells were investigated.
Abstract: Using the effective mass and parabolic band approximations, we investigated the position-dependent effective mass and non-resonant intense laser field effects on the first and third-order corrections of the absorption and relative changes of the refraction index coefficients for intersubband transition in Razavy-like quantum wells. Calculations have been extended to the spherical Razavy-like quantum dots electronic structure. We have shown that depending on the combinations of the Razavy-like potential parameters, the quantum wells can evolve from parabolic confinement in an isolated quantum well to a configuration of two coupled quantum wells. We have shown that in general the transition energies (dipole matrix elements) between the ground state and the first excited state: i) are decreasing (increasing) functions of M-parameter, ii) are increasing (decreasing) functions of A-parameter, iii) they increase (decrease) when considering the position-dependent effective mass effects, and iv) are increasing (decreasing) functions of the intense laser field parameter. In the case of the optical absorption and relative changes in the refractive index coefficients, we have shown blueshifts or redshifts by changing the A-, M-, and α0-parameters and by considering the effects of the position-dependent effective mass. In spherical quantum dots, we have shown that with an appropriate value of A- and M-parameters the system can evolve from a spherical quantum dot with infinite parabolic potential.

Journal ArticleDOI
02 Jun 2021-ACS Nano
TL;DR: In this article, a widened band gap and increased density of states are achieved via S alloying, resulting in 1.6 times enhancement of S (from 170 to 277 μV/K).
Abstract: As an eco-friendly thermoelectric material, Cu2SnSe3 has recently drawn much attention. However, its high electrical resistivity ρ and low thermopower S prohibit its thermoelectric performance. Herein, we show that a widened band gap and the increased density of states are achieved via S alloying, resulting in 1.6 times enhancement of S (from 170 to 277 μV/K). Moreover, doping In at the Sn site can cause a 19-fold decrease of ρ and a 2.2 times enhancement of S (at room temperature) due to both multivalence bands' participation in electrical transport and the further enhancement of the density of states effective mass, which allows a sharp increase in the power factor. As a result, PF = 9.3 μW cm-1 K-2 was achieved at ∼800 K for the Cu2Sn0.82In0.18Se2.7S0.3 sample. Besides, as large as 44% reduction of lattice thermal conductivity is obtained via intensified phonon scattering by In-doping-induced formation of multidimensional defects, such as Sn vacancies, dislocations, twin boundaries, and CuInSe2 nanoprecipitates. Consequently, a record high figure of merit of ZT = 1.51 at 858 K is acquired for Cu2Sn0.82In0.18Se2.7S0.3, which is 4.7-fold larger than that of pristine Cu2SnSe3.

Journal ArticleDOI
TL;DR: The Ag-rich nanoscale precipitates, discordant Ag atoms and Pb/Sr, P b/Ba point defects in the PbSe matrix work together to reduce the lattice thermal conductivity, resulting a record high average ZT avg.
Abstract: We present an effective approach to favorably modify the electronic structure of PbSe using Ag doping coupled with SrSe or BaSe alloying. The Ag 4d states make a contribution to in the top of the heavy hole valence band and raise its energy. The Sr and Ba atoms diminish the contribution of Pb 6s2 states and decrease the energy of the light hole valence band. This electronic structure modification increases the density-of-states effective mass, and strongly enhances the thermoelectric performance. Moreover, the Ag-rich nanoscale precipitates, discordant Ag atoms, and Pb/Sr, Pb/Ba point defects in the PbSe matrix work together to reduce the lattice thermal conductivity, resulting a record high average ZTavg of around 0.86 over 400-923 K.

Journal ArticleDOI
TL;DR: In this article, the authors showed the simultaneous enhancement of electrical transport and reduction of phonon propagation in p-type PbTe codoped with Tl and Na and demonstrated that the combination of impurity resonance scattering and crystal lattice softening can be a breakthrough concept for advancing thermoelectrics.
Abstract: In this work, we show the simultaneous enhancement of electrical transport and reduction of phonon propagation in p-type PbTe codoped with Tl and Na. The effective use of advanced electronic structure engineering improves the thermoelectric power factor S2σ over the temperature range from 300 to 825 K. A rise in the Seebeck coefficient S was obtained due to the enhanced effective mass m*, coming from the Tl resonance state in PbTe. Due to the presence of additional carriers brought by Na codoping, electrical conductivity became significantly improved. Furthermore, Tl and Na impurities induced crystal lattice softening, remarkably reducing lattice thermal conductivity, which was confirmed by a measured low speed of sound vm and high internal strain CeXRD. Eventually, the combination of both the attuned electronic structure and the lattice softening effects led to a very high ZT value of up to ∼2.1 for the Pb1-x-yTlxNayTe samples. The estimated energy conversion efficiency shows the extraordinary value of 15.4% (Tc = 300 K, Th = 825 K), due to the significantly improved average thermoelectric figure of merit ZTave = 1.05. This work demonstrates that the combination of impurity resonance scattering and crystal lattice softening can be a breakthrough concept for advancing thermoelectrics.

Journal ArticleDOI
TL;DR: In this paper, a generalized Bethe-Salpeter equation (BSE) was proposed to include dynamical screening from phonons at lowest order in the electron-phonon interaction.
Abstract: The ab initio Bethe-Salpeter equation (BSE) approach, an established method for the study of excitons in materials, is typically solved in a limit where only static screening from electrons is captured. Here, we generalize this framework to include dynamical screening from phonons at lowest order in the electron-phonon interaction. We apply this generalized BSE approach to a series of inorganic lead halide perovskites, CsPbX_{3}, with X=Cl, Br, and I. We find that inclusion of screening from phonons significantly reduces the computed exciton binding energies of these systems. By deriving a simple expression for phonon screening effects, we reveal general trends for their importance in semiconductors and insulators, based on a hydrogenic exciton model. We demonstrate that the magnitude of the phonon screening correction in isotropic materials can be reliably predicted using four material specific parameters: the reduced effective mass, static and optical dielectric constants, and frequency of the most strongly coupled longitudinal-optical phonon mode. This framework helps to elucidate the importance of phonon screening and its relation to excitonic properties in a broad class of semiconductors.

Journal ArticleDOI
TL;DR: In this paper, a cyclic and helical symmetry-adapted formulation and large-scale parallel implementation of real-space Kohn-Sham density functional theory for one-dimensional (1D) nanostructures, with application to the mechanical and electronic response of carbon nanotubes subject to torsional deformations.
Abstract: We present a cyclic and helical symmetry-adapted formulation and large-scale parallel implementation of real-space Kohn-Sham density functional theory for one-dimensional (1D) nanostructures, with application to the mechanical and electronic response of carbon nanotubes subject to torsional deformations. Specifically, employing a semilocal exchange correlation and a local formulation of the electrostatics, we derive symmetry-adapted variants for the energy functional, variational problem governing the electronic ground state, Kohn-Sham equations, atomic forces, and axial stress, all posed on the fundamental domain. In addition, we develop a representation for twisted nanotubes of arbitrary chirality within this framework. We also develop a high-order finite-difference parallel implementation capable of performing accurate cyclic and helical symmetry-adapted Kohn-Sham calculations in both the static and dynamic settings, and verify it through numerical tests and comparisons with established codes. We use this implementation to perform twist-controlled simulations for a representative set of achiral and chiral carbon nanotubes, in both the small and large deformation regimes. In the linear regime, we find that the torsional moduli are proportional to the cube of the diameter; metallic nanotubes undergo metal-insulator transitions; and both the band gap as well as effective mass of charge carriers are proportional to the shear strain and sine of three times the chiral angle. In the nonlinear regime, we find that there is significant Poynting effect, particularly at the ultimate strain, the value of which is determined by the chiral angle; torsional deformations provide a possible mechanism for the irreversible phase transformation from armchair to zigzag nanotubes; and both the band gap as well as effective mass have an oscillatory behavior, with the period for metal-insulator transitions being inversely proportional to the square of the diameter and sine of three times the chiral angle. Wherever available, the results are in good agreement with experimental observations and measurements. Overall, this opens an avenue for the highly accurate and efficient first-principles study of 1D nanostructures that have cyclic and/or helical symmetry, as well as their response to torsional deformations.

Journal ArticleDOI
TL;DR: In this article, the atomic structure, electronic and optical properties of CsPbX3 (X = I, Br, Cl, mixed-halide) perovskites using density functional theory calculations were reported.

Journal ArticleDOI
TL;DR: In this article, the authors used Co doping to achieve peak ZTs of 0.65 in n-type and p-type pseudo-ternary half-Heusler alloys.

Journal ArticleDOI
26 Apr 2021
TL;DR: In this paper, the effects of mixed-valence states of europium (Eu)-incorporated CH(NH2)2PbI3 (FAPbI), CH3NH3Pb I3 (MAPbI) perovskite crystals on electronic structures were investigated by first-principles calculation.
Abstract: Effects of mixed-valence states of europium (Eu)-incorporated CH(NH2)2PbI3 (FAPbI3) and CH3NH3PbI3 (MAPbI3) perovskite crystals on electronic structures were investigated by first-principles calculation. Partial replacements of europium ions into the perovskite crystal influenced the electronic structures and the effective mass related to carrier mobility. In the case of the FAPb(Eu+3)I3 crystal, there was wide distribution of the 5p orbital of iodine near the valence band, and the 3d orbital of the Eu3+ ion near the conductive band. The incorporation of Eu3+ ion into the crystal slightly caused to increase the effective mass ratio (me*/me, mh*/me) as compared with those of the FAPbI3 crystal, provided the wide distribution of 3d, 4f-5p hybrid orbitals near the valence band, and influenced the band dispersion with a decrease of me*/me and mh*/me, which is expected for improving the carrier mobility. The chemical shifts of 127I-NMR of the MAPb(Eu2+)I3 crystal indicated isotropic behavior. The chemical shifts of 157Eu-NMR and g-tensor depended on the quadrupole interaction based on the electron field gradient and asymmetry parameter in the coordination structure. The electronic correlation based on hybrization of the 3d, 4f-5p orbital in the Eu2+-iodine band promoted the carrier itinerary, which was expected to improve the carrier mobility related to the short circuit current density and the conversion efficiency as the photovoltaic performance.

Journal ArticleDOI
TL;DR: In this article, the authors explore the electrodynamic response of correlated metals at half filling for varying correlation strength upon approaching a Mott insulator and reveal persistent Fermi-liquid behavior with pronounced quadratic dependences of the optical scattering rate on temperature and frequency, along with a puzzling elastic contribution to relaxation.
Abstract: Landau suggested that the low-temperature properties of metals can be understood in terms of long-lived quasiparticles with all complex interactions included in Fermi-liquid parameters, such as the effective mass m⋆. Despite its wide applicability, electronic transport in bad or strange metals and unconventional superconductors is controversially discussed towards a possible collapse of the quasiparticle concept. Here we explore the electrodynamic response of correlated metals at half filling for varying correlation strength upon approaching a Mott insulator. We reveal persistent Fermi-liquid behavior with pronounced quadratic dependences of the optical scattering rate on temperature and frequency, along with a puzzling elastic contribution to relaxation. The strong increase of the resistivity beyond the Ioffe–Regel–Mott limit is accompanied by a ‘displaced Drude peak’ in the optical conductivity. Our results, supported by a theoretical model for the optical response, demonstrate the emergence of a bad metal from resilient quasiparticles that are subject to dynamical localization and dissolve near the Mott transition. Charge transport in strongly correlated electron systems is not fully understood. Here, the authors show that resilient quasiparticles at finite frequency persist into the bad-metal regime near a Mott insulator, where dynamical localization results in a ‘displaced Drude peak’ and strongly enhanced dc resistivity.

Journal ArticleDOI
Yijie Liu1, Liang Jin1, Hongfa Wang1, Dongying Liu1, Yingjing Liang1 
TL;DR: In this paper, the relationship among bandgap, effective mass and effective stiffness are analyzed for the mass-spring and continuum models, and the locations of stop-bands and negative effective parameters are manipulated by tuning the parameters of translational resonators.

Journal ArticleDOI
TL;DR: In this paper, the electrical and thermal transport properties of zone-melted Ag2Se were systematically investigated and compared with other typical low-temperature thermoelectric materials, such as Mg3Bi2, Bi2Te3, and BiSb.
Abstract: Silver selenide, Ag2Se, is a promising low-temperature thermoelectric material which can be used to harvest the low-quality waste heat for electrical power generation or cool the microelectronics. Currently, the investigation on Ag2Se and its derivatives has become a hot topic in the thermoelectric community, but the thermoelectric properties of Ag2Se below 300 K have been rarely investigated. In this study, we prepared Ag2Se by using the zone-melting method. The electrical and thermal transport properties of zone-melted Ag2Se from 5 to 380 K were systematically investigated and compared with the previously reported data of Ag2Se and other typical low-temperature thermoelectric materials, such as Mg3Bi2, Bi2Te3, and BiSb. Ag2Se shows intrinsic semiconductor features, ultrahigh carrier mobility, small density-of-state effective mass, and ultralow lattice thermal conductivity. At 300 K, the zT of zone-melted Ag2Se is 0.75. This study will shed light on the further investigation of Ag2Se.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the mechanical stability, electronic and optical properties, and redox potential of perovskite RbSr2Nb3O10 by using a first-principles density functional theory (DFT) calculations.

Journal ArticleDOI
TL;DR: In this article, the geometrical, electronic and optical properties of penta-PdX2 (X = As,P) using density functional calculation were examined, and it was shown that the PdAs2 and PdP2 are semiconductors with direct band gaps of 0.34 eV and 0.30 eV, respectively.
Abstract: In this study, we have examined the geometrical, electronic and optical properties of penta-PdX2 (X = As,P) using density functional calculation. The electronic structure calculations show that the penta-PdAs2 and PdP2 are semiconductors with direct band gaps of 0.34 eV and 0.30 eV, respectively. The dynamical stability of penta-PdX2 monolayer is proved by the absence of imaginary frequencies in the phonon dispersion curve. By applying a biaxial strain (for PdAs2: − 6% to + 6% and for PdP2: − 5.5% to + 5.5%) on the monolayer, the effective mass and band edges are tuned effectively. Remarkably, the range of penta-PdX2 carrier mobility was obtained in an extremely high order of 105 cm2 V−1 s−1 for holes and 104 cm2 V−1 s−1 for electrons. The optical properties of penta-PdX2 were also strained-tunable and exhibit outstanding absorption of infrared, visible and ultraviolet light. More importantly, the band edges alignment has tunable with implemented external electric field (V/A) along the z-direction. Our work would stimulate the fabrication of penta-PdX2 monolayer, and it is envisioned that it is an appropriate future candidate for optoelectronic and ultra-fast electronic applications.

Journal ArticleDOI
TL;DR: In this article, Ni and Coincorporated CH3NH3PbI3 perovskite solar cells were fabricated and characterized to optimize the photovoltaic and optical properties related to surface morphology, crystal growth and orientation, and electronic structures.
Abstract: Ni- and Co-incorporated CH3NH3PbI3 perovskite solar cells were fabricated and characterized to optimize the photovoltaic and optical properties related to surface morphology, crystal growth and orientation, and electronic structures. Partially replacing Pb with Ni or Co in the perovskite crystals improved the photovoltaic performance and carrier mobility based on the effective mass in the band structure. In particular, the addition of both Ni and Rb compounds to perovskite improved the long-term stability of the photovoltaic cells, which depended on surface modification and coverage, crystal growth, and the high (100) orientation in the perovskite layer. The short-circuit current density of the cells was increased by promoting the generation and mobility of photoinduced carriers, which were inversely proportional to the effective mass ratio. Electron correlation was associated with the promotion of charge transfer owing to the hybridization between the 3d orbitals of Ni and the 5p orbitals of the I atoms near the valence band state.

Journal ArticleDOI
TL;DR: In this paper, a theoretical study on pressure-dependent interfacial charge transfer excitons in WSe2-MoSe2 van der Waals heterostructures in near infrared region (NIR) is presented.
Abstract: In this letter, we report our theoretical study on pressure-dependent interfacial charge transfer excitons in WSe2-MoSe2 van der Waals heterostructures in near infrared region (NIR). It is found that pressure can control not only the degree of interfacial charge transfer, but also the orientation of interfacial charge transfer in 2D heterostructures. Pressure can efficiently promote the separation of interfacial charge transfer between two layers of the heterostructures. The variation of pressure results in the changing of band gap, the effective mass, as well as the intrinsic carrier concentration in WSe2-MoSe2 van der Waals heterostructures. The pressure-induced red shifted for interfacial charge transfer excitons is also obtained. Furthermore, the reason why pressure results in fluorescence quenching is strong van der Waals interactions. This discovery will provide a new method for reversible non-destructive control of the electrical properties of two-dimensional semiconductors.

Journal ArticleDOI
TL;DR: In this article, the authors have successfully synthesized thermally stable cubic phase cesium tin chloride (CsSnCl3) perovskite nanocrystals with improved surface morphology by adopting a rapid hot-injection technique.
Abstract: Lead-free metal halide perovskites have attracted great attention as light harvesters due to their promising optoelectronic and photovoltaic properties. In this investigation, we have successfully synthesized thermally stable cubic phase cesium tin chloride (CsSnCl3) perovskite nanocrystals with improved surface morphology by adopting a rapid hot-injection technique. The excellent crystalline quality of these cubic shaped nanocrystals was confirmed by high-resolution transmission electron microscopy imaging. The binding of organic ligands on the surface of the sample was identified and characterized using nuclear magnetic resonance spectroscopy. UV-visible spectroscopy confirmed that the CsSnCl3 nanocrystals have a direct band gap of ∼2.98 eV, which was further confirmed using steady-state photoluminescence spectroscopy. The band edge positions calculated using the Mulliken electronegativity approach predicted the potential photocatalytic capability of the as-prepared nanocrystals, which was then experimentally corroborated through the photodegradation of rhodamine-B dye under both visible and UV-visible irradiation. Our theoretical calculations employing experimentally obtained structural parameters within the generalized gradient approximation (GGA) and GGA+U methods demonstrated a 90% accurate estimation of the experimentally observed optical band gap when Ueff = 6 eV was considered. The ratio of the effective mass of the hole and electron expressed as was also calculated for Ueff = 6 eV. Based on this theoretical calculation and experimental observation of the photocatalytic performance of CsSnCl3 nanocrystals, we have proposed a rational interpretation of the “D” value. We think that a “D” value of either much smaller or much larger than 1 is an indication of the low recombination rate of the photogenerated electron–hole pairs and the high photocatalytic efficiency of the photocatalyst. We believe that this comprehensive investigation might be helpful for the large-scale synthesis of thermally stable cubic CsSnCl3 nanocrystals and also for a greater understanding of their potential in photocatalytic, photovoltaic and other prominent optoelectronic applications.

Journal ArticleDOI
Shizhe Zhang1, Chunying Zhu1, Huisheng Feng1, Taotao Fu1, Youguang Ma1 
TL;DR: In this paper, the authors investigated the effect of CO2 absorption by continuous sudden expansion units in microchannels and showed that the sudden expansion structure could remarkably enhance gas-liquid mass transfer.

Journal ArticleDOI
TL;DR: The desirable direct band gap and anisotropic effective mass of the th-C/th-BN heterostructure suggest that th-BN can be a suitable substrate for tetrahexcarbon.
Abstract: By performing first-principles calculations, a new two-dimensional (2D) boron nitride (th-BN) with perfectly ordered arrangements of tetragonal and hexagonal rings is predicted to be energetically, dynamically, thermally, and mechanically stable. The unique structure endows th-BN with anisotropic mechanical, electronic, and optical properties. Remarkably, th-BN exhibits exceptional mechanical properties such as high in-plane stiffness and sign-tunable Poisson's ratio (PR). The PR of th-BN gradually decreases with the increase of axial strain and even becomes negative at a very small strain (∼2%), which is novel, thereby offering the ability to become non-auxetic, auxetic, and partially auxetic 2D nanomaterials depending on the strain rate and direction. The structure can withstand tensile strain as large as 36%, and shows ultrahigh ideal strength that can even outperform graphene and hexagonal BN. The th-BN is a natural 2D semiconductor with an indirect wide band gap of 4.49 eV. The band gap can be tuned by applying lattice strain and hydrogenation. The full hydrogenated th-BN exhibits an indirect-to-direct band gap transition. The th-BN shows high optical absorption in the ultraviolet region. The optical absorption spectrum is highly direction-dependent and tunable by strain, suitable for high-performance optoelectronic device applications. Furthermore, th-BN can be stacked into two different configurations, and are dynamically stable and exhibit exotic electronic properties. The desirable direct band gap and anisotropic effective mass of the th-C/th-BN heterostructure suggest that th-BN can be a suitable substrate for tetrahexcarbon.