Strong Intervalley Scattering-Induced Renormalization of Electronic and Thermal Transport Properties and Selection Rule Analysis in 2D Tellurium.

The electron-phonon interaction (EPI) and phonon-phonon interactions are ubiquitous in promising two-dimensional (2D) semiconductors, determining both electronic and thermal transport properties. In this work, based on ab initio calculations, the effects of intervalley scattering on EPI and higher-order four-phonon interactions of α-Te and ß-Te are investigated. Through the proposed selection rules for scattering channels and calculations of full electron-phonon scattering rates, we demonstrate that multiple nearly degenerate local valleys/peaks produce more scattering channels, resulting in stronger intervalley scattering over intravalley scattering. The lattice thermal conductivities of α-Te and ß-Te are decreased by as much as 10.9% and 30.8% by considering EPI under the carrier concentration of 2 × 1013 cm-2 (n-type) at 300 K compared to those limited by three-phonon scattering, respectively. However, when further considering four-phonon scattering, EPI reduces the lattice thermal conductivities by 2.6% and 19.4% for α-Te and ß-Te, respectively. Furthermore, it is revealed that the four-phonon interaction is more dominant in phonon transport for α-Te than that for ß-Te due to the presence of an acoustic-optical phonon gap in α-Te. Finally, we demonstrate strong intervalley scattering induces significant renormalization effects from EPI on all the constituent parameters of thermoelectric performance. Our results show the contributions of intervalley scattering to the electronic properties as well as thermal transport properties in band-convergent thermoelectric materials are essential and highlight the potential of monolayer tellurium as a promising candidate for advanced thermoelectric applications.

Ultralow Lattice Thermal Transport and Considerable Wave-like Phonon Tunneling in Chalcogenide Perovskite BaZrS3.

Chalcogenide perovskites provide a promising avenue for nontoxic, stable thermoelectric materials. Here, the thermal transport and thermoelectric properties of BaZrS3 as a typical orthorhombic perovskite are investigated. An extremely low lattice thermal conductivity κL of 1.84 W/mK at 300 K is revealed for BaZrS3, due to the softening effect of Ba atoms on the lattice and the strong anharmonicity caused by the twisted structure. We demonstrate that coherence contributions to κL, arising from wave-like phonon tunneling, lead to an 18% thermal transport contribution at 300 K. The increasing temperature softens the phonons, thus reducing the group velocity of materials and increasing the scattering phase space. However, it simultaneously reduces the anharmonicity, which is dominant in BaZrS3 and ultimately improves the particle-like thermal transport. In addition, via replacement of the S atom with Se- and Ti-alloying strategy, the ZT value of BaZrS3 is significantly increased from 0.58 to 0.91 at 500 K, making it an important candidate for thermoelectric applications.

Spin-Orbit-Coupling-Induced Topological Transition and Anomalously Strong Intervalley Scattering in Two-Dimensional Bismuth Allotropes with Enhanced Thermoelectric Performances.

The convergence of multivalley bands is originally believed to be beneficial for thermoelectric performance by enhancing the charge conductivity while preserving the Seebeck coefficients, based on the assumption that electron interband or intervalley scattering effects are totally negligible. In this work, we demonstrate that ß-Bi with a buckled honeycomb structure experiences a topological transition from a normal insulator to a Z2 topological insulator induced by spin-orbit coupling, which subsequently increases the band degeneracy and is probably beneficial for enhancement of the thermoelectric power factor for holes. Therefore, strong intervalley scattering can be observed in both band-convergent ß- and aw-Bi monolayers. Compared to ß-Bi, aw-Bi with a puckered black-phosphorus-like structure possesses high carrier mobilities with 318 cm2/(V s) for electrons and 568 cm2/(V s) for holes at room temperature. We also unveil extraordinarily strong fourth phonon-phonon interactions in these bismuth monolayers, significantly reducing their lattice thermal conductivities at room temperature, which is generally anomalous in conventional semiconductors. Finally, a high thermoelectric figure of merit (zT) can be achieved in both bismuth monolayers, especially for aw-Bi with an n-type zT value of 2.2 at room temperature. Our results suggest that strong fourth phonon-phonon interactions are crucial to a high thermoelectric performance in these materials, and two-dimensional bismuth is probably a promising thermoelectric material due to its enhanced band convergence induced by the topological transition.

Discovery of Lead-Free Perovskites for High-Performance Solar Cells via Machine Learning: Ultrabroadband Absorption, Low Radiative Combination, and Enhanced Thermal Conductivities.

Exploring lead-free candidates and improving efficiency and stability remain the obstacle of hybrid organic-inorganic perovskite-based devices commercialization. Traditional trial-and-error methods seriously restrict the discovery especially for large search space, complex crystal structure and multi-objective properties. Here, the authors propose a multi-step and multi-stage screening scheme to accelerate the discovery of hybrid organic-inorganic perovskites A2 BB'X6 from a large number of candidates through combining machine learning with high-throughput calculations for pursuing excellent efficiency and thermal stability in solar cells. Followed by a series of screenings, the structure-property relationships mapping A2 BB'X6 properties are built and the predictions are close to reported experimental results. Successfully, four experimental-feasibly candidates with good stability, high Debye temperature and suitable band gap are screened out and further verified by density-functional theory calculations, in which the predicted efficiency for three lead-free candidates ((CH3 NH3 )2 AgGaBr6 , (CH3 NH3 )2 AgInBr6 and (C2 NH6 )2 AgInBr6 ) achieves 20.6%, 19.9% and 27.6% due to ultrabroadband absorption region ranging from UVC to IRC with excitonic radiative combination rates as low as 10 ps, large or intermediate polarons form with properties similar to CH3 NH3 PbI3 and the calculated thermal conductivities are 5.04, 4.39 and 5.16 Wm-1 K-1 , respectively, with Debye temperatures larger than 500 K, beneficial for suppression of both nonradiative combination and heat-induced degradation.

Thermoelectric performance of 2D materials: the band-convergence strategy and strong intervalley scatterings.

The strategy of band convergence of multi-valley conduction bands or multi-peak valence bands has been widely used to search or improve thermoelectric materials. However, the phonon-assisted intervalley scatterings due to multiple band degeneracy are usually neglected in the thermoelectric community. In this work, we investigate the (thermo)electric properties of non-polar monolayer ß- and α-antimonene considering full mode- and momentum-resolved electron-phonon interactions. We also analyze thoroughly the selection rules on electron-phonon matrix-elements using group-theory arguments. Our calculations reveal strong intervalley scatterings between the nearly degenerate valley states in both ß- and α-antimonene, and the commonly-used deformation potential approximation neglecting the dominant intervalley scattering gives inaccurate estimations of the electron-phonon scattering and thermoelectric transport properties. By considering full electron-phonon interactions based on the rigid-band approximation, we find that, the maximum value of the thermoelectric figure of merits zT at room temperature reduces to 0.37 in ß-antimonene, by a factor of 5.7 compared to the value predicted based on the constant relaxation-time approximation method. Our work not only provides an accurate prediction of the thermoelectric performances of antimonenes, which reveals the key role of intervalley scatterings in determining the electronic part of zT, but also exhibits a computational framework for thermoelectric materials.

Increasing Accessible Subsurface to Improving Rate Capability and Cycling Stability of Sodium-Ion Batteries.

Numerous studies have reported that the enhancement of rate capability of carbonaceous anode by heteroatom doping is due to the increased diffusion-controlled capacity induced by expanding interlayer spacing. However, percentage of diffusion-controlled capacity is less than 30% as scan rate is larger than 1 mV s-1 , suggesting there is inaccuracy in recognizing principle of improving rate capability of carbonaceous anode. In this paper, it is found that the heteroatom doping has little impact on interlayer spacing of carbon in bulk phase, meaning that diffusion-controlled capacity is hard to be enhanced by doping. After synergizing with tensile stress, however, the interlayer spacing in subsurface region is obviously expanded to 0.40 nm, which will increase the thickness of accessible subsurface region at high current density. So SRNDC-700 electrodes display a high specific capacity of 160.6 and 69.5 mAh g-1 at 20 and 50 A g-1 , respectively. Additionally, the high reversibility of carbon structure insures ultralong cycling stability and hence attenuation of SRNDC-700 is only 0.0025% per cycle even at 10 A g-1 for 6000 cycles. This report sheds new insight into mechanism of improving electrochemical performance of carbonaceous anode by doping and provides a novel design concept for doping carbon.

Pushing Optical Switch into Deep Mid-Infrared Region: Band Theory, Characterization, and Performance of Topological Semimetal Antimonene.

The existing pulsed laser technologies and devices are mainly in the infrared spectral region below 3 µm so far. However, longer-wavelength pulsed lasers operating in the deep mid-infrared region (3-20 µm) are desirable for atmosphere spectroscopy, remote sensing, laser lidar, and free-space optical communications. Currently, the lack of reliable optical switches is the main limitation for developing pulsed lasers in the deep mid-infrared region. Here, we demonstrate that topological semimetal antimonene possesses an ultrabroadband optical switch characteristic covering from 2 µm to beyond 10 µm. Especially, the topological semimetal antimonene shows a very low saturable energy fluence (only 3-15 nJ cm-2 beyond 3 µm) and an ultrafast recovery time of ps level. We also demonstrate stable Q-switching in fiber lasers at 2 and 3.5 µm by using topological semimetal antimonene as passive optical switches. Combined with the high environmental stability and easy fabrication, topological semimetal antimonene offers a promising optical switch that extends pulsed lasers into deep mid-infrared region.

Room Temperature Bound Excitons and Strain-Tunable Carrier Mobilities in Janus Monolayer Transition-Metal Dichalcogenides.

The successful synthesis of Janus transition metal dichalcogenides offers new opportunities in two-dimensional materials due to its novel properties induced by structural mirror asymmetry. Herein, by using the first-principle calculations, the thermodynamical stability for monolayers MoSSe and WSSe is demonstrated by phonon dispersion with no imaginary frequencies. No longitudinal optical-transverse optical (LO-TO) splitting exists at the Γ point and phonon frequencies are red-shifted, since the 2D Coulomb screening effect is introduced to eliminate the spurious interaction between adjacent layers. An indirect-direct-indirect transition in band gaps for both MoSSe and WSSe is observed, and tunable mobilities can be realized by uniaxial strain, reaching up to 106 cm2 V-1 s-1 when applying 2% tensile strain along the zigzag direction to monolayer MoSSe, which provides a good platform for flexible electronic devices. Large band gaps of 2.569 and 2.666 eV are predicted for monolayers MoSSe and WSSe when considering many-body quasiparticle corrections. The enhanced electron-hole interaction caused by a weak screening effect leads to considerable binding energies for both MoSSe and WSSe, and such tightly binding excitons with large oscillator strengths generate strong absorption peaks in visible region. The remarkable properties of Janus monolayers MoSSe and WSSe make them promising in next-generation microelectronic, optoelectronic, and valleytronic devices.

Atomic scale study of stress-induced misaligned subsurface layers in KDP crystals.

We carried out ab initio calculations to study the atomic configuration, band structure and optical absorption of the lattice misalignment structure (LMS) in a subsurface layer of a machined KH2PO4 (KDP) crystal. By varying the different degrees of misalignment, the changes in the corresponding atomic position and bond and energy are obtained, and their correlations are analysed in detail. The results indicate that in the LMS evolution, the variation in the proton distribution around the oxygen atoms plays an important role, and many local stable LMSs appear. Interestingly, at a certain misalignment value, the total system energy of the local stable LMS is near that of a perfect KDP crystal. For some local stable LMSs, the electronic and optical properties related to the laser damage threshold (LDT) of KDP are further studied. The results show that in comparison with a perfect KDP crystal, the band gaps of local stable LMSs at some certain misalignment values become narrow, and their optical absorption curves produce an obvious redshift. These facts demonstrate that the emergence of the LMS could have a significant impact on the optical absorption of the KDP material and thus affect the LDT of KDP under certain working conditions.

High intrinsic ZT in InP3 monolayer at room temperature.

Two-dimensional thermoelectric (TE) materials which have the figure of merit ZT that is greater than 1.5 at room temperature would be highly desirable in energy conversion since the efficiency is competitive to conventional energy conversion techniques. Here, we report that the indium triphosphide (InP3) monolayer shows a large ZT of 1.92 at 300 K, based on the quantum calculations within the ballistic thermal transport region. A remarkably low and isotropic phononic thermal conductivity is found due to the flat lattice vibration modes, which takes a major responsibility for the impressively high ZT at room temperature. Moreover, a large ZT of 1.67 can still be achieved even under a 1% mechanical tension on the lattice. These results suggest that the InP3 monolayer is a promising candidate for low dimensional TE applications.

In-Plane Anisotropic Thermal Conductivity of Few-Layered Transition Metal Dichalcogenide Td-WTe2.

2D Td-WTe2 has attracted increasing attention due to its promising applications in spintronic, field-effect chiral, and high-efficiency thermoelectric devices. It is known that thermal conductivity plays a crucial role in condensed matter devices, especially in 2D systems where phonons, electrons, and magnons are highly confined and coupled. This work reports the first experimental evidence of in-plane anisotropic thermal conductivities in suspended Td-WTe2 samples of different thicknesses, and is also the first demonstration of such anisotropy in 2D transition metal dichalcogenides. The results reveal an obvious anisotropy in the thermal conductivities between the zigzag and armchair axes. The theoretical calculation implies that the in-plane anisotropy is attributed to the different mean free paths along the two orientations. As thickness decreases, the phonon-boundary scattering increases faster along the armchair direction, resulting in stronger anisotropy. The findings here are crucial for developing efficient thermal management schemes when engineering thermal-related applications of a 2D system.

The role of Anderson's rule in determining electronic, optical and transport properties of transition metal dichalcogenide heterostructures.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) MX2 (M = Mo, W; X = S, Se, Te) possess unique properties and novel applications in optoelectronics, valleytronics and quantum computation. In this work, we performed first-principles calculations to investigate the electronic, optical and transport properties of the van der Waals (vdW) stacked MX2 heterostructures formed by two individual MX2 monolayers. We found that the so-called Anderson's rule can effectively classify the band structures of heterostructures into three types: straddling, staggered and broken gap. The broken gap is gapless, while the other two types possess direct (straddling, staggered) or indirect (staggered) band gaps. The indirect band gaps are formed by the relatively higher energy level of Te-d orbitals or the interlayer couplings of M or X atoms. For a large part of the formed MX2 heterostructures, the conduction band maximum (CBM) and valence band minimum (VBM) reside in two separate monolayers, thus the electron-hole pairs are spatially separated, which may lead to bound excitons with extended lifetimes. The carrier mobilities, which depend on three competitive factors, i.e. elastic modulus, effective mass and deformation potential constant, show larger values for electrons of MX2 heterostructures compared to their constituent monolayers. Finally, the calculated optical properties reveal strong absorption in the ultraviolet region.

Insights into High Conductivity of the Two-Dimensional Iodine-Oxidized sp2-c-COF.

A recent experiment [ Jin , E. ; Science 2017 , 357 , 673 - 676 ] shows that the conductivity of a two-dimensional (2D) sp2-carbon-hybridized π-conjugated covalent organic framework (sp2-c-COF) can be enhanced by as much as 12 orders of magnitude after iodine oxidation processing. To understand the physical mechanism underlying such a huge increase in the conductivity, we perform multiscale computations and find that the high conductivity of the iodine-oxidized 2D COF can be attributed to both hole transfer and ion transfer within the 2D COF. The computed dominant charge distribution corresponding to the valence band maximum (VBM) suggests that the delocalized π electrons occur mostly at the active reaction sites. The computed low ionization energy at the active reaction sites further supports that the 2D COF tends to lose electrons during iodine oxidation and to yield cationic COF and anionic triiodide I3-. Complementary classical molecular dynamics simulation shows a relatively high anion conductivity of 13.63 × 10-2 S m-1, consistent with the high conductivity measured from the experiment (7.1 × 10-2 S m-1). Meanwhile, we find that the cations in 2D COF can also induce a shift of the Fermi level to cross the valence band, thereby enhancing the hole mobility to 86.75 cm2 V-1 s-1. For proposing a potential application of the highly conductive iodine-oxidized 2D sp2-c-COF, we design a prototypical model of the 2D spirally wound lithium-ion battery and find that it exhibits enhanced stability than a typical electrolyte material.

The conflicting role of buckled structure in phonon transport of 2D group-IV and group-V materials.

Controlling heat transport through material design is one important step toward thermal management in 2D materials. To control heat transport, a comprehensive understanding of how structure influences heat transport is required. It has been argued that a buckled structure is able to suppress heat transport by increasing the flexural phonon scattering. Using a first principles approach, we calculate the lattice thermal conductivity of 2D mono-elemental materials with a buckled structure. Somewhat counterintuitively, we find that although 2D group-V materials have a larger mass and higher buckling height than their group-IV counterparts, the calculated κ of blue phosphorene (106.6 W mK-1) is nearly four times higher than that of silicene (28.3 W mK-1), while arsenene (37.8 W mK-1) is more than fifteen times higher than germanene (2.4 W mK-1). We report for the first time that a buckled structure has three conflicting effects: (i) increasing the Debye temperature by increasing the overlap of the pz orbitals, (ii) suppressing the acoustic-optical scattering by forming an acoustic-optical gap, and (iii) increasing the flexural phonon scattering. The former two, corresponding to the harmonic phonon part, tend to enhance κ, while the last one, corresponding to the anharmonic part, suppresses it. This relationship between the buckled structure and phonon behaviour provides insight into how to control heat transport in 2D materials.

First-Principles Prediction of Ultralow Lattice Thermal Conductivity of Dumbbell Silicene: A Comparison with Low-Buckled Silicene.

The dumbbell structure of two-dimensional group IV material offers alternatives to grow thin films for diverse applications. Thermal properties are important for these applications. We obtain the lattice thermal conductivity of low-buckled (LB) and dumbbell (DB) silicene by using first-principles calculations and the Boltzmann transport equation for phonons. For LB silicene, the calculated lattice thermal conductivity with naturally occurring isotope concentrations is 27.72 W/mK. For DB silicene, the calculated value is 2.86 W/mK. The thermal conductivity for DB silicene is much lower than LB silicene due to stronger phonon scattering. Our results will induce further theoretical and experimental investigations on the thermoelectric (TE) properties of DB silicene. The size-dependent thermal conductivity in both LB and DB silicene is investigated as well for designing TE devices. This work sheds light on the manipulation of phonon transport in two-dimensional group IV materials by dumbbell structure formed from the addition of adatoms.

Water-mediated cation intercalation of open-framework indium hexacyanoferrate with high voltage and fast kinetics.

Rechargeable aqueous metal-ion batteries made from non-flammable and low-cost materials offer promising opportunities in large-scale utility grid applications, yet low voltage and energy output, as well as limited cycle life remain critical drawbacks in their electrochemical operation. Here we develop a series of high-voltage aqueous metal-ion batteries based on 'M(+)/N(+)-dual shuttles' to overcome these drawbacks. They utilize open-framework indium hexacyanoferrates as cathode materials, and TiP2O7 and NaTi2(PO4)3 as anode materials, respectively. All of them possess strong rate capability as ultra-capacitors. Through multiple characterization techniques combined with ab initio calculations, water-mediated cation intercalation of indium hexacyanoferrate is unveiled. Water is supposed to be co-inserted with Li(+) or Na(+), which evidently raises the intercalation voltage and reduces diffusion kinetics. As for K(+), water is not involved in the intercalation because of the channel space limitation.

Low lattice thermal conductivity of stanene.

A fundamental understanding of phonon transport in stanene is crucial to predict the thermal performance in potential stanene-based devices. By combining first-principle calculation and phonon Boltzmann transport equation, we obtain the lattice thermal conductivity of stanene. A much lower thermal conductivity (11.6 W/mK) is observed in stanene, which indicates higher thermoelectric efficiency over other 2D materials. The contributions of acoustic and optical phonons to the lattice thermal conductivity are evaluated. Detailed analysis of phase space for three-phonon processes shows that phonon scattering channels LA + LA/TA/ZA ↔ TA/ZA are restricted, leading to the dominant contributions of high-group-velocity LA phonons to the thermal conductivity. The size dependence of thermal conductivity is investigated as well for the purpose of the design of thermoelectric nanostructures.

A first-principles study on the phonon transport in layered BiCuOSe.

First-principles calculations are employed to investigate the phonon transport of BiCuOSe. Our calculations reproduce the lattice thermal conductivity of BiCuOSe. The calculated grüneisen parameter is 2.4 ~ 2.6 at room temperature, a fairly large value indicating a strong anharmonicity in BiCuOSe, which leads to its ultralow lattice thermal conductivity. The contribution to total thermal conductivity from high-frequency optical phonons, which are mostly contributed by the vibrations of O atoms, is larger than 1/3, remarkably different from the usual picture with very little contribution from high-frequency optical phonons. Our calculations show that both the high group velocities and low scattering processes involved make the high-frequency optical modes contribute considerably to the total lattice thermal conductivity. In addition, we show that the sound velocity and bulk modulus along a and c axes exhibit strong anisotropy, which results in the anisotropic thermal conductivity in BiCuOSe.

High thermoelectric performance in two-dimensional graphyne sheets predicted by first-principles calculations.

The thermoelectric properties of two-dimensional graphyne sheets are investigated by using first-principles calculations and the Boltzmann transport equation method. The electronic structure indicates a semiconducting phase for graphyne, compared with the metallic phase of graphene. Consequently, the obtained Seebeck coefficient and the power factor of graphyne are much higher than those of graphene. The calculated phonon mean free path for graphene is 866 nm, which is in good agreement with the experimental value of 775 nm. Meanwhile the phonon mean free path of graphyne is only 60 nm, leading to two order lower thermal conductivity than graphene. We show that the low thermal conductivity of graphyne is due to its mixed sp/sp(2) bonding. Our calculations show that the optimized ZT values of graphyne sheets can reach 5.3 at intermediate temperature by appropriate doping.

Electronic Band Structure and Sub-band-gap Absorption of Nitrogen Hyperdoped Silicon.

We investigated the atomic geometry, electronic band structure, and optical absorption of nitrogen hyperdoped silicon based on first-principles calculations. The results show that all the paired nitrogen defects we studied do not introduce intermediate band, while most of single nitrogen defects can introduce intermediate band in the gap. Considering the stability of the single defects and the rapid resolidification following the laser melting process in our sample preparation method, we conclude that the substitutional nitrogen defect, whose fraction was tiny and could be neglected before, should have considerable fraction in the hyperdoped silicon and results in the visible sub-band-gap absorption as observed in the experiment. Furthermore, our calculations show that the substitutional nitrogen defect has good stability, which could be one of the reasons why the sub-band-gap absorptance remains almost unchanged after annealing.