Band engineering and precipitation enhance thermoelectric performance of SnTe with Zn-doping

We have systematically studied the thermoelectric properties in Zn-doped Sn Te.Strikingly,band convergence and embedded precipitates arising from Zn doping,can trigger a prominent improvement of thermoelectric performance.In particular,the value of dimensionless figure of merit z T has increased by 100% and up to ～ 0.5 at 775 K for the optimal sample with 2% Zn content.Present findings demonstrate that carrier concentration and effective mass play crucial roles on the Seebeck coefficient and power factor.The obvious deviation from the Pisarenko line（Seebeck coefficient versus carrier concentration） due to Zn-doping reveals the convergence of valence bands.When the doping concentration exceeds the solubility,precipitates occur and lead to a reduction of lattice thermal conductivity.In addition,bipolar conduction is suppressed,indicating an enlargement of band gap.The Zn-doped Sn Te is shown to be a promising candidate for thermoelectric applications.

Band engineering of double-wall Mo-based hybrid nanotubes

Hybrid transition-metal dichalcogenides （TMDs） with different chalcogens on each side （X-TM-Y） have attracted attention because of their unique properties. Nanotubes based on hybrid TMD materials have advantages in flexibility over conventional TMD nanotubes. Here we predict the wide band gap tunability of hybrid TMD double-wall nanotubes （DWNTs） from metal to semiconductor. Using density-function theory （DFT） with HSE06 hybrid functional, we find that the electronic property of X-Mo-Y DWNTs （X = O and S, inside a tube; Y = S and Se, outside a tube） depends both on electronegativity difference and diameter difference. If there is no difference in electron negativity between inner atoms （X） of outer tube and outer atoms （Y） of inner tube, the band gap of DWNTs is the same as that of the inner one. If there is a significant electronegativity difference, the electronic property of the DWNTs ranges from metallic to semiconducting, depending on the diameter differences. Our results provide alternative ways for the band gap engineering of TMD nanotubes.

Band engineering of honeycomb monolayer CuSe via atomic modification

Two-dimensional monolayer copper selenide(CuSe)has been epitaxially grown and predicted to host the Dirac nodal line fermion(DNLF).However,the metallic state of monolayer CuSe inhibits the potential application of nanoelectronic devices in which a band gap is needed to realize on/off properties.Here,we engineer the band structure of monolayer CuSe which is an analogue of a p-doped system via external atomic modification in an effort to realize the semiconducting state.We find that the H and Li modified monolayer CuSe shifts the energy band and opens an energy gap around the Fermi level.Interestingly,both the atomic and electronic structures of monolayer CuHSe and CuLiSe are very different.The H atoms bind on top of Se atoms of monolayer CuSe with Se-H polar covalent bonds,annihilating the DNLF band of monolayer CuSe dominated by Se orbitals.In contrast,Li atoms prefer to adsorb at the hexagonal center of CuSe,preserving the DNLF band of monolayer CuSe dominated by Se orbitals,but opening band gaps due to a slight buckling of the CuSe layer.The realization of metal-to-semiconductor transition from monolayer CuSe to CuXSe(X=H,Li)as revealed by first-principles calculations makes it possible to use CuSe in future electronic devices.

Electrically manipulating magnetization reversal via energy band engineering

Realization of a magnetization reversal by an external electric field is vital for developing ultra-low-power spintronic devices.In this report,starting from energy band engineering,a general design principle is proposed for achieving electrical manipulation of a nonvolatile 180°magnetization reversal.A half semiconductor(HSC)and a bipolar magnetic semiconductor(BMS)are selected as the model of magnetic layers,whose conduction-band minimum and valence-band maximum are in the same and opposite spin states,respectively.Based on the analysis of virtual hopping and tight-binding models,the interlayer coupling of HSC/insulator/BMS devices is successfully tuned between ferromagnetic and antiferromagnetic interactions by varying electric field directions.Moreover,the interlayer coupling nearly disappears after removing the electric field,proving the nonvolatile magnetization reversal.Using first-principles calculations,the feasibility of present design strategy is further confirmed by a representative device with the structure of CrBr3/h-BN/2H-VSe_(2).This design guideline and physical phenomena may open an avenue to explore magnetoelectric coupling mechanisms and develop next-generation spintronic devices.

Interface and energy band manipulation of Bi2O3-Bi2S3 electrode enabling advanced magnesium-ion storage

Rechargeable magnesium-ion(Mg-ion)batteries have attracted wide attention for energy storage.However,magnesium anode is still limited by the irreversible Mg plating/stripping procedure.Herein,a well-designed binary Bi_(2)O_(3)-Bi_(2)S_(3)(BO-BS)heterostructure is fulfilled by virtue of the cooperative interface and energy band engineering targeted fast Mg-ion storage.The built-in electronic field resulting from the asymmetrical electron distribution at the interface of electron-rich S center at Bi_(2)S_(3) side and electron-poor O center at Bi_(2)O_(3) side effectively accelerates the electrochemical reaction kinetics in the Mg-ion battery system.Moreover,the as-designed heterogenous interface also benefits to maintaining the electrode integrity.With these advantages,the BO-BS electrode displays a remarkable capacity of 150.36 mAh g^(−1) at 0.67 A g^(-1) and a superior cycling stability.This investigation would offer novel insights into the rational design of functional heterogenous electrode materials targeted the fast reaction kinetics for energy storage systems.

Band structures of strained kagome lattices

Materials with kagome lattices have attracted significant research attention due to their nontrivial features in energy bands.We theoretically investigate the evolution of electronic band structures of kagome lattices in response to uniaxial strain using both a tight-binding model and an antidot model based on a periodic muffin-tin potential.It is found that the Dirac points move with applied strain.Furthermore,the flat band of unstrained kagome lattices is found to develop into a highly anisotropic shape under a stretching strain along y direction,forming a partially flat band with a region dispersionless along ky direction while dispersive along kx direction.Our results shed light on the possibility of engineering the electronic band structures of kagome materials by mechanical strain.

Energy band and charge-carrier engineering in skutterudite thermoelectric materials

The binary CoSb_(3) skutterudite thermoelectric material has high thermal conductivity due to the covalent bond between Co and Sb, and the thermoelectric figure of merit, ZT, is very low. The thermal conductivity of CoSb_(3) materials can be significantly reduced through phonon engineering, such as low-dimensional structure, the introduction of nano second phases,nanointerfaces or nanopores, which greatly improves their ZT values. The phonon engineering can optimize significantly the thermal transport properties of CoSb_(3)-based materials. However, the improvement of the electronic transport properties is not obvious, or even worse. Energy band and charge-carrier engineering can significantly improve the electronic transport properties of CoSb_(3)-based materials while optimizing the thermal transport properties. Therefore, the decoupling of thermal and electronic transport properties of CoSb_(3)-based materials can be realized by energy band and charge-carrier engineering. This review summarizes some methods of optimizing synergistically the electronic and thermal transport properties of CoSb_(3) materials through the energy band and charge-carrier engineering strategies. Energy band engineering strategies include band convergence or resonant energy levels caused by doping/filling. The charge-carrier engineering strategy includes the optimization of carrier concentration and mobility caused by doping/filling, forming modulation doped structures or introducing nano second phase. These strategies are effective means to improve performance of thermoelectric materials and provide new research ideas of development of high-efficiency thermoelectric materials.

Realizing high thermoelectric performance in hot-pressed polycrystalline Al_(x)Sn_(1-x) Se through band engineering tuning

SnSe-based thermoelectric materials are being explored since they have potential high thermoelectric figure of merit.We synthesized polycrystalline Al_(x)Sn_(1-x)Se(x=0.01,0.02,0.03 and 0.04)by hot-pressing method,and combined theoretical estimation with experimental measurement to investigate the in-fluence of Al doping on thermoelectric properties of SnSe.It was found that dopant Al can effectively adjust the band structure of SnSe by introducing intermediate band.Al doping with low content(x=0.01 and 0.02)can introduce a single intermediate band close to the valence band maximum or conduction band minimum,achieving band engineering optimization.In high temperature region(498 K<T<823 K),the electronic transport properties significantly enhance with thermal excitation.The lattice thermal conductivity reduces with the Al atomic point defect scattering,leading to a low thermal conductivity of 0.47 W m^(-1) K^(-1) in Al_(0.04)Sn_(0.96)Se at 823 K.As a result,a high ZT of 0.84 at 823 K is obtained from the Al_(0.04)Sn_(0.96) Se perpendicular to the pressing direction,which is 58.5%larger than that of SnSe.In addition,dopant Al can adjust the anisotropy of polycrystalline SnSe.The anisotropy of electronic properties are enhanced with low doping level(x=0.01,0.02)and suppressed with high doping level(x=0.03,0.04).

Enhancing thermoelectric performance of SrFBiS_(2-x)Se_(x) via band engineering and structural texturing

SrFBiS_(2) is a quaternary n-type semiconductor with rock-salt-type BiS_(2) and fluorite-type SrF layers alternately stacked along the c axis.The tunability of the crystal and electronic structures as well as the intrinsically low thermal conductivity make this compound a promising parent material for thermo-electric applications.In the current work,we show that alloying of Se and S in SrFBi_(S) 2 reduces the optical band gap with the second conduction band serving as an electron-transport medium,simultaneously increasing the electron concentration and effective mass.In addition,the raw material Bi_(2)Se_(3) is shown to act as liquid adjuvant during the annealing process,favoring preferred-orientation grain growth and forming strengthen microstructural texturing in bulk samples after hot-pressed sintering.Highly ordered lamellar grains are stacked perpendicular to the pressure direction,leading to enhanced mobility along this direction.The synthetic effect results in a maximum power factor of 5.58 μm W cm^(-1) K^(-2) at 523 K for SrFBiSSe and a peak zT=0.34 at 773 K,enhancements of 180%compared with those of pristine SrFBiS_(2).

Impact of Oxygen Vacancy on Band Structure Engineering of n-p Codoped Anatase TiO2

Doping with various impurities is an effective approach to improve the photoelectrochemical properties of TiO2. Here, we explore the effect of oxygen vacancy on geometric and elec- tronic properties of compensated （i.e. V-N and Cr-C） and non-compensated （i.e. V-C and Cr-N） codoped anatase TiO2 by performing extensive density functional theory calculations. Theoretical results show that oxygen vacancy prefers to the neighboring site of metal dopant （i.e. V or Cr atom）. After introduction of oxygen vacancy, the unoccupied impurity bands located within band gap of these codoped TiO2 will be filled with electrons, and the posi- tion of conduction band offset does not change obviously, which result in the reduction of photoinduced carrier recombination and the good performance for hydrogen production via water splitting. Moreover, we find that oxygen vacancy is easily introduced in V-N codoped TiO2 under O-poor condition. These theoretical insights are helpful for designing codoped TiO2 with high photoelectrochemical performance.

Band structure engineering in metal halide perovskite nanostructures for optoelectronic applications

Metal halide perovskite nanostructures have emerged as low-dimensional semiconductors of great significance in many fields such as photovoltaics,photonics,and optoelectronics.Extensive efforts on the controlled synthesis of perovskite nanostructures have been made towards potential device applications.The engineering of their band structures holds great promise in the rational tuning of the electronic and optical properties of perovskite nanostructures,which is one of the keys to achieving efficient and multifunctional optoelectronic devices.In this article,we summarize recent advances in band structure engineering of perovskite nanostructures.A survey of bandgap engineering of nanostructured perovskites is firstly presented from the aspects of dimensionality tailoring,compositional substitution,phase segregation and transition,as well as strain and pressure stimuli.The strategies of electronic doping are then reviewed,including defect-induced self-doping,inorganic or organic molecules-based chemical doping,and modification by metal ions or nanostructures.Based on the bandgap engineering and electronic doping,discussions on engineering energy band alignments in perovskite nanostructures are provided for building high-performance perovskite p-n junctions and heterostructures.At last,we provide our perspectives in engineering band structures of perovskite nanostructures towards future low-energy optoelectronics technologies.

Doping-induced metal–N active sites and bandgap engineering in graphitic carbon nitride for enhancing photocatalytic H_(2 )evolution performance

Durable and inexpensive graphitic carbon nitride(g-C_(3)N_(4))demonstrates great potential for achieving efficient photocatalytic hydrogen evolution reduction(HER).To further improve its activity,g-C_(3)N_(4)was subjected to atomic-level structural engineering by doping with transition metals(M=Fe,Co,or Ni),which simultaneously induced the formation of metal-N active sites in the g-C_(3)N_(4)framework and modulated the bandgap of g-C_(3)N_(4).Experiments and density functional theory calculations further verified that the as-formed metal-N bonds in M-doped g-C_(3)N_(4)acted as an"electron transfer bridge",where the migration of photo-generated electrons along the bridge enhanced the efficiency of separation of the photogenerated charges,and the optimized bandgap of g-C_(3)N_(4)afforded stronger reduction ability and wider light absorption.As a result,doping with either Fe,Co,or Ni had a positive effect on the HER activity,where Co-doped g-C_(3)N_(4)exhibited the highest performance.The findings illustrate that this atomic-level structural engineering could efficiently improve the HER activity and inspire the design of powerful photocatalysts.

Band gap engineering of atomically thin two-dimensional semiconductors

Atomically thin two-dimensional （2D） layered materials have potential applications in nanoelectronics, nanophoton- ics, and integrated optoelectronics. Band gap engineering of these 2D semiconductors is critical for their broad applications in high-performance integrated devices, such as broad-band photodetectors, multi-color light emitting diodes （LEDs）, and high-efficiency photovoltaic devices. In this review, we will summarize the recent progress on the controlled growth of composition modulated atomically thin 2D semiconductor alloys with band gaps tuned in a wide range, as well as their induced applications in broadly tunable optoelectronic components. The band gap engineered 2D semiconductors could open up an exciting opportunity for probing their fundamental physical properties in 2D systems and may find diverse applications in functional electronic/optoelectronic devices.

Progress on band structure engineering of twisted bilayer and two-dimensional moiré heterostructures

Artificially constructed van der Waals heterostructures(vdWHs)provide an ideal platform for realizing emerging quantum phenomena in condensed matter physics.Two methods for building vdWHs have been developed:stacking two-dimensional(2D)materials into a bilayer structure with different lattice constants,or with different orientations.The interlayer coupling stemming from commensurate or incommensurate superlattice pattern plays an important role in vdWHs for modulating the band structures and generating new electronic states.In this article,we review a series of novel quantum states discovered in two model vdWH systems—graphene/hexagonal boron nitride(hBN)hetero-bilayer and twisted bilayer graphene(tBLG),and discuss how the electronic structures are modified by such stacking and twisting.We also provide perspectives for future studies on hetero-bilayer materials,from which an expansion of 2D material phase library is expected.

Improved photovoltaic effects in Mn-doped BiFeO3 ferroelectric thin films through band gap engineering

As a low-bandgap ferroelectric material, BiFeO3 has gained wide attention for the potential photovoltaic applications,since its photovoltaic effect in visible light range was reported in 2009. In the present work, Bi（Fe, Mn）O3thin films are fabricated by pulsed laser deposition method, and the effects of Mn doping on the microstructure, optical, leakage,ferroelectric and photovoltaic characteristics of Bi（Fe, Mn）O3 thin films are systematically investigated. The x-ray diffraction data indicate that Bi（Fe, Mn）O3 thin films each have a rhombohedrally distorted perovskite structure. From the light absorption results, it follows that the band gap of Bi（Fe, Mn）O3 thin films can be tuned by doping different amounts of Mn content. More importantly, photovoltaic measurement demonstrates that the short-circuit photocurrent density and the open-circuit voltage can both be remarkably improved through doping an appropriate amount of Mn content, leading to the fascinating fact that the maximum power output of ITO/BiFe（0.7）Mn（0.3）O3/Nb-STO capacitor is about 175 times higher than that of ITO/BiFeO3/Nb-STO capacitor. The improvement of photovoltaic response in Bi（Fe, Mn）O3 thin film can be reasonably explained as being due to absorbing more visible light through bandgap engineering and maintaining the ferroelectric property at the same time.

Band gap modulation of nanostructured WO_(3) nanoplate film by Ti doping for enhanced photoelectrochemical performance

Despite being a promising photoanode material for water splitting,WO_(3) has low conductivity,high onset potential,and sluggish water oxidation kinetics.In this study,we designed Ti-doped WO_(3) nanoplate arrays on fluoride-doped tin oxide by a seed-free hydrothermal method,and the effects of doping on the photoelectrochemical performance were investigated.The optimal Ti-doped WO_(3) electrode achieved a photocurrent density of 0.53 mA/cm^(2) at 0.6 V(vs Ag/AgCl),110%higher than that of pure WO_(3) nanoplate arrays.Moreover,a significant cathodic shift in the onset potential was observed after doping.X-ray photoelectron spectroscopy valence band and ultraviolet–visible spectra revealed that the band positions of Ti-doped WO_(3) photoanodes moved upward,yielding a lower onset potential.Furthermore,electrochemical impedance spectroscopy measurements revealed that the conductivities of the WO_(3) photoanodes improved after doping,because of the rapid separation of photo-generated charge carriers.Thus,we report a new design route toward efficient and low-cost photoanodes for photoelectrochemical applications.

Halogen-Driven Bandgap Opening in Graphdiyne for Overall Photocatalytic Water Splitting

In this work,we studied the electronic band structure of the halogen(F,Cl,and Br)functionalized graphdiynes(GDYs)by using hybrid density functional theory.The results revealed that the bandgap energies of modified GDYs increase as the number of halogen atoms increases.It is also found that the position of the valence band maximum(VBM)is influenced by the electronegativity of halogen atoms.The higher the electronegativity,the deeper the VBM of the GDYs modified by the same number of halogen atoms.Importantly,our results revealed that the bandgap of GDY could be effectively tuned by mixing types of halogen atoms.The new generated conduction band and valence band edges are properly aligned with the oxidation and reduction potentials of water.Further thermodynamic analysis confirms that some models with mixing types of halogen atoms exhibit higher performance of overall photocatalytic water splitting than non-mixing models.This work provides useful insights for designing efficient photocatalysts that can be used for overall water splitting.

Enhancing thermoelectric performance in P-type Mg_(3)Sb_(2)-based Zintls through optimization of band gap structure and nanostructuring

P-type Mg_(3)Sb_(2)-based Zintls have attracted considerable interest in the thermoelectric(TE)field due to their environmental friendliness and low cost.However,compared to their n-type counterparts,they show relatively low TE performance,limiting their application in TE devices.In this work,we simultaneously introduce Bi alloying at Sb sites and Ag doping at Mg sites into the Mg_(3)Sb_(2)to coopera-tively optimize the electrical and thermal properties for the first time,acquiring the highest ZT value of∼0.85 at 723 K and a high average ZT of 0.39 in the temperature range of 323-723 K in sample Mg_(2.94)Ag_(0.06)Sb_(1.9)Bi_(0.1).The first-principle calculations show that the codoping of Ag and Bi can shift the Fermi level into the valence band and narrow the band gap,resulting in the increased carrier concentration from 3.50×10^(17)cm^(-3)in the reference Mg 3 Sb 0.9 Bi 0.1 to∼7.88×10^(19)cm^(-3)in sample Mg 2.94 Ag 0.06 Sb 0.9 Bi 0.1.As a result,a remarkable power factor of∼778.9μW m^(-1)K^(-2)at 723 K is achieved in sample Mg 2.94 Ag 0.06 Sb 0.9 Bi 0.1.Meanwhile,a low lattice thermal conductivity of∼0.48 W m^(-1)K^(-1)at 723 K is also obtained with the help of phonon scattering at the distorted lattice,point defects,and nano-precipitates in sample Mg 2.94 Ag 0.06 Sb 0.9 Bi 0.1.The synergistic effect of using the multi-element co-doping/-alloying to optimize electrical properties in Mg_(3)Sb_(2)holds promise for further improving the TE performance of Zintl phase materials or even others.

Solute manipulation enabled band and defect engineering for thermoelectric enhancements of SnTe

With years of development, SnTe as a homologue of PbTe has shown great potentialfor thermoelectric applications in p-type conduction, and the most successfulstrategy is typified by alloying for maximizing the valence band degeneracy.Among the known alloy agents, MnTe has been found to be one of the most effectiveenabling a band convergence for an enhancement in electronic performance ofSnTe, yet its solubility of only ~15 at% unfortunately prevents a full optimizationin the valence band structure. This work reveals that additional PbTe alloying notonly promotes the MnTe solubility to locate the optimal valence band structure butalso increases the overall substitutional defects in the material for a substantialreduction in lattice thermal conductivity. In addition, PbTe alloying simultaneouslyoptimizes the carrier concentration due to the cation size effect. These features allenabled by such a solute manipulation synergistically lead to a very high thermoelectricfigure of merit, zT of ~1.5 in SnTe with a 20 at% MnTe and a 30 at% PbTealloying (Sn0.5Mn0.2Pb0.3Te), demonstrating the effectiveness of solute manipulationfor advancing SnTe and similar thermoelectrics.

Direct bandgap engineering with local biaxial strain in few-layer MoS2 bubbles

Nano Research volume 13,pages2072–2078(2020)Cite this article 211 Accesses Metrics details Abstract Strain engineering provides an important strategy to modulate the optical and electrical properties of semiconductors for improving devices performance with mechanical force or thermal expansion difference.Here,we present the investigation of the local strain distribution over few-layer MoS2 bubbles,by using scanning photoluminescence and Raman spectroscopies.We observe the obvious direct bandgap emissions with strain in the few-layer MoS2 bubble and the strain-induced continuous energy shifts of both resonant excitons and vibrational modes from the edge of the MoS2 bubble to the center(10μm scale),associated with the oscillations resulted from the optical interference-induced temperature distribution.To understand these results,we perform ab initio simulations to calculate the electronic and vibrational properties of few-layer MoS2 with biaxial tensile strain,based on density functional theory,finding good agreement with the experimental results.Our study suggests that local strain offers a convenient way to continuously tune the physical properties of a few-layer transition metal dichalcogenides(TMDs)semiconductor,and opens up new possibilities for band engineering within the 2D plane.