Two-dimensional(2D) magnetic crystals have attracted great attention due to their emerging new physical phenomena. They provide ideal platforms to study the fundamental physics of magnetism in low dimensions. In this ...Two-dimensional(2D) magnetic crystals have attracted great attention due to their emerging new physical phenomena. They provide ideal platforms to study the fundamental physics of magnetism in low dimensions. In this research,magnetic tunneling junctions(MTJs) based on XSe2(X = Mn, V) with room-temperature ferromagnetism were studied using first-principles calculations. A large tunneling magnetoresistance(TMR) of 725.07% was obtained in the MTJs based on monolayer MnSe2. Several schemes were proposed to improve the TMR of these devices. Moreover, the results of our non-equilibrium transport calculations showed that the large TMR was maintained in these devices under a finite bias.The transmission spectrum was analyzed according to the orbital components and the electronic structure of the monolayer XSe2(X = Mn, V). The results in this paper demonstrated that the MTJs based on a 2D ferromagnet with room-temperature ferromagnetism exhibited reliable performance. Therefore, such devices show the possibility for potential applications in spintronics.展开更多
Using self-consistent calculations of million-atom SchrSdinger-Poisson equations, we investigate the I-V characteristics of tunnelling and ballistic transport of nanometer metal oxide semiconductor field effect transi...Using self-consistent calculations of million-atom SchrSdinger-Poisson equations, we investigate the I-V characteristics of tunnelling and ballistic transport of nanometer metal oxide semiconductor field effect transistors （MOSFET） based on a full 3-D quantum mechanical simulation under nonequilibtium condition. Atomistic empirical pseudopotentials are used to describe the device Hamiltonian and the underlying bulk band structure. We find that the ballistic transport dominates the I-V characteristics, whereas the effects of tunnelling cannot be neglected with the maximal value up to 0.8mA/μm when the channel length of MOSFET scales down to 25 nm. The effects of tunnelling transport lower the threshold voltage Vt. The ballistic current based on fully 3-D quantum mechanical simulation is relatively large and has small on-off ratio compared with results derived from the calculation methods of Luo et al.展开更多
Topological phase transition in a single material usually refers to transitions between a trivial band insulator and a topological Dirac phase, and the transition may also occur between different classes of topologica...Topological phase transition in a single material usually refers to transitions between a trivial band insulator and a topological Dirac phase, and the transition may also occur between different classes of topological Dirac phases.It is a fundamental challenge to realize quantum transition between Z_2 nontrivial topological insulator（TI） and topological crystalline insulator（TCI） in one material because Z_2 TI and TCI have different requirements on the number of band inversions. The Z_2 TIs must have an odd number of band inversions over all the time-reversal invariant momenta, whereas the newly discovered TCIs, as a distinct class of the topological Dirac materials protected by the underlying crystalline symmetry, owns an even number of band inversions. Taking PbSnTe_2 alloy as an example, here we demonstrate that the atomic-ordering is an effective way to tune the symmetry of the alloy so that we can electrically switch between TCI phase and Z_2 TI phase in a single material. Our results suggest that the atomic-ordering provides a new platform towards the realization of reversibly switching between different topological phases to explore novel applications.展开更多
Metal oxides play an essential role in modern optoelectronic devices because they have many unique physical properties such as structure diversity, superb stability in solution, good catalytic activity, and simultaneo...Metal oxides play an essential role in modern optoelectronic devices because they have many unique physical properties such as structure diversity, superb stability in solution, good catalytic activity, and simultaneous high electron conductivity and optical transmission. Therefore, they are widely used in energy-related optoelectronic applications such as photovoltaics and photoelectrochemical（PEC） fuel generation. In this review, we mainly discuss the structure engineering and defect control of oxides for energy applications, especially for transparent conducting oxides（TCOs） and oxide catalysts used for water splitting. We will review our current understanding with an emphasis on the contributions of our previous theoretical modeling, primarily based on density functional theory. In particular, we highlight our previous work：（i） the fundamental principles governing the crystal structures and the electrical and optical behaviors of TCOs;（ii） band structures and defect properties for n-type TCOs;（iii） why p-type TCOs are difficult to achieve;（iv） how to modify the band structure to achieve p-type TCOs or even bipolarly dopable TCOs;（v） the origin of the high-performance of amorphous TCOs; and（vi） band structure engineering of bulk and nano oxides for PEC water splitting. Based on the understanding above, we hope to clarify the key issues and the challenges facing the rational design of novel oxides and propose new and feasible strategies or models to improve the performance of existing oxides or design new oxides that are critical for the development of next-generation energy-related applications.展开更多
First-principles approaches have recently been developed to replace the phenomenological modeling approaches with adjustable parameters for calculating carrier mobilities in semiconductors.However,in addition to the h...First-principles approaches have recently been developed to replace the phenomenological modeling approaches with adjustable parameters for calculating carrier mobilities in semiconductors.However,in addition to the high computational cost,it is still a challenge to obtain accurate mobility for carriers with a complex band structure,e.g.,hole mobility in common semiconductors.Here,we present a computationally efficient approach using isotropic and parabolic bands to approximate the anisotropy valence bands for evaluating group velocities in the first-principles calculations.This treatment greatly reduces the computational cost in two ways:relieves the requirement of an extremely denseκmesh to obtain a smooth change in group velocity,and reduces the 5-dimensional integral to 3-dimensional integral.Taking Si and SiC as two examples,we find that this simplified approach reproduces the full first-principles calculation for mobility.If we use experimental effective masses to evaluate the group velocity,we can obtain hole mobility in excellent agreement with experimental data over a wide temperature range.These findings shed light on how to improve the first-principles calculations towards predictive carrier mobility in high accuracy.展开更多
Introducing ferromagnetism into non-magnetic systems without the participation of magnetic elements is promising for all-electric spintronic devices[1,2].Many approaches have been pursued,such as non-magnetic defects ...Introducing ferromagnetism into non-magnetic systems without the participation of magnetic elements is promising for all-electric spintronic devices[1,2].Many approaches have been pursued,such as non-magnetic defects induced magnetization in layered materials[3–5]or the inversion symmetry breaking induced magnetization in magic-angle bilayer graphene[6–8],etc.However,these approaches have to tackle with the localization effects or the inevitable precise control of twist angle,which hinders the future application into large-scale spintronic information devices.Theorists also predicted that the spontaneous ferromagnetism could emerge in the quasi-2D crystals[9]like GaSe,but no experimental results have been reported.Here,we report the spontaneous ferromagnetism induced by van Hove singularity[9–13]in non-magnetic groupⅣGe_(1–x)Sn_(x)alloys grown by the molecular beam epitaxy(MBE)technique.Our findings experimentally open up an opportunity to realize spintronics in groupⅣsemiconductors.展开更多
Recent years have seen swift increase in the power conversion efficiency of perovskite solar cells(PSCs)Interface engineering is a promising route for further improving the performance of PSCs.Here we perform firstpri...Recent years have seen swift increase in the power conversion efficiency of perovskite solar cells(PSCs)Interface engineering is a promising route for further improving the performance of PSCs.Here we perform firstprinciples calculations to explore the effect of four candidate buffer materials(MACl,MAI,PbCl2and PbI2)on the electronic structures of the interface between MAPbI3absorber and TiO2.We find that MAX(X=Cl,I)as buffer layers will introduce a high electron barrier and enhance the electronhole recombination.Additionally,MAX does not passivate the surface states well.The conduction band minimum of PbI2is much lower than that of MAPbI3absorber,which significantly limits the band bending of the absorber and open-circuit voltage of solar cells.On the other side,suitable bandedge energy level positions,small lattice mismatch with TiO2surfaces,and excellent surface passivation make PbCl2a promising buffer material for absorber/electron-transport-layer interface engineering in PSCs.Our results in this work thus provide deep understanding on the effects of interface engineering with a buffer layer,which shall be useful for improving the performance of PSCs and related optoelectronics.展开更多
基金Project supported by the National Natural Science Foundation of China(Grant Nos.61571415 and 61622406)the National Key Research and Development Program of China(Grant No.2017YFA0207500)+1 种基金the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB30000000)Beijing Academy of Quantum Information Sciences,China(Grant No.Y18G04)
文摘Two-dimensional(2D) magnetic crystals have attracted great attention due to their emerging new physical phenomena. They provide ideal platforms to study the fundamental physics of magnetism in low dimensions. In this research,magnetic tunneling junctions(MTJs) based on XSe2(X = Mn, V) with room-temperature ferromagnetism were studied using first-principles calculations. A large tunneling magnetoresistance(TMR) of 725.07% was obtained in the MTJs based on monolayer MnSe2. Several schemes were proposed to improve the TMR of these devices. Moreover, the results of our non-equilibrium transport calculations showed that the large TMR was maintained in these devices under a finite bias.The transmission spectrum was analyzed according to the orbital components and the electronic structure of the monolayer XSe2(X = Mn, V). The results in this paper demonstrated that the MTJs based on a 2D ferromagnet with room-temperature ferromagnetism exhibited reliable performance. Therefore, such devices show the possibility for potential applications in spintronics.
基金Supported by the National Basic Research Program of China under Grant No G2009CB929300, and the National Natural Science Foundation of China under Grant Nos 60521001 and 60776061.
文摘Using self-consistent calculations of million-atom SchrSdinger-Poisson equations, we investigate the I-V characteristics of tunnelling and ballistic transport of nanometer metal oxide semiconductor field effect transistors （MOSFET） based on a full 3-D quantum mechanical simulation under nonequilibtium condition. Atomistic empirical pseudopotentials are used to describe the device Hamiltonian and the underlying bulk band structure. We find that the ballistic transport dominates the I-V characteristics, whereas the effects of tunnelling cannot be neglected with the maximal value up to 0.8mA/μm when the channel length of MOSFET scales down to 25 nm. The effects of tunnelling transport lower the threshold voltage Vt. The ballistic current based on fully 3-D quantum mechanical simulation is relatively large and has small on-off ratio compared with results derived from the calculation methods of Luo et al.
基金Supported by the Major State Basic Research Development Program of China under Grant No 2016YFB0700700the National Natural Science Foundation of China(NSFC)under Grants Nos 11634003,11474273,61121491 and U1530401+1 种基金supported by the National Young 1000 Talents Plansupported by the Youth Innovation Promotion Association of CAS(2017154)
文摘Topological phase transition in a single material usually refers to transitions between a trivial band insulator and a topological Dirac phase, and the transition may also occur between different classes of topological Dirac phases.It is a fundamental challenge to realize quantum transition between Z_2 nontrivial topological insulator（TI） and topological crystalline insulator（TCI） in one material because Z_2 TI and TCI have different requirements on the number of band inversions. The Z_2 TIs must have an odd number of band inversions over all the time-reversal invariant momenta, whereas the newly discovered TCIs, as a distinct class of the topological Dirac materials protected by the underlying crystalline symmetry, owns an even number of band inversions. Taking PbSnTe_2 alloy as an example, here we demonstrate that the atomic-ordering is an effective way to tune the symmetry of the alloy so that we can electrically switch between TCI phase and Z_2 TI phase in a single material. Our results suggest that the atomic-ordering provides a new platform towards the realization of reversibly switching between different topological phases to explore novel applications.
基金Project supported by the National Key Research and Development Program of China(Grant No.2016YFB0700700)the Science Challenge Project,China(Grant No.TZ20160003)+1 种基金the National Natural Science Foundation of China(Grant Nos.51672023,11474273,11634003,and U1530401)supported by the Youth Innovation Promotion Association of Chinese Academy of Sciences(Grant No.2017154)
文摘Metal oxides play an essential role in modern optoelectronic devices because they have many unique physical properties such as structure diversity, superb stability in solution, good catalytic activity, and simultaneous high electron conductivity and optical transmission. Therefore, they are widely used in energy-related optoelectronic applications such as photovoltaics and photoelectrochemical（PEC） fuel generation. In this review, we mainly discuss the structure engineering and defect control of oxides for energy applications, especially for transparent conducting oxides（TCOs） and oxide catalysts used for water splitting. We will review our current understanding with an emphasis on the contributions of our previous theoretical modeling, primarily based on density functional theory. In particular, we highlight our previous work：（i） the fundamental principles governing the crystal structures and the electrical and optical behaviors of TCOs;（ii） band structures and defect properties for n-type TCOs;（iii） why p-type TCOs are difficult to achieve;（iv） how to modify the band structure to achieve p-type TCOs or even bipolarly dopable TCOs;（v） the origin of the high-performance of amorphous TCOs; and（vi） band structure engineering of bulk and nano oxides for PEC water splitting. Based on the understanding above, we hope to clarify the key issues and the challenges facing the rational design of novel oxides and propose new and feasible strategies or models to improve the performance of existing oxides or design new oxides that are critical for the development of next-generation energy-related applications.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11925407 and 61927901)the Key Research Program of Frontier Sciences,Chinese Academy of Sciences(Grant No.ZDBS-LY-JSC019).
文摘First-principles approaches have recently been developed to replace the phenomenological modeling approaches with adjustable parameters for calculating carrier mobilities in semiconductors.However,in addition to the high computational cost,it is still a challenge to obtain accurate mobility for carriers with a complex band structure,e.g.,hole mobility in common semiconductors.Here,we present a computationally efficient approach using isotropic and parabolic bands to approximate the anisotropy valence bands for evaluating group velocities in the first-principles calculations.This treatment greatly reduces the computational cost in two ways:relieves the requirement of an extremely denseκmesh to obtain a smooth change in group velocity,and reduces the 5-dimensional integral to 3-dimensional integral.Taking Si and SiC as two examples,we find that this simplified approach reproduces the full first-principles calculation for mobility.If we use experimental effective masses to evaluate the group velocity,we can obtain hole mobility in excellent agreement with experimental data over a wide temperature range.These findings shed light on how to improve the first-principles calculations towards predictive carrier mobility in high accuracy.
基金supported by the National Natural Science Foundation of China(62125404,62004193,61922077,12274456,92163206,51991342,and 11874347)the Strategic Priority Research Program of Chinese Academy of Sciences(XDB43000000)+3 种基金the Guangdong Major Project of Basic and Applied Basic Research(2021B0301030002)the National Key R&D Program of China(2021YFA1400502 and 2022YFA1405600)supported by the Youth Innovation Promotion Association of Chinese Academy of Sciences(Y2021042)the Nanofabrication Laboratory in the National Centre for Nanoscience and Technology for the electron beam lithography。
基金the Key-Area Research and Development Program of Guangdong Province(2020B0303060001,and 2018B030327001)the National Natural Science Foundation of China(61874109,61922077,12004158,and 12074162)+1 种基金the National Key Research and Development Program of China(2018YFB2200100,and 2020YFA0309300)Guangdong Provincial Key Laboratory(2019B121203002)。
文摘Introducing ferromagnetism into non-magnetic systems without the participation of magnetic elements is promising for all-electric spintronic devices[1,2].Many approaches have been pursued,such as non-magnetic defects induced magnetization in layered materials[3–5]or the inversion symmetry breaking induced magnetization in magic-angle bilayer graphene[6–8],etc.However,these approaches have to tackle with the localization effects or the inevitable precise control of twist angle,which hinders the future application into large-scale spintronic information devices.Theorists also predicted that the spontaneous ferromagnetism could emerge in the quasi-2D crystals[9]like GaSe,but no experimental results have been reported.Here,we report the spontaneous ferromagnetism induced by van Hove singularity[9–13]in non-magnetic groupⅣGe_(1–x)Sn_(x)alloys grown by the molecular beam epitaxy(MBE)technique.Our findings experimentally open up an opportunity to realize spintronics in groupⅣsemiconductors.
基金financially supported by the National Natural Science Foundation of China(11804058,61571415,11674310 and 61622406)the financial support from RIE2020 AME Programmatic Grant A18A1b0045 funded by A*STARSERC,Singaporethe supports from the Agency for Science,Technology and Research(A*STAR)。
文摘Recent years have seen swift increase in the power conversion efficiency of perovskite solar cells(PSCs)Interface engineering is a promising route for further improving the performance of PSCs.Here we perform firstprinciples calculations to explore the effect of four candidate buffer materials(MACl,MAI,PbCl2and PbI2)on the electronic structures of the interface between MAPbI3absorber and TiO2.We find that MAX(X=Cl,I)as buffer layers will introduce a high electron barrier and enhance the electronhole recombination.Additionally,MAX does not passivate the surface states well.The conduction band minimum of PbI2is much lower than that of MAPbI3absorber,which significantly limits the band bending of the absorber and open-circuit voltage of solar cells.On the other side,suitable bandedge energy level positions,small lattice mismatch with TiO2surfaces,and excellent surface passivation make PbCl2a promising buffer material for absorber/electron-transport-layer interface engineering in PSCs.Our results in this work thus provide deep understanding on the effects of interface engineering with a buffer layer,which shall be useful for improving the performance of PSCs and related optoelectronics.
