We report a facile phase conversion method that can locally convert n-type SnSe_(2)into p-type SnSe by direct laser irradiation.Raman spectra of SnSe_(2)flakes before and after laser irradiation confirm the phase conv...We report a facile phase conversion method that can locally convert n-type SnSe_(2)into p-type SnSe by direct laser irradiation.Raman spectra of SnSe_(2)flakes before and after laser irradiation confirm the phase conversion of SnSe_(2)to SnSe.By performing the laser irradiation on SnSe_(2)flakes at different temperatures,it is found that laser heating effect induces the removal of Se atoms from SnSe_(2)and results in the phase conversion of SnSe_(2)to SnSe.Lattice-revolved transmission electron microscope images of SnSe_(2)flakes before and after laser irradiation further confirm such conversion.By selective laser irradiation on SnSe_(2)flakes,a pattern with SnSe_(2)/SnSe heteostructures is created.This indicates that the laser induced phase conversion technique has relatively high spatial resolution and enables the creation of micron-sized in-plane p-n junction at predefined region.展开更多
Furazan macrocyclic compound 3,4:7,8:11,12:15,16-tetrafurazan-1,9-dioxazo-5,13-diazocyclohexadecane(DOATF)is an ideal energetic material with high heat of formation.Here,using scanning tunneling microscopy(STM)and non...Furazan macrocyclic compound 3,4:7,8:11,12:15,16-tetrafurazan-1,9-dioxazo-5,13-diazocyclohexadecane(DOATF)is an ideal energetic material with high heat of formation.Here,using scanning tunneling microscopy(STM)and noncontact atomic force microscopy(nc-AFM),we investigated the adsorption structure of DOATF molecules on Au(111)surface,which shows the four furanzan rings in the STM images and a bright protrusion off the center of the molecule in the nc-AFM images.Combined with density functional theory(DFT)calculations,we confirmed that the bright feature in the nc-AFM images is an N-O coordinate bond pointing upwards in one of the two azoxy groups;while the other N-O bond pointing towards the Au(111)surface.Our work contributes for a deeper understanding of the adsorption structure of macrocyclic compounds,which would promote the designing of DOATF-metal frameworks.展开更多
Two-dimensional honeycomb lattices show great potential in the realization of Dirac nodal line fermions(DNLFs).Here,we successfully synthesized a gold telluride(AuTe)monolayer by direct tellurizing an Au(111)substrate...Two-dimensional honeycomb lattices show great potential in the realization of Dirac nodal line fermions(DNLFs).Here,we successfully synthesized a gold telluride(AuTe)monolayer by direct tellurizing an Au(111)substrate.Low energy electron diffraction measurements reveal that it is(2×2)AuTe layer stacked onto(3×3)Au(111)substrate.Moreover,scanning tunneling microscopy images show that the AuTe layer has a honeycomb structure.Scanning transmission electron microscopy reveals that it is a single-atom layer.In addition,first-principles calculations demonstrate that the honeycomb AuTe monolayer exhibits Dirac nodal line features protected by mirror symmetry,which is validated by angle-resolved photoemission spectra.Our results establish that monolayer AuTe can be a good candidate to investigate 2D DNLFs and provides opportunities to realize high-speed low-dissipation devices.展开更多
Two-dimensional(2 D) materials provide a platform to exploit the novel physical properties of functional nanodevices.Here, we report on the formation of a new 2 D layered material, a well-ordered monolayer TiTe_2, on ...Two-dimensional(2 D) materials provide a platform to exploit the novel physical properties of functional nanodevices.Here, we report on the formation of a new 2 D layered material, a well-ordered monolayer TiTe_2, on a Au(111) surface by molecular beam epitaxy(MBE). Low-energy electron diffraction(LEED) measurements of the samples indicate that the TiTe_2 film forms(√3 ×√7) superlattice with respect to the Au(111) substrate, which has three different orientations. Scanning tunneling microscopy(STM) measurements clearly show three ordered domains consistent with the LEED patterns.Density functional theory(DFT) calculations further confirm the formation of 2 H-TiTe_2 monolayer on the Au(111) surface with Te as buffer layer. The fabrication of this 2 D layered heterostructure expands 2 D material database, which may bring new physical properties for future applications.展开更多
Silicene is a promising 2D Dirac material as a building block for van der Waals heterostructures(vd WHs). Here we investigate the electronic properties of hexagonal boron nitride/silicene(BN/Si) vd WHs using first-pri...Silicene is a promising 2D Dirac material as a building block for van der Waals heterostructures(vd WHs). Here we investigate the electronic properties of hexagonal boron nitride/silicene(BN/Si) vd WHs using first-principles calculations.We calculate the energy band structures of BN/Si/BN heterostructures with different rotation angles and find that the electronic properties of silicene are retained and protected robustly by the BN layers. In BN/Si/BN/Si/BN heterostructure, we find that the band structure near the Fermi energy is sensitive to the stacking configurations of the silicene layers due to interlayer coupling. The coupling is reduced by increasing the number of BN layers between the silicene layers and becomes negligible in BN/Si/(BN)_3/Si/BN. In(BN)_n/Si superlattices, the band structure undergoes a conversion from Dirac lines to Dirac points by increasing the number of BN layers between the silicene layers. Calculations of silicene sandwiched by other 2D materials reveal that silicene sandwiched by low-carbon-doped boron nitride or HfO_2 is semiconducting.展开更多
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 flex...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.展开更多
Nanographenes(NGs)can be embedded with predesigned dopants or nonhexagonal rings to tailor the electronic properties and provide ideal platforms to study the unique physical and chemical properties.Here,we report the ...Nanographenes(NGs)can be embedded with predesigned dopants or nonhexagonal rings to tailor the electronic properties and provide ideal platforms to study the unique physical and chemical properties.Here,we report the on-surface synthesis of NBN-doped NG embedded with five-and seven-membered rings(NBN-575-NG)on Au(111)from a oligophenylene precursor preinstalled with a NBN unit and a heptagonal ring.Scanning tunneling microscopy and non-contact atomic force microscopy images elucidate the intramolecular cyclodehydrogenation and the existence of the five-and seven-membered rings.Scanning tunneling spectroscopy spectra reveal that the NBN-575-NG is a semiconductor,which agrees with the density functional theory calculation results on a freestanding NBN-575-NG with the same structure.This work provides a feasible approach for the on-surface synthesis of novel NGs containing non-hexagonal rings.展开更多
The linkage structures between monomers make great influence on the properties of polymers.The synthesis of some special linkage structures can be challenging,which is often overcome by employing special reaction cond...The linkage structures between monomers make great influence on the properties of polymers.The synthesis of some special linkage structures can be challenging,which is often overcome by employing special reaction conditions.Here,we build dihydropentalene linkage in poly-naphthalocyanine on Ag(110)surface.Scanning tunneling microscopy(STM)and non-contact atomic force microscopy(nc-AFM)measurements confirm the dihydropentalene linkage structure and a possible formation path with reconstruction steps is proposed.The controlled experiment on Ag(100)surface shows no dihydropentalene structures formed,which indicates the grooved substrate is necessary for the reconstruction.This work provides insights into the surface restricted reactions that can yield special structures in organic polymers.展开更多
The on-surface synthesis from predesigned organic precursors can yield graphene nanoribbons(GNRs)with atomically precise widths,edge terminations and dopants,which facilitate the tunning of their electronic structures...The on-surface synthesis from predesigned organic precursors can yield graphene nanoribbons(GNRs)with atomically precise widths,edge terminations and dopants,which facilitate the tunning of their electronic structures.Here,we report the synthesis of novel sulfur-doped cove-edged GNRs(S-CGNRs)on Au(111)from a specifically designed precursor containing thiophene rings.Scanning tunneling microscopy and non-contact atomic force microscopy measurements elucidate the formation of S-CGNRs through subsequent polymerization and cyclodehydrogenation,which further result in crosslinked branched structures.Scanning tunneling spectroscopy results reveal the conduction band minimum of the S-CGNR locates at 1.2 e V.First-principles calculations show that the S-CGNR possesses an energy bandgap of 1.17 e V,which is evidently smaller than that of an undoped cove-edged GNR(1.7 e V),suggesting effective tuning of the bandgap by introducing sulfur atoms.Further increasing the coverage of precursors close to a monolayer results in the formation of linear-shaped S-CGNRs.The fabrication of S-CGNRs provides one more candidate in the GNR toolbox and promotes the future applications of heteroatom-doped graphene nanostructures.展开更多
Epitaxial growth on transition metal surfaces is an effective way to prepare large-area and high-quality graphene.However,the strong interaction between graphene and metal substrates suppresses the intrinsic excellent...Epitaxial growth on transition metal surfaces is an effective way to prepare large-area and high-quality graphene.However,the strong interaction between graphene and metal substrates suppresses the intrinsic excellent properties of graphene and the conductive metal substrates also hinder its applications in electronics.Here we demonstrate the decoupling of graphene from metal substrates by germanium oxide intercalation.Germanium is firstly intercalated into the interface between graphene and Ir(111) substrate.Then oxygen is subsequently intercalated,leading to the formation of a GeO_(x) layer,which is confirmed by x-ray photoelectron spectroscopy.Low-energy electron diffraction and scanning tunneling microscopy studies show intact carbon lattice of graphene after the GeO_(x) intercalation.Raman characterizations reveal that the intercalated layer effectively decouples graphene from the Ir substrate.The transport measurements demonstrate that the GeO_(x) layer can act as a tunneling barrier in the fabricated large-area high-quality vertical graphene/GeO_(x)/Ir heterostructure.展开更多
Bulk iridium ditelluride(IrTe2)is a layered material and is known for its interesting electronic and structural properties,such as large spin-orbit coupling,charge ordering,and superconductivity.However,so far there ...Bulk iridium ditelluride(IrTe2)is a layered material and is known for its interesting electronic and structural properties,such as large spin-orbit coupling,charge ordering,and superconductivity.However,so far there is no experimental study about the fabrication of monolayer IrTe2.Here we report the formation of IrTe2 monolayer on Ir(111)substrate by direct tellurization method.Scanning tunneling microscope(STM)images show the coexistence of 1/5 phase and 1/6 phase structures of IrTe2 at room temperature.We also obtained STM images showing distorted stripe feature under low temperatures.This stripe feature is possibly induced by the strain between the IrTe2 monolayer and the metal substrate.Density functional theory(DFT)calculations show that the IrTe2 monolayer has strong interaction with the underlying Ir(111)substrate.展开更多
Two-dimensional(2D)materials received large amount of studies because of the enormous potential in basic science and industrial applications.Monolayer Pd2Se3 is a fascinating 2D material that was predicted to possess ...Two-dimensional(2D)materials received large amount of studies because of the enormous potential in basic science and industrial applications.Monolayer Pd2Se3 is a fascinating 2D material that was predicted to possess excellent thermoelectric,electronic,transport,and optical properties.However,the fabrication of large-scale and high-quality monolayer Pd2Se3 is still challenging.Here,we report the synthesis of large-scale and high-quality monolayer Pd2Se3 on graphene-SiC(0001)by a two-step epitaxial growth.The atomic structure of Pd2Se3 was investigated by scanning tunneling microscope(STM)and confirmed by non-contact atomic force microscope(nc-AFM).Two subgroups of Se atoms have been identified by nc-AFM image in agreement with the theoretically predicted atomic structure.Scanning tunneling spectroscopy(STS)reveals a bandgap of 1.2 eV,suggesting that monolayer Pd2Se3 can be a candidate for photoelectronic applications.The atomic structure and defect levels of a single Se vacancy were also investigated.The spatial distribution of STS near the Se vacancy reveals a highly anisotropic electronic behavior.The two-step epitaxial synthesis and characterization of Pd2Se3 provide a promising platform for future investigations and applications.展开更多
Nonvolatile electrical control of spin polarization in two-dimensional(2D)magnetic semiconductors is greatly appealing toward future low-dissipation spintronic nanodevices.Here,we report a 2D material VSeF,which is an...Nonvolatile electrical control of spin polarization in two-dimensional(2D)magnetic semiconductors is greatly appealing toward future low-dissipation spintronic nanodevices.Here,we report a 2D material VSeF,which is an intrinsic bipolar magnetic semiconductor(BMS)featured with opposite spin-polarized valence and conduction band edges.We then propose a general nonvolatile strategy to manipulate both spin-polarized orientations in BMS materials by introducing a ferroelectric gate with proper band alignment.The spin-up/spin-down polarization of VSeF is successfully controlled by the electric dipole of ferroelectric bilayer Al_(2)Se_(3),verifying the feasibility of the design strategy.The interfacial doping effect from ferroelectric gate also plays a role in enhancing the Curie temperature of the VSeF layer.Two types of spin field effect transistors,namely multiferroic memory and spin filter,are further achieved in VSeF/Al_(2)Se_(3) and VSeF/Al_(2)Se_(3)/Al_(2)Se_(3) multiferroic heterostructures,respectively.This work will stimulate the application of 2D BMS materials in future spintronic nanodevices.展开更多
Excitons dominate the photonic and optoelectronic properties of a material.Although significant advancements exist in understanding various types of excitons,progress on excitons that are indirect in both real-and mom...Excitons dominate the photonic and optoelectronic properties of a material.Although significant advancements exist in understanding various types of excitons,progress on excitons that are indirect in both real-and momentum-spaces is still limited.Here,we demonstrate the real-and momentum-indirect neutral and charged excitons(including their phonon replicas)in a multi-valley semiconductor of bilayer MoS_(2),by performing electric-field/doping-density dependent photoluminescence.Together with first-principles calculations,we uncover that the observed real-and momentum-indirect exciton involves electron/hole from K/Γvalley,solving the longstanding controversy of its momentum origin.Remarkably,the binding energy of real-and momentum-indirect charged exciton is extremely large(i.e.,~59 meV),more than twice that of real-and momentum-direct charged exciton(i.e.,~24 meV).The giant binding energy,along with the electrical tunability and long lifetime,endows real-and momentum-indirect excitons an emerging platform to study many-body physics and to illuminate developments in photonics and optoelectronics.展开更多
Two-dimensional semiconductors(2DSCs)with appropriate band gaps and high mobilities are highly desired for future-generation electronic and optoelectronic applications.Here,using first-principles calculations,we repor...Two-dimensional semiconductors(2DSCs)with appropriate band gaps and high mobilities are highly desired for future-generation electronic and optoelectronic applications.Here,using first-principles calculations,we report a novel class of 2DSCs,group-11-chalcogenide monolayers(M_(2)X,M=Cu,Ag,Au;X=S,Se,Te),featuring with a broad range of energy band gaps and high carrier mobilities.Their energy band gaps extend from 0.49 to 3.76 eV at a hybrid density functional level,covering from ultraviolet-A,visible light to near-infrared region,which are crucial for broadband photoresponse.Significantly,the calculated room-temperature carrier mobilities of the M_(2)X monolayers are as high as thousands of cm^(2)·V^(-1)·s^(-1).Particularly,the carrier mobilities ofε-Au_(2)Se and e-Au2Te are up to 104 cm^(2)·V^(-1)·s^(-1),which is very attracitive for electronic devices.Benefitting from the broad range of energy band gaps and superior carrier mobilities,the group-11-chalcogenide M_(2)X monolayers are promising candidates for future-generation nanoelectronics and optoelectronics.展开更多
As a new type of iron-based superconductor, CaKFe_(4)As_(4) has recently been demonstrated to be a promising platform for observing Majorana zero modes (MZMs). The surface of CaKFe_(4)As_(4) plays an important role in...As a new type of iron-based superconductor, CaKFe_(4)As_(4) has recently been demonstrated to be a promising platform for observing Majorana zero modes (MZMs). The surface of CaKFe_(4)As_(4) plays an important role in realizing the MZM since it hosts superconducting topological surface states. However, due to the complicated crystal structure, the terminal surface of CaKFe_(4)As_(4) has not been determined yet. Here, by using scanning tunneling microscopy/spectroscopy (STM/S), we find that there are two types of surface structure in CaKFe_(4)As_(4). Bias-dependent atomic resolution images show an evolvement from 2–√×2–√ superstructure with respect to the As lattice into 1 × 1 when the tip is brought close to the surface, revealing the sublattice of missing As atoms. Together with the first-principles calculations, we show that the surface As layer has a buckled structure. Our findings provide insight to future surface study of CaKFe_(4)As_(4) as well as other iron-pnictide superconductors.展开更多
Two-dimensional(2D)superconducting states have attracted much recent interest,especially when they coexist with nontrivial band topology which affords a promising approach towards Majorana fermions.Using first-princip...Two-dimensional(2D)superconducting states have attracted much recent interest,especially when they coexist with nontrivial band topology which affords a promising approach towards Majorana fermions.Using first-principles calculations,we predict van der Waals monolayered transition-metal monohalides MX(M=Zr,Mo;X=F,Cl)as a class of 2D superconductors with remarkable transition temperature(5.9–12.4 K).Anisotropic Migdal-Eliashberg theory reveals that ZrCl have a single superconducting gap∆~2.14 meV,while MoCl is a two-gap superconductor with∆~1.96 and 1.37 meV.The Z_(2)band topology of 2D MX is further demonstrated that MoF and MoCl are candidates for realizing topological superconductivity.Moreover,the Dirac phonons of ZrCl and MoCl contribute w-shape phononic edge states,which are potential for an edge-enhanced electron-phonon coupling.These findings demonstrate that 2D MX offers an attractive platform for exploring the interplay between superconductivity,nontrivial electronic and phononic topology.展开更多
Magnetic materials could realize the intriguing quantum anomalous Hall effect and metal-to-insulator transition when combined with band topology or electronic correlation,which have broad prospects in quantum informat...Magnetic materials could realize the intriguing quantum anomalous Hall effect and metal-to-insulator transition when combined with band topology or electronic correlation,which have broad prospects in quantum information,spintronics,and valleytronics.Here,we propose the approach of designing novel two-dimensional(2D)magnetic states via d-orbital-based superatomic lattices.Specifically,we chose triangular zirconium dichloride disks as superatoms to construct the honeycomb superatomic lattices.Using first-principles calculations,we identified a series of 2D magnetic states with varying sizes of superatoms.We found the non-uniform stoichiometries and geometric effect of superatomic lattice give rise to spin-polarized charges arranged in different magnetic configurations,containing ferromagnetic coloring triangles,antiferromagnetic honeycomb,and ferromagnetic kagome lattices.Attractively,these magnetic states are endowed with nontrivial band topology or strong correlation,forming an ideal Chern insulator or antiferromagnetic Dirac Mott insulator.Our work not only reveals the potential of d-orbital-based superatoms for generating unusual magnetic configurations,but also supplies a new avenue for material engineering at the nanoscale.展开更多
Zigzag graphene nanoribbons(ZGNRs)with spin-polarized edge states have potential applications in carbon-based spintronics.The electronic structure of ZGNRs can be effectively tuned by different widths or dopants,which...Zigzag graphene nanoribbons(ZGNRs)with spin-polarized edge states have potential applications in carbon-based spintronics.The electronic structure of ZGNRs can be effectively tuned by different widths or dopants,which requires delicately designed monomers.Here,we report the successful synthesis of ZGNR with a width of eight carbon zigzag lines and nitrogen-boronnitrogen(NBN)motifs decorated along the zigzag edges(NBN-8-ZGNR)on Au(111)surface,which starts from a specially designed U-shaped monomer with preinstalled NBN units at the zigzag edge.Chemical-bond-resolved non-contact atomic force microscopy(nc-AFM)imaging confirms the zigzag-terminated edges and the existence of NBN dopants.The electronic states distributed along the zigzag edges have been revealed after a silicon-layer intercalation at the interface of NBN-8-ZGNR and Au(111).Our work enriches the ZGNR family with a new dopant and larger width,which provides more candidates for future carbonbased nanoelectronic and spintronic applications.展开更多
基金supported by the National Key Research&Development Project of China(Grant Nos.2016YFA0202300 and 2018FYA0305800)the National Natural Science Foundation of China(Grant No.61888102)+1 种基金Strategic Priority Research Program of Chinese Academy of Sciences(Grant Nos.XDB30000000 and XDB28000000)Youth Innovation Promotion Association of Chinese Academy of Sciences(Grant No.Y201902)。
文摘We report a facile phase conversion method that can locally convert n-type SnSe_(2)into p-type SnSe by direct laser irradiation.Raman spectra of SnSe_(2)flakes before and after laser irradiation confirm the phase conversion of SnSe_(2)to SnSe.By performing the laser irradiation on SnSe_(2)flakes at different temperatures,it is found that laser heating effect induces the removal of Se atoms from SnSe_(2)and results in the phase conversion of SnSe_(2)to SnSe.Lattice-revolved transmission electron microscope images of SnSe_(2)flakes before and after laser irradiation further confirm such conversion.By selective laser irradiation on SnSe_(2)flakes,a pattern with SnSe_(2)/SnSe heteostructures is created.This indicates that the laser induced phase conversion technique has relatively high spatial resolution and enables the creation of micron-sized in-plane p-n junction at predefined region.
基金the National Key Research and Development Projects of China(Grant No.2019YFA0308500)the National Natural Science Foundation of China(Grant No.61888102)the Funds from the Chinese Academy of Sciences(Grant Nos.XDB30000000 and YSBR-003)。
文摘Furazan macrocyclic compound 3,4:7,8:11,12:15,16-tetrafurazan-1,9-dioxazo-5,13-diazocyclohexadecane(DOATF)is an ideal energetic material with high heat of formation.Here,using scanning tunneling microscopy(STM)and noncontact atomic force microscopy(nc-AFM),we investigated the adsorption structure of DOATF molecules on Au(111)surface,which shows the four furanzan rings in the STM images and a bright protrusion off the center of the molecule in the nc-AFM images.Combined with density functional theory(DFT)calculations,we confirmed that the bright feature in the nc-AFM images is an N-O coordinate bond pointing upwards in one of the two azoxy groups;while the other N-O bond pointing towards the Au(111)surface.Our work contributes for a deeper understanding of the adsorption structure of macrocyclic compounds,which would promote the designing of DOATF-metal frameworks.
基金Project supported by the National Key R&D Program of China (Grant No.2018YFA0305800)the National Natural Science Foundation of China (Grant Nos.61925111,61888102,and 52102193)+2 种基金the Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos.XDB28000000 and XDB30000000)CAS Project for Young Scientists in Basic Research (Grant No.YSBR-003)the Fundamental Research Funds for the Central Universities。
文摘Two-dimensional honeycomb lattices show great potential in the realization of Dirac nodal line fermions(DNLFs).Here,we successfully synthesized a gold telluride(AuTe)monolayer by direct tellurizing an Au(111)substrate.Low energy electron diffraction measurements reveal that it is(2×2)AuTe layer stacked onto(3×3)Au(111)substrate.Moreover,scanning tunneling microscopy images show that the AuTe layer has a honeycomb structure.Scanning transmission electron microscopy reveals that it is a single-atom layer.In addition,first-principles calculations demonstrate that the honeycomb AuTe monolayer exhibits Dirac nodal line features protected by mirror symmetry,which is validated by angle-resolved photoemission spectra.Our results establish that monolayer AuTe can be a good candidate to investigate 2D DNLFs and provides opportunities to realize high-speed low-dissipation devices.
基金Project supported by the National Key Research&Development Program of China(Grant Nos.2016YFA0202300 and 2018YFA0305800)the National Natural Science Foundation of China(Grant Nos.61504149,61725107,51572290,and 61622116)+2 种基金the Strategic Priority Research Program of Chinese Academy of Sciences(Grant Nos.XDB30000000 and XDB28000000)the University of Chinese Academy of Sciencesthe CAS Key Laboratory of Vacuum Physics
文摘Two-dimensional(2 D) materials provide a platform to exploit the novel physical properties of functional nanodevices.Here, we report on the formation of a new 2 D layered material, a well-ordered monolayer TiTe_2, on a Au(111) surface by molecular beam epitaxy(MBE). Low-energy electron diffraction(LEED) measurements of the samples indicate that the TiTe_2 film forms(√3 ×√7) superlattice with respect to the Au(111) substrate, which has three different orientations. Scanning tunneling microscopy(STM) measurements clearly show three ordered domains consistent with the LEED patterns.Density functional theory(DFT) calculations further confirm the formation of 2 H-TiTe_2 monolayer on the Au(111) surface with Te as buffer layer. The fabrication of this 2 D layered heterostructure expands 2 D material database, which may bring new physical properties for future applications.
基金Project supported by the National Key Research and Development Program of China(Grant No.2016YFA0202300)the National Natural Science Foundation of China(Grant Nos.61390501 and 61471337)+2 种基金the National Basic Research Program of China(Grant No.2013CBA01600)the CAS Pioneer Hundred Talents Programthe Beijing Nova Program,China(Grant No.Z181100006218023)
文摘Silicene is a promising 2D Dirac material as a building block for van der Waals heterostructures(vd WHs). Here we investigate the electronic properties of hexagonal boron nitride/silicene(BN/Si) vd WHs using first-principles calculations.We calculate the energy band structures of BN/Si/BN heterostructures with different rotation angles and find that the electronic properties of silicene are retained and protected robustly by the BN layers. In BN/Si/BN/Si/BN heterostructure, we find that the band structure near the Fermi energy is sensitive to the stacking configurations of the silicene layers due to interlayer coupling. The coupling is reduced by increasing the number of BN layers between the silicene layers and becomes negligible in BN/Si/(BN)_3/Si/BN. In(BN)_n/Si superlattices, the band structure undergoes a conversion from Dirac lines to Dirac points by increasing the number of BN layers between the silicene layers. Calculations of silicene sandwiched by other 2D materials reveal that silicene sandwiched by low-carbon-doped boron nitride or HfO_2 is semiconducting.
基金Project supported by the National Key Research and Development Program of China(Grant No.2016YFA0202300)the National Natural Science Foundation of China(Grant No.61390501)+3 种基金the National Basic Research Program of China(Grant No.2013CBA01600)Strategic Priority Research Program(B) of Chinese Academy of Sciences(Grant Nos.XDPB0601 and XDPB08-1)the CAS Pioneer Hundred Talents ProgramBeijing Nova Program,China(Grant No.Z181100006218023)
文摘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.
基金the National Natural Science Foundation of China(Grant Nos.51761135130 and 61888102)the National Key R&D Program of China(Grant Nos.2018YFA0305800 and 2019YFA0308500)+3 种基金DFG EnhanceNano(Grant No.391979941)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB30000000)the International Partnership Program of Chinese Academy of Sciences(Grant NO.112111KYSB20160061)the K.C.Wong Education Foundation.Part of the research was performed in the Key Laboratory of Vacuum Physics,Chinese Academy of Sciences.Computational resources were provided by the National Supercomputing Center in Tianjin Municipality,China.
文摘Nanographenes(NGs)can be embedded with predesigned dopants or nonhexagonal rings to tailor the electronic properties and provide ideal platforms to study the unique physical and chemical properties.Here,we report the on-surface synthesis of NBN-doped NG embedded with five-and seven-membered rings(NBN-575-NG)on Au(111)from a oligophenylene precursor preinstalled with a NBN unit and a heptagonal ring.Scanning tunneling microscopy and non-contact atomic force microscopy images elucidate the intramolecular cyclodehydrogenation and the existence of the five-and seven-membered rings.Scanning tunneling spectroscopy spectra reveal that the NBN-575-NG is a semiconductor,which agrees with the density functional theory calculation results on a freestanding NBN-575-NG with the same structure.This work provides a feasible approach for the on-surface synthesis of novel NGs containing non-hexagonal rings.
基金the National Natural Science Foundation of China(Grant No.61888102)the National Key Research and Development Program of China(Grant Nos.2018YFA0305800 and 2019YFA0308500)+1 种基金Chinese Academy of Sciences(Grant Nos.XDB30000000,YSBR003,and 112111KYSB20160061)the Fundamental Research Funds for the Central Universities,China.
文摘The linkage structures between monomers make great influence on the properties of polymers.The synthesis of some special linkage structures can be challenging,which is often overcome by employing special reaction conditions.Here,we build dihydropentalene linkage in poly-naphthalocyanine on Ag(110)surface.Scanning tunneling microscopy(STM)and non-contact atomic force microscopy(nc-AFM)measurements confirm the dihydropentalene linkage structure and a possible formation path with reconstruction steps is proposed.The controlled experiment on Ag(100)surface shows no dihydropentalene structures formed,which indicates the grooved substrate is necessary for the reconstruction.This work provides insights into the surface restricted reactions that can yield special structures in organic polymers.
基金the National Natural Science Foundation of China(Grant Nos.51761135130,61888102,and 21774076)the National Key Research and Development Program of China(Grant Nos.2018YFA0305800 and 2019YFA0308500)+3 种基金the DFG Enhance Nano(Grant No.391979941)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant Nos.XDB30000000)the International Partnership Program of Chinese Academy of Sciences(Grant No.112111KYSB20160061)the K C Wong Education Foundation and the Program of Shanghai Academic Research Leader(Grant No.19XD1421700)。
文摘The on-surface synthesis from predesigned organic precursors can yield graphene nanoribbons(GNRs)with atomically precise widths,edge terminations and dopants,which facilitate the tunning of their electronic structures.Here,we report the synthesis of novel sulfur-doped cove-edged GNRs(S-CGNRs)on Au(111)from a specifically designed precursor containing thiophene rings.Scanning tunneling microscopy and non-contact atomic force microscopy measurements elucidate the formation of S-CGNRs through subsequent polymerization and cyclodehydrogenation,which further result in crosslinked branched structures.Scanning tunneling spectroscopy results reveal the conduction band minimum of the S-CGNR locates at 1.2 e V.First-principles calculations show that the S-CGNR possesses an energy bandgap of 1.17 e V,which is evidently smaller than that of an undoped cove-edged GNR(1.7 e V),suggesting effective tuning of the bandgap by introducing sulfur atoms.Further increasing the coverage of precursors close to a monolayer results in the formation of linear-shaped S-CGNRs.The fabrication of S-CGNRs provides one more candidate in the GNR toolbox and promotes the future applications of heteroatom-doped graphene nanostructures.
基金Project supported by the National Key Research&Development Program of China(Grant Nos.2019YFA0308500,2016YFA0202300,and 2018YFA0305800)the National Natural Science Foundation of China(Grant Nos.61888102,61925111,and 21661132006)+1 种基金the Strategic Priority Research Program of Chinese Academy of Sciences(Grant Nos.XDB30000000 and XDB28000000)the CAS Key Laboratory of Vacuum Physics。
文摘Epitaxial growth on transition metal surfaces is an effective way to prepare large-area and high-quality graphene.However,the strong interaction between graphene and metal substrates suppresses the intrinsic excellent properties of graphene and the conductive metal substrates also hinder its applications in electronics.Here we demonstrate the decoupling of graphene from metal substrates by germanium oxide intercalation.Germanium is firstly intercalated into the interface between graphene and Ir(111) substrate.Then oxygen is subsequently intercalated,leading to the formation of a GeO_(x) layer,which is confirmed by x-ray photoelectron spectroscopy.Low-energy electron diffraction and scanning tunneling microscopy studies show intact carbon lattice of graphene after the GeO_(x) intercalation.Raman characterizations reveal that the intercalated layer effectively decouples graphene from the Ir substrate.The transport measurements demonstrate that the GeO_(x) layer can act as a tunneling barrier in the fabricated large-area high-quality vertical graphene/GeO_(x)/Ir heterostructure.
基金Project supported by the National Key Research&Development Project of China(Grant Nos.2019YFA0308500,2018YFA0305800,and 2016YFA0202300)the National Natural Science Foundation of China(Grant Nos.51991340,61888102,and 11888101)the Chinese Academy of Sciences(Grant Nos.XDB28000000 and XDB30000000).
文摘Bulk iridium ditelluride(IrTe2)is a layered material and is known for its interesting electronic and structural properties,such as large spin-orbit coupling,charge ordering,and superconductivity.However,so far there is no experimental study about the fabrication of monolayer IrTe2.Here we report the formation of IrTe2 monolayer on Ir(111)substrate by direct tellurization method.Scanning tunneling microscope(STM)images show the coexistence of 1/5 phase and 1/6 phase structures of IrTe2 at room temperature.We also obtained STM images showing distorted stripe feature under low temperatures.This stripe feature is possibly induced by the strain between the IrTe2 monolayer and the metal substrate.Density functional theory(DFT)calculations show that the IrTe2 monolayer has strong interaction with the underlying Ir(111)substrate.
基金Project supported by the National Key Research and Development Program of China(Grant Nos.2016YFA0202300,2018YFA0305800,and 2019YFA0308500)the National Natural Science Foundation of China(Grant Nos.51922011,51872284,and 61888102)+1 种基金the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant Nos.XDB30000000 and XDB28000000)the Science Fund from University of the Chinese Academy of Sciences.
文摘Two-dimensional(2D)materials received large amount of studies because of the enormous potential in basic science and industrial applications.Monolayer Pd2Se3 is a fascinating 2D material that was predicted to possess excellent thermoelectric,electronic,transport,and optical properties.However,the fabrication of large-scale and high-quality monolayer Pd2Se3 is still challenging.Here,we report the synthesis of large-scale and high-quality monolayer Pd2Se3 on graphene-SiC(0001)by a two-step epitaxial growth.The atomic structure of Pd2Se3 was investigated by scanning tunneling microscope(STM)and confirmed by non-contact atomic force microscope(nc-AFM).Two subgroups of Se atoms have been identified by nc-AFM image in agreement with the theoretically predicted atomic structure.Scanning tunneling spectroscopy(STS)reveals a bandgap of 1.2 eV,suggesting that monolayer Pd2Se3 can be a candidate for photoelectronic applications.The atomic structure and defect levels of a single Se vacancy were also investigated.The spatial distribution of STS near the Se vacancy reveals a highly anisotropic electronic behavior.The two-step epitaxial synthesis and characterization of Pd2Se3 provide a promising platform for future investigations and applications.
基金This work was supported by grants from the National Natural Science Foundation of China(61888102,52102193,52272172,11974045)the Major Program of National Natural Science Foundation of China(92163206)+2 种基金the National Key Research and Development Program of China(2021YFA1201501)the Strategic Priority Research Program of the Chinese Academy of Sciences(XDB30000000)the Fundamental Research Funds for the Central Universities.Computational resources were provided by the National Supercomputing Center in Tianjin.
文摘Nonvolatile electrical control of spin polarization in two-dimensional(2D)magnetic semiconductors is greatly appealing toward future low-dissipation spintronic nanodevices.Here,we report a 2D material VSeF,which is an intrinsic bipolar magnetic semiconductor(BMS)featured with opposite spin-polarized valence and conduction band edges.We then propose a general nonvolatile strategy to manipulate both spin-polarized orientations in BMS materials by introducing a ferroelectric gate with proper band alignment.The spin-up/spin-down polarization of VSeF is successfully controlled by the electric dipole of ferroelectric bilayer Al_(2)Se_(3),verifying the feasibility of the design strategy.The interfacial doping effect from ferroelectric gate also plays a role in enhancing the Curie temperature of the VSeF layer.Two types of spin field effect transistors,namely multiferroic memory and spin filter,are further achieved in VSeF/Al_(2)Se_(3) and VSeF/Al_(2)Se_(3)/Al_(2)Se_(3) multiferroic heterostructures,respectively.This work will stimulate the application of 2D BMS materials in future spintronic nanodevices.
基金This work was supported by the National Natural Science Foundation of China(NSFC)(12274447,61888102,11834017,61734001,and 12074412)the National Key Research and Development Program(2021YFA1202900 and 2021YFA1400502)+1 种基金the Strategic Priority Research Program of Chinese Academy of Sciences(XDB30000000)the Key-Area Research and Development Program of Guangdong Province(2020B0101340001).
文摘Excitons dominate the photonic and optoelectronic properties of a material.Although significant advancements exist in understanding various types of excitons,progress on excitons that are indirect in both real-and momentum-spaces is still limited.Here,we demonstrate the real-and momentum-indirect neutral and charged excitons(including their phonon replicas)in a multi-valley semiconductor of bilayer MoS_(2),by performing electric-field/doping-density dependent photoluminescence.Together with first-principles calculations,we uncover that the observed real-and momentum-indirect exciton involves electron/hole from K/Γvalley,solving the longstanding controversy of its momentum origin.Remarkably,the binding energy of real-and momentum-indirect charged exciton is extremely large(i.e.,~59 meV),more than twice that of real-and momentum-direct charged exciton(i.e.,~24 meV).The giant binding energy,along with the electrical tunability and long lifetime,endows real-and momentum-indirect excitons an emerging platform to study many-body physics and to illuminate developments in photonics and optoelectronics.
基金supported by the National Natural Science Foundation of China(No.61888102)the National Key Research and Development Projects of China(No.2016YFA0202300)+1 种基金the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDB30000000)the Fundamental Research Funds for the Central Universities.
文摘Two-dimensional semiconductors(2DSCs)with appropriate band gaps and high mobilities are highly desired for future-generation electronic and optoelectronic applications.Here,using first-principles calculations,we report a novel class of 2DSCs,group-11-chalcogenide monolayers(M_(2)X,M=Cu,Ag,Au;X=S,Se,Te),featuring with a broad range of energy band gaps and high carrier mobilities.Their energy band gaps extend from 0.49 to 3.76 eV at a hybrid density functional level,covering from ultraviolet-A,visible light to near-infrared region,which are crucial for broadband photoresponse.Significantly,the calculated room-temperature carrier mobilities of the M_(2)X monolayers are as high as thousands of cm^(2)·V^(-1)·s^(-1).Particularly,the carrier mobilities ofε-Au_(2)Se and e-Au2Te are up to 104 cm^(2)·V^(-1)·s^(-1),which is very attracitive for electronic devices.Benefitting from the broad range of energy band gaps and superior carrier mobilities,the group-11-chalcogenide M_(2)X monolayers are promising candidates for future-generation nanoelectronics and optoelectronics.
基金We thank Min Ouyang and Wu Zhou for helpful discussion. This work is supported by the National Key Research and Development Program of China (Nos. 2019YFA0308500 and 2018YFA0305800)the National Natural Science Foundation of China (Nos. 51922011, 51991340, and 61888102)+1 种基金the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDB28000000)A portion of the research was performed in CAS Key Laboratory of Vacuum Physics. G. H. C. is supported by Funds for the Central Universities and the National Key Research and Development Program of China (Nos. 2019FZA3004, 2017YFA0303002, and 2016YFA0300202).
文摘As a new type of iron-based superconductor, CaKFe_(4)As_(4) has recently been demonstrated to be a promising platform for observing Majorana zero modes (MZMs). The surface of CaKFe_(4)As_(4) plays an important role in realizing the MZM since it hosts superconducting topological surface states. However, due to the complicated crystal structure, the terminal surface of CaKFe_(4)As_(4) has not been determined yet. Here, by using scanning tunneling microscopy/spectroscopy (STM/S), we find that there are two types of surface structure in CaKFe_(4)As_(4). Bias-dependent atomic resolution images show an evolvement from 2–√×2–√ superstructure with respect to the As lattice into 1 × 1 when the tip is brought close to the surface, revealing the sublattice of missing As atoms. Together with the first-principles calculations, we show that the surface As layer has a buckled structure. Our findings provide insight to future surface study of CaKFe_(4)As_(4) as well as other iron-pnictide superconductors.
基金This work was supported by the National Key Research and Development Program of China(Grant Nos.2020YFA0308800)the National Natural Science Foundation of China(Grant No.11974045,61888102)+3 种基金Chinese Academy of Sciences(Grant Nos.XDB30000000)CAS Project for Young Scientists in Basic Research(YSBR-003)Beijing Institute of Technology Research Fund Program for Young Scholars(Grants No.3050011181909)the Fundamental Research Funds for the Central Universities.F.L.was supported by DOE-BES(Grant No.DE-FG02-04ER46148).
文摘Two-dimensional(2D)superconducting states have attracted much recent interest,especially when they coexist with nontrivial band topology which affords a promising approach towards Majorana fermions.Using first-principles calculations,we predict van der Waals monolayered transition-metal monohalides MX(M=Zr,Mo;X=F,Cl)as a class of 2D superconductors with remarkable transition temperature(5.9–12.4 K).Anisotropic Migdal-Eliashberg theory reveals that ZrCl have a single superconducting gap∆~2.14 meV,while MoCl is a two-gap superconductor with∆~1.96 and 1.37 meV.The Z_(2)band topology of 2D MX is further demonstrated that MoF and MoCl are candidates for realizing topological superconductivity.Moreover,the Dirac phonons of ZrCl and MoCl contribute w-shape phononic edge states,which are potential for an edge-enhanced electron-phonon coupling.These findings demonstrate that 2D MX offers an attractive platform for exploring the interplay between superconductivity,nontrivial electronic and phononic topology.
基金This work was supported by the National Basic Research Program of China (No.2013CBA01603),the National Natural Science Foundation of China (No.61335006),and Chinese Academy of Sciences (Nos.1731300500015 and XDB07030100).
基金supported in part by the Key R&D of the Ministry of Science and Technology(No.2022YFA1204103).
文摘Magnetic materials could realize the intriguing quantum anomalous Hall effect and metal-to-insulator transition when combined with band topology or electronic correlation,which have broad prospects in quantum information,spintronics,and valleytronics.Here,we propose the approach of designing novel two-dimensional(2D)magnetic states via d-orbital-based superatomic lattices.Specifically,we chose triangular zirconium dichloride disks as superatoms to construct the honeycomb superatomic lattices.Using first-principles calculations,we identified a series of 2D magnetic states with varying sizes of superatoms.We found the non-uniform stoichiometries and geometric effect of superatomic lattice give rise to spin-polarized charges arranged in different magnetic configurations,containing ferromagnetic coloring triangles,antiferromagnetic honeycomb,and ferromagnetic kagome lattices.Attractively,these magnetic states are endowed with nontrivial band topology or strong correlation,forming an ideal Chern insulator or antiferromagnetic Dirac Mott insulator.Our work not only reveals the potential of d-orbital-based superatoms for generating unusual magnetic configurations,but also supplies a new avenue for material engineering at the nanoscale.
基金The work was supported by grants from the National Key Research and Development Program of China(No.2019YFA0308500)the National Natural Science Foundation of China(No.61888102)+5 种基金the Chinese Academy of Sciences(Nos.XDB30000000 and YSBR-003)the EU Graphene Flagship(Graphene Core 3,No.881603)the H2020-MSCA-ITN(ULTIMATE,No.813036)the Center for Advancing Electronics Dresden(CfAED)the H2020-EU.1.2.2.-FET Proactive Grant(LIGHT-CAP,No.101017821)the DFG-SNSF Joint Switzerland-German Research Project(EnhanTopo,No.429265950).
文摘Zigzag graphene nanoribbons(ZGNRs)with spin-polarized edge states have potential applications in carbon-based spintronics.The electronic structure of ZGNRs can be effectively tuned by different widths or dopants,which requires delicately designed monomers.Here,we report the successful synthesis of ZGNR with a width of eight carbon zigzag lines and nitrogen-boronnitrogen(NBN)motifs decorated along the zigzag edges(NBN-8-ZGNR)on Au(111)surface,which starts from a specially designed U-shaped monomer with preinstalled NBN units at the zigzag edge.Chemical-bond-resolved non-contact atomic force microscopy(nc-AFM)imaging confirms the zigzag-terminated edges and the existence of NBN dopants.The electronic states distributed along the zigzag edges have been revealed after a silicon-layer intercalation at the interface of NBN-8-ZGNR and Au(111).Our work enriches the ZGNR family with a new dopant and larger width,which provides more candidates for future carbonbased nanoelectronic and spintronic applications.