Ce-doped titanium oxide nanoparticles were investigated in the paper. The surface structures of undoped and Ce-doped TiO2 nanoparticles were observed by scanning tunneling microscopy （STM）. The experimental results ...Ce-doped titanium oxide nanoparticles were investigated in the paper. The surface structures of undoped and Ce-doped TiO2 nanoparticles were observed by scanning tunneling microscopy （STM）. The experimental results of scanning tunneling spectroscopy （STS） show that the surface electronic structures of TiO2 nanoparticles are modified by introducing new electronic states in the surface band gap through cerium ion doping. The results are discussed in terms of the influence of doping concentration on the surface band gap of TiO2.展开更多
Growth and electronic properties of ultrathin Ga films on Cd(0001) are investigated by low-temperature scanning tunneling microscopy(STM) and density functional theory(DFT) calculations. It is found that Ga films exhi...Growth and electronic properties of ultrathin Ga films on Cd(0001) are investigated by low-temperature scanning tunneling microscopy(STM) and density functional theory(DFT) calculations. It is found that Ga films exhibit the epitaxial growth with the pseudomorphic 1×1 lattice. The Ga islands deposited at 100 K show a ramified shape due to the suppressed edge diffusion and corner crossing. Furthermore, the majority of Ga islands reveal flat tops and a preferred height of three atomic layers, indicating the electronic growth at low temperature. Annealing to room temperature leads to not only the growth mode transition from electronic growth to conventional Stranski–Krastanov growth, but also the shape transition from ramified islands to smooth compact islands. Scanning tunneling spectroscopy(STS) measurements reveal that the Ga monolayer exhibits metallic behavior. DFT calculations indicate that all the interfacial Ga atoms occupy the energetically favorable hcp-hollow sites of the substrate. The charge density difference analysis demonstrates that the charge transfer from the Cd substrate to the Ga atoms is negligible, and there is weak interaction between Ga atoms and the Cd substrate. These results shall shed important light on fabrication of ultrathin Ga films on metal substrates with novel physical properties.展开更多
When electrons are confined in a two-dimensional （2D） system, typical quantum-mechanical phenonl- ena such as Landau quantization can be detected. Graphene systems, including the single atomic layer and few-layer st...When electrons are confined in a two-dimensional （2D） system, typical quantum-mechanical phenonl- ena such as Landau quantization can be detected. Graphene systems, including the single atomic layer and few-layer stacked crystals, are ideal 2D materials for studying a variety of quantum-mechanical problems. In this article, we review the experimental progress in the unusual Landau quantized behav- iors of Dirac fernlions in monolayer and multilayer graphene by using scanning tunneling microscopy （STM） and scanning tulmeling spectroscopy （STS）. Through STS measurement of the strong mag- netic fields, distinct Landau-level spectra and rich level-splitting phenomena are observed in different graphene layers. These unique properties provide an effective method for identifying the number of layers, as well as the stacking orders, and investigating the fllndamentally physical phenomena of graphene. Moreover, in the presence of a strain and charged defects, the Landau quantization of graphene can be significantly modified, leading to unusual spectroscopic and electronic properties.展开更多
Revealing the structural/electronic features and interfacial interactions of monolayer MoS2 and WS2 on metals is essential to evaluating the performance of related devices.In this study,we focused on the atomic-scale ...Revealing the structural/electronic features and interfacial interactions of monolayer MoS2 and WS2 on metals is essential to evaluating the performance of related devices.In this study,we focused on the atomic-scale features of monolayer WS2 on Au（001） synthesized via chemical vapor deposition.Scanning tunneling microscopy and spectroscopy reveal that the WS2/Au（001） system exhibits a striped superstructure similar to that of MoS2/Au（001） but weaker interfacial interactions,as evidenced by experimental and theoretical investigations.Specifically,the WS2/Au（001） band gap exhibits a relatively intrinsic value of ～ 2.0 eV.However,the band gap can gradually decrease to ～ 1.5 eV when the sample annealing temperature increases from ～370 to 720 ℃.In addition,the doping level （or Fermi energy） of monolayer WS2/Au（001） varies little over the valley and ridge regions of the striped patterns because of the homogenous distributions of point defects introduced by annealing.Briefly,this work provides an in-depth investigation into the interfacial interactions and electronic properties of monolayer MX2 on metal substrates.展开更多
In recent years,topological quantum materials(TQMs)have attracted intensive attention in the area of condensed matter physics due to their novel topologies and their promising applications in quantum computing,spin el...In recent years,topological quantum materials(TQMs)have attracted intensive attention in the area of condensed matter physics due to their novel topologies and their promising applications in quantum computing,spin electronics and next-generation integrated circuits.Scanning tunneling microscopy/spectroscopy(STM/STS)is regarded as a powerful technique to characterize the local density of states with atomic resolution,which is ideally suited to the measurement of the bulk-boundary correspondence of TQMs.In this review,using STM/STS,we focus on recent research on bismuth-based TQMs,including quantum-spin Hall insulators,3D weak topological insulators(TIs),high-order TIs,topological Dirac semi-metals and dual TIs.Efficient methods for the modulation of the topological properties of the TQMs are introduced,such as interlayer interaction,thickness variation and local electric field perturbation.Finally,the challenges and prospects for this field of study are discussed.展开更多
Scanning tunneling microscopy/spectroscopy(STM/STS)at 4.8 K has been used to examine the growth of a double-decker bis(phthalocyaninato)yttrium(YP_(c2))molecule on a reconstructed Au(111)substrate.Local differential c...Scanning tunneling microscopy/spectroscopy(STM/STS)at 4.8 K has been used to examine the growth of a double-decker bis(phthalocyaninato)yttrium(YP_(c2))molecule on a reconstructed Au(111)substrate.Local differential conductance spectra(dI/dV)of a single YPc2 molecule allow the characteristics of the highest occupied molecular orbital(HOMO)and the lowest unoccupied molecular orbital(LUMO)to be identified.Furthermore,lateral distributions of the local density of states(LDOS)have also been obtained by dI/dV mapping and confirmed by first principles simulations.These electronic feature mappings and theoretical calculations provide a basis for understanding the unique STM morphology of YPc2,which is usually imaged as an eight-lobed structure.In addition,we demonstrate that bias-dependent STM morphologies and simultaneous dI/dV maps can provide a way of understanding the stability of two-dimensional YP_(c2) films.展开更多
Two-dimensional(2D)semiconductors,such as lead selenide(PbSe),locate at the key position of next-generation devices.However,the ultrathin PbSe is still rarely reported experimentally,particularly on metal substrates.H...Two-dimensional(2D)semiconductors,such as lead selenide(PbSe),locate at the key position of next-generation devices.However,the ultrathin PbSe is still rarely reported experimentally,particularly on metal substrates.Here,we report the ultrathin PbSe synthesized via sequential molecular beam epitaxy on Ag(111).The scanning tunneling microscopy is used to resolve the atomic structure and confirms the selective formation of ultrathin PbSe through the reaction between Ag5Se2 and Pb,as further evidenced by the theoretical calculation.It is also found that the increased accumulation of Pb leads to the improved quality of PbSe with larger and more uniform films.The detailed analysis demonstrates the bilayer structure of synthesized PbSe,which could be deemed to achieve the 2D limit.The differential conductance spectrum reveals a metallic feature of the PbSe film,indicating a certain interaction between PbSe and Ag(111).Moreover,the moirépattern originated from the lattice mismatch between PbSe and Ag(111)is observed,and this moirésystem provides the opportunity for studying physics under periodical modulation and for device applications.Our work illustrates a pathway to selectively synthesize ultrathin PbSe on metal surfaces and suggests a 2D experimental platform to explore PbSe-based opto-electronic and thermoelectric phenomena.展开更多
Interlayer coupling in layered semiconductors can significantly affect their optoelectronic properties.However,understanding the mechanisms behind the interlayer coupling at the atomic level is not straightforward.Her...Interlayer coupling in layered semiconductors can significantly affect their optoelectronic properties.However,understanding the mechanisms behind the interlayer coupling at the atomic level is not straightforward.Here,we study modulations of the electronic structure induced by the interlayer coupling in theγ-phase of indium selenide(γ-InSe)using scanning probe techniques.We observe a strong dependence of the energy gap on the sample thickness and a small effective mass along the stacking direction,which are attributed to strong interlayer coupling.In addition,the moirépatterns observed inγ-InSe display a small band-gap variation and nearly constant local differential conductivity along the patterns.This suggests that modulation of the electronic structure induced by the moirépotential is smeared out,indicating the presence of a significant interlayer coupling.Our theoretical calculations confirm that the interlayer coupling inγ-InSe is not only of the van der Waals origin,but also exhibits some degree of hybridization between the layers.Strong interlayer coupling might play an important role in the performance ofγ-InSe-based devices.展开更多
文摘Ce-doped titanium oxide nanoparticles were investigated in the paper. The surface structures of undoped and Ce-doped TiO2 nanoparticles were observed by scanning tunneling microscopy （STM）. The experimental results of scanning tunneling spectroscopy （STS） show that the surface electronic structures of TiO2 nanoparticles are modified by introducing new electronic states in the surface band gap through cerium ion doping. The results are discussed in terms of the influence of doping concentration on the surface band gap of TiO2.
基金Project supported by the National Natural Science Foundation of China (Grant Nos.11874304 and 11574253)。
文摘Growth and electronic properties of ultrathin Ga films on Cd(0001) are investigated by low-temperature scanning tunneling microscopy(STM) and density functional theory(DFT) calculations. It is found that Ga films exhibit the epitaxial growth with the pseudomorphic 1×1 lattice. The Ga islands deposited at 100 K show a ramified shape due to the suppressed edge diffusion and corner crossing. Furthermore, the majority of Ga islands reveal flat tops and a preferred height of three atomic layers, indicating the electronic growth at low temperature. Annealing to room temperature leads to not only the growth mode transition from electronic growth to conventional Stranski–Krastanov growth, but also the shape transition from ramified islands to smooth compact islands. Scanning tunneling spectroscopy(STS) measurements reveal that the Ga monolayer exhibits metallic behavior. DFT calculations indicate that all the interfacial Ga atoms occupy the energetically favorable hcp-hollow sites of the substrate. The charge density difference analysis demonstrates that the charge transfer from the Cd substrate to the Ga atoms is negligible, and there is weak interaction between Ga atoms and the Cd substrate. These results shall shed important light on fabrication of ultrathin Ga films on metal substrates with novel physical properties.
基金This work was supported by the National Natural Science Foundation of China （Grant Nos. 11674029, 11422430, and 11374035）, the National Basic Research Program of China （Grants Nos. 2014CB920903 and 2013CBA01603）, and the program for New Century Excellent Talents in University of the Ministry of Education of China （Grant No. NCET-13-0054）. L. He also acknowledges support from the National Program for Support of Top-notch Young Professionals.
文摘When electrons are confined in a two-dimensional （2D） system, typical quantum-mechanical phenonl- ena such as Landau quantization can be detected. Graphene systems, including the single atomic layer and few-layer stacked crystals, are ideal 2D materials for studying a variety of quantum-mechanical problems. In this article, we review the experimental progress in the unusual Landau quantized behav- iors of Dirac fernlions in monolayer and multilayer graphene by using scanning tunneling microscopy （STM） and scanning tulmeling spectroscopy （STS）. Through STS measurement of the strong mag- netic fields, distinct Landau-level spectra and rich level-splitting phenomena are observed in different graphene layers. These unique properties provide an effective method for identifying the number of layers, as well as the stacking orders, and investigating the fllndamentally physical phenomena of graphene. Moreover, in the presence of a strain and charged defects, the Landau quantization of graphene can be significantly modified, leading to unusual spectroscopic and electronic properties.
基金We acknowledge financial support by the National Natural Science Foundation of China （Nos. 51472008 and 51290272）, the National Key Research and Development Program of China （No. 2016YFA0200103）,the Beijing Municipal Science and Technology Planning Project （No. Z151100003315013）, the Open Research Fund Program of the State Key Laboratory of Low- Dimensional Quantum Physics （No. KF201601） and the ENN Energy Research Institute.
文摘Revealing the structural/electronic features and interfacial interactions of monolayer MoS2 and WS2 on metals is essential to evaluating the performance of related devices.In this study,we focused on the atomic-scale features of monolayer WS2 on Au（001） synthesized via chemical vapor deposition.Scanning tunneling microscopy and spectroscopy reveal that the WS2/Au（001） system exhibits a striped superstructure similar to that of MoS2/Au（001） but weaker interfacial interactions,as evidenced by experimental and theoretical investigations.Specifically,the WS2/Au（001） band gap exhibits a relatively intrinsic value of ～ 2.0 eV.However,the band gap can gradually decrease to ～ 1.5 eV when the sample annealing temperature increases from ～370 to 720 ℃.In addition,the doping level （or Fermi energy） of monolayer WS2/Au（001） varies little over the valley and ridge regions of the striped patterns because of the homogenous distributions of point defects introduced by annealing.Briefly,this work provides an in-depth investigation into the interfacial interactions and electronic properties of monolayer MX2 on metal substrates.
基金supported by the Beijing Municipal Natural Science Foundation(Grant No.Z180007)the National Natural Science Foundation of China(Grant Nos.11874003,11904015 and 52073006)the Australian Research Council(ARC)(LP180100722).
文摘In recent years,topological quantum materials(TQMs)have attracted intensive attention in the area of condensed matter physics due to their novel topologies and their promising applications in quantum computing,spin electronics and next-generation integrated circuits.Scanning tunneling microscopy/spectroscopy(STM/STS)is regarded as a powerful technique to characterize the local density of states with atomic resolution,which is ideally suited to the measurement of the bulk-boundary correspondence of TQMs.In this review,using STM/STS,we focus on recent research on bismuth-based TQMs,including quantum-spin Hall insulators,3D weak topological insulators(TIs),high-order TIs,topological Dirac semi-metals and dual TIs.Efficient methods for the modulation of the topological properties of the TQMs are introduced,such as interlayer interaction,thickness variation and local electric field perturbation.Finally,the challenges and prospects for this field of study are discussed.
基金The first author acknowledges the financial support of JSPS(Japan Society for the Promotion of Science)This work was also supported by an International Colla-borative Research Grant by the National Institute of Information and Communications Technology of Japan.
文摘Scanning tunneling microscopy/spectroscopy(STM/STS)at 4.8 K has been used to examine the growth of a double-decker bis(phthalocyaninato)yttrium(YP_(c2))molecule on a reconstructed Au(111)substrate.Local differential conductance spectra(dI/dV)of a single YPc2 molecule allow the characteristics of the highest occupied molecular orbital(HOMO)and the lowest unoccupied molecular orbital(LUMO)to be identified.Furthermore,lateral distributions of the local density of states(LDOS)have also been obtained by dI/dV mapping and confirmed by first principles simulations.These electronic feature mappings and theoretical calculations provide a basis for understanding the unique STM morphology of YPc2,which is usually imaged as an eight-lobed structure.In addition,we demonstrate that bias-dependent STM morphologies and simultaneous dI/dV maps can provide a way of understanding the stability of two-dimensional YP_(c2) films.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 12174096, 51772087, 51972106, 11904094, 11804089 and 12174095)the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB30000000)the Natural Science Foundation of Hunan Province, China (Grant Nos. 2019JJ50073 and 2021JJ20026)
文摘Two-dimensional(2D)semiconductors,such as lead selenide(PbSe),locate at the key position of next-generation devices.However,the ultrathin PbSe is still rarely reported experimentally,particularly on metal substrates.Here,we report the ultrathin PbSe synthesized via sequential molecular beam epitaxy on Ag(111).The scanning tunneling microscopy is used to resolve the atomic structure and confirms the selective formation of ultrathin PbSe through the reaction between Ag5Se2 and Pb,as further evidenced by the theoretical calculation.It is also found that the increased accumulation of Pb leads to the improved quality of PbSe with larger and more uniform films.The detailed analysis demonstrates the bilayer structure of synthesized PbSe,which could be deemed to achieve the 2D limit.The differential conductance spectrum reveals a metallic feature of the PbSe film,indicating a certain interaction between PbSe and Ag(111).Moreover,the moirépattern originated from the lattice mismatch between PbSe and Ag(111)is observed,and this moirésystem provides the opportunity for studying physics under periodical modulation and for device applications.Our work illustrates a pathway to selectively synthesize ultrathin PbSe on metal surfaces and suggests a 2D experimental platform to explore PbSe-based opto-electronic and thermoelectric phenomena.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.51772087,11804089,11574350,11904094,and 51972106)the Natural Science Foundation of Hunan Province,China(Grant Nos.2018JJ3025,2019JJ50034,and 2019JJ50073)+1 种基金the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB30000000)the Fundamental Research Funds for the Central Universities of China.
文摘Interlayer coupling in layered semiconductors can significantly affect their optoelectronic properties.However,understanding the mechanisms behind the interlayer coupling at the atomic level is not straightforward.Here,we study modulations of the electronic structure induced by the interlayer coupling in theγ-phase of indium selenide(γ-InSe)using scanning probe techniques.We observe a strong dependence of the energy gap on the sample thickness and a small effective mass along the stacking direction,which are attributed to strong interlayer coupling.In addition,the moirépatterns observed inγ-InSe display a small band-gap variation and nearly constant local differential conductivity along the patterns.This suggests that modulation of the electronic structure induced by the moirépotential is smeared out,indicating the presence of a significant interlayer coupling.Our theoretical calculations confirm that the interlayer coupling inγ-InSe is not only of the van der Waals origin,but also exhibits some degree of hybridization between the layers.Strong interlayer coupling might play an important role in the performance ofγ-InSe-based devices.
