Recently, researchers such as Chen Chunlin, Ma Xiuliang, researcher of Solid State Image Research Institute of Shenyang Institute of Materials Science, Shenyang Institute of Materials Science, IBM Johannes Georg Bednorz, winner of Nobel Prize in Physics, Yuichi Ikuhara, University of Tokyo, Japan, Quasi-one-dimensional conductive material SrnNbnO was established on the atomic scale 3n+2And the relationship between their electronic structures and the electronic structure of the three-dimensional conductor, the formation mechanism of quasi-one-dimensional conductive properties is revealed. Based on this, a research idea of preparing two-dimensional conductive materials by inserting insulating layers into three-dimensional conductors is proposed.
SrnNbnO 3n+2 (ie SrNbO x) Is a type of oxide with a layered perovskite structure that can be passed to SrNbO 3The introduction of excess oxygen and obtained.The structure and properties of these oxides are very sensitive to its oxygen content and distribution, the oxygen content of how much determines the lamellar structure of the sheet thickness and conductivity characteristics.As early as 20 years ago, The research team led by Bednorz has recognized SrnNbnO 3n+2(Along the a-axis conductivity is much greater than other crystallographic orientation), and its formation mechanism was discussed.However, the origin of quasi-one-dimensional conductivity has long been revealed on the atomic scale Still a challenging issue.
Chen Chunlin, etc. using scanning electron transmission microscopy and first-principles theoretical calculations combined with the method to systematically study the SrnNbnO 3n+2 (Including SrNbO 3.4, SrNbO 3.45And SrNbO 3.5Etc.) and its Nb ion valence state and NbO 6Octahedral morphology of the relationship between the electron energy loss spectrum of the results show that, SrnNbnO 3n+2The conductivity depends on the valence of Nb ions: the material contains Nb 4+The ionic area has good conductivity and contains Nb 5+Areas of the ions are not conductive. Excess oxygen accumulates on the SrnNbnO 3n+2The interlayer transition region (jagged region in the paper) forms an insulating layer, so that the conductive characteristic exhibits two-dimensional conduction characteristics.The results of the first-principles calculations further indicate that the valence state of Nb ions and its NbO 6Octahedral morphology is directly related to: Nb4 + ions are always located in NbO 6Octahedral center position, and Nb5 + ions are always far from the center position. SrnNbnO 3n+2The electrical properties of the local region of the material consist of its NbO 6The shape of the octahedron is determined by this study which reveals SrnNbnO 3n+2It is the origin of the first-quasi-one-dimensional conductive characteristics and provides the idea for the design and development of new quasi-one-dimensional conductive materials.Nowadays, the cooperative research and development for the new quasi-one-dimensional conductive materials is underway.
The research has been funded by key research projects of Chinese Academy of Sciences and the National Youth Thousand Talents Program, etc. Relevant results are published online at ACS Nano.
Figure 1, SrnNbnO 3n+2The HAADF image of the compound along the '010' zone axis. All compounds are composed of alternately arranged chain and zigzag regions (a) Type 2-2-2-2, corresponding to n = 4 (i.e.SrNbO 3.5); (b) 3-3-3-3 type structure, corresponding to n = 5 (i.e.SrNbO 3.4); (c) Type 2-3-2-3, corresponding to n = 4.5 (i.e.SrNbO 3.45); (d) Type 2-4-4-2, corresponding to n = 4,6,6,4 (i.e.SrNbO3.4).
Figure 2, SrnNbnO 3n+2(A) 2-2-2-2 type structure; (b) 3-3-3-3 type structure; (c) 2-3-2-3 type structure; ( d) Type 2-4-4-2. The ratio b / a of the b-peak to the a-peak in the OK edge reflects the valence of Nb ions in this region. For larger b / a values, Nb is a +4 valence, On the contrary, the value of +5. EELS results show 3-3-3-3 type structure, 2-3-2-3 type structure and 2-4-4-2 type structure for the two-dimensional conductor, 2-2-2- 2 type structure for the insulator.
Figure 3, SrnNbnO 3n+2(A) 2-2-2-2 type structure; (b) 3-3-3-3 type structure; (c) 2-3-2-3 type structure ; (d) 2-4-4-2 type structure. The result of DOS shows that 3-3-3-3 type structure, 2-3-2-3 type structure and 2-4-4-2 type structure are two-dimensional Conductor, 2-2-2-2 type structure for the insulator.
Figure 4, SrnNbnO 3n+2Representative compounds in the NbO 6(A) 2-2-2-2 type structure; (b) 3-3-3-3 type structure; (c) 2-3-2-3 type structure; (d) 2-4 -4-2 structure. The yellow background indicates a chain-like region. The Nb-O bond length ratio in the serrated region is larger, indicating that the Nb ion deviates from the center of the oxygen octahedron.
Figure 5, a schematic diagram of the preparation of two-dimensional conductive material: The introduction of three-dimensional insulation layer conductor, so that the three-dimensional conductive into two-dimensional conductive.
