Singapore Synchrotron Light Source, National University of Singapore, Singapore 117603, Singapore.
School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore.
Nanoscale. 2017 May 11;9(18):6094-6102. doi: 10.1039/c7nr00742f.
In this work, multifunctional oxide NdNiO (NNO) thin films grown on a SrTiO (STO) substrate using pulsed-laser deposition are studied. Temperature dependent resistivity measurements revealed that NNO/STO samples exhibit a sharp thickness dependent metal-insulator transition (MIT) over a range of 150-200 K. It is known that the electronic properties of correlated oxides are extremely complex and sensitive to changes in orbital occupancy. To evaluate the changes in the electronic and/or crystallographic structure responsible for the MIT, a site-selective (O, Ni and Nd) X-ray absorption near edge structure (XANES) analysis is performed above and below the transition temperature. Analysis of XANES spectra suggests that: (i) in NNO films nominally trivalent Ni ions exhibit multiple valency (bond disproportionation), (ii) intermetallic hybridization plays an important role, (iii) the presence of strong O 2p-O 2p hole correlation at low temperature results in the opening of the p-p gap and (iv) the valency of Nd ions matches well with that of Nd. For NNO films exhibiting a sharp MIT, Ni 3d electron localization and concurrent existence of Ni 3d and Ni 3dL[combining low line] states are responsible for the observed transition. At temperatures below the MIT the O 2p-O 2p hole correlation is strong enough to split the O 2p band stabilizing insulating phase. Temperature and thickness dependent differences observed in the site-selective XANES data are discussed in terms of possible mechanisms for the MIT (negative charge-transfer type).
在这项工作中，使用脉冲激光沉积在 SrTiO（STO）衬底上生长的多功能氧化物 NdNiO（NNO）薄膜。温度相关电阻率测量表明，NNO/STO 样品在 150-200 K 的范围内表现出明显的厚度相关金属-绝缘体转变（MIT）。众所周知，相关氧化物的电子性质极其复杂，并且对轨道占据的变化非常敏感。为了评估导致 MIT 的电子和/或结晶结构的变化，在转变温度以上和以下进行了位置选择性（O、Ni 和 Nd）X 射线吸收近边结构（XANES）分析。XANES 光谱分析表明：（i）在名义上为三价的 NNO 薄膜中，Ni 离子表现出多种价态（键离析），（ii）金属间杂化起着重要作用，（iii）低温下存在强 O 2p-O 2p 空穴相关，导致 p-p 带隙打开，（iv）Nd 离子的价态与 Nd 匹配良好。对于表现出明显 MIT 的 NNO 薄膜，Ni 3d 电子局域化和同时存在的 Ni 3d 和 Ni 3dL[组合低线]态是导致观察到的转变的原因。在 MIT 以下的温度下，O 2p-O 2p 空穴相关足够强，可分裂 O 2p 带，稳定绝缘相。根据 MIT（负电荷转移型）的可能机制，讨论了位置选择性 XANES 数据中观察到的温度和厚度相关差异。
