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自旋器件有望实现量子信息存储、传感和计算, 是下一代数据存储和通信的理想器件. 与无机自旋器件相比, 有机自旋器件不仅可以实现传统无机自旋器件的功能, 而且在同一有机自旋阀器件中会同时测到正负磁电阻信号, 这是因为有机分子与铁磁电极在界面会发生自旋杂化而产生独特的自旋界面. 通过控制自旋界面, 可以改变界面处分子能级展宽和偏移程度, 从而实现对磁电阻信号的可控调制. 有机自旋阀器件发展迅速, 但仍有一些问题亟待研究, 如对自旋界面进行识别和表征, 以及利用自旋界面对有机自旋阀信号进行操控等. 针对上述问题, 本文首先综述了有机自旋阀的基本原理, 通过对比无机有机材料能级结构的差异解释了有机自旋阀中自旋界面形成的原因, 对于有机自旋阀中磁电阻信号的增强和反转现象, 利用自旋界面模型中能级展宽和偏移进行了解释; 接着列举了自旋界面的实验识别案例, 如利用对表面敏感的表征技术对自旋界面进行识别以及设计新颖的器件结构验证自旋界面的存在等; 然后汇总了利用自旋界面调制自旋信号的相关工作, 自旋界面的调制可以通过电场调节铁电层的铁电极化、诱导铁磁电极相变、界面化学工程和磁交换相互作用等方式实现; 最后总结了有机自旋界面中仍需解决的问题, 并对有机自旋界面的识别和可控利用进行了展望.Spintronics are attractive to the utilization in next-generation quantum-computing and memory. Compared with inorganic spintronics, organic spintronics not only controls the spin degree-of-freedom but also possesses advantages such as chemical tailorability, flexibility, and low-cost fabrication process. Besides, the organic spin valve with a sandwich configuration that is composed of two ferromagnetic electrodes and an organic space layer is one of the classical devices in organic spintronics. Greatly enhanced or inversed magnetoresistance (MR) sign appearing in organic spin valve is induced by the unique interfacial effect an organic semiconductor/ferromagnetic interface. The significant enhancement or inversion of MR is later proved to be caused by the spin-dependent hybridization between molecular and ferromagnetic interface, i.e., the spinterface. The hybridization is ascribed to spin-dependent broadening and shifting of molecular orbitals. The spinterface takes place at one molecular layer when attaching to the surface of ferromagnetic metal. It indicates that the MR response can be modulated artificially in a specific device by converting the nature of spinterface. Despite lots of researches aiming at exploring the mechanism of spinterface, several questions need urgently to be resolved. For instance, the spin polarization, which is difficult to identify and observe with the surface sensitive technique and the inversion or enhancement of MR signal, which is also hard to explain accurately. The solid evidence of spinterface existing in real spintronic device also needs to be further testified. Besides, the precise manipulation of the MR sign by changing the nature of spinterface is quite difficult. According to the above background, this review summarizes the advance in spinterface and prospects future controllable utilization of spinterface. In Section 2, we introduce the basic principle of spintronic device and spinterface. The formation of unique spinterface in organic spin valve is clarified by using the difference in energy level alignment between inorganic and organic materials. Enhancement and inversion of MR sign are related to the broadening and shifting of the molecular level. In Section 3, several examples about identification of spinterface are listed, containing characterization by surface sensitive techniques and identification in real working devices. In Section 4 some methods about the manipulation of spinterface are exhibited, including modulation of ferroelectric organic barrier, interface engineering, regulation of electronic phase separation in ferromagnetic electrodes, etc. Finally, in this review some unresolved questions in spintronics are given, such as multi-functional and room-temperature organic spin valve and improvement of the spin injection efficiency. Spinterface is of great importance for both scientific research and future industrial interest in organic spintronics. The present study paves the way for the further development of novel excellent organic spin valves.
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Keywords:
- organic spin valve /
- spinterface /
- organic spintronics /
- magnetoresistance
1. 引 言
氨气(NH3)催化分解制氢技术是指在固体催化剂的作用下将NH3转化为清洁燃料氢气. 这项技术不仅可以消除氨气的大气污染, 而且提供了一种解决质子交换膜燃料电池电极中毒的有效途径. 鉴于氨气分解制氢在燃料电池领域潜在的应用价值, 近年来氨气在金属[1]、合金[2,3]和金属氮化物[4]等表面吸附和催化分解引起人们广泛关注. 其中金属Ru和Ir对氨气脱氢分解反应具有最高的催化活性, 但是由于贵金属成本高、资源稀缺, 难以大规模开发利用. 因此, 开发廉价且高效的氨分解催化剂具有重要的科学意义和实际应用价值.
过渡金属碳化物具有和Pt相似的d带电子密度态, 作为一种潜在的能够替代贵金属氨分解催化材料而被广泛研究. 目前文献报道了NH3在WC[5,6]、VC[7]和MoC[8]等催化剂表面吸附分解的实验研究, 然而关于氨气在金属碳化物表面上吸附与分解的理论研究还未有报道. 在相关文献查询中发现具有多孔结构的TaC对氨分解反应的催化活性远远超过商业Pt/C催化剂[9,10], 然而NH3在TaC表面上吸附的电子结构本质和催化分解机理尚不清楚. 本文采用密度泛函理论(DFT)方法研究NH3在TaC(0001)面上的吸附位点和几何结构, 并从电子结构层次上对吸附成因进行分析, 计算脱氢分解各基元反应的过渡态与活化能, 并找出脱氢反应的电子结构本质, 为进一步开发高性能的氨气分解TaC催化剂提供理论基础.
2. 计算模型和方法
本文所有计算均采用基于密度泛函理论的商业VASP软件进行. 采用了具有广义梯度近似(GGA)的Perdew-Burke-Ernzerh(PBE)泛函来描述核与电子之间的相互作用[11]. 单电子波函数采用平面波基组展开, 截断能为400 eV, 布里渊区积分采用Monkhorst-Pack方案划分K点网格, 其中K点网格密度取为5 × 5 × 1. 在结构优化过程中, 力和能量的收敛准则分别设定为0.02 eV/ Å(1 Å = 0.1 nm)和10–5 eV. 计算模型采用具有7个原子层和垂直于平板15 Å真空层的(3 × 3)超晶胞模型模拟TaC(0001)表面, 其中按照表面层原子种类可分为以碳为终止(C-TaC)和以Ta为终止(Ta-TaC)的结构模型. 在构建优化过程中底部5个原子层的原子坐标是固定的, 而顶层表面原子随吸附物分子共同驰豫.
首先计算体相TaC和气相NH3分子的性质, 计算得到六角晶系结构TaC的晶格参数值为3.12 Å和2.74 Å. 对于NH3分子, 在尺寸为15 Å × 15 Å × 15 Å的大单元格中计算得到的键长r (N—H) = 1.021 Å, 键角θ(H—N—H) = 106.6°. 这与文献报道的实验值1.017[12], 107.8[12]基本一致.
本文吸附能的计算公式为
Eads=Eall−Esurface−Eadsorbate, 其中
Eall ,Esurface 和Eadsorbate 分别代表吸附体系的总能量、清洁表面能量和吸附分子的能量. 如果吸附能为正值, 那么吸附为吸热过程, 吸附不是自发进行的; 反之吸附过程为放热, 反应可以自发进行, 且吸附能的绝对值越大, 吸附体系越稳定, 吸附越容易发生. 本论文中所有能量进行了零点自由能矫正.为了绘制最小能量路径并定位过渡态, 使用CI-NEB和dimer相结合方法寻找化学反应的过渡态. 每步脱氢反应的活化能和反应热分别为
Ea=Ets−Eis, ΔH=Efs−Eis, 其中
Eis ,Ets 和Efs 分别表示反应物、过渡态和产物的总能量.3. 结果与讨论
3.1 表面能的计算
根据非化学计量比表面的表面自由能定义, 计算TaC表面的表面能[13]
σ=12A[Eslab−NTaμTaC(bulk)+(NTa−Nc)μc], 其中A表示TaC模型的表面积;
ESlab 表示板层模型优化后的能量值;NTa ,NC 分别表示模型中钽原子和碳原子个数;μTaC(bulk) 表示体相TaC的化学势;μC 表示碳的表面相化学势. 图1展示了Ta-TaC和C-TaC的表面能与化学势之间关系. 从图1可以看出, 在整个能量范围内Ta-TaC表面能始终低于C-TaC表面能, 这说明以钽为终止的TaC(0001)表面更稳定.
图 1 TaC(0001)的表面能(σs Fig. 1. Relationships between surface energies of two TaC (0001) surfaces and chemical potentials.3.2 NHx在Ta-TaC表面的吸附
在上述TaC(0001)表面能及稳定性研究基础上, 选择以金属钽为终止的TaC(0001)表面作为吸附能和过渡态的平面模型, 同时考察了NHx及其他小分子在顶位(top)、三重空位(fcc和hcp)和桥位(bridge)的吸附构型(图2). 表1列出了优先吸附位置的吸附能和关键几何参数.
表 1 Ta-TaC表面的吸附位点、吸附能和关键几何参数Table 1. Adsorption site, adsorption energy and key geometric parameters of Ta-TaC surface.Species Sites Eads/eV d (N—Ta) d (N—H) θ(H—N—H) NH3 top 0.08 2.354 1.027 108.1 NH2 hcp –3.6 2.221 1.036 102.8 NH fcc –6.49 2.140 1.026 — N fcc –9.79 2.030 — — H hcp –1.1 — — — v-N2 top –0.37 2.157 — — 对于NH3来说, 由于其具有Cs点群对称性, 需要考虑两种构型来优化吸附几何结构: 一种结构是一个氢原子在氮原子的左边, 两个氢原子在氮原子右边(图3(a)); 另外一种结构是两个氢原子在氮原子的左边, 一个氢原子在氮原子的右边(图3(b)). 除NH2具有C2V对称外, 其他所有组成成分都具有C4V对称. 同时还考虑了平行(图3(c), p-N2)和垂直(图3(d), v-N2)两种吸附构型.
图 3 (a)(b) NH3和 (c)(d) N2的两种吸附构型Fig. 3. Two adsorption configurations of (a)(b) NH3 and (c)(d) N2.对于NH3在Ta-TaC表面的吸附, 对4个位置(top, hcp, fcc和bridge)进行优化. 其中top位最稳定, NH3分子吸附在top位, 然后把这个结构模型作为NH3分解催化剂的始态. NH3在Ta-TaC表面逐步脱氢过程中, 中间产物在该表面的最稳定吸附位点、吸附能及一些关键几何参数如表1所示. NH3优先吸附在顶部位置, N原子与Ta成键, H原子指向外. 任何试图在其他对称位点找到能量最小值的尝试都将在完全优化后得到顶部位置, 这与文献[14, 15]报道的NH3在过渡金属上的吸附一致. NH3, NH2, NH和N的吸附能分别是0.08 eV, 3.6 eV, 6.49 eV和9.79 eV. 在优先顶部几何结构中, N原子距离表面2.354 Å. N—H键长和N—H—N角分别为1.027 Å和108.1°, 与气相NH3分子的数值相接近. 这说明NH3分子的结构在吸附后没有明显改变, NH3-底物之间存在较弱的相互作用. 对于NH2, 吸附位置在hcp位置, N—H键长、H—N—H键角、N与Ta表面的垂直距离分别是为1.036 Å, 102.8°, 2.221 Å. N—H键长从片段NH2的1.037 Å下降到1.036 Å, 而H—N—H键角由102.5°增大到102.8°. 对于NH和N, 吸附位置都是在fcc位置, N与Ta表面的垂直距离分别为2.140 Å和2.03 Å. 综上所述, 随着NHx组分中H原子数的减少, N原子与TaC表面的垂直高度降低, Ta配位N原子数增加, NHx中的未成键的孤对电子和底物Ta原子之间相互成键, 导致NHx组分的吸附能逐步增大.
3.3 NH3在Ta-TaC表面的逐步脱氢分解机理
众所周知, NH3分子在催化剂表面分解脱氢生成N2, 包含以下4个基元反应过程:[16,17]
NH∗3+∗→NH∗2+H∗, (1) NH∗2+∗→NH∗+H∗, (2) NH∗+∗→N∗+H∗, (3) N∗+N∗→N2. (4) 对于NH3脱氢和复合反应的初态(IS)、过渡态(TS)和终态(FS)结构如图4所示, 其中NH3在Ta-TaC表面的第一步解离, 选择了NH3分子在平面模型表面的顶位吸附作为初始态(IS1). 随着N—H距离由1.027 Å增大至1.037 Å, 形成过渡态TS1. 通过相应的过渡态中N—H的裂解, 进一步解离成NH2和H. 这一步需要克服0.93 eV的能垒, 吸附放热2.1 eV. NH2 + H共吸附构型(IS2)被用作第二阶段分解的初始状态, 其中被吸附的NH2和H分别占据了hcp位置. NH2通过过渡态TS2使N—H键断裂, 脱氢分解至NH和H物种共吸附在Ta-TaC表面. 第二步需要克服1.02 eV的能垒, 吸附放热1.78 eV. NH和H物种的最稳定共吸附态(IS3, fcc)作为第三步脱氢反应的反应物, N—H键距离逐渐拉伸, 当N-H距离增大到3.116 Å时, TS3过渡态被确定. 第三步脱氢反应的能垒为2.03 eV, 放热为0.27 eV. 经过以上三步脱氢过程, 最终氮原子以最稳定的fcc位吸附在TaC表面, 两个N原子再进一步相互靠拢, 当N和N原子之间间距为1.37 Å时, 过渡态形成(TS4). 第四步复合反应的能垒为5.32 eV, 吸热为2.89 eV.
图 4 NH3脱氢和复合反应的初态、过渡态和终态结构Fig. 4. Structure of initial state, transition state and final state of NH3 dehydrogenation and recombination reaction.综上所述, 根据图5和表2的计算结果, 得出以下结论: 1)脱氢反应为放热反应, 说明氨气分子适合在Ta-TaC表面吸附分解, 其脱氢反应能垒与目前报道贵金属催化剂相接近[18,19]; 2)有且只有一个虚频, 说明搜索过渡态结构的唯一性; 3)随着NHx组分中H原子数的减少, NHx在其表面的吸附能逐渐增大, 导致N—H键的活化解离所需要的活化能也逐渐增大, 氮原子的复合反应脱附成为整个反应的限速步骤.
图 5 (a) NH3分解反应和 (b) N-N复合反应的能量图Fig. 5. Calculated potential energy diagram for (a) NH3 dehydrogenation and (b) N-N coupling reaction.表 2 各步基元反应的活化能、虚频和反应热Table 2. The activation energy、imaginary frequency and reaction heat of each step elementary reaction.reaction ΔE/eV ΔH/eV Freq./cm–1 NH3→NH2+H 0.93 –2.10 1154 NH2→NH+H 1.02 –1.78 502 NH→N+H 2.03 –0.27 1405 N+N→N2 5.32 2.89 136 3.4 NHx/Ta-TaC表面电子结构分析
为进一步阐明NH3在Ta-TaC表面吸附解离的机理, 本文从电荷密度分布和电子态密度角度分析最稳定吸附位置的电子结构. 如图6所示, 通过吸附N和H原子及其最邻近的表层原子截取电荷密度分布图, 其中原子周围的黄色区域表示成键区域电荷密度增强, 蓝色区域表示成键区域电荷密度减小, 轮廓等值面为0.015 e/Å3. 此外, 还使用Bader电荷分析方法计算了吸附体系中各原子电荷. 从图6(a)可知, NH3分子和底物表面之间的电荷转移非常小(只有0.01|e|, 从Ta原子到NH3分子). 随着第1次脱氢反应的进行, 0.6e从底物表面转移到NH2片段(图6(b)). 在第2次和第3次脱氢反应过程中, 电荷转移现象变得更加明显, 这个值分别增大到1.03e和1.33e (图6(c)和6(d)). 通过以上结果分析可知, 在NH3脱氢催化过程中吸附质和底物之间的电荷转移是其吸附能和活化能增加的根本原因.
图 6 (a) NH3, (b) NH2, (c) NH, (d) N在Ta-TaC表面的差分电荷密度示意图和Bader电荷分析Fig. 6. Schematic diagram of differential charge density of (a) NH3, (b) NH2, (c) NH, (d) N on the surface of Ta-TaC and Bader charge analysis.为了进一步了解NH3及其片段与Ta-TaC表面相互作用机理, 计算了吸附前后体系的态密度. 众所周知, NH3在气相中的电子结构可以表示为(σ2a1)2(σ1e)4(n3a1)2, 其中2a1轨道主要由N原子的2s轨道和H原子的1s轨道形成, 1e轨道由N原子的2Px及2Py轨道与H原子的1s轨道混合而成, 3a1轨道由N原子的2pz轨道组成[20,21]. 从图7(a)可以看到, 在能量范围为–20—0 eV之间出现3个不同峰, 分别代表σ2a1, σ1e和n3a1轨道. 从图7(a)—7(d)中可以看出, 在吸附前后及脱氢过程中2a1和1e轨道所对应的峰形貌变化不大, 说明2a1和1e轨道同表面态之间相互作用弱, 对NH3及其片段的吸附贡献小. 同气相NH3相比(Fig.7(a)), 3a1轨道所对应的峰出现明显展宽现象, 同时所对应的能量向低能方向发生了位移(图7(b)—7(e)), 说明在垂直方向上N原子的2pz轨道和Ta原子d轨道之间出现杂化行为, 这是NH3及其片段稳定吸附于表面的主要原因, 类似于NH3在其他体系吸附的成因[13,22].
图 7 (a) 吸附前NH3, (b) 吸附后NH3, (c) NH2, (d) NH, (e) N吸附于Ta-TaC表面态密度分布, 虚线代表费米能级Fig. 7. Local density of states (LDOS) for NH3 dehydrogenation: (a) NH3 before interaction, (b) NH3 after interaction, (c) NH2, (d) NH, (e) N The dashed line represents the Fermi level.4. 结 论
采用第一性原理的方法研究了NH3在Ta-TaC表面上的吸附和脱氢反应机理. 得到了相应的最优吸附位置、能量和几何结构, 确定了NH3逐步分解反应的过渡态结构及能垒. 主要结论归纳如下:
1) 表面能计算结果说明以钽为终止的TaC(0001)表面更稳定.
2) NH3和N2倾向于吸附在top位上, 而NH, N倾向于吸附在fcc位上, NH2和H倾向于hcp位上. 各组分在脱氢过程中的吸附能呈现如下趋势: NH3 < NH2 < NH < N.
3) 在NH3的整个分解反应中, N与N原子之间复合脱附反应活化能较高, 它决定了整个反应的快慢, 是整个反应的速率控制步骤.
4) 电子结构分析结果表明, NH3分子及其片段通过其N原子的2pz轨道与底物Ta的
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图 1 (a)有机自旋阀器件示意图; (b)有机自旋阀器件中随着施加磁场变化的理想磁电阻曲线
Fig. 1. (a) Schematic of organic spin valve device; (b) ideal MR curve when sweeping the applied magnetic field on organic spin valve devices.
图 2 (a)和(b)磁性隧道结中铁磁电极在平行和反平行磁化状态的结构示意图; (c)和(d)在平行和反平行磁化状态下铁磁层的能带结构; (e)和(f)在磁化方向平行和反平行状态下的双电阻网络模型[54]
Fig. 2. (a) and (b) Schematics shows different states of ferromagnetic electrode with parallel and antiparallel magnetization in MTJ; (c) and (d) band structure of ferromagnetic layer for parallel and antiparallel magnetization; (e) and (f) two-resistor network model for magnetization of parallel and antiparallel alignment[54].
图 4 自旋界面的示意图 (a)无机物和铁磁电极接触界面的导带和价带示意图; (b)自旋界面处当
Γ≫ΔE 时会诱导自旋极化的反转; (c)Γ≪ΔE 时会造成自旋极化增强[46]Fig. 4. Schematics of spinterface[46]: (a) Schematics of conduction and valence band structure at inorganic/FM interface; (b) inversed spin-polarization case of
Γ≫ΔE at the spinterface; (c) enhanced spin-polarization case ofΓ≪ΔE at the interface[46].
图 5 (a) H2Pc吸附在Fe上的SP-STM图[64]; (b) 两个吸附在Cr (001)表面的C60分子上的电导图[65]; 吸附于Co基底上不同厚度TNAP的UPS图, 其中(c)—(e)分别对应(c)二次电子截止边、(d)价带边、(e)价带的细节谱图; (f)在Co上TNAP吸附前后Co的L边XMCD图; (g) 单层和多层TNAP在Co上N元素的K边NEXAFS图[66]
Fig. 5. (a) SP-STM image of H2Pc absorbed on Fe[64]. (b) conductance maps measured over two C60 molecules absorbed on Cr (001) surface[65]. UPS spectra of TNAP with different thickness deposited on Co substrate: (c) Secondary electron cutoff; (d) valence band; (e) detail spectral features of valence band. (f) Co L-edge XMCD results before and after adsorption of TNAP on Co; (g) NEXAFS N K-edge spectra of monolayer and multilayer TNAP on Co[66].
图 6 (a) AlOx绝缘层对Co渗透的阻挡作用及渗透的Co和P3HT间形成自旋界面的示意图; (b) LSMO/P3HT/AlOx/Co器件中自旋依赖电子隧穿过程示意图[48]
Fig. 6. (a) Schematic drawing of blocking effect for the insulated AlOx to penetrated Co, and the formation of spinterface between penetrated Co and P3HT molecular; (b) schematics of spin-dependent electron tunneling in LSMO/P3HT/AlOx/Co junction[48].
图 7 (a)和(b)分别为器件A和器件B的磁输运测试; (c)无LiF层、具有反铁磁双氟层和LiF沉积在氧化铝上的器件结构以及对应的磁电阻信号示意图[49]
Fig. 7. (a) and (b) Magnetotransport measurements of device A and device B, respectively; (c) schematics of devices with no LiF layer, an anti-ferromagnetic difluoride layer and LiF deposited on an alumina and their respective MR curves [49].
图 8 (a) Fe3O4/P3HT/Co有机自旋阀器件示意图; (b) 不同电流下Fe3O4/P3HT/Co有机自旋阀器件和Fe3O4电极MR值与温度的关系; (c) 不同温度下孪晶界对自旋注入调制过程的模型图[70]
Fig. 8. (a) Schematic of organic spin valve device of Fe3O4/P3HT/Co; (b) relationship between MR ratio and temperature for Fe3O4/P3HT/Co OSV device and Fe3O4 electrode at different bias current; (c) model of twin boundary-modulated spin injection at different temperature[70].
图 9 (a) LSMO/PVDF/Co器件示意图; (b)在PVDF表面测得的PFM相图; (c) 极化后器件所测得的隧穿磁电阻信号; (d)在10 mV, 10 K条件下测得LSMO/PVDF/Co器件的隧穿磁电阻; (e)在10 mV, 10 K条件下测得LSMO/PVDF/MgO/Co器件的隧穿磁电阻[51]
Fig. 9. (a) Schematic of LSMO/PVDF/Co device; (b) PFM phase image measured on the PVDF surface; (c) tunneling magnetoresistance measured after polarizing the device; (d) tunneling magneto resistance of a LSMO/PVDF/Co device measured under 10 mV at 10 K; (e) tunneling magneto resistance of a LSMO/PVDF/MgO/Co device measured under 10 mV at 10 K[51].
图 10 (a) Au/Co/Alq3/PZT/LSMO有机自旋阀的器件示意图; (b)和(c)施加不同预设电压后MR的偏移; (d)当PZT的电极化向上和向下时器件的能级关系示意图[71]
Fig. 10. (a) Schematic of a Au/Co/Alq3/PZT/LSMO organic spin valve device; (b) and (c) MR shift after applying different ramping voltage; (d) the energy relationship schematic of device when the electric polarization of the PZT is “up” and “down”[71].
图 11 在LPCMO有机自旋阀中(a) FMM和COI相共存和(b)全FE相时的EPS调制自旋注入示意图; (c)在LPCMO有机自旋阀中不同预设磁场强度下的MR信号[72]
Fig. 11. Illustration of EPS-modulated spin current injection in the LPCMO-OSVs under the co-existed FM/COI phase (a) and fully FM phase (b) of the LPCMO thin film; (c) MR loops of the LPCMO-OSV device under different pre-set magnetic field strength[72].
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