Department of Chemistry and Biochemistry and ‡Department of Materials Science and Engineering, University of Arizona , Tucson, Arizona 85721, United States.
ACS Appl Mater Interfaces. 2017 Sep 13;9(36):31111-31118. doi: 10.1021/acsami.7b08899. Epub 2017 Aug 30.
Herein, we consider the heterojunction formation of two prototypical metal oxides: p-type NiO and n-type ZnO. Elementally abundant, low-cost metal oxide/oxide' heterojunctions are of interest for UV optical sensing, gas sensing, photocatalysis, charge confinement layers, piezoelectric nanogenerators, and flash memory devices. These heterojunctions can also be used as current rectifiers and potentially as recombination layers in tandem photovoltaic stacks by making the two oxide layers ultrathin. In the ultrathin geometry, understanding and control of interface electronic structure and chemical reactions at the oxide/oxide' interface are critical to functionality, as oxygen atoms are shared at the interface of the dissimilar materials. In the studies presented here the extent of chemical reactions and interface band bending is monitored using X-ray and ultraviolet photoelectron spectroscopies. Interface reactivity is controlled by varying the near surface composition of nickel oxide, nickel hydroxide, and nickel oxyhydroxide using standard surface-treatment procedures. A direct correlation between relative percentage of interface hydroxyl chemistry (and hence surface Lewis basicity) and the local band edge alignment for ultrathin p-n junctions (6 nm NiO/30 nm ZnO) is observed. We propose an acid-base formulism to explain these results: the stronger the acid-base reaction, the greater the fraction of interfacial electronic states which lower the band offset between the ZnO conduction band and the NiO valence band. Increased interfacial gap states result in larger reverse bias current of the p-n junction and lower rectification ratios. The acid-base formulism could serve as a future design principle for oxide/oxide' and other heterojunctions based on dissimilar materials.
在此，我们考虑了两种典型金属氧化物的异质结形成：p 型 NiO 和 n 型 ZnO。元素丰富、成本低的金属氧化物/氧化物异质结在紫外光传感、气体传感、光催化、电荷限制层、压电纳米发电机和闪存器件等方面具有应用价值。这些异质结还可以通过使两个氧化物层超薄，用作电流整流器，并有可能用作串联光伏堆叠中的复合层。在超薄结构中，为了实现功能，必须理解和控制氧化物/氧化物界面的界面电子结构和化学反应，因为在不同材料的界面处共享氧原子。在本文的研究中，使用 X 射线光电子能谱和紫外光电子能谱监测了化学反应和界面能带弯曲的程度。通过使用标准表面处理程序改变氧化镍、氢氧化镍和镍氢氧化物的近表面组成，控制界面反应性。观察到超薄 p-n 结（6nm NiO/30nm ZnO）的界面羟基化学（因此表面路易斯碱性）的相对百分比与局部能带边缘排列之间存在直接相关性。我们提出了一种酸碱公式来解释这些结果：酸碱反应越强，界面电子态的比例就越大，这会降低 ZnO 导带和 NiO 价带之间的能带偏移。增加的界面间隙态导致 p-n 结的反向偏置电流增大，整流比降低。酸碱公式可以作为基于不同材料的氧化物/氧化物和其他异质结的未来设计原则。
