Deciphering the Structure of Amorphous Functional Materials using 4D-STEM
Deciphering the Structure of Amorphous Functional Materials using 4D-STEM
复制标题
使用 4D-STEM 解读非晶功能材料的结构
DOI：
10.1093/micmic/ozad067.144
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发表时间：
2023
影响因子：
2.8
通讯作者：
Hwang, Jinwoo
中科院分区：
文献类型：
--
作者：
Ortiz, Gabriel Calderón;Im, Soohyun;Abbasi, Mehrdad;Islam, Minhazul;Hwang, Jinwoo
理解非晶材料不仅是一个基本问题，而且是一个实际问题，因为控制非晶材料中原子构型的灵活性可能会导致晶体材料无法实现的新特性。然而，与晶体材料不同，由于缺乏长程原子有序，表征和理解非晶材料的结构仍然具有巨大的挑战性。相反，几十年的理论和实验工作表明，在某些长度尺度上的原子有序，即短程有序和中程有序（MRO）可能是理解这些材料的重要性质的关键。涨落电子显微镜（FEM）是一种有效的方法来了解MRO和它如何属性的整体程度的结构波动在非晶材料。近年来，通过引入高动态范围像素化STEM检测器，FEM取得了重大进展[1]。这导致了对非晶材料结构的更深入的理解，通过实现扫描纳米衍射图案的全四维采集，这提供了关于MRO如何形成和影响其应用关键的重要特性的统计学重要信息[2，3]。在这项工作中，我们介绍了两种分析方法，使4D-STEM，角相关（AC）和MRO尺寸直方图分析，提供了新的详细信息的纳米级原子或分子在各种非晶材料的排序。AC分析计算作为每个纳米衍射图案内的方位角的函数的自相关函数（图1a），然后将其呈现为散射矢量幅度k的函数，并对100，000个纳米衍射图案进行平均以实现统计显著性。所得到的AC揭示了MRO内的结构对称性，其通过在倒易空间中添加新的维度来补充来自强度方差（也作为k的函数）的信息。另一种方法，MRO尺寸直方图分析，基于通过将4D-STEM数据重建到真实的空间中来直接映射MRO域，然后分析它们在大区域中的尺寸分布，这提供了作为k的函数的MRO尺寸的直方图。这种分析提供了独特的重要信息的类型的MRO（基于他们的k值）与他们的平均sizes.We目前的结果从三种类型的非常不同的无定形功能材料。首先，我们表明，从Zr-Cu-Al块体金属玻璃AC分析显示，当添加Al和Zr浓度逐渐增加时，它们的纳米级对称性发生了显着变化（图1b）。结果揭示了第一个实验证据，即Al的添加使FCC状富Cu MRO变小并增加玻璃形成能力，并且较高的Zr浓度增加了二十面体状有序，这可能对应于先前执行的结合FEM数据的反向蒙特卡罗（RMC）模拟生成的结构[4]（图1c）。其次，我们显示了通过原子层沉积生长的非晶TiO 2膜中MRO的演变作为ALD生长和后退火温度的函数，这最终导致纳米晶体的成核，这对于理解和优化新型光电化学电池的效率和寿命很重要[5，6]。最后，我们展示了高性能有机光伏（OPV）聚合物的研究，这些聚合物在纳米尺度上的分子有序性存在显着差异。比如说...
Understanding amorphous materials is not only a fundamental problem but also a practical one, since controlling the flexible nature of the atomic configuration in amorphous materials can potentially lead to novel properties that are not achievable in crystalline materials. However, unlike in crystalline materials, characterizing and understanding the structure of amorphous materials remain formidably challenging due to the lack of long-range atomic ordering. Instead, a few decades of theoretical and experimental works have revealed that the atomic ordering at certain length scales, namely short-range ordering and medium range-ordering (MRO) may be the key to understanding the important properties of these materials. Fluctuation electron microscopy (FEM) has been an effective way to understand the MRO and how it attributes to the overall degree of structural fluctuation in amorphous materials. In recent few years, significant advances have been made in FEM by the introduction of high-dynamic range pixelated STEM detector [1]. This led to a deeper understanding of the structure of amorphous materials by enabling the full 4-dimensional acquisition of scanning nanodiffraction patterns, which provides statistically significant information about how MRO forms and influence their important properties critical to their applications [2, 3]. In this work, we introduce two analysis methods enabled by 4D-STEM, angular correlation (AC) and MRO size histogram analyses, that provide new detailed information about the nanoscale atomic or molecular ordering in various amorphous materials. AC analysis calculates the autocorrelation function as a function of the azimuthal angle within each nanodiffraction pattern (Fig. 1a), which is then presented as a function of the scattering vector magnitude k, and averaged over∼ 100,000 nanodiffraction patterns to achieve statistical significance. The resulting AC reveals the structural symmetry within the MRO, which complements the information from intensity variance (also as a function of k) by adding a new dimension in the reciprocal space. The other method, MRO size histogram analyses, is based on directly mapping the MRO domains by reconstructing the 4D-STEM data into real space, and then analyzing the distribution of their sizes in a large area, which provides the histogram of the MRO size as a function of k. This analysis provides unique important information about how the types of the MRO (based on their k value) relate to their average sizes.We present the results from three types of very different amorphous functional materials. First, we show that AC analysis from Zr-Cu-Al bulk metallic glasses show remarkable changes in their nanoscale symmetry when Al is added and Zr concentration is progressively increased (Fig. 1b). The result reveals the first experimental evidence that the addition of Al alleviates the FCC-like Cu-rich MRO and increases the glass forming ability, and higher Zr concentration increases the icosahedral-like ordering that likely corresponds to the structure generated by the previously performed reverse Monte Carlo (RMC) simulation incorporating FEM data [4](Fig. 1c). Second, we show the evolution of MRO in amorphous TiO2 films grown by atomic layer deposition as a function of ALD growth and post-annealing temperatures, which eventually leads to nucleation of nano crystals that are important for understanding and optimizing the efficiency and lifetime of the novel photoelectrochemical cells [5, 6]. And finally, we show the investigation of the high performance organic photovoltaic (OPV) polymers that show dramatic differences in their molecular ordering at the nanoscale. For example, the …