Fast Pixel Detectors: a paradigm shift in STEM imaging

这里提出的研究旨在开发全新的电子显微镜成像方法，并将这些方法用于研究现实世界的材料问题。为了说明我们提出的方法的力量，我们从碳纳米技术、生命科学和电子器件结构领域选择了原理证明材料问题。在过去的几十年里，扫描透射式电子显微镜(STEM)因其高空间和高能量分辨率而变得越来越受欢迎。我们对宏观材料性质与原子结构和成键的关系，以及我们如何通过操纵这些性质来控制性质的大部分理解，都是在很短的尺度上表征材料的技术发展的结果。STEM不仅能够成像原子并观察材料的结构和晶体细节，而且还能够对单个原子进行光谱分析，从而能够逐个原子地确定化学成分。另一个关键的发展是球面像差校正器，它克服了电子透镜早期的限制，使这些仪器的性能发生了革命性的变化。STEM的原理是使用电子透镜聚焦一束电子，形成一个小的照明点或探测器。该探头可以使用光束偏转器扫描样品。使用薄的、电子透明的样品，并且可以探测到传输的电子，并且在二维扫描期间将被探测到的电子的强度作为探测器位置的函数来绘制以形成图像。最常用的探测器是环形暗场(ADF)探测器和明场(BF)探测器，ADF探测器是一种宽环形式的探测器，它检测相对较高的散射角，明场(BF)探测器收集未散射和低角度散射的电子。这两个探测器都收集一定范围的散射角上的总散射，并使用总强度来形成图像。这种方法忽略了作为散射角的函数出现的强度波动中所包含的丰富信息。构成当前建议的基础的主要目标是使用快速像素化探测器来记录作为STEM的探测器平面中的散射角的函数的强度，这实际上是一种衍射图案。通过在二维扫描中记录作为探测器位置的函数的每个二维衍射图，可以记录作为最终STEM成像实验的四维数据集。这样一个丰富的数据集包含有关传输引起的相移、关于样品的组成、样品中的应变和样品中的三维有序的信息。我们建议开发使用快速像素化探测器记录这种4D数据集的方法，并通过开发优化的直接检测系统，以及处理这些数据集的方法来进行物理上有用的测量。我们相信我们正在采取的方法将创造STEM成像的范式转变，并最终将成为在STEM中记录数据的标准方法。为了说明这种方法的力量，我们已经确定了关键的材料科学问题，我们将用我们开发的方法来解决这些问题。这些应用包括：(I)掺杂纳米材料中的电荷转移成像；(Ii)软材料和辐射敏感材料的成像；(Iii)磁性纳米结构中电场和磁场的成像；(Iv)陶瓷中的三维组成和结构有序效应；(V)陶瓷和半导体异质界面中的互扩散。开发的方法将得到广泛传播，特别是通过它们在EPSRC国家像差校正STEM(SuperSTEM)上的实施，广泛的用户将能够通过它获得新的方法。

The research proposed here aims to develop entirely new ways of imaging in the electron microscope, and to use these methods to study real-world materials problems. To illustrate the power of the methods we propose to develop, we have selected proof-of-principle materials problems from the areas of carbon nanotechnology, life sciences and electronic device structures.Over the past couple of decades a particular type of electron microscope, the scanning transmission electron microscope (STEM) has become increasingly popular due to its high-spatial and energy-resolution. Much of our understanding of how macroscopic materials properties relate to atomic structure and bonding, and how we can control properties by manipulating these, is a result of the development of techniques to characterise materials on very short length scales. The STEM is not only capable of imaging atoms and observing the structure and crystallographic details of materials, but also in performing spectroscopy on single atoms, allowing atom-by-atom chemistry to be determined. A further key development is the spherical aberration corrector, which has revolutionised the performance of these instruments by overcoming the earlier limitations of electron lenses.The principle of STEM is the use of electron lenses to focus a beam of electrons to form a small illuminating spot or probe. The probe can be scanned across a sample using a beam deflector. A thin, electron-transparent sample is used, and transmitted electrons can be detector, and the intensity of those detected plotted as a function of the probe position during a two-dimensional scan to form an image. The mostly commonly used detectors are an annular dark-field (ADF) detector which is a detector in the form of a broad annulus that detects relatively high angles of scatter, and bright-field (BF) detectors that collect the unscattered and low-angle scattered electrons. Both these detectors collect the total scattering over a range of scattering angles, and the total intensity is used to form an image. Such an approach neglects the rich information that is contained in the fluctuation in intensity that occurs as a function of scattering angle.The overarching aim that forms the basis of the current proposal is to use fast pixelated detectors to record the intensity as a function of scattering angle in the detector plane of a STEM, which is effectively a diffraction pattern. By recording each two-dimensional diffraction pattern as a function of probe position in a two-dimensional scan, a four-dimensional data set can be recorded that is the ultimate STEM imaging experiment. Such a rich dataset contains information about the phase shift that results from transmission, about the composition of the sample, the strain in the sample and the three-dimensional ordering in the sample. We propose to develop the methods to record this 4D data set, using fast pixelated detectors, and by developing an optimised direct-detection system, together with the methods to process such datasets to enable physically useful measurements to be made.We believe the approach we are taking will create a paradigm shift in STEM imaging, and in time will become the standard approach to record data in the STEM. To illustrate the power of the approach, we have identified key materials science questions that we will address with the methods we develop. The applications are: (i) imaging charge transfer in doped nanomaterials; (ii) imaging of soft and radiation sensitive materials, (iii) imaging of electric and magnetic fields in magnetic nanostructures, (iv) 3D composition and structural ordering effects in ceramics; (v) interdiffusion in ceramic and semiconductor heterointerfaces.The methods developed will be disseminated widely, particularly through their implementation at the EPSRC National Facility for Aberration-Corrected STEM (SuperSTEM) through which a wide range of users will be able to access the new methods.