Near atomistic tomographic imaging of PbX quantum-dot superlattices for improved electronic and structural order

量子点是非常小的颗粒，其性质可以通过改变点的大小、形状或组成来改变。 这项研究是关于理解这些量子点之间的相互作用，这些量子点被排列成有序的固体一旦量子点被组织成有序的固体，称为超晶格，那么固体就会表现出新的光学和电子特性，这些特性来自量子点之间的相互作用。量子点超晶格的性质可以通过改变量子点之间的耦合来控制。通过控制粒子之间的距离，通过用桥连接量子点，或者通过用另一种材料填充量子点之间的空间来改变电子耦合。这项研究旨在制造更有序的量子点超晶格，以探索材料特性，并用于太阳能电池，光电探测器和热电器件。然而，很难研究人们看不到的结构。为了克服这一障碍，将开发利用近原子直接空间成像的高分辨率扫描透射电子断层摄影术。这种新的高分辨率断层扫描数据将提供足够的细节，以提供样品制造和所得超晶格有序之间的反馈，从而能够制造具有更大超晶格畴、更均匀分布的桥和更少缺陷的更完美的样品。新的高分辨率数据还将使新的理论方法能够模拟固体中量子点之间的相互作用，从而使超晶格有序度的增加与光学和电子特性的特定变化联系在一起。长期目标是从量子点中开发出足够完美的固体，以将电荷迁移率提高约10倍。这项研究将通过在可公开下载的论坛上发布扫描透射电子断层扫描数据并创建有关将在互联网上发布的材料的非技术教育视频来与公众分享。向服务不足的社区进行宣传和教育将提供实践STEM培训。以超晶格形式组织的胶体量子点（QD）已经展示了跨中尺度维度的集体电子和激子行为。小程度的空间无序、表面化学缺陷和外延缺陷如何影响这种集体行为或如何制造更完美的超晶格结构的细节尚不清楚。该项目将使用分辨率为4-5 μ m的层析成像，超过1000个QD，以测量真实的空间中这些小程度的结构无序。本研究着重于改进成像技术以实现更高的分辨率，并改进重建技术以增加图像体积。图像质量的这些改进将实现所有量子点、颈部和缺陷的近原子映射，推动制造、结构控制和对电子结构/性质关系的理解的改进。近原子分辨率成像的反馈将能够改进制造，目标是100%颈部连接性和均匀性，超晶格晶粒尺寸至少为10 µm，电荷迁移率接近50 cm 2 V-1 s-1。改进的样品质量和高分辨率3D真实空间成像将促进理论方法，可以通过离域“迷你带”研究跳跃与电荷传输，并将通过变温霍尔效应测量进行验证。所提出的层析成像推动了分辨率/体积的极限，实现了具有高空间分辨率的大型中尺度样本的重建。预期的结果是多个超高分辨率的断层图像，其告知结构形成机制、改进的制造、形成QD-QD颈部的质量传输以及基于数据告知现实电子建模的空间分辨率。研究目标是多管齐下的制造设计规则，可应用于其他量子点超晶格，改进的扫描透射电子断层扫描技术，以提高断层空间分辨率和数据解释，以及使用真实的空间数据的离域输运的中尺度建模。通过结合这些方法，该项目将纳米结构、中尺度有序和散装材料性质联系起来。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Quantum dots are very small particles whose properties can be changed by changing the size, shape, or composition of the dot. This research is about understanding the interactions between these quantum dots that have been arranged into ordered solids Once the quantum dots are organized into ordered solids, called a super-lattice, then the solids exhibit new optical and electronic properties that arise from the interaction between the quantum dots. The properties of the quantum dot super-lattices are controllable by changing the coupling between the quantum dots. The electronic coupling is changed by controlling the distance between particles, by connecting the quantum dots with bridges, or by filling in the spaces between the dots with another material. This research seeks to fabricate more ordered quantum dot super-lattices to explore materials properties with utilization in devices like solar cells, photodetectors, and thermoelectrics. However, it is hard to investigate structures that one cannot see. To overcome this roadblock, the use of high-resolution scanning transmission electron tomography with near-atomic direct-space imaging will be developed. This new high-resolution tomographic data will provide sufficient detail to provide feedback between sample fabrication and resulting superlattice order to enable the fabrication of more perfect samples with larger super-lattice domains, more evenly distributed bridges, and fewer defects. The new high-resolution data will also enable new theoretical approaches to model the interaction between quantum dots in the solid so that increases in super-lattice order can be tied to specific changes in the optical and electronic properties. The long-term goal is to develop solids from quantum dots that are perfect enough to increase the charge mobility by about ten times. This research will be shared with the public by publishing the scanning transmission electron tomography data on a publicly downloadable forum and creating non-technical educational videos about the materials to be published on the internet. Outreach and education to underserved communities will provide hands-on STEM training.Colloidal quantum-dots (QDs), organized in a super-lattice, have demonstrated collective electronic and excitonic behavior across mesoscale dimensions. The specifics of how small degrees of spatial disorder, surface chemical defects, and epitaxial defects affect this collective behavior or how to fabricate more perfect super-lattice structures are not understood. This project will use tomographic imaging with a resolution of 4-5 Å over 1000s of QDs to measure these small degrees of structural disorder in real space. This research has a strong emphasis on improving the imaging technique to enable higher resolution and to improve the reconstruction technique to increase the image volume. These improvements to the image quality will enable near atomic mapping of all QDs, necks, and defects, driving improvement in fabrication, structural control, and understanding of electronic structure/property relationships. The feedback of near atomic resolution imaging will enable improved fabrication with the goals of 100% neck connectivity and uniformity with super-lattice grain sizes of at least 10 µm and charge mobility approaching 50 cm2 V-1 s-1. The improved sample quality and high-resolution 3D real-space imaging will facilitate theoretical approaches that can study hopping vs. charge transport through delocalized “mini-bands” and will be validated by variable-temperature Hall-effect measurements. The proposed tomography pushes the limits of resolution/volume achieving reconstructions of large mesoscale samples with high spatial resolution. The expected outcome is multiple ultra-high-resolution tomograms that inform the structure formation mechanism, improved fabrication, mass transport to form QD-QD necks, and spatial resolution to inform realistic electronic modeling based on data. The research goals are multi-pronged with focus on fabrication design rules that can be applied to other QD super-lattices, improved scanning transmission electron tomography techniques to enhance tomogram spatial resolution and data interpretation, and mesoscale modeling of delocalized transport using real spatial data. By combining these approaches this project connects between nanoscale structure, mesoscale order, and bulk materials properties.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.