Cold Injection Synthesis of Nanoheterostructures based on Cluster Decomposition

纳米晶体已经引起了相当大的兴趣，因为它们承诺了功能材料小型化的巨大潜力，并且具有取决于其尺寸的物理性质。它们已通过湿化学方法合成，结构越来越复杂。复杂性可以通过制造具有各向异性形状的颗粒（例如纳米棒）或不同材料的复杂异质结构（例如具有多个外壳的颗粒）来实现，或者两者兼而有之。新材料在纳米结构上的沉积通常需要两个前体分子的反应，这两个前体分子在种子颗粒的表面发生非均相成核反应。因此前体反应需要匹配。此外，还需要考虑晶格错配和表面配体的影响。因此，纳米颗粒的制备通常在高温下进行。如果复杂的形态暴露在热下，它们会迅速降解。各向异性的形状熔化成球形颗粒，而异质结构中的界面形成材料梯度。因此，高温限制了通过湿化学方法可以获得的纳米结构的复杂性。温和的反应条件、较低的温度以及前驱体分解反应的缺失将规避这些限制，并允许合成范围更广的纳米材料。在本项目中，我们的目标是建立一种基于“冷注射”方法形成纳米晶异质结构的新合成路线。在这里，化学触发器而不是物理参数启动纳米晶体的生长反应。我们打算采用超小型半导体簇作为种子生长的材料来源。这些团簇可以在非原位生产，并作为目标材料的储存库。由于它们的分子结构，它们可以在明确的条件下分解。通过将半导体化合物的形成与生长反应分开，一般方法将适用于广泛的种子材料。使用化学触发器来启动反应允许在非常温和的条件下和热力学控制下工作，这对于热不稳定的形态至关重要。因此，以更低的能源成本制造确定的异质结构将成为可能。我们将把该原理应用于两个半导体模型系统，即核/壳粒子中“硬”势步骤的形成和各向异性粒子仅在一个方向上的区域选择性生长。这两种体系都对纳米异质结构的基本理解和胶体纳米晶体在复杂材料和电子器件中的未来应用具有很高的兴趣。然而，所提出的反应将是非常有益的，超出了这个项目的范围，例如，用杂质掺杂修饰纳米晶体，其中掺杂物的扩散必须被抑制，或者纳米晶体已经组装成一个更大的上层结构。

Nanocrystals have attracted considerable interest, because they promise large potential for miniaturisation of functional materials and have physical properties that depend on their size. They have been synthesised by wet-chemical means with more and more complex architectures. Complexity is either achieved by making particles with anisotropic shape (e.g. nanorods) or in complex heterostructures of different materials (e.g. particles with multiple shells), or both. The deposition of a new material onto a nanostructure requires the reaction of usually two precursor molecules that react at the surface of a seed particle in a heterogeneous nucleation reaction. Therefore precursor reactivities need to be matched. In addition, lattice mismatch and effects of surface ligands need to be taken into account. Therefore, the fabrication of nanoparticles usually occurs at high temperatures. If complex morphologies are exposed to heat they quickly degrade. Anisotropic shapes melt into spherical particles, while interfaces in heterostructures form a material gradient. High temperature thus poses a limit to the complexity of nanostructures that are accessible by wet-chemical methods. Mild reaction conditions, low temperature, and the absence of precursor decomposition reactions would evade these restrictions and allow the synthesis of a much wider range of nanomaterials. In this project, we aim to establish a new synthetic route to form nanocrystalline heterostructures based on a "cold injection" approach. Here, chemical triggers rather than physical parameters initiate the growth reaction of a nanocrystal.We intend to employ ultrasmall semiconductor clusters as source of material for seeded growth. These clusters can be produced ex-situ and act as a reservoir of the target material. Due to their molecular structure they can be decomposed under well-defined conditions. By separating the formation of the semiconductor compound from the growth reaction the general approach will be applicable to a broad spectrum of seed materials. The use of a chemical trigger to initiate the reaction allows working under very mild conditions and thermodynamic control, which is crucial for thermally unstable morphologies. Fabrication of defined heterostructures will thus be possible at much reduced energy costs. We will apply the principle to two semiconductor model systems, the formation of "hard" potential steps in core/shell particles and regio-selective growth of anisotropic particles in only one direction. Both systems are of high interest for both a fundamental understanding of nanoheterostructures and future application of colloidal nanocrystals in complex materials and electronic devices. However, the proposed reaction will be highly beneficial beyond the scope of this project, e.g. for modification of nanocrystals with impurity doping, in which dopant diffusion must be suppressed, or for nanocrystals that have been assembled into a larger superstructure.