Cation Exchange in Single Nanowires

在宏观固体中，阳离子交换反应通常非常缓慢，因此常常是不利的。然而，纳米结构具有非常大的表面积与体积比，这使得阳离子交换的进展速度比相应的块体材料快许多个数量级。因此，阳离子交换反应已经成为一种非常流行和有效的方法来改变纳米结构的材料组成。然而，目前对单个纳米结构中的阳离子交换反应动力学知之甚少。在宏观固体和纳米粒子系综中，阳离子交换反应似乎持续进行。然而，在这些情况下，阳离子交换反应是在许多结构域、缺陷和晶面上平均的。因此，在单个纳米结构水平上的反应动力学可能会有明显的不同。我们的项目的目标是通过光学和电子测量的手段来控制和分析单个纳米线中的阳离子交换反应。作为模型系统，我们将研究从CdSe到Ag2Se和从CdS到Ag2S的众所周知的反应。在我们的实验室里，合成和接触单根纳米线已经是很成熟的程序了。我们将把我们的在线监测阳离子交换的方法扩展到单根纳米线，并包括光学测量技术。我们计划研究以下问题：a)在阳离子交换过程中，单根纳米线水平的电导、荧光和光吸收是如何变化的，关于反应动力学可以得出哪些结论？B)通过阳离子交换可以多精确地控制单个纳米线的电子和光学性质？C)在单根纳米线中，从CdSe到Ag2Se的阳离子交换有多均匀？D)在纳米线中从CdS到Ag2S的阳离子交换反应中出现的条状相在电导率中是否有相应的特征？除了材料科学方面的基本兴趣外，我们的结果还可以应用于半导体掺杂、电池电极和化学传感器。

Cation exchange reactions are typically very slow in macroscopic solids and are therefore often not favorable. Nanostructures, however, have very large surface to volume ratios, which enable cation exchange to progress many orders of magnitude faster than in the corresponding bulk materials. Cation exchange reactions have therefore become a very popular and efficient rout for modifying the material composition of nanostructures. However, currently relatively little is known about the kinetics of cation exchange reactions in individual nanostructures. In macroscopic solids and in ensembles of nanoparticles, cation exchange reactions appear to proceed continuously. However, in these scenarios cation exchange reactions are averaged over a multitude of domains, defects, and crystal facets. Reaction kinetics could therefore be markedly different at the single nanostructure level.The goal of our project is to control and analyze cation exchange reactions in individual nanowires in situ by means of optical and electronic measurements. As model systems we will study the well-known reactions from CdSe to Ag2Se and from CdS to Ag2S. Synthesizing and contacting single nanowires are already well established procedures in our labs. We will extend our method for in situ electrical monitoring of cation exchange on nanowire ensembles to single nanowires and to include optical measurement techniques.We plan to investigate the following questions: a) How does the electrical conductivity, fluorescence, and optical absorption change at the single nanowire level during cation exchange, and which conclusions can be drawn with respect to reaction kinetics? b) How precisely can the electronic and optical properties of individual nanowires be controlled through cation exchange? c) How homogeneous is cation exchange from CdSe to Ag2Se in single nanowires? d) Does the appearance of striped phases during the cation exchange reaction from CdS to Ag2S in nanowires have corresponding signatures in the electrical conductivity? In addition to being of basic interest in materials science, our results could find applications in semiconductor doping, battery electrodes, and in chemical sensors.