Materials World Network: Interplay Between Quantum Size Effect and Strain Effect on Growth of Nanoscle Metal Thin Films

当一种薄膜材料生长在不同晶格尺寸的衬底材料上时，应变效应在异质外延生长中普遍存在。另一方面，在半导体衬底上生长金属薄膜时，当金属薄膜非常薄时，量子尺寸效应(QSE)占主导地位。然而，在过去，这两个重要的影响被单独或单独研究，尽管它们在许多情况下都存在。材料世界网络项目启动了美国和中国之间的国际合作，首次对这两种效应进行了系统的研究，以揭示它们在影响半导体衬底上金属薄膜生长热力学和动力学方面的基本关系和相互作用。这项研究将极大地扩展我们控制金属纳米薄膜和纳米结构生长形态的知识基础。这个多壁纳米项目包括一项研究纳米厚金属薄膜生长的量子尺寸效应和应变效应的理论研究，并在北京清华大学开展了同一主题的协同实验研究的国际合作，中国。本研究主要以在硅衬底上生长铅薄膜为模型系统，旨在回答三个基本问题：(1)量子自旋效应和应变效应之间有什么关系？(2)这两种效应如何相互作用而影响金属薄膜的热力学稳定性，特别是改变其表面能及其厚度关系？(3)这两种效应如何共同作用来影响金属薄膜的生长动力学，例如通过共同改变表面扩散能垒？这项研究将为理解QSE和应变效应带来新的见解，不仅是个体的，而且是整体的，并为有望引起广泛兴趣的新的研究方向奠定基础。该项目的一个特殊成果是通过美国和中国之间的国际学生交换项目，不仅在基础研究方面而且在全球范围内培养具有全球视野的研究生。

The strain effect is ubiquitous in heteroepitaxial growth when one kind of thin film material is grown on a substrate material of different lattice size. On the other hand, in the growth of metal thin films on semiconductor substrates, the quantum size effect (QSE) effect becomes dominant when the metal film is very thin of a few nanometers thick. In the past, however, these two important effects have been studied alone or separately despite the fact they are both present in many cases. This Materials World Network (MWN) project launches an international collaboration between US and China to carry out the first systematic study of both these effects to reveal their fundamental relationship and interplay on affecting the growth thermodynamics and kinetics of metal thin films on semiconductor substrates. The study will greatly expand our knowledge base to control the growth morphology of metal nanofilms and nanostructures.This MWN project encompasses a theoretical study to investigate the quantum size effect (QSE) and strain effect on growth of nanometer-thick metal thin films, augmented by an international collaboration of synergistic experimental study on the same topic at Tsinghua University in Beijing, China. Using primarily the growth of Pb thin films on Si substrate as a model system, the proposed research aims to answer three fundamental questions: (1) What is the relationship between the QSE and strain effect? (2) How these two effects will interplay with each other to affect the thermodynamic stability of metal thin films, particularly changing the surface energy and its thickness dependence? (3) How these two effects will act together to affect the growth kinetics of metal thin films, e.g. by changing together the surface diffusion energy barrier? The study will bring out new insights to the understanding of the QSE and strain effect not only individually but also as a whole, and lay the groundwork for a new research direction that is expected to generate broad interest. A special outcome of this project is the education and training of graduate students not only in fundamental research but also with a global perspective through international student exchange program between US and China.