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

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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.