Precise assembling of thin films with nanoscale building blocks

Significance 

Thin films have a vast range of applications including microelectronics and energy conversion owing to their unique properties. Numerous technologies are available for fabricating thin films. They majorly rely on the assembly of molecules and atoms as their elemental building blocks. For example, growth and nucleation of thin films in chemical and physical vapor deposition methods depend on surface diffusion, adsorption, and chemical binding.  Unfortunately, the presence of defects in thin films impacts negatively on their functional and structural properties.   Therefore, proper understanding and control of these defects will give way to fabricating structures with efficient structural and functional properties.

Presently, clusters have been used as the building blocks for nanostructured thin films. This is due to the increasing interest in the gas phase deposition. Cluster beam deposition (CBD) technology, for example, allows deposition of clusters on a substrate produced in the gas phase thus enabling fabrication of efficient nanostructured thin films and devices. It does not alter the individual properties of the nanoscale building blocks during the assembly process. Consequently, supersonic cluster beam deposition (SCBD) is an improvement of the CBD to enhance deposition rate and lateral resolution. It also enables control of various parameters of nanostructured films such as the roughness which is a key consideration in tailoring their structural and functional properties.

However, microscopic mechanisms that influence the structuring and growth of the nanostructured thin films have not been fully explored experimentally.  Among the proposed theoretical models to describe the growth of the cluster-assembled films in the sub-monolayer level is the deposition diffusion and aggregation (DDA) model. Despite taking into consideration all the three main physical mechanisms, it still experiences challenges just like the other models.

Italian scientists: Dr. Francesca Borghi, Professor Alessandro Podestà, Dr. Claudio Piazzoni, and Professor Paolo Milani at Università degli Studi di Milano investigated the microscopic mechanisms influencing the growth and structural properties of cluster-assembled films using atomic force microscopy. The authors used cluster-assembled zirconia films deposited by the supersonic cluster beam deposition method. They purposed to investigate the influence of the dimensions of the building blocks on the growth mechanisms and the final surface roughness of the resulting nanostructured films. Furthermore, the growth process from the submonolayer to thin film regime was also investigated. The work is published in the journal, Physical Review Applied.

The authors observed that before deposition, the cluster sizes affect the growth dynamics. For instance, small cluster sizes favor nucleation and surface diffusion on silicon substrate leading to two-dimensional growth while the large cluster sizes favor three-dimensional growth mode by acting as nucleation sites. In addition, various morphological properties of the cluster-assembled films were determined by the growth dynamics in the submonolayer regime. However, roughness evolution with respect to the number of clustered deposited exhibited growth exponent equivalent to that of ballistic deposition model.

The study has successfully provided adequate information regarding the effects of the microscopic mechanisms on the structuring and growth of the nanostructured thin films and especially during the transition from the submonolayer to the thin-film regime. With the possibility to control the nanostructuring and mesostructuring of the nanostructured films, the study will advance its applications in large scales.

Precise assembling of thin films with nanoscale building blocks - Advanced Engineering

About the author

Francesca Borghi is a postdoc researcher at the Department of Physics of the University of Milano, and at the Interdisciplinary Centre for Nanostructured Materials and Interfaces; she graduated in Physics in 2011, and she obtained her PhD title in Physics in 2015. Her main work concerns the characterization of the interfacial chemico-physical properties of nanostructured thin films produced by low-energy clusters beam deposition in gas phase. Francesca Borghi is author of more than 16 publications and her H-index is 8.

About the author

Alessandro Podestà is Associate Professor at the Department of Physics of the University of Milano, where he got the MS in Physics in 1998 and the PhD in Materials Science in 2002; his main research interests pertain to the study and characterization of physical and chemical properties of interfaces at the nanometer scale using scanning probe techniques based on Atomic Force Microscopy (AFM). He works in an interdisciplinary research environment, collaborating with physicists, biologists, chemists, engineers. AP is author of more than 85 publications on peer-reviewed journals and his H-index (WoS) is 23. AP is member of the Editorial Board of Scientific Reports (Nature Springer). AP participates to several funded research projects, in some cases as Principal Investigator or local coordinator.

About the author

Claudio Piazzoni is EP technical staff at the Physics Department of Università degli Studi di Milano, where he graduated in 2001. CP is the site manager of CIMaINa laboratories. His technical activity spans through safety, maintenance, logistics, and organization of CIMaINa lab services and facilities

About the author

Paolo Milani is Full Professor at the Department of Physics of the University of Milano. He graduated in Physics from the University of Pavia (Italy) in 1984 and he received the PhD in 1991 from the Ecole Polytechnique Federale of Lausanne. He founded in 1992 the Molecular Beams and Nanocrystalline Materials Laboratory at the University of Milano.

His research focuses on cluster-assembled nanostructured materials and their integration on microfabricated platforms by exploiting the supersonic cluster beam deposition technology. He is active in the field of nanostructured systems and devices for stretchable electronics and optics, biotechnology and medicine, energy production and storage.

He has published more than 250 papers on refereed journals (h index=50), several review papers and a monograph on supersonic cluster beam deposition for the synthesis of nanostructured thin films. Currently, Milani serves as Director of the Interdisciplinary Centre for Nanostructured Materials and Interfaces of the University of Milano. He is co-founder of three start-up companies.

Reference

Borghi, F., Podestà, A., Piazzoni, C., & Milani, P. (2018). Growth Mechanism of Cluster-Assembled Surfaces: From Submonolayer to Thin-Film Regime. Physical Review Applied, 9(4).

Go To Physical Review Applied

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