Energetic ion-assisted nanocomposite film growth: sculpting matter in 3D at the nanoscale

Nanostructures dramatically influence materials properties due to size, shape and interface effects. Thus the control over the structure at the nanoscale is a key issue in nanomaterials science. The growth of nanostructured thin films is governed by the atomistic processes at the very surface of the growing film. The interaction range of hyperthermal ions with solid surface is confined to some nanometers. Therefore energetic ion assistance during film deposition is of primary relevance in the context of the thin film nanostructuring.
Our recent activities in the field of ion-assisted physical vapor deposition (PVD) of nanocomposite films will be summarized with the focus on the growth-structure relationship of carbon-transition metal films. This class of materials is relevant in the context of tribology, sensing, fusion, electrochemistry, information storage, spintronics, solar-thermal energy conversion or as metamaterials. We have employed two paradigms involving the use of energetic ions:

• ion assisted PVD (ia-PVD), where the growing film is irradiate with assisting ion beam
• ionized PVD (i-PVD), where the depositing species are energetic ions themselves.

The nanocomposite film growth is determined by surface diffusion assisted phase separation of immiscible species adsorbed from homogeneous mixture in the vapor phase. Ion irradiation intervenes with the interplay of thermodynamic forces and kinetic constraints as a third competing factor and has a dramatic effect on the film morphology. A large variety of structures is observed such as encapsulated nanoparticles, high aspect ratio nanocolumns or self-organized layered 3D nanoparticle arrays (see Fig 1.). In addition, the ion induced atomic mobility is not isotropic, as it would be in the case of thermally excited migration, but conserves to a large extent the initial direction of the incoming ions. Independently of the growth regime, it results in the morphology tilt: metal nanopatterns no longer align with the advancing surface but with the incoming ions. Such effects allow 3D sculpting of nanocomposites which is due to ion irradiation effects and does not require any glancing incidence conditions.
As the observed effects are of physical origin (ion-solid interactions), we believe that the presented results are applicable to other immiscible or partially miscible systems as well. This presents a possible path towards a material design approach based on material system independent tools to sculpt the morphology at the nanoscale in order to match the requirements of a wide range of applications.