Speaker
Description
Strain-engineering as a means to tune and control functional properties is extensively pursued in materials science today. In particular, epitaxial strain has been shown to allow the control of multiferroic properties (magnetic and ferroeletric transitions) as well as conduction states (metal-insulator or superconducting transitions) in thin films of various materials. We propose here a new way to engineer negative strain states using helium ion implantation. Helium is a noble gas, it does not form bonds, and it implants interstitially without modifying the chemistry of the host material, inducing a volume increase that can be understood as an experimental negative pressure. In thin films, due to the in-plane clamping of the film to its subtrate, helium implantation induces an out-of-plane lattice parameter expansion continuously tunable with dose [1].
In this communication, we will show that helium implantation at low doses is able to induce important structural changes and functional properties modifications while still preserving the crystallinity of the material. We could show that in epitaxial BiFeO$_3$ thin films (grown by PLD) helium implantation can trigger a transition towards the super-tetragonal polymorph, allowing for an increase in tetragonality with lattice parameterexpansion up to 8.9% on SrTiO$_3$ substrate [2]. Another study in sol-gel ceramic thin films showed that we could induce out-of-place lattice expansion of up to 3.2% without inducing structural cracking, which is unprecedented in polycrystalline films, whilealso preserving the ferroelectric properties [3]. Other materials will also be discussed, such as V$_2$O$_3$, in which we are able to tune the two metal-insulator transitions under helium implantation: the first one at low-temperature (LT MIT) from the low-temperature monoclinic insulating phase to the hexagonal metallic phase of room temperature, and the second isostructural one at room temperature (RT MIT).
[1]Guo et al., Phys. Rev. Lett., 114, 256801 (2015)
[2]Toulouse et al., Phys. Rev. Mat, 5 (2), 024404 (2021)
[3]Blázquez Martínez et al., APL Mat. 13 (2), 021111 (2025)