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Ultrasensitive optical probes made from doped Si nanocrystals

Silicon stands for plasmonics

par Guy Molénat - publié le

Plasmonics, which uses the collective oscillations of free electrons localized in doped metallic or semiconductor nanostructures (plasmon resonances), allows for the manipulation of light on a much smaller scale than its wavelength, down to nanometric dimensions.

Researchers from CEMES collaborated with four other French laboratories to demonstrate in a recently published study the appearance of surface plasmon resonances carried by small (10 nm) silicon nanocrystals doped with phosphorus and embedded in a silica matrix. These Si nanocrystals were fabricated in the lab using low energy ion implantation, and their plasmon resonance was measured using Fourier Transform IR spectroscopy in the mid-infrared (IR) (4-7 µm). This resonance can be tuned across a wide spectral range by simply adjusting the free carrier concentration via phosphorus doping.

Left : Energy filtered electron microscopy (EFTEM) image of P doped SiNCs. Inset, atom probe tomographic image showing the distribution of P atoms (black) within a SiNC (red). Right : Number of free electrons/NC as a function of the carrier concentration. Inset : FTIR measurements of the surface plasmon resonance for different implanted P doses.

By comparing these experimental results to numerical simulations, we were able to demonstrate a coupling between this plasmon mode and the IR phonons of the surrounding silica matrix. We also demonstrated that only ten free electrons inside the nanostructure are enough to generate a plasmon. Finally, the nanocrystals’ small size, well below the mean free path of electrons, drastically reduces their mobility, resulting in a broadening of the plasmon signature. The formation of phosphorus aggregates inside the nanocrystals exacerbates this phenomenon for high doping.

This work was funded by the ANR DONNA (Doping at the Nanoscale, ANR-18-CE09-0034).

Publication : M. Zhang, JM Poumirol, N. Chery, C. Majorel, R. Demoulin, E. Talbot, H. Rinnert, C. Girard, F. Cristiano, P. R. Wiecha, T. Hungria, V. Paillard, A. Arbouet, B. Pécassou, F. Gourbilleau and C. Bonafos
Nanophotonics 11 (15), 3485 (2022).

Contact : Caroline Bonafos, caroline.bonafos[at]cemes.fr
Jean-Marie Poumirol, jean-marie.poumirol[at]cemes.fr