Metallic ZnO nanocrystals for Mid IR plasmonics

International audiencePlasmonics has achieved tremendous improvements over the recent decade in sensing (e.g. SERS). However, plasmonics based on noble metal electron gas allows enhancing electromagnetic fields only in the visible and near-IR range. Unfortunately, many molecules exhibit vibrational modes in the mid-IR. In order to enhance the coupling of Mid-IR light with molecular vibrations and thus to improve the sensitivity of IR absorption spectroscopy (the so-called SEIRA: surface enhanced IR absorption), it is necessary to design materials with tunable electron gas density. This is possible in degenerate semiconductors [1, 2]. Among these, oxides and most specifically ZnO are second to none. In the present paper, we address the issue of the design of Ga or Al doped ZnO nanocrystals for Mid-IR plasmonics. We first demonstrate the tunability of the local surface plasmon resonance (LSPR) of the nanocrystals upon Ga (or Al) concentration [3]. The nanocrystals are synthesized by a physical route, relying on the adiabatic supersonic expansion of a plasma. The resulting nanocrystal films are studied structurally by TEM, XDR, chemically by RBS and XPS and optically by FTIR and PhotoAcoustic FTIR. The effect of high doping level in ZnO nanocrystals is investigated and the solubility of Ga and Al in our process is determined. The LSPR of oxide nanocrystals is characterized by a large FWHM as can be seen on figure 1. This characteristic is related to several parameters. Among them, the role of the dopant location and activation is crucial. Consequently we examine both experimentally [4] and numerically [5] their influence on our nanocrystals. We show that the dopant activation can be greatly enhanced if the nanocrystals are synthesized in an O-poor environment. However, no further post-deposition treatment, such as annealing, as proven to be efficient to further improve the dopant activation. This questions the dopant stability in degenerate nanocrystals.Embedding the nanocrystals in a dielectric matrix (Al2O3) does not improve the activation of the dopants. However, it forbids the self-organization of the crystals (a process known as Oriented Attachment) and consequently leads to a reduced FWHM of the plasmon resonance. Eventually, the conditions to observe SEIRA effect will be discussed