ZnO nanoparticles: from intense visible emission to Mid IR plasmon
Résumé
As far as optics is concerned, ZnO nanostructures have first been considered as potential interesting UV emitters to compete with GaN ones. Because of the difficulty to achieve stable p-doping in ZnO, other optical properties than the large bandgap (3.4 eV) and large UV exciton binding energy (~ 60 meV) have been harnessed. First, the size reduction in nanoparticles and the concomitant predominance of the surface increase the presence of specific crystalline defects (Zn vacancies and O interstitials). These defects happen to behave as very efficient light-emitters. Their emission can be so intense as to lead to single photon emission. However, the stability in time of their high photoluminescent quantum yield (PL QY) still remains a challenge. In the present contribution, we first show that by controlling the synthesis ambient in the hydrolysis process of diethyl zinc by the addition of some weak polymeric acid (PAA, polyacrilic acid) we can design hybrid organic/inorganic ZnO based nanomaterials which exhibit stable PL QY up to 70% in the green range of the visible spectrum. Therefore, by carefully controlling the nature and size of the polymeric chains, we are able to synthesize scalable amount of visible light emitters that can compete with the well-established chalcogenide nano-emitters. Second, ZnO can be doped by Al (AZO) or Ga (GZO) at large concentration (up to a few percent). This leads to an alloy which behaves as a good TCO (transparent conductive oxide). This property is currently exploited to design transparent electrodes for photovoltaic cells for instance. The resulting semiconductor being degenerate, it possesses a free electron gas which can sustain a plasmon resonance. The major interest is the tunability of the electron gas concentration through the tuning of the dopant (Al or Ga) concentration. Since the plasmon resonance frequency is proportional to the square root of the electron gas concentration, these materials are promising candidates to transfer the outstanding results of plasmonics achieved with noble metals (Ag and Au) in the visible range to the IR range (in the IR range, the noble metals do not allow the tuning of their electron gas concentration and exhibit very large losses). In particular, since most of the chemical molecules have vibration frequencies in the mid IR range, their resonant coupling with localized surface plasmons (LSPR) is particularly interesting to design highly sensitive gas sensors. Therefore, in the second part of this contribution, we will present how the dopant concentration (Al or Ga) can allow tuning the LSPR of nanoparticles in the mid IR range. Solutions to current issues of the LSPR damping and broadening will be addressed in relation with the dopant localization and chemical state. We will show that degenerate metal oxide nanoparticles, and in particular Ga or Al doped ZnO nanoparticles, are outstanding materials for mid IR plasmonics.