Degenerate metal oxide nanocrystals (NCs) are promising systems to expand the significant achievements of plasmonics into the infrared (IR) range¹. We report on the tunable mid IR Plasmon induced in degenerate Al and Ga doped ZnO (AZO and GZO) nanocrystals. The NCs were obtained by Low Energy Cluster Beam Deposition (LECBD). By varying the Al and Ga content from 3 to 9 at.% within the particles we are able to tune the plasmon wavelength from 3 to 4 μm.   However, the plasmon resonances are characterized by an unusually large damping, which originates from two mechanisms. The first one is the Oriented Attachment (OA) process (cf. figure 1, B)³. When NCs attach by epitaxy, the resulting structure has a lower symmetry, which induces a shift and broadening of the plasmon resonance. Embedding the particles in an Al₂O₃ matrix has prevented the OA, and hence the damping was reduced along with broadening. The second mechanism is the partial activation of the dopants. We have observed that less than half of the dopants actually participate to the electron gas². The cause of the partial activation is related to the position of the dopants within the particles. It has been proposed that the damping is larger if the dopants are homogeneously distributed⁴. In the present work we investigate the possibility of mapping the spatial distribution of dopants within the nanocrystals usingFEI-TITAN ETEM equipped with High resolution Gatan GIF (see Figure 2). We subsequently anneal the nanocrystals to let them reach the thermodynamic equilibrium. The distribution of dopants, and its consequences on the plasmon resonances is then investigated [5,6].

References
[1] G.V. Naik, V.M. Shalaev, A. Boltasseva, “Alternative Plasmonic Materials: Beyond Gold and Silver”, Adv. Mat. 2013, 25, 3264-3294.
[2] M. K. Hamza, J.-M. Bluet, K. Masenelli-Varlot, B. Canut, O. Boisron, P. Melinon and B. Masenelli. ”Tunable mid IR plasmon in GZO nanocrystals”. Nanoscale, 2015, 7, 12030
[3] D. Hapiuk, B. Masenelli, K. Masenelli-Varlot, D. Tainoff, O. Boisron, C. Albin and P. Melinon.”Oriented Attachment of ZnO Nanocrystals ”. J. Phys. Chem. C, 2013, 117, 10220–10227.
[4]  S. D. Lounis, E. L. Runnerstrom, A. Bergerud, D. Nordlund  and D. J. Milliron, J. Am. Chem. Soc., 2014, 136, 7110–7116.
[5] V. C. Holmberg, J. R..Helps, K. A. Mkhoyan, D. J. Norris “Imaging Impurities in Semiconductor Nanostructures”, Chem. Mater., 2013, 25 (8), pp 1332–1350
[6] Thanks are due to the CLYM (Centre Lyon – St-Etienne de Microscopie, www.clym.fr) for access to the microscope.

Figures:

Figure 1 A) FTIR reflectance spectrum of 3% doped GZO nanocrystals deposited on Si substrates (red curve), and nanocrystals embedded in an Al2O3 matrix (Blue curve). B) HRTEM image of GZO nanocrystals. The Oriented attachment is highlighted: two facetted uncapped nanocrystals can be distinguished, sharing a common face.

Figure 2 A) ADF-STEM image of a GZO Nanocrystal. B) EELS spectrum at the Zn-L23 edge of a GZO Nanocrystal.

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