(196i) Design Rules to Tailor the Localized Surface Plasmon Resonance Characteristic of Metal Oxide Nanocrystals

Localized surface plasmon resonance (LSPR) of degenerately doped semiconductor nanocrystals (NC) falls in the near- to mid-IR range of the electromagnetic spectrum (1). While synthetic work for this class of materials has progressed significantly, quantitative correlation between their intrinsic properties to the final LSPR properties such as extinction and near field enhancement remains poorly explored, thus prohibiting their use in applications from plasmon derived smart windows to sensors. Here, we illustrate how intrinsic NC attributes like its crystal structure, shape and size, along with band structure and surface properties affects the LSPR properties and its possible applications.

First, the interplay of NC shape and the intrinsic crystal structure on the LSPR was studied using model systems of In:CdO and Cs:WO₃, the latter of which has an intrinsic anisotropic crystal structure. For both systems, a change of shape from spherical to faceted NCs led to as anticipated higher near field enhancements around the particle (2). However, with Cs:WO₃, presence of an anisotropic hexagonal crystal structure leads to additional strong LSPR band-splitting into two distinct peaks with comparable intensities. This new insight demonstrated how the LSPR of semiconductor NCs can be controlled with shape, size and now crystal structure to tune the LSPR from near IR to mid-IR (3).

Second, plasmon-molecular vibration coupling, as a proof of concept for sensing applications, was shown using newly developed F and Sn codoped In₂O₃ NCs to couple to the C-H vibration of surface-bound oleate ligands. Electron energy loss spectroscopy was used to map the near field enhancement around these NCs responsible for coupling between the LSPR and molecular vibrations. A combined theoretical and experimental approach was employed to describe the observed plasmon-plasmon coupling, the influence of coupling strength and relative detuning between the molecular vibration and LSPR on the enhancement factor, and the observed Fano lineshape by deconvoluting the combined response of the LSPR and molecular vibration in transmission, absorption, and reflection (4).

References:

1) Ankit Agrawal, Robert Johns, and Delia Milliron, Control of Localized Surface Plasmon Resonances in Metal Oxide Nanocrystals. Annual Review of Materials Research, 47, 2017, 11.1-11.31.

2) Ankit Agrawal, Ilka Kriegel, and Delia J. Milliron, Shape-Dependent Field Enhancement and Plasmon Resonance of Oxide Nanocrystals. Phys. Chem. C, 119(11), 2015, 6227–6238

3) Jongwook Kim, Ankit Agrawal,Franziska Krieg,Amy Bergerud,and Delia J. Milliron, The Interplay of Shape and Crystalline Anisotropies in Plasmonic Semiconductor Nanocrystals. Nano Letters, 16(5), 2016, 3879−3884

4) Ankit Agrawal, Ajay Singh, Sadegh Yazdi, Amita Singh, Emilie Ringe, and Delia Milliron, Resonant Coupling between Molecular Vibrations and Localized Surface Plasmon Resonance of Faceted Metal Oxide Nanocrystals. Nano Letters, 17(4), 2017, 2611-2620.