Highly Responsive Plasmon Modulation in Dopant-Segregated Nanocrystals

Abstract

Electron transfer to and from metal oxide nanocrystals (NCs) modulates their infrared localized surface plasmon resonance (LSPR), revealing fundamental aspects of their photophysics and enabling dynamic optical applications. We synthesized and chemically reduced dopantsegregated Sn-doped In 2 O 3 NCs, investigating the influence of radial dopant segregation on LSPR modulation and near-field enhancement (NFE). We found that core-doped NCs show large LSPR shifts and NFE change during chemical titration, enabling broadband modulation in LSPR energy of over 1000 cm −1 and of peak extinction over 300%. Simulations reveal that the evolution of the LSPR spectra during chemical reduction results from raising the surface Fermi level and increasing the donor defect density in the shell region. These results establish dopant segregation as a useful strategy to engineer the dynamic optical modulation in plasmonic semiconductor NC heterostructures going beyond what is possible with conventional plasmonic metals.

How Depletion Layers Govern the Dynamic Plasmonic Response of In-Doped CdO Nanocrystals

“…Furthermore, the slight In surface segregation revealed by XPS (Tables S1 and S2) means that the substantial depletion layer widths we observed here might be even larger were the dopants uniformly distributed since surface segregation of dopants has been shown to diminish depletion layer widths. 16,20,21,57,58 Consistent with eq 1 and with previous results for ITO NCs, 28 we This is in stark contrast to ITO NCs, wherein the depletion layer is nearly eliminated in samples larger than 12 nm and NCs doped above 6% Sn. 14,28 The wide depletion layer in ICO NCs can be primarily attributed to the higher ϵ R in the material; however, a lower flat band E F and thus higher built-in potential, E BI , may also be significantly contributing, as has previously been hypothesized for CdO-based NCs.…”

“…Despite the myriad of beneficial properties noted in ICO as a low loss plasmonic material, the intrinsically wide depletion layers may pose a challenge for applications that require strong coupling; this limitation could be rectified with the depletion layer engineering strategies from molecular dipoles to redox doping, or, in the future, controlling the radial distribution of dopants. 21,58…”

How Depletion Layers Govern the Dynamic Plasmonic Response of In-doped CdO Nanocrystals

“…Furthermore, the slight In surface segregation revealed by XPS (Table S1-S2) means that the substantial depletion layer widths we observed here might be even larger were the dopants uniformly distributed since surface segregation of dopants has been shown to diminish depletion layer widths. 15,19,20,51,52 Consistent with Eq. 1 and with previous results for ITO NCs, 27 we observe that f e (inversely correlated to W d ) increases as a function of diameter, which we rationalize as an approximately constant W d but higher core volume fraction.…”

“…Despite the myriad of beneficial properties noted in ICO as a low loss plasmonic material, the intrinsically wide depletion layers may pose a challenge for applications that require strong coupling; this limitation could be rectified with the depletion layer engineering strategies from molecular dipoles to redox doping, or, in the future, controlling the radial distribution of dopants. 20,52…”

“…8,9 While the depletion layer can be considered a drawback in applications that rely on inter-NC charge transfer 10 or sensitivity to the surrounding dielectric environment, 11,12 it can also be leveraged as a charge storage region that enhances LSPR modulation range. [13][14][15] The effect of the depletion layer on optical and electronic properties has been extensively studied in ITO NCs [11][12][13][16][17][18][19][20][21][22][23][24] and to a much lesser extent, AZO NCs, 25 but understanding how its characteristics depend on the attributes of the host material will unlock the potential for depletion layer engineering to realize new and optimized applications of these emerging plasmonic nanomaterials. 15,19 The radial depletion layer width (W d ) in spherical NCs is well approximated by Poisson's equation for the depletion layer in planar electrodes, Eq.…”