Viscoelastic optical nonlocality of low-loss epsilon-near-zero nanofilms

Optical nonlocalities are elusive and hardly observable in traditional plasmonic materials like noble and alkali metals. Here we report experimental observation of viscoelastic nonlocalities in the infrared optical response of doped cadmium-oxide, epsilon-near-zero nanofilms. The nonlocality is detectable thanks to the low damping rate of conduction electrons and the virtual absence of interband transitions at infrared wavelengths. We describe the motion of conduction electrons using a hydrodynamic model for a viscoelastic fluid, and find excellent agreement with experimental results. The electrons elasticity blue-shifts the infrared plasmonic resonance associated with the main epsilon-near-zero mode, and triggers the onset of higher-order resonances due to the excitation of electron-pressure modes above the bulk plasma frequency. We also provide evidence of the existence of nonlocal damping, i.e., viscosity, in the motion of optically-excited conduction electrons using a combination of spectroscopic ellipsometry data and predictions based on the viscoelastic hydrodynamic model.Comment: 19 pages, 5 figure

Rationalizing the Impact of Surface Depletion on Electrochemical Modulation of Plasmon Resonance Absorption in Metal Oxide Nanocrystals

Dynamic control over the localized surface plasmon resonance (LSPR) makes doped metal oxide nanocrystals (NCs) promising for several optoelectronic applications including electrochromic smart windows and redox sensing. Metal oxide NCs such as tin-doped indium oxide display tunable infrared LSPRs via electrochemical charge injection and extraction as a function of the externally applied potential. In this work we have employed dispersion phase electrochemical charging/discharging to study the mechanism behind the optical modulation on an individual NC scale. The optical modulation of the LSPR is dominated by a sharp variation in intensity during reduction and oxidation along with an only modest shift in the LSPR frequency. With a core-shell modeling approach, in which an active NC core surrounded by a depleted shell is assumed, we were able to reproduce the trends in and main features of our experimental results. The shell thickness depends on the applied potential and we extracted the temporal evolution of the shell thickness together with the variation of the Drude parameters until equilibrium was reached. The variation of the core versus shell volume fraction as a function of electrochemical potential reinforces the importance of the depletion layer in highly doped NCs and uncovers important implications on their near and far field plasmonic properties

Rationalizing the Impact of Surface Depletion on Electrochemical Modulation of Plasmon Resonance Absorption in Metal Oxide Nanocrystals

Dynamic control over the localized surface plasmon resonance (LSPR) makes doped metal oxide nanocrystals (NCs) promising for several optoelectronic applications including electrochromic smart windows and redox sensing. Metal oxide NCs such as tin-doped indium oxide display tunable infrared LSPRs via electrochemical charge injection and extraction as a function of the externally applied potential. In this work we have employed dispersion phase electrochemical charging/discharging to study the mechanism behind the optical modulation on an individual NC scale. The optical modulation of the LSPR is dominated by a sharp variation in intensity during reduction and oxidation along with an only modest shift in the LSPR frequency. With a core-shell modeling approach, in which an active NC core surrounded by a depleted shell is assumed, we were able to reproduce the trends in and main features of our experimental results. The shell thickness depends on the applied potential and we extracted the temporal evolution of the shell thickness together with the variation of the Drude parameters until equilibrium was reached. The variation of the core versus shell volume fraction as a function of electrochemical potential reinforces the importance of the depletion layer in highly doped NCs and uncovers important implications on their near and far field plasmonic properties

Direct observation of narrow mid-infrared plasmon linewidths of single metal oxide nanocrystals.

Infrared-responsive doped metal oxide nanocrystals are an emerging class of plasmonic materials whose localized surface plasmon resonances (LSPR) can be resonant with molecular vibrations. This presents a distinctive opportunity to manipulate light-matter interactions to redirect chemical or spectroscopic outcomes through the strong local electric fields they generate. Here we report a technique for measuring single nanocrystal absorption spectra of doped metal oxide nanocrystals, revealing significant spectral inhomogeneity in their mid-infrared LSPRs. Our analysis suggests dopant incorporation is heterogeneous beyond expectation based on a statistical distribution of dopants. The broad ensemble linewidths typically observed in these materials result primarily from sample heterogeneity and not from strong electronic damping associated with lossy plasmonic materials. In fact, single nanocrystal spectra reveal linewidths as narrow as 600 cm(-1) in aluminium-doped zinc oxide, a value less than half the ensemble linewidth and markedly less than homogeneous linewidths of gold nanospheres

Electrolyte-gate-driven carrier density modulation and metal–insulator transition in semiconducting epitaxial CdO films

CdO has drawn much recent interest as a high-room-temperature-mobility oxide semiconductor with exciting potential for mid-infrared photonics and plasmonics. Wide-range modulation of carrier density in CdO is of interest both for fundamental reasons (to explore transport mechanisms in single samples) and for applications (in tunable photonic devices). Here, we thus apply ion-gel-based electrolyte gating to ultrathin epitaxial CdO(001) films, using transport, x-ray diffraction, and atomic force microscopy to deduce a reversible electrostatic gate response from −4 to +2 V, followed by rapid film degradation at higher gate voltage. Further advancing the mechanistic understanding of electrolyte gating, these observations are explained in terms of low oxygen vacancy diffusivity and high acid etchability in CdO. Most importantly, the 6-V-wide reversible electrostatic gating window is shown to enable ten-fold modulation of the Hall electron density, a striking voltage-induced metal–insulator transition, and 15-fold variation of the electron mobility. Such modulations, which are limited only by unintentional doping levels in ultrathin films, are of exceptional interest for voltage-tunable devices

Colloidal Nanocrystal Films Reveal the Mechanism for Intermediate Temperature Proton Conductivity in Porous Ceramics

Over the past few years, the observation of unexpected but significant proton conductivity in porous, nanocrystalline ceramics has generated substantial scientific interest mirroring the excitement surrounding ionic conduction in other nanostructured or porous materials. Here, we utilize colloidally synthesized ceramic nanocrystals of cerium oxide (CeO₂) and titanium oxide (TiO₂) to systematically study how grain size, microporosity, and composition influence proton conduction. By measuring the temperature-dependent impedance of porous thin films of these nanocrystals under dry and wet atmospheres, we find that both CeO₂ and TiO₂ display significant proton conductivity at intermediate temperatures between 100 and 350 °C. Furthermore, we investigate the effect of oxygen activity on proton transport, finding that using oxygen as a carrier gas drastically reduced the proton conductivity by up to 60 times. Together, these results suggest that the most likely source of mobile protons in these systems is dissociative adsorption of water at surface oxygen vacancies, with composition, nanocrystal size, and oxide defect equilibria influencing the surface activity toward this reaction and, hence, the proton conductivity

Influence of Dopant Distribution on the Plasmonic Properties of Indium Tin Oxide Nanocrystals

Doped metal oxide nanocrystals represent an exciting frontier for colloidal synthesis of plasmonic materials, displaying unique optoelectronic properties and showing promise for a variety of applications. However, fundamental questions about the nature of doping in these materials remain. In this article, the strong influence of radial dopant distribution on the optoelectronic properties of colloidal indium tin oxide nanocrystals is reported. Comparing elemental depth-profiling by X-ray photoelectron spectroscopy (XPS) with detailed modeling and simulation of the optical extinction of these nanocrystals using the Drude model for free electrons, a correlation between surface segregation of tin ions and the average activation of dopants is observed. A strong influence of surface segregation of tin on the line shape of the localized surface plasmon resonance (LSPR) is also reported. Samples with tin segregated near the surface show a symmetric line shape that suggests weak or no damping of the plasmon by ionized impurities. It is suggested that segregation of tin near the surface facilitates compensation of the dopant ions by electronic defects and oxygen interstitials, thus reducing activation. A core–shell model is proposed to explain the observed differences in line shape. These results demonstrate the nuanced role of dopant distribution in determining the optoelectronic properties of semiconductor nanocrystals and suggest that more detailed study of the distribution and structure of defects in plasmonic colloidal nanocrystals is warranted

Dynamically Modulating the Surface Plasmon Resonance of Doped Semiconductor Nanocrystals

Localized surface plasmon absorption features arise at high doping levels in semiconductor nanocrystals, appearing in the near-IR range. The surface plasmons of Sn-doped In oxide nanocrystal films can be dynamically and reversibly tuned by postsynthetic electrochem. modulation of the electron concn. Without ion intercalation and the assocd. material degrdn., a textgreater1200 nm shift in the plasmon wavelength and a factor of nearly 3 change in the carrier d. was induced