Highly Responsive Plasmon Modulation in Dopant-Segregated Nanocrystals.

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 dopant-segregated Sn-doped In2O3 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.

Plasmonic Response of Complex Nanoparticle Assemblies.

Optical properties of nanoparticle assemblies reflect distinctive characteristics of their building blocks and spatial organization, giving rise to emergent phenomena. Integrated experimental and computational studies have established design principles connecting the structure to properties for assembled clusters and superlattices. However, conventional electromagnetic simulations are too computationally expensive to treat more complex assemblies. Here we establish a fast, materials agnostic method to simulate the optical response of large nanoparticle assemblies incorporating both structural and compositional complexity. This many-bodied, mutual polarization method resolves limitations of established approaches, achieving rapid, accurate convergence for configurations including thousands of nanoparticles, with some overlapping. We demonstrate these capabilities by reproducing experimental trends and uncovering far- and near-field mechanisms governing the optical response of plasmonic semiconductor nanocrystal assemblies including structurally complex gel networks and compositionally complex mixed binary superlattices. This broadly applicable framework will facilitate the design of complex, hierarchically structured, and dynamic assemblies for desired optical characteristics.

Contact Conductance Governs Metallicity in Conducting Metal Oxide Nanocrystal Films.

Although colloidal nanoparticles hold promise for fabricating electronic components, the properties of nanoparticle-derived materials can be unpredictable. Materials made from metallic nanocrystals exhibit a variety of transport behavior ranging from insulators, with internanocrystal contacts acting as electron transport bottlenecks, to conventional metals, where phonon scattering limits electron mobility. The insulator-metal transition (IMT) in nanocrystal films is thought to be determined by contact conductance. Meanwhile, criteria are lacking to predict the characteristic transport behavior of metallic nanocrystal films beyond this threshold. Using a library of transparent conducting tin-doped indium oxide nanocrystal films with varied electron concentration, size, and contact area, we assess the IMT as it depends on contact conductance and show how contact conductance is also key to predicting the temperature-dependence of conductivity in metallic films. The results establish a phase diagram for electron transport behavior that can guide the creation of metallic conducting materials from nanocrystal building blocks.

Dual-Mode Infrared Absorption by Segregating Dopants within Plasmonic Semiconductor Nanocrystals.

When aliovalent dopants are sufficiently segregated to the core or near the surface of semiconductor nanocrystals, charge carriers donated by the dopants are also segregated to the core or near the surface, respectively. In Sn-doped indium oxide nanocrystals, we find that this contrast in free charge carrier concentration creates a core and shell with differing dielectric properties and results in two distinctly observable plasmonic extinction peaks. The trends in this dual-mode optical response with shell growth differ from core/shell nanoparticles composed of traditional plasmonic metals such as Au and Ag. We developed a model employing a core/shell effective medium approximation that can fit the dual-mode spectra and explain the trends in the extinction response. Lastly, we show that dopant segregation can improve sensitivity of plasmon spectra to changes in refractive index of the surrounding environment.

Surface Depletion Layers in Plasmonic Metal Oxide Nanocrystals.

Strong infrared (IR) light-matter interaction and spectral tunability combine to make plasmonic metal oxide nanocrystals (NCs) a compelling choice for IR applications. In particular, visible transparency paired with strong, dynamically tunable IR absorption has motivated their implementation in electrochromic smart windows, but these NCs hold promise for a far broader range of plasmonically driven processes such as surface-enhanced infrared sensing, photothermal therapy, and enhanced photocatalysis. These unique properties result from localized surface plasmon resonance (LSPR) sustained by a relatively low free charge carrier concentration, which in turn requires consideration of distinct materials physics relative to traditional plasmonic materials (i.e., metals). Particularly important is the formation of insulating shells devoid of charge carriers (depletion layers) near the NC surface. Surface states as well as applied surface potentials can give rise to a potential difference between the NC surface and its core that depletes free charge carriers from the surface, forming an insulating shell that reduces the conductivity in NC films, lowers the dielectric sensitivity of the LSPR, and diminishes the incident electric field enhancement. In this Account, we report recent investigations of depletion layers in plasmonic metal oxide NCs that have advanced understanding of the semiconductor physics underlying the optoelectronic properties of these NCs and the electrochemical modulation of their LSPR, establishing a conceptual framework with which to broaden their applicability and optimize their performance. As a result of surface depletion, larger, highly doped NCs have improved dielectric sensitivity compared with their smaller, lightly doped counterparts. Concentrating dopants near the NC surface compresses the depletion layer, resulting in improved conductivity of NC films. Moreover, atomic layer deposition of alumina to infill NC films enhances the film conductivity by more than 2 orders of magnitude, ascribed to the elimination of depletion effects by reactive removal of surface water species. At the conclusion, we reflect on how our newfound understanding of surface depletion in plasmonic metal oxide NCs is quickly leading to rational material design. This insight is already resulting in significant performance improvements, and the same principles can be applied to new, exciting opportunities in hot carrier extraction and resonant IR energy transduction.

Quantitative Analysis of Extinction Coefficients of Tin-Doped Indium Oxide Nanocrystal Ensembles.

The optical extinction coefficients of localized surface plasmon resonance (LSPR) in doped semiconductor nanocrystals (NCs) have intensities determined by the density and damping mechanisms of free charge carriers. We investigate the dependence of the extinction coefficient of tin-doped indium oxide (ITO) NCs on size and dopant concentration and find extinction coefficients as high as 56.6 µm-1 in the near-infrared for 20 nm diameter ITO NCs with 7.5 atomic% Sn. We find ITO NCs to be more efficient infrared light absorbers than metal nanoparticles or molecular dyes. We also find the intensive, volume-normalized extinction coefficient increases significantly with NC doping and NC diameter, but only up to the point of saturation in both cases. We qualitatively analyze trends in LSPR peak position and width to explain the effect of doping and size on extinction.

Tuning Nanocrystal Surface Depletion by Controlling Dopant Distribution as a Route Toward Enhanced Film Conductivity.

Electron conduction through bare metal oxide nanocrystal (NC) films is hindered by surface depletion regions resulting from the presence of surface states. We control the radial dopant distribution in tin-doped indium oxide (ITO) NCs as a means to manipulate the NC depletion width. We find in films of ITO NCs of equal overall dopant concentration that those with dopant-enriched surfaces show decreased depletion width and increased conductivity. Variable temperature conductivity data show electron localization length increases and associated depletion width decreases monotonically with increased density of dopants near the NC surface. We calculate band profiles for NCs of differing radial dopant distributions and in agreement with variable temperature conductivity fits find NCs with dopant-enriched surfaces have narrower depletion widths and longer localization lengths than those with dopant-enriched cores. Following amelioration of NC surface depletion by atomic layer deposition of alumina, all films of equal overall dopant concentration have similar conductivity. Variable temperature conductivity measurements on alumina-capped films indicate all films behave as granular metals. Herein, we conclude that dopant-enriched surfaces decrease the near-surface depletion region, which directly increases the electron localization length and conductivity of NC films.