ABSTRACT
Doped metal oxide nanocrystals that exhibit tunable localized surface plasmon resonances (LSPRs) represent an intriguing class of nanomaterials that show promise for a variety of applications from spectroscopy to sensing. LSPRs arise in these materials through the introduction of aliovalent dopants and lattice oxygen vacancies. Tuning the LSPR shape and energy is generally accomplished through controlling the concentration or identity of dopants in a nanocrystal, but the lack of finer synthetic control leaves several fundamental questions unanswered regarding the effects of radial dopant placement, size, and nanocrystalline architecture on the LSPR energy and damping. Here, we present a layer-by-layer synthetic method for core/shell nanocrystals that permits exquisite and independent control over radial dopant placement, absolute dopant concentration, and nanocrystal size. Using Sn-doped In2O3 (ITO) as a model LSPR system, we synthesized ITO/In2O3 core/shell as well as In2O3/ITO core/shell nanocrystals with varying shell thickness, and investigated the resulting optical properties. We observed profound influence of radial dopant placement on the energy and linewidth of the LSPR response, noting (among other findings) that core-localized dopants produce the highest values for LSPR energies per dopant concentration, and display the lowest damping in comparison to nanocrystals with shell-localized or homogeneously distributed dopants. Inactive Sn dopants present on ITO nanocrystal surfaces are activated upon the addition of a subnanometer thick undoped In2O3 shell. We show how LSPR energy can be tuned fully independent of dopant concentration, relying solely on core/shell architecture. Finally, the impacts of radial dopant placement on damping, independent of LSPR energy, are explored.
ABSTRACT
The ability to precisely control the composition of nanocrystals, similar to the way organic chemists control the structure of small molecules, remains an important challenge in nanoscience. Rather than dictating nanocrystal size through the nucleation event, growth of nanocrystals through continuous precursor addition would allow fine structural control. Herein, we present a method of growth for indium oxide nanocrystals that relies on the slow addition of an indium carboxylate precursor into hot oleyl alcohol. Nanocrystal size and structure can be governed at a subnanometer scale, and it is possible to precisely control core size over a range of three to at least 22 nm with dispersities as low as 7%. Growth can be stopped and restarted repeatedly without aggregation or passivation. We show that the volume of the nanocrystal core (and thus molecular weight) increases linearly with added monomer and the number of nanocrystals remains constant throughout the growth process, yielding an extremely predictable approach to size control. It is also possible to place metal oxide shells (e.g., Sn-doped In2O3 (ITO)) at various radial positions within the nanocrystal, and we use this approach to synthesize ITO/In2O3 core/shell nanocrystals as well as In2O3/ITO/In2O3 core/shell/shell nanocrystals.
