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 InO (ITO) as a model LSPR system, we synthesized ITO/InO core/shell as well as InO/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 InO 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.

Doped oxide nanocrystals hold promise for a wide variety of applications if dopant-induced properties can be appropriately harnessed. However, synthesis of doped nanocrystals with precise control over composition and structure presents a significant challenge. With traditional thermal decomposition synthetic methods, nanocrystal composition is hard to control due to the differing reactivities of dopant and host precursors. Under decomposition conditions, the variety of dopant atoms that can be introduced is limited, and the efficacy of dopant atom incorporation is variable. Here, we show the slow-addition of metal oleates into hot, long-chain alcohol permits >90% dopant incorporation efficacy for a variety of first-row transition-metal dopants into an In2O3 lattice at dopant concentrations up to 20 atom %. X-ray photoelectron spectroscopy analysis indicates that dopants are distributed throughout the nanocrystal. Elemental composition analysis, shifts and intensity changes in the X-ray diffraction peaks, and electronic absorbance spectroscopy suggest that the guest cations are substitutionally doping in the host matrix. We demonstrate that the synthetic method allows access to previously unobtainable compositions and structures without significant investment into synthetic optimization and precursor selection. Synthetic approaches with such attributes will not only lead to faster development of applications from these materials but also aid in our understanding and optimization of their properties.

We report the first demonstration of light-driven permanent charge separation across an ultrathin solid-state zerodimensional (0D)/2D hybrid interface by coupling photoactive Sn-doped In 2 O 3 nanocrystals with monolayer MoS 2 , the latter serving as a hole collector. We demonstrate that the nanocrystals in this device-ready architecture act as local light-controlled charge sources by quasi-permanently donating ∼5 holes per nanocrystal to the monolayer MoS 2 . The amount of photoinduced contactless charge transfer to the monolayer MoS 2 competes with what is reached in electrostatically gated devices. Thus, we have constructed a hybrid bilayer structure in which the electrons and holes are separated into two different solid-state materials. The temporal evolution of the local doping levels of the monolayer MoS 2 follows a capacitive charging model with effective total capacitances in the femtofarad regime and areal capacitances in the μF cm −2 range. This analysis indicates that the 0D/2D hybrid system may be able to store light energy at densities of at least μJ cm −2 , presenting new potential foundational building blocks for next-generation nanodevices that can remotely control local charge density, power miniaturized circuitry, and harvest and store optical energy.

Thin films made from transparent conducting oxide (TCO) nanocrystals are promising alternatives to traditional vacuum-sputtered films. However, the material properties of nanocrystal-derived thin films are dependent upon the doping levels and sizes of the nanocrystal building blocks. To date, a lack of deliberate and precise control over size in TCO nanocrystals has hindered the investigation of how nanocrystal size affects the optoelectronic properties of the resulting thin films. Here, this gap is addressed through the use of a synthetic approach that produces a series of uniform nanocrystals with tunable, well-defined sizes with nanometer resolution. A size ladder of Sn-doped In 2 O 3 (ITO) nanocrystals, containing seven samples ranging from 5 to 21 nm in diameter, was synthesized sequentially under the same reaction conditions in a single slow-injection reaction. The nanocrystals displayed constant dopant levels, homogeneous dopant distributions, and high carrier concentrations (∼10 21 cm −3 ) across all sizes produced. The ITO nanocrystals were solution-deposited into thin films and processed under mild conditions. For all nanocrystal sizes, the films were smooth and crack-free and exhibited >95% optical transparency. The resistivities of the thin films decrease over an order of magnitude, from 5.0 × 10 −2 to 4.5 × 10 −3 Ω cm for the 5.3 and 21.5 nm samples, respectively. Larger nanocrystals exhibit lowered thin film resistivities due to decreased coulombic charging energy, decreased electron surface scattering, and reduced interface density.

No abstract