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.

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.

The synthesis and characterization of turbostratically disordered (BiSe) 1.15 TiSe 2 is reported. Specular and in-plane x-ray diffraction studies indicate an alternating structure containing two planes of a distorted rock salt structured BiSe and a Se-Ti-Se trilayer of TiSe 2 with independent lattices. The title compound was found to be turbostratically (rotationally) disordered about the c-axis, and the BiSe layer displays an orthorhombic in-plane structure with a = 4.562(2) Å and b = 4.242(1) Å. Temperature dependent electrical resistivity reveals that the disordered compound is metallic, but with less temperature dependence than may be expected for a 3D crystal, which is attributed to the lack of coherent vibrations due to the turbostratic disorder. The room temperature resistivity was found to be ρ = 5.0 × 10 −6 m with a carrier concentration of n = 5 × 10 21 cm −3 . Comparing the carrier concentration to (PbSe) 1.16 TiSe 2 suggests that the bismuth is trivalent and donates an electron to the conduction band of the TiSe 2 constituent.

The important properties of polymers and nanocrystals both depend upon the precise control of three-dimensional structure. Living polymerization has transformed polymer chemistryproviding absolute control over molecular weights, yielding monodisperse chains, and enabling the production of copolymers with specifically tailored properties. Despite the apparent analogies between polymerization of organic monomers and nanocrystal growth from inorganic monomers, living growth approaches to nanocrystals have been slower to develop. Living nanocrystal growth methods promise to provide exquisite control over core size, size dispersion, doped composition, and core/shell structure. As a result, they have the potential to advance the development of predictive structure/property relationships and afford a finer level of structural control during nanomaterial synthesis. In this perspective, we outline the essential attributes of living nanocrystal syntheses and discuss prerequisites required to discover and develop reactions with these types of mechanisms. Examples from the literature are reviewed that share some attributes of living growth methods (e.g., seeded growth methods) in an attempt to identify existing approaches that might meet the living growth prerequisites. We describe recent findings from our laboratory on metal oxide nanocrystal synthesis that exhibit all the key attributes of living growth. We demonstrate the potential of this method for enhanced structural and compositional control in nanocrystal growth through examples involving efficient dopant incorporation into a metal oxide framework, precise control of the radial distribution of dopant atoms, and the production of core/shell metal oxide nanocrystals. Finally, we outline exciting future prospects for discovery and development of living growth systems and point out important research avenues critical for development of the field.

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.