Resolving the Growth of 3D Colloidal Nanoparticle Superlattices by Real-Time Small-Angle X‑ray Scattering

The kinetics and intricate interactions governing the growth of 3D single nanoparticle (NP) superlattices (SLs, SNSLs) and binary NP SLs (BNSLs) in solution are understood by combining controlled solvent evaporation and <i>in situ</i>, real-time small-angle X-ray scattering (SAXS). For the iron oxide (magnetite) NP SLs studied here, the larger the NP, the farther apart are the NPs when the SNSLs begin to precipitate and the closer they are after ordering. This is explained by a model of NP assembly using van der Waals interactions between magnetite cores in hydrocarbons with a ∼21 zJ Hamaker constant. When forming BNSLs of two different sized NPs, the NPs that are in excess of that needed to achieve the final BNSL stoichiometry are expelled during the BNSL formation, and these expelled NPs can form SNSLs. The long-range ordering of these SNSLs and the BNSLs can occur faster than the NP expulsion

Resolving the Growth of 3D Colloidal Nanoparticle Superlattices by Real-Time Small-Angle X‑ray Scattering

The kinetics and intricate interactions governing the growth of 3D single nanoparticle (NP) superlattices (SLs, SNSLs) and binary NP SLs (BNSLs) in solution are understood by combining controlled solvent evaporation and <i>in situ</i>, real-time small-angle X-ray scattering (SAXS). For the iron oxide (magnetite) NP SLs studied here, the larger the NP, the farther apart are the NPs when the SNSLs begin to precipitate and the closer they are after ordering. This is explained by a model of NP assembly using van der Waals interactions between magnetite cores in hydrocarbons with a ∼21 zJ Hamaker constant. When forming BNSLs of two different sized NPs, the NPs that are in excess of that needed to achieve the final BNSL stoichiometry are expelled during the BNSL formation, and these expelled NPs can form SNSLs. The long-range ordering of these SNSLs and the BNSLs can occur faster than the NP expulsion

Resolving the Growth of 3D Colloidal Nanoparticle Superlattices by Real-Time Small-Angle X‑ray Scattering

The kinetics and intricate interactions governing the growth of 3D single nanoparticle (NP) superlattices (SLs, SNSLs) and binary NP SLs (BNSLs) in solution are understood by combining controlled solvent evaporation and <i>in situ</i>, real-time small-angle X-ray scattering (SAXS). For the iron oxide (magnetite) NP SLs studied here, the larger the NP, the farther apart are the NPs when the SNSLs begin to precipitate and the closer they are after ordering. This is explained by a model of NP assembly using van der Waals interactions between magnetite cores in hydrocarbons with a ∼21 zJ Hamaker constant. When forming BNSLs of two different sized NPs, the NPs that are in excess of that needed to achieve the final BNSL stoichiometry are expelled during the BNSL formation, and these expelled NPs can form SNSLs. The long-range ordering of these SNSLs and the BNSLs can occur faster than the NP expulsion

Electrochemically Induced Transformations of Vanadium Dioxide Nanocrystals

Vanadium dioxide (VO₂) undergoes significant optical, electronic, and structural changes as it transforms between the low-temperature monoclinic and high-temperature rutile phases. Recently, alternative stimuli have been utilized to trigger insulator-to-metal transformations in VO₂, including electrochemical gating. Here, we prepare and electrochemically reduce mesoporous films of VO₂ nanocrystals, prepared from colloidally synthesized V₂O₃ nanocrystals that have been oxidatively annealed, in a three-electrode electrochemical cell. We observe a reversible transition between infrared transparent insulating phases and a darkened metallic phase by in situ visible–near-infrared spectroelectrochemistry and correlate these observations with structural and electronic changes monitored by X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, and conductivity measurements. An unexpected reversible transition from conductive, reduced monoclinic VO₂ to an infrared-transparent insulating phase upon progressive electrochemical reduction is observed. This insulator–metal–insulator transition has not been reported in previous studies of electrochemically gated epitaxial VO₂ films and is attributed to improved oxygen vacancy formation kinetics and diffusion due to the mesoporous nanocrystal film structure

Defect Engineering in Plasmonic Metal Oxide Nanocrystals

Defects may tend to make crystals interesting but they do not always improve performance. In doped metal oxide nanocrystals with localized surface plasmon resonance (LSPR), aliovalent dopants and oxygen vacancies act as centers for ionized impurity scattering of electrons. Such electronic damping leads to lossy, broadband LSPR with low quality factors, limiting applications that require near-field concentration of light. However, the appropriate dopant can mitigate ionized impurity scattering. Herein, we report the synthesis and characterization of a novel doped metal oxide nanocrystal material, cerium-doped indium oxide (Ce:In₂O₃). Ce:In₂O₃ nanocrystals display tunable mid-infrared LSPR with exceptionally narrow line widths and the highest quality factors observed for nanocrystals in this spectral region. Drude model fits to the spectra indicate that a drastic reduction in ionized impurity scattering is responsible for the enhanced quality factors, and high electronic mobilities reaching 33 cm²V^–1 s^–1 are measured optically, well above the optical mobility for tin-doped indium oxide (ITO) nanocrystals. We investigate the microscopic mechanisms underlying this enhanced mobility with density functional theory calculations, which suggest that scattering is reduced because cerium orbitals do not hybridize with the In orbitals that dominate the bottom of the conduction band. Ce doping may also reduce the equilibrium oxygen vacancy concentration, further enhancing mobility. From the absorption spectra of single Ce:In₂O₃ nanocrystals, we determine the dielectric function and by simulation predict strong near-field enhancement of mid-IR light, especially around the vertices of our synthesized nanocubes