Fermi level pinning in doped metal oxide (MO) nanocrystals (NCs) results in the formation of depletion layers, which affect their optical and electronic properties, and ultimately their application in smart optoelectronics, photocatalysis, or energy storage. For a precise control over functionality, it is important to understand and control their electronic bands at the nanoscale. Here, we show that depletion layer engineering allows designing the energetic band profiles and predicting the optoelectronic properties of MO NCs. This is achieved by shell thickness tuning of core–shell Sn:In2O3–In2O3 NCs, resulting in multiple band bending and multi-modal plasmonic response. We identify the modification of the band profiles after the light-induced accumulation of extra electrons as the main mechanism of photodoping and enhance the charge storage capability up to hundreds of electrons per NC through depletion layer engineering. Our experimental results are supported by theoretical models and are transferable to other core-multishell systems as well.
Understanding and tuning the ligand shell composition in colloidal halide perovskite nanocrystals (NCs) has been done systematically only for Pb-based perovskites, while much less is known on the surface of Pb-free perovskite systems. Here, we reveal the ligand shell architecture of Bi-doped Cs 2 Ag 1– x Na x InCl 6 NCs via nuclear magnetic resonance analysis. This material, in its bulk form, was found to have a photoluminescence quantum yield (PLQY) as high as 86%, a record value for halide double perovskites. Our results show that both amines and carboxylic acids are present and homogeneously distributed over the surface of the NCs. Notably, even for an optimized surface ligand coating, achieved by combining dodecanoic acid and decylamine, a maximum PLQY value of only 37% is reached, with no further improvements observed when exploiting post-synthesis ligand exchange procedures (involving Cs-oleate, different ammonium halides, thiocyanates and sulfonic acids). Our density functional theory calculations indicate that, even with the best ligands combination, a small fraction of unpassivated surface sites, namely undercoordinated Cl ions, is sufficient to create deep trap states, opposite to the case of Pb-based perovskites that exhibit much higher defect tolerance. This was corroborated by our transient absorption measurements, which showed that an ultrafast trapping of holes (most likely mediated by surface Cl-trap states) competes with their localization at the AgCl 6 octahedra, from where, instead, they can undergo an optically active recombination yielding the observed PL emission. Our results highlight that alternative surface passivation strategies should be devised to further optimize the PLQY of double perovskite NCs, which might include their incorporation inside inorganic shells.
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.
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