Manipulating particle size is a powerful means of creating unprecedented optical properties in metals and semiconductors. Here we report an insulator system composed of NaYbF4:Tm in which size effect can be harnessed to enhance multiphoton upconversion. Our mechanistic investigations suggest that the phenomenon stems from spatial confinement of energy migration in nanosized structures. We show that confining energy migration constitutes a general and versatile strategy to manipulating multiphoton upconversion, demonstrating an efficient five-photon upconversion emission of Tm3+ in a stoichiometric Yb lattice without suffering from concentration quenching. The high emission intensity is unambiguously substantiated by realizing room-temperature lasing emission at around 311 nm after 980-nm pumping, recording an optical gain two orders of magnitude larger than that of a conventional Yb/Tm-based system operating at 650 nm. Our findings thus highlight the viability of realizing diode-pumped lasing in deep ultraviolet regime for various practical applications.

This paper describes the synthesis, formation mechanism, and mechanical property of multilayered ultrathin Pd nanosheets. An anisotropic, Hanoi Tower-like assembly of Pd nanosheets was identified by transmission electron microscopy and atomic force microscopy (AFM). These nanosheets may contain ultrathin Pd layers, down to single unit cell thickness. Selected area electron diffraction and scanning transmission electron microscopy data show the interconnected atomically thick layers stacking vertically with rotational mismatches, resulting in unique diffractions and Moiré patterns. Density functional theory (DFT) calculation with van der Waals correction (DFT+vdW) shows the adsorption of Pd4(CO)4(OAc)4 on Pd(110) surface (Ead = -5.68 eV) is much stronger than that on Pd(100) (Ead = -4.72 eV) or on Pd(111) (Ead = -3.80 eV). The adsorption strength of this Pd complex is significantly stronger than that of CO on the same Pd surfaces. The DFT+vdW calculation results suggest a new mechanism for the observed anisotropic growth of nanosheets with unusually high aspect ratio, in which the competitive adsorptions between Pd4(CO)4(OAc)4 complex and CO on various surfaces result in a favored growth along the ⟨110⟩ directions and inhibition along ⟨111⟩ directions. The mechanical property of these multilayered Pd nanosheets was studied using AFM and nanoindentation techniques, which indicate multilayered nanosheets show more plastic deformation than the bulk in response to an applied force.

Two-dimensional metal oxide pseudocapacitors are promising candidates for size-sensitive applications. However, they exhibit limited energy densities and inferior power densities. Here, we present an electrodeposition technique by which ultrathin CeO 2− x films with controllable volumetric oxygen vacancy concentrations can be produced. This technique offers a layer-by-layer fabrication route for ultrathin CeO 2− x films that render Ce 3+ concentrations as high as ~60 at% and a volumetric capacitance of 1873 F cm −3 , which is among the highest reported to the best of our knowledge. This exceptional behaviour originates from both volumetric oxygen vacancies, which enhance electron conduction, and intercrystallite water, which promotes proton conduction. Consequently, simultaneous charging on the surface and in the bulk occur, leading to the observation of redox pseudocapacitive behaviour in CeO 2− x . Thermodynamic investigations reveal that the energy required for oxygen vacancy formation can be reduced significantly by proton-assisted reactions. This cyclic deposition technique represents an efficient method to fabricate metal oxides of precisely controlled defect concentrations and thicknesses.

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