Resonant Multiple-Phonon Absorption Causes Efficient Anti-Stokes Photoluminescence in CsPbBr3 Nanocrystals.

Lead halide perovskite nanocrystals, such as CsPbBr3, exhibit efficient photoluminescence (PL) up-conversion, also referred to as anti-Stokes photoluminescence (ASPL). This is a phenomenon where irradiating nanocrystals up to 100 meV below gap results in higher energy band edge emission. Most surprising is that ASPL efficiencies approach unity and involve single-photon interactions with multiple phonons. This is unexpected given the statistically disfavored nature of multiple-phonon absorption. Here, we report and rationalize near-unity anti-Stokes photoluminescence efficiencies in CsPbBr3 nanocrystals and attribute them to resonant multiple-phonon absorption by polarons. The theory explains paradoxically large efficiencies for intrinsically disfavored, multiple-phonon-assisted ASPL in nanocrystals. Moreover, the developed microscopic mechanism has immediate and important implications for applications of ASPL toward condensed phase optical refrigeration.

Mixed Ligand Passivation as the Origin of Near-Unity Emission Quantum Yields in CsPbBr3 Nanocrystals.

Key features of syntheses, involving the quaternary ammonium passivation of CsPbBr3 nanocrystals (NCs), include stable, reproducible, and large (often near-unity) emission quantum yields (QYs). The archetypical example involves didodecyl dimethyl ammonium (DDDMA+)-passivated CsPbBr3 NCs where robust QYs stem from interactions between DDDMA+ and NC surfaces. Despite widespread adoption of this synthesis, specific ligand-NC surface interactions responsible for large DDDMA+-passivated NC QYs have not been fully established. Multidimensional nuclear magnetic resonance experiments now reveal a new DDDMA+-NC surface interaction, beyond established "tightly bound" DDDMA+ interactions, which strongly affects observed emission QYs. Depending upon the existence of this new DDDMA+ coordination, NC QYs vary broadly between 60 and 85%. More importantly, these measurements reveal surface passivation through unexpected didodecyl ammonium (DDA+) that works in concert with DDDMA+ to produce near-unity (i.e., >90%) QYs.

Excitation Energy Dependence of Semiconductor Nanocrystal Emission Quantum Yields.

Accurate measurements of semiconductor nanocrystal (NC) emission quantum yields (QYs) are critical to condensed phase optical refrigeration. Of particular relevance to measuring NC QYs is a longstanding debate as to whether an excitation energy-dependent (EED) QY exists. Various reports indicate existence of NC EED QYs, suggesting that the phenomenon is linked to specific ensemble properties. We therefore investigate here the existence of EED QYs in two NC systems (CsPbBr3 and CdSe) that are possible candidates for use in optical refrigeration. The influence of NC size, size-distribution, surface ligand, and as-made emission QYs are investigated. Existence of EED QYs is assessed using two approaches (an absolute approach using an integrating sphere and a relative approach involving excitation spectroscopy). Altogether, our results show no evidence of EED QYs across samples. This suggests that parameters beyond those mentioned above are responsible for observations of NC EED QYs.

Universal Size-Dependent Stokes Shifts in Lead Halide Perovskite Nanocrystals.

Size-dependent photoluminescence Stokes shifts (ΔEs) universally exist in CsPbX3 (X = Cl-, Br-, or I-) perovskite nanocrystals (NCs). ΔEs values, which range from ∼15 to 100 meV for NCs with average edge lengths (l) from approximately 13 to 3 nm, are halide-dependent such that ΔEs(CsPbI3) > ΔEs(CsPbBr3) ≳ ΔEs(CsPbCl3). Observed size-dependent Stokes shifts are not artifacts of ensemble size distributions as demonstrated through measurements of single CsPbBr3 NC Stokes shifts (⟨ΔEs⟩ = 42 ± 5 meV), which are in near quantitative agreement with associated ensemble (l = 6.8 ± 0.8 nm) ΔEs values (ΔEs ≈ 50 meV). Transient differential absorption measurements additionally illustrate no significant spectral dynamics on the picosecond time scale that would contribute to ΔEs. This excludes polaron formation as being responsible for ΔEs. Altogether, the results point to an origin for ΔEs, intrinsic to the size-dependent electronic properties of individual perovskite NCs.

The synergistic effect of oxygen and water on the stability of the isostructural family of metal-organic frameworks [Cr3(BTC)2] and [Cu3(BTC)2].

The instability of most prototype metal-organic frameworks (MOFs) in the presence of water has limited their industrial scale development. The sensitivity of certain MOFs to humid conditions has been vigorously studied and most of the carrier gas used to make the humid conditions is inert gas. However, a large amount of industrial activities are carried out in air containing 21% oxygen, so it is important to study the effect of oxygen on the hydrostability of MOFs for future industrial applications. In this work, we have studied the stability of M3(BTC)2 (M = Cu, Cr; BTC = 1,3,5-benzenetricarboxylate) under controlled environments (pure oxygen environment; water vapor environment and mixed O2 and H2O environment). The stability was evaluated using water vapor and oxygen adsorption isotherms combined with powder X-ray diffraction (PXRD) experiments and surface area analysis. Our research shows that Cr3(BTC)2 has a relatively high stability under a single atmosphere of either oxygen or water vapor (with Ar). Interestingly, when it was placed under the mixed O2 and H2O environment, it rapidly lost approximately 96% of its original surface area. Cu3(BTC)2 is more stable; however, it was also degraded, especially under a mixed O2 and H2O environment. The experiments show that water molecules and oxygen molecules have a synergistic effect on the stability of MOFs. Computational simulations were used to provide insight into the mechanism governing these trends in the stability of the materials studied.

Protection of open-metal V(III) sites and their associated CO2/CH4/N2/O2/H2O adsorption properties in mesoporous V-MOFs.

Metal-organic frameworks with open metal site are potential sorbents for the separation of gas mixtures; however, low valence metal will bind to oxygen in the open air causing a decrease in adsorption ability. We now report open-metal sites V(III) on both MIL-100V(III/IV) and MIL-101V(III/IV) that can be protected with water molecules, and which associated CO2/CH4/N2/O2 adsorption properties on these two mesoporous V-MOFs were investigated. The protective properties of water were investigated and evaluated using density functional theory simulations. The binding energy of single O2 on open-metal V(III) site was 93.278 kJ/mol, which decreased to 26.5 kJ/mol when H2O occupies the site. When the water coating is removed, the X-ray photoelectron spectroscopy pattern of V2p showed that the V-MOF changes to MIL-100V(IV) and MIL-101V(IV) at 298 K because of the action of O2. Under these conditions, O2 binds strongly on the open V site significantly reducing the BET (Brunauer-Emmett-Teller) surface and CH4 adsorption volume of the V-MOFs. From the ideal adsorbed solution theory calculated, the adsorption selectivity of CH4/N2 is higher before than after binding of O2 (with V(III) site). In contrast, the adsorption selectivity of CO2/CH4 is higher after than before O2 binding (with no more V(III) sites).

[Comparison of friction and abrasion between six different dental materials and natural enamel].

PURPOSE: The purpose of this study was to evaluate and compare the friction and abrasion of six kinds of dental materials that was widely used clinically. The mechanisms were investigated to provide experimental reference for selecting proper restorative material to protect opposite natural enamel of the patients. METHODS: In the experimental groups,pure Ti, Ti alloy, Au-Pd alloy, Ni-Cr alloy, AgHg alloy,ceramic were fabricated into thirty uniform 8.0mmx8.0mmx2.0mm block. Five specimens in each group.In the control group,five recently extracted mandibular third molar of adults were fabricated into 8.0mmx8.0mmx2.0mm block. Steatites were fabricated into discs in the diameter of 25mm and thickness of 2.0mm to act as counter sample materials. The mass loss of each specimen and the opposing steatite were weighted by electric balance. The volume loss of the materials and opposing steatites were analyzed by SPSS11.5 software package and the difference was compared between each group. RESULTS: The ranking of the volume loss of six materials and enamel from the most to the least was group A, group E, group C, group D, group B, group F and group G. Significant difference was found between group A and the other groups. No significance was found between group G and F. The coefficient of friction of group A-G was 0.68, 0.12, 0.11, 0.41, 0.38, 0.48 and 0.65,respectively. CONCLUSION: The six materials in this study were less resistant to wear than natural tooth. The wear of pure Ti, AgHg alloy and Ni-Cr alloy are to enamel.