Mid-gap luminescence in copper (Cu)-doped semiconductor nanocrystals (NCs) involves recombination of delocalized conduction-band electrons with copper-localized holes. Silver (Ag)-doped semiconductor NCs show similar mid-gap luminescence at slightly (∼0.3 eV) higher energy, suggesting a similar luminescence mechanism, but this suggestion appears inconsistent with the large difference between Ag and Cu ionization energies (∼1.5 eV), which should make hole trapping by Ag highly unfavorable. Here, Ag-doped CdSe NCs (Ag:CdSe) are studied using time-resolved variable-temperature photoluminescence (PL) spectroscopy, magnetic circularly polarized luminescence (MCPL) spectroscopy, and time-dependent density functional theory (TD-DFT) to address this apparent paradox. In addition to confirming that Ag:CdSe and Cu:CdSe NCs display similar broad PL with large Stokes shifts, we demonstrate that both also show very similar temperature-dependent PL lifetimes and magneto-luminescence. Electronic-structure calculations further predict that both dopants generate similar localized mid-gap states. Despite these strong similarities, we conclude that these materials possess significantly different electronic structures. Specifically, whereas photogenerated holes in Cu:CdSe NCs localize primarily in Cu(3d) orbitals, formally oxidizing Cu to Cu, in Ag:CdSe NCs they localize primarily in 4p orbitals of the four neighboring Se ligands, and Ag is not oxidized. This difference reflects a shift from "normal" to "inverted" bonding going from Cu to Ag. The spectroscopic similarities are explained by the fact that, in both materials, photogenerated holes are localized primarily within covalent [MSe] dopant clusters (M = Ag, Cu). These findings reconcile the similar spectroscopies of Ag- and Cu-doped semiconductor NCs with the vastly different ionization potentials of their Ag and Cu dopants.

Wet-kneading is a technique commonly used for the synthesis of SiO 2 −MgO catalysts for the Lebedev ethanol-to-butadiene process, with catalyst performance known to depend heavily on the preparation parameters used in this method. Here, the large influence of Mg precursor and MgO content on morphology, chemical structure (as determined by TEM(-EDX), FT-IR, XRD and solid-state 1 H− 29 Si cross-polarized MAS NMR), and on catalyst performance is demonstrated. The Mg precursor used is found to influence the extent of magnesium silicate formation during wet-kneading, as estimated from TEM and FT-IR, which, in turn, was found to correlate with catalyst performance. Accordingly, the catalyst synthesized from a nanosized Mg(OH) 2 precursor (SiO 2 −MgO (III) nano ), showing the highest degree of chemical contact between the SiO 2 and MgO components, gave the highest butadiene yield. Variation of the Mg/Si ratio in a series of SiO 2 −MgO (III) nano materials showed a volcano-type dependence of the butadiene yield on MgO content. 1 H− 29 Si CP-MAS NMR studies allowed for the identification of the type and an estimation of the amount of magnesium silicates formed during wet-kneading. Here, we argue that the structural characteristics of the hydrous magnesium silicates, lizardite and talc, formed during catalyst preparation, together with the ratio of the magnesium silicates to MgO, determine the overall acid/base properties of the SiO 2 −MgO (III) nano catalyst materials and as a result, catalyst performance.

Ultrathin two-dimensional (2D) nanomaterials have attracted intense research efforts due to their extraordinary optoelectronic properties. However, the nucleation and growth mechanisms of 2D colloidal nanosheets are still poorly understood. Here, we follow the formation of ultrathin colloidal Cu2–x S nanosheets by in situ small-angle X-ray scattering. While thermal decomposition of copper–dodecanethiolates produces spheroidal Cu2–xS nanocrystals, the addition of chloride to the reaction mixture results in 2 nm thick Cu 2–x S nanosheets with well-defined shape and size. Our results show that chloride stabilizes stacks of lamellar copper–thiolate supramolecular complexes, so that they remain intact beyond the onset of Cu2–x S nucleation at 230 °C, leading to 2D-constrained stack-templated nucleation and growth. The face-to-face stacking of the nanosheets reinforces the 2D constraints imposed by the lamellar soft template, since it prevents internanosheet mass transport and nanosheet coalescence, thereby inhibiting growth in the thickness direction and allowing only for lateral growth. Our work thus provides novel insights into soft-templating formation mechanisms of ultrathin colloidal nanosheets, which may be exploited for other metal sulfide compositions.

The luminescence of CuInS 2 quantum dots (QDs) is slower and spectrally broader than that of many other types of QDs. The origin of this anomalous behavior is still under debate. Single-QD experiments could help settle this debate, but studies by different groups have yielded conflicting results. Here, we study the photophysics of single core-only CuInS 2 and core/shell CuInS 2 /CdS QDs. Both types of single QDs exhibit broad PL spectra with fluctuating peak position and single-exponential photoluminescence decay with a slow but fluctuating lifetime. Spectral diffusion of CuInS 2 -based QDs is qualitatively and quantitatively different from CdSe-based QDs. The differences reflect the dipole moment of the CuInS 2 excited state and hole localization on a preferred site in the QD. Our results unravel the highly dynamic photophysics of CuInS 2 QDs and highlight the power of the analysis of single-QD property fluctuations.

Hot-injection synthesis is renowned for producing semiconductor nanocolloids with superb size dispersions. Burst nucleation and diffusion-controlled size focusing during growth have been invoked to rationalize this characteristic yet experimental evidence supporting the pertinence of these concepts is scant. By monitoring a CdSe synthesis in-situ with X-ray scattering, we find that nucleation is an extended event that coincides with growth during 15−20% of the reaction time. Moreover, we show that size focusing outpaces predictions of diffusion-limited growth. This observation indicates that nanocrystal growth is dictated by the surface reactivity, which drops sharply for larger nanocrystals. Kinetic reaction simulations confirm that this so-called superfocusing can lengthen the nucleation period and promote size focusing. The finding that narrow size dispersions can emerge from the counteracting effects of extended nucleation and reaction-limited size focusing ushers in an evidence-based perspective that turns hot injection into a rational scheme to produce monodisperse semiconductor nanocolloids.