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The exciton binding energy (EBE) in CdSe quantum dots (QDs) has been determined using X-ray spectroscopy. Using X-ray absorption and photoemission spectroscopy, the conduction band (CB) and valence band (VB) edge shifts as a function of particle size have been determined and combined to obtain the true band gap of the QDs (i.e., without an exciton). These values can be compared to the excitonic gap obtained using optical spectroscopy to determine the EBE. The experimental EBE results are compared with theoretical calculations on the EBE and show excellent agreement.

The local structure and composition of Cu ions dispersed in CdSe nanocrystals is examined using soft x-ray absorption near edge spectroscopy (XANES). Using Cu L-edge XANES and X-ray photoelectron measurements (XPS), we find that the Cu ions exist in the Cu(I) oxidation state. We also find that the observed Cu L-edge XANES signal is directly proportional to the molar percent of Cu present in our final material. Se L-edge XANES indicates changes in the Se density of states with Cu doping, due to a chemical bonding effect, and supports a statistical doping mechanism. Photoluminescence (PL) measurements indicate the Cu ions may act as deep electron traps. We show that XANES, XPS, and PL are a powerful combination of methods to study the electronic and chemical structure of dopants in nanostructured materials.

In this study, we report structural, vibrational, and magnetic data providing evidence of random ion displacement in the core of CdSe quantum dots on the Cd(2+) sites by Co(2+) ions (between x = 0 and 0.30). Structural evidence for core doping is obtained by analyzing the powder X-ray diffraction (pXRD), data which exhibits a linear lattice compression with increasing Co(2+) concentration, in accord with Vegard's law. Correlated with the pXRD shift, a hardening of the CdSe longitudinal optical phonon mode and a new local vibrational mode are observed which track Co(2+) doping concentration. Consistent with the observed core doping, superconducting quantum interference device (SQUID) measurements indicate a surprising increase for the onset of spin glass behavior by an order of magnitude over bulk Co:CdSe. Correlation of SQUID results, pXRD, and Raman measurements suggests that the observed enhancement of magnetic superexchange between Co(2+) dopant ions in this confined system arises from changes in the nature of coupling in size-restricted materials.

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The introduction of dopants plays a key role in the physical properties of semiconductors for optoelectronic applications. However, doping is generally challenging for nanocrystals (NCs), especially for two-dimensional (2D) NCs, due to the self-annealing effect and high surface energies required for dopant addition. Here, we report an efficient doping strategy for Mn-doped 2D CsPbCl3 (i.e., Mn:CsPbCl3) nanoplatelets (NPLs) through a postsynthetic solvothermal process. While the original lightly doped 2D Mn:CsPbCl3 NPLs were obtained from growth doping, higher Mn doping efficiencies were achieved through diffusion doping under pressure-mediated solvothermal conditions, resulting in enhanced Mn photoluminescence (PL). Surprisingly, a new CsMnCl3 phase with complete dopant substitution by spinodal decomposition was observed with extended solvothermal treatment, which is confirmed by powder X-ray diffraction, X-ray absorption  fine structure, and electron paramagnetic resonance. Compared with Mn:CsPbCl3 NPLs, the pure CsMnCl3 NPLs give rise to shorter Mn PL lifetime, which is consistent with the short Mn–Mn distance within CsMnCl3 NPLs. This work provides an efficient strategy for doping inside NCs as well as new insights on the dopant concentration-dependent structural and optical properties of perovskite NCs.

Formation of biomineral structures is increasingly attributed to directed growth of a mineral phase from an amorphous precursor on an organic matrix. While many in vitro studies have used calcite formation on organothiol self-assembled monolayers (SAMs) as a model system to investigate this process, they have generally focused on the stability of amorphous calcium carbonate (ACC) or maximizing control over the order of the final mineral phase. Little is known about the early stages of mineral formation, particularly the structural evolution of the SAM and mineral. Here we use near-edge X-ray absorption spectroscopy (NEXAFS), photoemission spectroscopy (PES), X-ray diffraction (XRD), and scanning electron microscopy (SEM) to address this gap in knowledge by examining the changes in order and bonding of mercaptophenol (MP) SAMs on Au(111) during the initial stages of mineral formation as well as the mechanism of ACC to calcite transformation during template-directed crystallization. We demonstrate that formation of ACC on the MP SAMs brings about a profound change in the morphology of the monolayers: although the as-prepared MP SAMs are composed of monomers with well-defined orientations, precipitation of the amorphous mineral phase results in substantial structural disorder within the monolayers. Significantly, a preferential face of nucleation is observed for crystallization of calcite from ACC on the SAM surfaces despite this static disorder.

The detailed mechanism by which ethylene polymerization is initiated by the inorganic Phillips catalyst (Cr/SiO 2 ) without recourse to an alkylating co-catalyst remains one of the great unsolved mysteries of heterogeneous catalysis. Generation of the active catalyst starts with reduction of Cr VI ions dispersed on silica. A lower oxidation state, generally accepted to be Cr II , is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of Cr VI /SiO 2 was prepared using sol-gel chemistry in order to monitor the reduction process spectroscopically. Using in situ UV-vis spectroscopy, we observed a very clean, step-wise reduction by CO of Cr VI first to Cr IV , then to Cr II . Both the intermediate and final states show XANES consistent with these oxidation state assignments, and aspects of their coordination environments were deduced from Raman and UV-vis spectroscopies. The intermediate Cr IV sites are inactive towards ethylene at 80 °C. The Cr II sites, which have long been postulated as the endpoint of CO reduction, were observed directly by high-frequency/high-field EPR spectroscopy. They react quantitatively with ethylene to generate the organoCr III active sites, characterized by X-ray absorption and UV-vis spectroscopy, which initiate polymerization.