Example 1:

We reveal the existence of two different crystalline phases, i.e., the metastable rock salt and the equilibrium zinc blende phase within the CdS-shell of PbS/CdS core/shell nanocrystals formed by cationic exchange. The chemical composition profile of the core/shell nanocrystals with different dimensions is determined by means of anomalous small-angle X-ray scattering with subnanometer resolution and is compared to X-ray diffraction analysis. The highest emission was achieved for chemically pure CdS shells below 1 nm thickness with a dominant metastable rock salt phase fraction matching the crystal structure of the PbS core. The photoluminescence intensity depicts a constant decrease with decreasing metastable rock salt phase fraction but shows an abrupt drop for shells above 1.3 nm thickness. We relate this effect to two different transition mechanisms for changing from the metastable rock salt phase to the equilibrium zinc blende phase depending on the shell thickness. (R.T. Lechner, G. Fritz-Popovski, M. Yarema, W. Heiss, A. Hoell, T.U. Schülli, D. Primetzhofer, M. Eibelhuber, and O. Paris, Chem. Mater., ASAP (2014), DOI: 10.1021/cm502521q)

Example 2:

Wood is a hierarchical composite, consisting at its lowest hierarchy level of crystalline cellulose elementary fibrils with diameters of 2-4 nm embedded in a matrix of hemicelluloses and lignin. At the micrometer scale, it has a cellular architecture resembling a honeycomb structure. The transformation of the hierarchical wood structure into a silica replica has been reported recently. Its formation process and structural details are studied in this contribution. First, a silica/biopolymer composite is prepared by wood delignification and cell-wall modification, followed by silica precursor infiltration and condensation. The calcination process is monitored in-situ by small-angle x-ray scattering (SAXS) to gain insight into the structure development upon decomposition of the biopolymers. The material changes its architecture gradually from fibrillar structures of 10-20 nm in diameter with homogeneous electron density, into fibrils of 8-10 nm in diameter with inhomogeneous electron density, exhibiting internal sub-fibrillar structures of about 2 nm in diameter. Structural modeling of the SAXS data suggests that first the hemicelluloses are degraded at 250-300 °C, leaving a composite of amorphous silica fibrils with embedded cellulose elementary fibrils. At 350-450 °C the cellulose elementary fibrils disintegrate, leaving nanometer sized cylindrical parallel pores within the silica fibrils. Hence, this work demonstrates for the first time the steps of the successful replication of the cellulose elementary fibrils into nanopores of similar diameter and orientation in a fibrillar silica matrix. These nanopore replicas of the original cellulose are wound in a steep helix within the macropore walls. Besides a large potential of these advanced materials for lightweight structural applications, the nanopores might also be advantageous for molecular separation applications. (Fritz-Popovski, G.; Van Opdenbosch, D.; Zollfrank, C.; Aichmayer, B. & Paris, O. Development of the Fibrillar and Microfibrillar Structure During Biomimetic Mineralization of Wood Advanced Functional Materials, 2012, 23, 1265-1272 (DOI: 10.1002/adfm.201201675))