Donor-Acceptor transition was previously suggested as a mechanism for luminescence in (ZnS)1-x(AgInS2)x nanocrystals. Here we show the participation of delocalized valence/conduction band in the luminescence. Two emission pathways are observed: Path-1 involves transition between a delocalized state and a localized state exhibiting higher energy and shorter lifetime (∼25 ns) and Path-2 (donor-acceptor) involves two localized defect states exhibiting lower emission energy and longer lifetime (>185 ns). Surprisingly, Path-1 dominates (82% for x = 0.33) for nanocrystals with lower x, in sharp difference with prior assignment. Luminescence peak blue shifts systematically by 0.57 eV with decreasing x because of this large contribution from Path-1. X-ray absorption fine structure (XAFS) study of (ZnS)1-x(AgInS2)x nanocrystals shows larger AgS4 tetrahedra compared with InS4 tetrahedra with Ag-S and In-S bond lengths 2.52 and 2.45 Å respectively, whereas Zn-S bond length is 2.33 Å along with the absence of second nearest-neighbor Zn-S-metal correlation.

We report here a different kind of binary and ternary colloidal inorganic nanocrystals, where no capping ligand is used. The surfaces of these ligand-free nanocrystals were designed to exhibit negative charges and, therefore, electrostatically repel each other, forming a colloidal dispersion in a polar solvent. Undoped and Mn-doped ZnxCd1-xS nanocrystals were studied both in solution and close-packed films. While undoped samples exhibit poor luminescence because of surface defects, Mn-doped nanocrystals show strong luminescence both in solution (20% quantum efficiency) and the film. Strong Mn emission arises from the inner-core d electrons, which are less sensitive to nonradiative decay channels on the nanocrystal surface. Ligand-free AgInS2 nanocrystals exhibit donor-acceptor type luminescence. Our preliminary results suggest the possibility of electronic coupling between ligand-free nanocrystals in their film, and therefore, they are expected to be more suitable for electronic and optoelectronic applications compared to organic-capped nanocrystals.

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Sulfate erosion is one of the main durability issue of ordinary Portland cement (OPC) while used in a sulfate-rich environment. Two aluminate phases in OPC, tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF), are primarily responsible for sulfate attack but with different sulfate resistant performance. This paper therefore focuses on the internal sulfate invasion to hydration products of these two aluminate minerals and attempts to explain the role of irons in hydrates. Chemical shrinkage coupled with X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectra (EDS) are employed to determine the overall development of internal sulfate erosion of these hydrates. The results suggested that the doped iron in hydrogarnet (C3(A,F)­H6) dramatically changes crystal growth and leads to large size of polyhedral particle to tiny cubic grain of C3AH6, which contributes superior performance to sulfate erosion. As to another hydrate monosulfate, the introduction of Fe in C4(A,F)­S̅H12 causes a slightly worse sulfate resistance to C4AS̅H12. The substitution of iron to aluminum in hydrogarnet is the radical reason for better sulfate resistance of C4AF to C3A. This study greatly enhances the understanding of sulfate resistance to OPC.

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