The capacity of fluorescent colloidal semiconductor nanocrystals for commercial application has led to the development of nanocrystals with nontoxic constituent elements as replacements for the currently available Cd- and Pb-containing systems. CuInS2 is a good candidate material because of its direct band gap in the near-infrared spectral region and large optical absorption coefficient. The ternary nature, flexible stoichiometry, and different crystal structures of CuInS2 lead to a range of optoelectronic properties, which have been challenging to elucidate. In this Perspective, the optoelectronic properties of CuInS2 nanocrystals are described and what is known of their origin is discussed. We begin with an overview of their synthesis, structure, and mechanism of formation. A complete discussion of the tunable luminescence properties and the radiative decay mechanism of this system is then presented. Finally, progress toward application of these "green" nanocrystals is summarized.

CuInS2 nanocrystals with the wurtzite structure show promise for applications requiring efficient energy transport due to their anisotropic crystal structure. We investigate the source of photoluminescence in the near-infrared spectral region recently observed from these nanocrystals. Spectroscopic studies of both wurtzite CuInS2 itself and samples alloyed with Cd or Zn allow the assignment of this emission to a radiative point defect within the nanocrystal structure. Further, by varying the organic passivation layer on the material, we are able to determine that the atomic species responsible for nonradiative decay paths on the nanocrystal surface are Cu- or S-based. Density functional theory calculations of defect states within the material allow identification of the likely radiative species. Understanding both the electronic structure and optical properties of wurtzite CuInS2 nanocrystals is necessary for their efficient integration into potential biological, photovoltaic, and photocatalytic applications.

Nanocrystals of CuInS 2 with the hexagonal wurtzite structure hold great potential for applications requiring efficient energy transport, such as photocatalysis, due to their anisotropic crystal structure. However, thus far their optical properties have proven difficult to study, as luminescence from wurtzite nanocrystals has only recently been observed. In this work, we report the colloidal synthesis of single crystalline, luminescent CuInS 2 nanocrystals with both the cubic and hexagonal structures. The crystalline phase, optical properties and mechanism of formation of nanocrystals are controlled by changing the reaction temperature. Photoluminescence is observed in the visible and near-infrared spectral regions, which results from the cubic and hexagonal nanocrystals respectively. Synthetic studies combined with XRD, TEM and EDS mapping provide evidence for the mechanisms behind phase selection. † Electronic supplementary information (ESI) available: Complete Rietveld refinement parameters; Scherrer line broadening crystallite size compared to TEM size; EDS data; quantum yields; Fityk peak fitting parameters; absorbance spectra of hexanes; TEM images from aliquot study; complete EDS maps for aliquot study, including dark field and elemental analysis data. See structure formed at these temperatures. 32 The QY was o0.8% for all samples (Fig. S5, ESI †).The visible peak, attributed to ZB CuInS 2 , was fit to a single Gaussian (Fig. S6, ESI †), which showed a red shift as reaction temperature increased. This is most likely due to an increase in the size of the NCs and the relaxation of quantum confinement conditions. This is consistent with the trends observed in absorbance spectra and reported in literature examples. [56][57][58] The broad, multimodal shape of the NIR PL peak, centred at B950 nm, and the large Stokes shift (0.25 eV) indicate that Fig. 3 (a) XRD of CuInS 2 NCs prepared at various temperatures. Pure ZB and WZ spectra are digitized from the work of Chang et al.; 36 (b) Proportion of WZ (blue), ZB (red) and hiCC Cu 2 S (green) phases present in each sample plotted as a function of temperature, as determined by Rietveld refinement of XRD; (c) EDS map of CuInS 2 NCs prepared at 155 1C showing the presence of Cu 2 S NCs; (d), (e) High resolution TEM (HRTEM) images of CuInS 2 NCs prepared at 215 1C (plates) and 115 1C (spheres) respectively showing lattice fringing.Fig. 4 (a) Absorbance spectra of CuInS 2 NC dispersions in hexanes prepared at different temperatures; (b) Tauc plot of the predominantly WZ CuInS 2 NCs prepared at 215 1C; (c) PL spectra normalized to the QY of CuInS 2 NC dispersions in hexanes prepared at different temperatures.

The metastable and thermodynamically favored phases of CuFeS 2 are shown to be alternatively synthesized during partial cation exchange of hexagonal Cu 2 S using various phosphorus-containing ligands. Transmission electron microscopy and energy dispersive spectroscopy mapping confirm the retention of the particle morphology and the approximate CuFeS 2 stoichiometry. Powder X-ray diffraction patterns and refinements indicate that the resulting phase mixtures of metastable wurtzite-like CuFeS 2 versus tetragonal chalcopyrite are correlated with the Tolman electronic parameter of the tertiary phosphorus-based ligand used during the cation exchange. Strong L-type donors lead to the chalcopyrite phase and weak donors to the wurtzite-like phase. To our knowledge, this is the first demonstration of phase control in nanoparticle synthesis using solely L-type donors.

The ternary copper chalcogenide semiconductor nanoparticles have gained much attention as their optical properties make them ideal candidates for many applications ranging from photovoltaics to bioimaging. While their synthesis is well documented, there have been few reports on the synthesis of ternary copper chalcogenide−metal hybrid nanoparticles, which can further expand the list of potential applications through synergistic properties. To this end, Pt− CuInS 2 hybrids have been synthesized by a two-step approach in high boiling organic solvents. The hybrid nanostructures were characterized employing transmission electron microscopy, X-ray diffraction, UV−Vis spectroscopy, and energydispersive X-ray spectroscopy mapping. We find that during hybrid nanoparticle synthesis under conditions modified from typical Pt nanoparticle reaction schemes, a near-complete shell of Pt forms on the semiconductor nanoparticles. Careful control of the reactivity of the Pt precursor, through choice of organic reducing agent and Pt coordinating ligands, was successfully used to obtain controlled and isolated domains on the semiconductor nanoparticles. This strategy was further extended for the synthesis of Pd−CuInS 2 hybrids.