Matchstick-shaped Ag2S-ZnS heteronanostructures preserving both UV/blue and near-infrared photoluminescence.

In recent years, heterostructured nanomaterials have attracted intense research interest due to their integrated multifunctionality of disparate components. Such multifunctionality gives heterostructured nanomaterials great potential in different fields of diagnosis, sensors, catalysis, optoelectronic devices, and so on. In particular, enormous efforts have been devoted to synthesizing different heterodimer nanomaterials, including CoPt3–Au, [3] PbSe–Au, Fe3O4– Au, 5] PbS–Au, Fe3O4–Ag, [7,8] and Ag2S/Ag, [10] which combine optical and/or electrical, magnetic, catalytic properties. Matchstick-shaped heteronanostructures (HNSs) are an important kind of heterostructured nanomaterials which are very suitable for integrating into nanodevices for further applications. Metal-tipped semiconductor nanorod HNSs have been well studied in the last decade. Metal tips (Au, Pt, Co, etc.) were selectively grown on either top or side of CdS/CdSe nanorods, and the resulting metal–semiconductor interface facilitated charge separation, which favored their application in solar energy. Due to the flexibility of bandgap engineering, semiconductor–semiconductor HNSs have been considered to offer better opportunities for internal exciton separation and carrier transport and optoelectronic applications. Recently, we reported that Ag2S quantum dots (QDs) can be good candidates as near-infrared (NIR) emitters, and that ultrathin ZnS nanowires can emit in the UV/blue region. We therefore wondered how HNSs consisting of Ag2S QDs and ZnS nanowires would behave. Recently, Xu et al. prepared Ag2S–ZnS HNSs by a seeded-growth method in which Ag2S nanocrystals acted as catalyst for growth of ZnS nanorods. However, both Ag2S nanocrystals and ZnS nanorods of the as-prepared Ag2S–ZnS HNSs had large diameters of about 20 nm and their optical properties were not reported. Since the Bohr radius of ZnS is 2.4 nm (to the best of our knowledge, that of Ag2S is unknown), we expect that Ag2S–ZnS HNSs with smaller sizes will exhibit their intrinsic optical properties due to the quantum confinement effect. Therefore, the driving force for this work was to determine whether Ag2S–ZnS HNSs with smaller sizes preserve both the NIR and UV/blue emissions or not. Three merits of this work can be noted: 1) The as-prepared small Ag2S–ZnS HNSs exhibit both NIR and UV/blue emissions from Ag2S QDs and ZnS nanorods, respectively; 2) A facile one-pot method is utilized for Ag2S–ZnS HNSs synthesis by thermal co-decomposition of single-source precursors Ag(DDTC) and Zn(DDTC)2 (DDTC = diethyldithiocarbamate), which is much more convenient than the seededgrowth or catalyst-assisted growth method; 3) The size of the HNSs can be easily tuned by changing the reaction conditions, which is not possible for seeded-growth with given seeds. Figure 1a depicts a typical low-magnification TEM image of Ag2S–ZnS HNSs prepared with an Ag(DDTC)/Zn(DDTC)2 molar ratio of 2:1. The HNSs are of uniform matchstick shape with significant difference in the massthickness contrast between the spherical head (ca. 4.5 nm in diameter) and the stem (4 48 nm in diameter and length). The narrow size distribution of as-prepared Ag2S–ZnS HNSs facilitated their self-assembly into superlattice structures with hexagonal packing, which was supported by a selected-area fast Fourier transform (FFT) pattern (inset in Figure 1a). The Ag2S–ZnS HNS superlattices were perpendicular to the TEM grid, as was further confirmed by TEM tilting experiments (see Supporting Information), similar to a previously reported CoO nanorod superlattice. The mass-thickness contrast difference between the spherical head and stem indicated the various chemical compositions of the as-prepared HNSs. A high-resolution TEM (HRTEM) image of a typical Ag2S–ZnS HNS is shown in Figure 1b. The HNS is highly crystalline with a spherical head and a nanorodlike stem, and has a partially coherent interface between single-crystalline head and stem. Based on the analysis of the corresponding crystal lattices, the spherical head is composed of Ag2S and the stem of ZnS, and the conjunction interface consists of the ( 121) plane of the Ag2S head and the (008) plane of the ZnS stem with a lattice mismatch of 16% (Figure 1b). The (008) plane of ZnS was further confirmed by a higher quality HRTEM image (Figure 1c), in which hcp ABAB stacking of ZnS double layers along the [001] direction can be clearly observed. This is a strong evidence that d = 0.31 nm corresponds to the (008) plane of hexagonal ZnS. Detailed analysis of the local elemental composition of the Ag2S–ZnS HNSs was performed by line-scan energy-dispersive X-ray spec[*] Dr. S. Shen, Y. Zhang, L. Peng, Dr. Y. Du, Prof. Dr. Q. Wang Division of Nanobiomedicine andi-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences Suzhou, 215123 (China) Fax: (+ 86)512-6287-2620 E-mail: qbwang2008@sinano.ac.cn