We develop a nucleation-based model to explain the formation of the wurtzite (WZ) phase during the vapor-liquid-solid growth of free-standing nanowires of zinc-blende (ZB) semiconductors. Nucleation occurs preferentially at the edge of the solid/liquid interface, which entails major differences between ZB and WZ nuclei. Depending on the pertinent interface energies, WZ nucleation is favored at high liquid supersaturation. This explains our systematic observation of ZB during early growth.PACS numbers: 68.65. La,64.60.Qb,81.05.Ea,81.15.Kk,64.70.Nd Free-standing wires with diameters ranging from hundreds down to a few nanometers are nowadays commonly fabricated from a large range of semiconductor materials [1,2,3,4,5]. These nanowires (NWs) have remarkable physical properties and many potential applications. The present work deals with the epitaxial growth of NWs of III-V semiconductors on a hot substrate. Metal catalyst nanoparticles deposited on the substrate before growth define the wire diameter. According to the vapor-liquidsolid (VLS) growth mechanism, the atoms are fed from the vapor phase to the solid wire through this particle (or droplet), which remains liquid during growth [6].We consider III-V compounds which, under bulk form, adopt the cubic zinc-blende (ZB) crystal structure [7] (although some non-ZB high-pressure phases [8] may be metastable at atmospheric pressure [9]), leaving aside nitrogen-based NWs. We discuss the usual case of NWs grown on a [111]B (As-terminated) face of the ZB substrate. Probably the most surprising feature of these NWs is that, in contrast to their bulk counterparts, they often adopt the hexagonal wurtzite (WZ) structure. This was observed for most ZB III-V materials and growth techniques [1,3,4,10,11]. However, although often dominantly of WZ structure, the NWs usually contain stacking faults (SFs) and sequences of ZB structure. The coexistence of two phases is clearly a problem for basic studies as well as applications, so that phase purity control is one of the main challenges of III-V NW fabrication.The surprising prevalence of the WZ structure in III-V NWs has not been explained satisfactorily so far. Here, based on new experimental observations, we propose an explanation of the occurrence of the WZ structure and develop a model predicting quantitatively in which growth conditions it should form. We consider the specific case of gold-catalyzed GaAs NWs grown by molecular beam epitaxy (MBE) on a GaAs substrate but we expect our model and our conclusions to remain valid for any ZB III-V compound and any growth method.Let us start with briefly reviewing previously proposed explanations. Calculations give the difference δw in cohesive energy between ZB and WZ bulk GaAs as about 24 meV per III-V pair at zero pressure [7]. It has been argued that this favoring of the ZB form might be offset in NWs of small diameter by the large relative contribution to the total energy of either the lateral facets [12] or the vertical edges separating the latter [13] (provided the specifi...

We have recently synthesized atomically flat semiconductor colloidal nanoplatelets with quasi 2D geometry. Here, we show that core/shell nanoplatelets can be obtained with a 2D geometry that is conserved. The epitaxial growth of the shell semiconductor is performed at room temperature. We report the detailed synthesis of CdSe/CdS and CdSe/CdZnS structures with different shell thicknesses. The shell growth is characterized both spectroscopically and structurally. In particular, the core/shell structure appears very clearly on high-resolution, high-angle annular dark-field transmission electron microscope images, thanks to the difference of atomic density between the core and the shell. When the nanoplatelets stand on their edge, we can precisely count the number of atomic planes forming the core and the shell. This provides a direct measurement, with atomic precision, of the core nanoplatelets thickness. The constraints exerted by the shell growth on the core is analyzed using global phase analysis. The core/shell nanoplatelets we obtained have narrow emission spectra with full-width at half-maximum close to 20 nm, and quantum yield that can reach 60%.

We present the synthesis and the optical properties of a new type of two-dimensional heterostructure: core/crown CdSe/CdS nanoplatelets. They consist of CdSe nanoplatelets that are extended laterally with CdS. Both the CdSe core and the CdS crown dimensions can be controlled. Their thickness is controlled at the monolayer level. These novel nanoplatelet-based heterostructures have spectroscopic properties that can be similar to nanoplatelets or closer to quantum dots, depending on the CdSe core lateral size.

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We have synthesized atomically flat CdSe/CdTe core/crown nanoplatelets (NPLs) with thicknesses of 3, 4, and 5 monolayers with fine control of the crown lateral dimensions. In these type-II NPLs, the charges separate spatially, and the electron wave function is localized in the CdSe core while the hole wave function is confined in the CdTe crown. The exciton's recombination occurs across the heterointerface, and as a result of their spatially indirect band gap, an important emission red shift up to the near-infrared region (730 nm) is observed with long fluorescence lifetimes that range from 30 to 860 ns, depending on the type of interface between the core and the crown. These type-II NPLs have a high quantum yield of 50% that can be further improved to 70% with a gradient interface. We have characterized these novel CdSe/CdTe core/crown NPLs using UV-vis, emission, and excitation spectroscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy.