Core-shell nanocrystals InAs-ZnSe

Figure 5.6 shows tunneling-conductance spectra measured on two InAs-ZnSe core-shell nanocrystals with two- and six-monolayer (ML) shells, along with a typical curve for an InAs QD of radius similar to the nominal core radius 1.7 nm, deposited on Au with DT linkers. The general appearance of the spectra of the core and of the core-shell nanocrystals was similar, and the band-gap near-identical, as observed with optical absorption measurements [34]. In contrast, the s-p level separation is substantially reduced. Both effects were consistent with a model in which the s state was confined to the InAs core region, while the p level extended to the ZnSe shell. 

The hot-soup method has also been applied to the production of semiconductor core/shell nanostructures, where a semiconductor nanocrystal is coated with a second semiconductor of wider bandgap. Shown in Figure 23 is a scheme for the synthesis of CdSe/CdS core/shell nanocrystals. These nanostructures were synthesized via a high-temperature route in a mixture of TOP and TOPO (223,224) and via a low-temperature route in pyridine (225). Among the semiconductor core/shell nanomaterials produced were CdSe/ZnS, CdSe/CdS, InAs/InP, InAs/CdSe, InAs/ZnSe, and InAs/ZnS (223,225-228). In these structures, the shell type and thickness allow further control of the optical, electronic, and other properties of semiconductor nanocrystals. For example, the shell may be used to passivate the imperfect surface of the core semiconductor, resulting in significantly improved luminescence efficiency. 

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