Growth and Characterization of Strained and Alloyed Type-II ZnTe/ZnSe Core–Shell Nanocrystals

Abstract

We investigate the growth and the physical and optical properties of type-II heterostructured ZnTe/ZnSe colloidal nanocrystals, focusing on the role of the 7% lattice mismatch between the two materials in determining growth homogeneity and band structure. We find that the lattice mismatch between the two materials places limitations on the range of structures that can be grown, and for those in which coherent growth is achieved we present clear evidence that the low bulk modulus ZnTe cores are compressed by the higher modulus ZnSe shells, accentuating the red-shift of the excitonic state with increasing shell thickness. By employing a variety of characterization tools we build a clear picture of the core–shell architecture. We show how strain is manifested in structures with sharp core–shell interfaces and how intentional alloying of the interface can influence the growth and exciton energies. We show that a (2,6)-band effective mass model is able to distinguish between the as-grown “sharp” and “alloyed” interfaces, indicating that the alloyed structures incorporate reduced strain.

The passivating effect of cadmium in PbS/CdS colloidal quantum dots probed by nm-scale depth profiling

“…After synthesis the original oleic acid or oleylamine ligands were exchanged for either butylamine or 3-mercaptopropionic acid (3-MPA). This was done to avoid sample charging when measuring photoemission, 39 as the new ligands are shorter in length. The ligand exchange to butylamine and 3-MPA followed modified methods from Fairclough et al and Zhang et al respectively.…”

“…The ligand exchange to butylamine and 3-MPA followed modified methods from Fairclough et al and Zhang et al respectively. 39,40 The methods are outlined in the ESI. †…”

“…Discrepancies of this type have also been observed in other CQD systems and are generally diagnostic of alloying. 39 If the XPS-calculated total particle size is smaller than that observed in TEM, alloying in the core is indicated (as in ZnTe/ZnSe). 39 The reverse observation (where the shell size calculated from XPS leads to an overall particle size larger than that observed in TEM, as here) can be explained by alloying of CdS with PbS in the shell.…”

Switchable dissociation of excitons bound at strained CdTe/CdS interfaces

“…29 The tendency of the structure to minimise strain leads to formation of crystal defects and, for higher strain, anisotropic rather than epitaxial growth. 30,31 Of the II-VI semiconductors CdTe has one of the highest compressibilities 28 and forms a strongly mismatched type-II interface with CdS, for which large strain effects can be expected. [32][33][34] While this material combination is employed widely for thin film solar cells 35,36 relatively few studies are concerned with the CdTe/CdS interface in colloidal nanocrystalline systems and focus on core/shell systems.…”

Electronic structure of quantum dots: response to the environment and externally applied fields

“…In such a case, strain produces shifts of different magnitude in the band edges of core and shell materials, leading to transitions from one band alignment to another. This control mechanism has been investigated in CdTe/ZnSe, [152] CdSe/CdTe, [163] and ZnSe/ZnTe nanocrystals, [164] among others. However, it cannot be efficiently employed in CdSe/CdS QDs owing to their weak strain, hence the e-h separation in such structures has been traditionally controlled by quantum confinement.…”