Tuning the optical and electronic properties of colloidal nanocrystals by lattice strain

Abstract

Lattice strain is a structural parameter that has been exploited in microelectronic devices with great success, but its role in colloidal nanocrystals is still poorly understood. Here we have developed strain-tunable colloidal nanocrystals by using lattice-mismatched heterostructures that are grown by epitaxial deposition of a compressive shell (e.g., ZnSe or CdS) onto a soft and small nanocrystalline core (e.g., CdTe). This combination of a "squeezed" core and a "stretched" shell causes dramatic changes in both the conduction and valence band energies. As a result, we show that core-shell QDs with standard type-I behavior are converted into type-II nanostructures, leading to spatial separation of electrons and holes, extended excited state lifetimes, and giant spectral shifting. This new class of strain-tunable QDs exhibits narrow light emission with high quantum yield across a broad range of visible and near-infrared wavelengths (500 nm to 1050 nm).

One-pot aqueous synthesis of highly strained CdTe/CdS/ZnS nanocrystals and their interactions with cells

“…By using the lattice strain between a small so core (CdTe) and a thick compressive shell (ZnS, CdS, ZnSe, CdSe), Nie and coworkers presented a new class of core (small) /shell (thick) NCs displaying type-II behavior with greater photostability and giant spectral shis in comparison to traditional core (thick) /shell (thin) NCs. 1 When the small CdTe core is subjected to a large stress arising from the epitaxial overgrowth of a lattice-mismatched shell, the CdTe core can overcome this stress due to its elasticity while maintaining excellent spectral properties without causing any defect trap sites. The rst aqueous synthesis of CdTe/CdS so-called 'magic-core/thick-shell' NIR emitting NCs was reported by Deng and coworkers.…”

Outermost tensile strain dominated exciton emission in bending CdSe nanowires

“…Further modulation of the electronic structures and control of the transport dynamics of carriers in CdSe nanostructures are very important to optimize the optoelectronic properties. It has been demonstrated that the carrier and exciton transport dynamics can be effectively tuned in CdS/CdSe superlattices and core/shell nanostructures such as CdTe/ CdSe [4] and ZnSe/CdSe [5], due to the alignment of their band structures and bandgap offset. Most recently, it has been experimentally illustrated that the colloidal core/ crown CdSe/CdS nano-platelet based heterostructures can act as efficient exciton concentrators [6].…”

Graded Shells in Semiconductor Nanocrystals

“…Alternative band alignments include the type-II configuration with a staggered band alignment that induces charge separation into core and shell (CdSe/CdTe, ZnSe/CdS) and the intermediate quasi-type-II configuration in which one carrier is confined to the core and the other is delocalised over core and shell (CdSe/CdS) [9][10][11][12]. These materials replace the core surface with a sharp interface between core and shell, which brings a number of implications that arise from interfacing two different, crystalline materials: if the core has a larger lattice constant than the shell (as is generally the case for type-I structures) high tensile strain on the shell for thin shells and high compressive strain on the core for thicker shells is induced [13]. This is the case for one of the best studied systems, CdSe/ZnS, for which even moderately thick shells > 2 monolayers (ML) leads to dislocations, additional defects and interface states, which directly counteracts the aim to spatially separate exciton and surface [14].…”