Solution-Based II−VI Core/Shell Nanowire Heterostructures

Abstract

We demonstrate the solution-phase synthesis of CdS/CdSe, CdSe/CdS, and CdSe/ZnTe core/shell nanowires (NWs). On the basis of bulk band offsets, type-I and type-II heterostructures are made, contributing to the further development of low-dimensional heteroassemblies using solution-phase chemistry. Core/shell wires are prepared by slowly introducing shell precursors into a solution of premade core NWs dispersed in a noncoordinating solvent at moderate temperatures (215-250 degrees C). Resulting heterostructures are characterized through low- and high-resolution transmission electron microscopy, selected area electron diffraction, and energy dispersive X-ray analysis. From these experiments, initial shell growth appears to occur through either Stranski-Krastanov or Volmer-Weber island growth. However, beyond a critical shell thickness, nucleation of randomly oriented nanocrystals results in a polycrystalline coat. In cases where overcoating has been achieved, corresponding elemental analyses show spatially varying compositions along the NW radial direction in agreement with expected element ratios. Electronic interactions between the core and shell were subsequently probed through optical studies involving UV-vis extinction spectroscopy, photoluminescence experiments, and transient differential absorption spectroscopy. In particular, transient differential absorption studies reveal unexpected shell-induced changes in core NW Auger kinetics at high carrier densities. Previously seen three-carrier Auger kinetics in CdS (bimolecular in CdSe) NWs were suppressed by the presence of a CdSe (CdS) shell. These observations suggest the ability to influence NW optical/electrical properties by coating them with a surrounding shell, a method which could be important for future NW optical studies as well as for NW-based applications.

Carrier recombination dynamics in individual CdSe nanowires

“…This provides steric as well as electronic stabilization to the wires. Furthermore, solutionbased approaches allow for nanowires to be overcoated [10][11][12] , opening up the possibility of creating core/shell systems that are potentially important for a number of applications.…”

Synthesis and Application of Solution-Based II–VI and IV–VI Semiconductor Nanowires

“…Hence, the thermal stability of core NWs is important and is often tested by suspending them in a growth solvent and heating the suspension to temperatures compatible with the decomposition of the shell precursors. This has been discussed by Goebl et al for CdSe and CdS NWs [49].…”

“…The potential usefulness of such Type II heterostructures stems from their improved charge separation efficiencies upon photoexcitation [49,50]. Specifically, because of the nature of the band offsets, carriers of one type are forced to stay in [46].…”

“…As an illustration, the 3.9 % lattice mismatch between CdSe and CdS makes the creation of CdSe/CdS or CdS/CdSe core/shell morphologies possible, as first demonstrated with colloidal CdSe quantum dots (QDs) [52]. Analogous CdSe/CdS NW heterostructures have since been realized [48,49]. Similarly, CdSe/ZnTe NWs have been made by taking advantage of the 0.8 % lattice mismatch between CdSe and ZnTe [49].…”

“…Analogous CdSe/CdS NW heterostructures have since been realized [48,49]. Similarly, CdSe/ZnTe NWs have been made by taking advantage of the 0.8 % lattice mismatch between CdSe and ZnTe [49]. Although a small lattice mismatch between the core and shell is generally preferred, a tolerance for larger mismatches exists.…”