Excitonic emission of colloidal nano-crystals embedded in Molecular Beam Epitaxy grown ZnSe

We combine ZnSe layers grown by molecular beam epitaxy with colloidal CdSe core, CdSe/ZnSe and CdSe/ZnS core/shell nano-crystals (NCs) to achieve monolithic NC-semiconductor heterostructures. The NCs are prepared in solution and deposited by spray-coating on ZnSe buffer layers and subsequently overgrown by ZnSe. We find a blue shift of the photoluminescence of core/shell dots when they are overgrown by ZnSe. Rapid thermal annealing is used to improve the interface region between the NCs and the ZnSe matrix. The effect of different annealing temperatures on the optical properties of CdSe core, CdSe/ZnSe and CdSe/ZnS core/shell NCs overgrown with a cap layer of ZnSe is investigated. After annealing at 673 K the photoluminescence of these samples is red-shifted as compared to unprocessed samples. All photoluminescence results are explained by a model calculation with the following assumption about the 3D confining potential of the NCs: (i) the shell of core/shell NCs dissolves during ZnSe overgrowth, (ii) after overgrowth NCs are separated from the ZnSe matrix by an interface barrier, (iii) the height of this barrier is significantly reduced by annealing. For all three types of NCs we find an excellent quantitative agreement between the experimental and calculated NC transition energies. The absence of the barrier after annealing is further demonstrated by low temperature photoluminescence data of annealed samples which show enhanced diffusion of electron-hole pairs from ZnSe into the NCs.