Semiconductor nanocrystals, or quantum dots, are attracting significant attention due to their unique size-tunable properties that can be useful for a variety of applications in clinical diagnostics, nanoelectronics, sensors, and photovoltaics [1,2]. The size-dependent tunability of the properties arises when the size of the grain becomes smaller than the corresponding de Broglie wavelength or Bohr radius (mean separation of an optically excited electron-hole pair) causing quantum confinement of the exciton by the grain boundary.

Several liquid- and vapor-phase techniques for growing II-VI semiconductor nanocrystals, such as CdSe and ZnSe, have been reported in the literature. The use of templates in the synthesis of nanostructured materials allows precise control of shape and size, scalability, and stabilization for long-term storage. In previous work we have demonstrated the development of novel microemulsion and liquid crystal templates consisting of a polar phase, a non-polar phase and an amphiphilic block copolymer surfactant, and their subsequent use in the synthesis of ZnSe quantum dots, quantum rods and nanoplates with excellent optical properties, including size-dependent luminescence [3-5]. ZnSe quantum dots exhibit size dependent luminsescence in the UV, violet and blue parts of the spectrum (corresponding to wavelengths between 360 nm and 460 nm)

This presentation will focus on the synthesis of (Zn,Mn)Se and (Zn,Cu)Se quantum dots that exhibit luminescence peaks at longer wavelengths than the ZnSe bulk emission, due to the ions incorporated in the ZnSe lattice. We employ microemulsion templates formed by self-assembly of a poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (PEO-PPO-PEO) amphiphilic block copolymer in the presence of a polar and a non-polar solvent. For this work the water / PEO-PPO-PEO / p-xylene system has been used to form microemulsions with water as the dispersed phase and p-xylene as the continuous phase. The precursors for the metals were acetate salts dissolved in water. The initial concentration of Zn, Cu and Mn precursors in the dispersed aqueous phase can be been manipulated for tuning the size of the ZnSe nanocrystals and the amount of Cu and Mn incorporated in them. The formation of the quantum dots starts with a direct nucleation reaction between the Zn precursor and hydrogen selenide gas that is bubbled through the microemulsion and diffuses to the droplet interface where it undergoes a spontaneous, irreversible reaction with Zn acetate to form ZnSe nuclei. The ions of Mn and Cu diffuse into the ZnSe lattice and produce doped quantum dots that exhibit strong luminescence peaks in the orange and green parts of the spectrum, respectively. This enables the realization of multi-color quantum dots based on ZnSe. The incorporation of magnetic ions, such as Mn, in ZnSe may also lead to materials with optical properties that are tunable by an external magnetic field [6,7].

Ongoing experiments in our laboratory focus on extraction of the nanocrystals from the templates, surface functionalization using mercapto-carboxylic acids, and applications in clinical diagnostics, e.g. in DNA hybridization detection and in multiplexed immunoassays.

References

1. A.P. Alivisatos, Science 271, 933 (1996).

2. C.B. Murray, C.R. Kagan, and M.G. Bawendi, Annu. Rev. Mater. Sci. 30, 545 (2000).

3. G. N. Karanikolos, P. Alexandridis, G. Itskos, A. Petrou, and T. J. Mountziaris, Langmuir 20, 550 (2004).

4. G.N. Karanikolos, P. Alexandridis, R. Mallory, A. Petrou, and T. J. Mountziaris, Nanotechnology, 16, 2372-2380 (2005)

5. G.N. Karanikolos, N.-L. Law, R. Mallory, A. Petrou, P. Alexandridis, and T. J. Mountziaris, Nanotechnology, 17, 3121-3128 (2006).

6. D. J. Norris, N. Yao, F. T. Charnock, and T. A. Kennedy, Nano Letters 1, 3 (2001)

7. S. C. Erwin, L. Zu, M. I. Haftel, Al. L. Efros, T. A. Kennedy, and D. J. Norris, Nature 436, 91 (2005)