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We introduce a versatile and high precision three-dimensional optical tweezers setup with minimal optical interference to measure small forces and manipulate single molecules in the vicinity of a weak reflective surface. Our tweezers system integrates an inverted optical microscope with a single IR-laser beam that is spatially filtered in an appropriate way to allow force measurements in three dimensions with remarkably high precision when operated in backscattered light detection mode. The setup was tested by overstretching a lambda-DNA in x and z directions (perpendicular and along the optical axis), and by manipulating individual lambda-DNA molecules in the vicinity of a nanopore that allowed quantitative single molecule threading experiments with minimal optical interference.

Co nanocrystals have been prepared via two different chemical preparation routes. Microstructural characterization has been carried out by conventional and high-resolution transmission electron microscopy. Magnetic characterization reveals that Co nanocrystals with mean particle size distributions about 6 nm are ferromagnetic at room temperature. Superlattices of about 1 m 1 m can be reproduced. For the second preparation route the formation of an -Co phase was detected by X-ray diffraction. Whereas the pure Co nanocrystals form domains with a mixture of simple cubic and hexagonal packing, FeCo alloyed nanoparticles form long chain-like structures.

Recognizing the technological potential of magnetic nanocrystals for the next generation of high density data storage [S. Sun and C. B. Murray, J. Appl. Phys. 85, 4325 (1999)] and as markers in biological systems for detection and manipulation of attached molecules, [L. Lagae, R. Wirix-Speetjens, J. Das, D. Graham, H. Ferreira, P. P. F. Freitas, G. Borghs, and J. De Boeck, J. Appl. Phys. 91, 7445 (2002)] ligand stabilized nanoparticles prepared via high–temperature solution–phase synthesis [C. B. Murray, S. Sun, W. Gaschler, H. Doyle, T. A. Betley, and C. R. Kagan, IBM J. Res. Dev. 45, 47 (2001); V. F. Puntes, K. M. Krishnan, and A. P. Alivisatos, Science 291, 2115 (2001)] rapidly gain interest in research and development. Following a chemical preparation route recently proposed [V. F. Puntes, K. M. Krishnan, and A. P. Alivisatos, Science 291, 2115 (2001)] Co and FeCo alloyed nanocrystals have been synthesized with different nanocrystal size distributions employing different growth rates during preparation. The resulting magnetic properties are a function of the particle size distribution and can be tailored for superparamagnetic or ferromagnetic behavior at room temperature. In addition, the possibility of synthesizing FeCo alloyed nanoparticles is demonstrated and discussed.

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