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Electronic Structure of Cobalt Nanocrystals Suspended in Liquid Print


Advances in the synthesis of crystals of nanometer dimensions, narrow size distribution, and controlled shape have generated interest because of the potential to create novel materials with tailored physical and chemical properties. New properties arise from quantum confinement effects and from the increasing fraction of surface atoms with unique bonding and geometrical configurations. At the ALS, an international team of scientists has performed an electronic structure study of colloidal nanocrystals—nanocrystals suspended in the liquid solvent in which they were grown. A range of photon-in/photon-out spectroscopies, including x-ray absorption spectroscopy (XAS), was applied. These techniques are element-selective, as they involve core atomic levels and can thus probe the local electronic structure of selected species in complex systems.

Artificial Atoms

Nanocrystals are sometimes referred to as "artifical atoms" because many of their technologically important physical properties—such as melting temperature, band-gap size, and magnetic remanence—can be synthesized to order simply by adjusting the crystal size and shape. The combination of size- and shape-dependent physical properties and ease of fabrication and processing makes nanocrystals promising building blocks for materials with designed functions. The ability to control the uniformity of the size, shape, crystal structure and surface properties of the nanocrystals is not only of technological interest: access to defined nanoscale structures is essential for uncovering their intrinsic properties unaffected by sample heterogeneity. Rigorous understanding of the properties of individual nanocrystals will enable us to exploit them, making it possible to design and build novel electronic, magnetic, and photonic devices and other functional materials based on these nanostructures.

Top: Illustration of a colloidal nanocrystal with a cobalt center surrounded by a ligand layer made up of molecules of an organic surfactant (oleic acid). Bottom: Transmission electron microscope (TEM) image of cobalt nanocrystals. Scale bar = 50 nm.

Colloidal nanocrystals have an inorganic core (in this case, cobalt) surrounded by an organic surfactant, or ligand, layer (in this case, oleic acid) and are suspended in a liquid solvent (in this case, 1,2-dichlorobenzene). Cobalt nanocrystals display a wealth of size-dependent structural, magnetic, electronic, and catalytic properties. The challenge in making isolated cobalt nanocrystals is to overcome the large attractive forces between the nanoparticles due to surface tension and van der Waals interactions that tend to aggregate them. Using appropriate surfactants, however, cobalt nanocrystals can be grown with controlled shapes and sizes. It has been found, for example, that cobalt initially forms disks in a binary surfactant mixture and that these spontaneously transform into more thermodynamically stable spheres after heating for a sufficient period of time. Cobalt nanocrystals with various diameters can be prepared by simply adjusting the amount of oleic acid used in the synthesis.

To gain a fundamental understanding of how the properties of nanocrystals are affected by the growth process, it is necessary to obtain detailed information of the electronic structure as a function of size and of the presence and nature of the molecules bound to the surface. Because cobalt nanocrystals are extremely reactive and oxidize easily, it is important to use techniques that can interrogate the particles in their growth environment so that their electronic and chemical structures can be followed during growth and during catalytic reactions. Samples of cobalt nanocrystals with diameters of 3, 4, 5, 6, and 9 nm were grown and suspended in 1,2-dichlorobenzene, encapsulated in a small liquid cell, and transferred to ALS Beamline 7.0.1 for x-ray spectroscopic experiments.

The most notable feature of the cobalt L-edge XAS spectra is the new absorption peak, labeled A2, at about 6 eV above the main absorption edge, A1. This peak is absent in the reference spectra for Co3O4, CoO, CoCl2, and Co metal. Because the precursors in the synthesis materials contain only the elements Co, C, O, and Cl, no other absorption lines are expected near the cobalt L edges. The researchers believe that this peak is due to a metal-to-ligand charge transfer (MLCT) transition between the cobalt and the oleic acid or 1,2-dichlorobenzene. The MLCT satellite peak starts to appear in nanocrystals of 9 nm and shows an increasing intensity when the diameter decreases, as expected from the increasing proportion of cobalt–surfactant molecular interactions.

Left: The most notable feature in the spectra of the Co nanocrystals (red curves) is the absorption peak (A2) about 6 eV above the main absorption edge (A1) that is absent in the reference spectra for Co3O4, CoO, CoCl2, and Co metal (blue). Right: Illustration of MLCT transitions between cobalt and the oleic acid or 1,2-dichlorobezene.

Comparison of the spectra to simulations using the single-impurity Anderson model showed excellent agreement, and the analysis indicated that the surface cobalt atoms are aligned in an ordered fashion to connect the ligands perpendicularly to the nanocrystal surface. The results also suggested that the nanocrystals interact more strongly with solvent molecules in the initial stages of growth, while at a later stage, the interaction is dominated by the oleic acid surfactant. More generally, experimental and theoretical studies such as these show that the interaction between cobalt nanocrystals and surfactant and solvent molecules can be measured by in situ techniques, opening the way for more detailed studies of growth and reactivity.



Research conducted by H. Liu, Y. Lin, D.F. Ogletree, and M. Salmeron (Berkeley Lab); J. Guo (ALS); A. Augustsson (ALS and Uppsala University, Sweden); C. Dong (ALS and Tamkang University, Taiwan); J. Nordgren (Uppsala University, Sweden); C. Chang (Tamkang University, Taiwan); P. Alivisatos (Berkeley Lab and University of California, Berkeley); G. Thornton (University College London); F.G. Requejo (Universidad Nacional de La Plata, Argentina); and F. de Groot (Utrecht University, The Netherlands).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: H. Liu, J. Guo, Y. Yin, A. Augustsson, C. Dong, J. Nordgren, C. Chang, P. Alivisatos, G. Thornton, D.F. Ogletree, F.G. Requejo, F. de Groot, and M. Salmeron, "Electronic structure of cobalt nanocrystals suspended in liquid," Nano Lett. 7, 1919 (2007).