No abstract

Nanostructured materials are increasingly subject to nearly every type of chemical and physical analysis possible. Due to their small sizes, there is a significant focus on tools with high spatial resolution. It is also natural to characterize nanomaterials using tools designed to analyze surfaces, because of their high surface area. Regardless of the approach, nanostructured materials present a variety of obstacles to adequate, useful, and needed analysis. Case studies of measurements on ceria and iron metal-core/oxide-shell nanoparticles are used to introduce some of the issues that frequently need to be addressed during analysis of nanostructured materials. We use a combination of tools for routine analysis including X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and x-ray diffraction (XRD) and apply several other methods as needed to obtain essential information. The examples provide an introduction to other issues and complications associated with the analysis of nanostructured materials including particle stability, probe effects, environmental effects, specimen handling, surface coating, contamination, and time.

Magnetic nanoparticles in a liquid have two relaxation times, Néel relaxation τN and Brownian relaxation τB. For particle size larger than 25nm, τN quickly becomes much larger than τB and can be ignored. τB has a relaxation period from 10−1to10−5s, and related to the particle’s hydrodynamic volume, which includes coatings and biomolecules attached to the magnetic nanoparticle cores. This causes the imaginary part of the ac magnetic susceptibility to display a maximum at a frequency f=1∕2πτB, and can be used to create a sensor capable of detecting biomolecules. Because this is based on particle size, a size distribution will broaden the curve and reduce the sensitivity. Although the magnetic nanoparticles may have a narrow size distribution, this may not be true once coatings have been added and biomolecules have bonded to the magnetic cores. Our group has examined the effects of normal and lognormal size distributions on the ac magnetic susceptibility using several theoretical measurements, and we have found that the effect of size distributions on the ability to use τB and the ac magnetic susceptibility as the basis of a biosensor is not significant.

Nanoporous films of core-shell iron nanoclusters have improved possibilities for remediation, chemical reactivity rate, and environmentally favorable reaction pathways. Conventional methods often have difficulties to yield stable monodispersed core-shell nanoparticles. We produced core-shell nanoclusters by a cluster source that utilizes combination of Fe target sputtering along with gas aggregations in an inert atmosphere at 7 AE C. Sizes of core-shell iron-iron oxide nanoclusters are observed with transmission electron microscopy (TEM). The specific surface areas of the porous films obtained from Brunauer-Emmett-Teller (BET) process are size-dependent and compared with the calculated data.

We prepared 2% and 5% Co-doped ZnO nanocluster films at room temperature (RT) using doped ZnO nanoclusters as building blocks. The nanoclusters are produced by a third-generation magnetron-sputtering-aggregation source. Superconducting quantum interference device (SQUID), photoluminescence (PL), x-ray diffraction (XRD), x-ray photoelectron spectrometer (XPS), and atomic force microscopy (AFM) measurements were done on the samples. The average nanocrystallite size of the nanoclusters was ∼7.5nm. The 2% Co-doped ZnO nanocluster films exhibit significant ferromagnetism and ultraviolet (UV) photoluminescence (PL) at RT. The coercivity (Hc) doubled in the 2% Co-doped samples when compared to the 5% Co-doped samples. A strong UV-PL of ∼3.33eV was observed for the 2% Co-doped ZnO nanocluster film at RT. The 5% Co-doped ZnO nanocluster film showed a ferromagnetic behavior at RT but no UV luminescence.