The ability to fabricate nanostructures with interesting properties creates a need for new ways to characterize them on the nanoscale. Here we examine three case studies, showing how novel imaging techniques 1) reveal collective behavior in magnetic nanoparticles arrays, 2) can image the motion of single particles in solution, and 3) can potentially be used to read bits of information.

In many ways monodomain nanoparticles act like giant spins. It is not yet possible to image the spins of individual magnetic atoms within a crystal, but novel imaging techniques make it possible to “see” the magnetization directions of nanoparticles within an array. Here self-assembled monolayer arrays of Co and Fe3O4 nanoparticles are characterized using magnetic electron microscopy techniques. Electron holography reveals the domain structure and enables quantitative determination of the in-plane magnetic order parameter. These results directly confirm theoretical predictions of a ferromagnetic ground state without exchange interactions. Lorentz electron microscopy movies show the slow collective dynamics of these nano-domains as they switch due to thermal fluctuations, in accordance with an Arrhenius law.

Novel imaging tools are also needed for nanoparticles that are used in biomedicine and cell biology. Here we consider gold-coated iron oxide nanoparticles can be imaged optically through their surface plasmon resonance, even though they are well below the optical diffraction limit. The chemistry of the coating process and the dark field optical microscopy technique are described. Movies show Brownian motion of individual particles in an aqueous dispersion. The trajectories are analyzed and compared with random walk predictions to determine the particle sizes.

Electronic measurements on nanostructures have always been challenging due to the difficulty of attaching electrodes. Here we describe a new nanomasking strategy to make arrays of multilayer nanopillars, which can be read magnetoresistively with a scanning probe tip. Self-assembled nanoparticle arrays are used as a nanomask on top of a multilayer film, which is patterned by ion milling and/or reactive ion etching. High resolution scanning electron microscopy is used to track changes in the sample during the dry etching process. Scanning probe microscopy is used to determine the final aspect ratio of the structures, and to demonstrate the ability to address single nanopillar particles. The implications for single particle per bit patterned recording media are discussed.

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