Scientists announce first 3-D assembly of magnetic and semiconducting nanoparticles

A collection of iron oxide nanoparticles (blue) and smaller lead selenide nanoparticles (red) -- a.k.a. quantum dots -- beginning to interact and organize in solution on their way to crystallizing into a binary superlattice. The resulting assembly captures the magnetic properties of the iron oxide while retaining the distinct optical signature of the quantum dots.
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A space filling representation of a binary superlattice with the magnetic iron oxide nanoparticles shown in blue and the smaller lead selenide nanoparticles shown in red. Around each of the nanoparticle, the dark shell represents the layers' organic surfactants that coat the surface of the particles. This organic shell is critical in stabilizing the particles in solution and provide a matrix which fill in the voids in the structure and the superlattice forms.
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A schematic of a binary superlattice where thirteen small lead selenide quantum dots (red) are grouped together, filling the spaces between the 11 nm diameter iron oxide (blue). The distance between the iron oxide particle is exaggerated to allow a clear view of how the lead selenide particles pack together.
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This electron microscopy image has been colorized to highlight the positions of the magentic iron oxide particles in blue and the semiconducting lead selenide quantum dots in red.
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The animation shows the growth of a binary nanoparticle superlattice with the iron oxide (blue) and lead selenide particles (red) beginning to interact as the colloid becomes more concentrated. The random motion of the particles bring them in to contact as the dispersion is concentrated. When a particle finds a low energy position with respect to the neighboring particles, it experiences a slightly stronger attraction. The particle may sometimes stay long enough in this position for an additional particle to bind trapping, further stabilizing the particle and thus increasing the size of the ordered assembly. Particles that hit in a less favorable location have greater mobility and can move to find low energy positions on the surface or they can redissolve to try attaching later. Ordered superlattices are only attainable when the rate of deposition allows the particles the time and energetic freedom to make many attempts to find their optimal positions.
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