Peter Njoki - Processing Core/Alloy/Shell Nanoparticles: Tunable Optical Properties and Evidence for Self-Limiting Alloy Growth

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      Publication Details (including relevant citation   information):

      J. Phys. Chem. C, 2011, 115 (20), pp 9933–9942

      DOI: 10.1021/jp201151m


      The postsynthetic processing of nanomaterials may allow   researchers to reach specific properties, morphologies, or phase   regimes that are not accessible by simple synthesis alone. Here,   we take advantage of atomic interdiffusion at nanoparticle   interfaces to fabricate core/alloy and core/alloy/shell   nanoparticles. Modest temperature changes were found to have   profound effects for the interfacial alloying of the confined   nanosystem. The alloy formation and subsequent interdiffusion   allowed us to tailor nanoparticle composition and ultrastructure,   as well as surface plasmon response. This processing step, which   involves the layer-by-layer formation of a core/alloy/shell   morphology, utilizes hydrothermal annealing provided by automated   microwave irradiation to control solute deposition, as well as   alloy thickness. As a proof-of-principle system, we employed a   Au/AuxAg1–x/Ag   nanosystem, due, in large part, to its miscible phase diagram and   rich plasmonic behavior. Nanostructure morphology was   characterized by TEM and STEM, and compositional analysis was   performed via selective area EDX and XPS. The resulting surface   plasmon resonance signatures were modeled as a function of alloy   or monometallic shell thickness, as well as alloy composition,   using the discrete dipole approximation method. A proposed shell   growth mechanism is described, which involves the competition   between a classical ripening system at low temperature and a   self-limiting growth at high temperature, the latter of which may   be driven by an alloy order–disorder phenomena at the interface.   These optical and growth models strongly correlate with the   experimental results, namely, that the plasmon resonance is   highly tunable on shell thickness and alloy composition and that   alloy thickness and interdiffusion are highly tunable by the   thermal processing.

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