Formation of selenium superstructures: Self-assembly of monodispersed sub-micron spheres and nanorods

Abstract Self-assembly of monodispersed nanospheres and nanorods into nano- and micro-scale superstructures are important from the viewpoint of developing small scale devices. Synthesizing monodispersed particles with minimal polydispersity index is challenging due to the complexity involved in the nanoparticle growth from the constituent atoms. Monodispersed a-Se spherical particles, with polydispersity index much smaller than 0.1, are synthesized through the reduction of selenious acid using hydrazine in ethylene glycol. The size of the particle readily changed from 190 to 350 nm by varying the molar ratio of selenious acid to hydrazine. The optical extinction spectra obtained from the colloidal solution show peaks corresponding to magnetic and electric dipole resonances as verified from the Mie theory, as well as from the theoretical investigation of the local electric field using the finite element method. Interestingly, very small agitation of the a-Se nanospheres resulted in the formation of t-Se nanorods via crystal growth mechanism. Under ambient conditions, the nanorods self-assemble to form sub-micron spheres, which in turn grow into micron-sized superstructures. The mechanisms involved in the formation of such self-assembled superstructures are discussed.