Seeded growth of monodisperse and spherical silver nanoparticles
  Aiming at spherical and monodisperse silver nanoparticles with diameters up to 100 nm, the potential of heterogeneous nucleation of silver particles was explored. Gold seed particles, mainly produced with a spark discharge generator, were carried by nitrogen through a three-zone tube furnace. Silver was evaporated at 1210 {\deg}C in the first zone of the furnace and particle growth and shaping took place in the subsequent zones, heated to 730 {\deg}C and 390 {\deg}C respectively. The generated aerosol was monitored by a scanning mobility particle sizer (SMPS), while parameters, such as furnace temperature, seed particle size and concentration and nitrogen carrier gas flow, were investigated. Off-line atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to characterize the morphology of the silver nanoparticles in addition to the SMPS scans. Spherical silver nanoparticles with a mobility diameter of more than 115 nm and a geometric standard deviation of typically 1.09 or lower at concentrations as large as 5e5cm^{-3} could be produced. The mobility diameter of the monodisperse aerosol could be varied in the range of 50 nm to 115 nm by changing the furnace temperature or the gold seed particle size. Elemental analysis revealed that the gold from the seed particles formed a homogeneous alloy with the silver (< 3.5 atomic percent of gold). The growth mechanism could not be identified unambiguously since the obtained silver particles could both originate from heterogeneous nucleation of silver vapour on the seed particles or from coagulation and coalescence of the seed particles with smaller, homogeneously nucleated silver particles. Moreover, the narrow size distribution opens the opportunity to obtain an exclusively singly charged, monodisperse calibration aerosol at sufficient concentrations after and additional mobility selection process. 
Simon Zihlmann, Felix L\"u\"ond, Johanna K. Spiegel
cond-mat.mtrl-sci physics.ao-ph physics.chem-ph