 directed assembly of gold nanorods 
 through the use of dithiolated molecular linkers is one of the most 
 efficient methodologies for the morphologically controlled tip - to - tip 
 assembly of this type of anisotropic nanocrystals . however , in a direct 
 analogy to molecular polymerization synthesis , this process is characterized 
 by difficulties in chain - growth control over nanoparticle oligomers . 
 in particular , it is nearly impossible to favor the formation of one 
 type of oligomer , making the methodology hard to use for actual applications 
 in nanoplasmonics . 
 we propose here a light - controlled synthetic procedure 
 that allows obtaining selected plasmonic oligomers in high yield and 
 with reaction times in the scale of minutes by irradiation with low 
 fluence near - infrared ( nir ) femtosecond laser pulses . 
 selective inhibition 
 of the formation of gold nanorod n - mers ( trimers ) 
 with a longitudinal localized surface plasmon in resonance with a 
 800 nm ti : sapphire laser , allowed efficient trapping of the ( n  1)-mers ( dimers ) by hot spot mediated photothermal 
 decomposition of the interparticle molecular linkers . 
 laser irradiation 
 at higher energies produced near - field enhancement at the interparticle 
 gaps , which is large enough to melt gold nanorod tips , offering a 
 new pathway toward tip - to - tip welding of gold nanorod oligomers with 
 a plasmonic response at the nir . 
 thorough optical and electron microscopy 
 characterization indicates that plasmonic oligomers can be selectively 
 trapped and welded , which has been analyzed in terms of a model that 
 predicts with reasonable accuracy the relative concentrations of the 
 main plasmonic species . 
Supplementary Material