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Paper text:
the synthesis of pbse / cdse dumbbell nanostructures is detailed 
 in the supporting information .
tem specimens 
 were prepared by dropcasting 8 l of the nc colloidal solution 
 onto a mems microhot plate with electron - transparent sin membranes , 
 which was mounted onto a denssolutions low drift tem heating holder .
after dropcasting , the sample was plasma cleaned 
 for 10 s in order to remove deposits from the solution that prevent 
 high - resolution imaging in the tem .
the in situ experiments were performed 
 in a 80300 fei titan microscope equipped with a chemi - stem 
 edx detection system . during haadf - stem imaging ,
the camera length used in the experiments equals 
 91 mm in order to avoid diffraction effects and to guarantee z - contrast 
 imaging . in haadf - stem imaging ,
the intensity approximately scales 
 with z. as pb has a higher z number than cd , the pbse 
 domains appear with higher intensity in haadf - stem images in comparison 
 to the cdse domains .
the chemi - stem edx experiments were performed 
 using the same holder 
 and in the same 80300 fei titan microscope but operated at 
 a lower acceleration voltage of 200 kv to reduce beam damage during 
 mapping .
a representative spectrum is shown in supporting 
 information figure s20 . in the quantification of the elemental 
 maps ,
18 pbse nc maps were used to determine the cation / anion ratio 
 at the pbse tips at the initial state . for the pbse tips from where 
 cation exchange proceeded , the elemental composition of 10 different 
 pbse tips was quantified . for the nanorod domains attached to the 
 pbse tips where cation exchange took place , the elemental composition 
 of 10 different nanorod ( transformed)-domains was quantified .
additional 
 tem images , chemical maps , and quantitative analysis are provided 
 in figures s2s19 and tables s1s6 of the supporting information . for the force field 

md simulations , a new interaction potential 
 model for the pb  cd  se system was developed .
the potential 
 was found to accurately describe physical parameters such as lattice 
 parameters , elastic constants , and the relative stability of phases . 

details of the potential model ( supporting information table s7 ) and a description of the nanodumbbell models are given 
 in section e of the supporting information .
for simulations of the nanodumbbells , coulomb and short - range interactions 
 were calculated by taking into account all atom pairs .
the equations 
 of motion were integrated using the velocity verlet algorithm with 
 a time step of 1 fs .
periodic boundary conditions were not used and 
 the nanodumbbell models were isolated in vacuum .
simulations of 5 
 ns were carried out in the nvt ensemble and 1 ns was used for equilibration . all density functional theory ( dft ) calculations on defect energies 
 and energies of mixed pbse - cdse phases
were carried out using the 
 first - principles vienna ab initio simulation program ( vasp ) using the projector augmented wave ( paw ) method . the generalized gradient approximation ( gga ) 
 formulated by perdew , burke , and ernzerhof ( pbe )
the electronic 
 wave functions were sampled on a 4  4  2 grid using the 
 monkhorst and pack method with 8 to 20 k - points depending 
 on different symmetries of supercells ( 108 atoms ) .
different k - meshes and cutoff energies for waves 
 were tested to have a good convergence ( < 2 mev / atom ) .
here , 
 we show a novel solid  solid 
  vapor ( ssv ) growth 
 mechanism whereby epitaxial growth of heterogeneous semiconductor 
 nanowires takes place by evaporation - induced cation exchange . during 
 heating of pbse - cdse nanodumbbells inside a transmission electron 
 microscope ( tem ) 
 , we observed that pbse nanocrystals grew epitaxially 
 at the expense of cdse nanodomains driven by evaporation of cd . 
 analysis 
 of atomic - resolution tem observations and detailed atomistic simulations 
 reveals that the growth process is mediated by vacancies .