colloidal iv  vi semiconductor nanocrystals ( also known as quantum dots , qds ) are of increasing potential applications in telecommunication , photoelectronic device , and biomedical labeling , etc .
pbse qds are important materials because of the strong confinement effect due to their large bohr radius and the small band gap in near infrared region .
several approaches have been developed to prepare pbse qds with uniform size and high quantum yields [ 3 - 5 ] .
pbse / pbs   and pbse / sio2 core / shell structures have been synthesized to stabilize pbse qds .
but cdse should be a better shell material due to the higher stability under air condition , the lower lattice mismatch of ~1% , and the little change of the surface chemistry and physics .
it is difficult to grow cdse shells upon pbse cores using typical cadmium oleate anion precursor because of high reaction temperatures needed .
hollingsworth s group recently developed a method of ion exchange to form pbse / cdse core / shell structures in which cd atoms replaced pb atoms in the outlayers of large pbse qds .
however , it may not be easy to control the thickness of the cdse layers . in this work , we employed the  successive ion layer adsorption and reaction ( silar )  technology   to form air - stable pbse / cdse qds with strong photoluminescence
two solutions were prepared for cdse shell growth . a cadmium injection solution ( 0.04 m )
was prepared by heating cadmium cyclohexanebutyrate ( 0.1804 g ) in oleyamine ( 8.130 g ) at 60c under n2 flow to obtain a clear colorless solution . a selenium injection solution ( 0.04 m )
was prepared by mixing selenium powder ( 0.0316 g ) in octadecene ( 7.880 g ) at 220c under n2 flow until a clear yellow solution was obtained .
, pbse qds ( 4.8 nm in diameter , 1.01  10 mmol of particles ) dispersed in 5 ml of hexanes were loaded into a 25-ml three - neck flask and mixed with 1.500 g of octadecylamine and 5.000 g of octadecene .
a mechanical pump was employed at room temperature for 30 min to remove hexanes from the flask .
then , the predetermined amounts of the cadmium and selenium solutions were alternatively injected into the three - neck flask drop by drop with syringes using standard air - free procedures .
transmission electron microscope ( tem ) was used to characterize pbse and pbse / cdse core / shell qds as shown in fig .
1 . tem images and histograms of 4.8 nm pbse ( a , b ) and 6.2 nm pbse / cdse ( c , d ) qds .
the pbse and pbse / cdse qd samples shown here have narrow size distributions of 8.1% ( b ) and 6.9% ( d ) , respectively figure 2 shows the evolution of absorption and photoluminescence spectra of the pbse / cdse qds upon the series growth of three monolayers of cdse shells on the 4.8 nm pbse cores .
a consistent red shift of the peak wavelength was observed in both the absorption and pl spectra .
the red shifts of the first excitonic absorption peak for three layers were 11 , 10 , and 11 nm , respectively .
the red shift of absorption and pl spectra depends on several factors including ( 1 ) the connection between pbse and cdse renders the expansion of the carriers wavefunctions out of the core region with different expansion probabilities resulting in the increase of exciton distance and ( 2 ) the surface polarization due to the different dielectric constants of pbse and cdse materials . absorption ( a ) and photoluminescence ( b ) spectra recorded during cdse shell growth . a consistent red shift of the peak wavelength ( 11 , 10 , 11 nm ) was observed when one to three monolayers of cdse shells grown on 4.8 nm pbse cores the optical gap ( egap ) of pbse qds is the minimum energy needed to excite an electron from the valence band to the conduction band .
the optical gap is given by   where   are bulk material kinetic energies of the electron and hole , respectively ;   is band gap of bulk material ;   are the confinement kinetic energies of electron and hole , respectively ;   are the surface - polarization energies of the electron and hole , respectively ; and jeh is the direct electron 
the size of the pbse qds ( 4.8 nm ) is much smaller than the bohr radius ( 46 nm ) ; therefore , in the strong confinement realm , the energy difference between the electron and hole should be as followed   where r is the radius of quantum dot particle ; and mr is the reduced mass of the electron and hole .
the energy of coulomb attraction is given by where   are the wavefunctions of the electron and hole , respectively ; and   is the potential function of the electron and hole .
the surface - polarization energies of the electron and hole are where   is the surface polarization potential where   is the screened coulomb potential of the qd at point   due to a point charge located at   and   is the same quantity in the corresponding bulk material system .
when two semiconductors contact , both electron and hole will induce tunnelling effect and the wavefunctions will diffuse into cdse shells .
the transmission coefficient can be given as   where e is energy barrier at the interface of core / shell structure ; m is the effective mass of diffusing particle ; a is the thickness of energy barrier . because the wavefunctions diffuse into shells , the confinement energy will change with the increase in shell thickness . according to eq .
2 , where t is the difference of transmission coefficients of electron and hole . from the energy levels shown in fig .
3[16 - 18 ] , the expansion probability of electron is bigger than that of hole because of the lower barriers of electron according to the very closed effective masses of electron and hole ( me = 0.070 , mh = 0.068 ) . in this case
, one nanometer shell barrier will result in a decrease in 17.55 mev for the confinement energy .
we have also calculated that the increase in coulomb energy is 0.068 mev for one nanometer barrier .
correspondingly , the red shifts of the first excitonic absorption peak for three individual monolayer cdse shells are counted as 10.81 , 9.45 , and 8.36 nm , respectively .
they are in good agreement for the experimental data taking account of the effect of surface polarization energy .
lumo and homo structures for 4.8 nm pbse qds and 0.35 nm cdse shell the third term in eq .
1 is the surface - polarization energy that affects the gap energy egap , which is the stark effect .
a certain number of defect states are expected on the surface of unpassivated pbse qds .
therefore , charge carriers trapped on or near the surface of qds may generate localized electric fields , where delocalized exciton states within the qds can be highly polarizable .
the surface polarization energy   versus local electric field  is given by   where  and  are the resolved exciton dipole and polarizability , respectively .
according to muller et al.s work , the spectrum shift of cdse nanorods depends on the direction of the external electric field .
the positive electric field induces red shift , and the negative one leads to blue shift .
since the qds in this work are spherical ( zero dimensional ) , it is reasonable that their peak shifts are independent of the direction of the electric field .
both positive and negative electric field can cause the emission peak to red shift , and the red shift increases when the electric field is stronger .
it has been known that unpassivated pbse qds surface is a pb atom - rich shell .
therefore , there may be polarization charges on the surface of pbse qds which generate surface - polarization energy .
however , the polarization charges are neutralized , because pb atoms on the surface of pbse qds connect to oleic acid ( the organic ligand used in the synthesis ) .
when the pbse / cdse core / shell was synthesized , cdse contacted with pb atoms instead of oleic acid ; this induced the increase of surface polarization charges .
the spectra shift to red because of the enhancement of the stark effect ( fig .
different crystal lattices and thermal expansivities for pbse and cdse will more or less induce surface defects at the interface of the two materials . the carriers will be trapped and result in the enhancement of the stark effect .
such local fields cause the first exciton peak to shift to red and suppress the emission strength due to a reduced electron  hole wavefunction overlap .
unbalanced charges may also decrease the photoluminescence efficiency ( quantum yield ) via nonradiative auger recombination .
the new traps were induced by surface defects depend on the shell growth . compared with the photoluminescence of one monolayer core / shell qds ,
however , it was found that more shell layers resulted in a decrease in photoluminescence strength ( fig .
that is also because the tensile change at the interface is nonlinear with the shell thickness .
when pbse qds were covered with two layers of cdse , the good lattice tensility at the interface reduced the lattice mismatch and therefore increased the photoluminescence strength .
when pbse qds were covered by three layers of cdse , the lattice tensility was stronger and hence the photoluminescence strength decreased .
even so the quantum yield was still as high as 70% for our pbse / cdse core / shell qds ( ir-26 as the reference ) .
pbse qds are unstable even under the ambient conditions ( room temperature and room light in air ) ( fig .
this process for each particle induces the effective particle size decrease and the blue shift of the spectrum .
when the old oxidized surface is gone , the new surface will be oxidized quickly . for pbse
/ cdse core / shell structures , cdse shells effectively prevent pbse cores from the quick oxidation .
the lifetime of pbse qds under ambient conditions therefore can be extended from a few days to at least a month ( currently available data ) .
the stability of pbse ( a ) and pbse / cdse ( b ) qds .
the cdse shells prevent pbse core from the destructive oxidation . compared with the unstable pbse core ,
the pbse / cdse qds remained unchanged in conclusion , pbse / cdse core / shell qds with a quantum yield of 70% were synthesized .
the surface polarization and the expansion of carriers wavefunctions contributed to the spectral red shift .
the spectra red shifts during the formation of cdse shells were calculated , and they exhibited a good fit to the experimental data .
the funding supports from the state key laboratory on integrated optoelectronics , college of electronic science and engineering , jilin university , the worcester polytechnic institute , and the national 863 projects of china ( 2007aa03z112 , 2007aa06z112 ) are acknowledged .
this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
 pbse / cdse core / shell nanocrystals with quantum yield of 70% were obtained by the  successive ion layer adsorption and reaction  technology in solution . 
 the thickness of the cdse shell was exactly controlled . 
 a series of spectral red shifts with the cdse shell growth were observed , which was attributed to the combined effect of the surface polarization and the expansion of carriers wavefunctions . 
 the stability of pbse nanocrystals was tremendously improved with cdse shells . 