Optically active II-VI semiconductor nanocrystals via chiral phase transfer
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Optically active II-VI semiconductor nanocrystals via chiral phase transfer Maria V. Mukhina1, Vladimir G. Maslov1, Ivan V. Korsakov1, Finn Purcell Milton2, Alexander Loudon2, Alexander V. Baranov1, Anatoly V. Fedorov1 and Yurii K. Gun’ko1,2 1 2
ITMO University, Saint Petersburg, Russian Federation. Trinity College, University of Dublin, Dublin 2, Ireland.
ABSTRACT We report optically active ensembles of II-VI semiconductor nanocrystals prepared via chiral phase transfer, which is initiated by exchange of the original achiral ligands capping the nanocrystals surfaces for chiral L- and D-cysteine. We used this method to obtain ensembles of CdSe, CdS, ZnS:Mn, and CdSe/ZnS quantum dots and CdSe/CdS quantum rods exhibited Circular Dichroism (CD) and Circularly Polarized Luminescence (CPL) signals. The optically active nanocrystals revealed the CD and CPL bands strongly correlated with absorption and luminescence bands with unique band “pattern” for each material and the nanocrystal shape. INTRODUCTION An emerging class of optically active semiconductor nanocrystals envisages to play an important role in many applications in both medicine and biology. Using the chiral nanocrystals, it will be possible to enable naturally occurring biological mechanisms, for example, molecular recognition [1], in nanotechnology. In particular, the area of chiral nanocrystals has received a great deal of attention due to the range of potential applications offered by these materials in chiral chemo- and bio-sensing, nano-bio interfaces, and spintronics. It has been demonstrated that optically active nanocrystals can be obtained by various methods [2-14]. Initially chiral Quantum Dots (QDs) have been prepared by using microwave induced heating with chiral ligand penicillamine as stabilizer [2-9]. This method allows obtaining the nanocrystals with strong CD signal, but only with defect luminescence. More recently, it has been reported that chiroptical activity can be induced via postsynthetic ligand exchange of achiral ligands for chiral ligand cysteine on the surface of CdSe nanocrystals obtained by standard hot injection synthesis [1013]. The most obvious advantage of this procedure is that after postsynthetic ligand exchange the chiral QDs still display excitonic Photoluminescence (PL) unlike to the nanocrystals prepared in water [11]. Here we report the shape and material dependent CD and CPL from optically active ensembles of II-VI semiconductor nanocrystals prepared via procedure of chiral phase transfer of the nanocrystals after the hot injection synthesis. EXPERIMENT Synthesis of nanocrystals Semiconductor core/shell CdSe/ZnS quantum dots were synthesized by a method described previously [15] without using any chiral chemicals. The dots had a diameter of 2.5 nm and exhibit a PL band with a peak at 520 nm.
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For synthesis of ZnS QDs doped with Mn, 0.4g of ZnCl2 and 0.02g of MnCl2·4H2O were dissolved in 54mL of dibenzylamine and heated under vacuum at 120°C for 2 hours (named pot 1). The solution was then cooled to 50°C and re
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