A comparative study on the single particle optical properties of binary CdSe and ternary alloyed CdS x Se 1-x nanocrystals Swayandipta Dey, Shutang Chen, M.R.Shakil, Steven L.Suib & Jing Zhao , Department of Chemistry, University of Connecticut, 55 N. Eagleville Road, Storrs, CT 06269,USA Ternary alloyed (AB x C 1-x ) quantum dots (QDs) are recently emerging as a new class of promising optoelectronic materials due to the continuous tunability of their properties via gradual variation of their composition. This allows fine tuning of the energy bandgap of these materials which is independent of the overall nanocrystal volume. Therefore, they can be engineered to emit over a wide spectral range (UV to near IR) that are often not accessible with the regular binary QDs. In this work,we have synthesized a series of composition tunable CdS X Se 1-x quantum dots via a one step hot injection colloidal approach.Optical characterization of these materials at a single particle level were carried out using confocal fluorescence spectroscopy. Introduction Materials and Synthesis TOP injected into Cd-precursor/alkylamine mixture vacuum degassing rapid injection of (S+Se)-TOP into reaction mixture washed with acetone/hexane redispersed in hexane x min (A) (A) (B) Absorption and Emission spectra were taken for the above reaction aliquots pulled out from the same reaction batch.Prominent excitonic features are visible in the absorption spectra.The photoluminescence(PL) spectra are characterized by single,narrow peaks void of any emissions associated with deep trap surface states.The PL FWHM of the emission peaks were between 27-32nm. UV-Vis Absorption and Photoluminescence spectra TEM characterization and Size distribution XRD spectra of CdS X Se 1-x nanocrystals Intensity(a.u.) 20 30 40 50 60 2T(degree) CdS 0.24 Se 0.76 CdS 0.18 Se 0.82 Cd4d Se3d S2p SeLMM S2s C1s Cd3d O1s Normalized PL intensity(a.u.) Time(ns) 100 200 300 0 400 50 100 150 200 250 300 0 200 20 40 60 0 Time(s) Intensity(kcps) 50 100 150 200 250 0 500 40 30 20 10 0 Intensity(kcps) Time(s) (A) (B) Typical XPS survey scan spectra of CdS X Se 1-x nanocrystals Conclusion Acknowledgement The XRD peaks show the crystal structures as hexagonal wurtzite phase.The 2θ values from all the major diffraction peaks lie between that ofbulk CdSe and CdS which rules out the phase separation or any separate nucleation of CdSe and CdS domains. 10 20 30 40 50 60 70 80 90 100 0 5 10 15 20 25 30 Number of QDs % on time fraction CdSSe 10 20 30 40 50 60 70 80 90 100 0 2 4 6 8 10 12 14 16 18 20 Number of QDs % on time fraction CdSe (A) (B) PL emission traces and on-time distribution of CdSSe and CdSe nanocrystals Ensemble PL lifetime curves of CdSSe NCs At a single particle level,the alloyed CdSSe nanocrystals exhibit a higher ‘on’ time fraction as compared to their binary CdSe counterparts and they were relatively more photostable.This might be due to a chemical composition gradient core-shell like structure with a Se-enriched core and a S-rich shell arising due to the relative reactivity differences of the anionic precursors. Due to their relatively smaller size along with a wide spectral tunability, the CdSSe QDs can potentially be better fluorescent labels for bioimaging applications. MDA-MB-231 human breast tumor cells labeled with CdSSe NCs Representative emission traces of single (A) CdSSe and (B) CdSe NCs. The histograms represent the distribution of intensities observed in the time trace. The “on” time fraction of single CdSSe NCs as observed from the above histogram shows a much higher on time fraction as compared to CdSe NCs. 2 3 4 5 6 7 8 9 10 0 10 20 30 40 50 60 Number of particles Diameter(nm) 2 3 4 5 6 7 8 9 10 0 10 20 30 40 50 Number of particles Diameter(nm) 20nm 20nm The change in longer lifetime component for the QDs emitting in the red side can be attributed to the size effect or due to more S content as a result of more effective surface passivation on the Se-rich cores. heating under N 2 Nearly monodisperse nanocrystals were obtained from batch to batch.Typical size obtained from our synthesis varies between 3.96± 0.45nm to 6.5± 0.75nm. Such a narrow size distribution also indicates the uniformity of our sample. Laura Fajardo,Dr.Xiuling Liu,UConn School of Pharmacy,Storrs,CT,USA The gradual increase in S content with growth time indicates that these nanocrystals are grown with a chemical composition gradient with a Se-rich core and a S-rich shell.The schematic above shows the proposed growth of CdSSe nanocrystals.