A comparative study on the single particle optical properties of binary CdSe and ternary alloyed CdSxSe1-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 (ABxC1-x ) quantum dots (QDs) are recently emerging as a new class ofpromising optoelectronic materials due to the continuous tunability of their properties via gradual variation of their composition. This allows �ne 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 CdSXSe1-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 �uorescence spectroscopy.

Introduction

Materials and Synthesis

TOP injected into

Cd-precursor/alkylamine

mixture

vacuum degassingrapid injection of(S+Se)-TOP into reaction mixture

washed with acetone/hexaneredispersed in hexane

x min

(A)(A) (B)

Absorption and Emission spectra were taken for the above reaction aliquots pulled outfrom the same reaction batch.Prominent excitonic features are visible in the absorptionspectra.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 CdSXSe1-x nanocrystals

Inte

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.)

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CdS0.24Se0.76

CdS0.18Se0.82

Cd4d

Se3d

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O1s

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Typical XPS survey scan spectra of CdSXSe1-x nanocrystals

Conclusion

Acknowledgement

The XRD peaks show the crystal structures as hexagonal wurtzitephase.The 2θ values from all the major di�raction peaks liebetween that ofbulk CdSe and CdS which rules out the phase separation or any separate nucleation of CdSe and CdS domains.

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PL emission traces and on-time distribution of CdSSe and CdSe nanocrystalsEnsemble PL lifetime curves of CdSSe NCs

At a single particle level,the alloyed CdSSe nanocrystalsexhibit a higher ‘on’ time fraction as compared to theirbinary CdSe counterparts and they were relatively morephotostable.This might be due to a chemical compositiongradient core-shell like structure with a Se-enriched coreand a S-rich shell arising due to the relative reactivitydifferences 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 inthe time trace.

The “on” time fraction of single CdSSe NCs as observed from the above histogram shows a muchhigher on time fraction as compared to CdSe NCs.

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The change in longer lifetime component for the QDs emittingin the red side can be attributed to the size effect or due to moreS content as a result of more effective surface passivation on theSe-rich cores.

heating under N2

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 growthtime indicates that these nanocrystals are grownwith a chemical composition gradient with aSe-rich core and a S-rich shell.The schematicabove shows the proposed growth of CdSSenanocrystals.