1) The document reports on a study of the optical properties of binary CdSe and ternary alloyed CdSxSe1-x nanocrystals synthesized via a one-step hot injection colloidal approach. 2) Spectroscopic characterization of the nanocrystals showed prominent excitonic features in absorption spectra and single, narrow photoluminescence peaks devoid of surface state emissions. X-ray diffraction patterns confirmed a hexagonal wurtzite crystal structure. 3) At the single particle level, the ternary CdSxSe1-x nanocrystals exhibited higher 'on' time fractions and greater photostability than binary CdSe nanocrystals

1. A comparative study on the single particle optical properties of binary CdSe and ternary alloyed CdSx
Se1-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 (ABx
C1-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 CdSX
Se1-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 CdSX
Se1-x
nanocrystals
Intensity(a.u.)
20 30 40 50 60
2 (degree)
CdS0.24
Se0.76
CdS0.18
Se0.82
Cd4d
Se3d
S2p
SeLMM
S2s
C1s
Cd3d
O1s
NormalizedPLintensity(a.u.)
Time(ns)
100 200 3000 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 CdSX
Se1-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
NumberofQDs
% 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
NumberofQDs
% on time fraction
CdSe
(A) (B)
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 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
Numberofparticles
Diameter(nm)
2 3 4 5 6 7 8 9 10
0
10
20
30
40
50
Numberofparticles
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 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 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.