Optical transitions in polarized CdSe, CdSe/ ZnSe, and CdSe/ CdS / ZnSquantum dots dispersed in various polar solvents Ung Thi Dieu Thuy, Nguyen Quang Liem,a兲and Do Xuan Thanh Institute
Trang 1Optical transitions in polarized CdSe, Cd Se ∕ Zn Se , and Cd Se ∕ Cd S ∕ Zn S quantum
dots dispersed in various polar solvents
Ung Thi Dieu Thuy, Nguyen Quang Liem, Do Xuan Thanh, Myriam Protière, and Peter Reiss
Citation: Applied Physics Letters 91, 241908 (2007); doi: 10.1063/1.2822399
View online: http://dx.doi.org/10.1063/1.2822399
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/24?ver=pdfcov
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Trang 2Optical transitions in polarized CdSe, CdSe/ ZnSe, and CdSe/ CdS / ZnS
quantum dots dispersed in various polar solvents
Ung Thi Dieu Thuy, Nguyen Quang Liem,a兲and Do Xuan Thanh
Institute of Materials Science (IMS), Vietnamese Academy of Science and Technology (VAST), 18 Hoang
Quoc Viet, Cau Giay, Hanoi, Vietnam
Myriam Protière and Peter Reiss
DRT/LITEN/DTNM/L2 T and DSM/DRFMC/SPrAM (UMR 5819 CEA–CNRS–UJF 1)/LEMOH CEA
Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
共Received 7 September 2007; accepted 16 November 2007; published online 12 December 2007兲
The optical transitions in ensembles of colloidal CdSe-based quantum dots 共QDs兲 have been
systematically studied as a function of the net QDs’ polarity/polarization and of the solvent’s
polarity While the general trend observed for all QD systems dispersed in different solvents is
similar, the spectral shifts are more pronounced in core QDs than in core/shell structures Our results
can be rationalized by taking account of the electric field experienced by the QDs that results from
their effective polarization in solvents of different polarities共quantum confined Stark effect兲 as well
as from the effect of the external dielectric environment 共solvatochromatic effect兲 © 2007
American Institute of Physics. 关DOI:10.1063/1.2822399兴
Various QDs of II-VI and III-V semiconductors have
been studied intensively in the past decade for their
promis-ing applications in biological labelpromis-ing1 3 and optoelectronic
devices.4 8 CdSe-based core and core/shell QDs have been
prepared by different methods using organometallic
dimethylcadmium911or, more recently, nonpyrophoric CdO
or cadmium carboxylate precursors.12–14 High-quality CdSe
QDs are characterized by their narrow photoluminescence
共PL兲 spectra and a sharp excitonic absorption peak.9 14
We note that different solvents have been used to disperse
hydro-phobic colloidal CdSe QDs capped with long alkyl chain
containing organic surfactants In some reports, nonpolar
sol-vents such as n-hexane were used,11 while elsewhere more
polar solvents such as toluene,13,15 chloroform,16 or even
n-butanol9were applied This observation stands out against
the fact that the solubility parameter of QDs in solvents of
significantly different polarities cannot be the same
The growth of larger band gap semiconductor shells on
the surface of CdSe QDs is an established method for
in-creasing their PL quantum yield共QY兲 from some percent to
values in the range of 50%–85% The inorganic shell,
con-sisting, for example, of ZnSe,14ZnS,10,11or of a ZnSe/ZnS15
or CdS/ZnS double epilayer,17
efficiently passivates fast nonradiative decay channels originating from surface
states.18,19 However, core/shell systems still exhibit PL QY
variations upon dispersion in solvents of different polarities
Another phenomenon is at the origin of this behavior,
namely, the quantum confined Stark effect, occurring on the
nanometer scale with the contributions of the electric field
induced by polarization of QDs, ligand molecules, polarity
of solvent, and surface states.20 This effective electric field
on the QDs could imprints clearly on the optical transitions
with the shifts of the PL and absorption spectra For colloidal
CdSe QDs in solvents with different dielectric constants, one
should also take account of solvatochromism which induces
redshift of the absorption spectrum.21
The aim of this letter is to address in detail the spectral shifts caused by the Stark effect and solvatochromatic effect
on CdSe-based QDs capped with organic ligands and dis-persed in solvents of different polarities The peak shifts in the PL and absorption spectra taken from the polarized ligands capped CdSe QDs as a function of the solvent’s po-larity and therefore as a function of the number of remaining surface ligands have been measured and compared Further-more, particular attention has been paid to polarization ef-fects not only of bare CdSe QDs but also of core/shell 关CdSe/ZnSe 共CS兲兴 and core/double-shell QDs 关CdSe/CdS/ZnS 共CSS兲兴 Finally, we propose a model, which describes qualitatively the dispersibility of QDs in sol-vents of varying polarity
Monodisperse CdSe and CdSe/ZnSe QDs were prepared following the procedure described in Ref.14 The synthesis
of CdSe/CdS/ZnS CSS QDs was carried out using exclu-sively air-stable metal precursors, namely, cadmium 共zinc兲 ethylxanthate and stearate.22 Depending on the number of purification cycles共vide infra兲, the samples were dispersed in
n-hexane, toluene, chloroform, n-butanol, or water, which
have polarities共dielectric constants兲 of 0 共2兲, 0.37 共2.4兲, 1.01 共4.8兲, 1.66 共18兲, and 1.85 共80兲, respectively
Absorption spectra were recorded in the 1 cm cells using
a Cary 5000 UV-vis-NIR spectrometer In PL measurement, several excitation sources were used such as 370 or 460 nm light-emitting diode 共LEDs兲 The excitation power density was weak, about 100W/cm2 in all cases The PL signals were dispersed by a 0.6 m grating monochromator 共Jobin-Yvon HRD1兲 and then detected by a thermoelectric cooled Si charge-coupled device共CCD兲 camera 共Hamamatsu兲
As-prepared core, CS, or CSS QDs were separated from the reaction medium by precipitation with methanol, fol-lowed by centrifugation, resulting samples denominated 0-cleaning-cycle QDs Depending on the reaction conditions, the surface of these QDs was passivated by different types of molecular ligands such as trioctylphosphine共TOPO兲, hexa-decylamine 共HDA兲, or stearate Consequently, the QDs are soluble in rather nonpolar solvents共n-hexane and toluene兲, hardly soluble in chloroform, and insoluble in n-butanol or
a兲Author to whom correspondence should be addressed Also at: College of
Technology, Hanoi National University, 144 Xuan Thuy, Hanoi, Vietnam.
Electronic mail: liemnq@ims.vast.ac.vn.
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Trang 3water The PL QY from as-synthesized CdSe QDs is
typi-cally rather high共40%兲
Upon the use of a well-defined purification procedure to
remove organic surfactant molecules from the QDs surface,
the QDs could be dispersed in polar solvents such as
chloro-form, n-butanol, and even water due to the enhanced net
polarity of the QDs that induced by complex of QDs
polar-ization and surfactant molecules The applied procedure
con-sisted in the dispersion of⬃5 mg of the zero-cleaning-cycle
QDs in 5 ml of toluene, followed by an ultrasound treatment
共55 kHz, 5 min兲 and centrifugation 共6000 rpm, 5 min兲 to
eliminate any nonliquid residual Subsequently, 5 ml of
methanol were added, resulting in the flocculation of the
QDs, which were collected by centrifugation 共12000 rpm,
5 min兲 In the following, the obtained samples are
denomi-nated one-cleaning-cycle QDs and repeating the described
procedure led to two-cleaning-cycle QDs From the practical
point of view, it is important to note that with increasing
number of cleaning cycles QDs, they can be dispersed in
solvents of increasing polarity We attribute this behavior to
the fact that the net polarity of the inorganic part of the QDs
becomes more evident upon successive surface ligand
re-moval As a result, the one-cleaning-cycle QDs could easier
be dispersed in chloroform, which is a polar solvent with a
polarity of 1, than in n-hexane or in toluene After two cycles
of cleaning, all kinds of CdSe and ZnSe- or
CdS/ZnS-capped CdSe QDs could readily be dispersed in
chloroform We noticed that our two-cleaning-cycle QDs
could easily be dispersed in n-butanol, methanol, and even in
water upon a short共5 min兲 application of ultrasound This is
an important characteristic applicable to various experiments
where water-soluble QDs are needed, without necessity to do
ligand exchange共e.g., with mercaptocarboxylic acids兲
Along with the cleaning process comprising the partial
removal of surface ligands from the QDs, we observed
dis-tinct shifts of the spectral features in both PL and absorption
spectra Figure 1 summarizes the observed shifts for CdSe
QDs in different solvents after one and two cleaning cycles
The PL and absorption peaks for cleaned CdSe QDs
dispers-ing in chloroform are located at the shortest wavelength In
both kinds of solvents with higher or lower polarity with
respect to chloroform, the PL and absorption peaks from
cleaned CdSe QDs are located at lower energy This is di-rectly resulting from the electric dipoles’ cancellation be-tween the net polarization of the QD-ligands complex and the polarity of the solvent The resulting polarization is sup-posed to affect the optical transitions inside the QD in a similar way as the Stark effect In single CdSe QDs, the Stark shift of absorption induced by intentionally applying
an external electric field was studied by Empedocles and Bawendi.23
In order to study the Stark shifts without interference from surface states, the PL and absorption spectra for well-passivated two-cleaning-cycle CdSe/CdS/ZnS CSS QDs in different solvents were measured Figure2 shows the spec-tral shifts in different solvents that follow a similar trend as those of the CdSe QDs共Fig.1兲: The spectral shift in chloro-form is minimum, corresponding to the least electric field experienced by the QDs For the solvents of lower and higher polarities, the peaks were shifted to lower energy, indicating a stronger Stark effect Once again, we attribute the observed behavior to the net electric field applied to the QDs However, as compared to the bare CdSe QDs, the ob-served spectral shifts of CSS ones are much less pronounced This can be explained by the contribution of the shell thick-ness共1.5 nm兲 that reduces the influence of the solvent’s po-larity on the polarized CdSe core CdSe/ZnSe QDs of com-parable shell thickness exhibited a very similar behavior as the CSS QDs
Figure3shows a model proposed for the net polarization from the QD-ligands complex in solvent In fact, in CdSe QDs built up from different kinds of elements共Cd and Se兲, the number of the surface atoms is large as compared to the volume atoms Depending on the termination and stoichiom-etry at the QD surface, an electric dipole moment, or charge, polarization corresponding to the QD results.20,24,25 Several papers reported on the permanent dipole moment of nonme-tallic nanoparticles共ZnSe and CdSe兲 that always exists even
if they crystallized in the highly symmetric zinc-blende structure.24,26 Ligand molecules of surfactant type could act
as a neutralization buffer for the QD’s polarity The neutral-ization rate depends on the dipole moment共polarization兲 of the QD and on the number of ligand molecules on its sur-face Depending on the number of cleaning cycles and hence
FIG 1 共Color online兲 PL and absorption spectra of as synthesized CdSe QDs in hexane 共a兲 and the absorption 共Abs兲 and PL peak positions as functions of the solvent and the number of cleaning cycles 共b兲 In the inset, Abs 共PL兲 1 共2兲 means absorption 共PL兲 spectra for the one-共two兲 cleaning-cycle QDs The data points for Abs 共PL兲 1 corresponding to the QDs in water are not shown because these one-cleaning-cycle QDs are not dispersible in water.
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Trang 4the average number of ligand molecules per QD, the net
polarization varies that induces the shift of the QD energy
levels due to essentially the Stark effect The Stark shift
ob-served in the present experiment is very large, namely, about
60 meV in the case of CdSe QDs in hexane with respect to
those in chloroform, or at least of 30 meV in the case of
CdSe/CdS/ZnS CSS QDs in a similar comparison These
shifts are very large as compared to the solvatochromatic
shift that originates from the effect of the external dielectric
environment, which is usually below 10 meV.21
In conclusion, we have studied the PL and absorption
behaviors of CdSe, CdSe/ZnSe, and CdSe/CdS/ZnS QDs
dispersed in solvents of different polarities Our experiments
revealed that all types of QDs possess a natural polarization
that make them dispersible in polar organic solvents and
even in water upon partial removal of the organic surface
ligands by an appropriate cleaning procedure Due to the net
polarization of the QDs and ligands complex, Stark effect
induced shifts of the PL and absorption spectra as a function
of the solvent’s polarity took place These effects occurred
on CdSe QDs without inorganic shell and, to a lower extent,
on those with single 共CdSe/ZnSe兲 and double shells
共CdSe/CdS/ZnS兲
The authors thank Professor N.V Hieu and professor L.V Hong for helpful discussions and L.Q Phuong for tech-nical assistance The Basic Research Programme in Natural Science 共MOST Vietnam兲 and the Materials Science Direction共VAST兲 are gratefully acknowledged for financial support
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FIG 2 共Color online兲 PL and absorption spectra of the CdSe/CdS/ZnS CSS QDs in hexane 共a兲 and the absorption 共Abs兲 and PL peak positions after two cleaning cycles in solvents of different polarity 共b兲.
FIG 3 共Color online兲 Model proposed for the net polarization from the
QD-ligand complex in the solvent PQD, PLG, and Psol: the polarity/
polarization of bare QDs, of ligand molecules, and of the solvent,
respectively.
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