Lead-salt quantum-dot ionic liquids

Article abstract of DOI:10.1002/smll.200902218

PbS quantum dots (QDs) are functionalized using ionic liquids with thiol moieties as capping ligands. The resulting amphiphilic QD ionic liquids exhibit fluidlike behavior at room temperature, even in the absence of solvents. The photostability of the QDs

Full text of DOI:10.1002/smll.200902218

communications
Ionic liquids
Lead–Salt Quantum-Dot Ionic Liquids**
Liangfeng Sun,* Jason Fang, Jason C Reed, Luis Estevez, Adam C. Bartnik, Byung-
Ryool Hyun, Frank W. Wise, George G. Malliaras, and Emmanuel P. Giannelis*
The electronic energies of lead–salt quantum dots (QDs) are the QDs. This approach provides an effective method to tailor
determined primarily by quantum confinement due to their the physical properties of lead–salt QDs. We demonstrate the
[
1]
large exciton Bohr radii.
structure of the QDs (PbS and PbSe) has been worked out
The fundamental electronic approach using PbS QDs as a model system.
Colloidal PbS QDs were synthesized using organometallic
[
2]
[14,15]
by Kang et al., and now research with these materials is precursors.
turning towards applications. For instance, lead–salt QDs have protonating sodium 3-mercapto-1-propanesulfonate followed
Ionic liquid ligands were synthesized by
[
3,4]
been used as active materials in photovoltaic devices
due to by neutralizing with polyetheramine, as shown in Scheme 1.
their size-tunable infrared (IR) absorption. They are also 0.5 mL of a PbS QD suspension in toluene (concentration
[
5]
ꢁ1
efficient IR emitters and could be used in biomedical imaging
ꢀ50 mg mL ) was added to 0.5 mL of thiol-capped ionic liquid
[
6,7]
and in electroluminescent devices.
In order for QDs to in a centrifuge tube. The molar ratio of PbS QDs to the ionic
4
realize their full potential, their stability (e.g., photostability) liquid ligand was about 1:10 . The excess ligands prevent the
and compatibility with other materials must be improved. QDs from aggregating. The ionic liquid and QD toluene
Accordingly, much effort is devoted to surface passivation and solution werephaseseparated, as evidencedbya layerofbrown
functionalization of QDs, with increasing attention being paid QD toluene solution on top of a layer of transparent ionic
[
8–13]
to the use of ionic liquids to passivate the QD surface.
liquid.Thethiolmoietyintheionicliquidservesastheanchorto
Using certain ionic liquid ligands, solid materials can be bind ionic liquid ligands to the PbS QD surface. After vigorous
transferred to a new state that exhibits liquidlike behavior at shaking, the mixture became uniformly brown in color. The
[
12,13]
[12]
room temperature.
To date, metal nanoparticles
and solution was then centrifuged at a speed of 14500 rpm for five
[
9–11]
oxide nanoparticles
have been functionalized using a minutes. No separation or precipitation was observed. The
polymer ionic liquid. Some semiconductor nanoparticles (e.g., mixedsolutionwasthenloadedintoavacuumovenanddriedat
CdSe) functionalized using small-molecule ionic ligands have room temperature until all the toluene was removed. The final
[
13]
been reported.
PbS, PbSe, and PbTe) QD ionic liquid where polymer ionic the volume of the original ionic liquid. It is believed that the
In this work, we report the first lead–salt volume of the solution was about 0.5 mL, which is the same as
(
liquidligandsareusedascappingligandsforQDs. Theresulting QDs were capped by ionic liquid instead of oleic acid due to the
amphiphilic QD ionic liquids exhibit fluidlike behavior at room much stronger binding strength of a thiol than a carboxylic
[
5,16]
temperature, even in the absence of solvents. The ionic liquid group to a QD.
In a separate experiment, we observed that
capping ligands also dramatically improve the photostability of the original hydrophobic QDs capped by oleic acid were
attracted to water where more hydrophilic ionic liquid was
dissolved (see Supporting Information), showing the comple-
[
ꢂ] Dr. L. Sun
teness of ligand exchange. Figure 1c shows a transmission
electron microscopy (TEM) image of the original PbS QDs,
where QDs of about 2.5 nm in size are observed. After capping
the QDs with ionic liquid ligands, a much higher interparticle
Center for Nanoscale Systems
Cornell University
Ithaca, NY 14853 (USA)
E-mail: ls462@cornell.edu
Prof. E. P. Giannelis, J. Fang, J. C Reed, L. Estevez, Prof. G. G. Malliaras separation is observed in the TEM image (Figure 1c, left inset),
Materials Science and Engineering Department
Cornell University
Ithaca, NY 14853 (USA)
which is expected based on the sizes of the oleic acid and the
ionic liquid ligand. Larger QD clusters (size ꢀ7 nm) are also
observedinQDionicliquid, whichconsistofafewsmalldots, as
A. C. Bartnik, Dr. B.-R. Hyun, Prof. F. W. Wise
School of Applied and Engineering Physics
Cornell University, Ithaca, NY 14853 (USA)
resolved in the high-resolution TEM (HRTEM) image
(
Figure 1c, right inset). The small dots form a cluster but do
not fuse together. Thermal gravimetric analysis (TGA)
measurements (see Supporting Information) showed that there
was about 4.5% weight residue when the PbS QD ionic liquid
[
ꢂꢂ] L.S. and J.F. contributed equally to this work. We acknowledge
support from the National Science Foundation under Grant No.
EEC-0646547, NYSTAR and CCMR at Cornell University. We also
acknowledge partial support from Award No. KUS-C1-018-02, was heated to 540 8C, corresponding to the inorganic portion of
made by King Abdullah University of Science and Technology
(KAUST).
the hybrid. The resulting QD ionic liquid is a viscous, brown
fluid, as shown in Figure 1b. The fluidity of the material is
affected by both the electrostatic and the van der Waals forces
among the ions (shown in Figure 1a). By carefully tuning the
size of the ions, which in turn changes the relative magnitude of
:
Supporting Information is available on the WWW under http://
www.small-journal.com or from the author.
DOI: 10.1002/smll.200902218
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small 2010, 6, No. 5, 638–641

Scheme 1. Schematic image showing the synthesis of ionic liquid with a
thiol moiety.
[
16]
thetwoforces,thefluiditycanbevariedatfixedtemperature.
In other words, the glass-transition temperature T of the
material can be changed by tuning the size of the ions.
g
The amphiphilic nature of the PbS QD ionic liquid was
demonstrated using contact-angle measurements. As shown in
Figure 2a, the contact angle of PbS QD ionic liquid on a silicon
wafer is about 128, which is much less than that of water (588) on
thesamesubstrate. Thelowcontactangleindicates thatthePbS
QD ionic liquid easily wets a hydrophilic substrate. On a Teflon
film, the PbS QD ionic liquid shows a contact angle of 408, while
the contact angle of water on the same substrate is about 1068.
Thus the PbS QD ionic liquid wets reasonably well both a
hydrophilic and a hydrophobic substrate.
Both the ionic liquid used for surface functionalization and
the QD ionic liquid are electrically conducting, although their
g
conductivities are low. At temperatures above T measured by
differential scanning calorimetry (DSC; see Supporting
Information), the conductivity–temperature relation of the
pure ionic liquid follows the Vogel–Tammann–Fulcher (VTF)
[
22]
model,
as shown in Figure 3. However, the conductivity of
Figure 1. a) Schematic image of an ionically modified QD. b) A
photographofaPbSQDionicliquid.c)TEMimageoftheoriginalPbSQDs.
the QD ionic liquidhas more features (shown in Figure 3) and is
about an order of magnitude lower. The conductivity is Left inset: TEM image of the QD ionic liquid. Right inset: HRTEM of the QD
probably reduced because the heavy QD cores reduce the ionic liquid.
fluidity of the fluid, which in turn reduces the mobility of
g
the ligands binding to the QDs. At temperatures below T , the
conductivities of the pure ionic liquid and the QD ionic liquid
overlap (shown in Figure 3). Since the materials are in the solid
phase, the fluidity has no contribution to the conductivity. The
mechanismof theconductivity below T isexplained interms of
g
[
18,19]
many-body interactions
and is ‘‘universal’’ for a wide
[
18]
[18,19]
range of materials.
The theory
also predicts that the
dependence of the conductivity on frequency follows a power
n
law s(v)/v , with nꢀ 1, consistent with our experiment (see
Supporting Information). Further studies using different types
of ionic liquids and/or different sizes of QDs are underway to
elucidate charge-transport phenomena in these systems.
The optical properties of PbS QDs were preserved and in
some cases enhanced after being capped by the ionic liquid
ligands. These properties were investigated by measuring the
optical absorption, photoluminescence (PL), and absolute
quantum yield. A solid-state laser that emits 635-nm light was
used to excite the samples. The power of the laser was kept
Figure 2. ContactangleofPbSQDionicliquidonasiliconwafer(a)andon
lower than 100 mW to avoid degrading the sample. The QD Teflon (c); contact angle of water on a silicon wafer (b) and on Teflon (d).
small 2010, 6, No. 5, 638–641
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Table 1. Absolute quantum yield of each sample.
Absolute quantum yield [%]
QDs in toluene
QD ionic liquid
QD film
40 ꢄ 2
22 ꢄ 2
3 ꢄ 2
potential that the efficiency of PL from QD ionic liquids can be
improved, if cluster formation can be prevented during
synthesis.
The photostability of the PbS QD ionic liquid was measured
by shining a solid-state laser (4 mW, 635 nm) onto the sample in
ambient air, while PbS QDs in toluene were used as the control.
A small amount of the sample (ꢀ50 mL) was loaded into a
cuvette (3 ꢃ 3 ꢃ 30mm in width ꢃ length ꢃ height for the
interior dimensions). By using small volumes, most of the
sample can be exposed under the ꢀ3-mm-diameter laser spot
Figure 3. Temperature-dependent conductivity of ionic liquid (solid
squares) and QD ionic liquid (dots), measured at a frequency of 1 Hz.
ionic liquid was prepared by sandwiching a thin layer (ꢀ10 mm) while minimizing the convection effect in the liquid, which
ofthe liquid betweentwocover glassslides. Theoriginal QDsin brings/removes fresh/exposed QDs in/out of the laser spot. The
ꢁ
1
toluene (concentration is about 0.5 mg mL ) were prepared in PLfromthePbSQDsintoluenedecreasedrapidlyinthefirstfew
a glass cuvette (optical path, 3 mm). The QD film was prepared seconds and then slowly decreased in the following 20 minutes.
by spin-coating a layer of QDs on a glass slide. The purpose of In contrast, there is virtually no change in the PL intensity of the
the different sample preparation methods is to keep the optical PbS QD ionic liquid (Figure 5). For the QDs in toluene, the fast
density low (close to or less than 0.1 at the lowest energy- decrease is quite reversible, which is thought to be caused by the
[
25]
absorption peak) to reduce self-absorption on the PL spectra. trapping of photoexcited charge in the QDs, while the slow
Both the lowest energy-absorption peak (see Supporting decrease is not reversible on the timescale of a few hours (see
Information) and PL emission peak of the QD ionic liquid Supporting Information). We also observed spectrum blue-shift
werered-shiftedbyꢀ65 nmrelativetothoseoftheoriginalQDs afterlong-termlaserexposure.Theblue-shiftcouldbecausedby
(
shown in Figure 4), as observed previously with different thiol photo-oxidation, which causes a shrink in QD size, as suggested
[
16,5]
[25]
ligands.
No noticeable absorption or PL peak shift was by Krauss et al. It might also be caused by ionization of large
observed from a spin-coated QD film. The absolute quantum QDsfollowed bya long-term chargetrapping. ThePLspectraof
yield of each sample was measured using an integrating sphere, the PbS QD ionic liquid before and after laser exposure were
[
23]
which is based on the design proposed by Friend et al.
As almost identical (see Supporting Information), which indicates
shown in Table 1, the PL quantum yield from the original QDs that no photo-oxidation or long-term charge trapping occurred.
dispersed in toluene is about 40%, while it is only about 3% for This may be attributed to the surface passivation by the thiol
the QD film. The quantum yield from QD ionic liquid is about moieties of the ionic liquid. The QD ionic liquid could be
2
2%, which is lower than that of the original QDs but still larger developed further for a dye-sensitized solar cell, where QDs
[
24]
thanatypicalIRdye(e.g.,13%fromIR125 ).Thedecreaseof serve as the sensitizer dye molecules. It has an absorption band
thequantumyieldintheQDionicliquidmightbecausedbylow that is tunable from the visible to IR, ionic conductivity, and
levels of aggregation, as observed in the TEM image (Figure 1c, negligible vapor pressure, which are favorable for a practical
[
26]
right inset), or possibly the effect of the ligand on the electronic dye-sensitized solar cell.
properties. The small peak at around 1300 nm in the PL In summary, we have synthesized a new kind of ionic liquid
spectrum of the QD ionic liquid likely indicates cluster consisting of lead–salt semiconductor QDs. The method
formation after ligand exchange. If that is true, there is the
Figure 4. a) PL intensity of PbS QDs in toluene (solid squares), PbS QD
ionic liquid (solid circles), and PbS QD film (solid triangles).
Figure 5. Time-dependent PL from PbS QDs in toluene (solid squares)
and PbS QD ionic liquid (dots).
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ß 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2010, 6, No. 5, 638–641

c
developed is a simple yet general way to functionalize and P (area under PL peak), where the subscripts a, b, and c
nanoparticles. In ambient environment, the QD ionic liquids denote the measurements aforementioned. The quantum yield is
[
23]
aresolventfreeandmorestablethanthesameQDsdispersedin then calculated by the simple equation
a solvent.
PcLb ꢁ PbLc
h ¼
:
LaðLb ꢁ LcÞ
Experimental Section
We verified that the accuracy of this method is about ꢄ2%.
Synthesis of colloidal PbS QDs: Lead (II) oxide (220mg, 1 mmol,
9
9.999%), oleic acid (0.64 mL, 2 mmol, 90%), and 1-octadecene
ODE, 9.36 mL, 90%) are mixed and stirred at 150 8C under pure
nitrogen environment. At the same time, a 0.1 M hexamethyldisi-
lathiane ((TMS) S, 126 mL, 0.6mmol) in ODE (6mL) is stirred in a
glove box for at least 30 minutes. After the reaction solution
containing PbO) turns clear, the temperature is set to 908C and
allowed to stabilize. Then, 5 mL of the (TMS) S/ODE solution (0.5
Keywords:
(
.
.
.
.
conductivity ionic liquids lead salts photostability
quantum dots
2
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solution. After reacting for 1 min, the reaction solution is rapidly
cooled with an ice bath. The growth solution is then mixed with
[
[
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mL hexane. Excess oleic acid and unreacted precursors are
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in 4 mL deionized water. The solution is run through an ion
exchange column (Dowex, HCR-W2 ion exchange resin) to remove
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0). Polyetheramine (0.67g, 1.12 mmol, Jeffamine XTJ505, XTJ506) is
[
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[
Absolute quantum yield measurement: An integrating sphere
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diffusely reflecting material – PTFE – is used to redistribute the
light isotropically over the sphere interior surface. A solid-state
laser (wavelength 635 nm) beam enter the IS through the input
port. At the output port of the IS, an optical-fiber bundle is used to
collect and send the light to a spectrometer (SpectraPro 275,
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allows the sample to pass in and out of the laser beam. Three
spectra are taken: a) laser beam only, b) sample in IS but off the
laser beam, and c) sample in the laser beam. In each photon-
number spectrum (intensity ꢃ wavelength verus wavelength), five
[
Received: November 26, 2009
Published online: February 2, 2010
parameters are calculated, L
a
, L
b
, L
c
(area under laser peak), P
b
small 2010, 6, No. 5, 638–641
ß 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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