Photoluminescent Aerogels from Quantum Wells

Article abstract of DOI:10.1021/acs.chemmater.5b04872

Highly porous self-supported three-dimensional aerogel monoliths are synthesized from strongly quantum confined quantum wells, namely, 5 monolayer thick CdSe and CdSe/CdS core/crown nanoplatelets. The aerogels are synthesized by hydrogelation of the aqueous quantum well solutions and subsequent supercritical drying. The aerogels are optically characterized by UV-vis absorption spectroscopy, emission spectroscopy, and photoluminescence (PL) decay analysis. Morphological and structural characterizations are achieved by transmission electron microscopy, scanning electron microscopy, and X-ray diffractometry. The macroscopic aerogels exhibit extremely low densities of 0.038 ± 0.007 g·cm^-3 and a significantly high specific surface area of 219 m²·g^-1 with nearly entirely (111) as the exposed crystal facet. Moreover, the aerogels feature properties related to quantum confinement comparable to those of their original building blocks with photoluminescence quantum yields up to 10.3%.

Full text of DOI:10.1021/acs.chemmater.5b04872

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Article
Photoluminescent Aerogels from Quantum Wells
Suraj Naskar, Jan F. Miethe, Sara Sánchez-Paradinas, Nadeschda Schmidt,
Karthiga Kanthasamy, Peter Behrens, Herbert Pfnür, and Nadja C Bigall
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04872 • Publication Date (Web): 03 Mar 2016
Downloaded from http://pubs.acs.org on March 9, 2016
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Photoluminescent Aerogels from Quantum Wells
Suraj Naskar1, 4, Jan. F. Miethe1, 4, Sara SánchezꢀParadinas1, 4, Nadeschda Schmidt2, 4, Karthiga Kanꢀ
thasamy3, 4, Peter Behrens2, 4, Herbert Pfnür3, 4, Nadja C. Bigall1,4*
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1Institute of Physical Chemistry and Electrochemistry, Leibniz Universität Hannover, Callinstraße 3A, 30167 Hannover,
Germany.
2Institute for Inorganic Chemistry, Leibniz Universität Hannover, Callinstraße 9, 30167 Hannover, Germany.
3Institut für Festkörperphysik, Leibniz Universität Hannover, Appelstraße 2, 30167 Hannover, Germany.
4Laboratory for Nano and Quantum Engineering, Schneiderberg 39, 30167 Hannover, Germany.
ABSTRACT: Highly porous selfꢀsupported three dimensional aerogel monoliths are synthesized from strongly quantum confined
quantum wells, namely 5 monolayer thick CdSe and CdSe/CdS core/crown nanoplatelets. The aerogels are synthesized by hydroꢀ
gelation of the aqueous quantum well solutions and subsequent supercritical drying. The aerogels are optically characterized by
UVꢀVis absorption spectroscopy, emission spectroscopy, and PL decay analysis. Morphological and structural characterizations are
achieved by transmission electron microscopy, scanning electron microscopy, and Xꢀray diffractometry. The macroscopic aerogels
exhibit extremely low densities of 0.038 ± 0.007g·cmꢀ3 and a significantly high specific surface area of 219 m2·gꢀ1 with nearly enꢀ
tirely (111) as the exposed crystal facet. Moreover, the aerogels feature properties related to quantum confinement comparable to
those of their original building blocks with photoluminescence quantum yields up to 10.3%.
1. Introduction
nanoparticles were obtained by controlled destabilization of
the colloidal solution.19 Since then research efforts have been
dedicated to extend this technique to other colloidal semiconꢀ
ductor building blocks of various cadmium chalcogenide
compounds including quasiꢀspherical and rodꢀlike shapes.20ꢀ26
Regarding possible applications, metal chalcogenide aerogels
for capturing radioactive elements and enzyme encapsulated
Metalꢀchalcogenide nanoparticles, such as colloidal quantum
dots, rods, tetrapods, octapods, nanowires, nanoribbons, nanoꢀ
platelets (NPLs) or nanosheets (sometimes also referred to as
quantum wells), have attracted the attention of many scientists
due to their interesting size and shape dependent optoelectronꢀ
ic properties.1ꢀ6 By controlling the shape of the nanocrystals,
the exciton (electronꢀhole pair) can be confined in three diꢀ
mensions e.g. in quasiꢀspherical crystals, in two dimensions
such as in nanorods and nanowires and in one dimension,
namely in quantum wells such as NPLs. In recent years, a
variety of only few monolayer (ML) thick NPLs, core/shell
NPLs or core/crown NPLs of various compositions, such as
quantum dot hydrogels for the development of biosensors have
27
already been demonstrated.21,
More recently, also mixed
aerogels e.g. from Au and CdTe,28, 29 and aerogels solely from
metal nanoparticles (such as Pt, Au, Ag)30ꢀ36 have been reportꢀ
ed. Here, we present to the best of our knowledge for the first
time the assembly of fluorescent quantum wells to form selfꢀ
supported highly porous, extremely lightweight, macroscopic
3D aerogels with large specific surfaces. The destabilization of
the here employed anisotropic 5 ML thick CdSe and
CdSe/CdS core/crown NPL building blocks in aqueous mediꢀ
um was achieved by hydrogen peroxide induced ligand oxidaꢀ
tion. The impact of variable amounts of H2O2 on the gelation
process and on the optical properties of the resulting hydrogels
and aerogels is investigated. A diagram representing the synꢀ
thesis steps from the pristine colloidal NPLs in hexane soluꢀ
tion over phase transfer to aqueous solution, and over hydroꢀ
gelation to fluorescent aerogel monoliths is shown in
scheme 1.
7ꢀ10
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CdSe,4,
CdSe/CdS,5,
CdSe/CdS/CdZnS,12 PbSe/PbS,6
ZnSe/ZnS,13 and CdSe/CdTe14 have been reported, with
unique optoelectronic properties such as high photoluminesꢀ
cence quantum yields (PLQYs), extremely narrow emission
band widths, ultrafast fluorescence decays and reduced Stokes
shifts. As for applications, electroluminescent emitters, reꢀ
versible oxygen sensing, and room temperature lasing with
low energy threshold have already been reported.15ꢀ18 Furtherꢀ
more, the CdSe NPLs were assembled into closeꢀpacked pileꢀ
shaped products emitting polarized light.8 Instead, the assemꢀ
bly of the NPLs into selfꢀsupported, highly porous 3D bulk
architectures, which would be of interest for a variety of posꢀ
sible applications, such as in photovoltaics, sensors for detectꢀ
ing chemicals and photocatalysis, has not been reported so far.
The aerogels reported here exhibit intense photoluminescence
and might in future serve as an effective material for highly
sensitive sensing devices. For example, Lorenzon et al.17 have
observed that CdSe based NPLs have unique capability of
In earlier studies, porous aerogels from quasiꢀspherical CdS
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Scheme 1. Schematic diagram of the aerogel formation from CdSe/CdS core/crown NPLs. First, ligand exchange is per-
formed in order to transfer the quantum wells to aqueous solution. Controlled destabilization by means of hydrogen perox-
ide addition results in the formation of hydrogels, which are subsequently transferred to aerogels by supercritical drying.
The photographs on the left of the schemes display the quantum wells in the respective solutions and in the gels under UV
illumination.
photoluminescence based reversed oxygen sensing (and other
gases such as CO and CO2) attributing this property to the
large specific surface of the particle increasing the probability
for the creation of surface trap states which are necessary for
this type of reversed gas sensing. The assembly of NPLs into a
highly porous and accessible large surface superstructure (as
achieved in the present work) might help making gas sensing
devices from such nanoscopic building blocks, since the aeroꢀ
gels are selfꢀsupported in gaseous environments. The high
porosity and mesoporous character of the hydrogel and aerogel
materials developed in the present work might allow possible
reactant molecules to access the large specific surface area,
and hence such gels are potentially also applicable for superior
electroꢀcatalytic activity.37 A different example for possible
applications, in which the porous arrangement of CdSe based
NPLs, was suggested by the Dubertret group38 in their work
about the employment of NPLs in electrolyte gated field effect
transistors. It was found that porous arrangement leads to
much higher currents due to a better chemical interface, and it
was suggested that even more porous arrangements could
improve this effect. Therefore, a gelꢀtype assembly as syntheꢀ
sized in our present work would be of high interest also for
this type of applications. Moreover, aerogels of the NPLs
exhibit solely (111) as the exposed crystal facet, providing the
opportunity to perform facet dependent chemical reactions on
its surface. Furthermore, the here developed aerogel materials
might be applicable in optical sensing, photovoltaics, light
emitting diodes (LEDs), or as a catalyst.
2. Experimental Section
Reagents. Cadmium nitrate tetrahydrate (99.999 %), cadmium
acetate dihydrate (98.0 %), sulphur powder (> 99 %), sodium
myristate (> 99 %), 11ꢀmercaptoundecanoic acid (MUA)
(95 %), hydrogen peroxide (30 %), 1ꢀoctadecene (90 %)
(ODE), oleic acid (90 %), methanol (99.5 %), ethanol
(99.5 %), nꢀhexane (> 99 %), and acetone (> 99 %) were purꢀ
chased from Sigma Aldrich. Selenium powder 200 meshes
(99.999 %), nitric acid (> 69 %) and hydrochloric acid (>
37 %) were purchased from Fluka. Indium foil, thickness
0.1 mm, ≥ 99.995 % trace metal basis was purchased from
SigmaꢀAldrich.
Synthesis of 5 ML thick CdSe NPLs. Quasi 2D, 5 ML thick
CdSe NPLs were prepared according to the procedure deꢀ
39
scribed in literature with little modifications.5,
A total
amount of 170 mg cadmium myristate, Cd(myr)2 (0.3 mM)
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and 14 mL ODE were placed in a 25 mL three neck round distilled water. The CdSe/CdS NPLs were transferred to aqueꢀ
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bottom flask. The mixture was degassed at 100 °C for one
hour at constant stirring conditions. A clear solution of cadmiꢀ
um myristate dissolved in ODE was obtained. Under argon
flow, the temperature of the solution was raised to 250 °C.
Then 1 mL of a 0.15 mM Se solution (prepared by dispersing
12 mg Se powder in 1 mL ODE), was quickly injected into the
mixture. Subsequently, one minute after the selenium solution
injection, a total amount of 120 mg (0.9 mM) Cd(OAc)2·2H2O
was added to the mixture. The reaction was continued at a
constant temperature of 250 °C for 10 min. Finally, 0.5 mL
oleic acid was injected into the mixture, and the reaction temꢀ
perature was lowered down to room temperature by placing a
water bath under the flask. The NPLs were separated from all
other byproducts (mainly CdSe cubes and spheres) by adding
20 mL of a mixture of hexane : ethanol = 3 : 1 and centrifugꢀ
ing for 15 min at 3800 g (rcf). The dark red precipitate of the
CdSe NPLs was collected from the bottom of the centrifuge
vial and dispersed in 2 mL hexane.
ous medium following the same procedure as mentioned
above.
Hydrogelation of the CdSe/CdS core/crown and CdSe NPLs.
In the first set of experiment the effect of H2O2 concentration
on the time required for the appearance of hydrogel formation
was studied. Therefore, eight 2 mL microꢀcentrifuge tubes
were filled with 400 µL of the aqueous NPLs solution. The Cd
ion concentration of the NPLs solution was kept constant at
1.77 mM. Thereafter, a series of different concentration of
H2O2 solutions (0.1 %, 0.15 %, 0.25 %, 0.5 %, 0.75 %, 1 %,
1.5 % and 2 %) were prepared and 20 µL of each was added to
the vials containing NPLs, which led to molar ratios of
[H2O2] : [Cd2+] = 1.2 : 1, 1.8 : 1, 3.0 : 1, 5.8 : 1, 8.7 : 1,
11.6 : 1, 17.4 : 1 and 23.2 : 1 respectively. All the sample vials
were placed in an oven (already heated) at 80 °C for 2 min in
order to initiate the gelation process and subsequently placed
under ambient conditions in the dark. The time required for
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the first appearance of the hydrogel was 1.5
h for
Synthesis of 5 ML thick CdSe/CdS core/crown NPLs. The
growth of a CdS crown on these CdSe NPLs was performed
following a modified method of Tessier et.al.5 A synthesis
mixture for the growth of the CdS crown was prepared by
placing 480 mg Cd(OAc)2.2H2O together with 340 µL oleic
acid and 2 mL ODE in a three neck flask. The temperature of
the mixture was increased to 150 °C, and the reaction was
continued for 15 min under air with constant stirring and soniꢀ
cation. The light grey mixture of the cadmium oleate was
cooled down to room temperature, and subsequently 3 mL of a
0.1 M sulphur precursor solution prepared previously by disꢀ
solving 19 mg of elemental S in 6 mL ODE was added. The
whole mixture was then transferred to a syringe. The entire
product of the previously prepared 5 ML CdSe NPLs were
placed inside a 25 mL three neck round bottom flask with
10 mL ODE. The temperature of the reaction flask was inꢀ
creased to 100 °C, and the mixture was degassed under vacuꢀ
um for 30 min to remove the hexane completely. Under Ar
flow, the mixture was heated up to 240 °C. The crown growth
mixture (prepared previously) was continuously injected at a
rate of 8 mL/h for 7.5 min by means of a syringe pump. The
heating mantle was removed and the solution was allowed to
reach room temperature and after that 15 mL ethanol was
added. The whole mixture was transferred to a centrifuge vial,
and the NPLs were precipitated by centrifugation at 3800 g
(rcf) for 15 min. The supernatant was discarded and the
CdSe/CdS core/crown NPLs were dispersed in 3 mL hexane.
[H2O2] : [Cd2+] = 23.2 : 1 and (NPLs are completely bleached)
and more than 14 days for [H2O2] : [Cd2+] = 1.1 : 1. In case of
molar [H2O2] : [Cd2+] ratios 1.8 : 1, the gelation time was
approximately one week. In all other cases gelation time was
found to vary between 3ꢀ8 h. The extent of bleaching of the
NPLs was found to increase with higher molar ratios of
[H2O2] : [Cd2+]. Hence, the optimum gelation condition lies
between molar ratio [H2O2] : [Cd2+] = 3.0 : 1 to 5.5 : 1, where
the gelation time is 6ꢀ8 h and the NPLs are least oxidized.
In the second set of experiment, the effect of variable
[H2O2] : [Cd2+] molar ratio on the optical properties of the
resulting gel was investigated following the above mentioned
optimum reaction conditions. Therefore, 5 µL, 7 µL, 9 µL,
11 µL and 13 µL of a 0.5 % H2O2 solution were added to each
microꢀcentrifuge tubes containing 400 µL aqueous NPLs
solution (molar ratio [H2O2] : [Cd2+] = 3.0 : 1, 3.7 : 1, 4.3 : 1,
4.9 : 1 and 5.5 : 1 respectively). Once the appearance of the
hydrogels was observed, the supernatant solvent was exꢀ
changed with deionized water for 7ꢀ8 times in order to remove
excess H2O2 and reaction byproducts from the system. Subseꢀ
quently, the samples were kept at ambient conditions in the
dark for 3 days (aging). Thereafter, the solvent was gradually
exchanged with mixtures of 10 %, 20 %, 40 %, and 60 % (v/v)
acetone in water, and finally the samples were washed with
pure acetone (7 to 8 times). In order to remove residual water,
the vials were filled with dry acetone and were placed inside a
desiccator with anhydrous CaCl2 to avoid the contamination of
moisture from air. The desiccator was evacuated until the
acetone slightly boiled, and left at these conditions for 8 h.
The acetogels were then transferred to a supercritical drying
boat in anhydrous acetone environment. The boat was inserted
in the critical point dryer chamber (E3100 from Quorum
Technologies), where acetone was replaced with liquid CO2.
After storage in liquid CO2 for one day, the temperature of the
chamber was raised above the critical point of CO2 (31.5 °C
and 1100 psi), and the CO2 present in the system was released
as a gas. A volume shrinkage of roughly 30 % to 40 % was
observed during the conversion of hydrogels to aerogels. The
monolithic aerogels of the CdSe and CdSe/CdS core/crown
NPLs were stored under ambient conditions.
Phase transfer of 5 ML thick CdSe and CdSe/CdS NPLs. The
NPLs were transferred to aqueous medium following a proceꢀ
dure described by Kodanek et.al.40 Briefly, a mixture of 61 mg
MUA together with 4 mL methanol and 0.05 g KOH was
placed in a vial and sonicated for 3 min. The entire hexane
solution of the CdSe NPLs (Cd ion concentration is 25 mM as
determined from AAS) synthesized above was added to the
vial with the phase transfer reagent and shaken overnight. The
hexane phase was removed by means of a separation funnel
and the MUA capped CdSe NPLs in the methanolic phase
were precipitated by centrifugation. Afterwards, the whole
product was dispersed in 4 mL of a 0.1 M KOH solution.
Finally, the phase transferred NPLs were washed twice with
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Measurement of density of the aerogels. The aerogels are tion source to analyze the surface of the aerogel. A small
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irregular in shape, which makes it difficult to estimate the
exact volume. We have calculated the volume via inserting
small aerogel pieces in a cylindrical vial with known inner
diameter. From the filling height of the aerogel inside the vial
(several measurements are taken to obtain an average height)
the volume has been calculated. From the mass of the aerogel
(measured with a Sartorius balance measurement accuracy
± 0.0001g) the density has been evaluated. We have taken into
account a maximum of 20 % error in the measurement.
amount of aerogel sample was pressed onto an indium foil
(~1.7 cm2). The conductive, soft surface of the indium foil
holds the sample adequately and prevents specimen chargꢀ
ing.21 The samples were incubated in vacuum (10ꢀ8 mbar) for
18 h before the measurements. All the binding energy peaks
were calibrated with measured carbon 1s peak (binding energy
284.6 eV). The raw data from XPS analysis was fitted with
Gaussian/Lorentzian curve fitting using XPS 4.1 software.
9
Nitrogen physisorption measurements. Nitrogen sorption
isotherms were measured at 77 K on a Quantachrome Autoꢀ
sorbꢀ3 instrument. The samples were degassed under vacuum
at 25 °C for 24 h before the start of the sorption measurement.
Surface areas were estimated by applying the Brunauerꢀ
EmmettꢀTeller (BET) equation.41 The pore size distribution
was calculated using the BarrettꢀJoynerꢀHalenda (BJH) methꢀ
od.42 The total pore volume was estimated using the single
point method at p/p0 = 0.99.
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Atomic absorption spectroscopy. The Cd ion concentration of
the CdSe NPLs and the CdSe/CdS NPLs solution was deterꢀ
mined by atomic absorption spectroscopy (AAS) using a Variꢀ
an AA140 instrument equipped with an air/acetylene flame
atomizer. The samples were prepared by digestion with aqua
regia. Five standard Cd2+ solutions were measured to obtain a
calibration curve. A 1 % HNO3 solution was used in order to
correct the background signal.
Scanning electron microscopy. Scanning electron microscopy
(SEM) was carried out on a JEOL JSM 6700F equipped with
cold field emission gun electron source. The samples for SEM
imaging were prepared by deposition of small pieces of the
aerogels on adhesive carbon films.
3. Results and discussion
After the NPL synthesis and prior to gelation, both CdSe
NPLs and the CdSe/CdS core/crown NPLs were transferred to
water following a procedure described in a previous work.40
The TEM micrographs of the anisotropic NPLs as well as the
optical characterizations, measurements from UVꢀVis absorbꢀ
ance, emission and PL decay are shown in figure SIꢀ1 and
figure SIꢀ2 and discussed in detail in the Supporting Inforꢀ
mation. These NPLs in aqueous solution were employed as
building blocks for the gel synthesis.
Transmission electron microscopy. The morphologies of the
gels were investigated by transmission electron microscopy
(TEM) using a FEI Tecnai G2 F20 TMP (Cs = 2 mm, CC
= 2 mm), equipped with a 200 kV field emission gun. Microꢀ
graphs were taken in bright field mode. Each sample was
purified 3 to 4 times by precipitation with ethanol (CdSe NPLs
and CdSe/CdS NPLs) followed by centrifugation and reꢀ
dispersion in hexane, prior to TEM grid preparation. Subseꢀ
quently, 10 µL of the purified sample were dropcasted on a
300 mesh carbon coated copper TEM grid, purchased from
Quantifoil. In case of aqueous solution, 10 µL of the sample
was dried on the TEM grid using an IR lamp for 20 min. The
TEM samples of the aerogels were prepared by carefully
touching the aerogels with the TEM grid.
Prior studies reveal that hydrogels of conventional (such as
dots or rods) metal chalcogenide nanoparticles are obtained by
controlled destabilization of the system employing H2O2 or
22, 26, 43ꢀ45
tetranitromethane as the oxidizing agents.21,
The
gelation process is initiated via partial removal of the surface
ligands by oxidation, which reduces repulsion between the
nanoparticles and results in partial aggregation to form the gelꢀ
like network.25, 26 In the present article, hydrogelation of the
CdSe NPLs and the CdSe/CdS NPLs was achieved employing
the same technique using H2O2 as chemical oxidizing agent.
The effect of H2O2 amount on the CdSe/CdS (aq) NPLs gelaꢀ
tion process was investigated by adding a defined amount of
H2O2 in concentrations varying from 0.1 % to 2 %, while
keeping the amount of MUA ligands and NPLs constant (see
experimental section). With 0.1 % concentration (a molar
[H2O2] : [Cd2+] ratio of 1.2 : 1) the initial gelation effect, i.e. a
voluminous monolithic aggregation with a clear supernatant,
was only observed after 2 weeks with reduced fluorescence
intensity (PLQY = 4.3 % ± 0.2 %). For larger molar
[H2O2] : [Cd2+] ratios below 3.0 : 1, hydrogelation took also
relatively long time (approximately one week). For such low
hydrogen peroxide amounts probably only insufficient ligands
were removed from the surface of the NPLs. Furthermore, for
these low hydrogen peroxide concentrations the hydrogels
obtained exhibited low photoluminescence, and the gel
consistency is not enough to enable transfer to the supercritical
drying boat for further processings. In case of higher oxidant
concentrations (i.e. molar [H2O2] : [Cd2+] ratios of 11.6 to
23.2 : 1), a rapid gelation of the NPLs (within 1.5 h) was obꢀ
Spectroscopic characterization. UVꢀVis absorption spectra of
the samples were recorded using an Agilent Cary
5000 absorption spectrophotometer. Emission spectra of the
samples were recorded using a Horiba Fluoromaxꢀ4 spectromꢀ
eter. The PLQY (in absolute mode) of the samples in solution,
the hydrogels and the aerogels were measured with a Horibaꢀ
DUAL FL spectrophotometer equipped with a Quantaꢀφ inteꢀ
grating sphere (Horiba). All absorption and emission spectra
of the samples were measured in a 1 cm quartz cuvette using
toluene, hexane (UVꢀVis spectroscopy grade), water or 0.1 M
KOH as solvents. Timeꢀcorrelated single photon counting
(TCSPC) measurements were performed using a Fluoromaxꢀ4
combined with the Fluorohub TCSPC unit. A nanosecond
pulsed LED with full width at half maxima (FWHM) around
1.2 ns was used as the excitation source. The excitation waveꢀ
length was 368 nm for CdSe NPLs in hexane medium and
454 nm for all other samples.
Xꢀray photoelectron spectroscopy (XPS). A Leybold Heraeus
XPS analyzer with Al Kα xꢀray beam was used as the excitaꢀ
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served. However, with these larger amounts of H2O2, the emisꢀ stable network (as was suggested for conventional
nanoparticles25). During the aging period, a little volume loss
(around 5 %) can be observed. The hydrogels obtained in this
way are stable monoliths as shown in figure 1. In order to
obtain voluminous aerogels, the hydrogels were supercritically
dried following Kistler’s method46 (see experimental section).
This method allows the network to retain its structure without
any major changes since cappillary forces do not occur during
this drying process. The aerogels obtained after supercritical
drying were monoliths of interconnected NPLs. However, it
should be noted that careful handling is necessary to avoid
mechanical stress and electrostatic charging which might
break the monoliths into smaller pieces. The resulting aerogels
of CdSe/CdS core/crown NPLs are highly fluorescent (PLQY
up to 10.3 %, which is only slightly lower than that of the
corresponding hydrogel with 12.1 %). However, we observe a
loss in PLQY during gelation (the PLQY of the aqueous
solution is 18 %). We explain the loss of PLQY as follows:
The addition of H2O2 leads to three effects: i) The peroxide
radical can generate active Se·ꢀ (in case of CdSe) or S·ꢀ and Se·ꢀ
(in case of CdSe/CdS) at the surface of the NPLs, which in
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sion of the wet gel was found to be completely quenched. The
optimum gel formation was found to take place with molar
[H2O2] : [Cd2+] ratios of 3.0 : 1 to 5.5 : 1. In these cases, also
bright photoluminescence was observed. Photographs of the
hydrogels and the corresponding aerogels under daylight and
upon UV light irradiation are shown in figure 1. In this paramꢀ
eter range, the gelation time was 6ꢀ8 h, which is slightly longꢀ
er, but at the same time there was only negligible loss of the
deep orange color and fluorescence properties with respect to
the original aqueous solution. The hydrogel and the correꢀ
sponding aerogel with a molar [H2O2] : [Cd2+] ratio of 3.0 : 1
exhibited the highest photoluminescence intensities. For furꢀ
ther increasing of hydrogen peroxide content, the photolumiꢀ
nescence intensity was found to decrease more and more.
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turn facilitates to form the interconnection of the NPLs
22
through chalcogenide bonding.20,
ii) The peroxide radical
can help to generate a disulphide bonding between two MUA
surface ligands, which is a soluble byproduct. When ligands
desorb from the NPLs, unsaturated surface states can cause
22
lowering of the PLQY.20, iii) Excess amount of H2O2 can
also cause the oxidation of CdSe to form a soluble oxide
species, (e.g. CdSeO4) which is washed out after hydrogelation
by solvent exchange (see XPS measurements proving the
absence of other Se components than Se2ꢀ in the aerogels).
This explains the dissolution of the CdSe core of the
CdSe/CdS core crown NPL (which is observed by TEM by the
generation of NPLs rings for higher H2O2 concentrations, see
figure 6). With higher oxidant concentration the CdSe
crystallites are more and more dissolved. Kinks, lattice defects
and hence unsaturated surface states are therefore more and
more created. Since the PL emission of the platelets only
comes from the CdSe part (for both CdSe and CdSe/CdS
NPLs) due to the specific band structures of the two materials,
dissolution of the CdSe also goes along with reducing the
PLQY of the samples. Therefore, effects ii) and iii) can
explain why the PLQY decreases with increasing hydrogen
peroxide concentrations.
Figure 1. Photographs of the hydrogels (A and B) and aerogels (C
and D) obtained from the CdSe/CdS (aq) NPLs with variable
molar ratios of [H2O2] : [Cd2+] ranging from 3.0 : 1 to 5.5 : 1.(left
to right, respectively). All monoliths (both hydrogels and aeroꢀ
gels) exhibit intense fluorescence upon irradiation with a UV
lamp (wavelength 365 nm). The hydrogel and the corresponding
aerogel with [H2O2] : [Cd2+] = 3.0 : 1 and 5.5 : 1 have the highest
and lowest PLQY, respectively. The scale bar is in centimeter.
Notably, the surfaces of the hydrogels inside the micro centrifuge
tubes are curved inward which means that the volumes of the
aerogel and hydrogel cannot be compared directly.
The aerogels from CdSe/CdS core/crown NPLs exhibit
extremely low densities of 0.038 ± 0.007 g·cmꢀ3 representing
0.8 % of the density of bulk CdS (4.82 g·cmꢀ3) and 0.69 % of
bulk CdSe (5.816 g·cmꢀ3) respectively. Similarly, also the
aerogels from CdSe NPLs exhibit low densities of
0.036 ± 0.007 g·cmꢀ3 corresponding to 0.62 % of the bulk
CdSe. A small piece of the CdSe/CdS core/crown NPL aerogel
(molar ratio [H2O2 : Cd2+] = 3.0 : 1) was investigated by SEM
to visualize the morphology of the solid structure. Figure 2 (Aꢀ
C) shows the SEM micrographs of the CdSe/CdS NPLs
aerogel in different magnifications. At lower magnifications, it
is evident that the aerogel superstuctures have pores in the
macroporous regime (with pore size > 50 nm). The higher
resolution SEM micrograph also proves the presence of an
enormously voluminous and branched 3D network having
many pores with diameters extending from the mesoporous
As a reference experiment, the gelation of CdSe (aq) NPLs
was also investigated following the same reaction conditions.
We observed similar time scales for the hydrogel formation
even though the photoluminescence was found to be
completely quenched (a fact which is already observed for the
aqueous CdSe NPL building block solutions caused by the
phase transfer via ligand exchange).
In all gelation experiments, after the formation of the initial
wet gel, a washing step is crucial to remove excess oxidant
44
and soluble byproducts e.g. disulphides.26,
Consecutive
washing and aging allows the NPLs with their partially bare
facets to come closer to each other and hence to form the
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Page 6 of 13
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Figure 2. Morphological characterization of an aerogel. (A, B and C) SEM and (D) TEM micrographs of the aerogel from CdSe/CdS
core/crown NPLs in different magnifications. Inset of figure (D) shows the orientation of the NPLs network inside the aerogel. The pore
distribution ranges from mesopores (2ꢀ50 nm) to macropores (> 50 nm).
(2 to 50 nm) to the macroporous (> 50 nm) regime. From the
comparison of the lower and higher resolution SEM images, it
can be derived that these aerogels consist of a fractalꢀtype
network. The SEM and TEM micrographs of the aerogels
from solely CdSe (aq) NPLs also show highly porous
morphology similar to the core/crown NPLs (see Supporting
Information figure SIꢀ3). We have also investigated the
morphology of xerogels which are obtained by drying the
hydrogels under ambient conditions (see figure SIꢀ4). It should
be noticed that the volumes of the xerogels are drastically
reduced to only 1ꢀ2 % of their corresponding hydrogels. This
is most likely due to capillary forces, generated during the
liquid evaporation process, breaking down the porous
morphology to a much more compact structure.
are assembled in a nonꢀordered 3D arrangement, where the
NPLs coalesce in a random fashion. The NPLs are mostly
bended, and sometimes rectangular sheets with edgeꢀtoꢀedge
connections can be found. A closer look on the high resolution
TEM (HRTEM) micrographs (figure SIꢀ5) of the gelated
NPLs (through fast fourier transformation, FFT) reveals a
distance of ~ 0.34 nm between the lattice planes, which
accounts for the presence of (111) facets of the NPLs. Due to
the extremely low thickness (~1.5 nm) of the building blocks
and rather random connections between the NPLs, by HRTEM
we are unable to find out whether the interparticle connections
are crystallineꢀtype or not. Indeed, we assume the interplatelet
connections to be of nonꢀcrystalline type. The assumption of
an amorphous connection is based on the outcome of a previꢀ
ous work on aerogels from CdSe core CdS shell nanorods,26 in
which prolonged PL lifetimes resulted from crystalline type
connections, which is not the case in the present work (as
shown later). If the connections between the neighboring
NPLs would be crystallineꢀtype, we would expect the possibilꢀ
The TEM micrographs of the aerogel from CdSe/CdS
core/crown NPLs are shown in figure 2D. In accordance with
the previous observations, here also a porous interconnected
network of the NPLs is observed. Most of the CdSe/CdS NPLs
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ity that excited electrons could travel to their next neighbors not significantly changed compared to their aqueous building
1
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8
which would lead to a lower recombination probability and
therefore a prolonged radiative lifetime.26
blocks, which means that the quantum confinement properties
are retained, even though the discrete heavy and light holes are
not distinguishable and smeared out. A saturation effect in the
shorter wavelength regime can be observed due to the inhoꢀ
mogeneity of the samples inside the cuvette, where the excitaꢀ
tion beam cannot be adjusted to only hit the sample, rather a
part of the light passes besides the gels. The emission maxima
of the hydrogels and aerogels likewise those of the NPLs in
solution (figure 4B) can be attributed to recombination from
the CdSe band edge. The emission peak of the CdSe/CdS
hydrogels (black line) is 6 nm shifted towards the lower enerꢀ
gy regime with an increased FWHM of 24 nm in comparison
to the CdSe/CdS (aq) NPLs building block solution (with a
FWHM of 17 nm). This effect could be due to the assembly of
the CdSe/CdS NPLs, which allows energy transfer processes
to occur between neighboring NPLs causing a slight shift of
the emission peak to the lower energy regime.24, 26 Also the
emission peak of the hydrogels is broadened in comparison to
that of the corresponding aqueous solution. In addition, for the
hydrogels and aerogels, an elongated tail in the emission specꢀ
trum in the longer wavelength regime can be observed, which
is attributed to trap state emission. The trap state generation
could be due to partial surface dissolution of the NPLs by
peroxide radical, producing unsaturated valences on the Cd
atoms present on the surface. Noticeably, there was no trap
state emission of the NPL solutions observed, neither in aqueꢀ
ous nor in organic medium. It should also be noted that the
emission maximum of the aerogel is shifted to shorter waveꢀ
lengths with a slight further broadening of the FWHM comꢀ
pared to the corresponding hydrogel. While the phenomena of
shifting to shorter wavelengths and broadening of absorption
and emission spectra is not yet perfectly understood, it could
In order to characterize the surface of the aerogel, whether
H2O2 causes any surface oxidation to produce CdO or any
other complex oxides, the aerogel sample (with molar
[H2O2] : [Cd2+] = 4.9 : 1 and 17.4 : 1 respectively) are anaꢀ
lyzed using XPS spectroscopy and is shown in figure 3.
9
3d
2p3/2
4.9 : 1
17.4 : 1
A
B
Se
C
S
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Cd
3d5/2
3d3/2
48 50 52 54 56 58 60
Bindingenergy(eV)
402 405 408 411 414
Bindingenergy(eV)
156158160162164166168
Bindingenergy(eV)
Figure 3. XPS spectra of the CdSe/CdS aerogel with molar ratios
[H2O2] : [Cd2+] = 4.9 : 1 (red line) and 17.4 : 1 (blue line). The
position of the characteristic peaks confirms the presence of Cd is
in +2 oxidation state (A) and Se and S are in –2 oxidation state (B
and C, respectively). High amount of oxidant causes dissolution
of CdSe from the centre of the CdSe/CdS NPL and hence the
intensity of binding energy peak for Se is near to noise level for
the aerogel with molar [H2O2] : [Cd2+] = 17.4 : 1.
The binding energy peaks at ≈ 405.2 eV and at ≈ 412 eV can
be attributed to Cd 3d5/2 and 3d3/2 doublets from CdSe respecꢀ
tively. The absence of a binding energy peak (3d5/2) near 404
eV confirms there are no CdO or other complex oxides e.g.
CdSeO4 present on the surface of the aerogel.21, 48 The 3d and
2p3/2 binding energy peaks appearing at 163 eV and at 54.2 eV
(figure SIꢀ6B and C) confirm the presence of sulphur and
selenium in ꢀ2 oxidation state respectively. A very weak enerꢀ
gy peak ≈ 58 eV might be due to the presence of tiny amount
of SeO2 that resulted due to the etching of the CdSe NPL by
H2O2. Notably, in case of higher oxidant concentration (molar
the [H2O2] : [Cd2+] = 17.4 : 1) the binding energy peak from
Se2ꢀ is absent which confirms nearly complete dissolution of
the CdSe core of the NPL under this conditions, which is
consistent with the TEM micrographs shown in figureꢀ7. We
assume that such kind of oxidized products are washed out
due to their water solubility and therefore cannot be deterꢀ
mined in XPS. However, the results from optical spectra and
TEM (dissolution of the CdSe domains for higher H2O2 conꢀ
centration) are in good agreement with the missing CdSe
signal in XPS.
CdSe/CdS (hex)
CdSe/CdS (aq)
hydrogel
CdSe/CdS (hex)
CdSe/CdS (aq)
hydrogel
A
B
aerogel
aerogel
0.0
650 675 700 725 750
0.0
300
0.0
400
Wavelength (nm)
CdSe/CdS (aq)
500
600
700
500 550 600 650 700 750
Wavelength (nm)
CdSe/CdS (aq)
CdSe/CdS (film)
aerogel
104
103
102
D
C
CdSe/CdS (film)
aerogel
0.0
400
500
600
700
0
10 20 30 40 50 60
Time (ns)
For an overall comparison of the optical properties of the
resulting hydrogels and aerogels with respect to their initial
building blocks, UVꢀVis absorption and emission spectra were
recorded. The high optical density and strong scattering beꢀ
havior of the gels do not allow one to measure the optical
properties in transmission mode. Therefore, absorption and
emission spectra and absolute PLQY measurements were
performed using an integrating sphere. A comparison of the
absorption spectra of the initial oleic acid capped CdSe/CdS
core/crown NPLs in hexane (CdSe/CdS (hex)), the phase
transferred MUA capped CdSe/CdS core/crown NPLs in
aqueous medium (CdSe/CdS (aq)) and the resulting hydrogel
and aerogel (with molar ratio [H2O2] : [Cd2+] = 3.0 : 1) is
shown in figure 4A. From the absorption onset of the hydroꢀ
gels and aerogels, it can be inferred that the CdS band gap is
Wavelength (nm)
Figure 4. (A and B) Comparison of the absorption and emission
spectra of the CdSe/CdS NPLs before and after gelation. Inset of
(B) shows the magnification of the selected region of the spectra.
(C) comparison of the absorption and emission spectra and (D)
PL decay of the CdSe/CdS NPLs in aqueous solution, as thin film
and as an aerogel.
be caused by an interplay of partial dissolution of platelets,
partial loss of ligands, the change in dielectric constant in
combination with tilting, overlaying and broadening of the
nanoplatelets. The broadening might be explained by partial
dissolution, removing parts of the CdSe crystallites, which
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results in slight variations of the thickness of the quantum
Page 8 of 13
Hydrogel
Aerogel
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wells.
2+
[H O ] : [Cd ]
A
B
2
2
3.0 : 1
3.7 : 1
4.3 : 1
4.9 : 1
At this point it is interesting to compare the optical properties
of the highly porous selfꢀsupported network of the same NPLs
with those of denser packed NPLs. In order to do so, the optiꢀ
cal properties of a NPL film with similar optical density as the
aerogel was investigated (1.1 and 1.2 at 300 nm, respectively,
measured in scattering free absorption mode at similar areas
exposed to the beam). The film was prepared by drying the
CdSe/CdS (aq) NPLs solution on a glass slide. The spectra are
shown in figure 4C and D. Here, we observe that the shape of
the absorption and the emission spectra of the film and of the
aqueous solution are very similar. In case of aerogel the CdS
absorption onset appears in the same wavelength regime
(compared to the film and the solution), while the CdSe abꢀ
sorption band is broadened. Compared to the film, the emisꢀ
sion spectrum of the aerogel was also found to be broadened.
However, it should be noticed that the PLQY of the film is
only 5.5 % with photoluminescence lifetime (τ) of 3.6 ns
whereas, the aqueous solution has 18 % (τ = 11 ns) and the
aerogel has the PLQY of 10.3 % (τ = 9.5 ns). This short lifeꢀ
time and low PLQY of the film is most likely due to the dense
packing of the NPLs which increases the occurrence of variꢀ
ous nonradiative recombination pathways in the system. Nevꢀ
ertheless, the aerogels from the core/crown NPLs, as shown in
the manuscript, have higher PLQY (10.3 % for the lowest
amount of H2O2 used) as well as longer PLꢀdecay times comꢀ
pared to the densely packed film. We attribute this to the highꢀ
ly porous morphology, where the NPLs are loosely packed
(compared to the film) and the interactions with the neighborꢀ
ing NPLs are limited.
0.0
0.0
400
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Wavelength (nm)
Wavelength (nm)
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650 675 700 725 750
650 675 700 725 750
0.0
0.0
500 550 600 650 700 750
Wavelength (nm)
500 550 600 650 700 750
Wavelength (nm)
104
103
102
104
F
E
103
102
0
10 20 30 40 50
Time (ns)
0
10 20 30 40 50
Time (ns)
Figure 5. Optical characterization of the hydrogels and the correꢀ
sponding aerogels with four different [H2O2 ] : [Cd2+] molar ratios
of 3.0 : 1, 3.7 : 1, 4.3 : 1, and 4.9 : 1 (black, red, blue, and green
line, respectively). (A and B) and (C and D) represent UVꢀVis
absorption and emission spectra of the corresponding hydrogels
and aerogels, respectively. Insets of (C and D) represent magniꢀ
fied spectra of the selected region. (E and F) represents the PL
decay curves of the hydrogels/ aerogels.
In order to investigate the impact of the H2O2 amount used
during gelation on the optical properties of the resulting hyꢀ
drogels and the corresponding aerogels, we have compared the
spectra of the samples with four different [H2O2] : [Cd2+]
molar ratios, namely 3.0 : 1, 3.7 : 1, 4.3 : 1, and 4.9 : 1 (figure
5, black, red, blue and green line, respectively). It can be
clearly derived that the UVꢀVis absorption spectra have sharp
absorption onsets and have contribution from both CdSe and
CdS with band gaps considerably larger than the correspondꢀ
ing bulk semiconductors. In case of the emission spectra it
should be noticed that there is a small shift of the emission
maximum towards the shorter wavelength region with increasꢀ
ing H2O2 amount. This can partly be explained by the fact that
larger amounts of H2O2 can oxidize more thiol ligands (MUA)
from the surface of the CdSe/CdS (aq) NPLs leading to a
decrease of the exciton probability function in the thickness
direction. Furthermore simultaneously the dissolution of the
CdSe domain occurs. This occurrence of a parallel reaction
effecting the CdSe domains is proven by employing a higher
molar ratio of [H2O2] : [Cd2+] = 17.4 : 1. From the TEM imagꢀ
es of the corresponding gel shown in figure 6A and B it can be
derived that the CdSe is nearly completely vanished from the
centre of the NPLs leaving holes inside the residual CdS NPL
rings. Hence it is likely that also with the small amounts of
H2O2 which we employ in our fluorescent quantum well gelaꢀ
tion, we can expect that partial local thinning (not the comꢀ
plete monolayer) of the CdSe platelets occurs, which leads to a
slightly reduced (and inhomogeneous) thickness. This results
in the shift of the emission peak to shorter wavelengths. In
addition, it should be noted that with higher H2O2 concentraꢀ
tion, the trap state emission increases, which is in agreement
to an increased degree of ligand oxidation and the creation of
e.g. dangling bonds (imperfect saturation of surface). The
lifetime of the (presumably) trap state emission was found to
be in the order of tens of microseconds, which is in agreement
with previous observations for trap state emission of assemꢀ
bled CdSe nanocrystals (see figure SIꢀ6).47
Figure 6. (A and B) TEM micrographs of CdSe/CdS aerogel in
two different magnifications with [H2O2] : [Cd+2] = 17.4 : 1.
Higher amount of H2O2 causes etching of CdSe and produces
holes inside the rectangular NPLs. Schematic models inside the
figures are drawn to guide the eye.
The measured PLQYs and the corresponding PL lifetimes for
both hydrogels and aerogels are shown in figure 7A and B.
The QY (in case of monoexponential PL decay) can be exꢀ
pressed as the ratio of the rate constants of the radiative and
nonradiative processes as QY = krad / (krad + ∑knonꢀrad) where
krad and knonꢀrad being radiative and nonꢀradiative rate constants,
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respectively. The PLQY of the CdSe/CdS NPLs in aqueous
solution is 18 % with corresponding PL lifetime of 11 ns.
analyzed the porosity and the specific surface area of the aeroꢀ
1
2
3
4
5
6
7
8
gels obtained from the CdSe NPLs and the CdSe/CdS
core/crown NPLs using nitrogen physisorption experiments.
The measured isotherms are shown in figure 8 and analyzed
by the BrunauerꢀEmmettꢀTeller (BET) method. The isotherms
for N2 gas adsorption and desorption mainly show a multilayer
adsorption behavior and can be designated with mixed IUPAC
type II and type IV behavior. The multipoint BET measureꢀ
ments show very large specific surface areas and high porosiꢀ
ties. The aerogels have BET surface areas of 219 m2 gꢀ1 and
189 m2 gꢀ1 for the CdSe/CdS NPLs and CdSe NPLs, respecꢀ
tively. These values are comparable with that of previously
reported metal chalcogenide aerogels (typically in the range of
90 to 240 m2 gꢀ1).20ꢀ22 However, care must be taken when comꢀ
paring massꢀspecific surface areas of materials of different
composition. As noted earlier49, such comparisons are biased
by the influence of the molar mass of the compounds. For
example, a typical silica aerogel with a massꢀspecific surface
area of 500 m2 gꢀ1 has a moleꢀspecific surface area of 30.000
m2 molꢀ1 for comparison, a CdSe aerogel with a massꢀspecific
surface area of 200 m2 gꢀ1 has a moleꢀspecific surface area of
29.000 m2 molꢀ1.
A
12
B
13
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11
10
9
12
10
8
12
11
10
9
hydrogel
aerogel
PLQY
Lifetime
PLQY
Lifetime
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3.0 3.5 4.0 4.5 5.0
3.0 3.5 4.0 4.5 5.0
2+
2+
Molarratio[HO] : [Cd ]
Molarratio[HO] : [Cd ]
2
2
2
2
Figure 7. (A and B) PLQY and fluorescence lifetime of CdSe/CdS
hydrogels and aerogels with variable molar ratios of
[H2O2] : [Cd2+].
It should be noticed from figure 7A, that the PLQY as well as
the measured fluorescence lifetime of the hydrogels decreases
with increasing H2O2 amount. Similar trend was also observed
for the corresponding aerogels (figure 7B). This decrease in
PLQY is most likely due to dissolution of the CdSe with highꢀ
er amounts of hydrogen peroxide and due to the presence of
less and less ligands on the surface, both effects resulting in
the creation of unsaturated surface states and hence nonꢀ
radiative recombination pathways (or lowerꢀenergetic trap
state emission).26 The decay curves can be reasonably fitted
with a monoꢀexponential decay, measured with the lifetime
τmeasured = 1 / (krad + ∑knonꢀrad). The extent of decrease in lifeꢀ
time of the exciton is quite consistent with the decrease in
PLQY. Hence, we can conclude that the system undergoes a
homogeneous quenching mechanism: which means that all
individual fluorescent NPLs of a given ensemble undergo
similar quenching upon H2O2 addition (for a brief explanation
of the terms homogeneous and inhomogeneous quenching,
please see Supporting Information).
This comparison shows that the semiconductor NPL aerogels
described here has equally high specific surfaces as in classiꢀ
cal oxide aerogels. It should be noted that the maximum surꢀ
face area achievable (from rough geometrical estimations)
with this system is ~ 290 m2·gꢀ1 (considering all the NPLs are
present separately). Hence, the specific surface area reported
here is around 70 % of the maximum possible value. That
means roughly 30 % of the surface is covered by overlaying of
the NPLs surface to form 3D network. We have also evaluated
the pore size distributions from the desorption branch of the
isotherms, applying BJH (BarrettꢀJoynerꢀHalenda) theory.42
The broad pore size distributions obtained, ranging from mesꢀ
opores to macropores, are consistent with a highly porous
disordered gel structure, as revealed by the SEM and TEM
microscopic analysis. The cumulative pore volumes V3.0ꢀ245
(lower subscript denotes the pore diameter) calculated for the
CdSe/CdS core/crown NPL and CdSe NPL aerogels (at relaꢀ
tive pressure P/P0 = 0.99) using BJH theory are 2.27 cm3·gꢀ1
and 1.65 cm3·gꢀ1, respectively. The total pore volume Vt can be
calculated using the equation Vt = (1/ρa) – (1/ρb), where Vt is
the total pore volume and ρa and ρb represent the density of
aerogel and bulk solid material.50 The total pore volume for
the CdSe/CdS and for the CdSe aerogel are 26.14 cm3gꢀ1 and
27.60 cm3gꢀ1, respectively. The average pore diameters estiꢀ
mated using the equation 4 x Vt/σ (σ represents the calculated
BET specific surface area of the aerogel) are 477 nm and
584 nm for CdSe/CdS and CdSe aerogel, respectively.50, 51 So,
the average pore diameters calculated using the equation
Vt = (1/ρa) – (1/ρb) are significantly higher than the average
diameters as they were determined from the nitrogen adsorpꢀ
tion data. This fact is an indication that the porosity results
predominantly from the macropores, which is in accordance
with the measured isotherms.
In a recent publication of our group, exceptionally long radiaꢀ
tive lifetime (56 ns with a PLQY of 29 %) of aerogels from
CdSe seeded CdS nanorods have been observed.26 In that case,
according to our interpretations, tipꢀtoꢀtip connections of the
CdSe/CdS nanorod crystallites open up the possibility for the
excited electrons to travel to the connected nanoparticles while
the holes are confined in the CdSe leading to a delay in the
recombination of the exciton and hence longer radiative lifeꢀ
time. Instead, in the present article, we do not observe that
kind of tipꢀtoꢀtip connections; rather here the NPLs are ranꢀ
domly attached with their neighbors. We assume that the
connections of the NPLs are not through crystal points and
together with that bending of the NPLs reduces the probability
of the electron delocalization. In addition, an overall increase
in the nonradiative recombination pathways results in shorter
photoluminescence lifetime. However, also this work shows
similarly to the aboveꢀmentioned article that the PLQY can be
high in aerogels; in the present case it is e.g. approximately
twice higher than in the denser packed films of similar optical
densities.
The crystallographic characterizations of the CdSe/CdS aeroꢀ
gel and the NPLs in solution (before and after phase transfer)
were investigated using powder xꢀray diffractometry (XRD)
and are shown in the Supporting Information.
Aerogels with very high porosity and large specific surface
area are highly promising for catalytic as well as for sensor
applications, as the reactant molecules or the analyte can easiꢀ
ly diffuse through the pores with minimum hindrance. We
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adsorption
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Pore diameter (nm)
Figure 8. Nitrogen physisorption isotherms for (A) CdSe/CdS core/crown NPL and (B) CdSe NPL aerogels, as well as their pore size
distributions derived from these data. (C) and (D) BJH cumulative pore volume distribution of the CdSe/CdS aerogel and of the CdSe
aerogel.
core/crown aerogel, PL decay analysis of the surface trap states,
photograph of the film prepared by drying CdSe/CdS NPLs on a
glass slide, Xꢀray diffraction pattern of the CdSe/CdS (aq) NPLs
and of the hydrogels and aerogels obtained from CdSe/CdS NPLs.
This material is available free of charge via the Internet at
http : //pubs.acs.org.
4. Conclusions
In conclusion, we have successfully developed a method to
prepare highly porous aerogels from CdSe NPLs and
CdSe/CdS core/crown NPLs with high PLQY up to 10.3 %.
This is the first report on aerogels from colloidal quantum well
building blocks. Notably, the aerogels of the NPLs have solely
(111) as the exposed facet in the aerogel, which might be of
high interest for facet selective catalysis or sensing. The aeroꢀ
gels from the NPLs partially retain the quantum confinement
properties of the building blocks and have very large inner
specific surface area of 219 m2/g which is close to the highest
values reported so far for nanocrystalꢀbased aerogels. Furꢀ
thermore, the highly porous character would allow molecules
to pass through with minimum hindrance, making them promꢀ
ising candidates e.g. detecting chemical agents.
AUTHOR INFORMATION
Corresponding Author
*Dr. Nadja C. Bigall, Institute of Physical Chemistry and Electroꢀ
chemistry, Leibniz University Hannover, Callinstr. 3A, Dꢀ30167
Hannover, Germany, nadja.bigall@pci.uniꢀhannover.de.
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the manuꢀ
script.
ASSOCIATED CONTENT
Funding Sources
Supporting Information. TEM micrographs of the 5 ML
CdSe/CdS core/crown NPLs before and after phase transfer,
optical characterizations (UVꢀVis absorbance, emission and PL
decay) of the CdSe and CdSe/CdS core/crown NPLs in organic
and in aqueous medium, , SEM micrographs of the aerogels from
CdSe (aq) NPLs, SEM micrographs of the xerogel from
CdSe/CdS core/crown NPLs, HRTEM of the aerogel from
CdSe/CdS core/crown NPLs, XPS spectra of the CdSe/CdS
German Federal Ministry of Education and Research (BMBF)
support code 03X5525.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
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Stimulated Emission Using Colloidal Quantum Wells. Nano Lett.
2014, 14, 2772ꢀ2777.
N.B., as well as S.N, J.F.M., and S.S.ꢀP. are grateful for financial
1
2
3
4
5
6
7
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support from the German Federal Ministry of Education and
Research (BMBF) within the framework of the program Nanoꢀ
MatFutur, support code 03X5525. We like to acknowledge D.
Dorfs for discussions.
17.
Lorenzon, M.; Christodoulou, S.; Vaccaro, G.; Pedrini, J.;
Meinardi, F.; Moreels, I.; Brovelli, S., Reversed oxygen sensing using
colloidal quantum wells towards highly emissive photoresponsive
varnishes. Nat. Commun. 2015, 6, 6434.
18.
Grim, J. Q.; Christodoulou, S.; Di Stasio, F.; Krahne, R.;
ABBREVIATIONS
Cingolani, R.; Manna, L.; Moreels, I., Continuousꢀwave biexciton
lasing at room temperature using solutionꢀprocessed quantum wells.
Nat. Nanotechnol. 2014, 9, 891ꢀ895.
NPL, nanoplatelet; MUA, 11ꢀmercaptoundecanoic acid; PLQY,
photoluminescence quantum yield; TEM, transmission electron
microscopy; SEM, scanning electron microscopy; BET, Brunauꢀ
erꢀEmmettꢀTeller; BJH, BarrettꢀJoynerꢀHalenda; XRD, xꢀray
diffractometry; FWHM, full width at half maximum; XPS, Xꢀray
photoelectron spectroscopy.
19.
Mohanan, J. L.; Brock, S. L., A new addition to the aerogel
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community: unsupported US aerogels with tunable optical properties.
J. NonꢀCryst. Solids 2004, 350, 1ꢀ8.
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Arachchige, I. U.; Brock, S. L., Solꢀgel assembly of CdSe
nanoparticles to form porous aerogel networks. J. Am. Chem. Soc.
2006, 128, 7964ꢀ7971.
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