Incorporation of a highly luminescent semiconductor quantum dot in

ARTICLE
Chemistry School, University of Melbourne, Parkville, VIC, 3010, Australia
INFM, Dip. di Fisica, Università di Padova, via Marzolo 8, I35131 Padova, Italy
c
CNR-IFN Istituto di Fotonica e Nanotecnologie, CSMFO group, via Sommarive 14,
I-38050 Povo (Trento), Italy
d
INSTM, Dip. Ingegneria Meccanica S. Materiali, Università di Padova, via Marzolo 9,
I35131 Padova, Italy. E-mail: [email protected]; Fax: 0039 049 8275505;
Tel: 0039 049 8275506
Journal of
a
Materials
Chemistry
Craig Bullen,a Paul Mulvaney,a Cinzia Sada,b Maurizio Ferrari,c Alessandro Chiaserac
and Alessandro Martucci*d
www.rsc.org/materials
Incorporation of a highly luminescent semiconductor quantum
dot in ZrO2–SiO2 hybrid sol–gel glass film
b
Received 26th November 2003, Accepted 7th January 2004
First published as an Advance Article on the web 17th February 2004
In this paper we describe a method for transferring semiconductor quantum dots, produced in non-polar
solvents by an organometallic approach, into sol–gel matrices. ZrO2–SiO2 hybrid sol–gel glass films have been
homogeneously doped with different semiconductor quantum dots (CdSe, CdSe@CdS and CdSe@ZnS). Both
the absorption and the emission properties of the semiconductor nanocrystals are only slightly affected by the
incorporation into the sol–gel matrix. The doped films showed sufficiently high refractive index for the
realization of planar waveguides.
Introduction
DOI: 10.1039/b315332k
In the last 20 years different chemical approaches have been
developed to synthesize semiconductor quantum dots (QDs)
of controlled composition, size, shape and surface states and
extensive reviews have been published elsewhere.1,2 One of
the most studied chemical syntheses of semiconductor QDs
involves the decomposition of organometallic precursors in hot
coordinating solvents.3 With this approach, highly luminescent
semiconductor nanoparticles can be prepared with controlled
size, size distribution and surface states. Such nanocrystals
have many potential applications in photonics and electronics
components4 but it is necessary to embed the nanocrystals into
a solid matrix which allows optical information or signals to be
transmitted with high propagation efficiency.
Several approaches have been developed for producing bulk
or thin film materials doped with semiconductor QDs: melt
glasses,5,6 polymers7,8 and sol–gel glasses9 have all been doped
with different semiconductor nanocrystals. In all these cases,
the nanoparticles precipitate or grow directly inside the structured media, which may offer micro(nano)-reaction chambers.10
However in situ growth does not allow for control of surface
states, particle shape or aspect ratio, or passivation of the
nanocrystal dangling bonds by shell layers. Few studies have
been reported on the incorporation of semiconductor QDs
obtained by colloidal synthesis into a structured media.4,11–14
In this paper we describe a method for transferring highly
luminescent semiconductor QDs produced in non-polar
solvents by organometallic approaches into ZrO2–SiO2
hybrid sol–gel glass films. We demonstrate using direct
measurement of the m-line that these materials are suitable
for optical waveguides.
1112
Experimental methods
The incorporation of nanoparticles in sol–gel film involved
three stages:
Step 1. Highly luminescent and monodisperse core (e.g.
CdSe) or core-shell (CdSe@CdS or CdSe@ZnS) nanoparticles
J. Mater. Chem., 2004, 14, 1112–1116
were prepared by the reaction of organometallic reagents in
organic surfactants.15 In a typical CdSe core synthesis, an
injection solution of 0.1 mL dimethylcadmium (1.4 mmol), and
0.079 g selenium (1 mmol) dissolved in (5 mL) trioctylphosphine was rapidly injected into the reaction vessel containing
vigorously stirred trioctylphosphine oxide heated to 355 uC.
Subsequent growth at 280 uC led to a slow growth of nanocrystals to the desired size. CdS or ZnS shells were grown on
the core nanocrystals in organic surfactants using published
methods based on the organometallic precursors dimethylcadmium or diethylzinc in combination with bis(trimethylsilyl)
sulfide.16,17
Step 2. The nanoparticles were capped with aminoethylaminopropyltrimethoxysilane (AEAPTMS) allowing their dissolution in polar solvents (such as propanol or ethanol).
Typically 1 mL of 1.9 mM QDs hexane solution is mixed with
3 mL of ethanol allowing precipitation of the nanoparticles.
After centrifugation and separation, 20 mL of AEAPTMS are
added to the precipitated nanocrystals allowing their dissolution in 1 mL of propanol (doping solution).
Step 3. The capped particles were mixed with the sol–gel
solution leading to a final sol that can be used for depositing
thin films on different substrates (SiO2 glass, soda-lime glass,
silicon, plastic) by spinning or dipping. The matrix solution was
synthesized by reacting 3-(trimethoxysilyl)propylmethacrylate
(TMSPM), methacrylic acid (MA) and zirconium n-propoxide
(Zr(OPrn)4), using a procedure similar to that reported in
ref. 18. The TMSPM was first prehydrolyzed at room temperature for one hour (TMSPM : H2O : HCl ~ 10 : 7.6 : 1.4).
MA and Zr(OPrn)4 (MA : Zr ~ 10 : 10) were mixed for one
hour at room temperature and then added to the TMSPM sol.
Further water was added (TMSPM : H2O ~ 10 : 10.4) and the
sol was stirred for half an hour at room temperature. The
deposition solution was typically prepared by mixing 1 mL of
the matrix solution with 1 mL of the doping solution. After
deposition the films were dried at different temperatures up to
150 uC.
Linear absorption spectra in the UV-visible region (300–
800 nm) were taken at room temperature using a Cary 5 UV-vis
This journal is ß The Royal Society of Chemistry 2004
spectrophotometer. Photoluminescence measurements were
carried out using a Hitachi spectrofluorometer equipped with
a 150 W xenon lamp.
Transmission Electron Microscope (TEM) characterization
was conducted at 200 kV. Sol–gel doped films were deposited
on holey-carbon copper grids by spin coating.
SIMS measurements were carried out by means of an IMS 4f
mass spectrometer (Cameca, Padova, Italy) using a 10 kV Cs1
primary beam and by negative secondary ion detection (the
sample potential was fixed at 24.5 kV) with a final impact
energy of 14.5 keV. The SIMS spectra were carried out in ultra
high vacuum conditions at a primary beam intensity of 50 nA,
rastering over a nominally 125 6 125 mm2 area. Beam blanking
mode was used to improve the depth resolution, interrupting
the sputtering process during magnet stabilization periods. The
dependence of the erosion speed on the matrix composition was
taken into account by measuring the erosion speed at various
depths for each sample. The erosion speed was then evaluated
by measuring the depth of the erosion crater at the end of each
analysis by means of a Tencor Alpha Step profilometer with a
maximum uncertainty of a few nanometers. The measurements
were performed in High Mass Resolution configuration to avoid
mass interference artifacts. The charge build-up while profiling
the insulating samples was compensated by an electron gun
without any need to cover the surface with a metal film.
The waveguide properties of the films and their refractive
index at 632.8 and at 543.5 nm were measured in TE and TM
polarization by an m-line apparatus based on the prism
coupling technique.
Fig. 2 Absorption and photoluminescence (excitation at 400 nm) of
sol–gel film doped with CdSe core particles deposited on silica and
heated at 100 uC.
In Fig. 1 are reported the absorption (Fig. 1a) and the photoluminescence (excitation at 400 nm) spectra (Fig. 1b) of three
solutions: the CdSe core particles in hexane, the propanol
solution of the CdSe functionalized with AEAPTMS and the
sol–gel doped solution used for film deposition. Both the
absorption exciton peaks and the PL peak are slightly shifted to
lower wavelengths when the QDs are transferred from the
hexane solution to the propanol and sol–gel solutions. The sol–
gel solution gives the highest shift. No appreciable variation in
either absorption or emission peak position has been observed
for the doped, sol–gel solution after deposition, as can be seen
from Fig. 2 where the absorption and PL spectra of sol–gel
films doped with CdSe core are reported.
The same features have been observed also for CdSe@CdS
core-shell particles as can been seen in Fig. 3 where the absorption (Fig. 3a) and the photoluminescence (Fig. 3b) spectra of
three solutions: the CdSe@CdS core-shell in hexane, the
Fig. 1 Absorption (a) and photoluminescence (b) (excitation at
400 nm) spectra of CdSe core solutions.
Fig. 3 Absorption (a) and photoluminescence (b) (excitation at
400 nm) spectra of CdSe@CdS core-shell solutions.
Results
J. Mater. Chem., 2004, 14, 1112–1116
1113
Fig. 4 Absorption and photoluminescence (excitation at 400 nm) of
sol–gel glass film doped with CdSe@CdS core-shell particles deposited
on silica and heated at 100 uC.
propanol solution of the CdSe@CdS functionalized with
AEAPTMS and the sol–gel doped solution are reported.
Also in this case both the absorption exciton peaks and the PL
peak are slightly shifted to lower wavelengths when the QDs
are transferred from the hexane solution to the propanol and
sol–gel solution. The sol–gel solution gives the highest shift. In
Fig. 4 are reported the absorption and PL spectra of sol–gel
films doped with CdSe@CdS core-shell particles. Again,
neither the absorption or PL peak positions shifted with
respect to those of the parent sol–gel doped solution (Fig. 3).
The values reported in Table 1 have been obtained by fitting
the spectra with a Gaussian function.
Transparent doped films have been deposited on different
substrates and heated up to 150 uC. The coating thickness,
measured with a profilometer on a step made by scratching the
film after deposition, was between 0.1 and 7 mm, depending on
deposition technique (spinning or dipping), rotation or withdrawal speed, and sample heat treatments. No significant
differences in adhesion and thickness were found among the
different substrates.
Fig. 5 shows two pictures of a sol–gel film doped with
CdSe@CdS and CdSe@ZnS core-shell particles under UV
illumination. All the doped films (both core and core-shell
doped) showed very bright emission with colors tunable
through the particle size.
A TEM micrograph of a doped sol–gel film is shown in
Fig. 6. Homogeneously dispersed nanometer sized particles are
clearly recognizable.
The SIMS profiles (Fig. 7) of films doped with CdSe and
CdSe@CdS particles confirmed that the distribution of all the
elements (Cd, Se, S, Se, Si, Zr, O, C, H) was constant
throughout the film depth.
The waveguide properties of the doped sol–gel films have
been tested by m-line spectroscopy measurements. Using a
ratio SiO2/ZrO2 ~ 2.5 the sol–gel doped film showed a
refractive index of 1.5224 at 632.8 nm, while a tunable higher
refractive index could be obtained by increasing the ZrO2
Fig. 5 Sol–gel glass film doped with CdSe@CdS (top) and
CdSe@ZnS (bottom) core-shell particles deposited on silica substrate
(2.5 6 2.5 cm) under UV illumination.
content. In Fig. 8 are reported the m-lines of 1 mm thick sol–gel
films doped with CdSe@ZnS core-shell particles. The absorption and emission spectra are shown in the inset. At 632.8 nm
the waveguide supports two modes (for both TE and TM
polarization) but the coupled light only propagates for a few
mm due to reabsorption.
Discussion
Since QDs synthesized via organometallic thermolysis are
single crystals, there is no need to anneal the final matrix at the
high temperatures utilized in NC doped glasses. In this work,
we wanted to be able to prepare QD doped matrices that have
tunable compositions and refractive indices. Hybrid organic–
inorganic sol–gel glass matrices were chosen as hosts for the
QD. Trimethoxysilylpropylmethacrylate and zirconium(IV)propoxide were used to tailor the refractive index of the host
material. This particular class of sol–gel zirconia-based glasses
allows the realization of waveguides19 and single layer films
Table 1 Absorption (ABS) and photoluminescence (PL) peak positions (nm) of particles dispersed in hexane, in propanol (after surface
functionalization with AEAPTMS), in sol–gel solution and in sol–gel films. The estimated21 diameter D (nm) is also reported.
CdSe
Hexane
Propanol
Sol–gel
Film
1114
CdSe@CdS
CdSe@ZnS
ABS
D
PL
ABS
D
PL
ABS
D
PL
500
497
496
497
2.34
2.32
2.31
2.32
516
513
512
512
514
506
498
498
2.49
2.40
2.33
2.33
527
519
513
512
615
616
616
616
5.32
5.38
5.38
5.38
619
619
620
619
J. Mater. Chem., 2004, 14, 1112–1116
Fig. 8 m-line at 632.8 nm with TE and TM polarization. Inset:
absorption and photoluminescence (excitation at 400 nm) spectra of
CdSe@ZnS QDs.
Fig. 6 TEM micrograph of CdSe@ZnS doped sol–gel glass film
heated at 100 uC for 1 hour.
Fig. 7 SIMS profiles of sol–gel glass film doped with CdSe (top) and
CdSe@CdS (bottom) particles heated at 100 uC for 1 hour.
with thickness up to 10 mm can be easily obtained.19 The
organic part of the glass confers good flexibility on the film and
the films can be deposited also onto plastic substrates that can
be bent; moreover the presence of CLC bonds within the matrix
allow UV patterning of the final doped sol–gel films.19
Among the different amines that can be used to facilitate
phase transfer into ethanol or propanol, we chose AEAPTMS
because it is a bifunctional ligand bearing hydrolysable siloxy
groups, which can undergo hydrolysis and condensation like
the silyl group of TMSPM sol–gel matrix precursors.
The surface functionalization with AEAPTMS of both core
and core-shell particles allows the transfer of nanoparticles into
propanol and only slightly affects the optical properties of the
QDs. A shift to shorter wavelength of between 2 and 8 nm has
been observed for both the absorption and emission peaks
(Figs. 1 and 3 and Table 1), while this shift is much smaller
when the QD propanol solution is mixed with the sol–gel
solution. For all the investigated samples no appreciable
variation in the peak positions have been observed between the
sol–gel doped solution and the doped films.
If it is assumed that the blue-shift is due to a decrease in QD
size, then from the values reported in Table 1, the variations are
always less than 0.1 nm, which represents less than one monolayer. For this reason we think that such variations are due only
to changes related to the surface functionalization with amines,
and not to a real variation of particle size. After passivation of
the surface with AEAPTMS subsequent peak position changes
are almost absent. Shifts in absorption and PL peak positions
caused by interactions between the CdSe surface and ligands
have already been observed by Balogh et al.20
CdSe@ZnS core-shell particles did not show any variation in
peak position (see Table I). We think this is mainly due to the
improved passivation offered by the ZnS shell. This assumption could also explain a higher stability of the luminescence
properties found in film doped with CdSe@ZnS core-shell
particles. In fact while the luminescence of films doped with
CdSe or CdSe@CdS particles lasted for no more than a few
days, CdSe@ZnS core-shell particle doped films showed the
same bright luminescence after 1 year of ageing at room
temperature in air. A decay of the PL intensity with time of
CdSe@ZnS core-shell nanocrystals embedded in solid matrices
has observed by other groups and suggests that long term
stability is related to the quality and homogeneity of the shell
layer. Patchy coatings do not offer low term protection against
chemical degradation by acidic components in the matrix or
oxidizing agents present.13
TEM pictures of the sol–gel doped films (Fig. 6) showed
homogeneously dispersed, spherical particles. Because the
SIMS profiles showed that the distribution of all the elements
is constant throughout the film, this suggests that the particles
are homogeneously dispersed throughout the film and that
there is no segregation of particles or clustering within the
matrix.
The doped films showed also good waveguide properties.
The propagation and emission in waveguides depend strongly
on the absorption and emission properties of the embedded
particles. For example, films doped with CdSe@ZnS particles,
J. Mater. Chem., 2004, 14, 1112–1116
1115
having an exciton absorption peak at 616 nm, exhibit no
propagation modes at 543.5 nm, due to the very high
re-absorption, while a short stripe of red light can be seen at
632.8 nm, probably indicating propagation of the emitted light.
At 632.8 nm the waveguides can support modes even if the
particles still slightly absorb.
4
5
6
7
8
Conclusions
Highly luminescent CdSe, CdSe@CdS and CdSe@ZnS
colloidal nanoparticles have been successfully transferred
into sol–gel ZrO2–SiO2 hybrid, sol–gel glass films. The reported
procedure allows dispersal of the QDs homogenously in the
film without affecting their emission properties. The refractive
index of the doped film can be adjusted to realize planar
waveguides.
9
10
11
12
13
14
Acknowledgements
16
References
18
1 P. Alivisatos, P. F. Barbara, A. W. Castleman, J. Chang,
D. A. Dixon, M. L. Klein, G. L. McLendon, J. S. Miller,
M. A. Ratner, P. J. Rossky, S. I. Stupp and M. E. Thompson, Adv.
Mater., 1998, 10, 1297.
2 T. Trindade, P. O’Brien and N. L. Pickett, Chem. Mater., 2001, 13,
3843.
3 C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. Soc.,
1993, 115, 8706.
1116
15
This research was performed in the framework of the bilateral
academic staff exchange program between Padova and
Melbourne University.
J. Mater. Chem., 2004, 14, 1112–1116
17
19
20
21
S. V. Kershaw, M. T. Harrison and M. G. Burt, Philos. Trans.
R. Soc. London, Ser. A, 2003, 361, 331.
L. Liu and S. H. Risbud, J. Appl. Phys., 1990, 68, 28.
P. A. M. Rodrigues, G. Tamulaitis, P. Y. Yu and S. H. Risbud,
Solid State Commun., 1995, 94, 583.
M. Y. Gao, Y. Yang, B. Yang, F. L. Bian and J. C. Shen, Chem.
Commun., 1994, 2779.
S. W. Haggata, X. Li, D. J. Cole-Hamilton and J. R. Fryer,
J. Mater. Chem., 1996, 6, 1771.
M. Guglielmi, SPIE Crit. Rev. Proc., 1997, CR68, 25.
D. Crouch, S. Norager, P. O’Brien, J. Park and N. Pickett, Philos.
Trans. R. Soc. London, Ser. A, 2003, 361, 297.
T. Rajh, M. I. Vucemilovic, N. M. Dimitrijevic, O. I. Micic and
A. J. Nozik, Chem. Phys. Lett., 1988, 143, 305.
S. T. Selvan, C. Bullen, M. Ashokkumar and P. Mulvaney, Adv.
Mater., 2001, 13, 985.
J. Rodriguez-Viejo, H. Mattoussi, J. R. Heine, M. K. Kuno,
J. Michel, M. G. Bawendi and K. F. Jensen, J. Appl. Phys., 2000,
87, 8526.
V. C. Sundar, H.-J. Eisler and M. G. Bawendi, Adv. Mater., 2002,
14, 739.
C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. Soc.,
1993, 115, 8706.
M. A. Hines and P. Guyout-Sionnest, J. Phys. Chem., 1996, 100,
468.
X. Peng, M. G. Schlamp, A. V. Kadavanich and A. P. Alivisatos,
J. Am. Chem. Soc., 1997, 119, 7019.
R. Nass, H. Schmidt and E. Arpac, Proc. SPIE: Sol-Gel Optics,
1990, 1328, 258.
H. Schmidt, in: Sol-gel optics. Processing and applications, ed.
L.C. Klein, Kluwer Academic Publishers, 1994, p. 451.
L. Balogh, C. Zhang, S. O’Brien, N. J. Turro and L. Brus, Chim.
Oggi, 2002, 45.
W. W. Yu, L. Qu, W. Guo and X. Peng, Chem. Mater., 2003, 15,
2854.

Download Report