Identi fication of Optical Transitions in Colloidal CdSe Nanotetrapods Nguyen Xuan Nghia,*, † Le Ba Hai,† Nguyen Thi Luyen,‡ Pham Thu Nga,† Nguyen Thi Thuy Lieu,§ and The-Long Phan∥ †Inst
Trang 1Identi fication of Optical Transitions in Colloidal CdSe Nanotetrapods Nguyen Xuan Nghia,*, † Le Ba Hai,† Nguyen Thi Luyen,‡ Pham Thu Nga,† Nguyen Thi Thuy Lieu,§ and The-Long Phan∥
†Institute of Materials Science, Vietnam Academy of Science and Technology, Cau Giay, Hanoi, Vietnam
‡University of Engineering and Technology, Hanoi National University, Cau Giay, Hanoi, Vietnam
§Posts and Telecommunications Institute of Technology, Vietnam Post and Telecommunication Group, Thanh Xuan, Hanoi, Vietnam
∥BK-21 Physics Program and Department of Physics, Chungbuk National University, Cheongju 361-763, Korea
*S Supporting Information
ABSTRACT: Though many different results associated with the optical properties
of semiconductor nanotetrapods were reported, the relation between the spectral
characteristics and change in energy offsets across core/arm interfaces of tetrapods
has not been clarified yet Particularly, the origin of an emission peaked at the
high-energy region is still an issue of debate To get more insight into these topical
problems, we have studied systematically the optical properties of CdSe tetrapods
synthesized with various precursor concentrations Absorption and emission
transitions in CdSe tetrapods were identified by means of their spectroscopic
characteristics We have identified the high-energy emission peak originating from
spatially direct recombination of photogenerated carriers, which are located in the
core of tetrapods The relative-intensity increase of this emission is mainly related to
an increase in the potential barrier for electrons and to a decrease in the potential
barrier for holes The differences in spectroscopic characteristics of CdSe tetrapods
have also been further discussed in relation to their shape evolution in the one-pot synthesis
In recent years, tetrapod-structured semiconductors have been
of intensive interest because of their potential applications in
lighting sources,1,2 photovoltaic devices,3−7 nanoscale
transis-tors,8electromechanical devices,9probe tips for high-resolution
atomic force microscopy,10optical strain gauge,11and so forth
Remarkable advances in synthetic methods have enabled the
fabrication of both tetrapod-shaped homostructures12−19 and
heterostructures.1,19−21
Geometrically, a tetrapod consists of four rod-like wurtzite
arms grown from the {111} facets of the dot-like zinc blende
core.12,13,16The optical properties of this branched
nanostruc-ture are governed by the band alignment, which is determined
by chemical compositions, crystal structure, size, and con
fine-ment dimensionality of the core and arms.20,22 Earlier works
pointed to differences in the spectroscopic characteristics of
both tetrapod-shaped homostructures and heterostructures A
comparative investigation indicated no qualitative difference
between the optical spectra of CdSe tetrapods and dots,16while
other studies revealed the appearance of a new optical
absorption peak in CdSe tetrapods17or a double-peak structure
in luminescent spectra of CdTe tetrapods.23 In the case of
CdSe(core)/CdS(arm) heterotetrapods, a strong emission peak
due to the radiative recombination of carriers located in the
core was studied by steady-state luminescence spectroscopy.1
Besides this type-I radiative transition, the spatially indirect
emission across core/arm interfaces of a CdSe/CdS tetrapod
also was recorded by the so-called single-particle luminescence spectroscopy.24,25 More recently, the luminescent spectra with
a double-peak structure also have been observed for type-II CdSe/CdTe tetrapods.26It is believed that the elucidation of the nature of optical transitions in tetrapod-shaped nanostruc-tures is thus essential to control effectively their optical properties
For CdTe tetrapods, theoretical calculations based on the envelope-function approximation proved that the emissions peaked at lower and higher energies originated from the spatially indirect transition across core/arm interfaces and spatially direct transition in the arms, respectively.23A similar result also has been reported for CdSe/CdTe heterotetra-pods.26However, the localization of the lowest-energy exciton
in the core region was found by means of the femtosecond time-resolved transient absorption for CdTe tetrapods27and on
a comparative study of temperature-dependent spectroscopic properties for CdSe dots, rods, and tetrapods.28
In fact, the true structure of tetrapods might be different from the structure modeled in theoretical calculations An additional potential barrier could be present at core/arm interfaces of tetrapod-shaped heterostructures because of the interfacial strain caused by the large lattice mismatch of two
Received: May 16, 2012
Revised: October 25, 2012
Published: October 29, 2012
Article
pubs.acs.org/JPCC
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Trang 2material types24,29 or of the formation of a thin-shell layer
around the core before growing the arms.26 In the case of
tetrapod-shaped homostructures, their tetrahedral symmetry
could be broken because of the change in stacking order
between the zinc blende and wurtzite phases at core/arm joint
areas.30 Consequently, the electronic structure of
tetrapod-shaped nanostructures would be changed and is different from
assumptions proposed in the theoretical calculations
The present work focuses on the identification of the nature/
origin of optical transitions in colloidal CdSe tetrapods We
investigated systematically the steady-state absorption and
photoluminescence (PL) properties of two series of CdSe
tetrapods synthesized with the different precursor
concen-trations Here, a continuous change in the spectroscopic
characteristics of CdSe tetrapods in the growth process and an
opposite change trend in degradation process were recorded
The origins of optical transitions in CdSe tetrapods were
suggested based on their size-dependent spectroscopic
characteristics The excitation-power dependence of PL spectra
was investigated to confirm the nature of optical transitions and
elucidate the competition of different radiative transitions in
CdSe tetrapods
Materials Initial materials and chemicals including
cadmium oxide (CdO, 99.99%), selenium powder (Se,
99.999%), oleic acid (OA, 90%), trioctylphosphine (TOP,
97%), and octadecene (ODE, 90%) were purchased from
Aldrich and used as received without further purification
Synthesis of CdSe Tetrapods To investigate the
size-dependent absorption and PL spectra for CdSe tetrapods, we
prepared two sample series (denoted by T1 and T2) by the
colloidal chemical method The synthesis of series T1 can be
found elsewhere.31 A mixture of powdered Se (0.079 g, 1
mmol), TOP (2 mL, 4.48 mmol), and ODE (8 mL, 25 mmol)
was stirred for 60 min (min) at 80 °C in a vessel with a
nitrogenflow After Se powder was completely dissolved, the Se
solution cooled to room temperature was loaded into a syringe
Meanwhile, 0.2568 g (2 mmol) of CdO, 3.8 mL (12 mmol) of
OA, and 46.2 mL (144 mmol) of ODE were put in a three-neck
flask, which was then heated to 300 °C for 180 min, under the
condition of the nitrogenflow, to form an optically clear Cd
solution After that, the Se solution in the syringe was swiftly
injected into the Cd solution when it was cooled to 200 °C
The aliquots of reaction solution containing CdSe tetrapods
were taken from the reaction flask at various durations/times
ranging from 0.5 to 180 min and then quickly cooled to room
temperature Different from series T1, series T2 was
synthesized with initial Cd and Se concentrations decreased
by a factor of 0.3 In this case, the other reaction conditions
were kept the same as those for the preparation of series T1
Synthesis of CdSe Dots A series of CdSe dots (denoted
by D1) were prepared by using a phosphine-free procedure
The Se solution was directly obtained by dissolving Se powder
(0.063 g, 0.8 mmol) in ODE (8 mL, 25 mmol) at 180°C for
300 min, while a mixture of CdO (0.137 g, 1.07 mmol), OA (1
mL, 3 mmol), and ODE (49 mL, 150 mmol) was heated to 280
°C for 180 min to form an optically clear Cd solution The
preparation of Se and Cd solutions was carried out in the
nitrogen-flow conditions The Se solution was swiftly injected
under vigorous stirring The aliquots of the reaction solution
containing CdSe dots were also collected from the reaction
flask at different times ranging from 0.5 to 60 min and then quickly cooled to room temperature
Purification of CdSe Tetrapods and Dots The crude solutions obtained after the preparation of nanocrystals were mixed with isopropanol (according to the ratio of 1:3 in volume) Dot samples were isolated by centrifugation for 3 min
at the speed of 5000−10 000 rpm, depending on the reaction time, while CdSe tetrapods were obtained by the centrifugation for 3 min at lower speeds of 2000−5000 rpm to separate them from rods, bipods, and tripods (if any)
After the fabrication, a part of the obtained products in powder checked the crystal structure by using an X-ray
diffractometer, while the other part was dispersed in toluene for morphology analyses and steady-state spectroscopic measure-ments at room temperature Notably, to minimize the reabsorption, a small amount of CdSe nanocrystals in toluene was used for spectroscopic measurements
Measurements Transmission electron microscopy (TEM) images of CdSe dots and tetrapods were recorded by using a Joel-JEM 1010 microscope, operated at 80 kV The samples were mounted on a carbon-coated cooper-mesh grid X-ray
diffraction (XRD) patterns were obtained from an X-ray
diffractometer (Siemen, D5005), using a Cu Kα radiation source with λ = 1.5406 Å Optical absorption spectra were recorded with a Jasco 670 spectrometer The PL properties were studied by using LABRAM-1B spectrometers (Horiba Jobin-Yvon), where an argon laser with a wavelength of 488 nm and with a maximum power of 30 mW was used In our experiment, the diameter of laser spots on the samples was maintained at about 1 mm
Morphological and Structural Characterizations of CdSe Tetrapods and Dots Typical XRD patterns and TEM images of CdSe dots and tetrapods are shown in Figure 1 The
zinc blende structure of CdSe dots is confirmed by the appearance of diffraction peaks centered at 25.3, 42.1, 49.5, 61.0, and 66.7° corresponding to the Miller indices (111), (220), (311), (400), and (331), respectively Meanwhile, an XRD pattern of CdSe tetrapods indicates the wurtzite structure with diffraction peaks centered at 23.9, 25.5, 27.0, 35.5, 42.1,
Figure 1 XRD patterns of (a) CdSe dots and (b) CdSe tetrapods with the Miller indices showing the zinc blende and wurtzite structures The insets on the right side are TEM images of CdSe tetrapods (top right) and CdSe dots (bottom right) used in XRD studies, and the scale bars correspond to 20 nm.
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(100), (002), (101), (102), (110), (103), (112), and (202),
respectively One can see that it is difficult to identify an
existence of zinc blende cores if only basing on the XRD
patterns of CdSe tetrapods because of the similarity of both
zinc blende and wurtzite structures and a large volume fraction
of the arms in comparison with that of the core However, the
model associated with the zinc blende core epitaxial growth of
wurtzite arms has been widely accepted for tetrapod structures
synthesized by the colloidal chemical method.12,13,16,30
Figure 2 shows TEM images with a higher magnification for
typical CdSe tetrapods (series T1) synthesized for 6, 12, 30, 60,
and 180 min It appears from Figure 2 that the average length
of the arms increases with increasing reaction time (tr) It
reaches the maximum length of about 50 nm for the sample
with tr= 30 min and then slightly decreases with tr The growth
of tetrapod arms can be similar to that of rods in the one-pot
synthesis.32Therefore, we believe that the reaction time interval
from 30 to 180 min is the one-dimension to two-dimension
(1D-to-2D) ripening stage However, it is hard to determine
precisely diameters of the core and arms based on these TEM images because of the resolution limit of the TEM system used
To observe more clearly changes in the diameter of the cores and arms, an additional series of CdSe tetrapods was synthesized at a higher temperature (220 °C) Their TEM images (see Supporting Information, Figure S1) also indicate that the average length of the arms increases with increasing tr The length of the arms reaches the maximum value of∼32 nm for tr = 20 min and then decreases for longer reaction times Simultaneously, the core diameter quickly increases if compared to that of the arms as changing trfrom 20 to 60 min Absorption and PL Spectra of CdSe Tetrapods Previous works indicated that spectroscopic properties of tetrapod-shaped homostructures depend on not only the crystal structure and confinement dimensionality but also the size of the core and arms The size of tetrapods is very sensitive to the reaction conditions, such as the ligand and precursor concentrations, reaction temperature and time, and so forth
To observe all the possible spectral features of tetrapods, we need tofind suitable reaction conditions and then investigate the temporal evolution of spectroscopic properties of tetrapods with respect to the growth process Having based the proposed mechanism related to the shape evolution of rods in the one-pot synthesis, it is expected that different changes in the diameter of the core and arms in the 1D-to-2D ripening stage lead to clear changes of the absorption and PL spectra of tetrapods A decrease in precursor concentration promotes the Ostwald ripening process For this reason, more of our attention is given to two series of CdSe nanotetrapods with different precursor concentrations
In Figure 3, the absorption and PL spectra of the series T1, T2, and D1 are shown in detail One can see that all the spectral peaks shift toward lower energies with increasing tr,
reflecting a decrease in the quantum confinement energy It is
different from the variation in characteristic spectra of CdSe dots, where there are some noticeable changes in absorption and PL spectra of CdSe tetrapods with respect to the growth process: (i) the presence of an absorption tail (labeled as ALE), (ii) the splitting of thefirst absorption peak (labeled as AHE) into two distinct peaks at lower and higher energies (labeled as
AHE1and AHE2, respectively), (iii) the increase in the distance
Figure 2 TEM images of five representative tetrapod samples (series
T1) synthesized for (a) 6 min, (b) 12 min, (c) 30 min, (d) 60 min,
and (e) 180 min Scale bars are 20 nm.
Figure 3 Dependences of the characteristic absorption (the solid lines) and PL (the dotted lines) spectra on trfor (a) series T1, (b) series T2, and (c) series D1 In these figures, the reaction time in minutes is shown with numbers on the left side of the spectra.
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Trang 4between two absorption peaks AHE1and AHE2with increasing tr,
(iv) the appearance of a new emission peak at a higher energy
(denoted by PHE), and (v) the relative-intensity decrease of the
low-energy emission peak (denoted by PLE) when tr is
increased
A similar variation tendency of the absorption and PL spectra
also is found in tetrapod samples additionally synthesized at a
higher temperature (see Supporting Information, Figure S2)
We note that it is difficult to identify the appearance of the
emission peak PHEdue to the broad size distribution and the
rapid growth of CdSe tetrapods in the first reaction minutes
However, the appearance of this emission peak was observed
clearly for another series of CdSe tetrapods synthesized with
the nucleation temperature of 260°C and growth temperature
of 200 °C (see Supporting Information, Figure S3) Besides
two emissions PLE and PHE, a broad emission band below 1.9
eV due to the surface state emission is observed in both series
T1 and T2, with tr= 0.5−60 min (see Figures 3(a) and 3(b))
It has been known that the precursor concentration strongly
affects the spectral characteristics of CdSe tetrapods Detailed
comparisons in Figures 3(a) and 3(b) reveal that series T2
exhibits an earlier separation of the absorption peaks AHE1and
AHE2and a faster decrease in the intensity of the emission peak
PLE Such changes in the absorption and PL spectra also are
clearly observed for the series of tetrapods synthesized at 220
°C (see Figure S2, Supporting Information)
Figure 4 shows the energy positions of the peaks AHE, AHE1,
AHE2, and PHEof series T1 and of the absorption and emission
peaks (labeled as Adot and Pdot, respectively) of series D1
varying as a function of tr It should be noticed that the values
of absorption energies are only approximate because of the
overlap of absorption bands The emission peak positions were
obtained by fitting the PL spectra to the convolution of
Gaussian and Lorentzian functions The features of the curves
shown in Figure 4 indicate that the absorption and emission
energies decrease very quickly when tris changed from 0.5 to
30 min (denoted by region 1) and then slowly in the trrange of
30−180 min (denoted by region 2) This result is directly
related to the depletion of monomer concentration
Interest-ingly, the separation of the absorption peak AHE1 from AHE2 occurs in region 2, followed by a strong shift of peak AHE1 toward low energies Together with the variations in the shape and feature of the absorption and PL spectra shown in Figures S1 and S2 (Supporting Information), we come to the conclusion that the changes in spectral characteristics are the consequence of a quick increase in the core diameter, as compared with that of the arms in region 2
In the previous studies, it was shown that the photooxidation accompanied by the dissolution of nanocrystals leads to the particle size shrinking.33−35 Accordingly, to confirm the size-dependent spectral characteristics of CdSe tetrapods, we take into account the variation of the absorption and PL spectra of a tetrapod sample in series T1, which was synthesized for 180 min in its degradation process CdSe tetrapods were dispersed
in toluene and sealed in an optically transparent cuvette The sample was then stored at room temperature and exposed to room light (i.e, fluorescent lamp light) Figure 5 shows a
reverse change of the absorption and PL spectra compared to that presented in Figure 3(a) The shift of absorption and PL peaks toward higher energies indicates that the quantum confinement is increased because of the decrease in the core and arm sizes Simultaneously, the spectra are broadened due to the increased size distribution of tetrapods As expected, the emission intensity of PLE gradually increases as compared to that of PHE if the degradation time increases The opposite change trends in the spectral characteristics of CdSe tetrapods for the growth and degradation/aging processes reflect their size-dependent optical properties This can explain the reason why the observed spectroscopic properties of tetrapod-shaped homostructures are different in previous reports.16,17,23,28
Considering carefully the variation in the absorption and PL spectra of CdSe tetrapods shown in Figures 3(a) and 3(b), a continuous change from one absorption peak and one emission peak (for tr = 0.5 min) to one absorption peak and two emission peaks (for tr = 0.5−30 min), then two absorption peaks and two emission peaks (for tr = 30−180 min), and finally to two absorption peaks and one emission peak (for tr=
Figure 4 Variations of the absorption and emission energies for series
T1 and D1 as a function of the reaction time tr Regions 1 and 2 are
separated by the vertical dashed line.
Figure 5 Evolutions of the absorption (the solid lines) and PL (the dotted lines) spectra for a CdSe tetrapod sample (belonging to series T1) synthesized for 180 min; the spectra for (a) the as-prepared sample and for those measured after (b) one year, (c) two years, and (d) three years.
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Trang 5180 min) can be seen Though all of these possibilities were
observed in previous reports,16,17,23,28their variation trend is as
not clear and systematical as the results indicated in our present
work The observed spectral characteristics of CdSe tetrapods
are thus related to their shape evolution in the growth process
Origin of Optical Transitions in CdSe Tetrapods
Basically, the electron and hole wave functions partly extend
across core/arm interfaces of tetrapods The spread degree of
the wave functions depends on the potential barrier at core/
arm interfaces.22,36 Additionally, the electronic structure in
nanostructures with the shape anisotropy such as tetrapods can
also be significantly affected by large perturbations generated
from the crystal field.1
Therefore, to identify the origin of optical transitions in CdSe tetrapods, we consider energy offsets
across core/arm interfaces as an overall result of all possible
effects In this case, the localization of the carriers is interpreted
as that the wave functions largely reside on one component of
the tetrapod.22 Simultaneously, the terms “size” and “Stokes
shift” need to be understood as the “effective exciton size” and
“quasi-Stokes shift”.22,37−39
The appearance of the tail ALEin the absorption spectra of
CdSe tetrapods (see Figures 3(a) and 3(b)), indicates the
type-II band alignment of investigated branched
nanostruc-ture,22,36,40−42 where the lowest energy electron state is
localized in the core, the highest energy hole state is in the
arms, and thefirst excited electron and hole states are localized
in the arms and core.20,23,43Some previous reports showed that
the changes in the absorption and PL spectra of tetrapods were
due to the change in the diameters of the core and arms, rather
than the length of the arms.1,13,26Therefore, the faster increase
in the core diameter (see Figure S1, Supporting Information)
and the stronger shift of the absorption peak AHE1toward lower
energies (see Figure S2, Supporting Information) in the
1D-to-2D ripening stage reveal that the AHE1 and AHE2 peaks are
originated from the absorption transitions in the core and arms,
respectively
On the basis of the energy positions of the emission peak PLE
and the absorption tail ALEin Figures 3(a) and 3(b), we assign
the peak PLEto the spatially indirect emission transition across
core/arm interfaces of CdSe tetrapods Meanwhile, it is difficult
to assign the emission peak PHEto the radiative recombination
of carriers located in the core or in the arms of tetrapods To
date, the origin of this emission peak has not been clarified yet
Previous theoretical calculations showed that an intraband
transition of the electrons from the first excited state to the
ground state is not allowed because of different symmetry of
these states Therefore, the peak PHE was ascribed to the
spatially direct radiative recombination of carriers located in the
arms.23 However, some experimental studies revealed the
localization of the lowest-energy exciton in the center of
tetrapod-shaped homostructures.27,28 Recently, Liu and
co-workers have found atom-resolved evidence of the stacking
order change at core/arm interfaces of ZnS tetrapods and
concluded that the alternate stacking of the zinc blende and
wurtzite phases at core/arm interfaces could induce the carrier
delocalization due to the breaking of crystal symmetry of
tetrapods.30
Here, we identified the origin of the emission peak PHEbased
on the comparison of the quasi-Stokes shift of CdSe tetrapods
and the Stokes shift of CdSe dots with the zinc blende
structure Because of the shape anisotropy, the size dependence
of the Stokes shift for rods and dots is different The Stokes
shift of the dots decreases with increasing their size,44−46
whereas the Stokes shift of the rods increases with increasing the length/diameter ratio.47−49As mentioned above, a tetrapod consist of a dot-like core and four rod-like arms If the peak PHE originates from the recombination of carriers located in the core, the quasi-Stokes shift of the tetrapod is dot-like In contrast, if this emission peak arises from the exciton recombination in the arms, the quasi-Stokes shift must be rod-like The absorption and PL spectra of two tetrapod samples synthesized for 140 min (series T1) and 180 min (series T2) are compared in Figure 6(a) These two samples
were chosen since they have the same position of the emission peak PHE As can be seen in Figure 6(a), two samples have the approximate energy distance PHE−AHE1, but the distance PHE−
AHE2 is different This indicates that the emission peak PHE originates from the spatially direct radiative recombination of carriers located mainly in the core, not in the arms of tetrapods
We also compared the energy distance PHE−AHE1 of CdSe tetrapods to the Stokes shift of CdSe dots All the tetrapod samples of both series T1 and T2 having the separation of the absorption peaks AHE1 and AHE2 were utilized for the comparison Instead of the size dependence of the quasi-Stokes shift, Figure 6(b) reveals the emission energy as a function of absorption energy The quasi-Stokes shift of CdSe tetrapods is consistent with the Stokes shift of CdSe dots Once again, the results obtained confirm that the emission peak PHEoriginates from the spatially direct transition of carriers located in the core
of CdSe tetrapods
It should be noted that because photogenerated carriers are not separated completely into different spatial regions of tetrapods,22the spatially direct radiative transition of carriers located in the arms can occur The peak PHE is the superimposition of the emission peaks generated from the core and arms However, the emission from the arms is weak, and it is difficult to detect it in steady-state PL spectra of CdSe tetrapods
It is now necessary to discuss the evolutions of absorption and PL spectra for CdSe tetrapods Figures 3(a) and 3(b)
Figure 6 (a) Absorption (the solid lines) and emission (the dotted line) spectra of two CdSe tetrapod samples synthesized for (a1) 140 min (series T1) and (a2) 180 min (series T2) The absorption and emission peaks are shown by the vertical arrows and lines, respectively (b) PHEemission energy versus AHE1absorption energy for both series T1 and T2 The correlation between the emission and absorption energies for dot samples with di fferent sizes is also inserted for comparison purposes The solid curve shows the change trend of the emission energy with respect to the absorption energy of CdSe dots (i.e., series D1).
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Trang 6exhibit that a stronger change in intensity of two emission
peaks PLEand PHEis accompanied with a larger separation of
the absorption peaks AHE1and AHE2 Furthermore, the peak PLE
even disappears in the PL spectrum of the tetrapod sample
(series T2) having the largest separation of peaks AHE1 and
AHE2 (see Figure 6(a)) This reflects that the size-dependent
change in the energy offset across the core/arm interfaces alters
the carrier delocalization in the tetrapods In the 1D-to-2D
ripening stage, the fast increase in the diameter of the core leads
to the increase in the height of the potential barrier for
electrons and also leads to the decrease in the height of the
potential barrier for holes This change in the energy offsets
increases the probability that photogenerated electrons and
holes are located in the core, which enhances the intensity of
the emission peak PHE and decreases the intensity of the
emission peak PLE, and even results in the disappearance of the
peak PLE, as seen in Figure 6(a)
Excitation-Power-Dependent Photoluminescence
Spectra of CdSe Tetrapods We also investigated the
excitation-power dependence of PL spectra for CdSe tetrapods
to assess the nature and competition of two emission
transitions PLE and PHE Three samples of series T1 with
different intensities of peaks PLE and PHE were chosen for
comparison, namely, the samples with tr= 6, 12, and 30 min
Their PL spectra varied as a function of the excitation power
ranging from 3 × 10−4to 18 mW and are shown in Figures
7(a)−7(c) Careful analyses of these PL spectra lead to the results plotted in Figures 7(d)−7(f) In Figure 7(d), it shows the evolution of PLEemission energies for three samples versus the cubic root of excitation power The blue shift of the PLE emission with increasing excitation power was observed for all the samples The linear dependence of this shift on the cubic root of excitation power indicates the type-II emission nature of the emission peak PLE.26,50 Meanwhile, no shift was observed for PHEemission energy of all three samples, as shown in Figure 7(e), reflecting its type-I nature.26
Such results support more evidence to confirm the nature and origin of the emissions PLE and PHE, with the reasons stated above
Figure 7(f) shows the integrated intensity ratio of the PHEto
PLEemissions (IPHE/IPLE) on the log scale versus the excitation power for the samples The curves exhibit a slight increase in the value of the ratio IPHE/IPLEto a certain excitation power At higher excitation powers, a strong increase in IPHE/IPLEvalues is observed (see Figure 7(f)) Such features indicate the competition between spatially direct and indirect recombina-tion channels in CdSe tetrapods Because both the peaks PHE and PLE are originated from the radiative transitions of the electrons located in the core, the competition between these recombination channels is dependent on the hole delocalization
in tetrapods It is also known that the spatially indirect emission transition across core/arm interfaces is characterized by a small
Figure 7 Excitation-power dependence of PL spectra for three tetrapod samples (series T1) synthesized for (a) 6 min, (b) 12 min, and (c) 30 min The vertical arrows show the increase of excitation power (d) Type-II emission energies versus the cubic root of excitation power, (e) excitation-power dependences of type-I emission energies, and (f) the I P HE /I P LE ratio of integrated intensities for three investigated samples The solid lines are guides to the eye.
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Trang 7oscillator strength.26,51 Under the continuous excitation
condition, accordingly, the state-filling effect for the hole states
localized in the arms increases the hole amount in the core
region of the tetrapod, leading to a strong increase in the ratio
IPHE/IPLE Moreover, the increase in IPHE/IPLE value with
increasing reaction time exhibits the size-dependent change of
the energy offsets and is dependent on the growth process, as
discussed above
We systematically investigated the steady-state absorption and
PL properties of colloidal CdSe tetrapods, and observed their
different spectral characteristics The appearance of the
absorption tail indicates the type-II band alignment due to
the difference in crystal structures of the core and arms Two
absorption peaks separated at lower and higher energies are
attributed to the absorption transitions in the core and arms,
respectively The emission peak at the low energy is assigned to
the spatially indirect transition across core/arm interfaces, and
the high-energy emission peak is assigned to the spatially direct
recombination of the carriers located in the core The
intensities of these two emission peaks are strongly dependent
on the diameters of the core and arms On the basis of the
spectroscopic characteristics of CdSe tetrapods, we found that
the intensity increase of the high-energy emission peak is due to
the height decrease of the potential barrier for holes and the
height increase of the potential barrier for electrons
Addition-ally, the different spectroscopic characteristics of CdSe
tetrapods are closely related to their shape evolution in the
one-pot synthesis The separation of two absorption peaks in
the 1D-to-2D ripening stage and the intensity change of two
observed emission peaks are strongly dependent on the
reaction time and excitation power, which have been assigned
to the changes in the diameter of the core and arms Our
experimental results revealed clearly the relation between the
spectral characteristics and change in energy offsets across
core/arm interfaces of tetrapod-shaped nanostructures
*S Supporting Information
TEM images of the tetrapod samples synthesized at 220 °C;
Temporal evolutions of the absorption and PL spectra of CdSe
tetrapods synthesized at 220°C; Evolutions of the absorption
and PL spectra of CdSe tetrapods synthesized with the
nucleation temperature of 260°C and growth temperature of
200°C This material is available free of charge via the Internet
at http://pubs.acs.org
Corresponding Author
*E-mail: nghianx@ims.vast.ac.vn Tel.: +84 43 756 4693 Fax:
+84 43 836 0705
Notes
The authors declare no competingfinancial interest
This work is supported by National Foundation for Science and
Technology Development of Vietnam (Grant No
103.06.63.09)
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