Identification of Optical Transitions in Colloidal ... - ACS Publications

Institute of Materials Science, Vietnam Academy of Science and Technology, Cau Giay, Hanoi, Vietnam. ‡ University of Engineering and Technology, Han...
1 downloads 4 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Identification 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 highenergy 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.

■

INTRODUCTION 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 transistors,8 electromechanical devices,9 probe tips for high-resolution atomic force microscopy,10 optical strain gauge,11 and 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.12,13,16 The optical properties of this branched nanostructure are governed by the band alignment, which is determined by chemical compositions, crystal structure, size, and confinement 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,16 while other studies revealed the appearance of a new optical absorption peak in CdSe tetrapods17 or 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 © 2012 American Chemical Society

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.26 It is believed that the elucidation of the nature of optical transitions in tetrapod-shaped nanostructures 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.23 A similar result also has been reported for CdSe/CdTe heterotetrapods.26 However, the localization of the lowest-energy exciton in the core region was found by means of the femtosecond time-resolved transient absorption for CdTe tetrapods27 and 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 25517

dx.doi.org/10.1021/jp304784w | J. Phys. Chem. C 2012, 116, 25517−25524

The Journal of Physical Chemistry C

Article

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 measurements 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.

material 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 tetrapodshaped 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 concentrations. 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.

■

EXPERIMENTAL SECTION 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 sizedependent 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 nitrogen flow. 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 nitrogen flow, 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

■

RESULTS AND DISCUSSION 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

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.

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, 25518

dx.doi.org/10.1021/jp304784w | J. Phys. Chem. C 2012, 116, 25517−25524

The Journal of Physical Chemistry C

Article

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 tr from 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 to find 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 onepot 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 the first absorption peak (labeled as AHE) into two distinct peaks at lower and higher energies (labeled as AHE1 and AHE2, respectively), (iii) the increase in the distance

46.0, 49.8, and 57.1°, which correspond to the Miller indices (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,

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.

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.32 Therefore, 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

Figure 3. Dependences of the characteristic absorption (the solid lines) and PL (the dotted lines) spectra on tr for (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. 25519

dx.doi.org/10.1021/jp304784w | J. Phys. Chem. C 2012, 116, 25517−25524

The Journal of Physical Chemistry C

Article

between two absorption peaks AHE1 and AHE2 with 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 PHE due 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 AHE1 and AHE2 and 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 PHE of series T1 and of the absorption and emission

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 sizedependent 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

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.

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.

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 tr is changed from 0.5 to 30 min (denoted by region 1) and then slowly in the tr range of 30−180 min (denoted by region 2). This result is directly related to the depletion of monomer concentration. Interest25520

dx.doi.org/10.1021/jp304784w | J. Phys. Chem. C 2012, 116, 25517−25524

The Journal of Physical Chemistry C

Article

180 min) can be seen. Though all of these possibilities were observed in previous reports,16,17,23,28 their 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 ALE in the absorption spectra of CdSe tetrapods (see Figures 3(a) and 3(b)), indicates the typeII band alignment of investigated branched nanostructure,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 the first excited electron and hole states are localized in the arms and core.20,23,43 Some 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.1,13,26 Therefore, the faster increase in the core diameter (see Figure S1, Supporting Information) and the stronger shift of the absorption peak AHE1 toward lower energies (see Figure S2, Supporting Information) in the 1D-to2D 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 ALE in Figures 3(a) and 3(b), we assign the peak PLE to the spatially indirect emission transition across core/arm interfaces of CdSe tetrapods. Meanwhile, it is difficult to assign the emission peak PHE to 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 coworkers 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 PHE based 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.47−49 As 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

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) PHE emission energy versus AHE1 absorption energy for both series T1 and T2. The correlation between the emission and absorption energies for dot samples with different 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).

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 PHE originates 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,22 the 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) 25521

dx.doi.org/10.1021/jp304784w | J. Phys. Chem. C 2012, 116, 25517−25524

The Journal of Physical Chemistry C

Article

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) excitationpower dependences of type-I emission energies, and (f) the IPHE/IPLE ratio of integrated intensities for three investigated samples. The solid lines are guides to the eye.

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 PLE emission 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 PHE emission 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 PHE to PLE emissions (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/IPLE to a certain excitation power. At higher excitation powers, a strong increase in IPHE/IPLE values is observed (see Figure 7(f)). Such features indicate the competition between spatially direct and indirect recombination 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

exhibit that a stronger change in intensity of two emission peaks PLE and PHE is accompanied with a larger separation of the absorption peaks AHE1 and 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 × 10−4 to 18 mW and are shown in Figures 25522

dx.doi.org/10.1021/jp304784w | J. Phys. Chem. C 2012, 116, 25517−25524

The Journal of Physical Chemistry C

■

oscillator 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.

CONCLUSIONS 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. Additionally, 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. ASSOCIATED CONTENT

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.

■

REFERENCES

(1) Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.; Aloni, S.; Sadtler, B.; Alivisatos, A. P. Nano Lett. 2007, 7, 2951−2959. (2) Lutich, A. A.; Mauser, C.; Da Como, E.; Huang, J.; Vaneski, A.; Talapin, D. V.; Rogach, A. L.; Feldmann, J. Nano Lett. 2010, 10, 4646− 4650. (3) Zhou, Y.; Li, Y.; Zhong, H.; Hou, J.; Ding, Y.; Yang, C.; Li, Y. Nanotechnology 2006, 17, 4041−4047. (4) Zhong, H.; Zhou, Y.; Yang, Y.; Yang, C.; Li, Y. J. Phys. Chem. C 2007, 111, 6538−6543. (5) Lee, Y. −L.; Chi, C. −F.; Liau, S. −Y. Chem. Mater. 2010, 22, 922−927. (6) Li, Y.; Mastria, R.; Fiore, A.; Nobile, C.; Yin, L.; Biasiucci, M.; Cheng, G.; Cucolo, A. M.; Cingolani, R.; Manna, L.; Gigli, G. Adv. Mater. 2009, 21, 4461−4466. (7) Martinez-Ferrero, E.; Albero, J.; Palomares, E. J. Phys. Chem. Lett. 2010, 1, 3039−3045. (8) Cui, Y.; Banin, U.; Bjork, M. T.; Alivisatos, A. P. Nano Lett. 2005, 5, 1519−1523. (9) Fang, L.; Park, J. Y.; Cui, Y.; Alivisatos, A. P.; Shcrier, J.; Lee, B.; Wang, L. −W.; Salmeron, M. J. Chem. Phys. 2007, 127, 184704− 184709. (10) Nobile, C.; Ashby, P. D.; Schuck, P. J.; Fiore, A.; Mastria, R.; Cingolani, R.; Manna, L.; Krahne, R. Small 2008, 4, 2123−2126. (11) Choi, C. L.; Koski, K. J.; Sivasankar, S.; Alivisatos, A. P. Nano Lett. 2009, 9, 3544−3549. (12) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700−12706. (13) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382−385. (14) Peng, X. Adv. Mater. 2003, 15, 459−463. (15) Yu, W. W.; Wang, Y. A.; Peng, X. Chem. Mater. 2003, 15, 4300− 4308. (16) Pang, Q.; Zhao, L.; Cai, Y.; Nguyen, D. P.; Regnault, N.; Wang, N.; Yang, S.; Ge, W.; Ferreira, R.; Bastard, G.; Wang, J. Chem. Mater. 2005, 17, 5263−5267. (17) Mohamed, M. B.; Tonti, D.; Al Salman, A.; Chergui, M. Chem. Phys. Chem. 2005, 6, 2505−2507. (18) Cho, J. W.; Kim, H. S.; Kim, Y. J.; Jang, S. Y.; Park, J.; Kim, J. −G.; Kim, Y. −J.; Cha, E. −H. Chem. Mater. 2008, 20, 5600−5609. (19) Huang, J.; Kovalenco, M. V.; Talapin, D. V. J. Am. Chem. Soc. 2010, 132, 15866−15868. (20) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L. −W.; Alivisatos, A. P. Nature 2004, 430, 190−195. (21) Fiore, A.; Mastria, R.; Lupo, M. G.; Lanzani, G.; Giannini, C.; Carlino, E.; Morello, G.; De Giorgi, M.; Li, Y.; Cingolani, R.; Manna, L. J. Am. Chem. Soc. 2009, 131, 2274−2282. (22) de Mello Donega, C. Phys. Rev. B 2010, 81, 165303. (23) Tari, D.; De Giorgi, M.; Della Sala, F.; Carbone, L.; Krahne, R.; Manna, L.; Cingolani, R. Appl. Phys. Lett. 2005, 87, 224101. (24) Borys, N. J.; Walter, M. J.; Huang, J.; Talapin, D. V.; Lupton, J. M. Science 2010, 330, 1371−1374. (25) Choi, C. L.; Li, H.; Olson, A. C. K.; Jain, P. K.; Sivasankar, S.; Alivisatos, A. P. Nano Lett. 2011, 11, 2358−2362. (26) Morello, G.; Fiore, A.; Mastria, R.; Falqui, A.; Genovese, A.; Creti, A.; Lomascolo, M.; Franchini, I. R.; Manna, L.; Della Sala, F.; Cingolani, R.; De Giorgi, M. J. Phys. Chem. C 2011, 115, 18094− 18104. (27) Malkmus, S.; Kudera, S.; Manna, L.; Parak, W. J.; Braun, M. J. Phys. Chem. B 2006, 110, 17334−17338. (28) Al Salman, A.; Tortschanoff, A.; Mohamed, M. B.; Tonti, D.; van Mourik, F.; Chergui, M. Appl. Phys. Lett. 2007, 90, 093104. (29) Smith, A. M.; Nie, S. Acc. Chem. Res. 2010, 43, 190−200. (30) Liu, W.; Wang, N.; Wang, R.; Kumar, S.; Duesberg, G. S.; Zhang, H.; Sun, K. Nano Lett. 2011, 11, 2983−2988. (31) Luyen, N. T.; Hai, L. B.; Nghia, N. X.; Nga, P. T.; Lieu, N. T. T. Int. J. Nanotechnol. 2011, 8, 214−226. (32) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 1389−1395.

■

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +84 43 756 4693. Fax: +84 43 836 0705. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by National Foundation for Science and Technology Development of Vietnam (Grant No. 103.06.63.09). 25523

dx.doi.org/10.1021/jp304784w | J. Phys. Chem. C 2012, 116, 25517−25524

The Journal of Physical Chemistry C

Article

(33) Stouwdam, J. W.; Shan, J.; van Veggel, F. C. J. M.; PattantyusAbraham, A. G.; Young, J. F.; Raudsepp, M. J. Phys. Chem. C 2007, 111, 1086−1092. (34) Moreels, I.; Fritzinger, B.; Martins, J. C.; Hens, Z. J. Am. Chem. Soc. 2008, 130, 15081−15086. (35) Dai, Q.; Wang, Y.; Zang, Y.; Li, X.; Li, R; Zou, B.; Seo, J. T.; Wang, Y.; Liu, M.; Yu, W. W. Langmuir 2009, 25, 12320−12324. (36) Ivanov, S. A.; Piryatinski, A.; Nanda, J.; Tretiak, S.; Zavadil, K. R.; Wallace, W. O.; Werder, D.; Klimov, V. I. J. Am. Chem. Soc. 2007, 129, 11708−11719. (37) Kim, J.; Nair, P. S.; Wong, C. Y.; Scholes, G. D. Nano Lett. 2007, 7, 3884−3890. (38) Smith, E. R.; Luther, J. M.; Johnson, J. C. Nano Lett. 2011, 11, 4923−4931. (39) She, C.; Demortiere, A.; Shevchenko, E. V.; Pelton, M. Phys. Chem. Lett. 2011, 2, 1469−1475. (40) Chin, P. T. K.; de Mello Donega, C.; van Bavel, S. S.; Meskers, S. C. J.; Sommerdijk, N. A. J. M.; Janssen, R. A. J. J. Am. Chem. Soc. 2007, 129, 14880−14886. (41) Oron, D.; Kazes, M.; Banin, U. Phys. Rev. B 2007, 75, 035330. (42) Kirsanova, M.; Nemchikov, A.; Hewa-Kasakarage, N. N.; Schmall, N.; Zamkov, M. Chem. Mater. 2009, 21, 4305−4309. (43) Li, J.; Wang, L. -W. Nano Lett. 2003, 3, 1357−1363. (44) Liptay, T. J.; Marshall, L. F.; Rao, P. S.; Ram, R. J.; Bawendi, M. G. Phys. Rev. B 2007, 76, 155314. (45) Quyang, J.; Kuijper, J.; Brot, S.; Kingston, D.; Wu, X.; Leek, D. M.; Hu, M. Z.; Ripmeester, J. A.; Yu, K. J. Phys. Chem. C 2009, 113, 7579−7593. (46) Liu, L.; Zhuang, Z.; Xie, T.; Wang, Y.-G.; Li, J.; Peng, Q.; Li, Y. J. Am. Chem. Soc. 2009, 131, 16423−16429. (47) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59−61. (48) Hu, J.; Li, L.-S.; Yang, W.; Manna, L.; Wang, L.-W.; Alivisatos, A. P. Science 2001, 292, 2060−2063. (49) Carbone, L.; Nobile, C.; De Giorgi, M.; Della Sala, F.; Morello, G.; Pompa, P.; Hytch, M.; Snoeck, E.; Fiore, A.; Franchini, I.; Nadasan, M.; Silvestre, A. F.; Chiodo, L.; Kudera, S.; Cingolani, R.; Krahne, R.; Manna, L. Nano Lett. 2007, 7, 2942−2950. (50) Wang, C. H.; Chen, T. T.; Tan, K. W.; Chen, Y. F.; Cheng, C. T.; Chou, P. T. J. Appl. Phys. 2006, 99, 123521. (51) Talapin, D. V.; Koeppe, R.; Gtzinger, S.; Kornowski, A.; Lupton, J. M.; Rogach, A. L.; Benson, O.; Feldmann, J.; Weller, H. Nano Lett. 2003, 3, 1677−1681.

25524

dx.doi.org/10.1021/jp304784w | J. Phys. Chem. C 2012, 116, 25517−25524