Photoluminescence properties of CdTe/CdSe core-shell type-ii

Photoluminescence properties of CdTe/CdSe core-shell type-ii quantum dots C. H. Wang, T. T. Chen, K. W. Tan, and Y. F. Chen * Department of Physics, National Taiwan University, Taipei 106, Taiwan Abstract We report investigations on the optical properties of type-ii CdTe/CdSe core-shell quantum dots. By varying the core size, we provide an elegant way to verify that the detected emission signal indeed arises from type-ii band alignment. The photoluminescence (PL) peak energy increases with a third root of the excitation power. Both of the PL peak energy and linewidth exhibit unique temperature dependence. All these observations can be rationalized by the band bending effect resulting from the spatially separated photo-excited carriers in a type-ii band alignment. 1

I. Introduction Type-II quantum dots (QDs) are expected to have many interesting properties that are fundamentally different from type-i QDs because of the spatial separations of confined electrons and holes. The staggered band lineup provides a novel opportunity to obtain optical transition with a photon energy smaller than the band gap of both semiconductors forming the heterostructures, giving a new way for color tuning. In addition, it has been found that a type-ii quantum well possesses several unique properties, such as giant bending effect, 1-2 long radiative lifetime, 3 and giant optical anisotropy. 4 Therefore, on top of their fundamental interests, type-ii QDs have a great deal of potential applications operating at lower currents and higher temperatures, including lasers, 5 and memory elements. 6 Specially, the separation of photo-excited electrons and holes in type-ii heterostructures makes these materials suitable for photovoltaic applications, in which one of the photocarriers contributes directly to the photoinduced current before recombination. In spite of their importance, the optical properties of type-ii QDs have not been clearly understood due to their difficulties in growing high quality samples as well as the detection of type-ii luminescence. 7,8 In this article, a detailed study on the optical properties of CdTe/CdSe type-ii nanoparticles has been performed, and several peculiar behaviors have been discovered. For example, we have observed a large blue shift of the peak position of the emission spectra under a moderate excitation power. The peak energy decreases with increasing temperature in the low temperature region, whereas an opposite trend occurs in the high temperature region. In addition, the variation of the photoluminescence (PL) line width also shows a similar peculiar behavior. All these observations can be well interpreted in term of the band bending model resulting from the carrier separation in a type-ii heterostructure. 2

II. Experiment The CdTe/CdSe type-ii quantum dots were synthesized by the chemical colloidal method. To obtain the CdTe/CdSe core/shell nanoparticles, the precipitated CdTe QDs (0.020 g) were dispersed in tri-n-octylphosphine oxide(topo, 2.20 g) and hexadecylamine (HAD, 1.26 g) before being heated to 190. In addition, CdCl 2 (0.092 g) was dissolved in 2.5 ml of tri-n-butylphosphine (TBP) upon gentle heating (ca. 80 ). After being cooled to room temperature, the resulting 0.2 M solution was mixed with 2.5 ml of a 0.2 M solution of Se in TBP. With a syringe pump, this mixture was injected within 1h into the reaction flask containing the core nanocrystals at 190 ~ 200. After the addition was completed, the crystals were annealed at 190 for an additional 11.5 h. The prepared CdTe/CdSe QDs were further purified by centrifugation and twice re-precipitation from methanol. The structure of the sample consists of CdTe (core) with a radius of 2.65 nm, CdSe (shell) with a thickness of 1 nm and TOPO. X-ray photoelectron spectroscopy (XPS) has been performed to confirm the formation of the core-shell structure. In XPS measurement, we have observed that the peak intensities for Se versus Te depend on the electron takeoff angle, which show our samples were a core-shell structure. 9,10 For the PL measurement, we used a Ar-ion laser with 514.5 nm wavelength as the excitation source. The PL spectra were recorded by a Spectra Pro 300i monochromator and detected by an InGaAs detector. The sample exhibits a bright near-ir emission at room temperature indicating the excellent quality of our studied materials. III. Results and discussion We show the PL spectra of CdTe QDs with different core sizes and corresponding CdTe/CdSe QDs with the same core size as CdTe QDs, depicted in Fig. 1. The CdTe QDs (CdTe/CdSe QDs) sizes, measured by transmission electron 3

microscope (TEM), were calculated to be 5.3 (6.3), 6.1 (7.1) and 6.9 (7.8) nm. Accordingly, the CdSe shell thickness can be treated as constant values, while the core size is changed. According to the high energy PL bands shown in Fig.1, we observe that the red shift of the CdTe QDs is 114 mev, while the shift in the core-shell CdTe/CdSe QDs located at the low energy PL bands is only 42 mev upon increasing in the CdTe core size. This result can be well understood in terms of the band alignment for both materials reported previously as shown in the inset of Fig. 2. 2 The shift of the peak energy for the type-ii emission in CdTe/CdSe QDs is mainly determined by the change of the quantum confinement energy of holes in the CdTe core, which can be estimated by the equation: 2 2 h π 1 1 E = 2 2 m1 hr1 m2hr Δ 2 2, (1) = m [ + ( E E ) E ], m m [ + ( E E )/ E ] m / 1h h 1 1 g g 2h h 1 2 =, (2) where the hole effective mass m h = 0.35m0, the radius of CdTe R = 2.65 1 nm, and R 2 = 3.45 nm with the corresponding PL peak energy 1 g g E, E 2 and the corresponding hole effective mass m 1 h, m 2 h. 11,12 Note that the E g is the bulk band gap of CdTe. The estimated shift of the peak energy is 37 mev, which is in agreement with the observed value. We thus point out that the detection of the emission spectra under different core size can provide a very good way to verify the type-ii transition. Figure 2 depicts the normalized PL spectra of CdTe/CdSe type-ii QDs under different excitation power at 15 K. It is found that the PL integrated intensity is linearly proportional to the pumping power (not shown here), indicating that the emission originates from an excitonic transition. The PL peak occurs at around 1.2 ev, which is much smaller than both of the bulk band gap energies of CdTe (1.50 ev) and 4

CdSe (1.74 ev). Besides, the PL signal arising from the CdTe core disappears in the PL measurement with CdTe/CdSe QDs. These photophysical behaviors also render a signature for a type-ii band alignment. Quite interestingly, we observe a large blue shift of the PL peak positions under a moderate excitation power as shown in Fig. 2. For example, the PL peak exhibits a higher energy shift of about 14 mev when the pumping intensity changes from 1 to 800 2 mw/cm. The large blue shift will eventually saturate when the excitation power increases further as shown in Fig. 3. Firstly, this peculiar behavior cannot be due to radiation heating, because it will cause a red shift in the band gap. Secondly, the large blue shift is too large to be described by the state filling of the localized states due to interface roughness or alloy potential fluctuations. 13 Thirdly, the possibility of the enhanced electron-electron interaction due to the increase of excitation power can also be ruled out, because the coulomb interaction cancels out due to electron-electron and electron-hole coupling, and the exchange interaction will cause a red shift in the energy of the system. 14 Therefore, we believe that the observation can be more plausibly rationalized by the band bending effect of the type-ii band alignment associated with CdTe/CdSe quantum dots. For the clarity of elucidation, an underlying mechanism based on a band bending model is illustrated in the inset of Fig. 2. In contrast to type-i semiconductor structures, in a type-ii structure, the energy minima for electrons and holes lie in different materials. Due to the special type-ii band alignment, electrons and holes accumulate at the core-shell interface when the laser excites the sample. The separation of photo-excited carriers builds up an internal electric field and the conduction band and valence band start to bend and subsequently alter the overlapping of the electrons and holes wave function. 1,2 Since the PL is generated from the recombination of the accumulated electrons and holes at 5

the interface, an increase in the excitation power raises the steepness of the confining potential, and consequently, the quantization energy is enhanced, resulting in a spectral blue shift. In order to analyze this result quantitatively, we consider the change of the non-equilibrium carrier density due to photo-excitation. 15 The electron density n w and hole density p w generated in the thin layer region by a light beam with intensity I are characterized by the relation n w p w 2 α (L + l) = n w = I (3) γ where α denotes the absorption coefficient, L the thickness of the CdSe layer, l the diameter of CdTe, and γ the radiative recombination coefficient. The strongly localized carriers near the interface form a charged surface and correspondingly, produce an approximately triangular well with the electric-field strength of ε 2π en ε w 1/2 = I. (4) 0 The ground electron state E in such a well is proportional toε 2/3, and thus the electron quantization energy is expected to increase proportionally with the third root of the excitation power 15. Experimentally, the PL peak energy versus the third root of the excitation power is depicted in the inset of Fig. 3. The plot is sufficiently straight, consistent with the behavior for the electron quantization predicted theoretically. We further consider the temperature dependence of the PL properties. Figure 4 depicts the integrated PL intensity as a function of temperature. It is found that the PL intensity is quenched with increasing temperature and such behavior can be well described by the equation expressed as: 6

I0 I(T) = E / kt (1 + Ce a. (5) ) The activation energy E a obtained from the above fitting is 17.5 mev. According to the exciton model for type-ii quantum dots, we can estimate the exciton binding energy of our sample by using the parameters: m e = 0.13m0 for CdSe, m h = 0.35m0 for CdTe, the radius of core R = 2.65 nm and the average dielectric constant ε = 8.37ε. 16 The obtained binding energy of 16.4 mev is in good agreement with the 0 activation energy extracted experimentally, affirming our belief that the quenching of the PL integrated intensity is caused by non-radiative recombination process due to the dissociation of excitons. Nevertheless, as shown in Fig. 5, the peak energy of PL as a function of temperature displays an anomalous behavior, in which a red shift of the peak energy is observed with increasing temperature from 15 K to 160 K, whereas a blue shift is apparent in the range from 160 K to 300K. This behavior has been observed in many semiconductor alloys and heterostructures. 17 According to these studies, we can attribute the observed phenomenon to the fact that the photo-generated carriers transfer and subsequently relax into the energetically low-lying states, giving rise to the red shift of the emitted energy 17. When temperature exceeds 160 K, the giant blue shift can be interpreted by thermal excitation, in which the carriers absorb thermal energy, dissociate from exciton states, and jump to the ground state of the conduction band. 16 The value of the blue shift up to 300 K is about 30 mev, which is in agreement with the combination of the exciton binding energy (~17.5 mev) and thermal energy of 3 k Δ T (~13 mev). 2 We also perform the temperature dependence of the full width at half maximum 7

(FWHM) of the PL spectra with the results shown in Fig. 5. Unlike most of the measurements for semiconductors, the result does not exhibit a monotonous increase with increasing temperature. The FWHM increases and then decreases upon varying the temperature from 15 K-300 K. The turning point is at around 200 K, which is similar to that of the temperature dependence of the peak energy. We thus believe that the increase of FWHM at low temperature region is a result of thermal broadening, while the decrease at high temperature is due to the fact that most of the carriers are now accumulated in the ground states of the QDs and emit photons with a similar energy. IV. Conclusion In conclusion, we have reported several remarkable optical properties of CdTe/CdSe core/shell type-ii quantum dots, the results of which lead to a confirmation that the emission indeed involves a type-ii band alignment. These unique behaviors include a large blue shift at moderate excitation power, the change of peak energy and the variation of FWHM with temperature. All these results can be interpreted in terms of the proposed band bending model, due to a type-ii band lineup. Thus, our results should be very useful for a better understanding of the optical properties of type-ii QDs and for paving the path toward their applications in optoelectronic devices. This work was supported by the Education of Ministry and National Science Council of the Republic of China. 8

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Figure captions Fig. 1: The photoluminescence spectra of CdTe core and CdTe/CdSe core-shell type-ii quantum dots in toluene with different sizes, (a) 5.3 nm (CdTe), (a ) 6.3 nm (CdTe/CdSe), (b) 5.3 nm (CdTe), (b ) 6.1 nm (CdTe/CdSe), (c) 6.9 nm (CdTe), (c ) 7.8 nm (CdTe/CdSe). Fig. 2: Photoluminescence of CdTe/CdSe core/shell type-ii quantum dots under different excitation power at 15 K. The inset shows the type-ii band structure under the influence of the band-bending effect. The band bending can trap electrons and holes near the interface, and the quantum confinement will cause the blue shift. Fig. 3: Dependence of the photoluminescence peak energy on the excitation power. The inset shows the dependence of the photoluminescence peak energy on the cubic root of the excitation power. Fig. 4: The photoluminescence integrated intensity vs. reciprocal temperature together with the activation energy fit. Fig. 5: Dependence of the photoluminescence peak energy and photoluminescence full width at half maximum on temperature. 11

PL Intensity (a. u.) 1.0 1.25 1.5 1.75 2.0 Photon Energy (ev)

Normalized Intensity (a. u.) CdTe CdSe Organic 0.42 ev CB 0.57 ev VB 300 mw/cm 2 20 mw/cm 2 1 mw/cm 2 0.8 0.9 1.0 1.1 1.2 1.3 Photon Energy (ev)

Peak Energy (ev) 1.152 1.149 1.146 1.143 1.140 1.137 Peak Energy (ev) 1.148 1.146 1.144 1.142 1.140 1.138 1.136 1 2 3 4 ( Excitation Power ) 1/3 (mw/cm 2 ) 1/3 0 200 400 600 800 Excitation Power (mw/cm 2 )

Integrated Intensity (a. u.) 0.0027 0.0024 0.0021 0.0018 0.0015 E a = 17.5 mev 0 10 20 30 40 50 60 70 1000/T (1/K)

Peak Energy (ev) 1.155 1.150 1.145 1.140 1.135 1.130 0.18 0.17 0.16 0.15 FWHM (ev) 1.125 0 50 100 150 200 250 300 Temperature (K) 0.14