Control of the Morphology of Complex Semiconductor Nanocrystals

Jun 29, 2007 - Department of Chemistry & Biochemistry, UniVersity of Arkansas, FayetteVille, ... LLC, FayetteVille, Arkansas 72703, Joint Materials Re...
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Chem. Mater. 2007, 19, 3815-3821

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Control of the Morphology of Complex Semiconductor Nanocrystals with a Type II Heterojunction, Dots vs Peanuts, by Thermal Cycling Bridgette Blackman,†,‡ David M. Battaglia,‡,§ Tetsuya D. Mishima,|,⊥ Matthew B. Johnson,|,⊥ and Xiaogang Peng*,†,| Department of Chemistry & Biochemistry, UniVersity of Arkansas, FayetteVille, Arkansas 72701, NN-Labs LLC, FayetteVille, Arkansas 72703, Joint Materials Research Science & Engineering Center at the UniVersity of Oklahoma and the UniVersity of Arkansas, and the Homer L. Dodge Department of Physics and Astronomy, UniVersity of Oklahoma, Norman, Oklahoma 73019 ReceiVed February 16, 2007. ReVised Manuscript ReceiVed April 6, 2007

Type II CdSe/CdTe core/shell nanocrystals with a dot shape were synthesized using a modified SILAR technique that incorporates cycling of the reaction temperature (thermal cycling). Conversely, experimental results revealed that the standard SILAR alone produced type II core/shell nanocrystals in a peanut shape (1D). Despite their differences in shape, the optical properties observed for the type II dot- and peanutshaped core/shell nanocrystals were similar. The dot-shaped nanocrystals were confirmed as core/shell structures with an abrupt type II heterojunction within the experimental accuracy, and the peanut-shaped ones were found to be consistent with CdSe and CdTe separated on the two ends of the rods. Similar techniques were used for the synthesis of CdS/CdSe/CdTe type II colloidal quantum well heterostructures with dot and peanut shapes. For these type II colloidal quantum well structures, the PL peak positions were shown to be readily tunable by varying the CdSe and CdTe shell thickness, something not typically seen for the quantum dots. The PL quantum yield of these nanocrystals were found to range between 30 and 60%.

Introduction In recent years, synthetic chemistry of colloidal semiconductor nanocrystals has been a major area for materials chemistry and this field has advanced dramatically.1,2 The emphasis in terms of materials development has gradually shifted from simple composition with a regular dot shape to complex composition/morphology. One of the present frontiers is “band gap and composition engineering in solution”. This direction is expected to yield nanocrystals with properties otherwise not available from the corresponding individual materials. Here, band gap engineering refers to the control of the behavior of the photogenerated carriers, both electrons and holes, by means of epitaxial growth of various semiconductors with different band structures. Doping is another key technology in the semiconductor field, which may be considered here as another type of “composition engineering”. Without band gap engineering and doping, it would be difficult to imagine today’s semiconductor industry.3,4 Band gap and composition engineering in solution is much less developed. This report introduces a new synthetic * Corresponding author. Phone: 479-575-4612. Fax: 479-575-4049. Email: xpeng@ uark.edu. † University of Arkansas. ‡ These authors contributed equally to this work. § NN-Labs LLC. || Joint MRSEC at the University of Oklahoma and University of Arkansas. ⊥ University of Oklahoma.

(1) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Ann. ReV. Mater. Sci. 2000, 30, 545. (2) Peng, X.; Thessing, J. Struct. Bonding 2005, 118, 79. (3) Kittel, C. Introduction to Solid State Physics, 6th ed.; Wiley: New York, 1988; Vol. 1. (4) Herman, M. A.; Sitter, H. Molecular Beam Epitaxy: Fundamentals and Current Status, 2nd ed.; Springer: New York, 1996.

concept, thermal cycling, to further this development. This work used the model system based on CdSe-CdTe type II heterostructures, but it might also be used for band gap and composition engineering in solution for other systems. Type II heterojunctions refer to heterostructures with both the conduction and valence bands of one semiconductor higher than those of its neighboring semiconductor. Growth of core/shell semiconductor nanocrystals represents the first and most well-known example of band gap engineering in solution. Although the quality of the core nanocrystals and growth chemistry was not as ideal as it is today and the resulting core/shell nanocrystals generally did not have a well-controlled size distribution, outstanding emission properties of the core/shell nanocrystals with type I band gap offsets (meaning the conduction band (valence band) of the core semiconductor is lower (higher) than those of the shell semiconductor) were well-demonstrated.5 Today, high-quality plain-core semiconductor nanocrystals have become more or less standard for many different semiconductor compositions.1,2 Two well-known epitaxial growth schemes on solid substrates, successive ionic layer adsorption and reaction (SILAR)6,7 and atomic layer epitaxy (ALE),4 were modified and developed for the application in solution growth of colloidal heterostructured nanocrystals, which is known as either SILAR or solution ALE (SALE).8 The key (5) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468. (6) Li, J. J.; Wang, Y. A.; Guo, W.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. J. Am. Chem. Soc. 2003, 125, 12567. (7) Park, S.; Clark, B. L.; Keszler, D. A.; Bender, J. P.; Wager, J. F.; Reynolds, T. A.; Herman, G. S. Science 2002, 297, 65. (8) Battaglia, D.; Li Jack, J.; Wang, Y.; Peng, X. Angew. Chem., Int. Ed. 2003, 42, 5035.

10.1021/cm0704682 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/29/2007

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feature of SILAR for colloidal nanocrystal systems is the elimination in the homogeneous nucleation of the shell materials by introducing the anionic and cationic precursors of the shell materials in separated injections. This synthetic technology, borrowed from solid-state materials growth, has brought the size and size distribution control of a few model core/shell systems to the level of plain-core nanocrystals.8-10 It was somewhat surprising when Bawendi’s group reported that type II CdTe/CdSe core/shell nanocrystals could emit with a decent efficiency, with about 4% for the asprepared nanocrystals and up to more than 20% photoluminescence (PL) quantum yield (QY) after they were coated with a ZnS shell.11 Likely, this is due to the extremely large junction area in a core/shell nanocrystal, which enables significant wavefunction overlap between the spatially separated electron and hole.11 After Bawendi’s initial report, colloidal semiconductor nanocrystals with type II band offsets have been reported repeatedly in the literature. 12-18 The extended emission wavelength into the near-infrared window of these new emitters make them suitable for high penetration depth imaging in living tissues.19 One-dimensional (1D) type-II nanocrystals have also been reported by different research groups.20-23 For instance, the CdSe/CdTe system can grow as either spherical or rod/ peanut-shaped nanocrystals.22,23 Yet, to the best of our knowledge, each type of morphology was always formed using a specific system and starting materials. Among all type-II systems mentioned above, most of them are based on CdTe/CdSe core/shell nanocrystals. Presumably, it would be interesting to explore the inverse system, CdSe/ CdTe core/shell structures. Ideally, the bulk band offsets should be the same, but the location of the photogenerated electron and hole would be inverse. According to the band offsets,11 the photogenerated electron (hole) should always be pushed to CdSe (CdTe). If the reasonably efficient PL (9) Battaglia, D.; Blackman, B.; Peng, X. J. Am. Chem. Soc. 2005, 127, 10889. (10) Xie, R.; Kolb, U.; Li, J.; Basche, T.; Mews, A. J. Am. Chem. Soc. 2005, 127, 7480. (11) Kim, S.; Fisher, B.; Eisler, H.-J.; Bawendi, M. J. Am. Chem. Soc. 2003, 125, 11466. (12) Balet, L. P.; Ivanov, S. A.; Piryatinski, A.; Achermann, M.; Klimov, V. I. Nano Lett. 2004, 4, 1485. (13) Chen, C.-Y.; Cheng, C.-T.; Lai, C.-W.; Hu, Y.-H.; Chou, P.-T.; Chou, Y.-H.; Chiu, H.-T. Small 2005, 1, 1215-1220. (14) Cheng, C.-T.; Chen, C.-Y.; Lai, C.-W.; Liu, W.-H.; Pu, S.-C.; Chou, P.-T.; Chou, Y.-H.; Chiu, H.-T. J. Mater. Chem. 2005, 15, 3409. (15) Li, J. J.; Tsay, J. M.; Michalet, X.; Weiss, S. Chem. Phys. 2005, 318, 82. (16) Xie, R.; Zhong, X.; Basche, T. AdV. Mater. 2005, 17, 2741. (17) Yu, K.; Zaman, B.; Romanova, S.; Wang, D.-s.; Ripmeester, J. A. Small 2005, 1, 332. (18) Wang, C. H.; Chen, T. T.; Tan, K. W.; Chen, Y. F.; Cheng, C. T.; Chou, P. T. J. Appl. Phys. 2006, 99, 123521/1. (19) Kim, S.; Lim, Y. T.; Soltesz, E. G.; De Grand, A. M.; Lee, J.; Nakayama, A.; Parker, J. A.; Mihaljevic, T.; Laurence, R. G.; Dor, D. M.; Cohn, L. H.; Bawendi, M. G.; Frangioni, J. V. Nat. Biotechnol. 2004, 22, 93. (20) Talapin, D. V.; Koeppe, R.; Goetzinger, S.; Kornowski, A.; Lupton, J. M.; Rogach, A. L.; Benson, O.; Feldmann, J.; Weller, H. Nano Lett. 2003, 3, 1677. (21) Kudera, S.; Carbone, L.; Casula, M. F.; Cingolani, R.; Falqui, A.; Snoeck, E.; Parak, W. J.; Manna, L. Nano Lett. 2005, 5, 445. (22) Kanaras, A. G.; Soennichsen, C.; Liu, H.; Alivisatos, A. P. Nano Lett. 2005, 5, 2164. (23) Halpert, J. E.; Porter, V. J.; Zimmer, J. P.; Bawendi, M. G. J. Am. Chem. Soc. 2006, 128, 12590.

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QY of the CdTe/CdSe core/shell nanocrystals is indeed due to the spatial confinement of the photogenerated and fieldsplit carriers, one would speculate that the CdSe/CdTe core/ shell nanocrystals work better as a type II nanocrystal system. Because holes are heavier than electrons and thus have a larger radius, a hole in the shell semiconductor should be more confined to the interface because of the electrostatic attraction of the electron in the core. In addition, the welldeveloped CdSe core nanocrystals are known to be much more stable than CdTe core nanocrystals and are probably the most utilized core nanocrystals for core/shell nanocrystal growth. Our results discussed below reveal that the simple SILAR approach for the growth of CdTe onto CdSe nanocrystals resulted in CdSe/CdTe peanut structures, a quasi-1D structure. Fortunately, this problem could be resolved by introducing a new modification of SILAR, thermal cycling. Different from the original SILAR synthesis, a sufficient amount of precursors for the growth of one monolayer of a CdTe shell were introduced at a relatively low temperature. After the precursors were uniformly distributed into the bulk solution and at least partially adsorbed onto the surface of the existing nanocrystals, the reaction temperature was increased for the growth of the CdTe shell. Similar techniques also worked for the controlled growth of both peanutand dot-shaped CdS/CdSe/CdTe quantum well heterostructures with a type II band offset between the CdSe and CdTe interface. Preliminary results also indicate that thermal cycling works well for the growth of other types of highquality core/shell nanocrystals, such as CdS/ZnSe type II dotshaped nanocrystals. Experimental Section Chemicals. Cadmium oxide (99.99%), zinc oxide (99.9%), selenium (99.5%, 100 mesh), tellurium (99.8%, 200 mesh), tributylphosphine (TBP, 97%), 1-octadecene (ODE), oleic acid (OA, 90%), and benzoyl peroxide were purchased from Aldrich. Octadecylamine (ODA, 98+%) was purchased from Lancaster. CdSe and CdS cores were purchased from NN-Labs and prepared through the known alternate precursor methods for nanocrystal synthesis. All organic solvents were purchased from EM Sciences. All chemicals were used directly without any further purification unless otherwise stated. Preparation of Precursor Solutions. The cadmium precursor solution (0.04 M) was prepared by adding CdO (0.051 g) and oleic acid (0.904 g) in a 1:8 molar ratio to a 25 mL 3-neck roundbottomed flask followed by 7.085 g of octadecene (ODE). The CdO mixture was sealed, purged with argon, and then heated to 240 °C until the solution turned clear. While being cooled, the Cd precursor solution (0.04 M) was transferred to a 20 mL glass vial with a rubber septum and stored at room temperature. In a glove box, the Te precursor (0.04 M) was prepared by adding pure Te powder (0.061 g) to a 20 mL glass vial and then dissolving it in tributylphoshine (TBP, 1.554 g) at a 1:16 molar ratio. ODE (7.947 g) was then added and the mixture was sealed with a rubber septum, removed from the glove box, sonicated, and if necessary, heated at 100 °C to dissolve any remaining Te. The Se precursor solution (0.04 M) was prepared similarly to the Te precursor solution in a glove box. Pure Se powder (0.379 g) was dissolved in TBP (1.5538 g, 1:16 molar ratio) and ODE (7.948 g) in a 20 mL glass vial and sealed with a rubber septum.

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Figure 1. Growth of CdSe/CdTe heterojunction nanocrystals under different reaction conditions.

Typical SILAR Synthesis of CdSe/CdTe Nanocrystals by Thermal Cycling. To a 25 mL 3-neck round-bottomed reaction flask equipped with a stir bar were added 3.0 g of octadecylamine (ODA) and 3.0 g of ODE. About 2.5 × 10-5 mmol of CdSe core nanocrystals (4.8 nm) in hexanes was also added to the flask. The flask was then sealed and put under a vacuum for the removal of air and hexanes while heating to 100 °C with constant stirring. The reaction was purged with argon for at least 10 min and further heated to 190 °C. The Cd and Te precursor solutions (0.14 mL each) were added consecutively via syringe to the reaction flask containing the CdSe cores, waiting 5 min between each injection. The temperature was increased immediately to 260 °C for 20 min to allow growth of the first CdTe monolayer and then decreased back to 190 °C. Next, 0.18 mL of the Cd and Te precursor solutions were injected as before using the same time intervals and temperature changes for growth of the second monolayer. For the third and fourth monolayers, the temperature cycling was continued by injecting 0.22 mL (third) and 0.27 mL (fourth), respectively, of the precursor solutions. Aliquots (∼1 mL) were taken between each injection to monitor the shell growth. Typical SILAR Synthesis of CdS/CdSe/CdTe Nanocrystals by Thermal Cycling. A 3.4 nm CdS core mixture (2.5 × 10-5 mmol) was put under a vacuum, purged with argon, and heated to 190 °C in the same manner as described above for the CdSe core. The Cd and Se precursor solutions (0.09 mL each) were added consecutively via syringe to the reaction flask containing the CdS cores, waiting 5 min between each injection. The mixture was stirred for 30 min at 190 °C to allow growth of the first CdSe monolayer. Next, 0.12 mL of the Cd and Se precursor solutions were injected and mixed for 30 min at 190 °C to allow growth of the second CdSe monolayer. For the third monolayer, 0.16 mL of the Cd and Se precursor solutions were injected and mixed for another 30 min at 190 °C. Following this step, 0.21 mL of the Cd and Se precursor solutions were injected a fourth time and the temperature was increased to 240 °C to react as much of the Cd and Se monomers as possible. Without further purification, the outer CdTe shell was layered in the same manner as stated above for the synthesis of CdSe/CdTe nanocrystals. Four monolayers of the CdTe shell were grown onto the CdS/CdSe nanocrystals by adding 0.22, 0.27, 0.33, and 0.39 mL of the Cd and Te precursor solutions, sequentially. Aliquots (∼1 mL) were taken between each injection to monitor the shell growth. Purification of the Nanocrystals. When the desired synthesis was completed, the reaction was cooled down to 70 °C, transferred to a 125 mL separatory funnel, and diluted with hexanes (∼5 mL), followed by an excess of methanol (∼10 mL). The top hexane layer was transferred to a 25 mL centrifuge tube and methanol was added until the solution became cloudy (indicating nanocrystal precipitation). The precipitate was spun down in a centrifuge at 3000 RPM for 10 min, the supernatant was decanted, and 1-2 mL of toluene was added to the resulting nanocrystals. If necessary, the solubility of the nanocrystals can be improved by heating and/or small additions of octylamine. Etching of CdSe/CdTe Nanocrystals. To the purified CdSe/ CdTe nanocrystal solution (0.5 mL) was added 1.0 mL of

benzylamine, and the resulting suspension was sonicated for 20 min until the solution turned clear. This allowed time for an exchange of the ODA surface ligands to benzylamine. Subsequently, 0.2 mL of the sonicated CdSe/CdTe nanocrystal solution was transferred to a 1 mL quartz cuvette (equipped with a micro stir bar), followed by 0.2 mL of methanol and 0.3 mL of toluene. The cuvette was sealed, placed in a UV-vis sample holder, and stirred. To initiate the etching process, 0.05 mL of a 0.3 M benzoyl peroxide solution in a 2:1 toluene:methanol mixture was added to the cuvette by syringe. This process was monitored in real time at range of 500-800 nm for up to 200 s using an Ocean Optics USB2000 UV-vis spectrometer. Transmission Electron Microscopy (TEM). The low-resolution TEM images were taken on a JEOL 100CX transmission electron microscope with an acceleration voltage of 80 kV. All of the samples were purified by acetone precipitation from chloroform solution or hexanes/methanol extraction. Formvar film-coated copper grids were dipped in the hexanes or toluene solutions to deposit nanocrystals onto the film. Randomly oriented nanocrystals on the TEM substrate were obtained using a diluted nanocrystal solution, with an absorbance of the first absorption peak of the nanocrystals below about 0.05. If the absorbance was above 0.2, densely packed monolayers and multilayers of nanocrystals were observed. Selected area electron diffraction patterns (SAED) were taken with a camera length of 120 cm. High-resolution TEM (HRTEM) pictures were taken using a JEOL 2000 FX microscope. The nanocrystals were deposited onto ultrathin carbon film, attached to TEM mesh grids. Optical Measurements. Absorption spectra were measured on a HP 8453 diode array spectrophotometer. Photoluminescence (PL) was measured on a Spex Fluorolog 3-111 using a PMT detector for spectra between 400 and 800 nm and a liquid-nitrogen-cooled InGaAs photodiode detector for emission in the NIR (800-1100 nm). PL quantum yields (QYs) of the samples were determined through comparison using organic dyes with known quantum yields as standards. All samples for measurement consisted of nanocrystals dissolved in 1 mL of solvent. The excitation wavelength was set at 400 nm for samples emitting in the visible spectrum and 600 nm for those emitting the NIR region.

Results and Discussions CdSe/CdTe. CdSe/CdTe nanocrystals grown through the standard SILAR approach were always peanut-shaped (Figure 1, left). The dimension along the short axis of the CdSe/ CdTe peanuts was the same as the diameter of the core nanocrystals. To solve this shape problem, we varied the reaction conditions, such as the concentration of the core nanocrystals, the concentration of the precursors, the reaction temperature, and the composition of the reaction solutions, within the ranges used for the growth of CdSe/CdS, CdSe/ ZnS, CdS/CdSe, CdSe/CdS/CdSe, and CdSe/ZnS/CdSe core/ shell nanocrystals.8,9 After taking into account many different reaction schemes, none of these variations yielded dot-shaped

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CdSe/CdTe core/shell nanocrystals; thus, the resulting nanocrystals were either peanut-shaped, or no significant growth was observed. For instance, if the reaction temperature was below 180 °C, the reactivity of the precursors was very low, and negligible growth of CdTe onto the existing CdSe core nanocrystals was observed. An insufficient concentration of monomers would also result in no observable growth of the CdTe shell. CdTe nanocrystals are known to readily form elongated structures in comparison to either CdS or CdSe.24,25 This was probably why the growth of CdTe onto the existing CdSe core nanocrystals preferentially yielded peanut-shaped composite nanocrystals, instead of the dot-shaped structures observed for other types of composite nanocrystals using either CdS or CdSe as the shell material. This fact allows the intentional branching of CdSe-CdTe composite nanocrystals.22 It was also known that elongated nanocrystals would be preferred when the solution has a relatively high monomer concentration because this offers a solution environment with high chemical potential.26 On the other hand, as discussed above, simply reducing the monomer concentration and core nanocrystal concentration did not help. However, it was observed that growth of the elongated CdSe/CdTe peanuts occurred rapidly after the injection event. This suggested the possibility that the growth of the peanut-shaped CdSe/CdTe nanocrystals was due to a local high concentration of the precursors right after the injection. Therefore, if the monomers could homogeneously spread out into the entire reaction solution before the growth reaction took place, one might be able to synthesize CdSe/CdTe core/shell dots. Thermal Cycling. Thermal cycling was considered with the above analysis. The homogeneous distribution of the precursors prior to growth could be realized in the current system by reducing the injection temperature down to about 180 °C. After each injection cycle for SILAR, the reaction temperature was promptly boosted to about 260 °C for efficient growth of a CdTe shell onto the existing nanocrystals. This low injection temperature might also offer the monomers sufficient time to be adsorbed onto the surface of the existing nanocrystals in the solution prior to the growth of CdTe shell onto the existing nanocrystals at a relatively high temperature. Both effects would reduce the monomer concentration, and thus the chemical potential, in the solution. Subsequently, it should prevent the peanut growth. Surprisingly, this simple idea worked very well. As shown in Figure 1 (right), nearly perfect dot-shaped nanocrystals were produced using SILAR coupled with the thermal cycling process. The resulting dot-shaped nanocrystals (Figure 1, right) were nearly monodispersed, which was similar to that of the starting core nanocrystals (Figure 1, middle). The increase of the average size before and after the CdTe shell growth matched the targeted growth thickness of the CdTe shell. It should be pointed out that the final growth temperature for the dots and peanuts complex (24) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382. (25) Yu, W. W.; Wang, Y. A.; Peng, X. Chem. Mater. 2003, 15, 4300. (26) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2002, 124, 3343.

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Figure 2. Left: Evolution of the absorption spectrum of the CdSe/CdTe core/shell dots upon the growth of a CdTe shell with a different thickness. Right: absorption, emission, and contrast-enhanced HRTEM images of CdSe/CdTe core/shell dots with four monolayers of CdTe (bottom) and the corresponding core nanocrystals (top).

nanocrystals could be identical, and the only difference was the injection temperature used. Crystal Structure. The crystal structure revealed by electron diffraction (Figure 1) and HRTEM images for both peanuts and dots was the same, in spite of their shape difference. The diffraction patterns resembled that of a typical mixed wurtzite and zinc-blende structure. Because of the small energy difference between wurtzite and zinc blende, stacking faults along the c-axis of the wurtzite structure (or the axis of the zinc-blende structure) was found to be common for II-VI semiconductor nanocrystals.27 The optical properties, both absorption and emission, of the CdSe/ CdTe peanuts and dots were found to be similar as well. Below, the optical properties of the CdSe/CdTe core/shell dots will be discussed in detail. As the thickness of the CdTe shell increased for the dotshaped nanocrystals, the sharp absorption features of the original CdSe core nanocrystals gradually broadened (Figure 2, left), although the size of the CdSe/CdTe core/shell nanocrystals increased constantly with the increase in the thickness of the CdTe shell (see TEM images in Figure 1 and HRTEM images in Figure 2 (right, insets)). Although no significant red shift of the first exciton absorption peak of the CdSe core nanocrystals was observed, a substantial tail at the low-energy side of the first exciton absorption peak of the CdSe core nanocrystals gradually developed with the growth of the CdTe shell. The PL spectrum of the CdSe/ CdTe core/shell dots was greatly shifted to red in comparison to that of the original CdSe core nanocrystals (Figure 2, right). CdSe/CdTe Core/Shell Structure with a Type II Heterojunction. A CdSe/CdTe core/shell structure with a type II heterojunction for the dots, instead of a CdTexSe(1-x) alloy, would be consistent with the optical properties discussed above according to the literature. To confirm the core/shell structures, we subjected the resulting CdSe/CdTe core/shell dots to oxidative etching. The protocol for examination of (27) Wickham, J. N.; Herhold, A. B.; Alivisatos, A. P. Phys. ReV. Lett. 2000, 84, 923.

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Figure 4. Growth of CdS/CdSe/CdTe dots and peanuts.

Figure 3. UV-vis spectra displaying etching of CdSe/CdTe (a) peanuts and (b) dots. TEM images of the etched (d) peanuts and (e) dots are also displayed. (c) The original UV-vis spectrum of the CdSe core and the (f) TEM image were included as reference.

core/shell nanocrystals through controlled etching, reported previously using organic peroxides,9 was adopted (see Experimental Section for details). The etching process was monitored in situ by a UV-vis absorption spectrophotometer (see the Supporting Information, Figure S1). The temporal evolution of the UV-vis spectra implied that the etching of CdSe/CdTe core/shell dots followed more-or-less the reverse pattern for the growth of the corresponding core/shell nanocrystals. If the etching process was sufficiently slow, it was possible to stop the etching when the absorption spectrum of the nanocrystals (Figure 3b) was similar to that of the original CdSe core nanocrystals (Figure 3c). The etched nanocrystals at this stage were isolated and checked under TEM (Figure 3e). Consistent with the matched absorption spectra, the TEM image of the etched dots showed practically the same size and size distribution as the original CdSe core nanocrystals, within experimental errors. It has been reported that CdSexTe1-x alloy nanocrystals should have a substantially different absorption spectrum in comparison to a pure CdSe nanocrystal with the same size.28 Therefore, it is reasonable to conclude that, after etching, the CdSe core nanocrystals were recovered and remain intact for the case of CdSe/CdTe dots. In addition, the success of the etching process demonstrates that there were no pinholes in the CdTe shell, as pinholes would allow etching of the interior CdSe core.9 With the above analysis, one would picture that the controlled etching process proceeds from the outer surface in a layer-by-layer fashion, which gradually removed the CdTe shell. Although the CdTe shell was etched thinner and thinner, the feature of the type II heterojunction (the long red tail below the first exciton absorption peak) became less and less pronounced (Figure 3b and the Supporting Information, Figure S1). Once the CdTe shell was completely (28) Bailey, R. E.; Nie, S. J. Am. Chem. Soc. 2003, 125, 7100.

removed, the nanocrystals were recovered as pure CdSe dots, indicated by their characteristic absorption features and the size (panels b and e in Figure 3). The resolution of this etching experiment did not allow us to fully conclude that there was not any alloying between the CdTe shell and CdSe core. However, if there was any, it must be within the interface area. The CdSe/CdTe peanuts, however, behaved differently from the CdSe/CdTe dots during the etching process. Because both of the CdSe and CdTe sections were exposed to the oxidation reagents, both sections were etched. As a result, we never observed the recovery of the absorption spectrum of the original CdSe cores. As shown in Figure 3a, even after the first exciton absorption peak blue-shifted significantly past that of the original CdSe core nanocrystals, there was still a noticeable type II tail to the absorption spectrum. The corresponding TEM images of the etched CdSe/CdTe peanuts given the absorption spectrum in Figure 3a were more or less dot-shaped (Figure 3d), implying that the etching rate of the peanuts was higher at the two ends. Quantum-Well-Based Type II Heterojunction. A quantum-well-based type II heterojunction CdS/CdSe/CdTe nanocrystal system was also explored. This was achieved by a one-pot, two-step process. The growth of CdS/CdSe quantum shells (Figure 4, top) was accomplished in the first step using the procedure reported previously.8 Because the band gap of CdS is substantially wider than that of CdSe and the band offsets between these two semiconductors are type I, the photogenerated electrons and holes should be confined in the CdSe shell layer, which is why such core/shell nanocrystals are called quantum shells. The second step added CdTe onto the existing CdS/CdSe quantum shells, which should form a type II heterojunction between the CdSe and CdTe. A possible advantage of quantum well nanocrystals with a type II heterojunction, in comparison to the corresponding quantum dots discussed above, lies in the spatial confinement of the photogenerated electrons in the CdSe layer. Because CdSe exists as a thin shell in the quantum well structure, the electron wavefunction would be more closely confined to the CdSe/CdTe type II junction area, in comparison to the case of CdSe dots. As a result, it might provide a better

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Figure 5. Absorption and PL spectra of CdS/CdSe/CdTe quantum well type II nanocrystals. For reference, absorption and PL spectra of CdS quantum dots and CdS/CdSe quantum shell nanocrystals are also illustrated.

attraction to the photogenerated holes located in the CdTe outer shell, and the PL QY would thus be higher than that of the quantum-dot-based type II nanocrystals. Once again, a straight SILAR approach yielded only peanuts for the CdS/CdSe/CdTe nanocrystals (Figure 4, bottom left). Thermal cycling, as described above, solved this problem for the synthesis of the quantum well type II composite nanocrystals as it did for the type II quantum dots. The narrow size distribution of the resulting composite dots is evident in Figure 4 (bottom right). The TEM images in Figure 4 also illustrate that the average size of the final dotshaped composite nanocrystals increased substantially upon the growth of the CdSe and CdTe layers. Optical Spectra. The optical spectra of the quantum well type II composite nanocrystals of CdS/CdSe/CdTe were found to be similar to those of the CdSe/CdTe quantum dot type II nanocrystals, Figure 5. A long red tail in the absorption spectrum was observed that was due to the type II heterojunction. Related to this, a near-infrared PL peak that was well-separated from the main absorption peak was recorded (Figure 5). The PL spectrum of the CdS/CdSe/CdTe dots was broader than that for both CdS quantum dots and CdS/CdSe quantum shells in Figure 5. However, if the fullwidth-at-half-maximum was measured in energy (eV), it would be about 120 meV for all three samples (see the Supporting Information, Figure S2). This is consistent with the narrow size distribution of the CdS/CdSe/CdTe type II dots (Figure 4). PL Peak Position. PL peak position of the type II quantum dots and quantum wells, in principle, should be tunable by varying the size/thickness of CdSe and CdTe in the composite nanocrystals. Experimental results, however, revealed that the tuning range of the PL peak position of type II CdSe/ CdTe core/shell dots was insubstantial upon changing the CdTe shell thickness (Figure 6, top). When the size of the CdSe core nanocrystals was changed, the variation of PL peak position of CdSe/CdTe type II dots was also insubstantial (data not shown). However, the tunable range of the PL peak position was substantial in the case of CdS/CdSe/ CdTe quantum well type II dots. This tuning was accomplished by varying the thickness of either the CdSe inner shell or CdTe outer shell, especially of the CdSe inner shell (Figure 6, top). The differences between the type II quantum

Figure 6. PL peak position and PL QY of quantum dots (labeled as CdSe dots) and quantum wells with different numbers of monolayers of the CdSe shell (labeled as QW 1 Layer, QW 2 Layer, QW 3 Layer, QW 4 Layer).

dots and quantum wells observed here can mainly be attributed to the tuning of the energy levels of the corresponding CdSe quantum dots and quantum shells. Experimentally, it was observed that small CdSe core nanocrystals were unstable under the growth conditions (260 °C) in the current system. In contrast, it was quite straightforward to grow an extremely thin CdSe quantum shell, one and two monolayers in thickness, onto the CdS core nanocrystals. PL QY. The PL QY of the CdSe/CdTe quantum dots and CdS/CdSe/CdTe quantum well type II nanocrystals was quite high, in comparison to that of reported CdTe/CdSe type II quantum dots. The PL QY of CdTe/CdSe type II dots was reported to be below 4%, and an additional shell coating of high band gap material could boost it up to about 20%.11 Measured using the same standard organic dye, the new type II nanocrystals reported here had a PL QY in the range between 30 and 60% (Figure 6, bottom). Generally speaking, if the CdTe thickness was at least two monolayers, the PL QY of the CdSe/CdTe quantum dots and CdS/CdSe/CdTe quantum well type II nanocrystals was about the same. However, the first monolayer of the CdTe shell gave an unusually low PL QY for the quantum dot type II nanocrystals, which was only about 30%. The PL QY values of the CdSe/CdTe and CdS/CdSe/CdTe nanocrystals shown here (Figure 6, bottom) were substantially higher than that of the CdTe/CdSe case reported in the literature.11 It seems to be consistent with our hypothesis that the relatively heavy hole in the CdTe outer layer might be better confined close to the junction interface by the electrostatic attraction provided by the electrons confined in the CdSe core or inner layer. Another factor, however, could be the better-controlled synthesis offered by a SILAR process. The latter explanation seems to be consistent with the fact that the PL QY of the peanuts was not much lower than that of the corresponding dot-shaped type II nanocrystals discussed above. Although the PL QY of the CdSe/CdTe and CdS/CdSe/ CdTe type II nanocrystals had a decent PL QY for their nearinfrared emission, the PL QY was observed to decrease when the CdTe shell thickness increased from one monolayer to

Shape Control in Type II Core/Shell Nanocrystals Via Thermal Cycling

four monolayers (except the unusually low PL QY for the one-monolayer quantum dot case). This agrees with the general trend observed by Bawendi’s group for CdTe/CdSe type II dots.11 Their explanation was that the split charge in the outer shell should have more overlap with the opposite charge in the core, which should boost the PL QY. For the most part, this explanation seems to be consistent with our experiments. Photochemical and Chemical Stability. The photochemical and chemical stability of the CdSe/CdTe type II quantum dots and CdS/CdSe/CdTe type II quantum wells were found to be strongly dependent on the thickness of the CdTe outer layer. Although a thin layer of CdTe generally gave a high PL QY, such QWs were found to be sensitive to photochemical and chemical oxidations. When stored in air, with or without optical radiation, the type II quantum dots and quantum wells with one or two monolayers of a CdTe outer shell were found to gradually oxidize. However, their stability was still better than pure CdTe dots, which oxidized almost immediately after being taken out of the reaction flask and dissolved in organic solvents that were not degassed. Overall, quantum well and quantum dot type II nanocrystals with four monolayers of a CdTe shell were found to be stable. After the nanocrystals were stored under ambient conditions for about 1 month, the PL QY retained the original value and the optical spectra were not changed significantly. This stability makes the type II nanocrystals with a thick CdTe shell a better choice for potential applications, even though the PL QY of these nanocrystals was not extremely high (about 30%, see Figure 6, bottom).

Chem. Mater., Vol. 19, No. 15, 2007 3821

Conclusions In summary, a controlled synthesis of CdSe/CdTe type II core/shell and CdS/CdSe/CdTe type II quantum well nanoparticles using the SILAR technique incorporating thermal cycling has been described. It was shown that the standard SILAR technology yielded a quasi-1D structure, whereas the new modified SILAR yielded core/shell nanocrystals, both of which have an abrupt type II heterojunction. This synthetic process contributes to the advancement of band gap engineering in solution for the production of nanocrystals that are ideal for various practical applications. The PL QY of the resulting type II nanocrystals was found to be significantly higher than that for the CdTe/CdSe core/shell type II nanocrystals mentioned in the same way. This increased PL QY is consistent with the spatial confinement argument originally proposed by Bawendi’s group.11 On the basis of this argument, one would conclude that the holes in the outer shell have a spatial confinement closer to the heterojunction interface than that of the electrons. Acknowledgment. Financial support from the NIH, NSF, and Arkansas Biomedical Institute is acknowledged. Supporting Information Available: Figures S1 and S2 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM0704682