Morphology-controlled Lead Selenide ... - ACS Publications

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J. Phys. Chem. C 2008, 112, 11218–11226

Morphology-controlled Lead Selenide Nanocrystals and Their In Situ Growth on Carbon Nanotubes Yoon Ju Na, Han Sung Kim, and Jeunghee Park* Department of Chemistry, Korea UniVersity, Jochiwon 339-700, Korea ReceiVed: March 14, 2008; ReVised Manuscript ReceiVed: April 28, 2008

High-quality colloidal lead selenide (PbSe) nanocrystals (NCs) were synthesized with various morphologies in a controlled manner by changing the growth temperature and capping ligand. When oleic acid was used as a ligand and activating agent for the Pb precursor, the evolution of the NCs from nanodots to nanocubes was achieved. The nanocubes can be progressively transformed into nanosized flowers, stars, hexapods, and tetrapods by decreasing the growth temperature, when using phosphonic acid instead of oleic acid. The tetrapods, whose branches are grown along the [111] direction, were produced at the lowest temperature. These systematic morphology-control strategies were applied to the in situ growth of PbSe NCs on carbon nanotubes, showing the successful production of dot, octahedron, tetragon, zigzagged rod, and tapered hexapod shaped PbSe NCs. We suggest that the carbon nanotubes would enhance the [100] growth of PbSe NCs, forming unique morphologies, which were not observed without carbon nanotubes. 1. Introduction Colloidal semiconductor nanocrystals (NCs) have attracted considerable interest in recent years, due to their unique physical and chemical properties depending on their size and shape (i.e., quantum confinement effects) and various potential applications as the building blocks of future nanodevices using “bottom up” approaches. As one of the important IV-VI semiconductor materials, rock-salt (cubic) structured lead selenide (PbSe) has been the object of particular attention because of its narrow band gap (in bulk) of 0.28 eV (at room temperature) and strong quantum confinement effect due to its large Bohr radius (ca. rB ) 46 nm).1–19 The wide range of applications of PbSe nanostructures utilizing such size/shape dependent properties includes leading optical switches and communication devices, photovoltaic cells, biological imaging, and photodetectors. Therefore, the ability to produce the desired architecture of nanoscale building blocks with a well-defined morphology has been the most important issue. Since the Murray group first synthesized monodispersed PbSe quantum dots, a number of groups have reported various shapes of PbSe NCs such as nanocubes, octahedrons, hexapods, nanorods/nanowires, and nanotubes, over the past decade.2–30 However, the morphologycontrolled synthesis of colloidal PbSe NCs has received relatively less attention compared to that of other chalcogenides (e.g., CdSe, CdTe). Recently, significant interest has been directed toward the design of NC (including PbSe, CdS, CdSe, CdTe, ZnS, ZnSe, PbS, Cu2S) and carbon nanotube (CNT) hybrid nanostructures, in order to extend the range of applications.16g,31–38 In particular, the binding of pregrown PbSe NCs with surface-modified CNTs allows for the highly efficient generation of photocurrents via the interaction between the excited PbSe NCs and conductive CNTs, thus demonstrating their importance as building blocks for light-harvesting assemblies.16g However, there are few works on the in situ growth of PbSe NCs on CNTs, so the morphologycontrolled growth of NCs in the CNT hybrid nanostructure system still remains a challenging subject. * Corresponding author e-mail: [email protected].

Herein, we report the morphology-controlled solvothermal synthesis of colloidal PbSe NCs, using various growth conditions. Trioctylphosphine (TOP) was used as an activating agent for Se, forming the TOP/Se complex in a noncoordinating solvent, 1-octadecene (ODE). Two different acids, oleic acid (OA) and tetradecylphosphonic acid (TDPA), were used as activation agents for the Pb precursor, lead oxide (PbO). We monitored the morphology evolution of the monodispersed PbSe NCs, from dots to cubes, stars, flowers, hexapods, and tetrapods, which was achieved by varying the growth temperature, for these ligands. Importantly, an in situ synthetic route to prepare the PbSe NC and CNT hybrid nanostructures was developed, in order to show the remarkable morphology evolution of the PbSe NCs grown on the surface of single-walled (SWCNTs) and multiwalled CNTs (MWCNTs). 2. Experimental Section 2.1. Materials. Lead oxide (PbO, 99.999%), selenium (Se) powders (99.99%), trioctylphosphine (TOP, 90%), oleic acid (OA, C8H17dC8H15-COOH, 90%), tetradecylphosphonic acid (TDPA, C14H29-H2PO4, 98%), and 1-octadecene (ODE, C17H34dCH2, tech grade, 90%) were purchased from SigmaAldrich or Alfa Co. All chemicals were used without further purification. Purified SWCNTs (Iljin, 99%) and MWCNTs (Aldrich) were used. 2.2. Synthetic Procedure. Typical Synthesis of PbSe NCs Using OA as ActiWating Agent for PbO. The procedure was divided into three steps. (1) PbO (0.80 mmol, 0.179 g) and OA (2.0 mmol, 0.61 mL) were mixed with 3.1 mL of ODE in a 50 mL three-neck flask equipped with a condenser, and the mixture was heated to 180 °C under argon flow. (2) A solution of Se (1.62 mmol, 0.128 g or 0.81 mmol, 0.064 g) dissolved in 2 mL (4.0 mmol) of TOP was swiftly injected into the heated solution, and the reaction mixture was then cooled to 150 °C. (3) The mixture was maintained at 150 °C for 1∼15 min to allow the growth of the PbSe NCs. Typical Synthesis of PbSe NCs Using TDPA as ActiWating Agent for PbO. (1) PbO (0.08 mmol, 0.018 g) and TDPA (0.04 mmol, 0.011 g) were mixed with 1.6 mL of ODE in a 50 mL

10.1021/jp802224c CCC: $40.75  2008 American Chemical Society Published on Web 07/09/2008

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SCHEME 1: PbSe NCs

SCHEME 2: PbSe NCs-CNTs Hybrid

three-neck flask equipped with a condenser, and the mixture was heated to 300 °C under argon flow. (2) A solution of Se (0.162 mmol, 0.0128 g or 0.081 mmol, 0.0064 g) dissolved in 0.5 mL (1.0 mmol) or 0.2 mL (0.40 mmol) of TOP was quickly injected into the heated solution, and the reaction mixture was then cooled to 270 °C. (3) The mixture was maintained at 270 °C for 1 min to allow the growth of the PbSe NCs. For the other growth temperatures, the temperature of the reaction solution was lowered at a rate of 7 °C/min by removing the heating mantle. Growth temperatures of 240, 200, 170, 160, and 150 °C were kept for 1 min. Typical Synthesis of PbSe NCs Attached to CNTs. Two growth methods using OA and TDPA were employed. In step (1), 1 mg of SWCNTs or MWCNTs was added. The remainder of the procedure is identical to that described above. After the reaction mixture had cooled to approximately 40 °C, the purification of the nanocrystals was carried out by repeatedly extracting the unreacted precursors from the reaction mixture dissolved in hexane by adding methanol, and the PbSe NCs were isolated by precipitation with acetone. 2.3. Characterization. The products were characterized by field-emission transmission electron microscopy (FE TEM, FEI TECNAI G2 200 kV and Jeol JEM 2100F), and high-voltage TEM (HVEM, Jeol JEM ARM 1300S, 1.25 MV). Highresolution X-ray diffraction (XRD) patterns were obtained using the 8C2 beam line of the Pohang light source (PLS) with monochromatic radiation (λ ) 1.54520 Å) and the Cu KR line (λ ) 1.5406 Å) of a laboratory-based diffractometer (Philips X’Pert PRO MRD). The X-ray photoelectron spectroscopy (XPS) measurements were performed at the U7 beam line of the PLS and using a laboratory-based spectrometer (XPS, ESCALAB 250, VG Scientifics) with a photon energy of 1486.6 eV (Al KR). Raman spectroscopy (Horiba Jobin-Yvon HR-800 UV) was measured using the 514.5 nm line of an argon ion laser. UV-visible-near IR spectroscopy (Varian, Cary V) was used to identify the growth of the PbSe NCs.

3. Results and Discussion The synthetic procedure and various shapes of the labeled products are summarized in Schemes 1 and 2. The growth condition and size/shape of the PbSe NCs synthesized in this study are listed in Table 1. 3.1. Size and Shape Evolution of Morphology of PbSe NCs Using OA. Figures 1a-c display the TEM images of the dot- and cube-shaped PbSe NCs synthesized at 150 °C, for 1, 10, and 15 min, using OA with a ratio of Pb/Se/OA/TOP ) 1:2:2.5:2.7, respectively. The NCs are denoted as NC1∼NC3, respectively. As the growth time increases, the shape of the NCs evolves from spherical dots to cubes, and the average size increases from 6.0 ( 0.5 (NC1) to 10.5 ( 0.2 (NC2) and 14 ( 1 nm (NC3), respectively. The nanocubes can also be grown using half of the Se concentration (i.e., Pb/Se/OA/TOP ) 1:1: 2.5:1.3) for 10 min. The inset of Figure 1c shows the latticeresolved TEM image and corresponding FFT ED (zone axis ) [011]) pattern of NC3. The facet of the cubes is matched to the {100} planes of the cubic unit cell. The EDX analysis confirms that the NCs were comprised of Pb and Se at a ratio of 1 ( 0.1, as shown in Supporting Information Figure S1. The growth condition and the dot-shaped PbSe NC products are similar to those of the work of Colvin and co-workers.26 Upon the injection of the Se/TOP solution into the hot Pb precursor solution, the monomers are immediately nucleated, and many nuclei started to rapidly evolve into spherical-shaped nanocrystals with the stabilization provided by the weak capping ligands (OA). As a result, the concentration of the monomers remaining in the bulk solution should be extremely low (nearly depleted) at a longer growth time or lower Se concentration. Then, the growth would be expected to occur slowly under the thermodynamically controlled condition, and such an incubationlike process induces the formation of thermodynamically stable cube-shaped NCs.39 Because the surface energy of a cubic structure is suggested to be in the order γ{111} < γ{100} < γ{110},

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TABLE 1: The Growth Condition and Shape/Size of the PbSe NCs Synthesized in This Study without CNTs Pb/Se/acid/TOP

acid

1:2:2.5:2.7

OA

(1:1:2.5:1.3)a 1:2:0.5:14

1:1:0.5:7

TDPA

TDPA

growth temperature (°C) 150 150 150

growth time (min) 1 10 15 (10)

NC

shape

1 2 3

dot dot cube

270 240 200 170

1 1 (4)b 1 (10) 1 (14)

4 5 6 7

cube flower star hexapod

160 150

1 (16) 1 (17)

270

1

8

hexapod

170

1 (14)

9

multipod

with CNTs

av size of NCs (nm) 6.0 ( 0.5 10.5 ( 0.2 14 ( 1 (12 ( 1)

NC

shape

av size of NCs (nm)

10 11

dot octahedron

7(1 12 ( 1

15 ( 1 35 ( 3 85 ( 5 60 ( 3 (D ) 15 ( 0.5; L ) 19 ( 1)c

12

13

tetragon tetragon tetragon zigzagged rod

50 50 50 D ) 50 ( 10; L ) 100 ( 20

14 15

hexapod multipod

70 ( 5 (D ) 8 ( 1; L ) 30 ( 5) 20

35 ( 3 (D ) 8 ( 0.2; L ) 12 ( 1) 30 ( 3 (D ) 6 ( 1; L) 14 ( 1)

a The ratio, growth time, and size in parentheses indicate the NCs grown using Pb/Se ) 1:1 and 10 min. b The time in parentheses indicate the time taken for the decrease to the respective temperature. c The diameter (D) and length (L) of the branches of the hexapod and other multipods (rod, dipod, tripod, and tetrapod) structures.

Figure 1. TEM images of PbSe NCs grown using OA at 150 °C for (a) 1 min (NC1), (b) 10 min (NC2), and (c) 15 min (NC3). The average size is 6.0 ( 0.5, 10.5 ( 0.2, and 14 ( 1 nm, respectively. The insets of panel c correspond to their magnified images and FFT ED pattern (zone axis ) [011]), revealing the highly crystalline rock-salt PbSe crystal. The distance between the adjacent (200) planes is 0.31 nm.

the thermodynamically controlled growth preferentially induces growth along the {111} facets, forming the cube-shaped NCs.40 The UV-visible absorption spectra are displayed in Supporting Information Figure S2. The peaks appear at 1405, 1719, and 2215 nm for NC1, NC2, and NC3, respectively, which is consistent with the results of previous works.3,9a,26 3.2. Morphology Evolution of PbSe NCs Using TDPA. Figure 2a displays the TEM images of the cube-shaped PbSe NCs (NC4; av size ) 15 ( 1 nm), synthesized at 270 °C for 1 min, using TDPA with a ratio of Pb/Se/TDPA/TOP ) 1:2:0.5: 14. The inset shows a magnified view of the cubes. In order to study the morphology evolution as a function of the growth temperature, the temperature of the reaction mixture was decreased at a rate of 7 °C/min after the injection of the Se/ TOP mixture into the PbO/TDPA/ODE solution heated at 300 °C. The size/shape of the NCs was monitored for the growth at 240, 200, and 170 °C. The temperature decrease took about 4, 10, and 14 min after the injection, respectively, and the reaction solution was kept at that temperature for 1 min. Panels b-d of Figure 2 correspond to the TEM images of the PbSe NCs grown at the respective temperatures. The insets show a magnified view of each NC. At 240 °C, the growth of the round-shaped

nanocrystals on the facet of the cubes yields a flower-like structure (NC5; av size ) 35 ( 3 nm). At 200 °C, the growth of the sharp-edged nanocrystals on the facet of the cubes is enhanced to produce the star shape (NC6; av size ) 85 ( 5 nm). As the temperature decreases to 170 °C, the six branches are grown with a much longer length, forming a hexapod-shaped structure (NC7; av size ) 60 ( 3 nm). The average diameter (D) and length (L) of the branches are 15 ( 0.5 and 19 ( 1 nm, respectively. Below 170 °C, no distinctive-shaped PbSe NCs were formed. To develop more versatile shapes, the PbSe NCs were synthesized using a ratio of Pb/Se/TDPA/TOP ) 1:1:0.5:7 (with a lower concentration Se monomer). Figure 2e displays the TEM images of the hexapod-shaped PbSe NCs (NC8; av size ) 35 ( 3 nm) synthesized at 270 °C for 1 min, following the injection of the Se/TOP mixture into the reaction solution at 300 °C. The average diameter and length of the branches are 8 ( 0.2 and 12 ( 1 nm, respectively (inset). The temperature of the reaction mixture was decreased to 170 °C. Then, branched PbSe NCs such as rods, V-shaped dipods, tripods, and tetrapods are grown as a mixture (NC9; av size ) 30 ( 3 nm), as shown in Figure

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Figure 2. TEM images of PbSe NCs grown using TDPA when the temperature is decreased to (a) 270 °C (cubes, NC4), (b) 240 °C (flowers, NC5), (c) 200 °C (stars, NC6), and (d) 170 °C (hexapods, NC7), after injecting the Se/TOP mixture into the hot Pb precursor solution heated at 300 °C. TEM images of PbSe NCs grown at (e) 270 °C (hexapods, NC8) and (f) 170 °C (multipods, NC9), using a lower concentration Se monomer.

2f. The average diameter and length of these branches are 6 ( 1 and 14 ( 1 nm, respectively (inset). The lattice-resolved TEM image of the hexapod (NC8) is shown in Figure 3a. Its corresponding FFT ED pattern (zone axis ) [001]) shows its single-crystalline nature and the growth of branches along the [100] direction (inset). The (200) plane fringes are separated by about 0.31 nm. Figure 3b and the insets show the lattice-resolved TEM image and corresponding FFT ED pattern (zone axis ) [011]) of the selected tetrapod (NC9), respectively. The branches are grown along the [111] direction. The (111) plane fringes are separated by a distance of about 0.35 nm, which is consistent with that of rock salt PbSe. Panels c and d of Figure 3 show the lattice-resolved images of the

tripod and rod, respectively. The highly crystalline (220) and (111) planes are separated with d ) 0.22 and 0.35 nm, respectively. For tripods, the electron beam projection along the [111] zone axis shows that three branches are aligned toward the [110] direction (insets of Figure 3c). If they were pyramidal shaped, the growth direction of the branches would be [111]. The corresponding FFT ED pattern (zone axis ) [011]) of the nanorods also show the [111] growth direction (insets of Figure 3d). In contrast to the case of OA, a remarkable morphology evolution is observed in the case where TDPA is used. We first demonstrate that phosphonic acid did promote the formation of elongated PbSe NCs in the noncoordinating solvent, ODE,

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Figure 3. HRTEM image of (a) hexapod (NC8), (b) tetrapod (NC9), (c) tripod, (NC9), and rod (NC9), produced using a lower concentration of Se monomers. The TEM images and the corresponding FFT ED patterns are shown in the insets. The zone axis is (a) [001], (b) [011], (c) [111], and (d) [011], respectively. The branches of the hexapods are grown along the [100] direction, whereas those of the tetrapods and rods are grown along the [111] direction. The highly crystalline (200), (111), and (220) planes are separated with d ) 0.31, 0.35, and 0.22 nm, respectively.

which have not previously been reported. The excess TOP (Se/ TOP ) 1:7) would play the role of a coordinating coligand along with TDPA. The progression of the morphology can be basically due to the strong ligand coordination of the monomers in bulk solution, which decreases their reactivity at the nucleation/ growth stages. On the basis of the TEM images, the temporal shape evolution of the PbSe NCs can be described as follows. At the highest temperature (270 °C), nanocubes are formed due to the incubation process that occurred after the rapid formation of the nuclei and depletion of monomers, as described in the case of OA (see Section 3.1). As the growth temperature decreases, the lower consumption of monomers at the nucleation stage (a lower number of nuclei is formed) increases the monomer concentration (activity) at the growth stage, making the elongated growth favorable.41 The growth of the higher surface energy {100} facets becomes preferred under such kinetically controlled growth conditions. The growth along the six [100] directions of the nanocubes progressively produces the longer six-branched structures, ranging from flowers (NC5) to stars (NC6) and hexapods (NC7). When the activity of the monomers is decreased (by decreasing the Se concentration by 1/2), the growth of elongated structures becomes favorable even at 270 °C, which can be ascribed to the slower nucleation and growth (which is a similar growth condition to that of NC7). The thinner branches of the hexapods compared to those of NC7 would have resulted from the initial growth of smaller sized seeds (probably spherical

shape). As the growth temperature decreases to 170 °C, the number of branches decreases, and a mixture of rods, V-shaped dipods, tripods, and tetrapods is formed. It is noteworthy that these multipod structures have branches mainly grown along the [111] direction, which has not been previously reported for PbSe NCs. The growth along the [111] direction, which is presumably more thermodynamically controlled growth, would be responsible for the extremely low monomer activity at the growth stage during the sufficiently long growth period. Therefore, the nanocubes are produced by the thermodynamically controlled incubation process at the higher growth temperature, subsequently to faster nucleation/growth. The sixbranched NCs (flowers, stars, and hexapods) are formed under kinetically controlled growth conditions, due to the insufficient thermal energy as well as the high monomer activity at the growth stage. As the monomer activity decreases (at the lower temperature), the 2-4 numbered branched structures would be grown under the thermodynamically controlled growth conditions. The kinetically controlled [100] growth forms the sixbranched structure, whereas the thermodynamically controlled [111] growth produces the cubes at the higher temperature or lower numbered branched multipod structures at the lower temperature. Our result is consistent with the work of other groups showing that the growth of nanocubes becomes favorable at a higher temperature or longer growth time.7a,21c 3.3. Morphology Evolution of PbSe NCs on CNTs. We also performed the in situ synthesis of PbSe NCs attached to

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Figure 4. TEM images of PbSe NCs grown on MWCNTs or SWCNTs using OA for (a) Pb/Se ) 1:2 (NC10) and (b) Pb/Se ) 1:1 (NC11) at 150 °C; using TDPA at (c) 270 °C (NC12), (d) 170 °C (NC13), 150 °C (NC14), and 140 °C (NC15). The inset displays the HRTEM image and corresponding FFT ED pattern of an individual PbSe NC. The morphology of the PbSe NCs is (a) dots, (b) octahedrons, (c) tetragons, (d) zigzagged rods consisted of linked octahedrons, (e) tapered hexapods, and (f) multipods.

the SWCNTs or MWCNTs, using OA and TDPA. The TEM image of the PbSe-MWCNT nanostructures (NC10), synthesized using OA at 150 °C for 10 min (the growth condition is the same as that of NC2), is displayed in Figure 4a. The CNTs are homogeneously decorated with the dot-shaped PbSe NCs. The size range of the PbSe NCs is 6-8 nm with an average value of 7 ( 1 nm (inset). The HRTEM image and corresponding FFT ED pattern (inset) with the [001] zone axis reveal that the single-crystalline PbSe NCs are tightly bound to the graphite layers of the CNTs. As the concentration of Se monomers

decreases (or the growth time increases), the shape becomes an octahedron.Figure4bshowstheTEMimageofthePbSe-MWCNT hybrid nanostructures (NC11), synthesized using a reduced Se concentration, at 150 °C for 10 min (the growth condition is the same as that of NC3). The HRTEM image and FFT ED pattern (zone axis ) [011]), as shown in the insets, reveal the diagonal axis along the [100] direction, indicating the singlecrystalline octahedrons (av size ) 12 ( 1 nm). The use of TDPA produces tetragonal-shaped PbSe NCs attached along the long axis of the SWCNTs (NC12; av size )

11224 J. Phys. Chem. C, Vol. 112, No. 30, 2008 50 nm), as shown in Figure 4c. They were grown using a ratio of Pb/Se/TDPA/TOP ) 1:2:0.5:14, at 270 °C for 1 min (under the same growth condition as that of NC4). The HRTEM image and FFT ED pattern (zone axis ) [001]) reveal the singlecrystalline tetragonal shaped PbSe NCs (insets). The long axis ([100]) of the tetragonal NCs is aligned along the lateral planes of the CNTs. As the temperature decreases to 170 °C (under the same condition of that of NC7), zigzagged PbSe nanorods attached along the long axis of the SWCNTs (NC13) are produced, as shown in Figure 4d. The HRTEM image and FFT ED pattern (zone axis ) [001]) reveal that 2-3 octahedrons are connected along their [100] direction (insets). Their average diameter and length are 50 ( 10 nm and 100 ( 20 nm, respectively. As the temperature decreases to 160 °C, tapered hexapod-shaped PbSe NCs are grown on the CNTs (Figure 4e). The average diameter of the sharp branches is 8 ( 1 nm, and their length is 30 ( 5 nm. As the temperature decreases to 150 °C, a mixture of tripod-, tetrapod-, and hexapod-shaped PbSe NCs is grown on the CNTs (Figure 4f). The size of the NCs is not uniform, with an average value of about 20 nm. The corresponding FFT ED patterns (zone axis ) [001]) reveal that the growth direction of the branches of hexapods is [100], for both NC14 and NC15. The efficient growth of CdS and Cu2S NCs on CNTs was explained by the lattice matching of the cubic CdS (111) and hexagonal Cu2S (001) planes with the (002) graphite planes of the CNTs.31b,38 In the present case, the interlayer distances of the (100) planes of PbSe (3.5 Å) match with that of the (002) graphite planes of the CNTs (3.4 Å), with only small lattice mismatches of 3%. So the lattice matching may contribute to the high-yield, in situ growth of the PbSe NCs with the [100] direction. Furthermore, the surface defect sites of the purified CNTs contain oxygenated functional groups (e.g., -OH or -COOH), which can act as templates and/or ligands for the nucleation and growth of NCs.32b The adsorption on the defect sites of the CNTs may increase the monomer activity by eliminating the steric hindrance of the coordinating ligands (OA and TDPA), thus increasing the concentration of nuclei compared to the growth without CNTs. In the case of OA, the CNTs cannot significantly influence the spherical shape of the NCs, because of the fast enough nucleation/growth, even without CNTs. The size tends to decrease, compared to the NCs grown without CNTs (NC2), indicating that the increased number of nuclei reduces the amount of monomer remaining in the bulk solution. The octahedron-shaped NCs are grown due to the sufficient growth toward the [100] direction, under the condition that the nanocubes are grown without CNTs. The enhanced [100] growth may be ascribed to their lattice matching with the graphitic layers. The addition of CNTs would more significantly influence the morphology in the case where the stronger coordinating ligand, TDPA, is used. The enhanced kinetically controlled [100] growth produced the unique shapes such as octahedrons, tetragons, and zigzagged nanorods, which were observed without CNTs. The formation of tetragonal-shaped NCs aligned along the CNT axis (over the wide temperature range of 200-270 °C) definitely indicates that the CNTs play a significant role in enhancing the nucleation and growth. As the growth temperature decreases to 170 °C, the six equivalent growth of the {100} facets of cubes would produce the octahedrons. These octahedrons are grown attached along the tube axis and probably have the chance to connect to each other to form the zigzagged rods grown along the [100] direction, whose structure is similar to

Na et al.

Figure 5. XRD patterns of the PbSe powders (Aldrich, 99.99%), 10.5 nm-size nanodots (NC2), 60 nm-size hexapods (NC7), and 7 nm-size PbSe NC-CNTs hybrid nanostructures (NC10).

that reported in the works by other groups.7a,21b The octahedronshaped NCs are driven to attach to the end of the growing nanorods, since they have the largest dipole moment along the [100] direction. The adsorption of monomers on the CNTs facilitates the nucleation/growth even at temperatures below 170 °C, where no growth was observable without CNTs. The tapered hexapods, which were produced at 170 °C without CNTs, are grown on the CNTs at 160 °C. The branched multipods are produced by the improved nucleation/growth rate, even at 150 °C. The reduction in the number of branches also occurs due to the low monomer concentration at the growth stage. 3.4. XRD. The XRD patterns of the PbSe powders (Aldrich, 99.99%), 10.5 nm diameter nanodots (NC2), hexapods (NC7), and 7 nm PbSe-CNT hybrid structures (NC10) are shown in Figure 5. They show peaks corresponding to rock-salt PbSe crystal (JCPDS No. 78-1903; a ) 6.1213 Å). Using the Debye-Scherrer equation (for (200) and (220) peaks), the average sizes of the nanodots were estimated to be 12 and 9 nm for NC2 and NC10, respectively. The XPS spectra of the PbSe NCs (NC2 and NC10) are shown in Supporting Information Figure S3, confirming the presence of pure and highly crystalline PbSe NCs. 4. Conclusion We demonstrate the unique morphology control of PbSe NCs by changing the growth temperature and capping ligands of the solvothermal reaction. Two different ligands, OA and TDPA, were used as the activating agent for PbO in the noncoordinating solvent, ODE. The morphology of the monodispersed PbSe NCs can be varied from dots to cubes when using OA as the ligand. The cubes were progressively transformed into flowers, stars, hexapods, and tetrapods by decreasing the growth temperature, when using TDPA instead of OA. The tetrapods (including rods, dipods, and tripods), whose branches are grown along the [111] direction, would be produced under thermodynamically controlled condition, due to the greatly reduced monomer activity at the growth stage. The morphology-controlled PbSe NCs—that is, dots, octahedrons, tetragons, zigzagged rods, and tapered hexapods—were grown in situ on SWCNTs or MWCNTs, under the same growth condition as that of the monodispersed NCs. The CNTs would enhance the kinetically controlled [100] growth of PbSe NCs along the CNTs axis, forming distinctive morphologies that were not observed without CNTs. This result provides very useful insight into the strategy to adopt to control

Morphology-controlled Lead Selenide Nanocrystals the morphology of PbSe NCs, in dispersed and CNT-attached forms, by selecting the growth temperature and ligands. Acknowledgment. This work was supported by KRF grants (R14-2003-033-01003-0; R02-2004-000-10025-0; 2003-015 C00265) and BK21. The SEM, HVEM, XRD, and XPS measurements were performed at the Korea Basic Science Institute. The experiments at the PLS were partially supported by MOST and POSTECH. Supporting Information Available: EDX, UV-visible absorption spectra, and XPS spectra of PbSe NCs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Wise, F. W. Acc. Chem. Res. 2000, 33, 773. (b) Rogach, A. L.; Eychmuller, A.; Hickey, S. G.; Kershaw, S. V. Small 2007, 3, 536. (2) Olkhovets, A.; Hsu, R.-C.; Lipovskii, A.; Wise, F. W. Phys. ReV. Lett. 1998, 81, 3539. (3) Du, H.; Chen, C.; Krishnan, R.; Krauss, T. D.; Harbold, J. M.; Wise, F. W.; Thomas, M. G.; Silcox, J. Nano Lett. 2002, 2, 1321. (4) Redl, F. X.; Cho, K. S.; Murray, C. B.; O’Brien, S. Nature 2003, 423, 968. (5) Talapin, D. V.; Murray, C. B. Science 2005, 310, 86. (6) (a) Shevchenko, E. V.; Talapin, D. V.; O’Brien, S.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 8741. (b) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55. (c) Shevchenko, E. V.; Talapin, D. V.; Murray, C. B.; O’Brien, S. J. Am. Chem. Soc. 2006, 128, 3620. (d) Talapin, D. V.; Shevchenko, E. V.; Murray, C. B.; Titov, A. V.; Kra´l, P. Nano Lett. 2007, 7, 1213. (7) (a) Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140. (b) Talapin, D. V.; Black, C. T.; Kagan, C. R.; Shevchenko, E. V.; Afzali, A.; Murray, C. B. J. Phys. Chem. C 2007, 111, 13244. (c) Talapin, D. V.; Yu, H.; Shevchenko, E. V.; Lobo, A.; Murray, C. B. J. Phys. Chem. C 2007, 111, 14049. (8) (a) Nedeljkovif, J. M.; Nenadovif, M. T.; Mic´ic´, O. I.; Nozik, A. J. J. Phys. Chem. 1986, 90, 12. (b) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Nano Lett. 2005, 5, 865. (c) Nozik, A. Inorg. Chem. 2005, 44, 6893. (d) Murphy, J. E.; Beard, M. C.; Nozik, A. J. J. Phys. Chem B 2006, 110, 25455. (e) Luther, J. M.; Beard, M. C.; Song, Q.; Law, M.; Ellingson, R. J.; Nozik, A. J. Nano Lett. 2007, 7, 1779. (9) (a) Wehrenberg, B. L.; Wang, C.; Guyot-Sionnest, P. J. Phys. Chem. B 2002, 106, 10634. (b) Wehrenberg, B. L.; Guyot-Sionnest, P. J. Am. Chem. Soc. 2003, 125, 7806. (c) Wehrenberg, B. L.; Yu, D.; Ma, J.; Guyot-Sionnest, P. J. Phys. Chem. B 2005, 109, 20192. (10) (a) Schaller, R. D.; Petruska, M. A.; Klimov, V. I. J. Phys. Chem. B 2003, 107, 13765. (b) Schaller, R. D.; Klimov, V. I. Phys. ReV. Lett. 2004, 92, 186601–1. (c) Pietryga, J. M.; Schaller, R. D.; Werder, D.; Stewart, M. H.; Klimov, V. I.; Hollingsworth, J. A. J. Am. Chem. Soc. 2004, 126, 11752. (d) Schaller, R. D.; Agranovich, V. M.; Klimov, V. I. Nat. Phys. 2005, 1, 189. (e) Schaller, R. D.; Pietryga, J. M.; Goupalov, S. V.; Petruska, M. A.; Ivanov, S. A.; Klimov, V. I. Phys. ReV. Lett. 2005, 95, 196401. (f) Schaller, R. D.; Petruska, M. A.; Klimov, V. I. Appl. Phys. Lett. 2005, 87, 253102. (g) Schaller, R. D.; Klimov, V. I. Phys. ReV. Lett. 2006, 96, 097402. (h) Maskaly, G. R.; Petruska, M. A.; Nanda, J.; Bezel, I. V.; Schaller, R. D.; Htoon, H.; Pietryga, J. M.; Klimov, V. I. AdV. Mater. 2006, 18, 343. (i) Klimov, V. I. Appl. Phys. Lett. 2006, 89, 123118. (j) Klimov, V. I. J. Phys. Chem. B 2006, 110, 16827. (11) (a) Steckel, J. S.; Coe-Sullivan, S.; Bulovic, V.; Bawendi, M. G. AdV. Mater. 2003, 15, 1862. (b) Steckel, J. S.; Yen, B. K. H.; Oertel, D. C.; Bawendi, M. G. J. Am. Chem. Soc. 2006, 128, 13032. (12) (a) Du, H.; Chen, C.; Krishnan, R.; Krauss, T. D.; Harbold, J. M.; Wise, F. W.; Thomas, M. G.; Silcox, J. Nano Lett. 2002, 2, 1321. (b) Peterson, J. J.; Huang, L.; Delerue, C.; Allan, G.; Krauss, T. D. Nano Lett. 2007, 7, 3827. (13) (a) Springholz, G.; Holy, V.; Pinczolits, M.; Bauer, G. Science 1998, 282, 734. (b) Springholz, G.; Pinczolits, M.; Mayer, P.; Holy, V.; Bauer, G.; Kang, H. H.; Salamanca-Riba, L. Phys. ReV. Lett. 2000, 84, 2669. (c) Springholz, G.; Wiesauer, K. Phys. ReV. Lett. 2002, 88, 015507. (d) Raab, A.; Lechner, R. T.; Springholz, G. Appl. Phys. Lett. 2002, 80, 1273. (e) Springholz, G.; Raab, A.; Lechner, R. T.; Holy, V. Appl. Phys. Lett. 2003, 82, 799. (f) Abtin, L.; Springholz, G.; Holy, V. Phys. ReV. Lett. 2006, 97, 266103. (g) Springholz, G.; Abtin, L.; Holy, V. Appl. Phys. Lett. 2007, 90, 113119. (14) (a) Franceschetti, A.; An, J. M.; Zunger, A. Nano Lett. 2006, 6, 2191. (b) An, J. M.; Franceschetti, A.; Dudiy, S. V.; Zunger, A. Nano Lett. 2006, 6, 2728. (c) An, J. M.; Zunger, A.; Franceschetti, A. Nano Lett. 2007, 7, 2129.

J. Phys. Chem. C, Vol. 112, No. 30, 2008 11225 (15) (a) Kamisaka, H.; Kilina, S. V.; Yamashita, K.; Prezhdo, O. V. Nano Lett. 2006, 6, 2295. (b) Kilina, S. V.; Craig, C. F.; Kilin, D. S.; Prezhdo, O. V. J. Phys. Chem. C 2007, 111, 4871. (16) (a) Choudhury, K. R.; Sahoo, Y.; Prasad, P. N. AdV. Mater. 2005, 17, 2877. (b) Choudhury, K. R.; Sahoo, Y.; Ohulchanskyy, T. Y.; Prasad, P. N. Appl. Phys. Lett. 2005, 87, 073110. (c) Yong, K. T.; Sahoo, Y.; Choudhury, K. R.; Swihart, M. T.; Minter, J. R.; Prasad, P. N. Nano Lett. 2006, 6, 709. (d) Shi, W.; Zeng, H.; Sahoo, Y.; Ohulchanskyy, T. Y.; Ding, Y.; Wang, Z. L.; Swihart, M.; Prasad, P. N. Nano Lett 2006, 6, 875. (e) Shi, W.; Sahoo, Y.; Zeng, H.; Ding, Y.; Swihart, M. T.; Prasad, P. N. AdV. Mater. 2006, 18, 1889. (f) Choudhury, K. R.; Kim, W. J.; Sahoo, Y.; Lee, K. S. ; Prasad, P. N. Appl. Phys. Lett. 2006, 89, 051109. (g) Cho, N.; Choudhury, K. R.; Thapa, R. B.; Sahoo, Y.; Ohulchanskyy, T.; Cartwright, A. N.; Lee, K. S.; Prasad, P. N. AdV. Mater. 2007, 19, 232. (h) Thapa, R.; Choudhury, K. R.; Kim, W. J.; Sahoo, Y.; Cartwright, A. N.; Prasad, P. N. Appl. Phys. Lett. 2007, 90, 252112. (17) (a) Cui, D.; Xu, J.; Zhu, T.; Paradee, G.; Ashok, S.; Gerhold, M. Appl. Phys. Lett. 2006, 88, 183111. (b) Xu, J.; Cui, D.; Zhu, T.; Paradee, G.; Liang, Z.; Wang, Q.; Xu, S.; Wang, A. Y. Nanotechnology 2006, 17, 5428. (18) Bo¨berl, M.; Kovalenko, M. V.; Gamerith, S.; List, E. J. W.; Heiss, W. AdV. Mater. 2007, 19, 3574. (19) Hyun, B. R.; Chen, H.; Rey, D. A.; Wise, F. W; Batt, C. A. J. Phys. Chem. B 2007, 111, 5726. (20) (a) Lifshitz, E.; Bashouti, M.; Kloper, V.; Kigel, A.; Eisen, M. S.; Berger, S. Nano Lett. 2003, 3, 857. (b) Sashchiuk, A.; Amirav, L.; Bashouti, M.; Krueger, M.; Sivan, U.; Lifshitz, E. Nano Lett. 2004, 4, 159. (c) Brumer, M.; Kigel, A.; Amirav, L.; Sashchiuk, A.; Solomesch, O.; Tessler, N.; Lifshitz, E. AdV. Funct. Mater. 2005, 15, 1111. (d) Lifshitz, E.; Brumer, M.; Kigel, A.; Sashchiuk, A.; Bashouti, M.; Sirota, M.; Galun, E.; Burshtein, Z.; Le Quang, A. Q.; Ledoux-Rak, I.; Zyss, J. J. Phys. Chem. B 2006, 110, 25356. (21) (a) Ji, T.; Jian, W. B.; Fang, J. J. Am. Chem. Soc. 2003, 125, 8448. (b) Lu, W.; Gao, P.; Jian, W. B.; Wang, Z. L.; Fang, J. J. Am. Chem. Soc. 2004, 126, 14816. (c) Lu, W.; Ding, Y.; Wang, Z. L.; Fang, J. J. Phys. Chem. B 2005, 109, 19219. (22) (a) Kudera, S.; Carbone, L.; Casula, M. F.; Cingolani, R.; Falqui, A.; Snoeck, E.; Parak, W. J.; Manna, L. Nano Lett. 2005, 5, 445. (b) Carbone, L.; Kudera, S.; Giannini, C.; Ciccarella, G.; Cingolani, R.; Cozzoli, P. D.; Manna, L. J. Mater. Chem. 2006, 16, 3952. (23) (a) Houtepen, A. J.; Koole, R.; Vanmaekelbergh, D.; Meeldijk, J.; Hickey, S. G. J. Am. Chem. Soc. 2006, 128, 6792. (b) Klokkenburg, M.; Houtepen, A. J.; Koole, R.; de Folter, J. W. J. De.; Erne´, B. H.; van Faassen, E.; Vanmaekelbergh, D. Nano Lett. 2007, 7, 2931. (24) (a) Wang, W.; Geng, Y.; Qian, Y.; Ji, M.; Liu, X. AdV. Mater. 1998, 10, 1479. (b) Zhu, W.; Wang, W.; Shi, J. J. Phys. Chem. B 2006, 110, 9785. (nanowires, sphere). (25) (a) Ge, J. P.; Li, Y. D. J. Mater. Chem. 2003, 13, 911. (b) Xu, J.; Ge, J. P.; Li, Y. D. J. Phys. Chem. B 2006, 110, 2497. (c) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Langmuir 2006, 22, 7364. (26) Yu, W. W.; Falkner, J. C.; Shih, B. S.; Colvin, V. L. Chem. Mater. 2004, 16, 3318. (27) Hull, K. L.; Grebinski, J. W.; Kosel, T. H.; Kuno, M. Chem. Mater. 2005, 17, 4416. (28) Zhu, J.; Peng, H.; Chan, C. K.; Jarausch, K.; Zhang, X. F.; Cui, Y. Nano Lett. 2007, 7, 1095. (29) Bierman, M. J.; Albert Lau, Y. K.; Jin, S. Nano Lett. 2007, 7, 2907. (30) Tong, H.; Yang, L. X.; Li, L.; Zhang, L.; Zhu, Y. J. Angew. Chem., Int. Ed. 2006, 45, 7739. (31) (a) Shi, J.; Qin, Y.; Li, X.; Guo, Z. X.; Zhu, D. Carbon 2004, 42, 423. (b) Huang, Q.; Gao, L. Nanotechnology 2004, 15, 1855. (c) SheeneyHaj-Ichia, L.; Basnar, B.; Willner, I. Angew. Chem., Int. Ed. 2005, 44, 78. (d) Robel, I.; Bunker, B. A.; Kamat, P. V. AdV. Mater. 2005, 17, 2458. (e) Liu, B.; Lee, J. Y. J. Phys. Chem. B 2005, 109, 23783. (32) (a) Banerjee, S.; Wong, S. S. J. Am. Chem. Soc. 2003, 125, 10342. (b) Banerjee, S.; Wong, S. S. AdV. Mater. 2004, 16, 34. (c) Guldi, D. M.; Aminur Rahman, G. M.; Sgobba, V.; Kotov, N. A.; Bonifazi, D.; Prato, M. J. Am. Chem. Soc. 2006, 128, 2315. (d) Grzelczak, M.; Correa-Duarte, M. A.; Salgueirin˜o-Maceira, V.; Giersig, M.; Diaz, R.; Liz-Marza´n, L. M. AdV. Mater. 2006, 18, 415. (e) Li, W.; Gao, C.; Qian, H.; Yan, D. J. Mater. Chem. 2006, 16, 1852. (f) Engtrakul, C.; Kim, Y. H.; Nedeljkovic, J. M.; Ahrenkiel, S. P.; Gilbert, K. E. H.; Alleman, J. L.; Zhang, S. B.; Micic, O. I.; Nozik, A. J.; Heben, M. J. J. Phys. Chem. B 2006, 110, 25153. (g) Jia, N.; Lian, Q.; Shen, H.; Wang, C.; Li, X.; Yang, Z. Nano Lett. 2007, 7, 2976. (33) (a) Banerjee, S.; Wong, S. S. Nano Lett. 2002, 2, 195. (b) Haremza, J. M.; Hahn, M. A.; Krauss, T. D.; Chen, S.; Calcines, J. Nano Lett. 2002, 2, 1253. (c) Chaudhary, S.; Kim, J. H.; Singh, K. V.; Ozkan, M. Nano Lett. 2004, 4, 2415. (d) Landi, B. J.; Castro, S. L.; Ruf, H. J.; Evans, C. M.; Bailey, S. G.; Raffaelle, R. P. Sol. Energy Mater. Sol. Cells 2005, 87, 733. (e) Li, Q.; Sun, B.; Kinloch, I. A.; Zhi, D.; Sirringhaus, H.; Windle, A, H. Chem. Mater. 2006, 18, 164. (f) Juarez, B. H.; Klinke, C.; Kornowski, A.; Weller, H. Nano Lett. 2007, 7, 3564.

11226 J. Phys. Chem. C, Vol. 112, No. 30, 2008 (34) (a) Banerjee, S.; Wong, S. S. J. Am. Chem. Soc. 2003, 125, 10342. (b) Banerjee, S.; Wong, S. S. AdV. Mater. 2004, 16, 34. (c) Guldi, D. M.; Aminur Rahman, G. M.; Sgobba, V.; Kotov, N. A.; Bonifazi, D.; Prato, M. J. Am. Chem. Soc. 2006, 128, 2315. (d) Grzelczak, M.; Correa-Duarte, M. A.; Salgueirin˜o-Maceira, V.; Giersig, M.; Diaz, R.; Liz-Marza´n, L. M. AdV. Mater. 2006, 18, 415. (e) Li, W.; Gao, C.; Qian, H.; Yan, D. J. Mater. Chem. 2006, 16, 1852. (f) Engtrakul, C.; Kim, Y. H.; Nedeljkovic, J. M.; Ahrenkiel, S. P.; Gilbert, K. E. H.; Alleman, J. L.; Zhang, S. B.; Micic, O. I.; Nozik, A. J.; Heben, M. J. J. Phys. Chem. B 2006, 110, 25153. (g) Jia, N.; Lian, Q.; Shen, H.; Wang, C.; Li, X.; Yang, Z. Nano Lett. 2007, 7, 2976. (35) (a) Kim, H.; Sigmund, W. J. Cryst. Growth 2003, 255, 114. (b) Du, J.; Fu, L.; Liu, Z.; Han, B.; Li, Z.; Liu, Y.; Sun, Z.; Zhu, D. J. Phys.

Na et al. Chem. B 2005, 109, 12772. (c) Shan, Y.; Gao, L. J. Am. Ceram. Soc. 2006, 89, 759. (d) Gu, F.; Li, C.; Wang, S. Inorg. Chem. 2007, 46, 5343. (36) Li, B.; Xie, Y.; Xu, Y.; Wu, C.; Zhao, Q. J. Phys. Chem. B 2006, 110, 14186. (37) Yu, D.; Chen, Y.; Li, B.; Chen, X.; Zhang, M. Appl. Phys. Lett. 2007, 90, 161103. (38) Lee, H. J.; Yoon, S. W.; Kim, Y. J.; Park, J. Nano Lett. 2007, 7, 778. (39) Jun, Y.; Choi, J.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414. (40) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (41) (a) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183. (b) Peng, X. AdV. Mater. 2003, 15, 459.

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