Letter pubs.acs.org/JPCL
Zinc Blende 0D Quantum Dots to Wurtzite 1D Quantum Wires: The Oriented Attachment and Phase Change in ZnSe Nanostructures Suresh Sarkar,†,‡ Shinjita Acharya,†,‡ Arup Chakraborty,§ and Narayan Pradhan*,†,‡ †
Department of Materials Science, ‡Centre for Advanced Materials and §Department of Solid State Physics, Indian Association for the Cultivation of Science, Kolkata, 700032, India S Supporting Information *
ABSTRACT: Oriented attachment of nanocrystals has been recently studied as one of the important tools to organize the nanocrystals in a regular array to design new nanostructures. This is mostly a thermodynamically driven process where the nanocrystals align in a certain crystallographic direction and merge, minimizing the interfacial energy of the system during the course of reaction. While this has been widely studied for several group II−VI semiconductor nanocrystals, we explore herein ZnSe 0D quantum dots which on merging change to 1D quantum nanowires. Importantly, the phase of the nanocrystals is found to be transformed from zinc blende to wurtzite after the fusion. To understand this, we have analyzed the intermediate samples and studied the high-resolution transmission electron microscopy (HRTEM) of single, twin, and triple connected dots as well as the final nanowires and address the phase change during the shape conversion. Additionally, we have provided density functional theory (DFT) calculation to support our experimental observations. SECTION: Physical Processes in Nanomaterials and Nanostructures
Z
crystallographic orientation and annihilate their sharp facet boundaries, leading to different shapes of nanostructures.11,14,25−29 Several theoretical calculations are also performed to support the experimental observations of this phenomenon.24,30,31 Herein, we explore it in ZnSe, where the 0D dots transform into 1D quantum wires, and, interestingly, a twisting in the crystal phase has been observed during this crystal fusion. The ZB phase of the dots is changed to the WZ phase after the complete attachment of the dots to wires. Although such phase change during oriented attachment has been reported in several other materials,25,26,32,33 to our knowledge this has not been reported for ZnSe semiconductor nanomaterial. This has been addressed by analyzing different intermediate twin and triple fused ZnSe dots along with the final product of solid nanowires. In addition, several physical parameters that control this oriented attachment have been discussed. Further, our experimental results of the oriented attachment and subsequent phase transformation have been supported by density functional theory (DFT) calculations. To study the oriented attachment in ZnSe nanostructure, we have synthesized ZnSe quantum wires following the modified literature method.18 Exploiting Zn-carboxylate as Zn and selenourea as Se precursors and annealing their mixture in polar alkylamine solvent at 150 °C, ∼1.2 nm nanowires are obtained (transmission electron microscopic (TEM) images shown in Figure S1 in the Supporting Information, SI). The
nSe is a high bandgap semiconducting material with bulk bandgap ∼2.7 eV.1 Unlike other leading group II−VI semiconductor nanomaterials, synthesis of high-quality ZnSe nanocrystals remains very sensitive and requires hectic air-free synthetic procedures.1−5 A literature search reveals that the design of different ZnSe nanostructures and investigation of their solution phase crystal growth have neither been thoroughly understood nor achieved. Quantum confined tunable one-dimensional (1D) nanostructures, self-assembled nanorods, oriented attachment of dots to form different complex structures, tetrapod-like multiphase nanostructures, and so on are well established for different semiconductors,6−14 but to date ZnSe mostly remains unexplored. For ZnSe, while the traditional crystal growth of zero-dimensional (0D) nanocrystals in solution mostly occurs in zinc blende (ZB),2,4,5,15−21 the rods or wires are preferably grown in wurtzite (WZ) phase.18 Literature survey reveals that even more than two decades have passed since the development of high-quality semiconductor nanocrystals,22 the physics and chemistry in designing various shapes of ZnSe nanostructures are still lagging behind relative to the dominating cadmiumbased semiconductor nanomaterials. Motivated by the current progress in crystal growth and designing of various shapes of semiconductor nanostructures, we study here the facet-directed and polarity-driven oriented attachment of ZnSe 0D quantum dots. Oriented attachment is commonly understood as the fusion of nanocrystals by sharing its common crystallographic orientation, thermodynamically driven by the reduction of interfacial energy.11,14,23−26 Under certain reaction conditions, the nanocrystals align in a definite © 2013 American Chemical Society
Received: August 24, 2013 Accepted: September 16, 2013 Published: September 16, 2013 3292
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range, and to obtain wider quantum wires, the seed dots must be grown larger. To achieve this, we have adopted the hot injection protocol where the mixture of Zn-stearate and selenourea in alkylamine at 100 °C is swiftly injected to a preheated alkylamine solvent kept in a reaction flask at 250 °C. With a molar ratio of 1:3 of Zn (0.1 mmol of ZnSt2) and Se (0.3 mmol of selenourea), the nanocrystals are observed to be grown to ∼3.2 nm and the obtained nanowires exhibit a similar diameter (details of the experimental procedures are provided in the Supporting Information). However, at the initial stage of reaction, the collected aliquots are mostly populated with connected dots, but aliquots collected in later stages of the reaction with due course of annealing show solid nanowires only. Figure 2a,b shows the TEM images of different resolutions of the 3.2 nm diameter 0D dots, connected dots, and the solid 1D nanowires. Magnified images shown in Figure 2b−d show the crystallographically oriented alignment of dots, fused dots, and the long nanowires. Figure 2e shows the absorption spectrum of these mixture of nanostructures, and it reflects that the band edge of the nanocrystals remains below the bulk ZnSe, supporting the quantum confinement in these nanocrystals. Further, to verify whether the nanowires are formed via oriented attachment of seed dots or if it is the mixture of dots and wires as observed in the TEM images, HRTEM images at different regions are analyzed. Figure 3a−c shows the HRTEM images of the connected dots. These images clearly reflect that the dots are indeed fused (arrow marks), and these are expected to be the prestage of the nanowire formation. Figure 3d shows a single ZnSe dot. An atomic model for this single dot is shown in the inset. Figure 3e,f presents the twin and fused tripled dots. The calculated d-spacing of 0.28 and 0.328 nm corresponds to the {002} and {111} planes of the ZB phase of ZnSe, respectively. Accordingly, the dots in Figure 3e,f are fused through the [111] direction. This suggests that the ZB
high-resolution TEM (HRTEM) analysis for the formation of these nanostructures from the seed nanocrystals has not yet been established. However, careful analysis of the intermediate stage reveals that these nanowires are formed from the oriented attachment of the magic size ∼1.2 nm ZnSe dots. The sample collected at the early stage of the reaction mixture at 150 °C suggests that these wires are the linear array of small ZnSe 0D dots. The TEM image presented in Figure 1a of the
Figure 1. (a) Typical TEM image of connected magic size dots on their way to form nanowires with magic size diameter. An enlarged image of this TEM is shown in Figure S2. (b) Absorption spectrum showing the dual peaks.
intermediate stage shows connected dots. The magic size nature of these nanowires is further reflected from the sharp dual absorption peak in their UV−visible optical spectra (Figure 1b). The narrow and dual absorption peaks are the signature of magic size clusters, which has been reported for several other magic size nanostructures including ZnSe.6,18,25,34−37 However, due to the small dimension of the magic size nanocrystals, the HRTEM analysis and the oriented attachment could not be studied for these nanowires. These wires possess a width of ∼1.2 nm with length in the micrometer
Figure 2. (a) TEM images showing ZnSe dots, connected dots, and the nanowires. (b−d) Magnified TEM images obtained from different regions of the TEM grid. (e) Absorption spectrum of the mixture of connected dots and wires. 3293
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Figure 3. (a−c) HRTEM images showing the nanowires and fused ZnSe dots. These are observed at different regions on the TEM grid. HRTEM image of a (d) single and (e,f) fused twin and triple ZnSe dots, respectively. Inset of panel d shows an atomic model of a single ZB dot, and that of panel f presents the selected area FFT pattern.
the interface of the seed nanocrystals to allow the oriented attachment. Here, the phenomenon is monitored based on the anchoring capability of ligands: the strong versus weak dynamic ligands where the former keeps the ZB ZnSe particles isolated and the later aligns them to form nanowires. However, a little twist in optimized synthetic protocol can also lead to connected dots only in short-range. The precursors Zn-stearate and selenourea and their ratio have been observed to play a pivotal role in this oriented attachment process. Figure S4a,b shows the TEM and XRD of the ZnSe nanocrystals obtained with a Zn to Se ratio of 2:1 instead of the optimized ratio of 1:3. In this case, with the relatively lower amount of selenium precursor, the overall reactivity of the precursors slows down, and hence only short-range connected dots are observed. Even further increase of reaction temperature or annealing for longer time does not lead to the formation of long wires. The results clearly suggest that almost all the dots are connected, and the XRD peaks corroborate with the retention of the ZB phase at this connected dot stage. It has also been further observed that with the higher ratio of Se to Zn, the rate of formation of WZ nanowires increases. Decomposition of selenourea in alkyl amine results in H2Se gas, which is the reactive Se-precursor.38 Hence, with increasing the relative amount of selenourea, the population of active anions increases in the reaction system, and, consequently, the growth rate of ZnSe increases. We have observed that with higher relative ratio of selenourea instead of the optimized Zn:Se molar ratio of 1:3, trapping of connected dots is very much difficult as the dots are merged to nanowires faster, leading to the formation of solid WZ nanowires (Figure S5). On the contrary, the relative Zn-rich system allows the reaction more time for the formation of nanowires, and clear connected dots can be obtained as the intermediate in this case. However, the important factor here is the polarity-induced driving force that allows the seed ZB dots to align in particular crystallographic orientation. For the ZB dots, Zn and Se atoms are alternatively placed along the [111] direction (shown in the scheme in Figure 5a), and this makes the [111] direction polar. This polarity preferably drives the nanocrystals to arrange linearly along this direction. Once the seed particles are aligned, the
seeds are linearly aligned along [111] direction before the fusion to form nanowires. Interestingly, when the HRTEM images of the nanowires are analyzed, their planes and crystal structure resemble that of WZ ZnSe. Figure 4a shows an HRTEM image of a section of the nanowires. The selected area fast Fourier transform (FFT) is shown in Figure 4b, and the simulated HRTEM from the FFT is presented in Figure 4c. From the FFT in Figure 4b, it is clearly observed that these wires are in WZ phase and they grow along the polar [001] direction. Figure 4d also represents the HRTEM of the section of another set of nanowires having crystal structure similar to that of ZnSe. A simulated image of the nanowire with marked direction has been shown in Figure 4e showing the WZ structure. Importantly, the WZ phase of the nanowires is confirmed by X-ray diffraction (XRD) analysis. The powder XRD pattern (Figure 4f) shows the presence of mixed phase structures where (210) and (200) peaks exhibit much higher intensity than that of (103). The former two peaks resemble (022) and (113) peaks of ZB ZnSe, respectively. These results are also compatible with the TEM data, which shows the presence of a mixture of ZB single dots and connected dots, and nanowires of WZ phase. Hence, from all these observations we can conclude that the quantum dots change their phase from ZB to WZ during the oriented attachment forming quantum wires. Further, we study different controlling physical parameters associated with the oriented attachment of these dots. The nanocrystal fusion is observed in the alkylamine solvent where the fatty amine also acts as capping ligand. Interestingly, when alkylthiol or trioctylphosphine are added to the reaction system, the ZB dots remain isolated throughout the reaction and do not undergo either oriented attachment or any kind of phase transformation (Figure S3a,b). The presence of strong ligands on the surface of nanocrystals prevents their fusion and restricts the atomic deformation to change the phase from ZB to WZ. It is expected that the ligands from the approaching facets need to be removed dynamically during the attachment.34 Consequently, in case of thiol or phosphine ligands, the reaction does not lead to the nanowire formation as observed in alkylamine ligands. Hence, the labile surface ligands are essential to be at 3294
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with another having a Se terminate of the ZB nanocrystals. This is schematically shown in Figure 5a. However, the most important and unique observation here in ZnSe is the phase alteration from ZB to WZ during the oriented attachment process. To address this, we have analyzed the atomic patterns of the ZB and WZ structure along their [111] and [001] directions, respectively, and these are shown schematically in Figure 5a,b. Here, ABCABC atomic arrangements of the ZB phase along its [111] direction (Figure 5a) changes to ABABA of the WZ along the [001] direction (Figure 5b). It is also well known that the energy difference between the (111) facets of ZB and (002) facets of WZ is minimum.39 For our case, here in ZnSe, this minimum energy barrier is crossed during the thermal annealing, and the ZB dots are linearly merged as observed in CdTe even in room temperature.25 Moreover, the WZ phase of ZnSe is stable in the 1D nanostructure, and hence ultimately the dots change the atomic arrangement from ZB to WZ phase after the oriented attachment and subsequent wire formation. Further, to support this oriented attachment and phase transformation, we have calculated the formation energy for the assembled ZB ZnSe dots and the WZ 1D nanowire/nanorod in the framework of DFT, as implemented in Vienna ab initio simulation package (VASP)40,41 (more computational details are provided in the Supporting Information). For this, we have simulated the coupled dots of ZnSe in ZB structure, made of two single quantum dots attached along the [111] direction and a WZ ZnSe nanorod with c-axis along the [001] direction. The radius of each quantum dot is taken as 0.6 nm (excluding the passivator atoms), and it contains 20 Zn and 20 Se atoms. Similarly, the radius of the cross-section of the WZ rod is taken as 0.7 nm and keeping the length equivalent to that for the coupled dot along the [111] direction (excluding the passivator atoms) containing 72 Zn and 72 Se atoms. Both atomic models are schematically shown in Figures S6−S8. The formation energy (EF) in each case has been extracted from the total energy calculations for the coupled dots and corresponding rod. The following formula42 has been followed for the calculation of EF. E F = E(total) − nZn × μ Zn − nSe × μSe − E H
Here, E (total) is the total energy of the passivated systems (coupled dots or nanorod), and nZn, nSe, μZn, and μSe are the number of Zn atoms and Se atoms in the system and their chemical potential, respectively. The EH is the contribution to the total energy by the passivator hydrogen atoms and is calculated from the difference in energies between passivated and unpassivated systems. Following this equation, the calculated formation energy per pair of ZnSe for the ZB coupled dots and the WZ rod are found to be −2.320 eV and −2.663 eV, respectively. Details of the calculated values are provided in Table S1 in the Supporting Information. This suggests that the formation energy of the WZ rod is less than the coupled ZB dots. It reflects that the WZ nanorod of ZnSe formation is more energetically favorable than the assembled quantum dots in the ZB phase. We have also extended the number of unit cells in the case of the nanorod and observed that the formation energy per ZnSe is lower in the case of the WZ 1D nanostructure than that of the ZB connected 0D dots. To our understanding, these results are in good agreement with experimental observations. In summary, we address here the oriented attachment in ZnSe during the shape and phase transformation of the seed
Figure 4. (a) HRTEM image of a section of a nanowire. (b) Selected area FFT and (c) simulated HRTEM obtained from the simulated FFT (inset). (d) HRTEM image of another section of the nanowires and (e) atomic model of the WZ nanowire. (f) XRD of the sample, which shows the mixed phase with existence of both the ZB and WZ phase.
dipole moment along the same direction is further enhanced, which in turn attracts even more dots to align in the same crystallographic orientation, leading to the formation of a long nanowire.14 The lattice plane matching is pivotal for this kind of fusion of dots to nanowire transformation. With the approach of two facets of two different nanocrystals, the energy of the system needs to be minimized, and this is possible when the appropriate facets approach each other. This has been observed here where the (111) facet of one having Zn terminate fuses 3295
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Figure 5. (a) Atomic model of coupled ZnSe ZB dots and (b) Models WZ rod in different viewing directions. After the oriented attachment and phase change, the (111) planes of ZB dots change to the (002) plane of WZ, and, accordingly, the ABCABC atomic arrangement pattern changes to ABABA type. (3) Reiss, P. ZnSe based Colloidal Nanocrystals: Synthesis, Shape Control, Core/Shell, Alloy and Doped Systems. New J. Chem. 2007, 31, 1843−1852. (4) Pradhan, N.; Goorskey, D.; Thessing, J.; Peng, X. An Alternative of CdSe Nanocrystal Emitters: Pure and Tunable Impurity Emissions in ZnSe Nanocrystals. J. Am. Chem. Soc. 2005, 127, 17586−17587. (5) Acharya, S.; Sarma, D. D.; Jana, N. R.; Pradhan, N. An Alternate Route to High-Quality ZnSe and Mn-Doped ZnSe Nanocrystals. J. Phys. Chem. Lett. 2010, 1, 485−488. (6) Peng, Z. A.; Peng, X. Nearly Monodisperse and Shape-Controlled CdSe Nanocrystals via Alternative Routes: Nucleation and Growth. J. Am. Chem. Soc. 2002, 124, 3343−3353. (7) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Controlled Growth of Tetrapod-Branched Inorganic Nanocrystals. Nat. Mater. 2003, 2, 382−385. (8) Li, L. -S.; Walda, J.; Manna, L.; Alivisatos, A. P. Semiconductor Nanorod Liquid Crystals and Their Assembly on a Substrate. Adv. Mater. 2003, 15, 408−411. (9) Deka, S.; Miszta, K.; Dorfs, D.; Genovese, A.; Bertoni, G.; Manna, L. Octapod-Shaped Colloidal Nanocrystals of Cadmium Chalcogenides via “One-Pot” Cation Exchange and Seeded Growth. Nano Lett. 2010, 10, 3770−3776. (10) Deng, Z.; Yan, H.; Liu, Y. Controlled Colloidal Growth of Ultrathin Single-Crystal ZnS Nanowires with a Magic-Size Diameter. Angew. Chem., Int. Ed. 2010, 49, 8695−8698. (11) Yu, J. H.; Joo, J.; Park, H. M.; Baik, S. -I.; Kim, Y. W.; Kim, S. C.; Hyeon, T. Synthesis of Quantum-Sized Cubic ZnS Nanorods by the Oriented Attachment Mechanism. J. Am. Chem. Soc. 2005, 127, 5662− 5670. (12) Deng, Z.; Tong, L.; Flores, M.; Lin, S.; Cheng, J. -X.; Yan, H.; Liu, Y. High-Quality Manganese-Doped Zinc Sulfide Quantum Rods with Tunable Dual-Color and Multiphoton Emissions. J. Am. Chem. Soc. 2011, 133, 5389−5396. (13) Cho, K. -S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. Designing PbSe Nanowires and Nanorings through Oriented Attachment of Nanoparticles. J. Am. Chem. Soc. 2005, 127, 7140−7147. (14) Koh, W.-k.; Bartnik, A. C.; Wise, F. W.; Murray, C. B. Synthesis of Monodisperse PbSe Nanorods: A Case for Oriented Attachment. J. Am. Chem. Soc. 2010, 132, 3909−3913. (15) Cozzoli, P. D.; Manna, L.; Curri, M. L.; Kudera, S.; Giannini, C.; Striccoli, M.; Agostiano, A. Shape and Phase Control of Colloidal ZnSe Nanocrystals. Chem. Mater. 2005, 17, 1296−1306.
0D quantum dots into 1D quantum wires and address different associated physical parameters associated with this process. Importantly, here the phase change has been observed during this shape conversion, which is unique and different from several other reports. Further, this has been supported by the DFT calculation of the formation energy per Zn−Se using coupled ZB ZnSe dots and a WZ nanowire. As ZnSe remains one of the least explored semiconducting nanomaterials among the group II−VI semiconductors, this observation of the oriented attachment would help physical and material chemists to understand more about associated physical parameters for solution-based crystal growth and shape conversion in the nanodimension.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Experimental details, instrumentation, and supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS DST of India is acknowledged for funding. S.S. and S.A. acknowledge CSIR-India for fellowship. N.P. Acknowledges DST Swarnajaynti Fellowship. We are thankful to Prof Indra Dasgupta for valuable discussion. Authors S.S., S.A., and A.C. equally contributed to this work.
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