pubs.acs.org/NanoLett
Nanowire Transformation by Size-Dependent Cation Exchange Reactions Bin Zhang,‡,§ Yeonwoong Jung,§ Hee-Suk Chung, Lambert Van Vugt, and Ritesh Agarwal* Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 ABSTRACT The unique properties of nanostructured materials enable their transformation into complex, kinetically controlled morphologies that cannot be obtained during their growth. Solution-phase cation-exchange reactions can transform sub-10 nm nanocrystals/nanorods into varying compositions and superlattice structures; however, because of their small size, cation-exchange reaction rates are extremely fast, which limits control over the transformed products and possibilities for obtaining new morphologies. Nanowires are routinely synthesized via gas-phase reactions with 5-200 nm diameters, and their large aspect ratios allow them to be electrically addressed individually. To realize their full potential, it is desirable to develop techniques that can transform nanowires into tunable but precisely controlled morphologies, especially in the gas-phase, to be compatible with nanowire growth schemes. We report transformation of single-crystalline cadmium sulfide nanowires into composition-controlled ZnxCd(1-x)S nanowires, core-shell heterostructures, metal-semiconductor superlattices (Zn-ZnxCd(1-x)S), single-crystalline ZnS nanotubes, and eventually metallic Zn nanowires by utilizing size-dependent cation-exchange reaction along with temperature and gas-phase reactant delivery control. This versatile synthetic ability to transform nanowires offers new opportunities to study size-dependent phenomena at the nanoscale and tune their chemical/physical properties to design reconfigurable circuits. KEYWORDS Cation exchange, nanostructure transformation, size-dependent reaction
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anion sublattice tends to get destroyed due to the rapid reaction. Furthermore, fast reaction rates make it extremely difficult to achieve arbitrary chemical compositions and precisely control the morphology of the nanocrystals.7,9 In addition, most of the cation-exchange research has focused on the solution-phase process in nanocrystals/nanorods of very small size (10 nm length scales. A detailed explanation of the transformations will be discussed later. As we performed the cation-exchange reaction with CdS nanowires of various sizes at different temperatures, we observed an intrinsic size-effect on the reaction that determines the structure/composition of resultant nanowires. The size-dependence of the cation-exchange reaction with wurtzite CdS nanowires (substrate temperature, 220 °C) can be visualized from scanning transmission electron microscopy (STEM) images (Figure 2) of elemental distribution of the resulting nanowires after introduction of a large quantity of DMZn supplied to ensure that the overall process is not limited by the amount of precursor. All the products under these conditions are single-crystalline and some important observations are as follows: thin CdS nanowires (120 nm) display cation exchange localized to a region of 25-30 nm from the surface leading to formation of core-shell heterostructures with CdS and ZnxCd(1-x)S in the core and the shell regions, respectively (Figure 2a-d). An energy-dispersive X-ray spectroscopy (EDS) line-scan profile (Figure 2d) across the 130 nm thick nanowire shows bimodal distribution of Zn with Cd localized toward the core, while the distribution of S is uniform throughout the structure, which indicates that cation exchange initiates from the surface and proceeds
dimethylzinc (DMZn) were chosen as the reactants to construct a model cation-exchange system. Precise control of the amount of DMZn is a critical component for the cationexchange reaction, which was obtained by using a homebuilt atomic layer deposition (ALD) setup equipped with a high-speed valve to supply the precursor pulses. The silicon substrate with CdS nanowires of uniform diameters was placed at the center of a horizontal tube furnace. The furnace temperature was slowly raised to reaction temperatures, and the quartz tube was pumped down to a base pressure of 0.01 Torr with pure Ar (99.99% purity) introduced at a flow rate of 20 sccm. Subsequently, vapor-phase DMZn was introduced into the higher temperature zone of the furnace, supplied from a bubbler (Epichem) maintained at room temperature (vapor pressure, 160 Torr). The specific amount of DMZn was controlled by a high-speed ALD switch (National Instruments Co.), which was responsive to millisecond pulses (fastest pulse delivers DMZn for 10 ms). This step was automated by a LabView software. After the application of DMZn, the furnace was maintained for 20 min to allow for the cation-exchange reaction, after which it was cooled to room temperature. The as-prepared samples were immediately transferred to a transmission electron microscopy (TEM) grid for the electron microscopy analysis. By controlled delivery of DMZn, we transform the CdS nanowire into new morphologies with controlled composition, illustrating novel nanoscale chemical properties (overview in Figure 1). Depending on the extent of the cation-exchange reaction (exchange of Cd2+ with Zn2+) controlled by the amount of DMZn, the nanowire is transformed into various morphologies: with increasing the amount of DMZn, the transformation proceeds in a sequence of CdxZn(1-x)S alloyed nanowires, Zntube-CdxZn(1-x)S superlattices, ZnS nanotubes, © 2010 American Chemical Society
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DOI: 10.1021/nl903059c | Nano Lett. 2010, 10, 149-155
ent activation energies. To ensure that the reaction with different sized CdS nanowires is not limited by the amount of precursor, we applied 500 ms pulses (no significant change beyond 500 ms) of DMZn at temperatures ranging from 80 to 180 °C, and examined the EDS spectra of the compositionally uniform ZnxCd(1-x)S alloyed nanowires. The Arrhenius-type plot of the atomic ratio of zinc to cadmium (Zn2+/Cd2+) of nanowires with different diameters (Figure 2g) clearly displays that the vapor-phase cation exchange is a thermally activated process, and the activation energies (sum of the diffusion and the exchange reaction), determined from the slopes of the plots, decrease with the nanowire diameter (Ea (90 nm) ) 0.212 eV to Ea (30 nm) ) 0.143 eV). The measured size-dependent activation energies for the cation-exchange reaction are much less than the diffusion activation energies of group II cations in II-VI bulk semiconductors (∼1.8-2.0 eV),19,20 demonstrating that the overall cation-exchange process at the nanoscale is much faster than diffusion in bulk materials. This observation demonstrates that both the diffusion and the exchange reaction are strongly size dependent, although we cannot separate their contribution to the overall measured activation energies. However, in the size regime of 30-100 nm and temperatures above 80 °C, the partially exchanged nanowires are compositionally uniform, suggesting that the overall process is not diffusion-limited under these conditions. If the process were diffusion-limited, then we expect to observe radial composition gradient, which is indeed the case for either thick nanowires (>125 nm), which form core-shell heterostructures, or reaction at very low temperatures when we only observe a roughened surface with little Zn. In the case of ion-exchange processes, the overall reaction reflects the combined process of diffusion and reaction, with the activation energy of reaction typically higher than that of diffusion,21 in agreement with our nanowire results. Once the diffusion barrier is overcome (T > 80 °C, and nanowire size < 100 nm), any increase in temperature increases the reaction rate much higher than diffusion, and the effect is reflected in our size-dependent activation energy measurements. The size-dependent diffusion and alloying in nanocrystals has been previously reported,22-27 which has been attributed to various factors such as suppressed melting temperatures,23 enhanced interfacial reactivity due to defects,24-26 and increase in the vibrational amplitude of surface atoms,26,27 possibly further promoted by the enhanced radial strain.28 As demonstrated above, nanowires in the 5-200 nm size-regime allow detailed investigations of size-dependent reaction and diffusion mechanisms, bridging the gap between the bulk and the nanocrystals range where the kinetics are extremely fast to be resolved. We now present the morphological and compositional variation of CdS nanowires undergoing the cation-exchange process at higher temperatures with systematically increasing the amount of DMZn. Since the thickness of nanowires
FIGURE 2. Size-dependent cation-exchange reaction in CdS nanowires. (a-d), STEM elemental mapping images of transformed CdS nanowires of various thickness after introduction of DMZn (500 ms pulse) at 220 °C. The spatial distribution of each element is shown in different colors: (a) Zn, (b) Cd, (c) S, and (d) overall reconstructed elemental distribution. Thin nanowires (125 nm) display partial cation exchange to form CdS/ZnCdS core-shell heterostructures. Inset in panel d shows a line-scan EDS profile obtained along the red dotted-line from the thick core-shell nanowire, revealing bimodal distribution of Zn and S with Cd localized toward the core. (e) High-resolution TEM (HRTEM) of an intermediate sized CdS nanowire (70 nm) transformed to ZnxCd(1-x)S with x ∼ 0.7 in singlecrystalline structure (top inset) and the corresponding EDS line scans along the radial direction (bottom inset). (f) HRTEM of thin nanowires (6 nm) after being completely transformed to single-crystalline ZnS nanowires at 60 °C with the corresponding EDS spectrum (bottom inset). Top inset; transformed ZnS nanowires with stacking faults and catalysts. (g) Nanowire thickness-dependent Arrhenius plots of the natural logarithm of the atomic ratio of Zn2+ to Cd2+ (ln(Zn2+/Cd2+)) versus 1/kT of CdxZn1-xS alloy nanowires obtained from quantitative EDS point scan analysis, measured from 80 to 180 °C. The activation energies are 0.143, 0.171, and 0.212 eV for 30, 50, and 90 nm, respectively.
toward the core. Complete transformation of extremely thin CdS nanowires (800 °C), and did not transform into any of the complex morphologies as observed with DMZn. All our cation-exchange reactions were performed below 350 °C, where DMZn pyrolyzes to form methyl and methylzinc radicals31 which may also play a role in the formation of complex morphologies obtained due to the reaction of methyl radicals with sulfur, further facilitating its outward diffusion in addition to the role played by strain. It is known that S2- reacts rapidly with methyl radicals in solution-phase,32 but its reactivity in the solid-state is low; however, since the reaction activation energies are much lower for nanostructures than in bulk, it is possible that S2- in the CdS nanowire reacts easily with the radicals promoting its outward diffusion, especially becoming more pronounced at very high DMZn concentration. We also note that reaction with DMZn (>100 ms pulses) at 400 °C produced nanowires with large voids throughout the structures, while reaction at 450 °C leads to complete distortion of the original nanowires (see Supporting Information Figure S12), suggesting a different mechanism possibly driven by radical-based reactions compared to simple cation exchange with ZnCl2 or Zn performed at much higher temperatures. The ability to precisely control the amount of vapor-phase reactant is critical to transform CdS nanowires into unusual morphologies via cation-exchange process at 5-200 nm length scales, not typically accessible for smaller nanocrystals. A unique strength of our method is its versatility, since different morphologies and compositions can be obtained by using the same chemical precursors and utilizing the sizeand temperature-dependent cation-exchange reaction rates. The interplay of strain and radical chemistry-driven outward diffusion of elements in controlling the composition and morphology of the nanostructures needs further investigation and will stimulate new theoretical and experimental studies to understand, predict, and control the formation of complex nanostructures. The observed morphological evolution of CdS nanowires opens up new ways to transform inorganic nanowires into chemically diverse 154
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structures, which cannot be obtained during their growth. Furthermore, it may be possible to reconfigure preassembled nanowire-based circuits into different functionalities by controlling nanowire morphology and composition via vapor-phase cation-exchange reactions.
(10) Kovalenko, M. V.; Talapin, D. V.; Loi, M. A.; Cordella, F.; Hesser, G.; Bodnarchuk, M. I.; Heiss, W. Angew. Chem., Int. Ed. 2008, 47, 3029. (11) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (12) Henkes, A. E.; Vasquez, Y.; Schaak, R. E. J. Am. Chem. Soc. 2007, 129, 1896. (13) Fan, H. J.; Knez, M.; Scholz, R.; Nielsch, K.; Pippel, E.; Hesse, D.; Zacharias, M.; Gosele, U. Nat. Mater. 2006, 5, 627. (14) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science 2004, 304, 1787. (15) Backhaus-Ricoult, M. Annu. Rev. Mater. Res. 2003, 33, 55. (16) Park, W., II; Kim, H. S.; Jang, S. Y.; Park, J.; Bae, S. Y.; Jung, M.; Lee, H.; Kim, J. J. Mater. Chem. 2008, 18, 875. (17) Lee, J. Y.; Kim, D. S.; Park, J. Chem. Mater. 2007, 19, 4663. (18) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 189. (19) Martin, W. E. J. Appl. Phys. 1973, 44, 5639. (20) Woodbury, H. H. Phys. Rev. 1964, 134, A492. (21) Helfferich, F. G. Ion Exchange; Dover Publications: New York, 1995. (22) Horvath, J.; Birringer, R.; Gleiter, H. Solid State Commun. 1987, 62, 319. (23) Dick, K.; Dhanasekaran, T.; Zhang, Z.; Meisel, D. J. Am. Chem. Soc. 2002, 124, 2312. (24) Shibata, T.; Bunker, B. A.; Zhang, Z.; Meisel, D.; Vardeman, C. F., II; Gezelter, J. D. J. Am. Chem. Soc. 2002, 124, 11989. (25) Ouyang, G.; Tan, X.; Wang, C. X.; Yang, G. W. Chem. Phys. Lett. 2002, 420, 65. (26) Shimizu, Y.; Ikeda, K. S.; Sawada, S.-I. Phys. Rev. B. 2001, 64, No. 075412. (27) Buffat, P.; Borel, J.-P. Phys. Rev. A 1976, 13, 2287. (28) Nichols, F. A.; Mullins, W. W. Trans. Metall. Soc. AIME 1965, 233, 1840. (29) Cabot, A.; Smith, R. K.; Yin, Y.; Zheng, H.; Reinhard, B. M.; Liu, H.; Alivisatos, A. P. ACS Nano 2008, 2, 1452. (30) Zhdanov, V. P.; Kasemo, B. Nano Lett. 2009, 9, 2172. (31) Price, S. J. W.; Trotman-Dickenson, A. F. Trans. Faraday Soc. 1957, 53, 1208. (32) Romeo, R.; Wozniak, L. A.; Chatgilialoglu, C. Tetrahedron Lett. 2000, 41, 9899.
Acknowledgment. This work was supported by the NSFCAREER award (ECS-0644737) and Penn-MRSEC seed grant (DMR05-20020). Electron microscopy experiments were performed at the Penn Regional Nanotechnology Facility at the University of Pennsylvania. Supporting Information Available. Additional SEM/TEM images and EDS spectra as described in the text. This material is available free of charge via the Internet at http:// pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9)
Whitesides, G. M. Small 2005, 1, 172. Lieber, C. M.; Wang, Z. L. MRS Bull. 2007, 32, 99. Law, M.; Sirbuly, D. J.; Johnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. Science 2004, 305, 1269. Nam, J. M.; Thaxton, C. S.; Mirkin, C. A. Science 2003, 301, 1884. Yin, Y.; Erdonmez, C.; Aloni, S.; Alivisatos, A. P. J. Am. Chem. Soc. 2006, 128, 12671. Jeong, U.; Camargo, P. H. C.; Lee, Y. H.; Xia, Y. J. Mater. Chem. 2006, 16, 3893. Son, D. H.; Hughes, S. M.; Yin, Y.; Alivisatos, A. P. Science 2004, 306, 1009. Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, L-W.; Alivisatos, A. P. Science 2007, 317, 355. Wark, S. E.; Hsia, C. H.; Son, D. H. J. Am. Chem. Soc. 2008, 130, 9550.
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