REPORTS the polycrystalline films used in todays crystal and thin films. This has been seen previou LJ. Belenky, D M. Kim, and H P Sun for their help This leads to an upper bound in film thickness ingle crystals, fibers, and powders of BaTiO, and 000 A. The thinness of the ferroelectric lso constrained. First, there is electric phase of the crystal that locally break the a thickness of about 100 A(17, 22-24)and inversion symmetry (30) 30. G.R. Fox, J.K. Yamamoto, D. V Miller, L. E. Cross, S.K supported in part by the Postdoctoral Fellowship banishes for thicknesses in the 10 to 30 rogram of Korea Science and Engineering Foundation ts ( pinholes and nonuniform thickness over the 31. C. B. Eom et al., Science 258, 1766 pporting Online Material amics(Academic Press, London, 1971), p .sciencemag org/cgi/content/full/ 306/5698/1005 26. J w. Matthews, A E Blakeslee, J. Cryst. Growth 27, 33. v Nagarajan et al. /. Appl. Ph 4. R. Ramesh et aL., Appl. Phys. Lett. 61, 1537(1992 29. A small amount of symmetry forbidden SHG signal 37. We thank L E Cross, S.K. Streiffer, and S. Troller is observed even above Te in both the BaTiO, sing McKinstry for useful discussions. We also thank 26 July 2004: accepted 6 October 2004 Cation Exchange Reactions in 000 kJ/mol in the bulk(20, 21). Ag, Se al hibits an interesting temperature-dependent polymorphism: The superionic conducting lonic Nanocrystals phase transition occurs at a relatively low temperature of 133 C in the bulk phase(22) Dong Hee Son,Steven M. Hughes, Yadong Yin, Thus, it may be possible to prepare Ag,Se A. Paul Alivisatos with unusually high cation mobility. Theses two factors favor complete cation exchange Cation exchange has been investigated in a wide range of nanocrystals of nanocrystals and may prove sufficient to varying composition, size, and shape. Complete and fully reversible exchange overcome the fact that the exchange reaction occurs, and the rates of the reactions are much faster than in bulk cation is completely kinetically hindered at ambient exchange processes. A critical size has been identified below which the shapes temperature and pressure in the bulk of complex nanocrystals evolve toward the equilibrium shape with lowest We investigated the reaction by mixing energy during the exchange reaction. Above the critical size, the anion a solution of CdSe nanocrystals(diameter sublattice remains intact and the basic shapes of the initial nanocrystals are 4.2 nm)in toluene with a small amount of retained throughout the cation exchange. The size-dependent shape change ethanolic solution of Ag NO, under ambien can also be used to infer features of the microscopic mechanism. onditions The volume fraction of methanol in the solution mixture is about 1%: the solution contains Agt ion in a slightly larger Chemical transformations from one solid to can be accompanied by a lowering of phase amount than necessary to replace all the another via insertion and exchange of atoms transition temperatures(17, 18). With the Cd2+ ions in the nanocrystals. Methanol can be used to modify the properties of decrease in the volume, statistical averaging more strongly binds to any free binary crystalline materials (1). Recent develop of the kinetics and mechanisms over a cations in solution and thus favors the ments have enabled the production of many distribution of heterogeneous reaction sites forward reaction. A rapid (<l s) change of technologically important crystalline materi- intrinsic to the bulk solid is also reduced, solution color and complete disappearance of als in nanometer sizes, with a wide range of leading to more homogeneous molecule-like fluorescence is observed upon mixing the size-and shape-tunable properties(2-8). Of reaction kinetics and even different reaction solutions. Measurements of the x-ray diffrac particular interest is the creation of nano- mechanisms in nanocrystals(19). The opti- tion (XRD) patterns and optical absorption crystals with nonequilibrium shapes and with mal use of various chemical transformation spectra confirm that the reaction product is higher structural and compositional complex- methods to broaden the range of nanocrystal- Ag, Se(Fig. 1). The reverse reaction is done ity(9-13). In extended solids, reactions in- line materials depends on an understanding under ambient conditions by mixing Ag, Se volving chemical transformation are in general of how chemical transformations in a crys- nanocrystals with an excess amount(typical very slow because of high activation ener- talline solid will be affected by a reduction ly 50 to 100 times the initial Cd+ content )of gies for the diffusion of atoms and ions inin size. We show that cation exchange re- Cd(NO, mixture of toluene and the solid. For this reason, typical solid-phase actions can occur completely and reversibly in methanol in the presence of tributylphosphine reactions require very high temperatures or ionic nanocrystals at room temperature with (volume fraction <3%) A slower color change pressures(14-16)and therefore would seem unusually fast reaction rates. We also show back to that of CdSe nanocrystals and the to be incompatible with kinetically con- that the crystal structure and morphology of reappearance of fluorescence are observed trolled nonequilibrium nanostructures. the reaction products are strongly dependent over a period of I min. XRD patterns, optical However, in crystals only a few nano- on the size and shape of the nanocrystals. absorption, and fluorescence spectra all indi- meters in size, both the thermodynamics and The prototypical semiconductor nano- cate that CdSe is recovered from the reverse kinetics of reactions can change with size. crystal system of CaSe reacts with Agt ions cation exchange. The XRD linewidths of the For example, a large surface-to-volume ratio to yield Ag, Se nanocrystals by the forward initial and recovered case are nearly identical cation exchange reaction, and vice versa for Moreover, the absorption and fluorescence Materials Sciences Divisio the reverse cation exchange reaction. We ak positions, which show strong size wrence Berkeley chose to work with CdSe nanocrystals dependence due to the quantum confinemer National Laboratory, Berkeley, CA 94720. USA Department of Chemistry, University of California, because of the high degree of control over effect(2), are also nearly identical for the size and shape that has been achieved (2, 3). initial and recovered CaSe nanocrystals. Fi should be addressed. The conversion to Ag, Se is strongly favored nally, transmission electron micrograph E-mail: alvis@berkeley. edu by a thermodynamic driving force of about (TEM) images of the initial and recovered vwwsciencemag org SCIENCE VOL 306 5 NOVEMBER 2004 1009well as the polycrystalline films used in today’s FeRAM. This leads to an upper bound in film thickness of about 1000 A˚. The thinness of the ferroelectric film is also constrained. First, there is an intrinsic finite-size effect in which the Tc begins to decrease at a thickness of about 100 A˚ (17, 22–24) and even￾tually vanishes for thicknesses in the 10 to 30 A˚ range (23, 24). The second reason is that extrinsic effects (pinholes and nonuniform thickness over the capacitor area) lead to unacceptably high leakage currents for FeRAM device operation. 26. J. W. Matthews, A. E. Blakeslee, J. Cryst. Growth 27, 118 (1974). 27. J. Schubert et al., Appl. Phys. Lett. 82, 3460 (2003). 28. M. D. Biegalski et al., unpublished data. 29. A small amount of symmetry-forbidden SHG signal is observed even above Tc in both the BaTiO3 single crystal and thin films. This has been seen previously in single crystals, fibers, and powders of BaTiO3 and is suggested to arise from metastable micropolar re￾gions (compositional or physical defects) in the para￾electric phase of the crystal that locally break the inversion symmetry (30). 30. G. R. Fox, J. K. Yamamoto, D. V. Miller, L. E. Cross, S. K. Kurtz, Mater. Lett. 9, 284 (1990). 31. C. B. Eom et al., Science 258, 1766 (1992). 32. B. Jaffe, W. R. Cook Jr., H. Jaffe, Piezoelectric Ceramics (Academic Press, London, 1971), p. 78. 33. V. Nagarajan et al., J. Appl. Phys. 86, 595 (1999). 34. R. Ramesh et al., Appl. Phys. Lett. 61, 1537 (1992). 35. C. B. Eom et al., Appl. Phys. Lett. 63, 2570 (1993). 36. A. Sharan et al., Phys. Rev. B 69, 214109 (2004). 37. We thank L. E. Cross, S. K. Streiffer, and S. Trolier￾McKinstry for useful discussions. We also thank L. J. Belenky, D. M. Kim, and H. P. Sun for their help with the experiments. Supported by NSF through grants DMR-0313764, ECS-0210449, DMR-0103354, and DMR-0122638 and a David and Lucile Packard Fellowship (C.B.E.). K.J.C. acknowledges that this work was supported in part by the Postdoctoral Fellowship Program of Korea Science and Engineering Foundation (KOSEF). Supporting Online Material www.sciencemag.org/cgi/content/full/306/5698/1005/ DC1 Materials and Methods Figs. S1 and S2 References 26 July 2004; accepted 6 October 2004 Cation Exchange Reactions in Ionic Nanocrystals Dong Hee Son,1 Steven M. Hughes,2 Yadong Yin,1 A. Paul Alivisatos1,2* Cation exchange has been investigated in a wide range of nanocrystals of varying composition, size, and shape. Complete and fully reversible exchange occurs, and the rates of the reactions are much faster than in bulk cation exchange processes. A critical size has been identified below which the shapes of complex nanocrystals evolve toward the equilibrium shape with lowest energy during the exchange reaction. Above the critical size, the anion sublattice remains intact and the basic shapes of the initial nanocrystals are retained throughout the cation exchange. The size-dependent shape change can also be used to infer features of the microscopic mechanism. Chemical transformations from one solid to another via insertion and exchange of atoms can be used to modify the properties of crystalline materials (1). Recent develop￾ments have enabled the production of many technologically important crystalline materi￾als in nanometer sizes, with a wide range of size- and shape-tunable properties (2–8). Of particular interest is the creation of nano￾crystals with nonequilibrium shapes and with higher structural and compositional complex￾ity (9–13). In extended solids, reactions in￾volving chemical transformation are in general very slow because of high activation ener￾gies for the diffusion of atoms and ions in the solid. For this reason, typical solid-phase reactions require very high temperatures or pressures (14–16) and therefore would seem to be incompatible with kinetically con￾trolled nonequilibrium nanostructures. However, in crystals only a few nano￾meters in size, both the thermodynamics and kinetics of reactions can change with size. For example, a large surface-to-volume ratio can be accompanied by a lowering of phase transition temperatures (17, 18). With the decrease in the volume, statistical averaging of the kinetics and mechanisms over a distribution of heterogeneous reaction sites intrinsic to the bulk solid is also reduced, leading to more homogeneous molecule-like reaction kinetics and even different reaction mechanisms in nanocrystals (19). The opti￾mal use of various chemical transformation methods to broaden the range of nanocrystal￾line materials depends on an understanding of how chemical transformations in a crys￾talline solid will be affected by a reduction in size. We show that cation exchange re￾actions can occur completely and reversibly in ionic nanocrystals at room temperature with unusually fast reaction rates. We also show that the crystal structure and morphology of the reaction products are strongly dependent on the size and shape of the nanocrystals. The prototypical semiconductor nano￾crystal system of CdSe reacts with Agþ ions to yield Ag2Se nanocrystals by the forward cation exchange reaction, and vice versa for the reverse cation exchange reaction. We chose to work with CdSe nanocrystals because of the high degree of control over size and shape that has been achieved (2, 3). The conversion to Ag2Se is strongly favored by a thermodynamic driving force of about –1000 kJ/mol in the bulk (20, 21). Ag2Se also exhibits an interesting temperature-dependent polymorphism: The superionic conducting phase transition occurs at a relatively low temperature of 133-C in the bulk phase (22). Thus, it may be possible to prepare Ag2Se with unusually high cation mobility. These two factors favor complete cation exchange in nanocrystals and may prove sufficient to overcome the fact that the exchange reaction is completely kinetically hindered at ambient temperature and pressure in the bulk. We investigated the reaction by mixing a solution of CdSe nanocrystals (diameter 4.2 nm) in toluene with a small amount of methanolic solution of AgNO3 under ambient conditions. The volume fraction of methanol in the solution mixture is about 1%; the solution contains Agþ ion in a slightly larger amount than necessary to replace all the Cd2þ ions in the nanocrystals. Methanol more strongly binds to any free binary cations in solution and thus favors the forward reaction. A rapid (¡1 s) change of solution color and complete disappearance of fluorescence is observed upon mixing the solutions. Measurements of the x-ray diffrac￾tion (XRD) patterns and optical absorption spectra confirm that the reaction product is Ag2Se (Fig. 1). The reverse reaction is done under ambient conditions by mixing Ag2Se nanocrystals with an excess amount (typical￾ly 50 to 100 times the initial Cd2þ content) of Cd(NO3)2 in a mixture of toluene and methanol in the presence of tributylphosphine (volume fraction G3%). A slower color change back to that of CdSe nanocrystals and the reappearance of fluorescence are observed over a period of 1 min. XRD patterns, optical absorption, and fluorescence spectra all indi￾cate that CdSe is recovered from the reverse cation exchange. The XRD linewidths of the initial and recovered case are nearly identical. Moreover, the absorption and fluorescence peak positions, which show strong size dependence due to the quantum confinement effect (2), are also nearly identical for the initial and recovered CdSe nanocrystals. Fi￾nally, transmission electron micrograph (TEM) images of the initial and recovered 1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. 2 Department of Chemistry, University of California, Berkeley, CA 94720, USA. *To whom correspondence should be addressed. E-mail: alivis@berkeley.edu R EPORTS www.sciencemag.org SCIENCE VOL 306 5 NOVEMBER 2004 1009 on February 4, 2008 www.sciencemag.org Downloaded from