letters to nature used by new types of algorithms (4)Aside from the input and nanospheres, such as anisotropic metal nanostructures with out lines no further means are necessary for the logic operation, interesting optical properties -2. Here we demonstrate that for example, additional magnetic fields, switching thresholds, or the previously described photoinduced method for converting voltages(5)The switching frequency of magnetic films can be silver nanospheres into triangular silver nanocrystals-so-called pushed to several GHz, in principle allowing fast operation".(6) nanoprisms-can be extended to synthesize relatively mono- Since only short current pulses are required to rotate the magnetiza- disperse nanoprisms with desired edge lengths in the 30- tion, the energy consumption is further reduced. (7) The size of an 120 nm range. The particle growth process is controlled using MRAM cell-commercially available in 2004(ref. 12)-can easily be dual-beam illumination of the nanoparticles, and appears to be ed down to less than 100 nm(refs 13, 14)to further increase the driven by surface plasmon excitations. we find that, depending integration density.(8)Our approach a priori does not necessarily on the illumination wavelengths chosen, the plasmon excitations require semiconductors to fabricate a logic gate. (9) The run-time lead either to fusion of nanoprisms in an edge-selective manner programmability will increase the computational efficiency or to the growth of the nanoprisms until they reach their light Our new approach has the potential to induce a paradigm shift controlled final size. from transistor-based logic to magneto-logic, where the program- The photoinduced synthesis of silver(Ag)nanoprisms involves mable functionality and non-volatility are as important as minia- the preparation of a colloidal suspension of Ag spheres(diameter turization and clock speed 0 <10nm), followed by conversion of the spheres to larger prism structures with visible light. In a typical experiment, colloidal Ag Received 12 February: accepted I September 2003: doi:lo 1038/nature02014. nanoparticles passivated with sodium citrate and bis(p-sulphonato- Sctence282,1660-1663(1998) 2. Sidhu, R. P S, Mei, A& Prasanna, V K in Field-programmable logic and Applications(eds Lysaght, P, (eds lysaght, &. phenyl)phenylphosphine dihydrate dipotassium(BSPP)(diameter Irvine,L&Hartenstein.R.W)301-312(Lecture Notes in Computer Science 1673. Springer, Berlin, 4.8+ 1.1nm, standing solution)were irradiated with a narrow-I band light source(using a 150 W xenon lamp with a light output 3.Parkin,SSPet al. Exchange-biased magnetic tunned junctions and application to nonvolatile "12 W)with an optical bandpass filter(centre wavelength 550nm 4. Grunberg P Layered magnetic structures history, highlights, applications. Phys. Today 54, 31-37(2001). width 40 nm)for -50h(see Supplementary Information). Trans- 5. Balbach,且.N.td Lett. 61 5(IleA agnetoresistance of (oon)Fe/(001)Cr magnetic superlattices Plys Rex mission electron microscopy (TEM) shows that the colloid formed 6. Binasch, G,Grinberg P, Saurenbach, E&Zinn, W.Enhanced magnetoresistance in layered magnetic and inset), with the smaller particles( designated as type 1)and the structures with antiferromagnetic interlayer exchange. Phys. Rev. B 39, 4828-4830(1989) 7. Moodera, I.S., Kinder, L R, Wong, T. M. Meservey, R. Large magnetoresistance at larger particles(type 2)having average edge lengths of 70 12 nr netic thin film tunnel junctions. Phys. Rev. Left. 74, 3273-3276(1995). and 150 16 nm, respectively. These structures tend to form stacks, 8. Black, w. C. Ir Das, B. Progr able logic using siant-magnetoresistance and spin-dependent so that edge-on views allow the precise determination of nanoprism tunneling devices. I. Appl. Phys. 87, 6674-6679(2000). thickness". Although the average edge lengths for the type 1 and 9. Richter, R et al. ield programmable spin-logic based on magnetic tunneling elements. A Magn. type 2 nanoprisms are significantly different, their thicknesses are oceedings of SSGRR 2000, Intermational Conference on Advances in Infrastructure for almost identical (9.8+1.01 ElectronicBuesiness,Sciernece,andEducatiomontheiNternet,at(htprl/www.sgrtiten/ssgr2000/papersThebimodalparticlegrowthprocessalsohasbeenmonitored 11. Gerrits, Th, van den Berg. H. A M, Hohlfeld, L, Bar, L& Rasing, Th. Ultrafast Precessional by ultraviolet-visible-near-infrared(UV-vis-NIR)spectroscopy (Fig. 2a). During the reaction, one sees the disappearance of the 12. Motorola Inc. Press release. IEEE International Solid State Circuits Conference(San Francisco, June plasmon band at -395 nm(characteristic of the spherical silver particles )and the formation of two new, strong plasmon bands at 680 nm and 1,065 nm that are associated with the type I and type 2 egel, R w. Hu, E& Roco, M. C)67-92(National Science and Technology Council (NSTC Committee on Technology and The Interagency Working Group on NanoScience, Enginering and nanoprisms, respectively(see below). The band for the type I Technology (wGN). 1999): at Http://ww wec orgy/oyola/nano/05-01. htm)(199). Copyright is prisms is initially centred at Amax=680 nm and gradually blue- held by WTEC, Loyola College(Maryl shifts toλ 640nm. This blue-shifting corre 14.comPaia,r.techmelogiCalRoadmapforeUrepeamnNanocdectroricsat(ftp//ftpcordis.Ju/pub/ist/doasharpnessofthenanoprismfeaturesroundingisknowntoleadto blue-shifting. The second strong band at Amax= 1,065 nm is Competing interests statement The authors declare that they have no competing financial assigned to type 2 particles(see below). In addition to the two strong surface plasmon bands, one can observe two other weak Correspondence and requests for materials should be adressed to RK( kochepdi-berlindel resonances at 340 and 470 nm, respectively(Fig. 2a, spectrum 6). To gain further insight into the optical spectra of the solution with the bimodal particle distribution, we carried out theoretical modellin using a finite-element-based method known as the discrete dipole approximation(DDA)--. The calculated spectrum shows plas mon bands that reproduce the experimentally observed spectrum (compare Fig. 2b and Fig. 2a, spectrum 6), confirming our peak Controlling anisotropic nanoparticle assignments. The first three peaks in the spectrum of the colloid growth through plasmon excitation of-plane quadrupole resonance ) 470 nm(in-plane quadrupole Rongchao Jin, Y. Charles Cao, Encai Hao, Gabriella S Metraux, the type l nanoprisms; in the case of the type 2 nanoprisms, only the George C Schatz & Chad A. Mirkin strong dipole resonance at 1,065 nm is clearly observed Quadrupole resonances, which occur at 340 nm and 600 nm(weak) in the Department of Chemistry and Institute for Nanotechnology Northwestern spectrum of the solution of the type 2 nanoprisms, are overlapped University, Evanston, Illinois 60208, USA with plasmon bands from the type I nanoprisms(Fig. 2b). The time-dependent optical spectra thus suggest that the process is Inorganic nanoparticles exhibit size-dependent properties that bimodal, rather than unimodal as would be expected in the case of are of interest for applications ranging from biosensing-and conventional Ostwald ripening 3. 14. catalysis to optics and data storage. They are readily available The bimodal growth of Ag nanoprisms is not caused by the in a wide variety of discrete compositions and sizes- 4. Shape- wavelength dispersity of the excitation beam(550 20 nm) selective synthesis strategies now also yield shapes other than Indeed, when a monochromatic laser beam(A= 532.8 nm, the NaturEVol42512october2003www.nature.com/nature e 2003 Nature Publishing Groupstep) used by new types of algorithms. (4) Aside from the input and output lines no further means are necessary for the logic operation, for example, additional magnetic fields, switching thresholds, or voltages. (5) The switching frequency of magnetic films can be pushed to several GHz, in principle allowing fast operation11. (6) Since only short current pulses are required to rotate the magnetiza￾tion, the energy consumption is further reduced. (7) The size of an MRAM cell—commercially available in 2004 (ref. 12)—can easily be scaled down to less than 100 nm (refs 13, 14) to further increase the integration density. (8) Our approach a priori does not necessarily require semiconductors to fabricate a logic gate. (9) The run-time programmability will increase the computational efficiency. Our new approach has the potential to induce a paradigm shift from transistor-based logic to magneto-logic, where the program￾mable functionality and non-volatility are as important as minia￾turization and clock speed. A Received 12 February; accepted 1 September 2003; doi:10.1038/nature02014. 1. Prinz, G. A. Magnetoelectronics. Science 282, 1660–1663 (1998). 2. Sidhu, R. P. S., Mei, A. & Prasanna, V. K. in Field-programmable Logic and Applications(eds Lysaght, P., Irvine, J. & Hartenstein, R. W.) 301–312 (Lecture Notes in Computer Science 1673, Springer, Berlin, 1999. 3. Parkin, S. S. P. et al. Exchange-biased magnetic tunnel junctions and application to nonvolatile magnetic random access memory. J. Appl. Phys. 85, 5828–5833 (1999). 4. Gru¨nberg, P. Layered magnetic structures: history, highlights, applications.Phys. Today 54, 31–37 (2001). 5. Baibich, B. N. et al. Giant magnetoresistance of (001)Fe/(001)Cr magnetic superlattices. Phys. Rev. Lett. 61, 2472–2475 (1988). 6. Binasch, G., Gru¨nberg, P., Saurenbach, F. & Zinn, W. Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange. Phys. Rev. B 39, 4828–4830 (1989). 7. Moodera, J. S., Kinder, L. R., Wong, T. M. & Meservey, R. Large magnetoresistance at room temperature in ferromagnetic thin film tunnel junctions. Phys. Rev. Lett. 74, 3273–3276 (1995). 8. Black, W. C. Jr & Das, B. Programmable logic using giant-magnetoresistance and spin-dependent tunneling devices. J. Appl. Phys. 87, 6674–6679 (2000). 9. Richter, R. et al. Field programmable spin-logic based on magnetic tunneling elements. J. Magn. Magn. Mater. 240, 127–129 (2002). 10. Martin, A. J. Proceedings of SSGRR 2000, International Conference on Advances in Infrastructure for Electronic Business, Science, and Education on the Internet; at khttp://www.ssgrr.it/en/ssgrr2000/papers/ 185.pdfl (2000). 11. Gerrits, Th., van den Berg, H. A. M., Hohlfeld, J., Ba¨r, L. & Rasing, Th. Ultrafast precessional magnetization reversal by picosecond magnetic field pulse shaping. Nature 418, 509–511 (2002). 12. Motorola Inc. Press release. IEEE International Solid State Circuits Conference (San Francisco, June 2001); at khttp://www.motorola.com/mot/document/content/0,1028,372,00.docl (2000). 13. Goronkin, H., von Allmen, P., Tsui, R. K. & Zhu, T. X. Nanostructure Science and Technology (eds Siegel, R. W., Hu, E. & Roco, M. C.) 67–92 (National Science and Technology Council (NSTC) Committee on Technology and The Interagency Working Group on NanoScience, Engineering and Technology (IWGN), 1999); at khttp://www.wtec.org/loyola/nano/05_01.html (1999). Copyright is held by: WTEC, Loyola College (Maryland). 14. Compan˜o´, R. Technological Roadmap for European Nanoelectronics at kftp://ftp.cordis.lu/pub/ist/docs/ fetnidrm.zipl (2000). Competing interests statement The authors declare that they have no competing financial interests. Correspondence and requests for materials should be addressed to R.K. (koch@pdi-berlin.de). .............................................................. Controlling anisotropic nanoparticle growth through plasmon excitation Rongchao Jin, Y. Charles Cao, Encai Hao, Gabriella S. Me´traux, George C. Schatz & Chad A. Mirkin Department of Chemistry and Institute for Nanotechnology, Northwestern University, Evanston, Illinois 60208, USA ............................................................................................................................................................................. Inorganic nanoparticles exhibit size-dependent properties that are of interest for applications ranging from biosensing1–5 and catalysis6 to optics7 and data storage8 . They are readily available in a wide variety of discrete compositions and sizes9–14. Shape￾selective synthesis strategies now also yield shapes other than nanospheres, such as anisotropic metal nanostructures with interesting optical properties15–23. Here we demonstrate that the previously described photoinduced method23 for converting silver nanospheres into triangular silver nanocrystals—so-called nanoprisms—can be extended to synthesize relatively mono￾disperse nanoprisms with desired edge lengths in the 30– 120 nm range. The particle growth process is controlled using dual-beam illumination of the nanoparticles, and appears to be driven by surface plasmon excitations. We find that, depending on the illumination wavelengths chosen, the plasmon excitations lead either to fusion of nanoprisms in an edge-selective manner or to the growth of the nanoprisms until they reach their light￾controlled final size. The photoinduced synthesis of silver (Ag) nanoprisms involves the preparation of a colloidal suspension of Ag spheres (diameter ,10 nm), followed by conversion of the spheres to larger prism structures with visible light. In a typical experiment, colloidal Ag nanoparticles passivated with sodium citrate and bis(p-sulphonato￾phenyl)phenylphosphine dihydrate dipotassium (BSPP) (diameter 4.8 ^ 1.1 nm, standing solution) were irradiated with a narrow￾band light source (using a 150 W xenon lamp with a light output ,12 W) with an optical bandpass filter (centre wavelength 550 nm, width 40 nm) for ,50 h (see Supplementary Information). Trans￾mission electron microscopy (TEM) shows that the colloid formed consists of two different size distributions of nanoprisms (Fig. 1a and inset), with the smaller particles (designated as type 1) and the larger particles (type 2) having average edge lengths of 70 ^ 12 nm and 150 ^ 16 nm, respectively. These structures tend to form stacks, so that edge-on views allow the precise determination of nanoprism thickness23. Although the average edge lengths for the type 1 and type 2 nanoprisms are significantly different, their thicknesses are almost identical (9.8 ^ 1.0 nm) (Fig. 1b, c). The bimodal particle growth process also has been monitored by ultraviolet–visible–near-infrared (UV–vis.–NIR) spectroscopy (Fig. 2a). During the reaction, one sees the disappearance of the plasmon band at ,395 nm (characteristic of the spherical silver particles) and the formation of two new, strong plasmon bands at 680 nm and 1,065 nm that are associated with the type 1 and type 2 nanoprisms, respectively (see below). The band for the type 1 prisms is initially centred at l max ¼ 680 nm and gradually blue￾shifts to l max ¼ 640 nm. This blue-shifting correlates with the tip sharpness of the nanoprism features; rounding is known to lead to blue-shifting24. The second strong band at l max ¼ 1,065 nm is assigned to type 2 particles (see below). In addition to the two strong surface plasmon bands, one can observe two other weak resonances at 340 and 470 nm, respectively (Fig. 2a, spectrum 6). To gain further insight into the optical spectra of the solution with the bimodal particle distribution, we carried out theoretical modelling using a finite-element-based method known as the discrete dipole approximation (DDA)24–26. The calculated spectrum shows plas￾mon bands that reproduce the experimentally observed spectrum (compare Fig. 2b and Fig. 2a, spectrum 6), confirming our peak assignments. The first three peaks in the spectrum of the colloid containing both type 1 and type 2 particles, centred at 340 nm (out￾of-plane quadrupole resonance), 470 nm (in-plane quadrupole resonance) and 640 nm (in-plane dipole resonance)24, result from the type 1 nanoprisms; in the case of the type 2 nanoprisms, only the strong dipole resonance at 1,065 nm is clearly observed. Quadrupole resonances, which occur at 340 nm and 600 nm (weak) in the spectrum of the solution of the type 2 nanoprisms, are overlapped with plasmon bands from the type 1 nanoprisms (Fig. 2b). The time-dependent optical spectra thus suggest that the process is bimodal, rather than unimodal as would be expected in the case of conventional Ostwald ripening13,14. The bimodal growth of Ag nanoprisms is not caused by the wavelength dispersity of the excitation beam (550 ^ 20 nm). Indeed, when a monochromatic laser beam (l ¼ 532.8 nm, the letters to nature NATURE | VOL 425 | 2 OCTOBER 2003 | www.nature.com/nature © 2003 Nature PublishingGroup 487