letters to nature Figure 1 The bimodal growth of Ag nanoprisms. a, TEM image of a sample of Ag showing that nanoprisms have nearly identical thicknesses 9.8+ 1.0nm). d, Schematic nanoprisms formed using single-beam excitation (550+ 20 nm); inset, histograms used diagram of the proposed light-induced fusion growth of Ag nanoprisms. to characterize the size distribution as bimodal. b, c, TEM images of nanoprism stacks second harmonic of a Nd: YAG laser, CW, light output -0.2 W)is precursors(AgNO3, CH_CO2Ag, AgC1O4 and Ag2SO4) used to photolyse the Ag colloids, bimodal growth is still observed We propose that the observed bimodal growth process occurs (see Supplementary Information). The bimodal growth was also through an edge-selective particle fusion mechanism, with four observed with other excitation wavelengths (500-700 nm). type I nanoprisms coming together in step-wise fashion to form a Additional experiments where the surfactant BSPP was absent type 2 nanoprism(Fig. 1d). The following observations are con- from the reaction mixture and initial Ag colloid yielded comparable sistent with this mechanism. First, bimodal growth results in type 1 results, demonstrating that BSPP is not critical for effecting the and type 2 prisms where four of the former prisms can fit together to bimodal growth process and nanoprism formation. In addition, the form a prism with dimensions (cumulative edge length, bimodal growth process was observed for a variety of BSPP: sodium 140+ 17 nm) that compare well with the latter(150# 16 nm) citrate ratios(investigated molar ratios ranged from 0: 1 to 1: 1, with (Fig. 1). Second, edge-selective growth occurs with no apparent sodium citrate fixed at 0.3 mM) and with different silver salt change in nanostructure thickness in going from the type l to type 2 820 810 0 395nm Wavelength(nm) Figure 2 The optical of Ag nanoprisms. a, Time evolution of UV-vis-N nanoprisms(model parameters: edge length of type 1, 70 nm, type 2, 150 nm; thickness, of a Ag spheres) under single-beam excitation 550+ 10nm). Note that the tip truncation of nanoprisms, which leads to a blueshift of the dipole after 10h: 3. after resonance from 770 to 640 nm, has been taken into account in the modelling. 6, after 55h. b, Theoretical modelling of the optical spectra of two different-sized e 2003 Nature Publishing Group AtuReiVol4252OctoBer2003www.nature.com/naturesecond harmonic of a Nd:YAG laser, CW, light output ,0.2 W) is used to photolyse the Ag colloids, bimodal growth is still observed (see Supplementary Information). The bimodal growth was also observed with other excitation wavelengths (500–700 nm). Additional experiments where the surfactant BSPP was absent from the reaction mixture and initial Ag colloid yielded comparable results, demonstrating that BSPP is not critical for effecting the bimodal growth process and nanoprism formation. In addition, the bimodal growth process was observed for a variety of BSPP:sodium citrate ratios (investigated molar ratios ranged from 0:1 to 1:1, with sodium citrate fixed at 0.3 mM) and with different silver salt precursors (AgNO3, CH3CO2Ag, AgClO4 and Ag2SO4). We propose that the observed bimodal growth process occurs through an edge-selective particle fusion mechanism, with four type 1 nanoprisms coming together in step-wise fashion to form a type 2 nanoprism (Fig. 1d). The following observations are con￾sistent with this mechanism. First, bimodal growth results in type 1 and type 2 prisms where four of the former prisms can fit together to form a prism with dimensions (cumulative edge length, 140 ^ 17 nm) that compare well with the latter (150 ^ 16 nm) (Fig. 1). Second, edge-selective growth occurs with no apparent change in nanostructure thickness in going from the type 1 to type 2 Figure 2 The optical spectra of Ag nanoprisms. a, Time evolution of UV–vis.–NIR spectra of a Ag colloid (4.8 ^ 1.1 nm spheres) under single-beam excitation (550 ^ 20 nm). Spectrum 1, initial colloid; 2, after 10 h; 3, after 15 h; 4, after 19 h; 5, after 24 h; and 6, after 55 h. b, Theoretical modelling of the optical spectra of two different-sized nanoprisms (model parameters: edge length of type 1, 70 nm, type 2, 150 nm; thickness, 10 nm). Note that the tip truncation of nanoprisms, which leads to a blueshift of the dipole resonance from 770 to 640 nm, has been taken into account in the modelling. Figure 1 The bimodal growth of Ag nanoprisms. a, TEM image of a sample of Ag nanoprisms formed using single-beam excitation (550 ^ 20 nm); inset, histograms used to characterize the size distribution as bimodal. b, c, TEM images of nanoprism stacks showing that nanoprisms have nearly identical thicknesses (9.8 ^ 1.0 nm). d, Schematic diagram of the proposed light-induced fusion growth of Ag nanoprisms. letters to nature 488 © 2003 Nature PublishingGroup NATURE | VOL 425 | 2 OCTOBER 2003 | www.nature.com/nature