Photochemical Synthesis and Multiphoton Luminescence of Monodisperse Silver Nanocrystals
- First Online:
- Cite this article as:
- Kempa, T., Farrer, R.A., Giersig, M. et al. Plasmonics (2006) 1: 45. doi:10.1007/s11468-006-9008-5
- 235 Views
A rapid, photochemical solution-phase synthesis has been developed for the production of monodisperse, nanometer-sized silver particles. The stabilizer used in the synthesis can be used to control the average diameter of the particles over a range from 1 to 7 nm. The same reaction mixture can also be employed to deposit patterns of nanoparticles with a laser via multiphoton absorption. The particles exhibit strong multiphoton absorption-induced luminescence when irradiated with 800-nm light, allowing emission from single nanoparticles to be observed readily.
Key wordsSilver nanoparticlePhotochemical synthesisMultiphoton absorptionLuminescence
Applications of noble-metal nanoparticles have grown exponentially over the past decade. Silver is one of the least expensive of the noble metals and is only weakly reactive in the bulk. In nanoparticle form, silver has properties that can be exploited for applications such as surface-enhanced Raman spectroscopy (SERS) , even down to the single-molecule level [2,3]. In addition, subnanometer silver clusters show considerable promise as luminescent media for single-molecule studies [4,5]. Although the preparation of monodisperse nanoclusters of metals such as gold and platinum with average diameters that can be less than 1 nm is well documented [6,7], the synthesis of monodisperse silver nanoclusters is generally a considerably more difficult prospect, and it has been difficult to attain diameters of less than 3 nm in solution [8,9]. Here we report a facile solution-phase photochemical synthesis that, in a matter of minutes, produces monodisperse silver nanoclusters with controllable average diameters that can range from 1 to 7 nm. We further show that two-photon absorption (TPA) can be used to pattern the nanoparticles on a substrate. In addition, the particles exhibit strong multiphoton-absorption-induced luminescence (MAIL) upon irradiation with ultrafast pulses of 800-nm light, allowing the emission of individual particles to be monitored.
The role of kinetics in determining the average size and dispersity of semiconductor nanoparticles has long been recognized and used to advantage . Many such schemes require the rapid mixing of precursors to initiate the synthesis of nanoparticles. However, this type of strategy has received considerably less attention in the synthesis of noble-metal nanoparticles. A key element in such an approach is the ability to reduce silver cations quickly, such that nanoparticles with small diameters can be trapped kinetically. Photochemical reduction is an attractive means of accomplishing this end, as it allows for complete mixing of the reagents before the synthesis is initiated. Whereas a number of groups have reported photochemical syntheses of silver nanoparticles [11–26], in all of this previous work, the resultant nanoparticles have been polydisperse and often relatively large as well. The polydispersity of the particles produced is probably related to the fact that these syntheses involve ultraviolet exposure times of many minutes.
Average diameter, diameter polydispersity, and mass polydispersity of nanoparticles with different stabilizers.
The photochemical nature of the synthesis can be used to advantage to pattern silver particles on a microscopic scale. Rather than excite the photobase resonantly with single-photon excitation, we employed two-photon excitation using the 800-nm output of a Ti:sapphire laser. Because there is no transition in the photobase that is resonant with this light, two photons must be absorbed simultaneously to cause electronic excitation, and the absorption probability therefore scales as the square of the laser intensity. As a result, TPA can be localized to within the tight focal volume that results from sending the laser beam through a microscope objective , which, in turn, should localize the production of nanoparticles.
Our TPA apparatus has been described in detail elsewhere . Briefly, a commercial Ti:sapphire laser (Coherent Mira Basic) was used to produce pulses with a center wavelength of 800 nm and a repetition rate of 76 MHz. After dispersion compensation, the pulse length was ∼100 fs. The laser output was introduced into the reflected-light port of an upright microscope (Zeiss AxioPlan 2) and focused using a 20×, 0.5-NA, nonimmersion objective. The laser spot size was large enough to overfill the back aperture of the objective. The position of the sample was controlled in three dimensions with a motorized stage (Ludl BioPrecision) and the focusing drive of the microscope. Transmitted light was used to view the sample on a charge-coupled device camera during the deposition process.
A solution was prepared consisting of 10-mL portions of 5.00 mM AgClO4, 3.75 mM N-methylnifedipine, and 5.00 mM sodium malonate dissolved in THF. The sample cell was constructed from two clean glass microscope slides with a 1-mm rubber O-ring serving as a spacer and seal. A 300-mesh copper TEM grid with a thin coating (10 nm) of amorphous carbon was secured to the surface of one of the microscope slides such that it was in the center of the O-ring. A few drops of the solution were placed inside the O-ring, and the second microscope slide was brought in contact with it to seal the liquid sample above the TEM grid. Teflon tape was used to secure the sample tightly. The microscope was focused on the carbon-film surface of the TEM grid, and a shutter was opened to allow the laser output to reach the sample. The stage was then scanned to produce two-dimensional patterns. An average power of 3 mW at the sample was sufficient for the observation of material deposition.
Noble metals are known to luminesce upon multiphoton excitation in the infrared [35–39]. Although this MAIL is generally weak, we have recently demonstrated that gold nanoparticles can exhibit highly efficient visible MAIL when excited with ultrafast pulses in the near infrared . The high efficiency of MAIL is believed to be associated with large multiphoton absorption cross sections arising from the strong electric field enhancements generated at nanoparticles that have asperities .
The same particles exhibit different emission intensities in Figure 6a and b, indicating that the emission is polarized, as was observed for gold nanoparticles. The silver nanocrystals are also highly photostable; they neither blink nor photobleach. The MAIL intensity of a particle does fluctuate to some extent over time, which may result from orientational dynamics or may be akin to the spectral fluctuations observed previously in silver nanoclusters . As was the case for gold nanoparticles, the MAIL spectrum spans much of the visible spectrum, although the emission from the two metals is distinguishable.
In summary, we have demonstrated a simple photochemical synthesis of monodisperse silver nanoparticles with controllable average diameters in the range of a few nanometers. Nanoparticles not only can be created in the bulk using single-photon absorption, but also can be deposited in a controlled fashion on surfaces using TPA of 800-nm light. The ability to pattern these particles with high resolution may have a number of applications, including the fabrication of SERS devices. The nanoparticles also exhibit efficient MAIL upon absorption of two photons of 800-nm light and are highly photostable. As a result, these particles should prove to be excellent photolabels for observation of single-molecule dynamics.
This work was supported by the National Science Foundation, Grant ECS-0088438. J.T.F. is a Research Corporation Cottrell Scholar and a Camille Dreyfus Teacher-Scholar. T.K. is a Beckman Scholar and thanks the Hahn-Meitner Institute for a Summer Student Fellowship. We thank Michael Hilgendorff for assistance with some of the TEM measurements reported here.