**5.2 Typical examples**

Experiments were carried out with an ellipsoidal cell having an exit hole diameter of 2 mm, a long axis of 30 mm, and a short axis of 19 mm. An Nd:YAG laser with second harmonic generator (λ = 532 nm) was used, and the sample was oxygen-free copper. Helium was chosen as the ambient gas, and the cell pressures were 100, 500, and 1000 Pa. The energy of the laser pulse was 256 mJ, with a pulse duration of 8 ns. The laser irradiated area was measured to be 3.88 mm2 from observation of a laser trace on the sample surface.

An image of the copper nanoparticles after 30 laser pulses obtained with transmission electron microscopy (TEM) is shown in Figure 9. Although some grain growth due to aggregation of nanoparticles after generation is recognized in the case of gas pressure 1000 Pa, monodisperse nanoparticles less than 10 nm are easily obtainable by properly controlling the ambient gas pressure.

Nanoparticle size distributions analyzed using TEM with a 0.5 μm square field of vision are shown in Figure 10. Using gas pressures of 100 and 500 Pa, nanoparticles with average diameters below 10 nm were obtained. In both cases, the particle size distribution can be approximated using a lognormal distribution function. The geometric standard deviation, σ, ranges from 1.09 to 1.12. Furthermore, it was confirmed in other experiments that the standard deviation can be further reduced by reducing the diameter of the exit hole of the ellipsoidal cell.

Fig. 9. TEM images of Cu nanoparticles formed under (a)100, (b)500 and (c)1000 Pa of helium gas pressure.

An electron diffraction pattern and corresponding TEM image of copper nanoparticles is shown in Figure 11. By comparing the diffraction pattern of the copper nanoparticles with that of only the carbon film on which the nanoparticles were collected, we confirmed that the copper nanoparticles are as crystalline. Debye Scherrer rings are observed in the electron diffraction pattern where most of Laue spots are very small, suggesting that the crystallized nanoparticles are facing various directions with respect to the nanoparticle crystal axis.

The plume then collides with the ambient gas, which is considerably denser because of the shock wave focusing in the vicinity of this focal point. A mixture region, where the thermodynamic states are uniform, is formed in the boundary between the ambient gas of high density and the plume front and where the mono-disperse nanoparticles are formed.

Experiments were carried out with an ellipsoidal cell having an exit hole diameter of 2 mm, a long axis of 30 mm, and a short axis of 19 mm. An Nd:YAG laser with second harmonic generator (λ = 532 nm) was used, and the sample was oxygen-free copper. Helium was chosen as the ambient gas, and the cell pressures were 100, 500, and 1000 Pa. The energy of the laser pulse was 256 mJ, with a pulse duration of 8 ns. The laser irradiated area was measured to be 3.88 mm2 from observation of a laser trace on the

An image of the copper nanoparticles after 30 laser pulses obtained with transmission electron microscopy (TEM) is shown in Figure 9. Although some grain growth due to aggregation of nanoparticles after generation is recognized in the case of gas pressure 1000 Pa, monodisperse nanoparticles less than 10 nm are easily obtainable by properly

Nanoparticle size distributions analyzed using TEM with a 0.5 μm square field of vision are shown in Figure 10. Using gas pressures of 100 and 500 Pa, nanoparticles with average diameters below 10 nm were obtained. In both cases, the particle size distribution can be approximated using a lognormal distribution function. The geometric standard deviation, σ, ranges from 1.09 to 1.12. Furthermore, it was confirmed in other experiments that the standard deviation can be further reduced by reducing the diameter of the exit hole of the

Fig. 9. TEM images of Cu nanoparticles formed under (a)100, (b)500 and (c)1000 Pa of

An electron diffraction pattern and corresponding TEM image of copper nanoparticles is shown in Figure 11. By comparing the diffraction pattern of the copper nanoparticles with that of only the carbon film on which the nanoparticles were collected, we confirmed that the copper nanoparticles are as crystalline. Debye Scherrer rings are observed in the electron diffraction pattern where most of Laue spots are very small, suggesting that the crystallized nanoparticles are facing various directions with respect to the nanoparticle crystal axis.

**5.2 Typical examples** 

sample surface.

ellipsoidal cell.

helium gas pressure.

controlling the ambient gas pressure.

Fig. 10. Histograms showing the diameters and the logarithmic normal distributions of the nanoparticles.

Fig. 11. TEM images and corresponding the electron diffraction patterns of Cu nanoparticles.
