**8. Illumination-stimulated reshaping of metal nanoparticles**

At present, metal nanoparticles are used in various fields of science and engineering. Their optical properties associated with collective electronic excitations are of particular interest. In most cases, ensembles of nanoparticles obtained on dielectric surfaces by means of the self-organization of atoms adsorbed from a vapor phase are investigated and used. The shapes of the particles thus obtained are often nonequilibrium and vary with the time. The shapes vary more rapidly when the substrate is heated (Ivlev et al., 1988). These facts are well known and reported in detail in papers devoted to electron microscopy investigations, atomic force microscopy data, and the optical extinction spectra of metal island films (Warmack & Humphrey, 1986).

Although it is clear that the equilibrium shape of nanoparticles should depend on the temperature (Combe et al., 2000, as far as we know, this dependence has not yet been studied systematically. A change in the shapes of the particles is usually treated as an irreversible transition to the equilibrium state, and heating accelerates the transition process. We observed reversible changes in the optical extinction spectra of silver and sodium films on dielectric substrates under repeated cyclic variations of their temperatures. Moreover, it was found that the illumination of sodium films noticeably accelerates the transition of their spectra to a stable state corresponding to room temperature with the negligibly small heating of nanoparticles by light. The nonthermal photoevaporation of atoms from nanoparticles, i.e., photoatomic emission (Abramova et al., 1984; Bonch-Bruevich et al., 1998; Hoheisel 1988; Burchianti et al. 2009) was also insignificant owing to the choice of the wavelength of light near the threshold of this relatively low probable process. Light-induced changes in the shapes of metal nanoparticles are actively investigated at present. The most well-known works in this field are separated into two groups. In the first group(Huang et al., 2005; Stietz, 2000; Habenicht et al., 2005), the effect of light is reduced to the thermal effect, owing to which individual nanoparticles are either rounded or displaced on the substrate and coagulate when meeting each other. In the second group (Sun et al., 2003; Jin et al., 2001; Kim et al., 2009), light induces physicochemical processes in colloids of metal nanoparticles, which result first in their transformation usually from spheres to prisms and,

Light-Induced Surface Diffusion 433

Fig. 6. Optical density of the annealed sodium film (1) and the same after an hour of

Fig. 7. Reshaping of sodium nanoparticles under laser illumination. Optical density differences in the dark region (1) and in the illuminated region (2). The laser wavelength is

illumination

indicated by an arrow

then, in the formation of complex aggregates of the latter. The nature of the effect of light on the formation of metal nanocrystals in liquids remains unclear. Note that in all of the mentioned works, irradiation induced significant irreversible changes in the nanoparticles, which were manifested as a substantial change in their optical spectra.

Here, we consider the observation of the effect of light on the formation of nanocrystals of absolutely different nature than that mentioned above. The observed light-induced acceleration of the relaxation process is due to the nonthermal photostimulation of the diffusion of metal atoms on the surface of metal nanoparticles. The light-induced acceleration of the relaxation of the shapes of nanoparticles is due to the same processes as photoatomic emission (Abramova et al. 1984). An attempt to observe photostimulated surface mass transport was reported in (Vartanyan et al., 2009a).

#### **8.1 Observation of the illumination accelerated nanoparticle reshaping**

The experiments were performed with silver and sodium films. The sodium films were obtained in sealed off evacuated cells with quartz, sapphire, or glass windows. The probing and illumination of the films were performed through the transparent windows of the cells. The silver films were obtained by thermal sputtering in a vacuum. The substrates were maintained at room temperature during the growth of the films. The optical probing of the silver films was the films from the vacuum setup.

The films formed under the indicated conditions are metastable. The kinetics of their morphology, which can be seen, e.g., in a change in the optical extinction spectra, is complex and long term. The annealing of the films a ccelerated the processes resulting in the stabilization of their structure. The sodium and silver films were annealed at temperatures of 50 and 200°C for 10 and 30 min in a vacuum, respectively. Annealing was followed by the fast cooling of the films. Further, variations in the temperature induced small, but regular and reversible changes in the extinction spectra.

The changes in the spectrum of the annealed sodium film were investigated at room temperature. Fig. 6 shows (1) the optical extinction spectrum of two separated regions of the film immediately after annealing. Then, the film was aged for an hour at room temperature and one of the regions was illuminated by a cw laser with a power density of 4 mW/cm2 (1016 photons per centimeter squared per second) and a wavelength of 810 nm (hν=1.53 eV), whereas the other region remained unilluminated. The spectrum of the unilluminated region changed very slightly and almost coincided with initial spectrum 1. The spectrum of the illuminated region changes noticeably (spectrum 2 in Fig. 1).

Fig. 7 shows the differences between the spectra of the unilluminated and illuminated regions after and before illumination. It is seen that the integral of the difference extinction spectrum is much smaller than a change in extinction. For this reason, it can be assumed that the light-induced change in the volume of nanoparticles is insignificant and the observed effect is attributed to a change in their shape. Note that an increase in the irradiation intensity, which did not noticeably heat the particles, was accompanied by an increase in photoatomic emission and a noticeable decrease in the size of the illuminated particles. This effect is not considered below.

A reversible change in the extinction spectra upon a variation of the temperature of the substrate was also observed in annealed silver films. Changes in the spectra were detected immediately after the cooling of the film and lasted for 40–60 min; after that, the extinction spectrum was stabilized. Fig. 8 shows the extinction spectra (1) immediately after deposition, (2) after annealing and fast cooling to room temperature, and (3) after aging at room

then, in the formation of complex aggregates of the latter. The nature of the effect of light on the formation of metal nanocrystals in liquids remains unclear. Note that in all of the mentioned works, irradiation induced significant irreversible changes in the nanoparticles,

Here, we consider the observation of the effect of light on the formation of nanocrystals of absolutely different nature than that mentioned above. The observed light-induced acceleration of the relaxation process is due to the nonthermal photostimulation of the diffusion of metal atoms on the surface of metal nanoparticles. The light-induced acceleration of the relaxation of the shapes of nanoparticles is due to the same processes as photoatomic emission (Abramova et al. 1984). An attempt to observe photostimulated

The experiments were performed with silver and sodium films. The sodium films were obtained in sealed off evacuated cells with quartz, sapphire, or glass windows. The probing and illumination of the films were performed through the transparent windows of the cells. The silver films were obtained by thermal sputtering in a vacuum. The substrates were maintained at room temperature during the growth of the films. The optical probing of the

The films formed under the indicated conditions are metastable. The kinetics of their morphology, which can be seen, e.g., in a change in the optical extinction spectra, is complex

stabilization of their structure. The sodium and silver films were annealed at temperatures of 50 and 200°C for 10 and 30 min in a vacuum, respectively. Annealing was followed by the fast cooling of the films. Further, variations in the temperature induced small, but regular

The changes in the spectrum of the annealed sodium film were investigated at room temperature. Fig. 6 shows (1) the optical extinction spectrum of two separated regions of the film immediately after annealing. Then, the film was aged for an hour at room temperature and one of the regions was illuminated by a cw laser with a power density of 4 mW/cm2 (1016 photons per centimeter squared per second) and a wavelength of 810 nm (hν=1.53 eV), whereas the other region remained unilluminated. The spectrum of the unilluminated region changed very slightly and almost coincided with initial spectrum 1. The spectrum of

Fig. 7 shows the differences between the spectra of the unilluminated and illuminated regions after and before illumination. It is seen that the integral of the difference extinction spectrum is much smaller than a change in extinction. For this reason, it can be assumed that the light-induced change in the volume of nanoparticles is insignificant and the observed effect is attributed to a change in their shape. Note that an increase in the irradiation intensity, which did not noticeably heat the particles, was accompanied by an increase in photoatomic emission and a noticeable decrease in the size of the illuminated particles. This

A reversible change in the extinction spectra upon a variation of the temperature of the substrate was also observed in annealed silver films. Changes in the spectra were detected immediately after the cooling of the film and lasted for 40–60 min; after that, the extinction spectrum was stabilized. Fig. 8 shows the extinction spectra (1) immediately after deposition, (2) after annealing and fast cooling to room temperature, and (3) after aging at room

ccelerated the processes resulting in the

which were manifested as a substantial change in their optical spectra.

surface mass transport was reported in (Vartanyan et al., 2009a).

silver films was the films from the vacuum setup.

and long term. The annealing of the films a -

and reversible changes in the extinction spectra.

effect is not considered below.

the illuminated region changes noticeably (spectrum 2 in Fig. 1).

**8.1 Observation of the illumination accelerated nanoparticle reshaping** 

Fig. 6. Optical density of the annealed sodium film (1) and the same after an hour of illumination

Fig. 7. Reshaping of sodium nanoparticles under laser illumination. Optical density differences in the dark region (1) and in the illuminated region (2). The laser wavelength is indicated by an arrow

Light-Induced Surface Diffusion 435

data. It is seen on the microphotographs (Vartanyan et al., 2009a) of our annealed silver films that the nanoparticles with a mean diameter of 14 nm are separated at large distances. Moreover, all of the changes associated with the mass transport on the substrate should be irreversible and manifested in the deviation of the spectra from the stable shapes, which

We do not know any consistent theoretical description of the shape of nanoparticles. The numerical simulation (Combe et al., 2000) indicates that the rate of the relaxation of the shapes of the crystal nanoparticles owing to surface mass transport is limited not by the displacement of atoms through terraces, which is a very fast process, but by the escape of atoms from relatively stable positions near the steps and the attachment of diffusing atoms at the positions corresponding to the final equilibrium shapes. The activation energy of the escape of the atoms from the positions attached to the steps to a terrace is obviously lower than the evaporation heat, but can be comparable with the latter. The activation energy of the incorporation of atoms into stable positions can be low, but the incorporation process can be very long owing to the complexity of the path of the assembly of the stable final

In the light of the above discussion, the observed light-induced acceleration of the relaxation of the shapes of sodium nanoparticles is explained by the fact that photons trigger the mechanism of surface diffusion by separating atoms from the steps and their transfer to terraces. It is assumed that light excites electrons quasilocalized near the atoms situated on the surface at irregular positions, and the energy of the excitations is converted into the energy of the displacement of atoms on the surface. A similar, but low probable process results in the separation of atoms from the surface, which is manifested as the photoatomic

In the absence of illumination, the attachment of atoms diffusing on the terraces to the steps decelerates the relaxation of the shape, and nanoparticles are "frozen" in metastable shapes, so that the mean thermal energy is much lower than the energy of the separation of an atom from a step. In the presence of the illumination of sodium nanoparticles, the energy of a photon is sufficient for initiating separation. The efficiency of the light effect can be estimated as follows. In particular, if the cross section for the absorption of photons by atoms attached to the steps is taken to be a molecular value of 10–16 cm–2 and the quantum efficiency of the separation of the atom from the step is taken to be 0.01, the frequency of the photostimulated acts of the separation of the atom from the step for the radiation intensity corresponding to a photon flux of 1016 cm–2 s–1 is 0.01 s–1. This is the frequency of the thermal activation of the separation at room temperature and at an activation energy of 0.8 eV (a sodium evaporation heat of 1.14 eV) of the transfer of the attached sodium atom from the step to the terrace. According to these estimates, light can noticeably affect the rate of the relaxation of the shape of the sodium nanoparticles. The evaporation heat of silver is larger than that for sodium. For this reason, high energies of photons are required to transfer attached silver atoms from steps to terraces. This circumstance can explain why illumination did not accelerate the relaxation of the shapes of the silver nanoparticles in our experiments.

Photostimulated mass transport over the solid surfaces is a wide and promising area of research in which only the very first steps are already made (Leonov et al. 2010) . Several directions of the future development may be identified right now. First, the electronic

increases in time; these features were not observed in our experiments.

shapes of the nanoparticles.

**9. Conclusion** 

emission (Bonch-Bruevich et al., 1998).

temperature for an hour. Note that the extinction maximum at the last stage was shifted toward the opposite side with respect to the annealing induced shift. A change in the spectrally integrated absorption in the case under consideration is due not to the evaporation of nanoparticles, which is insignificant at the indicated heating. The change is caused by the interband transition induced di - fference of the dispersion of the relative permittivity of silver from that accepted in the Drude model. This is confirmed by the fact that the repeated annealing at 200°C in a vacuum for 5 min or a slightly longer time returned the extinction spectrum to the initial position (spectrum 2 in Fig. 8). The cycles of changes in the shape of the spectrum under the heating and cooling of the film were repeated many times. In this case, the almost complete reversibility of the changes in the extinction spectrum was observed, indicating a direct relation of the shapes of the islands with the temperature of the substrate. The illumination of silver films by available laser sources did not noticeably accelerate the relaxation processes.

Fig. 8. Reshaping of silver nanoparticles in the dark

#### **8.2 Sodium versus silver nanoparticles reshaping**

The optical extinction spectra of metal nanoparticles are determined primarily by their shapes. The dependence of the shape of the spectrum on the size of the nanoparticles is not very noticeable. The extinction spectra of ensembles of particles are determined by the shape distribution of the particles. For this reason, the revealed reversible changes in the extinction spectra of silver and sodium films with the variation of their temperature can be attributed to changes in the shapes of individual islands, i.e., autocoalescence (Ivlev et al. 1988).

Other possible mechanisms of a change in the shapes of the particles, which are associated with the displacement of the material between the particles or the motion of particles themselves on the substrate, can berejected in view of the transmission electron microscopy

temperature for an hour. Note that the extinction maximum at the last stage was shifted toward the opposite side with respect to the annealing induced shift. A change in the spectrally integrated absorption in the case under consideration is due not to the evaporation of nanoparticles, which is insignificant at the indicated heating. The change is

permittivity of silver from that accepted in the Drude model. This is confirmed by the fact that the repeated annealing at 200°C in a vacuum for 5 min or a slightly longer time returned the extinction spectrum to the initial position (spectrum 2 in Fig. 8). The cycles of changes in the shape of the spectrum under the heating and cooling of the film were repeated many times. In this case, the almost complete reversibility of the changes in the extinction spectrum was observed, indicating a direct relation of the shapes of the islands with the temperature of the substrate. The illumination of silver films by available laser

The optical extinction spectra of metal nanoparticles are determined primarily by their shapes. The dependence of the shape of the spectrum on the size of the nanoparticles is not very noticeable. The extinction spectra of ensembles of particles are determined by the shape distribution of the particles. For this reason, the revealed reversible changes in the extinction spectra of silver and sodium films with the variation of their temperature can be attributed

Other possible mechanisms of a change in the shapes of the particles, which are associated with the displacement of the material between the particles or the motion of particles themselves on the substrate, can berejected in view of the transmission electron microscopy

to changes in the shapes of individual islands, i.e., autocoalescence (Ivlev et al. 1988).

fference of the dispersion of the relative


caused by the interband transition induced di -

sources did not noticeably accelerate the relaxation processes.

Fig. 8. Reshaping of silver nanoparticles in the dark

**8.2 Sodium versus silver nanoparticles reshaping** 

data. It is seen on the microphotographs (Vartanyan et al., 2009a) of our annealed silver films that the nanoparticles with a mean diameter of 14 nm are separated at large distances. Moreover, all of the changes associated with the mass transport on the substrate should be irreversible and manifested in the deviation of the spectra from the stable shapes, which increases in time; these features were not observed in our experiments.

We do not know any consistent theoretical description of the shape of nanoparticles. The numerical simulation (Combe et al., 2000) indicates that the rate of the relaxation of the shapes of the crystal nanoparticles owing to surface mass transport is limited not by the displacement of atoms through terraces, which is a very fast process, but by the escape of atoms from relatively stable positions near the steps and the attachment of diffusing atoms at the positions corresponding to the final equilibrium shapes. The activation energy of the escape of the atoms from the positions attached to the steps to a terrace is obviously lower than the evaporation heat, but can be comparable with the latter. The activation energy of the incorporation of atoms into stable positions can be low, but the incorporation process can be very long owing to the complexity of the path of the assembly of the stable final shapes of the nanoparticles.

In the light of the above discussion, the observed light-induced acceleration of the relaxation of the shapes of sodium nanoparticles is explained by the fact that photons trigger the mechanism of surface diffusion by separating atoms from the steps and their transfer to terraces. It is assumed that light excites electrons quasilocalized near the atoms situated on the surface at irregular positions, and the energy of the excitations is converted into the energy of the displacement of atoms on the surface. A similar, but low probable process results in the separation of atoms from the surface, which is manifested as the photoatomic emission (Bonch-Bruevich et al., 1998).

In the absence of illumination, the attachment of atoms diffusing on the terraces to the steps decelerates the relaxation of the shape, and nanoparticles are "frozen" in metastable shapes, so that the mean thermal energy is much lower than the energy of the separation of an atom from a step. In the presence of the illumination of sodium nanoparticles, the energy of a photon is sufficient for initiating separation. The efficiency of the light effect can be estimated as follows. In particular, if the cross section for the absorption of photons by atoms attached to the steps is taken to be a molecular value of 10–16 cm–2 and the quantum efficiency of the separation of the atom from the step is taken to be 0.01, the frequency of the photostimulated acts of the separation of the atom from the step for the radiation intensity corresponding to a photon flux of 1016 cm–2 s–1 is 0.01 s–1. This is the frequency of the thermal activation of the separation at room temperature and at an activation energy of 0.8 eV (a sodium evaporation heat of 1.14 eV) of the transfer of the attached sodium atom from the step to the terrace. According to these estimates, light can noticeably affect the rate of the relaxation of the shape of the sodium nanoparticles. The evaporation heat of silver is larger than that for sodium. For this reason, high energies of photons are required to transfer attached silver atoms from steps to terraces. This circumstance can explain why illumination did not accelerate the relaxation of the shapes of the silver nanoparticles in our experiments.

#### **9. Conclusion**

Photostimulated mass transport over the solid surfaces is a wide and promising area of research in which only the very first steps are already made (Leonov et al. 2010) . Several directions of the future development may be identified right now. First, the electronic

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excited states of the atoms adsorbed onto the surface and interacting with their neighbours are to be investigated thoroughly (Vartanyan et al. 2009b). Of prime interest here are such questions as the localization of the electronic excitation on the metal and semiconductor surfaces, their decay times, the pathways and probabilities of the transformation of the electron energy into atomic displacements. Second, photo-induced surface transport is to be studied on a wider range of objects, in particular, those with a metastable structure because in the case the one can expect more pronounced changes (Vartanyan et al. 2010). Finally, practical implementations of photo-induced surface transport may find their way into industry for non-thermal modification of the surfaces at low temperatures (Vartanyan et al., 2011).
