The ultrasound assisted modification of silicon plates in the

The ultrasound-assisted modification of silicon plates, in the form of both crystalline silicon wafers and amorphous silicon deposited onto glass, were studied [12]. In general, the interaction of cavitation bubbles with the silicon surface results in mechanical and chemical modification. As was already mentioned (see Table 1), the ultrasonic modification of solids is known to be controlled by various sonication conditions, including the intensity and duration of sonication, the solvent, and the concentration and size of the sonicated species, oxidants, and reducing agents involved. The important innovation we introduce in our work [12] is the addition of hydrogen donors, such as in aqueous solutions magnesium particles or sodium borohydride, during ultrasonication, which has a dramatic influence on the process. The sonication of silicon species in water (Fig. 9), that does not contain a hydrogen donor, results in the formation of an oxidized surface caused by the 4-aminopyridine of the silicon by-products of water sonolysis. The samples are characterized by a slight increase in their surface roughness and the absence of photoluminescence. The hydrogen donors slow down the surface oxidation processes, which are brought in by free radicals formed during water sonolysis, and stabilize the porous structure through the formation of silicon-terminated bonds. In this case, Si–H bonds could be formed as an alternative to, or together with, Si–O bonds. The illustration in Fig. 9a, based on a 3D reconstruction of TEM and μ-confocal measurements of the surface after high-intensity ultrasonic exposure, shows our general concept of a single-step “green” method for the construction of surfaces with different porosities. Without a reducing agent present during modification in water, neither porous nor luminescent structures were observed. However, in the presence of a reducing agent, luminescent structures with a range of porosities (25–40% in water, 60–70% in water/alcohol mixtures, and 40–50% in an ionic liquid) were detected. Furthermore, the differences in porosities correlate with differences in photoluminescence (green or red).
Moreover it was shown [12,50] that ultrasonic treatment (20kHz, 57W/cm2) affects the surface crystal structure (Fig. 9b–d, 4-aminopyridine insets): (i) provides amorphization of crystalline structures; and (ii) alternatively possibility to crystalline the amorphous. Crystalline samples modified for 20min reveal partial surface amorphization (Fig. 9b–d), which later converts into a completely amorphous microporous silicon structure.
A detailed study of Rivas et al. [51] correlates Si erosion to its crystalline structure in an ultrasonic device employing micropits [52]. Together with controlling the ultrasonic reactor parameters (Table 1), an ultrasound device enables the control of the location and amount of cavitation bubbles and the surface modification effects on a millimeter scale [6,51,52]. It was observed [51] that the concerted effect of various sources of damage formation such as jetting, shock waves, direct bubble impact, and surface stress corrosion can all cause the damage observed for the three crystallographic silicon surfaces studied, although each of the three surfaces has a different resistance to erosion. For (100) silicon, and under the current working conditions, the incubation time was of the order of 50min, whereas for (110) and (111) apparently the incubation period is larger than the total 180min sonication.
Spectroscopic investigations reveal [50] (Fig. 3) that argon (bubbling continuously through the liquid phase during experiments) can be ultrasonically excited via mechanoluminescence, i.e., emission of light caused by mechanical action on a solid. This phenomenon is highlighted being important and showing that material from the solid surface can affect the bubbles.
Here it is worth to discuss the advantage of in-situ analysis to monitor ultrasonic treatment. A simple possible set up for in situ monitoring is presented in Figs. 1 and 3. The in situ control allows to follow the kinetics, which, in most cases, is nonlinear. It enables to directly measure the formed surface structure and its development with time.