Origin of Plasticity in Nanostructured Silicon

The mechanism of plasticity in nanostructured Si has been intensively studied over the past decade but still remains elusive. Here, we used in situ high-pressure radial x-ray diffraction to simultaneously monitor the deformation and structural evolution of a large number of randomly oriented Si nanoparticles (SiNPs). In contrast to the high-pressure $\beta$-Sn phase dominated plasticity observed in large SiNPs ($\sim 100\mathrm{nm}$), small SiNPs ($\sim 9\mathrm{nm}$) display a high-pressure simple hexagonal phase dominated plasticity. Meanwhile, dislocation activity exists in all of the phases, but significantly weakens as the particle size decreases and only leads to subtle plasticity in the initial diamond cubic phase. Furthermore, texture simulations identify major active slip systems in all of the phases. These findings elucidate the origin of plasticity in nanostructured Si under stress and provide key guidance for the application of nanostructured Si.

Figure 1

2D unrolled diffraction images of the (a) 100 nm SiNPs and (b) 9 nm SiNPs during compression. At the azimuth angle $\eta =0°$ (360°) and 180° (along the DAC compression axis, maximum stress direction), the Debye-Scherrer rings shift to larger $2\theta$ (smaller $d$ spacing). The x-ray wavelength is 0.4959 Å.

Figure 2

(a) 2D unrolled image of 100 nm SiNPs at 9.5 GPa at which the Si-I and the Si-II phases coexist (bottom) and the corresponding Rietveld refinement result using MAUD (top). (b) Texture indexes of the Si-I (solid symbols), Si-II (open circles), and Si-V (open squares) phases as functions of pressure.

Figure 3

(a) The $d$ spacing of the Si-I (111) planes of the 9 nm SiNPs as a function of azimuth angle at different pressures. (b) Lattice strain parameter ($Q$) of the Si-I (111) plane as a function of pressure for large (100 nm) and small (9 nm) SiNPs. The arrows mark the pressure where the phase transition starts.

Figure 4

Selected experimental inverse pole figures for the (a) Si-I, (b) Si-II (large SiNPs at 9.5 GPa), and (c) Si-V (small SiNPs at 14.7 GPa) phases along the compression direction (normal direction). The corresponding simulation results for the (d) Si-I, (e) Si-II, and (f) Si-V phases obtained from vpsc are shown in the right column. Because of different crystalline symmetry, different portions of the IPFs are needed to represent the orientation distribution for Si-I (a half quadrant), Si-II (45°), and Si-V (30°) phases. The pole densities are displayed on a color scale. The top color scale is for Si-I and the bottom scale is for Si-II and Si-V. Equal area projection and a linear scale are used. For the vpsc simulation, the strain was set to 4% for Si-I and 30% for both the Si-II and Si-V phases.