Size dependent anisotropic strain and optical properties of strained Si nanocrystals.

We report on the growth of strained Si nanocrystals (NCs) of sizes in the range 5-43 nm and analyze the detailed nature of strain and its influence on the optical properties of the NCs as a function of size. Freestanding Si NPs were prepared in a controlled way using a contamination free mechanical ball milling for duration 2-40 hrs. Structural analysis based on X-ray diffraction (XRD) pattern and high resolution transmission electron microscopy (HRTEM) confirms the good crystalline nature of these Si NCs. A detailed analysis of XRD line profile reveals that nature of the strain is anisotropic and the screw type dislocations are the main contributors to the lattice strain. The dislocation density and corresponding strain changes non-monotonically, while the crystallite size changes monotonically with milling time. Direct evidence of dislocations is shown from HRTEM images. The UV-vis-NIR absorption spectra of the Si NCs show an enhanced absorption band in the visible region that shows a systematic blue shift with reduced NC sizes. Si NCs with size approximately 5-10 nm exhibits a distinct photoluminescence (PL) band in the visible region at 580-585 nm at room temperature, while higher size NCs does not exhibit any visible emission. PL excitation measurement shows a very small Stokes shift for the visible emission band indicating no involvement of defects/interface in the emission. We argue that the observed absorption and emission can be explained based on the enhanced confinement effect on the strained Si NCs due to the combined effect of strain and size quantization.