Supporting Information

Quantitative Analysis of the Phonon Confinement Effect in Arbitrary Shaped Si Nanocrystals Decorated on Si Nanowires and Its Correlation with the Photoluminescence Spectrum

Ramesh Ghosh1, Arindam Pal2, P. K. Giri1, 3[(]

1Department of Physics, Indian Institute of Technology Guwahati, Guwahati -781039, India
2Department of Material Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India

3Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati -781039, India

Table S1. Details of the growth parameters and dimensions of Si NWs.

Sample code / Etching solution / Etching duration (mins) / Si NWs length (µm)
M1 / Ag assisted etching in 4.6 M HF & 1.422 M H2O2 / 5 / 3.9 ± 0.2
M2 / 10 / 6.5 ±0.2
M3 / 15 / 9.5 ± 0.2
M4 / 20 / 11.6 ±0.2

Laser heating effect of Si NCs/NWs in the Raman Spectrum:

Laser exposer of solid sample can cause inhomogeneous heating resulting in a down shift and broadening of the Raman peaks. In order to understand the effect of laser power on the Raman spectra, we have performed the Raman measurements with different laser powers ranging from 0.083 mW to 5 mW. The laser spot diameter is about 6 µm, when the sample was focused by a lens with 50X magnification. Fig. S2 shows the 1st order Raman spectra of the sample M3 at different laser powers. With the increase of the laser power: (i) the peak intensity increases; (ii) peak position shifts downwards; (iii) the line width (FWHM) increases; and (iv) the asymmetry increases. Note that vertically ordered Si NWs array grown by reactive ion etching did not show any significant change in Raman spectra at different laser power[1]. However, in the present case, the MACE grown Si NWs decorated with Si NCs are very sensitive to the laser heating as compared to its bulk counterpart. This is primarily due to size effect, giving a much lower thermal conductivity of the Si NCs than bulk Si and poor thermal contact with the substrate. Further, Si NC has higher absorption than bulk Si in the entire visible region. This may cause a local rise in temperature. Thus, the rise in local temperature must be taken into account when interpreting the Raman spectra. Note that, in case of laser power 0.083 mW, the corresponding laser power density is ~293 W/cm2. This is sufficiently low and comparable to that used by Ilker et al.[2] Note that a fraction of the laser radiation is specularly reflected, and a significant amount of the power absorbed by the specimen is reradiated. Thus, only a fraction of the incident power is actually converted to heat[3]. On the other hand, since the Si NWs/NCs are not free standing, maximum amount of heat was dissipated to the Si wafer beneath the Si NWs/NCs. Thus, the contribution of the local heating in the Raman spectra is not considerable, though not fully

eliminated at this power. Note that, high laser powers can create free Carriers. These carriers can interfere with the Si phonon line and create an asymmetric Fano line shape and this should show an increase of the low wavenumber asymmetry in the Si NWs/NCs spectrum at higher power[4, 5]. In order to minimize such effects on Raman spectra, we have performed the Raman measurements at a very low laser power i.e. 0.083 mW.

References

[1] M. Khorasaninejad, J. Walia, S. S. Saini, Nanotechnology, 2012, 23, 275706.

[2] D. Ilker, V. d. Sanden, C. M. Mauritius J. Appl. Phys, 2013, 114, 134310.

[3] T. R. Hart, R. L. Aggarwal, B. Lax, Phys. Rev. B, 1970, 1, 638.

[4] A. Compaan, M. C. Lee, G. J. Trott, Phys. Rev. B, 1985, 32, 6731.

[5] S. Piscanec, M. Cantoro, A. C. Ferrari, J. A. Zapien, Y. Lifshitz, S. T. Lee, S. Hofmann, J. Robertson, Phys. Rev. B, 2003, 68, 241312.

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