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Porous silicon nanocrystals in a silica aerogel matrix.

Amonkosolpan J, Wolverson D, Goller B, Polisski S, Kovalev D, Rollings M, Grogan MD, Birks TA - Nanoscale Res Lett (2012)

Bottom Line: Samples with a wide range of concentrations were prepared, resulting in aerogels that were translucent (but weakly coloured) through to completely opaque for visible light over sample thicknesses of several millimetres.No sensitivity to oxygen was observed from the nanoparticles which had partially H-terminated surfaces before incorporation, and so we conclude that the silicon surface has become substantially oxidised.Finally, the FTIR and Raman scattering spectra of the composites were studied in order to establish the presence of crystalline silicon; by taking the ratio of intensities of the silicon and aerogel Raman bands, we were able to obtain a quantitative measure of the silicon nanoparticle concentration independent of the degree of optical attenuation.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics, University of Bath, Claverton Down, Bath, BA2 7AY, UK. d.wolverson@bath.ac.uk.

ABSTRACT
Silicon nanoparticles of three types (oxide-terminated silicon nanospheres, micron-sized hydrogen-terminated porous silicon grains and micron-size oxide-terminated porous silicon grains) were incorporated into silica aerogels at the gel preparation stage. Samples with a wide range of concentrations were prepared, resulting in aerogels that were translucent (but weakly coloured) through to completely opaque for visible light over sample thicknesses of several millimetres. The photoluminescence of these composite materials and of silica aerogel without silicon inclusions was studied in vacuum and in the presence of molecular oxygen in order to determine whether there is any evidence for non-radiative energy transfer from the silicon triplet exciton state to molecular oxygen adsorbed at the silicon surface. No sensitivity to oxygen was observed from the nanoparticles which had partially H-terminated surfaces before incorporation, and so we conclude that the silicon surface has become substantially oxidised. Finally, the FTIR and Raman scattering spectra of the composites were studied in order to establish the presence of crystalline silicon; by taking the ratio of intensities of the silicon and aerogel Raman bands, we were able to obtain a quantitative measure of the silicon nanoparticle concentration independent of the degree of optical attenuation.

No MeSH data available.


The ratio of the silicon to silica integrated Raman intensities. The ratio of the Raman intensities as a function of nanoparticle concentration (in the initial gel preparation) for LH, LO and SO particles (red squares, blue dots and green triangles respectively). Unconstrained linear least-squares fits to each dataset are also shown (solid lines).
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Figure 3: The ratio of the silicon to silica integrated Raman intensities. The ratio of the Raman intensities as a function of nanoparticle concentration (in the initial gel preparation) for LH, LO and SO particles (red squares, blue dots and green triangles respectively). Unconstrained linear least-squares fits to each dataset are also shown (solid lines).

Mentions: Figure 3 shows a test of this idea: the ratio of the silicon Raman band to the aerogel D2 band is plotted as a function of the mass density of Si NPs introduced into the gel preparation, for each type of nanoparticle. Linear fits to each set of data are shown; these were not constrained to pass through the origin and do not do so exactly. It can be seen, however, that the fits converge near the origin and that the assumption of a linear relationship is reasonable. It is also apparent that the gradient of the linear fits depends on the type of nanoparticle, and this can easily be understood. Firstly, we assume that, in the case of the SO nanoparticles, a significant proportion of the particles is likely to be lost during the gel preparation process, because their mean diameter is much less than the aerogel pore size (typically 10 to 50 nm), and so they are less easily immobilised in the gel network. This accounts both for the weak degree of colouration of these composites and the low rate of increase of the Raman band with concentration. Secondly, the reduction in Raman strength of about a factor of 2 between the LO particles and the LH particles (for the same initial concentration) is qualitatively consistent with our earlier conclusion that the porous shell of the LH particles becomes oxidised, leading to a loss of scattering cross-section of the silicon phonon mode. Of course, the fundamental Raman scattering cross-section for these different types of particle is not necessarily identical, and any variation of this will also lead to a difference in slope of the fits in Figure 3. To estimate the importance of this effect, we plan to investigate the dependence of the Raman spectra on excitation energy.


Porous silicon nanocrystals in a silica aerogel matrix.

Amonkosolpan J, Wolverson D, Goller B, Polisski S, Kovalev D, Rollings M, Grogan MD, Birks TA - Nanoscale Res Lett (2012)

The ratio of the silicon to silica integrated Raman intensities. The ratio of the Raman intensities as a function of nanoparticle concentration (in the initial gel preparation) for LH, LO and SO particles (red squares, blue dots and green triangles respectively). Unconstrained linear least-squares fits to each dataset are also shown (solid lines).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3475076&req=5

Figure 3: The ratio of the silicon to silica integrated Raman intensities. The ratio of the Raman intensities as a function of nanoparticle concentration (in the initial gel preparation) for LH, LO and SO particles (red squares, blue dots and green triangles respectively). Unconstrained linear least-squares fits to each dataset are also shown (solid lines).
Mentions: Figure 3 shows a test of this idea: the ratio of the silicon Raman band to the aerogel D2 band is plotted as a function of the mass density of Si NPs introduced into the gel preparation, for each type of nanoparticle. Linear fits to each set of data are shown; these were not constrained to pass through the origin and do not do so exactly. It can be seen, however, that the fits converge near the origin and that the assumption of a linear relationship is reasonable. It is also apparent that the gradient of the linear fits depends on the type of nanoparticle, and this can easily be understood. Firstly, we assume that, in the case of the SO nanoparticles, a significant proportion of the particles is likely to be lost during the gel preparation process, because their mean diameter is much less than the aerogel pore size (typically 10 to 50 nm), and so they are less easily immobilised in the gel network. This accounts both for the weak degree of colouration of these composites and the low rate of increase of the Raman band with concentration. Secondly, the reduction in Raman strength of about a factor of 2 between the LO particles and the LH particles (for the same initial concentration) is qualitatively consistent with our earlier conclusion that the porous shell of the LH particles becomes oxidised, leading to a loss of scattering cross-section of the silicon phonon mode. Of course, the fundamental Raman scattering cross-section for these different types of particle is not necessarily identical, and any variation of this will also lead to a difference in slope of the fits in Figure 3. To estimate the importance of this effect, we plan to investigate the dependence of the Raman spectra on excitation energy.

Bottom Line: Samples with a wide range of concentrations were prepared, resulting in aerogels that were translucent (but weakly coloured) through to completely opaque for visible light over sample thicknesses of several millimetres.No sensitivity to oxygen was observed from the nanoparticles which had partially H-terminated surfaces before incorporation, and so we conclude that the silicon surface has become substantially oxidised.Finally, the FTIR and Raman scattering spectra of the composites were studied in order to establish the presence of crystalline silicon; by taking the ratio of intensities of the silicon and aerogel Raman bands, we were able to obtain a quantitative measure of the silicon nanoparticle concentration independent of the degree of optical attenuation.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics, University of Bath, Claverton Down, Bath, BA2 7AY, UK. d.wolverson@bath.ac.uk.

ABSTRACT
Silicon nanoparticles of three types (oxide-terminated silicon nanospheres, micron-sized hydrogen-terminated porous silicon grains and micron-size oxide-terminated porous silicon grains) were incorporated into silica aerogels at the gel preparation stage. Samples with a wide range of concentrations were prepared, resulting in aerogels that were translucent (but weakly coloured) through to completely opaque for visible light over sample thicknesses of several millimetres. The photoluminescence of these composite materials and of silica aerogel without silicon inclusions was studied in vacuum and in the presence of molecular oxygen in order to determine whether there is any evidence for non-radiative energy transfer from the silicon triplet exciton state to molecular oxygen adsorbed at the silicon surface. No sensitivity to oxygen was observed from the nanoparticles which had partially H-terminated surfaces before incorporation, and so we conclude that the silicon surface has become substantially oxidised. Finally, the FTIR and Raman scattering spectra of the composites were studied in order to establish the presence of crystalline silicon; by taking the ratio of intensities of the silicon and aerogel Raman bands, we were able to obtain a quantitative measure of the silicon nanoparticle concentration independent of the degree of optical attenuation.

No MeSH data available.