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The role of the surfaces in the photon absorption in Ge nanoclusters embedded in silica.

Cosentino S, Mirabella S, Miritello M, Nicotra G, Lo Savio R, Simone F, Spinella C, Terrasi A - Nanoscale Res Lett (2011)

Bottom Line: The optical absorption of Ge nanoclusters has been measured by spectrophotometry analyses, evidencing an optical bandgap of 1.6 eV, unexpectedly independent of the QDs size or of the solid phase (amorphous or crystalline).A simple modeling, based on the Tauc law, shows that the photon absorption has a much larger extent in smaller Ge QDs, being related to the surface extent rather than to the volume.These data are presented and discussed also considering the outcomes for application of Ge nanostructures in photovoltaics.PACS: 81.07.Ta; 78.67.Hc; 68.65.-k.

View Article: PubMed Central - HTML - PubMed

Affiliation: MATIS-IMM-CNR and Dipartimento di Fisica e Astronomia, Università di Catania, Via Santa Sofia 64, 95123 Catania, Italy. mirabella@ct.infn.it.

ABSTRACT
The usage of semiconductor nanostructures is highly promising for boosting the energy conversion efficiency in photovoltaics technology, but still some of the underlying mechanisms are not well understood at the nanoscale length. Ge quantum dots (QDs) should have a larger absorption and a more efficient quantum confinement effect than Si ones, thus they are good candidate for third-generation solar cells. In this work, Ge QDs embedded in silica matrix have been synthesized through magnetron sputtering deposition and annealing up to 800°C. The thermal evolution of the QD size (2 to 10 nm) has been followed by transmission electron microscopy and X-ray diffraction techniques, evidencing an Ostwald ripening mechanism with a concomitant amorphous-crystalline transition. The optical absorption of Ge nanoclusters has been measured by spectrophotometry analyses, evidencing an optical bandgap of 1.6 eV, unexpectedly independent of the QDs size or of the solid phase (amorphous or crystalline). A simple modeling, based on the Tauc law, shows that the photon absorption has a much larger extent in smaller Ge QDs, being related to the surface extent rather than to the volume. These data are presented and discussed also considering the outcomes for application of Ge nanostructures in photovoltaics.PACS: 81.07.Ta; 78.67.Hc; 68.65.-k.

No MeSH data available.


Transmittance and reflectance spectra. Transmittance spectra for the bare substrate (quartz, continuous line) and for the as-deposited and annealed SiGeO samples (symbols). The reflectance spectrum (R) for the SiGeO sample after annealing at 800°C is also reported (dotted line) (color online).
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Figure 3: Transmittance and reflectance spectra. Transmittance spectra for the bare substrate (quartz, continuous line) and for the as-deposited and annealed SiGeO samples (symbols). The reflectance spectrum (R) for the SiGeO sample after annealing at 800°C is also reported (dotted line) (color online).

Mentions: In Figure 3, the transmittance (T) spectra of some SiGeO samples are plotted (symbols) together with that of the quartz substrate (T ~ 90%, the missing 10% being due to reflection by the quartz surface, not reported here). The presence of Ge QDs induces, in the 200 to 1000 nm range, a strong decrease of T which is modulated with the annealing temperature. On the other hand, the reflectance (R) spectrum does not depend on the temperature (thus, only the 800°C-annealed sample was reported) and R is quite low (approximately 10%) and constant, except for the typical oscillations caused by the beam interference at the air-SiGeO and SiGeO-quartz interfaces. The decrease of T for wavelengths smaller than approximately 1000 nm shows the absorption of light related to the presence of Ge QDs embedded in the film. On the other hand, the blueshift of T for higher annealing temperatures cannot be straightforwardly related to the Ostwald ripening of Ge QDs, since a redshift should be expected basing on the QCE (the larger QD, the lower the optical bandgap). Thus, the optical transmittance of this SiGeO film is clearly affected by the thermal treatments, but to find a relationship with the structural changes, the absorption spectra should be calculated.


The role of the surfaces in the photon absorption in Ge nanoclusters embedded in silica.

Cosentino S, Mirabella S, Miritello M, Nicotra G, Lo Savio R, Simone F, Spinella C, Terrasi A - Nanoscale Res Lett (2011)

Transmittance and reflectance spectra. Transmittance spectra for the bare substrate (quartz, continuous line) and for the as-deposited and annealed SiGeO samples (symbols). The reflectance spectrum (R) for the SiGeO sample after annealing at 800°C is also reported (dotted line) (color online).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211182&req=5

Figure 3: Transmittance and reflectance spectra. Transmittance spectra for the bare substrate (quartz, continuous line) and for the as-deposited and annealed SiGeO samples (symbols). The reflectance spectrum (R) for the SiGeO sample after annealing at 800°C is also reported (dotted line) (color online).
Mentions: In Figure 3, the transmittance (T) spectra of some SiGeO samples are plotted (symbols) together with that of the quartz substrate (T ~ 90%, the missing 10% being due to reflection by the quartz surface, not reported here). The presence of Ge QDs induces, in the 200 to 1000 nm range, a strong decrease of T which is modulated with the annealing temperature. On the other hand, the reflectance (R) spectrum does not depend on the temperature (thus, only the 800°C-annealed sample was reported) and R is quite low (approximately 10%) and constant, except for the typical oscillations caused by the beam interference at the air-SiGeO and SiGeO-quartz interfaces. The decrease of T for wavelengths smaller than approximately 1000 nm shows the absorption of light related to the presence of Ge QDs embedded in the film. On the other hand, the blueshift of T for higher annealing temperatures cannot be straightforwardly related to the Ostwald ripening of Ge QDs, since a redshift should be expected basing on the QCE (the larger QD, the lower the optical bandgap). Thus, the optical transmittance of this SiGeO film is clearly affected by the thermal treatments, but to find a relationship with the structural changes, the absorption spectra should be calculated.

Bottom Line: The optical absorption of Ge nanoclusters has been measured by spectrophotometry analyses, evidencing an optical bandgap of 1.6 eV, unexpectedly independent of the QDs size or of the solid phase (amorphous or crystalline).A simple modeling, based on the Tauc law, shows that the photon absorption has a much larger extent in smaller Ge QDs, being related to the surface extent rather than to the volume.These data are presented and discussed also considering the outcomes for application of Ge nanostructures in photovoltaics.PACS: 81.07.Ta; 78.67.Hc; 68.65.-k.

View Article: PubMed Central - HTML - PubMed

Affiliation: MATIS-IMM-CNR and Dipartimento di Fisica e Astronomia, Università di Catania, Via Santa Sofia 64, 95123 Catania, Italy. mirabella@ct.infn.it.

ABSTRACT
The usage of semiconductor nanostructures is highly promising for boosting the energy conversion efficiency in photovoltaics technology, but still some of the underlying mechanisms are not well understood at the nanoscale length. Ge quantum dots (QDs) should have a larger absorption and a more efficient quantum confinement effect than Si ones, thus they are good candidate for third-generation solar cells. In this work, Ge QDs embedded in silica matrix have been synthesized through magnetron sputtering deposition and annealing up to 800°C. The thermal evolution of the QD size (2 to 10 nm) has been followed by transmission electron microscopy and X-ray diffraction techniques, evidencing an Ostwald ripening mechanism with a concomitant amorphous-crystalline transition. The optical absorption of Ge nanoclusters has been measured by spectrophotometry analyses, evidencing an optical bandgap of 1.6 eV, unexpectedly independent of the QDs size or of the solid phase (amorphous or crystalline). A simple modeling, based on the Tauc law, shows that the photon absorption has a much larger extent in smaller Ge QDs, being related to the surface extent rather than to the volume. These data are presented and discussed also considering the outcomes for application of Ge nanostructures in photovoltaics.PACS: 81.07.Ta; 78.67.Hc; 68.65.-k.

No MeSH data available.