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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.


Tauc constant, Ge content, and the surfaces of Ge QDs. Comparison between the Tauc constant (B, triangles) as obtained from Tauc fits, the Ge content (D, circles) as measured by RBS, and the surfaces of Ge QDs (D/r, squares). All the values have been normalized to that of the as-deposited sample (color online).
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Figure 5: Tauc constant, Ge content, and the surfaces of Ge QDs. Comparison between the Tauc constant (B, triangles) as obtained from Tauc fits, the Ge content (D, circles) as measured by RBS, and the surfaces of Ge QDs (D/r, squares). All the values have been normalized to that of the as-deposited sample (color online).

Mentions: The reduction of α with temperature (Figure 4a) can be instead ascribed to a significant decreasing of the Tauc constant (B) as evident from the falling slopes of fits in Figure 4b. In fact, the B values, normalized to the as-deposited case, are reported as open triangles in Figure 5, revealing that after 800°C annealing, the DOS in Ge QDs involved in the light absorption (proportional to B) is strongly reduced to about one third, independently of the Ge QDs phase (c or a, open or closed triangles, respectively). If the DOS was related only to the density of Ge-Ge bonds, the B trend would decrease as much as the Ge content in the film (D, circles in Figure 5, as measured by RBS and normalized to the as-deposited case), but this is not the case. Instead, the photon absorption could be related to Ge bonds near the QD surfaces. If so, given a fixed amount of clustered Ge, the B value would be larger the smaller is r. Since the surface to volume ratio is proportional to 1/r and the volume is proportional to D, the total area of the surfaces of Ge QDs should decrease as D/r, reported in Figure 5 as squares. The patent correlation between B and D/r trends clearly suggests that the light absorption in Ge QDs embedded in SiO2 is strongly influenced by the surface of Ge QDs. In addition, such an evidence can account also for independence of Eoptg on the QDs size or phase, since the photon absorption seems to be mediated by surface electronic states, not related to the volume of QDs.


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)

Tauc constant, Ge content, and the surfaces of Ge QDs. Comparison between the Tauc constant (B, triangles) as obtained from Tauc fits, the Ge content (D, circles) as measured by RBS, and the surfaces of Ge QDs (D/r, squares). All the values have been normalized to that of the as-deposited sample (color online).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 5: Tauc constant, Ge content, and the surfaces of Ge QDs. Comparison between the Tauc constant (B, triangles) as obtained from Tauc fits, the Ge content (D, circles) as measured by RBS, and the surfaces of Ge QDs (D/r, squares). All the values have been normalized to that of the as-deposited sample (color online).
Mentions: The reduction of α with temperature (Figure 4a) can be instead ascribed to a significant decreasing of the Tauc constant (B) as evident from the falling slopes of fits in Figure 4b. In fact, the B values, normalized to the as-deposited case, are reported as open triangles in Figure 5, revealing that after 800°C annealing, the DOS in Ge QDs involved in the light absorption (proportional to B) is strongly reduced to about one third, independently of the Ge QDs phase (c or a, open or closed triangles, respectively). If the DOS was related only to the density of Ge-Ge bonds, the B trend would decrease as much as the Ge content in the film (D, circles in Figure 5, as measured by RBS and normalized to the as-deposited case), but this is not the case. Instead, the photon absorption could be related to Ge bonds near the QD surfaces. If so, given a fixed amount of clustered Ge, the B value would be larger the smaller is r. Since the surface to volume ratio is proportional to 1/r and the volume is proportional to D, the total area of the surfaces of Ge QDs should decrease as D/r, reported in Figure 5 as squares. The patent correlation between B and D/r trends clearly suggests that the light absorption in Ge QDs embedded in SiO2 is strongly influenced by the surface of Ge QDs. In addition, such an evidence can account also for independence of Eoptg on the QDs size or phase, since the photon absorption seems to be mediated by surface electronic states, not related to the volume of QDs.

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.