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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.


Thermal evolution of the mean diameter (2r) of Ge nanostructures. Measured by TEM (diamond) or GI-XRD (squares). Line is a guide for eyes (color online).
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Figure 2: Thermal evolution of the mean diameter (2r) of Ge nanostructures. Measured by TEM (diamond) or GI-XRD (squares). Line is a guide for eyes (color online).

Mentions: A high density of Ge precipitates within the SiO2 matrix is revealed by the STEM images (at the same magnification) in Figure 1, just after the deposition (a) and after thermal annealing at 750°C (b). The bright patches represent Ge nanoclusters whose density and mean size noticeably change after annealing (the mean diameter increasing from 2.5 to 7.5 nm). Although Ge QDs are already present in the as-deposited films, as recently found also by Zhang et al. [22], the deposition temperature was not high enough to induce the formation of crystalline QDs in our case. SiGeO film deposited by sputtering can be described as a mixture of Ge, GeO2, and SiO2 units, according to a random matrix model, similarly to what occurs for silicon-rich oxide [32]. During annealing, Ge QDs undergo an Ostwald ripening mechanism, similar to the Si QD case [33], leading to a size increasing of precipitates with a concomitant a-c transition occurring in the 600°C to 800°C range [20]. The inset in Figure 1b reports an HR-TEM image of the annealed sample, evidencing a clear crystalline phase for Ge QD with the fringes due to crystalline planes (indicated by red lines and separated by 0.33 nm, as the (111) planes of c-Ge bulk). In Figure 2, the mean QD diameter (2r) measured by TEM (diamond) and by GI-XRD (crossed squares, line is a guide for eyes) is reported as a function of the annealing temperature. Even if GI-XRD gives information only on c-QDs, the reasonable agreement between the two techniques observed at 750°C is supporting the idea that the size distribution of c-QDs does not significantly deviate from that of a-QDs. The overall variation of r can be extracted by joining the two techniques, showing a clear QD enlargement in the 400°C to 800°C range compatible with an Ostwald ripening mechanism.


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)

Thermal evolution of the mean diameter (2r) of Ge nanostructures. Measured by TEM (diamond) or GI-XRD (squares). Line is a guide for eyes (color online).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 2: Thermal evolution of the mean diameter (2r) of Ge nanostructures. Measured by TEM (diamond) or GI-XRD (squares). Line is a guide for eyes (color online).
Mentions: A high density of Ge precipitates within the SiO2 matrix is revealed by the STEM images (at the same magnification) in Figure 1, just after the deposition (a) and after thermal annealing at 750°C (b). The bright patches represent Ge nanoclusters whose density and mean size noticeably change after annealing (the mean diameter increasing from 2.5 to 7.5 nm). Although Ge QDs are already present in the as-deposited films, as recently found also by Zhang et al. [22], the deposition temperature was not high enough to induce the formation of crystalline QDs in our case. SiGeO film deposited by sputtering can be described as a mixture of Ge, GeO2, and SiO2 units, according to a random matrix model, similarly to what occurs for silicon-rich oxide [32]. During annealing, Ge QDs undergo an Ostwald ripening mechanism, similar to the Si QD case [33], leading to a size increasing of precipitates with a concomitant a-c transition occurring in the 600°C to 800°C range [20]. The inset in Figure 1b reports an HR-TEM image of the annealed sample, evidencing a clear crystalline phase for Ge QD with the fringes due to crystalline planes (indicated by red lines and separated by 0.33 nm, as the (111) planes of c-Ge bulk). In Figure 2, the mean QD diameter (2r) measured by TEM (diamond) and by GI-XRD (crossed squares, line is a guide for eyes) is reported as a function of the annealing temperature. Even if GI-XRD gives information only on c-QDs, the reasonable agreement between the two techniques observed at 750°C is supporting the idea that the size distribution of c-QDs does not significantly deviate from that of a-QDs. The overall variation of r can be extracted by joining the two techniques, showing a clear QD enlargement in the 400°C to 800°C range compatible with an Ostwald ripening mechanism.

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.