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Structural origin of light emission in germanium quantum dots.

Little W, Karatutlu A, Bolmatov D, Trachenko K, Sapelkin AV, Cibin G, Taylor R, Mosselmans F, Dent AJ, Mountjoy G - Sci Rep (2014)

Bottom Line: Two sets of nanoparticles were studied, with oxygen and hydrogen terminated surfaces.We found that in oxygen terminated nanoparticles its the oxide-rich regions that are responsible for the light emission.In hydrogen terminated nanoparticles we established that structurally disordered Ge regions contribute to the luminescence.

View Article: PubMed Central - PubMed

Affiliation: Center for Condensed Matter and Materials Physics, School of Physics and Astronomy, Queen Mary, University of London, Mile End Road, London E1 4NS, UK.

ABSTRACT
We used a combination of optically-detected x-ray absorption spectroscopy with molecular dynamics simulations to explore the origins of light emission in small (5 nm to 9 nm) Ge nanoparticles. Two sets of nanoparticles were studied, with oxygen and hydrogen terminated surfaces. We show that optically-detected x-ray absorption spectroscopy shows sufficient sensitivity to reveal the different origins of light emission in these two sets of samples. We found that in oxygen terminated nanoparticles its the oxide-rich regions that are responsible for the light emission. In hydrogen terminated nanoparticles we established that structurally disordered Ge regions contribute to the luminescence. Using a combination of molecular dynamics simulations and optically-detected x-ray absorption spectroscopy we show that these disordered regions correspond to the disordered layer a few Å thick at the surface of the simulated nanoparticle.

No MeSH data available.


Results of the fitting of molecular dynamics based models to OD-XAS derived data.(a), Experimental data and the best fit. (b), Goodness of fit for various RDFs calculated as a function of distance from the surface towards the center of the QD.
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f4: Results of the fitting of molecular dynamics based models to OD-XAS derived data.(a), Experimental data and the best fit. (b), Goodness of fit for various RDFs calculated as a function of distance from the surface towards the center of the QD.

Mentions: It is not possible to observe the level of structural detail obtained from MD in OD-XAS since MD data give positions of nuclei while EXAFS is sensitive to the electron density distribution. As a consequence, the same level of detail cannot be recovered in the magnitude of the FT for direct comparison to the MD data. However, if the XEOL emission is associated with surface states only, a shortening of the average interatomic distance down to ≈2.42 Å should be observed in the data extracted from OD-XAS (see Fig. 3, b). We observe no obvious shortening of the interatomic distance in our OD-XAS experimental data and hence conclude that the OD-XAS signal does not originate from the surface. However, the cumulant analysis of the first neighbour Ge-Ge peak (R = 2.44 ± 0.01 Å) in the OD-XAS data indicates a non-zero value of the third cumulant (0.005 ± 0.003 Å3, skewness of the peak) while third cumulants in the bulk crystalline reference sample and in the transmission signal are close to zero (0.0003 ± 0.0018 Å3 and 0.0009 ± 0.0021 Å3 respectively). At the temperature of our experiment (T = 100 K) this peak skewness suggests that there is a static contribution to the Ge-Ge peak associated with structures with distinctly different interatomic distances. This is not inconsistent with the bi-modal distribution of distances we observe in the RDFs obtained from the MD simulation (Fig. 3, b). Therefore, we used information from RDFs to constrain the refinement of OD-XAS data in order to investigate the origins of peak skewness and to recover the sub-structure responsible for the observed OD-XAS signal. Specifically, the average number of neighbours associated with the first (surface-core interface) peak, the second (core) peak and the ratio of the corresponding peaks were used as set parameters. This information was then used to generate an EXAFS signal and to compare the model with the experiment for a number of RDFs between 2 Å and 25 Å. This allowed us to reduce the number of variable parameters to just two during a refinement: a single nearest neighbour number and a single Debye-Waller factor. The result of this comparison can be seen in Fig. 4. We found a clear minimum in the value of the fit index (which characterizes the “goodness” of a fit) for the RDF corresponding to the 5 Å layer. This result suggests that the origin of the light emission can be localized to a layer extending to around 5 Å from the surface towards the centre of the particle generated by MD.


Structural origin of light emission in germanium quantum dots.

Little W, Karatutlu A, Bolmatov D, Trachenko K, Sapelkin AV, Cibin G, Taylor R, Mosselmans F, Dent AJ, Mountjoy G - Sci Rep (2014)

Results of the fitting of molecular dynamics based models to OD-XAS derived data.(a), Experimental data and the best fit. (b), Goodness of fit for various RDFs calculated as a function of distance from the surface towards the center of the QD.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4260222&req=5

f4: Results of the fitting of molecular dynamics based models to OD-XAS derived data.(a), Experimental data and the best fit. (b), Goodness of fit for various RDFs calculated as a function of distance from the surface towards the center of the QD.
Mentions: It is not possible to observe the level of structural detail obtained from MD in OD-XAS since MD data give positions of nuclei while EXAFS is sensitive to the electron density distribution. As a consequence, the same level of detail cannot be recovered in the magnitude of the FT for direct comparison to the MD data. However, if the XEOL emission is associated with surface states only, a shortening of the average interatomic distance down to ≈2.42 Å should be observed in the data extracted from OD-XAS (see Fig. 3, b). We observe no obvious shortening of the interatomic distance in our OD-XAS experimental data and hence conclude that the OD-XAS signal does not originate from the surface. However, the cumulant analysis of the first neighbour Ge-Ge peak (R = 2.44 ± 0.01 Å) in the OD-XAS data indicates a non-zero value of the third cumulant (0.005 ± 0.003 Å3, skewness of the peak) while third cumulants in the bulk crystalline reference sample and in the transmission signal are close to zero (0.0003 ± 0.0018 Å3 and 0.0009 ± 0.0021 Å3 respectively). At the temperature of our experiment (T = 100 K) this peak skewness suggests that there is a static contribution to the Ge-Ge peak associated with structures with distinctly different interatomic distances. This is not inconsistent with the bi-modal distribution of distances we observe in the RDFs obtained from the MD simulation (Fig. 3, b). Therefore, we used information from RDFs to constrain the refinement of OD-XAS data in order to investigate the origins of peak skewness and to recover the sub-structure responsible for the observed OD-XAS signal. Specifically, the average number of neighbours associated with the first (surface-core interface) peak, the second (core) peak and the ratio of the corresponding peaks were used as set parameters. This information was then used to generate an EXAFS signal and to compare the model with the experiment for a number of RDFs between 2 Å and 25 Å. This allowed us to reduce the number of variable parameters to just two during a refinement: a single nearest neighbour number and a single Debye-Waller factor. The result of this comparison can be seen in Fig. 4. We found a clear minimum in the value of the fit index (which characterizes the “goodness” of a fit) for the RDF corresponding to the 5 Å layer. This result suggests that the origin of the light emission can be localized to a layer extending to around 5 Å from the surface towards the centre of the particle generated by MD.

Bottom Line: Two sets of nanoparticles were studied, with oxygen and hydrogen terminated surfaces.We found that in oxygen terminated nanoparticles its the oxide-rich regions that are responsible for the light emission.In hydrogen terminated nanoparticles we established that structurally disordered Ge regions contribute to the luminescence.

View Article: PubMed Central - PubMed

Affiliation: Center for Condensed Matter and Materials Physics, School of Physics and Astronomy, Queen Mary, University of London, Mile End Road, London E1 4NS, UK.

ABSTRACT
We used a combination of optically-detected x-ray absorption spectroscopy with molecular dynamics simulations to explore the origins of light emission in small (5 nm to 9 nm) Ge nanoparticles. Two sets of nanoparticles were studied, with oxygen and hydrogen terminated surfaces. We show that optically-detected x-ray absorption spectroscopy shows sufficient sensitivity to reveal the different origins of light emission in these two sets of samples. We found that in oxygen terminated nanoparticles its the oxide-rich regions that are responsible for the light emission. In hydrogen terminated nanoparticles we established that structurally disordered Ge regions contribute to the luminescence. Using a combination of molecular dynamics simulations and optically-detected x-ray absorption spectroscopy we show that these disordered regions correspond to the disordered layer a few Å thick at the surface of the simulated nanoparticle.

No MeSH data available.