Abstract
We report on the ultrafast vibrational response of single Ge-Si core-shell nanowires obtained by epitaxial growth and investigated by femtosecond transient reflectivity and coherent x-ray diffraction measurements. The oscillations of the sample reflectivity are correlated with the fundamental breathing mode for wires with a diameter ranging from 150 to 350 nm and compared with solutions of the Navier equation. Taking advantage of a free standing geometry, we are able to get a mechanical quality factor of higher than 80. Coupling electron microscopy and pump and probe investigations with a very high spectral resolution performed on the same wire, we demonstrate that both shell and core diameter fluctuations are revealed and quantified. X-ray coherent diffraction measurements on individual nanowires evidence changes in the Ge-core diameter and different strain states along a single structure.
Highlights
Discrete vibrational modes, confined in semiconductor coreshell nano-objects, remain rarely investigated using a standard pump and probe scheme[1] compared to metal-insulating two-component nano-objects,[2] metal-metal,[3–8] or mono-component,[9–14] whereas they appear as good candidates for a wide variety of applications
Vibrational modes in single Ge-Si core shell nanowires, with a diameter of around 150–300 nm are monitored in the time domain using ultrafast optical pump and probe spectroscopy
Time resolved elastic measurements coupled with electron microscopy investigations give us the opportunity to identify nanowires exhibiting both shell and core diameter fluctuation in the same batch
Summary
Discrete vibrational modes, confined in semiconductor coreshell nano-objects, remain rarely investigated using a standard pump and probe scheme[1] compared to metal-insulating two-component nano-objects,[2] metal-metal,[3–8] or mono-component,[9–14] whereas they appear as good candidates for a wide variety of applications. Finite element simulations, which were carried out to quantify the effect of the real hexagonal shape, show a difference of only 2–3 GHz in the studied diameter range This discrepancy has the same magnitude than the accuracy of the frequency determination on oscillations exhibiting a quality factor around 20, the cylindrical hypothesis will be considered in the following to reach a semianalytical model. The change of the total diameter is mainly associated with the core’s diameter variation Selecting the latter nanowires, we can estimate the fluctuation of the silicon shell thickness deposited on the germanium core. We can estimate the fluctuation of the silicon shell thickness deposited on the germanium core The fluctuation of the shell thickness is large from 10 up to 45 nm, with a modal value equal to 23.1 nm (see Fig. 5) in good agreement with the nominal value of 20 nm
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