Nanocrystal-induced line narrowing of surface acoustic phonons in the Raman spectra of embeddedGexSi1−xalloy nanocrystals

Abstract

Raman scattering was performed on Ge x Si 1−x ͑x = 0.54 or 0.28͒ alloy nanocrystals embedded in amorphous Si oxide. An asymmetric, depolarized, and size-dependent low-frequency Raman peak was observed and identified as the superposition of two surface acoustic vibration modes of the alloy nanocrystals. The current theoretical models can be used to explain the mode frequencies but not the dampings observed experimentally. Based on energy-dispersive x-ray microanalysis and density-functional-theory total energy optimization of structures, a modified core-shell-matrix model in which the effects of neighboring nanocrystals in the matrix are taken into account is in good agreement with experiments. This work provides good insight into the frequencies and dampings of acoustic vibrations of the nanocrystals embedded in the matrix.

Damping of surface acoustic vibration induced by electrons trapped on SnO2 nanocrystal surface

“…[1][2][3][4][5][6][7][8][9][10][11] Recently, it has been utilized to kill virus by enhancing NC surface vibration 10 or absorbing microwave under resonant conditions. 11 In most systems consisting of free and embedded NCs, Lamb's theory 12 is suitable for explaining the frequencies of surface acoustic modes 1,2,4-7 but the damping ͓full width of half maximum ͑FWHM͔͒ cannot be obtained by this theory.…”

“…Hence, this peak can be inferred to be composed of single peak. After deducting the contribution from the size distribution, 8,17 we fit these Raman spectra using Lorentzian lineshape and the frequency and damping as ͑ exp = 54.8 cm −1 , ⌫ exp = 51.1 cm −1 ͒, ͑ exp = 31.9 cm −1 , ⌫ exp = 19.2 cm −1 ͒, and ͑ exp = 28.8 cm −1 , ⌫ exp = 17.8 cm −1 ͒ for samples A, B, and C, respectively.…”

Morphology-dependent low-frequency Raman scattering in ultrathin spherical, cubic, and cuboid SnO2 nanocrystals

“…Our results are new and interesting because a larger size should cause a downshift in the confined acoustic mode according to Lamb's theory and our previous results. 6,16 In the spherical ZnO NCs, a double-peak LF Raman spectrum was observed, but each subpeak consistently downshifted with increasing NC size. 5 Hence, a new physical process is involved here.…”

“…In this case, the action of a finite matrix and coherent excitation of phonons in different NCs must be taken into account in order to explain the observed mode frequency and linewidth. 6 Recently, interesting effects such as Terahertz radiation have been observed from self-assembled NC systems. 3,7 These NCs are connected by strong or weak molecular bonding, and the distances between NCs have the nanometer scale.…”

“…The high resolution and rejection rate of this measurement system allowed the observation of the vibration signals close to the Rayleigh line to less than 5 cm À1 . 6,7 The acquired spectra were the same as those taken under excitation by the 488 nm line.…”