Spectroscopy phonon band range

The first investigation of the phonon modes in binary InN was an extrapolation of the Gai-xInxN (0 < x < 1) alloy modes in reflection towards the binary compound [1], A typically high free carrier concentration in the mid 1020 cm 3 range controls the absorption (Drude absorption) in the infrared and must also account for the broadened Reststrahlen band in pure InN films (e.g. in [1]). In this case infrared active phonons couple to the plasma of the free electrons forming phonon-plasmon coupled modes [10,11], However, layers of low carrier concentration have been achieved and pure LO phonon energies have been derived in Raman spectroscopy. Resonant Raman spectroscopy at 514 nm has been performed, assigning five of the six Raman allowed zone centre phonon modes [8,9] (TABLE 1). 

In addition to the already mentioned materials. Si is used for IREs in ATR-FTIR spectroscopy. It is an inert material and solely affected by strong bases and oxidizing agents in combination with fluorine compounds only. Major drawbacks of Si for MIR optical appUcations with traditional IREs are (1) its high refractive index, which causes reflection losses at interfaces and (2) some strong absorption bands in the range between 1500 and 300 cm caused by phonon vibrations (Hind et al., 2001 Lau, 1998). 

In the spectra of CdSe, three modes at 258, 359 and 950 cm are clearly observed. In general, the LO and TO phonons are observed along with the surface modes in polar nanocrystals in resonance Raman spectra and/or surface enhanced Raman spectra [275]. However, LO and TO modes are observed simultaneously only in randomly oriented nanoparticles. Resonance Raman Spectra (RRS) of CdTe nanoparticles give band due to Longitudinal optical (LO) phonons at 170 cm (LO), 340 cm (2LO) and 510 cm (3LO) mode frequency is found to shift due to quantum confinement effect and confined phonons are observed using surface enhanced Raman spectroscopy [275]. Transverse optic (TO) phonon is reported at 145 cm and its position is invariant with decreasing particle size as the dispersion curve for TO phonon branch is almost fiat [275]. In CdSe nanoparticles, LO phonons are reported in the range 180- 200 cm, wheras in ZnSe at 140 cm [Ref. 275 and references therein]. Thus, the Raman spectra observed in the present work well identifies the phonons in these nanoparticles. 

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