Swift Heavy Ion Induced Modification Studies of C60 Thin Films

Modification induced by 110 MeV Ni ion irradiated thin film samples of C60 on Si and quartz substrates were studied at various fluences. The pristine and irradiated samples were investigated using Raman spectroscopy, electrical conductivity and optical absorption spectroscopy. The Raman data and band gap measurements indicate that swift ions at low fluences result in formations that involve multiple molecular units like dimer or polymer. High fluence irradiation resulted in sub-molecular formations and amorphous semiconducting carbon, indicating overall damage of the fullerene molecules. These sub-molecular units have been identified with nanocrystalline diamond and nanocrystalline graphite like formations.Comment: 7 pages, 29 references and 9 figures submitted to J. Appl. Phy

Ultrafast optical generation of coherent phonons in CdTe1-xSex quantum dots

We report on the impulsive generation of coherent optical phonons in CdTe0.68Se0.32 nanocrystallites embedded in a glass matrix. Pump probe experiments using femtosecond laser pulses were performed by tuning the laser central energy to resonate with the absorption edge of the nanocrystals. We identify two longitudinal optical phonons, one longitudinal acoustic phonon and a fourth mode of a mixed longitudinal-transverse nature. The amplitude of the optical phonons as a function of the laser central energy exhibits a resonance that is well described by a model based on impulsive stimulated Raman scattering. The phases of the coherent phonons reveal coupling between different modes. At low power density excitations, the frequency of the optical coherent phonons deviates from values obtained from spontaneous Raman scattering. This behavior is ascribed to the presence of electronic impurity states which modify the nanocrystal dielectric function and, thereby, the frequency of the infrared-active phonons

Raman scattering from coupled plasmon-phonon modes in HgTe

An inelastic-light-scattering experiment from the (100) face of p-type HgTe, a zero-band-gap semiconductor, is reported. The spectral features, which involve coupling of the longitudinal phonon mode with the multicomponent plasma comprised of light electrons in the Γ8c conduction band and heavy holes in the Γ8v valence band, are analyzed by calculating Im[ε−1(q,ω)], where ε(q,ω) is the frequency- and wave-vector-dependent dielectric function of the medium. The sharp peak at 138 cm−1 is ascribed to "forbidden" LO-phonon scattering. The partially screened allowed LO phonon appears as a broad peak around 127 cm−1, which is higher than the TO-phonon frequency (~118 cm−1). The hole carriers give rise to additional features in the spectrum around 160 cm−1. The results of infrared-reflectivity measurements by Grynberg et al. are also discussed in light of Raman data. While the ir spectra can be analyzed using ε( ω), it is seen that a finite wave vector involved in the light-scattering experiment modifies the phonon-plasmon spectrum profoundly

Anomalous mode in the Raman and ir spectra of mercury telluride

We discuss the origin of the low-frequency mode occurring at ~108 cm-1 (at 77 K) in HgTe and Hg-rich Hg1-xCdxTe. Through temperature- and wavelength-dependent Raman spectra and their analysis, we show that this feature is a combination phonon and acquires significant Raman (ir) activity as a consequence of a Fermi resonance with the TO phonon. Within this framework we are able to explain its TO-phonon-like behavior, and the variation of its frequency, width, and intensity with temperature