Raman scattering in cluster-deposited nanogranular silicon films

We study nanograin size confinement effects, and the effect of the increase of local temperature on the first-order Raman spectrum in silicon nanogranular films obtained by cluster deposition. The local temperature was monitored by measuring the Stokes/antiStokes peak ratio with the laser power up to similar to20 kW/cm(2). We find large energy shifts, up to 30 cm(-1), and broadenings, up to 20 cm(-1), of the Raman-active mode, which we attribute to both laser heating and confinement effects. The phonon softening and phonon linewidth are calculated using a phenomenological model which takes into account disorder effects through the breakdown of the k = 0 Raman-scattering selection rule, and also anharmonicity, which is incorporated through the three- and four- phonon decay processes. Very good agreement with experimental data is obtained for calculated spectra with nanograin sizes of about 10 nm, and with an increase in the anisotropy constants with respect to those of bulk silicon

Phase-dependent desorption from biphenyl-substituted alkanethiol self-assembled monolayers induced by ion irradiation

Using laser ionization in combination with time-of-flight mass spectrometry, we have studied ion-induced desorption of neutral particles from self-assembled monolayers (SAMs) of omega-(4 '-methylbiphenyl-4-yl) alkane thiols (CH3(C6H4)(2)(CH2)(n)SH, BPn, n = 2, 4 6) formed on Au(111) substrates. Because BPn/Au(111) SAMs with it = even exhibit polymorphism, the effect of purely structural changes on emission yield and fragmentation pattern could be studied without interference from changes in the chemical composition. In spite of the high energy of the primary ion beam (15 keV), the mass spectra reveal a striking sensitivity of the desorption process to rather subtle changes in the structure of the layer. Depending on the SAM structure, substantial differences in the ratio between the cleavage of the molecule-substrate and the C-S bonds are observed. For applications of SAMs as resists in ion beam lithography, the results demonstrate that well-defined removal of molecules requires exact control of the SAM structure.</p