We report two first-order Raman modes in the spectra of few-layer MoS 2 at 286 and 471 cm −1 that are only observed at excitation energies above 2.4 eV. We show that these normally not observed modes are interlayer modes related to symmetry-forbidden modes of the single layer. Based on group theory, we provide a general treatment and systematic classification of all phonon modes in few-layer crystals with inversion symmetry and/or horizontal reflection symmetry. The results can thus be applied to different materials like few-layer graphene, transition-metal dichalcogenides, or BN. Moreover, the few-layer specific Raman modes are strongly resonant with the C optical transition in MoS 2 . We conclude that the corresponding exciton wave function is extended over all layers of the few-layer MoS 2 , in contrast to the A and B excitons.

Raman spectroscopy on the radial breathing mode is a common tool to determine the diameter d or chiral indices (n,m) of single-wall carbon nanotubes. In this work we present an alternative technique to determine d and (n,m) based on the high-energy G(-) mode. From resonant Raman scattering experiments on 14 highly purified single chirality (n,m) samples we obtain the diameter, chiral angle, and family dependence of the G(-) and G(+) peak position. Considering theoretical predictions we discuss the origin of these dependences with respect to rehybridization of the carbon orbitals, confinement, and electron-electron interactions. The relative Raman intensities of the two peaks have a systematic chiral angle dependence in agreement with theories considering the symmetry of nanotubes and the associated phonons.

We present Raman measurements of mono-and few-layer WS2. We study the monolayer A 1 mode around 420 cm −1 and its evolution with the number of layers. We show that with increasing layer number there is an increasing number of possible vibrational patterns for the out-of-plane Raman mode: in N -layer WS2 there are N Γ-point phonons evolving from the A 1 monolayer mode. For an excitation energy close to resonance with the A excitonic transition energy, we were able to observe all of these N components, irrespective of their Raman activity. Density functional theory calculations support the experimental findings and make it possible to attribute the modes to their respective symmetries. The findings described here are of general importance for all other phonon modes in WS2 and other layered transition metal dichalcogenide systems in the few layer regime.

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We investigated the changes in electronic structures induced by chemical functionalization of the five smallest diamondoids using valence photoelectron spectroscopy. Through the variation of three parameters, namely functional group (thiol, hydroxy, and amino), host cluster size (adamantane, diamantane, triamantane, [121]tetramantane, and [1(2,3)4]pentamantane), and functionalization site (apical and medial) we are able to determine to what degree these affect the electronic structures of the overall systems. We show that unlike, for example, in the case of halobenzenes, the ionization potential does not show a linear dependence on the electronegativity of the functional group. Instead, a linear correlation exists between the HOMO-1 ionization potential and the functional group electronegativity. This is due to localization of the HOMO on the functional group and the HOMO-1 on the diamondoid cage. Density functional theory supports our interpretations.