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Crystals of the layered two-dimensional transition-metal dichalcogenide 1T-VSe 2 are investigated by pointcontact spectroscopy in the charge-density wave state. Relevant characteristics such as mean, root-meansquare, and Debye phonon frequencies are obtained. The Debye temperature is ⌰ D ϭ220Ϯ5 K, close to that of 2H-NbSe 2 . The point-contact function g pc () of the electron-phonon interaction reveals a moderate coupling constant pc ϭ0.27. Consequently, the Coulomb potential, the density of states at the Fermi level, and the deformation potential are estimated to be of order 0.22 eV, 2.5 states/eV, and 0.22 eV/Å , respectively.

Several layered transition-metal dichalcogenides are studied using conducting probe AFM aiming to investigate the probe-mediated thermal processes likely to arise in the probe-substrate vicinity due to the high-current densities involved. A signature of local heating is found in the shape of current-potential (i/V) curves. The latter allows straightforward detection of a charge-density-wave (CDW) transition for 1T-TaSe(2) samples exhibiting it above room temperature. This is an illustration of a new use of conducting probe AFM to investigate solid-state bulk characteristics owing to a distinctive nanoscale Joule heating.

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We report on the determination of monomers, dimers, and alternating states of tetracyanoquinodimethane (TCNQ) species in molybdenum complexes using optical absorption and Raman spectroscopy. The molecular crystals investigated are [MoVIO(dtc)3](TCNQ) 1a, [MoV(dtc)4](TCNQ) 2a and 2b, and [MoV(dtc)4](TCNQ)2 3a (a, dtc = Et2NCS2; b, dtc = Me2NCS2). Optical and micro-Raman experiments performed at 50, 78, and 293 K yield information about the electronic states and the mode of packing of TCNQ moieties in these crystals. The optical spectrum of 2a presents the characteristic bands of the isolated monomeric species TCNQ•- at 1.46, 1.63, and 3.02 eV. In complex 1a, the dimerized state is evidenced by the presence of three absorptions:  two excitonic transitions LE1 at 1.9 eV and LE2 at 3.4 eV and an intermolecular charge transfer band (CT) near 1.38 eV. The bands LE1 and LE2 are blue-shifted by Davydov effect with respect to their position in the monomer states. Furthermore, it turns out that dimers are totally eclipsed in this system. Despite the fact that complex 2b differs only from 2a by a methyl substitution in the organometallic cation, we find that this salt presents dimer species which are well characterized by Raman spectroscopy. A coupling between the CT and LE1 bands establishes the presence of slipped dimers. At low temperature, however, Raman and optical spectra indicate a probable transition towards an eclipsed dimer state. Our optical spectra show that a perfect dimerization state is not possible in complex 3a since there is no excitonic band at 1.9 eV. However, the occurrence of a CT band at 1.23 eV corresponding to a charge transfer between TCNQ0 and TCNQ•- suggests the presence of TCNQ0/TCNQ•-/TCNQ•-/TCNQ0-stacked tetramers.

PACS 74.25.Fy, 74.25.Ha, 74.72.Bk, 81.20

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