We demonstrate the alignment-preserving transfer of parallel graphene nanoribbons (GNRs) onto insulating substrates. The photophysics of such samples is characterized by polarized Raman and photoluminescence (PL) spectroscopies. The Raman scattered light and the PL are polarized along the GNR axis. The Raman cross section as a function of excitation energy has distinct excitonic peaks associated with transitions between the one-dimensional parabolic subbands. We find that the PL of GNRs is intrinsically low but can be strongly enhanced by blue laser irradiation in ambient conditions or hydrogenation in ultrahigh vacuum. These functionalization routes cause the formation of sp defects in GNRs. We demonstrate the laser writing of luminescent patterns in GNR films for maskless lithography by the controlled generation of defects. Our findings set the stage for further exploration of the optical properties of GNRs on insulating substrates and in device geometries.

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Layered α-RuCl3 has been discussed as a proximate Kitaev spin liquid compound. Raman and THz spectroscopy of magnetic excitations confirm that the low-temperature antiferromagnetic ordered phase features a broad Raman continuum, together with two magnon-like excitations at 2.7 and 3.6 meV, respectively. The continuum strength is maximized as long-range order is suppressed by an external magnetic field. The state above the field-induced quantum phase transition around 7.5 T is characterized by a gapped multi-particle continuum out of which a two-particle bound state emerges, together with a well-defined single-particle excitation at lower energy. Exact diagonalization calculations demonstrate that Kitaev and off-diagonal exchange terms in the Fleury-Loudon operator are crucial for the occurrence of these features in the Raman spectra. Our study firmly establishes the partially-polarized quantum disordered character of the high-field phase. arXiv:1908.11617v1 [cond-mat.str-el]

Graphene nanoribbons (GNRs) are promising materials for the production of nanoscale devices. [1][2][3][4][5] In contrast to graphene, GNRs can be metallic or semiconducting with a tunable band gap that depends on the ribbon width and the edge configuration. [6][7][8][9][10] Carbon nanotubes are a similar 1D material but despite a 20 year history, the large-area synthesis of monochiral carbon nanotubes remains challenging. [11] On the other hand, nanoribbons can be fabricated with atomically controlled precision thanks to the bottom-up nanofabrication techniques. [12][13][14][15][16][17][18][19][20] Thus, GNRs combine the best attributes of the nanotube and graphene worlds.Engineering of GNR-based optoelectronic devices requires an understanding of the charge transfer effect on the A semiconductor-to-metal transition in N = 7 armchair graphene nanoribbons causes drastic changes in its electron and phonon system. By using angle-resolved photoemission spectroscopy of lithium-doped graphene nanoribbons, a quasiparticle band gap renormalization from 2.4 to 2.1 eV is observed. Reaching high doping levels (0.05 electrons per atom), it is found that the effective mass of the conduction band carriers increases to a value equal to the free electron mass. This giant increase in the effective mass by doping is a means to enhance the density of states at the Fermi level which can have palpable impact on the transport and optical properties. Electron doping also reduces the Raman intensity by one order of magnitude, and results in relatively small (4 cm −1 ) hardening of the G phonon and softening of the D phonon. This suggests the importance of both lattice expansion and dynamic effects. The present work highlights that doping of a semiconducting 1D system is strikingly different from its 2D or 3D counterparts and introduces doped graphene nanoribbons as a new tunable quantum material with high potential for basic research and applications.

The tetradymite compound BiSbTeSe2 is one of the most bulk-insulating three-dimensional topological insulators, which makes it important in the topological insulator research. It is a member of the solid-solution system Bi2−xSbxTe3−ySey, for which the local crystal structure, such as the occupation probabilities of each atomic site, is not well understood. We have investigated the temperature and polarization dependent spontaneous Raman scattering in BiSbTeSe2, revealing a much higher number of lattice vibrational modes than predicted by group theoretical considerations for the space group R3m corresponding to an ideally random solid-solution situation. The density functional calculations of phonon frequencies show a very good agreement with experimental data for parent material Bi2Te3, where no disorder effects were found. In comparison to Bi2Te3 the stacking disorder in BiSbTeSe2 causes a discrepancy between theory and experiment. Combined analysis of experimental Raman spectra and DFT calculated phonon spectra for different types of atomic orders showed coexistence of different sequences of layers in the material and that those with Se in the center and a local order of Se-Bi-Se-Sb-Te, are the most favored.