The second harmonic generation (SHG) produced from two-dimensional atomic crystals have been utilized recently in studying the grain boundaries and electronic structure of such ultra-thin materials. However, the SHG in many of these crystals, such as transition metal dichalcogenides (TMDCs), only occur in odd numbered layers with limited intensity due to their noncentrosymmetric nature. Here, we probe the SHG from the bulk noncentrosymmetric molybdenum disulfide (MoS2). Whereas the commonly studied 2H crystal phase’s anti-parallel nonlinear dipoles in adjacent layers give an oscillatory SH response, the parallel nonlinear dipoles of each atomic layer in the 3R phase constructively interfere to amplify the nonlinear light. Due to this interference, we observed the atomically phase-matched condition yielding a quadratic dependence between the intensity and layer number. Additionally, we probed the layer evolution of the A and B excitonic transitions in 3R-MoS2 using SHG spectroscopy and found distinct electronic structure differences arising from the crystal geometry. These findings demonstrate the dramatic effect of the symmetry and layer stacking of these atomic crystals.
Organic single-crystal ambipolar light-emitting transistors show a great interest due to their unique features, spectral
matching with their active material spectra and the quantum efficiency preservation during ambipolar operation at high
current density operation in kA/cm<sup>2</sup> order. The development of ambipolar light emitting transistor based on high
photoluminescent material, α,ω-bis(biphenylyl)terthiophene (BP3T) single crystal is reported. By using bottom-gated
top-contact configuration, with Ca and Au opposed metal electrodes, high value of hole and electron mobility were
obtained. Extremely bright light emission observed during ambipolar operation shows prospect for electrically driven
amplified spontaneous emission from organic materials.
Formation of C<SUB>60</SUB> aggregates has been found in toluene, benzene, and carbon disulfide (CS<SUB>2</SUB>) solutions by means of photoluminescence spectroscopy. From the detailed investigation of the temperature dependence of luminescence, it has been confirmed that the C<SUB>60</SUB> aggregates are formed at freezing temperature of these solvents in the cooling process and are weakly bound clusters which are decomposed into C<SUB>60</SUB> molecules at temperatures higher than ca. 220 K in toluene and CS<SUB>2</SUB> solutions. The 0-0 transition energy of their luminescence has been found to be blue-shifted in comparison with that of C<SUB>60</SUB> crystal. It has been also found that the irradiation of ultraviolet light upon the C<SUB>60</SUB> aggregates in benzene at approximately 260 K transforms them to stable substances under atmosphere, which look like round-shaped nanoscale particles in the high-resolution transmission electron microscopy images.