A constant height of gallium nitride (GaN) nanowires with graphene deposited on them is shown to have a strong enhancement of Raman scattering, whilst variable height nanowires fail to give such an enhancement. Scanning electron microscopy reveals a smooth graphene surface which is present when the GaN nanowires are uniform, whereas graphene on nanowires with substantial height differences is observed to be pierced and stretched by the uppermost nanowires. The energy shifts of the characteristic Raman bands confirms that these differences in the nanowire height has a significant impact on the local graphene strain and the carrier concentration. The images obtained by Kelvin probe force microscopy show clearly that the carrier concentration in graphene is modulated by the nanowire substrate and dependent on the nanowire density. Therefore, the observed surface enhanced Raman scattering for graphene deposited on GaN nanowires of comparable height is triggered by self-induced nano-gating to the graphene. However, no clear correlation of the enhancement with the strain or the carrier concentration of graphene was discovered.
Bulk-related conduction electron spin resonance and conduction hole spin resonance were investigated in Bi2Se3, a three-dimensional topological insulator. Electrons in the conduction band and holes in the valence band both have spin ½. The effective g-factors for conduction electrons are equal to 27.3 ± 0.15 for magnetic field parallel to the c axis and 19.48 ± 0.07 for magnetic field perpendicular to the c axis, whereas for conduction holes 29.90 ± 0.09 for magnetic field parallel and 18.96 ± 0.04 for magnetic field perpendicular to the c axis, respectively. Nonparabolicity effects were not observed in the investigated lowcarrier concentration range, below 8×1017 cm−3.Large g-factors, higher by an order of magnitude than the free electron value, are due to strong spin-orbit interactions in Bi2Se3. The striking similarity of the spin resonances due to conduction electrons and holes confirms the peculiar symmetry between the conduction and valence bands of Bi2Se3, both having similar effective masses and spin character.
Photoinduced charge transfer is a crucial process in the operation of organic solar cells. In this work we used two spectroscopic techniques, namely, time-resolved photoluminescence and light-induced electron spin resonance, which directly detect this phenomenon in blends composed of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and two air-stable polyazomethines with different chemical structures, one with thiophene ring and cardo moieties (25Th-cardo) and one with three thiophene rings (2252Th-DMB). For both polymers we observed a photoluminescence with a maximum around 2 eV. In a mixture with PCBM, the photoluminescence of both polymers was quenched. At the same time, the photoluminescence of PCBM was enhanced in a mixture with 25Th-cardo, whereas it was similar to the photoluminescence of pure PCBM in the mixture with 2252Th-DMB. Moreover, for 2252Th-DMB:PCBM, two overlapping lines of electron spin resonance were detected under illumination, one originating from the positive polaron and another from the negative polaron. No trace of the analogue lines was seen for illuminated 25Th-cardo:PCBM. These differences were reflected also in an external quantum efficiency which increased from 0.3% for 25Th-cardo to 3% for 2252Th-DMB mixtures with PBCM. Experimental results are supported by the theoretical calculations of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels using the density functional theory method, and the presented observations are discussed within HOMO–LUMO models of the studied materials in the context of charge-transfer processes. Finally, the results addressed above are compared with those obtained for the P3HT:PCBM mixture typically used as donor–acceptor active layer in bulk heterojunction polymer solar cells.
In this paper, contactless microwave spectroscopy measurements of weak localization in as-grown and hydrogen intercalated quasi-free-standing graphene (QFSG) grown on SiC are presented. Delamination from the substrate is observed by the change from substrate dominated to grain boundaries dominated intervalley elastic scattering in QFSG comparing to epitaxial graphene. In the case of as-grown graphene, the finite coherence length at 0 K caused by an additional inelastic scattering is observed. This additional scattering mechanism vanishes for hydrogen intercalated QFSG, and the significant enhancement of coherence length comparing to as-grown QFSG and epitaxial graphene is observed. The coherence length is comparable to that observed in free-standing graphene. The conditions under which the quantum corrections produce weak localization or weak antilocalization behavior in conductivity are also discussed.
X-band microwave spectroscopy is applied to study the cyclotron resonance in Bi2Te3 exposed to ambient conditions. With its help, intraband transitions between Landau levels of relativistic fermions are observed. The Fermi velocity equals to 3260 m/s, which is much lower than has been reported in the literature for samples cleaved in vacuum. Simultaneous observation of bulk Shubnikov–de Haas oscillations by contactless microwave spectroscopy allows determination of the Fermi-level position. Occupation of topological surface states depends not only on bulk Fermi level but also on the surface band bending.
