Charge Transfer Evidence between Carbon Nanotubes and Encapsulated Conjugated Oligomers

International audienceA hybrid system consisting of quaterthiophene derivative inserted into carbon nanotubes is studied. Encapsulation efficiency of the conjugated oligomers in the hollow core of nanotubes is investigated by transmission electron microscopy and spatial-resolved electron energy loss spectroscopy. Infrared spectroscopy showed evidence of a significant positive charge transfer on the inserted oligothiophene. Raman spectra display different behaviors depending on the excitation energy and correlated to the quaterthiophene optical absorption energy. At high excitation wavelength (far from the oligomer resonance), radial breathing modes exhibit a significant upshift consistent with an encapsulation effect. At low excitation wavelength (close to the oligomer resonance), both the G-band shift and the low-frequency modes vanishing suggest a significant charge transfer between the quaterthiophene and the nanotube

Fermi level shift in carbon nanotubes by dye confinement

International audienceDye confinement into carbon nanotube significantly affects the electronic charge density distribution of the final hybrid system. Using the electron-phonon coupling sensitivity of the Raman G-band, we quantify experimentally how charge transfer from thiophene oligomers to single walled carbon nanotube is modulated by the diameter of the nano-container and its metallic or semiconducting character. This charge transfer is shown to restore the electron-phonon coupling into defected metallic nanotubes. For sub-nanometer diameter tube, an electron transfer optically activated is observed when the excitation energy matches the HOMO-LUMO transition of the confined oligothiophene. This electron doping accounts for an important enhancement of the photoluminescence intensity up to a factor of nearly six for optimal confinement configuration. This electron transfer shifts the Fermi level, acting on the photoluminescence efficiency. Therefore, thiophene oligomer encapsulation allows modulating the electronic structure and then the optical properties of the hybrid system

Piezoelectric Response in α‑Quartz-Type GeO2

International audienceDensity functional perturbation theory is used to study the mechanism of the piezoelectric response in α-GeO2 and to estimate its magnitude. To date, its piezoelectric-stress elements have never been calculated using this level of theory nor measured on a high-quality water-free single crystal. Our calculations predict that the value of e11 piezoelectric-stress is about 50% higher than in α-quartz. This enhancement is due to larger tetrahedra distortions and/or a lower frequency of the dominant polar E-mode. In contrast to α-quartz, α-GeO2 has also a sizable e14 value that reaches 0.169 C/m2

Trigonal BiPO4: A potential piezoelectric for low-temperatures

International audienceDensity functional theory is used to estimate the e11 and d11 piezoelectric elements in the trigonal form of BiPO4. The calculated values, e11 = -1.46 C/m2 and d11 = -22.84 pC/N at 0 K, are significantly higher than those predicted in a-quartz-type materials but require experimental confirmation. The polar E-modes below 100 cm-1 are at the origin of the piezoelectric efficiency