Systematic conversion of single walled carbon nanotubes into n-type thermoelectric materials by molecular dopants.

Thermoelectrics is a challenging issue for modern and future energy conversion and recovery technology. Carbon nanotubes are promising active thermoelectic materials owing to their narrow bandgap energy and high charge carrier mobility, and they can be integrated into flexible thermoelectrics that can recover any waste heat. We here report air-stable n-type single walled carbon nanotubes with a variety of weak electron donors in the range of HOMO level between ca. -4.4 eV and ca. -5.6 eV, in which partial uphill electron injection from the dopant to the conduction band of single walled carbon nanotubes is dominant. We display flexible films of the doped single walled carbon nanotubes possessing significantly large thermoelectric effect, which is applicable to flexible ambient thermoelectric modules.

Efficient excitation-energy transfer in ion-based organic nanoparticles with versatile tunability of the fluorescence colours.

Organic nanoparticles consisting of 3,3'-diethylthiacyanine (TC) and ethidium (ETD) dyes are synthesized by ion-association between the cationic dye mixture (10 % ETD doping) and the tetrakis(4-fluorophenyl)borate (TFPB) anion, in the presence of a neutral stabilizing polymer, in aqueous solution. Doping with ETD makes the particle size smaller than without doping. Size tuning can also be conducted by varying the molar ratio (ρ) of the loaded anion to the cationic dyes. The fluorescence spectrum of TC shows good overlap with the absorption of ETD in the 450-600 nm wavelength region, so efficient excitation-energy transfer from TC (donor) to ETD (acceptor) is observed, yielding organic nanoparticles whose fluorescence colours are tunable. Upon ETD doping, the emission colour changes significantly from greenish-blue to reddish or whitish. This change is mainly dependent on ρ. For the doped nanoparticle sample with ρ=1, the intensity of fluorescence ascribed to ETD is ∼150-fold higher than that from pure ETD nanoparticles (efficient antenna effect). Non-radiative Förster resonance-energy transfer (FRET) is the dominant mechanism for the ETD fluorescence enhancement. The organic nanoparticles of a binary dye system fabricated by the ion-association method act as efficient light-harvesting antennae, which are capable of transferring light energy to the dopant acceptors in very close proximity to the donors, and can have multi-wavelength emission colours with high fluorescence quantum yields.