Nowadays, the materials commonly used to fabricate thermoelectric devices are tellurium, lead and germanium. These materials ensure from one side the best thermoelectric performances, but exhibit drawbacks in terms of availability, sustainability, cost and manufacturing complexity. Moreover, they do not guarantee a safe and cheap implementation in wearable thermoelectric applications. Here, p-and n-type flexible thermoelectric textiles are produced with sustainable and low-cost materials through green and scalable processes. Cotton is functionalized with inks made with biopolyester and carbon-nanomaterials. Depending on the nanofiller, i.e. graphene nanoplatelets, carbon nanotubes or carbon nanofibers, positive or negative Seebeck coefficient values are obtained, achieving also a remarkable value of electrical conductivity of 55 S cm -1 using carbon nanotubes. The best bending and washing stability are registered for the carbon nanofibers-based biocomposites, which increase their electrical resistance by 5 times after repeated bending cycles and only of the 30% after washing. Finally, in-plane flexible thermoelectric generators are fabricated and characterized coupling the best p-and n-type materials, achieving an output voltage of ~ 1.65 mV and a maximum output power of ~ 1.0 nW by connecting only 2 p/n thermocouples at a temperature difference of 70°C.

International audienceWe report on the ultrafast vibrational response of single copper nanowires investigated by femtosecond transient reflectivity measurements. The oscillations of the sample reflectivity are correlated with individual modes of resonance for wires with a diameter ranging from 100 to 500 nm and are compared with 2D finite element simulation. Fluctuation of the sample-substrate coupling is illustrated through its effect on the damping rate. We demonstrate elastic confinement in free standing wires which allowed the detection of up to the third harmonic of the breathing mode. By removing the energy relaxation channel towards the substrate, we obtained nano-oscillators with quality factors up to 130. Finally, taking advantage of the very high spectral resolution achieved on free standing wires, we could observe the elastic coupling between two close wires via their polymer cladding. (C) 2013 AIP Publishing LLC

We report on gigahertz acoustic phonon waveguiding in free-standing single copper nanowires studied by femtosecond transient reflectivity measurements. The results are discussed on the basis of the semianalytical resolution of the Pochhammer and Chree equation. The spreading of the generated Gaussian wave packet of two different modes is derived analytically and compared with the observed oscillations of the sample reflectivity. These experiments provide a unique way to independently obtain geometrical and material characterization. This direct observation of coherent guided acoustic phonons in a single nano-object is also the first step toward nanolateral size acoustic transducer and comprehensive studies of the thermal properties of nanowires.

The synthesis of a naphthalene diimide bithiophene copolymer P­(EO-NDIT2) with branched, base-stable, and purely ether-based side chains is presented. Stille polycondensation leads to high molecular weights that are limited by methyl transfer and eventually T2 homocouplings. While extensive solution aggregation hampers molecular weight determination by conventional methods, NMR spectroscopy allows identification of both T2- (H and methyl) and NDI-related (methyl) end groups, enabling the determination of absolute number average molecular weights larger than M n,NMR ∼100 kg/mol. Solvent- and temperature-dependent aggregation in solution is investigated by NMR and UV–vis spectroscopy. These results are used for solution doping of P­(EO-NDIT2) with N-benzimidazole-based n-dopants. Spin coating from heated chlorobenzene solutions and using 4-(2,3-dihydro-1,3-dimethyl-1H-benzoimidazol-2-yl)-N,N-diisopropylaniline (N-DiPrBI) as the dopant leads to homogeneous films with highest conductivities up to 10–2 S/cm. Generally, N-DiPrBI concentrations as low as ∼5 wt % are sufficient to increase conductivity by orders of magnitude. Strikingly, maximum power factors up to 0.11 μW/mK2, although limited by conductivity, are achieved for the highest molar mass sample at a low dopant concentration of 2 wt % N-DiPrBI only.

The monochromatic and geometrically anisotropic acoustic field generated by 400 nm and 120 nm diameter copper nanowires simply dropped on a 10 µm silicon membrane is investigated in transmission using three-dimensional time-resolved femtosecond pump-probe experiments. Two pump-probe time-resolved experiments are carried out at the same time on both side of the silicon substrate. In reflection, the first radial breathing mode of the nanowire is excited and detected. In transmission, the longitudinal and shear waves are observed. The longitudinal signal is followed by a monochromatic component associated with the relaxation of the nanowire's first radial breathing mode. Finite Difference Time Domain (FDTD) simulations are performed and accurately reproduce the diffracted field. A shape anisotropy resulting from the large aspect ratio of the nanowire is detected in the acoustic field. The orientation of the underlying nanowires is thus acoustically deduced.

The Seebeck coefficient and electrical resistance of BiSb nanowire arrays electrodeposited in etched ion-track membranes have been investigated as a function of wire diameter (40-750 nm) and composition (0 ≤ x ≤ 1). The experimental data reveal a non-monotonic dependence between thermopower and wire diameter for three different compositions. Thus, the thermopower values decrease with decreasing wire diameter, exhibiting a minimum around ∼60 nm. This non-monotonic dependence of the Seebeck coefficient is attributed to the interplay of surface and bulk states. On the one hand, the metallic properties of the surface states can contribute to decreasing the thermopower of the nanostructure with increasing surface-to-volume ratio. On the other hand, for wires thinner than ∼60 nm, the relative increase of the thermopower can be tentatively attributed to the presence of quantum-size effects on both surface and bulk states. These measurements contribute to a better understanding of the interplay between bulk and surface states in nanostructures, and indicate that the decrease of Seebeck coefficient with decreasing diameter caused by the presence of surfaces states can possibly be overcome for even thinner nanowires.

Molecular doping of conjugated polymers is extremely desirable to control charge density gradients and shape the electric field across polymer electronic devices, including highly efficient organic solar cells. It is also a fundamental requirement for organic thermoelectrics and a powerful strategy to boost charge injection and transport properties in transistors. Yet, currently available doping approaches are far from offering a suitable level of control, particularly in the case of n-type doping. We here reveal that part of this limitation lies in the lack of understanding of dominant factors in doping efficiency. In particular, we highlight the key role played by very small amounts of a specific decomposition product formed during processing of the widely used molecular dopant 4-(2,3-dihydro-1,3dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (DMBI-H) in influencing the n-type conductivity in polymer blends. We show that such an overlooked decomposition product acts as a nucleating agent for a new crystalline phase of DMBI-H, with the overall effect of boosting the electrical conductivity of the final doped polymer films. Such results, confirmed by control experiments performed with a different nucleating agent, focus on the crucial role played by the solid-state microstructure in molecular doped semiconductors and offer ground for a significant change in design guidelines for molecular doping strategies.

Doped conjugated organic semiconductors are suitable materials to be used as building blocks of flexible and cost‐effective thermoelectric generators. While several efficient solution‐processable p‐type organic thermoelectric materials are reported, n‐doped materials are fewer because of lack of good electron‐transporting materials and stable n‐dopants. Here, n‐doping process is investigated on a relevant n‐type class, namely naphthalene‐diimide‐based copolymers. Among these, copolymer incorporating bithiophene (T2) donor units, largely studied poly{N,N″‐bis(2‐octyl‐dodecyl)‐1,4,5,8‐naphthalenedicarboximide‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (PNDI‐T2), is one of first polymers reported to achieve good solution‐based n‐doping. By substituting T2 with thienylenevinylene‐thienylene (TVT), resulting PNDI‐TVT copolymer exhibits improvements on both structural and transport properties, offering a suitable basis to improve thermoelectric properties upon doping. When doped with 1H‐benzimidazoles, PNDI‐TVT achieves maximum in‐plane electrical conductivity at room temperature of 2.4 × 10−2 S cm−1, being the highest value for PNDI‐T2 solution‐doped derivatives excluding those with oligoethylene‐glycol chains. Electron paramagnetic resonance and variable temperature electrical conductivity measurements relate this enhancement to more efficient charge‐transfer between n‐dopant molecules and host polymeric matrix, and easier charge carrier transport within the system. This electrical conductivity large enhancement also improves in‐plane power factor of almost three times with respect to similar doped PNDI‐T2 films.