Measurements of the frequency shift of optical phonons in Si with uniaxial stress have been performed by Raman spectroscopy using a laser frequency in the region of transparency. A mean deviation of 25% has been found in the phonon deformation potentials as compared with previous results obtained with laser frequencies above the absorption edge. The new values are p/${\mathrm{\omega }}_0^2$=-1.85±0.06, q/${\mathrm{\omega }}_0^2$=-2.31±0.06, and r/${\mathrm{\omega }}_0^2$=-0.71±0.02, and 1.08±0.06 for the Grüneisen parameter ${\gamma }_{\mathit{G}}$. A valence-force-field model with five anharmonic parameters provides a consistency check between the phonon deformation potentials and the third-order elastic constants of Si. On the basis of this model, existing uniaxial-stress data for the optical phonons of diamond and for the pressure dependence of the second-order elastic constants have been reanalyzed and the corresponding five anharmonic parameters have been determined. In this manner, reliable values for the third-order elastic constants have been obtained for diamond.

We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature ( T _c ) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO ₃ layers in BaTiO ₃ /SrTiO ₃ superlattices are not only ferroelectric (with T _c as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO ₃ layers adjacent to them. T _c was tuned by ∼500 kelvin by varying the thicknesses of the BaTiO ₃ and SrTiO ₃ layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.

This work reports on the synthesis of the intrinsically conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with several counter-ions, ClO₄, PF₆ and bis(trifluoromethylsulfonyl)imide (BTFMSI), by electro-polymerization and its thermoelectric properties. We show that, depending on the counter-ion size, the thermoelectric efficiency of PEDOT can be increased up to two orders of magnitude. A further chemical reduction with hydrazine optimizes the power factor (PF). By changing the counter-ions, we were able to increase the electrical conductivity (σ) of PEDOT by a factor of three, while the Seebeck coefficient remains at the same order of magnitude in the three polymers. The best thermoelectric efficiency has been observed in PEDOT:BTFMSI. From the measurement of the Seebeck coefficient and σ, a PF of 147 μW m⁻¹ K⁻² has been deduced, while the measured thermal conductivity is κ = 0.19 W m⁻¹ K⁻¹, resulting in a ZT ∼ 0.22 at room temperature, one of the highest values reported in the literature for polymers. The increase in σ with the change of the counter-ion is mainly due to the stretching of the polymer chains. In this work, we provide a chemical route to further improve ZT in polymers and demonstrate a method of synthesis based on the electro-polymerization on gold. After removing the gold layer, a very thin semiconducting polymer film can be isolated.

In this review, we report the state-of-the-art of polymers in thermoelectricity. Classically, a number of inorganic compounds have been considered as the best thermoelectric materials. Since the prediction of the improvement of the figure of merit by means of electronic confinement in 1993, it has been improved by a factor of 3–4. In the mean time, organic materials, in particular intrinsically conducting polymers, had been considered as competitors of classical thermoelectrics, since their figure of merit has been improved several orders of magnitude in the last few years. We review here the evolution of the figure of merit or the power factor during the last years, and the best candidates to compete with inorganic materials. We also outline the best polymers to substitute classical thermoelectric materials and the advantages they present in comparison with inorganic systems.

The thermoelectric properties in one- and two-dimensional silicon and germanium structures have been investigated using first-principles density functional techniques and linear response for the thermal and electrical transport. We have considered here the two-dimensional silicene and germanene, together with nanoribbons of different widths. For the nano ribbons, we have also investigated the possibility of nano structuring these systems by mixing silicon and germanium. We found that the figure of merit at room temperature of these systems is remarkably high, up to 2.5.

The progressive stacking of chalcogenide single layers gives rise to two-dimensional semiconducting materials with tunable properties that can be exploited for new field-effect transistors and photonic devices. Yet the properties of some members of the chalcogenide family remain unexplored. Indium selenide (InSe) is attractive for applications due to its direct bandgap in the near infrared, controllable p- and n-type doping and high chemical stability. Here, we reveal the lattice dynamics, optical and electronic properties of atomically thin InSe flakes prepared by micromechanical cleavage. Raman active modes stiffen or soften in the flakes depending on which electronic bonds are excited. A progressive blue-shift of the photoluminescence peaks is observed for decreasing flake thickness (as large as 0.2 eV for three single layers). First-principles calculations predict an even larger increase in the bandgap, 0.40 eV, for three single layers, and as much as 1.1 eV for a single layer. These results are promising from the point of view of the versatility of this material for optoelectronic applications at the nanometer scale and compatible with Si and III-V technologies.