Polymers are lightweight, flexible, solution-processable materials that are promising for low-cost printed electronics as well as for mass-produced and large-area applications. Previous studies demonstrated that they can possess insulating, semiconducting or metallic properties; here we report that polymers can also be semi-metallic. Semi-metals, exemplified by bismuth, graphite and telluride alloys, have no energy bandgap and a very low density of states at the Fermi level. Furthermore, they typically have a higher Seebeck coefficient and lower thermal conductivities compared with metals, thus being suitable for thermoelectric applications. We measure the thermoelectric properties of various poly(3,4-ethylenedioxythiophene) samples, and observe a marked increase in the Seebeck coefficient when the electrical conductivity is enhanced through molecular organization. This initiates the transition from a Fermi glass to a semi-metal. The high Seebeck value, the metallic conductivity at room temperature and the absence of unpaired electron spins makes polymer semi-metals attractive for thermoelectrics and spintronics.

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We study the dynamics of exciton-spin injection, transfer, and relaxation in self-assembled quantum dots ͑QDs͒ of CdSe coupled with a diluted magnetic semiconductor ͑DMS͒ layer of Zn 0.80 Mn 0.20 Se, where spinpolarized excitons can be injected from the DMS into the QDs. The degree of circular polarization P of excitonic photoluminescence ͑PL͒ at 5 T in the coupled QDs exhibits a rapid increase with increasing delay time, up to +0.3 at 25 ps after the pulse excitation of the DMS by a linearly polarized light. This development of a positive P value directly reflects the spin-injection dynamics from the DMS, since the intrinsic polarization of the QD excitons due to Zeeman splitting is P ϳ −0.1 when only the QDs are selectively excited. The P value gradually decays with time after reaching its maximum, as a result of the exciton-spin relaxation with a time constant of 800 ps in the QDs. Time-resolved circularly polarized PL spectra immediately after the pulse excitation directly show the exciton-energy dependence of the spin-injection dynamics in the QD ensemble, where two-dimensional-like QDs with higher exciton energies show higher receptivity to the spin-polarized excitons than three-dimensional-like dots with lower exciton energies. A rate equation analysis reveals all time constants responsible for the spin-injection dynamics. We deduce a time constant of 10 ps for the spin injection. The spin-injection efficiency of 0.94 is also obtained, which corresponds to the ratio between the number of the spin-polarized excitons responsible for the rise of the positive P value in the QD emission and the total number of the excitons injected from the DMS. Moreover, we observe that interdot exciton transfer significantly affects the P value within the QD emission band after the fast spin injection, in addition to the spin relaxation within the QDs.

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Magneto-optical spectroscopy in combination with tunable laser spectroscopy is employed to study optical spin injection from a diluted magnetic semiconductor ͑DMS͒ ZnMnSe into nonmagnetic CdSe quantum dots ͑QDs͒. Observation of a DMS feature in the excitation spectra of the QD photoluminescence polarization provides clear evidence for optical spin-injection from the DMS to the QDs. By means of a rate equation analysis, the injected spin polarization is deduced to be about 32% at 5 T, decreasing from 100% before the injection. The observed spin loss is shown to occur during the spin injection process including crossing the heterointerfaces and energy relaxation within the QDs.