We demonstrate a facile route to obtain high and broad-band circular polarization of electroluminescence in single-layer polymer OLEDs. As a light-emitting material we use a donor-acceptor polyfluorene with enantiomerically pure chiral side-chains. We show that upon thermal annealing the polymer self-assembles into a multidomain cholesteric film. By varying the thickness of the polymer emitting layer, we achieve high levels of circular polarization of electroluminescence (up to 40% excess of right-handed polarization), which are the highest reported for polymer OLEDs not using chiral dopants or alignment layers. Mueller matrix ellipsometry shows strong optical anisotropies in the film, indicating that the circular polarization of luminescence arises mainly after the photon has been generated, through selective scattering and birefringence correlated in the direction of the initial linear polarization of the photon. Our work demonstrates that chirally substituted conjugated polymers can combine photonic and semiconducting properties in advanced optoelectronic devices.
The effect of spin polarization in conduction and in electric field-induced polarization was measured for double-stranded DNA oligonucleotides and oligopeptides of different lengths. These measurements were conducted using magnetic contact AFM, spin-dependent electrochemistry, spin-dependent polarization, and magnetoresistance studies. It was established that the spin-dependent conduction through chiral molecules depends on the voltage applied with a power of d, when d is larger than unity, and that there is a different voltage threshold for conducting each of the spin polarizations. In addition, there is no spin flipping during the conduction through the chiral system. The spin polarization depends linearly on the length, within the range of lengths studied, and it seems to scale like the optical activity. These results suggest the importance of the electric polarizability in the chiral-induced spin selectivity process. It was also shown that the preferred spin back scattering is suppressed, compared with the nonpreferred spin, probably as a result of the coupling between the electron's linear momentum and its spin as a result of the chiral potential.
Spin‐dependent conduction and polarization in chiral polymers were studied for polymers organized as self‐assembled monolayers with conduction along the polymer backbone, namely, along its longer axis. Large spin polarization and magnetoresistance effects were observed, showing a clear dependence on the secondary structure of the polymer. The results indicate that the spin polarization process does not include spin flipping and hence it results from backscattering probabilities for the two spin states.
We study GaAs/AlGaAs
devices hosting a two-dimensional electron
gas and coated with a monolayer of chiral organic molecules. We observe
clear signatures of room-temperature magnetism, which is induced in
these systems by applying a gate voltage. We explain this phenomenon
as a consequence of the spin-polarized charges that are injected into
the semiconductor through the chiral molecules. The orientation of
the magnetic moment can be manipulated by low gate voltages, with
a switching rate in the megahertz range. Thus, our devices implement
an efficient, electric field-controlled magnetization, which has long
been desired for their technical prospects.
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