All organic molecule spin valve

I remember in my previous entry I mentioned a spin filtering effect of DNA molecules. Here comes another molecule with similar effect and can operate as a spin valve at low temperatures[http://www.nature.com/nmat/journal/v10/n7/full/nmat3061.html?WT.ec_id=NMAT-201107].

Now, writing in Nature Materials, Urdampilleta and co-workers¹ report that a single-walled carbon nanotube decorated with magnetic molecules can act in just the same way as a conventional spin valve, albeit only at low temperature.
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How is it possible for a single molecule to perform as efficiently as 10 nm of iron? The key is the ability of a chemical bond to modify the magnetic properties of a surface, which has been studied under the suggestive name of 'spinterface science'⁵. It has already been shown that an attached molecule can alter the spin-polarization of the electrons emerging from a magnetic surface^{6, 7}; the experiments of Urdampilleta and co-workers now prove the opposite effect — namely that a magnetic molecule can alter the spin polarization of the current flowing in a non-magnetic material. Two particular features make this possible. First, the magnetic centre must be sufficiently close to the conduction channel. In this respect, the case of bis-phthalocyaninato-terbium(III) is rather peculiar, because the Tb³⁺ ion (Tb³⁺ carries a total angular momentum, J = 6) is sandwiched between two phthalocyanine ligands, and it is at least 1 nm away from the nanotube — too far to transfer any magnetic information. However, there is a second source of spin in this molecule, namely a S = 1/2 radical delocalized over the two phthalocyanine ligands. These are likely to participate in the bond and help to spin-polarize the electron current. Second, the conduction channel must be sufficiently sensitive to the local magnetic moment. All the atoms in a single-walled carbon nanotube reside on the surface, so that a surface modification results in an alteration of the entire electronic structure. It is an extreme surface sensitivity that makes this spin valve work.

Trions detected in CNTs

An exciton is a bound pair of one electron and one hole, analogous to a hydrogen atom, but with much smaller binding energy. It can be excited in many semiconductors by light. An electron in valence band can be lifted by a photon to the conduction band and in the meanwhile a hole-quasiparticle- will be left behind. This electron shall attract with the hole due to electrostatic interaction and make an exciton. A trion is like an ionized hydrogen molecule, containing one electron and two holes. Such objects are of course less stable than an exciton, but have been observed. Now some scientists from Kyoto University observed these trions in carbon nanotubes. They seem much stable with binding energy of a tenth of an electron volt [Phys. Rev. Lett. 106, 037404 (2011) ]. A view point can be found here [Physics 4, 5 (2011)] :

The smoking gun of the trions in the experiment of Matsunaga et al. is the appearance of an additional peak in the optical spectra on the low-energy end of the exciton peak. Matsunaga et al. work hard to show that this peak indeed originates from trions and not, for example, from defects introduced through doping. They first investigate the influence of different dopants and doping concentrations and find that new peaks associated with trions in nanotubes appear at the same energy, regardless of the dopant species. They also become stronger with increasing doping concentration, along with a reduction of the exciton peak. Moreover, they find that excitons in nanotubes with different diameters and twist angles all come along with a corresponding trion partner and show clear “family patterns,” similar to those known for excitons in nanotubes. All this provides strong evidence for trions.