Molecular spintronics: Accessing 4f states

Nature Commun. 4, 2425 (2013)

Credit: © 2013 NPG

In molecular spintronics, magnetic molecules are interfaced with metallic electrodes to measure their spin state. Accessing electrons in spin-polarized states, such as 4f states, can, however, be difficult when those states lie far away from the Fermi level and do not contribute to charge transport, which is the case for molecules that include late lanthanides. Moreover, the orbital hybridization between molecular and electrode states can affect the magnetic properties of the molecule, to the extent that its magnetic moment can be quenched. Nicolae Atodiresei, Claire Besson and colleagues at Forschungszentrum Jülich and RWTH Aachen University have now studied a molecule–electrode model system in which spin-polarized 4f states can be accessed by using a scanning tunnelling microscope and the magnetic moment of the molecule can be retained.

The researchers measure the density of states of a single bis(phthalocyaninato)-neodymium(III) (NdPc2) molecule adsorbed on a Cu(100) surface using scanning tunnelling spectroscopy. The three 4f electrons of neodymium are less localized than the 4f electrons in late lanthanides, and have a higher degree of hybridization with the Pc ligands. Moreover, they are all in the same spin channel, resulting in a net magnetic moment. By using the tip of the microscope to take spectra at different positions above the molecule — above the centre and above the Pc ligand — and by comparing the obtained spectra with ab initio calculations, the researchers conclude that the 4f states contribute to the tunnelling current and carry a magnetic moment. ED

Fluorescence microscopy: Imaging on the go

ACS Nano http://doi.org/pb3 (2013)

Fluorescent microscopes are rarely thought of as portable devices due to their relatively bulky and expensive components. Smaller devices can impose limitations on the strength of the fluorescent signal and can have increased background noise due to leakage of the excitation light and detection noise.

Aydogan Ozcan and colleagues at the University of California, Los Angeles have now developed a lightweight attachment for a smartphone that can serve as a mobile fluorescent microscope capable of imaging nanoparticles. The device uses a 450-nm laser diode at a high illumination angle, which is instrumental in reducing the background noise. The noise is further suppressed by using a thin-film interference filter. Samples, which can be solid or liquid, are imaged on a small platform inserted into the phone.

The researchers evaluated the performance of the device by imaging fluorescently labelled polystyrene beads and were able to detect individual beads with diameters of around 100 nm. They also showed that the mobile microscope could be used to image single human cytomegaloviruses that were fluorescently labelled. Although these virus particles are larger than the polystyrene beads (with diameters of 150 to 300 nm), they can be difficult to detect because they contain a low density of fluorescent labels.

Aydogan Ozcan and colleagues suggest that the portable device could be used in various field settings, including point-of-care applications. SB

Carbon nanostructures: Carbyne shows its strength

ACS Nano http://doi.org/pb4 (2013)

Carbyne is an all-carbon linear polymer and has been predicted to have interesting mechanical properties. Under normal conditions, the most stable form is made of carbon atoms linked by alternate single and triple bonds. Boris Yakobson and colleagues at Rice University have now carried out a comprehensive theoretical study of the mechanical properties of carbyne using first-principles calculations and provide insights that could be valuable for future practical applications of the material.

Under tension, carbyne can maintain its structural integrity until a force of about 10 nN is applied, outperforming graphene, carbon nanotubes and diamond. The bandgap depends on the tensile stress applied, increasing by about 80% under a 10% strain. This extreme sensitivity should make carbyne an attractive material for opto- and electromechanical applications.

Owing to its axial symmetry, the mechanical properties of carbyne under torsion can only be defined when it is chemically modified by an end group. The bandgap then becomes a function of the torsional angle and the researchers show that at near 90° angles, carbyne becomes a magnetic semiconductor, a property that could be exploited in spintronic devices. AM

Plasmonics: Graphene fills the gap

Nano Lett. http://doi.org/pb5 (2013)

The coupling of plasmonic nanostructures can lead to intense local optical fields, which can be used in devices such as sensors and photodetectors. To create and control the coupling it is necessary to place the nanostructures at a short distance from each other, with a typical gap of a few tens of nanometres. Jeremy Baumberg and colleagues at the University of Cambridge, the Nokia Research Center in Cambridge and the Center of Materials Physics in San Sebastián have now used graphene to generate well-controlled gaps between gold nanospheres and a gold substrate.

Nanospheres with diameters of 80 nm were deposited on a silicon substrate covered by a thin gold layer and a monolayer of graphene. Single particle scattering from the samples revealed a spectral doublet, which the researchers attributed to the mixing of two modes: a dipole resonance due to the electric polarization between the charge in the nanosphere and an equal and opposite charge in an image nanosphere formed in the gold layer; and a plasmon mode that is localized near the graphene layer. When the graphene layer was removed, only a peak associated with the coupling of the sphere with its image remained. Conversely, when more layers of graphene were used to create the gap only a peak associated with the gap survived.

Baumberg and colleagues also observed that although the separation between the two peaks was the same for every gold nanosphere studied, their spectral position varied accordingly to the local conductivity of the graphene layer. This suggests that a voltage applied to the graphene layer to vary its conductivity could be used to finely tune the plasmonic resonances. FP

Written by Sarah Brown, Elisa De Ranieri, Alberto Moscatelli and Fabio Pulizzi.