F. Schedin

Fluorographene: A Two-Dimensional Counterpart of Teflon

A stoichiometric derivative of graphene with a fluorine atom attached to each carbon is reported. Raman, optical, structural, micromechanical, and transport studies show that the material is qualitatively different from the known graphene-based nonstoichiometric derivatives. Fluorographene is a high-quality insulator (resistivity >10(12) Ω) with an optical gap of 3 eV. It inherits the mechanical strength of graphene, exhibiting a Youngs modulus of 100 N m(-1) and sustaining strains of 15%. Fluorographene is inert and stable up to 400 °C even in air, similar to Teflon.

Proton transport through one-atom-thick crystals

Graphene is increasingly explored as a possible platform for developing novel separation technologies. This interest has arisen because it is a maximally thin membrane that, once perforated with atomic accuracy, may allow ultrafast and highly selective sieving of gases, liquids, dissolved ions and other species of interest. However, a perfect graphene monolayer is impermeable to all atoms and molecules under ambient conditions: even hydrogen, the smallest of atoms, is expected to take billions of years to penetrate graphene’s dense electronic cloud. Only accelerated atoms possess the kinetic energy required to do this. The same behaviour might reasonably be expected in the case of other atomically thin crystals. Here we report transport and mass spectroscopy measurements which establish that monolayers of graphene and hexagonal boron nitride (hBN) are highly permeable to thermal protons under ambient conditions, whereas no proton transport is detected for thicker crystals such as monolayer molybdenum disulphide, bilayer graphene or multilayer hBN. Protons present an intermediate case between electrons (which can tunnel easily through atomically thin barriers) and atoms, yet our measured transport rates are unexpectedly high and raise fundamental questions about the details of the transport process. We see the highest room-temperature proton conductivity with monolayer hBN, for which we measure a resistivity to proton flow of about 10xa0Ωxa0cm2 and a low activation energy of about 0.3xa0electronvolts. At higher temperatures, hBN is outperformed by graphene, the resistivity of which is estimated to fall below 10−3xa0Ωxa0cm2 above 250xa0degrees Celsius. Proton transport can be further enhanced by decorating the graphene and hBN membranes with catalytic metal nanoparticles. The high, selective proton conductivity and stability make one-atom-thick crystals promising candidates for use in many hydrogen-based technologies.

Magnetic properties of stoichiometric and nonstoichiometric ultrathin Fe3O4(111) films on Al2O3(0001)

A detailed characterization of magnetic oxide films is essential to enable their use in magnetoresistive devices since their properties depend critically on stoichiometry and structural order. Here, the composition and magnetic properties of ultrathin iron oxide films grown epitaxially on Al2O3(0001) have been characterized using x-ray magnetic circular dichroism XMCD and magnetoresistance (MR) measurements. The XMCD data show by comparison with theoretical calculations that we have successfully found growth conditions for well ordered epitaxial films with Fe3O4 stoichiometry. Nonstoichiometric films exhibit, in addition to a relative reduction in Fe2+ ions, a net transfer of Fe3+ from tetrahedral to octahedral sites. The in-plane MR for both these films is found to be 1% at room temperature in a field of 1 T even though the electrical conductivity differs by a factor of 5.

Windows and photocathodes for a high resolution solid state bandpass ultraviolet photon detector for inverse photoemission

We have measured the absolute quantum yield for alkali halides and the spectral transmission for alkaline earth fluoride windows to find an optimized bandpass combination for a solid state ultraviolet (UV) photon detector for inverse photoemission. The best resolution achieved is 0.33 eV (full width at half-maximum), being obtained with the NaCl photocathode–BaF2 window combination. This, however, leads to a rather low quantum efficiency of 0.3%. The combination NaCl–SrF2 chosen for our detector offers a resolution of 0.42 eV (full width at half-maximum) with a maximum quantum efficiency of 2.5% at 9.50 eV photon energy.

Magnetic properties of ultrathin epitaxial Fe3O4 films on Pt(1 1 1)

Abstract The magnetic properties of ultrathin Fe 3 O 4 (1xa01xa01) films grown epitaxially on Pt(1xa01xa01) have been studied by X-ray magnetic circular dichroism (XMCD) and conversion electron Mossbauer spectroscopy (CEMS). Both the XMCD and CEMS spectra exhibit features characteristic of Fe 3 O 4 . In addition, a paramagnetic contribution, which is interpreted as originating from the interface layer, is observed in the Mossbauer spectrum. Furthermore, the CEMS data indicate that all the magnetic moments lie in the plane of the film. The relative occupancies of tetrahedral and octahedral sites in the lattice derived from the two techniques are compared and yield a higher value for the tetrahedral site from the CEMS data, probably because it was recorded from a thinner film.

Magnetic Moment in an Ultrathin Magnetite Film

We have investigated the magnetic properties of a Cu capped thin film of magnetite (Fe3O4) grown epitaxially on Pt(111). Conversion electron Mossbauer spectroscopy data show good agreement with those from bulk Fe3O4, evidencing a good degree of structural order. The data point to in-plane ferrimagnetic alignment of the magnetic moment in the Fe3O4 layer. Polarized neutron reflectivity (PNR) data determines the layer thinknesses to be 53±6 A for the magnetite film and 106±5 A for the Cu capping layer. The average magnetic moment determined by PNR for the Fe3O4 layer is 2.8±0.3u200aμB, smaller than the value of 4.1u200aμB for bulk Fe3O4. It is suggested that the reduced moment is in part a result of a reduced ordering temperature in the ultrathin film.

Dual-Scattering Near-Field Microscope for Correlative Nanoimaging of SERS and Electromagnetic Hotspots

Surface-enhanced Raman spectroscopy (SERS) enables sensitive chemical studies and materials identification, relying on electromagnetic (EM) and chemical-enhancement mechanisms. Here we introduce a tool for the correlative nanoimaging of EM and SERS hotspots, areas of strongly enhanced EM fields and Raman scattering, respectively. To that end, we implemented a grating spectrometer into a scattering-type scanning near-field optical microscope (s-SNOM) for mapping of both the elastically and inelastically (Raman) scattered light from the near-field probe, that is, a sharp silicon tip. With plasmon-resonant gold dimers (canonical SERS substrates) we demonstrate with nanoscale spatial resolution that the enhanced Raman scattering from the tip is strongly correlated with its enhanced elastic scattering, the latter providing access to the EM-field enhancement at the illumination frequency. Our technique has wide application potential in the correlative nanoimaging of local-field enhancement and SERS efficiency as well as in the investigation and quality control of novel SERS substrates.

Resonance control of mid-infrared metamaterials using arrays of split-ring resonator pairs.

We present our design, fabrication and characterization of resonance-controllable metamaterials operating at mid-infrared wavelengths. The metamaterials are composed of pairs of back-to-back or face-to-face U-shape split-ring resonators (SRRs). Transmission spectra of the metamaterials are measured using Fourier-transform infrared spectroscopy. The results show that the transmission resonance is dependent on the distance between the two SRRs in each SRR pair. The dips in the transmission spectrum shift to shorter wavelengths with increasing distance between the two SRRs for both the back-to-back and face-to-face SRR pairs. The position of the resonance dips in the spectrum can hence be controlled by the relative position of the SRRs. This mechanism of resonance control offers a promising way of developing metamaterials with tunability for optical filters and bio/chemical sensing devices in integrated nano-optics.

Bolt-on source of spin-polarized electrons for inverse photoemission

We have developed a portable spin-polarized electron gun which can be bolted on to an ultrahigh vacuum chamber. The gun has been successfully operated with an electron gun to target distance of about 150 mm. This allows accommodation of other surface science equipment in the same vacuum system. The spin-polarized electrons are obtained via photoemission from a negative electron affinity GaAs(001) surface with circularly polarized light. A transversely polarized beam is achieved with a 90° electrostatic deflector. A set of two three-element electrostatic tube lenses are employed to transport and to focus the electrons onto a target. The measured transmission through the electron optics is >70% for electron energies in the range 7–20 eV. This is achieved by using large diameter electron transport lenses. The energy resolution of the electron beam is measured to be better than 0.27 eV and the polarization is determined to be 25±5%.

Use of Supramolecular Assemblies as Lithographic Resists

A new resist material for electron beam lithography has been created that is based on a supramolecular assembly. Initial studies revealed that with this supramolecular approach, high-resolution structures can be written that show unprecedented selectivity when exposed to etching conditions involving plasmas.