Atomically thin mica flakes and their application as ultrathin insulating substrates for graphene

We show that it is possible to deposit, by mechanical exfoliation on SiO2/Si wafers, atomically thin mica flakes down to a single monolayer thickness. The optical contrast of these mica flakes on top of a SiO2/Si substrate, which depends on their thickness, the illumination wavelength and the SiO2 substrate thickness, can be quantitatively accounted for by a Fresnel law based model. The preparation of atomically thin insulating crystalline sheets will enable the fabrication of ultrathin defect-free insulating substrates, dielectric barriers or planar electron tunneling junctions. Additionally, we show that few-layer graphene flakes can be deposited on top of a previously transferred mica flake. Our transfer method relies on viscoelastic stamps, as those used for soft lithography. A Raman spectroscopy study shows that such an all-dry deposition technique yields cleaner and higher quality flakes than conventional wet-transfer procedures based on lithographic resists.Comment: 11 pages, 5 figures, 1 graphical abstrac

Temperature dependent transport characteristics of graphene/n-Si diodes

Realizing an optimal Schottky interface of graphene on Si is challenging, as the electrical transport strongly depends on the graphene quality and the fabrication processes. Such interfaces are of increasing research interest for integration in diverse electronic devices as they are thermally and chemically stable in all environments, unlike standard metal/semiconductor interfaces. We fabricate such interfaces with n-type Si at ambient conditions and find their electrical characteristics to be highly rectifying, with minimal reverse leakage current ($<$10$^{-10}$ A) and rectification of more than $10^6$. We extract Schottky barrier height of 0.69 eV for the exfoliated graphene and 0.83 eV for the CVD graphene devices at room temperature. The temperature dependent electrical characteristics suggest the influence of inhomogeneities at the graphene/n-Si interface. A quantitative analysis of the inhomogeneity in Schottky barrier heights is presented using the potential fluctuation model proposed by Werner and G\"{u}ttler.Comment: 5 pages, 5 figure

Large yield production of high mobility freely suspended graphene electronic devices on a PMGI based organic polymer

The recent observation of fractional quantum Hall effect in high mobility suspended graphene devices introduced a new direction in graphene physics, the field of electron-electron interaction dynamics. However, the technique used currently for the fabrication of such high mobility devices has several drawbacks. The most important is that the contact materials available for electronic devices are limited to only a few metals (Au, Pd, Pt, Cr and Nb) since only those are not attacked by the reactive acid (BHF) etching fabrication step. Here we show a new technique which leads to mechanically stable suspended high mobility graphene devices which is compatible with almost any type of contact material. The graphene devices prepared on a polydimethylglutarimide based organic resist show mobilities as high as 600.000 cm^2/Vs at an electron carrier density n = 5.0 10^9 cm^-2 at 77K. This technique paves the way towards complex suspended graphene based spintronic, superconducting and other types of devices.Comment: 14 pages, 4 figure

Tertiary development of the Polish and eastern Slovak parts of the Carpathian accretionary wedge : insights from balanced cross-sections

During Eocene-Sarmatian, a Polish-eastern Slovak portion of the Outer West Carpathian accretionary wedge was deformed in front of the ALCAPA terrane. This portion advanced into the area of the subducting remnant Carpathian Flysch Basin, a large oceanic tract left in front of the Alpine orogen. Western parts of the wedge were characterized by a noticeable lack of involvement of thick-skin thrusting and by a predominant development of fault-propagation folds. Eastern parts of the wedge were characterized by the involvement of thick-skin thrusting, triangle zones and back-thrusts. The frontal portion of the wedge was characterized by a décollement formed along the shale and gypsum formations of the Badenian molasse sediments, which resulted in the increased width of the thrust sheets. Forelandward thinning of foreland basin sediments indicates that the portion of the European Platform attached to the subducting oceanic lithosphere flexed underneath the advancing Carpathians as early as the Eocene. Oligocene sediments record syn-depositional thrusting by abrupt thickness changes over short distances. Younger periods of the thrusting are documented by the Eggenburgian-Karpatian piggy-back basin carried by thrust sheets in the frontal portion of the ALCAPA terrane, the Early Miocene age of the youngest sediments in the central portion of the wedge and involvement of the middle Badenian molasse sediments in the frontal portion of the wedge. The end of the shortening is documented by the lower Sarmatian end of the strike-slip fault activity behind the wedge, by the middle Sarmatian transgression over the deformed wedge in the Orava-Nowy Targ Basin, which is located in the rear portion of the wedge, and by the Sarmatian undeformed sediments sealing the wedge front. The existence of the forebulge in front of the advancing Carpathians is documented by local Eocene, Oligocene and Lower Miocene unconformities in the frontal portion of the wedge

A road to hydrogenating graphene by a reactive ion etching plasma

We report the hydrogenation of single and bilayer graphene by an argon-hydrogen plasma produced in a reactive ion etching (RIE) system. Electronic transport measurements in combination with Raman spectroscopy are used to link the electric mean free path to the optically extracted defect concentration. We emphasize the role of the self-bias of the graphene in suppressing the erosion of the akes during plasma processing. We show that under the chosen plasma conditions the process does not introduce considerable damage to the graphene sheet and that hydrogenation occurs primarily due to the hydrogen ions from the plasma and not due to fragmentation of water adsorbates on the graphene surface by highly accelerated plasma electrons. For this reason the hydrogenation level can be precisely controlled. The hydrogenation process presented here can be easily implemented in any RIE plasma system.Comment: 7 page

Electronic Spin Transport in Dual-Gated Bilayer Graphene

The elimination of extrinsic sources of spin relaxation is key in realizing the exceptional intrinsic spin transport performance of graphene. Towards this, we study charge and spin transport in bilayer graphene-based spin valve devices fabricated in a new device architecture which allows us to make a comparative study by separately investigating the roles of substrate and polymer residues on spin relaxation. First, the comparison between spin valves fabricated on SiO2 and BN substrates suggests that substrate-related charged impurities, phonons and roughness do not limit the spin transport in current devices. Next, the observation of a 5-fold enhancement in spin relaxation time in the encapsulated device highlights the significance of polymer residues on spin relaxation. We observe a spin relaxation length of ~ 10 um in the encapsulated bilayer with a charge mobility of 24000 cm2/Vs. The carrier density dependence of spin relaxation time has two distinct regimes; n<4 x 1012 cm-2, where spin relaxation time decreases monotonically as carrier concentration increases, and n>4 x 1012 cm-2, where spin relaxation time exhibits a sudden increase. The sudden increase in the spin relaxation time with no corresponding signature in the charge transport suggests the presence of a magnetic resonance close to the charge neutrality point. We also demonstrate, for the first time, spin transport across bipolar p-n junctions in our dual-gated device architecture that fully integrates a sequence of encapsulated regions in its design. At low temperatures, strong suppression of the spin signal was observed while a transport gap was induced, which is interpreted as a novel manifestation of impedance mismatch within the spin channel

Isotope Shift Measurements of Stable and Short-Lived Lithium Isotopes for Nuclear Charge Radii Determination

Changes in the mean-square nuclear charge radii along the lithium isotopic chain were determined using a combination of precise isotope shift measurements and theoretical atomic structure calculations. Nuclear charge radii of light elements are of high interest due to the appearance of the nuclear halo phenomenon in this region of the nuclear chart. During the past years we have developed a new laser spectroscopic approach to determine the charge radii of lithium isotopes which combines high sensitivity, speed, and accuracy to measure the extremely small field shift of an 8 ms lifetime isotope with production rates on the order of only 10,000 atoms/s. The method was applied to all bound isotopes of lithium including the two-neutron halo isotope Li-11 at the on-line isotope separators at GSI, Darmstadt, Germany and at TRIUMF, Vancouver, Canada. We describe the laser spectroscopic method in detail, present updated and improved values from theory and experiment, and discuss the results.Comment: 34 pages, 24 figures, 14 table