U. Starke

Quasi-free-standing epitaxial graphene on SiC obtained by hydrogen intercalation.

Quasi-free-standing epitaxial graphene is obtained on SiC(0001) by hydrogen intercalation. The hydrogen moves between the (6 square root(3) x 6 square root(3))R30 degrees reconstructed initial carbon layer and the SiC substrate. The topmost Si atoms which for epitaxial graphene are covalently bound to this buffer layer, are now saturated by hydrogen bonds. The buffer layer is turned into a quasi-free-standing graphene monolayer with its typical linear pi bands. Similarly, epitaxial monolayer graphene turns into a decoupled bilayer. The intercalation is stable in air and can be reversed by annealing to around 900 degrees C.

Charge neutrality and band-gap tuning of epitaxial graphene on SiC by molecular doping

Epitaxial graphene on SiC(0001) suffers from strong intrinsic n-type doping. We demonstrate that the excess negative charge can be fully compensated by non-covalently functionalizing graphene with the strong electron acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ). Charge neutrality can be reached in monolayer graphene as shown in electron dispersion spectra from angular resolved photoemission spectroscopy (ARPES). In bilayer graphene the band gap that originates from the SiC/graphene interface dipole increases with increasing F4-TCNQ deposition and, as a consequence of the molecular doping, the Fermi level is shifted into the band gap. The reduction of the charge carrier density upon molecular deposition is quantified using electronic Fermi surfaces and Raman spectroscopy. The structural and electronic characteristics of the graphene/F4-TCNQ charge transfer complex are investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The doping effect on graphene is preserved in air and is temperature resistant up to 200 C. Furthermore, graphene non-covalent functionalization with F4-TCNQ can be implemented not only via evaporation in ultra-high vacuum but also by wet chemistry.

Raman spectra of epitaxial graphene on SiC and of epitaxial graphene transferred to SiO2.

Raman spectra were measured for mono-, bi-, and trilayer graphene grown on SiC by solid state graphitization, whereby the number of layers was preassigned by angle-resolved ultraviolet photoemission spectroscopy. It was found that the only unambiguous fingerprint in Raman spectroscopy to identify the number of layers for graphene on SiC(0001) is the line width of the 2D (or D*) peak. The Raman spectra of epitaxial graphene show significant differences as compared to micromechanically cleaved graphene obtained from highly oriented pyrolytic graphite crystals. The G peak is found to be blue-shifted. The 2D peak does not exhibit any obvious shoulder structures, but it is much broader and almost resembles a single-peak even for multilayers. Flakes of epitaxial graphene were transferred from SiC onto SiO2 for further Raman studies. A comparison of the Raman data obtained for graphene on SiC with data for epitaxial graphene transferred to SiO2 reveals that the G peak blue-shift is clearly due to the SiC substrate. The broadened 2D peak however stems from the graphene structure itself and not from the substrate.

Structural and electronic properties of epitaxial graphene on SiC(0 0 0 1): a review of growth, characterization, transfer doping and hydrogen intercalation

Graphene, a monoatomic layer of graphite, hosts a two-dimensional electron gas system with large electron mobilities which makes it a prospective candidate for future carbon nanodevices. Grown epitaxially on silicon carbide (SiC) wafers, large area graphene samples appear feasible and integration in existing device technology can be envisioned. This paper reviews the controlled growth of epitaxial graphene layers on SiC(0 0 0 1) and the manipulation of their electronic structure. We show that epitaxial graphene on SiC grows on top of a carbon interface layer that—although it has a graphite-like atomic structure—does not display the linear π-bands typical for graphene due to a strong covalent bonding to the substrate. Only the second carbon layer on top of this interface acts like monolayer graphene. With a further carbon layer, a graphene bilayer system develops. During the growth of epitaxial graphene on SiC(0 0 0 1) the number of graphene layers can be precisely controlled by monitoring the π-band structure. Experimental fingerprints for in situ growth control could be established. However, due to the influence of the interface layer, epitaxial graphene on SiC(0 0 0 1) is intrinsically n-doped and the layers have a long-range corrugation in their density of states. As a result, the Dirac point energy where the π-bands cross is shifted away from the Fermi energy, so that the ambipolar properties of graphene cannot be exploited. We demonstrate methods to compensate and eliminate this structural and electronic influence of the interface. We show that the band structure of epitaxial graphene on SiC(0 0 0 1) can be precisely tailored by functionalizing the graphene surface with tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) molecules. Charge neutrality can be achieved for mono- and bilayer graphene. On epitaxial bilayer graphene, where a band gap opens due to the asymmetric electric field across the layers imposed by the interface, the magnitude of this band gap can be increased up to more than double its initial value. The hole doping allows the Fermi level to shift into the energy band gap. The impact of the interface layer can be completely eliminated by decoupling the graphene from the SiC substrate by a hydrogen intercalation technique. We demonstrate that hydrogen can migrate under the interface layer and passivate the underlying SiC substrate. The interface layer alone transforms into a quasi-free standing monolayer. Epitaxial monolayer graphene turns into a decoupled bilayer. In combination with atmospheric pressure graphitization, the intercalation process allows the production of quasi-free standing epitaxial graphene on large SiC wafers and represents a highly promising route towards epitaxial graphene based nanoelectronics.

Automated determination of complex surface structures by LEED

Conventional surface crystallography by low-energy electron diffraction (LEED) employs a trial-and-error search controlled at each step by human effort. This trial-and-error approach becomes very cumbersome and unreliable to solve complex surfaces with a large number of unknown structural parameters. We discuss automatic optimization procedures for LEED, which combine numerical search algorithms with efficient methods of determining the diffracted intensities for varying structures. Such approaches can reduce the computer time required for an entire structure determination by many orders of magnitude, while fitting many times more unknown structural parameters. Thereby, relatively complex structures, with typically 10 adjustable atoms (or 30 adjustable coordinates), can be readily determined on todays workstations. These include non-symmetrically relaxed structures, surface reconstructions and adsorbate-induced substrate distortions. We also address the theoretical and experimental requirements for an accurate structural determination.

Experimental analysis of charge redistribution due to chemical bonding by high-resolution transmission electron microscopy

The electronic charge density distribution or the electrostatic atomic potential of a solid or molecule contains information not only on the atomic structure, but also on the electronic properties, such as the nature of the chemical bonds or the degree of ionization of atoms. However, the redistribution of charge due to chemical bonding is small compared with the total charge density, and therefore difficult to measure. Here, we demonstrate an experimental analysis of charge redistribution due to chemical bonding by means of high-resolution transmission electron microscopy (HRTEM). We analyse charge transfer on the single-atom level for nitrogen-substitution point defects in graphene, and confirm the ionicity of single-layer hexagonal boron nitride. Our combination of HRTEM experiments and first-principles electronic structure calculations opens a new way to investigate electronic configurations of point defects, other non-periodic arrangements or nanoscale objects that cannot be studied by an electron or X-ray diffraction analysis.

Charge-transfer-induced structural rearrangements at both sides of organic/metal interfaces

Organic/metal interfaces control the performance of many optoelectronic organic devices, including organic light-emitting diodes or field-effect transistors. Using scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure spectroscopy and density functional theory calculations, we show that electron transfer at the interface between a metal surface and the organic electron acceptor tetracyano-p-quinodimethane leads to substantial structural rearrangements on both the organic and metallic sides of the interface. These structural modifications mediate new intermolecular interactions through the creation of stress fields that could not have been predicted on the basis of gas-phase neutral tetracyano-p-quinodimethane conformation.

Snapshots of non-equilibrium Dirac carrier distributions in graphene

The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all of these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy- and momentum-dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme-ultraviolet pulses to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. Whereas excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find any indication of carrier multiplication, questioning the applicability of graphene for light harvesting.

Strong Performance Improvement of La0.6Sr0.4Co0.8Fe0.2O3 − δ SOFC Cathodes by Electrochemical Activation

It is shown that the electrochemical resistance of mixed conducting solid oxide fuel cell (SOFC) model cathodes can be reduced drastically by a short but strong dc polarization of the cell. The samples investigated are dense thin-film microelectrodes of La 0 . 6 Sr 0 . 4 Co 0 . 8 Fe 0 . 2 O 3 - δ on a yttria-stabilized zirconia solid electrolyte. Relative performance improvements of more than two orders of magnitude can be achieved with a cathodic dc bias of the order of 1 V, applied for a few minutes at fuel cell operating temperature. The positive effect on the electrode performance corresponds to an acceleration of the oxygen surface exchange reaction, initially the resistance determining process. For this surface-related resistance, absolute values as low as 0.065 Ω cm 2 at 700°C have been obtained. After such an activation, the resistance slowly increases again on a much larger time scale, indicating the possibility of a steady performance enhancement by a periodic activation with short dc pulses. X-ray photoelectron spectra show that a strong cathodic polarization severely changes the cation concentrations within the outermost surface layer of the electrode, and these field-induced surface compositional changes are assumed to be the main cause of the performance improvement.

Reliability of detailed LEED structural analyses: Pt(111) and Pt(111)-p(2×2)-O

As surface structures are being examined in more detail than ever before, the reliability of structural details becomes an important issue. To discuss necessary components of a high quality, reliable dynamical LEED studies, detailed dynamical LEED analyses of the clean Pt(111) and the Pt(111)-p(2×2)-O structures have been carried out utilizing different computer programs and search methods applied to a common set of experimental LEED I-V data. We have investigated the effects of various non-structural parameters, in particular those involved in the construction of the phase shifts, on the resulting structures of the clean Pt(111) and the Pt(111)-p(2×2)-O surfaces. The use of relativistic Pt phase shifts is found to be important in order to determine the adsorption structure accurately. For clean Pt(111), we find the top interlayer spacing is noticeably expanded by 0.025 ± 0.01 A with a low Rp-factor value of 0.15. For the Pt(111)-p(2×2)-O system we find an Rp-factor of 0.18. O atoms in the p(2×2)-O overlayer are found to adsorb in fcc-hollow sites and induce buckling in both the first and second metal layers. In addition, there is an expansion of the first metal-metal interlayer spacing and a small contraction of the second.