T. Mashoff

Wave-function mapping of graphene quantum dots with soft confinement.

Using low-temperature scanning tunneling spectroscopy, we map the local density of states of graphene quantum dots supported on Ir(111). Because of a band gap in the projected Ir band structure around the graphene K point, the electronic properties of the QDs are dominantly graphenelike. Indeed, we compare the results favorably with tight binding calculations on the honeycomb lattice based on parameters derived from density functional theory. We find that the interaction with the substrate near the edge of the island gradually opens a gap in the Dirac cone, which implies soft-wall confinement. Interestingly, this confinement results in highly symmetric wave functions. Further influences of the substrate are given by the known moiré potential and a 10% penetration of an Ir surface resonance into the graphene layer.

Bistability and Oscillatory Motion of Natural Nanomembranes Appearing within Monolayer Graphene on Silicon Dioxide

The truly two-dimensional material graphene is an ideal candidate for nanoelectromechanics due to its large strength and mobility. Here we show that graphene flakes provide natural nanomembranes of diameter down to 3 nm within its intrinsic rippling. The membranes can be lifted either reversibly or hysteretically by the tip of a scanning tunneling microscope. The clamped-membrane model including van-der-Waals and dielectric forces explains the results quantitatively. AC-fields oscillate the membranes, which might lead to a completely novel approach to controlled quantized oscillations or single atom mass detection.

Electrical transport and low-temperature scanning tunneling microscopy of microsoldered graphene

Using the recently developed technique of microsoldering, we perform systematic transport studies of the influence of polymethylmethacrylate on graphene revealing a doping effect with a n-type dopant density Δn of up to Δn=3.8×1012 cm−2 but negligible influence on mobility and hysteresis. Moreover, we show that microsoldered graphene is free of contamination and exhibits very similar intrinsic rippling as found for lithographically contacted flakes. Characterizing the microsoldered sample by scanning tunneling spectroscopy, we demonstrate a current induced closing of the phonon gap and a B-field induced double peak attributed to the 0 Landau level.

Hydrogen storage with titanium-functionalized graphene

We report on hydrogen adsorption and desorption on titanium-covered graphene in order to test theoretical proposals to use of graphene functionalized with metal atoms for hydrogen storage. At room temperature, titanium islands grow on graphene with an average diameter of about 10 nm. Samples were then loaded with hydrogen, and its desorption kinetics was studied by thermal desorption spectroscopy. We observe the desorption of hydrogen in the temperature range between 400 K and 700 K. Our results demonstrate the stability of hydrogen binding at room temperature and show that hydrogen desorbs at moderate temperatures in line with what is required for practical hydrogen-storage applications.

A low-temperature high resolution scanning tunneling microscope with a three-dimensional magnetic vector field operating in ultrahigh vacuum

We present a low-temperature ultrahigh vacuum (UHV) scanning tunneling microscope setup with a combination of a superconducting solenoid coil and two split-pair magnets, providing a rotatable magnetic field up to 500 mT applicable in all spatial directions. An absolute field maximum of B=7 T(3 T) can be applied perpendicular (parallel) to the sample surface. The instrument is operated at a temperature of 4.8 K. Topographic and spectroscopic measurements on tungsten carbide and indium antimonide revealed a z-noise of 300 fm(pp), which barely changes in magnetic field. The microscope is equipped with a tip exchange mechanism and a lateral sample positioning stage, which allows exact positioning of the tip with an accuracy of 5 microm prior to the measurement. Additional contacts to the sample holder allow, e.g., the application of an additional gate voltage. The UHV part of the system contains versatile possibilities of in situ sample and tip preparation as well as low-energy electron diffraction and Auger analysis.

Scanning tunneling spectroscopy of a dilute two-dimensional electron system exhibiting Rashba spin splitting

Using scanning tunneling spectroscopy (STS) at 5 K in B-fields up to 7 T, we investigate the local density of states of a two-dimensional electron system (2DES) created by Cs adsorption on p-type InSb(110). The 2DES, which in contrast to previous STS studies exhibits a 2D Fermi level, shows standing waves at B = 0 T with corrugations decreasing with energy and with wave numbers in accordance with theory. In magnetic field percolating drift states are observed within the disorder broadened Landau levels. Due to the large electric field perpendicular to the surface, a beating pattern of the Landau levels is found and explained quantitatively by Rashba spin splitting within the lowest 2DES subband. The Rashba splitting does not contribute significantly to the standing wave patterns in accordance with theory.

Revealing the Multibonding State between Hydrogen and Graphene-Supported Ti Clusters

Hydrogen adsorption on graphene-supported metal clusters has brought much controversy due to the complex nature of the bonding between hydrogen and metal clusters. The bond types of hydrogen and graphene-supported Ti clusters are experimentally and theoretically investigated. Transmission electron microscopy shows that Ti clusters of nanometer size are formed on graphene. Thermal desorption spectroscopy captures three hydrogen desorption peaks from hydrogenated graphene-supported Ti clusters. First-principles calculations also found three types of interaction: two types of bonds with different partial ionic character and physisorption. The physical origin for this rests on the charge state of the Ti clusters: when Ti clusters are neutral, H2 is dissociated, and H forms bonds with the Ti cluster. On the contrary, H2 is adsorbed in molecular form on positively charged Ti clusters, resulting in physisorption. Thus, this work clarifies the bonding mechanisms of hydrogen on graphene-supported Ti clusters.