Markus Morgenstern

Intrinsic and extrinsic corrugation of monolayer graphene deposited on SiO 2

Using scanning tunneling microscopy in an ultrahigh vacuum and atomic force microscopy, we investigate the corrugation of graphene flakes deposited by exfoliation on a Si/SiO2 (300 nm) surface. While the corrugation on SiO2 is long range with a correlation length of about 25 nm, some of the graphene monolayers exhibit an additional corrugation with a preferential wavelength of about 15 nm. A detailed analysis shows that the long-range corrugation of the substrate is also visible on graphene, but with a reduced amplitude, leading to the conclusion that the graphene is partly freely suspended between hills of the substrate. Thus, the intrinsic rippling observed previously on artificially suspended graphene can exist as well, if graphene is deposited on SiO2.

The Ice Bilayer on Pt(111): Nucleation, Structure and Melting

The H20-adsorption on Pt(l 11) at 120 K—140 is investigated by temperature-variable scanning tunneling microscopy. At 140 the adsorption kinetics of the first bilayer is determined by heterogeneous nucleation at the upper side and the lower side of step edges as well as by homogeneous island nucleation on the terraces. Depending on the preparation conditions the bilayer exhibits three different phases. Phase I can be characterized as an ideal ice bilayer rotated ±7° with respect to the (112)-direction of the Pt(lll)-surface. As a result the STM-images show a Moirée pattern. The second phase, phase IIh, is less dense than phase I and appears to be a regular arrangement of regions of , -molecules with the fix J3 _/?30°-distance of the Pt(l 1 l)-substrate and regions with higher density in between. This leads to two superstructure domains differing in orientation and periodicity from the domains of phase I. Contrary to phase I, the superstructure of phase IIb is not a Moirée pattern. The third phase, phase IIU, shows a superstructure with the same periodicity as phase IIh, but with a different orientation. It has probably a similar atomic arrangement as phase IL, and is also not a Moirée pattern. The three solid phases can be transformed into each other either by changing the temperature and/or by applying a FLO-pressure at elevated temperatures. It turns out that solid-solid-phase transformations are only possible, if the ice layer is partially molten. From the pressure dependence of the phase transitions an order of density of the different solid phases can be deduced. It is in agreement with the densities concluded from the structural analysis. All phase transformations can be described consistently, if one as-

Quantum Hall transition in real space: from localized to extended states.

Using scanning tunneling spectroscopy in an ultrahigh vacuum at low temperature (T=0.3 K) and high magnetic fields (B<or=12 T), we directly probe electronic wave functions across an integer quantum Hall transition. In accordance with theoretical predictions, we observe the evolution from localized drift states in the insulating phases to branched extended drift states at the quantum critical point. The observed microscopic behavior close to the extended state indicates points of localized quantum tunneling, which are considered to be decisive for a quantitative description of the transition.

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.

A 300 mK ultra-high vacuum scanning tunneling microscope for spin-resolved spectroscopy at high energy resolution

We describe the design and development of a scanning tunneling micoscope (STM) working at very low temperatures in ultra-high vacuum (UHV) and at high magnetic fields. The STM is mounted to the 3He pot of an entirely UHV compatible 3He refrigerator inside a tube which can be baked out to achieve UHV conditions even at room temperature. A base temperature of 315 mK with a hold time of 30 h without any recondensing or refilling of cryogenics is achieved. The STM can be moved from the cryostat into a lower UHV-chamber system where STM-tips and -samples can be exchanged without breaking UHV. The chambers contain standard surface science tools for preparation and characterization of tips and samples in particular for spin-resolved scanning tunneling spectroscopy (STS). Test measurements using either superconducting tips or samples show that the system is adequate for performing STS with both high spatial and high energy resolution. The vertical stability of the tunnel junction is shown to be 5 pmpp and the ene...

Nucleation and morphology of homoepitaxial Pt(111)-films grown with ion beam assisted deposition

Abstract The nucleation and morphology of thin Pt-films grown with ion beam assisted deposition (IBAD) on Pt(111) have been studied by scanning tunneling microscopy and low energy electron diffraction. In comparison to conventional vapor phase deposition it is found that the simultaneous ion bombardment with Ar + (400eV)- and Ar + (4keV)-ions during deposition drastically increases the island number density at T ≥200 K. The increase is due to nucleation at ion impact induced adatom clusters. As a consequence of the increased island number density, the lateral dimensions of the film microstructure are decreased. In contrast, at T ≤70 K where the adatom diffusion is thermally inhibited the simultaneous ion bombardment leads to a decreased island number density due to ion impact induced adatom mobility. Thin Pt-films grown with ion assistance at T = 50 K exhibit an improved epitaxy compared to the rather amorphous Pt-films grown by conventional vapor phase deposition at this temperature.

Molecular structure of the H2O wetting layer on Pt(111)

The molecular structure of the wetting layer of ice on Pt(111) is resolved using scanning tunneling microscopy (STM). Two structures observed previously by diffraction techniques are imaged for coverages at or close to completion of the wetting layer. At 140K only a sqrt(37) x sqrt(37) R25.3{\deg} superstructure can be established, while at 130K also a sqrt(39) x sqrt(39) R16.1{\deg} superstructure with slightly higher molecular density is formed. In the temperature range under concern the superstructures reversibly transform into each other by slight changes in coverage through adsorption or desorption. The superstructures exhibit a complex pattern of molecules in different geometries.

Direct comparison between potential landscape and local density of states in a disordered two-dimensional electron system.

The local density of states (LDOS) of the adsorbate-induced two-dimensional electron system (2DES) on n-InAs(110) is studied by scanning tunneling spectroscopy. In contrast to a similar 3DES, the 2DES LDOS exhibits 20 times stronger corrugations and rather irregular structures. Both results are interpreted as consequences of weak localization. Fourier transforms of the LDOS reveal that the k values of the unperturbed 2DES still dominate the 2DES, but additional lower k values contribute. To clarify the origin of the LDOS patterns, we measure the potential landscape of the 2DES area. We use it to calculate the expected LDOS and find reasonable agreement between calculation and experiment.

Giant Rashba‐Type Spin Splitting in Ferroelectric GeTe(111)

Photoelectron spectroscopy in combination with piezoforce microscopy reveals that the helicity of Rashba bands is coupled to the nonvolatile ferroelectric polarization of GeTe(111). A novel surface Rashba band is found and fingerprints of a bulk Rashba band are identified by comparison with density functional theory calculations.