Armin Rettenberger

New design of a variable-temperature ultrahigh vacuum scanning tunneling microscope

We present the design of a variable-temperature ultrahigh vacuum (UHV) scanning tunneling microscope which can be operated between 20 and 400 K. The microscope is mounted directly onto the heat exchanger of a He continuous flow cryostat without vibration isolation inside the UHV chamber. The coarse approach is performed with an inertial slider driven by the same piezo tube that is also used for scanning. The performance of the instrument is demonstrated by two different kinds of measurements: First we show topographic images of close packed metal surfaces with atomic resolution. Then, we present results from scanning tunneling spectroscopy measurements on WSe2 under illumination at variable temperatures

The influence of tip sample interaction on step fluctuations on Ag(111)

The fluctuations of the position of monatomic steps on Ag(111) are investigated by scanning tunneling microscopy (STM). We analyze the influence of tip?sample interaction by varying the gap impedance over more than two orders of magnitude. For tunneling tips providing a weak tip?sample interaction, we show that the step position autocorrelation function remains essentially unaltered. In this unperturbed case, the kinetics of step fluctuations are found to be dominated by one-dimensional mass transport. For larger variations of the tip sample distance or for less favorable tip configurations, we observe a tip-induced increase of the step fluctuations. Our measurements suggest that this effect is caused rather by short-range forces than by the electric field in the tunneling gap.

STM investigation of the island growth of gold on WS2 and WSe2

We have investigated the island growth of Au on the van der Waals surfaces of WS2 and WSe2 by STM. We show how the STM can be used to image as well as to manipulate nm-sized crystallites on these atomically smooth non-reactive substrates. The islands on the van der Waals planes are triangular in shape and are well aligned with the substrate lattice. Statistical analysis of our data yield subtle differences in the growth of Au on WS2 compared to WSe2.

Step roughness on Ag (111) investigated by STM : a systematic study of tip influence

We have investigated monatomic steps on Ag(111) by STM at different temperatures. At room temperature, the rough appearance of these steps is usually attributed to thermal step fluctuations. We have investigated the influence of the tip systematically. Applying a new test, we demonstrate that even subtle influences can lead to wrong results in statistical analysis.

Investigation of photoinduced tunneling current and local surface photovoltage by STM

Photoassisted scanning tunneling microscopy was used to simultaneously image topoyraphy, photoinduced mnneling current and local surface photovoltage on an nm-scale. A novel interrupted z-feedback technique is presented which overcomes the limitations of previously reported techniques, that were restricted to semiconductor surfaces with a high density of surface states. As an example, measurements on the van der Waals surface of WS2 are shown. This semiconductor surface is known to be free of intrinsic surface states. In the vicinity of monolayer steps an enhanced minority charge carrier recombination and a reduced photovolta,oe was observed. G I998 Elsevier Science B.V.

Thermovoltages in vacuum tunneling investigated by scanning tunneling microscopy

By heating the tunneling tip of a scanning tunneling microscope the thermoelectric properties of a variable vacuum barrier have been investigated. The lateral variation of the observed thermovoltage will be discussed for polycrystalline gold, stepped surfaces of silver, as well as for copper islands on silver.

PLASMON-INDUCED TUNNELING CURRENTS: THE INFLUENCE OF TIP MODES

The interaction of propagating surface plasmons (PSP) excited in a silver film with the tunneling junction of a scanning tunneling microscope has been investigated. Under particular conditions we find a strong plasmon-induced current (PIC) superimposed on the tunneling current. The generation of the PIC is in our view due to the rectification of the optical electric field by the nonlinearity of the tunneling junction. To obtain a PIC with detectable magnitude an enhancement of the optical field between tip and sample by several orders of magnitude is required. The observed dependence of the field enhancement (FE) on the tip material and the geometry of the tunneling junction will be discussed in terms of localized surface plasmons.