Rene Heimbuch

Research Update: Molecular electronics: The single-molecule switch and transistor

In order to design and realize single-molecule devices it is essential to have a good understanding of the properties of an individual molecule. For electronic applications, the most important property of a molecule is its conductance. Here we show how a single octanethiol molecule can be connected to macroscopic leads and how the transport properties of the molecule can be measured. Based on this knowledge we have realized two single-molecule devices: a molecular switch and a molecular transistor. The switch can be opened and closed at will by carefully adjusting the separation between the electrical contacts and the voltage drop across the contacts. This single-molecular switch operates in a broad temperature range from cryogenic temperatures all the way up to room temperature. Via mechanical gating, i.e., compressing or stretching of the octanethiol molecule, by varying the contacts interspace, we are able to systematically adjust the conductance of the electrode-octanethiol-electrode junction. This two-terminal single-molecule transistor is very robust, but the amplification factor is rather limited.

In-situ spectroscopy of intrinsic Bi2Te3 topological insulator thin films and impact of extrinsic defects

Combined in situ x-ray photoemission spectroscopy, scanning tunneling spectroscopy, and angle resolved photoemission spectroscopy of molecular beam epitaxy grown Bi 2 Te 3 on lattice mismatched substrates reveal high quality stoichiometric thin films with topological surface states without a contribution from the bulk bands at the Fermi energy. The absence of bulk states at the Fermi energy is achieved without counterdoping. We observe that the surface morphology and electronic band structure of Bi 2 Te 3 are not affected by in vacuo storage and exposure to oxygen, whereas major changes are observed when exposed to ambient conditions. These films help define a pathway towards intrinsic topological devices.

Manipulating transport through a single-molecule junction

Molecular Electronics deals with the realization of elementary electronic devices that rely on a single molecule. For electronic applications, the most important property of a single molecule is its conductance. Here we show how the conductance of a single octanethiol molecule can be measured and manipulated by varying the contacts interspace. This mechanical gating of the single molecule junction leads to a variation of the conductance that can be understood in terms of a tunable image charge effect. The image charge effect increases with a decrease of the contacts interspace due to a reduction of the effective potential barrier height of 1.5 meV/pm.

Interfering Bloch waves in a 1D electron system

Using low-temperature scanning tunnelling spectroscopy we have studied the spatial variation of confined electronic states between neighbouring atomic chains on a Ge(001)/Pt surface. The quasi-one-dimensional electronic states reside in the troughs between the atomic chains and exhibit a profound Bloch character along the chain direction. In the proximity of defects an enhancement of the oscillatory standing wave pattern in the density of states is found. The spatial variation of the standing wave pattern can be explained by an interference of incoming and reflected Bloch waves.

Electron‐Induced Dynamics of Heptathioether β‐Cyclodextrin Molecules

Variable-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) measurements are performed on heptathioether β-cyclodextrin (β-CD) self-assembled monolayers (SAMs) on Au. The β-CD molecules exhibit very rich dynamical behavior, which is not apparent in ensemble-averaged studies. The dynamics are reflected in the tunneling current-time traces, which are recorded with the STM feedback loop disabled. The dynamics are temperature independent, but increase with increasing tunneling current and sample bias, thus indicating that the conformational changes of the β-CD molecules are induced by electrons that tunnel inelastically. Even for sample biases as low as 10 mV, well-defined levels are observed in the tunneling current-time traces. These jumps are attributed to the excitations of the molecular vibration of the macrocyclic β-CD molecule. The results are of great importance for a proper understanding of transport measurements in SAMs.

Single-Crystal Au Triangles as Reconfigurable Contacts for Atomically Smooth Surfaces: Ultra-High Vacuum Transfer-Printing

In a planar atomic-scale device, defined nano-contacts to liaise between the atomic scale and the macroscale via the intermediate micron scale are crucial to ensure the atomic-scale circuit performances by minimizing the leakage current, minimizing cross talks, and minimizing contact resistance. Whether the macroscale contact is via a multi-probe system or via doped back interconnects preparing for a nano-packaging, defined and easily recognizable contact nano-pads are necessary. A new approach is presented here based on the in situ transfer of single Au crystals grown on a MoS2 surface in ultra-high vacuum (UHV) onto another surface where such metallic nano-crystal growth is usually not compatible. The molybdenite surface is particularly well suited to grow single Au nano-crystal triangles, and transfer-printing in ambient environment has already been successfully demonstrated. A dedicated transfer-printing tool was designed and constructed to perform this transfer printing task in a UHV environment. Preliminary results from scanning electron microscopy and low-temperature scanning tunneling microscopy are presented.