Toggling the Local Electric Field with an Embedded Adatom Switch

Torque-Induced Change in Configuration of a Single NO Molecule on Cu(110)

We demonstrated that a nitric oxide (NO) molecule on Cu(110) acts as an "ON-OFF-ON toggle switch" that can be turned on and off by repulsive force and electron injection, respectively. On the surface, NO molecules exist in three configurations: flat along the [001] direction (ON), upright (OFF), and flat along [001[over ¯]] (ON). An NO-functionalized tip, which was characterized by scanning tunneling microscopy and inelastic electron tunneling spectroscopy, can convert an upright NO adsorbate into a flat-lying NO. Atomic force microscopy and a simulation of the interactions between the NO molecules reveal that a repulsive force not aligned with the N-O bond provides the torque that detrudes the NO toggle; i.e., the upright NO adsorbate is tilted away from the tip. Therefore, the NO adsorbate behaves as a nonvolatile sensor for the detection of locally applied repulsive torque.

Structure and local charging of electromigrated Au nanocontacts

We study the structure and the electronic properties of Au nanocontacts created by controlled electromigration of thin film devices, a method frequently used to contact molecules. In contrast to electromigration testing, a current is applied in a cyclic fashion and during each cycle the resistance increase of the metal upon heating is used to avoid thermal runaway. In this way, nanometer sized-gaps are obtained. The thin film devices with an optimized structure at the origin of the electromigration process are made by shadow evaporation without contamination by organic materials. Defining rounded edges and a thinner area in the center of the device allow to pre-determine the location where the electromigration takes place. Scanning force microscopy images of the pristine Au film and electromigrated contact show its grainy structure. Through electromigration, a 1.5 μm-wide slit is formed, with extensions only on the anode side that had previously not been observed in narrower structures. It is discussed whether this could be explained by asymmetric heating of both electrodes. New grains are formed in the slit and on the extensions on both, the anode and the cathode side. The smaller structures inside the slit lead to an electrode distance below 150 nm. Kelvin probe force microscopy images show a local work function difference with fluctuations of 70 mV on the metal before electromigration. Between the electrodes, disconnected through electromigration, a work function difference of 3.2 V is observed due to charging. Some of the grains newly formed by electromigration are electrically disconnected from the electrodes.