Silvia Karthäuser

Resistive switching and data reliability of epitaxial (Ba,Sr)TiO3 thin films

We report on resistive switching of capacitor-like SrRuO3∕Ba0.7Sr0.3TiO3∕Pt thin films epitaxially grown on SrTiO3 substrates. We observe a weak but stable hysteresis in the current-voltage curve. By applying short voltage pulses, a high or low resistive state as well as intermediate states can be addressed even at room temperature. We demonstrate a multiple-branch hysteresis curve corresponding to multilevel switching modus revealing different subloops for different write voltages. Furthermore reliability issues such as cycling endurance and data retention are presented. Read-write operations over 10000cycles show a fatigue-like drift of both resistance states. No data loss is found upon continuous readout.

Control of molecule-based transport for future molecular devices

In this review, possibilities to modify intentionally the electronic transport properties of metal/molecule/metal devices (MMM devices) are discussed. Here especially the influence of the metal work function, the metal-molecule interface, the molecule dipole and different tunneling mechanisms are considered. A route to evaluate the effective surface work function of metal-molecule systems is given and, based on experimental results, an exemplary estimation is performed. The electron transport across different metal-molecule interfaces is characterized by relating transmission coefficients extracted from experimentally derived molecular conductances, decay constants or tunneling barrier heights. Based on the reported results the tunneling decay constant can be assumed to be suitable to characterize intrinsic molecular electron transport properties, while the nature of the metal-molecule contacts is properly described by the transmission coefficient. A clear gradation of transmission efficiencies of metal-anchoring group combinations can be given.

NHC-Based Self-Assembled Monolayers on Solid Gold Substrates

Thin films of 1,3-diethylbenzimidazol-2-ylidene (BIEt) were fabricated from THF solution on solid gold substrates and characterised by high-resolution X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopy. The surface-analytical data are in accord with the formation of self-assembled monolayers of BIEt molecules exhibiting an approximately vertical orientation on the substrate. The crystal structure of (BIEt)(2) was also determined.

Resistive switching of rose bengal devices: A molecular effect?

The resistive switching behavior of devices consisting of aluminum top electrode, molecular layer (rose bengal), and bottom electrode (zinc oxide and indium tin oxide) is examined. By measuring the current versus voltage dependence of these devices for various frequencies and by systematically varying the composition of the device, we show that the switching is an extrinsic effect that is not primarily dependent on the molecular layer. It is shown that the molecular layer is short circuited by filaments of either zinc oxide or aluminum and that the switching effect is due to a thin layer of aluminum oxide at the zinc oxide/aluminum interface.

Early self-assembled stages in epitaxial SrRuO3 on LaAlO3

The stress-induced self-assembled growth of SrRuO3 on LaAlO3 was studied by atomic force microscopy and x-ray diffraction. SrRuO3 epitaxially grown on LaAlO3 by pulsed laser deposition shows two types of out-of-plane arrangements and four in-plane matches. The lattice mismatch (stress) produced by these arrangements was estimated and correlated with the SrRuO3 growth dynamics. After 1 nm, the SrRuO3 film surface exhibits a ripple structure, which serves as a template for the development of a nanopattern of flat islands. These islands coalesce anisotropically resulting in a regular array of “infinite” wires. The wire coalescence for the 12–20 nm thick film nullifies the surface symmetry, while SrRuO3 keeps growing in three dimensions.

Reliable fabrication of 3?nm gaps between nanoelectrodes by electron-beam lithography

The reliable fabrication of nanoelectrode pairs with predefined separations in the few nanometer range is an essential prerequisite for future nanoelectronic devices. Here we demonstrate a fine-tuned electron-beam lithographic (EBL) fabrication route which is suitable for defining nanoelectrode pairs with a gap size down to 3 ± 1 nm and with a yield of 55%. This achievement is based on an optimized two-layer resist system in combination with an adopted developer system, as well as on an elaborated nanoelectrode pattern design taking into consideration the EBL inherent proximity effect. Thus, even a structural control in the nanometer scale is achieved in the EBL process.

Field-Emission Resonances at Tip/α,ω-Mercaptoalkyl Ferrocene/Au Interfaces Studied by STM

The electrical properties of alpha,omega-mercaptoalkyl ferrocenes with different alkyl chain lengths embedded in a self-assembled host matrix of alkanethiols on Au(111) are studied by scanning tunneling microscopy and spectroscopy. Based on current-distance spectroscopy, as well as on the evaluation of Fowler-Nordheim tunneling current oscillations, the apparent barrier height of ferrocene is determined independently by two methods. The electronic coupling of the ferrocene moiety to the Au(111) substrate is shown to depend on the length of the alkane-spacer chain. In a double tunnel junction model our experimental findings are explained, addressing the role of the different molecular moieties of the mercaptoalkyl ferrocenes.

Cu-adatom-mediated bonding in close-packed benzoate/Cu(110)-systems.

Using UHV-STM investigations and density-functional theory calculations we prove the contribution of Cu-adatoms to the stabilization of a new high-density phase of benzoate molecules on a Cu(110) substrate. We show that two different chemical species, benzoate and benzoate Cu-adatoms molecules, build the new close-packed structure. Although both species bind strongly to the copper surface, we identify the benzoate Cu-adatoms molecules as the more mobile species on the surface due to their reduced dipole moment and their lower binding energy compared to benzoate molecules. Therefore, the self-assembly process is supposed to be mediated by benzoate Cu-adatom species, which is analogous to the gold-thiolate species on Au(111) surfaces.

Differential Adsorption of Gold Nanoparticles to Gold/Palladium and Platinum Surfaces

Integration of molecule-capped gold nanoparticles (AuNP) into nanoelectronic devices requires detailed knowledge about the AuNP-electrode interface. Here, we report the pH-dependent adsorption of amine or carboxylic acid-terminated gold nanoparticles on platinum or gold/palladium (30% Pd) alloy, respectively. We synthesized amine-terminated AuNP, applying a new solid phase supported approach, as well as AuNP exhibiting carboxylic acid as terminal groups. The pH-induced agglomeration of the synthesized AuNP was investigated by UV-vis, DLS, and ζ-potential measurements. Depending on the pH and the ionic strength of the AuNP solution a preferential adsorption on the different metals occurred. Thereby, we demonstrate that by choosing the appropriate functional group and adjusting the pH as well as the ionic strength a directed binding can be achieved, which is an essential prerequisite for applications of these particles in nanoelectronics. These findings will pave the way for a controlled designing of the interface between molecule-capped AuNP and metallic electrodes for applications in nanoelectronics.

Fabrication of arrays of SrZrO3 nanowires by pulsed laser deposition

SrZrO3 thin films grown by pulsed laser deposition on SrRuO3-buffered SrTiO3 substrates have been investigated by means of x-ray diffraction, atomic force microscopy and Rutherford backscattering spectroscopy. The compressive stress originated in the coherent SrZrO3/SrRuO3 interface due to the lattice mismatch () forces epitaxial SrZrO3 to grow in a three-dimensional habit. So-deposited SrZrO3 is used to fabricate nanoarrays of dielectric wires from a nanopatterned SrRuO3 surface. The influence of the geometric shadowing effect produced by the non-perpendicular incidence of the ablated particles on the SrZrO3 growth habit is discussed.