Alexander Bietsch

Current-driven insulator–conductor transition and nonvolatile memory in chromium-doped SrTiO3 single crystals

Materials showing reversible resistive switching are attractive for today’s semiconductor technology with its wide interest in nonvolatile random-access memories. In doped SrTiO3 single crystals, we found a dc-current-induced reversible insulator–conductor transition with resistance changes of up to five orders of magnitude. This conducting state allows extremely reproducible switching between different impedance states by current pulses with a performance required for nonvolatile memories. The results indicate a type of charge-induced bulk electronic change as a prerequisite for the memory effect, scaling down to nanometer-range electrode sizes in thin films.

Conformal contact and pattern stability of stamps used for soft lithography

Patterning in soft lithography techniques such as microcontact printing or light-coupling mask lithography is mediated by surface topographical patterns of elastomeric stamps: intimate contact with the substrate is achieved locally at the protruding areas, whereas a gap remains between the substrate and recessed zones. This principle challenges the properties of the stamp, especially when printing high-resolution or extreme aspect-ratio patterns with high accuracy. On the one hand, the stamp must be soft enough to enable conformal contact with the substrate, which means that it must adapt elastically without leaving voids created by the natural roughness of the substrate. On the other hand, a precise definition of micropatterns requires a rigid material. In this article, we analyze the conditions of elasticity, roughness, and energy of adhesion to establish conformal contact between an elastomer and the target surface. Furthermore, we address questions of replication accuracy and evaluate local elastic de...

Rapid functionalization of cantilever array sensors by inkjet printing

The controlled deposition of functional layers is the key to converting nanomechanical cantilevers into chemical or biochemical sensors. Here, we introduce inkjet printing as a rapid and general method to coat cantilever arrays efficiently with various sensor layers. Self-assembled monolayers of alkanethiols were deposited on selected Au-coated cantilevers and rendered them sensitive to ion concentrations or pH in liquids. The detection of gene fragments was achieved with cantilever sensors coated with thiol-linked single-stranded DNA oligomers on Au. A selective etch protocol proved the uniformity of the monolayer coatings at a microscopic level. A chemical gas sensor was fabricated by printing thin layers of different polymers from dilute solutions onto cantilevers. The inkjet method is easy to use, faster and more versatile than coating via microcapillaries or the use of pipettes. In addition, it is scalable to large arrays and can coat arbitrary structures in non-contact.

Size and grain-boundary effects of a gold nanowire measured by conducting atomic force microscopy

The resistivities of thin metal films and wires are highly sensitive to their polycrystalline structure and surface morphology because grain boundaries and surfaces provide additional scattering sites compared to bulk materials. Here, we investigated polycrystalline gold wires of nanometer-scale diameter that were—at some locations—connected through single grain boundaries. A detailed topography of the wires was recorded by atomic force microscopy. A Pt-coated tip in a conducting atomic force microscopy setup served as a mobile electrode to probe the resistance of a wire. Analyzing the topographical cross section and the resistance data allowed us to evaluate the effective specific resistivity of the wire as well as reflection coefficients of single grain boundaries.

Electrical testing of gold nanostructures by conducting atomic force microscopy

We devised a method for the reliable electrical testing of nanoscale wire arrays using conducting probe atomic force microscopy (AFM) in ambient conditions. A key requirement of this approach is the formation of highly reproducible electrical contacts between the conducting tip and the sample. We discuss the basic mechanical and electrical criteria of nanocontacts and derive a force-controlled protocol for the formation of low-ohmic contacts. Tips sputter coated with platinum provided the mechanical stability for both tapping-mode imaging and the formation of low-ohmic contacts on gold samples. Nanostructures on the sample were identified by topographic imaging and subsequently probed using the AFM tip as a mobile electrode. We measured resistivities in arrays of nanowires or local potentials of wires within electrical circuits, and detected electrical failures, thermal gradients, and small geometrical variations. The ability of this instrument to address electrical characteristics with high spatial resol...

A tunnelling displacement sensor based on a squeezable molecular bilayer

For measuring nanoscale displacements, air/vacuum tunnelling is the most sensitive method. However, the alignment mechanism is difficult to scale down for integrated devices. Here, we present a tunnelling displacement sensor based on a squeezable molecular bilayer, which was built from two stacked self-assembled monolayers of mercaptohexadecanoic acid. The bilayer provides an inherent vertical alignment between the tunnelling electrodes. Squeezing of the bilayer leads to an exponential change in the tunnel current. Nanometre displacement sensitivity was achieved.

Functionalization of Cantilever Array Sensors Using Ink Jet Deposition

Micromechanical cantilever arrays can be used as versatile chemical sensors when coated with a layer that absorbs specific molecules leading to changes of surface stress, mass or temperature [1-4]. After their microfabrication processes, reproducible coating procedures are essential to functionalize the cantilevers individually. Here we introduce the use of ink jet to deposit analyte solutions onto cantilevers for functionalization (Figure 1). The method is fast, reliable and versatile. After evaporation of an Au layer, self-assembled mono-layers of alkanethiols or thiol-linked single-stranded DNA can be formed locally within fractions of a second by spotting droplets of solutions onto selected cantilevers. By selective etch diagnostics, we show down to a microscopic level that the quality of the monolayers is similar to those achieved by microcontact printing [5]. We also spotted thin polymer layers from dilute solutions onto cantilevers. The quality of the coatings and the performance were compared with cantilever sensors coated by previous methods.