Ch. Gerber

Surface Studies by Scanning Tunneling Microscopy

Surface microscopy using vacuum tunneling is demonstrated for the first time. Topographic pictures of surfaces on an atomic scale have been obtained. Examples of resolved monoatomic steps and surface reconstructions are shown for (110) surfaces of CaIrSn 4 and Au.

Tunneling through a controllable vacuum gap

We report on the first successful tunneling experiment with an externally and reproducibly adjustable vacuum gap. The observation of vacuum tunneling is established by the exponential dependence of the tunneling resistance on the width of the gap. The experimental setup allows for simultaneous investigation and treatment of the tunnel electrode surfaces.

Reproducible switching effect in thin oxide films for memory applications

Thin oxide films with perovskite or related structures and with transition metal doping show a reproducible switching in the leakage current with a memory effect. Positive or negative voltage pulses can switch the resistance of the oxide films between a low- and a high-impedance state in times shorter than 100 ns. The ratio between these two states is typically about 20 but can exceed six orders of magnitude. Once a low-impedance state has been achieved it persists without a power connection for months, demonstrating the feasibility of nonvolatile memory elements. Even multiple levels can be addressed to store two bits in such a simple capacitor-like structure.

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.

A chemical sensor based on a microfabricated cantilever array with simultaneous resonance-frequency and bending readout

Abstract We present a chemical sensor based on a microfabricated array of eight silicon cantilevers actuated at their resonance-frequency and functionalized by polymer coatings. The operating principle relies on transduction of chemical or physical processes into a mechanical response. After exposure to analyte vapor, analyte molecules diffuse into the cantilever coating, which begins to swell. Jointly with the mass increase, a change of interfacial stress between coating and cantilever occurs, resulting in a bending of the cantilevers. Our setup allows the simultaneous detection of cantilever oscillation and bending of eight cantilevers by time-multiplexed optical beam deflection readout. The ac component of the cantilever response is demodulated, and the cantilever resonance-frequency is tracked by a custom-built phase-locked loop. By filtering out the ac component (oscillation), the dc signal (bending) is extracted, yielding information on mass as well as surface stress changes simultaneously. Detection results of water, primary alcohols, alkanes and perfumes are presented.

Atomic Resolution with Atomic Force Microscope

The atomic force microscope (AFM) is a promising new method for studying the surface structure of both conductors and insulators. In mapping a graphite surface with an insulating stylus, we have achieved a resolution better than 2.5 A.

Observation of a chemical reaction using a micromechanical sensor

We describe a new form of calorimeter designed for use in gaseous and vacuum environments which can sense chemical reactions with an estimated sensitivity limit of approximate to 1 pJ. The device is based on a micromechanical Si lever coated with a thick layer of Al upon which a sample in the form of a thin layer is fixed or deposited. Heat fluxes are detected by measuring the cantilever deflection induced by the differential thermal expansion of the lever (bimetallic effect) using the optical position sensor from a force microscope. The limit of sensitivity to local temperature changes is approximate to 10(-5) K at 300 K. Using this technique the catalytic conversion of H-2 + O-2 to form H2O over a thin Pt overcoated layer is observed to exhibit self-sustained oscillations in the reaction rate on the macroscopic scale.

A cantilever array-based artificial nose

We present quantitative and qualitative detection of analyte vapors using a microfabricated silicon cantilever array. To observe transduction of physical and chemical processes into nanomechanical motion of the cantilever, swelling of a polymer layer on the cantilever is monitored during exposure to the analyte. This motion is tracked by a beam-deflection technique using a time multiplexing scheme. The response pattern of eight cantilevers is analyzed via principal component analysis (PCA) and artificial neural network (ANN) techniques, which facilitates the application of the device as an artificial chemical nose. Analytes tested comprise chemical solvents, a homologous series of primary alcohols, and natural flavors. First differential measurements of surface stress change due to protein adsorption on a cantilever array are shown using a liquid cell.

Rapid and label-free nanomechanical detection of biomarker transcripts in human RNA

The availability of entire genome sequences has triggered the development of microarrays for clinical diagnostics that measure the expression levels of specific genes. Methods that involve labelling can achieve picomolar detection sensitivity, but they are costly, labour-intensive and time-consuming. Moreover, target amplification or biochemical labelling can influence the original signal. We have improved the biosensitivity of label-free cantilever-array sensors by orders of magnitude to detect mRNA biomarker candidates in total cellular RNA. Differential gene expression of the gene 1-8U, a potential marker for cancer progression or viral infections, has been observed in a complex background. The measurements provide results within minutes at the picomolar level without target amplification, and are sensitive to base mismatches. This qualifies the technology as a rapid method to validate biomarkers that reveal disease risk, disease progression or therapy response. We foreseee cantilever arrays being used as a tool to evaluate treatment response efficacy for personalized medical diagnostics.

An artificial nose based on a micromechanical cantilever array

A novel chemical sensor based on a micromechanical array of silicon cantilevers is presented. Chemical reactions are transduced by sensitization of cantilevers with coatings such as metals, self-assembled monolayers, or polymers into a mechanical response. This is read out using an optical beam-deflection technique by a sequential readout scheme. Reference cantilever sensors permit subtraction of background signals (differential measurement). Coating of each cantilever sensor with a different sensitive layer allows operation of the array-device as a new form of chemical nose. Detection of hydrogen, primary alcohols, natural flavors, and water vapor is demonstrated. We show that the magnitude of sensor response is proportional to the amount of analyte present.