Achim Voigt

Hydrogen sensing with diameter- and chirality-sorted carbon nanotubes.

The work function of palladium is known to be sensitive to hydrogen by the formation of a surface dipole layer or Pd hydride. One approach to detect such a change in the work function can be based on the formation of a Schottky barrier between the palladium metal and a semiconductor. Here, we study the hydrogen sensitivity of Schottky barrier field-effect transistors made for the first time from diameter- and chirality-sorted semiconducting single-walled carbon nanotubes (s-SWNTs) in contact with Pd electrodes. We observe an unrivaled 100-fold change in the on-state conductance at 100 ppm H2 compared to air for devices with s-SWNT and diameters between 1 and 1.6 nm. Hydrogen sensing is not observed for devices of Pd-contacted few-layer graphene (FLG), as expected due to the absence of a significant Schottky barrier. Unexpectedly, we observe also a vanishing sensitivity for small-diameter SWNTs. We explain this observation by changes in the nanotube work function caused by spillover and chemisorption of atomic hydrogen onto small-diameter nanotubes. We also observe that long-term sensing stability is only achieved if the gate voltage is inverted periodically. Under constant gate bias, the sensitivity reduces with time, which we relate to gate screening by accumulated charges in the substrate.

A scalable, CMOS-compatible assembly of ambipolar semiconducting single-walled carbon nanotube devices

Semiconducting single-walled carbon nanotubes are integrated into high-density arrays using dielectrophoresis, which is a CMOS-compatible, bottom-up assembly technique. The devices are statistically analyzed by voltage-contrast scanning electron microscopy and electron transport measurements. Annealing and the choice of parylene substrate are shown to improve device performance.

Selective detection of HFC and HCFC refrigerants using a surface acoustic wave sensor system

Halogenated hydrocarbons are the generic base of most refrigerants. They are known to be greenhouse gases; some of them are even suspected to have an ozone depleting potential. Thus, an urgent need exists to detect and identify these compounds. However, refrigerants usually have very low boiling points as well as low electrochemical activities. The latter problem is a serious obstacle to the development of appropriate electrochemical sensors. It is circumvented by using mass sensitive methods of detection. To solve the former problem, the analytical system has to be optimized with regard to effective collection and detection of the refrigerants. In this work, a system for detection and identification of refrigerants is presented, based on a surface acoustic wave sensor array. The system is using a two-step analyte preprocessing unit containing a molecular sieve filter to minimize humidity and a carboxen trap for preconcentration of the analytes. Refrigerants R22, R134a, and R507 have been selected for ana...

Detection of aromatic compounds in artificial gasoline with hybrid surface acoustic wave sensor array and a short packed column (SAW-GC)

A self-developed and newly re-designed SAW sensor system composed by 4 polymer coated and 4 differently modified nano-diamond coated SAW sensors was applied to measure aromatic compounds in gasoline in a low cost, fast and easy way. An additional short packed column at system inlet improved the selectivity. The short packed column separated selectively the aromatics from the rest of the sample. Since all employed sensors show linearity for the concentration range tested an easy quantification of single fuel components was possible.

Long-Term Stability of Polymer-Coated Surface Transverse Wave Sensors for the Detection of Organic Solvent Vapors

Arrays with polymer-coated acoustic sensors, such as surface acoustic wave (SAW) and surface transverse wave (STW) sensors, have successfully been applied for a variety of gas sensing applications. However, the stability of the sensors’ polymer coatings over a longer period of use has hardly been investigated. We used an array of eight STW resonator sensors coated with different polymers. This sensor array was used at semi-annual intervals for a three-year period to detect organic solvent vapors of three different chemical classes: a halogenated hydrocarbon (chloroform), an aliphatic hydrocarbon (octane), and an aromatic hydrocarbon (xylene). The sensor signals were evaluated with regard to absolute signal shifts and normalized signal shifts leading to signal patterns characteristic of the respective solvent vapors. No significant time-related changes of sensor signals or signal patterns were observed, i.e., the polymer coatings kept their performance during the course of the study. Therefore, the polymer-coated STW sensors proved to be robust devices which can be used for detecting organic solvent vapors both qualitatively and quantitatively for several years.

Towards Displaying Graphics on a Cheap, Large-Scale Braille Display

Large-scale Braille displays will make participation in modern media society easier for visually impaired people. At the moment extensive research is done on developing new technologies for affordable refreshable Braille displays. However, the developed displays often do not match the user requirements. This is because most of the engineers entrusted with the development know little to nothing about the potential users of their systems. To bridge that gap we carried out an online survey. Within this survey 69 people who either are visually impaired themselves or take care of someone who has lost his/her sight stated their opinion on how a large-scale refreshable Braille display should be designed. The results of this survey were used to build a first prototype of a large-scale refreshable braille display for displaying text and tactile graphics. This prototype relies on cheap, energy efficient microfluidic phase change actuators.

Suspended Liquid Subtractive Lithography: One-step generation of 3D channel geometries in viscous curable polymer matrices

The miniaturization of synthesis, analysis and screening experiments is an important step towards more environmentally friendly chemistry, statistically significant biology and fast and cost-effective medicinal assays. The facile generation of arbitrary 3D channel structures in polymers is pivotal to these techniques. Here we present a method for printing microchannels directly into viscous curable polymer matrices by injecting a surfactant into the uncured material via a steel capillary attached to a 3D printer. We demonstrate this technique using polydimethylsiloxane (PDMS) one of the most widely used polymers for the fabrication of, e. g. microfluidic chips. We show that this technique which we term Suspended Liquid Subtractive Lithography (SLSL) is well suited for printing actuators, T-junctions and complex three dimensional structures. The formation of truly arbitrary channels in 3D could revolutionize the fabrication of miniaturized chips and will find broad application in biology, chemistry and medicine.

Modular Optoelectronic Microfluidic Backplane for Fluid Analysis Systems

We report on the development of backplane modules with integrated microvalves and optical switches enabling custom-made system design of analysis systems. The backplane modules are reversibly interconnected by magnetostatic connectors and provide optical and fluidic coupling to four neighboring backplane modules and one optical sensor module, which is mounted on top. This concept allows for selectively guiding fluids and light to the sensor modules to be operated. We integrated shape memory alloy (SMA) microvalves in different designs and two kinds of optical switches, i.e., one linearly actuated assembly of optical elements and one based on an electrostatically deflectable mirror. We manufactured the modules in polymers and carried out optical and fluidic characterization. The functionality of the backplane is demonstrated by interconnecting two optical sensor modules with integrated spectrometer and photodiode color sensor, respectively. Herewith, we carried out fluorescence transmission experiments.

Fast SAW Based Sensor System for Real-Time Analysis of Volatile Anesthetic Agents

We present first studies of an exceedingly fast and cost-efficient sensor system for analysis of organic gases and vapors. Our new sensor concept combines specially designed surface acoustic wave (SAW) resonator devices with a new and fast electronic evaluation circuitry in a specially developed SAW sensor micro array. Coating of SAW devices with different functionalized polymers facilitates high reliable qualitative and quantitative detection of organic vapors and multivapor mixtures. We illustrate the capability of our system by the analysis of established volatile anesthetics (halothane, isoflurane, desflurane, sevoflurane and enflurane). The new sensor concept fulfills all criteria needed for rapid, versatile and facile monitoring of the anesthetics in real-time with low costs per analysis. Particularly, the rapidity of our system permits direct and continuous real-time measurement of anesthetic gas components of patients inhaled and exhaled breath during surgical operation.

In-Situ Soil Gas Analysis with a Robust Sawsensor System Integrated in Percussion Driven Penetration Cones

We present the first integration of a SAW sensor based sensor system (electronic nose) into a percussion driven penetration cone for in situ soil gas analysis. The system includes an 8-fold SAW sensor array with different polymers as sensitive coatings, sampling system with absorbent materials for the selective enrichment of analytes, inlet system for probing ambient soil, rechargeable battery pack and a remote control to an external PC on ground via Bluetooth. By means of an integrated spring based mechanical decoupling system measurements could be performed even during penetration of the cone via a pneumatic hammer! Proper operation of sensor system was possible until soil depths of 8m!