Roland Pohle

In situ infrared emission spectroscopic study of the adsorption of H2O and hydrogen-containing gases on Ga2O3 gas sensors

Abstract Infrared emission spectroscopy (IRES) was used for studying in situ gas-surface interactions and adsorbed species on Ga 2 O 3 at elevated temperatures used for gas sensors. Regarding the permanent presence of humidity in most gas sensor applications, the interaction of H 2 O with the sensor surface was observed under working conditions in order to emphasize the benefits of IRES for the understanding of gas sensing mechanisms. The coadsorption of water and H 2 ,C 2 H 4 , acetone and ethanol was investigated to point out the role of humidity in the sensitive reactions to organic gases. Screen printed Ga 2 O 3 films were studied in the temperature range from 250°C up to 650°C. These results were correlated with simultaneous conductivity measurements. The evident influence of the addition of water on the surface chemistry of Ga 2 O 3 during the adsorption of hydrogen-containing gases is confirmed by this work.

Low-power gas sensors based on work-function measurement in low-cost hybrid flip–chip technology ☆

To implement low-power gas sensors with low component costs, the principle of work-function read out via a hybrid suspended gate FET (SGFET) is being pursued, whereby a freely selectable sensor film undergoes a reversible work-function change corresponding to the build-up of a potential difference on the surface in response to gas adsorption/reaction. This is read out via an ISFET structure. An innovative design which allows cheap manufacturing will be described for the principle that has already been successfully demonstrated. The starting point of the design is a ceramic Al2O3 substrate coated with conductor patterns and sensitive materials onto which the FET is mounted in flip–chip technology. By means of the freely selectable sensor film and its preparation method, a wide range of applications can be opened up.

Pulsed gate sweep strategies for hysteresis reduction in carbon nanotube transistors for low concentration NO2 gas detection

Carbon-nanotube-based field effect transistors (CNFETs) have been employed as highly sensitive chemical sensors. Often used as the sensor output signal, the gate threshold voltage (V(th)) is subject to concentration-dependent shifts upon exposure to target analytes. However, an unambiguous determination of the intrinsic V(th) is usually hampered by substantial hysteresis in CNFET gate characteristics. In this study we show that short gate voltage (V(gd)) pulses can be used for hysteresis reduction in CNFETs as chemical sensors, in particular for NO(2) detection. In the pulsed operation regime, even small shifts of V(th) upon sub-ppm NO(2) exposure remain resolvable. Furthermore, the hysteretic behaviour is systematically investigated by varying the pulse waveforms and timing parameters. Finally, we use an adapted hysteresis model for pulsed V(gd) and employ it to discuss the measurement data.

TiN in work function type sensors: a stable ammonia sensitive material for room temperature operation with low humidity cross sensitivity

Abstract In this work, it was found that exposure of TiN to ammonia (400 ppb–100 ppm) in wet synthetic air at room temperature leads to a reversible decrease of Δ Φ . There is a change of the work function of −30 meV/decade of increasing ammonia concentration at room temperature. The response time ( t 90 ) to 1 ppm is ≤1 min. The sensitivity is a typical room temperature effect as it decreases at higher temperatures up to 100°C. The reaction has a low cross sensitivity to humidity, as — first — the sensitivity to ammonia does not depend on the relative humidity and — second — the work function change due to humidity changes is as low as −2 meV for an increase of 10% in relative humidity. The Diffuse Reflectance Infrared Fourier Transform Spectra (DRIFT-spectra) indicate that the adsorption of ammonia is correlated with a decrease of OH species on the surface. All data are compared to the gas reactions of TiO 2 .

Analyte Detection with Cu-BTC Metal–Organic Framework Thin Films by Means of Mass-Sensitive and Work-Function-Based Readout

Metal-organic frameworks (MOFs) constitute a new generation of porous crystalline materials, which have recently come into focus as analyte-specific active elements in thin-film sensor devices. Cu-BTC--also known as HKUST-1--is one of the most theoretically and experimentally investigated members of the MOF family. Its capability to selectively adsorb different gas molecules renders this material a promising candidate for applications in chemical gas and vapor sensing. Here, we explore details of the host-guest interactions between HKUST-1 and various analytes under different environmental conditions and study the vapor adsorption mechanism by mass-sensitive and work-function-based readouts. These complementary transduction mechanisms were successfully applied for the detection of low ppm (2 to 50 ppm) concentrations of different alcohols (methanol, ethanol, 1-propanol, and 2-propanol) adsorbed into Cu-BTC thin films. Evaluation of the results allows for the comparison of the amounts of adsorbed vapors and the contribution of each vapor to the changes of the electronic properties of Cu-BTC. The influence of the length of the alcohol chain (C1-C3) and geometry (1-propanol, 2-propanol) as well as their polarity on the sensing performance was investigated, revealing that in dry air, short chain alcohols are more likely adsorbed than long chain alcohols, whereas in humid air, this preference is changed, and the sensitivity toward alcohols is generally decreased. The adsorption mechanism is revealed to differ for dry and humid atmospheres, changing from a site-specific binding of alcohols to the open metal sites under dry conditions to weak physisorption of the analytes dissolved in surface-adsorbed water reservoirs in humid air, with the signal strength being governed by their relative concentration.

Sub-ppm NO2 detection by Al2O3 contact passivated carbon nanotube field effect transistors

We investigate carbon nanotube field effect transistors (CNFETs) with aluminum oxide (Al2O3) passivated contacts for NO2 detection. For the CNFETs, consisting of one individual pristine single walled carbon nanotube (SWNT), the measurements indicate a strong influence of adsorbed NO2 gas molecules on the exposed CNFET channel and NO2 concentrations as low as 100 ppb were detected. Applied to the contact-SWNT interfaces, Al2O3 is a suitable material to protect the metal contacts from NO2 molecules and other undesired environmental influences. We discuss the effect of the different processing steps on the CNFET characteristics and show device recovery after short heat treatment.

Realization of a new sensor concept: improved CCFET and SGFET type gas sensors in Hybrid Flip-Chip technology

A wide range of emerging applications in the consumer sector is envisioned for low-cost gas sensors with a high degree of adaptability to special measuring applications. The readout of gas induced work function changes via hybrid suspended gate field effect devices is already accepted as a promising technique for the realisation of a versatile, low-cost sensor platform. However, the industrialisation is still in the beginning. We report the realisation of the innovative, cheaply manufacturable Hybrid Flip-Chip (HFCFET) setup which was introduced. Using different modifications of improved ISFET and CCFET type transducers we compare the performance of these readout devices. Additionally, the design of the readout structures was improved compared to other reported devices. Profiting from the use of a standard CMOS process an analogue readout circuit was integrated in a CCFET type transducer enabling an on-chip reference and compensation of signal drifts.

Fractional exhaled nitric oxide measurement with a handheld device.

A sensing system for fractional exhaled nitric oxide (FeNO) measurement is presented, which is characterized by a compact setup and a cost potential to be made available for the patient at home. The sensing is based on the work function measurement of a phthalocyanine-type sensing material, which is shown to be sufficiently sensitive for NO(2) in the ppb range. The transducer used to measure the work function is a field effect transistor with a suspended gate electrode. Selectivity is given with respect to other breath components including typically metabolic by-products. The measurement system includes breath treatments in a simple setup, which essentially are dehumidification and a quantitative conversion of NO to NO(2) with a conversion rate of approx. 95%, using a disposable oxidation catalyst. The accomplishment of the correct exhalation maneuver and feeding of the suited portion of exhaled air to the sensor is provided by breath sampling means. The sensor is not gas consuming. This allows us to fill the measurement chamber once, instead of establishing a gas flow for the measurement. This feature simplifies the device architecture. In this paper, we report on sensor characteristics, system architecture and measurement with artificial breath-gas as well as with human breath with the device.

Infrared emission spectroscopic study of the adsorption of oxygen on gas sensors based on polycrystalline metal oxide films

Abstract Infrared emission spectroscopy (IRES) was used for studying in situ gas-surface interactions and adsorbed species on semiconducting metal oxides based gas sensors under working conditions. Regarding the key position of oxygen for the surface chemistry of oxides, the adsorption and reaction of oxygen was studied on screen printed films of polycrystalline WO 3 , AlVO 4 and Co 3 O 4 in the temperature range between 300 and 600°C. The role of the ionization of oxygen vacancies on the conductivity mechanisms and on the reaction to hydrocarbons will be discussed correlating the results of IRES and conductivity measurements. This work shows that IRES offers a direct access to the generation of free charge carriers due to changes in oxygen partial pressure and reactions with reducing gases.

NO2 Gas Sensors Based on Individual Suspended Single-Walled Carbon Nanotubes

We report on the fabrication and measurements of the first NO2 gas sensors based on individual suspended single-walled carbon nanotubes (SWNTs). We analyze the transient current response of two types of SWNTs when exposed to NO2 in dry and humid air. Furthermore, we analyze the change in the gate characteristic of the SWNTs due to NO2 and propose two different transduction mechanisms in suspended SWNTs.