Techniken der Oberflchenphysik Techniques of Surface Physics 5

Techniken der Oberflächenphysik (Techniques of Surface Physics) 5. Übung im WS 15/16, 20. 01. 2016 Prof. Yong Lei & Stefan Bösemann (& Liying Liang) Fachgebiet 3 D-Nanostrukturierung, Institut für Physik Contact: yong. lei@tu-ilmenau. de; stefan. boesemann@tu-ilmenau. de; liying. liang@tu-ilmenau. de Office: Heliosbau 1102, Prof. Schmidt-Straße 26 (tel: 3748) www. tu-ilmenau. de/nanostruk Vorlesung: Übung: Mittwochs (G), 9 – 10: 30, C 108 Mittwochs (U), 9 – 10: 30, C 108

1. What is a sensor? What are the requirements for a good sensor? Please introduce and explain the specific sensors that response to two or more kinds of the following stimulus: acoustic, biological & chemical, electric, magnetic, optical, thermal, and mechanical.

What are sensors? • American National Standards Institute (ANSI) Definition: a device which provides a usable output in response to a specified stimulus. • A sensor is a converter that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument.

A good sensor: • Is sensitive to the measured property only • Is insensitive to any other property likely to be encountered in its application • Does not influence the measured property

Oxygen sensor (resistive) An oxygen sensor is an electronic device that measures the proportion of oxygen in the gas or liquid being analysed. • Zr. O 2: solid electrolyte • Difference of oxygen pressure on two sides • Potential difference The oxygen sensor is based on a solid-state electrochemical fuel cell called the Nernst cell. Its two electrodes provide an output voltage corresponding to the quantity of oxygen in the exhaust relative to that in the atmosphere. The voltage produced by the sensor is linear with the difference between the amount of oxygen in the exhaust gas and the amount of oxygen in air. The most common application is to measure the exhaust gas concentration of oxygen for internal combustion engines in automobiles and other vehicles. Divers also use a similar device to measure the partial pressure of oxygen in their breathing gas.

Hygrometer (capacitive) Capacitive hygrometer sensors use a capacitor that is sensitive to the amount of water vapor in the air to measure humidity. There are two types of capacitance hygrometer sensors: thin-film polymer sensors and aluminum-oxide sensors. Thin-film polymer sensors generally have four layers. Thin-film polymer sensor The bottom layer is a glass or silicon substrate that acts as the support for the capacitor. A metal electrode is placed on the substrate and covered with a thin film of polymer (dielectric). The thin-film polymer is then coated with a porous metal layer, which is the top electrode. The water vapor travels through the porous metal layer and is absorbed by the polymer, changing its capacitance. The change in capacitance is proportional to the change in the relative humidity.

Hygrometer (resistive) ØVariations in ambient relative humidity produce variations in resistance. This occurs in certain moisture-sensitive materials such as hygroscopic salts and carbon powder. ØIn resistive hygrometer sensors, these materials are applied as a film over an insulating substrate and are terminated by metal contacts. The components of a resistive hygrometer sensor are represented above. As air passes over the film, changes in resistance vary with changes in relative humidity. Humidity measurement and control is important in textile and paper industries.

2. What is the type of Zn. O sensor for NO 2 detection? What is the working mechanism? A figure of its performance was presented in the class. What are the facts from this figure can demonstrate that Zn. O nanotube arrays as a good gas sensor?

Semiconductor gas sensor (resistive gas sensor) • Metal oxide sensor: chem-resistor (Zn. O) • Detection of change in resistance upon the absorption of the target gas (concentration of target gas) • Gas-solid interaction affects the density of electronic species in metal oxide. 01. 12. 2020 www. tu-ilmenau. de/nanostruk Seite 9

High sensitivity Good stability Insensitive to nonmeasured gas Low testing limitation

3. Fe 2 O 3 and Ti. O 2 are two n-type semiconductors which have been widely studied as photoanode in solar water splitting. Do you know other n-type semiconductor that can be used as photoanode? How do they work?

N-type semiconductors There are two types of semiconductor carriers, namely holes in the valence band electrons in the conduction band. In n type semiconductors, the number of electrons is more than holes, so electrons are measured as majority charge carriers, and holes are referred as minority charge carriers. N-type semiconductors: Zn. O、Ta 2 O 5 P-type semiconductors: Cu 2 O、Ni. O、VO 2

Band energetics of a semiconductor (n-type)/ liquid contact A/A-: redox pair in electrolyte qφb: barrier height How do P-type semiconductors work? Voc: photovoltage

1. The first step (i) is absorption of photons to form electron–hole pairs. Semiconductors absorbs suitable photons, and then electrons are excited from the valence band to the conduction band. Moreover electron-hole pairs are formed inside the material. 2. The second step (ii) consists of charge separation and migration of photogenerated carriers.

A simplest N-type PEC cell N type P type

4. We have introduced tandem structure consisting of an n-type (photoanode) and a p-type (photocathode) semiconductor which are separated by ion exchange membrane to split water into oxygen and hydrogen, respectively, at the same time. If two types of semiconductor are contacted directly to form a p-n junction, what would happen using the junction as photoelectrode for water splitting? What are the advantages of the junction structure compared with a single semiconductor photoelectrode?

P-N junction A p–n junction is a boundary or interface between two types of semiconductor material, p-type and n-type, inside a single crystal of semiconductor. It is created by doping, for example by ion implantation. If two separate pieces of material were used, this would introduce a grain boundary between the semiconductors that would severely inhibit its utility by scattering the electrons and holes.

P-N junction • Build-in field • Hole-electron separation

Thanks for listening Any questions? Das Lösung wird heute Abend online gestellt http: //www. tu-ilmenau. de/nanostruk/teaching/ 19