Transcript Prof. Yong Lei & Stefan Bösemann (& Liying Liang)

Techniken der Oberflächenphysik
(Techniques of Surface Physics)
5. Übung im WS15/16, 20.01.2016
Prof. Yong Lei & Stefan Bösemann (& Liying Liang)
Fachgebiet 3D-Nanostrukturierung, Institut für Physik
Contact: [email protected];
[email protected]; [email protected]
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.
• ZrO2: 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.
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
Thin-film polymer sensor
relative humidity.
Hygrometer (resistive)
Variations in ambient relative humidity produce
variations in resistance. This occurs in certain
moisture-sensitive materials such as hygroscopic
salts and carbon powder.
In 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 ZnO sensor for NO2 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 ZnO nanotube arrays as a good gas sensor?
Semiconductor gas sensor
(resistive gas sensor)
• Metal oxide sensor: chem-resistor (ZnO)
• 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.
04.04.2016
www.tu-ilmenau.de/nanostruk
Seite 9
High sensitivity
Good stability
Insensitive to nonmeasured gas
Low testing
limitation
3. Fe2O3 and TiO2 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 and 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: ZnO、Ta2O5
P-type semiconductors: Cu2O、NiO、VO2
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.
Photo-generated electron-hole pairs separate, and then the electrons and holes migrate to
the surface of material.
3. The final step (iii) involves the surface chemical reactions.
When the electrons and holes reach the surface, the oxidation and reduction reaction occurs.
Water molecules are reduced by the electrons to form H2 and are oxidized by the holes to
form O2 for overall water splitting.
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/
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