New applications of organic polymers in chemical gas sensors [Elektronische Ressource] = Neue Einsatzmöglichkeiten organischer Polymere in chemischen Gassensoren / vorgelegt von Mika Harbeck

Mika Harbeck, University Oftübingen

Informations

• Publié par: eberhard_karls_universitat_tubingen
• Publié le: 01 janvier 2005
• Nombre de lectures: 24
• Langue: English
• Poids de l'ouvrage: 5 Mo

Extrait

NewApplicationsofOrganicPolymersin
ChemicalGasSensors
NeueEinsatzmöglichkeitenorganischer
PolymereinchemischenGassensoren
Dissertation
der Fakultät für Chemie und Pharmazie
der Eberhard-Karls-Universität Tübingen
zur Erlangung des Grades eines Doktors
der Naturwissenschaften
2005
vorgelegt von
Mika HarbeckTag der mündlichen Prüfung: 18.11.2005
Dekan: Prof. Dr. S. Laufer
1. Berichterstatter: PD Dr. U. Weimar
2. Prof. Dr. G. GauglitzContents
1. Introduction 1
1.1. Introduction to the Field...................... 1
1.2. Motivation and Scope ....................... 4
1.3. Overview of the Presented Work ................. 5
2. Theoretical Background and Related Work 7
2.1. Sorption Processes ......................... 7
2.2. Electrochemical Aspects of Interfaces .............. 11
2.3. TheChemicalandPhysicalStructureoftheElectricalDouble
Layer................................. 16
2.4. Measuring the Work Function and Surface Potentials ..... 30
2.5. Chemical Sensing with Field Effect Devices........... 41
3. Experimental Details 51
3.1. Instrumental Equipment...................... 51
3.2. Materials for the Preparation of the Sensing Layers ...... 59
3.3. Polymer Deposition......................... 64
3.4. Measurement Procedure 70
4. Sensitive Layer Morphology: Characterisation and Optimization 73
4.1. Polyacrylic Acid Layers 74
4.2. Polystyrene Layers 81
4.3. Poly-(4-vinylphenol) Layers .................... 84
4.4. Poly-(acrylonitrile-co-butadiene) Layers ............. 86
4.5. Poly-(cyanopropyl-phenyl-siloxane) Layers ........... 87
4.6. Summary............................... 87
5. Response to Analyte Gases 89
5.1. Inert Reference Material and Uncoated Substrates ....... 89
5.2. Polyacrylic Acid Coated Substrates................ 91
5.3. Polystyrene Coated Substrates ..................114
5.4. Poly-(4-vinylphenol) Coated Substrates .............122
5.5. Poly-(acrylonitrile-co-butadiene) Coated Substrates ......129
5.6. Poly-(cyanopropyl-phenyl-siloxane) Coated ....133
iContents
5.7. Summary of the KP and QMB Experiments...........135
5.8. A Model for the Origin of the Observed KP Signals ......141
6. Conclusion and Outlook 149
6.1. General Conclusions ........................149
6.2. Outlook................................150
A. The Grahame Equation 151
iiList of Acronyms
AFM Atomic Force Microscope
BAW Bulk Acoustic Wave
CCFET Capacitively Coupled Field Effect Transistor
ChemFET Chemical Field Effect Transistor
CMOS Complementary Metal Oxide Semiconductor
CVD Chemical Vapour Deposition
DCM Dichloromethane
DL (Electrical) Double Layer
FET Field Effect Transistor
GasFET Gas Sensitive Field Effect Transistor
IC Integrated Circuit
IGFET Isolated Gate Field Effect Transistor
IR Infrared
ISFET Ion Selective Field Effect Transistor
ITO Indium Tin Oxide
KFM Kelvin Force Microscope
KP Probe
KPFM Kelvin Probe Force Microscope
LB Langmuir-Blodgett
LOD Limit of Detection, i.e. lower limit
MFC Mass Flow Controller
MIS Metal Insulator Semiconductor
MISCAP S Capacitor
MISFET Metal Insulator S Field Effect Transistor
MOSFET Oxide Semiconductor Field Effect T
iiiList of Acronyms
OM Optical Microscope
PAA Polyacrylic Acid
PAB Poly(acrylonitrile-co-butadiene)
PCPhS Poly-(cyanopropyl-phenyl-siloxane)
PDMS Polydimethylsiloxane
PEG Polyethylenglycol
PES Photoelectron Emission Spectroscopy
PMAA Poly(methyl acrylic acid)
PMMA methacrylate)
PS Polystyrene
PTFE Poly(tetrafluoroethylene)
PVA Polyvinylalcohol
PVPh Poly(4-vinylphenol)
pzc Point of Zero Charge
QMB Quartz (Crystal) Micro Balance
SAM Self-Assembled Monolayer
SAW Surface Acoustic Wave
SEM Scanning Electron Microscope
SGFET Suspended Gate Field Effect Transistor
SPV Surface Photo Voltage
TLM Triple Layer Model
UHV Ultra High Vacuum
UPS Ultraviolet Photon Spectroscopy
UV Ultra
VOC Volatile Organic Compound
ivNotation
List of the most common symbols used throughout the text.
C Concentration
χ Surface potential
e Electron charge
E Conduction band edgeC
E Fermi energy levelF
E Potential energypot
ε Relative permittivity
ε Permittivity of vacuum0
E Valence band edgeV
E Vacuum energy levelvac
F Faraday’s constant
h Planck’s constant
K Equilibrium constant
k Boltzmann’s
µ˜ Electrochemical potentiali
µ Chemical potentiali
ν Frequency of electro-magnetic radiation
p Partial pressure
p Saturation vapour pressure0
Φ Galvani potential
φ Work function
ϕ Work (potential)
Ψ Volta potential
R General gas constant
R Surface profile: average roughnessa
vNotation
R Surface profile: height of peaksp
R profile: rms average roughnessq
R Surface profile: depth of valleysv
R ;R profile: peak-to-peak roughnesstzi
T Absolute temperature
t Temperature
T Glass transition temperatureg
θ Relative surface coverage
τ Recovery time constantoff
τ Response timeon
V KP backing potentialb
V Contact potentialCPD
V Sum of contact potential and KP backing potentialK,S
V Band bendingS
ξ Electron affinity
z Charge numberi
vi1. Introduction
1.1. Introduction to the Field
1.1.1. Chemical Micro Gas Sensors
Chemical gas sensors have been of much scientific and commercial interest
already for several decades. Metal oxide semiconductor sensors based on
ZnOand SnO werefirst developedbySeiyamaet al.[1]and Taguchi[2]in2
thebeginning1960sasdetectorsforliquidpetroleumgases(LPG)inhomes.
This sensor type was soon followed by others, e.g. quartz micro balance
(QMB) sensors for the detection of volatile organic compounds (VOC) as
reported by King [3]. Since then, chemical gas sensors have undergone
a long development and they are nowadays in use in several application
fields.
Newapplicationareasprovokeasteadydevelopmentofnewsensortypes
to meet the additional requirements. At the same time, new types
open up new applications areas. Hence, much effort has been spent among
others on the development of microsensors. The miniaturization of sen-
sor devices offers substantial advantages such as low power consumption,
smallsize,andbatchfabricationatindustrialstandardslikecomplementary
metal oxide semiconductor (CMOS) compatible processes. Those are very
crucial issues, for example, in battery-operated systems and infer low costs
to the sensor user. Besides, the production technique allows easy addition
of signal acquisition and data evaluation circuitry, partly a necessity due to
the small signal of the transducer and partly an advantageous step in inte-
gration. Nevertheless, the very common CMOS microsensors inherit some
disadvantagesofCMOSdeviceslikealimitedoperationtemperaturerange
and high development costs. The development and production of CMOS
microsensors only pays off in mass market applications.
Micrbasedonseveraltransductionprinciplesareknownandde-
veloped, as there are cantilevers, thermopiles, or capacitive/resistive struc-
tures for polymer layers or micro hotplates for metal oxides. A very promi-
nent exampleof a microsensoris the chemical field effecttransistor (Chem-
FET),namelythegassensitivefieldeffecttransistor(GasFET)asintroduced
by Lundström et al. [4] for the detection of hydrogen. Meanwhile, many
other designsof theGasFEThave beendeveloped anda widerange ofnew
sensitive materials has been tested. A common derivation of the Lund-
11. Introduction
ström field effect transistor (FET) is the suspended gate field effect tran-
sistor (SGFET), known for almost twenty years. Several SGFET types are
describedintheliterature, buttherehasbeennosignificantcommercializa-
tion of this sensor type, as yet. However, for SGFETs a prospective future
is envisioned, as they allow cheap production, low-power operation, and
are flexible in the choice of the sensitive material, and suitable for many
new applications [5, 6]. A very personal view on the future of the GasFET
together with a summary of its possible application is given by Janata [7].
1.1.2. Polymeric Sensitive Layers
The performance of a chemical gas sensor is generally judged on several
criteria. The most common performance criteria are the classical factors of
sensitivity,selectivity,andstabilityaswellasreversibilityandfastresponse
& recovery. If a sensor device can keep to the actual limits depends on
the overall performance of the transducer and the chemically active layer.
Sensitive layers based on organic polymers inhibit many positive features
and, consequently, are of wide interest and widely used in chemical gas
sensors. The existing experience in the field is a proof that they fulfil many
of the aforementioned requirements.
Polymersareavailableinmanykindshavingdifferentchemicalandsorp-
tion properties. The main approach for tuning sensitivity and selectivity is
a chemical modification of side groups attached to the polymer backbone.
Moreadvancedstrategiesliketheuseofcavestructuresorchiralmolecules,
to name a few, are pursued in special cases. The size exclusion principle
of analyte molecules and specific interactions are in those cases the main
reason for a selectivity towards certain analytes.
The versatilit