New Applications of Organic Polymers in Chemical Gas Sensors Neue Einsatzmöglichkeiten organischer Polymere in chemischen Gassensoren Dissertation der Fakultät für Chemie und Pharmazie der Eberhard-Karls-Universität Tübingen zur Erlangung des Grades eines Doktors der Naturwissenschaften 2005 vorgelegt von Mika Harbeck Tag der mündlichen Prüfung: 18.11.2005 Dekan: Prof. Dr. S. Laufer 1. Berichterstatter: PD Dr. U. Weimar 2. Berichterstatter: Prof. Dr. G. Gauglitz Contents 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. The Chemical and Physical Structure of the Electrical Double 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 Substrates .... 133 i Contents 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 ii List 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 Kelvin 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 Metal Insulator Semiconductor Capacitor MISFET Metal Insulator Semiconductor Field Effect Transistor MOSFET Metal Oxide Semiconductor Field Effect Transistor iii List 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 Poly(methyl 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 Ultraviolet VOC Volatile Organic Compound iv Notation List of the most common symbols used throughout the text. C Concentration χ Surface potential e Electron charge EC Conduction band edge EF Fermi energy level Epot Potential energy ε Relative permittivity ε 0 Permittivity of vacuum EV Valence band edge Evac Vacuum energy level F Faraday’s constant h Planck’s constant K Equilibrium constant k Boltzmann’s constant µ ˜i Electrochemical potential µ i Chemical potential ν Frequency of electro-magnetic radiation p Partial pressure p0 Saturation vapour pressure Φ Galvani potential φ Work function ϕ Work function (potential) Ψ Volta potential R General gas constant Ra Surface profile: average roughness v Notation Rp Surface profile: height of peaks Rq Surface profile: rms average roughness Rv Surface profile: depth of valleys Rzi; Rt Surface profile: peak-to-peak roughness T Absolute temperature t Temperature Tg Glass transition temperature θ Relative surface coverage τ off Recovery time constant τon Response time constant Vb KP backing potential VCPD Contact potential VK,S Sum of contact potential and KP backing potential VS Band bending ξ Electron affinity zi Charge number vi 1. 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 ZnO and SnO2 were first developed by Seiyama et al. [1] and Taguchi [2] in the beginning 1960s as detectors for liquid petroleum gases (LPG) in homes. 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. New application areas provoke a steady development of new sensor types to meet the additional requirements. At the same time, new sensor 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, small size, and batch fabrication at industrial standards like complementary 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 disadvantages of CMOS devices like a limited operation temperature range and high development costs. The development and production of CMOS microsensors only pays off in mass market applications. Microsensors based on several transduction principles are known and de- veloped, as there are cantilevers, thermopiles, or capacitive/resistive struc- tures for polymer layers or micro hotplates for metal oxides. A very promi- nent example of a microsensor is the chemical field effect transistor (Chem- FET), namely the gas sensitive field effect transistor (GasFET) as introduced by Lundström et al. [4] for the detection of hydrogen. Meanwhile, many other designs of the GasFET have been developed and a wide range of new sensitive materials has been tested. A common derivation of the Lund- 1 1. Introduction ström field effect transistor (FET) is the suspended gate field effect tran- sistor (SGFET), known for almost twenty years. Several SGFET types are described in the literature, but there has been no significant commercializa- 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, and stability as well as reversibility and fast response & 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. Polymers are available in many kinds having different chemical and sorp- tion properties. The main approach for tuning sensitivity and selectivity is a chemical modification of side groups attached to the polymer backbone. More advanced strategies like the use of cave structures or chiral molecules, 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 versatility of polymers makes them usable in many sensor types based on different transduction principles for the detection of a variety of analytes. Most commonly, polymers are used in bulk acoustic wave (BAW) and surface acoustic wave (SAW) sensors, cantilevers, capacitive and re- sistive sensors, and thermopiles