Ashok K. Batra + Department of Physics, PO Box 1268, Alabama A&M University, Normal, AL 35762

1. Overview of Chemical Gas Sensors • Ashok K. Batra+ • Department of Physics, PO Box 1268, • Alabama A&M University, Normal, AL 35762 NSF/RISE Workshop/Short Course on Development and Study of Advanced Sensors and Sensor Materials July 9 - July 13 2007 + ashok.batra@aamu.edu

2. Overview of Chemical Gas Sensors Outline • Chemical Sensor • Categories of Sensors • How does it work? • What we are doing…

3. Nature’s Creation Five Senses: Eyes, Ears, Skin, Nose and Tongue Camera……………………………………mimics………Eyes Microphone and Tape Recorder………mimics…………..Ears Tactile Sensors……………………………………mimic………Skin Chemical Sensors………………….mimic……….. Nose & Tongue

4. A Typical Sensor… • An interactive material which interacts with environment and generates a response. • + • Device which reads the response and converts it into an interpretable and quantifiable term.

5. Current Voltage Light Intensity Mass Refractive Index Capacitance Resistance

7. Some Definitions and Terms Adsorption: is a process that occurs when a gas or liquid or solute (called adsorbate) accumulates on the surface of a solid or more rarely a liquid (adsorbent), forming a molecular or atomic film (adsorbate). Chemisorption: a type of adsorption whereby a molecule adheres to a surface through the formation of a chemical bond.

8. Expected Qualities of an Excellent Chemical Sensor 3Ss • Sensitivity • Stability • Selectivity • Minimum Hardware Requirements • Good Reversibility • Identification and Quantification of Multiple Species • Quick Response

9. Application of Gas Sensors • Safety • Indoor Air Quality • Environmental Control • Food • Industrial Production • Medicine • Automobiles Chemical sensors play an increasingly important role in our everyday life: environmental monitoring, industrial process control, quality control of food and beverages, hazardous chemicals, explosives detection and workplace monitoring are just a few examples of their widespread use. In all cases the driving force behind the development of sensor technology is the need for immediate and accurate analyses.

10. Four General Groups Chemical Sensors • Electrochemical Sensors • Mass Sensors • Optical Sensors • Chromatography and Spectrometry

11. Substrate Sensing Material Electrode Electrochemical Sensor Based on Metal Oxides - Stoichiometry - Microstructure - Thickness - Phase - Temperature Sensitivity = Rgas / Rair

12. Design of a Typical Thick Film Semiconductor Sensor “Heater-substrate-film-combination electrode” structure. - Tin Oxide - Tungsten oxide - Zinc Oxide - Indium oxide ------------- The combination of the sensor operating temperature and composition of the metal oxide yields different responses to various gases Materials Science & Engineering B 139 (2007) 1-23

13. chemical Sensors

14. Chemical Sensors New Silicon-Based Metal-Oxide Chemical Sensor Microfabricated Metal-Oxide Chemical Sensor

15. Chemical Sensors A Scheme of SnO2 acting as Semiconductor Sensing Material 2e- + O2 2 O- O- + CO CO2

16. Chemical Sensors Role of Additives (Dopants) Additives are used for sensitizing and increase the response to particular gases i.e. enhance the sensitivity, selectivity, decrease the response time and operating temperature of sensitive layer. Sensitizing SnO2 with Cu; under oxidizing condition Cu is present as CuO which is p-type; p-n junction is formed which results in electron depletion at interface. Exposure to H2S converts CuO to Cu2O that exhibits metallic character and thus increases the conductivity of the system

17. Electrochemical Sensors Additives Distribution Ways on Semiconductor Gas Sensors

18. Electrochemical Sensors Chemical sensitive absorbent is deposited on a solid phase that acts as an electrode When chemical vapors come in contact with the absorbent. The chemical absorbs into the polymer, causing it to swell. The swelling changes the resistance of electrode, which can be measured The amount of swelling corresponds to the concentration of the chemical vapors

19. Catalytic Bead Sensor It is comprised of a passive and active element. The active element is coated with a catalyst platinum and passive is coated with an inert glass to act as a reference element Both the elements are heated to a prescribed temperature. When a combustible gas contacts the elements, the vapor combusts on the active element, and the active element increases in temperature. As a result, the resistance of the platinum coil changes. Two elements are connected to a Wheatstone bridge circuit, so changes are measured in voltage ActivePassive

20. Mass Sensors Surface Acoustic Wave Sensor The velocity and attenuation of the signal are sensitive to the viscoelasticity and mass of the thin film which can allow for the identification of the contaminant. Heating element under the chemical film can also be used to desorb chemicals from the device. A Signal pattern recognition system is needed.

21. Mass Sensors

22. Mass Sensors Based on Cantilever: Nanotechnology

23. Optical Sensors Infrared Sensors Infrared sensors can be used to detect gases, which have unique infrared absorption signatures in the 2-14 μm range. The uniqueness of the gas absorption spectra enables identification and quantification

24. Optical Sensors Colorimetry Work by analyzing the color of the contaminated water that has been mixed with a particular reagent

25. Surface Plasmon Resonance (SPR) Sensors SPR- A Charge Density Oscillation that may exist at the interface of two media. The SPR technique is an optical method for measuring the refractive index of very thin layer of material adsorbed on a metal Optical Setup for SPR P-reflectivity Photons at certain angle are able to excite SP on the adsorbate side of the metallic slab; whenever plasmon is excited, one photon disappears, producing a dip in the reflected light; angle is dependent on refractive index of the adsorbate. Angle of Incident SPR Curves for Different Molecules

26. Surface Enhances Raman Spectroscopy (SERS) • Optical Sensors SERS is based on finding the chemical composition of a sample by irradiating it with laser and measuring the light that scatters from it. Surface Enhanced ( ~1014) Raman Scattering is observed for molecules found close to silver or gold nanoparticles because of surface plasmon resonance. Thus sensitivity increases many folds. Plasmons are collective oscillations of the free electron density, often at optical frequencies

27. Chromatography Chromatography: Separation of Molecules Liquid Chromatography Sorbent Gas Chromatography

28. Spectrometry Ion Mobility Spectrometry • Time-of-Flight Measurement When the gas has entered the spectrometer, it will be ionized by a radioactive source The resulting positive and negative charged species are accelerated over a short distance Time-of-Flight is determined

29. Spectrometry Mass Spectrometry The principle is similar to the ion mobility spectrometer, except vacuum is required Gas mixture is ionized, and charged fragments are produced These fragments are sorted in a mass filter according to their mass to charge ratio. The ions are detected as electrical signal with an electron multiplier

30. Recent Advances

31. Nano Structured Materials Next Generation of Sensors ? Because of the small size of nanotubes, nanowires, or nanoparticles, a few gas molecules are sufficient to change the electrical properties of the sensing elements. This allows the detection of a very low concentration of chemical vapors. Nanotechnology based chemical sensors provide high sensitivity ( 3-4 orders), low power and low cost portable tools for in-situ chemical analysis. Operate at room temperature. SnO2; ZnO; In2O3; WO3; SnO2:Pd; TiO2 Sensors & Actuators B 122 (2007) 659-671

32. Optochemical Sensors For H2, O2, O3, CO, CO2 and H2O detection in Air • Absorbance and Reflectance • Refractive Index • Photoluminescence • Photothermal • Photoacoustic and related • Surface Plasmon Resonance (SPR) • Chemiluminescence Trends in Analytical Chemistry 25(2006) 937-948

34. Development of Nanoparticles-based Chemical Gas Sensor • A. K. Batra+, J. R. Currie*, Anup D. Sharma and R. B. Lal • Department of Physics, PO Box 1268, • Alabama A&M University, Normal, AL 35762 • *Instrumentation & Advanced Sensor Group, • NASA/Marshall Space Flight Center, AL 35812 NSF/RISE Workshop/Short Course on Development and Study of Advanced Sensors and Sensor Materials July 9-July 13, 2007 + ashok.batra@aamu.edu

35. A Typical Semiconductor Sensor… An interactive material which interacts with environment and generates a response. + Device which reads the response and converts it into an interpretable and quantifiable term. Sensing material captures a molecule of vapor with a certain selectivity that induces physical change in the material because of captured molecule's chemical interaction with the material.

36. Semiconductor Gas Sensors: Mechanisms Detect Gases Due to Change in Their Resistance or Conductance Changes in Conductance can result from combination of several physical properties of film: • Bulk defects ( interstitials and oxygen vacancies) • Surface defects ( donor type oxygen vacancies) • Catalytic elements ( breakdown of an incoming gases by catalyst on the surface of the sensing film) • Microstructure and grain boundaries ( smaller grains, large number of grain boundaries, high surface to volume ratio) • Interface and three phase boundaries (changes in interface conductance due to incoming gases at triple point)

37. Our Approach • Microstructure and grain boundaries ( smaller grains, large number of grain boundaries, high surface to volume ratio). By use of Nanoparticles in fabrication of the thick-films. …high surface reactivity + larger density of molecules which can adsorb on the surface … contribute to larger effect on electrical conductivity… enhances sensor sensitivity. ●Binary composites: SnO2:WO3; SnO2:In2O3; SnO2:ZnO

38. Weighing Raw Materials Mixing And Milling Powder Pressed to Discs Sintering Lapping And Grinding Electrical Characterization Ultrasonic Cleaning Electroding SnO2 Sample Processing Steps Pellet / Sample Preparation

39. Tablet Pressing Fixture 2.5 cm 2.5 cm Pressing Sleeve Pressing Sleeve 1.3 cm 1.3 cm Anvil Anvil 5 mm 5 mm 5 mm 5 mm Die Die 4 cm Piston Piston 2.9 cm 2.9 cm Pressing Ram 1.4 cm 1.4 cm 1.2 cm 1.2 cm 1.2 cm 1.2 cm

40. electrodes Sensor Configuration The SnO2 Sensor (pellet) shows shaded electroded regions on the top surface having a finite gap between these two physical regions.

41. Test Sample GPIB-USB2 Interface Keithley 617 Electrometer Agilent 34401A 6 ½ Digit Multimeter Gateway E-3301 CPU running National Instruments LabVIEW Ver. 6.0 Hot Plate w/ Thermal Chamber QuadTech 7600 LCR Meter Test Facility for VOC Chemical Sensors

42. Our Study • Sensor elements were bombarded with helium particles to change the surface characteristics. In our study, a VOC sensor pellets of Tin Dioxide (SnO2) are fabricated then bombarded at various (2 MeV ) helium doses. • A noteworthy result has been observed that the device has a decrease in response time when bombarded at about 1016 ion/cm2. • The response time decreases with increase of fluence. • A trend is seen whereby capacitance tends to decrease as fluence is increased.

43. Decrease in Response Time Response Time is based on 1 (62.3% of saturation).

44. IPA Response Time Decrease in Response Time Observed IPA Response Time* *Response Time is based on 1 or (62.3% of saturation).

45. Why look at Thick Films? • Bulk • Not stable; consume power; not compatible with silicon technology; and high operating cost. • Thin-Film • Compatible with micro-sensors; rapid response; compact; low operating cost; mechanically weak; Dopants homogeneity???. • Thick-Film • More Robust than Thin Films. • Lighter than Bulk Materials. • Dopants Homogeneity. • Consume less power. • Compatible with silicon technology.

46. Thick Film Deposition Techniques Screen Printing Liquid Phase Epitaxy Melt Spinning Dip Coating Solution Casting

47. Why Screen Printing? • Low Cost of Production. • Ease of Fabrication. • Compatible with semiconductor technology. • Films with large surface to volume ratio. • Porous films.

48. Nanopowder Solvent Mixing (Paste) Screen-printing Drying Annealing Thick-Film Functional & Porous Film Process of Thick-film Preparation by Screen-printing Organic Binder

49. Squeegee Ink/Paste Nylon Screen Frame Substrate Vacuum Chuck* Hinge Dowel Pins Base Plate 45° Vacuum Pump *Fabricatedby Mr. Garland Sharp Fabrication of Thick-Film using Screen Printing Technology Screen Printing Set-up

50. Design of a Typical Semiconductor Sensor “Heater-substrate-film-combination electrode” structure. S. Film Electrode