1. SENS-ERA project, 7/12/2012 Metal-oxide Semiconductor Gas Sensors Dimitris Davazoglou NCSR Demokritos, Institute of Advanced Materials, Physico-Chemical Processes and Micro- Nano- Electronics

2. SENS-ERA project, 7/12/2012 I. INTRODUCTION Applications – Gas Sensors II. CATALYTIC MATERIALS Metallic oxides III. RELATED PHENOMENA Resistance variations, gas-chromism, electro-chromism IV. APPLICATIONS 1. In shipyards, 2. Food, 3. Seamless sensor-display systems

3. SENS-ERA project, 7/12/2012 I.INTRODUCTION APPLICATIONS OF GAS SENSORS AND TARGET GASES MINES: (Methane, Flammable and Explosive Hydrocarbons) SHIPYARDS: (Same) SPACE: (Hydrogen) MILITARY: (Corrosive, Bio, …) SAFETY: Airports (Explosives, Bio, …) Subways (Same) Public Buildings (Same) CHEMICAL Industry (Hydrogen, VOCs, …) OIL Industry (Same) FOOD Industry (VOCs, Ammoniac, Amines, Hydrogen Sulfide, …_ DOMESTIC: Air Conditioners (Water Vapors, CO x, CFCs, …) Microwave Ovens (VOCs, Vapors) AUTOMOBILE: (CO x, NO x, …)

4. SENS-ERA project, 7/12/2012 FIRST GAS DETECTORS Canaries, Controlled combustion, Heated Platinum Wires THE PELLISTOR Resistance Measurement Problem: High Power Consumption ( ̴̴ 1-10 W)

5. SENS-ERA project, 7/12/2012 MINIATURIZATION OF DEVICES USING SCREEN PRINTING Power Consumption: Down to 50 mW Resistance of the sensing element affected by the chemical environment. Necessity of high temperature (150-350 o C).

6. SENS-ERA project, 7/12/2012 Power Consumption: Down to 5-10 mW Still very high for many applications (e.g., in safety and fire detection systems 1 mW or less desired) IN ALL DEVICES THE CATALYTIC MATERIAL PLAYS THE MAIN ROLE FURTHER MINIATURIZATION BY MICROELECTRONIC TECHNIQUES (SURFACE MICROMACHINING)

7. SENS-ERA project, 7/12/2012 II. CATALYTIC MATERIALS: METAL-OXIDES (SnO 2, WO 3, MoO 3, …) superficially doped with noble metals (functionalized ) W OBasic Building Block: Octahedron W at the center O at the corners Octahedra connected by sharing the O ions Other basic building blocks possible dependent on the oxide (e.g., tetrahedra for TiO 2, pyramides for MoO 3, etc.). All kinds of building blocks are connected by sharing the oxide ions in various ways (corners, acmes, planes).

8. SENS-ERA project, 7/12/2012 e-e- Adsorption on surface Interchange of electrons Possible gas dissociation Possible out-diffusion (oxygen loss) Possible in-diffusion of ions Desorption Example: Electronic structure of cubic WO 3 Band gap of 3 eV => Insulator, transparent Surface interaction of a Metal-oxide with a reactive gas phase

9. SENS-ERA project, 7/12/2012 Influence of the loss of oxygen and (of charge interchange) on the electronic structure of various oxides. They are now conductors. Also, some oxides exhibit coloration. Influence of the doping on the electronic structure of WO 3. All these processes are reversible. After removal of the gas phase and re-exposure of the oxide to ambient air the electronic structure of the oxide returns to its initial state (insulating and transparent).

10. SENS-ERA project, 7/12/2012 By monitoring the electrical conductivity => Gas Sensing By taking advantage of the in-diffusion => Gas-chromic smart windows An electrochromic Display By taking advantage of the in-diffusion and electronic interchange => Electro-chromic smart windows and displays III. RELATED PHENOMENA

11. SENS-ERA project, 7/12/2012 Characteristics of Gas Sensors 1. Sensitivity = R s /R 0 R s = Sensor Resistance in various gas concentrations R 0 = Sensor Resistance in a specific concentration of a target gas 2. Sensor Response. Sensor resistance drops very quickly when exposed to gas, and when removed from gas its resistance will recover to its original value after a short time. The speed of response and reversibility vary according to sensing oxide and the gas involved. 3.Selectivity (sensors arrays) 4.Initial action 5.long term stability 6.heater stability 7.heater voltage dependency New catalytic materials needed to improve all the above characteristics

12. SENS-ERA project, 7/12/2012 IV. APPLICATIONS 1.Detection of Explosive Gases in Shipyards (EPET II Program) In the tanks of remain hydrocarbons, which during welding explode. Movement in the tank very difficult, so continuous inspection impossible. Proposed solution: Numerous simple sensors positioned in the tank able to emit IR when triggered. IR cannot escape from the metallic tank so it will be detected by one (expensive) IR detector to give an alarm. IR LED IR DetectorTank (all metal)

13. SENS-ERA project, 7/12/2012 2. Electronic Detection of Fish Freshness (EPET II Program) A Sensing system was devised to detect Ammoniac and other Amines based on WO 3 and In 2 O 3. Ammoniac seemed to be the most efficient indicator. Many kinds of fishes were tested by measuring the emitted gases with standard chemical analysis and with electronic gas sensors for a time period of 5 days. Excellent correlations between the results of the chemical analysis and of the electronic detection were obtained.

14. SENS-ERA project, 7/12/2012 3. A seamless System Gas Sensor-Display based on WO 3 films (GoodFood Project) The gas sensor was used as a variable resistor, whose value was dropping in contact with a reducing gas (e.g., butane) so the electrochromic display was turning on.

15. SENS-ERA project, 7/12/2012 OUTLOOK & CONCLUSIONS Metal-Oxide Gas Sensors mainly based on SnO 2 are mature devices Open questions: Power, selectivity Many other oxides remain to be tested Many deposition methods remain to be tested (low temperature, large area) The rapidly developing technologies of large area and transparent electronics offer new opportunities for the further development of this technology with the vision of seamless systems integrating sensors, electronics and displays based on the same metal-oxide film. The number of possible applications is limited only by our imagination (food, vehicles, packages, clothing, glazing, portable electronic devices, …)

16. SENS-ERA project, 7/12/2012 THANK YOU FOR YOUR ATTENTION