Raman Spectroscopy of Tungsten Trioxide and COMSOL© Computer Simulation in Gas Sensor Technology What: C. Craig Mentor: Dr. P. Misra REU Team Members: R. Garcia-Sanchez, Dr. D. Casimir, S. Bartley Summer 2015 REU Program Howard University Department of Physics and Astronomy

Outline Motivation for research Background: What are semiconducting metal oxide gas sensors? How are optical gas sensors used today? What is Raman Spectroscopy? Research methodology and Results Comsol© Simulation Conclusions and Future Research

Why use spectroscopy in gas sensing? Semi-conducting metal oxide gas sensors are Small Portable Cheap But inefficient Optical gas sensors will Improve time-efficiency Improve precision of concentration detection To improve gas sensing technology by miniaturizing and making available gas sensors using spectroscopy. Most commonly used gas sensors are semiconducting metal oxide gas sensors. Problems: These sensors are not time-efficient And cannot detect the amount of gas in an area with precision Optical gas sensors that use spectroscopy would fix these problems if they could be miniaturized and made available. Spectroscopy allows for almost instantaneous measurement And the intensity of the spectral lines communicates the amount of gas in the area.

Semiconducting Metal-Oxide Gas Sensors [1] Composition: Thin film layer of semiconducting metal oxide Substrate Heating track Fine et al. 2010 Carbon Dioxide Sensor http://www.futurlec.com/Gas_Sensors.shtml

Changes in Resistivity Semiconducting metal-oxide gas sensors use changes in resistivity to detect the presence of certain gases http://www.ipm.fraunhofer.de/content/dam/ipm/en/PDFs/Product%20sheet/GP/ISS/semiconductor-gas-sensors.pdf

Optimal Operating Temperature Optimal gas detecting temperatures differ depending on the gas ZnO detects Chlorobenzene at ~200°C Ethanol at ~380°C

Optical Gas Sensors Used Today Mars Land Rover http://news.rpi.edu/content/2013/09/26/nasa-mars-rover-curiosity-finds-water-first-sample-planet-surface

What is Raman Spectroscopy? Study of the interactions of matter and light (visible and invisible) Raman Spectroscopy uses monochromatic light to identify molecules based on light scattering from the vibration that occurs between bonded atoms in lattice structures.

Tungsten Trioxide Monoclinic Lattice Structure *Tungsten oxide has different structures monoclinic, triclinic, orthorhombic, and tetragonal. These form at different temperatures between -263 and 900 degrees Celsius. Monoclinic forms at room temperature and up to approx. 300 degrees Celsius. It is the most common form and the one we have been studying. Bignozzi et al. 2012 Atoms bond in a lattice structure to form solids. Bonds vibrate at different frequencies. Vibration-laser beam interaction creates spectral lines.

Fingerprint of WO3 The major Raman peaks of Tungsten Trioxide are 808, 719, and 274 cm-1. These peaks result from the W-O stretching mode, the W-O bending mode, and the W-O-W deformation mode, respectively, in the lattice structure. https://www.mdsp.org/ Talk about the fingerprint But the fingerprint isn’t always exactly the same. There are many different factors that affect the fingerprint.

Methodology A DXR Smart Raman spectrometer 780 nm laser Temperature Controlled Environmental Chamber Objective Lens Sample Collection Notch Filter Imaging Spectrometer CCD Detector CW Laser ( 780 nm) 780 nm Narrowband Mirror A DXR Smart Raman spectrometer 780 nm laser OmnicTM Specta Software Ventacon H-4-200 Sealed Hot Cell Tungsten Trioxide Sample P. Misra et al. 2015 P. Misra et al. 2015 www.ventacon.com/hotcell/hotcell2.htm

Results The peaks exhibited a slight red-shift in frequency as the temperature increased from 30 to 200°C. P. Misra et al. 2015 *Insert picture from Raul’s thing. Maybe ask Raul to do one for me.

Red-Shift in Frequencies Slopes: 808 peak: -0.006, -0.038 718 peak: -0.0043, -0.0038 275 peak: -0.0179, -0.0172 *More pronounced for the 808 and 274 peaks. For the 718 peak the data is much more scattered by the general trend still has a downward slope. *There are two sets of data because the experiment was conducted increasing the temperature from 30 to 200 and decreasing from 200 to 80. The laser intensity for the increasing temperature was 24mW. This gave poor results at higher temperature (beginning at 110). The spectra became saturated. The second experiment conducted with decreasing temperature used a 12mW laser because this did not saturate the spectra at any tested temperature.

Discussion of Results The decrease in frequency Thermal expansion Phonon Interactions Temperature uncertainty at extremity of hot cell. Use of Comsol© Simulation to resolve uncertainty.

Comsol© Simulation of Hot Cell Build geometry Apply materials and physics Run simulations

Simulation Results Surface Temperature (K) Multislice Electric Potential (V) Isosurface Temperature (K)

Future Work and Goals Tungsten Trioxide samples will be exposed to SO2 and NO gas and the resulting Raman spectra will be taken. These spectra will be compared to the WO3 spectra previously gathered. Relate intensity to concentration. Break into the gas sensor industry with optical sensors Miniaturized Fingerprint indicates the gas Intensity of spectral lines indicates concentration

Acknowledgments NSF Funding My mentor, Dr. P. Misra My REU Team Members: R. Garcia, Dr. D. Casimir, S. Bartley

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