1. 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

2. 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

3. 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.

4. Semiconducting Metal-Oxide Gas Sensors [1]
Composition:
Thin film layer of semiconducting metal oxide
Substrate
Heating track
Fine et al. 2010
Carbon Dioxide Sensor

5. Changes in Resistivity
Semiconducting metal-oxide gas sensors use changes in resistivity to detect the presence of certain gases

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

7. Optical Gas Sensors Used Today
Mars Land Rover

8. 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.

9. 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.

10. 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.
Talk about the fingerprint
But the fingerprint isn’t always exactly the same. There are many different factors that affect the fingerprint.

11. 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 Sealed Hot Cell
Tungsten Trioxide Sample
P. Misra et al. 2015
P. Misra et al. 2015

12. 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.

13. Red-Shift in Frequencies
Slopes:
808 peak: ,
718 peak: ,
275 peak: ,
*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.

14. 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.

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

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

17. 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

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

19. Bibliography
[1]
[2] Raul Garcia. Ph.D. Dissertation
[3] inphotonics.
Carbon Dioxide Sensor
Bignozzi CA, Caramori S, Cristino V, Argazzi R, Meda L, Tacca A Nanostructured photoelectrodes based on WO3: applications to photooxidation of aqueous electrolytes. Royal Society of Chemisty 42, 2228–2246.
Misra P, Casimir D, Garcia-Sanchez R, Balinga S. Raman spectroscopic characterization of carbon nanotubes & tungsten oxide of relevance to energy storage and gas sensing applications. Poster session presented at: Name of Convention. Number of conference; 2015 June 15; Lake Forest, CA.
Wang C, Yin L, Zhang L, Xiang D, Gao R. 15 March Metal oxide gas sensors: sensitivity and influencing factors. Sensors (10):
Shimizu Y, Egashira M Basic aspects and challenges of semiconductor gas sensors. MRS Bulletin
Xie S, Inglesia E, Bell AT Effects of Temperature on the Raman Spectra and Dispersed Oxides. J. Phys. Chem B. 105(22):
Liu X, Cheng S, Hong L, Hu S, Zhang D, Ning H A survey on gas sensing technology. Sensors.
Fine GF, Cavanagh LM, Afonja A, Binions R Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring. Sensors (10): Basel, Switzerland, 5469–5502.
Lou, LF Introductions to Phonons and Electrons. Singapore: World Scientific Publishing Company.
Author(s). Date. Title. Edition. Place of publication: publisher. Extent. Notes.
Leboffe MJ, Pierce BE Microbiology: laboratory theory and application. Englewood (CO): Morton Publishing Company.
(Leboffe and Pierce 2010)