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REU in Physics at Howard University
Raman Spectroscopy, COMSOL Multiphysics and Molecular Dynamics Simulation Studies of
Tungsten Oxide (WO3) as a Potential Metal-Oxide Gas SensorLarkin Sayre
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Metal-Oxide Gas Sensors (MOGS)
• The basic principle
• The conductivities of metal oxides change when they undergo reversible reactions with the gases we are trying to detect
• This conductivity change can be measured and used to identify the gases present
• 4 components of MOGS: gas sensing material, substrate, electrodes, heater.
• Applications:
• Environmental – gases associated with climate change
• Safety – sensing harmful gases - NOx
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Overview of the projectMain goal: Look at behavior of WO3 and its interaction
with NOx
3 main aspects of my project:•Raman Spectroscopy – the molecular
structure of WO3
•COMSOL modelling – the macro side
• LAMMPS simulations – the nano side
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What is Raman Spectroscopy?
The basic principle:
A laser is directed towards the molecule and the scattered light is detected and interpreted.
Key points:
• Rayleigh Scattering
• Raman Scattering
• Equipment
• Thermo-Scientific DXR SmartRaman Spectrometer
• Interpretation of the spectra produced
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Using the Equipment - Procedure
• The sensors must first be calibrated
• The sample is placed in a plastic holder
• Short test iterations to ensure laser is hitting the sample
Silicon substrate
Top view
LaserWO3 deposit
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Analyzing the Spectrum
Examples of peak assignments:
• Peaks at 1002, 1602, 1583 and 620 cm-1 correspond to benzene ring vibrations
• 1002 – “ring breathing mode”
• 2800-3100 – C-H stretching vibrations
Sample of polystyrene used
Units are “wavenumber” – 1/wavelength
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Effect of heating on the Raman Spectrum of WO3
Raman spectra increasing temperature from 30 Celcius to 190 Celcius.
Raman spectra decreasing temperature from 190 Celcius to 30 Celcius
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Using COMSOL Multiphysics to model Metal Oxide on Silicon Substrate
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Results
My model outputs plots for:• Temperature• Electric Potential• Isothermal Contours
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LAMMPS and Molecular Dynamics Simulation
• Large-scale Atomic/Molecular Massively Parallel Simulator
• LAMMPS is a program that carries out molecular dynamics simulations
• It predicts how the system of atoms will behave using classical mechanics approximations (Newton’s Equations of Motion)
• How does molecular dynamics relate to research using Raman Spectroscopy?
• Simulating the vibrational modes of the molecules
• Using trajectories to model Raman spectrum
LAMMPS Citation: S. Plimpton, Fast Parallel Algorithms for Short-Range Molecular Dynamics, J Comp Phys, 117, 1-19 (1995), http://lammps.sandia.gov/
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Visualizing the resultsVMD – Visual Molecular Dynamics
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ConclusionWhere do I go from here?
• Continue to improve my LAMMPS and COMSOL models
• COMSOL workshop in Greenbelt July 8th
• Carry out exposures of WO3 to NO and investigate effect on Raman Spectrum
• Continue to Investigate behavior of WO3
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Acknowledgements
• Raul Garcia and Daniel Casimir
• Professor Misra
• NSF for REU funding