Transcript Final 7/31/14 presentation

REU in Physics at Howard
University
Raman Spectroscopy and COMSOL Multiphysics Studies of
Tungsten Oxide (WO3) as a Potential Metal-Oxide Gas Sensor
Larkin Sayre
Metal-Oxide Gas Sensors (MOGS)
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The basic principle
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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:
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Environmental – gases associated with climate change
Safety – sensing harmful gases - NOx
Overview of the project
Main goal: Look at behavior of WO3 under different temperatures
2 main aspects of my project:
• Raman Spectroscopy – the molecular structure of WO3
• COMSOL modeling – the macro side
Side project – LAMMPS and Molecular Dynamics
What is Raman Spectroscopy?
The basic principle:
A laser is directed towards the molecule and the scattered light is detected
and interpreted.
Key points:
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Rayleigh Scattering
Raman Scattering
Equipment
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Thermo-Scientific DXR SmartRaman Spectrometer
Interpretation of the spectra produced
Silicon substrate
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
Top view
WO3 deposit
Laser
Sample of
polystyrene used
Analyzing the Spectrum
Examples of peak assignments:
• Peaks at 1002, 1602, 1583 and 620
cm-1 correspond to benzene ring
vibrations
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1002 – “ring breathing mode”
• 2800-3100 – C-H stretching
vibrations
Units are “wavenumber” – 1/wavelength
Effect of heating on the Raman Spectrum of WO3
190 degrees
30 degrees
Raman spectra increasing temperature from 30
Celcius to 190 Celcius.
190 degrees
30 degrees
Raman spectra decreasing temperature from 190
Celcius to 30 Celcius
Peak Reduction Over Time
0 hours
24 hours
48 hours
72 hours
Using COMSOL
COMSOL is a CAD
modeling software that
creates simulations of realworld systems. It is heavily
used by researchers and
academics and it is a
valuable skill for me to pick
up during my REU.
The classic simulation example is the busbar with DC current running through it
producing Joule heating. This heating can be mapped by COMSOL and displayed as
below. The bar section is copper while the pins attached are titanium.
Using COMSOL Multiphysics to model Metal
Oxide on Silicon Substrate
Results
My model outputs plots for:
• Temperature
• Electric Potential
• Isothermal Contours
Credit to: Raul Garcia for the geometry of the
heated cell
LAMMPS Citation: S. Plimpton, Fast Parallel Algorithms for Short-Range Molecular Dynamics, J
Comp Phys, 117, 1-19 (1995), http://lammps.sandia.gov/
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?
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Simulating the vibrational modes of the molecules
Using trajectories to model Raman spectrum
Using LAMMPS to visualize graphene sheets
lmp_serial.exe < graphene_attempt_2.txt
Produced 108 atoms – no fixes defined,
atoms won’t move
Information on computational cost
Visualizing the results
Software used – VMD and Ovito
Both software packages produce visualizations from the
‘dump’ file created by LAMMPS.
Graphene sheet
Experimentation
with lattice structure
using VMD
Conclusions
• Peak formed at 1500cm-1 is unaffected by time spent at that
temperature
• Only after a period of days does the peak start to decrease
• COMSOL is a useful software package for macro modelling and
optimization
• Continued investigation of behavior of WO3 would be valuable
• LAMMPS would be a good extension of the project as evidenced by
graphene modelling example
What I learned during my 10 weeks at Howard
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The theory behind Raman Spectroscopy
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How to heat and handle silicon substrates with
metal-oxide deposits
The basics of exposing materials to NOx
Extended reading of publications concerning
metal-oxide gas sensors and/or tungsten oxide
The basics of Molecular Dynamics
The theory behind MD models
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Applications of LAMMPS
Visualization in Ovito and VMD
Modelling of the structure of graphene
Distinction between models and simulations
How to use the command-line interface
The directory and file systems in computers
The basics of Ubuntu Linux
COMSOL Multiphysics Modelling Software
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Its power as a detection and characterization tool
How to use a DXR SmartRaman Spectrometer to
take the Raman spectra of diverse
nanomaterials
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Geometries and heat transfer module
Modelling metal-oxide gas sensors
Attended workshop in Greenbelt by COMSOL
Webinar on post-processing and displaying
results
Current cutting-edge research and possible
careers in nanotechnology at the University of
Maryland NanoDay
The Howard University programs in
Atmospheric Science research
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Attended NASA Goddard Science Jamboree
2014 – learned about coronal mass ejections and
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the melting of ice in Antarctica
Valuable graduate school application advice and •
insight into the Howard Graduate School
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Technical writing workshop – how to formulate
an abstract, thesis, cover letter
How to give a brief research presentation – lots
of practice public speaking
The DC Metro system
The Howard University campus
How to scull on the Potomac
Museums on the Mall
U Street
The best place to sit for 4th July fireworks
on the Mall
The Georgetown Materials Physics REU
program
All the Potbelly Sandwich and FroZenYo
locations in the Greater DC area
The C&O Hiking trail near Georgetown
How to cook for myself (not so easy!)
The 14th Street Trader Joe’s
What I learned!
Condensed Version
Acknowledgements
• Raul Garcia and Daniel Casimir
• Professor Misra
• NSF for REU funding
• COMSOL Multiphysics for Heat Transfer Simulation Workshop and module
trials