reu in physics at howard university
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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). The basic principle - PowerPoint PPT PresentationTRANSCRIPT
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)
• 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
Overview of the projectMain goal: Look at behavior of WO3 under different
temperatures2 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:
• Rayleigh Scattering
• Raman Scattering
• Equipment
• Thermo-Scientific DXR SmartRaman Spectrometer
• Interpretation of the spectra produced
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
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
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
30 degrees
190 degrees
190 degrees
30 degrees
Peak Reduction Over Time
0 hours24 hours48 hours72 hours
Using COMSOL
COMSOL is a CAD modeling software that creates simulations of real-world 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 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/
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 resultsSoftware 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
• The theory behind Raman Spectroscopy
• Its power as a detection and characterization tool
• How to use a DXR SmartRaman Spectrometer to take the Raman spectra of diverse nanomaterials
• 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
• 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
• 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
• Attended NASA Goddard Science Jamboree 2014 – learned about coronal mass ejections and the melting of ice in Antarctica
• Valuable graduate school application advice and insight into the Howard Graduate School
• 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