1. 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

2. 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

3. 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

4. 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

5. Using the Equipment - Procedure
Silicon substrate
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

6. Analyzing the Spectrum
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
1002 – “ring breathing mode”
– C-H stretching vibrations
Units are “wavenumber” – 1/wavelength

8. Effect of heating on the Raman Spectrum of WO3
190 degrees
190 degrees
30 degrees
30 degrees
Raman spectra increasing temperature from 30 Celcius to 190 Celcius.
Raman spectra decreasing temperature from 190 Celcius to 30 Celcius

9. Peak Reduction Over Time
0 hours
24 hours
48 hours
72 hours

10. 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.

11. Using COMSOL Multiphysics to model Metal Oxide on Silicon Substrate

12. Results My model outputs plots for: Temperature Electric Potential
Isothermal Contours

13. Credit to: Raul Garcia for the geometry of the heated cell

14. LAMMPS and Molecular Dynamics Simulation
LAMMPS Citation: S. Plimpton, Fast Parallel Algorithms for Short-Range Molecular Dynamics, J Comp Phys, 117, 1-19 (1995),
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

15. 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

16. 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

17. 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

18. Extensions of the project:
Exposure of WO3 to NOx
The hypothesis is that the reaction between WO3 and NO is NON-REVERSIBLE and therefore will produce a permanent change in the Raman spectrum of the WO3 when NOx is absorbed
Analyze Raman spectrum of damp WO3
Produce models of WO3 gas interactions in LAMMPS
Analyze Raman spectrum of WO3 below room temperature

19. What I learned during my 10 weeks at Howard
The theory behind Raman Spectroscopy
COMSOL Multiphysics Modelling Software
The DC Metro system
Its power as a detection and characterization tool
Geometries and heat transfer module
The Howard University campus
How to use a DXR SmartRaman Spectrometer to take the Raman spectra of diverse nanomaterials
Modelling metal-oxide gas sensors
How to scull on the Potomac
Attended workshop in Greenbelt by COMSOL
Museums on the Mall
Webinar on post-processing and displaying results
How to heat and handle silicon substrates with metal-oxide deposits
U Street
Current cutting-edge research and possible careers in nanotechnology at the University of Maryland NanoDay
The best place to sit for 4th July fireworks on the Mall
The basics of exposing materials to NOx
Extended reading of publications concerning metal-oxide gas sensors and/or tungsten oxide
The Georgetown Materials Physics REU program
The Howard University programs in Atmospheric Science research
The basics of Molecular Dynamics
All the Potbelly Sandwich and FroZenYo locations in the Greater DC area
Attended NASA Goddard Science Jamboree – learned about coronal mass ejections and the melting of ice in Antarctica
The theory behind MD models
The C&O Hiking trail near Georgetown
Applications of LAMMPS
Visualization in Ovito and VMD
Valuable graduate school application advice and insight into the Howard Graduate School
How to cook for myself (not so easy!)
Modelling of the structure of graphene
The 14th Street Trader Joe’s
Technical writing workshop – how to formulate an abstract, thesis, cover letter
Distinction between models and simulations
How to use the command-line interface
How to give a brief research presentation – lots of practice public speaking
The directory and file systems in computers
The basics of Ubuntu Linux

20. What I learned! Condensed Version

21. Acknowledgements Raul Garcia and Daniel Casimir Professor Misra
NSF for REU funding
COMSOL Multiphysics for Heat Transfer Simulation Workshop and module trials

22. Infrared vs. Raman Spectroscopy
Both detect photons emitted
Elastic vs. inelastic scattering
Rayleigh and Raman scattering
IR is absorption spectroscopy – photons absorbed have the SAME wavelength as those emitted
Why do we use Raman spectroscopy for these experiments?
Raman active transition – change in polarizeability of the molecule
IR active transition – dipole moment change
This is why IR and Raman spectra are often complimentary

23. Polarizability
For a vibration to be Raman-active there must be a change in polarizability
Polarizability is the ease with which an electron cloud is distorted by an external electric field.
The electric field of the incident laser acts interacts with the electron cloud

24. Polystyrene
To help me learn how to use the equipment and interpret the data I took the Raman Spectrum of polystyrene which has a polymer structure of:
Phenyl and CH2 groups – The bonds within the molecule dictate how the spectrum will look
Picture source:

25. What does the Raman Spectrum of a molecule show?
Some photons strike the molecule inelastically
These interactions cause vibrations in the bonds of the molecules (e.g. stretching and rotation) and changes in energy levels
The Raman shift is the signature of these vibrations detected by the machine

26. The importance of laser intensity and frequency
By varying laser wavelength different spectra can appear for the same molecule
For instance: Raman Spectrum of glucose is studied at two different laser wavelengths and the resulting spectrum has peaks in different places even though the sample is unchanged
This is because different laser intensities will excite different vibrational modes within the molecule and give different spectra
The choice of laser wavelength has an important impact on experimental capabilities:
Sensitivity - Raman scattering intensity is directly related to wavelength
Spatial resolution

27. Analogy – Hooke’s Law
When visualizing the vibrations in the molecule caused by the laser, classical mechanics analogies can be used
Hooke’s Law F = -kx
Lighter portions of the molecule – higher frequency vibrations – and vice versa
Examples:
Comparing C=C (put sample wavenumber here) and C-H (and here)
Ring-breathing mode of benzene ring

28. Analyzing the spectrum continued…
The peaks I found (in cm-1):
– v(6b) radial ring stretching mode
795.84
– ring breathing mode
– CH2 stretching mode
– ring vibrational mode
– ring vibrational mode
The peaks in the 3000cm-1 range are characteristic of the C-H vibrational modes in the polystyrene. Polystyrene has repeated CH2 groups. Heavier portions of the polymer have lower frequencies while the lighter portions (C-H) have higher frequencies.

29. Comparison to standard Raman Spectrum of Polystyrene (courtesy of Thermo-Fisher)
Spectrum from Thermo-Scientific library contained in the DXR SmartRaman Machine
Spectrum that I collected
The ThermoScientific DXR SmartRaman equipment – built-in libraries of standard spectra
The minor differences in wave numbers is most likely due to variation in resolution and frequency of the incident light and artifacts from impurities on the sample
Similarity between spectra gives confidence that my technique was correct

30. Comparison to polystyrene spectrum found online
Source:
The internet source gives a spectrum with three fewer peaks than my spectrum. They are circled in blue. This may be simply due to the resolution of my spectrum being better. It is easier to see the peaks especially in the approximately 3000cm-1 range and therefore easier to assign them.

31. Daily Spectrum Quiz This spectrum shows CO2
The top left spectrum is of CCl4 and the bottom is of C2Cl4. The difference is the C=C double bond that gives the shift in the spectrum and the peak at The peak at 1576 is weak because the C=C bond is strong.
These two spectra are closely related. The major difference is the peak at 1576 on the right hand spectrum. What does this represent?

32. Detailed Analysis of CCl4 Spectrum
3N – 6 normal modes
= 9 normal modes
Peak at 454cm-1 is from the symmetric stretching of the C-Cl bonds
From literature ( 89/raman/raman.pdf) I found that the 770 peak in fact contains two peaks at approximately 760 and 790cm-1
The resolution of this spectrum was not sufficient to distinguish the two
Source:

33. Mystery Spectrum of 3 DIFFERENT Molecules
Solution
My thought process:
All three spectra MUST be related (peaks correspond closely) - indicates trend in molecular structure.
I recognized some of the characteristics of phenyl groups that were present in polystyrene.
Therefore, top spectrum is benzene.
The second and third spectra therefore definitely have phenyl groups plus extra groups attached
Simplest addition to the benzene ring is successive H and CH3 groups.
Third spectrum is of toluene
Image source:

34. H H H C C O H H H Mystery Spectrum Molecular Structure of Ethanol:
How I figured out what molecule the spectrum shows:
1460 peak is characteristic of CH2 stretching
1055 is an in-plane bending in the CH3 group and 1280 is a C-C stretch
Therefore it must have CH3 and multiple carbons
I guessed that it has two carbons based on the relatively few peaks present
Next I considered functional groups to add to CH3-CH2
Adding OH to make ethanol - works as a solution H H

35. Using COMSOL Tetrahedron and helix difference
Created using toroid and multiple cone difference and union functions
Tetrahedron and helix difference
Using Geometry tools to create more complex shapes:
Tools such as union and difference under Boolean operations can build complex geometries from simple primitives.

36. Results of busbar joule heating simulation

37. Mapping of electric potential of busbar
Demonstrates charge distribution of the busbar

38. Example Geometry using a workplane
Credit: Raul Garcia

39. 3D results showing Electric Displacement Field Norm
Credit: Raul Garcia

40. Metal oxide deposited onto substrate via CVD

41. Iron electrodes

42. Heated cell
Heated cell with silicon substrate placed on top for heating

43. Tutorial with MEMS Joule Heating

44. Tutorial with Heating Circuit

45. Input Text File

46. LAMMPS Commands

47. Aspect I found most interesting:
Maryland NanoCenter
Talks by scientists working in nanotechnology
Poster sessions for researchers to present their investigations
Aspect I found most interesting:
Researchers talking about piezoelectric materials – applications of nanotechnology