1. 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 Sensor
Larkin Sayre

2. About my project About me
Rising sophomore
Interested in majoring in physics and mechanical engineering
My project is in Professor Misra’s Spectroscopy Lab
Working with Raul Garcia and Daniel Casimir
Project focused on tungsten oxide and its interaction with NOx

3. 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

4. Overview of the project Main 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

5. 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

6. 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

8. 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

9. 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

10. Raman Spectrum of WO3
WO3 is the most used metal oxide for sensing NOx molecules
Investigating the effects of exposure to NO on the bonding in WO3 using Raman Spectroscopy
WO3 is layered on a silicon substrate and exposed (for a set amount of time) to NO
Exposure time must be adjusted and controlled to attain results

11. Exposure of WO3 to NO
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
NOx – Environmental pollutants
Why is WO3 a good NOx sensor?
Large range of operational temperatures ( degrees Celcius)
N-type, transition

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

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

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. LAMMPS Output lmp_serial.exe < wo3_attempt_larkin.txt
Produced 250 atoms
Information on computational cost

16. Visualizing the results
VMD – Visual Molecular Dynamics

17. Conclusion Where 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

18. Acknowledgements Raul Garcia and Daniel Casimir Professor Misra
NSF for REU funding

19. 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

20. 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

21. 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:

22. 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

23. 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

24. 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

25. 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.

26. 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

27. 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.

28. 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?

29. 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:

30. 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:

31. 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

32. 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.

33. Using COMSOL
COMSOL is a CAD modelling 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.

34. Results of busbar joule heating simulation

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

36. Example Geometry using a workplane
Credit: Raul Garcia

37. 3D results showing Electric Displacement Field Norm
Credit: Raul Garcia

38. Metal oxide deposited onto substrate via CVD

39. Iron electrodes

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

41. Tutorial with MEMS Joule Heating

42. Tutorial with Heating Circuit

43. Input Text File

44. LAMMPS Commands

45. 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

46. Planetarium and Telescope Observatory

47. Technical Writing Workshop