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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.

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Presentation on theme: "REU in Physics at Howard University Raman Spectroscopy, COMSOL Multiphysics and Molecular Dynamics Simulation Studies of Tungsten Oxide (WO3) as a Potential."— Presentation transcript:

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 - NO x

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 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 Laser WO3 deposit


8 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 Sample of polystyrene used 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 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),

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 CH 2 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 in the 3000cm-1 range are characteristic of the C-H vibrational modes in the polystyrene. Polystyrene has repeated CH 2 groups. Heavier portions of the polymer have lower frequencies while the lighter portions (C-H) have higher frequencies. The peaks I found (in cm-1): – v(6b) radial ring stretching mode – ring breathing mode – CH 2 stretching mode – ring vibrational mode – ring vibrational mode

26 Comparison to standard Raman Spectrum of Polystyrene (courtesy of Thermo-Fisher) 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 Spectrum from Thermo-Scientific library contained in the DXR SmartRaman Machine Spectrum that I collected Similarity between spectra gives confidence that my technique was correct

27 Comparison to polystyrene spectrum found online 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. Source:

28 Daily Spectrum Quiz These two spectra are closely related. The major difference is the peak at 1576 on the right hand spectrum. What does this represent? This spectrum shows CO 2 The top left spectrum is of CCl 4 and the bottom is of C 2 Cl 4. 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.

29 Detailed Analysis of CCl 4 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 89/raman/raman.pdf The resolution of this spectrum was not sufficient to distinguish the two Source: antum.html antum.html

30 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 Mystery Spectrum of 3 DIFFERENT Molecules Solution Image source: /2008/cp/b713965a /2008/cp/b713965a

31 Mystery Spectrum How I figured out what molecule the spectrum shows: 1460 peak is characteristic of CH 2 stretching 1055 is an in-plane bending in the CH 3 group and 1280 is a C-C stretch Therefore it must have CH 3 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 CH 3 -CH 2 Adding OH to make ethanol - works as a solution CC H H H H H H O Molecular Structure of Ethanol:

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

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 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

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