Presentation on theme: "REU in Physics at Howard University"— Presentation transcript:
1REU in Physics at Howard University Raman Spectroscopy, COMSOL Multiphysics and Molecular Dynamics Simulation Studies of Tungsten Oxide (WO3) as a Potential Metal-Oxide Gas SensorLarkin Sayre
2About my project About me Rising sophomoreInterested in majoring in physics and mechanical engineeringMy project is in Professor Misra’s Spectroscopy LabWorking with Raul Garcia and Daniel CasimirProject focused on tungsten oxide and its interaction with NOx
3Metal-Oxide Gas Sensors (MOGS) The basic principleThe conductivities of metal oxides change when they undergo reversible reactions with the gases we are trying to detectThis conductivity change can be measured and used to identify the gases present4 components of MOGS: gas sensing material, substrate, electrodes, heater.Applications:Environmental – gases associated with climate changeSafety – sensing harmful gases - NOx
4Overview 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 WO3COMSOL modelling – the macro sideLAMMPS simulations – the nano side
5What is Raman Spectroscopy? The basic principle:A laser is directed towards the molecule and the scattered light is detected and interpreted.Key points:Rayleigh ScatteringRaman ScatteringEquipmentThermo-Scientific DXR SmartRaman SpectrometerInterpretation of the spectra produced
6Using the Equipment - Procedure Silicon substrateThe sensors must first be calibratedThe sample is placed in a plastic holderShort test iterations to ensure laser is hitting the sampleTop viewWO3 depositLaser
8Analyzing the Spectrum Sample of polystyrene usedAnalyzing the SpectrumExamples of peak assignments:Peaks at 1002, 1602, 1583 and 620 cm-1 correspond to benzene ring vibrations1002 – “ring breathing mode”– C-H stretching vibrationsUnits are “wavenumber” – 1/wavelength
9Effect 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
10Raman Spectrum of WO3WO3 is the most used metal oxide for sensing NOx moleculesInvestigating the effects of exposure to NO on the bonding in WO3 using Raman SpectroscopyWO3 is layered on a silicon substrate and exposed (for a set amount of time) to NOExposure time must be adjusted and controlled to attain results
11Exposure of WO3 to NOThe 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 absorbedNOx – Environmental pollutantsWhy is WO3 a good NOx sensor?Large range of operational temperatures ( degrees Celcius)N-type, transition
12Using COMSOL Multiphysics to model Metal Oxide on Silicon Substrate
13Results My model outputs plots for: Temperature Electric Potential Isothermal Contours
14LAMMPS 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 SimulationLarge-scale Atomic/Molecular Massively Parallel SimulatorLAMMPS is a program that carries out molecular dynamics simulationsIt 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 moleculesUsing trajectories to model Raman spectrum
15LAMMPS Output lmp_serial.exe < wo3_attempt_larkin.txt Produced 250 atomsInformation on computational cost
16Visualizing the results VMD – Visual Molecular Dynamics
17Conclusion Where do I go from here? Continue to improve my LAMMPS and COMSOL modelsCOMSOL workshop in Greenbelt July 8thCarry out exposures of WO3 to NO and investigate effect on Raman SpectrumContinue to Investigate behavior of WO3
18Acknowledgements Raul Garcia and Daniel Casimir Professor Misra NSF for REU funding
19Infrared vs. Raman Spectroscopy Both detect photons emittedElastic vs. inelastic scatteringRayleigh and Raman scatteringIR is absorption spectroscopy – photons absorbed have the SAME wavelength as those emittedWhy do we use Raman spectroscopy for these experiments?Raman active transition – change in polarizeability of the moleculeIR active transition – dipole moment changeThis is why IR and Raman spectra are often complimentary
20PolarizabilityFor a vibration to be Raman-active there must be a change in polarizabilityPolarizability 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
21PolystyreneTo 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 lookPicture source:
22What does the Raman Spectrum of a molecule show? Some photons strike the molecule inelasticallyThese interactions cause vibrations in the bonds of the molecules (e.g. stretching and rotation) and changes in energy levelsThe Raman shift is the signature of these vibrations detected by the machine
23The importance of laser intensity and frequency By varying laser wavelength different spectra can appear for the same moleculeFor 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 unchangedThis is because different laser intensities will excite different vibrational modes within the molecule and give different spectraThe choice of laser wavelength has an important impact on experimental capabilities:Sensitivity - Raman scattering intensity is directly related to wavelengthSpatial resolution
24Analogy – Hooke’s LawWhen visualizing the vibrations in the molecule caused by the laser, classical mechanics analogies can be usedHooke’s Law F = -kxLighter portions of the molecule – higher frequency vibrations – and vice versaExamples:Comparing C=C (put sample wavenumber here) and C-H (and here)Ring-breathing mode of benzene ring
25Analyzing the spectrum continued… The peaks I found (in cm-1):– v(6b) radial ring stretching mode795.84– ring breathing mode– CH2 stretching mode– ring vibrational mode– ring vibrational modeThe 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.
26Comparison to standard Raman Spectrum of Polystyrene (courtesy of Thermo-Fisher) Spectrum from Thermo-Scientific library contained in the DXR SmartRaman MachineSpectrum that I collectedThe ThermoScientific DXR SmartRaman equipment – built-in libraries of standard spectraThe 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 sampleSimilarity between spectra gives confidence that my technique was correct
27Comparison 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.
28Daily 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?
29Detailed Analysis of CCl4 Spectrum 3N – 6 normal modes= 9 normal modesPeak at 454cm-1 is from the symmetric stretching of the C-Cl bondsFrom literature (http://www.physics.rutgers.edu/ugrad/3 89/raman/raman.pdf) I found that the 770 peak in fact contains two peaks at approximately 760 and 790cm-1The resolution of this spectrum was not sufficient to distinguish the twoSource:
30Mystery Spectrum of 3 DIFFERENT Molecules SolutionMy 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 attachedSimplest addition to the benzene ring is successive H and CH3 groups.Third spectrum is of tolueneImage source:
31H 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 stretching1055 is an in-plane bending in the CH3 group and 1280 is a C-C stretchTherefore it must have CH3 and multiple carbonsI guessed that it has two carbons based on the relatively few peaks presentNext I considered functional groups to add to CH3-CH2Adding OH to make ethanol - works as a solutionHH
32Using COMSOL Tetrahedron and helix difference Created using toroid and multiple cone difference and union functionsTetrahedron and helix differenceUsing Geometry tools to create more complex shapes:Tools such as union and difference under Boolean operations can build complex geometries from simple primitives.
33Using COMSOLCOMSOL 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.
45Aspect I found most interesting: Maryland NanoCenterTalks by scientists working in nanotechnologyPoster sessions for researchers to present their investigationsAspect I found most interesting:Researchers talking about piezoelectric materials – applications of nanotechnology