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