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 SensorLarkin 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 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

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

Using COMSOL Multiphysics to model Metal Oxide on Silicon Substrate

Results

My model outputs plots for:• Temperature• Electric Potential• Isothermal Contours

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/

Visualizing the resultsVMD – Visual Molecular Dynamics

ConclusionWhere 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

Acknowledgements

• Raul Garcia and Daniel Casimir

• Professor Misra

• NSF for REU funding