This document discusses temperature programmed desorption (TPD) experiments on TiO2 surfaces to study the behavior of subsurface argon atoms. Scanning tunneling microscopy images of the TiO2 surface are shown at different scales. TPD experiments are performed by heating a sample with buried argon atoms while monitoring the desorbed gas with a mass spectrometer. Sharp TPD peaks indicate argon is desorbing from subsurface sites rather than just the surface. The results are modeled using equations that include argon diffusion and an "attractive force" between argon atoms and the lattice. Increasing the strength of this attractive force in the model better matches the experimental TPD curves.

1. Temperature Programmed Desorption of
Ar Gas from TiO2: A Surface Science
Perspective
Oliver Heywood
Primary Investigator: Richard M. Osgood
Senior Scientist: Denis V. Potapenko

2. Why study TiO2?
• Solar cells
• Gas sensors
• Anti-corrosion
coatings
• Biocompatibility of
bone implants
• White pigments
Surface effects critical to many of these applications

3. Scanning Tunneling Microscopy
M A

4. (110) Face of Rutile TiO2
110
101
011
100
One Monolayer = 350 pm
40 nm

5. ‘Terrace’ Structure
Atomic Rows
Tip Sharpness Changes
The Same Surface Zoomed In
Metal Impurity

6. Even More Zoom!
Ti
O
Metal Impurity

7. Cleaning a Sample for the STM
1. Sputtering – bombardment
with inert gas ions
2. Annealing – healing the
crystal by heating
Ar+
900 k

8. Ar Blisters Formed During Annealing
0.3 nm
~ 15 nm
~ 5nm
How can we investigate these features further?

9. TiO2
Heating Plate
Thermocouple
0 100 200 300 400
0
200
400
600
800
Time (Seconds)
Temperature(K)
Mass Spectrometer
Important Surface Technique:
Temperature Programed Desorption
Desorbed Ar
Vacuum Chamber
Will it work on a sub-surface species?

10. We call it: Buried Atom TPD!
The sharpness of these features is surprising

11. Modeling using Interstitial Diffusion
C – concentration of Ar
T – temperature
D – diffusion constant
500 600 700 800 900
0
0.02
0.04
0.06
0.08
TPD simulation
Temperature, K
RelativestrengthofArsurfaceflux
Surface
0 2 4 6 8 10
0
0.2
0.4
0.6
0.8
1
Depth x, nm
ArConcentration,0to1scale
Concentration Profiles
T=500
T=800
T=850
T=900
T=950

12. A New Model with Interstitial Attractive Force
0 0.2 0.4 0.6 0.8 1
0
0.2
0.4
0.6
0.8
1
Concentration
PotentialEnergy
crystal lattice unchanged
local distortion begins
distortion complete

13. Simulations with the New Model
(dE/dC = -9)
0 2 4 6 8 10
0
0.2
0.4
0.6
0.8
1
Depth x, nm
ArConcentration,0to1scale
Concentration Profiles
T=500
T=800
T=850
T=900
T=950
500 600 700 800 900
0
0.02
0.04
0.06
0.08
0.1
TPD simulation
Temperature, K
RelativestrengthofArsurfaceflux

14. Increase the Strength of the ‘Attractive Force’
(dE/dC = -18)
300 400 500 600 700 800 900 1000
0
2
4
6
8
x 10
-3 TPD simulation
Temperature, K
RelativestrengthofArsurfaceflux
0 2 4 6 8 10
0
0.2
0.4
0.6
0.8
1
Depth x, nm
ArConcentration,0to1scale
Concentration Profiles
T=500
T=550
T=600
T=650
T=700
T=750
T=800
T=850
T=900
T=950
Partial Success!

15. Acknowledgements
Many thanks go to...
Principle Investigator Professor Richard Osgood
Senior Scientist Denis Potapenko
Graduate Student Zhisheng Li
Program Coordinator Dario Vasquez

16. Questions

17. TPD Simulation
300 400 500 600 700 800 900 1000
-0.01
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
Temperature, K
RelativestrengthofArsurfaceflux
dE/dC = -0
dE/dC = -3
dE/dC = -6
dE/dC = -9
dE/dC = -12
dE/dC = -15
dE/dC = -18