1. Carbon Nanotube Electronics
Presented By: Yu-Jin Chen
Mentor: Professor Philip Collins
IM-SURE 2007

2. My Work: COMSOL model of a CNT device
Tested two methods of depositing SiO2:
Electron beam evaporation (E-Beam)
Plasma-enhanced chemical vapor deposition (PECVD; BMR)
Also tested etching metal dots from the surface of a chip (with and without oxide)

3. Carbon Nanotubes
A carbon nanotube can be modeled as a hexagonal lattice (graphene) “rolled up” into a cylinder.
The nanotubes we work with are single-walled and typically 1 nm in diameter.

4. Motivation sensitive to gases modified for biosensing
The Collins Group researches carbon nanotubes because there are potential applications for biosensors, sensing dangerous gases, studying proteins, etc.

5. Nanotube Devices
The carbon nanotubes are grown on silicon wafers by chemical vapor deposition (CVD).
Lithographically-deposited electrodes allow us to measure nanotube wires and transistors.

6. CNT Devices: Typical Device Sizes
~10 um
~150 um
~1 um

7. Nanotubes with Defects
Defects can be added to a nanotube and the defects can be chemically modified.
Defects are important because they are very sensitive to changes in the environment.

8. The Big Picture
First, we have a device with a metal dot deposited on the nanotube sidewall.
Next, we deposit oxide on this device.
Finally, we etch the metal dot, and are left with a nanotube device that has an isolated defect.
The ultimate goal is to develop a working recipe for making oxide-covered nanotube devices that take advantage of the sites of defects.

9. COMSOL
The model consists of a silicon substrate, a silicon oxide layer, two titanium electrodes and 105 cylindrical segments that represent the nanotube.

10. E-Beam Evaporator
This uses an electron beam to evaporate materials (silicon oxide in our case). The resulting gas then expands from the crucible and hits the sample mounted at the top of the machine.
This method is very controllable, capable of depositing a film 1 Angstrom thick.

11. BMR PECVD
The BMR PECVD system flows gases into a chamber containing our sample. Then, a plasma is created, which causes the other gases to react, forming oxide. The oxide then sticks to the surface of the sample.
This method creates a very uniform and dense film.

12. Comparing Oxides
Initial tests showed that the BMR film was much better in quality compared to the E-Beam film.

13. Nanotube Test Results
The results show that the nanotubes survive the electron beam evaporator but not the plasma CVD.

14. Metal Etch
We use samples in which gold dots have been placed on the surface so that we can test our ability to etch metal dots from under oxide.

15. Metal Etch Results
The sample on the left has oxide deposited on it. The sample on the right has been etched in aqua regia.

16. Images

17. Problems with Etching
The image on the left shows a sample before it was etched. The right image shows the sample after it was etched. In both cases, this was a bare surface with gold dots.

18. Breaking the Monolayer
We discovered that the monolayer surrounding the particles protected them from etching.
After consulting the literature and other groups, we tried exposing the samples to UV as well as different etching solutions to solve this problem. However, we have not had success.
λ= 365 nm Au

19. Summary What we can do: Future work:
Deposition of metal dots on defects
Controlled deposit of very thin oxide films
Etch very large metal dots from under oxide
Future work:
Adding more to the COMSOL model
More tests to etch away metal nano-dots
Characterizing the oxide films deposited by e-beam evaporation

20. Special Thanks Collins Group:
Professor Phil Collins
Alex Kane
Bucky Khalap
Brett Goldsmith
John Coroneus*
Danny Wan
Steve Hunt
Tatyana Sheps
Phil Haralson
Images courtesy of the Collins Group and the INRF website.
INRF:
David Crosley
Vu Phan
IM-SURE:
Said Shokair
The NSF and UROP

21. Questions?

22. Electronic Properties
Carbon nanotubes act as one-dimensional wires. They can be either metallic or semiconducting depending on the chirality of the tube, the “twist” of the hexagonal lattice.

23. Chirality
The chiral vector for a (2,4) nanotube (left) and an unrolled (5,0) nanotube with one unit cell highlighted in red.

24. Gate Electrode
The gate electrode is capacitively coupled to the nanotube, so it is able to increase or decrease the number of charge carriers in a semiconducting tube.

25. Goals
The ultimate goal is to develop a working recipe for making oxide-covered nanotube devices that take advantage of the sites of defects.