1. Nanotube as a gas Sensor

2. Outline What are Carbon nanotubes? Types Properties Applications
Motivation
Approach
Results
Preliminary results
Symmetry and
Basis set Effect
NO2 +CNT
Future Work
Conclusion
Acknowledgements
References

3. What are carbon nanotube
Carbon nanotubes, long, thin cylinders of carbon, were discovered in 1991 by S. Iijima
They can be considered as rolled up graphene tubes of carbon
There are two types: SWNT and MWNT.
It has very strong C-C chemical bonding.

4. Types of SWNT
A chiral vector Ch characterizes the nanotubes Ch=na1+na2, where a1 and a2 are lattice vectors of the 2D hexagonal lattice, and n and m are integers

5. Properties of nanotubes
They can be either metals or semiconductors with different size energy gaps, depending diameter and helicity of the tubes, on the indices (n,m)
A SWNT is considered metallic if the value n - m is divisible by three. Otherwise, the nanotube is semi conducting.
Ultra-small SWNTs (diameter 4Å) exhibit Superconductivity below 20K.
Nanotubes are very strong with very high Youngs Modulus and extremely flexible
High thermal conductivity.
High sensitivity to gas adsorption

6. Applications Micro-electronics / semiconductors
Controlled Drug Delivery/release
Field Effect transistors and Single electron transistors
Nano electronics
Nanogear
Hydrogen Storage

7. Motivation
Use of carbon nanotubes as chemical sensors for gases like NH3 and NO2 was first demonstrated by Kong et al.
Electrical conductance of an SWNTs increases by three orders of magnitude when exposed to NO2 and to decrease by 2 orders of magnitude in the presence of Ammonia.
In general all the papers have predicted physisorption between NO2, followed by charge transfer from tube to molecule
There is very little or no interaction between NH3 and SWNT . The interaction between NH3 and SWNT was studied by photoemission spectroscopy and it was found out that the tube is sensitive to NH3 even though the sensitivity is lesser as cmpd to NO2

8. Approach
The motivation for out study was to study the nanotube gas interaction by approximating the nanotube as a molecule of specific length and using semi empirical method (PM3) to predict the change in properties.
A 10, 0 semiconducting nanotube was used for our study.
To study the change in the electronic structure of the tube
when NO2 physisorbs
NO2 chemisorbs
PES with varying C-N bond length
Rotational PES for NO2 in chemisorbed well and physisorbed well
Effect of adsorption of 2 NO2 molecules

9. Effect of symmetry D10d symmetry D10h symmetry
The symmetry of the nanotube depends on the number of hexagons along the tube axis and along the circumference.
They to, 2 different types of point groups, Dnh (with horizontal mirror planes) and Dnd (with dihedral mirror planes)

10. Effect of symmetry on the LUMO and HOMO
Single point calculations using DFT(B3PW91 ) and HF

11. Effect of Symmetry
LUMO and HOMO of D10h tube

12. Band gap and dipole moment

13. Basis set effect
The DFT and HF calculations done using the STO- 3G basis set concentrate the frontier orbitals of the carbon nanotube on the edge carbon atoms While the Hf / 3-21G split valence basis set distributes the orbitals along the axis of the nanotubes resulting in delocalized HOMO- LUMO orbitals.

14. Length effect energy band gap of N= 5 tube = 0.255
HOMO of (10,0 )N=5 and N= 7
energy band gap of N= 5 tube = 0.255

15. LUMO 10,0 N=5 ,and 7

16. HOMO and LUMO ,CNT +NO2
HOMO and LUMO orbital for NO2 at a dist of 1.8 and 2.61A

17. PES wrt C-N

18. PES of Rotation
C-N = 1.8A
C-N = 2.61A

19. NO2 in Type Energy band gap plain CNT 2.2320 0.045 Physisorbed 1.6374
0.255
Chemisorbed
1.6481
0.261
No2in
1.6105
0.272

20. Model Chemistry Type Band gap CNT CNT+NO2(PM3) CNT+NO2(ROHF/3-21g
0.045
CNT+NO2(PM3)
0.255
CNT+NO2(ROHF/3-21g
0.016
CNT +NO2 in (PM3)
0.272
CNT +NO2 in (ROHF/3-21g)
0.011
2NO2(PM3)
0.06

21. Binding energy BE = (Emolecule+CNT – ECNT –ENO2 ) Type binding energy
Physisorbed
Chemisorbed
NO2 in
-0.618
2NO2
0.0326

22. Future Work
To evaluate the binding energies of the structures using ROHF/3-21G level of theory.
To determine the amt and type of charge transfer in the system
To study the method for regeneration process
Either by the route of Chemical reaction or
By investigating the Energy difference for 2 NO2 molecules on the surface
To study the behavior of the tube in the presence of the electric field
To compare the more favourable position for NO2 inside or outside the nanotube

23. Conclusion
Symmetry of the nanotube fragment affects the nature of the frontier orbitals
As the length of the nanotube increases , the orbitals are less delocalized
PM3 introduces a spin contamination which can be potentially solved by doing a single point at higher level of theory
From the current calculation NO2 prefers to be inside the Nanotube
Between chemisorbed and Physisorbed region, physisorption seems to be the preferred state.

24. Dr Schlegel Dr Goldfield Dr Hratchian Dr Anand Dr Knox Jie Lie
Acknowledgment
Dr Schlegel
Dr Goldfield
Dr Hratchian
Dr Anand
Dr Knox
Jie Lie
Stan Smith
Barbara Munk

25. References http://www.pa.msu.edu/cmp/csc/ntproperties/
Teri Wang Odom; Jin-Lin Huang; Philip Kim; Charles M. Lieber, J. Phys. Chem. B, 2000, 104,
L.G. Bulusheva; A.V. Okotrub; D.A. Romanov; D. Tomanek, J. Phys. Chem.A, 1998, 102,
M.J.Frisch et al., GAUSSIAN 03, Revision B.05, Gaussian Inc., Pittsburgh PA, 2003.
Shu Peng a,, Kyeongjae Cho a, Pengfei Qi , Hongjie Dai, Chemical Physics Letters 387 (2004) 271–276
Wai-Leung Yim, X. G. Gong, and Zhi-Feng LiuJ. Phys. Chem. B 2003, 107,