Nanotube as a gas Sensor

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

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.

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

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

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

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

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

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)

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

Effect of Symmetry LUMO and HOMO of D10h tube

Band gap and dipole moment

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.

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

LUMO 10,0 N=5 ,and 7

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

PES wrt C-N

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

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

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

Binding energy BE = (Emolecule+CNT – ECNT –ENO2 ) Type binding energy Physisorbed -0.5916 Chemisorbed -0.5787 NO2 in -0.618 2NO2 0.0326

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

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.

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

References http://www.pa.msu.edu/cmp/csc/ntproperties/ http://dagotto.phys.utk.edu/condensed/noppi.carbon.2.pdf http://physicsweb.org/articles/world/11/1/9 http://www.e-nanoscience.com/application.html Teri Wang Odom; Jin-Lin Huang; Philip Kim; Charles M. Lieber, J. Phys. Chem. B, 2000, 104, 2794-2809. L.G. Bulusheva; A.V. Okotrub; D.A. Romanov; D. Tomanek, J. Phys. Chem.A, 1998, 102, 975-981. 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, 9363-9369