NSF Nanoscale Science and Engineering Grantees Conference, Dec 3-6, 2007
Grant # : 0609050
NIRT-GOALI: Solution-Based Dispersion, Sorting, and Placement of Carbon
Nanotubes
NSF NIRT Grant 0609050
PIs: A. Jagota*, M. Zheng#, Y-M. Chiang+, S.V. Rotkin*, C. Kiely*
*
Lehigh University; # CR&D, The DuPont Company; + Massachusetts Institute of Technology
The overall goal of our project is to study and develop solution-based processing routes
for carbon nanotubes [1]. Critical problems are effective dispersion, sorting according to length,
diameter, and electronic character, and controlled placement on a substrate. Our work involves
complementary theory and experiments that address these issues using DNA-CNT hybrids. In
this annual report we describe briefly our progress over the last year.
Our effort is organized in the following categories.
1) Controlled Placement of Carbon Nanotubes on a Substrate
Most of the promising early applications for carbon nanotubes require pure dispersed
material deposited in controlled (usually aligned) arrays on a substrate. We have been
working on controlled deposition of DNA-CNT onto modified Silicon substrates. The
experimental work is based on two phenomena. The first is based on alignment of carbon
nanotubes near a Si surface modified by a hydrophobic monolayer [2]. This phenomenon
has been interpreted as being mediated by the formation of a nematic liquid crystal sheet near
the surface which can be distorted by applied external conditions. Based on this suggestion
we have studied theoretically a more general problem of how elasticity in a sheet is affected
when its constituents can re-arrange themselves to minimize free energy [2,3]. The second
phenomenon is one in which nanotubes deposited from a drop are aligned by motion of a
meniscus. We have shown that deposition kinetics are governed by activated hopping over an
electrostatic potential barrier.
2) Solution-based sorting of carbon nanotubes
A critical enabling step is the ability to sort carbon nanotubes based on electronic
character and length. DNA-CNT hybrids have been singularly successful in this regard and
we have recently shown the ability to separate (9,1) tubes from (6,5): a difficult task because
they have essentially the same diameter and, to first order, the same bandgap [4]. We are
also working on optimization of the ion exchange process for separation. Our understanding
of the separation mechanism is based on modulation of electrostatic interactions by screening
charges [5]. However, it is clear from experiments that hydrophobic interactions and specific
anion type also play an important role. We are designing new ion-exchange stationary
phases through novel surface chemistry with well-controlled chemical structures. Our
strategy includes: 1) control of pore structure; 2) control of the polymer ion-exchange chain
length and density; and 3) control of hydrophobicity.
3) DNA-CNT Structure
Solution-based processing relies on the ability to form stable and robust hybrids with a
dispersing agent, DNA in our case. The structure of the hybrid is a critical determinant of its
behavior, in particular, its ability to allow separation. We have undertaken a detailed molecular
NSF Nanoscale Science and Engineering Grantees Conference, Dec 3-6, 2007
Grant # : 0609050
dynamics study of the structure of homopolymer DNA-CNT hybrid materials [6]. One of our
findings is that while single base binding is considerably greater than thermal energy so that they
remain stacked to the CNT surface, in a chain a large fraction of bases un-stack and move into
solution (Figure 2). This reduces the effective adhesion between DNA and CNT and because
this effect is different for different bases, it can alter the binding strength of the polymer in
comparison to that of individual bases.
4) DNA-CNT Electrostatics and device behavior
Understanding and quantitative modeling of electrostatic and van der Waals interactions is a
fundamental problem that recurs throughout our proposed work and we have devoted
considerable energies to it. In one project, we have performed quantum mechanical modeling of
the DNA-nanotube hybrids [7-10], specifically,
 electronic structure of the DNA-NT hybrid; modulation of the bandgap due to the
Coulomb potential of the charged backbone of the wrapped DNA
 symmetry breaking and subsequent modulation of the optical properties of the bare NT
due to the interaction in the hybrid material
 interactions between the NT and the DNA, between the two latter and the surrounding
water molecules in solution.
We obtained the following main results:
 numerical and analytical methods have been developed to compute the electronic
structure of the DNA-NT hybrids with helical wrap around the tube;
 the electronic charge densities induced on the NT by the Coulomb potential of the DNA
were computed with the polarization of the environment included phenomenologically,
via a screening constant (dielectric permittivity);
 the knowledge of the polarization charge allowed us to compute the polarization
component of the DNA-NT cohesion energy;
 analytical models were employed to understand qualitatively the symmetry breaking due
to the helical warp of the DNA and the optical modulation of the nanotube material
following the symmetry lowering;
 special attention has been paid to the band gap opening (metal-insulator transition) in the
armchair nanotubes that are known to have the highest symmetry and being stable against
such a transition for a number of various perturbations; we obtained a map of the
commensurability of the NT and its wrap which leads (or does not lead) to the metalinsulator transition;
 We have also modeled behavior of CNT device as an electrochemical sensing element
[11].
5) van der Waals interactions: Theory
We have worked on the accurate formulation/calculation of the van der Waals – London
dispersion (vdW-Ld) interaction energies between SWCNTs and other entities in a colloidal
suspension. This is a critical interaction between nanotubes and particularly between them and a
substrate. The theoretical work of vdW-Ld spectra and interaction energies involves
1. Calculation of ab initio optical properties.
2. Development of anisotropic cylinder-substrate formulations.
3. Data mining and optical property analysis.
NSF Nanoscale Science and Engineering Grantees Conference, Dec 3-6, 2007
Grant # : 0609050
4. Advanced multi-layer + retardation formulations
We have demonstrated the utility of using ab initio codes to calculated optical properties that
would be nearly impossible to obtain experimentally because of geometry and size
considerations [12,13]. We note the anisotropic nature of the SWCNTs and how the properties
vary considerably between chirality and axial/radial directions. We then demonstrated that the
degree of anisotropy changed with the metal/semiconducting nature of the SWCNT, with the
metal having the greater potential for a vdW-Ld “torque” because of the large difference in
properties between the radial and axial direction. We also demonstrated that the properties of
graphite/graphene were unable to accurately give the same qualitative results and therefore
shouldn’t be used as a replacement for the SWCNT properties. We have further developed the
full formulations in both the near and far limits (i.e. the vdW-Ld energies at contact and at
distances greater than 2 SWCNT diameters away). The results show many new phenomena and
considerations, and essentially stress the importance of the spectral difference between each
particular set of chirality’s AND the huge impact that the specific formulation has on the final
calculation.
References (10 point font)
1. For further information about this project email anj6@lehigh.edu
2. R.S. McLean, X. Huang, C. Khripin, A. Jagota, M. Zheng, “Controlled Two-Dimensional Pattern of
Spontaneously Aligned Carbon Nanotubes”, Nanoletters, 6 [1] 55-60 (2006).
3. C. Khripin, T. Tang, A. Jagota, “Deformation of a Sheet with Coupling Between Elasticity and
Concentration”, Journal of Physical Chemistry B (in the press, 2007).
4. Ming Zheng, Ellen D. Semke, “Enrichment of Single Chirality Carbon Nanotubes”, Journal of the
American Chemical Society, in the press (2007).
5. Steven Raymond Lustig, Anand Jagota, Constantine Khripin, Ming Zheng, “Theory of Structure-Based
Carbon Nanotube Separations by Ion-Exchange Chromatography of DNA/CNT Hybrids”, J. Phys. Chem B
109 2559-2566 (2005).
6. Suresh Manohar, Tian Tang, Anand Jagota, “Structure of Homopolymer DNA-CNT Hybrids”, Journal of
Physical Chemistry C, in the press (2007, invited for special issue honoring R. Smalley).
7. Vadim Puller, and Slava V. Rotkin, "Helicity and Broken Symmetry in DNA-Nanotube Hybrids",
Europhysics Letters 77 (2), 27006--1-6 (Jan 2007).
8. Stacy E. Snyder, and Slava V. Rotkin, “Polarization component of the cohesion energy in the complexes of
a single-wall carbon nanotube and a DNA", JETP Letters 84 (6), 348-351, (Pis'ma v ZhETF 84 (6), 411415), (2006).
9. Alexey G. Petrov, Slava V. Rotkin, “Hot carrier energy relaxation in single-wall carbon nanotubes via
surface optical phonons of the substrate” JETP Letters 84 (3), 156-160 (Pis'ma v ZhETF 84 (3), 185-189),
(2006).
10. Vadim Puller, and Slava V. Rotkin, “Characterization of DNA-Carbon Nanotube Hybrids by their van
Hove Singularities”, in Nanomodeling, Akhlesh Lakhtakia, Sergey A. Maksimenko, Editors; Proceedings
of SPIE, vol. 6328, 63280D, 2006.
11. T. Tang, A. Jagota, “Model for modulation of conductance in a carbon nanotube field effect transistor by
electrochemical gating”, Journal of Computational and Theoretical Nanotechnology accepted (2007).
12. R.F. Rajter, R.H. French, W.Y. Ching, W.C. Carter, Y.M. Chiang, “Calculating van der Waals-London
dispersion spectra and Hamaker coefficients of carbon nanotubes in water from ab initio optical
properties”, Journal of Applied Physics 101 054303 (2007).
13. R. Rajter, R. Podgornik, V.A. Parsegian, R.H. French, W.Y. Ching, “van der Waals-London Dispersion
Interactions for Optically Anisotropic Cylinders: Metallic and Semiconducting Single Wall Carbon
Nanotubes”, Physical Review B (under review 2007).