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Project 1: Theoretical and Computer Simulation Studies of Nanoscale Materials

Description

Structural, electronic, magnetic, mechanical,optical and transport properties of silicon and carbon based nanostructures are examined. Their growth mechanisms are studied using molecular dynamics calculations that employ linear scaling algorithms. Specific structures investigated include: (i) silicon and Si-Ge nanoclusters and the role played by hydrogen-passivation, (ii) self-assembled nanostructures in organic thin films with ethylene molecular bombardment, (iii) Ge ad-dimers on Si(100) and Ge(100) surfaces, (iv) single-wall carbon nanotubes and effects of aromaticity on their electronic structures, (v) metallic carbon nanotori and their paramagnetic moments, and (vi) silicon nanoclusters.

Contacts

Dr. Chakram S. Jayanthi (PI)
Department of Physics
University of Louisville
Louisville, KY 40292
Email: csjaya01@gwise.louisville.edu

Dr. Shi-Yu Wu (Co-PI)
Department of Physics
University of Louisville
Louisville, KY 40292
Email: sywu0001@gwise.louisville.edu

Highlight 1

Colossal Paramagnetic Moments in Metallic Carbon Nanotori
Recent reports on the observation of ring-shaped carbon nanostructures suggest that it might be interesting to investigate the electronic properties and magnetic response of carbon nanotubes bent into the form of rings. Because of the geometry of these carbon nanotori, they seem to be ideal candidates for enhancing the effect of the interplay between the delocalized -electrons and the geometrical structure of the torus, and thus one may anticipate an enhanced magnetic response of these systems in the presence of an applied magnetic field. We have carried out an exploratory investigation of the magnetic response of carbon nanotori under an applied magnetic field (with the field perpendicular to the plane of tori) based on a -orbital tight-binding theory. We find that, although not all carbon nanotori formed from metallic carbon nanotubes are metallic, but those that are metallic exhibit a surprising three orders of magnitude larger paramagnetic moment compared to the diamagnetic moment of graphite at an applied field of ~0.1 Tesla and at 0 K. The result has been published in Physical Review Letters (Physical Review Letters 88, 217206-1 (2002).

Ring current showning a 5,5 metallic nanotorus

Figure(a). The ring current (denoted by arrows) is shown for a (5,5) metallic nanotorus containing 1200 carbon atoms, where the applied magnetic field is pointing outwards. The direction of the induced magnetic moment is in the same direction as the applied field; hence this system is a paramagnet.

Ring current showning a 7,4 semiconducting nanotorus

Figure (b). The ring current (denoted by arrows) is shown for a (7,4) semi-conducting nanotorus containing 1200 carbon atoms, where the applied magnetic field is pointing outwards. The direction of the induced magnetic moment is in the opposite direction to the applied field; hence this system is a diamagnet.

(Highlighted in May 2002 Physics Web News:
http://physicsweb.org/article/news/6/5/13/1)

Carbon nanotube bent into rings are the latest nanostructures to display surprising properties, according to new calculations. Shi-Yu Wu of the University of Louisville and colleagues found that the magnetic moments of some metallic nanotori were thousands times stronger when the rings had certain magic radii. The researchers believe that such unexpected properties could be explained by the unusual behavior of the electrons when they circulate in the ring-shaped structures.

Highlight 2

Effects of Transparency and Equivalency of Contacts on the Electrical Conductance of Carbon Nanotori Contacted by Single-Wall Carbon Nanotubes
The recent discoveries by the Louisville group on the unusual electrical and magnetic properties of metallic carbon nanotori (Phys. Rev. Lett. 88, 217206, 2002) have prompted the present project on efficient contacting of carbon nanotori. An in-depth understanding of contacting a carbon nanotorus is required for incorporating these nanostructures into nanoscale devices.

Metallic carbon nanotori

In the present project, we have carried out a series of calculations on the electrical conductance of carbon nanotori contacted by single-wall carbon nanotubes to shed light on the effects of the geometry as well as the chemistry of the contacts. The relaxed structures of the contacted nanotori were determined by a generalized tight-binding molecular dynamics scheme. The conductance was calculated using the Landauer-BÜttiker formula based on a p-orbital Hamiltonian. We found that the conductance of the contacted carbon nanotorus is very sensitive to the transparency (chemistry) of the contacts. We also found that the equivalence (chemistry as well as geometry) of the contacts plays an important role in the transport properties. For example, a difference in the right-contact and left-contact diminishes the constructive quantum interference of the transmission as compared to the situation when the two contacts are equivalent.

Recent Publications

  1. S.Y. Wu and C.S. Jayanthi, Order-N Methodologies and their applications, Phys. Report 358-1, 1 (2002). (Invited Review).
  2. Y. Hu, S. Shen, L. Liu, C.S. Jayanthi, S.Y. Wu, and S. Sinnott, Thin-film Nucleation through Molecular Cluster Beam Deposition: Comparison of Tight-binding and Many- body empirical Potential Molecular Dynamics Simulations, J. Chem. Phys. 116, 6738 (2002).
  3. L. Liu, C.S. Jayanthi, and S.Y. Wu, Manifestation and Effects of Aromaticity on the Electronic Structure of Single-wall Carbon Nanotubes, Chem. Phys. Lett. 357, 91 (2002).
  4. L. Liu, G.-Y. Guo, C.S. Jayanthi, and S.Y. Wu, Colossal Paramagnetic Moments in Metallic Carbon Nanotori, Phys. Rev. Lett. 88, 217206-1 (2002).
  5. Q.M. Zhang, S.Y. Wu, and Z. Zhang, Energetics of Ge ad-dimers on the Si(100) and Ge(100) Surfaces: A comparative Study, Comput. Mat. Sci. 23, 48 (2002).
  6. L.Liu, C.S. Jayanthi, and S.Y. Wu, Linking Vibrational Dynamics to Local Electronic Structure: Local Analysis of Dynamics of Relaxed Si87 Cluster, in press, to be published in Phys. Rev. B.
  7. M. Yu, C.S. Jayanthi, D. Drabold, and S.Y. Wu, Enhanced Radiative Transition in SinGem Nanoclusters, in press, to be published in Phys. Rev. B.
  8. L. Liu, C.S. Jayanthi, and S.Y. Wu, Dynamics of the 7x7 DAS-Reconstructed Silicon (111) Surface, submitted to Phys. Rev. Lett.
  9. S. Liu, C.S. Jayanthi, Z. Zhang, and S.Y. Wu, Stability and Mechanical Properties of Silicon Nanorods, submitted to Phys. Rev. B.
  10. S. Chakraborty, K. Walsh, B.W. Alphenaar, L. Liu, and K. Tsukagoshi, Temperature mediated switching of magnetoresistance in Co-contacted multiwall carbon nanotubes, submitted to Applied Physics Letters.
  11. L. Liu, C.S. Jayanthi, S. Chakraborty, B.W. Alphenaar, and S.Y. Wu, Tunneling conductance anomaly in multiwall carbon nanotubes, submitted to Science.

Recognition

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