Single-wall carbon nanotubes and nanotube-derived hybrid
nanoscopic materials are exceptionally promising molecular
circuitry components. I will discuss experiments that
explore the basic physics and device capabilities of
nanotube-based transistors, diodes, interconnects, and
memory cells at room temperature. We use combinations of
transport measurements and advanced microscopies to probe
local properties of these devices with nanoscale resolution.
We measure the Fermi energy at individual defects along
carbon nanotube field effect transistors (CNFETs) by
combining transport with simultaneous scanning-probe based
imaging (Scanning Gate Microscopy and Scanning Impedance
Microscopy). SGM/SIM are used to how transport in the CNFET
evolves from defect-limited to diffusive to quasi-ballistic
as the carrier density is increased by elecrostatic doping.
This work was supported by the Laboratory for Research on
the Structure of Matter, Penn's NSF-supported MRSEC, DMR
00-79909.
[A4.002] High-Mobility Semiconducting Nanotubes
Michael S. Fuhrer (Department of Physics and Center for Superconductivity Research, University of Maryland, College Park, MD 20742-4111)
Recently, exceptionally long (tens of microns), clean semiconducting single-walled nanotubes (SWNTs) have been prepared by chemical vapor deposition. We have studied the electronic properties of devices consisting of individual long semiconducting SWNTs contacted by metal source and drain electrodes, with a conducting silicon back gate. The gate-voltage dependence of the conductance places a lower bound on the hole mobility in these nanotubes of 20,000 cm^2/Vs at room temperature(Paul L. McEuen, Michael S. Fuhrer, Hongkun Park, IEEE Transactions on Nanotechnology, 1, 78 (2002).), rivaling the best known semiconductors. Electron transport remains delocalized at very low temperatures; the Coulomb charging energy may be pushed below 1 K in these very long nanotubes, allowing exploration of the low-energy physics of the system. By analogy to the two-dimensional electron system in semiconductor heterostructures, this high-mobility, tunable one-dimensional electron system should serve as an excellent laboratory for studying one-dimensional electron physics. High mobility semiconducting SWNTs have also been exploited to create single-charge-sensitive transistors and memory cells(M. S. Fuhrer, B. M. Kim, T. Dürkop, and T. Brintlinger, Nano Letters 2, 755 (2002).) operating up to room temperature.
[A4.003] Mapping the Electronic States of One Dimensional Peapod Structures
D.J. Hornbaker (Department of Physics and Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign)
A key issue of potential technological importance is how the electronic properties of single wall carbon nanotubes are altered by their interactions with other molecules. We address this question by examining the properties of carbon heterostructures formed by the encapsulation of C_60 molecules within the hollow interiors of single wall nanotubes.^1 We study the properties of these novel macromolecules (dubbed 'peapods') using a low temperature, ultra-high vacuum scanning tunneling microscope (STM). Our experiments reveal that while no discernable change in the atomic structure of the encapsulating nanotubes is evident, the presence of interior C_60 molecules can dramatically affect the electronic structure of the nanotube cage. Constant current STM images of peapods display pronounced spatial modulation of the electronic density at sample biases greater than 1V, with a periodicity consistent with the intermolecular spacing of close-packed C_60 molecules inside the nanotube. This effect has been observed on peapods displaying both semiconducting and metallic densities of states. Coincident with this modulation is the appearance of characteristic features in the electronic band structure measured via tunneling spectroscopy. Theoretical modeling^2,3 indicates these features arise from coupling between the valence states of the encapsulated fullerenes, leading to the formation of a hybrid electronic band. Our experiments demonstrate that encapsulation of molecules is a viable route for selectively altering the electronic properties of carbon nanotubes.
^1B.W. Smith and D.E. Luzzi, Chem. Phys. Lett. 321, 169 (2000).
^2D.J. Hornbaker et al. Science 295, 828 (2002).
^3C. Kane et al. Phys. Rev. B (submitted).
[A4.004] Electronic structures of carbon nanotube peopods
Young Kuk (Department of Physics, Seoul National University, Seoul,151-747 Korea)
Carbon nanotubes have been successfully used for nanometer-sized devices such as diodes and transistors. These discrete devices utilize the spatially varying electronic structures of processed nanotubes by creating defect junctions or introducing substitutional or interstitial dopants. It was recently found that adsorption or insertion of molecules inside or outside of a nanotube modifies the electronic structure as observed with a low-temperature scanning tunneling microscope (LTSTM). We report a method to form spatial variation of the electronic structure by inserting various molecules such as fullerenes, metallo-fullerenes, metals or insulators. The results suggest that one can synthesize this band gap-engineered 1-dimensional wire by self-assembly instead of epitaxial growth. We propose a new type of device structure made by this processing technology.
1. J. Lee, H.J. Kim, G. Kim, Y.-W. Son, J. Ihm, S.J. Kahng, H. Kato, Z.W. Wang, T. Okazaki, H. Shinohara, and Y. Kuk, Nature, 415, 1005 (2002)