Theoretical Solid State PhysicsMarvin L. Cohen and Steven G. Louie, University of California at Berkeley,DMR-0087088
Carbon nanotubes possess novel properties thatare of fundamental scientific interest andsignificant technological potential. For example,their unique optical properties make thempromising candidates as nanoscale opto-electronic devices or photo-markers in variousapplications. However, quantum many-electroneffects often dramatically modify the propertiesof reduced dimensional systems.
We have performed first-principles calculationsto investigate electron-hole interaction (excitonic)effects on the optical spectra of single-walledcarbon nanotubes. We discovered that excitoniceffects are orders of magnitude larger in thenanotubes than bulk carbon materials and theyqualitatively alter the optical response of bothsemiconducting and metallic tubes. These largemany-electron effects elucidate recentexperimental findings and explain thediscrepancies between previous theories andexperiments.
Optical absorption spectrum (top panel) andspatial correlation of the photo-excited electron-hole pair (exciton) for state A1’ (bottom panel) ofa (8,0) carbon nanotube. Spataru, Ismail-Beigi,Benedict & Louie, Phys. Rev. Lett. (2004).
Theoretical Solid State PhysicsMarvin L. Cohen and Steven G. Louie, University of California at Berkeley,DMR-0087088
Structurally, boron nitride (BN)nanotubes are identical to carbonnanotubes except that the carbon atomsare alternatively replaced by boron andnitrogen atoms. This class of nanotubes,whose existence was first predicted bytheory, can now be synthesized inabundance. Unlike carbon nanotubes,the BN nanotubes however are large gapsemiconductors.
Our ab initio calculations show that theband gap of BN nanotubes can besignificantly reduced by a transverseelectric field. This gap reduction arisesfrom a giant Stark effect. The gapmodulation increases with tube diameterand is nearly independent of chirality.This effect provides a possible way totune the band gap of BN nanotubes forvarious applications.
Tuning the band gap of BN nanotube withtransverse electric field. Khoo, Mazzoni, andLouie, Phys. Rev. B (2004).
Theoretical Solid State PhysicsMarvin L. Cohen and Steven G. Louie, University of California at Berkeley,DMR-0087088
The energetics of face-centered-cubicCs3C60, which is a material of greatinterest as a possible high transition-temperature superconductor, is studied.At the optimized lattice constant thevolume per C60 is found to be smallerthan the the most-stable hexagon-coordination A15 phase, while the totalenergy of the fcc phase is higher than theA15 phase. These results indicate that alow-temperature and high-pressuresynthesis method might be a possibleway to produce the fcc Cs3C60 phase. Inaddition, it is also found that the A15Cs3C60 should show a phasetransformation from a hexagon-coordination phase to a pentagon-coordination phase under hydrostaticpressure.
Total energies of the fcc phase (solid line), two bcophases (+ and x) and two A15 phases (dashed lines) asa function of the volume per C60. Saito, Umemoto,Louie & Cohen, Solid Stat. Commun. (2003).
Theoretical Solid State PhysicsMarvin L. Cohen and Steven G. Louie, University of California at Berkeley,DMR-0087088
How mechanical energy dissipates (orfriction) at the nanoscale remains an openissue which is central to the understandingand efficient operation of nanomachines. Wehave carried out molecular dynamicssimulations to study dissipation in carbonnanotube oscillators of lengths of tens ofnanometers. The principal source of frictionis found to be the ends of the tubes and hencedynamical friction is virtually independent ofthe overlap area between tubes. As a result ofthis, tube commensuration does not lead tosignificantly increased frictional forces. Thefriction force is found to depend strongly andnonlinearly on the relative velocity of thetubes. It is suggested that a strong velocitydependence and strong contributions fromsurface edges may be quite general features offriction at the nanoscale.
Molecular dynamics simulation of energy dissipationin double-walled carbon nanotube oscillators.Tangney, Louie and Cohen, Phys. Rev. Lett. (2004).
