Intercalated nanostructures

The best method to alter the electronic properties of nanostructures is to dope them. By doping is meant the addition to removal of electrons from the fullerene molecules or nanotubes. There are various methods by which one can achieve doping. This page deals with the most commonly used form of doping known as intercalation (or exohedral doping).
For instance, alkali metal intercalated C₆₀ compounds exhibit metallic conductivity and superconductivity at transition temperatures only bettered by those of the high temperature superconductors. A pre-requisite to the understanding of these phenomena is a detailed knowledge of the electronic structure of the compound in question.

Solid C₆₀ is an insulator - this means that there is an energy gap between the occupied electronic states and the lowest lying unoccupied states. However, C₆₀ is no ordinary solid - it is built up of discrete molecular C₆₀ units, which interact only weakly with one another. In this respect it is similar to other van der Waals solids such as solidified rare gases. The molecular nature of solid C₆₀, which makes itself felt in the spectroscopic investigation of pristine fullerenes, has important consequences for the fullerene's electronic states lying close to the energy gap. Considering that the archetypical fullerene C₆₀ forms a solid with a face centered cubic (fcc) structure, we have at our disposal two tetrahedral and one octahedral interstitial site per C₆₀ ball. Metal ions can be inserted into these interstitial sites forming a host-guest system which is referred to as an intercalation compound, thus leading to a controlled doping of the fullerene electron system. The alkali metals (Na, K, Rb, Cs) are typical electron donors and much of the work on doped fullerenes to date has been carried out on the alkali metal intercalation compounds of C₆₀.

For the single wall carbon nanotubes (SWCNT), which form a triangular bundle lattice intercalation happens within the channels of the bundle lattice. In these nanotube intercalation compounds in contrast to C₆₀ n- and p-type doping is possible.

In the work carried out at the IFW, we have been studying alkali metal intercalated fullerenes and nanotubes with an aim of developing a full and all-round picture of their electronic structure. We are involved in the investigation of the electronic structure of many of the different phases in the A-C₆₀ , A-SWCNT system (A=alkali metal) and Ba intercalation as well as on the FeCl₃-SWCNT system. Some of the highlights of this work include studies of :

Intercalated fullerenes:

• superconducting A₃C₆₀
• anomalous lineshape of the high resolution photoemission spectra of these phases understood as sign of coupling of the electron system to vibrons and the charge carrier plasmon
• non-dispersive nature charge carrier plasmon in K₃C₆₀ understood
• insulating A₄C₆₀
• dependence of low-lying transitions between the occupied and unoccupied states on the lattice constant and number of nearest neighbors shows that the energy gap in the A₄ materials has its origin in electron correlation (these compounds are Mott-Hubbard insulators)
• 'superdoping' - Na_xC₆₀ x>6
• special 'electron pockets' formed in the center of the Na₄ aggregates located at the octahedral site 'soak up' the electrons donated from the Na between x = 6 and 8. Between these intercalation stages, the charge on the C₆₀ ball doesn't change, although the Na ions are practically fully ionized.

Intercalated bundles of single wall carbon nanotubes:

• Structural properties:
  Intercalation A_xSWCNT up to C/A=7 possible, similar to graphite intercalation compounds (GIC).
  
  

  

• Optical properties:
  All nanotubes get metallic with a Drude plasmon at 1.5 eV, 1 eV lower than in GIC
  
  

  

• Electronic structure:
  • p-doping is possible. FeCl₃ doping possible up to the same charge transfer as for the strongest n-type donors (alkali metals).
    
    

    

  • In addition we studied the doping dependence of the nature of the metallic ground state in bundles of SWCNT by photoemission spectroscopy. A direct evidence for a transition from a 1D metal (Tomonaga-Luttinger liquid) to a normal 3D Fermi liquid was observed.
    
    

Combination of intercalation and other doping methods:

• Intercalation of Heterofullerenes/Heteronanotubes
• Intercalation of Metallofullerenes
• Intercalation of C₆₀@SWCNT:
• Examples:
  Formation of a new metallic 1-dim polymer chain inside a SWCNT
  Proof by Raman, XRD and electron diffraction.
  
  

Some of this work is contained in the papers listed below. If you'd like a copy of any of these or more information about our work, please contact Thomas Pichler.

Literature:

• X. Liu, T. Pichler, M. Knupfer, J. Fink
  Electronic properties of barium-intercalated single-wall carbon nanotubes
  Physical Review B 70 (2004) Nr. 24, S. 245435/1-7
• X. Liu, T. Pichler, M. Knupfer, J. Fink, H. Kataura
  Electronic properties of FeCl₃-intercalated single-wall carbon nanotubes
  Physical Review B 70 (2004) Nr. 20, S. 205405/1-5
  
• H. Rauf, T. Pichler, M. Knupfer, J. Fink, H. Kataura
  Transition from a Tomonaga-Luttinger Liquid to a Fermi Liquid in Potassium-Intercalated Bundles of Single-Wall Carbon Nanotubes
  Physical Review Letters 93 (2004) Nr. 9, S. 96805/1-4
  
• X. Liu, T. Pichler, M. Knupfer, J. Fink, H. Kataura
  Electronic properties of potassium-intercalated C60 peapods
  Physical Review B 69 (2004) Nr. 7, S. 75417/1-7
  
• A. Kukovecz, T. Pichler, R. Pfeiffer, C. Kramberger, H. Kuzmany
  Diameter selective doping of single wall carbon nanotubes
  Physical Chemistry, Chemical Physics (2003) Nr. 5, S. 582-587.
  
• T. Pichler, A. Kukovecz, H. Kuzmany, H. Kataura, Y. Achiba
  Quasicontinuos electron and hole doping in C60 peapods
  Phys. Rev. B 67, (2003) 125416
  
• X. Liu, T. Pichler, M. Knupfer and J. Fink
  Electronic and optical properties of alkali-metal intercalated single-wall carbon nanotubes
  Phys. Rev. B 67, 125403 (2003)
  
• H. Kuzmany, A. Kukovecz, C. Kramberger, T. Pichler, M. Holzinger, H. Kataura
  Exohedral and endohedral functionalization of single wall carbon Nanotubes
  Synthetic Metals 135-136 (2003), S. 791-793
  
• T. Pichler, A. Kukovecz, H. Kuzmany, H. Kataura
  Charge transfer in doped single wall carbon nanotubes
  Synthetic Metals 135-136 (2003), S. 717-719
  
• A. Kukovecz, T. Pichler, R. Pfeiffer, H. Kuzmany
  Diameter selective charge transfer in p- and n-doped single wall carbon nanotubes synthesized by the HiPCO method
  Chem. Comm., 1730 (2002)
  
• T. Pichler, H. Kuzmany, H. Kataura, Y. Achiba
  Metallic polymers of C60 inside single-walled carbon nanotubes
  Physical Review Letters 87 (2001) Nr. 26, S. 267401/1-4
  
• T. Pichler, M. Knupfer, M.S. Golden, J. Fink, A. Rinzler, R.E. Smalley
  The loss function and optical conductivity of potassium intercalated bundles of single wall carbon nanotubes
  Synthetic Metals 103 (1999) Nr. 1-3, S. 2515-2516
  
• T. Pichler, M. Sing, M. Knupfer, M.S. Golden, and J. Fink
  Potassium intercalated bundels of single-wall carbon nanotubes: electronic structure and optical properties
  Solid State Commun. 109, 721 (1999)
  
• J. F. Armbruster, M. Knupfer, and J. Fink
  Electron energy-loss studies of Na_xC₆₀ compounds
  Z. Phys. B 102 (1997) 55
  
• M. Knupfer and J. Fink
  Mott-Hubbard-like behaviour of the energy gap of A₄C₆₀ compounds (A=Na,K,Rb,Cs) and Na₁₀C₆₀
  Phys. Rev. Lett. 79 (1997) 2714
  
• O. Gunnarsson, A. I. Lichtenstein, V. Eyert, M. Knupfer, J. Fink, and J. F. Armbruster
  Plasmon dispersion and broadening in A₃C₆₀ (A = K, Rb)
  Phys. Rev. B 53 (1996) 34557
  
• A. I. Lichtenstein, O. Gunnarsson, M. Knupfer, J. Fink, and J. F. Armbruster
  Plasmon damping and response function in doped C₆₀ compounds
  J. Phys. Cond. Matter 8 (1996) 4001
  
• O. Gunnarsson, V. Eyert, M. Knupfer, J. Fink, and J. F. Armbruster
  Plasmon dispersion in A₃C₆₀ (A = K, Rb)
  J. Phys. Cond. Matter 8 (1996) 2557
  
• M. Knupfer, J. Fink, and J. F. Armbruster
  Splitting of the electronic states near E_F in A₄C₆₀ compounds (A=alkali metal)
  Z. Phys. B 101(1996) 57
  
• W.Andreoni, P. Giannozzi, J. F. Armbruster, M. Knupfer, and J. Fink
  Anomalous electronic behavior of Na superfullerides: theory and experiment
  Europhys. Lett. 34 (1996) 699
  
• M. Knupfer, J. Fink, J. F. Armbruster, and H. A. Romberg
  Preparation and electronic structure of phase pure K₃C₆₀
  Z. Phys. B 98 (1995) 9
  
• M. Knupfer, J. F. Armbruster, H. A. Romberg, and J. Fink
  Electronic structure of K-C₆₀ compounds studied using electron energy-loss spectroscopy
  Synth. Metals 70 (1995) 1321
  
• M. S. Golden, M. Knupfer, J. Fink, J. F. Armbruster, T. R. Cummins, H. A. Romberg, M. Roth, M. Sing, M. Schmidt, E. Sohmen
  The electronic structure of fullerenes and fullerene compounds from high-energy spectroscopy
  [this paper gives an overview of our work in the field up to 1994]
  J. Phys. Cond. Matter 7 (1995) 8219
  
• M. Merkel, M. Knupfer, M. S. Golden, J. Fink, R. Seemann and R. L. Johnson
  Photoemission study of the electronic structure of C₆₀ and K_xC₆₀
  Phys. Rev. B 47 (1993) 11 470
  
• M. Knupfer, M. Merkel, M. S. Golden, J. Fink, O. Gunnarsson and V. P. Andropov
  Satellites in the photoemission spectra of A₃C₆₀ (A=K and Rb)
  Phys. Rev. B 47 (1993) 13 944