The top panel shows a TEM (transmission electron microscopy) image of a fullerene peapod: C_60 encapsulated within a single wall nanotube. The lower two panels are STM images taken by Dan Hornbaker and Ali Yazdani (UIUC) at positive sample bias (b) where electrons tunnel into the sample and negative bias (c) where electrons tunnel out of the sample. The encapsulated lattice is clearly resolved in case (b) but not in case (c) showing a strong energy dependence of the scattering of electrons on the nanotube from the peapod lattice.
The electronic properties of a carbon nanotube can be modified
by incorporation of molecular species inside the tube. This is demonstrated in recent work carried out in collaboration
with Ali Yazdani's scanning tunneling microscopy group at the University of Illinois.
STM measurements show that a lattice
of encapsulated C_60 (buckyballs) can be detected inside a semiconducing tube by tunneling into the exterior tube walls when the relative electric potentials of the tip and sample allow electrons to tunnel from the tip into the sample. This is shown in the accompanying graphic where the differential conductance is represented in the color scale as a function of the sample bias (horizontal scale) and position (vertical scale). The modulations at positive
sample bias are due to the encapsulated species.
The theory considers the scattering of propagating electronic states on the nanotube by an encapsulated molecular species. The
graphic illustrates the geometry for a bucky dimer, an isolated pair of C_60 molecules.
Symmetry constrains the allowed mixing between the tube and each ball, so that only special molecular orbitals on the buckyba
ll couple to the tube. This gives a strong energy dependence to the scattering, with the strongest coupling when the
energy of the propagating tube mode is matched to the energy of the orbital on the encapsulated species.
For scattering from a bucky-dimer (an isolated pair of
buckyballs) we obtain the spectrum plotted as a local density of states in the grayscale plot on the left.
Each bucky ball produces a bound state on the tube wall, and these mix to form a bonding-antibonding pair of
in the spectrum that produce the pair of low energy peaks that are localized in the spatial coordinate (y-axis). The prominent peak in the scattering spectrum above 1.5 eV is due to a Fabry-Perot like resonance, where the propagating electronic state on the tube wall is multiply backscattered between the two impurities. The bright streaks in this plot track the energy dependence of the
maxima in a standing wave pattern that is produced by backscattering the tube states from the dimer.
The energy dependence of the wavelength of these modulations probes the dispersion of the propagating states on the tube
walls.
When this method is applied to study the spectrum
for an ordered peapod lattice, each of the features in the dimer
model generalize to electronic bands. These bands are split by
gaps due to the hybridization of the tube and ball degrees of freedom ("hybridization gap") and due
to the periodicity of the encapsulated lattice ("Bragg gaps"). The former can be clearly resolved in the
experimental data as a suppression of the differential tunneling conductance in a range of positive sample bias.
These results suggest a new route to tuning the electronic properties of a nanotube, by controlling
their interactions with various encapsulated species.
These results were
reported as the cover story in the Science magazine (Feb. 2002). The cover is a snazzy graphic constructed by
Dan Hornbaker.
C.L. Kane, E.J. Mele, A.T. Johnson, D.E. Luzzi, B.W. Smith, D.J. Hornbaker and A. Yazdani, Physical Review B 66, 235423 (2002)