Transcript Solids - UW-Madison Department of Physics

The many forms of carbon
Carbon is not only the basis of life, it also provides an
enormous variety of structures for nanotechnology.
This versatility is connected to the ability of carbon
to form two stable bonding configurations (sp2, sp3)
with different bond geometry (planar, tetrahedral).
sp2
sp3
+
pz
-bonds
-bonds
Diamond
3D
sp3
Graphite, Graphene (= single sheet)
2D
sp2
Fullerene
0D
Nanotube
1D
Diamondoids: The smallest possible diamonds
Diamondoids are small nanocrystals of diamond
with the surface passivated by hydrogen atoms.
Fullerenes
Buckminster Fuller,
father of the geodesic dome
Buckminsterfullerene C60 has the same hexagon + pentagon pattern
as a soccer ball. The pentagons (highlighted) provide the curvature.
C60 solution
in toluene
Fullerenes with increasing size
Fewer pentagons produce less curvature.
Symmetry
Production of fullerenes
Plasma generation of fullerenes in
a Krätschmer-Huffman apparatus.
Mass spectrum showing the
different fullerenes generated.
Formation of fullerenes during cooling of the plasma.
Carbon clusters smaller than C60 are often short chains.
Molecular orbitals of C60
The LUMO (lowest unoccupied
molecular orbital) is located at the
five-fold rings:
The high symmetry of C60 leads
to highly degenerate levels. i.e.,
many distinct wave functions
have the same energy. Up to 6
electrons can be placed into the
LUMO of a single C60 (see next).
Empty orbitals of C60 from X-ray absorption spectroscopy (XAS)
The continuous of * and * bands of
graphite (top) become quantized into
discrete levels (bottom).
LUMO, located at the strained five-fold rings
Terminello et al., Chem. Phys. Lett. 182, 491 (1991).
core
level
C60 can be charged with up to 6 electrons
The ability to take up that much charge makes C60 a popular electron
acceptor for molecular electronics, for example in organic solar cells.
Endo-fullerenes
An endofullerene is a fullerene with an atom (or molecule) inside.
Terminology: Ti @ C60
Carbon nanotubes grown free-standing between pillars
Controlling the location of nanowires is a difficult task, but critical for the wiring
of nano-devices. These nanotubes start at catalytic metal clusters (Ni, Co, Fe,…).
Lefebvre et al., PRL 90, 217401 (2003)
Carbon nanotube electronics
Atomic force microscopy image of an isolated carbon nanotube deposited
onto seven Pt electrodes by spin-coating from dichloroethane solution. An
auxiliary electrode is used as electrostatic gate (upper right).
Device containing several transistors on a single nanotube
Transistors with a nanotube channel work better than silicon,
but are difficult to mass-produce.
Chen et al., Science 311, 1735 (2006)
Artist’s view of a futuristic transistor made of
graphene, a single sheet of graphite
gate
channel
source
drain
Geim and McDonald,
Physics Today, August 2007, p. 35
Electrons
Standard silicon transistor:
A positive gate voltage draws electrons into
the channel. These electrons carry a current
between source and drain. The switch is on.
Vibrations of a single wall nanotube (SWNT)
A. Longitudinal acoustic mode
B. Transverse acoustic mode
C. Twisting (acoustic) mode
D. E2g(2) mode
E. A1g mode (radial breathing mode)
Calculated sound velocities are
given for the acoustic modes.
D,E are Raman active (see next).
Observing vibrations of nanotubes by Raman spectroscopy
Intense laser light excites vibrations in a nanotube. The photon energy hphoton
reduced by the energy hphoton of a vibrational mode.
Raman spectrum of acid purified nanotube material. The Raman
active E2g(2) and A1g modes (G-band and D-band) are observed.
is
Filling of nanotubes
TEM image of a multi wall nanotube (MWNT) filled with Sm2O3.
The horizontal lines are the concentric nanotubes, viewed edge-on.
The Sm2O3 crystal can be seen at the center.
Nano-peapods: Nanotubes filled with fullerenes
Graphene: A single sheet of graphite