The current Hot Ten is again replete with papers on
graphene, of
which there are five. Of these, #8 and #9 are new to the list and
both report
graphene nanoribbons which were produced by unzipping carbon
nanotubes. The two methods produce materials of very different
behavior, and they might open up new uses for graphene as
electronic components, sensors, and even body implants. Depending
on their width, graphene nanoribbons display either metallic
conductivity or semiconductivity. Those less than 10 nanometers
wide behave as semiconductors.
These papers appeared in the same issue of Nature, one
report being from the group headed by Liying Jiao and Xinran Wang
at Stanford University (#8) and the other from Dmitry Kosynkin and
colleagues at Rice University, Houston, Texas (#9). The former
group unzipped their nanotubes by means of plasma etching and then
flattened them out to give tapes, while the latter group used
chemical methods to make them, which involved treating the
nanotubes with concentrated sulfuric acid followed by potassium
permanganate.
This figure of graphene nanoribbons is from an
interview with
Zhihong Chen, a scientist featured in the
ScienceWatch.com Special Topic of
Graphene.
What Kosynkin’s paper delivers are nanoribbons several layers
thick—they are produced from multi-walled nanotubes—and
surprisingly they are soluble in both organic solvents and water.
At the edge of these nanotubes are oxygen atoms, and it appears to
be these which disrupt the flow of charge along the ribbon. When
they were removed by chemical reduction, for example by heating in
an atmosphere of hydrogen, the resulting ribbons behaved like
metallic conductors, as might be expected for graphene.
Jiao and Wang’s method of opening up nanotubes was more like
delicate surgery. They embedded them in a polymer film and then
sliced them with an argon plasma, after which they were removed
from the polymer and heated at 300oC. These nanoribbons
could be as thin as a single layer and were consequently much
narrower than Kosynkin’s nanoribbons—and they were
definitely not conductors but semiconductors.
Also in the current Hot Ten is paper #7 on a long-established
semiconductor: silicon. This reports a way of making this versatile
element behave as a thermoelectric material, in other words of
having the ability to convert heat to electricity.
Generating electricity by conventional means results in a lot of
energy being wasted; more than half ends up being lost to the
environment as heat. If that energy could be tapped into, enormous
benefits could accrue. Not that silicon has ever seemed a likely
route to this because silicon is a good conductor of heat and so
very unlikely to produce the necessary temperature gradient which
thermoelectric devices must have.
And so it appeared until Allon Hochbaum and Renkun Chen of the
University of California, Berkeley, showed that silicon could, in
fact, display exactly this kind of behavior.
The researchers took silicon nanowires that were round and, by
modifying them, observed thermoelectric efficiencies comparable to
that of the best currently available: bismuth tellurium
(Bi2Te3) and its alloys with antimony and
selenium.
The silicon ones were made by inserting wafer-scale arrays of
silicon nanowires into a bath of silver nitrate and hydrofluoric
acid (HF). There the silver ions were reduced to silver atoms at
the silicon surface, a process which creates a "hole" in the
lattice, and this then acted as a site which the HF could attack
and which produced a roughened surface. The residual silver atoms,
which clustered as nanoparticles on the wires, were washed off by
immersion in a bath of nitric acid.
The resulting "rough" silicon nanowires were 20 to 300 nanometers
in diameter, and against all expectations they behave as
thermoelectric materials. The thermal conductivity of this form of
silicon was a hundred times less than conventional silicon for
reasons yet unexplained.
A related paper from Hochbaum and Chen (Physical Review
Letters, 101: no. 105501, 2008) reports their observations of
thin silicon wires at low temperatures. They graphically describe
the range of conductance behavior as varying from "the nearly
ballistic to the completely diffusive."
Clearly there are still things to be learned about the two elements
which come at the head of group 14 of the periodic
table.
Dr. John Emsley is based at the Department of
Chemistry, Cambridge University, U.K.
Chemistry Top 10
Papers
Rank
Paper
Citations
This Period
(Nov-Dec 09)
Rank
Last Period
(Sep-Oct 09)
1
Y. Kamihara, et al., "Iron-based
layered superconductor
La[O1-xFx]FeAs (x = 0.05-0.12)
with Tc = 26 K,"J. Am. Chem.
Soc., 130(11): 3296-7, 19 March 2008. [Tokyo Inst.
Technol., Yokohama, Japan] *273SL
102
1
2
X.L. Li, et al., "Chemically derived,
ultrasmooth graphene nanoribbon
semiconductors,"Science, 319(5867):
1229-32, 29 February 2008. [Stanford U., CA] *267SX
44
3
3
C. de la Cruz, et al., "Magnetic order
close to superconductivity in the iron-based layered
LaO1-xFx FeAs systems,"Nature, 453(7197): 899-902, 12 June 2008. [6
U.S. and China institutions] *311WV
40
2
4
D.C. Elias, et al., "Control of
graphene’s properties by reversible
hydrogenation: Evidence for graphane,"Science, 323(5914): 610-3, 30 January 2009.
[U. Manchester, U.K.; Inst. Microelectronics Tech.,
Chernogolovka, Russia; Cambridge U., U.K.; U. Nijmegen,
Netherlands] *400JB
32
6
5
C. Lee, et al., "Measurement of the
elastic properties and intrinsic strength of monolayer
graphene,"Science, 321(5887): 385-8,
18 July 2008. [Columbia U., New York, NY] *327FB
31
4
6
H. Takahashi, et al.,
"Superconductivity at 43 K in an iron-based
layered compound LaO1-xFx
FeAs,"Nature, 453(7193): 376-8, 15
May 2008. [Nihon U., Tokyo, Japan; Tokyo Inst.
Technol., Japan] *301AI
29
5
7
A.I. Hochbaum, et al., "Enhanced
thermoelectric performance of rough silicon
nanowires,"Nature, 451(7175): 163-7,
10 January 2008. [U. Calif., Berkeley; Lawrence
Berkeley Natl. Lab., CA] *249GA
28
†
8
L.Y. Jiao, et al., "Narrow graphene
nanoribbons from carbon nanotubes,"Nature, 458(7240): 877-80, 16 April 2009.
[Stanford U., CA] *433CS
24
†
9
D.V. Kosynkin, et al., "Longitudinal
unzipping of carbon nanotubes to form graphene
nanoribbons,"Nature, 458(7240):
872-6, 16 April 2009. [Rice U., Houston, TX] *433CS
24
†
10
R. Banerjee, et al., "High-throughput
synthesis of zeolitic imidazolate frameworks and
application to CO2 capture,"Science, 319(5865): 939-43, 15 February 2008.
[U. Calif., Los Angeles; Arizona St. U., Tempe] *262RM