2 research outputs found
Networks of Semiconducting SWNTs: Contribution of Midgap Electronic States to the Electrical Transport
ConspectusSingle-walled carbon nanotube (SWNT) thin films
provide a unique platform for the development of electronic and photonic
devices because they combine the advantages of the outstanding physical
properties of individual SWNTs with the capabilities of large area
thin film manufacturing and patterning technologies. Flexible SWNT
thin film based field-effect transistors, sensors, detectors, photovoltaic
cells, and light emitting diodes have been already demonstrated, and
SWNT thin film transparent, conductive coatings for large area displays
and smart windows are under development. While chirally pure SWNTs
are not yet commercially available, the marketing of semiconducting
(SC) and metallic (MT) SWNTs has facilitated progress toward applications
by making available materials of consistent electronic structure.
Nevertheless the electrical transport properties of networks of separated
SWNTs are inferior to those of individual SWNTs. In particular, for
semiconducting SWNTs, which are the subject of this Account, the electrical
transport drastically differs from the behavior of traditional semiconductors:
for example, the bandgap of germanium (<i>E</i> = 0.66 eV)
roughly matches that of individual SC-SWNTs of diameter 1.5 nm, but
in the range 300–100 K, the intrinsic carrier concentration
in Ge decreases by more than 10 orders of magnitude while the conductivity
of a typical SC-SWNT network decreases by less than a factor of 4.
Clearly this weak modulation of the conductivity hinders the application
of SC-SWNT films as field effect transistors and photodetectors, and
it is the purpose of this Account to analyze the mechanism of the
electrical transport leading to the unusually weak temperature dependence
of the electrical conductivity of such networks. Extrinsic factors
such as the contribution of residual amounts of MT-SWNTs arising from
incomplete separation and doping of SWNTs are evaluated. However,
the observed temperature dependence of the conductivity indicates
the presence of midgap electronic states in the semiconducting SWNTs,
which provide a source of low-energy excitations, which can contribute
to hopping conductance along the nanotubes following fluctuation induced
tunneling across the internanotube junctions, which together dominate
the low temperature transport and limit the resistivity of the films.
At high temperatures, the intrinsic carriers thermally activated across
the bandgap as in a traditional semiconductor became available for
band transport. The midgap states pin the Fermi level to the middle
of the bandgap, and their origin is ascribed to defects in the SWNT
walls. The presence of such midgap states has been reported in connection
with scanning tunneling spectroscopy experiments, Coulomb blockade
observations in low temperature electrical measurements, selective
electrochemical deposition imaging, tip-enhanced Raman spectroscopy,
high resolution photocurrent spectroscopy, and the modeling of the
electronic density of states associated with various defects.Midgap states are present in conventional semiconductors, but what
is unusual in the present context is the extent of their contribution
to the electrical transport in networks of semiconducting SWNTs. In
this Account, we sharpen the focus on the midgap states in SC-SWNTs,
their effect on the electronic properties of SC-SWNT networks, and
the importance of these effects on efforts to develop electronic and
photonic applications of SC-SWNTs
Effect of Atomic Interconnects on Percolation in Single-Walled Carbon Nanotube Thin Film Networks
The formation of covalent bonds to
single-walled carbon nanotube
(SWNT) or graphene surfaces usually leads to a decrease in the electrical
conductivity and mobility as a result of the structural rehybridization
of the functionalized carbon atoms from sp<sup>2</sup> to sp<sup>3</sup>. In the present study, we explore the effect of metal deposition
on semiconducting (SC-) and metallic (MT-) SWNT thin films in the vicinity of the percolation threshold and
we are able to clearly delineate the effects of weak physisorption,
ionic chemisorption with charge transfer, and covalent hexahapto (η<sup>6</sup>) chemisorption on these percolating networks. The results
support the idea that for those metals capable of forming bis-hexahapto-bonds,
the generation of covalent (η<sup>6</sup>-SWNT)ÂMÂ(η<sup>6</sup>-SWNT) interconnects provides a conducting pathway in the
SWNT films and establishes the transition metal bis-hexahapto organometallic
bond as an electronically conjugating linkage between graphene surfaces