Both fullerenes and single-walled carbon nanotubes (SWNTs) exhibit many advantageous properties. Despite the similarities between these two forms of carbon, there have been very few attempts to physically merge them. We have discovered a novel hybrid material that combines fullerenes and SWNTs into a single structure in which the fullerenes are covalently bonded to the outer surface of the SWNTs. These fullerene-functionalized SWNTs, which we have termed NanoBuds, were selectively synthesized in two different one-step continuous methods, during which fullerenes were formed on iron-catalyst particles together with SWNTs during CO disproportionation. The field-emission characteristics of NanoBuds suggest that they may possess advantageous properties compared with single-walled nanotubes or fullerenes alone, or in their non-bonded configurations.
We use noninvasive atomic force microscopy
to probe the spatial
electrical conductivity of isolated junctions of pristine and nitric
acid treated single-walled carbon nanotube networks (SWCNT-N). By
analyzing the local IV curves of SWCNTs and bundles with various diameters,
the resistance per unit length and the contact resistance of their
junctions are estimated to be 3–16 kΩ/μm and 29–532
kΩ, respectively. We find that the contact resistance decreases
with increasing SWCNT or bundle diameter and depends on the contact
morphology, reaching a value of 29 kΩ at a diameter of 10 nm.
A nitric acid treatment moderately dopes SWCNTs and reduces their
average contact resistance by a factor of 3 while the resistance of
the nanotubes remains largely unaltered. Remarkably, the same treatment
on an SWCNT-N shows similar reduction in the sheet resistance by a
factor of 4. These results suggest that the resistance reduction mechanism
is related to the contact modulation with no major impact on conductance
of SWCNTs.
Characterization of electronic properties of nanomaterials usually
involves fabricating field effect transistors and deriving materials
properties from device performance measurements. The difficulty in
fabricating electrical contacts to extremely small-sized nanomaterials
as well as the intrinsic heterogeneity of nanomaterials makes it a
challenging task to measure the electronic properties of large numbers
of individual nanomaterials. Here, we utilize a scanning probe technique,
the dielectric force microscopy (DFM) to address the challenges. The
DFM technique measures the low frequency dielectric response of nanomaterials,
which is intrinsically related to their electrical conductivity. The
incorporation of a gate bias voltage in DFM measurements allows for
charge carrier density modulation, which is exploited to determine
the carrier type in nanomaterials such as semiconducting single-walled
carbon nanotubes (SWNTs) and ZnO nanowires (ZnO NWs). This technique
avoids the need of electrical contacts and inherits the spatial mapping
capability of scanning probe microscopy, as manifested in the imaging
of intratube metallic/semiconducting junctions in SWNTs. We expect
the DFM technique to find broad applications in the characterization
of various nanoelectonic materials and nanodevices.
Low-frequency dielectric polarization of single-walled carbon nanotubes (SWNTs) not only affects charge carrier transport in SWNT-based nanoelectronic devices but also determines their interaction with molecules, other nanomaterials, and external fields. Differential dielectric responses of metallic and semiconducting SWNTs are critical in electronic-type sorting of SWNTs. Here, we describe the measurement of low-frequency dielectric polarization of individual SWNTs without making electrical contacts to the nanotubes. Qualitative contrast is observed between metallic and semiconducting SWNTs due to drastically different longitudinal polarizabilities. This is developed into a facile assay for metallic and semiconducting contents in SWNT samples.
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