Biomedical Engineering Reference
In-Depth Information
Yet, the researchers have reported a method to isolate, with 99% of purity,
SWCNTs with speciic chirality (10, 5).
78
Such result was achieved through
ion exchange chromatography (IEC), by conjugating HiPco tubes of 1.03 nm
with the single-stranded DNA (TTTA)
3
T sequence, which can recognise only
(10, 5) tubes. This also represents the irst time that SWCNT-FETs with single-
chirality SWCNTs have been achieved.
With the aim of obtaining SWCNTs for potentially high performance
electronics, the authors have carried out separation of DNA-functionalised
HiPco SWCNTs by (i) length, using size-exclusion chromatography (SEC),
and (ii) diameter, metallicity and chirality, utilising IEC.
79
For SEC, longer
nanotubes were eluted earlier than shorter nanotubes, while IEC method was
effective in separating semiconducting SWCNTs by chirality and diameter.
SEC and IEC were essentially independent of each other for length and
metallicity separations, respectively. Photoluminescence excitation/emission
(PLE) spectroscopy was used for the irst time to characterise the (
n
,
m
)
chiralities of SWCNTs following the combination of SEC/IEC separations. The
subsequent fabrication of FETs with these separated tubes displayed high
I
on
/
I
off
ratios, owing to semiconducting SWCNTs enriched in only a few (
n
,
m
) chiralities, thus conirming successful separation and paving the way for
high-performance nanoelectronics.
Besides showing interesting properties for the fabrication of advanced
FETs, SWCNTs possess the intrinsic characteristic of emitting light,
by assuming a wide number of electronic states (known as van Hove
singularities) within narrow ranges of energy. In particular, in semiconducting
tubes interband transitions and excitons could be observed by photo- and
electroluminescence, while they are not detected in case of metallic SWCNTs
due to their non-radiative relaxation. However, it is also possible to measure
the emission of light from a suspended quasi-metallic SWCNT (QM-SWCNT)
under low bias voltages, owing to Joule heating.
80
As a consequence of this
phenomenon, strong peaks could be identiied in the visible and IR regions,
corresponding to interband transitions. This is a result of thermal light
emission in a 1D system, which could not be observed in case of large bundles
of SWCNTs or multi-walled nanotubes.
Additionally, it was possible to convert metallic tubes into semiconducting
SWCNTs by co-doping the tubes with boron and nitrogen (BCN-SWCNTs).
81
The obtained FETs showed pure semiconducting behaviour, indicating a great
advance in comparison with the merely 70% achieved by undoped SWCNTs.
Normally, raw CNTs are characterised by a huge aspect ratio, due to their
narrow diameter (of a few nanometres) in comparison with their length
(typically 1-5 μm). In a new investigation,
82
the researchers have instead
considered ultra-short SWCNTs about 7.5 nm in length, obtained with
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