Building electronics from the ground up14 January 2013, by
Steven Powell
Nanodots of iron oxide were laid out in a highly orderedpattern
without the use of templates. The averagediameter of the particles
was 25 nanometers, withregular spacing of 45 nm.
(Phys.org)There's hardly a moment in modern lifethat doesn't
involve electronic devices, whetherthey're guiding you to a
destination by GPS ordeciding which incoming messages merit a
beep,ring or vibration. But our expectation that the nextshopping
season will inevitably offer an upgrade tomore-powerful gadgets
largely depends on size namely, the ability of the industry to
shrinktransistors so that more can fit on ever-tinier
chipsurfaces.
Engineers have been up to the task of electronicsminiaturization
for decades now, and the principlethat the computer industry will
be able to do it on aregular schedule as codified in Moore's Law
won't come into doubt any time soon, thanks toresearchers like the
University of South Carolina'sChuanbing Tang.
Tang is a leader in constructing minisculestructures from the
bottom up, rather than the topdown. Currently, modern electronics
are primarilyfabricated by the latter method: the smooth surfaceof
a starting material say, a wafer of silicon isetched through micro-
or nanolithography to
establish a pattern on it.
The top-down method might involve a prefabricatedtemplate, such
as a photomask, to establish thepattern. But the approach is
becoming more andmore challenging, because reducing the size of
thefeatures on the requisite templates is gettingextremely
expensive as engineers work their wayfurther down the nanoscale.
"Going from 500 tosub-30 nanometers is cost prohibitive for
large-scale production," said Tang, an assistant professorin the
department of chemistry and biochemistry inUSC's College of Arts
and Sciences.
Chuanbing Tang (right) and Christopher Hardy usedatomic force
microscopy to characterize the nanoscalepatterns they built from
the bottom up.
As a chemist, Tang uses a bottom-up approach: heworks with the
individual molecules that go onto asurface, coaxing them to
self-arrange into thepatterns needed. One established method of
doingthis involves block copolymers, in which a polymerchain is
made up of two or more sections ofdifferent polymerized
monomers.
If the different block sections are properly designed,the blocks
will self-aggregate when placed on asurface, and the aggregation
can be harnessed tocreate desirable patterns on the nanoscale
withoutthe need for any templates. Di-block copolymers
ofpoly(ethylene oxide) and polystyrene, for example,
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have been used to construct highly ordered arraysof
perpendicular cylinders of nanoscale materials.Solvent evaporation,
or annealing, of thesepolymers on surfaces exerts an external
directionalfield that can enhance the patterning process andcreate
nearly defect-free arrays.
Tang's laboratory just published a paper for thespecial
"Emerging Investigators 2013" issue of thejournal Chemical
Communications that takes thismethod to a new level. Working
together withgraduate student Christopher Hardy, Tang led ateam
that fabricated nanoparticles of pure,crystalline iron oxide with
controlled size andspacing on silicon wafers by
covalentlyincorporating a ferrocene moiety into a
tri-blockcopolymer.
Incorporating metals into nanoscale designs iscrucial for
fabricating electronic devices, and Tang'smethod is a step forward
for the field. Becauseferrocene is covalently bonded to the
blockcopolymer, there is no need for a complexationstep to add a
metal-containing compound to thesurface a burdensome requirement of
mostprevious methods. Moreover, their technique is astep beyond
related polymer systems that containcovalent ferrocenylsilane
linkages, in whichremoval of the organic components leaves
behindsilicon oxide as an impurity in the metal oxide.
The technique is a promising addition to theavailable tools for
addressing the chronic need todecrease the size of electronic
components. "Theindustry won't replace top-down methods," Tangsaid,
"but they plan to use bottom-up together withthe existing top-down
methods soon."
There's versatility in the technique as well. "Herewe use a
ferrocene-containing polymer, which weconvert into the inorganic
iron oxide. But if wereplace the ferrocene in the polymer with
carbonprecursor, we could make a perpendicular carbonnanorod, which
would have a lot of potential uses,"Tang said. "Or we can
incorporate a semi-conducting polymer, like polythiophene,
whichwould be very useful in solar cell applications."
More information:dx.doi.org/10.1039/C2CC36756D
Provided by University of South Carolina
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