New quantum dot technique combines best ofoptical and electron
microscopy12 June 2013
Much like in an old tube television where a beam ofelectrons
moves over a phosphor screen to createimages, the new microscopy
technique works byscanning a beam of electrons over a sample that
hasbeen coated with specially engineered quantum dots.The dots
absorb the energy and emit it as visible lightthat interacts with
the sample at close range. Thescattered photons are collected using
a similarly closelyplaced photodetector (not depicted), allowing an
imageto be constructed. Credit: Dill/NIST
It's not reruns of "The Jetsons", but researchersworking at the
National Institute of Standards andTechnology have developed a new
microscopytechnique that uses a process similar to how an oldtube
television produces apicturecathodoluminescenceto image
nanoscalefeatures. Combining the best features of opticaland
scanning electron microscopy, the fast,versatile, and
high-resolution technique allowsscientists to view surface and
subsurface featurespotentially as small as 10 nanometers in
size.
The new microscopy technique, described in thejournal AIP
Advances, uses a beam of electrons to
excite a specially engineered array of quantum dots,causing them
to emit low-energy visible light veryclose to the surface of the
sample, exploiting so-called "near-field" effects of light. By
correlating thelocal effects of this emitted light with the
position ofthe electron beam, spatial images of these effectscan be
reconstructed with nanometer-scaleresolution.
The technique neatly evades two problems innanoscale microscopy,
the diffraction limit thatrestricts conventional optical
microscopes toresolutions no better than about half thewavelength
of the light (so about 250 nm for greenlight), and the relatively
high energies and samplepreparation requirements of electron
microscopythat are destructive to fragile specimens like
tissue.
NIST researcher Nikolai Zhitenev, a co-developerof the
technique, had the idea a few years ago touse a phosphor coating to
produce light for near-field optical imaging, but at the time, no
phosphorwas available that was thin enough. Thickphosphors cause
the light to diverge, severelylimiting the image resolution. This
changed whenthe NIST researchers teamed with researchersfrom a
company that builds highly engineered andoptimized quantum dots for
lighting applications.The quantum dots potentially could do the
same jobas a phosphor, and be applied in a coating bothhomogenous
and thick enough to absorb the entireelectron beam while also
sufficiently thin so that thelight produced does not have to travel
far to thesample.
The collaborative effort found that the quantumdots, which have
a unique core-shell design,efficiently produced low-energy photons
in thevisible spectrum when energized with a beam ofelectrons. A
potential thin-film light source in hand,the group developed a
deposition process to bind
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them to specimens as a film with a controlledthickness of
approximately 50 nm.
Much like in an old tube television where a beam ofelectrons
moves over a phosphor screen to createimages, the new technique
works by scanning abeam of electrons over a sample that has
beencoated with the quantum dots. The dots absorb theelectrons'
energy and emit it as visible light thatinteracts with and
penetrates the surface overwhich it has been coated. After
interacting with thesample, the scattered photons are collected
using aclosely placed photodetector, allowing an image tobe
constructed. The first demonstration of thetechnique was used to
image the naturalnanostructure of the photodetector itself.
Becauseboth the light source and detector are so close tothe
sample, the diffraction limit doesn't apply, andmuch smaller
objects can be imaged.
"Initially, our research was driven by our desire tostudy how
inhomogeneities in the structure ofpolycrystalline photovoltaics
could affect theconversion of sunlight to electricity and how
thesedevices can be improved," says Heayoung Yoon,the lead author
of the paper. "But we quicklyrealized that this technique could
also be adaptedto other research regimes, most notably imaging
forbiological and cellular samples, wet samples,samples with rough
surfaces, as well as organicphotovoltaics. We are anxious to make
thistechnique available to the wider researchcommunity and see the
results."
More information: H. Yoon, Y, Lee, C. Bohn, S.Ko, A.
Gianfrancesco, J. Steckel, S. Coe-Sullivan,A. Talin and N.
Zhitenev. High-resolutionphotocurrent microscopy using
near-fieldcathodoluminescence of quantum dots. AIPAdvances.
Published online 10 June 2013.
Provided by National Institute of Standards andTechnology
APA citation: New quantum dot technique combines best of optical
and electron microscopy (2013, June 12)
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