Download 2011170051 금가연 화공생명공학과 Article Review Nanosphere

2011170051 금가연
화공생명공학과
Article Review
Nanosphere lithography: Fabrication of Large Area Ag Nanoparticle Arrays by
Convective Self-Assembly and Their Characterization by Scanning UV-Visible
Extinction Spectroscopy
Overall Summary:
This article, published in Langmuir in 2004, is focused on how to produce large area (about
1cm3) silver nanoparticle array and their characterization using Scanning UV-Visible Extinction
Spectroscopy. Article first introduces various methods of sample preparation to utilize nanoparticle
arrays, but authors focus on Nanosphere Lithography (NSL). Though NSL is only applicable to few
substrate materials, it shows good control over particles’ size, shape, and especially inter-particle
spacing. Nanoparticle shape is controlled by precision of template, the angle of deposition, or by post
deposition steps like thermal annealing.
To build metal nanoparticle array using NSL method, hexagonally close-packed nanosphere
arrays should be formed first. This is the most critical step to produce highly ordered and uniform
metal nanoparticle array. The colloidal crystal template is formed from suspension of nanospheres
drop-coated onto the substrate where they self-assemble into a hexagonally close-packed array. Then
the colloidal crystal templates are then mounted into the chamber where vapor deposition of Ag or Au
happens. This article applied convective self-assembly (CSA) method to further improve the process.
Authors implement Scanning UV-Vis extinction spectroscopy to better understand the
nanoparticle arrays produced. Traditional ways such as AFM has limitations, but using Scanning UVVis extinction spectroscopy and observing the sensitivity of Localized Surface Plasmon Resonance
(LSPR) can be an excellent way to assess quality and morphology of nanosphere packing. This
spectroscopy allows for analysis or large areas of sample in a reasonable time with reliable
measurements.
Experiment
1. Materials: Ag, Glass substrate, pre –treatment of glass substrate (H2SO4, H2O2, NH4OH),
surfactant free, white carboxyl-substituted polystyrene latex nanospheres suspended in water,
absolute ethanol.
2. Nanosphere Mask formation by CSA:
The glass substrate was placed on a translational stage, and 10mL drop of nanosphere
solution was placed onto the edge of the substrate. Using a 1mm-thick-Teflon sheet attached
to a rectangular glass coverslip placed at a distance h, above the substrate surface. The
position of the spreader created two zones, the dry zone in the front and the wet zone in the
back. Monolayer formation occurs in dry zone whereas in wet zone consists of bulk droplet
solution. Humidity and temperature not controlled.
3. Preparing Periodic Particle Arrays (PPAs)
After formation of nanosphere template, the samples were mounted in Vapor deposition
system, and the vapor deposited at a rate of 0.1nm s-1. After deposition, nanosphere mask was
removed via sonication of the sample in absolute ethanol for 2 min.
4. AFM/SEM
Images were taken from AFM and SEM. Authors used etched Si nanoprobe tips in AFM
which had resonance frequencies between 280 and 320 kHz.
5. Scanning UV-Vis Macro-Extinction Spectroscopy
UV-Vis extinction spectra were taken using white light transmitted through 200nm core
multimode optical fiber. The light was focused on a sample with a spot size of 0.5mm in
diameter. The light emitted through the sample was collected with a collimating lens attached
to 200mm core multimode fiber connected to CCD detector.
Result and Discussion:
The formation of a monolayer array is controlled by three factors: capillary force, convective
transfer, and water evaporation. When solution drops to the substrate, a pressure differential is created
which causes a capillary effect that transfers solution to the dry zone, forming a thin film. Then
evaporation plays key factor; as water evaporates, spheres form hexagonally close-packed layer. To
evenly distribute particles, the stage should be moved in the opposite direction to the monolayer
formation at a constant velocity. Also, withdrawal rate should be same with evaporation rate. If not, it
can cause multiple empty zones when withdrawal rate is faster and nanoparticle accumulation when
withdrawal rate is slower than evaporation rate. The distance between spreader and substrate is also
important; this can be aided with Teflon membrane equalizing the capillary forces across the sample.
Because Teflon membrane is highly flexible, it does not apply enough downward force onto dry zone
to disrupt the layer formation.
Authors points out several limitations of traditional imaging methods such as AFM:
restricted access to sample sites, unacceptable time required to resolve image of large area of sample,
and difficulty in distinguishing between monolayers, bilayers, and multiple layers. Authors
implemented Scanning UV-Vis extinction spectroscopy, which can observe the sensitivity of LSPR of
the nanosphere packing. Well ordered templates produce highly uniform PPAs with characteristic
spectra. Defects in mask result in defects in the metal nanoparticle array, which cause perturbations in
LSPR spectrum. A broad spectrum happens when large defects exist, but the type and the exact
number of defects cannot be obtained. However, the overall quantity of each spot can be addressed on
the basis of spectral shape, which can provide relative amounts of defects overall.
After reading the first article above, I looked up another article to better understand nanosphere
lithography. I’ll briefly summarize what I’ve focused on while reading.
Nanosphere Lithography: A Versatile Nanofabrication Tool for Studies of SizeDependent Nanoparticle Optics
Brief Summary:
This article was published in The Journal of Physical Chemistry B in 2001. Author tells us
that the signature optical property of a metallic nanoparticle is LSPR. This resonance occurs when the
correct wavelength of light strikes a metallic nanoparticle, causing plasma of conducting electrons to
oscillate. This collective oscillation is localized near surface region, hence called localized surface
Plasmon resonance (LSPR). Excitation of LSPR can bring about selective photon absorption and
generation of locally enhanced or amplified electromagnetic field. Author explains that unlike
conventional ways to produce nanoparticles such as photolithography and electron beam lithography,
Nanosphere lithography (NSL) can be a good alternative with low production cost and high
throughput.
Every NSL structure starts with self-assembly of size-monodisperse nanoshperes to form 2-D
colloidal crystal deposition mask.
Ways to deposit nanospheres include spin coating, drop coating, and thermoelectrically cooled angle
coating. Nanospheres should be able to freely disperse across the substrate, and this can be achieved
by chemically modifying the surface of the nanosphere with negatively charged functional groups. As
solvent evaporates, capillary forces draw nanospheres together, and they form a hexagonally closedpacked pattern. Then a metal or other material is deposited by thermal evaporation, electron beam
deposition, or pulsed laser deposition. With NSL, double layers or 3-D structures of metal
nanoparticle arrays can be built, not only single layer. More complex nanoparticle assemblies can be
built by covalently attaching chemically synthesized Ag or Au colloids to SL PPA template with
bifuctional self-assembled monolayer linkers. Various shapes can be achieved altering Θdep, which is
the angle between the nanosphere mask and the beam of material being deposited.
The relationship between nanoparticle size and LSPR extinction maximum, λmax, has been
recognized, and since NSL can acquire any wanted size of nanoparticle by changing nanosphere size,
It is easy to observe that relationship/
Reference:
Hicks, E.C.M. ( 1 ), et al. "Nanosphere Lithography: Fabrication Of Large-Area Ag Nanoparticle Arrays By
Convective Self-Assembly And Their Characterization By Scanning UV - Visible Extinction Spectroscopy." Langmuir
20.16 (2004): 6927-6931. Scopus®. Web. 17 Nov. 2014.
Haynes, C.L., and R.P. Van Duyne. "Nanosphere Lithography: A Versatile Nanofabrication Tool For Studies Of SizeDependent Nanoparticle Optics." Journal Of Physical Chemistry B 105.24 (2001): 5599-5611. Scopus®. Web. 17 Nov.
2014.