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A ‘‘Nanoprism’’ Probe for Nano-optical Applications
By Taekyeong Kim, Deok-Soo Kim, Byung Yang Lee, Zee Hwan Kim, and
Seunghun Hong*
Nanoscale probes terminated with thin coatings and peculiar
nanostructures have been a key tool for advanced scanning probe
microscopy (SPM) applications.[1–21] For example, nanoprobes
terminated with spherical nanoparticles were extensively utilized
for near-field scanning optical microscopy.[22–24] Recently, a
‘‘nanoprism’’ structure has drawn attention as an ‘‘optical
nanoantenna’’ due to its exotic optical properties,[25,26] while it
has been extremely difficult, if not impossible, to prepare a probe
terminated with a nanoprism for nano-optical applications.
Herein, we report a method to mass-produce pristine nanoprism
probes. We performed apertureless near-field scanning optical
microscopy (ANSOM) on gold nanoparticles using a nanoprism
probe, revealing the significant field localization at the torn-off
sharp edges and vertices of the nanoprism. As a proof of concept,
we demonstrate the fabrication of multiple nanoprism probes in a
parallel fashion.[27] This method is quite a versatile technique,
which allowed us to prepare probes terminated with virtually any
nanostructure, such as nanoprisms, ZnO nanorods, and metallic
nanoparticles. This method should be a major breakthrough and
provide tremendous flexibility for SPM applications, as it allows
one to mass-produce nanoprobes terminated with virtually any
nanostructure.
Figure 1 shows the schematic diagram depicting our
fabrication method. First, the atomic force microscopy (AFM)
probe (Olympus) was coated with Al (20 nm) via thermal
evaporation (base pressure 6.7 10 4 Pa, Fig. 1A). Then, the
probe was installed on the AFM system equipped with a humidity
controller, and it was scanned repeatedly over 1 mm long lines in
contact with a silicon oxide substrate for 60 s. The process
resulted in a probe with a bare Si surface exposed at its end
(Fig. 1B and S1 in Supporting Information). Here, the shape of
the grinded surface was determined by the cross-section of the
AFM probe. An AFM probe with a triangular cross-section was
used to prepare a nanoprism probe. Afterwards, thin films
comprised of desired materials or nanostructures were deposited
onto the probe (Fig. 1C). Note that we can deposit virtually any
material or nanostructure (e.g., metal thin films, nanowires, and
nanoparticle layers) depending on the specific application of the
[*] Prof. S. Hong, T. Kim, B. Y. Lee
Department of Physics and Astronomy
Seoul National University
Shillim-Dong, Kwanak-Gu
Seoul 151-747 (Korea)
E-mail: seunghun@snu.ac.kr
D. S. Kim, Prof. Z. H. Kim
Department of Chemistry
Korea University
Anam-Dong, Seongbuk-Gu
Seoul 136-701 (Korea)
DOI: 10.1002/adma.200801528
1238
probe. Finally, the sacrificial Al layer was removed in a basic
solution for 20 min, resulting in a probe with a small
nanostructure patch on its end (Fig. 1D).
Figure 2A and B show nanoprobes terminated with Au
nanoprisms, with their side lengths of 40 and 250 nm,
respectively. Initially, the untreated probe-end exhibited a
tetrahedral shape, and a triangular-shape probe-end face resulted
from grinding off the tetrahedral probe-end (Fig. 1B and S1 in
Supporting Information). After grinding off the probe-end, thin
films of Cr (8 nm thick) and Au (30 nm thick) were successively
deposited via thermal evaporation (Fig. 1C). The shape of the
evaporated thin metal film on the probe-end followed the
triangular shape of the truncated tetrahedral horn, as expected.
This process allowed us to precisely control the shape and
thickness of the nanostructures at the end of the probe. After
removal of the Al sacrificial layer on the probe side, the metal
patch remained at the probe-end. It should be noted that this Au
nanoprism was prepared only via thin-film deposition followed by
lift-off, without the use of any organic linker molecules, thus
minimizing the contamination of the probe.
The top and bottom graphs in Figure 2C are the energydispersive X-ray (EDX) spectra from the Au nanoprism (marked
by a circular mark in Fig. 2B) and the probe-side (marked by a
square mark in Fig. 2B), respectively. The EDX spectrum on the
Au nanoprism clearly shows the presence of Si, Au, and Cr, where
the signal peaks can be attributed to the silicon cantilever, Au
nanoprism (30 nm), and Cr adhesion layer (8 nm), respectively. In contrast, the EDX spectrum from the side of the probe
shows no indication of Au, Cr, or Al, implying that our process
allows us to fabricate desired nanostructures only at the end of the
probe with high definition. We could also use our process to
prepare Ag-nanoprism probes (Fig. S2 in Supporting Information). The high definition is one of the major advantages of this
process. Significantly, this method can be easily combined with
other fabrication strategies to prepare nanostructure-terminated
probes with a high yield. For example, we can combine our
method with the directed-assembly strategy[21] to assemble a
nanoparticle only at the end of the probe without any adsorption
on the probe side (Fig. S3 in Supporting Information).
Since this method relies on simple thin-film deposition
followed by a lift-off process, it can be utilized to attach virtually
any nanostructure at the end of SPM probes. Figure 2D shows the
ZnO nanorod probe fabricated using our method. In this case, a
probe with the sacrificial Al layer grinded off from its end was
placed in aqueous solution of 25 mM zinc nitrate hexahydrate
[Zn(NO3)2 6H2O] and 25 mM hexamethylenetetramine (C6H12N4)
at constant temperature of 95 8C for 1 h.[28] In the solution, ZnO
nanorods were grown on the entire probe surface. After the
removal of the sacrificial Al layer in a basic solution, we can obtain
ZnO nanorods only at the end of the Si probe (Fig. 2D). The result
indicates that our method is truly versatile.
ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2009, 21, 1238–1242
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We performed scattering-type ANSOM imaging of gold
nanoparticles (30–50 nm in diameter) using a Au-nanoprism
probe (side length 250 nm). The instrumental set-up for
ANSOM and its operating principles were similar to the ones
by Kim et al. (Fig. 3A).[7] Briefly, the nanoprism probe was
dithered near the resonant frequency V (300 kHz) of the
cantilever with a full amplitude of 20–100 nm above the sample
surface. In this set-up, the nanoprism surface was tilted to a 18 8
angle with respect to the substrate surface (Fig. 3A). Since we
employed different AFM equipments for the tip-grinding and the
ANSOM measurement, the same nanoprism probe had different
cantilever angles with respect to the sample surface, which caused
18 8 tilt of the nanoprism plane for the ANSOM measurement.
Since the vertex ‘‘a’’ of the nanoprism was further away from the
substrate than ‘‘b’’ and ‘‘c’’, only the b–c side of the nanoprism
interacts with the sample, which was reflected in the topography
and the corresponding ANSOM images (Fig. 3A and B). The
p-polarized light from a laser (HeNe, Melles Griot, 632.8 nm) was
focused onto the tip–sample junction with an angle of 30 8 with
respect to the sample surface via an objective lens (0.42 NA).
Back-scattered light from the tip–sample junction was collected
by the same lens, and homodyne-amplified to give the intensity
information of the coupled tip–nanoparticle system. The far-field
background was rejected by the third harmonic (3 V 900 kHz)
demodulation technique via a lock-in amplifier. The ANSOM map
and AFM topography images were obtained simultaneously by
raster-scanning the sample stage and recording the scattering and
the height information as a function of the sample position.
Figure 2. SEM images of Au-nanoprism probes with one triangle side of
A) 40 nm and B) 250 nm. C) EDX spectrum from the Au nanoprism
(marked by a circular mark in (B)) and probe side (marked by a square mark
in (B)). D) SEM image of a ZnO-nanorod probe.
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Figure 1. Schematic diagram depicting the fabrication method for nanoprism probes.
Figure 3B displays a third harmonic
demodulation ANSOM approach curve (scattered optical-signal intensity as a function of
our metal tip–sample distance) above the Au
nanoparticles. The short decay length
(20 nm) of the ANSOM signal with probe
height above the Au nanoparticles corresponds
to the distance scale at which the oscillating
dipole formed on the nanoparticle and its
image dipole on the surface of the nanoprism
Figure 3. A) Schematic diagram showing the ANSOM imaging set-up. The
labels ‘‘a’’, ‘‘b’’, and ‘‘c’’ indicate the three vertices of the nanoprism. The
nanoprism with a side length of 250 nm was used as a probe to image Au
nanoparticles (30–50 nm in diameter) on silicon substrates. In this case,
since the nanoprism was much larger than the nanoparticle, the nanoparticle was actually imaging the nanoprism. The nanoprism was tilted with
a 18 8 angle with respect to the substrate, so that only two vertices, ‘‘b’’
and ‘‘c’’, were engaged with the nanoparticles on the substrate. B) The
demodulated scattering signals as a function of probe–sample distance
(approach curve). C) AFM topography image on a single Au nanoparticle.
Since only two vertices, ‘‘b’’ and ‘‘c’’, were engaged with the sample surface
with an angle, only part of the triangular shape of the nanoprism appeared
in topography and in ANSOM images. D) ANSOM image taken on a single
nanoparticle simultaneously with the topography in C). It revealed two hot
spots near the vertices ‘‘b’’ and ‘‘c’’. Top and bottom insets show the
expected location of the nanoparticle relative to the nanoprism when the
field enhancement spots near ‘‘b’’ and ‘‘c’’ occurred, respectively. E) AFM
topography image of two Au nanoparticles. It exhibited dual features, each
of which is similar to that caused by a single nanoparticle in C). It clearly
indicates that the image was generated by two nanoparticles. F) ANSOM
image on two Au nanoparticles. It revealed two hot spots. The two hot
spots can be explained as a superposition of two ANSOM images
similar to the Figure 3D. The dual triangular images and the hot-spot
locations of each triangle were marked by dotted triangles and solid circles,
respectively.
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produce mutual perturbation via the dipole–image dipole
coupling, which verifies that the nanoprism formed at the end
of the probe was optically active.
Figure 3C shows the tapping-mode AFM topography image
taken on a single Au nanoparticle (diameter 30 nm, judged from
the AFM height information) using a nanoprism with its side
length of 250 nm. Since the nanoparticle was much smaller
than the nanoprism, in this case the nanoparticle was actually
imaging the nanoprism. In particular, we expect that the gold
nanoparticle samples the local field formed around the
nanoprism probe-end, in analogy with the report by Naber
et al.[29] Also, note that the topography shows only the partial
triangular shape of the nanoprism, because only two vertices ‘‘b’’
and ‘‘c’’ of the nanoprism were engaged with the nanoparticle.
The topography image showing multiple features similar to the
one in Figure 3C are presented in the Supporting Information, to
support the idea that in this case the nanoparticles on the
substrate were imaging the nanoprism probe (Fig. S4 in the
Supporting Information). It was also possible to obtain
topography images of full triangles by adjusting the angle of
the probe so that the nanoprism engaged with the sample without
much tilting (Fig. S5 in the Supporting Information). Figure 3D
shows the corresponding ANSOM image (third harmonic
intensity) taken simultaneously with the topography image in
Figure 3C, which shows a strongly enhanced local field at the ‘‘b’’
and ‘‘c’’ ends of the nanoprism probe when the nanoparticle was
placed just below these ends of the prism (Fig. 3D and insets).
The topography and ANSOM images shown in Figure 3E and F
were obtained with the same probe used at different location in
the same sample. Upon careful comparison with Figure 3C, we
find (see the triangles in Fig. 3C and E) that the features in
Figure 3E originated from the two nanoparticles being scanned
underneath the nanoprism probe. The dimensions of the features
in Figure 3C and E are also similar to that of the nanoprism
measured via SEM (Fig. 2B). Likewise, the corresponding
ANSOM image (Fig. 3F and inset) appears as a superposition
of two ANSOM images similar to Figure 3D. The qualities of the
electric field mapping and AFM images confirmed that the metal
patch remained attached on the probe-end even after tappingmode imaging (Fig. S6 in Supporting Information). It further
shows that the nanoprism probe can be employed for AFM and
NSOM experiments in general.
One major advantage of a nanoprism probe compared with
previous nanoprobes for nano-optical applications can be the
strong field-enhancement by its torn-off sharp edges. Since
nanoprisms were prepared by the lift-off process, it usually had
sharp edges originated from torn-off films (Figs. 2 and 5).
Figure 4 shows the section profiles of the ANSOM and AFM
topography images in the Figure 3C and D. Since in this case a Au
nanoparticle is actually imaging the nanoprism, the AFM
topography represents the shape of the nanoprism at the end
of the probe. Significantly, strong field localization was observed
on the edge of the nanoprism. Presumably, the torn-off sharp
edges of the nanoprism enhanced the local field around it. Since
nanoprobes with uniform coatings or spherical nanoparticles
cannot have such torn-off sharp edges, this result clearly shows
that the nanoprism probes are advantageous for ANSOM
applications. It also should be noted that the field enhancement
was somewhat localized in a specific part of the nanoprism edges.
Figure 4. Section profiles of ANSOM (upper) and AFM topography
(lower) images of Figure 3C and D. The ANSOM signal A) by the sharpened
edge and corner of the nanoprism probe is enhanced much more than that
B) by the flat-surface part of the nanoprism.
In addition, we found that different excitation-polarization
directions (s- and p-polarized light) produce different ANSOM
images (Fig. S5 in Supporting Information).[30]
Another important advantage of this method is that it can be
scaled up for mass-production of nanoprism probes. In this case,
a major challenge might be grinding multiple probes in a parallel
fashion, because all other steps, such as thin-film deposition and
Al etching, can be easily performed for multiple probes. As a
proof of concept, we demonstrated the fabrication of multiple
probes in a parallel fashion, by grinding multiple probes
simultaneously using the method previously reported
(Fig. 5A).[27] At first, a multiple-probe array was prepared by
simply physically separating an array of cantilevers from a
commercially available wafer block containing 375 individual
cantilevers (Olympus). For the sake of simplicity, we will describe
experiments involving only two cantilevers in the array. The
Al-coated probes were installed on the AFM system equipped
with a humidity controller. The AFM system had a common
force-detection system based on a laser and a quadrant
photodetector. The laser was focused onto a single cantilever
and the probes were tilted so that both probes were in contact with
the substrate (Fig. 5A). The multiple probe array was scanned
repeatedly over a 1-mm-long line in contact with a silicon oxide
substrate for 120 s (speed 2 mm s 1, contact force 100 nN,
relative humidity (RH) 60%) so that a small end portion of the
probes was removed. After Au/Cr (30 nm/8 nm) deposition
followed by the lift-off step, we achieved multiple probes with a
similarly sized Au nanoprisms at their ends (Fig. 5B and C). This
result shows that our process is suitable for the fabrication of
multiple nanoprism probes in a parallel fashion. Furthermore,
advanced grinding methods, such as chemical mechanical
polishing, should enable larger-scale fabrication of uniform
probe arrays.
In summary, we report a simple but versatile method to
mass-produce pristine nanoprism probes and other probes
terminated with peculiar nanostructures such as ZnO nanorods
ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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fabrication. The tips had a nominal radius of curvature of less than 10 nm
and tetrahedral end shape. The spring constants and resonance
frequencies of the levers were 42 N m 1, 300 kHz and 2 N m 1, 70 kHz,
respectively.
Tip Grinding for the Nanoprism-Probe Preparation: The total grinding
amount of the probe apex was controlled by the scanning conditions. The
common grinding conditions, unless specified otherwise, were scan speed
2 mm s 1, contact force 50 nN, and RH 30%. The conditions for the
40 nm nanoprism probe in Figure 2A were scan speed 2 mm s 1, contact
force 20 nN, and RH 30%, while those for the 250 nm nanoprism probe
in Figure 2B were scan speed 2 mm s 1, contact force 300 nN, and RH
60%.
Nanoparticle Sample Preparation: To prepare Au-nanoparticle samples
on a flat substrate, a Au colloidal solution (purchased from BBI) was
drop-cast onto a piranha cleaned (with 1:3 mixture of concentrated H2SO4
and 30% H2O2) silicon-substrate surface and air-dried.
Acknowledgements
The work was supported by the NRL grant funded by the KOSEF (no.
R0A-2004-000-10438-0). T. K. was supported by Seoul Science Fellowship.
S. H. acknowledges partial support from the NSI-NCRC program of the
KOSEF, and the NBIT program of KICOS. Z. H. K. acknowledges the
support from the KOSEF through the Nano R&D program (grant
M1070300103208M030003211), and KRF through the Basic Research
Promotion grant 2008-331-C00134 by the Ministry of Education, Science
and Technology (MEST). Supporting Information is available online from
Wiley InterScience or from the author.
Received: June 4, 2008
Revised: August 29, 2008
Published online: January 14, 2009
Figure 5. Parallel fabrication of multiple nanoprism probes. A) Schematic
diagram depicting the grinding process of multiple probes coated with an
Al thin film. B,C) SEM images of multiple nanoprim probes after grinding,
Cr (8 nm) and Au (30 nm) deposition, and Al removal steps were performed.
and nanoparticles. In the case of nanoprism probes, our
fabrication process resulted in nanoprisms with sharp edges
originated from the torn-off film during the lift-off process. Such
a sharp edge significantly enhanced the field around the particle,
and rendered nanoprism probes ideal for nano-optical applications. Furthermore, as a proof of concept, we demonstrated the
fabrication of multiple nanoprism probes in a parallel fashion.
This versatile method should provide tremendous flexibility for
researchers, and may open up various new SPM applications.
Experimental
Instrumentation: The wearing of the tips was carried out using a CP
Research AFM purchased from Veeco equipped with a humidity controller.
SEM images were obtained using a Hitachi 4800 field-emission electron
microscope. The ANSOM map and AFM topography images were obtained
by raster-scanning the sample stage using the Physikal Instrumente,
PI-500.
Cantilevers: Silicon sharpened-tip cantilevers (OMCL-AC160TS and
AC240TS, purchased from Olympus) were used for nanoprism-probe
Adv. Mater. 2009, 21, 1238–1242
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