Investigation of coupled cobalt–silver nanoparticle system by plan view TEM

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Investigation of coupled cobalt–silver
nanoparticle system by plan view TEM
Daniel Foxa, Ruggero Verrea, Brendan J. O’Dowda, Sunil K. Aroraa, Colm C. Faulknerb, Igor V.
Shvetsa, Hongzhou Zhanga a,*
a. School of Physics and CRANN, Trinity College Dublin, Dublin 2, Ireland
b. CRANN Advanced Microscopy Laboratory, Trinity College Dublin, Dublin 2, Ireland
*. Corresponding author. E-mail address: [email protected] (H. Zhang)
KEYWORDS
Focused ion beam; Transmission electron microscopy; Lift-out; Co; Ag; Nanoparticle
Abstract We present a transmission electron microscopy (TEM) investigation of a coupled
cobalt and silver nanoparticle system. A plan view in situ lift-out method for preparing
samples for TEM using the focused ion beam (FIB) microscope was used. This technique is
used to prepare high quality TEM samples with site specificity in a short time and with a high
success rate. We demonstrate the ability of the plan view sample preparation technique to
provide information about an ordered system of nanoparticles which could not be observed
using standard FIB cross sectioning of the sample. High resolution TEM and energy
dispersive X-ray spectroscopy mapping of both cross sectional and plan view samples are
presented, clearly showing the significant benefit of plan view TEM analysis for certain
samples.
1. Introduction
High resolution imaging and analysis are required for a broad
range of materials and devices. As the development of
semiconductor devices with ever smaller features [1], and the
study of nanomaterials become more common, the need for
high resolution imaging and analysis is more prevalent than
ever [2–4]. The transmission electron microscope (TEM) can
be used to provide a wealth of information about the structure
and composition of materials at the atomic scale [5,6]. One
requirement of TEM analysis is that the samples must be
sufficiently thin, typically around 100 nm [7]. Conventional
TEM sample preparation is achieved by a series of time
consuming steps involving grinding, dimpling and ion milling
of samples in order to thin them sufficiently [8]. A more
efficient method of TEM sample preparation is achieved by
the gallium focused ion beam (FIB).
The FIB can extract thin sections of material from bulk
samples. Modern FIBs have both an ion beam for selective
removal of material by milling, and an electron beam which
provides high resolution imaging and charge compensation [9].
Regions of interest on the bulk sample can be located with the
electron beam before being cross sectioned by the ion beam,
allowing site-specific TEM analysis of samples. An ex situ lift
out method was initially employed in FIB sample preparation
[10]. In this technique a lamella of material is cut free from the
sample with the FIB. The sample is then placed under an
optical microscope. The electrostatic attraction between a
glass rod and the lamella is used to transfer the lamella to a
TEM grid. This method had inherent limitations such as the
inability to re-thin samples after lift out, it also had a relatively
high failure rate (50–90% success yield) when compared with
more recent techniques [11].
The FIB in situ lift-out technique, whereby the TEM
lamella is removed from the sample and transferred to a
TEM grid within the FIB chamber, has reduced the time and
failure rates (90–100% success yield) traditionally associated
with conventional TEM sample preparation. However, this
cross sectioning technique can only provide a limited view of
the sample. When analysing features which reside on or near
the surface, a plan view of the sample would prove far more
useful. Many previous attempts to prepare plan view TEM
samples have involved polishing and chemical etching [12] or
ex situ FIB lift-out [13]. We propose the use of the plan view
in situ lift-out [14], this technique provides greater surface
sampling and a view direction which cannot be achieved by
typical cross sectioning.
In this article the plan view FIB in situ lift-out technique has
been successfully demonstrated on a system consisting of Ag and
Co nanoparticle arrays previously deposited on an optically
transparent substrate. Bimetallic nanoparticle systems are currently
under intense research in a range of fields due to their
unique properties [15]. Such a system is of extraordinary interest
as it supports localised plasmon resonances, i.e. oscillations of the
free carrier within the nanoparticles [16]. As the system is also
ferromagnetic, coupling between the magnetic and plasmonic
properties of the nanocomposite layer can also be realized. The
advanced functionalities have been previously demonstrated to
produce novel interesting phenomena, such as plasmon enhanced
magneto-optic activity [17,18]. Such a system is of potential
interest for modulated sensing, imaging of magnetic fields and
miniaturised magneto-optical devices and for enhanced spectroscopy
due to the presence of ‘‘hot spots’’ in the interstitial space
between the nanoparticles [19]. The work reported in this paper
was used to locate and identify each individual nanoparticle
within the prepared region. From this information the distribution
and proximity of the two deposited materials was identified,
giving a greatly enhanced understanding of the system.
2. Experimental
The sample investigated was produced using a novel selfassembly procedure named ATLAS (Atomic Terrace Low
Angle Shadowing). In this method material is deposited using
an e-beam evaporator in ultra-high vacuum (base pressure of
5x10^-7 Pa) at a glancing angle onto a stepped substrate [20].
The stepped substrate was produced by annealing single
crystal Al2O3 (0001) with a 6° miscut in the [1-2 1 0] direction
as previously described [21]. The sample was then loaded in
the deposition chamber and inclined at 6° with respect to the
flux of evaporated material. Co was deposited at a rate at
normal incidence of 0.15 nm/minute for a nominal thickness
of 1 nm. Subsequently, Ag was deposited at 11° using the same
rate and nominal thickness. The result of the process consisted
of nanoparticle arrays of both Co and Ag, deposited in an
ordered fashion as shown in Fig. 1. This image has been taken
using a Carl Zeiss Ultra Plus scanning electron microscope
(SEM). However, the SEM utilised cannot establish where
each material was exactly placed due to the small dimensions
of the structures produced. TEM analysis was rendered
necessary to establish where the Co and Ag were placed and
how they interact due to their close proximity.
In order to protect the sample surface before FIB milling a
thin gold coating would usually be sputtered onto the surface
of the sample. However, the similar atomic mass of the gold
coating and the deposited silver would lead to poor image
contrast. Instead the surface was protected by coating the
sample with a 75 nm layer of carbon using a Cressington 108
carbon/A carbon coater. A 5 nm layer of gold was finally
deposited in a Cressington 108 auto sputter coater.
The FIBs used in these experiments were an FEI Strata 235
and a Carl Zeiss Auriga. In the Strata FIB the ion beam axis is
orientated 521 relative to the electron beam, in the Auriga FIB
the angle between the beams is 541. In both microscopes a
working distance of 5 mm was used. The Auriga FIB is equipped
with a Kleindeik Nanotechnik micromanipulator needle and a
Picoprobe GIS system for the lift-out procedure. The Auriga FIB
can also mill with an ion beam energy as low as 2 keV which,
when used as a final milling step, provides high quality samples
with a damage layer as thin as 2–3 nm [22].
The TEM used for high resolution analysis was an FEI Titan
80–300 (S)TEM operating at an accelerating voltage of 300 kV.
2.1. Cross section lift-out
The sample was brought to the eucentric height at 5 mm working
distance and the electron and ion beams were aligned to coincidence
at 01 relative to the ion beam. The first step in any liftout procedure is to deposit a protective layer of platinum on the
region of interest in the FIB. Electron beam induced decomposition
of an organo-metallic precursor was used to deposit a 300 nm
thick layer of platinum over an area of 4x1.5 mm. Ion beam
induced deposition was then used to deposit a further 1 mm thick
layer of platinum over an 8x2 mm area (Fig. 2(a)). A 10 nA ion
beam was then used to mill a trench to a depth of 10 mm either
side of a 1.5 mm thick section of material. This lamella was then
thinned to 1 mm with a 2 nA ion beam (Fig. 2(b)). One side of the
lamella was freed from the substrate with a 2 nA ion beam. The
lamella was undercut at 49° relative to the ion beam, leaving it
supported on only one side. A micromanipulator needle was then
brought into contact with the free side of the lamella and affixed
with platinum. The lamella was then cut free from the substrate
with a 2 nA ion beam and transferred to an Omniprobe TEM liftout grid mounted vertically on a sample holder (Fig. 2(c)). The
sample was thinned to 300 nm with an ion beam current of
200 pA at an angle of 1° relative to the ion beam, and then to
100 nm with 20 pA at an angle of 0.7°. Finally a 5 keV gallium
beam energy at a beam current of 20 pA was used to reduce FIB
induced damage by scanning the beam at 2° to the face of the
sample on either side for one minute each. The final lamella is
shown in Fig. 2(d). The sample was plasma cleaned for 3 min in a
Fischione model 1020 plasma cleaner to further remove contamination
before TEM analysis.
2.2. Plan view lift-out
For the plan view lift-out the sample was again brought to the
eucentric height and the beams co-aligned. A protective
platinum layer was deposited on a larger area of 5x9 mm to
protect the surface area for TEM analysis. A trench was milled
either side of the protective region, except this time the ion
beam was at 52° to the surface normal in order to cut under
the region. The beam current used was 10 nA and the milling
time allowed for each trench was 15 min. The sample was then
tilted 15° such that the ion beam was at 67° to the surface
normal. A 10 nA beam was used to mill each side of the
protected region for a further 5 min in order to ensure the
sample was fully undercut. The sample was milled to the
wedge shape illustrated in Fig. 3(a) and observed in Fig. 3(b).
One final trench was milled with a 10 nA beam for 10 min to
free one side of the sample from the substrate, and also to
observe undercutting had completed successfully.
As in the cross section lift-out the micromanipulator needle
was brought into contact with the free side of the sample and
fixed with platinum, the sample was then cut free from the
substrate. This time the sample was transferred to an Omniprobe TEM lift-out grid which was mounted horizontally on
the sample holder. This was achieved by overhanging the grid
over the edge of carbon tape, as shown in Fig. 3(c). The
sample was affixed to the grid with platinum and the micromanipulator needle was cut free. The grid was then unloaded
from the microscope and the mounted in the usual vertical
position as shown in Fig. 2(c).
The sample was thinned on the side with the wedge shape
(the side observed in Fig. 3(d)) with a 200 pA ion beam in
order to make the front and rear sides of the lamella parallel.
A 20 pA beam was used to further thin this side until the
nanoparticles could be observed with the electron beam. The
rear side of the sample (observed in Fig. 3(e)) was thinned with
a 20 pA beam until the sample was observed to be electron
transparent at an electron beam energy of 5 keV, an indication
that the sample is thin enough for TEM analysis. Finally a
5 keV ion beam energy at a beam current of 20 pA was again
used to reduce the FIB induced damage on the sample. The
finished sample is shown in Fig. 3(f). The sample was plasma
cleaned for 3 min.
3. Results
An overview of the cross section TEM lamella is shown in
Fig. 4(a). In the TEM analysis of the sample prepared in cross
section view the location of the nanoparticles can be clearly
identified, as seen in the STEM–high angle annular dark field
(HAADF) image in Fig. 4(b). The nanoparticles have high
contrast with the surrounding carbon which we deposited on the
sample. The Al2O3 substrate was tilted onto the [2 1 0] zone axis
before imaging. A high magnification bright field TEM image of
a particle on the substrate step is shown in Fig. 4(c). One of the
problems we need TEM to solve is the geometrical distribution
of the cobalt and silver on the sample. We used EDX mapping to
analyse the distribution of the concentrations of each element
and generated an elemental colour map to illustrate the results,
shown in Fig. 4(d) (top right). From this map the cobalt and
silver appeared to have come together to form a composite
particle. There was very little separation between the locations of
cobalt and silver in this view direction, suggesting close proximity
between the two materials deposited on the steps of the substrate.
Critically, in this view direction our sample orientation is such
that we are observing a projection of approximately 2 to 3
nanoparticles; we cannot obtain information about an individual
nanoparticle from this sample.
When imaged in plan view, rows of individual nanoparticles
were observed (see Fig. 5) and the number of particles
observable is greatly increased over the cross section view, thus
allowing for much greater sampling using a single lamella. The
contrast between the particles and the surrounding material is
high as shown in the bright field TEM image in Fig. 5(a) and
HAADF image in Fig. 5(b). A high magnification bright field
TEM image is shown in Fig. 5(c). We note that, due to this new
viewing direction, we can use EDX mapping to observe the
exact distribution of cobalt and silver on the sample, without
the complication of multiple nanoparticles overlapping. The
EDX map is shown in the second image in Fig. 5(d). This map
shows a clear spatial separation between the two elements
deposited on our sample. This information gives a clear
understanding of the distribution of the deposited elements
on the sample, thus revealing the exact location of each
nanoparticle. This TEM measurement can help to establish
the origin of any coupling effect between the magnetic Co and
the plasmonic Ag nanoparticles produced.
4. Discussion
Cross sectional TEM analysis using FIB sample preparation is
an invaluable microscopy technique for the characterisation of
chip based or solid samples. The in situ FIB lift-out technique
provides reproducible sample fabrication with high quality,
good throughput, site-specificity, thickness control, sample
dimension control, and a high sample yield. Critically, the
orientation of the FIB prepared sample has typically been
limited to a cross sectional view and may not provide sufficient
information about the sample regardless of the TEM analysis
technique employed.
An in situ lift-out for plan view TEM samples has been
demonstrated to provide information beyond that observed in
a conventional cross sectional sample. The self-assembled
nanoparticle arrays on the stepped template, investigated
herein, is just one sample type which benefits from plan view
TEM. Plan view TEM analysis can potentially be of great
interest for a wide range of samples such as: sub 10 nm
featured wire [23] and dot arrays [24] fabricated using block
copolymer techniques, nanowires, nano-contacts, and a large
variety of structures and geometries patterned using top down
lithography [25]. This will lead to higher resolution imaging
and enhanced understanding and control of these physical
systems.
We have noted that a wide range of samples can be
explored using plan view TEM samples. Furthermore, the
FIB plan view TEM sample preparation technique will allow
advanced TEM imaging and analytic techniques to be used on
samples that could not previously be probed by these techniques.
For example one can imagine tomography, revealing
3-D relationships between features, or Lorentz microscopy,
giving micro-magnetic understanding [26] of appropriate TEM
samples prepared in this manner.
With this technique the success rate is similar to that of the
standard in situ lift-out of 90–100%. Finally, we note that the
time required to produce a plan view TEM sample in the FIB
is similar to that of the standard cross sectional TEM sample
geometry, typically 3 h for a skilled operator.
5. Conclusion
We have demonstrated the utility of site-specific plan view
TEM analysis for our material system. Plan view analysis of
our sample was required in order to gain a complete
understanding of the material distribution and structure of the
sample. As the distribution of the materials in the system can
now be clearly observed, further modifications to the sample
preparation process can be made in order to tune the properties
of the nanoparticle system and the interactions within. The
information provided by this view direction is complimentary
to the typical cross section lift-out and can provide information
which may not be available from other lift-out geometries.
Plan view sample TEM sectioning can provide insight into a
range of nano-structured surfaces which would be inaccessible
by standard FIB cross sectioning techniques.
Acknowledgements
The work at the School of Physics and the Centre for
Research on Adaptive Nanostructures and Nanodevices at
Trinity College Dublin is supported by Science Foundation
Ireland under Grant 07/SK/I1220a. The TEM work was
conducted under the framework of the INSPIRE program,
funded by the Irish Government’s Program for Research in
Third Level Institutions, Cycle 4, National Development Plan
2007–2013
Fig. 1 SEM image of the cobalt and silver nanoparticles
deposited on a stepped sapphire substrate
Fig. 2 (a) Platinum strap deposited on the substrate surface by
electron beam and ion beam induced deposition. (b) Plan view of the
two trenches cut with the ion beam leaving a thin section of material
isolated at the centre. (c) Omniprobe TEM lift-out grid mounted
vertically on a sample holder. (d) SEM image of the final lamella.
Fig. 3 (a) Illustration of the ion beam angles used to undercut
a wedge of material from the sample surface. (b) The wedge of material
(coated with platinum) after undercutting. (c) Omniprobe TEM liftout grid mounted horizontally on carbon tape on an SEM stub.
(d) The bottom of the wedge of material after lift-out and transfer
to the TEM grid. (e) The top of the wedge, the opposite side to (d).
(f) The final lamella with a region thinned for TEM analysis.
Fig. 4 (a) Bright field TEM image of an overview of the cross section lamella.
(b) HAADF image of the region indicated in (a). The nanoparticles (NPs) are observed at the
steps of the substrate.
(c) High magnification image of a nanoparticle on a sapphire step.
(d) Top left: HAADF image of a nanoparticle. Top right: Elemental colour map of the
distribution of silver (cyan) and cobalt (red). Bottom left and bottom right: individual
maps of the silver and cobalt locations, respectively.
Fig. 5 (a) Bright field TEM image of an overview of the plan view lamella.
(b) HAADF image of the nanoparticle array.
(c) High magnification image of individual nanoparticles.
(d) First image: HAADF image of nanoparticles. Second image: Elemental colour map
of the distribution of silver (cyan) and cobalt (red). Third and fourth images: individual maps
of the silver and cobalt locations, respectively. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
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