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Simultaneous Scanning Transmission Electron Microscopy,
Cathodoluminescence Imaging and EELS of Quantum Dot in Rods
George R. Fern*, Jack Silver*, Terry G. Ireland*, Ashley Howkins**, Tobias Jochum***,
Jan S. Niehaus***, Frank Schröder-Oeynhausen*** and Horst Weller***.
* Centre for Phosphors and Display Materials, Wolfson Centre for Materials Processing, Brunel University
London, Uxbridge, Middlesex, UB8 3PH, UK.
** Experimental Techniques Centre, Brunel University London, Uxbridge, Middlesex, UB8 3PH, UK.
*** Centrum für Angewandte Nanotechnologie (CAN) GmbH, Grindelallee 117, 20146 Hamburg, Germany.
Abstract
We show the simultaneous scanning transmission electron
microscopy
high
angle
annular
dark
field,
cathodoluminescence imaging and Se electron energy loss
spectroscopy maps of a single CdSe/CdS core/shell
quantum dot-in rod. The maps shown align well with
significant red light emission within 2.7nm of the Se core.
Keywords: Dot-in-Rod; Cathodoluminescence; TEM;
EELS
1.
Objective and Background
Currently one-dimensional quantum rods or dot-in-rods
(DRs) have reached a point where the development of their
synthetic methods have led to outstanding chemical and
photo stability, intense light emission, high quantum
efficiency, large absorption cross-section and suppressed
blinking [1]. When compared to spherical quantum dots
(QDs) DRs have been shown to have superior absorption
cross-sections and optical gain lifetimes making them
contenders for laser applications [2]. These structures offer
potential efficiency improvements due to their favorable
emission of light for display applications [1], lighting,
photovoltaics [3], light emitting diodes [4] and luminescent
probes for in vivo medical diagnostics [5].
We have not previously been able to show our findings
where simultaneous scanning transmission electron
microscopy (STEM) using high angle annular dark field
(HAADF) and simultaneous cathodoluminescent (CL)
imaging and simultaneous electron energy loss
spectroscopy (EELS) have been collected. Instead machine
setup and sample have only allowed sequential
measurements. In this work we are now showing these
measurements which will facilitate more detailed and
scientific study of this class of important new display
materials on a more routine basis.
The objective of this work was to:
Add to our portfolio of CL imaging data for luminescent
materials with supplementary evidence supplied from
simultaneous EELS/CL/HAADF data on CdSe/CdS
core/shell quantum dot-in-rods.
2.
Experimental
Combined STEM-CL-EELS was performed on a
JEOL2100F FEG TEM (JEOL, Japan), equipped with a
Gatan VulcanTM CL detector (Gatan, USA) for
cathodluminescence imaging and spectroscopy, and Gatan
GIF Quantum SE (Gatan, USA) for EELS spectroscopy.
The TEM was set to STEM mode with the STEM images
captured using a Gatan HAADF detector (Gatan, USA).
Prior to analysis, samples were cooled to 103K (-170oC)
within the STEM, using a built in cryostat connected to the
Vulcan sample holder. Once the temperature was stable,
image acquisition and analysis was performed using Gatan
Microscopy Suite (GMS) (Gatan, USA).
Using GMS enabled the combined acquisition of visible
light, acquired from the Vulcan system set to imaging
mode, simultaneously with the collection of HAADF
STEM images from individual particles. Switching the
Vulcan to spectroscopy mode enabled the acquisition of
CL-spectrum as well as the simultaneous acquisition of
EELS spectra from the GIF. Moreover, having the TEM in
STEM mode, as well as the Vulcan and GIF systems set to
spectroscopy modes, enabled mapping of the individual
particles and identification of areas of interest.
3.
Results
Figure 1a shows the high angle annular dark field
(HAADF) STEM image and in Figure 1b is the CL image
the DRs where the dark speckle is due to random noise
from the photomultiplier tube detector. The heavier shaded
areas show good alignment to the position of the dot-inrods on close examination. Electronic overlay of the files
helps to make this clear but it is very difficult to represent
this in printed format. Our prior work on DRs [6,7],
quantum dots [8], dot-in-rods [9] and nano-phosphors [10]
using this TEM-CL technique show perfect overlap of the
dark field (DF) STEM and CL images and this is consistent
here. To help identify the CL emissions the Figure 1c
shows the intensity of the CL emission as a histogram that
clearly shows the position and distance of 12.61nm over
which the CL emission occurs. This is taken from top to
bottom of the blue rectangle shown in (b) and from left to
right in (c). The dark blue arrows also highlight the overlay
between these two images. We have previously shown such
images from QD Vision red quantum dots at this conference
[8] but it has been our quest to demonstrate the potential to
simultaneously collect the EELS data to show that the CL is
involved in the emission of this visible light.
Previously we have shown the CL emission spectrum for this
materials collected in the STEM-CL[6]. Due to the high
beam current in a very small area it is not possible to hold the
beam on a single DR and collect a CL emission spectrum. The
observed 622 nm emission peak has a full width at half height
of 59 nm and was obtained whilst scanning the beam across a
small ensemble of particles (~50 particles). It is likely that we
are observing some beam damage to the sample, local heating
and surface coating removal leading to modification of the
emission spectrum.
The particles are very beam sensitive but it has been possible
due to the increased size of some emissive areas (wider
sections of the particles) of these DRs to carry out a second
scan of the CL emissive areas to collect the red filtered photomultiplier tube image.
Hence the rectangular area highlighted in Figure 2 (spectrum
image) was selected due to the strong red filtered CL signal
that was observed after the first scan (see Figure 1b).
Previously it has not been possible with our instrumentation to
observe the signal after one full scan of any particles, be it
quantum dots or DRs because the CL emission intensity was
rapidly lost. This loss of signal could be due to ligand or
adsorbed molecule loss at the surface. It is also apparent that
these particles are very beam sensitive. The circle (Figure 2)
highlights the position of a particle that has almost completely
been evaporated away due to the electron beam damage after
an attempt to unsuccessfully map this particle (scanning from
left to right). It is therefore found that it is important to scan
across the short axis of the particle than along the rod so that
the beam damage is reduced as much as practicable, similarly
it was felt necessary to use sub-pixel scanning whilst mapping
the particles shown in Figure 2.
Figure 1. (a) Dark field STEM image, (b) Image collected using
a 625(50) nm band pass filter of the CL signal, (c) histogram
representation extracted from (b) showing the visible light
emission.
Figure 2, Composite overlay of previous images in figure 1.
Figure 3 is shows the ultimate goal that we have been
seeking for nearly 3 years in terms of data collection. It
shows side by side the simultaneously collected HAADFSTEM, Se EELS map (X-ray removed and edge extracted)
and 625(50)nm band pass filtered CL map. This has been
possible to achieve even after the initial scan that was taken
to enable the microscope setup to be made. This confirms
the result that we have previously alluded to in the fact that
we are now able to confirm the position of the Se
containing dot in the DR whilst at the same time being able
to collect the HAADF map and the CL map.
Figure 4 illustrates that this is not the case, see the red
arrow to show the weak CL emission position (this is weak
due to this being from a second scan of the electron beam)
and the green arrow that shows the Se is located away from
the position of where the visible light emission is observed.
EELS is a technique based on the direct beam and therefore
is not subject to loss of spatial resolution. The CL signal
however is a secondary effect and therefore much more
care needs to be taken over the interpretation of this result.
This is why the simultaneous collection of the HAADFCL-EELS data is so important and is why we feel that this
data needs reporting at this stage.
4.
Impact
We have shown that in DRs the simultaneous HAADF, CL
imaging and EELS scan that can be used to improve the
understanding of the energy transfer process between the
dot and rod.
Acknowledgements
We are grateful to the EPSRC and Technology Strategy
Board
(TSB)
for
funding
the
PURPOSE
(TP11/MFE/6/1/AA129F; EP-SRC TS/G000271/1) and
CONVERTED (JeS no. TS/1003053/1), PRISM
(EP/N508974/1), HTRaD (EP/L504671/1) and FAB3D
programs. We are also grateful to the TSB for funding the
CONVERT program.
References
Figure 3, a) HAADF-STEM, b) Se EELS map and c) CL
intensity map of a single DR shown in figure 2 in the
rectangle highlighted as the spectrum image.
Figure 4 expresses Figure 3 in a graphical form to show the
HAADF, CL and EELS signal. It is possible to observe the
onset of the CL emission before the electron beam has
made contact with the dot. We are still trying to understand
why this occurs, possible explanations include the transfer
of energy in the CdS rod or a more innocent explanation
could be due to the true electron beam spot position being
larger than expected. However the latter idea might not be
relevant since the simultaneous collection should have
shown a Se signal at the same position.
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