Red quantum rods under the electron microscope

George R. Fern, Jack Silver, Tobias Jochum*, Jan S. Niehaus*, Frank Schröder-Oeynhausen* and Horst Weller*

Centre for Phosphors and Display Materials, Wolfson Centre for Materials Processing, Brunel University, Uxbridge, Middlesex UB8 3PH

* Centrum für Angewandte Nanotechnologie (CAN) GmbH, Grindelallee 117, 20146 Hamburg

Keywords: Quantum Dot Display, Quantum Rod, Dot-in-Rod, Cathodo-luminescence imaging

ABSTRACTCathodoluminescent (CL) imaging of the

visible light emitted from quantum dot in rods (DRs) is reported. Their shape and uniformity is observed in the images and the image of the particles created from their visible light collected simultaneously is shown. The CL emission spectrum collected from the DRs is reported.

1. IntroductionWhilst the battle continues for the successor to current display technologies one candidate is possible that utilizes CdSe quantum dots with an elongated shell of CdS, the so called quantum rods or dot-in-rod (DR) with a potential for a 3 fold increase in brightness.1 Hence this property has the potential to yield significant improvements to mobile device battery performance. These materials are believed to possess the same useful features such as observed in quantum dots whereby a large colour gamut can be achieved and hence should meet the performance requirements of ultra-high definition televisions e.g. Rec 2020, with a wide colour gamut and high colour purity (colour saturation). Whilst quantum dots are already commercialized, e.g. Kindle Fire, Sony Bravia LCD televisions, etc2, 3 no quantum rod displays are commercially available. However DRs are able to emit directional and polarized light which means their use in backlighting for liquid crystal displays is an obvious candidate for investigation. The DRs have also been successfully functionalized for use in aqueous media for use as biological imaging agents.4,5

The objectives of this work were to compare: The cathodoluminescence spectra of DR to

see how they vary from dot to dot. How the shape of the DR influence the

emission properties.

2. ExperimentalThe DR were synthesized by the published procedure.6 A dilute dispersion of the DR sample was prepared and dropped onto a holey carbon 300 mesh copper sample grid. The sample was allowed to air dry without further cleaning so as not to damage any surface coating on the particles.The sample was observed in a JEOL 2100F Transmission Electron Microscope (TEM) at 100 kV at a temperature of -170 oC. The TEM was operated in scanning mode with a spot size of 0.7 nm. To reduce X-ray excitation of the sample the hard X-ray aperture was inserted which reduces the background noise in CL imaging mode. The TEM was fitted with the Gatan Vulcan CL imaging and spectroscopic detectors. This spectroscopic analysis was carried out using a Czerny-Turner spectrometer and back-illuminated CCD and a grating with 150 lines/mm. Light was collected from the sample using a mirror above and below the sample which gives a solid angle of about 5 sr. The high solid angle makes light collection highly efficient and makes it possible to collect the light at very low intensity. By collecting the visible light with the Vulcan system simultaneously with the JEOL high angle anullar dark field (HAADF) detector it was possible to observe the visible light that was emitted from the particles. Photo luminescent spectra were collected using a Horiba Fluorolog spectrometer.

Figure 1 Photoluminescent emission from DRs.

3. ResultsFigure 1 shows the emission from the quantum rods demonstrating a range of colours suitable for displays (in combination with the direct blue LED emission). Figure 2 shows the absorption and emission of the DRs. The gigantic absorption at wavelengths smaller than 450 nm is based on the elongated CdS

shell, whereas the first absorption maximum at 618 nm is due to the spherical CdSe core. The emission

maximum is located at 637 nm (excitation wavelength was 530 nm) and offers an FWHM value of 36 nm. The quantum yield was measured against Rhodamine 6G and offers 63 %. Figure 3 shows the low magnification TEM image of the DR

nanometre-sized dispersion and shows the uniform size (6 nm x 60 nm) and ease with which the sample is dispersed onto the support film. These DRs are consistent with the many and varied shapes that can be formed by using various modifications during the synthetic procedure.7 In Figure 4 the high resolution TEM image is shown. It is noticeable that the quantum rods are all lying in a well separated manner by about 2 nm,

Figure 2 Absorption and emission spectra from DRs. Excitation wavelength is 530 nm.

Figure 3 TEM image of red light emitting DRs on a holey carbon grid

Figure 4 HRTEM image of red DRs.

Figure 5 DRs Simultaneous dark field STEM image (top) and CL light

image (middle) and (bottom) where both the STEM and CL images are overlaid.

probably due to the surface organic coating of the rods used to influence rod shaped growth which is transparent in the 100 kV electron beam used.The HAADF STEM image is shown in Figure 5 (top) and the DRs are observed with a tadpole-like shape. In Figure 5 (middle) the cathodoluminescent image is shown that was collected simultaneously with 5 (top) and a large number of the DRs are seen to be CL emissive. Careful observation shows that this image is formed, (CL light detected) when the electron beam is close to the wider section of the quantum rod and hence shows that the light is generated in the rod from this region only. When the electron beam is further from the core then CL emission is absent and the DR is not fully observed in the image 5 (middle). Our previous publications on quantum dots8 and nano-phosphors9, 10 using this TEM-CL technique show perfect overlap of the HAADF STEM and CL images. These DRs depart from this behavior and clearly illustrate the power of this technique in being able to distinguish the distance along the rod from the shell that is able to interact with the excited state to yield CL emission from the core. Figure 5 (bottom) shows 5 (top and middle) images overlain where 5 (middle) has been coloured red and unlike these previous publications only part of the particle is shown, however the CL image shows perfect alignment to the HAADF image with just the tail of the shell being absent in the CL active particles. The results induce the assumption that the emission is mainly generated from the quantum dots and its narrow surrounding. Still the absorption cross-

section of the respective material does have an influence as well. Different thickness measurements of further DR systems are necessary to explain that effect in detail. In Figure 6 the sharp red line represents the cathodo-luminescent emission spectrum collected in the electron microscope. The spectrum has been fitted to a Lorentzian line shape and shows only slight asymmetry with the centre observed at 622 nm and the cathodo-luminescent full line width at half maximum is 59 nm. This broadening from the PL data shown in figure 2 is in keeping with broadening seen in other quantum dot materials that we have studied.8

4. Impact The shape and uniformity of individual

particles can be identified. Particles can be identified and the visible light

emission observed from individual quantum rods.

Quantum rods are separated by the organic coating and do not show evidence for energy transfer between particles.

The -170 oC CL emission spectrum is reported for the DR ensemble.

This work represents a significant advance towards reconciling the visible light emission properties of individual quantum rods with the size and shape of the particles.

5. AcknowledgmentsWe are grateful to the Technology Strategy Board (TSB) (UK) for substantial financial funding in the form of TSB Technology programs for the PLACES, FAB3D, ACTIVEL, SHAPEL, BEDS and HTRaD programs and to our many industrial collaborators on these programs that have allowed us to develop our knowledge and capability in this field. Thanks are also due to Ashley Howkins, TEM technician in Brunel ETC.

6. References1. “Merck on Quantum Rod-based Displays,

Dertinger, S., Interview with N. Tanaka, Nikkei Electronics”, 8 May 2014, http://techon.nikkeibp.co.jp/english/NEWS_EN/20140508/350462/?P=1 (25/6/2014)

2. “Quantum dots go on display: Adoption by TV makers could expand the market for light-emitting nanocrystals”, Bourzac, K., Nature, 493, 283 (2013).

3. “Quantum Dots: The Ultimate Down-Conversion Material for LCD Displays”,

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Wavelength (nm)Figure 6 CL emission spectrum of the DRs taken at -170 oC in the electron microscope. The red line represents the

Lorentzian line fit to the data with a centre channel at 622nm and a full

width at half height of 59nm

Steckel, J.S., Ho, J., Hamilton, C., Breen, C., Liu, W., Allen, P., Xi, J. & Coe-Sullivan, S., SID Symposium Digest of Technical Papers, (2014).

4. “Production and biofunctionalization of elongated semiconducting nanocrystals for ex-vivo applications” T. Jochum, D. Ness, M. Dieckmann, K. Werner, J. Niehaus and H. Weller, MRS Online Proceedings Library, 1635 (2014).

5. “CdSe/CdS-quantum rods: fluorescent probes for in vivo two-photon laser scanning microscopy”, J. Dimitrijevic, L. Krapf, C. Wolter, C. Schmidtke, J.-P. Merkl, T. Jochum, A. Kornowski, A. Schüth, A. Gebert, G. Hüttmann, T. Vossmeyer, H. Weller, Nanoscale, 6, 10413-10422 (2014).

6. “Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/CdS Nanorods Prepared by a Seeded Growth Approach” L. Carbone, C. Nobile, M. De Giogi, F. Della Sala, G. Morello, P. Pompa, M. Hytch, E. Snoeck, A. Fiore, I. R. Franchine, M. Nadasan, A. F. Silvestre, L. Chiodo, S. Kudera, R. Cingolani, R. Krahne, and L. Manna, Nano Lett. 7, 2942-2950 (2007).

7. “Colloidal Quantum Rods and Wells for Lighting and Lasing Applications”, C. She, I. Fedin, D. Dolzhnikov, D. Talapin, R. Schaller M. Pelton, M. Boles SID Symposium Digest of Technical Papers, (2014).

8. “Red Quantum Dots under the electron microscope.” G.R. Fern, J. Silver, S. Coe-Sullivan and J.S. Steckel, SID Symposium Digest of Technical Papers, (2014).

9. “Cathodoluminescence spectra of single Y2O3:Tb3+ nanometer sized phosphor crystals excited in a field emission scanning transmission microscope.” J. Silver, X. Yan, G.R. Fern and N. Wilkinson, Proc. IDW, 823-826, (2013).

10.“Cathodoluminescence Spectra of Single Gd2O2S:Tb3+ Nanometer Sized Phosphor Crystals Excited in a Field Emission Scanning Transmission Electron Microscope” G.R. Fern, X. Yan, N. Wilkinson and J. Silver, Proc. IDW, 820-822, (2013).