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Silicon QDs and SiLEDs

In close collaboration with the groups of Prof. Geoffrey Ozin, Prof. Uli Lemmer, and Prof. Annie Powell we have been characterizing (size selected) silicon QDs as an approach for green nanochemistry to explore their potential applications, e.g. as active components in organic LEDs (SiLEDs) or as non-toxic labels.  

 

1. Characterization of size selected silicon quantum dots

Silicon QDs, prepared by disproportionation of a sub-oxide, followed by HF etching and covalent capping with an organic ligand, have been size separated by ultracentrifugation or fractionated crystallization resulting in highly mono-disperse ncSi fraction. The core-shell structure of the particles can be directly visualized by EFTEM imaging.

Silicon and Carbon EFTEM maps of Si-C18 Si QD HRTEM

Fig 1.1: EFTEM maps for Si (left) and carbon (middle) confirming the core/shell structure of the Si SQs. The HRTEM image (right) shows a twined nanocrystal often observed for the larger particles.

 

The sized measured by HAADF-STEM are in good agreement with the optical spectra of the size selected particles. 

 Size fractionation of ncSi

Fig. 2.1: Ultracentrifugation of the core/shell silicon nanocrystals resulting in size selected fractions clearly visible directly from their color. The HAADF-STEM images quantitatively confirm the size differences.

 

Details to this work have been published at

  • M. Mastronardi, F. Maier-Flaig, D. Faulkner, E. Henderson, C. Kübel, U. Lemmer, G.A. Ozin, "Size-Dependent Absolute Quantum Yields for Size-Separated Colloidally-Stable Silicon Nanocrystals", Nano Lett., 2012, 12, 337-342; DOI: 10.1021/nl2036194.
  • F. Maier-Flaig, E. Henderson, S. Valouch, S. Klinkhammer, C. Kübel, G. Ozin, U. Lemmer, "Photophysics of Organically-Capped Silicon Nanocrystals – A Closer Look into Silicon Nanocrystal Luminescence Using Low Temperature Transient Spectroscopy", Chem. Phys., 2012, 405, 175-180; DOI: 10.1016/j.chemphys.2012.07.009
  • M. Mastronardi, F. Hennrich, E. Henderson, F. Maier-Flaig, C. Blum, J. Reichenbach, U. Lemmer, C. Kübel, D. Wang, M. Kappes, G. Ozin, "Preparation of Monodisperse Silicon Nanocrystals Using Density Gradient Ultracentrifugation", J. Am. Chem. Soc., 2011, 133(31), 11928-11931, DOI: 10.1021/ja204865t.

 

2. Organic LEDs with silicon QDs (SiLEDs)

The structure and composition of SiLEDs had been characterized by a combination of HAADF-STEM, EDX and EFTEM. Despite the beam sensitivity of the organic materials, it was possible to directly image all different layers and to look at their composition, clearly revealing the layer structure in these devices.

SiLED Structure

 Fig. 1.2: HAADF-STEM overview image with STEM-EDX and EFTEM maps of the elemental distribution of the different organic and Si QD layers.

During operation at high operation voltage, the overall structure stays intact, but there is high diffusion of the silicon nanoparticles into the TPBi layer, in agreement with the fact that the highest potential drop occurs across this interface.

SiLED - diffusion of ncSi SiLED non-size selected

Fig 2.2: HAADF-STEM overview image of the Si QDs after diffusion at high applied current (let) and ultra-small Si QD nanoparticle clouds diffusing into the TBPi layer in case of non-size separated SiLEDs.

When comparing the structure and performance of size selected and not-size selected SiLEDs, that typically exhibit a ~10x reduced life time, it became clear that ultra-small Si QDs present without size-selection are one of the limiting factors as they quickly diffuse into the TPBi layer and can cause short circuits.

 

Details of this work have been published at

  • F. Maier-Flaig, C. Kübel, J. Rinck, T. Bocksrocker, T. Scherer, R. Prang, A.K. Powell, G.A. Ozin, U. Lemmer, "Looking Inside a Working SiLED", Nano Letters, 2013, 13(8), 3539–3545; DOI: 10.1021/nl400975u.
  • F. Maier-Flaig, M. Stephan, T. Bocksrocker, M. Bruns, C. Kübel, J. Rinck, A.K. Powell, G.A. Ozin, U. Lemmer, "Multicolour Silicon Light Emitting Diodes (SiLEDs)", Nano Letters, 2013, 13, 475-480; DOI: 10.1021/nl3038689.