Interfacial alloying in CdSe/CdS heteronanocrystals, a Raman spectroscopy analysis

Transcription

1 Interfacial alloying in CdSe/CdS heteronanocrystals, a Raman spectroscopy analysis Norman Tschirner,, Holger Lange, Andrei Schliwa, Amelie Biermann, Christian Thomsen, Karel Lambert,, Raquel Gomes,, and Zeger Hens,, Institut für Festkörperphysik, Technische Universität Berlin, Germany, Physics and Chemistry of Nanostructures, Ghent University, Belgium, and Center for Nano- and Biophotonics (NB-Photonics), Ghent University, Belgium notsch359@yahoo.de; zeger.hens@ugent.be Supporting Information Optical properties of CdSe/CdS heteronanocrystals Monitoring CdSe/CdS shell growth by absorption spectroscopy Figure 1(a) displays a series of absorbance spectra, normalized at the wavelength λ 0 of the first exciton peak, of aliquots taken during a SILAR coating where 5 CdS layers have been grown. In line with published results, the CdS shell deposition leads to a dramatic increase of the absorbance below 500 nm, corresponding with the onset of the absorption in bulk CdS (around 512 nm). 1 3 In addition, λ 0 (Figure 1(b)) exhibits a progressive redshift upon CdS shell deposition, especially To whom correspondence should be addressed Institut für Festkörperphysik, Technische Universität Berlin, Germany UGent Center for Nano- and Biophotonics (NB-Photonics), Ghent University, Belgium 1

2 Figure 1: (a) Normalized absorbance spectra taken during the progressive growth of 5 CdS shells around a zincblende CdSe core. (b) Wavelength λ 0 of the first exciton peak after each step in the SILAR procedure. (c) Overlay of the absorbance spectrum of CdSe QDs (blue) as-synthesized and (red) after annealing in octadecylamine at 225 C. for the first two layers. These shifts can be attributed to the larger extension of the electronic wave function, which leaks into the CdS shell. 1,3 Although λ 0 shows a clear redshift after each CdS layer added, the effects for the Cd and S additions are not the same. Opposite to what was reported for wz-cdse, we find that this redshift occurs almost exclusively after the Cd additions (see Fig. Figure 1(b)). Elemental analysis by x-ray fluorescence confirms that the addition of the S precursor leads to the adsorption of sulfur at the CdSe surface, regardless of the addition of the Cd precursor. Hence, the lack of redshift upon S addition does not reflect a lack of reactivity of the S precursor. Possibly, S diffuses somewhat into the CdSe core which apart from interfacial alloying leads to an enhanced confinement of the hole and thus a reduced redshift. Changes in the absorbance spectrum are also observed during the degassing process, before any precursor addition. Figure 1(c) shows the normalized spectra of 4.0 nm CdSe QDs dissolved in an ODE-ODA mixture after 1 h degassing at 100 C, and 10 min after the temperature was raised to 225 C. Whereas λ 0 remains unchanged, the position of the second exciton transition (2S (h) - 1S (e) ) shifts towards lower energies upon heating. This change has been observed before, and was attributed to the occurence of wurtzite/zincblende polytypism induced by primary amines. 4 2

3 Figure 2: PL spectra and quantum yields of (bc) bare CdSe QDs and (csn) different CdSe/nCdS core/shell quantum dots. Photoluminescence enhancement by CdS shell growth Figure 2(a) displays photoluminescence spectra of CdSe core QDs and CdSe/nCdS core/shell QDs with n=2,3 and 5. In line with the redshift of λ 0, also the PL spectrum shows a systematic redshift with increasing CdS shell thickness. More importantly, the PLQY systematically increases with n, up to a value of 35% after five CdS layers (see Figure 2(b)). Unlike the uncoated Q-CdSe, the CdSe/CdS QDs are highly stable against oxidation. CdSe/CdS monolayers stored in air are luminescent for weeks. s Characterization of Cd(Se,S) alloy nanocrystals CdS 0.5 Se 0.5 was synthesized by co-injection of equal amounts of Se and S ODE precursors as outlined in the experimental information. Figure 3 represents the basic materials characteristics of the purified material. A number of elements indicate that the material corresponds to Cd(Se,S) alloy quantum dots: 1. The powder XRD diffractogram (Figure 3(c)) confirms that the resulting material has the zincblende crystal structure with reflections in between those of CdSe (blue) and CdS (red). 2. The size histogram (Figure 3(d)) yields an average diameter of 2.92 nm. On the other hand, 3

4 Figure 3: (a) Powder XRD diffractogram of CdSe0.5 S0.5 QDs, giving the expected reflections for (blue) zinc blende CdSe and (red) CdS as a reference. (b) Overview TEM image, the scale bar corresponds to 10 nm. (c) Size histogram, yielding an average diameter of 2.92 nm. (d) Normalized absorbance and emission spectra. (e) X-ray fluorescence spectrum of CdSe0.5 S0.5 dissolved in toluene using Cu as a secondary target. The regions of the Cu and the Mo emission are not shown for clarity. the first exciton transition has its peak at 488 nm (Figure 3(a)). This is in between the wavelength expected for CdSe (535 nm) 5 or CdS (385 nm) 6 nanocrystals of this size and agrees with literature data on the relation between band gap, size and composition of Cd(Se,S) alloys The X-ray fluorescence spectrum demonstrates the presence of Cd, Se and S (seefigure 3(e)). A quantitative analysis using the Rigaku fundamental parameters software package and XRF spectra obtained with all secondary targets yields a S/Se ratio of

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