Tunable band structure in coreshell quantum dots through alloying of the core

Transcription

1 Tunable band structure in coreshell quantum dots through alloying of the core A. Guille ǀ D. Mourad ǀ T. Aubert ǀ A. Houtepen ǀ R. Van Deun ǀ E. Brainis ǀ Z. Hens

2 1. Introduction Semiconductor nanocrystals High tunability Different materials available (CdSe, PbSe, CdS, ZnS, ) Influence of size (quantum confinement) Possibility to synthesize core/shell structures CdSe QDs : Increasing size Additional degree of freedom : Composition

3 1. Introduction Tunability of CdS (1-x) Se x \ZnSe QDs Homogeneously alloyed CdS (1-x) S x core Tuning of overlap of charge carriers wavefunctions Single exciton gain demonstrated in Type II CdS / ZnSe

4 1. Introduction Quantum dots as gain medium : Advantages of type II QDs Absorption Stimulated emission Gain requires more than one exciton per dot : High excitation fluence Klimov et al., Nature 2007, 447, Reduced reabsorption : Reduced excitation fluence

5 1. Introduction Band offsets (ev) Overlap of charge carriers Valence band-offset CdS (1-x) S x \ ZnSe Calculated numerically with tight binding approach (D. Mourad) From type II to type I 1/2 Se ratio: Valence band Conduction band Se ratio

6 1. Introduction Overlap of charge carriers and gain threshold Calculation in effective mass approximation with calculated band offsets Electron-hole overlap CdS CdSe 0.5 S 0.5 CdSe Gain threshold

7 Introduction QDs synthesis and characterisation Results and discussion Conclusion

8 2. Synthesis Hot injection synthesis of CdS (1-x) Se x QDs T. Aubert et al., Chem. Mater. 2013, 25, Precursors: Se powder dispersed in ODE S dissolved in ODE Cd oleate Balanced reactivity of S and Se precursors Composition measured by EDX Alloying checked with Raman spectroscopy Controlled Se/S ratio in ODE ODE + CdOA 270 C

9 2. Synthesis Growth of ZnSe shell Cores in ODE and octadecylamine Continuous injection of Zn oleate and TOP-Se Stored in hexane Core Core-shell CdS 0.6 Se 0.4 CdS 0.6 Se 0.4 / ZnSe

10 2. Characterisation Normalized absorbance (a.u.) Intensity (a.u.) Normalized absorbance (a.u.) Absorption and emission spectra Core Sulfur ratio x: ,0 0,8 0,6 0,4 Core-shell Sulfur ratio x: , , Core radius: 1.5 nm Shell thickness: 1.5 nm Trap emission in S rich core QDs Excitonic peak appears in Se rich cores Transition from type II to type I 1/2 1,0 Sulfur ratio x: 1 0, , ,4 0 0,2 0,

11 2. Characterisation Intensity (a.u.) Normalized absorbance Origin of trap emission in CdS/ZnSe Larger cores : trap emission visible Different PLE spectra and decays Absorption Emission PLE exciton emission PLE trap emission Increased absorption : Effect on gain? Exciton emission Trap emission Time (ns)

12 Introduction QDs synthesis and characterisation Results and discussion Conclusion

13 3. Results and discussion Transient absorption spectroscopy Absorption spectrum at different pump-probe delays CdS\ZnSe (2) (1) CdS Core (2) (1) ZnSe Shell Evidence for the type II structure

14 3. Results and discussion Transition Type II to type I 1/2 CdS 1.0 Se 0.0 CdS 0.8 Se 0.2 CdS 0.7 Se 0.3

15 Absorbance log(i 0 /I) Absorbance log(i 0 /I) 3. Results and discussion Absorption spectrum of excited QDs : Optical gain 0.12 Core : CdS 1.0 Se 0.0 Core : CdS 0.8 Se 0.2 Core : CdS 0.6 Se Emission + Emission + Emission Core : CdS 0.4 Se 0.6 Core : CdS 0.2 Se 0.8 Core : CdS 0.0 Se Emission + Emission + Emission

16 Introduction QDs synthesis and characterisation Results and discussion Conclusion

17 4. Conclusion Synthesis of CdS (1-x) Se x / ZnSe QDs Absorption and emission from traps at the interface Excess of absorbance at the emission wavelength : synthesis to improve Tunability of QDs band structure Continuous transition from type II to type I ½ demonstrated Very large tunability of emission wavelength and overlap Gain observed with transient absorption spectroscopy Core : CdS 0.6 Se 0.4 Core : CdS 0.0 Se Emission + Emission

18 Thank you for your attention!

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