MBE overgrowth of ex‐situ prepared CdSe colloidal nanocrystals

Abstract

We present a growth technique, which combines molecular beam epitaxy of ZnSe and externally wet‐chemically prepared, colloidal NCs of CdSe to achieve fully integrated monolithic epitaxial heterostructures. Our results show that wet‐chemically prepared semiconductor nanocrystals can be incorporated in an epitaxally grown crystalline cap layer. We investigated CdSe core, CdSe/ZnSe and CdSe/ZnS core/shell nanocrystals (NCs) overgrown with cap layers of ZnSe, where the thickness was varied between 20‐40 nm. In this paper we discuss PL measurements of overgrown NCs as a function of the cap layer thickness and compare the results with the PL of NCs in solution. A distinct blue shift of the PL is observed when the core/shell dots are overgrown by ZnSe. We present a model which explains this blue shift as resulting from dissolution of the shell of the dots during the overgrowth (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced photoluminescence of colloidal nanocrystals embedded in epitaxially grown semiconductor microstructures

“…However, the quantitative amount of the redshift is significantly different among the NC types. 14 The PL peak energy shifts (i.e., blueshift between solution and as-grown samples in case of core/shell NCs and redshift between as-grown and annealed samples for all NC types) can be explained by modeling the band diagram of spherical NCs embedded in ZnSe epitaxial layers, based on the following three assumptions: (I) the solution around the core/shell NC is considered as a quasivacuum and provides a confinement potential of about 12 eV for the NCs, 22 which is equally distributed relative to the valence band edge of the NCs. (II) During overgrowth of deposited core/shell type NCs the 2 ML thick shell degenerates most likely due to interdiffusion or segregation of excess selenium atoms into the NCs.…”

“…13 We have recently demonstrated the deposition of CdSe NCs on top of epitaxially grown ZnSe layers with a sophisticated spray-coating method followed by the overgrowth with ZnSe by molecular beam epitaxy (MBE) to achieve complete integration of the NCs in semiconductor material. 14 The PL emission from such epitaxial-chemical hybrid (ECH) structures was observed, which demonstrates the improvement of the stability of the optical properties when the colloidal NCs are integrated in the ZnSe host crystal. 1 Although the larger band-gap energy of ZnSe versus CdSe provides a strong confinement potential for the CdSe NCs in the ECH structures, the PL emission intensity from the NCs was not substantially amplified.…”

“…In this context a quantitative analysis of the PL intensity of ECH structures with different cap layer thicknesses confirmed that the main fraction of the PL emission results from direct absorption of excitation power in the NCs. 14 In addition a blueshift of the PL energy of core/shell type NCs embedded in the ECH structure relative to that in solution was observed. These two facts indicate a disturbance of the interface between the epitaxial layer and the NCs, providing a barrier that prevents carrier diffusion and radiative recombination in the NCs.…”

Doping MAPbBr$_3$ hybrid perovskites with CdSe/CdZnS quantum dots: from emissive thin-films to hybrid single-photon sources

“…As a consequence, substantial research effort has been dedicated to CdSe QDs in LEDs, 18 solar cells, 19,20 solar concentrators 21 or as SPS and building blocks for quantum-information devices. [22][23][24][25] Unfortunately, the fragility of this class of emitters during encapsulation process 26,27 and the lack of compatible semiconductor host matrices has rendered difficult their incorporation in crystalline, fast charge-transfer devices analog to what was achieved with state-of-the-art InGaAs diodes. 28 The semiconductor nature of hybrid perovskites and the process compatibility between CdSe-QD synthesis and perovskite thinfilm elaboration (soft chemistry) suggests that combining the remarkable properties of these two materials is an intrinsically interesting route for applications such as light converters (solar concentrator or LEDs at high doping level) and single photon sources (at low doping level).…”

Exciton States in Free-Standing and Embedded Semiconductor Nanocrystals

“…It means that the tail of the wavefunction inside the barrier (the first shell) is smaller than 3ML. Finally, when NCs are embedded into an epitaxial semiconductor matrix, a blue shift of the PL peak is observed, compared to the emission spectrum of the same NCs in organic solvent Rashad et al (2010); Woggon et al (2005). This effect is not straightforward to explain, because, in a simple view, the replacement of TOPO by a semiconductor matrix with a band structure similar to that of the QD materials should result in lower barriers at the interface and, consequently, in a weaker confinement and a red shift of the exciton transition.…”