Insight into Strain Effects on Band Alignment Shifts, Carrier Localization and Recombination Kinetics in CdTe/CdS Core/Shell Quantum Dots

This work shows that ZnTe/CdSe core/shell quantum dots synthesized by standard literature procedures in actuality have an alloyed CdxZn1-xTe core. We employ X-ray absorption spectroscopy (XAS) at all four K-shell ionization edges (Zn, Te, Cd, Se) and perform a global fitting analysis in order to extract the first-shell bond distances. We combine our XAS results with transmission electron microscopy (TEM) sizing and elemental analyses, which allows us to propose models of the internal particle structure. Our multimodal characterization approach confirms (1) the presence of Cd-Te bonds, (2) cation alloying in the particle core (and the absence of anion alloying), and (3) a patchy pure-phase CdSe shell. We synthesize particles of different shell thicknesses and performed synthetic control studies that allowed us to discard a ZnTe/CdTe/CdSe core/shell/shell structure and confirm the alloyed core/shell structure. Our structural analysis is extended with electronic band structure calculations and UV/vis absorption spectroscopy, demonstrating that the alloyed CdxZn1-xTe/CdSe core/shell quantum dots exhibit a direct band gap, different from the predicted type-II band alignment of the intended ZnTe/CdSe core/shell quantum dots. This study highlights the challenges with synthesizing II-VI quantum dot heterostructures and the power of XAS for understanding the internal structure of heterogenous nanoparticles.

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Internal Atomic-Scale Structure Determination and Band Alignment of II-VI Quantum Dot Heterostructures

10.26434/chemrxiv.11349713 ◽

2019 ◽

Author(s):

Cecilia Gentle ◽

Yuanheng Wang ◽

Tyler N. Haddock ◽

Conner P. Dykstra ◽

Renske M. van der Veen

Keyword(s):

Quantum Dots ◽

Quantum Dot ◽

Absorption Spectroscopy ◽

Shell Structure ◽

Electronic Band Structure ◽

Band Alignment ◽

Atomic Scale ◽

Core Shell ◽

Cdse Core ◽

Electronic Band

This work shows that ZnTe/CdSe core/shell quantum dots synthesized by standard literature procedures in actuality have an alloyed CdxZn1-xTe core. We employ X-ray absorption spectroscopy (XAS) at all four K-shell ionization edges (Zn, Te, Cd, Se) and perform a global fitting analysis in order to extract the first-shell bond distances. We combine our XAS results with transmission electron microscopy (TEM) sizing and elemental analyses, which allows us to propose models of the internal particle structure. Our multimodal characterization approach confirms (1) the presence of Cd-Te bonds, (2) cation alloying in the particle core (and the absence of anion alloying), and (3) a patchy pure-phase CdSe shell. We synthesize particles of different shell thicknesses and performed synthetic control studies that allowed us to discard a ZnTe/CdTe/CdSe core/shell/shell structure and confirm the alloyed core/shell structure. Our structural analysis is extended with electronic band structure calculations and UV/vis absorption spectroscopy, demonstrating that the alloyed CdxZn1-xTe/CdSe core/shell quantum dots exhibit a direct band gap, different from the predicted type-II band alignment of the intended ZnTe/CdSe core/shell quantum dots. This study highlights the challenges with synthesizing II-VI quantum dot heterostructures and the power of XAS for understanding the internal structure of heterogenous nanoparticles.

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Influence of carrier localization at the core/shell interface on the temperature dependence of the Stokes shift and the photoluminescence decay time in CdTe/CdS type-II quantum dots

Physical Review B ◽

10.1103/physrevb.96.035305 ◽

2017 ◽

Vol 96 (3) ◽

Cited By ~ 9

Author(s):

Taichi Watanabe ◽

Kohji Takahashi ◽

Kunio Shimura ◽

DaeGwi Kim

Keyword(s):

Quantum Dots ◽

Temperature Dependence ◽

Decay Time ◽

Stokes Shift ◽

Core Shell ◽

Type Ii ◽

Carrier Localization ◽

The Core ◽

Photoluminescence Decay

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Strain Effects on the Band Gap and Diameter of CdSe Core and CdSe/ZnS Core/Shell Quantum Dots at Any Temperature

Advances in Materials Science and Engineering ◽

10.1155/2019/3764395 ◽

2019 ◽

Vol 2019 ◽

pp. 1-10 ◽

Cited By ~ 4

Author(s):

Md. Rezaul Karim ◽

Mesut Balaban ◽

Hilmi Ünlü

Keyword(s):

Quantum Dots ◽

Band Gap ◽

Compressive Strain ◽

Fluorescence Emission ◽

Core Shell ◽

Average Particle ◽

Cdse Core ◽

Strain Effects ◽

The Core ◽

Core Band

We present the results of an experimental study about strain effects on the core band gap and diameter of spherical bare CdSe core and CdSe/ZnS core/shell quantum dots (QDs) synthesized by using a colloidal technique at varying temperatures. Structural characterizations were made by using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques. Optical characterizations were made by using UV-Vis absorption and fluorescence emission spectroscopies. The XRD analysis suggests that the synthesized bare CdSe core and CdSe/ZnS core/shell QDs have zinc blende crystal structure. HRTEM results indicate that the CdSe core and CdSe/ZnS QDs have average particle sizes about 3.50 nm and 4.84 nm, respectively. Furthermore, compressive strain causes an increase (decrease) in the core band gap (diameter) of spherical CdSe/ZnS core/shell QDs at any temperature. An elastic strain-modified effective mass approximation (EMA) predicts that there is a parabolic decrease (increase) in the core band gap (diameter) of QDs with temperature. The diameter of spherical bare CdSe core and CdSe/ZnS core/shell QDs calculated by using the strain-modified EMA, with core band gap extracted from absorption spectra, are in excellent agreement with the HRTEM data.

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