Effect of ZnSe and CdS Shells on the Structural and Photophysical Behavior of CdSe Nanocrystals in a PVA Matrix

Abstract

The CdSe nanocrystals implanted in a polyvinyl alcohol (PVA) matrix are examined in this work to determine the influence of ZnSe and CdS shells on their structural and photophysical behavior. By using a controlled experimental methodology, we were able to synthesis four different sets of samples using the same base conditions and systematically varying just the shell composition. There were a total of 125 samples distributed throughout the categories, which comprised bare CdSe cores, CdSe/ZnSe single-shell, CdSe/CdS single-shell, and CdSe/ZnSe/CdS double-shell nanocrystals. The structural investigation, which included TEM and XRD, verified that the particle size and crystallite domains increased gradually during shelling, and that lattice strain decreased, suggesting an improvement in crystal quality. Quantum yield rose dramatically from bare CdSe to double-shelled samples, and optical measurements showed that absorption and emission peaks shifted red as the photoluminescence full width at half maximum (FWHM) shrank. The results of the time-resolved PL analysis demonstrated that the improved surface passivation led to longer lifetimes and the suppression of non-radiative decay channels. Shelling also significantly enhanced PVA\'s photostability, thermal resistance, film smoothness, and environmental robustness, according to stability evaluations. In addition to improving the structural integrity and environmental stability of CdSe nanocrystals, our results show that ZnSe and CdS shells increase luminescence efficiency, suggesting that these materials might be used in optoelectronic and photonic applications.

Introduction

1. Background & Motivation

Semiconductor nanocrystals, especially cadmium selenide (CdSe) quantum dots (QDs), are widely studied due to their size-tunable optical and electronic properties, such as:

• High photoluminescence (PL) efficiency

• Strong quantum confinement effects

• Variable bandgaps

However, bare CdSe nanocrystals suffer from limitations, including:

• Surface defects

• Photo-oxidation

• Poor chemical stability
  → Resulting in PL quenching and reduced device performance

2. Core/Shell Strategy for Enhancement

To improve performance, researchers use core/shell nanostructures, where a CdSe core is coated with a shell material like:

• ZnSe (Zinc Selenide) – wider bandgap (2.7 eV)

• CdS (Cadmium Sulfide) – intermediate bandgap (2.42 eV)

Each shell type offers benefits:

• ZnSe: Sharp emission, better lattice match (lower strain), reduced trap recombination

• CdS: Higher quantum yield, red-shifted emission, thicker shell possible for more protection

3. Role of PVA Matrix

Embedding nanocrystals in polyvinyl alcohol (PVA) matrix offers:

• Better dispersion and reduced agglomeration

• Enhanced environmental and thermal stability

• Transparent, solution-processable thin films for optoelectronics and bio-imaging

4. Literature Review Highlights

• Various synthesis methods (wet chemical, non-coordinating solvents, SILAR) used to grow CdSe with ZnO, ZnS, and CdS shells

• Key findings from past studies:

  • Shelling improves PL intensity and quantum yield

  • Shell thickness and material composition significantly affect emission wavelength and lifetime

  • PVA improves dispersion and overall film quality

5. Experimental Methodology

• Controlled study with 125 samples across 4 groups:

  • CdSe (core only)

  • CdSe/ZnSe

  • CdSe/CdS

  • CdSe/ZnSe/CdS (double shell)

• Shelling performed via SILAR, with reaction conditions optimized per shell material

• Characterization techniques: TEM, XRD, UV-Vis, PL, TRPL, TGA, AFM

6. Key Findings from Structural & Optical Data

A. Structural Changes

• Shelling increased particle size from ~3.8 nm to ~5.8 nm (double-shell)

• XRD peaks shifted due to lattice expansion from shell growth

• Lattice strain decreased, indicating better structural stability

• Shell thickness measured: ZnSe ~0.4 nm, CdS ~0.6 nm

B. Optical Performance

• Red-shift in PL with increasing shell thickness

• Narrower PL peaks (better spectral purity)

• Higher QY and intensity due to reduced non-radiative recombination

C. Time-Resolved PL (TRPL)

• Longer exciton lifetimes with shelling (12 ns → 30 ns)

• Biexponential decay models show reduced trap-related fast decay

• Slow decay components (radiative recombination) dominate in shelled samples

7. Stability Improvements

• PVA films of shelled nanocrystals show:

  • Better photostability (UV/sunlight resistance)

  • Higher thermal decomposition temperatures

  • Improved morphology (smoother, better dispersed)

  • Greater water resistance

Conclusion

The comparison of ZnSe and CdS shells on CdSe nanocrystals embedded in a PVA matrix highlights the delicate balance between structural compatibility and photo physical performance. ZnSe shells, owing to their wider bandgap and smaller lattice mismatch with CdSe, contribute to efficient passivation of surface states, leading to strong, sharp photoluminescence and high structural coherence. CdS shells, on the other hand, allow for thicker over layer formation, enhancing environmental and photo-stability, while simultaneously inducing strain and partial carrier delocalization that manifest as red-shifted and broadened emission features. The incorporation of these core/shell Nano crystals into a PVA matrix further stabilizes the nanostructures, prevents aggregation, and enables processable thin films for device applications. Taken together, the structural and photo physical outcomes suggest that ZnSe shells are advantageous for applications requiring narrow and intense emission, whereas CdS shells are better suited for scenarios demanding long-term stability and broader spectral response. The interplay between shell composition, lattice effects, and polymer encapsulation thus offers a versatile platform for tuning the optical properties of CdSeNano crystals, providing valuable insights for the design of next-generation optoelectronic and photonic devices.

References

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Copyright

Copyright © 2025 Pisal Mahadeo Bhiku, Dr. Shailendra Jain. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.