Colloidal semiconductor nanocrystals (NCs) and composite metal-semiconductor (M-S) structures are emerging as a promising class of nanomaterials for the development of solid state optical applications. Some of these applications require colloidal nanocrystals in form of thin films (matrices), possessing both thermal and chemical stability, and demonstrating high energy transfer efficiency. In this study, we addressed the issues of low emission yield and instability of NC matrices through the use of ligand-free encapsulation of colloidal nanocrystals within all-inorganic crystalline solids. The present methodology relies on solution-processing of CdSe nanocrystals into a crystalline matrix ofa wide bandgap semiconductor (CdS, ZnS), which replaces the original molecular ligands on nanocrystal surfaces with an inorganic medium. Such matrices efficiently protect nanoparticles from the surrounding environment and preserve the quantum confinement of electrical charges in embedded NCs.
Assembly of thin film matrices from NC “inks” is cost-effective approach for the development of next-generation materials with high efficiency photoluminescence. Specifically, the main accomplishments of this work included:
(i) hot-injection synthesis of CdSe/CdS core/shell semiconductor NCs;
(ii) processing of NCs into light emitting solid films through spin-coating with consecutive ligand exchange;
(iii) optimization of film morphology towards improving the emission yield and stability;
(iv) further enhancement of developed morphology via introducing the M-S composites:
a. development of chemical strategies for the fabrication of M-S nanocomposites that demonstrate an efficient and potentially tunable energy exchange between excitons in semiconductor and plasmons in metal nanoparticles;
b. investigation of the nature of underlying exciton-plasmon interactions and the ultrafast electron processes in the fabricated M-S heteronanocrystals, using modern spectroscopy methods.
A general strategy for the low-temperature processing assembly of all-inorganic light-emitting nanocrystal films with emission quantum yield (QY) inthe 30-52% range is reported. This technology exceeds the traditional ligand-interlinking scheme and is based on the ligand-free encapsulation of morphologically-defined nanocrystalarrays into a wide bandgap
semiconductor matrix. This approach preserves optoelectronic properties of individual nanoparticles and maintains properties of the photoconductive nanocrystal film. Fabricated material shows outstanding thermal stability due to the heteroepitaxial structure of nanocrystal-matrix interfaces. In addition to strong emission, fabricated films show excellent thermal and
chemical stability, and a large refractive index, which avails their integration into emerging
solid-state NC devices, including light emitting diodes, solar concentrators, and quantum dot
lasers.
Improving the efficiency of prototype NC light emitting films was achieved through material
optimization, adjustment of the thickness of semiconductor layers and the study of ligand exchange process. Light absorbance and energy transfer performance of fabricated films can also be improved by introducing M-S composites as dopant into the NC matrix inks substance.