TEM and SAXS Investigation of Self-Assembled 2D Structures of PbS Quantum-Dots Deposited onto Different Underlying Substrates

Colloidal semiconductor nanocrystals (NCs) are widely explored as functional materials for photonics and energy applications including solar cells, light emitting diodes, photodetectors, field-effect transistors, memory elements and thermoelectric elements. Key properties of NCs revolve around synthesis methods, solution processability, high-photoluminescence quantum yield, good photo-stability, and the possibility to control their electronic structure by engineering their composition, size, and shape. Among various kinds of NCs, inorganic lead sulfide quantum dots (QD) have tunable band gap and high absorption coefficients suitable for harvesting a large fraction of solar spectrum photons. Moreover, Pb and S are major industrial elements with suitable natural abundance for large-scale production. Our interest in PbS is to enhance the solar energy conversion process as it is desirable to extend the light absorption edge of the photo-electrodes.

In this work, we present a systematic analysis of the structural and morphological properties of selfassembled lead sulfide (PbS) quantum-dots (PbS-QDs) in super lattice like structure in the 2D arrangement. High resolution transmission electron microscopy (HRTEM) using FEI TALOS X, and small-angle x-ray scattering (SAXS) using Rigaku S- Max 3000 were used to investigate the assembly trends of PbS-QDs attempted on different substrates. PbS-QDs of 1200 nm PL emission peak encapsulated in Oleic Acid (OA) and suspended in toluene were used. Samples for TEM imaging were prepared by depositing a small drop of diluted nanoparticles solution onto Lacy/ Carbon thin film on copper grid and allowed to evaporate. Samples for SAXS analysis were prepared by depositing a drop of nanoparticles solution onto Kapton tape and allowed to evaporate (Kapton tape is highly transparent to X-rays in the SAXS region).