Near infrared heteronanocrystals with enhanced electro-optical properties : from synthesis to layer formation

Synthesis of nanocrystals actived in the near infrared and study of their opto-electronic properties

Absorption enhancement in 2D nanocrystal superlattices through near-field dipolar coupling: a novel optical phenomenon at the nanoscale

We demonstrate giant and broadband enhancement of the nanocrystal absorption cross section in close packed nanocrystal superlattices, which is the first report on a collective optical phenomenon in this type of self-assembled metamaterials to date

Multiple Dot-in-Rod PbS/CdS heterostructures with high photoluminescence quantum yield in the near-infrared

Pb cations in PbS quantum rods made from CdS quantum rods by successive complete cationic exchange reactions are partially re-exchanged for Cd cations. Using STEM-HAADF, we show that this leads to the formation of unique multiple dot-in-rod PbS/CdS heteronanostructures, with a photoluminescence quantum yield of 45-55%. We argue that the formation of multiple dot-in-rods is related to the initial polycrystallinity of the PbS quantum rods, where each PbS crystallite transforms in a separate PbS/CdS dot-in-dot. Effective mass modeling indicates that electronic coupling between the different PbS conduction band states is feasible for the multiple dot-in-rod geometries obtained, while the hole states remain largely uncoupled

Short-wave infrared colloidal quantum dot photodetectors on silicon

In this paper, two kinds of colloidal quantum dots, PbS and HgTe, are explored for SWIR photodetectors application. The colloidal dots are prepared by hot injection chemical synthesis, with organic ligands around the dots keeping them stable in solution. For the purpose of achieving efficient carrier transport between the dots in a film, these long organic ligands are replaced by shorter, inorganic ligands. We report uniform, ultra-smooth colloidal QD films without cracks realized by dip-coating and corresponding ligand exchange on a silicon substrate. Metal-free inorganic ligands, such as OH- and S2-, are investigated to facilitate the charge carrier transport in the film. Both PbS and HgTe-based quantum dot photoconductors were fabricated on interdigitated gold electrodes. For PbS-based detectors a responsivity of 200A/W is measured at 1.5μm, due to the large internal photoconductive gain. A 2.2μm cut-off wavelength for PbS photodetectors and 2.8μm for HgTe quantum dot photodetectors are obtained

Phase transformation of PbSe/CdSe nanocrystals from core-shell to Janus structure studied by photoemission spectroscopy

Photoelectron spectroscopic measurements have been performed, with synchrotron radiation on PbSe/CdSe heteronanocrystals that initially consist of core-shell structures. The study of the chemical states of the main elements in the nanocrystals shows a reproducible and progressive change in the valence-band and core-level spectra under photon irradiation, whatever the core and shell sizes are. Such chemical modifications are explained in light of transmission electron microscopy observations and reveal a phase transformation of the nanocrystals: The core-shell nanocrystals undergo a morphological change toward a Janus structure with the formation of semidetached PbSe and CdSe clusters. Photoelectron spectroscopy gives new insight into the reorganization of the ligands anchored at the surface of the nanocrystals and the modification of the electronic structure of these heteronanocrystals

Langmuir-Blodgett monolayers of colloidal lead chalcogenide quantum dots: morphology and photoluminescence

Monolayers of PbSe and PbS quantum dots and PbSe/CdSe core/shell quantum dots made by Langmuir-Blodgett deposition are compared, with a focus on the formation, the morphology and the photoluminescence properties of the films. It is shown that PbSe quantum dots suffer from oriented attachment and a complete quenching of their photoluminescence after Langmuir-Blodgett processing. While the oriented attachment can be resolved by growing a CdSe shell around the PbSe core QDs, the photoluminescence of PbSe/CdSe Langmuir-Blodgett monolayers remains largely quenched. In the case of PbS quantum dots, the formation of a close-packed monolayer is more difficult, yet the resulting films show no sign of oriented attachment and their photoluminescence is comparable to that of the original, suspended quantum dots. In spite of their slow oxidation in a matter of weeks, these results mark PbS quantum dots as the preferred material for light-emitting applications in the near IR based on Langmuir-Blodgett monolayers of lead chalcogenide quantum dots

Mechanistic insights in seeded growth synthesis of colloidal core/shell quantum dots

We analyze the mechanism of seeded growth reactions used to synthesize colloidal core/shell nanocrystals. Looking at the formation of CdSe/CdS and CdSe/ZnSe using both zinc blende and wurtzite CdSe seeds with a different surface termination, we show that the formation, rate of the shell material does not depend on the presence of the seed nanocrystals. This suggests that shells grow by inclusion of CdS or ZnSe initially formed in the reaction mixture, possibly under the form of reactive monomers, and not by successive adsorption and reaction of metal and chalcogen precursors. This insight makes balancing homogeneous nucleation and heterogeneous growth of the shell material key to suppressing spurious secondary nucleation. Through a combination of experimental work and reaction simulations, we argue that this can be effectively achieved by raising the monomer Solubility through the concentration of carboxylic acid used in the seeded growth reaction