Colloidal quantum dots based on lead sulfide (PbS) are very attractive materials for the realization of novel image sensors. They offer low cost synthesis, compatibility with a variety of substrates ...and processing on large area. The tunable band gap enables selective light detection from the visible wavelengths up to the short-wave-infrared (SWIR). This work describes the roadmap towards the integration of quantum dot photodiodes (QDPD) on top of a Si based CMOS read-out circuit. Photodiodes using an n-p junction architecture are fabricated on Si substrates, showing a dark current of 30 nA/cm 2 at −1 V reverse bias, EQE above 20% and specific detectivity above 10 12 cm Hz 1/2 W −1 at the wavelength of 940 nm. Efficiency is improved by reducing absorption in the top contact through optical design. Furthermore, photolithographic patterning of the thin-film stack is introduced for the first time, showing the feasibility of pixel pitches down to <inline-formula> <tex-math notation="LaTeX">40~\mu \text{m} </tex-math></inline-formula>, opening the way towards high resolution monolithic infrared imagers and the incorporation of infrared and visible sensitive pixels side by side.
PbS Qdots are synthesized using PbCl2 and elemental sulfur as precursors. The available size range is significantly expanded using tri-n-octylphosphine (TOP), enabling the synthesis of monodisperse ...suspensions of Qdots with a mean size varying between 3 and 10 nm. The ligand composition and dynamics are investigated with nuclear magnetic resonance (NMR) spectroscopy. We show that the Qdots are passivated solely by highly dynamic OlAm ligands, even when TOP is employed during synthesis. In this respect, TOP is a compound strongly modifying the Qdot synthesis, without affecting the final Qdot surface chemistry. Next, the OlAm ligands are exchanged for oleic acid (OlAc). NMR data show that the OlAc ligands are tightly bound to the Qdot surface, with a coverage of 3.0 ± 0.4 nm−2. In addition, we demonstrate that they are bound as oleate ions. Combining this with the inorganic Qdot composition, we observe that charge-neutral Qdots are obtained when taking into account the charge of the stoichiometric PbS Qdot core, the surface excess of Pb ions, the surface-adsorbed Cl ions and the oleate ligands. The Qdot suspensions are stable under atmospheric conditions, showing no changes in the NMR and absorbance spectra for several weeks. Finally, we determine the photoluminescence quantum yield (PL QY) for OlAc-capped PbS Qdots, synthesized either with or without TOP. In both cases, they are highly luminescent, with PL QY values varying between 20 and 90%, depending on the Qdot size.
Aminophosphines have recently emerged as economical, easy-to-implement precursors for making InP nanocrystals, which stand out as alternative Cd-free quantum dots for optoelectronic applications. ...Here, we present a complete investigation of the chemical reactions leading to InP formation starting from InCl3 and tris(dialkylamino)phosphines. Using nuclear magnetic resonance (NMR) spectroscopy and single crystal X-ray diffraction, we demonstrate that injection of the aminophosphine in the reaction mixture is followed by a transamination with oleylamine, the solvent of the reaction. In addition, mass spectrometry and NMR indicate that the formation of InP concurs with that of tetra(oleylamino)phosphonium chloride. The chemical yield of the InP formation agrees with this 4 P(+III) → P(−III) + 3 P(+V) disproportionation reaction occurring, since full conversion of the In precursor was only attained for a 4:1 P/In ratio. Hence it underlines the double role of the aminophosphine as both precursor and reducing agent. These new insights will guide further optimization of high quality InP quantum dots and might lead to the extension of synthetic protocols toward other pnictide nanocrystals.
Although solvent–ligand interactions play a major role in nanocrystal synthesis, dispersion formulation, and assembly, there is currently no direct method to study this. Here we examine the ...broadening of 1H NMR resonances associated with bound ligands and turn this poorly understood descriptor into a tool to assess solvent–ligand interactions. We show that the line broadening has both a homogeneous and a heterogeneous component. The former is nanocrystal-size dependent, and the latter results from solvent–ligand interactions. Our model is supported by experimental and theoretical evidence that correlates broad NMR lines with poor ligand solvation. This correlation is found across a wide range of solvents, extending from water to hexane, for both hydrophobic and hydrophilic ligand types, and for a multitude of oxide, sulfide, and selenide nanocrystals. Our findings thus put forward NMR line-shape analysis as an indispensable tool to form, investigate, and manipulate nanocolloids.
Colloidal core/shell InP/ZnSe quantum dots (QDs), recently produced using an improved synthesis method, have a great potential in life-science applications as well as in integrated quantum photonics ...and quantum information processing as single-photon emitters. Single-particle spectroscopy of 10 nm QDs with 3.2 nm cores reveals strong photon antibunching attributed to fast (70 ps) Auger recombination of multiple excitons. The QDs exhibit very good photostability under strong optical excitation. We demonstrate that the antibunching is preserved when the QDs are excited above the saturation intensity of the fundamental-exciton transition. This result paves the way toward their usage as high-purity on-demand single-photon emitters at room temperature. Unconventionally, despite the strong Auger blockade mechanism, InP/ZnSe QDs also display very little luminescence intermittency (“blinking”), with a simple on/off blinking pattern. The analysis of single-particle luminescence statistics places these InP/ZnSe QDs in the class of nearly blinking-free QDs, with emission stability comparable to state-of-the-art thick-shell and alloyed-interface CdSe/CdS, but with improved single-photon purity.
The accurate determination of the dimensions of a nano-object is paramount to the development of nanoscience and technology. Here we provide procedures for sizing quasi-spherical colloidal ...nanocrystals (NCs) by means of small-angle X-ray scattering (SAXS). Using 2.5 to 10 nm PbS NCs as a model system, the protocols outline the extraction of the net NC SAXS pattern by background correction and address the calibration of scattered X-ray intensity to an absolute scale. The NC size distribution is retrieved by fitting the corrected SAXS pattern either to parametrized analytical distributions or to a distribution constructed through a Monte Carlo approach. We compare the two methods and show that they yield nearly identical estimates of the NC diameter in the case of an NC ensemble with a monodisperse and monomodal size distribution. Extending the analysis to PbSe, CdSe, and CdS NCs, we provide SAXS-calibrated sizing curves ranging from 2.5 to 7 nm, which relate the NC diameter and the NC band-gap energy as determined using absorbance spectroscopy. In comparison with sizing curves calibrated by means of transmission electron microscopy (TEM), we systematically find that SAXS calibration assigns a larger diameter than TEM calibration to NCs with a given band gap. We attribute this difference to the difficulty of accurately sizing small objects in TEM images. To close, we demonstrate that NC concentrations can be directly extracted from SAXS patterns normalized to an absolute scale and that SAXS-based concentrations differ less than 10% from concentrations obtained using absorbance spectroscopy.
Femtosecond pump-probe spectroscopy reveals ultrafast carrier dynamics in mid-infrared (MIR) colloidal HgTe nanoparticles with a bandgap of 2.5 μm. We observe intraband relaxation processes after ...photoexcitation ranging from resonant excitation up to the multi-exciton generation (MEG) regime by identifying initially excited states from atomic effective pseudopotential calculations. Our study elucidates the earliest dynamics below 10 ps in this technologically relevant material. With increasing photon energy, we find carrier relaxation times as long as 2.1 ps in the MEG regime close to the ionization threshold of the particles. For all photon energies, we extract a constant mean carrier energy dissipation rate of 0.36 eV ps
−1
from which we infer negligible impact of the density of states on carrier cooling.
Femtosecond pump-probe spectroscopy reveals ultrafast carrier dynamics in mid-infrared (MIR) colloidal HgTe nanoparticles with a bandgap of 2.5 μm at a mean energy dissipation rate of 0.36 eV ps
−1
irrespective of excitation wavelength.
We use solution NMR techniques to analyze the organic/inorganic interface of CdSe quantum dots (Q-CdSe) synthesized using oleic acid as a surfactant. It is shown that the resulting Q-CdSe are ...stabilized by tightly bound oleic acid species that only exchange upon addition of free oleic acid. The NMR analysis points toward a two-step exchange mechanism where free ligands are initially physisorbed within the ligand shell to end up as bound, chemisorbed ligands in a second step. Importantly, we find that every ligand is involved in this exchange process. By addition of oleic acid with a deuterated carboxyl headgroup, we demonstrate that the bound ligands are oleate ions and not oleic acid molecules. This explains why a dynamic adsorption/desorption equilibrium only occurs in the presence of excess free oleic acid, which donates the required proton. Comparing the number of oleate ligands to the excess cadmium per CdSe quantum dot, we find a ratio of 2:1. This completes the picture of Q-CdSe as organic/inorganic entities where the surface excess of Cd2+ is balanced by a double amount of oleate ligands, yielding overall neutral nanoparticles.
Surface chemistry is a key enabler for colloidal nanocrystal applications. In this respect, metal oxide nanocrystals (NCs) stand out from other NCs as carboxylic acid ligands adsorb on their surface ...by dissociation to carboxylates and protons, the latter proving essential in electron transfer reactions. Here, we show that this binding motif sets the stage for chemically driven ligand displacement where the binding of amines or alcohols to HfO2 NCs is promoted by the conversion of a bound carboxylic acid into a non-coordinating amide or ester. Furthermore, the sustained ligand displacement, following the addition of excess carboxylic acid, provides a catalytic pathway for ester formation, whereas the addition of esters leads to NC-catalysed transesterification. Because sustained, chemically driven ligand displacement leaves the NCs-including their surface composition-unchanged and preserves colloidal stability, metal oxide nanocrystals are thus turned into effective nanocatalysts that bypass the tradeoff between colloidal stability and catalytic activity.
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