Yolanda Justo

Size-Tunable, Bright, and Stable PbS Quantum Dots: A Surface Chemistry Study

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.

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.

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.

Giant and Broad-Band Absorption Enhancement in Colloidal Quantum Dot Monolayers through Dipolar Coupling

The absorption cross section of colloidal quantum dots in close-packed monolayers shows a 4 (CdSe) to 5-fold (PbS) enhancement compared to quantum dots in a dilute dispersion. Quantitative agreement is demonstrated between the value and the size dependence of the enhancement and theoretical model predictions based on dipolar coupling between neighboring quantum dots. This collective optical behavior offers a new degree of freedom in the custom design of optical properties for electro-optical devices.

Carboxylic‐Acid‐Passivated Metal Oxide Nanocrystals: Ligand Exchange Characteristics of a New Binding Motif

Ligand exchange is central in the processing of inorganic nanocrystals (NCs) and requires understanding of surface chemistry. Studying sterically stabilized HfO2 and ZrO2 NCs using (1) H solution NMR and IR spectroscopy as well as elemental analysis, this paper demonstrates the reversible exchange of initial oleic acid ligands for octylamine and self-adsorption of oleic acid at NC surfaces. Both processes are incompatible with an X-type binding motif of carboxylic acids as reported for sulfide and selenide NCs. We argue that this behavior stems from the dissociative adsorption of carboxylic acids at the oxide surface. Both proton and carboxylate moieties must be regarded as X-type ligands yielding a combined X2 binding motif that allows for self-adsorption and exchange for L-type ligands.

Air-stable short-wave infrared PbS colloidal quantum dot photoconductors passivated with Al2O3 atomic layer deposition

A PbS colloidal quantum dot photoconductor with Al2O3 atomic layer deposition (ALD) passivation for air-stable operation is presented. Two different types of inorganic ligands for the quantum dots, S2− and OH−, are investigated. PbS/S2− photoconductors with a cut-off wavelength up to 2.4 μm are obtained, and a responsivity up to 50 A/W at 1550 nm is reported. The corresponding specific detectivity is ∼3.4 × 108 Jones at 230 K. The 3-dB bandwidth of the PbS/S2− and PbS/OH− photodetectors is 40 Hz and 11 Hz, respectively.

Slow recombination in quantum dot solid solar cell using p–i–n architecture with organic p-type hole transport material

The interfaces between different materials in the heterojunction colloidal quantum dot (QD) solar cell play an important role for charge carrier separation, recombination and collection. Here, an organic–inorganic hybrid p–i–n architecture for the heterojunction PbS QD solid solar cell is constructed to increase the charge extraction and reduce charge recombination. Heavily doped poly(3-hexylthiophene-2,5-diyl) (P3HT) is applied as hole transport interlayer between the QD film and metal contact electrode. The results show that the P3HT interlayer diminishes the charge carrier recombination at the QD film/metal contact electrode interface leading to increased open-circuit voltage and increased electron life time. Furthermore, after incorporation of P3HT interlayer an additional p–i heterojunction might form at P3HT/QD film interface resulting in increased depletion region, which promotes charge carrier extraction under working conditions. Two other organic p-type interlayers are also investigated, however, the results indicate that a barrier for charge extraction is formed for these devices, which is explained by the difference in energy levels. The solar cell with the P3HT interlayer exhibits a power conversion efficiency of 5.1% at 1 sun of illumination and ambient atmosphere, which is ∼20% higher compared to the solar cell without any hole transport interlayer.

Enhancement of the photovoltaic performance in P3HT: PbS hybrid solar cells using small size PbS quantum dots

Different approaches of surface modification of the quantum dots (QDs), namely, solution-phase (octylamine, octanethiol) and post-deposition (acetic acid, 1,4-benzenedithiol) ligand exchange were used in the fabrication of hybrid bulk heterojunction solar cell containing poly (3-hexylthiophene) (P3HT) and small (2.4 nm) PbS QDs. We show that replacing oleic acid by shorter chain ligands improves the figures of merit of the solar cells. This can possibly be attributed to a combination of a reduced thickness of the barrier for electron transfer and an optimized phase separation. The best results were obtained for post-deposition ligand exchange by 1,4-benzenedithiol, which improves the power conversion efficiency of solar cells based on a bulk heterojunction of lead sulfide (PbS) QDs and P3HT up to two orders of magnitude over previously reported hybrid cells based on a bulk heterojunction of P3HT:PbS QDs, where the QDs are capped by acetic acid ligands. The optimal performance was obtained for solar cells with 69 wt. % PbS QDs. Besides the ligand effects, the improvement was attributed to the formation of an energetically favorable bulk heterojunction with P3HT, when small size (2.4 nm) PbS QDs were used. Dark current density-voltage (J-V) measurements carried out on the device provided insight into the working mechanism: the comparison between the dark J-V characteristics of the bench mark system P3HT:PCBM and the P3HT:PbS blends allows us to conclude that a larger leakage current and a more efficient recombination are the major factors responsible for the larger losses in the hybrid system.

A phonon scattering bottleneck for carrier cooling in lead chalcogenide nanocrystals.

The cooling dynamics of hot charge carriers in colloidal lead chalcogenide nanocrystals is studied by hyperspectral transient absorption spectroscopy. We demonstrate a transient accumulation of charge carriers at a high energy critical point in the Brillouin zone. Using a theoretical study of the cooling rate in lead chalcogenides, we attribute this slowing down of charge carrier cooling to a phonon scattering bottleneck around this critical point. The relevance of this observation for the possible harvesting of the excess energy of hot carriers by schemes such as multiexciton generation is discussed.

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.