PbSe Core Research Articles

Spin-exchange carrier multiplication in manganese-doped colloidal quantum dots.

Carrier multiplication is a process whereby a kinetic energy of a carrier relaxes via generation of additional electron-hole pairs (excitons). This effect has been extensively studied in the context of advanced photoconversion as it could boost the yield of generated excitons. Carrier multiplication is driven by carrier-carrier interactions that lead to excitation of a valence-band electron to the conduction band. Normally, the rate of phonon-assisted relaxation exceeds that of Coulombic collisions, which limits the carrier multiplication yield. Here we show that this limitation can be overcome by exploiting not 'direct' but 'spin-exchange' Coulomb interactions in manganese-doped core/shell PbSe/CdSe quantum dots. In these structures, carrier multiplication occurs via two spin-exchange steps. First, an exciton generated in the CdSe shell is rapidly transferred to a Mn dopant. Then, the excited Mn ion undergoes spin-flip relaxation via a spin-conserving pathway, which creates two excitons in the PbSe core. Due to the extremely fast, subpicosecond timescales of spin-exchange interactions, the Mn-doped quantum dots exhibit an up-to-threefold enhancement of the multiexciton yield versus the undoped samples, which points towards the considerable potential of spin-exchange carrier multiplication in advanced photoconversion.

Efficient Infrared Solar Cells Employing Quantum Dot Solids with Strong Inter‐Dot Coupling and Efficient Passivation

AbstractLead chalcogenide quantum dot (QD) infrared (IR) solar cells are promising devices for breaking through the theoretical efficiency limit of single‐junction solar cells by harvesting the low‐energy IR photons that cannot be utilized by common devices. However, the device performance of QD IR photovoltaic is limited by the restrictive relation between open‐circuit voltages (VOC) and short circuit current densities (JSC), caused by the contradiction between surface passivation and electronic coupling of QD solids. Here, a strategy is developed to decouple this restriction via epitaxially coating a thin PbS shell over the PbSe QDs (PbSe/PbS QDs) combined with in situ halide passivation. The strong electronic coupling from the PbSe core gives rise to significant carrier delocalization, which guarantees effective carrier transport. Benefited from the protection of PbS shell and in situ halide passivation, excellent trap‐state control of QDs is eventually achieved after the ligand exchange. By a fine control of the PbS shell thickness, outstanding IR JSC of 6.38 mA cm−2 and IR VOC of 0.347 V are simultaneously achieved under the 1100 nm‐filtered solar illumination, providing a new route to unfreeze the trade‐off between VOC and JSC limited by the photoactive layer with a given bandgap.

Thermodynamic Model for Quantum Dot Assemblies Formed Because of Charge Transfer

Two initially neutral semiconductor quantum dots with appropriate band offsets can participate in a ground state charge transfer process. The charge transfer manifests itself in the form of bleaching of optical transitions and also causes the quantum dots to precipitate from solution, giving rise to assemblies with unusual properties. As this represents a postsynthetic modification of the electronic structure of quantum dots, it holds tremendous potential for improving the characteristics of quantum dot devices. Here, we study the dependencies of the properties of these assemblies on the structure of the participating quantum dots. In particular, we find that for assemblies formed out of Cu:CdS and ZnTe/CdS quantum dots, the composition of the assembly varies from 1:1.26 to 1:0.23 ZnTe/CdS to Cu:CdS as the shell thickness of CdS in ZnTe/CdS is increased. In contrast, the composition changes from 1:1.1 to 1:15 for PbSe/CdSe and Cu:CdS quantum dots, as the size of the PbSe core is increased. These observations are explained on the basis of a phenomenological thermodynamic model. The applicability of thermodynamics to this example of self-assembly is verified empirically.

Tuning Optical Activity of IV–VI Colloidal Quantum Dots in the Short-Wave Infrared (SWIR) Spectral Regime

The achievement of tunable optical properties across a wide spectral range, along with an efficient surface passivation of lead chalcogenide (PbSe) colloidal quantum dots (CQDs), has significant importance for scientific research and for technological applications. This paper describes two comprehensive pathways to tune optical activities of PbSe CQDs in the near-infrared (NIR, 0.75–1.4 μm) and the short-wave infrared (SWIR, 1.4–3 μm) ranges. A one-pot procedure enabled the growth of relatively large PbSe CQDs (with average sizes up to 14 nm) exploiting programmable temperature control during the growth process. These CQDs showed optical activity up to 3.2 μm. In addition, PbSe/PbS core/shell CQDs prepared by an orderly injection rate led to an energy red-shift of the absorption edge with the increase of the shell thickness, whereas a postannealing treatment further extended the band-edge energy toward the SWIR regime. A better chemical stability of the CQDs with respect to that of PbSe core CQDs was atta...

Design and synthesis of heterostructured quantum dots with dual emission in the visible and infrared.

The unique optical properties exhibited by visible emitting core/shell quantum dots with especially thick shells are the focus of widespread study, but have yet to be realized in infrared (IR)-active nanostructures. We apply an effective-mass model to identify PbSe/CdSe core/shell quantum dots as a promising system for achieving this goal. We then synthesize colloidal PbSe/CdSe quantum dots with shell thicknesses of up to 4 nm that exhibit unusually slow hole intraband relaxation from shell to core states, as evidenced by the emergence of dual emission, i.e., IR photoluminescence from the PbSe core observed simultaneously with visible emission from the CdSe shell. In addition to the large shell thickness, the development of slowed intraband relaxation is facilitated by the existence of a sharp core-shell interface without discernible alloying. Growth of thick shells without interfacial alloying or incidental formation of homogeneous CdSe nanocrystals was accomplished using insights attained via a systematic study of the dynamics of the cation-exchange synthesis of both PbSe/CdSe and the related system PbS/CdS. Finally, we show that the efficiency of the visible photoluminescence can be greatly enhanced by inorganic passivation.

PbSe-Based Colloidal Core/Shell Heterostructures for Optoelectronic Applications.

Lead-based (IV–VI) colloidal quantum dots (QDs) are of widespread scientific and technological interest owing to their size-tunable band-gap energy in the near-infrared optical region. This article reviews the synthesis of PbSe-based heterostructures and their structural and optical investigations at various temperatures. The review focuses on the structures consisting of a PbSe core coated with a PbSexS1–x (0 ≤ x ≤ 1) or CdSe shell. The former-type shells were epitaxially grown on the PbSe core, while the latter-type shells were synthesized using partial cation-exchange. The influence of the QD composition and the ambient conditions, i.e., exposure to oxygen, on the QD optical properties, such as radiative lifetime, Stokes shift, and other temperature-dependent characteristics, was investigated. The study revealed unique properties of core/shell heterostructures of various compositions, which offer the opportunity of fine-tuning the QD electronic structure by changing their architecture. A theoretical model of the QD electronic band structure was developed and correlated with the results of the optical studies. The review also outlines the challenges related to potential applications of colloidal PbSe-based heterostructures.

Anisotropic Cation Exchange in PbSe/CdSe Core/Shell Nanocrystals of Different Geometry

We present a study of Cd2+-for-Pb2+ exchange in PbSe nanocrystals (NCs) with cube, star, and rod shapes. Prolonged temperature-activated cation exchange results in PbSe/CdSe heterostructured nanocrystals (HNCs) that preserve their specific overall shape, whereas the PbSe core is strongly faceted with dominance of {111} facets. Hence, cation exchange proceeds while the Se anion lattice is preserved, and well-defined {111}/{111} PbSe/CdSe interfaces develop. Interestingly, by quenching the reaction at different stages of the cation exchange new structures have been isolated, such as core–shell nanorods, CdSe rods that contain one or two separated PbSe dots and fully zinc blende CdSe nanorods. The crystallographically anisotropic cation exchange has been characterized by a combined HRTEM/HAADF-STEM study of heterointerface evolution over reaction time and temperature. Strikingly, Pb and Cd are only intermixed at the PbSe/CdSe interface. We propose a plausible model for the cation exchange based on a layer-by-layer replacement of Pb2+ by Cd2+ enabled by a vacancy-assisted cation migration mechanism.

Three-Dimensional Atomic Imaging of Colloidal Core–Shell Nanocrystals

Colloidal core-shell semiconductor nanocrystals form an important class of optoelectronic materials, in which the exciton wave functions can be tailored by the atomic configuration of the core, the interfacial layers, and the shell. Here, we provide a trustful 3D characterization at the atomic scale of a free-standing PbSe(core)-CdSe(shell) nanocrystal by combining electron microscopy and discrete tomography. Our results yield unique insights for understanding the process of cation exchange, which is widely employed in the synthesis of core-shell nanocrystals. The study that we present is generally applicable to the broad range of colloidal heteronanocrystals that currently emerge as a new class of materials with technological importance.

Beneficial effect of tributylphosphine to the photoluminescence of PbSe and PbSe/CdSe nanocrystals

A small quantity of tributylphosphine was employed to improve the surface state of fresh and oxidized PbSe nanocrystals after the formation of nanocrystals. Experimental results showed that the photoluminescence intensity increased in both situations. However, an excessive amount of tributylphosphine exhibited negative effects of decreasing photoluminescence and particle aggregation. A suitable amount of tributylphosphine added before the synthesis of a CdSe shell on PbSe core (PbSe/CdSe) also exhibited a photoluminescence intensity increase for these core/shell nanocrystals.

Anomalous Independence of Multiple Exciton Generation on Different Group IV−VI Quantum Dot Architectures

Multiple exciton generation (MEG) in PbSe quantum dots (QDs), PbSe(x)S(1-x) alloy QDs, PbSe/PbS core/shell QDs, and PbSe/PbSe(y)S(1-y) core/alloy-shell QDs was studied with time-resolved optical pump and probe spectroscopy. The optical absorption exhibits a red-shift upon the introduction of a shell around a PbSe core, which increases with the thickness of the shell. According to electronic structure calculations this can be attributed to charge delocalization into the shell. Remarkably, the measured quantum yield of MEG, the hot exciton cooling rate, and the Auger recombination rate of biexcitons are similar for pure PbSe QDs and core/shell QDs with the same core size and varying shell thickness. The higher density of states in the alloy and core/shell QDs provide a faster exciton cooling channel that likely competes with the fast MEG process due to a higher biexciton density of states. Calculations reveal only a minor asymmetric delocalization of holes and electrons over the entire core/shell volume, which may partially explain why the Auger recombination rate does not depend on the presence of a shell.

Thermally induced atomic reconstruction of PbSe/CdSe core/shell quantum dots into PbSe/CdSe bi-hemisphere hetero-nanocrystals

The properties of hetero-nanocrystals (HNCs) depend strongly on the mutual arrangement of the nanoscale components. In this work we have investigated the structural and morphological evolution of colloidal PbSe/CdSe core/shell quantum dots upon annealing under vacuum. Prior to annealing the PbSe core has an approximately octahedral morphology with eight {111} facets, and the CdSe shell has zinc-blende crystal structure. Thermal annealing under vacuum at temperatures between 150 °C and 200 °C induces a structural and morphological reconstruction of the HNCs whereby the PbSe core and the CdSe shell are reorganized into two hemispheres joined by a common {111} Se plane. This thermally induced reconstruction leads to considerable changes in the optical properties of the colloidal PbSe/CdSe HNCs.

The Different Nature of Band Edge Absorption and Emission in Colloidal PbSe/CdSe Core/Shell Quantum Dots

We present a quantitative analysis of the absorption and luminescence of colloidal PbSe/CdSe core/shell quantum dots (QDs). In absorption, both the energy and the oscillator strength of the first exciton transition coincide with that of plain PbSe QDs. In contrast, luminescence lifetime measurements indicate that the oscillator strength of the emitting transition is reduced by at least a factor of 4 compared to PbSe core QDs. Moreover, the addition of an electron scavenger quenches the PbSe/CdSe emission, while a hole scavenger does not. This implies that the electron wave function reaches the QD surface, while the hole is confined to the PbSe core. These observations are consistent with calculations based on the effective mass model, which show that PbSe/CdSe QDs are at the boundary between the type-I and quasi-type-II regime, where the electron spreads over the entire nanoparticle and the hole remains confined in the PbSe core. However, as this only leads to a minor reduction of the oscillator strength, it follows that the drastic reduction of the oscillator strength in emission cannot be explained in terms of electron delocalization. In combination with the increased Stokes shift for PbSe/CdSe QDs, this indicates that the emission results from lower energy states that are fundamentally different from the absorbing states.

Temperature dependence of the ground‐state exciton in PbSe core and relevant core‐shell colloidal quantum dot structures

AbstractThe present study describes a thorough investigation of the temperature dependence of the photoluminescence (PL) and the PL decay processes in PbSe/PbS core‐shell and PbSexS1‐x/PbSeyS1‐y‐alloy core‐shell colloidal quantum dots (CQDs). The obtained data reveal a reduction of the temperature coefficient of the energy band‐gap and a slight decrease of exciton and optical phonon coupling in PbSe/PbS core‐shell, PbSexS1‐x/PbSeyS1‐y alloy core‐shell CQDs with respect to the coupling in the corresponding PbSe core. The PL decay process of core‐shell CQDs is characterized by single exponential behaviour similar to that of PbSe core CQDs. Also, the core‐shell structures lifetime is on the µs scale, and reflects substantially longer lifetimes (by a factor of 1.4‐1.8) with respect to the corresponding core samples. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Langmuir–Blodgett monolayers of colloidal lead chalcogenide quantum dots: morphologyand photoluminescence

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

Photoluminescence properties of PbSe/PbS core‐shell quantum dots

AbstractThe present study describes the investigation of the ground‐state exciton emission in the PbSe/PbS core‐shell colloidal quantum dots (CQDs) with different core‐radius/shell‐thickness ratio, by recording temperature dependence of ground‐state exciton photoluminescence (PL) properties, including line‐width, integrated PL intensity, and the temperature coefficient of energy band‐gap, over a temperature range from 1.4 K to 300 K. The obtained data reveal a reduction of the temperature coefficient of the energy band‐gap as well as slightly decrease of exciton and optical phonon coupling in the PbSe/PbS core‐shell CQDs with respect to that in the corresponding PbSe core. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Emission processes in colloidal PbSe/PbS core-shell quantum dots

The present study describes a thorough investigation of the ground-state exciton emission of PbSe/PbS core-shell colloidal quantum dots (CQDs) with different shell-thickness/core-radius ratio, by recording the photoluminescence (PL) spectra and the corresponding emission decay processes over a temperature range from 1.4 K to 300 K. These spectra were compared with that of the corresponding PbSe core CQDs. All PL spectra showed variation in the emission intensity and peak energy, including two distinct abrupt transitions, upon the increase of the temperature. However, the transitions appeared at different temperatures in the core-shell structures with respect to that of the core. Further on, the PL-decay processes in the core materials showed a single exponent behavior, while the emission of the core-shell CQDs, decayed as a stretched exponent, with an effective lifetime of ∼1 μs, at room temperature, slightly longer than that of the corresponding core.

Wear life and adhesion of solid lubricant films on laser-textured steel surfaces

Incorporation of solid lubricant into micro-reservoirs produced by laser surface texturing (LST) and its effect on the adhesion and wear life of burnished films is studied. In order to improve the adhesion between the ground steel and the MoS 2 burnished film, a sub-layer of nanoparticles was burnished on a virgin steel surface. Ultrasonic radiation (sonochemical synthesis) was used for synthesis of nanostructured materials. Some types of nanoparticles of sulfides and selenides as MoSe, MoSe 2/C core–shells, PbSe, ZnSe, WSe, CdZnSe, MoS 2, WS 2 and Ni core–carbon shells were chosen for the analysis of their friction and adhesion properties. The adhesion and wear life of solid lubricant films were evaluated. Some types of the friction and wear rigs were used for the evaluation of the adhesion and the wear life of burnished layers. The burnished CdZnSe film showing maximum value of the friction coefficients was chosen as the materials providing the best adhesion. MoS 2 particles were burnished on the surface of CdZnSe sub-layer. The adhesion between MoS 2 and CdZnSe films was analyzed. The strong transfer of the MoS 2 layers on the surface of the CdZnSe film was observed. The wear life of a MoS 2 film on a steel substrate was compared with similar films on the CdZnSe sub-layer. The wear life of the MoS 2 film on a sub-layer was more than twice as long as that for a MoS 2 film on a steel substrate. The study of the friction and wear behaviour of CdZnSe + MoS 2 films on LST surfaces confirm the results of the experiments on ground steel surfaces. The increase in the wear life is explained by the high adhesion and density of MoS 2 film burnished on a CdZnSe sub-layer.

Size-Dependent Composition and Molar Extinction Coefficient of PbSe Semiconductor Nanocrystals

Atomic compositions and molar extinction coefficients of PbSe semiconductor nanocrystals were determined by atomic absorption spectrometry, UV-vis-NIR spectrophotometry, and transmission electron microscopy. The Pb/Se atomic ratio was found to be size-dependent with a systematic excess of Pb atoms in the PbSe nanocrystal system. Experimental results indicated that the individual PbSe nanocrystal was nonstoichiometric, consisting of a PbSe core and an extra layer of Pb atoms. For these nonstoichiometric PbSe semiconductor nanocrystals, we proposed a new computational approach to calculate the total number of Pb and Se atoms in different sized particles. This calculation played a key role on the accurate determination of the strongly size-dependent extinction coefficient, which followed a power law with an exponent of approximately 2.5.

The ground-state exciton lifetime of PbSe nanocrystal quantum dots

We have investigated the ground-state exciton lifetime of PbSe nanocrystal quantum dots (NQDs). We found, under variation NQDs sizes, surrounding temperature and media, that the ground-state exciton lifetime of PbSe core NQDs showed a steep decrease of the lifetime from 6 to 1 μs when the temperature is changed from 7 to 150 K, while above that temperature, there is only a minor change in the measured lifetime, which are influenced of the surrounding medium.

Effect of PbS shell on the optical and electrical properties of PbSe core nanoparticles doped in PVA

Nanoparticles (NPs) of PbSe were synthesized in polyvinyl alcohol polymers with different Pb/Se ratio. It is crucial that the PbSe NPs were covered by a PbS sheet layer. Absorption and photoluminescence under excitation by 441.5 nm He–Cd laser were investigated. The structure of the nanocomposites was investigated using X-ray diffraction (XRD) measurements and transmission electronic microscopy (TEM). Conductivity measurements of the PbSe NP and PbSe/PbS sheets were performed.