Lead Chalcogenide Quantum Dots Research Articles

Defect Passivation Results in the Stability of Cesium Lead Halide Perovskite Nanocrystals

Nanoheterostructures (NHSs) based on lead halide perovskites (LHPs) and chalcogenide quantum dots have proved to be promising candidates for photovoltaic device applications. However, understanding the defect chemistry at the interfaces of LHPs and chalcogenides is essential to stabilize them and further tune their optoelectronic properties. Here, we demonstrate a route for designing CsPbBr3–PbSe NHSs and other derivatives of LHP-based NHSs using defect-rich MoSe2 nanosheets (NSs) and study the effect of the size of PbSe NPs on their optical properties. In this synthesis route, PbSe nanoparticles (NPs) are formed at an early stage of the reaction through a unique cation displacement reaction, over which CsPbBr3 nanocrystals (NCs) are epitaxially grown. Using this methodology, a nearly 3-fold enhancement in photoluminescence (PL) is achieved, whereas other selenium precursors, which form larger PbSe NPs, result in negligible PL enhancement with respect to the pure CsPbBr3 NCs. Detailed density functional theory (DFT) calculations suggest that the PbSe NPs are responsible for passivating the surface defects that consequently enhance the PL intensity. However, in the case of larger PbSe NPs, the associated valence and conduction bands lie within the band-gap region of CsPbBr3, creating a type-I heterostructure between the two materials, thereby affecting the luminescence properties. Strong passivation of surface defects in CsPbBr3–PbSe NHSs is also evidenced from low-temperature PL studies. Furthermore, the resulting CsPbBr3–PbSe NHSs demonstrate enhanced stability in the presence of water and do not degrade under ambient conditions for several months.

Precipitation and optical properties of PbSexS1-x quantum dots in glasses

Lead chalcogenide quantum dots have particular optoelectronic properties induced by the quantum confinement effects and have promising potentials towards applications in infrared spectral range. Modulation of the optoelectronic properties of lead chalcogenide quantum dots is mainly based on the thermal treatment. In this work, precipitation and optical properties of alloyed PbSexS1-x quantum dots in glasses are investigated. Composition, size, absorption, and photoluminescence properties of PbSexS1-x quantum dots are modulated by chalcogen elements in glasses and thermal treatment. By adjusting the concentration of ZnSe and ZnS in the glasses, average diameters of the PbSexS1-x quantum dots are tuned in the range of 8.1–16.1 nm, their absorption peak wavelengths are tuned in the range of 724–2497 nm, and photoluminescence bands are tuned in the range of 1016–2165 nm with full width at half maximum in the range of 167–517 nm. Fraction of Se in the PbSexS1-x quantum dots precipitated in glasses is dependent on the concentrations of ZnSe and ZnS in the glass and heat-treatment temperature. Increase in concentration of S in the glasses results in the reduction of relative fraction of Se in the PbSexS1-x quantum dots formed in glasses. More importantly, high heat-treatment temperature promotes the enrichment of Se in PbSexS1-x quantum dots, and Se fraction in PbSexS1-x quantum dots increases from 0.69 to 0.89, from 0.53 to 0.85, from 0.47 to 0.79, and from 0.45 to 0.76 for those precipitated in glasses with ZnSe/(ZnSe+ZnS) molar ratios of 0.545, 0.375, 0.286, and 0.231, respectively. Increase in ZnSe/(ZnSe+ZnS) molar ratios in glasses also leads to reduction in diameter of PbSexS1-x quantum dots formed in glasses. These alloyed PbSexS1-x quantum dots also exhibit improved photoluminescence quantum yields and enhanced resistance to thermal quenching of photoluminescence.

Optical transitions, exciton radiative decay, and valley coherence in lead chalcogenide quantum dots

We propose the concept of valley coherence and superradiance in the reciprocal space and show that it leads to an $N$-fold decrease of the bright exciton radiative lifetime in quantum dots (QDs) of an $N$-valley semiconductor. Next we explain why, despite this, the exciton radiative lifetimes in PbX (X = S, Se, Te) QDs, measured from the photoluminescence decay, are in the microsecond range. We also address peculiarities of the light-matter interaction in nanostructures made of narrow-gap materials with strong inter-band coupling.

Studies on copper indium selenide/Zinc sulphide semiconductor quantum dots for solar cell applications

Despite dedicated efforts to develop efficient quantum dot sensitized (QDS) photovoltaic cells, the efficiency of these cells still lags behind their theoretical value. In order to increase photo conversion efficiency, the extant methods are predominantly focus on modifying the band gaps of quantum dots and optimizing the interfaces of cell components to increase light utilization capacity. In this study, we have designed and investigated QDS solar cells using Copper Indium Selenide (CuInSe2 or simply CIS) as a quantum dot absorber. In order to achieve tunable bandgap, increased photoluminescence, reduced density of surface defect state and higher light-harvesting efficiency, the CuInSe2 is alloying with Zinc sulfide (ZnS) to design Copper Indium Selenide-Zinc sulfide (CISZS) quantum dots. The resulting CISZS sensitizer exhibits improved photoelectric characteristics and greater chemical stability. The performance of the CIS and CISZS solar cells is evaluated individually through Silvaco-Atlas simulation software in terms of measures such as power conversion efficiency, open-circuit voltage (Voc), the density of short-circuit current (Jsc) and fill-factor (FF). The CISZS-based solar cells show an average conversion efficiency of 23.5% (i.e., 4.94% higher than the efficiency of CIS solar cell) with Voc = 0.596V, Jsc = 23.61mA/cm2 and FF = 0.84 under AM 1.5G with a power density of 100mW/cm2 . The achieved power conversion efficiency indicates the greatest performances of the QDS solar cells. These non-toxic photovoltaic devices reveal better optical and electrical properties than toxic lead and cadmium chalcogenide quantum dots absorbers.

Hot excitons cooling and multiexcitons Auger recombination in PbS quantum dots

In the past few years, lead chalcogenide quantum dots (QDs) have attracted attention as a new system with a strong quantum confinement effect. In this paper, the hot-excitons cooling and Auger recombination of multiexcitons in PbS QDs are investigated by the femtosecond time-resolved transient absorption spectroscopy. The results show that the excitons dynamics in PbS QDs are closely related to the pump-photon energy and pump-pulse energy. Multiexcitons generate when the excess energy of the absorbed photons is larger than the bandgap energy in PbS QDs. The hot-excitons cooling lifetime increases but the Auger recombination lifetime decreases as the pump-photon energy and the pump-pulse energy increase. Besides, there is a competitive relation between multiple-excitons generation and hot-excitons cooling. The dynamics results of the formation and relaxation of multiexcitons in PbS QDs would shed light on the further understanding of the interaction between excitons and photons in the optoelectronic application based on PbS QDs.

Exciton Fine Structure in Lead Chalcogenide Quantum Dots: Valley Mixing and Crucial Role of Intervalley Electron-Hole Exchange.

We study the exciton fine structure in quantum dots of multivalley lead chalcogenides. We demonstrate that intervalley electron-hole exchange interaction, ignored in previous studies, dramatically modifies the exciton fine structure and leads to appearance of the ultrabright valley-symmetric spin-triplet exciton state dominating interband optical absorption. Valley mixing leads to brightening of other symmetry-allowed spin-triplet states which dominate low-temperature photoluminescence.

Stability enhancement of PbS quantum dots by site-selective surface passivation for near-infrared LED application

Infrared lead chalcogenide quantum dots (QDs) suffer fast degradation due to the easy oxidation of surface chalcogen atoms. Here, we report a trioctylphosphine-mediated surface passivation method to improve the air stability of PbS QDs. Surface mechanism study reveals an in situ surface reaction, which leads to site-selective passivation of surface S atoms with lead mono-carboxylate. The surface capping motif sufficiently protects PbS QDs from oxygen and improves their stability as well as quantum efficiency regardless of the QD size and original ligands on surface cations. The modified PbS QDs display no obvious fluorescence quenching and surface oxidization after 30 days of air exposure. The robust surface capping also provides high compatibility of PbS QDs with polymers for optoelectronic device fabrication. The near-infrared LEDs based on the modified PbS QDs display a slight degradation of only 1.47% from the maximum intensity after continuous operation in air for 250 hours (lifetime > 10,000 h) at 0.5 W/cm2 power density, indicating the surface passivation route is promising strategy for promoting practical optoelectronic application of PbS QDs.

Modulation of the optical properties of transition metal doped PbSe quantum dots in silicate glasses

Optical properties of lead chalcogenide quantum dots (QDs) can be controlled by tuning its bandgaps. We prepared silicate glasses using the melt-quenching method and heat-treated 510 °C–540 °C for 10 h to precipitate transition metal ions into PbSe QDs. Transition metals oxide such as TiO, MnO & NiO were used in our experiment due to relative ease of incorporation of Ti2+, Mn2+, or Ni2+ ions in PbSe crystal structure lattice compared to rare-earth ions. Emission spectra of PbSe QDs were tuned in a wide wavelength range through changing concentration of transition metals oxide in glasses. For instance, photoluminescence bands of Pb1−xTixSe QDs QDs moved from 1890 nm to 1625 nm with increasing TiO from 0.0 to 0.5 mol% at heat treatment 530 °C for 10 h. Absorption bands of QDs also shifted to shorter wavelength sides as concentration of TiO or MnO increased under constant thermal treatment conditions mainly due to the change in the bandgap of QDs. However, absorption & PL peaks moved to longer wavelength sides (red-shifted) with increasing the duration of thermal annealing while keeping the amount of transition metal oxides constant mainly due to enlargement of the QDs sizes with thermal treatment. Tunable optical properties of QDs make them a promising candidate for various photoelectric devices, for example, fiber-optics amplifiers in optical communication and fiber lasers.

Intrinsic Exciton Photophysics of PbS Quantum Dots Revealed by Low-Temperature Single Nanocrystal Spectroscopy.

With a tunable size-dependent photoluminescence (PL) over a wide infrared wavelength range, lead chalcogenide quantum dots (QDs) have attracted significant scientific and technological interest. Nevertheless, the investigation of intrinsic exciton photophysics at the single-QD level has remained a challenge. Herein, we present a comprehensive study of PL properties for the individual core/shell PbS/CdS QDs emissive near 1.0 eV. In contrast to the sub-meV spectral line widths observed for II/VI QDs, PbS/CdS QDs are predicted to possess broad homogeneous line widths. Performing spectroscopy at cryogenic (4 K) temperatures, we provide direct evidence confirming theoretical predictions, showing that intrinsic line widths for PbS/CdS QDs are in the range of 8-25 meV, with an average of 16.4 meV. In addition, low-temperature, single-QD spectroscopy reveals a broad low-energy side emission attributable to optical as well as localized acoustic phonon-assisted transitions. By tracking single QDs from 4 to 250 K, we were able to probe temperature-dependent evolutions of emission energy, line width, and line shape. Finally, polarization-resolved PL imaging showed that PbS/CdS QDs are characterized by a 3D emission dipole, in contrast with the 2D dipole observed for CdSe QDs.

Improved Reproducibility of PbS Colloidal Quantum Dots Solar Cells Using Atomic Layer-Deposited TiO2.

Thanks to their broadly tunable bandgap and strong absorption, colloidal lead chalcogenide quantum dots (QDs) are highly appealing as solution‐processable active layers for third‐generation solar cells. However, the modest reproducibility of this kind of solar cell is a pertinent issue, which inhibits the exploitation of this material class in optoelectronics. This issue is not necessarily imputable to the active layer but may originate from different constituents of the device structure. Herein, the deposition of TiO2 electron transport layer is focused on. Atomic layer deposition (ALD) greatly improves the reproducibility of PbS QD solar cells compared with the previously optimized sol–gel (SG) approach. Power conversion efficiency (PCE) of the solar cells using atomic layer–deposited TiO2 lies in the range between 5.5% and 7.2%, whereas solar cells with SG TiO2 have PCE ranging from 0.5% to 6.9% with a large portion of short‐circuited devices. Investigations of TiO2 layers by atomic force microscopy and scanning electron microscopy reveal that these films have very different surface morphologies. Whereas the TiO2 films prepared by SG synthesis and deposited by spin coating are very smooth, TiO2 films made by ALD repeat the surface texture of the fluorine‐doped tin oxide (FTO) substrate underneath.

(Invited) Triplet Energy Transfer at Interfaces between Molecules and Perovskite or Metal Chalcogenide Nanostructures

The movement of energy and charge at nanoscale interfaces between organics and inorganics could enable novel schemes for photoconversion. We have been exploring ligand exchange and functionalization for nanoscale semiconductors with tunable properties. Lead chalcogenide quantum dots (QDs) have near-infrared band gaps that are ideal for solar harvesting and can be tuned to resonance with molecular ligands that harbor triplet excitons. Photoexcitation of either ligand or QD sets in motion energy transfer processes that have kinetics defined by nanocrystal band gap, triplet energy, and the arrangement and distance of molecules on the QD surface. Energy or charge transfer on a ps timescale can inhibit production of triplets by singlet fission, but strategies for enhancing the singlet fission rate or isolating the molecules from direct contact with the QD surface are being studied to circumvent this problem. In CsPbBr3 nanocrystals, ligand exchange with pentacene derivatives produces complementary absorption spectra for organic and inorganic components. Excitation of the strongly absorbing nanocrystal leads to energy transfer to the pentacene singlet, which undergoes fast singlet fission. The yield of triplets compared to absorbed photons is greater 100%, and the triplet lifetime is into the microsecond time range. Further dependence on the preparation conditions of the system (e.g., solution vs. film) is being explored. Other investigations have involved incorporating specialized molecules for attachment to transition metal dichalcogenides (TMDCs). The focus is on producing a uniform layer of bound molecules on the planar surfaces and understanding the interplay between interfacial charge-transfer and other photophysical properties like singlet fission.

Near-Infrared Active Lead Chalcogenide Quantum Dots: Preparation, Post-Synthesis Ligand Exchange, and Applications in Solar Cells.

Quantum dots (QDs) of lead chalcogenides (e.g. PbS, PbSe, and PbTe) are attractive near-infrared (NIR) active materials that show great potential in a wide range of applications, such as, photovoltaics (PV), optoelectronics, sensors, and bio-electronics. The surface ligand plays an essential role in the production of QDs, post-synthesis modification, and their integration to practical applications. Therefore, it is critically important that the influence of surface ligands on the synthesis and properties of QDs is well understood for their applications in various devices. In this Review we elaborate the application of colloidal synthesis techniques for the preparation of lead chalcogenide based QDs. We specifically focus on the influence of surface ligands on the synthesis of QDs and their solution-phase ligand exchange. Given the importance of lead chalcogenide QDs as potential light harvesters, we also pay particular attention to the current progress of these QDs in photovoltaic applications.

Blinking Suppression in Highly Excited CdSe/ZnS Quantum Dots by Electron Transfer under Large Positive Gibbs (Free) Energy Change.

Semiconductor quantum dots with stable photoluminescence are necessary for next generation optoelectronic and photovoltaic devices. Photoluminescence intensity fluctuations of cadmium and lead chalcogenide quantum dots have been extensively investigated since the first observation of blinking in CdSe nanocrystals in 1996. In a quantum dot, blinking originates from stochastic photocharging, nonradiative Auger recombination, and delayed neutralization. So far, blinking is suppressed by defect passivation, electron transfer, and shell preparation, but without any deep insight into free energy change of electron transfer. We report real-time detection of significant blinking suppression for CdSe/ZnS quantum dots exposed to N, N-dimethylaniline, which is accompanied by a considerable increase in the time-averaged photoluminescence intensity of quantum dots. Although the Gibbs (free) energy change (Δ Get = +2.24 eV), which is estimated electrochemically and from density functional theory calculations, is unfavorable for electron transfer from N, N-dimethylaniline to a quantum dot in the minimally excited (band-edge) state, electron transfer is obvious when a quantum dot is highly excited. Nonetheless, Δ Get crosses from the positive to negative scale as the solvent dielectric constant exceeds 5, favoring electron transfer from N, N-dimethylaniline to a quantum dot excited to the band-edge state. Based on single-molecule photoluminescence and ensemble electron transfer studies, we assign blinking suppression to the transfer of an electron from N, N-dimethylaniline to the hot hole state of a quantum dot. In addition to blinking suppression by electron transfer, complete removal of blinking is limited by short-living OFF states induced by the negative trion.

Pulsed laser irradiation synthesis of lead selenide quantum dots from lead and selenium salts in various surfactants

HypothesisSemiconductor nanocrystals with response in the near-infrared region include lead chalcogenide quantum dots (QDs), and these materials show a strong quantum confinement effect due to their large Bohr radius compared with the group II–VI QDs. In the present study, a facile synthesis of lead selenide (PbSe) QDs using a pulsed laser irradiation in liquid (PLIL) is presented. ExperimentsPbSe QDs were produced using a pulsed Nd:YAG laser (532 nm, 10 Hz, 7 ns) irradiation of the Pb and Se mixed precursor solutions using the following three surfactants at various concentrations: cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and polyvinylpyrrolidone (PVP). FindingsInterestingly, it was observed that the synthesis and particle size of PbSe QDs obtained via the PLIL technique were strongly influenced by the type and concentration of the surfactants and by the varying wavelength and power of the laser. A secondary irradiation of the prepared sample in PVP solution resulted in the formation of rock salt crystalline PbSe QDs that were approximately 6.83 nm in size.

Bright Colloidal PbS Nanoribbons

Colloidal lead sulfide (PbS) nanoribbons are synthesized using organometallic precursors with chloroalkane cosolvents. The few-atom-thick nanoribbons have a typical width 20 nm and a length more than 50 nm. Different from a nanosheet where the quantum confinement energy is mainly determined by the thickness, the narrow width of the nanoribbon has an additional contribution to the increase of energy gap. In contrast to nanosheets, the nanoribbons are much brighter. At room temperatures, well-passivated nanoribbons have achieved more than 30% photoluminescence quantum yield in the infrared spectrum, competing with the well-developed colloidal lead chalcogenide quantum dots of the similar energy gap.

Ligand-induced symmetry breaking, size and morphology in colloidal lead sulfide QDs: from classic to thiourea precursors

Colloidal lead chalcogenide quantum dots (CQDs) exhibit promising optoelectronic properties for applications in solar cell devices and as thermoelectrics. Herein, we report and discuss a ferroelectric structural distortion, at the picometer scale resolution, in PbS CQDs prepared using both classic and new synthetic pathways. The investigation was performed using synchrotron X-ray total scattering data and advanced methods of analysis that rely on a homo-core-shell model and evaluate the atomic arrangement, stoichiometry, size and morphology of nanocrystals. The CQDs show comparable size-dependent relative elongation, up to 0.7 % of one body diagonal of the cubic rock-salt structure, which corresponds to a rhombohedral lattice deformation. The findings suggest a joint role for the oleate ligands (which induce surface tensile strain) and the Pb(II) lone pair as the driving forces of the deformation. Pb displacements along the [111] direction, which provoke a ferrolectric distortion related to the lattice change, fall in the 0.0 – 0.1 Å. Overall, the findings suggest the local nature of the metal off-centering, leading to different average displacements which depend on the synthetic conditions.

Luminescence and photoelectrochemical properties of size-selected aqueous copper-doped Ag-In-S quantum dots.

Ternary luminescent copper and silver indium sulfide quantum dots (QDs) can be an attractive alternative to cadmium and lead chalcogenide QDs. The optical properties of Cu–In–S and Ag–In–S (AIS) QDs vary over a broad range depending on the QD composition and size. The implementation of ternary QDs as emitters in bio-sensing applications can be boosted by the development of mild and reproducible syntheses directly in aqueous solutions as well as the methods of shifting the photoluminescence (PL) bands of such QDs as far as possible into the near IR spectral range. In the present work, the copper-doping of aqueous non-stoichiometric AIS QDs was found to result in a red shift of the PL band maximum from around 630 nm to ∼780 nm and PL quenching. The deposition of a ZnS shell results in PL intensity recovery with the highest quantum yield of 15%, with almost not change in the PL band position, opposite to the undoped AIS QDs. Size-selective precipitation using 2-propanol as a non-solvent allows discrimination of up to 9 fractions of Cu-doped AIS/ZnS QDs with the average sizes in the fractions varying from around 3 to 2 nm and smaller and with reasonably the same composition irrespective of the QD size. The decrease of the average QD size results in a blue PL shift yielding a series of bright luminophors with the emission color varies from deep-red to bluish-green and the PL efficiency increases from 11% for the first fraction to up to 58% for the smallest Cu-doped AIS/ZnS QDs. The rate constant of the radiative recombination of the size-selected Cu-doped AIS/ZnS QDs revealed a steady growth with the QD size decrease as a result of the size-dependent enhancement of the spatial exciton confinement. The copper doping was found to result in an enhancement of the photoelectrochemical activity of CAIS/ZnS QDs introduced as spectral sensitizers of mesoporous titania photoanodes of liquid-junction solar cells.

Oxygen aided photoresponse enhancement of air-stable PbSe quantum dot based photoconductors

The recent success of the surface passivation of lead chalcogenide quantum dots for enhancing ambient stability offers further investigation of surface states in air. Here we studied the photoresponses due to surface passivation and oxygen, thus providing the relations of surface states and the photoresponse of PbSe QD films by monitoring the changes in QD film with air exposure. A dramatic near-infrared photoresponse was observed when the PbSe film stabilized through surface passivation was exposed to air. As a result of estimating the density of trap states from the FET characteristics, it was confirmed that the iodide-passivated PbSe film exposed to the air had more trap states than the nitrogen atmosphere. The increase of the trap state due to oxygen adsorption led to the increase of the trap-captured photogenerated electrons, which increased the photoconduction gain. Even though some trap states have a negative impact on device parameters such as charge mobility and response time, controlling the oxygen-related trap states on QD surface is expected to enhance the photoresponsivity on the QD-based photoconductive films without performance degradation in air.

Phase-Transfer Ligand Exchange of Lead Chalcogenide Quantum Dots for Direct Deposition of Thick, Highly Conductive Films.

The use of semiconductor nanocrystal quantum dots (QDs) in optoelectronic devices typically requires postsynthetic chemical surface treatments to enhance electronic coupling between QDs and allow for efficient charge transport in QD films. Despite their importance in solar cells and infrared (IR) light-emitting diodes and photodetectors, advances in these chemical treatments for lead chalcogenide (PbE; E = S, Se, Te) QDs have lagged behind those of, for instance, II-VI semiconductor QDs. Here, we introduce a method for fast and effective ligand exchange for PbE QDs in solution, resulting in QDs completely passivated by a wide range of small anionic ligands. Due to electrostatic stabilization, these QDs are readily dispersible in polar solvents, in which they form highly concentrated solutions that remain stable for months. QDs of all three Pb chalcogenides retain their photoluminescence, allowing for a detailed study of the effect of the surface ionic double layer on electronic passivation of QD surfaces, which we find can be explained using the hard/soft acid-base theory. Importantly, we prepare highly conductive films of PbS, PbSe, and PbTe QDs by directly casting from solution without further chemical treatment, as determined by field-effect transistor measurements. This method allows for precise control over the surface chemistry, and therefore the transport properties of deposited films. It also permits single-step deposition of films of unprecedented thickness via continuous processing techniques, as we demonstrate by preparing a dense, smooth, 5.3-μm-thick PbSe QD film via doctor-blading. As such, it offers important advantages over laborious layer-by-layer methods for solar cells and photodetectors, while opening the door to new possibilities in ionizing-radiation detectors.

Role of Nd3+ ions on the nucleation and growth of PbS quantum dots (QDs) in silicate glasses

AbstractThe influence of Nd2O3 addition on the precipitation kinetics of lead chalcogenide (PbS) quantum dots (QDs) in silicate glasses was investigated. Energy dispersive X‐ray spectroscopy (EDS) indicated that the Nd3+ ions are preferentially located inside the PbS QDs rather than in the glass matrix. Changes in diameter (D) of PbS QDs exhibited smaller time dependencies (i.e., D≈t0.270‐0.286) than that predicted by the classical Lifshitz–Slyozov–Wagner (LSW) theory. This is due to the limited concentrations of Pb2+ and S2− ions and the large diffusion distance inside the glass matrix. In addition, extended X‐ray absorption fine structure (EXAFS) results indicated that the formation of PbS QDs was retarded due to the presence of Nd2O3 in the glasses, as the large NdOx polyhedra interrupt the diffusion of Pb2+ and S2− ions. We believe that these Nd3+ ions are primarily located in PbS QDs in the form of Nd–O clusters, and that the PbS QDs are built on top of these clusters.